Study of short-lived particles with emulsion techniques

PHYSICS REPORTS (Review Section of Physics Letters) 82. No. 1 (1982) 85—106. North-Holland Publishing Company

Study of Short-Lived Particles with Emulsion Techniques J.D. PRENTICE University of Toronto, Toronto, Canada

1. Introduction The discovery of charmed and beautiful quarks and the r meson have stimulated the development of new techniques for the measurement of lifetimes of about 10_13 sec. The understanding of the weak decays of these new quarks and leptons is crucial to their interpretation in terms of the current models of strong and electroweak interactions. Since flavour changing decays occur with appreciable strength only through the charged current weak interactions, the r and some of the lightest beautiful and charmed hadrons are expected to decay weakly. Observations of charmed hadron decays have confirmed some of these predictions. Measurements of r-decay branching ratios have already shown that strong interaction corrections to the semi-leptonic decays are fairly small and thus allow us to predict the T lifetime rather accurately by relating the pure leptonic decay widths r~ i~+ ii,. (e~ Pc) to the equivalent decay ~ Using the well known mass dependence Fr..e/F,~..e= (M,IM~)5,the observed Br_.e = 0.17 and the muon lifetime, r~,gives a mean-life TT = 2.8 x 1013 seconds. The observation of hadrons in just over 60% of the final states together with the accessibility of two pure leptonic decay channels (see fig. 1) and a number of hadron decay channels equal to the number of quark colours gives striking evidence for the existence of 3 colour degrees of freedom. With the simple ideas of the quark model and the Glashow—Weinberg—Salam model so clearly supported it will be very interesting to confirm the lifetime prediction experimentally. While the new emulsion techniques described below have clearly demonstrated the capability of measuring this lifetime the difficulty lies in producing a detectable number of tauons in a fixed target reaction. They have so far only been convincingly observed in e~e annihilation. There is some hope of observing the decay F~ r~v~ followed by r decay in emulsions. The preliminary results for the F lifetime, discussed below, suggest however that even though the proximity of the r and F masses substantially reduces the helicity suppression of this channel the hadronic F decays are sufficiently enhanced to render this branching ratio unfortunately small. While initial predictions for charmed hadron decays suggested a similarly simple analysis in which the -~

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Lifetime measurements in the 10-133 range ~.c’(e) (u)

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charmed quark was assumed to decay independently [2] (fig. 2(a)) both lifetime and branching ratio data indicate that such a simple model is inadequate. A flood of theoretical papers continues to explore the strong interaction corrections which make substantial changes in the simple predictions. While some aspects of this complex and fascinating interplay of strong and electroweak interactions can be tested by measurements of branching ratios, the addition of lifetime measurements for all the weakly decaying charmed hadrons will determine a multitude of absolute partial widths that can rigourously confront and distinguish between, the many models that have been proposed. The results already obtained for charm decays imply that much can thus be learned about the effective masses of the quarks, the contribution of W exchange (fig. 2(b)) and annihilation diagrams (fig. 2(c)) and the related effects of gluon radiation in both the initial and final states. With a beauty quark to charm quark mass ratio of mb/mC 4.5/1.5 one might expect b decay lifetimes approximately (3)5 times shorter than the charmed particles. However since Mb
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2. Lifetime measurements in emulsion chambers and emulsion 2.1.

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The IT0 lifetime is the shortest ever determined by means of track length measurement. As early as 1963 the sequential decay of stopped kaons K~

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~ e~ey was measured to obtain a lifetime of (2.8±0.9)x1016 seconds [4]. Subsequent measurements by other techniques [5] and a remeasurement of the same decays [6] gave shorter lifetimes and the currently accepted value is (0.828 ±0.057) x 1016 seconds. The substantial systematic error in the emulsion measurements done before the photoproduction results [5] were obtained suggests that measurements of extremely short decay lengths may be misleading. Using the accepted lifetime value and the /3y = 1.55 for the iT° in K~ ~ir° yields a mean decay length 1 = /3ycr = 3.9 x 10~jim. This suggests that decay lengths of <0.1 jim cannot be measured reliably even with large angle secondaries and with several tracks fitted to both the primary and decay vertices. —~

2.2.

Early observations of charmed particles in emulsion

The observation of short decay tracks in emulsions exposed to cosmic rays and accelerator beams [7] may well have provided the first experimental detection of charmed hadrons. The first such short decay observed in emulsion chambers exposed to cosmic rays [8] was interpreted as a decay of a charmed particle [9] as early as 1972. While these and later observations in emulsions or emulsion chambers have clearly established both the existence of particles with decay lifetimes in the range ~iO’~ sec and also the associated production of pairs of such particles in hadronic interactions, the limited momentum resolution and particle identification of these detectors has restricted the analysis of these events. Bare emulsions provide only lower bounds for the momenta of most of the decay products, no charged particle identification above ‘=1 GeV and very inefficient detection of IT°,~ or neutral hadrons with r>1011sec. The emulsion chamber technique [10] utilizes thin layers of emulsion on thick plastic backings to provide longer track measurements and interspersed layers of lead or tungsten to improve electron identification, ir° detection and multiple scattering momentum measurements for charged tracks (fig. 3). These improvements substantially enhance the decay reconstruction efficiency but positive identification of the decaying particle is nevertheless seldom, if ever, possible. In addition to the early examples of charmed decays observed in cosmic ray experiments [7,8], more recent exposures of emulsion chambers to accelerator hadron beams have yielded several examples of associated production of pairs of short-lived particles. Two pairs of neutral decays were observed in an exposure to 400 GeV protons [10]. A second experiment using a 340 GeV/c i~ beam produced two events each containing one charged and one neutral decay and two with pairs of neutral decays [11]. This background free sample of charmed hadron events is useful in confirming associated production by hadron beams and in contributing to our knowledge of these cross sections. The ambiguities in the decay particle momenta and identification that result from incomplete measurement of the decay products, however, make them of marginal use for measuring lifetimes. The full benefit of the high

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J.D. Prentice, Study of short-lived particles with emulsion techniques

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precision decay length measurements made possible by emulsions has only been realized by combining them with other track detectors and spectrometers in hybrid experiments. Furthermore the addition of downstream detection reduces the labour of locating events in the emulsion thus permitting the use of larger volume targets needed for neutrino or photon beam exposures. 2.3.

Development of the hybrid emulsion technique

The motivation for the early uses of external detectors with emulsion was primarily the detection of rare events [12]. The feasibility of using an emulsion target in a neutrino beam was demonstrated by Burhop et al. [13] who found 4 events produced in 10 litres of emulsion by the CERN proton synchrotron neutrino beam (fig. 4). Tracks located in optical spark chambers triggered on neutral interactions during the neutrino beam spill, were searched for and when located followed back to the neutrino interactions in the emulsion. Though this experiment demonstrated, as early as 1964, the feasibility of observing neutrino interactions in emulsion the larger mass available in other neutrino targets outweighed the spatial resolution advantage of emulsion until the advent of charm and the concomitant desire to observe the short tracks of charmed hadrons.

3. Short lifetime measurements using the hybrid emulsion technique 3.1. Emulsion and spark chambers in the Fermilab neutrino beam The first example of the production and decay of a charmed hadron in a hybrid emulsion experiment resulted from the exposure of 16.6 litres of emulsion to the Fermilab broad band neutrino beam V S

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(Fermilab E-247) [14]. Events were located by reconstructing charged tracks in two wide-gap optical spark chambers (fig. 5). A total of 37 neutrino induced events were found in the emulsion. Tracks of relativistic charged particles, from neutrino interactions were completely scanned for interactions or decays. A cone of semivertical angle 30° and 2 mm long, downstream of each neutrino event was scanned for neutral decays. One charged track was found to decay after 182 jim yielding 3 charged particles and one or more neutral particles. As none of the decay particles could be uniquely identified several hypotheses were acceptable for the decay yielding proper times of =~6x 10_13 seconds. The importance of good emulsion quality and of the spatial connection between downstream detectors and the target and the need for more complete event reconstruction were made clear by these results. Several of the succeeding experiments which were at an early stage in 1976 benefitted from this experience. 3.2. Emulsions and large bubble chambers Both the CERN BEBC and the Fermilab 15 foot B.C. have been used as downstream detectors for large emulsion targets in neutrino beams. 3.2.1. The Big European Bubble Chamber In the CERN experiment (WA 17) emulsion stacks were placed outside the BEBC beam entrance window as shown in fig. 6 [15].A multi-wire proportional chamber (MWPC) between the target and the window aided in the connection of bubble chamber tracks to emulsion tracks. The four-strip scintillation

