Search for strange quark matter in high-energy heavy-ion collisions

Volume 252, number 4 PHYSICS LETTERS B 27 December 1990

Search for strange quark matter in high-energy heavy-ion collisions

J. Barrette a R. Bellwied b, p. Braun-Munzinger b, W.E. Cleland c, G. David b E. Duek d, M. Fatyga d, D. FOX e, S.V. Greene r, J.R. Hall g, R. Heifetz h, T.K. Hemmick f, N. Herrmann b.~, R.W. Hogue d, G. Ingold b, K. Jayananda °, D. Kraus c, A. Legault a, D. Lissauer d, W.J. Llope b, T. Ludlam d, R.D. Majka f, D. Makowiecki d, S.K. Mark a, J.T. Mitchell f M. Muthuswamy b, E. O'Brien d, L. Olsen ~", V. Polychronakos d, M. Rawool-Sull ivan i, F.S. Rotondo f, J. Sandweiss f, B. Shivakumar f, J. S imon i, A.J. Slaughter f, U. Sonnadara c, j. Stachel b, J. Sullivan i, j. Sunier e, H. Takai d, T. Throwe d, H. Van Hecke e, L. Waters b, K. Wolf i, D. Wolfe g and C. Woody d a McGill University, Montreal, Canada H3A 27"8 b State University o f New York Stony Brook, N Y 11794, USA c University o f Pittsburgh, Pittsburgh, PA 15260, USA a Brookhaven National Laboratory, Upton. N Y 11973, USA e Los Alarnos National Laboratory, Los Alamos. NM 87545, USA f Yale University, New Haven, C T 06511, USA

University o f New Mexico, Albuquerque, N M 87131, USA h University ofTelAviv, TelAviv, Israel

Texas A&M University, College Station, TX 77843, USA

Received 10 May 1990; revised manuscript received 24 September 1990

We have initiated a search for strange quark matter in the products of 14.6 GeV/c per nucleon 28Si collisions with nuclei, utilizing the apparatus of E-814 at Brookhaven National Laboratory's AGS. We report the first results of this search, which is sensitive to particles with charge-to-mass ratios between 0.1 and 0.3 ( G e V / c 2) -~ that are produced within approximately _+ 0.5 units of the center of mass rapidity.

A number of authors [1-8] have discussed the possible existence of stable or metastable massive mult iquark states involving u, d, and s quarks. In the context of the MIT bag model of hadrons, such states are made plausible by the Pauli exclusion principle, which asserts that a system with three quark flavors can be more tightly bound than a comparably sized system where only two flavors are available. Under appropriate conditions, this effect can more than compensate for the larger rest mass ( ~ 150MeV/c 2)

of the s quark. Although approximate theories such as those based on the bag model provide useful guid- ance, reliable calculations for such systems are be- yond the present capability of QCD theory and the

Present address: Physikalisches Institut, University of Heidel- berg, W-6900 Heidelberg, Germany.

existence of these systems is an open experimental question.

The use of high-energy heavy-ion reactions to produce such states, which following current practice we call strangelets, was suggested earlier [ 1,5] and is supported by recent results [ 9 ], which show that large positive kaon multiplicities ( > 10) and high local densities (several times ordinary nuclear density) are produced in these collisions. This paper reports the first result of a search for such states produced in collisions of a high-energy (14.6 GeV/c per nucleon) 28Si beam with a copper target. This study is part of the experimental E-814 research program at Brookhaven National Laboratory's Alternating Gra- dient Synchrotron.

550 0370-2693/90/$ 03.50 © 1990 - Elsevier Science Publishers B.V. ( North-Holland )


Calculations based on the MIT bag model #1 indicate that metastable strangelets with lifetimes in the 1 0 - 4 - 1 0 -5 S range could exist with the ratio of charge Z to baryon number A (A = ~ (nu + nd + ns), where nq is the number of quarks with flavor q) in the range 0.3 >/Z/A >/-- 0.1. The most stable strangelets are expected to have Z/A >~0. Because of surface energy contributions, these calculations indicate that A > 10 would be required for such states. The upper bound of the energy per unit A (E/A) of a metastable strangelet is determined by the requirement that strong decay be energetically forbidden. Thus, if one defines f s - ns/A, the maximum E/A for metastability is

(E/A)ma,,=fsMA+(1-fs)M n forf~ ~ 1. (1)

Eq. ( 1 ) is uncertain to a few tenths of a percent because of binding energies in the final decay product nuclei and hyperfragments. Typical values o f ~ range from 0.5 to 0.8 for strangelets with A between 10 and 30. On the low side, even the most optimistic calculations give E/A>9OOMeV for bulk strange matter, and larger values for strangelets with finite A. Thus the energy per baryon number A for strangelets is likely to lie within 4-_ 10% of 1.0 GeV, so that a measurement of strangelet mass would give a + 10% or better measurement of A.

