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Low pT phenomena observed in high energy nuclear collisions
NUCLEAR PHYSICS A
Nuclear Physics A566 (1994) 175c-182~ North-Holland, Amsterdam
Low
PT phenomena
Jeha.nne
Simon-Gillo
“Physics
Division,
observed
in high
energy
nllclear
collisions
a MS D-156, Los Alamos
National
Lab, Los Alamos,
Nhl S7545
For several decades, experiments have reported an enhancement in the pion invariant cross section at low IIT ( < 1300I\IIev/ c ) 11 ‘1ICII co11~parrt1 to a thermal fit or minimum bias pp results. Experiment.al results from pp reactions at the ISR to heavy ion collisions at CERN will he reviewed t,hen compared to est ract trends. Several theoretical explanations will he discussed. An emphasis will be placed on the rolr of resonances and a. comparison of data to models will be presentetl. 1. INTRODUCTION An excess of pions at low PT was first observed at the ISR when high multiplicity events from pp and cycl collisions were compared to minimum bias pp distributions [l]. Low pi enhancement was la6er seen in cosmic ray data [2], pA (Fermilab, CERN) [s-6] 15 0 f euperiment,s which have studied low 117 and AA results (AGS. CERN) [5-121. A l’,t ._ behavior are listed in Table 1.
Table 1 Experiments
that
have studied
low 117 beha\.iol
Experiment
Project,ile
CERN-Heidelberg-Lund (ISR) Atwater et al., [2] Garhutt et al., (Fermilab) [4] Chaney et al., (Fermilab) [:3] NA4.34 [5] NA.35 [6] EMU5 [24, 121 N444 [25, 111 ES10 [7] ES02/ES59/ES66 [9, 29. :30] ES14 [S, 32, 311
[l]
Target
p. 0 coslnic 1’ 11
p.0.s p.0.S s 1’. s Si Si, .=\II p,Si
ne,
p. 0 Ag. RI C, R !\I. Cu. Sn, PI, \\’ :\\I. s PI, PI) Au, Cu Au. PI1 Pb, .41
Rapidity
1.0 0.6 1.0 3.1 2.2 0.6 2.s
< < < < < < <
y < $/ < .1!< y < y < y < 7J <
1.9 5.0 4.0 4.1 2.6 1.4 3.6
Several int,riguing esplanat,ions 11avr attempted t,o describe this behavior, including exotic behavior in dense ha.dronic matter [13&lC] ) and the decay of quark niatt,er droplets [17]. The excess of low 1)~ particles could also bc a reslllt of collrct,ive effects [a, lS] or a
0375-9474/94/$07.00
0 1994 - Elsevier Science B.V. All rights reserved.
176c
J. Simon-Gillo
I Low pr phenomena
observed
reflection of the dyna.mics of the hot collision mesonic resonances [IO, l!&%l]. 2.
EXPERIMENTAL
in high energy
nuclear
zone in the reaction,
collisions
such as baryonic
and
RESULTS
The CERN-Heidell,erg-Luntl Collaboration st,udied cro and pp collisions at the ISR [l]. Where previous ISR experiments found that t.he inclusive cross sections either turned over or were well described by an exponeiit,ial in ???T [a’2, ‘231, they observed an excess of low pT particles with increasing multiplicity when they compared high multiplicity to minimum bias events. This excess is small compared to that observed in pA and AA collisions and may IIC c111rto a tlifffvnt p1~e110111cnon. I,ow 1’~ rnhancement was also seen in pA collisions at Fcrmilab [3. 11 ailtl ill lligli llrultil)licity cosiiiic rays ill f-mulsion [?I. There are four CERN heavy ion CS~WI~~III~~II~St llat have st.lldied single hadron dist,ribritions at low 1’~ in a varict), of rapidit!, 12iiiges: iV.431 [‘,I. N.435 [ci], EhllT5 [34, 121, and NA44 [25. II]. N:\:)3 sttldied 200(:r\‘/c (,(311t ral (high ET) pr\ and AA collisions (1.0 < y < 1.9). :\t (‘ERN, the mitl-ral)iflit,>. is at :3 and IW~III rapidity is at 6. The resultsing negative Iladron cross scctiolls. correct cd for elcctroll colrtamina.t,ioir, are shown in figure 1: a paramrterizatioiI of thr niiilillillni bias l)p rrsults arc also included for romparison. The magnitude of tllr enIlallf.f’lllr‘llt at 101~1~‘ in pi\ is similar to that observed in the AA collisions. Ra.tios of cross sect ious front various reactions indicat,e a weak dependence on project,ile a11cl no tlcpe~~dctrcc~ OII c(,Ilt rality ill the rapidity range covered [5]. Figure 2 sho\vs Ilrgat i1.r II~~IIVII slwct ra III(~~SIIIYY~1,~. K;1:3.5 ill tliffrrc,nt rapidity regions; pAu da.ta sllo\v a sitliilar iiiagriit~itlr of f~iiliaiicrinf~iit [12. 61. ‘I’llf~se tlata have been corrected for electroil contaiiiiiiatioil. ‘l’lif, lo\\. I,,. beliavior sI~o\vs a tlrpeiidrncr on rapidity [12]. New NA% reslllts also sl~o\v a st IIIII~ r;rl)i(lit\. clq)el~drncc and an enhancement in pA collisions [26]. EhlIJ5 measures Ilegat ivr IIH~I.OIIS in t hc rapidity IYIII~~~ of 1.0 to 4.0 and historically, has claimed no low /)7. eiiliancr~ncnt ill t lif’ii. tlat a [2-l]. II 01w\:e1‘. whrn their data is plotted with NA1l.5 results, 011~ SPPS gootl agrcc‘lllf’tlt [I?]