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counter C, immediately outside the chamber, provided a trigger for the external muon identifier (EMI) downstream of the bubble chamber and for the read-out of the MWPC. The presence or absence of a coincident pulse from the veto counter (V) was also recorded for each trigger. The two targets, of 10.5 ~‘ and 21 ~ respectively were irradiated by the wide band neutrino flux resulting from 0.3 x 1018 and 0.7 x 1018 protons of 350 GeV/c momentum incident on the neutrino production target. The neutrino energy spectrum peaked at about 25 GeV. The chamber was filled with liquid hydrogen. The exposure produced 206 000 bubble chamber pictures which were scanned for events with at least 3 tracks which appeared to originate from a common point in the emulsion and for which at least one track had a momentum greater than 3 GeV/c. The bubble chamber tracks were reconstructed and followed back into the emulsion using the BEBC magnetic field, calculated ionization energy loss and with uncertainties due to multiple scattering included. The 935 neutrino events with acceptable fitted vertices in the emulsion volume indicated typical errors in the vertex position of 0.7 mm transverse to and 7.0 mm along, the beam direction. From the neutrino events the charged current interactions were selected by an acceptable EMI-vetochamber combination (483 events) or by the requirement that the highest longitudinal and transverse momentum track in the bubble chamber be negatively charged and exit without interacting (40 events). These 523 candidates for the reaction p~N-+~sXwere searched for in the emulsion, in a volume surrounding the predicted vertex, at magnifications between 200x and 300x. Those accepted as charged current neutrino events had at least two minimum ionizing tracks that agreed in dip and azimuth within 3°with the angles predicted from the bubble chamber measurements and one of which was the muon track selected by the EMI or the momentum criterion. Charmed hadron decays were searched for by following all charged tracks from the charged current neutrino interactions for 4 mm or until they interacted or left the emulsion. In addition a cone of 30° semi-opening angle was examined at high magnification for 2 mm downstream of the neutrino interactions to search for neutral decay vertices. The search for the 523 charged current predictions yielded 169 neutrino events in the emulsion for a mean finding efficiency of 32%. A search for about 60% of the remaining 412 predictions yielded 45 neutrino vertices in the emulsion (---18%). The charm scan found 5 charged and 3 neutral decay candidates with track lengths from 54 to 1595 jim. All of these eight events, except one with a neutral decay candidate had a j~ identified by the EMI. The latter event was found while scanning for a different prediction and has no corresponding bubble chamber tracks. The three events with 3 prong charged decay candidates have been described in detail in refs. [16] and [17].One of the events [17]has a fully fitted decay identified as A~—8pKIT~with a proper time of 7.3 x 10 sec, and a mass [15] of M,.-~= 2.26 ±0.022 GeV/c2. The other two 3 prongs are consistent with charmed meson or baryon decays if an unobserved neutral decay particle is assumed. The resulting zero constraint fits to the decays yield proper times in the range (0.5 to 5.3) x 10- 13 sec. Two single prong decay candidates are shown to be incompatible with K~or decays and to have a low probability (<10~)of being due to nuclear scattering. These are therefore assumed to be charmed decays but cannot be kinematically fitted. All three of the neutral decay candidates have 2 prong topologies in which the plane of the V does not contain the primary vertex suggesting the existence of an unobserved neutral particle among the decay products. In one event the V is well-associated with the primary vertex as tracks from both are matched with those in the bubble chamber. For the other two events the absence of bubble chamber .~

Lifetime measurements in the 10

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tracks matching the decay tracks allows the possibility that the V has been incorrectly associated with the primary vertex. This hypothesis is, however, ruled out by the authors because the probability of an accidental association, based on the number of Vs found and the volume scanned, is less than iO~. If the events are all classified as decays the assumption of unobserved neutrals in each of them permits a number of acceptable zero constraint D°decay hypotheses and an associated set of proper decay times ranging from (0.2 to 13.6) x iO’~sec. Thus the only event that has yielded a unique proper decay time is the A ~ pK~, with no neutral particles in the decay. The importance of good neutral particle measurements and the concomitant disadvantages of using bubble chambers filled with H 2 or D2 as the downstream detector are clearly evident. A bubble chamber filling of a heavy Ne—H2 mixture may greatly improve this situation. An attempt has been made to extract lifetime information from the seven unfitted events by extrapolating the decay tracks back and analysing statistically their distances of closest approach to the primary vertex of the event. For a given transverse momentum of the decay particles the increase in the decay track length due to the relativistic time dilation factor f3y is approximately cancelled by the corresponding reduction in the angle between the decay track and that of the decaying parent. The statistical distribution of distances of nearest approach can thus be related to the lifetime [18]. Making use of this technique and assuming that all the charged decays except the A ~ are D~decays and that all the neutral decays are D° mesons, mean lifetimes r’ = (2.5~:~) x iO~sec and ~ = (0.53~:~) x i0~sec were deduced [15].The maximum likelihood fit used to obtain these values requires a knowledge of the variation of detection efficiency with distance of the decay from the production vertex. In view of the low and variable grain density of the tracks in these emulsions a considerable correction may be required, particularly for the D°lifetime. The importance of emulsion quality was well demonstrated by this experiment. The low grain density severely hampered the volume scan for neutrino events and eliminated the possibility of finding them by following back individual tracks. Its effect on the efficiency of finding neutral decays may also have affected the maximum likelihood estimate of the neutral lifetime. The need for large angular acceptance for K°and A°decays and for a higher conversion probability for ir° decay y rays than is provided by a hydrogen bubble chamber are indicated by the small fraction of fitted events and the resultant uncertainty in the identification of the particles for lifetime measurements. —~

3.2.2. Emulsion in the Fermilab 15 foot Bubble Chamber In Fermilab experiment E564, 22 litres of cryogenic emulsion were mounted in steel boxes inside the Fermilab 15’ B.C. filled with liquid D2 and equipped with a two plane external muon identifier [19]. In an exposure to the broad band neutrino beam with 5 x 1018, 350 GeV protons incident on the primary target 320 000 bubble chamber pictures were taken. The pictures were scanned for tracks leaving the emulsion boxes and the measured tracks were projected back to a common vertex in the 5 cm thick emulsion target. Events were located in the emulsion by volume scanning in the vicinity of the predicted vertex. About 20% of the expected neutrino events have been found and scanned for charm decay candidates. One event, with a 3 prong decay candidate 504 jim from the primary vertex, has been fully reconstructed. Two of the decay tracks entered the bubble chamber and were found to be positive. The third was emitted at too large an angle to enter the B.C. Its momentum was measured in the emulsion, and its charge assumed to be was negative. latter required for consistency with the 4 was 7r~IT’~IT IT° which found The to best fit was the event. If the wide angle track haddecay been hypothesischarged F positively the decaying track would have been 9 times minimum ionizing whereas its measured —*