This search was therefore designed to detect strangelets with mass M>~ 10GeV/c 2 and Z/M in the range 0.1 to 0.3 (GeV/c2) -~. Our approach is to measure the charge and mass of particles produced with rap-

Metastable strangelet lifetimes of about 10 ~°s are expected when weak neutron decay is allowed and lifetimes of 10 -4- 10 Ss when leptonic decay (leading to the emission of an electron and a neutrino) is the only energetically allowed weak decay. Calculations [4,8] indicate that the latter could be the case for substantial regions of the strangelet ( Z, A ).

idities approximately within a half unit of the rapidity of the collision center of mass. Such central rapidities would be expected qualitatively on the basis of either a coalescence model or an intermediate- quark-gluon plasma model [ 7 ] of strangelet production. We take the center of mass to be that of the pro- jectile and the zero-impact-parameter cylinder pro- jected on the target nucleus.

The flight time through the entire apparatus, for a particle of center of mass rapidity (fl=0.902), is 1.34× 10-7s. Decay losses will therefore be negligi- ble for strangelets with lifetimes > 10-6s .

The apparatus is shown schematically in fig. 1. The heavy-ion beam, 14.6GeV/c per nucleon 28Si, is in- cident upon a copper target 2.52 mm thick (4% of a nuclear interaction length). Scintillation counters define and select good beam particles and provide a fiducial timing signal for time-of-flight measure- ments. Pulse heights from the beam scintillators are digitized and recorded for each event.

The target scintillators consist of 52 strips of plastic scintillator that cover the inner four side walls of the target calorimeter.

The multiplicity array is composed of two annular 300gm thick silicon detectors, with a total angular coverage from 2 ° to 33 °. Each detector is divided into 512 independently recorded pad segments. The target scintillator and multiplicity array are used together in the pretrigger, as discussed below.

Magnets M 1 and M2 are both operated at approximately 1.2T. The field strength is adjusted to center the primary 28Si beam 93cm from the neutral line at the location of the downstream calorimeter stacks.

The forward scintillator array consists of 37 plastic scintillation counters, each I cm thick, 10cm wide, with a vertical size of 1.2 m. The counters are viewed,

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Fig. 1. Schematic diagram of the E-814 strangelet search configuration.

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through a light pipe, at the top and the bot tom by EMI 9954 photomultiplier tubes. Digitized pulse heights and timing signals from each tube are recorded. In addition, the dynode signals from the top and bottom tube of each counter are used to form an- alog sums and the 37 resulting signals are digitized within 5.61as by a LeCroy FERA system and are available for the second-level trigger. With offiine corrections for slewing because of pulse height vari- ation, a root mean square (RMS) timing resolution of 385ps is obtained from the forward scintillator system.

The 4.2 nuclear interaction length uranium/copper/scintillator calorimeter (UCAL) consists of ten stacks, each with an active area 20cm wide by 120cm high. Stacks are split into 12 optically decoupled vertical towers, which are read out by two waveshifter bars and two photomultiplier tubes. Pulse heights from the tube anodes are digitized and recorded. Ref. [10] gives further details of the UCAL and gives its energy resolution to be (AE/E)rms=0.4/ x / [ E ( G e V ) ] for hadronic showers. The timing and pulse height of the summed signals from each vertical stack are digitized for trigger purposes and also recorded. The 1.2 ns timing resolution of the stack sums provides a useful constraint in the offiine analysis.

The calorimeter energy resolution, together with the forward scintillator timing resolution of 385 ps, yields a mass resolution varying from 1.2 GeV/c 2 for A = 10 to 2.3 GeV/c 2 for A = 30.

The target calorimeter, participant calorimeter, and drift chambers DC1 and DC2 were not used in this search, although the hole (5.1cm vertical by 6.4cm horizontal) in the participant calorimeter defined the horizontal acceptance while the 15.2cm gap in mag- net M2 defined the vertical acceptance.

The trigger system selects two types of data events for the recording. Both types require a "pretrigger", which insures a good beam particle and a nuclear interaction producing at least 19 charged particles as detected by the silicon and target scintillators. For both trigger types, at least one hit in the "trigger region" of the forward scintillator counters (see fig. 1 ) is also required. The trigger region and the magnetic field are chosen to be sensitive to particles with Z / M in the range 0.1-0.3 (GeV/c2) - ~ with rapidities in the central region.

