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Current status of quark search experiments
CURRENT STATUS OF QUARK SEARCH EXPERIMENTS L. LYONS Nuclear Physics Laboratory Oxford U.K.
Abstrae¢--The quark model is successful in the fields of hadronic spectroscopy and high energy scattering processes, but the question of whether free quarks exist remains of fundamental importance. The experimental situation was reviewed in 1977 by Jones (11 who concluded that there was little evidence for free quarks. Developments since then are presented, with special emphasis on whether experiments claiming positive results are compatible with others seeing no effect. Some suggestions for the future are made.
CONTENTS 157 I60 161 162 163 163 164 165 165 166
1. 2. 3. 4. 5.
INTRODUCTION THE LA RUE EXPERIMENT YOCK EXPERIMENT OTHER RECENT EXPERIMENTS ARE DIFFERENT EXPERIMENTSCONSISTENT.9 5.1. Modal 1 5.2. Model 2 5~3. M,odel 3 6. CONCLUS[QNS, REFERENCESAND NOTES
t. I N T R O D U C T I O N The success of the quark model in the fields of hadronic spectroscopy (<~ and various aspects of scattering phenomenology (42~ has provided the impetus for the search for free quarks in a wide range of experimental situations. The diversity of methods used and potential sources investigated are summarised in Fig. 1, The lack of success of the majority of these experiments has given rise to the concept of confinement--that the interaction between quarks is strong and/or so iong ranged such that they cannot be separated from each other. If this is correct and confinement is absolute, then of course quarks wiI1 never appear except inside hadrons, and hence the search for free quarks would be doomed to failure. Since, however, no-one has yet even demonstrated that, for example, confinement is a necessary consequence of quantum chromodynamics (QCD), the search for fractional charge or other quark signatures continues. The situation concerning searches for quarks at accelerators, in cosmic rays, iv_ various 157
158
L. Lyons
l ao~rk searchesI Sectrch for already
l Produced in interactions I
existing quarks i
I From eisewhere! Niobium {G S, 9,/+3} eg. Tungsten(7,B,9,15} Iron {9,16,39} Ocean sed meat Oysters
eg. Cosmic ray 1 primaries 112) Solar spectrum i Noon rock
Meteorites
i
Lava Etc, Fig. 1. Summary of quark search techniques. Reference numbers are to recent articles listed in Table 1.
types of spectroscopic experiments and in modified Millikan oil drop searches was comprehensively reviewed by Jones (1) in 1977, with the conclusion that there was then little experimental evidence in favour of quarks. Here we update that review with the more recent experimental information; the new results are summarised in T a N e 1. Table 2 contains some of the suggestions advanced to explain why, even if quarks are not confined, most experiments to date have failed to detect them. There is one correction to the Jones review. The experiment of Hillas and Cranshaw (2) set an upper limit on the net charge of argon and of nitrogen atoms (3) by measuring the change in charge of a cylinder of high pressure gas as the gas was released. R is claimed that their result of a charge change of less than ~ 105 electronic charges can subsequently be re-interpreted as determining the quark/r~ucleon ratio to be lower than i0 .22 (although the derivation of this number from the experimental results is dubious). The experimer~t, however, included an ion trap at the outlet of the cylinder; this would presumably have retained any free quarks, which would thus n o t have contributed to the charge change during the gas effluxo Also an arbitrarily 1argo concentration of positive quarks coutd have had its charge largeiy neutralised by an appropriate admixture of electrons and/or negatively charged quarks, and again escape detection. Thus this experiment cannot be used to set an experimental limit on quark concentrations. The removal of this number is significant in that it was based on one of the few experiments which did not involve uncertain quark enrichment and transportation procedures, and which gave an upper limit on the quark concentration below the density estimated from the positive result of the La Rue experiment (see Table i and Section 2). Simitar remarks apply to earlier and ~ater versions of this type of experiment. We now discuss some of the experimer~ts listed ia Table 1~ starting with the two which claim positive results.
159
Current Status of Quark Search Experiments Table 1. Recent experimental searches for fractional charges Reference
Source
Bland (s) Boyd (71
Tungsten particles Tungsten ions
Putt(is) La Rue (4,s'a3/
Tungsten particles Niobium spheres
Marinelli (16'39) Steel spheres Schiffer (9)
Niobium, iron and tungsten
Yock (12'14~
Cosmic rays with fl < 0.59 400 GeV/c p + Cu at large Pr
Antreasyan/~Si
Method (a) Millikan Van der Graafas spectrometer Millikan Superconducting levitation electrometer Magnetic levitation electrometer Heated filament, accelerating voltage Pulse height + T O F Paise height + magnetic spectrometer
Cutts (1v)
400 GeV/c p + Be
TOF, C, calorimeter and magnetic spectrometer
Basile{~9)
pp at ,,/s = 62 and 52GeV
Pulse height + T O F
Bussiere( 4 ° ) Basile (2°)
200-240 GeV/c p N at 0 ° vN
TOF, C, Pulse height and magnetic spectrom, Pulse height + T O F
Morrison (37)
vN
ionisation
Basile ~4~)
vN and ,7N
Streamer chamber, T O F + pulse height
e+e at ,j's ~ 30 GeV
Pulse height+magnetic spectrometer
ADE (21~
Limit/b!
Comments
Q / N < 5 10 -~s Q / N < 5 10 ,~5 Q / N < 5 10-15 Q / N ~ 10 ao
Positive signal. See Section 2 Q / N < i0-21 After allowing for magneto-electric effect Q / N < 10- 21 for ci Uncertain assumptions concerning quark properties 0 ~ 3.10-9 Positive signal but see Section 3 No candidate in For [q[ = e/3, 5.104 hadrons % < 10 9cr~at PT of 2 GeV/c For Iq[ = 2e/3, a q < 10 6a~at Pr of 4 GeV/c No candidate in Sensitive to negatives 10 ~1 hadrons with q >~ 2e/3 a_~d Mq = 4-10GeV Q/charged particle Sensitive to fi > 0.i, ratio < 5.10 1! Iql = e/3 and (at 62GeV) and M < 26 GeV. Assumes < 2.10 9 (at 52GeV) ( P r ) ~ 0.4GeV/c %/or= ~, 10 9 Iq] = 2e/3. Mq < 6 GeV p = 6 40 GeV/c < 5.10 3 quarks per interaction < 3 10 a quarks per For Iql = e/3 interaction < 3.10- 5 q/int for v Correction for thick < 6.10- 5 q/int for v target uses aq = ax/3 for quarks not fl > 0.4 for ]ql = e/3 within hadronic jets fi > 0.8 for Iq[ = 2e/3 R < ~0.01 for Kinematic cuts require q = +_2e/'3, for model dependent qc7 + anything or correction. forq~i M < 12GeV
(~/TOF = time of flight; C = Cerenkov. (b)Q/N = quarks/nucleon; ~b = quark/cm2/sec/sterad; R = ratio of quark cross-section to that for t l ' # production. Table 2. Conjectures concerning absence of observed free quarks Reference QCD J. Orear (33)
Conjecture
Conclusion
Confinement is absolute
Give up looking for free quarks ?