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grain density was consistent with a singly charged relativistic particle. The ionization of the wide angle track showed it to be caused by a particle of f3 = 0.65 ±0.06. The three charged decay products showed a clear transverse momentum imbalance indicating a missing neutral particle and many zero constraint fits were possible. Three bubble chamber tracks not seen in the emulsion were consistent with having originated from two gamma conversions in the wall of the emulsion container and could be fitted to a ir°decay that provided the missing momentum. Using this IT0 the only acceptable 3 constraint fit was F~ IT~1T~1TIT°. A two constraint fit leaving the decaying particle mass free gave a fitted value MF+ = 2017±25MeV. The stainless steel wall of the box is only 0.6 cm or about ~radiation length thick thus the conversion of both IT0 decay gamma rays was quite fortuitous. The event is significant because, together with 3 other F events from another hybrid emulsion experiment (see below), it provides confirmation of the F~ meson through its mass and weak decay. The previous evidence for F~production in e~e collisions at DORIS and its decay to ~ir [20], were not confirmed at SPEAR [21]. Other evidence from a CERN photoproduction experiment shows mass enhancements in a number of channels containing an ~ and 1, 2 or 3 pions [22]. The mass of IT~IT IT° combination was 808 ±20MeV_suggesting a possible unr~final state. As this contains no evidence for final state s quarks in either a KK pair or a neutral resonance with known large sg content it offers some support to the view that c~annihilation may play an important role in F decay. A second run of this experiment is in progress. The heavy Ne—H 2 mixture (60% Ne atoms) in the bubble chamber will provide high efficiency gamma detection which should produce a much higher proportion of fully reconstructed charm decay events. In the second run two of the twenty-two emulsion stacks have the planes of the emulsion pellicles perpendicular to the beam direction. In the remainder, the pellicle planes are mainly parallel to the beam but each module has 3 pellicles perpendicular to it at the exit end. The distortion at the edges of pellicles, produced by development and drying, makes it more difficult to locate individual tracks from the bubble chamber in parallel pellicles than in those perpendicular to the beam. The new arrangement of the emulsion is expected to improve the ability to correlate individual tracks in the bubble chamber and the emulsion thus permitting the location of both neutrino events and charm decays by following back individual tracks from the bubble chamber to the emulsion. Both the neutrino event finding efficiency and the charm decay yield should be substantially increased. —~

4. Emulsion-spectrometer hybrid experiments Emulsions have been combined with downstream spectrometers in experiments using muon, taggedphoton and broad band neutrino beams. 4.1. Virtual and real photon beams Both virtual and real, high energy photons are expected to couple to quark anti-quark pairs in the target hadron. The ~e charge of the charmed quark suggests that it will contribute substantially to high energy photoproduction. The indications of charm contributions to the high energy increase in the hadronic photon total cross section [23] have been confirmed by recent observations of photoproduction of F [22] and D mesons [24] and A ~ charmed baryons [25].The possibility that more than 1% of the hadronic photon total cross section results in charm production makes photon beams more attractive for

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Lifetime measurements in the i0~~ s range

emulsion studies of charmed particles than hadron beams, in which the fraction of events containing charm appears to be nearer 0.1%. Virtual photons from deep inelastic muon scattering have the further advantage that they do not produce the large e~e~ background in the emulsion that inevitably accompanies a real photon beam. 4.1.1. An emulsion-spectrometer hybrid experiment in a muon beam

An attempt to observe the decay of charmed particles produced by deep inelastic muon scattering in emulsion was made in Fermilab experiment 382 [26]. Twenty-six stacks of emulsion, each containing 85—100 pellicles of dimensions 7.5 x 5.0 x 0.06 cm3 were successively exposed to the Fermilab muon beam. The emulsion pellicles were horizontal with the 5 cm edge parallel to the beam. A total of about (3—5) x iO~muons was accumulated in each stack. Multi-wire proportional chambers and drift chambers were used to measure both the incoming muon beam and the secondary particles. The scattered muon was identified by its penetration of a 2 m iron muon shield and then momentum analyzed in a spark chamber and iron toroid spectrometer as shown in fig. 7. The diffractive cross section for charm is expected to fall more slowly with increasing Q2 than the cross section for production of hadrons composed of the light quarks. One can thus expect a higher ratio of charmed hadrons in high Q2 events. A scan was therefore made for muons scattered by more than 6.5 mrad and the resulting 1200 event sample was further selected by a visual examination, by physicists, of the drift chamber—proportional chamber reconstruction of the downstream tracks. The resulting sample of 269 events had a (Q2) 4.6 GeV2 and muon energy loss (i.’) 62 GeV. The emulsion was searched for these 269 events. The beam track was used to predict x and y transverse to the beam direction; the downstream tracks gave an estimate of z for the vertex that was about 30 times less precise owing mainly to the large proportion of secondary scatters and interactions in the target. The high density of tracks prevented the identification and following of the individual beam tracks of the events and a resultant loss of almost all the events with 0 or 1 heavy tracks from nuclear fragments. The rather low grain density of 19—20/100 jim for minimum ionizing tracks enhanced the difficulty of finding events with the number of heavy tracks NH < 3.

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The overall event finding efficiency was 38% and for the expected fraction of events with N~ 3 it was 65%. In a search for short-lived decays all grey and minimum tracks were followed for 3 mm. Eight single prong kinks were found but none showed evidence of a second, associated short-lived decay as would be expected in production by a virtual photon. Only a partial scan was made for neutral decays (53 events). One unusual event with a kink, a 2 prong V, each containing an electron track and a further associated electron track that did not originate from the primary vertex, was found and reported at the Hamburg Photon Lepton conference [27]. This experiment demonstrated the feasibility of a tagged emulsion experiment in a muon beam but showed that both a larger volume of emulsion and a higher event finding efficiency would be required to observe a substantial number of charmed decays. With a charged beam the possibility of identifying the particular beam track producing the event in the emulsion is particularly attractive as it would lead to a much higher event finding efficiency. In order to achieve a sufficiently high event rate it is necessary to accept a high density of beam tracks but a more precise beam detector could provide the necessary resolution to identify the individual tracks with adequate accuracy. 4.1.2. Photoproduction of charm in a hybrid emulsion-spectrometer The photoproduction of associated pairs of charmed hadrons in emulsion targets exposed to the super proton synchrotron (SPS) tagged photon beam and using the Omega prime spectrometer for event finding and reconstruction has been observed in CERN experiment WA58 [28,29]. A previous experiment, using a similar technique with the Omega spectrometer found a well fitted D°decay [30]. In the earlier experiment (WA45) about 5000 Ilford emulsion pellicles, of dimensions 15 x 3.5 x 0.06 cm3, were each exposed, at an angle of 11°to the beam, to about 106 tagged photons between 20 and 70 GeV. The hadronic interactions of the photons in the emulsion were recorded in the Omega spectrometer. Events with four or more charged secondaries were reconstructed and the emulsion was searched in a volume around the predicted vertex. About 1000 events were found and were examined for evidence of short lived decaying particles. One well fitted D°—~ K’irIT~IT decay, with a length of 123 p. and a proper decay time of (0.226 ±0.005) x 10_13 sec was observed. The K~and one of the IT were identified and assuming the other two decay tracks to be pions a fitted mass of 1866±8MeV/c2 was obtained. The invariant mass of the K~and one IT was 911 ±5 MeV/c2 and that of the other ir and the ir~’ was 727 ±3 MeV/c2 consistent with a decay D° K*OpO. All the other fast secondary particles from the photon interaction vertex were followed to their exit points from the emulsion but no associated decay was found. The total effective thickness of the emulsion along the beam direction was 3 mm and the resultant probability of a D~leaving the emulsion before decaying was substantial. As an analysis of the production vertex revealed a missing mass of —2.5 GeV/c2 a search for neutral decays was made but none was found. A possible explanation of the event is that a D°was produced in association with the observed D°but decayed to all neutral decay products. In the second experiment the spectrometer was significantly improved and 6000 larger pellicles (20 x 5 x 0.06 cm3) were irradiated at 5°, to give an effective thickness of 6 mm. Each pellicle received 106 tagged photons of energies between 20 and 70 GeV. In the Omega prime spectrometer, with drift chambers replacing the optical spark chambers, events with multiplicity greater than or equal to three were reconstructed and searched for if a vertex was predicted in the emulsion. The experimental layout is shown schematically in fig. 8. In the scan of the first 2000 of 160000 hadronic triggers, 1400 events were found in the emulsion and —~

