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Comparative analysis of strangeness production at AGS-BNL energies and SPS energies
. Suppl .) 298 (1991) 269-272 Nuclear Physics B (îc-,c North-Holland
COMPARATIVE ANALYSIS OF STRANGENESS PRODUCTION AT AGS-BNL ENERGIES AND SPS ENERGIES N-S . AMELIN, E.F. STAUBO and L.P. CSERNAI Fysisk institu+t, Universitetet i Bergen A11ègaten 55, N-5007 Bergen, Norway Experimental data from AGS-BNL at 14.6 A-GeV and SPS at 200 A-GeV are analysed with the quark-gluon string model . The strangeness yields at the ACTS can be explained with ordinary hadronic physics . However, for SPS energies the model underestimates the transverse mass spectra by a factor of two . 1. INTRODUCTION Heavy-ion collisions at ultra-relativistic energies offers an opportunity to study the strong interaction in shatter at high energy densities . The experiments have beer- large:-,- r titivated by the prospect of creating a Quark-Gluon Plasma (QGP), adeconfined state of quarks and gluons. At the AGS-BNL, for beam energy 14.6 A-GeV, full stopping has been concluded from analysis of the transverse energy spectra for central Si + Au collisions [1, 2; 3, 4]. Also at SPS energies different CERN experiments (NA35, WA80) dc.nonstrate a substantial stopping power in central events (60% , NA35 in S + S collisions [5]). Estimates from particle production density (WA80) hrdicate energy densities above critical Values for QGP formation. Calculations with the Quark-gluor, String Model (AGSM) reproduce the transverse energy spectra of the AGS-BIN experiments [61. The baryon-rich matter is dominated by the 0 resonance . and the high density Legion persists fov ä considerable time. The QGSM also agree well with the grass features of collisions at SPS energies . The energy densities are above the estimated threshold for a QGP. For D+Au collisions, the high de-nsity region only exists for a short time [7] . The situation is improved for larger projectiles as S + S and in particulaï for Pb .f- Pb. where the QGSM predicts almost complete stopping . A pion dominated fireball is created for tinries perhaps sufficient for QGP formation . For these scenarios, an enhanced production of sirangeness has been considered as a pogsible gignal for for0920-5632/91/$0350 Q 1991 - Elsevier Science Publishers B .V.
matron of a QGP [8, 9]. However, before definite conclusions can be drawn, a proper study of the collisions in a hadronic collision model must be made. In this article we present comparisons of data from reactions at the AGSB"., at 14 lz'-:' ". Gev and at the SPS at 200 A-GeV, to calculations with the Quark-Gluon String Model (Q(r-.'SM) . 2. HADRONIC COLLISION MODEL The QGSM is a detwied kinetic string model of hadronic and nuclear collisions. The parameters are fitted to available h + h aD+1 is + A data. Secondary particles are allowed to scatter further, and the leading partoûs can rescatter with zerro formation time. For details see refs. [10, 11, 12,13,14] . Bei:"_g a purely hadronic model, the QGSM does not include the possibility of QGP formation . Strange particles are produced by string decay. The production rate is determined by the strange quark content of the colliding hadrons, and by the strangeness suppression probability at string break-sip. 3. ANALYSIS OF AGS-BNL EXPERIMEN'T'S Strangeness production for Si + Au at 14.6 A-GeV at AGS-BNL have been previously analysed in detail with k:netic models in refs. [15, 16] . In rig. 1 we rinOW the experimental rapidity distributions of the E802 collaboration for p, 7r*, and Kt at 14.6 A-GeV alongside with the predictions of the QGSM for two reactions, p + Be ,nd v -f- Au. We approximately reproduce not only the shapes but also the absolute values of dN/dy in the p+rse All rights reserved .
270
V .S Amelin et al. /Comparatlve analysis
of strangeness production
at AGS-13 ."JI, energies and SE'S energies
reaction . For p + Au we have a similarly good fit e:ccept for the pion number which is overestimaded a± low rapidities. in Fig.