. ‘1’1 lri’r f>sists a discrepancy between NA.?4 alItI IChlT:T, at thr tnost I)acl\-\~al~tls rapitlit!. region of overlap which may be an indication of t,hcBrapidit!. tlcl)~t~(lo~~c~*of t IIC lo\v ,P,- bolla\.ior. The N-l.74 low 1’~ acceptance is c.oncrllt ratrtl Ilear ra1)itlit!. ol’ I .O. \vl~ilc t II~ IlhllT.5 data are distributed within the l.O-2.,! ri\pi(lit!. rallgr [‘L;]. N.444 measures itlultifirtl pioiis iI1 I):\ a11(1I\:\ collisions iiear niitl-rapidit),. eliminating electron contaiilinatioii oilliiie \villi a gas cl~f~i~~~i~l~o\~ coliiltrr [25. I I]. Figure 3 shows pPb and pRe cross sections and tll(> ratio pl’b/1)lk (:j.l < !I < -1.1 ). ;\ small eiiliallccment is indicated at low 1)~ and is est imatetl to 1)~ < 7%. ‘1’11(~ Sl’b/pIk shows all enhancement similar in magnitutlr. 1\;h.l-I oI~ser~~5 IIO tliffc~rf~ilcc~I,rt.\vc>c>1i111~ shapf~ of the TT+and 7idistributions, no strong tlrpentlf~i~cr 011 criit relit!‘. ant1 ii0 c~iillailcriilent in kaon spectra at low 1)~ [I I. 281. There arc t llwr :\(:S c~rl)cl~i~~lc~nts t llat ha\-c* III(~~SIIIYYI 11atlro11 sprctra at. low IIT. ES10 measured ncgativf, Itadi~ons iii AI\ at I l.(Kk~\‘/ c iii t lie rapidit\, range of 2.2-2.6. At thr ACS, mitl-rapit1it.y is z I .7 atltl I)f~am ral)iflit\. is zz :j.-1. ‘1’11~~ spectra are well described by a sum of t\vo c~xponf~ilt.ials ai~d t IIO>.(1stiinat (~1 t list approximately :30% of the pions
J. Simon-Gillo I Low pT phenomena observed in high energy nuclear collisions
centralp*w
200
177c
GeV/N
I 1.0 C y C 1.9 1 min.
bias
pp
par.
“,
11..
NA35
200
.
0
. .
.
. l
1 pI
0.4
0.8
. l
c 0
”
1
a
0 y-0.6-1.6
GeV/N
.
100
”
I
O-Au
1.2 Pt (GeV/c)
(I
2
(GeV/cl
Figure 1. NA34 I)T spectra (reprinted [s]). R.QMD calculations of O+Au (1.0 < y < 1.9) a.re included in the O+W plot.
Figure 2. NA35 O+Au pT specl,ra. (reprinted [12]). RQMD calculatiorr of O+Au (2.0 < y < 3.0) are included. 2
Preliminary
1.5
1
0 Pt (GeV/c) Figure 3. NA44 pion PT distributions from p.4 collisions. for variations in dN/dy within the NA44 acceptance.
0.08
0.16
0.24
0.32
Pt (GeV/c) These da.ta have heen corrected
178~
J. Simon-Gillo I Low pr phenomena observed in high energy nuclear collisions
are contained in the low temperature exponent ial [7]. ES02/E859/ES66 ha.s studied low PT pions in the rapidity range 1.2-1.4. Preliminary studies from ES02 indicate that there may be a slight low pr enhancement in new data sets which measure to IIT z 2OOMeV/c in Si+A reactions [29, SO]. E866 is the third generation ES02 experiment and studies Au+Au reactions at ll.(iGeV/c. They report an enhancement at low mT which seems to show a. difference between the 7r+ and the 7riTspectra; only the 7riT-distributions show an enhancement. There is no strong dependence on y. It has been estimated that approximateI> 25% of the nega.tive pions are contained in the low temperature exponential [O]. Experiment E814 measures identified pions at 14.6Ge\i/c in the rapidity range 2.8-3.6. An enhancement is seen in the data when con~parcd (0 l301tzlnanii a.nd mT csponent.ia~ fits. The da.ta show no strong tlcpe11~1r~~c~ 011 targrt or rapitlii!. [S. :$‘L]. Tlley also l)ct\veeii their 7r+ and 7ireport no enhancemeilt in pA resrllt,s [Sl] and 110 tlifrercncc distributions [32]. In comparing the C’ERN and :1GS results. sc,\,rraI intcrcst,ing espcrimental trends can be extracted. Both t,hc C’ERN and .:\GS hadron sljectra show a low IQ- enhancement when compared to pp minimum bias results. ~I?T rspone~~tials. or a theunal fit. p.4 collisions studied at CERN show an enhancement similal~ ilr Inagllitlltlc to tllr AA results. There is a wrak clepenclence on t.argrt autl projectile. 13. .Iacak leas fit the CERN results with a sum of two exponeiit,ials ii1 1117’ to estiiiiatcx 1I1r fractioii of soft pions contained in the low pT region [2O]. :~lt]iough the clloicc of a fit tiiig pro’.~‘~lll’.e can ilifiueilce the absolute results, this exercise ~110~s a gelreral trclltl: t IIC nlagnit utlr of the enhancement of soft pT hadrons at CERN cuergies colnparrd to ttlinimunl 1)ias pp result,s is strongest at backwards rapidities. ES66 WCS a rise ill tlleir ncgati\rc Ijut llot positive pion spectra. N:\:34 alld N.443 report ES14 and NA44 see no difference between TT+and in- results. no strong dependence on centrality. l\‘hilc tlicrr is no str011g tlrpendence on rapidity at AGS energies. the C:ERN data show an ob\-ious tlq~~~tlcncr. wit 11tile low ])T enhancement more significant at backwards rapitlitirs [ 12. 201. 3.