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1000 of these were matched by track angles to events in the spectrometer. Three matched events were found with apparent pairs of decaying particles. In addition, three of the unmatched events contained pairs of short decay candidates. The latter group comprised 2 events each containing a charged 3 prong and a charged 1 prong decay candidate and one event with a charged one prong and a neutral 2 prong candidate. With only emulsion data for these events it is impossible to fit the decays to any particular charmed hadron. The matched events include the first observation of a A ~D° photoproduction event [29].The A ~ was seen to decay in the emulsion as a one prong kink after a flight path of 50 p.m. The decay track was not seen in the spectrometer but a neutral V reconstructed therein could be fitted to a A pir decay and was coplanar with the two tracks of the kink. Assuming the charged decay track to be a ir~yielded a parent mass of 2.33 ±0.05 GeV/c2. The charged decay is thus concluded to be a A ~ A 0IT~decay with momentum P(A ~)= 6.73 GeV/c and a proper decay time of (0.57 ±0.02) x 10_13 sec. The other decay in this event is a 4 prong neutral which was found 124 jim from the primary vertex. All the decay tracks were measured in the spectrometer and the small missing transverse momentum indicates that there were no missing neutral decay products. The charged decay particles were all of too low momentum to be identified in the Cerenkov counters but a ITK~I7-IT~ hypothesis gave a fitted mass of 1.847± 0.007GeV/c2 indicating a Cabibbo favoured decay of a D°with proper decay time of (0.86±0.01)x iO’~sec. The remaining matched events [281have a charged 3 prong with a neutral 2 prong decay and two charged 3 prongs respectively. In the former case all the charged decay tracks are seen in the spectrometer but assumed missing neutrals are needed to balance transverse momentum or fit a charmed particle mass. The resulting zero constraint fits are consistent with a D and D°—*Kir~(IT°) hypothesis with lifetime estimates of (0.88 or 0.57) x 10_13 sec and (0.45—0.85) x iO~sec respectively. In the third matched event one or more secondary tracks from each of the decays missed the spectrometer and no fits were possible. More recently the number of interactions located in the emulsion has been increased to —2500 and a fitted D°D° event has been found [31].The latter and other new decay events are under further study. —~

—~

—~ Ir~ITIT(K°)

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The maximum likelihood estimate of TD°= (0.58i1ig:~) 10~sec, based on the first 3 fitted neutral decays, depends on the assumption that is unaffected by variations of the neutral event finding efficiency with distance from the primary vertex. Estimates of the lifetime of D~and D°from the unfitted decays were made by the same statistical treatment of the offset of decay tracks from the primary vertex as used in WA17 (see above) [18] yielding results r~ 4.4 x 1013 sec and T0 0.43 x i0—’~sec. These estimates are subject to uncertainty due to both the unknown admixture of other particles with different lifetimes such as A ~, F~or A°(cds) and the unmeasured scanning efficiencies. The large fraction of the data (>90%) still to be analysed and the improvements being made in ir° fitting and estimates of scan efficiencies promise interesting further results from these experiments. X

4.2. Hybrid emulsion spectrometers in neutrino beams Two Fermilab experiments have exposed emulsion targets to the broad band neutrino beam using downstream spectrometers to find and analyse the neutrino events and charmed particle decays. 4.2.1. Fermilab experiment 553

In this experiment 15 of Kodak NTB3 emulsion in 8 x 2 X 0.06 cm3 pellicles was exposed to the broad band neutrino beam with the 2cm dimension parallel to the beam. The total neutrino flux resulted from 3.5 x 1018, 350 GeV protons on the primary target. The expected yield of neutrino interactions in the target was 200 to 300 events. Event location was accomplished by a new technique in which each emulsion module (8 X 2 X 45 cm3) was followed by a trigger counter and a self-marking spark chamber. One electrode of each spark chamber gap was an aluminum film deposited on a glass plate. Each spark produced a small hole in the metal film and these spots could be located visually. The downstream spectrometer employed optical spark chambers and a large aperture magnet. A flash tube calorimeter identified electrons and gamma ray showers in its upstream lead plate section and

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TWo

2

IN METERS

I

I LEAD SHEET

I

W2 iY4 FLASH TUBE

—1-~. STEEL PLATE

Fig. 9. The E553 emulsion target and spectrometer.

I

I /

i~ FLASH TUBE

~ STEEL PLATE

FLASJI TUBE

98

Lifetime measurements in the 1013s range

provided both hadron calorimetry and muon identification in its downstream steel plate section. The flash tube planes were recorded photographically. The apparatus is shown schematically in fig. 9 and described more fully in ref. [32]. Some 200 events observed in the spectrometer appeared to originate in the emulsion. Of these 47 neutrino events were located and scanned for decay candidates. One single prong kink, of length 200 p.m is a charm decay candidate and is being analysed. A possible decay of a particle of mass greater than those of known charmed hadrons is being studied. It was produced with very large transverse momentum (PT>7 GeV) and decayed or interacted after 32 p.m [33]. 4.2.2. Fermilab experiment 531 Another emulsion-spectrometer experiment (E531) was located further upstream in the broad band neutrino beam thus benefitting from a 2.5 times higher neutrino intensity. In the first data taking run 23 1? of emulsion were exposed to the neutrinos from 7.8 x 1018 protons on the production target. Lifetime measurements of the D°,D~,F~and A ~ from this experiment have been published [34]. The spectrometer which served to locate the events in the emulsion and analyse the reaction and decay products is shown schematically in fig. 10 and described more fully in ref. [34]. The 12 upstream and 8 downstream drift chambers together with the large aperture, 0.65 Tesla central field magnet provided momentum analysis of 80% of the charged secondaries with z~p= ±[(O.OO5p2)2+(0.O13p)2]”2 resolution. The momentum of wider angle tracks that failed to traverse the spectrometer was measured in the upstream chambers and the fringe field of the magnet or by multiple scattering in the emulsion. The time of flight system, with a resolution for the 30 downstream counters of 120 ps gave a lu separation of IT and K up to 3 GeV/c and K and proton up to 6 GeV/c. Emulsion ionization measurements separated ir and K up to 0.8 GeV/c and K and p up to 1.5 GeV/c. An array of 68 lead glass blocks 19 cm x 19 cm by at least 10 radiation lengths long provided electron identification and approximate reconstruction and energy measurement for ir°and ~° as well as single photon detection and momentum measurement. A simple hadron calorimeter with 5 layers of iron each 10 cm thick interleaved with planes of 4 vertical scintillators gave some sensitivity to neutral hadrons (n and K~.) and, combined with two hodoscopes behind 1.2 m and 2.9 m of Fe, served to separate charged hadrons from muons.

________

MUON STEEL SCM -04 MAGNET EMULOON

DCII

VETO

-

H--—

CALORIMETER LEAD GLASS

---TIME OF FJGHT H000SCOPES---

/.LI ~MUON~ HODOSCOPES

-—~

IOO~~

Fig. 10. The E531 spectrometer.

/.LU

ID. Prentice, Study of short-lived particles with emulsion techniques

99

The emulsion target consisted of 12 modules each containing 177 emulsion pellicles 14 x 5 x 0.06 cm3 with the 5 cm dimension parallel to the beam and 27 modules of 68 films of polystyrene 12 X 9.5 cm2, 70 jim thick and coated on both sides with 330 jim of emulsion. The planes of the latter were perpendicular to the beam. The emulsion target is shown in fig. 11 together with the fiducial sheet of lucite coated on both sides with emulsion. The latter covered all the emulsion stacks, was located relative to them to better than 100 p.m by the marks left on it by collimated X-ray sources in the modules, was changed every two days to maintain a low background and thus served to couple individual tracks from the drift chambers to the main emulsion target. This technique substantially improved neutrino event finding as well as enhancing and calibrating the finding efficiency for charmed decays. A knowledge of the variation of the latter with decay length is essential to a maximum likelihood calculation of lifetimes from a small sample of decay time measurements. Reconstruction of events in the spectrometer predicted 2190 vertices in the vicinity of the emulsion target and after fiducial cuts around support structures 1800 remained which were searched for either by a volume scan or by track following into the emulsion. A total of 1251 were found for an overall efficiency of 70%. The charged decay search, performed by following for 6 mm all tracks with angles from the beam less than 0.2 radians, located 23 multiprong decay candidates and 26 single prong kinks of which 5, with Pt >0.250 GeV/c, are good candidates for charm decay. A search for neutral decays, at high magnification, made in a cylinder of radius 300 jim and 1000 p.m along the beam direction yielded 21 neutral decay candidates. In addition tracks found in the spectrometer that extrapolated to within 2 mm of the vertex but which were not matched to emulsion tracks were followed back. This procedure not only found several decay candidates beyond the search regions but also helped to determine the efficiency of the scan for neutral decays. Identification of the decaying charmed hadrons was accomplished by kinematic fitting and by means of decay particle identification in the spectrometer and emulsion. In each of six of the charged decays and one neutral there is an identified baryon in the final state, either a proton identified by time of flight or ionization or an observed and fitted A°decay. All six of Hexcel Plate

~

__

.4

Lucite Post

X-Ray source

Emulsion Films

(68 total

)~.

