A search for slow massive particles with Z ⩽ 2 at the CERN intersecting storage rings

Volume 56B, number 1

31 March 1975

PHYSICS LETTERS

A SEARCH FOR SLOW MASSIVE PARTICLES AT THE CERN INTERSECTING

STORAGE

WITH Z < 2 R I N G S ¢r

J.V. JOVANOVICH*, T.W. MILLAR and P. O'CONNOR

Department of Physics, University of Manitoba, I¢innipeg, Canada and M. BOZZO, A. B()HM, H. FOETH, M. HANSROUL, G. MADERNI, C. RUBBIA, A. STAUDE and P. STROLIN

Aachen-CERN-Genova-Harvard- Torino Collaboration Received 2 February 1975 A large scintillation counter telescope positioned at right angles to the crossing beams at the CERN ISR was used in a search for slow (stopping) massive particles with charges Z < 2. No events which could be unambiguously identified as being due to unknown particles were seen among 10a charged particles which passed through the telescope.

Since the CERN Intersecting Storage Rings started operation, several searches for quarks and other unknown particles have been undertaken and some o f the results have been published [1 ]. In these experiments the main emphasis was on the detection o f fast particles with charge less than one. As the dynamics o f production o f hypothetical massive particles (including quarks) is unknown, it might be possible that they are produced with such low energies that they could not be detected in the above experiments. For instance, the statistical model o f Hagedorn [2] predicts that the mean transverse moment u m for sufficiently massive particles is equal to 0.5 x / ~ (3,/is the particle mass in GeV/c 2) and that the angular distribution is essentially isotropic ~:1 in the c.m. system. This might serve as a hint that quarks (or other massive particles) could be produced with ~, This experiment was supported in part by the Canadian Institute of Particle Physics and the University of Manitoba. * On sabbatical leave at CERN, during the first part o f this work. ,1 We are grateful to Dr. C. Daum for giving us the program for computing the expected angular and momentum distributions of massive particles in the ISR laboratory frame of references. For 26 + 26 GeV ISR beam energies the calculation showed that particles with masses above 12 GeV are produced fairly isotropicaUy and with the mean longitudinal momentum nearly equal to the mean transverse momentum.

small (non-relativistic) energies in the c.m. system , 2 . At the ISR where the laboratory system is nearly identical to the c.m. system, it is, therefore, o f interest to search for slow massive particles, in addition to searching for fast ones. In this paper, we present the results o f an experimental search for slow massive particles produced perpendicularly to the plane o f the intersecting beams at the CERN ISR. A plastic scintillation counter telescope (fig. 1) was designed so as to maximize the detection efficiency for Z = 1/3 and Z = 2/3 particles having momenta approximately equal to the mean (transverse) momenta as predicted b y the statistical model [2]. The range of particle velocities (/3 = v/c) accepted in the telescope was between 0.08 and 0.4. The five counters (D1, ..., DS) measured the energy loss o f particles traversing the telescope and the sixth (E counter) measured the energy o f particles stopping in it. Counters V 1 to V 4 were arranged in the form o f a frame around the D 4 counter. Three times o f flight (TOF's) were measured as indicated. Four wire

,2 The statistical model predicts unmeasurably small [2] cross-sections for the production of massive particles. Therefore, if it is considered to be a realistic model, all the attempts to discover quarks with masses greater than about 4 GeV/c 2 must be fruitless even if quarks exist. One of us (JVJ) is greatly indebted to Dr. R. Hagedorn for several discussions on this subject. 105

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r

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Fig. 1. Layout of the telescope. Wire chamber gaps 2 and 4 had wires oriented at 45* relative to wires in gaps 1 and 3. chamber gaps giving a total of eight track coordinates were used except in the beginning of the experiment. The telescope subtended a solid angle of 0.11 sr to the beam intersection region. The trigger D1D2D3D4D 5 A V 1 V2V3V 4 was subject to the condition that the pulses of the D counterswere 1.8 or 3 times larger than for minimum ionizing particles. Whenever a trigger occurred, all TOF's, pulse heights, a live time clock and - if chambers were used - spark coordinates were recorded on magnetic tape. The experiment ran parasitically with another (pp elastic) ISR experiment [3] by sharing data-acquisition equipment. The electronics were organized to open the gates for the telescope whenever the gates in the main experiment were closed. The charge (Z) of the detected particles was determined three times from the TOF's (ti) and energy loss measurements (ZkEi) using the relation [4] AE = const Z2t 2 In If(t)]. The masses were computed from the expression

M=(E+z2~E5+Z~E4/2)(1-O2+~/1-~2)H32.

(1)