THEORETICAL
POSSIBILITIES
Several intriguing exp1allationS ha\rf’ h’ll ])l’O]~Od t0 c’S]I]aill the lOw IIT h=ha\riOr. that this coultl he tlw th collective effects; Lee, Heinz [18], and Atwater [2] 1iavc> suggeskl Lee and Heinz compared transverse flow could lead to a concavr-shaped cross section. NA35 S+S data (mid-rapidity) to a global fit of a tllrrmal model that included ra.tlial collective flow. They found good agrermeut rscept. a( vrry low ]JT. ~Iowevrr. t,his thCOI’y does not explain wily the CI~~RIIC~WCII~ a. (‘l?RN ill p.A z .;\.A or \vll,v the (-‘ERN data show a ra.pidit,_v dependence. VanHove suggested we cor~ltl be obsrr\.ing tlrc tlcca~. of small plasnla droplets into pions [17]. A.3 in . t ri‘g um g as t,liis sounds, this is an cffcd that would Iw peaked at, mitl-rapidity, contrary t,o the C’ERN results. Several theorists have suggrst,ed that. the systcni coriltl be out. of chemical equilibrium. Kataja. and Ruuskanen state t.hat tllr cspcrilllental I’T spectra can be described by a thermal distribut,ion if one assumes that t.herc is a strong excess of pions lvith respect to chemical equilibrium at freezrolit: a positive clicmical pot,eutial would lead to the
J. Simon-Gillo I LOWpr phenomena observed in high energy nuclear collisions
119c
furt,her claim that the system production of more pions [14]. G avin and Ruuskanen may only be in partial thermal equilibrium due to a longitudinal expansion leading a monte car10 solution of a to additional soft pions [15]. B aiz et al., have calculated Boltzmann ecluation for a meson gas including resonances and incomplete thermalization; they find good agreement with NA35 midrapidity 0+.4u results [lG]. These theories ignore the influence of the haryons in the system and do not attempt to explain low PT behavior other than at mid-rapidity. Nor do they predict a similar enhancement in p.4 and A.4 collisions. Shuryak proposed that we could he ohserving a modification of the pion dispersion relation [13]. Attractive interact,ions between constituents could form a surfa.ce making it, difficult, for constituents to Icavc the system. trapping the low 11~ pions in a potential well. A search for t,his effect at Lt\hIPF intlicatcd its ahsrnce. at least in t,he relat)ively cold nuclear matter present in I>.+\collisioiis [:3:3]. I1 is crrtaitl l,hat, this mean field effect a.s proposed by Shnryak must exist. at some Icvel: the quest ioii is whether it is large enough to exp1a.m the observed features in the data. lioch and Bcrtsch have found collective mean fields insufficient t.0 esplaiil t lie magnitude of the ol~servetl enhancement unless there is an extremely slow adiabatic expansion which they find unrealistic [:36]. Sullivan and Simon-Gillo have sttttlied secontlaq. pion production in the target. Geant (ZOOGeV/A S+Pb and wa.s used to track particles generated from RQr\lB calctilations 14.6GeV/A Si+Pb) and the srcontlarics creat,etl in the target through various target thicknesses[34,%]. The calculat.ion indicated that thick targets could lead to a significant low pT enhancement peaked at backwards rapitlitirs hoth at (:ERN and the AGS (figure 4). However. t.hr rsperiinents rit I~er ~~secl thitl targets or st utlictl a rapidity region where a thick target llatl negligible rlfect. Many theorisb and cxl’erimr,llt.alists (lleiliz, 13row11, (‘olc. Iacak. Koch, Sollfrank, Stachel, and Welkr) ha\,e recognized tlie itillueticr of resonancrs on the IIT distribution of hadrons [19, 21, 10. 201. RQMB calcttlations a11t1 experimental results indicate that mesonic resonancrs dominate at nlitl-raI)itlit\. at (‘ERN. wllile .4GS collisions produce a more baryonic fnvironmeiit,. 4.
COMPARISON
OF
DATA
TO
MODELS
Figure 5a (top) sho\vs RQnil) calcrilatiotls at (‘ERN and r\GS energies to illustrate the cont.rihution of pions from \.arious resonances. x- arising from primary collisions, strings and ropes are s~unmetl together; RQnIB ropes are interactions between strings [:3.5]. In this calculat,ion, the resonances arc grouped a.s follows: haryonic: a, N’. a*; st,range: I<:, A, 2: niesonic: II> 11’. -3; pions from 0’s are kept, separately. Most of t,he pions at mid-rapidity at GERN energies are from p’s and primary collisions, while the baryonic resonances play a. more intport.atrt mle near t,arget rapidity. The mesonic and strange resonances play a comparal)lr role and peak at mid-rapidity. At AGS energies, the baryonic resonances dominate at all ral)itlities. Figure 51, (bot,tom) shows rapidity density plots for pions witli jq- < 300 lie\‘/ c f,tom RQhln. At (‘ERN energies. baryonic resonances d0minat.e at, target rapidit).. ‘I’hr other contrihritions become compara.ble and peak at mid-rapidit\.. Tllis plot explains Ilo\\ a rapidity dependence of the low 11~ enhancement, at (IERN co~~ltl rsist. ‘Il~c sl~a))r of the rapidity density at the AGS does
18Oc
J. Simon-Gillo I Low pr phenomena observed in high energy nuclear collisions
P, &V/c)
6 &V/c)
Figure 4. RQMD/GEANT calculation showing the affect of secondary pions created in the target on the pi distribution of pions at target rapidity at CERN energies. “in” are the particles generated by RQMD for 200GeV/A S+Pb. Particles and the secondaries created in a 1Omm Pb target are tracked with Geant. “out” are all resulting negative pions only in pions tracked by GEANT found in a given y range. “2nd” are secondary the same y range.