Lucite Polystyrene Emulsion ~FiduciaI Sheet

________

5cm

~-

Fig. 11. Schematic of emulsion target and fiducial sheet.

100

Lifetime measurements in the 10’~srange Table 1 A ~ decay candidates

Event number

Decay length Decay (~m) hypothesis

1 2 3 4

41 175 180 221

2C fit mass 2) (MeV!c

3C fit CL.

~ (GeV/c)

Decay time (10’s)

2131±63 2204 ±200 A~—eA°1T1T~1T 2274±41 A~—vA°,r~i~2374±63

0.27 0.97 0.41 0.05

5.73 5.80 8.40 4.67

0.54 2.30 1.63 3.60

2.9

0.73

5.0

0.42

AA°i~ir A ~ —e pK~

5

21

~

—

—

6

28

A~~p~ir(K°)

—

—

these charged decays have acceptable L~C= L~Shypothesis for A ~ decays and in four cases for which there are no unobserved decay particles a two constraint fit with an adjustable decay mass is possible. The weighted mean of the fitted masses for these events is 2263±27MeV. The length of the decaying track, the decay hypothesis, the two constraint fitted mass, the confidence level of the three constraint fit, the momentum of the charmed baryon and the proper decay time for each event is shown in table 1. Events 1, 4, 5 and 6 have been published [34]; the remaining two were reported at the sixteenth Rencontre de Moriond [35]. The particles underlined in the decay hypotheses have been identified by time of flight, ionization, range or decay kinematics. The proton from each of the A°decays was identified by time of flight. For two events the particles visible in the spectrometer implied some missing momentum that could be accounted for by assuming that a neutral decay particle had missed the Idetectors. In these cases a zero constraint calculation allowed two possible momenta and associated proper decay times. The fits shown in table 2 were similarly obtained for the D~and F~mesons while none of the F candidates has an acceptable D fit with confidence level >1% the probability of the F contamination in the D sample is somewhat higher. Events 1, 3 and 5 have acceptable fits to ~S = ~Q, F decay hypotheses but they are disfavoured by time of flight or OZI rule suppression of IT~1T tp final states. Most of the neutral decay candidates have acceptable D°fits or, in the case of the i induced events with an identified /L~ from the primary interaction vertex, D°fits. These are shown in table 3. D~ and

Event number 1 2 3 4

5 6 1 2 3

Decay length (~sm) 457 1802 2150 22.03 13000 2307 130 132 670

Decay

Table 2 P decay candidates 2C fit mass

2) (MeV/c uD*K*~+~.O 1829±35 D~—eKK~ir~sr° 1862±25 mD*K_~+~+(p) (at mm) D*_s~,r*1r_(K~) (at miii) *D+-eK_~,.+e+(v) (at miii) D * ~(i~) — P decays F~—e K~7T~1TKL 2057±111 P—eK~Kir~ir° 2050±45

Decay 3C fit CL.

P

(GeV/c) 0j,

time (1013)

hypothesis

F—vs-rrr°

2026± 56

0.07

10.0

0.52

17.4

— — —

16.6 11.0 111.0

2.85 6.44 8.06 12.5 6.9

—

8.9/10.0

0.68 0.46

9.33

0.94

5.93 12.25

1.51

0.51

16.5/14.4

3.90

ID. Prentice, Study of short-lived particles with emulsion techniques

101

Table 3 D°decay candidates

Event number 1 2 3

Decay length

Decay

(km) 6.5

hypothesis D°—eK

125 27

4

2C fit mass

5 6 7

4053

D°—eK7T~,f D° eK or°

8

5472

D°—eK~t~if(p

9 10

116

1.)

184 326 703 590 67 187 2646 4374

11 13 12 14 15 16 17

P (GeV/c) 0h

0.06(lc)

19.2 8.85 9.18

0.021

0.12

15.43 12.3

0.17 1.29

1876±122 1859±38

0.78 0.45

13.53 23.6

3.23 10.69

—

—

1939±117 1857±76

0.88 0.99

36/59 30.08 20.89

9.5/5.8 0.24 0.52

2000± 1911±76 130 1859±34

0.61 0.94 0.30

11.23 13.23

—

—

2.77 1.80 3.54 0.37

—

—

—

—

—

—

2) (MeV/c ~~1r,r 1923±46 D° rir(IT°) (at mm) ~°—K~1roT° 1766±48 D°-eKsr°ir~irir° 1855±43 D°—eKir~ir~i~ 1816±40

42 256 749

Decay 3C fit CL.

D°—eKir~ir°ir° Doi°ir° D°—eii~i~K~ 1Ar° DO~eiriif~K~ D°—*Kii D°—r~ir(K°) D°—eK~7r(1T°) D°_*K.sc~(v~) D°—eKe~(pe)

—

0.05

0.99

time

(jØ13)

0.88 0.18

12.35 11.3 6.8/9.5

1.7/1.2

23/39 30/20

7.2/4.2 9.2/4.3

A maximum likelihood method was used to obtain lifetimes from the sample of charmed decays. This requires a knowledge of the charm scan efficiency which is shown in fig. 12. Near the neutrino vertex, decay events can be obscured by primary tracks; 200 events were examined to estimate the average loss of decays as a function of distance. At large distances the efficiency was measured by comparing the results of the track following and neutral searches with the scan back of spectrometer tracks not seen at the primary vertex. CHARMED DECAY SEARCH EFFICIENCY I

II

I

Charged

Scan

II

I

‘‘‘I

II

maxinnuni possible

Efficiency

decoy Iengthv5cnn

Neutral Scan Efficiency 00 80

-i

I 2

5

10

20

50

I 100

Iii

T

500

Decay Distance (gm) Fig. 12. The chann scan efficiency for E531.

IT

2000

10000

3srange

Lifetime measurements in the 10~

102

Table 4 Fitted lifetimes and masses for charged charmed hadrons

Particle

Fitted lifetime (10’s s)

Fitted mass (MeV/c2)

Number of events

~

D~ P

9.5~ ~

2263±27 1851 ±20

6 6

2042±33

3

The weighted means of the fitted masses and the maximum likelihood fits for the A F4 and D~are given in table 4. Seven of the neutral events in table 3 (events 1, 2, 4, 9, 14, 15, 16) gave a lifetime of l.00~i~ x iO’~sec [34]. A straightforward application of the maximum likelihood method to the full sample [35]gives 3.2it~x iO’~sec with a probability >30% that the distribution of proper decay times is a statistical sample of a single exponential decay. There are, nevertheless, a number of reasons for treating this lifetime value with some caution. The semi-leptonic branching ratio for the D°as measured at SPEAR is at least three times smaller than that of the D~which also has a longer lifetime. The maximum likelihood estimate for the three semi-leptonic neutral decays in table 3 is 8.5i~i~ x 10~sec and for the 14 hadronic decays is ~ If this is taken as an indication of two lifetime components in the neutral sample, what then might one expect? Although much has been written theoretically about D°D° mixing there is no experimental evidence for it from e~e collisions [36]. Even if the D°and D°mixed substantially it would be hard to devise a scheme to cause a large lifetime difference between D? and D~since most of the final states resulting from D11 decay have S = I and are therefore not eigenstates of CP. A more likely scenario is that the sample is contaminated with weakly decaying neutral charmed strange baryons or charmed strange exotic mesons [37]or baryons [38].Although there are theoretical predictions for the masses of possible weakly decaying neutral hadrons with quark content such as (cds) [39], (csud) [37] and (c~ddd)[38]there is no experimental evidence for their existence. Kinematic fits of the events of table 4 to such decays as (cds) KIT4pKIT°are at best speculative; this is however, a consistent interpretation of the longest lived entry in the D°section (event 7) giving a fitted mass of 2583±26MeV/c2. Furthermore one neutral decay candidate (not shown in table 3) has a well identified proton in its final state and fits (cds)—~pirK~ (m = 2459 MeV/c2) or (css)—~pKK~(m = 2658 MeV/c2) if one is willing to accept Cabbibo unfavoured decay [35]. This event has a very long proper time —80 x 10 sec which might suggest that it is not a decay but an estimate of the probability that it results from a neutral particle interaction gives
—