Quarks have large interaction a, and hence interact and/or recombine before being detected
e +e - ~ qq at colliding beam accelerators, or in early Universe
A. Zichichi (341
Quarks produced only in primary interactions with large Pz
Look for q production in v N interactions
M. J. Longo (28)
Quarks can exist in nuclei but not free
Look for quarks in fission fragments produced by high energy interactions in large nuclei
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L. Lyons 2. T H E LA R U E E X P E R I M E N T
This experiment (4's'43~ measures the charge on small niobium spheres (of mass ~ 90 gg each) which are levitated in a superconducting magnet, by observing the oscillations produced with an alternating voltage. In the first publication, 8 balls are reported as being examined, of which two (balls 3 and 6) had charges consistent with - e / 3 and + e/3. [n the second, more measurements were made on these spheres and on an extra one; the last (ball 9) was also found to be consistent with a charge of + e/3. The report to the Madison Conference (~3~ records a total (including the earlier results) of 39 measurements on 13 balls. Of these, t3 measurements on 5 different balls observe fraction charges consistent with ± e/3. The i3 independent measurements on ball 6 include 9 observed changes of charge by amounts _+e/3 (or ~2e/3, which are of course indistinguishable from _+e/3). All 39 measurements are consistent (at the ~ 2 standard deviation level) with values of 0 or ± e/3 for the residual charge. Several features of this experiment are worthy of comment. (i) A measurement extending over several hours can achieve an accuracy of ~0.01e. A typical run occupies several days, with the charge changing by _+e every ~ 20 minutes. The final accuracy for a measurement of the residual charge on a ball is quoted typically as ±0.02< although corrections of < e have to be applied to the measurements in order to deduce the true residual charge. It is claimed that these corrections are known to better than 0.05e. The effect of these corrections is less important in the case of repeated measurements being used to obtain the change in residual charge on a single ball. (ii) Of the nine spheres tested in Refs. 4 and 5, 4 had been heat treated on a tungsten substrate prior to measurement; the 3 spheres having fractional charge measurements came from this subsample. (The third publication (43~ makes no mention of the nature of the heat treatment used on the rest of the spheres.) tf the fractional charges originate from the tungsten layer which coats the nickel spheres, the Q/N (quarh/nucleon) concentration in the tungsten would have been considerably higher than the 10-20 deduced for the spheres as a whole. Thus a layer of monatomic thickness completely coating the 4 spheres contains a factor ~ 2 . i 0 s less nucleons than the spheres, and Q/N would be increased accordingl),36) Boyd, (7~ Bland, (s) Putt (~s) and Schiffer (9> have searched for quarks in fungsten. The first three experiments set upper limits that are not good enough to rule out the possibility that the La Rue quarks are in a monatomic layer of tungsten; Schiffer's limit of Q/N < 10 -2a for quarks of charge + e/3 is more stringent, but because of the experimental techniques involved, it is necessary to make uncertain assumptions about the physical and chemical properties of quarks in order to deduce this limit. However, even if direct experimental tests are not inconsistent with the quarks coming from the tungsten, the resulting large quark concentration would be much larger than direct limits set in other materials, and may be difficult to produce in any plausible model. (iii) The residual charges on balls 3, 6 and 11 changed by ± e/3 during the course of the measurements; in the case of ball 6 this happened no less than 9 times. Orear (~°~ points out that, to explain a ball acquiring a fractional charge in this way, it is more plausible to assume that it started with several fractional charges which it Iost in stages, rather than that it picked up a quark during the course of the experiment. tf ball 6 (and perhaps the others too) did indeed start with several quarks which were progressively removed during the course of the experiment, we are again led to the
Current Status of Quark Search Experiments
161
conclusion that the Q/N ratio could be larger than ~ 10 2o, which was derived by assuming :hat there were 5 quarks among the t3 balls. Jones (1.) points out tha~ if quarks can be lost so readily from these spheres, there may be a problem with other concentration-type experiments which set low upper limits on the {2/N ratio. These could be reconciled with tke larger Q/N of La R'ee by assuming ':hat quarks interact with nuclei through a relativeiy long range force, a~_~d he:~ce are more difficult to dislodge from matter than had beer~ assumed. This idea seems inconsistent with the relatively frequent charge changes observed by La Rue. (iv) Marinelli and Morpurgo (39) explain how an appare:~t magneto-electric effect can mimic fractional charges--see Section 4(iii) below. La Rue e t a l . claim that tiqis phenomenon cannot be relevant for their results as the measured residual charges do not spread uniformly between +_e/2, but are alI consistent wkh 0 or + e/3. Further measurements will nevertheless be undertaken to confirm that this effect is unimportan': for their apparatus. (v) Orear (~°) surmises that in the early universe, charge exchange processes off protons would convert negatively charged quarks into positive ones. Then if the fractional charges observed by La Rue are due to quarks which were produced in the very early universe, the observed charge changes on the spEeres require the presence of both u and d quarks. The stability against decay of both types of quarks would imp!y that the mass difference be'~ween up and down quarks is less than the electron mass. ~n view of the extreme significance of the result and the apparent ease of producing spheres with fractional charge, it is of the greatest interest to ensure tha: magneto-electric. surface effects are unimportant, to clarify the role of the tungsten substrate, and to pin down the possible range of Q/N values. ~t would also be worthwhile for this experiment to be repeated by another group with different apparatus, :o help eliminate ti~e possibiD:y of the effect being instrumental. Another possibility is to try to transfer ~he fractional charge frorc_ one of the Stanford niobium bails to a steel one that could be used in the apparatus of Marinelli. 3. YOCK E X P E R I M E N T The other published result claiming a positive effect is due to Yock. (la~ He determines the velocity, charge and mass of cosmic ray particles by measuring ti~eir time of flight over 2 metres, and their pulse heights in 3 scintillators before an absorber and in 3 more after it. tn 4000 hours running, 23 tracks were observed which were interpreted as having velocities less than 0.59c and masses greater than ---2 Mp. Of these, 2 gave pulse heights corresponding to a charge [ql ~ 2e/3 (giver. their observed vetockies) and M > 4 Mp, and one gave ]q[ ~ e/3 and M > 20Mp. If these are quarks, they correspond to a flux (13~ of 3 10-9 quarks/(cm 2 sec sterad). On the other hand, the relatively modest apparatus 5as very little experimental redundancy built in~:o it. Thus, for example, t~'~e contribution of multiple tracks passing through the apparatus and producing spuriously long times of flight (and hence apparently low velocities) is not clearly ruled out. Indeed these 3 candidates do have possible accompanying tracks which make their interpretation dubious. Zn 2 subsequent runs of 4000 hours each, (14~Yock finds that if he rejects candidates which have possible accompanying tracks, there are no more examples of fractionally charged
162
L. Lyons
particles. He does, however, have 3 new candidates in each run with mass measured to be 4l~/lp (with an estimated error of ~ ½Mp) but with integral charge. Yock himsdf ack:.~owledges ~14~ that, in order to co~firm this effect, it would be desirable to repeat his experiment with a more sophisticated apparatus. 4. O T H E R R E C E N T E X P E R I M E N T S We comment briefly on some of the other recent experiments appearing in Table t. (i) La Rue observed fractional charges oa niobium balls. Schiffer ~9) '~es~ed a sample of niobium filament for the presence of quarks of charge +e/3. His limit of Q/N ~ 10 2~, while lower than the La Rue value, depends on uncertain assumptions about the p'~qysics and chemistry of such quarks. (ii) The La Rue positive results could be associated with the tungsten sufstrate, in which case Q/N is muc5 larger, per~aaps of order 10 is. Schiffer's limit on Q/N for tungsten is < 10 -21, again subject to uncertain assumptions. Bland ~s~ and ?~:tt ~5~ ~dse more direc~ methods of measurement, and together with Boyd (v~nil have limits of < 5 ~0 ~s (iiil The other stab!e matter search uses steel spheres. (.6'39'44~ tni~ial measurements (~6~ gave a fairly uniform distribution of residual charges between ±e/2. This was later (39~ ascribed to a surface effect involving a magneto-eiectric force which varied from ball to ball, and also drifted with time. Assuming that this was the cause of the observed uniform spectrum of residual charge, Marinelli and Morpurgo were abie to measure its effect, and correct for iL They then measured 47 spheres and found that all had a residual charge co~asistent with zero (within experimental errors of O.05e-O. le). This then sets Q/N < 10- 2 ~. This is below even the Iowes': estimate of Q/N for ~he La Rue experiment, albeit in a different material. (iv-) Accelerator experiments !ooking ~or the production of fractionally charged particles have been run at small momenturn transfer, (tv'4°l at large mo~aentum transfer, (~s~ at the largest available energies, (.9) in neutrino beams (2°'37"4s) and in e+e collisions. (2~ No positive effect has been see~. (v) Some of the null experimen;s ~.7'z~'4°~ of (iv) above were also sensitive to integrally charged particles of unusual mass. Other specific searches for such integraliy charged particles have also been performed. ThL~.s VidaI e t a I . (4s~ have looked at secondaries produced at ~3'° and at approxima'~ely zero c.m.s, longitudinal m o m e n t u m in 400 GeV/c pN interactions; among 5.10 *° ligi-~t particles, 3 track signals (of mass ~ 5 GeV) were observed, and considered as background. G o o d m a n e t a l . {46~ investigated cosmic ray showers followed by hadrons; 3 events with hadrons delayed by ~ 3 0 n s e c s could correspond to a flux of .-4.10 ~* particles/(cm 2 secsterad) and mass >/-5GeV. Bhat et aZ. (47) also observe 3 events with tracks at time delays of ~ 30nsecs behind the main cosmic ray shower; tfese are said to correspond to M ~> 10 GeV. It is to be remembered tha': Yock also claimed 3 non-relativistic tracks of integral charge and mass ~ 4 GeV in each of his 2 lates~ runs. Finally, of the experiments in (iv), Bussiere etal. <4°) observed 3 signals corresponding to integrally charged particles of n:ass ~ 4.4 GeV in one of their runs, but this effect could v_ot be reproduced in subsequent data. tt appears that in sensitive search experimen:s for heavy particles of uni~ charge, there are eventually backgrounds which are hard to remove. It does seem highly desirable, 5owever,
Curren': Status of Quark Search Experiments
163
that an experiment should be performed to provide a dearer answer to the question of whether there are interesting, new, relatively stable particles of integral charge and heavier ~:han the triton. (vi) Electron positron experiments may be the most significant in that the cross section for quark pair production is ca!culable; relative to that for/x +/~- pairs it should be ~ 3e~. This estimate, however, includes events where the produced quarks ere_it hadrons and in general subsequently annihilate rather than appearing as free quarks, i.e. the QCD (almost) confining interaction reduces the cross section for the production of free quarks toy an uncertain factor. (22'3s) In addition, the quarks may interact in the beam-pipe, and the current experiment (2~1 detects possible quarks only in restricted momentum ranges, and is sensitive only to quarks of charge ± 2e/3 (wi,~ileperhaps it is those of Iql = e/3 which are the stable ones}. It zhus seems desirable to try to achieve greater sensitivity in an experiment of this ~ype.
