Searches for new particles

Nuclear Physics B (Proc. Suppl.) 13 (1990) 221-226 North-Holland

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SEARCHES FOR NEW PARTICLES Michel SPIRO DPhPE/SEPh, CEN Saclay, 91191 Gif-sur-Yvette, France Abstract: this is a short overview of the many searches for new particles which are being undertaken with and without particle accelerators. I 238U+ 232Th [

There is a wide variety of new particles which are being 80

searched for. All these searches are as yet negative or inconclusive. The only positive claim concerns particles or

411

peaks which are totally unexpected (e + e- sum-energy lines

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in heavy ion collisions at Darmstadt). Mini review talks on these searches have been given in parallel sessions and the

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written reports are inserted in these proceedings. This contribution will then be a short overview of the present

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status of the searches for new particles. 1. ~ 3 S T U N E X P ~

PEAKS (DARMSTADI(~S) 0 500 10~ 1500

These peaks were reported by K. Stiebing. In 1985 the EPOS collaboration, working at Darmstadt near the GSI

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accelerator, discovered a narrow peak at 750 keV (width ~

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80 keV) in the sum of the energy of the e + and e- in the laboratory (fig.l) , from collisions of 6 MeV/nuclei

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Uranium or Thorium heavy ions on Uranium fixed target.

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Furthermore the difference of energy of the e + and e- was

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peaked at zero. This strongly supported the idea of a

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roughly 1.8 MeV particle produced at rest in the laboratory and decaying into an electron and a positron. The apparatus

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is a large acceptance spectrometer which can afford very high counting rates, which has good energy resolution, but poor angular resolution. This last feature, unfortunately,

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0 500 1000 1500 Ee++E , [keV]

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Fig.2 Summary of the EPOS Enes from the new data

Much more data are now available since the 1987 runs (fig. 2). By applying cuts on target and projectile (quality,

222

M. Sp~ro/ Convenor's summary: Searches for new particles

energy), on the diffusion angles of the target and projectile, nuclei and window cuts on the electron and positron emission angles, the EPOS collaboration demonstrated again the existence of narrow peaks in the e + e- sum-energy

EPOS LINES

peaks again are associated to a zero energy difference, but

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The ORANGE collaboration, running also at Darmstadt

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One must also add that there are no known origins for such peaks. It cannot be known atomic or nuclear effects. It cannot be point like particles (from g-2 and beam dump

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with a somewhat different spectrometer, has only marginal evidence for similar kind of peaks (fig.3). Furthermore none of them fall at the exact place of the EPOS peaks.

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were surprisingly found at 610 and 807 keV and not at 750

various cuts, the 750 keV peak finally shows up, but this time, with a very broad energy difference, centered around 150 keV.

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(30 keV width with a 10 keV energy resolution). These

keV (fig.2). The non appearance of the 750 keV peak is a problem which the collaboration is aware of. By applying

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experiments). Before getting too excited, it is fair to say that we all wait for a convincing confirmation of these peaks which have not yet been shown to be really reproducible 1. 2. MOST HOLY NEGATIVE SEARCH: THE HIGGS PARTICLES Many new results concerning searches for light Higgs particles are now available. The experimental search for light Higgs boson was reviewed by R. Cahn 2 and stands as follows. In the minimal standard model where there is only one isospin doublet and consequently only one Higgs particle (scalar and neutral), we can already put severe experimental limits on a light candidate:

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1. Very light Higgs have been searched for through the

Fig.3 The OrangeCollaborationdata

energy spectrum of X-rays emitted by muonic atoms. The agreement between theory and experiment at the level of three parts per million on muonic transition in 24Mg and 28Si places a limit on the Higgs mass: mH > 3 MeV, which means that the Higgs particle can be searched for through its decay into e + e2. L~gh¢Higgs decaying only into electron positron pair have been searched for in forbidden nuclear transition such as: 160* (6.05 MeV, 0+)--> 160 (g.s.) + e + e" 4He (20 MeV, 0+)-->4He (g.s.) +e+e -

From the non observation of anomalies in the spectra one is able to fairly exclude: 1.03 MeV < mH< 11.5 MeV 3. The search for the decay K0L --> ~0 + e+ e-excludes the range 10 MeV < mH < 240 MeV unless there is a conspiracy between a phase parameter and the top mass (see fig. 4). 4. The CLEO collaboration looked for the possible following decays:

M. Spiro / Convenor's summary: Searches for new particles

K°L ~ ~;°H

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6. Finally the Linde Weinberg cosmological bound

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arguments, provided that the to quark mass is not around

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In conclusion, a light Higgs below 5 GeV seem~ to be

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excluded in the minimal standard model, unless there is some very peculiar conspiracy in various parameters. With more than one Higgs doublet, all these limits have to be

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3.MOST PROMISING SEARCH: THE TOP Three proton antiproton experiments (UA 1, UA2 and

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CDF) reported negative results in the search for the top

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Mass of Top (GeV)

1.From the rate of isolated mnons accompanied by two, or more, jets, the UA1 collaboration sets a lower limit on

Fig.4 Rate of K.., ~ H as a function of the top mass

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quark. This was covered by D. Denegri.