—~

—

ID. Prentice, Study of short-lived particles with emulsion techniques

103

S. Lifetime summary Six constrained fit A ~ decays have been reported from CERN and Fermilab (WA17, WA58 and E531). If one assumes that the scanning efficiencies are similar for the three experiments then all six events have equal weight and the maximum likelihood fit for the lifetime will be the same as the mean i.e. TA~= 2.6 X 10 seconds. The weighted mean of the 6 fitted masses is MA~= 2268 ±13 MeV/c2. In each of these events there is a positively identified baryon in the final state so there is little probability of contamination in the sample. The four F~events (experiments E531 and E564) have similarly good fits and none of them has a D~ or A ~ fit with confidence level >1%. The mean fitted mass and lifetime are MF~= 2026±20MeV/c2 and TF~= 1.9~i~ x 10_13 seconds. For the D~there are only two fitted events (E531). Three of the semi-leptonic, zero constraint fits, however, occur at the minimum of the curve of missing momentum versus parent mass, thus determining unique values for decaying particle’s momentum and proper time. The mass for the two fitted events is 1850±20MeV/c2and the lifetime for events ito 5 is 8.9~gxi0’~seconds. It is clear that both the A ~ and F~have considerably shorter lifetimes than the D~.Both the CERN experiments have used the statistical, track offset method [18] to deduce lifetimes for a sample of unfitted charged decays that they assume to be mainly D~mesons. In view of the unknown mixtures of the shorter lived A ~ and the F~hadrons in these event samples it is a little difficult to assess the reliability assigning the values 2.5 x i0’~seconds (WA58) and 4.4 x i0~~ seconds to the mean life of the D4 meson. Although the charged D mesons are probably the most copiously produced of the three charged charmed hadrons the difficulty of obtaining an uncontaminated sample of D~events may make their mean life the hardest of the three to measure. The problems inherent in determining the D°lifetime from the present E531 neutral decay sample were described at length in the last section. Until the final analysis of the new events is completed it is appropriate to quote, in summary, the published value of 1.0i~i?:~ x iO’~sec. The values of 0.5~i~ X 1013 sec and 0.43 x i0’~sec for the neutral lifetime based on the track offset analysis [18] applied to sets of unidentified neutral decays by the WA17 and WA58 groups respectively appear to be subject to some doubt due both to possible decays of neutral particles other than D°and to unknown scanning efficiencies. 6. Conclusions The high precision of emulsion track detectors is extremely useful for the measurement of very short particle lifetimes. The i-~.o 0.8 x 1016 sec is probably just beyond the short lifetime limit for accurate results even with high statistics and a well understood easily recognized decay channel. For the particles of greatest current interest with masses >Mr = 1784 MeV, the complexity of the decays and the high energies involved render bare emulsions or even emulsion chambers ineffectual for lifetime studies. They have been used successfully to observe charmed particle decays and to establish the approximate range of the lifetimes but provide insufficient detail to identify individual lifetimes. The combination of emulsion targets with larger scale downstream detectors makes possible the detailed kinematic fits and particle identification that are essential in determining accurate lifetimes of particular particles. In complex events, reasonable scanning efficiency can be maintained only down to about 10 p.m.

104

Lifetime measurements in the 1013s range

Detection of shorter decay tracks, down to about 1 jim, can be effected by measuring offsets of tracks from the primary vertex. The added precision that one might expect from the larger time dilation factors associated with higher energies is not, in fact, obtained because, for a given decay energy, the impact parameters of the decay tracks extrapolated back to the primary vertex are almost independent of the decaying particle momentum. Choice of energy and beam particle thus depends mainly on production cross sections for the particle of interest and the total cross section for the beam which determine overall yield and the ratio of signal to background. Efficient matching of emulsion and spectrometer tracks is essential not only to maximize the yield of primary events but also in finding decays and in calibrating the efficiency of the scan for decays. The last is a crucial input to a maximum likelihood fit for obtaining lifetimes from a limited set of proper decay times. Study of charm decays has already demonstrated the importance of wide angular acceptance, good particle identification and high resolution in identifying individual decaying particles. A very large sample of events is needed to separate two comparable lifetimes by the shape of the decay time distribution alone. Good angular acceptance and neutral particle detection efficiency are of primary importance since a single missing decay particle prevents a kinematic fit. Good momentum resolution providing accurate fitted masses is helpful in identifying known particles but with the present uncertainty about charmed strange baryons and exotics, identification of decay products is important. The observation of a weak decay is a powerful tool in eliminating backgrounds and even small statistics but extremely pure samples of charmed hadrons may be useful for studying production mechanisms and spectroscopy. The discovery of charm and the third generation of quarks and leptons has inspired a successful marriage of venerable emulsion detectors with youthful electronic spectrometers. New developments in semiconductor-detectors and partial automation of emulsion measurement [40] may extend the useful lifetime of these techniques through the area not only of charm but of beauty as well.

Acknowledgements Support of the Natural Sciences and Engineering Research Council through the Institute of Particle Physics of Canada is gratefully acknowledged. I should like to thank particularly all the members of the E531 collaboration [34]from some of whom I have learnt all that I know about the emulsion technique. Thanks also to many other colleagues for enlightening discussions and in some cases pre-publication data and in particular B. Conforto, M. Conversi, G. Coremans-Bertrand, G. Diambrini-Palazzi, W. Frisken, L. Hand, A.L. Read, L. Voyvodic and D. Wagoner. Finally I should like to honour the memory of E.H.S. Burhop to whom we are all grateful for his pioneering work in the hybrid emulsion technique. References [1] Particle Data Group, Review of Particle Properties, Rev. Mod. Phys. 52 (1980) 105. [2] M.K. Gaillard, B.W. Lee and J. Rosner, Rev. Mod. Phys. 47 (1975) 277. [3] M. Kobayashi and T. Maskawa, Prog. Theor. Phys. 49 (1973) 652. [41EL. Koller, S. Taylor and T. Huetter, Nuovo Cimento 27 (1963) 1405 (references to other early emulsion measurements of r,,o are listed in [1]). [5]G. Belletuni, C. Bemporad, P.L. Braccini and L. Foà, Nuovo Cimento 40 (1965) 1139 (see also other references in [1]). [6] P. Stamer, S. Taylor, EL. Koller, T. Huetter, J. Grauman and D. Pandoulas, Phys. Rev. 151 (1966)1108.