L we show 6116 tapidity distributions of Si+Au from the same experiment with the centra' trigger, and the QGSM predictions for impact parameter `, fm . By
comparing QGSM and experimental E802 proton data we conclude that central events in the experiment should correspond roughly to b < 3 jm. We see further that the model overestimates the pion yield and the negative iaons . The protons and the positive kaon yields are well reproduced . The origin of the excess rnesons can be seen
Y FIGURE 2 Rapidity distributions, divided by 28, in central Si -IAu reactions at 14 .6 A-GeV. Symbols are the ûata from the E802 collaboration, wb ;! the histograms are AGSM predictions at b = 2 fm . Notation is the same as in Fig. 1. from Fig. 3 where we show the transverse mass distrib,:j~ion of negatives from the E810 collaboration and the negative pions frown the QGSM . The model overestimates the pion excess at !^w transverse momenta. The different behaviour of K+ can be explained by different absorp-
tion mechanisms from the varying quark content of the mesons . For the K4- /7r± ratios we get a smaller enhancement, (1 ,4_ 1 .0 (-}-), and no enhancement (-)) than the experimental measurements. However, when divided by the experimental pion numbers :ve obtain similar results as the experiments . vèc conclude that the QGSM satis-
factorily reproduces most basic features of the Si + Au data. There is no compelling reason to introduce QGP effects to explain K+ . This conclusion is in agreement
FIGURE 1 Rapidity distributions, dN/dy for identiled parttciCs per trigger in (a) p -f- Be and (b) p + Au at 14 .6 A-GeV. Different symbols are the results of the E802 collaboration. Histograms are. the QGSM predictions . The full line and filled squares correspond to rr - (multiplied by 10), dashed line and filled circles to p, the hashed dotted line and open squares to K+, and the dotted line and diamonds to K° .
with fluid dynamic results, which predict a negligible QCP formatiGn in S + Pb reactions at --the same energy [17] .
4. ANALYSIS OF SPS EXLERIMENTS
In Fig. 4 we present transverse mass spectra obtained by the VVA85 collaboration, compared to QGSM calculations for the reactions p+ W and S+ W at 200 A-GeV. The data show an enhancement of A and t1 relatively to
N.S. Amelin et al./Comparative analysis of strangeiess production at AGS-13NL energies and SPS energies
271
10e 106 104 103 1d 10 0.0
02
0.4
0.6 0A 1.0 Tt = rnt - rno [GM
10'
11
FIGURE 3 Transverse mass spectra for central Si -i- Au collisions . The squares are measured negative particles by E810 for y = 2.2 -- 2.4 . The histogram is the prediction for ?r' for the QGSM . the negatives for central S -l. W compared to the yields in p+ W collisions . The ratio A/P4 stays constant . From
106 -L Q Z v P
104 103
1d 10' 1d°
1.0
12
1.4
1.6
1.8
the figure we observe that the ratio can he reproduced
N4T [GeV.
tives is not reproduced . The slope of the distribution is correctly described, but the model underestimates the
FIGURE 4
20
2 .^ . 24
by the model ; but the increase relatively to the nega-
number of A's and A's by
ïactor of tivo . Note that the pe range is different from the AGS experiment., thus, excess mesons at low transverse momenta do not influence it
our conclusion .
The same conclusion has been made form a detailed
study of data from the NA35 collaboration with the QGSM model in ref. [7]. The model reproduces the transverse mass distributions and the rapidity distributions or negatives and protons. However, it underestimates the transverse mass spectra of KO, A, and  by the same factor ._- for S 1- W. 5. CONCLUSIONS
',alculations with the QGSM indicate that there i5 no
need to introduce a QGP to explain strangeness yields in Si + .4u reactions at 14 .6 A-GeV. On the other hand,
the model i.-nderpredicts data for strangeness production in S -1- S and 5 -t- W collisions at 200 A-GeV. Thus, new
Transverse mass spectra obtained 1" y the WA85 collaboration in central events of p -l- W (a) and S + W (b at 2q0 A.Uc the histograms are QGSM calculations . The full line and the filled squares are negatives, the dashed line and the crosses are A's, while the dashed-dotted line and the filled dots are A's. features in the reaction dynamics are present at the higher beam energies at the SPS for these collisions. The calculations with the QGSM sho-v h,rge baryon and energy densities far the SPS reactions E10, 16]. Furt1-c .-mofe, the string density aibains vaILr nt -hich the assumption of independent strings shouid be questioned . For a better description of reactions at 200 A GeV stringstring interactions must be taken into âc(.ount. ACKNOWLEDGEMENTS Contributions from K.K . Gudima and V.D . Toneev are aknowledged .