For one trigger type, ETRIG, there is a further re-

quirement that at least 15 GeV of energy be depos- ited in the UCAL stacks, which are shadowed by the trigger region forward scintillators. The ETRIG is designed to be sensitive to strangelets of charge one and the energy cut is needed to reject triggers arising from fast protons. For the other type, ZTRIG, there is a requirement that at least one of the forward scintillator counter pulse heights be greater than twice the minimum ionizing signal, the ZTRIG is sensitive to strangelets of Z>~ 2, and no additional energy cut is needed to produce an acceptable trigger rate. The ETRIG events are downscaled by a factor of seven relative to ZTRIG events in order to record roughly equal numbers of each trigger.

A total of about 95 000 ETRIG events and 128 000 Z T R I G events were recorded during the two-day run in December 1988.

The offline analysis proceeds by applying a series of cuts to the data. The first of these is a time-of-flight cut, which selects particles produced within one unit of rapidity about the center of mass (Ycr~ --+ 0.5, where Ycm = 1.51 ). This window about the center of mass corresponds to particles that arrive at the forward scintillator between 4 ns and 32 ns later than v= c particles. We thus apply a cut, called the "2 ns timing cut", that requires at least one of the hits in the trigger region forward scintillators to be at least 2ns but not more than 30ns late relative to v=c particles. The 2ns cut-off, corresponding to y = 2.33, is conservative. The 30ns cut-off, corresponding to y-- 1.04, is a few percent inefficient for strangelets with Z/A~O.I but necessary to eliminate albedo from the UCAL 5m downstream. These timing cuts eliminate approximately 95% of all the events, leaving 2013 ETRIG and 7297 ZTRIG events.

The events that pass the timing cuts are analyzed by a program that calculates the vertical positions of hits in the forward scintillator counters (as determined by the time difference between the top and bot tom photomultipliers), and also identifies single hadronic showers in the UCAL and fits their positions and energies. Fig. 2a shows a scatter plot of the difference between the positions determined by the forward scintillator and UCAL shower centroid al- gorithms for events where only a single hit was found in the forward scintillator and the UCAL. Fig. 2b shows the same scatter plot for events where the multiplicity is greater than or equal to one. The spread of

552


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Fig. 2. The difference in X (horizontal) and Y (vertical) positions determination of the UCAL and forward scintillator for events where both detectors had (a) multiplicity one, and (b) multiplicity > 1.

points in fig. 2a is consistent with the expected per- formance of these detectors. Based on this resolution, the UCAL and forward scint i l lator posi t ions are required to agree within + 1 l c m horizontal ly and + 10 cm vertically. This procedure, called the "box" cut, reduced the da ta sample to 175 E T R I G and 421 Z T R I G events.

Events passing the box cut are next required to show a t ime-of-fl ight difference from the forward scintillator to the U C A L that is consistent, within the reso-

lution, with the t ime-of-flight measured from the beam scint i l lator to the forward scintillator. This cut reduces the sample to 78 E T R I G and 202 Z T R I G events, of which 24 ETRIG and 46 Z T R I G events have reconstructed masses greater than 1 0 G e V / c 2.

For the remaining sample of candidates with masses greater than lOGeV/c 2, we require that the pulse heights from the beam scinti l lators be consistent with a single 288i beam particle. This el iminates acciden- tals in which two beam part icles arrive within about 30ns of each other. Such an occurrence, with the first beam part icle ini t iat ing the t iming sequence and the second one interact ing and simulat ing late part icles from the first, is a potent ia l background. A second background occurs when two forward scinti l lator hits overlap within the posi t ion resolut ion of the UCAL. Such events are also rejected. These last two cuts e l iminate all the remaining E T R I G and Z T R I G events. Thus, we see that using the full power o f the scinti l lators and the calorimeters suffices to el iminate all the backgrounds.

The system "examined" 2.8 X l0 s interactions (IE) for E T R I G events and 2.0X 106 interact ions ( Iz) for Z T R I G events. Thus, if the strangelet product ion cross section in these collisions were described by d2a/ df2dy, the number of E T R I G ( Z T R I G ) events we

E T R I G ( Z T R 1 G ) would observe, N obs is

N E T R I G ( Z T R I G ) obs

f dg2dy, (2) _/E(Iz)m ~acc(0, ¢, Y)~s. .... dOd--~ d2a

#2),

where ¢ represents the interact ion cross section; eacc (0, ¢, y) is the geometrical efficiency and ~sh . . . . is the efficiency for identifying showers in the UCAL.