ID. Prentice, Study of short-lived particles with emulsion techniques

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[7] For a review of early works see K. Niu, Proc. 19th ut. Conf. on High Energy Physics, Tokyo 1978, p. 447. [8] K. Niu, E. Mikumo and Y. Maeda, Prog. Theoret. Phys. 46 (1971) 1644. [9] T. Hayashi, E. Kawai, M. Matsuda, S. Ogawa and S. Shigeeda, Prog. Theoret. Phys. 47 (1972) 280, 1998. [10] N. Ushida, H. Fuchi, K. Hoshino, S. Kuramata, K. Niu, K. Niwa, H. Shibuya, S. Takaka, Y. Yanagisawa, Y. Maeda and H. Kimura, Lett. Nuovo Cimento 23 (1978) 577; H. Fuchi, K. Hoshino, S. Kuramata, K. Niu, K. Nura, H. Shibuya, S. Tasaka and Y. Yanagisawa, Phys. Lett. 85B (1979) 135. [11] H. Fuchi, K. Hoshino, M. Miyanishi, K. Niu, K. Niwa, H. Shibuya, Y. Yanagishawa, N. Ushida, S. Tasaka, Y. Maeda and H. Kimura, Lett. Nuovo Cimento (to be published). [12] AS. Dyoretsky, V.A. Kazakov, IV. Koleskov, Yu. Oravets, 1.1. Skrill, V.F. Sikolenko, LV. Silvestnov, N.S. Frolov and MS. Khvastunov, NucI. Instr. Meth. 20 (1963) 277; P.K.F. Grieder, Suppl. Nuovo Cimento 26 (1962) 271; NB. Mistry, G.T. Murthy, P.V. Ramana Murthy and By. Streekantan, Nuovo Cimento 17 (1960) 429. [13] E.H.S. Burhop, W. Busza, D.H. Davis, B.G. Duff, D.A. Garbutt, F.F. Heymann, KM. Potter, J.H. wickens, C. Brickman, J. Lemonne, J. Sacton, G. Schorochoff, MA. Roberts and WA. Toner, Nuovo Cimento 39 (1965)1037. [14] E.H.S. Burhop, D.H. Davis, D.N. Tovee, D. Petersen, AL. Read, G. Coremans-Bertrand, J. Sacton, P. Vilain, A. Breslin, A. Montwill, MA. Roberts, D.A. Garbutt, FR. Stannard, G. Blaes, R. Klein, F.M. Schmitt, G. Baroni, G. Ceradini, M. Conversi, L. Federici, ML. Ferrer, S. Gentile, S. DiLiberto, S. Petrera, G. Romano, R. Santonico, G. Bassompierre, M. Jung, N. Kurtz, M. Paty and M. Schneegans, Phys. Lett. 65B (1976) 299; AL. Read, G. Coremans-Betrand, J. Sacton, P. Vilain, A. Breslin, A. Montwill, D.A. Garbutt, E.H.S. Burhop, D.H. Davis, D.H. Tovee, FR. Stannard, G. Blaes, R. Klein, F.M. Schmitt, G. Baroni, F. Ceraduni, M. Conversi, S. DiLiberto, L. Federici, ML. Ferrer, S. Gentile, S. Petreta, G. Romano, R. Santonico, G. Bassompierre, M. Jung, N. Kurtz, M. Paty and M. Schneegans, Phys. Rev. D19 (1979) 1287. [15] D. Allasia, C. Angelini, P. Bagnaia, G. Baroni, J.H. Bartley, G. Bertrand-Coremans, V. Bisi, A. Breslin, E.H.S. Burhop, F. Carena, R. Casali, G. Ciapetti, M. Conversi, DII. Davis, S. Di LiberIa, R. Fantechi, ML. Ferrer, C. Franzinetti, D. Gamba, L. Godfrey, D. Keane, E. Lamanna, A. Marzari, F. Marzano, A. Montwill, A. Nappi, C. Palazzi-Cerruna, R. Pazzi, S. Petrera, G.M. Pierazzuni, G. Romano, A. Romero, J. Sacton, R. Santonico, R. Sever, FR. Stannard, P. Tolun, D.N. Tovee, P. Vilain, J.H. Wickens and G. Wilquet, NucI. Phys. B176 (1980)13. [16] C. Angetini, P. Bagnaia, G. Baroni, J.H. Bartley, G. Bertrand-Coremans, V. Bisi, A. Breslin, E.H.S. Burhop, F. Carena, R. Casali, G. Ciapetti, M. Conversi, D.H. Davis, S. DiLiberto, ML. Ferrer, C. Franzinetti, D. Gamba, L. Godfrey, D. Keane, E. Lamanna, A. Marzari, F. Marzano, V. Moggi, A. Montwill, A. Nappi, C. Palazzi-Cerrina, R. Pazzi, S. Petrera, G.M. Piera.zzini, G. Romano, A. Romeno, J. Sacton, R. Santonico, R. Sever, FR. Stannard, P. Tolun, D.N. Tovee, P. Vilain, J.H. Wickens and G. Wilquet, Phys. Lett. 80B (1979) 428. [17] C. Angelini, P. Bagnaia, G. Baroni, J.H. Bartley, G. Bertrand-Coremans, V. Bisi, A. Breslin, E.H.S. Burhop, F. Carena, R. Casali, G. Ciapetti, M. Conversi, D.H. Davis, S. DiLiberto, ML. Ferrer, C. Franzinetti, D. Gamba, L. Godfrey, D. Keane, E. Lamanna, A. Marzari, F. Marzano, A. Montwill, R. Morganti, A. Nappi, C. Palazzi-Cernina, R. Pazzi, S. Petrera, G.M. Pierazzini, L. Riccati, G. Romano, A. Romero, J. Sacton, R. Santonico, R. Sever, FR. Stannard, P. Tolun, D.N. Tovee, P. Vilaun, J.H. wickens and G. Wilquet, Phys. Lett. 84B (1979) 150. [18] 5. Petrera and G. Romano, NucI. Instr. Meth. 174 (1980) 61. [19] R. Amman, D. Coppage, R. Davis, N. Kwak, R. Riemer, R. Stump, H. Kautzky, w. Smart, S. Velen, L. Voyvodic, V. Ammosov, V. Baranov, A. Belkov, V. Gapienko, V. Klyukhin, V. Koreshev, P. Pitukhin, V. Sirotenko, V. Yarba, V. Efremenko, 0. Egorov, P. Goritchev, V. Kaftanov, N. Kolganova, M. Kubantsev, I. Makhluyeva, E. Poxharova, V. Shevchenko, V. Smirnitsky, A. Weissenberg, J. Babecki, B. Furmanska, R. Holynski, A. Jurak, S. Krzywdzinski, G. Nowak, H. Wilczynski, W. Wolter, B. Woseik, S. Bunyatov, M. Ivanova, 0. Kuznetsov, V. Lyukov, V. Sidonov, H. Tchernev, K. Bogomolov, T. Burnett, J. Lord, R. Rosenbladt and R.J. Wilkes, Phys. Lett. 94B (1980) 118. [20] R. Brandelik, W. Braunschweig, H-U. Martyn, HG. Sander, D. Schmitz, W. Sturm, W. Wallraff, D. Cords, R. Felst, R. Fries, E. Gadermann, H. Hultschig, P. Joos, W. Koch, U. Kotz, H. Krehbiel, D. Kreinick, H.L. Lynch, WA. McNeely, G. Midenberg, K.C. Moffeit, D. Notz, M. Schliwa, A. Shapira, B.H. Wiik, G. Wolf, J. Ludwig, K.H. Mess, A. Petersen, G. Poelz, J. Ringel, 0. Romer, R. Rusch, K. Sauerbert, P. Schmuser, W. de Boer, G. Buschhorn, W. Fues, Ch. von Gagern, G. Grindhammer, B. Gunderson, R. Kotthaus, H. Lierl, H. Oberlack, S. Onto, 1. Suds, Y. Totsuka and S. Yamada, Phys. Lett. 80B (1979) 412. [21] V. Luth, Proc. IXth Intern. Symp. on Lepton and Photon interactions at High Energies Fermilab (1979) p. 78; J. Kirkby, ibid. p. 107. [22] D. Aston, M. Atkinson, R. Bailey, A.H. Ball, B. Bouquet, G.R. Brooks, J. BrOring, P.J. Bussey, D. Clarke, A.B. Clegg, B. D’Almagne, G. De Rosny, B. Diekmann, A. Bonnachie, M. Draper, B. Drevillon, I.P. Duerdoth, J.-P. Dufey, RJ. Ellison, D. Ezra, P. Feller, A. Ferrer, P.J. Flynn, F. Friese, W. Gaibraith, R. George, S.D.M. Gill, M. Goldberg, S. Goodman, M. Górski, W. Graves, B. Grossetete, PG. Hampson, K. Heunloth, RE. Hughes-Jones, J.S. Hutton, M. Ibbotson, M. Jung, S. Katsanevas, M.A.R. Kemp, F. Kovacs, BR. Kumar, GD. Lafferty, J.B. Lane, J.-M. Levy, V. Liebenau, J. Litt, G. London, D. Mercer, J.V. Morris, K. MOller, D. Newton, E. Paul, P. Petroff, Y. Pons, C. Raine, F. Richard, R. Richter, J.H.C. Roberts, P. Roudeau, A. Rouge, M. Rumpf, M. Sené, J. Six, 1.0. Skillicorn, J.C. Sleeman, KM. Smith, C. Steunhauer, K.M. Stonr, R.T. Thompson, D. Treulle, Cli. Dc La Vaissiere, H. Videau, I. Videau, A.P. Waite, A. Wijangco, W. Wojcik, i-P. Wuthnick and T.P. Yiou, Photoproduction of charmed F mesons at y energies of 20—70 GeV, Phys. Lett. 100B (1981) 91. [23] DO. Caldwell, J.P. Cumalat, AM. Eisner, R.M. Egloff, M.E.B. Franklin, G.J. Luste, J.F. Martin, J.D. Prentice and 1. Nash, Phys. Rev. Lett. 40 (1978) 1122.

