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Lawrence Berkeley National LaboratoryLBL PublicationsTitleProton Rapidity Distributions from 60 GeV/n 16O+Au CollisionsPermalinkhttps://escholarship.org/uc/item/6rx909b5AuthorsTonse, Set alPublication Date1990-05-01Copyright InformationThis work is made available under the terms of a Creative Commons Attribution License, available at https://creativecommons.org/licenses/by/4.0/eScholarship.org Powered by the California Digital LibraryUniversity of California' f, LBL-29578 Rev. Lawrence Berkeley Laboratory UNIVERSITY OF CALIFORNIA Presented at the Quark Matter '90 Conference, Menton, France, May 7-11, 1990, and to be published in the Proceedings Proton Rapidity Distributions from 60 Ge V /n 160+Au Collisions S. Tonse et al. May 1990 Prepared for the U.S. Department of Energy under Contract Number DE-AC03-76SF00098. ---""ilCJ 0 ...... r ...... ;; 0 J I') D f(• ~ z ..... ~ !lJ ('j I'D rro I'D I'D "0 A"lfl -< Ill ---ttl ...... a. I.CJ . r ttl Ul r 1$1 I fi) r ...0 ...... (J] o-n --.J ;; 0 Q) !lJ"' "1"< :::0 "< I'D . f!.i < DISCLAIMER This document was prepared as an account of work sponsored by the United States Government. While this document is believed to contain conect information, neither the United States Government nor any agency thereof, nor the Regents of the University of California, nor any of their employees, makes any wananty, express or implied, or assumes any legal responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by its trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof, or the Regents of the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof or the Regents of the University of California. ..... '' 1:;;.\ \ . Proton Rapidity Distributions from 60 GeV/n 16Q+Au Collisions S. Tonse Nuclear Science Division Lawrence Berkeley Laboratory, University of California Berkeley, California 94 720, USA LBL-29578 Rev. This work was supported by the Director, Office of Energy Research, Division of Nuclear Physics of the Office of High Energy and Nuclear Physics of the U.S. Department of Energy under Contract DE-AC03-76SF00098 () PROTON RAPIDITY DISTRIBUTIONS FROM 60 GEV /N 160+Au COLLISIONS S.R. Tonse Lawrence Berkeley Lab., Berkeley CA 94720 USA and The N A35 Collaboration: J.Baechler,l J. Bartke,3 H. Bialkowska,4 R. Bock,5 R. Brockmann,5 P.Buncic,9 S.l. Chase,6 I. Derado,8 V. Eckardt,8 J. Eschke,11 C. Favuzzi,7 D. Ferenc,9 B. Fleischmann,5•11 M. Fuchs,5•11 M. Gazdzicki,10 H.J. Gebauer,8 E. Gladysz,3 C. Guerra,5 O.Hansen,15 J.W. Harris/ W. Heck,11 M.Hoffman,1 T. Humanic,5 S.Kabana,11 K. Kadija,8 •9 A. Karabarbounis,12 R. Keidel,13 J. Kosiec,11 •14M. Kowalski,3 •8 A.Kiihmichel,11 M.Lahanas,11 Y. Lee 11 M. LeVine,2 A. Ljubicic,9 S. Margetis,11 E. Nappi,7 G. Odyniec,6 G. Paic,9 A. Panagiotou,12 A. Petridis/ 2 A. Piper,13 F. Posa,7 H.G. Pugh,6 F. Piihlhofer/ 3 G. Rai,6 W. Rauch,8 R. Renfordt,11 D. Rohrich,11 H. Rothard,11 K. Runge/ A. Sandoval,5 E. Schmidt11 N. Schmitz,8 E. Schmoetten/ I.Schneider,11 P. Seyboth,8 J. Seyerlein,8 E. Skrzypczak,14 P. Spinelli/ R. Stock,11 H. Strobele,11 L. Teitelbaum,6 S. Tonse,6 G. Vassileiadis,12 G. Vesztergombi,8 D. Vranic,9 and S. Wenig11 1 Fakultat fiir Physik, Universitat Freiburg, D-7800 Freiburg, Germany 2Brookhaven National Laboratory, Upton, NY 11973, USA 3 lnstitute of Nuclear Physics, PL-30055 Cracow, Poland 4 Institute of Nuclear Studies, PL-00681 Warszawa, Poland 5Gesellschaft fiir Schwerionenforschung, D-6100 Darmstadt 11, Germany· 6 Lawrence Berkeley Laboratory, University of California, Berkeley CA 94720, USA 7 Dipartimento di Fisica, U niversita di Bari, and INFN, 1-70126 Bari Italy 8 Max-Planck-Institut fiir Physik und Astrophysik, D-8000 Miinchen, Germany 9 Rudjer Boskovic Institute, 41001 Zagreb, Yugoslavia 10lnstitut fiir Hochenergiephysik, Universitat Heidelberg, D-6900 Heidelberg, Germany 11 Fachbereich Physik, Universitat Frankfurt, D-6000 Frankfurt, Germany 12Physics Department, University of Athens, GR-157-71 Athens, Greece 13Fachbereich Physik, Universitat Marburg, D-3550 Marburg, Germany 14Institute of Experimental Physics, University of Warsaw, PL-00681 Warszawa, Poland Abstract: An analysis of the proton rapidity distribution in central 160+Au collisions at 60 GeV /n measured in the NA35 streamer chamber detector at the CERN SPS is presented. The