If we assume that the s trangelet-producing events have the same character as ordinary E T R I G or Z T R I G events, we est imate from the da ta that eshower is 70% for showers, which would satisfy the box cut as descr ibed above.

The angular range subtended by the spectrometer is + 12.3mrad vertically by + 1 8 . 4 m r a d horizon- tally. These angles are small enough so that dZa/df2dy is not expected to vary over their range.

Fig. 3 shows the acceptance E(0, ~, y) normal ized to A#2o = 9 X 10-4 sr integrated over angle, as a function o f y and Z/A. The bands in fig. 3 correspond to var ia t ions of the mass per unit A by + 10% around 1.0 G e V / c 2.

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covery in this experiment. The absence of heavy nuclei with low Z/A (such as

l 'Li) in our data does not preclude the possibility o f producing strangelets if the production mechanism involves the decay of the quark-gluon plasma. It has been suggested that a hot quark-gluon plasma pref- erentially radiates anti-s quarks, leaving a more stable plasma with a large fraction ofs quarks. It may be unlikely for a non-strange fragment to emerge from that deconfined state. Berger and Jaffe [ 4 ] also point out that u and d quarks in a deconfined state (such as the quark-gluon plasma) are not bound as nu- cleons, and so the overlap of their wavefunctions with that of a normal nucleus is unlikely. This implies that the emission of even small nuclei, such as an alpha particle, is suppressed. A nucleus like '~Li would be even less probable because of its large A-value and neutron excess.

Eq. (2) and fig. 3 allow any model for d2a/df2dy to be tested against our result, Nobs=0, for both singly and multiply charged strangelets. To gain a qual- itative estimate of the sensitivity of the measurement we assume: (a) that the width of the y-distribution of strangelets would be similar to that for other cen- trally produced hadrons, and (b) that (p , ) would be characteristic of a sequential "accretion" process so that ~2 (p , ) ~ 0.7x/~ GeV/c. Thus, we take a simpli- fied strangelet production model with y flat within Ycm + 0.5 and d a / d O flat out to 80 mrad and zero be- yond. Then our results correspond to a 90% confi- dence limit of less than 1.2 × 10 -4 strangelets per interaction for multiply charged strangelets and less than 8.3 × 10 -4 strangelets per interaction for singly charged strangelets. This is a conservative estimate; models with lower values of (P t ) would yield a greater sensitivity.

These limits are considerably above the estimates of strangelet production from a coalescence picture but have relevance to the quark-gluon plasma model of strangelet production [ 5-7 ], where they constrain some of the possible parameters. For example, if a quark-gluon plasma were formed in 1% of the central collisions, and if strangelets existed, some of the parameter sets ofref. [ 5 ] would have led to their dis-

~2 We have in mind a "'random walk" in p~ in which A ° hyperons (with typical values of <p,) of 0.7 GeV/c) are accreted to make up a strangelet.

We acknowledge the excellent support of the BNL AGS and Tandem staffs and thank Dr. Y. Makdisi and Dr. H. Brown for expert help in the design and running of our beam line. We thank Dr. W.J. Willis for important contributions in the initial phase of this experiment and acknowledge support by CERN in the construction of the uranium calorimeter and the target calorimeter. We also gratefully acknowledge the help of Yousaf Butt in calculating the acceptance of the apparatus. This research is supported, in part, by the US Department of Energy, the National Science Foundation, and the Natural Sciences and Engineer- ing Research Council of Canada.

References

[ 1 ] S.A. Chin and A.K. Kerman, Phys. Rev. Lett. 43 (1979) 1292.

[21E. Farm and R.L. Jaffe, Phys. Rev. D 30 (1984) 2379. [31E. Witten, Phys. Rev. D 30 (1984) 272. [4] M.S. Berger and R.L. Jaffe, Phys. Rev. C 35 (1986) 213. [5] H.C. Liu and G .L Shaw, Phys. Rev. D 30 (1984) 1137. [6] C. Greiner et al,, Phys. Rev. Len. 58 (1987) 1825. [7] C. Greiner et aL, Z. Phys. C 38 (1988) 283. [81 G.L. Shaw, M. Shin, R.H. Dalitz and M. Desai, Nature 337

(1989) 436. [9] See, for example, opening talk Seventh Intern. Conf. on

Ultra-relativistic nucleus-nucleus collisions (Quark matter '88 ), and references therin: M. Jacob, Nucl. Phys. A 498 (1989) lc.

[ 10] M. Fatyga, D. Makowiecki and W. Llope, Nucl. lnstrum. MethodsA 284 (1989) 323.

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Search for strange quark matter in high-energy heavy-ion collisions