Here E is the kinetic energy detected in the E counter 106

31 M a r c h 1 9 7 5

and/33 is the velocity of the detected particle in the third TOF region. The terms AE 5 and AE4/2 are corrections introduced to compensate for the fact that /33 is the average velocity measured between D 3 and E counters (see fig. 1), rather than the velocity of the particle at the entrance to the E counter. The validity of this expression was checked for all values of Z and particle energies of interest. The telescope was calibrated with cosmic rays. In addition to this, in a number of runs, the constants for conversion of pulse heights to energy losses and TOF's were obtained and their self-consistency was checked by determining the charge and mass of beambeam produced protons #3. The correct charge and mass were obtained with HWHM measured as 8% and 15%, respectively. For particles more massive and slower than detected protons, the accuracy improves by a factor of two to four. Information from the wire chambers was used to reject certain types of background events (multiple tracks, particles going through counter edges, etc.). The spark chamber efficiency was estimated to be greater than 90% for singletracks and in the range between 60% and 80% for double tracks. Of all the data, 72% were taken at 26 + 26GeV energy (~ of these data were taken without wire chambers), and 28% at 22 + 22 GeV. About ~ of the data were taken with the lower (1.8 times minimum ionizing particles) triggering bias. The luminosity-time product was calculated for each data-taking run using the live (open gates) time of this experiment and the luminosity as measured by the Van der Meer method. The total integrated luminosity-time product was 1.2 X 1035 cm -2. The uncertainty on this value was estimated to be 10%-20%. In the analysis of data taken without wire chambers, all events passing some trivial constraints (like pulse heights being within the linearity range of electronics, etc.) were accepted. If wire chambers were used, then the following additional constraints were imposed: (i) one and only one track was reconstructed with existing spark chamber information; (ii) the track ,3 Although protons are not able to simulate completely very slow and very massiveparticles with charges different from one, we estimate that the above procedure was adequate since particular attention was paid to the saturation of photomultipliers and the width of coincidence delay CHIVES.

Volume 56B, number 1

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originated in the beam intersection region and did not pass within 1.5 cm of the edges of any D counter. The E counter was sufficiently large so that it was not necessary to apply any constraint on it. For events passing these constraints, the mass M, the charge Z and a goodness of fit parameter defined as

cross-section was computed, using the measured luminosity-time product and the momentum acceptance of the telescope. The limits obtained are shown as solid lines in fig. 4. No limits are given for chargei

~

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were calculated. In eq. (2), the subscript i designates

,4 We tried a minimizing procedure on a small number of events, but the improvements were unimportant as far as practical rejection of background was concerned. ,s Pions stopping in the E counter are not visible as most of them were rejected by the triggering condition requiring particles to be more than three times minimum ionizing.

I

300

m2 =~(ti-tci~ 2 5 /~Ei-~Eci\2

measured quantities and ci the corresponding values computed (taking energy losses into account) on the assumption that the particle had the calculated charge and mass and the detected kinetic energy. The parameter m 2 is very similar to the commonly used X2, except that it is always larger than X2 for two reasons. Firstly, in the routine analysis of data we have not minimized #4 m 2 by Changing values of M, Z, and E; secondly, the measured energy losses and velocities are not Gaussian distributed. Fig. 2a shows the m 2distribution for protons #s (more precisely: the events below line A in fig. 3). From this distribution, which has qualitatively the shape of a X2 distribution for six degrees of freedom, a cut m 2 ~< 30 was assigned to accept events as candidates for single particles in the spectrometer. The scatter plot in fig. 3 gives the mass versus the charge for events having m 2 < 30. The figure displays a huge proton peak (for clarity of the figure, below line A only a small sample is shown) whose tails extend up to the somewhat arbitrarily chosen line B. Beyond this line there are still six events. However, from the m 2 distribution of all events (before the m2-cut) above line B (fig. 2b) we see that these six events with m 2 < 30 are compatible with background and no enhancement at low m 2 is visible. It is coneluded that no evidence is provided for production of new particles. For the charge-mass region of fig. 3, where no events were detected, the upper limit (90% confidence level) for the double-differential production

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Volume 56B, number 1

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sufficiently heavy masses the production of heavy. particles is qualitatively as in the statistical theory, the upper limits for the total cross-sections range between 3 × 10 -33 and 10 -32 cm 2, depending on particle mass and charge. However, while the limits in fig. 4 represent the result of the measurement, these limits for total cross-sections are highly model dependent and should therefore not be given intrinsic quantitative significance, particularly in comparison with limits obtained in experiments which covered different regions of phase-space or where cross-sections were computed using different production models.

2

Fig. 4. Upper limits (solid curves) for double differential cross-sections as a function of mass and charge. Dashed curves represent mean momentum of particles accepted by the telescope. mass ranges for which events, although compatible with background, were observed. The over-all systematic error to the limits is estimated to be in the 30% range. The dashed curves in fig. 4 (scale to the right) represent the mean momentum recorded by the telescope as a function of particle mass and charge. Denoting by P2 the maximum and by Pl the minimum momentum of particles detected by the telescope, the momentum range is given by P.2/Pl for all charges and masses, except Z = ~ and M ~ 15 GeV/c 2. In this case the momentum range decreases as the mass decreases, being P2/Pl = 1.8 and 1.5 for M = 10 and 5 GeV/c 2, respectively. Using the measured upper limits on double differential cross-sections from fig. 4 and assuming that for

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31 March 1975

We thank the ISR Department, its Experimental Support Staff, and the ISR coordinator for their efforts. Dr. G. Barbiellini designed and tested part of the telescope for use in a previous experiment. Finally, we acknowledge a moderate, but timely, grant from the Canadian Institute for Particle Physics, as without it the experiment could not have been carried out.

References [1] M. Bott-Bodenhausen et al., Phys. Lett. 40B (1973) 693; B. Alper et al., Phys. Lett. 46B (1973) 265; D.D. Caldwell, C.W. Fabjan, C.R. Gruhn, L.S. Peak, L.S. Rochester, F. Sauli, U. Stiedin, R. Tirler, B. Winstein and D. Zahniser, paper to be sumitted to Nucl. Phys; C.W. Fabjan, private communication. [2] R. Hagedorn, Thermodynamics of strong interactions, CERN 71-72 (1972). [3] A. B6hm et al., Phys. Lett. 49B (1974) 491. [4] L.C.L. Yuan and C.S. Wu (eds.), Methods of experimental physics, VoL 5A (Academic Press, N.Y., 1961-63).