5. ARE D W F E R E N T EXPER][MENTS C O N S I S T E N T ? The majority of experiments to date fail to detect any signal of quarks, the only recent exceptions being those of La Rud ~'s'43/and of Yock. ~2/As already noted, the latter result really needs to be confirmed in a more sophisticated experimental set-up, and in the latest runs, the anomalous signals do not have fractional charge. As far as the La Rue result is concerned, we can either attempt to explain it away as some form of experimental bias or statistical fluctuation, or e!se accept it at face value (and either assume that the fractional charges have nothing to do with quarks, or give up the idea of absolute confir.ement). In the latter case we should see who'&or the La Rue effect can be reconciied with "the many other negative results. The oMy other search in both niobium and tungsten is that of Schiffer.(9~ ~f his assumptions concerning the physical and chemical behaviour of quarks in matter are correct, his limits for J quarks are lower than the La Rue resuk. On the other hand, if Schiffer's assumptions are incorrect or if quarks other than J are involved in the La Rue experimer& the only directly comparable measurements refer to tungsten, (7'8' ~s~ and the limits there do not necessarily contradict the possible quark density in tungsten required to give the La Rue resuk. In order to compare the La Rue positive result with experiments in other materials or in cosmic rays, it is unfortunately necessary to resort to models for the source of quarks, with the inherent uncertainty that this entails (see Table 3). Models 1 and 2, however, do not necessarily include detailed predictions concerning the history of q~.arks from the time of their production until the present day. The physics and/or chemistry of quarks could thus result in their being concentrated in regions of space that do not include the Earth (relevant for model 2), or in oniy specific elements or minerals ~5°/ (relevant for models 1 and 2). Thus while it is simple to assume that if free quarks exist, they pervade all matter at approximately the same density, this may be grossly incorrect.
Apart ~rom the Yock experiment, there have been no significant claims for quarks in cosmic ray experiments. The best limits on the quark flux are in the region of
t64
L. L y o n s Table 3. (a) Models of quark production
Model
Problems
t. Quarks on Earth come solely from cosmic rays
Except for Yock, no experiment sees quarks in cosmic rays. Limit on quark flux too smalI to give La Rue's Q/N
2. Quarks produced in early Universe
3. Cosmic rays produce fission fragments conraining quarks
Advantages
Test
Depth of quark distribution in Earth's crust. Quark distribution among chemical substances. Constancy of cosmic ray flux over Earth's lifetime Compatible w[th non-observation of quarks in cosmic rays, and in hadronic reactions at accelerators
La Rue effect, Jf quarks come from tungsten substrate
Uncertainties
Quark distribution in Universe and within substances on Earth
Does not produce free quarks outside Jargish nuclei
e +e- ~ free quarks at accelerators
Produce quarked fission fragments at accelerators
(b) Compatibility of negative results o; previous quark searches with the La Rue experiment (via Models of Quark Production) Experimen't type
Typical reference
Typical limit
Cosmic ray searches
Cox (35~
~ < 10 -1~)
Concentration experiments
Stevens(36)
Q/N < ~0 zz
Direct Q/N Hadronic collisions
Marine]]i<39) Cutts(~ v/
Q/N < 10 21
vN interactions
Basile('.8)
e+e coI1isions
JADE ('"~/
<3A0 "~quarks per interaction R < 0.01
See Table l
Comments 6) too low to produce Q/N - 10 a0
throughout Earth's surface Compatible with Q/N in cosmic rays - La Rue's Q/N Limits depend on very uncertain assumptions concerning physical and chemical properties of qLiarks For steel. Allows for magneto-electric effect Quarks not produced because of Orear or Zichichi explanations (see Table 2) Maybe not sensitive enough, and/or Mq too large. Quarks may be reabsorbed in target Not sensitive enough and/or mass range not high enough
] 0 - ~ 1 q u a r k s / s q , c m / s e c / s t e r a d i a n . If w e a s s u m e t h a t t h i s is t h e o n l y s o u r c e o f q u a r k s i n material on the earth and that quarks are then distributed uniformly among the earth's s u r f a c e i'nateriais, t h i s l i m i t is a l r e a d y t o o s m a l l t o give t h e L a R u e q u a r k d e n s i t y / s 3 ) If, h o w e v e r , t h e q u a r k s a p p e a r p r e f e r e n t i a l l y i n n i o b i u m a n d / o r tLtngsten, n o d i s c r e p a n c y exists.
5.2. 1MIedet 2 Theoretical estimates of the Q/'N ratio from cosmological sources vary widely; Z e i d o v i c h .(24) p r e d i c t e d ! 0 - 9 - i 0 18, F e i n b e r g ( 2 5 ) s u g g e s t e d ~ _ 0 - 1 ° - 1 0 - t3, w h i l e W a g o n e r a n d S t e i g m a n (26) o b t a i n a v a l u e of 10 2o for a q u a r k m a s s of 1 5 - 3 . 0 G e V . Since, h o w e v e r , t h e y m a y h a v e a c c u m u l a t e d p r e f e r e n t i a l l y in r e g i o n s o f s p a c e t h a t d o n o t (or a l t e r n a t i v e l y
Current Status of Quark Search Experiments
165
do) contain the earth, it is difficult to make use of terrestrial measurements to test these predictions. Furthermore, even within the Earth, it seems unlikety that the quark concentration is material independent, if we make that dubious assumption, then the Marinelli limit (2v~ is already incot'~sistent with the La Rue result. On the other hand, the upper limit on the cosmic ray quark flux, re-expressed as a limit on the qv.ark/nucleon ratio in cosmic rays, is easily compatible with a universal Q/N ratio of ~ 10 -2°, so negative resuF,:s frozn cosmic ray searches are no problem. It is atso possible to conceive of mechanisms which would result in quark production in the early Universe (e.g. by e+e -+ q~) but which would not necessarily result in significant free quark production in hadron collisions. This would then remove any problem with ::he negative results of all accelerator experiments; searches at colliding e+e - beams in principle could test this model, but as yet have :not reached sufficiently high sensitivity nor probably high enough quark masses. 5.3. Mede~ 3 It is possible that euarks inside nuclei exlsibit the Archimedes effect: their effective mass is small in large nuclei, but the mass of individual free quarks is very heavy. Thus while direct prod'action of a free q-:.ark pair would be strongly suppressed, cosmic rays could produce quark pairs inside large nuclei, which could then undergo fission to give fragments containing single quan~s.' ' Estimates of the effective mass of quarks required to give La Rue's observed abundance are very low (2 10 GeV), but are sensitive to the parameters of the model. (~~ Although this model does give the appearance of having been constructed to explain tke detection of quarks in niobium but not in lighter nuclev - . ,29/ the low e£r~dive ....... mass of q~:arks does mean that it should be possible to check the model by seeing whether such q':.arked nuclei can be produced at current accelerators. Finally, it is difficult to see how such a model could explain the La Rr.e result if the fractional charges turn out to be associated with the tungsten substrate, since tungsten is hardly a fission fragment. Thus we conclude that models exist which would enable us to reconcile the La Rue positive result and the many other negative ones, especially if we are prepared to accep': that qL-~arks may not be uniformly distribwted throughout the Universe and among the various chemical materials on Earth. 6. CONCLUSIONS Most experiments searching for quarks have produced negative results. The most significant exception is the La Rue experiment, although it is not clear whether the possible signal is associated with the niobium and/or the tungsten, and the quark concentration is not well delimited by the measurements to dab:e. The only experiment in direct cor.tradiction with the La Rue result is ihat of Schiller, which involves uncertain assv.mptions concerning quark behaviour in solids, and searches on!y for positive q'darks of charge e/3. The only experiment not involving such ass,eruptions and having a greater sensitivity than La Rue's is that of Marine]li and Morpurgo ; they see no fractional charges. Comparisons with other experiments involve recourse to zmodets; largely because of the uncertainties involved in quark history during cosmological and/or geophysical processes, no discrepancy is
!66
L. Lyons
necessary, (49~ although experiments in the near future shouid be able to test some aspects of these modeis. In view of the extreme significance of the possibiIity tt~at free quarks may exist, it is of the greatest interest to see the La Rue experiment repeated by another experimenta2 group and to attempt to transfer a fractional charge from niobium to steel, in order to measure it in the Marinelli apparatus. In the meantime, this fundamental question remains open. Perhaps the most exciting scenario would be for the existence of free quarks to be confirmed, and for quark confinement to be proved. (3°~ would like to thank the various experimental groups involved in quark searches who have communicated 11o me their unpunished results, and for useful correspondence and discussions.