the top mass of 50 GeV. 2. By studying the shape of the electron neutrino

--> H + X with H --> ~t+ ~t- and --> K + H with H -->~t+lx - or x+x" The combined

limit for the Higgs mass is:

transverse mass distribution, the UA2 collaboration sees no indication at the expected level for a top in the range 40 to

0.2 GeV < mH < 3.6 GeV practically parameter independent in the minimal standard model.

60 GeV. 3.Finally the CDF collaboration reported no event in which an electron and a muon are simultaneously emitted.

THE LINDE-WEINBERG BOUND

A lower limit for the top mass is then deduced around 60 MH 2 >

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GeV. By combining all these non observations of events at the expected rate, it is very tempting to conclude that the top mass is above some 70 GeV, which in turn implies that it is very likely that the top quark will not be produced through

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W decay but rather in pairs through quark or gluon fusion and that it will have an appreciable branching ratio for the decay into W + b quark.A search for the top quark in events with 2 W's might then be very promising. Note on the other hand, that the rate of 2 W's events in pp and p~ 0

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Fig.5 Summary of some limits on a light Higgs particle in the minimal standard model

collider at very high energies might be dominated by the production of t2 pairs and this might obscure the search for heavy Higgs or for strong interactions of W particles. 4. LEAST PROMISING SEARCH; THE FOURTH

Y->VY +H with H-->tt+[t -o Although slightly model

GENERATION The existence of a fourth generation implies in the

dependent (QCD corrections), the limit on the branching

standard model the existence of a fourth neutrino. When

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these proceedings are issued, it is very likely that we shall

5. The CUSB collaboranon studied the branching ratio

/if. Spiro/Convenor's summary: Se~clies for new particles

224

know uliimately the number of neutrino species from SL(~

supersymmetric particle, quark or gluon fusion processes

and LEP experiments. Presently, there are various methods

will produce a pair of squarks which will decay into

which can be used to determine the number of neutrino species Nv, or an upper limit on this number, within the

ordinary quarks plus photinos. Depending on the mass of

standard models of stellar evolution, cosmology and

this will result in events with 4 or 2 jets plus missing

particle

transverse energy. From the number of events observed with missing Et>40 GeV (36 events) or 60 GeV (5 events)

physics 3. In fig.6 one can see the best

determinations based on 1. / .~-~,,^' ° I"

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the gluino (whether it is lighter or heavier than the squark)

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and from the comparison with the expected number of

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eeents from "conventional" sources (heavy quark decay, W or Z decays), it is possible to set a lower limit to the squark

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mass of 150 GeV (the previous limit also from CDF was 80

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GeV). It is likely that in the future this limit can be brought

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collider. 6. SEARCH IN THE DARK (HIGHLIGHTS) ,

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and who believe in the standard cosmological scenario of nucleosynthesis (which implies that flbaryons cannot be

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For those who believe that the energy density of the universe must be equal to the critical energy density (fl=l),

greater than 0.1), the universe must be dominated by non0

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baryonic dark matter. There are then two possible scenarios:

Fig.6 Summary on the various determinations of the number of neutrino species

1.the neutrino burst from SN1987A 2.the cosmological constraint

1. Hot dark matter scenario (like, for instance, 30 eV neutrinos) in which the particles which close the universe,decouple from thermal equilibrium when they

from primordial

abundances of light nuclei 3. the single photon production in e+ e- collisions 4. the ratio of the W-->ev to Zo-->e+e - partial cross sections in proton antiproton collisions.

were still relativistic 2. Cold dark matter scenario (Weakly Interactive Massive Particles like a massive photino, a few GeV new Dirac neutrino ...) in which particles were non relativistic at the time of decoupling.

The consistency between all these determinations

This second scenario is favoured from arguments which

represents an astounding success. Combining all

deal with small scale structure formation (galaxy

determinations, we obtain however a rather strange result: Nv = 2.0+0-6-0.4

formation). Furthermore in this second scenario, the dark

However the g 2 is so good that one must not really take ~l~sresult at face value! For more details on the physics implications of a fourth generation, see the review by V. Burger. 5. MOST SIGNIFICANT NEGATIVE SEARCH

matter would naturally explain the formation of dark halos around galaxies and account for the flat rotation curves of stars and gas around the center of the ~alaxies. New limits are now available on the masses of some candidates for this last scenario. 1. Photino

This goes to the search for squarks from the CDF

Residual cosmological density of photinos having

collaboration preliminary analysis (1988 data) reported by

survived annihilation can be reliably estimated if one

R. Wagner. Assuming, that the photino is the lightest

assumes that the lightest supersymmewie particle is a pure

M. Spiro / Convenor's summary: Searches for new particles

225

photino (no mixing with higgsino or others). The

SLC and LEP experiments will give a definite answer so~n.

decoupling time is determined by their low-energy

For sneutrinos the cosmological ranges depend on many parameters ( Zino, photino, higgsino masses).

annihilation cross section which can be calculated and which depends almost only on the photino mass and the lightest scalar-fermion mass. In many models, the scalarcurve in the m~" m"~ diagram which con'esponds to fl= 1 for photinos 4. From m~'< m ~

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fermion masses are degenerate (=m'~'). Fig. 7 shows the

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m q > 150 GeV, one can see that the photino mass is

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restricted in a range 30 to 300 GeV, the upper limit depending on the new channels which might bt, open to the annihilation (top, Wino masses...). Again this also assumes

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no mixing with Higgsinos or others and assumes some

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degeneracy in the scalar-fermion masses. One can see that

Fig.8 Limits on heavy neutrino masses from the Frejus experiment

in this simple minded scenario, CDF might well be able in a rather near future to close the remaining window.

