, p , and p ¯ in relativistic Au+Pt, Si+Pt, and p +Pt collisions

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  • PHYSICAL REVIEW C VQLUME 48, NUMBER 6 DECEMBER 1993

    Production of m+, K+, p, and pin relativistic Au + Pt, Si + Pt, and p + Pt collisions

    G. E. Diebold, B. Bassalleck, " T. Burger, M. Burger, R. Chrien, H. En'yo,G. Franklin, J. Franz, T. Iijima, K. Imai, 3. Lowe, R. Magahiz,

    A. Masaike, F. Merrill, J. M. Nelson, K. Okada, P. H. Pile, 2 B. Quinn,E. Rossle, A. Rusek, N. Saito, R. Sawafta, H. Schmitt, R. A. Schurnacher,

    R. L. Stearns, R. Sukaton, R. Sutter, F. Takeutchi, D. M. Wolfe,S. Yokkaichi, V. Zeps, and R. Zybert

    (E886 Collaboration)Department of Physics, University of Birmingham, Birmingham B15 2TT, United Kingdom

    Brookhaven National Iaboratory, Upton, Negro York 11978Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania 15218

    Fakultat fiir Physik, University of Freiburg, D 79104 F-reiburg, GermanyDepartment of Physics, Kyoto University, Sakyo Ku, Ky-oto 606, Japan

    Faculty of Science, Kyoto Sangyo University, Kyoto 609, JapanDepartment of Physics, University of wMexico, Albuquerque, New Mexico 87191

    Department of Physics and Astronomy, Vassar College, Poughkeepsie, wYork 12601Physics Deparment, Yale University, Ne'er Haven, Connecticut 06522

    (Received 15 June 1993)

    During the recent commissioning of Au beams at the Brookhaven Alternating Gradient Syn-chrotron facility, experiment 886 measured production cross sections for sr+, K+, p, and p in min-imum bias Au + Pt collisions at 11.5A GeV/c. Invariant difFerential cross sections, F z s, weredp3 'measured at several rigidities (p/Z ( 1.8 GeV/c) using a 5.7' (fixed-angle) focusing spectrometer.For comparison, particle production was measured in minimum bias Si + Pt collisions at 14.6AGeV/c using the same apparatus and in p + Pt collisions at 12.9 GeV/c using a similar spectrome-ter at KEK. When normalized to projectile mass, Ap, , the measured sr+ and K+ cross sections arenearly equal for the p + Pt and Si + Pt reactions. In contrast to this behavior, the vr cross sectionmeasured in Au + Pt shows a signiIIicant excess beyond Ap, scaling of the p + Pt measurement.This enhancement suggests collective phenomena contribute significantly to vr production in thelarger Au + Pt colliding system. For the Au + Pt reaction, the sr+ and K+ yields also exceed Ap, .scaling of p + Pt collisions. However, little significance can be attributed to these excesses due tolarger experimental uncertainties for the positive rigidity Au beam measurements. For antiprotons,the Si + Pt and Au + Pt cross sections fall well below Ap, ; scaling of the p + Pt yields indicatinga substantial fraction of the nuclear projectile is inefI'ective for p production. Comparing with p +Pt multiplicities, the Si + Pt and Au + Pt antiproton yields agree with that expected solely from"first" nucleon-nucleon collisions (i.e. , collisions between previously unstruck nucleons). In light ofexpected p annihilation in the colliding system, such projectile independence is unexpected withoutadditional (projectile dependent) sources of p production. In this case, the data indicate an ap-proximate balance exists between absorption and additional sources of antiprotons. This balance isremarkable given the wide range of projectile mass spanned by these measurements.