Y
Y
Figure 5. a) (top) RQMD rapidity density plot of pions from various sources at both CERN and AGS energies: 0 from collisions, strings and ropes, 0 baryonic, 0 mesonic, * strange, A p ‘s. b) (bottom) same plot for low pi pions (
not change significantly for low 1)~ pions alone; the bxyonic resonances 611 dominate at all rapidities. Consequently, one might expect a weak dependence of low 17~ enhancement on rapidity at AGS energies. Several experimental groups have compared t,o models that inrorpomtc resonance contributions. ES10 and ES14 pion pr spectra are well drscribed by RQMD and/or ARC 3’s to RQMD predictions and found results [S, 371. ES14 a 1so compa,red rwonst~ructed good agreement [S]. N.444 S+Pb pion spectra (3.1 < y < 4.1) are also well described by to NA34 is shown in figure 1 RQMD. The comparison of a.n RQMD O+A II calculation and the compa.rison to NA35 is in figure 2. (The Ii, and A are inch&d in hot,h of these da.ta sets a.nd therefore allowed t,o decay in RQ~~~.) The RQMD results are scaled in or&r to compare t.0 tlre data which are presentetl here in arbit.rary units. It is interest,ing, however. that RQMD cloes not reproduce t.hc magnitude of the low ,wr rnhancrmrnt at ba,ckwards rapiditirs. 5.
CONCLUSION
Low pr enhancement has hren nl~w~tl ill :\GS and CRRN I~ca\,y ion result,s. At, CERN a. similar behavior is obser\wl ill ]):l antI :\:I collisiolls. The CERN resuks show a strong rapidity depentlenct~ which is wilected in 1.1~ rapidity distribut~ion of pions from resonances. The AGS results are well drscribrtl by models incorporating resonances; these data, as well as the ~Iistril~~ltioI~s of the pions from lh~ co~~~~ril~~lt~i~~g resona.nces, do not show a strong rapidity rlrpcndcnre. RQMJ, results do not fully reproduce the magnitude of thr enhancenwnt bacl;wards of Ilti(l-rapidity a.t (“ERN. leaving room for further speculation. In order to undcrst~antl tlw (‘ERN wsr~lts. thcor.ri.ical motlt~ls should incorporate the influence of resonances, atltlwss t Iw fact t lrat thr II:\ 22 .-\.A results, and expla.in the rapidit,?; cl~pe~~dcnceolxx~r\~~l ill 1.11(> da1 a.
REFERENCES 1. 2. 3.
4. 5. 6. 7. S.
W. Bell et, al., Z. Phys. C 27( J!)Y5)27, T.W. Atwater et al.. Phys. Let,t. 13 lu!r(l!,s;):to. D. Chancy et. al., fhys. Rev. D I l~l~~~~~):~~i~. D.A. Garbutt et al., Phys. Lctt. 6X3( 1!)7’i):t.55. T. Akcsson et al.. Z. J’hys. (: .ICi(l~j~~O):~Oi. +J. Harris et, al.. Nurl. Phys. :\-I!)$( i!)$!))i:$:fc. S. Ahmatl c+ al.. Phys. 1,et.t. B :‘$I ( I!)!):!)?!!. ‘I’. Hemmick. HIP:1GS’!l3. (‘allll)ritige. RI:\. .Janl~ar!;. I!)!):1 also we the-se proceedings. M. Gonin, HIPAGS’S.?. Cambridge. \l:\, .Janua~.y, 1993, also see these proceedings.
9. 10. 13. Cole, HIP:2GS’!X3. C’ambritlgc. All-\, .Jan~~ar!,, 19!1:1. 11. h4. Murray, “Single Particle Spectra from N:\+l at t,he ClERN SW”. ings. 12. J. Schukraft, CERN-PPE/SI-O-1. .Januar!. 16. l!)!)l. 13. E.V. Shuryak, Pltys. Rev. D 42( i!t!)O)l7G~l. 14. I\I. Kataja and I’. Ruusl~anrn, Pl,ys. I,cxtt. 132 I:%(i9W)lSl. 15. S. Gavin and P. R~~IIs~;~II~~~~. P11ys. Mt. B%i2( 1~1 ):I%
t,hese proreetl-
182c
16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31.
J. Si~n-~i~lo
/ I!.OW pT p~e~~ena
observed in high energy nuclear co&ions
W.W. Barz et al., Phys. L&t. B2S7(1992)40. L. Van Hove, CERN preprint, ‘W-5236/85( 19SS). IX. Lee et al., 2. Phys. C 48( 1990).5%5. G.E. Brown, et al., Phys. Lett. l3 2X3( 1991)19. W. Jacak, Proceedings of NATO A cI vancrtl . Study Institute, I1 Ciocco, Italy, July, 1992, to be published. J. Sollfrank et al., Phys. I&t,. R 252( 1990)256. I<. Guettler et al., Phys. Lett. ~,~1~(197~)11 I. B. Alper et al,, Nucl. Phys. BlO~(l~~~~)2:~7. Y. Taka,hashi et al., P;ucl. Phys. .~~2~(19~~~).~9lc. M. Saralxira ct. al.. ?iirtl. Pltys. :\:i.l.:I( i!f!tL!)l:‘T,c. D. Rollrich et al., “IIntlron I’rotl~~ction in S+>\g. S+.Au C’ollisiom at, 2OOGeV per Nucleon Studirtl in .47r Acccpt,ance“, tlmr procrcdings. B. Jacak, Los Alamos Nat,ional I,almratory. private conlm~~tlicatio~r. H. Kalechofsky. PhD. Disscrtatiorl. I’. Pittshr~rgl~ B. Cole. Nrvis I,almratoric~s. (!olulnl)ia IT~li\x~rsit>.. pri\xtc conltlr~lnicatioll. Charles Parso~rs. 1’1111. Dissrrtat ioll, h~la~sac1lnsc~t.1~ Inst itut,e of T~hnolog):, June. 199”. M. Rosat,i. “Particle Procluctiotl in p+A (‘ollisions at, I-l.(i(:c\‘/c”, see these proceedings. t*hese proceedings.
33. J. McClelIancf et a.l.. Phys. Rev. Mt.. W( 1!@2)55”. 3.1. Geant User’s Guide, version 3.15, (‘ERN (‘ompntc~r (‘cntcr. 3.5. Il. Serge, et al., Nucl. Ph>x ,bI!)S( I!fS!)).%iTc. 136. V. I\;orh, SITX1. Sto~l!;Brool;. pri\:at~~ c.ol~lnrlttlicatiol1. 37. T-l. Sorgr, Phys. L&t. BXl( 1991 ):1T.