REFERENCES AND NOTES 1. L. W. JONES, Rev. Mod. Phys. 49, 717 (1977). 2. A. M. HILLAS and T. E. CRANSHAW,Nature t84, 892 (1959). 3. The motivation for this experiment to look for a possible inequality of the magnitude of the charges on the proton and the e~ectron, as a possible mechanism for producing the expansion of the Universe as a Coulomb repulsion effect -derives from an article by Bondi and Lyttleton published in the Manchester Guardian (13th May 1959). 4. G.S. LA RuE, W. M. FAIRBANKand A. F. HEBARD, Phys. Roy. Lett. 38, 1011 (1977). 5. G. S. LA RUE, W. M. FA~RBANKand J. D. PHILLIPS, Phys. Roy. Lett. ,¢~2,142 (1979); and errata on p. 1019. 6. In fact the number of 2 10 - i s for Q/N wouId be oniy a lower limit. No matter how large the quark concentration, we would expect that not more than 32of any random sample of objects tested would exhibit fractional charge (since 3n quarks of charge e/3 would not exhibit fractional charge). Thus even the overall result of 5 balls out of a total of 13 exhibiting fractional charge, corresponding to Q/N in niobium of I0 20, is consistent at the 4 % level with an infinite value of Q/N. 7. R. N. BOYD, D. PLAGUE, A. L. MELLlSS~_NOSand E. SVGARBAKEI~,Phys. Rev. Lett. 40, 216 (i978). 8. R. BLAND, D. BOCOBO,M. EUBANK and J. ROYER, Phys. Roy. Lett. 39, 369 (1977). 9. J. P. SCEIFFFR, T. R. RENNER, D. S. GEMMELLand F. P. MOORING, Phys. Rev. t)117, 2241 (1978). i0. J. ©REAR, Phys. Rev. D20, i736 (!979). 11. L. W. JONES, Phys. Rev. Dt7, 1462 (1978). 12. P . C . M . YOCK, Phys. Rev. D18, 641 (1978). 13. Previous experiments had set limits on the flux of < 10-~° quarks/(cm x secsterad), although these other experiments searched for quarks moving at relativistic velocities and/or for quarks accompanied by a shower of other particles. ~n contrast, Yock's candidates would correspond to single quarks at low velocities. 14. P. C. M. YocI¢, Phys. Rev. D22, 61 (t980); "Further evidence for heavy particles in the Cosmic Radiation" (submitted to Phys. Rev. Lett.); and private communication. 15. G. D. P u r r and P. C. M. YOCK, Phys. Rev. DI7, 1466 (1978). 16. G. GALLINARO,M. MARINELLIand G. MORPUI~OO,4th EPS General Conf., p. 562 (1979). 17. D. CUTTS et al., Phys. Rev. Lett. 4~, 363 (1978). 18. D. ANTREASYANel al., Phys. Rev. Lett. 39, 513 (1977). 19. M. BASILEet al., Nuovo Cimento 4®A, 41 (1977); and 45A~ 171 (1978). 20. M. BASILEet al., Nuovo Cimento 45A, 281 (1978). 21. First Resultsfi'om JADE Collaboration, Presented by S. ORITO at F N A L Symposium on Lepton and Photon Interactions at High Energy 11979), p. 52; and W. BARTELet al., Z. Phys. C6, 295 (1980). 22. On the other hand there is the possibility of a slight enhancement of the cross-section from the production of heavier quarks (especially of charge _+2e/3) and which decay rapidly into the quarks of interest. 23. This comparison does, however, require an estimate of the thickness of the earth's crust in which quarks accumulate, assumptions concerning the constancy of the cosmic ray flux over cosmological times, etc. 24. Y. B. ZELDOVICH, L. B. OKUN and S. B. P~KELNER,Phys. Lett. 117,164 (1965). 25. E. L. FEINBERO, Soy. Phys. Usp. ~0, 256 (1967). 26. R. V. WAOONERand G. ST~IG~AN, Phys. gev. D20, 825 (1979). 27. Other experiments to date do not reach the required sensitivity, provided we dismiss all experiments utilising quark enrichment or quark transference techniques (as involving uncertain assumptions concerning quark behaviour); and the Hillas and Cranshaw experiment 12) (for the reason mentioned in the introduction). 28. M.J. LONGO, Univ. of Michigan, Report No. UM-HE-77-3 (unpublished).
Current Status of Quark Search Experiments
167
29. As mentioned in Note 27, however, most searches on lighter elements do not reach the sensitivity of the La Rue experiment. 30. This may not be self-contradictory. For example Terezawa °1) has mentioned the possibility that even if confinement is a consequence of QCD, it m a y no'L remain so when the electro-weak interactions are atso considered. Also Zee (32) has speculated that echo quarks, with fractional charge but which are colour singlets (and hence perhaps unconfined), may exist in addition ~o the conventiona! coiour triplet (and possibly confined) quarks. 31. H. TEP,EZAWA, Prog. Th. Phys. 60, 1521 (1978). 32. A. ZEE, Phys. Lett. g41K 91 (1979}. 33. J. OREAR, ghys. Rev. Dt8, 3504 (1978). 34. A. Z~CH~'CH~,European Physical Society Conference, p. 335 (1977). 35. A.J. Cox, W. T. BEAUCHAMP,T. BOWEN and R. M. KALBACK,Phys. Rev. D6, 1203 (1972). 36. C. M. STEVENS,J. P. SCmFFER and W. CHUPKA,Phys. Rev. Dig, 716 (I976). 37. Neutrino Hydrogen Interactions in BEBC, presented by D. R. O. MORR1SON~ X][X lmernationai Conference on High Energy Physics, Tokyo, p. 354 (1978). 38. A. DE RUJULA, R. C. GILES and R. L. JAFFE [Phys. Rev. D17, 285 (1978)~, estimate that the quark pair production cross-section in e+e - collisions is suppressed relative to e~a~ by a factor of ~107-1013 for q~ark masses of 5 GeV. 39. M. MARINELLI and G. MORPURGO (preprints, April i980), Ne~,, results in the search of quarks in matter by the magnetic levitation electrometer, and Confirmation of the electric neutrality of matter at the milligram level; A ne~*, independent determination of the magneto electric forces in the experimental search for quarks by the magnetic levitation electrometer (June 1980); Search for quarks in matter. Presented at the XX lnternational Conference on High Energy Physics (Madison, 1980). 40. A. BUSS~EREet al., NucL Phys. ~174, 1 (1980). 41. S. D. PROTOPOPESCUand N. P. SAMIOS,Ann. Rev. QfNucl. Particle Phys. 29, 339 (!979). 42. P. V. LANDSI-tOEF,Progress in Particle and Nuclear Physics, VoL 3, p. 1 (1980). 43. G. S. LA RUE, J. D. PIc5SLLIPSand W. M. FA1RBANK,Additional evidence for fi"aetional charges oje/3 in matter. Presented at the XX International Conferertce on High Energy Physics (Madison, 1980). 44. The development of levitation experiments is described by G. MOaPUROO, Acta Physica Austriaca, Suppl. XXL 5 (1979). 45. R. VITAL et al., Phys. Lett. 77I~, 344 (1978). 46. J. A. GOODMANet al., Phys. Rev. D19, 2572 (1979). 47. P. N. BHAT et al., Search for new particles among high energy hadrons in extensive air showers, Kyoto Cosmic Ray Conf., Vol. 13, 8 (1979). 48. M. BASILE et al., Results of the quark search experiment in high energy r interactions, Preprint (CERN/EP
80-169). 49. Morpurgo points out, however, that even if initially quarks were to congregate preferentiaily in niobium, the ease with which the La Rue spheres lose their charge does suggest that they should be fairly abundant in other materials as well. 50. K. S. LACKNER and G. ZWEIO,The chemistry offi'ee quarks, Preprint CALT-68-781 and submitted to X X Mternational Co@rence on High Energy Physics (Madison, 1980).