charge excess of positive particles over negative particles was measured. The rapidity distribution of the charge excess which can be associated with the primordial protons in the collision is studied in terms of the nuclear stopping power and is compared to the predictions of various models. Central 160+Au collisions at 60 GeV /n were studied using the .NA35 streamer chamber detector. The emphasis of this study was to determine the degree of nuclear stopping at an 1 incident energy intermediate between 14.6 GeV /n and 200 GeV /n since these two energies have been studied extensively [1,2,3] and may be representative of different stopping regimes. Most fixed-target experiments at relativistic energies lack good acceptance forward of mid-rapidity due to high track densities. Therefore information derived from transverse energy distributions is used to measure the degree of stopping. The simplest method uses calorimetry to determine the total transverse energy of particles per event (E~t = I:Esin Btab)· This method compares E~t to what is expected from a fully stopped projectile, which results in isotropic particle emission with a transverse energy sum Epa:z: = 7r/4(y'S- mi), where mi is the total mass of all initial state particles. The measure of stopping in an event is then given by E~t jEpa"'. Using this approach a study of 28Si+Au collisions at 14.6 GeV /n reveals complete stopping[1], whereas at 60 GeV /n the data exhibit 0.6 stopping in 160+Au[4] and 0.9 in 160+W[2]. At 200 GeV /n, experiments report 0.5 stopping in 160+Au[3], and 0.7 in 160+ W reactions [2]. A more sophisticated model [5] allows a broader width for the rapidity distribution of produced particles, as observed experimentally, than that obtained for isotropic emission. This results in a smaller Epa"' than obtained from the previous method since more energy is allowed to propagate longitudinally and hence there is less transverse energy available. In this approach the amount of stopping in 160+Au collisions is estimated to be 1.0, 0.93 and 0.84 at incident energies of 14.5 GeV /n, 60 GeV /nand 200 GeV /n respectively. The data used in this analysis consist of 89 central 0-Au events (10,500 positive tracks and 7400 negative tracks) taken at a beam momentum of 60 GeV/n (y'S = 10.7 GeV/n). A high transverse energy calorimeter trigger was used to select the most central collisions corresponding to 1% of the total inelastic cross-section. In a geometrical model it is estimated that the 160 nucleus interacts on the average with a cylinder of 55 target nucleons (25p + 30n). Particle trajectories were recorded on film and measured offiine to yield the momentum and charge sign of each particle. In this analysis one camera view of a possible three was used. Therefore the measurement consists of the projected quantities: the longitudinal component of momentum along the beam and one transverse component py of the momentum. To compensate for the missing component of PT in the rapidity calculation, ( 1r /2)py is substituted for PT, since the average 2-dimensionsal projection of a 3-dimensional vector is diminished in magnitude by 2/7r. The resulting quantity is labelled quasi-rapidity. From simulation studies the distribution of the difference between true rapidity and quasi-rapidity was found to have a width of o-=0.09 units in the kinematic region of interest. Measuring in one view only, compared to conventional 3-view processing, increases statistics by greater than a factor of three since the 3-view practice of matching tracks from different views is eliminated .. The overall efficiency for the data sample is 2::99%. The proton quasi-rapidity distribution is obtained by subtracting the quasi-rapidity distri-bution of negative particles from positive ones, both calculated using the proton mass. This assumes a cancellation of produced 7r+ with 1r-, K+ with K- and protons with antiprotons, leaving the original non-produced protons. In isospin-symmetric systems equal numbers of produced 7r+ and 1r- are expected. Since 160 is isospin-symmetric 7r+ and 1r- production are assumed to be equal for rapidities forward of the center-of-mass. The data is displayed for the region forward of mid-rapidity (Y cM=l.8 in the 16-on-55 system). The integrated contents furnish an estimate of 10.88±0.68 protons, larger than the 8 protons