Nuclear Physics A566 (1994) 175c-182~ North-Holland, Amsterdam
Low
PT phenomena
Jeha.nne
Simon-Gillo
“Physics
Division,
observed
in high
energy
nllclear
collisions
a MS D-156, Los Alamos
National
Lab, Los Alamos,
Nhl S7545
For several decades, experiments have reported an enhancement in the pion invariant cross section at low IIT ( < 1300I\IIev/ c ) 11 ‘1ICII co11~parrt1 to a thermal fit or minimum bias pp results. Experiment.al results from pp reactions at the ISR to heavy ion collisions at CERN will he reviewed t,hen compared to est ract trends. Several theoretical explanations will he discussed. An emphasis will be placed on the rolr of resonances and a. comparison of data to models will be presentetl. 1. INTRODUCTION An excess of pions at low PT was first observed at the ISR when high multiplicity events from pp and cycl collisions were compared to minimum bias pp distributions [l]. Low pi enhancement was la6er seen in cosmic ray data [2], pA (Fermilab, CERN) [s-6] 15 0 f euperiment,s which have studied low 117 and AA results (AGS. CERN) [5-121. A l’,t ._ behavior are listed in Table 1.
Table 1 Experiments
that
have studied
low 117 beha\.iol
Experiment
Project,ile
CERN-Heidelberg-Lund (ISR) Atwater et al., [2] Garhutt et al., (Fermilab) [4] Chaney et al., (Fermilab) [:3] NA4.34 [5] NA.35 [6] EMU5 [24, 121 N444 [25, 111 ES10 [7] ES02/ES59/ES66 [9, 29. :30] ES14 [S, 32, 311
[l]
Target
p. 0 coslnic 1’ 11
p.0.s p.0.S s 1’. s Si Si, .=\II p,Si
ne,
p. 0 Ag. RI C, R !\I. Cu. Sn, PI, \\’ :\\I. s PI, PI) Au, Cu Au. PI1 Pb, .41
Rapidity
1.0 0.6 1.0 3.1 2.2 0.6 2.s
< < < < < < <
y < $/ < .1!< y < y < y < 7J <
1.9 5.0 4.0 4.1 2.6 1.4 3.6
Several int,riguing esplanat,ions 11avr attempted t,o describe this behavior, including exotic behavior in dense ha.dronic matter [13&lC] ) and the decay of quark niatt,er droplets [17]. The excess of low 1)~ particles could also bc a reslllt of collrct,ive effects [a, lS] or a
0375-9474/94/$07.00
0 1994 - Elsevier Science B.V. All rights reserved.
176c
J. Simon-Gillo
I Low pr phenomena
observed
reflection of the dyna.mics of the hot collision mesonic resonances [IO, l!&%l]. 2.
EXPERIMENTAL
in high energy
nuclear
zone in the reaction,
collisions
such as baryonic
and
RESULTS
The CERN-Heidell,erg-Luntl Collaboration st,udied cro and pp collisions at the ISR [l]. Where previous ISR experiments found that t.he inclusive cross sections either turned over or were well described by an exponeiit,ial in ???T [a’2, ‘231, they observed an excess of low pT particles with increasing multiplicity when they compared high multiplicity to minimum bias events. This excess is small compared to that observed in pA and AA collisions and may IIC c111rto a tlifffvnt p1~e110111cnon. I,ow 1’~ rnhancement was also seen in pA collisions at Fcrmilab [3. 11 ailtl ill lligli llrultil)licity cosiiiic rays ill f-mulsion [?I. There are four CERN heavy ion CS~WI~~III~~II~St llat have st.lldied single hadron dist,ribritions at low 1’~ in a varict), of rapidit!, 12iiiges: iV.431 [‘,I. N.435 [ci], EhllT5 [34, 121, and NA44 [25. II]. N:\:)3 sttldied 200(:r\‘/c (,(311t ral (high ET) pr\ and AA collisions (1.0 < y < 1.9). :\t (‘ERN, the mitl-ral)iflit,>. is at :3 and IW~III rapidity is at 6. The resultsing negative Iladron cross scctiolls. correct cd for elcctroll colrtamina.t,ioir, are shown in figure 1: a paramrterizatioiI of thr niiilillillni bias l)p rrsults arc also included for romparison. The magnitude of tllr enIlallf.f’lllr‘llt at 101~1~‘ in pi\ is similar to that observed in the AA collisions. Ra.tios of cross sect ious front various reactions indicat,e a weak dependence on project,ile a11cl no tlcpe~~dctrcc~ OII c(,Ilt rality ill the rapidity range covered [5]. Figure 2 sho\vs Ilrgat i1.r II~~IIVII slwct ra III(~~SIIIYY~1,~. K;1:3.5 ill tliffrrc,nt rapidity regions; pAu da.ta sllo\v a sitliilar iiiagriit~itlr of f~iiliaiicrinf~iit [12. 61. ‘I’llf~se tlata have been corrected for electroil contaiiiiiiatioil. ‘l’lif, lo\\. I,,. beliavior sI~o\vs a tlrpeiidrncr on rapidity [12]. New NA% reslllts also sl~o\v a st IIIII~ r;rl)i(lit\. clq)el~drncc and an enhancement in pA collisions [26]. EhlIJ5 measures Ilegat ivr IIH~I.OIIS in t hc rapidity IYIII~~~ of 1.0 to 4.0 and historically, has claimed no low /)7. eiiliancr~ncnt ill t lif’ii. tlat a [2-l]. II 01w\:e1‘. whrn their data is plotted with NA1l.5 results, 011~ SPPS gootl agrcc‘lllf’tlt [I?]