[24] P. Avery, J. Wiss, J. Butler, G. Gladding, M.C. Goodman, T. O’Halloran, JJ. Russell, A. Wattenberg, M. Binkley, J. Cumalat, I. Gaines, M. Gormley, R.L. Loveless, J. Peoples, MS. Atiya, S.D. Holmes, B.C. Knapp, W. Lee and WJ. Wisniewski, Phys. Rev. Lett. 44 (1980) 1309.

106

Lifetime measurements in the 1013 s range

[25] B. Knapp, W. Lee, P. Leung, S.D. Smith, A. Wijangco, J. Knauer, D. Yount, J. Bronsteun, R. Coleman, G. Gladdung, M. Goodman, M. Gormley, R. Messner, T. O’Halloran, J. Sarracuno, A. Watterberg, M. Binkley, I. Gaines and J. Peoples, Phys. Rev. Lett. 37 (1976) 882. [26] L. Hand, D. Petersen, H. Scott, M. Atac, AL. Read, L. Voyvodic, A. Dwurazny, B. Furmansk, AR. Holynski, A. Jurak, S. Krzywdzinski, R. Ball, J. Kiley, A. Kotlewski, L. Litt, J. Florian, ii. Lord and R.J. Wilkes, NucI. Instr. Meth. 167 (1979) 261. [27] L. Hand, Proc. Intern. Symp. on Lepton and Photon Interactions at High Energies, Hamburg (1977) p. 417. [28] MI. Adamovich, Y.A. Alexandrov, J.M. Bolta, L. Bravo, AM. Cartacci, MM. Chernyavski, A. Conti, G. Crosetti, MG. Dagliana, M. Dameri, G. Diambrini-Palazzi, G. Di Caporiacco, A. Forino, R. Gessaroli, E. Highon, S.P. Kharlamov, V.G. Lanionova, R. Llosa, J. Lory, A. Mattei, C. Meton, B. Monteleoni-Conforto, G.I. Orhova, B. Osculati, G. Parrini, N.G. Peresadko, A. Quareni Vignudehhi, A. Ruiz, KM. Romanovskaya, M. Sannino, D. Schune, G. Tomasuni, MI. Tretyakova, Tsai Chu, G. Vanderhaeghe, E. Villar, B. Willot, M. Atkinson, T. Bnodbeck, G.R. Brookes, P.J. Bussey, M. Davenport, J.-P. Duffey, R.J. Ellison, W. Galbraith, K. Heunloth, iS. Hutton, RE. Hughes-Jones, M. Ibbotson, BR. Kumar, GD. Lafferty, J.B. Lane, iC. Lassalle, R. McClatchey, D. Mercer, J.V. Morris, D. Newton, G.N. Patrick, C. Raune, A. Schlósser, KM. Starr and A.P. Waite, Phys. Lett. 99B (1981) 271. [29] A. Fiorino, R. Gessaroli, A. Quareni-Vignudelli, G. Vanderhaeghe, AM. Cartacci, B. Conforto, A. Conti, MG. Dagliana, G. Di Caporiacco, G. Parnini, M. Dameni, G. Diambnuni-Palazzi, B. Osculati, C. Persano, M. Sannino, G. Tomasini, R. Llosa, MI. Adamovich, Y.A. Alexandrov, MM. Chernyavski, S.P. Kharlamov, V.G. Larinova, G.I. Orlova, N.G. Peresadko, KM. Romanovskaya, MI. Tretyakova, J. Lory, C. Meton, D. Schune, Tsai Chu, B. Willot, L. Bravo, A. Rutz, E. Villar, J.M. Bolta, E. Higon, K. Heinloth, A. Schulósser, i-P. Buffer, GD. Lafferty, i-C. Lassale, P.J. Bussey, G.N. Patrick, C. Raine, T. Brodbeck, D. Newton, KM. Storr, Ri. Elhison, RE. Hughes-Jones, M. Ibbotson, J.B. Lane, D. Mercer, A.P. Waite, M. Atkinson, M. Davenport, J.S. Hutton, BR. Kumar, J.V. Morris, G.R. Brookes, W. Galbraith and R. McCiatchey, Lett. Nuovo Cimento 30 (1981) 166. [30] MI. Adamovich, Y.A. Alexandrov, J.M. Bolta, L. Bravo, AM. Cartacci, D. Conforto, A. Conti, MM. Chernyavski, MG. Dagliana, M. Dameni, G. Diambruni-Palazzi, G. Di Caponiacco, A. Forino, R. Gessaroli, E. Higon, S.D. Kharlamov, V.G. Lanionova, R. Llosa, J. Lory, C. Meton, B. Osculati, G. Parrini, A. Quareni, M. Sannino, D. Schune, G. Tomasini, MI. Tretyakova, Tsai Chu, G. Vanderhaeghe, E. Villar, B. Willot, D. Aston, M. Atkinson, R. Bailey, A. Ball, G.R. Brookes, J. Broning, P.J. Bussey, D. Clark, G. de Rosny, B. Diekmann, B. Drevillon, M. Draper, I.P. Duerdoth, J.-P. Dufey, R.J. Ellison, D. Ezra, P. Feller, P.J. Flynn, F. Friese, R. George, 5DM. Gill, M. Goldberg, W. Graves, B. Grossetete, PG. Hampson, J. Hutton, M. Ibbotson, R.E.H. Jones, M. Jung, S. Katsanevas, MAR. Kemp, F. Kovacs, R. Kumar, GD. Lafferty, J.B. Lane, J.-M. Levy, V. Liebenau, J. Litt, G. London, D. Mercer, K. Muller, D. Newton, E. Paul, P. Petroff, Y. Pans, C. Raune, R. Richard, R. Richter, J.C.H. Roberts, P. Roudeau, A. Rouge, M. Rumpf, J. Sleeman, C. Steinhauer, KM. Storr, Ri. Thompson, D. Treille, Cli. de ha Vaissiene, H. Videau, I. Videau, A.P. Waite, A. Wijangco, W. Wojcik, J.-P. Wuthrick and T.P. Yiou, Phys. Lett. 89B (1980) 427. [31] B. Conforto, private communication, March 1981. [32] J. Florian, L. Hand, R. Kass, D. Wagoner, B. Andersson, G. Claesson, G. Ingelman, I. Otterlund, K. Sóderstróm, Z. Osborne, W. Cooper, E. Engles, P. Shepard, J. Thompson, A. Bakich, L. Peak, W. Frisken, W. de Rosanio and W. Schappert, Proceedings of Neutrino 79, Bergen (1979) p. 36. [33] D. Wagoner, private communication, March 1981. [34] N. Ushida, T. Kondo, G. Fujioka, H. Fujushima, Y. Takahashi, C. Yokoyama, Y. Homma, Y. Tsuzuki, S. Bahk, C. Kim, J. Park, J. Song, D. Bailey, S. Conetti, J. Fischer, J. Trischuk, H. Fuchi, K. Hoshuno, M. Miyanishi, K. Niu, K. Niwa, H. Shibuya, Y. Yanagisawa, S. Ernede, M. Gutzwiller, S. Kuramata, NW. Reay, K. Reibel, TA. Romanowski, R. Sidwell, N.R. Stanton, K. Moriyama, H. Shibata, T. Hana, 0. Kusumoto, Y. Noguchi, M. Teranaka, H. Okabe, J. Yokota, J. Harnis, C. Hebert, J. Hebert, S. Lokanathan, B. McLeod, S. Tasaka, P. Davis, J. Martin, D. Pitman, iD. Prentice, P. Sinervo, T.S. Yoon, H. Kimura and Y. Maeda, Phys. Rev. Lett. 45 (1980) 1049; Phys. Rev. Lett. 45 (1980) 1053. [35] Proceedings of the Sixteenth Rencontre de Moriond, March 1981 (to be published). [36] See for example RH. Schindler, Charmed Meson Production and Decay Properties at the ~i (3770), thesis, SLAC 219, unpublished; RH. Schindler et al., Phys. Rev. D, to be published. [37] Hi. Lipkin, Phys. Lett. 70B (1977) 113; N. Isgur and Hi. Lipkin, Phys. Left. 99B (1981) 151. [38] i. Rosner, private communication. [39] K. Mailman and N. Isgur, Phys. Rev. D22 (1980) 1701. [40]Fermilab proposal P653 (Spokesperson, NW. Reay).