    PACS number(s): 25.75.+r, 13.85.Ni, 25.40.Ve

    I. INTR.ODU CTI(3N

    The recent successful acceleration of Au ions to 11.5A GeV/c at the Brookhaven Alternating Gradient Syn-chrotron (AGS) represents nearly an order of magnitudeincrease in projectile mass for ions accelerated to highlyrelativistic velocities. This event marks the beginningof an era of truly heavy relativistic ion physics at theAGS. As part of this new era, experiment 886 (E886)conducted a study of positive and negative particle pro-duction in the collisions of Au ions with a similarlyheavy Pt target. Presented here are results of inclu-sive particle production measurements made during theerst Au beam run at the AGS. For immediate compar-

    ison, particle production in Si + Pt and p + Pt colli-sions at similar beam momenta per nucleon (14.6A and12.9A GeV/c, respectively) were also measured and arereported here. The Si + Pt reaction was studied usingthe same apparatus as the Au + Pt measurements, whilethe p + Pt measurements were performed at KEK usinga similar spectrometer.

    This paper will focus on the projectile dependence ofparticle production as demonstrated by the minimumbias reactions studied. Of particular interest is the pro-jectile dependence of the p multplicities. These are ex-pected to be sensitive probes of the hadronic systemforined in nucleus-nucleus (A + A) collisions owing toa potentially large annihilation probability in a dense,

    0556-2813/93/48(6)/29844, '1 l)/$06. 00 1993 The American Physical Society

  • 48 PRODUCTION OF m +, K+, p, AND p IN RELATIVISTIC. . .

    TABLE I. Acceptance characteristics of the D6 and T3 focusing spectrometers.

    Spectrometer

    D6 (AGS)T3 (KEK)

    Productionangle (mrad)

    99.099.0

    ~~enp

    (msr %)

    11.10.45

    (%)+2.1+0.8

    AO

    (mrad)

    +30+ 3.1

    (mrad)

    + 4.6+8.9

    baryon-rich environment [1]. However, despite severalmeasurements of antiproton production in Si + A col-lisions [24], the degree to which the collision. environ-ment affects p yields is not well established or under-stood. Aside &om the complex nature of A + A colli-sions, this pursuit is hampered by very small productioncross sections at AGS energies. As a consequence, ob-taining accurate and precise measurements of p produc-tion has proven to be a particularly challenging task. Forexample, although previous measurements in Si + A col-lisions at the AGS [24] difFer by factors of 24 [5], suchdifrerences are not grossly beyond the associated experi-mental (statistical + systematic) uncertainties. Further,as there are relatively limited p + A data at suitable ener-gies, these experiments have had to generally rely on ex-trapolations of higher-energy p + A data for comparisonpurposes. As a result, even a qualitative understandingof A + A antiproton yields relative to simple expectationshas not been firmly established (e.g. , relative to "first"collision scaling of p + p or p + A data; see Sec. IVD).

    In the results and discussion to follow, emphasis will beplaced on the measured p yields from the Au + Pt and Si+ Pt reactions in comparison with those measured in p +(A 200) collisions at similar hearn energies per nucleonand in similar regions of phase space. As will be seen,by focusing on p production within the narrow phasespace acceptance of these measurements (0.8 ( y & 1.4,p| ( 0.180 GeV/c) a quantitative comparison of p yieldsin p + A and A + A collisions can be made with min-imal extrapolation uncertainty. From such comparisonsone can begin to address the efI'ects of the collision envi-ronment on observed p yields. However, as is also true forprevious narrow-acceptance p measurements [3, 4], suchcomparisons cannot strictly distinguish difI'erences in pyields due to absorption from nonabsorptive aspects ofthe collision (such as phase space redistribution of an-tiprotons). Consequently, it is difficult to draw singularconclusions based solely on these comparisons.