. ‘1’1 lri’r f>sists a discrepancy between NA.?4 alItI IChlT:T, at thr tnost I)acl\-\~al~tls rapitlit!. region of overlap which may be an indication of t,hcBrapidit!. tlcl)~t~(lo~~c~*of t IIC lo\v ,P,- bolla\.ior. The N-l.74 low 1’~ acceptance is c.oncrllt ratrtl Ilear ra1)itlit!. ol’ I .O. \vl~ilc t II~ IlhllT.5 data are distributed within the l.O-2.,! ri\pi(lit!. rallgr [‘L;]. N.444 measures itlultifirtl pioiis iI1 I):\ a11(1I\:\ collisions iiear niitl-rapidit),. eliminating electron contaiilinatioii oilliiie \villi a gas cl~f~i~~~i~l~o\~ coliiltrr [25. I I]. Figure 3 shows pPb and pRe cross sections and tll(> ratio pl’b/1)lk (:j.l < !I < -1.1 ). ;\ small eiiliallccment is indicated at low 1)~ and is est imatetl to 1)~ < 7%. ‘1’11(~ Sl’b/pIk shows all enhancement similar in magnitutlr. 1\;h.l-I oI~ser~~5 IIO tliffc~rf~ilcc~I,rt.\vc>c>1i111~ shapf~ of the TT+and 7idistributions, no strong tlrpentlf~i~cr 011 criit relit!‘. ant1 ii0 c~iillailcriilent in kaon spectra at low 1)~ [I I. 281. There arc t llwr :\(:S c~rl)cl~i~~lc~nts t llat ha\-c* III(~~SIIIYYI 11atlro11 sprctra at. low IIT. ES10 measured ncgativf, Itadi~ons iii AI\ at I l.(Kk~\‘/ c iii t lie rapidit\, range of 2.2-2.6. At thr ACS, mitl-rapit1it.y is z I .7 atltl I)f~am ral)iflit\. is zz :j.-1. ‘1’11~~ spectra are well described by a sum of t\vo c~xponf~ilt.ials ai~d t IIO>.(1stiinat (~1 t list approximately :30% of the pions
J. Simon-Gillo I Low pT phenomena observed in high energy nuclear collisions
centralp*w
200
177c
GeV/N
I 1.0 C y C 1.9 1 min.
bias
pp
par.
“,
11..
NA35
200
.
0
. .
.
. l
1 pI
0.4
0.8
. l
c 0
”
1
a
0 y-0.6-1.6
GeV/N
.
100
”
I
O-Au
1.2 Pt (GeV/c)
(I
2
(GeV/cl
Figure 1. NA34 I)T spectra (reprinted [s]). R.QMD calculations of O+Au (1.0 < y < 1.9) a.re included in the O+W plot.
Figure 2. NA35 O+Au pT specl,ra. (reprinted [12]). RQMD calculatiorr of O+Au (2.0 < y < 3.0) are included. 2
Preliminary
1.5
1
0 Pt (GeV/c) Figure 3. NA44 pion PT distributions from p.4 collisions. for variations in dN/dy within the NA44 acceptance.
0.08
0.16
0.24
0.32
Pt (GeV/c) These da.ta have heen corrected
178~
J. Simon-Gillo I Low pr phenomena observed in high energy nuclear collisions
are contained in the low temperature exponent ial [7]. ES02/E859/ES66 ha.s studied low PT pions in the rapidity range 1.2-1.4. Preliminary studies from ES02 indicate that there may be a slight low pr enhancement in new data sets which measure to IIT z 2OOMeV/c in Si+A reactions [29, SO]. E866 is the third generation ES02 experiment and studies Au+Au reactions at ll.(iGeV/c. They report an enhancement at low mT which seems to show a. difference between the 7r+ and the 7riTspectra; only the 7riT-distributions show an enhancement. There is no strong dependence on y. It has been estimated that approximateI> 25% of the nega.tive pions are contained in the low temperature exponential [O]. Experiment E814 measures identified pions at 14.6Ge\i/c in the rapidity range 2.8-3.6. An enhancement is seen in the data when con~parcd (0 l301tzlnanii a.nd mT csponent.ia~ fits. The da.ta show no strong tlcpe11~1r~~c~ 011 targrt or rapitlii!. [S. :$‘L]. Tlley also l)ct\veeii their 7r+ and 7ireport no enhancemeilt in pA resrllt,s [Sl] and 110 tlifrercncc distributions [32]. In comparing the C’ERN and :1GS results. sc,\,rraI intcrcst,ing espcrimental trends can be extracted. Both t,hc C’ERN and .:\GS hadron sljectra show a low IQ- enhancement when compared to pp minimum bias results. ~I?T rspone~~tials. or a theunal fit. p.4 collisions studied at CERN show an enhancement similal~ ilr Inagllitlltlc to tllr AA results. There is a wrak clepenclence on t.argrt autl projectile. 13. .Iacak leas fit the CERN results with a sum of two exponeiit,ials ii1 1117’ to estiiiiatcx 1I1r fractioii of soft pions contained in the low pT region [2O]. :~lt]iough the clloicc of a fit tiiig pro’.~‘~lll’.e can ilifiueilce the absolute results, this exercise ~110~s a gelreral trclltl: t IIC nlagnit utlr of the enhancement of soft pT hadrons at CERN cuergies colnparrd to ttlinimunl 1)ias pp result,s is strongest at backwards rapidities. ES66 WCS a rise ill tlleir ncgati\rc Ijut llot positive pion spectra. N:\:34 alld N.443 report ES14 and NA44 see no difference between TT+and in- results. no strong dependence on centrality. l\‘hilc tlicrr is no str011g tlrpendence on rapidity at AGS energies. the C:ERN data show an ob\-ious tlq~~~tlcncr. wit 11tile low ])T enhancement more significant at backwards rapitlitirs [ 12. 201. 3.