272
N.S. Amelir. ct al. /Comparative analysis oîstrangeness prodnction at AGS-BNL enemies and SPS energies
REFERENCES [1] T. Abbot,, et GI., Phys. Lett. B197 (1987) 285 . [2] M . S. Tannenbaum, et al., Nzcl. Phys. A488 (1988) 555c. [3] P. Braunmùnzinger, et al. ?. Phys. C38 (1988) 45. [4] J. Stachel, RRpporteur's talk, PANIC 111, Int. Conf. can Particle and Nuclei, MIT, June 25-29, 1990[5] II. Str5be1e, Nucl - Phys . A525 (1991) 59c . [S] N.S. Amelin, K .K. Gudima, V.D. Toneev and S.Yu . Sivoklokov, Bergen Scientific/Technical Report 224/1990 . [7] N. S. Amelin, K. K. Gudima, S. Yu. Sivoklokov and V. D, Toneev, Invited talk presented at the 29th Spring School, Holzhau, Germany, April 8-12, 1991. . 48 ltl J. Rafelski and D. Willer, Phys. Rev. fett ,1982) 1066; (E) 56 09%) 23U . [9] J. Rafelski, Phys. Rep . 88 (1982) P?! ;, [10] N .S. Amelin, K.K. Gudima and V.D. Toneev, in The Yuclon- P;uation of State, Pd. by W. Greiner and
H. Stôcker, NATO ASI 218B, (Plenum, NY, 1989) p. 473; V.D. Toneev, N. S. Arvelin, K. K. Gudima, S. Yu. Sivoklokov Nucl. Phys. A519 (1990) 483-. [11] N.S. Ainelin, K.K. Gudima and V.D. Toneev, Yad. Fiz. 51 (1990) 512 ; Sov . J. Nucl. Phys. 51 (1990) 327. [12] N.S. Amelin, K.K. Gudima, S.Yu. Sivoklokov and V.D. Toneev, Yad . Fiz. 52 (1990) 272 ; Sov . J . Nucl. Phys. 5 2 (1990) 172. [13] N.S. Amelin, L .B. Bravina, L.I. Sarycheva, and L.V. Smirnova, Sov. J. Nucl. Phys., 51 (1990) 535. [14] N.S. 11% melin and L.V. Bravina, Sov . J. Nucl. Phys. 5 1 (1990) 133. [15] R. Matiello, H. Sorge, H. St6cker and W. Greiner, Phys. Rev. Lett . 63 (1989) 1459. [lfi] N .S. Amelin, E.F. Staub:,3, L.P. Csernai, V. Toneev and K.K. Gudima, Bergen Scientific/Technical Report 243/1P90, submitted to Phys. Rev. C. [1?] E.F. Staubo, AX Holme, L.P. Csernai, M. Gong and D. Strottman, Phys. Lett. B229 (1989) 351 .
COMPARATIVE ANALYSIS OF STRANGENESS PRODUCTION AT AGS-BNL ENERGIES AND SPS ENERGIES N-S . AMELIN, E.F. STAUBO and L.P. CSERNAI Fysisk institu+t, Universitetet i Bergen A11ègaten 55, N-5007 Bergen, Norway Experimental data from AGS-BNL at 14.6 A-GeV and SPS at 200 A-GeV are analysed with the quark-gluon string model . The strangeness yields at the ACTS can be explained with ordinary hadronic physics . However, for SPS energies the model underestimates the transverse mass spectra by a factor of two . 1. INTRODUCTION Heavy-ion collisions at ultra-relativistic energies offers an opportunity to study the strong interaction in shatter at high energy densities . The experiments have beer- large:-,- r titivated by the prospect of creating a Quark-Gluon Plasma (QGP), adeconfined state of quarks and gluons. At the AGS-BNL, for beam energy 14.6 A-GeV, full stopping has been concluded from analysis of the transverse energy spectra for central Si + Au collisions [1, 2; 3, 4]. Also at SPS energies different CERN experiments (NA35, WA80) dc.nonstrate a substantial stopping power in central events (60% , NA35 in S + S collisions [5]). Estimates from particle production density (WA80) hrdicate energy densities above critical Values for QGP formation. Calculations with the Quark-gluor, String Model (AGSM) reproduce the transverse energy spectra of the AGS-BIN experiments [61. The baryon-rich matter is dominated by the 0 resonance . and the high density Legion persists fov ä considerable time. The QGSM also agree well with the grass features of collisions at SPS energies . The energy densities are above the estimated threshold for a QGP. For D+Au collisions, the high de-nsity region only exists for a short time [7] . The situation is improved for larger projectiles as S + S and in particulaï for Pb .f- Pb. where the QGSM predicts almost complete stopping . A pion dominated fireball is created for tinries perhaps sufficient for QGP formation . For these scenarios, an enhanced production of sirangeness has been considered as a pogsible gignal for for0920-5632/91/$0350 Q 1991 - Elsevier Science Publishers B .V.