    II. EXPERIMENTAL APPARATUSAND TECHNIQUE

    For each reaction, a thin Pt target of approximately67% of an interaction length was used together with a

    5.7' (fixed-angle) beam-line spectrometer system. TheAu + Pt and Si + Pt reactions were studied at theAGS using the D6 secondary beam line [6] and associ-ated open-geometry dipole spectrometer constructed forthe E813/836 H particle search [7]. The p + Pt reactionswere studied at KEK using the T3 beam line. Ordinarily,these beam lines are used to produce secondary pion andkaon beams for a variety of intermediate-energy physicsexperiments. The characteristics of each beam line aresummarized in Table I. For the Au + Pt data, parti-cle production was measured for eight magnetic rigidities(p/Z = +0.8, +1.2, +1.5, +1.8 GeV/c) while only tworigidities (p/Z = +1.8 GeV/c) were studied for the Si +Pt reaction. The p + Pt measurements included a total ofsix rigidities (p/Z = +1.2, +1.5, +1.8 GeV/c). Table IIsummarizes the reaction, target thicknesses, beam mo-menta, and secondary magnetic rigidities for each mea-surement reported here.

    A schematic diagram of the D6 beam line and E886 ex-perimental apparatus is shown in Fig. 1. For the presentmeasurements, the electrostatic separators were not used.Hence, all particle species produced within the momen-tum and solid angle acceptance of the beam line weretransported from the target to the final focus approxi-mately 31 m away. For each rigidity, particles were iden-tified by their time of Bight (TOF) measured betweentwo plastic scintillation counters, MTS and IE1, sepa-rated by approximately 15 m. The MTS counter waslocated just downstream of the erst mass slit while theIE1 counter was placed at the final focus of the beamline. As a hardware trigger, a delayed coincidence be-tween these counters selected particles by time of fIight.Additionally, an aerogel Cerenkov counter, IC (index ofrefraction, n =- 1.03), was used as a vr veto to enhance thesample of K and p triggers written to tape. Dead timein the data acquisition system was typically 2040 %.

    As an example of the clean particle identificationachieved, Fig. 2 shows the recorded MTS-IE1 time ofBight for the highest positive rigidity tune (1.8 GeV/c).For the Au beam data, this measurement alone was sufB-cient for efEcient particle identification. However, duringthe most intense Si beam runs, particle misidentificationdue to accidental hits in the MTS counter became sig-nificant ( 2030%). In this case, a second TOF mea-

    TABLE II. Summary of incident beam momenta and secondary rigidities studied for each reaction.

    Reaction

    Au+ PtSi+ Ptp+ pt

    Targetthickness (cm)

    0.150.280.50

    Beam momentum(GeV/c per nucleon)

    11.514.612.9

    Secondaryrigidity (GeV/c)

    +0.8, +1.2) +1.5) +1.8+1.8

    +1.2, +1.5, +1.8

  • 2986 G. E. DIEBOLD et al. 48

    Mass Slit 1

    9~ME~ I6 L'JELL'll UL'll

    Separator B

    MTS

    10 Meters

    FIG. 1. Schematic diagram of the D6 beam line and E886 experimental apparatus.

    10 4

    C3C3

    10 3

    U3 102

    0O 10

    Ill HII, I i, s I,IIT=2 0 2 4 6 8 10

    Time of Flight (ns)

    FIG. 2. Time-of-flight difference spectra (MTS-IE1) of 1.8GeV/c positive particles produced in Au + Pt collisions at11.5A GeV/c. Plotted is the difference between the recordedBight times and that of a v = c particle.

    surement was made in the E813/836 dipole spectrometerto identify and correct such rate-dependent identificationinc Kciencies.

    To obtain invariant differential production cross sec-3

    tions, Ed, , the identified particle yields were normalizeddp3 'to the integrated beam on the target and the differen-tial invariant phase space volume accepted by the beamline, d p/E. This phase space volume was determined byMonte Carlo simulation using the program DEGAY TUR-TLE [8]. Based on this simulation, the nominal accep-tance of the D6 beam line was found to be ""dO (:"P)p p'= 11.1 msr%. Depending on the precise position of theincident beam on the production target, the spectrom-eter acceptance may vary by as much as 10% from thisnominal value. In such a case, the variation in targetingproduces an observable change in the phase space distri-bution of particles transported by the beam line. Thisphase space distribution was monitored using three driftchambers located just upstream of the final focus. Tocompensate for this effect, the spectrometer acceptancewas calculated run by run assuming an incident beamposition on target which reproduced the observed phasespace distribution.