Abstrae¢--The quark model is successful in the fields of hadronic spectroscopy and high energy scattering processes, but the question of whether free quarks exist remains of fundamental importance. The experimental situation was reviewed in 1977 by Jones (11 who concluded that there was little evidence for free quarks. Developments since then are presented, with special emphasis on whether experiments claiming positive results are compatible with others seeing no effect. Some suggestions for the future are made.
CONTENTS 157 I60 161 162 163 163 164 165 165 166
1. 2. 3. 4. 5.
INTRODUCTION THE LA RUE EXPERIMENT YOCK EXPERIMENT OTHER RECENT EXPERIMENTS ARE DIFFERENT EXPERIMENTSCONSISTENT.9 5.1. Modal 1 5.2. Model 2 5~3. M,odel 3 6. CONCLUS[QNS, REFERENCESAND NOTES
t. I N T R O D U C T I O N The success of the quark model in the fields of hadronic spectroscopy (<~ and various aspects of scattering phenomenology (42~ has provided the impetus for the search for free quarks in a wide range of experimental situations. The diversity of methods used and potential sources investigated are summarised in Fig. 1, The lack of success of the majority of these experiments has given rise to the concept of confinement--that the interaction between quarks is strong and/or so iong ranged such that they cannot be separated from each other. If this is correct and confinement is absolute, then of course quarks wiI1 never appear except inside hadrons, and hence the search for free quarks would be doomed to failure. Since, however, no-one has yet even demonstrated that, for example, confinement is a necessary consequence of quantum chromodynamics (QCD), the search for fractional charge or other quark signatures continues. The situation concerning searches for quarks at accelerators, in cosmic rays, iv_ various 157
158
L. Lyons
l ao~rk searchesI Sectrch for already
l Produced in interactions I
existing quarks i
I From eisewhere! Niobium {G S, 9,/+3} eg. Tungsten(7,B,9,15} Iron {9,16,39} Ocean sed meat Oysters
eg. Cosmic ray 1 primaries 112) Solar spectrum i Noon rock
Meteorites
i
Lava Etc, Fig. 1. Summary of quark search techniques. Reference numbers are to recent articles listed in Table 1.
types of spectroscopic experiments and in modified Millikan oil drop searches was comprehensively reviewed by Jones (1) in 1977, with the conclusion that there was then little experimental evidence in favour of quarks. Here we update that review with the more recent experimental information; the new results are summarised in T a N e 1. Table 2 contains some of the suggestions advanced to explain why, even if quarks are not confined, most experiments to date have failed to detect them. There is one correction to the Jones review. The experiment of Hillas and Cranshaw (2) set an upper limit on the net charge of argon and of nitrogen atoms (3) by measuring the change in charge of a cylinder of high pressure gas as the gas was released. R is claimed that their result of a charge change of less than ~ 105 electronic charges can subsequently be re-interpreted as determining the quark/r~ucleon ratio to be lower than i0 .22 (although the derivation of this number from the experimental results is dubious). The experimer~t, however, included an ion trap at the outlet of the cylinder; this would presumably have retained any free quarks, which would thus n o t have contributed to the charge change during the gas effluxo Also an arbitrarily 1argo concentration of positive quarks coutd have had its charge largeiy neutralised by an appropriate admixture of electrons and/or negatively charged quarks, and again escape detection. Thus this experiment cannot be used to set an experimental limit on quark concentrations. The removal of this number is significant in that it was based on one of the few experiments which did not involve uncertain quark enrichment and transportation procedures, and which gave an upper limit on the quark concentration below the density estimated from the positive result of the La Rue experiment (see Table i and Section 2). Simitar remarks apply to earlier and ~ater versions of this type of experiment. We now discuss some of the experimer~ts listed ia Table 1~ starting with the two which claim positive results.
159
Current Status of Quark Search Experiments Table 1. Recent experimental searches for fractional charges Reference
Source
Bland (s) Boyd (71
Tungsten particles Tungsten ions
Putt(is) La Rue (4,s'a3/
Tungsten particles Niobium spheres
Marinelli (16'39) Steel spheres Schiffer (9)
Niobium, iron and tungsten
Yock (12'14~
Cosmic rays with fl < 0.59 400 GeV/c p + Cu at large Pr
Antreasyan/~Si
Method (a) Millikan Van der Graafas spectrometer Millikan Superconducting levitation electrometer Magnetic levitation electrometer Heated filament, accelerating voltage Pulse height + T O F Paise height + magnetic spectrometer
Cutts (1v)
400 GeV/c p + Be
TOF, C, calorimeter and magnetic spectrometer
Basile{~9)
pp at ,,/s = 62 and 52GeV
Pulse height + T O F
Bussiere( 4 ° ) Basile (2°)
200-240 GeV/c p N at 0 ° vN
TOF, C, Pulse height and magnetic spectrom, Pulse height + T O F
Morrison (37)
vN
ionisation
Basile ~4~)
vN and ,7N
Streamer chamber, T O F + pulse height
e+e at ,j's ~ 30 GeV
Pulse height+magnetic spectrometer
ADE (21~
Limit/b!
Comments
Q / N < 5 10 -~s Q / N < 5 10 ,~5 Q / N < 5 10-15 Q / N ~ 10 ao
Positive signal. See Section 2 Q / N < i0-21 After allowing for magneto-electric effect Q / N < 10- 21 for ci Uncertain assumptions concerning quark properties 0 ~ 3.10-9 Positive signal but see Section 3 No candidate in For [q[ = e/3, 5.104 hadrons % < 10 9cr~at PT of 2 GeV/c For Iq[ = 2e/3, a q < 10 6a~at Pr of 4 GeV/c No candidate in Sensitive to negatives 10 ~1 hadrons with q >~ 2e/3 a_~d Mq = 4-10GeV Q/charged particle Sensitive to fi > 0.i, ratio < 5.10 1! Iql = e/3 and (at 62GeV) and M < 26 GeV. Assumes < 2.10 9 (at 52GeV) ( P r ) ~ 0.4GeV/c %/or= ~, 10 9 Iq] = 2e/3. Mq < 6 GeV p = 6 40 GeV/c < 5.10 3 quarks per interaction < 3 10 a quarks per For Iql = e/3 interaction < 3.10- 5 q/int for v Correction for thick < 6.10- 5 q/int for v target uses aq = ax/3 for quarks not fl > 0.4 for ]ql = e/3 within hadronic jets fi > 0.8 for Iq[ = 2e/3 R < ~0.01 for Kinematic cuts require q = +_2e/'3, for model dependent qc7 + anything or correction. forq~i M < 12GeV
(~/TOF = time of flight; C = Cerenkov. (b)Q/N = quarks/nucleon; ~b = quark/cm2/sec/sterad; R = ratio of quark cross-section to that for t l ' # production. Table 2. Conjectures concerning absence of observed free quarks Reference QCD J. Orear (33)
Conjecture
Conclusion
Confinement is absolute
Give up looking for free quarks ?