matron of a QGP [8, 9]. However, before definite conclusions can be drawn, a proper study of the collisions in a hadronic collision model must be made. In this article we present comparisons of data from reactions at the AGSB"., at 14 lz'-:' ". Gev and at the SPS at 200 A-GeV, to calculations with the Quark-Gluon String Model (Q(r-.'SM) . 2. HADRONIC COLLISION MODEL The QGSM is a detwied kinetic string model of hadronic and nuclear collisions. The parameters are fitted to available h + h aD+1 is + A data. Secondary particles are allowed to scatter further, and the leading partoûs can rescatter with zerro formation time. For details see refs. [10, 11, 12,13,14] . Bei:"_g a purely hadronic model, the QGSM does not include the possibility of QGP formation . Strange particles are produced by string decay. The production rate is determined by the strange quark content of the colliding hadrons, and by the strangeness suppression probability at string break-sip. 3. ANALYSIS OF AGS-BNL EXPERIMEN'T'S Strangeness production for Si + Au at 14.6 A-GeV at AGS-BNL have been previously analysed in detail with k:netic models in refs. [15, 16] . In rig. 1 we rinOW the experimental rapidity distributions of the E802 collaboration for p, 7r*, and Kt at 14.6 A-GeV alongside with the predictions of the QGSM for two reactions, p + Be ,nd v -f- Au. We approximately reproduce not only the shapes but also the absolute values of dN/dy in the p+rse All rights reserved .
270
V .S Amelin et al. /Comparatlve analysis
of strangeness production
at AGS-13 ."JI, energies and SE'S energies
reaction . For p + Au we have a similarly good fit e:ccept for the pion number which is overestimaded a± low rapidities. in Fig.
L we show 6116 tapidity distributions of Si+Au from the same experiment with the centra' trigger, and the QGSM predictions for impact parameter `, fm . By
comparing QGSM and experimental E802 proton data we conclude that central events in the experiment should correspond roughly to b < 3 jm. We see further that the model overestimates the pion yield and the negative iaons . The protons and the positive kaon yields are well reproduced . The origin of the excess rnesons can be seen
Y FIGURE 2 Rapidity distributions, divided by 28, in central Si -IAu reactions at 14 .6 A-GeV. Symbols are the ûata from the E802 collaboration, wb ;! the histograms are AGSM predictions at b = 2 fm . Notation is the same as in Fig. 1. from Fig. 3 where we show the transverse mass distrib,:j~ion of negatives from the E810 collaboration and the negative pions frown the QGSM . The model overestimates the pion excess at !^w transverse momenta. The different behaviour of K+ can be explained by different absorp-
tion mechanisms from the varying quark content of the mesons . For the K4- /7r± ratios we get a smaller enhancement, (1 ,4_ 1 .0 (-}-), and no enhancement (-)) than the experimental measurements. However, when divided by the experimental pion numbers :ve obtain similar results as the experiments . vèc conclude that the QGSM satis-
factorily reproduces most basic features of the Si + Au data. There is no compelling reason to introduce QGP effects to explain K+ . This conclusion is in agreement
FIGURE 1 Rapidity distributions, dN/dy for identiled parttciCs per trigger in (a) p -f- Be and (b) p + Au at 14 .6 A-GeV. Different symbols are the results of the E802 collaboration. Histograms are. the QGSM predictions . The full line and filled squares correspond to rr - (multiplied by 10), dashed line and filled circles to p, the hashed dotted line and open squares to K+, and the dotted line and diamonds to K° .
with fluid dynamic results, which predict a negligible QCP formatiGn in S + Pb reactions at --the same energy [17] .
4. ANALYSIS OF SPS EXLERIMENTS
In Fig. 4 we present transverse mass spectra obtained by the VVA85 collaboration, compared to QGSM calculations for the reactions p+ W and S+ W at 200 A-GeV. The data show an enhancement of A and t1 relatively to
N.S. Amelin et al./Comparative analysis of strangeiess production at AGS-13NL energies and SPS energies
271
10e 106 104 103 1d 10 0.0
02
0.4
0.6 0A 1.0 Tt = rnt - rno [GM
10'
11
FIGURE 3 Transverse mass spectra for central Si -i- Au collisions . The squares are measured negative particles by E810 for y = 2.2 -- 2.4 . The histogram is the prediction for ?r' for the QGSM . the negatives for central S -l. W compared to the yields in p+ W collisions . The ratio A/P4 stays constant . From
106 -L Q Z v P
104 103
1d 10' 1d°
1.0
12
1.4
1.6
1.8
the figure we observe that the ratio can he reproduced
N4T [GeV.
tives is not reproduced . The slope of the distribution is correctly described, but the model underestimates the
FIGURE 4
20
2 .^ . 24
by the model ; but the increase relatively to the nega-
number of A's and A's by
ïactor of tivo . Note that the pe range is different from the AGS experiment., thus, excess mesons at low transverse momenta do not influence it
our conclusion .