    The beam intensity during the Si run was often as highas 10 ions per spill (spill length 1 s) while duringthe Au run the intensity reached as high as 10 ions

    per spill. At such high rates, the integrated beam ontarget could not be accurately measured using pulse-counting techniques. Instead, the time-integrated chargefrom two ionization chambers was used. The first cham-ber was placed in the primary beam (in-beam chamber)and monitored the total beam delivered to the D targetarea. This chamber was calibrated in place using a plasticscintillation counter during periods of low beam intensity(( 5 x 10 per spill). The second chamber was placed out-of-beam near the production target (out-of-beam cham-ber) and monitored the average flux of produced parti-cles. Together, the ratio of the ionization collected bythese chambers provided a relative measure of incidentbeam actually on target. This ratio was used to correctfor variations throughout the runs due primarily to hor-izontal instability (i.e. , "sweeping") of the beam spot onthe target. The beam position was monitored. visuallyusing a fluorescent flag mounted 10 cm upsteam of thetarget. This ensured that the beam spot was centered onthe target during most of the data taking periods. Thesize of the beam was small compared to the target size,and so the maximum observed out-of-beam to in-beamchamber ratio should correspond to nearly all of the inci-dent beam on target. This maximum ratio provided theabsolute normalization, of the fractional beam on target.

    Because of the high beam intensities used, no attemptwas made to select the ion species incident on the tar-get. Consequently, the target interactions sampled willinclude collisions of projectile fragments resulting fromprimary beam interactions upstream of the target. ForE886, material upstream of the target amounts to about5% (1%) of an interaction length for the Au (Si) beam.This includes contributions from the vacuum window atthe end of the beam pipe, air, the target flag, and thebeam flux monitor. The resulting error in the cross sec-tion measurements is limited to the difference in the par-ticle yield from upstream fragmentation products hittingthe target versus that from a primary ion collision. Thisdifference is relatively small and in the worst case (Aubeam) introduces less than 2% error in the measuredcross sections.

    In the T3 beam line at KEK, similar experimentaland analysis techniques were used to determine invari-ant cross sections for the p + Pt reaction. Additionaldetails of the T3 apparatus and data analysis for the p+ Pt measurements will be discussed in a forthcomingpaper [9].

  • PRODUCTION OF m.+, E+, p, AND p IN RELATIVISTIC. . . 2987

    810 ~I r r

    r o' (o)10 6

    10 5

    104

    10

    102

    0.5

    OAl)

    (UC3

    A

    E

    C!p

    U

    104

    I

    pj's

    X

    +~ ~ 7TA A Pe ~ K+

    o o K

    10 =

    I r I I I I I I

    1.5 2

    I

    I

    I r I rI

    I f I rI

    r 108

    ro'= (r)

    106

    105

    104

    10

    102

    10r I I

    0

    AAA

    x10 p7T

    ppp

    ~y

    ~ y& K'

    opo K

    rI

    r I I II

    r I I I

    FIG. 3. Invariant differential productioncross sections for particles produced at 5.7in Au + Pt collisions (11.5A GeV/c) plottedas a function of (a) rigidity and (b) rapidity.Note that the m cross sections have beenmultiplied by a factor of 10.

    Rigidity (GeV/c) Rapidity

    III. B.ESUj TSd3The invariant differential cross sections, Ed, , for a+,

    K+, p, and p production in minimum bias Au + Pt colli-sions are plotted in Fig. 3 as a function of rigidity and asa function of rapidity, y. Note the transverse momentumacceptanc...