3. Cosmions and direct detection 5

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M~ (MeV) Fig.7 Exclusion zonesfrom CDF andfrom cosmology

2. Heavy Dirac or Majorana neutrino and sneutrino

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20 40 60 80 Kinectic Energy (keV)

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F i g s Ratio of ionization deposition Nucleus~electron

Assuming now that WIMPs constitute the dark halo of our galaxy, WIMPs can be captured by the sun via elastic

By using underground well-shielded detectors (like

scattering. The abundance of WIMPS can then reach an

Germaoium or Silicon diodes), one can look at the elastic

equilibrium between capture and annihilation. The high

scattering of WIMPs on Ge or Si nuclei. By assuming a

energy neutrinos resulting from those annihilations can then

mass and a cross section one can compare the observed and

be detected in deep underground detectors. This method

expected rates of events per keV per kg and per day. Here

places the best limits on heavy Dirac neutrinos and on

k e V means equivalent electron energy, which means that

sneutrinos. The limits are shown on fig. 8.Heavy Dirac

one has to measure the relative amount (compared to

neutrinos are excluded above 3.5 GeV, Majorana neutrinos

electrons) of ionisation produced in a Ge or Si diode as a

above 5 GeV and sneutrinos above 3.5 The interesting cosmological ranges (f~=l) for Dirac neutrinos is 2 to 4

function of the energy of the nucleus recoil energy. This

GeV, for Majorana neutrinos 4 to 8 GeV. One can see that

Silicon by using neutron beams (see fig. 9). By using these

there is not much room left for these ranges in that scenario.

results and the observed Dreliminarv rate in a well shielded

has been done recently (UCSB, UCB, LBL, Saclay) for

M. Spiro / Convenor's summary: Searches for new particles

226

60 grams Silicon detector (less than 20 events per kg per day and per keV equivalent electron energy), one can rule out (fig.10) a quite popular class of dark matter particles, namely cosmions. The cosmions are "ad hoe" particles which can solve both the dark matter and the solar neutrino emblem. These particles get captured in the sun through

g-,

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elastic scat,~ring and cool down the center of the sun via again elastic scattering along their orbits. Consequently

10-22

10-26

10-30

their masses have to be in the range of 4 to 10 GeV, and their cross section on hydrogen in the few picobarn range. If one assumes some kind of vector coupling, which means some kind of coherent interactions on heavy nuclei, their cross sections on Silicon is expected to be in the 10-34 to 10-33 era2 range and are then consequently (fig. 11) almost

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ruled out. One way out of this scenario is to imagine that these particles interact axially, which means that they couple to spin, and consequently cannot be ruled out with a silicon experiment (zero spin).

2511 i ~g [-

~r...~ SILICON ~. ~"4GeV/c2 -_

/ RABY-WEST ~ ~.~¢/7GeV/c2 MODEL-

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1. In the minimal standard model the Higgs particle has to be heavier than 5 GeV 2. The top mass is heavier than 60 GeV 3. There is not much room for a fourth generation. 4. The squark mass is above 150 GeV. 5. Where has all the matter gone? We are still desperately searching for the dark matter. REFERENCES 1. For more details on this subject, see the Proceedings

t.\\ u~ 511 \ ~'~:2. m GeV/c2-0 ~~ e ~ " - r " 0

1

2 3 4 5 EQUIVALENT ELECTRON ENERGY (KeV) Fig.lO Observedevent rate compared to that expected from Cosmions in the Raby-West model

of the Rencontres de Moriond, January 1989, edited by O. Fackler and J. Tran Thanh Van, Publisher Editions Frontieres. 2. See also R.N. Cahn, Reports on Progress in Physics 52, 1989, 389 and M.S. Chanowitz in Annual Review of Nuclear Science 38, 1988, 323. 3. D. Denegri, B. Sadoulet, M. Spiro, The Number of Neutrino Species, preprint CERN 89, LBL 26014, DPHPE

7. CONCLUSIONS Although slightly disappointing for the moment, many searches for new particles are under way. The most significant negative limits are:

88-12. 4. T.K. Gaisser et al., Phys. Rev. D, 34, 1986, 2206. 5.See for instance J. Rich, Proceedings of the Rencontre de Moriond on Dark Matter, March 1988, Ed. by J. Audouze :'ad J. Tran Thanh Van, Publisher Ed. Fronti~res, p43. and J. Primack, D. Seckel and B. Sadoulet in Ann. Rev. Nucl. Pan. Sci. 38, 1988, 751.