Quarks have large interaction a, and hence interact and/or recombine before being detected
e +e - ~ qq at colliding beam accelerators, or in early Universe
A. Zichichi (341
Quarks produced only in primary interactions with large Pz
Look for q production in v N interactions
M. J. Longo (28)
Quarks can exist in nuclei but not free
Look for quarks in fission fragments produced by high energy interactions in large nuclei
160
L. Lyons 2. T H E LA R U E E X P E R I M E N T
This experiment (4's'43~ measures the charge on small niobium spheres (of mass ~ 90 gg each) which are levitated in a superconducting magnet, by observing the oscillations produced with an alternating voltage. In the first publication, 8 balls are reported as being examined, of which two (balls 3 and 6) had charges consistent with - e / 3 and + e/3. [n the second, more measurements were made on these spheres and on an extra one; the last (ball 9) was also found to be consistent with a charge of + e/3. The report to the Madison Conference (~3~ records a total (including the earlier results) of 39 measurements on 13 balls. Of these, t3 measurements on 5 different balls observe fraction charges consistent with ± e/3. The i3 independent measurements on ball 6 include 9 observed changes of charge by amounts _+e/3 (or ~2e/3, which are of course indistinguishable from _+e/3). All 39 measurements are consistent (at the ~ 2 standard deviation level) with values of 0 or ± e/3 for the residual charge. Several features of this experiment are worthy of comment. (i) A measurement extending over several hours can achieve an accuracy of ~0.01e. A typical run occupies several days, with the charge changing by _+e every ~ 20 minutes. The final accuracy for a measurement of the residual charge on a ball is quoted typically as ±0.02< although corrections of < e have to be applied to the measurements in order to deduce the true residual charge. It is claimed that these corrections are known to better than 0.05e. The effect of these corrections is less important in the case of repeated measurements being used to obtain the change in residual charge on a single ball. (ii) Of the nine spheres tested in Refs. 4 and 5, 4 had been heat treated on a tungsten substrate prior to measurement; the 3 spheres having fractional charge measurements came from this subsample. (The third publication (43~ makes no mention of the nature of the heat treatment used on the rest of the spheres.) tf the fractional charges originate from the tungsten layer which coats the nickel spheres, the Q/N (quarh/nucleon) concentration in the tungsten would have been considerably higher than the 10-20 deduced for the spheres as a whole. Thus a layer of monatomic thickness completely coating the 4 spheres contains a factor ~ 2 . i 0 s less nucleons than the spheres, and Q/N would be increased accordingl),36) Boyd, (7~ Bland, (s) Putt (~s) and Schiffer (9> have searched for quarks in fungsten. The first three experiments set upper limits that are not good enough to rule out the possibility that the La Rue quarks are in a monatomic layer of tungsten; Schiffer's limit of Q/N < 10 -2a for quarks of charge + e/3 is more stringent, but because of the experimental techniques involved, it is necessary to make uncertain assumptions about the physical and chemical properties of quarks in order to deduce this limit. However, even if direct experimental tests are not inconsistent with the quarks coming from the tungsten, the resulting large quark concentration would be much larger than direct limits set in other materials, and may be difficult to produce in any plausible model. (iii) The residual charges on balls 3, 6 and 11 changed by ± e/3 during the course of the measurements; in the case of ball 6 this happened no less than 9 times. Orear (~°~ points out that, to explain a ball acquiring a fractional charge in this way, it is more plausible to assume that it started with several fractional charges which it Iost in stages, rather than that it picked up a quark during the course of the experiment. tf ball 6 (and perhaps the others too) did indeed start with several quarks which were progressively removed during the course of the experiment, we are again led to the
Current Status of Quark Search Experiments
161
conclusion that the Q/N ratio could be larger than ~ 10 2o, which was derived by assuming :hat there were 5 quarks among the t3 balls. Jones (1.) points out tha~ if quarks can be lost so readily from these spheres, there may be a problem with other concentration-type experiments which set low upper limits on the {2/N ratio. These could be reconciled with tke larger Q/N of La R'ee by assuming ':hat quarks interact with nuclei through a relativeiy long range force, a~_~d he:~ce are more difficult to dislodge from matter than had beer~ assumed. This idea seems inconsistent with the relatively frequent charge changes observed by La Rue. (iv) Marinelli and Morpurgo (39) explain how an appare:~t magneto-electric effect can mimic fractional charges--see Section 4(iii) below. La Rue e t a l . claim that tiqis phenomenon cannot be relevant for their results as the measured residual charges do not spread uniformly between +_e/2, but are alI consistent wkh 0 or + e/3. Further measurements will nevertheless be undertaken to confirm that this effect is unimportan': for their apparatus. (v) Orear (~°) surmises that in the early universe, charge exchange processes off protons would convert negatively charged quarks into positive ones. Then if the fractional charges observed by La Rue are due to quarks which were produced in the very early universe, the observed charge changes on the spEeres require the presence of both u and d quarks. The stability against decay of both types of quarks would imp!y that the mass difference be'~ween up and down quarks is less than the electron mass. ~n view of the extreme significance of the result and the apparent ease of producing spheres with fractional charge, it is of the greatest interest to ensure tha: magneto-electric. surface effects are unimportant, to clarify the role of the tungsten substrate, and to pin down the possible range of Q/N values. ~t would also be worthwhile for this experiment to be repeated by another group with different apparatus, :o help eliminate ti~e possibiD:y of the effect being instrumental. Another possibility is to try to transfer ~he fractional charge frorc_ one of the Stanford niobium bails to a steel one that could be used in the apparatus of Marinelli. 3. YOCK E X P E R I M E N T The other published result claiming a positive effect is due to Yock. (la~ He determines the velocity, charge and mass of cosmic ray particles by measuring ti~eir time of flight over 2 metres, and their pulse heights in 3 scintillators before an absorber and in 3 more after it. tn 4000 hours running, 23 tracks were observed which were interpreted as having velocities less than 0.59c and masses greater than ---2 Mp. Of these, 2 gave pulse heights corresponding to a charge [ql ~ 2e/3 (giver. their observed vetockies) and M > 4 Mp, and one gave ]q[ ~ e/3 and M > 20Mp. If these are quarks, they correspond to a flux (13~ of 3 10-9 quarks/(cm 2 sec sterad). On the other hand, the relatively modest apparatus 5as very little experimental redundancy built in~:o it. Thus, for example, t~'~e contribution of multiple tracks passing through the apparatus and producing spuriously long times of flight (and hence apparently low velocities) is not clearly ruled out. Indeed these 3 candidates do have possible accompanying tracks which make their interpretation dubious. Zn 2 subsequent runs of 4000 hours each, (14~Yock finds that if he rejects candidates which have possible accompanying tracks, there are no more examples of fractionally charged
162
L. Lyons
particles. He does, however, have 3 new candidates in each run with mass measured to be 4l~/lp (with an estimated error of ~ ½Mp) but with integral charge. Yock himsdf ack:.~owledges ~14~ that, in order to co~firm this effect, it would be desirable to repeat his experiment with a more sophisticated apparatus. 4. O T H E R R E C E N T E X P E R I M E N T S We comment briefly on some of the other recent experiments appearing in Table t. (i) La Rue observed fractional charges oa niobium balls. Schiffer ~9) '~es~ed a sample of niobium filament for the presence of quarks of charge +e/3. His limit of Q/N ~ 10 2~, while lower than the La Rue value, depends on uncertain assumptions about the p'~qysics and chemistry of such quarks. (ii) The La Rue positive results could be associated with the tungsten sufstrate, in which case Q/N is muc5 larger, per~aaps of order 10 is. Schiffer's limit on Q/N for tungsten is < 10 -21, again subject to uncertain assumptions. Bland ~s~ and ?~:tt ~5~ ~dse more direc~ methods of measurement, and together with Boyd (v~nil have limits of < 5 ~0 ~s (iiil The other stab!e matter search uses steel spheres. (.6'39'44~ tni~ial measurements (~6~ gave a fairly uniform distribution of residual charges between ±e/2. This was later (39~ ascribed to a surface effect involving a magneto-eiectric force which varied from ball to ball, and also drifted with time. Assuming that this was the cause of the observed uniform spectrum of residual charge, Marinelli and Morpurgo were abie to measure its effect, and correct for iL They then measured 47 spheres and found that all had a residual charge co~asistent with zero (within experimental errors of O.05e-O. le). This then sets Q/N < 10- 2 ~. This is below even the Iowes': estimate of Q/N for ~he La Rue experiment, albeit in a different material. (iv-) Accelerator experiments !ooking ~or the production of fractionally charged particles have been run at small momenturn transfer, (tv'4°l at large mo~aentum transfer, (~s~ at the largest available energies, (.9) in neutrino beams (2°'37"4s) and in e+e collisions. (2~ No positive effect has been see~. (v) Some of the null experimen;s ~.7'z~'4°~ of (iv) above were also sensitive to integrally charged particles of unusual mass. Other specific searches for such integraliy charged particles have also been performed. ThL~.s VidaI e t a I . (4s~ have looked at secondaries produced at ~3'° and at approxima'~ely zero c.m.s, longitudinal m o m e n t u m in 400 GeV/c pN interactions; among 5.10 *° ligi-~t particles, 3 track signals (of mass ~ 5 GeV) were observed, and considered as background. G o o d m a n e t a l . {46~ investigated cosmic ray showers followed by hadrons; 3 events with hadrons delayed by ~ 3 0 n s e c s could correspond to a flux of .-4.10 ~* particles/(cm 2 secsterad) and mass >/-5GeV. Bhat et aZ. (47) also observe 3 events with tracks at time delays of ~ 30nsecs behind the main cosmic ray shower; tfese are said to correspond to M ~> 10 GeV. It is to be remembered tha': Yock also claimed 3 non-relativistic tracks of integral charge and mass ~ 4 GeV in each of his 2 lates~ runs. Finally, of the experiments in (iv), Bussiere etal. <4°) observed 3 signals corresponding to integrally charged particles of n:ass ~ 4.4 GeV in one of their runs, but this effect could v_ot be reproduced in subsequent data. tt appears that in sensitive search experimen:s for heavy particles of uni~ charge, there are eventually backgrounds which are hard to remove. It does seem highly desirable, 5owever,
Curren': Status of Quark Search Experiments
163
that an experiment should be performed to provide a dearer answer to the question of whether there are interesting, new, relatively stable particles of integral charge and heavier ~:han the triton. (vi) Electron positron experiments may be the most significant in that the cross section for quark pair production is ca!culable; relative to that for/x +/~- pairs it should be ~ 3e~. This estimate, however, includes events where the produced quarks ere_it hadrons and in general subsequently annihilate rather than appearing as free quarks, i.e. the QCD (almost) confining interaction reduces the cross section for the production of free quarks toy an uncertain factor. (22'3s) In addition, the quarks may interact in the beam-pipe, and the current experiment (2~1 detects possible quarks only in restricted momentum ranges, and is sensitive only to quarks of charge ± 2e/3 (wi,~ileperhaps it is those of Iql = e/3 which are the stable ones}. It zhus seems desirable to try to achieve greater sensitivity in an experiment of this ~ype.