The same conclusion has been made form a detailed
study of data from the NA35 collaboration with the QGSM model in ref. [7]. The model reproduces the transverse mass distributions and the rapidity distributions or negatives and protons. However, it underestimates the transverse mass spectra of KO, A, and  by the same factor ._- for S 1- W. 5. CONCLUSIONS
',alculations with the QGSM indicate that there i5 no
need to introduce a QGP to explain strangeness yields in Si + .4u reactions at 14 .6 A-GeV. On the other hand,
the model i.-nderpredicts data for strangeness production in S -1- S and 5 -t- W collisions at 200 A-GeV. Thus, new
Transverse mass spectra obtained 1" y the WA85 collaboration in central events of p -l- W (a) and S + W (b at 2q0 A.Uc the histograms are QGSM calculations . The full line and the filled squares are negatives, the dashed line and the crosses are A's, while the dashed-dotted line and the filled dots are A's. features in the reaction dynamics are present at the higher beam energies at the SPS for these collisions. The calculations with the QGSM sho-v h,rge baryon and energy densities far the SPS reactions E10, 16]. Furt1-c .-mofe, the string density aibains vaILr nt -hich the assumption of independent strings shouid be questioned . For a better description of reactions at 200 A GeV stringstring interactions must be taken into âc(.ount. ACKNOWLEDGEMENTS Contributions from K.K . Gudima and V.D . Toneev are aknowledged .
272
N.S. Amelir. ct al. /Comparative analysis oîstrangeness prodnction at AGS-BNL enemies and SPS energies
REFERENCES [1] T. Abbot,, et GI., Phys. Lett. B197 (1987) 285 . [2] M . S. Tannenbaum, et al., Nzcl. Phys. A488 (1988) 555c. [3] P. Braunmùnzinger, et al. ?. Phys. C38 (1988) 45. [4] J. Stachel, RRpporteur's talk, PANIC 111, Int. Conf. can Particle and Nuclei, MIT, June 25-29, 1990[5] II. Str5be1e, Nucl - Phys . A525 (1991) 59c . [S] N.S. Amelin, K .K. Gudima, V.D. Toneev and S.Yu . Sivoklokov, Bergen Scientific/Technical Report 224/1990 . [7] N. S. Amelin, K. K. Gudima, S. Yu. Sivoklokov and V. D, Toneev, Invited talk presented at the 29th Spring School, Holzhau, Germany, April 8-12, 1991. . 48 ltl J. Rafelski and D. Willer, Phys. Rev. fett ,1982) 1066; (E) 56 09%) 23U . [9] J. Rafelski, Phys. Rep . 88 (1982) P?! ;, [10] N .S. Amelin, K.K. Gudima and V.D. Toneev, in The Yuclon- P;uation of State, Pd. by W. Greiner and
H. Stôcker, NATO ASI 218B, (Plenum, NY, 1989) p. 473; V.D. Toneev, N. S. Arvelin, K. K. Gudima, S. Yu. Sivoklokov Nucl. Phys. A519 (1990) 483-. [11] N.S. Ainelin, K.K. Gudima and V.D. Toneev, Yad. Fiz. 51 (1990) 512 ; Sov . J. Nucl. Phys. 51 (1990) 327. [12] N.S. Amelin, K.K. Gudima, S.Yu. Sivoklokov and V.D. Toneev, Yad . Fiz. 52 (1990) 272 ; Sov . J . Nucl. Phys. 5 2 (1990) 172. [13] N.S. Amelin, L .B. Bravina, L.I. Sarycheva, and L.V. Smirnova, Sov. J. Nucl. Phys., 51 (1990) 535. [14] N.S. 11% melin and L.V. Bravina, Sov . J. Nucl. Phys. 5 1 (1990) 133. [15] R. Matiello, H. Sorge, H. St6cker and W. Greiner, Phys. Rev. Lett . 63 (1989) 1459. [lfi] N .S. Amelin, E.F. Staub:,3, L.P. Csernai, V. Toneev and K.K. Gudima, Bergen Scientific/Technical Report 243/1P90, submitted to Phys. Rev. C. [1?] E.F. Staubo, AX Holme, L.P. Csernai, M. Gong and D. Strottman, Phys. Lett. B229 (1989) 351 .