THEORETICAL
POSSIBILITIES
Several intriguing exp1allationS ha\rf’ h’ll ])l’O]~Od t0 c’S]I]aill the lOw IIT h=ha\riOr. that this coultl he tlw th collective effects; Lee, Heinz [18], and Atwater [2] 1iavc> suggeskl Lee and Heinz compared transverse flow could lead to a concavr-shaped cross section. NA35 S+S data (mid-rapidity) to a global fit of a tllrrmal model that included ra.tlial collective flow. They found good agrermeut rscept. a( vrry low ]JT. ~Iowevrr. t,his thCOI’y does not explain wily the CI~~RIIC~WCII~ a. (‘l?RN ill p.A z .;\.A or \vll,v the (-‘ERN data show a ra.pidit,_v dependence. VanHove suggested we cor~ltl be obsrr\.ing tlrc tlcca~. of small plasnla droplets into pions [17]. A.3 in . t ri‘g um g as t,liis sounds, this is an cffcd that would Iw peaked at, mitl-rapidity, contrary t,o the C’ERN results. Several theorists have suggrst,ed that. the systcni coriltl be out. of chemical equilibrium. Kataja. and Ruuskanen state t.hat tllr cspcrilllental I’T spectra can be described by a thermal distribut,ion if one assumes that t.herc is a strong excess of pions lvith respect to chemical equilibrium at freezrolit: a positive clicmical pot,eutial would lead to the
J. Simon-Gillo I LOWpr phenomena observed in high energy nuclear collisions
119c
furt,her claim that the system production of more pions [14]. G avin and Ruuskanen may only be in partial thermal equilibrium due to a longitudinal expansion leading a monte car10 solution of a to additional soft pions [15]. B aiz et al., have calculated Boltzmann ecluation for a meson gas including resonances and incomplete thermalization; they find good agreement with NA35 midrapidity 0+.4u results [lG]. These theories ignore the influence of the haryons in the system and do not attempt to explain low PT behavior other than at mid-rapidity. Nor do they predict a similar enhancement in p.4 and A.4 collisions. Shuryak proposed that we could he ohserving a modification of the pion dispersion relation [13]. Attractive interact,ions between constituents could form a surfa.ce making it, difficult, for constituents to Icavc the system. trapping the low 11~ pions in a potential well. A search for t,his effect at Lt\hIPF intlicatcd its ahsrnce. at least in t,he relat)ively cold nuclear matter present in I>.+\collisioiis [:3:3]. I1 is crrtaitl l,hat, this mean field effect a.s proposed by Shnryak must exist. at some Icvel: the quest ioii is whether it is large enough to exp1a.m the observed features in the data. lioch and Bcrtsch have found collective mean fields insufficient t.0 esplaiil t lie magnitude of the ol~servetl enhancement unless there is an extremely slow adiabatic expansion which they find unrealistic [:36]. Sullivan and Simon-Gillo have sttttlied secontlaq. pion production in the target. Geant (ZOOGeV/A S+Pb and wa.s used to track particles generated from RQr\lB calctilations 14.6GeV/A Si+Pb) and the srcontlarics creat,etl in the target through various target thicknesses[34,%]. The calculat.ion indicated that thick targets could lead to a significant low pT enhancement peaked at backwards rapitlitirs hoth at (:ERN and the AGS (figure 4). However. t.hr rsperiinents rit I~er ~~secl thitl targets or st utlictl a rapidity region where a thick target llatl negligible rlfect. Many theorisb and cxl’erimr,llt.alists (lleiliz, 13row11, (‘olc. Iacak. Koch, Sollfrank, Stachel, and Welkr) ha\,e recognized tlie itillueticr of resonancrs on the IIT distribution of hadrons [19, 21, 10. 201. RQMB calcttlations a11t1 experimental results indicate that mesonic resonancrs dominate at nlitl-raI)itlit\. at (‘ERN. wllile .4GS collisions produce a more baryonic fnvironmeiit,. 4.
COMPARISON
OF
DATA
TO
MODELS
Figure 5a (top) sho\vs RQnil) calcrilatiotls at (‘ERN and r\GS energies to illustrate the cont.rihution of pions from \.arious resonances. x- arising from primary collisions, strings and ropes are s~unmetl together; RQnIB ropes are interactions between strings [:3.5]. In this calculat,ion, the resonances arc grouped a.s follows: haryonic: a, N’. a*; st,range: I<:, A, 2: niesonic: II> 11’. -3; pions from 0’s are kept, separately. Most of t,he pions at mid-rapidity at GERN energies are from p’s and primary collisions, while the baryonic resonances play a. more intport.atrt mle near t,arget rapidity. The mesonic and strange resonances play a comparal)lr role and peak at mid-rapidity. At AGS energies, the baryonic resonances dominate at all ral)itlities. Figure 51, (bot,tom) shows rapidity density plots for pions witli jq- < 300 lie\‘/ c f,tom RQhln. At (‘ERN energies. baryonic resonances d0minat.e at, target rapidit).. ‘I’hr other contrihritions become compara.ble and peak at mid-rapidit\.. Tllis plot explains Ilo\\ a rapidity dependence of the low 11~ enhancement, at (IERN co~~ltl rsist. ‘Il~c sl~a))r of the rapidity density at the AGS does
18Oc
J. Simon-Gillo I Low pr phenomena observed in high energy nuclear collisions
P, &V/c)
6 &V/c)
Figure 4. RQMD/GEANT calculation showing the affect of secondary pions created in the target on the pi distribution of pions at target rapidity at CERN energies. “in” are the particles generated by RQMD for 200GeV/A S+Pb. Particles and the secondaries created in a 1Omm Pb target are tracked with Geant. “out” are all resulting negative pions only in pions tracked by GEANT found in a given y range. “2nd” are secondary the same y range.