5. ARE D W F E R E N T EXPER][MENTS C O N S I S T E N T ? The majority of experiments to date fail to detect any signal of quarks, the only recent exceptions being those of La Rud ~'s'43/and of Yock. ~2/As already noted, the latter result really needs to be confirmed in a more sophisticated experimental set-up, and in the latest runs, the anomalous signals do not have fractional charge. As far as the La Rue result is concerned, we can either attempt to explain it away as some form of experimental bias or statistical fluctuation, or e!se accept it at face value (and either assume that the fractional charges have nothing to do with quarks, or give up the idea of absolute confir.ement). In the latter case we should see who'&or the La Rue effect can be reconciied with "the many other negative results. The oMy other search in both niobium and tungsten is that of Schiffer.(9~ ~f his assumptions concerning the physical and chemical behaviour of quarks in matter are correct, his limits for J quarks are lower than the La Rue resuk. On the other hand, if Schiffer's assumptions are incorrect or if quarks other than J are involved in the La Rue experimer& the only directly comparable measurements refer to tungsten, (7'8' ~s~ and the limits there do not necessarily contradict the possible quark density in tungsten required to give the La Rue resuk. In order to compare the La Rue positive result with experiments in other materials or in cosmic rays, it is unfortunately necessary to resort to models for the source of quarks, with the inherent uncertainty that this entails (see Table 3). Models 1 and 2, however, do not necessarily include detailed predictions concerning the history of q~.arks from the time of their production until the present day. The physics and/or chemistry of quarks could thus result in their being concentrated in regions of space that do not include the Earth (relevant for model 2), or in oniy specific elements or minerals ~5°/ (relevant for models 1 and 2). Thus while it is simple to assume that if free quarks exist, they pervade all matter at approximately the same density, this may be grossly incorrect.
Apart ~rom the Yock experiment, there have been no significant claims for quarks in cosmic ray experiments. The best limits on the quark flux are in the region of
t64
L. L y o n s Table 3. (a) Models of quark production
Model
Problems
t. Quarks on Earth come solely from cosmic rays
Except for Yock, no experiment sees quarks in cosmic rays. Limit on quark flux too smalI to give La Rue's Q/N
2. Quarks produced in early Universe
3. Cosmic rays produce fission fragments conraining quarks
Advantages
Test
Depth of quark distribution in Earth's crust. Quark distribution among chemical substances. Constancy of cosmic ray flux over Earth's lifetime Compatible w[th non-observation of quarks in cosmic rays, and in hadronic reactions at accelerators
La Rue effect, Jf quarks come from tungsten substrate
Uncertainties
Quark distribution in Universe and within substances on Earth
Does not produce free quarks outside Jargish nuclei
e +e- ~ free quarks at accelerators
Produce quarked fission fragments at accelerators
(b) Compatibility of negative results o; previous quark searches with the La Rue experiment (via Models of Quark Production) Experimen't type
Typical reference
Typical limit
Cosmic ray searches
Cox (35~
~ < 10 -1~)
Concentration experiments
Stevens(36)
Q/N < ~0 zz
Direct Q/N Hadronic collisions
Marine]]i<39) Cutts(~ v/
Q/N < 10 21
vN interactions
Basile('.8)
e+e coI1isions
JADE ('"~/
<3A0 "~quarks per interaction R < 0.01
See Table l
Comments 6) too low to produce Q/N - 10 a0
throughout Earth's surface Compatible with Q/N in cosmic rays - La Rue's Q/N Limits depend on very uncertain assumptions concerning physical and chemical properties of qLiarks For steel. Allows for magneto-electric effect Quarks not produced because of Orear or Zichichi explanations (see Table 2) Maybe not sensitive enough, and/or Mq too large. Quarks may be reabsorbed in target Not sensitive enough and/or mass range not high enough
] 0 - ~ 1 q u a r k s / s q , c m / s e c / s t e r a d i a n . If w e a s s u m e t h a t t h i s is t h e o n l y s o u r c e o f q u a r k s i n material on the earth and that quarks are then distributed uniformly among the earth's s u r f a c e i'nateriais, t h i s l i m i t is a l r e a d y t o o s m a l l t o give t h e L a R u e q u a r k d e n s i t y / s 3 ) If, h o w e v e r , t h e q u a r k s a p p e a r p r e f e r e n t i a l l y i n n i o b i u m a n d / o r tLtngsten, n o d i s c r e p a n c y exists.
5.2. 1MIedet 2 Theoretical estimates of the Q/'N ratio from cosmological sources vary widely; Z e i d o v i c h .(24) p r e d i c t e d ! 0 - 9 - i 0 18, F e i n b e r g ( 2 5 ) s u g g e s t e d ~ _ 0 - 1 ° - 1 0 - t3, w h i l e W a g o n e r a n d S t e i g m a n (26) o b t a i n a v a l u e of 10 2o for a q u a r k m a s s of 1 5 - 3 . 0 G e V . Since, h o w e v e r , t h e y m a y h a v e a c c u m u l a t e d p r e f e r e n t i a l l y in r e g i o n s o f s p a c e t h a t d o n o t (or a l t e r n a t i v e l y
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do) contain the earth, it is difficult to make use of terrestrial measurements to test these predictions. Furthermore, even within the Earth, it seems unlikety that the quark concentration is material independent, if we make that dubious assumption, then the Marinelli limit (2v~ is already incot'~sistent with the La Rue result. On the other hand, the upper limit on the cosmic ray quark flux, re-expressed as a limit on the qv.ark/nucleon ratio in cosmic rays, is easily compatible with a universal Q/N ratio of ~ 10 -2°, so negative resuF,:s frozn cosmic ray searches are no problem. It is atso possible to conceive of mechanisms which would result in quark production in the early Universe (e.g. by e+e -+ q~) but which would not necessarily result in significant free quark production in hadron collisions. This would then remove any problem with ::he negative results of all accelerator experiments; searches at colliding e+e - beams in principle could test this model, but as yet have :not reached sufficiently high sensitivity nor probably high enough quark masses. 5.3. Mede~ 3 It is possible that euarks inside nuclei exlsibit the Archimedes effect: their effective mass is small in large nuclei, but the mass of individual free quarks is very heavy. Thus while direct prod'action of a free q-:.ark pair would be strongly suppressed, cosmic rays could produce quark pairs inside large nuclei, which could then undergo fission to give fragments containing single quan~s.' ' Estimates of the effective mass of quarks required to give La Rue's observed abundance are very low (2 10 GeV), but are sensitive to the parameters of the model. (~~ Although this model does give the appearance of having been constructed to explain tke detection of quarks in niobium but not in lighter nuclev - . ,29/ the low e£r~dive ....... mass of q~:arks does mean that it should be possible to check the model by seeing whether such q':.arked nuclei can be produced at current accelerators. Finally, it is difficult to see how such a model could explain the La Rr.e result if the fractional charges turn out to be associated with the tungsten substrate, since tungsten is hardly a fission fragment. Thus we conclude that models exist which would enable us to reconcile the La Rue positive result and the many other negative ones, especially if we are prepared to accep': that qL-~arks may not be uniformly distribwted throughout the Universe and among the various chemical materials on Earth. 6. CONCLUSIONS Most experiments searching for quarks have produced negative results. The most significant exception is the La Rue experiment, although it is not clear whether the possible signal is associated with the niobium and/or the tungsten, and the quark concentration is not well delimited by the measurements to dab:e. The only experiment in direct cor.tradiction with the La Rue result is ihat of Schiller, which involves uncertain assv.mptions concerning quark behaviour in solids, and searches on!y for positive q'darks of charge e/3. The only experiment not involving such ass,eruptions and having a greater sensitivity than La Rue's is that of Marine]li and Morpurgo ; they see no fractional charges. Comparisons with other experiments involve recourse to zmodets; largely because of the uncertainties involved in quark history during cosmological and/or geophysical processes, no discrepancy is
!66
L. Lyons
necessary, (49~ although experiments in the near future shouid be able to test some aspects of these modeis. In view of the extreme significance of the possibiIity tt~at free quarks may exist, it is of the greatest interest to see the La Rue experiment repeated by another experimenta2 group and to attempt to transfer a fractional charge from niobium to steel, in order to measure it in the Marinelli apparatus. In the meantime, this fundamental question remains open. Perhaps the most exciting scenario would be for the existence of free quarks to be confirmed, and for quark confinement to be proved. (3°~ would like to thank the various experimental groups involved in quark searches who have communicated 11o me their unpunished results, and for useful correspondence and discussions.