initially forward of mid-rapidity, but smaller than the 16.5 protons expected from 2 :"1 I \ -.I calculations for a fully stopped projectile in a 16-on-55 collision. A comparison of the proton quasi-rapidity distribution measured in 160+Au collisions with those predicted by various models is displayed in Fig.l. The dashed line represents the prediction of the Landau hydrodynamical model [6] with complete stopping for the system 16-on-55. It has a maximum at mid-rapidity which is far above the data points. This excess of protons forward of mid-rapidity compared to the data is a result of isotropic emission in the model. Comparisons were also made varying the number of target participants in the Landau model but a reasonable fit could not be made to the data. The rapidity distribution of protons from 60 GeV /n 160+Au collisions in a multiple nucleon-nucleon collision model, MARC0[7], is displayed in Fig.l. Using a linearly increas-ing x distribution ( a=2), where x is the fraction of energy plus longitudinal 1nomentum retained per nucleon-nucleon collision the MARCO calculations reproduce the data. How-ever, MARCO requires a flat x distribution (a=1) to fit transverse energy spectra from the same data set [7]. The rapidity spectrum from FRITIOF is also displayed in Fig.l. It has a broad peak centered at Ypro=3.3 whereas the data continue to rise toward mid-rapidity. It thus appears that there is more stopping of protons in the data, despite the fact that FRITIOF was generated with zero impact parameter, i.e. maximum centrality. Our substitution of the charge-excess distributon for the proton distribution neglects any inequality between the K+ and· K- production rates. This can be caused when the contribution of K+ 's from associated production is not cancelled by negative particles in the same rapidity bin. The expected excess of K+ production cannot be precisely estimated as sufficient kaon data have not been measured at 60 Ge V /c. In the present analysis it is neglected, since the effect is estimated to be approximately 10% compared to the differences observed between the data and the models in Fig.l. This estimate is derived from the FRITIOF model which predicts only 0.5 K+ forward of center of mass, spread over about 2 units of rapidity. A comparison of the proton and rr- rapidity distributions for the same data set is displayed in Fig.2. The data plotted forward of mid-rapidity show similar shapes for the two. The protons are lower by a factor of 0.35. A similarity of shape can be interpreted as emission fr01n a com1non source. In summary, a comparison of our data with the Landau hydrodynamical model[6] indicates that the 160 projectile does not stop completely in the Au target for central collisions at 60 GeV /n. The MARCO multiple collision model does not simultaneously describe the transverse energy distribution and the proton rapidity distribution. These discrepancies indicate that measurements of stopping derived from transverse energy spectra alone may not be reliable and that rapidity distributions of primordial baryons must be considered. This work was supported in part by the United States Dept. of Energy under contract DE-AC03-76SF00098. References [1] J. Barrete et al, Phys.Rev.Lett.64,1219 (1990). [2] J. Schukraft et al, Nucl.Phys.A 498,79c (1989). 3 [3] J. W. Harris et al, Nucl.Phys.A 498,133c (1989). [4] W. Heck et al, Z.Phys.C 38,19 (1988). [5] J. Stachel and P. Braun-Munzinger, Nucl.Phys.A 498,577c (1989). [6] Von Gersdorff et al, Phys.Rev.C 39,1385 (1989). [7] C.Y. Wong et al, Phys.Rev.D 39,2606 (1989). Figure Captions: 1) Proton rapidity distributions for the data and several models 2) Rapidity distributions of protons and 1r- from 160+Au collisions at 60 GeV /n 5 >- 20 " '-z " 10 I. I ' 16 0+Au 60 GeV (NA35) ' ' ' ' ' • ' ' 3 4 Fritiof Landau Marco ex. = 2 protons Rapidity (Quasi-rapidity for protons) • • I prolons 7T I 5 o~~~~~--~~-l~--L-~-L~~L_~~~ 2 3 4 5 Quasi-rapidity for protons, rapidity for 7T-4 '\ \i ~--LA~NCEBERKELEYLABORATORY UNIVERSITY OF CALIFORNIA INFORMATION RESOURCES DEPARTMENT BERKELEY, CALIFORNIA 94720 ~-

Proton Rapidity Distributions from 60 GeV/n 16O+Au Collisions

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Proton Rapidity Distributions from 60 GeV/n 16O+Au Collisions

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