Y
Y
Figure 5. a) (top) RQMD rapidity density plot of pions from various sources at both CERN and AGS energies: 0 from collisions, strings and ropes, 0 baryonic, 0 mesonic, * strange, A p ‘s. b) (bottom) same plot for low pi pions (
not change significantly for low 1)~ pions alone; the bxyonic resonances 611 dominate at all rapidities. Consequently, one might expect a weak dependence of low 17~ enhancement on rapidity at AGS energies. Several experimental groups have compared t,o models that inrorpomtc resonance contributions. ES10 and ES14 pion pr spectra are well drscribed by RQMD and/or ARC 3’s to RQMD predictions and found results [S, 371. ES14 a 1so compa,red rwonst~ructed good agreement [S]. N.444 S+Pb pion spectra (3.1 < y < 4.1) are also well described by to NA34 is shown in figure 1 RQMD. The comparison of a.n RQMD O+A II calculation and the compa.rison to NA35 is in figure 2. (The Ii, and A are inch&d in hot,h of these da.ta sets a.nd therefore allowed t,o decay in RQ~~~.) The RQMD results are scaled in or&r to compare t.0 tlre data which are presentetl here in arbit.rary units. It is interest,ing, however. that RQMD cloes not reproduce t.hc magnitude of the low ,wr rnhancrmrnt at ba,ckwards rapiditirs. 5.
CONCLUSION
Low pr enhancement has hren nl~w~tl ill :\GS and CRRN I~ca\,y ion result,s. At, CERN a. similar behavior is obser\wl ill ]):l antI :\:I collisiolls. The CERN resuks show a strong rapidity depentlenct~ which is wilected in 1.1~ rapidity distribut~ion of pions from resonances. The AGS results are well drscribrtl by models incorporating resonances; these data, as well as the ~Iistril~~ltioI~s of the pions from lh~ co~~~~ril~~lt~i~~g resona.nces, do not show a strong rapidity rlrpcndcnre. RQMJ, results do not fully reproduce the magnitude of thr enhancenwnt bacl;wards of Ilti(l-rapidity a.t (“ERN. leaving room for further speculation. In order to undcrst~antl tlw (‘ERN wsr~lts. thcor.ri.ical motlt~ls should incorporate the influence of resonances, atltlwss t Iw fact t lrat thr II:\ 22 .-\.A results, and expla.in the rapidit,?; cl~pe~~dcnceolxx~r\~~l ill 1.11(> da1 a.
REFERENCES 1. 2. 3.
4. 5. 6. 7. S.
W. Bell et, al., Z. Phys. C 27( J!)Y5)27, T.W. Atwater et al.. Phys. Let,t. 13 lu!r(l!,s;):to. D. Chancy et. al., fhys. Rev. D I l~l~~~~~):~~i~. D.A. Garbutt et al., Phys. Lctt. 6X3( 1!)7’i):t.55. T. Akcsson et al.. Z. J’hys. (: .ICi(l~j~~O):~Oi. +J. Harris et, al.. Nurl. Phys. :\-I!)$( i!)$!))i:$:fc. S. Ahmatl c+ al.. Phys. 1,et.t. B :‘$I ( I!)!):!)?!!. ‘I’. Hemmick. HIP:1GS’!l3. (‘allll)ritige. RI:\. .Janl~ar!;. I!)!):1 also we the-se proceedings. M. Gonin, HIPAGS’S.?. Cambridge. \l:\, .Janua~.y, 1993, also see these proceedings.
9. 10. 13. Cole, HIP:2GS’!X3. C’ambritlgc. All-\, .Jan~~ar!,, 19!1:1. 11. h4. Murray, “Single Particle Spectra from N:\+l at t,he ClERN SW”. ings. 12. J. Schukraft, CERN-PPE/SI-O-1. .Januar!. 16. l!)!)l. 13. E.V. Shuryak, Pltys. Rev. D 42( i!t!)O)l7G~l. 14. I\I. Kataja and I’. Ruusl~anrn, Pl,ys. I,cxtt. 132 I:%(i9W)lSl. 15. S. Gavin and P. R~~IIs~;~II~~~~. P11ys. Mt. B%i2( 1~1 ):I%
t,hese proreetl-
182c
16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31.
J. Si~n-~i~lo
/ I!.OW pT p~e~~ena
observed in high energy nuclear co&ions
W.W. Barz et al., Phys. L&t. B2S7(1992)40. L. Van Hove, CERN preprint, ‘W-5236/85( 19SS). IX. Lee et al., 2. Phys. C 48( 1990).5%5. G.E. Brown, et al., Phys. Lett. l3 2X3( 1991)19. W. Jacak, Proceedings of NATO A cI vancrtl . Study Institute, I1 Ciocco, Italy, July, 1992, to be published. J. Sollfrank et al., Phys. I&t,. R 252( 1990)256. I<. Guettler et al., Phys. Lett. ~,~1~(197~)11 I. B. Alper et al,, Nucl. Phys. BlO~(l~~~~)2:~7. Y. Taka,hashi et al., P;ucl. Phys. .~~2~(19~~~).~9lc. M. Saralxira ct. al.. ?iirtl. Pltys. :\:i.l.:I( i!f!tL!)l:‘T,c. D. Rollrich et al., “IIntlron I’rotl~~ction in S+>\g. S+.Au C’ollisiom at, 2OOGeV per Nucleon Studirtl in .47r Acccpt,ance“, tlmr procrcdings. B. Jacak, Los Alamos Nat,ional I,almratory. private conlm~~tlicatio~r. H. Kalechofsky. PhD. Disscrtatiorl. I’. Pittshr~rgl~ B. Cole. Nrvis I,almratoric~s. (!olulnl)ia IT~li\x~rsit>.. pri\xtc conltlr~lnicatioll. Charles Parso~rs. 1’1111. Dissrrtat ioll, h~la~sac1lnsc~t.1~ Inst itut,e of T~hnolog):, June. 199”. M. Rosat,i. “Particle Procluctiotl in p+A (‘ollisions at, I-l.(i(:c\‘/c”, see these proceedings. t*hese proceedings.
33. J. McClelIancf et a.l.. Phys. Rev. Mt.. W( 1!@2)55”. 3.1. Geant User’s Guide, version 3.15, (‘ERN (‘ompntc~r (‘cntcr. 3.5. Il. Serge, et al., Nucl. Ph>x ,bI!)S( I!fS!)).%iTc. 136. V. I\;orh, SITX1. Sto~l!;Brool;. pri\:at~~ c.ol~lnrlttlicatiol1. 37. T-l. Sorgr, Phys. L&t. BXl( 1991 ):1T.