REFERENCES AND NOTES 1. L. W. JONES, Rev. Mod. Phys. 49, 717 (1977). 2. A. M. HILLAS and T. E. CRANSHAW,Nature t84, 892 (1959). 3. The motivation for this experiment to look for a possible inequality of the magnitude of the charges on the proton and the e~ectron, as a possible mechanism for producing the expansion of the Universe as a Coulomb repulsion effect -derives from an article by Bondi and Lyttleton published in the Manchester Guardian (13th May 1959). 4. G.S. LA RuE, W. M. FAIRBANKand A. F. HEBARD, Phys. Roy. Lett. 38, 1011 (1977). 5. G. S. LA RUE, W. M. FA~RBANKand J. D. PHILLIPS, Phys. Roy. Lett. ,¢~2,142 (1979); and errata on p. 1019. 6. In fact the number of 2 10 - i s for Q/N wouId be oniy a lower limit. No matter how large the quark concentration, we would expect that not more than 32of any random sample of objects tested would exhibit fractional charge (since 3n quarks of charge e/3 would not exhibit fractional charge). Thus even the overall result of 5 balls out of a total of 13 exhibiting fractional charge, corresponding to Q/N in niobium of I0 20, is consistent at the 4 % level with an infinite value of Q/N. 7. R. N. BOYD, D. PLAGUE, A. L. MELLlSS~_NOSand E. SVGARBAKEI~,Phys. Rev. Lett. 40, 216 (i978). 8. R. BLAND, D. BOCOBO,M. EUBANK and J. ROYER, Phys. Roy. Lett. 39, 369 (1977). 9. J. P. SCEIFFFR, T. R. RENNER, D. S. GEMMELLand F. P. MOORING, Phys. Rev. t)117, 2241 (1978). i0. J. ©REAR, Phys. Rev. D20, i736 (!979). 11. L. W. JONES, Phys. Rev. Dt7, 1462 (1978). 12. P . C . M . YOCK, Phys. Rev. D18, 641 (1978). 13. Previous experiments had set limits on the flux of < 10-~° quarks/(cm x secsterad), although these other experiments searched for quarks moving at relativistic velocities and/or for quarks accompanied by a shower of other particles. ~n contrast, Yock's candidates would correspond to single quarks at low velocities. 14. P. C. M. YocI¢, Phys. Rev. D22, 61 (t980); "Further evidence for heavy particles in the Cosmic Radiation" (submitted to Phys. Rev. Lett.); and private communication. 15. G. D. P u r r and P. C. M. YOCK, Phys. Rev. DI7, 1466 (1978). 16. G. GALLINARO,M. MARINELLIand G. MORPUI~OO,4th EPS General Conf., p. 562 (1979). 17. D. CUTTS et al., Phys. Rev. Lett. 4~, 363 (1978). 18. D. ANTREASYANel al., Phys. Rev. Lett. 39, 513 (1977). 19. M. BASILEet al., Nuovo Cimento 4®A, 41 (1977); and 45A~ 171 (1978). 20. M. BASILEet al., Nuovo Cimento 45A, 281 (1978). 21. First Resultsfi'om JADE Collaboration, Presented by S. ORITO at F N A L Symposium on Lepton and Photon Interactions at High Energy 11979), p. 52; and W. BARTELet al., Z. Phys. C6, 295 (1980). 22. On the other hand there is the possibility of a slight enhancement of the cross-section from the production of heavier quarks (especially of charge _+2e/3) and which decay rapidly into the quarks of interest. 23. This comparison does, however, require an estimate of the thickness of the earth's crust in which quarks accumulate, assumptions concerning the constancy of the cosmic ray flux over cosmological times, etc. 24. Y. B. ZELDOVICH, L. B. OKUN and S. B. P~KELNER,Phys. Lett. 117,164 (1965). 25. E. L. FEINBERO, Soy. Phys. Usp. ~0, 256 (1967). 26. R. V. WAOONERand G. ST~IG~AN, Phys. gev. D20, 825 (1979). 27. Other experiments to date do not reach the required sensitivity, provided we dismiss all experiments utilising quark enrichment or quark transference techniques (as involving uncertain assumptions concerning quark behaviour); and the Hillas and Cranshaw experiment 12) (for the reason mentioned in the introduction). 28. M.J. LONGO, Univ. of Michigan, Report No. UM-HE-77-3 (unpublished).
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29. As mentioned in Note 27, however, most searches on lighter elements do not reach the sensitivity of the La Rue experiment. 30. This may not be self-contradictory. For example Terezawa °1) has mentioned the possibility that even if confinement is a consequence of QCD, it m a y no'L remain so when the electro-weak interactions are atso considered. Also Zee (32) has speculated that echo quarks, with fractional charge but which are colour singlets (and hence perhaps unconfined), may exist in addition ~o the conventiona! coiour triplet (and possibly confined) quarks. 31. H. TEP,EZAWA, Prog. Th. Phys. 60, 1521 (1978). 32. A. ZEE, Phys. Lett. g41K 91 (1979}. 33. J. OREAR, ghys. Rev. Dt8, 3504 (1978). 34. A. Z~CH~'CH~,European Physical Society Conference, p. 335 (1977). 35. A.J. Cox, W. T. BEAUCHAMP,T. BOWEN and R. M. KALBACK,Phys. Rev. D6, 1203 (1972). 36. C. M. STEVENS,J. P. SCmFFER and W. CHUPKA,Phys. Rev. Dig, 716 (I976). 37. Neutrino Hydrogen Interactions in BEBC, presented by D. R. O. MORR1SON~ X][X lmernationai Conference on High Energy Physics, Tokyo, p. 354 (1978). 38. A. DE RUJULA, R. C. GILES and R. L. JAFFE [Phys. Rev. D17, 285 (1978)~, estimate that the quark pair production cross-section in e+e - collisions is suppressed relative to e~a~ by a factor of ~107-1013 for q~ark masses of 5 GeV. 39. M. MARINELLI and G. MORPURGO (preprints, April i980), Ne~,, results in the search of quarks in matter by the magnetic levitation electrometer, and Confirmation of the electric neutrality of matter at the milligram level; A ne~*, independent determination of the magneto electric forces in the experimental search for quarks by the magnetic levitation electrometer (June 1980); Search for quarks in matter. Presented at the XX lnternational Conference on High Energy Physics (Madison, 1980). 40. A. BUSS~EREet al., NucL Phys. ~174, 1 (1980). 41. S. D. PROTOPOPESCUand N. P. SAMIOS,Ann. Rev. QfNucl. Particle Phys. 29, 339 (!979). 42. P. V. LANDSI-tOEF,Progress in Particle and Nuclear Physics, VoL 3, p. 1 (1980). 43. G. S. LA RUE, J. D. PIc5SLLIPSand W. M. FA1RBANK,Additional evidence for fi"aetional charges oje/3 in matter. Presented at the XX International Conferertce on High Energy Physics (Madison, 1980). 44. The development of levitation experiments is described by G. MOaPUROO, Acta Physica Austriaca, Suppl. XXL 5 (1979). 45. R. VITAL et al., Phys. Lett. 77I~, 344 (1978). 46. J. A. GOODMANet al., Phys. Rev. D19, 2572 (1979). 47. P. N. BHAT et al., Search for new particles among high energy hadrons in extensive air showers, Kyoto Cosmic Ray Conf., Vol. 13, 8 (1979). 48. M. BASILE et al., Results of the quark search experiment in high energy r interactions, Preprint (CERN/EP
80-169). 49. Morpurgo points out, however, that even if initially quarks were to congregate preferentiaily in niobium, the ease with which the La Rue spheres lose their charge does suggest that they should be fairly abundant in other materials as well. 50. K. S. LACKNER and G. ZWEIO,The chemistry offi'ee quarks, Preprint CALT-68-781 and submitted to X X Mternational Co@rence on High Energy Physics (Madison, 1980).