A sizable component of stopped baryons is predicted for pp and PbPb collisions at LHC. Based on an analysis of RHIC data within framework of our multi-chain Monte Carlo DPMJET-III the LHC predictions for pp and PbPb are presented.Predviđamo poveću komponentu zaustavljenih bariona u pp i PbPb u LHC mjerenjima. Predstavljamo LHC predviđanja za pp i PbPb sudare dana nasim više-niznim Monte Carlo DPMJET-III simulacijama zasnovanim na analizama RHIC podataka

Bopp, F.

Engel, R.

Ranft, J.

Roesler, S.

English

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Printed ISSN 1330–0016Online ISSN 1333–9133CD ISSN 1333–8390CODEN FIZBE7LETTERANTIBARION TO BARION PRODUCTION RATIOS IN Pb-Pb AND p-pCOLLISION AT LHC ENERGIES OF THE DPMJET-III MOMTE CARLOF. BOPPa, J. RANFTa, R. ENGELb and S. ROESLERCaSiegen University, Siegen, GermanybForschungszentrum Karlsruhe, Karlsruhe, GermanycCERN, Geneva, SwitzerlandReceived 23 October 2007; Accepted 20 February 2008Online 18 June 2008A sizable component of stopped baryons is predicted for pp and PbPb collisionsat LHC. Based on an analysis of RHIC data within framework of our multi-chainMonte Carlo DPMJET-III the LHC predictions for pp and PbPb are presented.PACS numbers: 13.85.-t, UDC 539.171Keywords: pp and PbPb collisions, LHC energy, stopped baryonsDPMJET is a Monte Carlo program for the study of scattering of hadrons ornuclei. It utilizes PHOJET for the scattering of individual hadrons and parts ofPYTHIA for the decay of partonic strings. The present version is DPMJET III.For the most recent general LHC prediction we refer to the Ranft’s CERN talk [1].Here the focus is on a particular aspect and the baryon stopping is addressed.There are different components to baryon stopping. We consider the componentwithout actual leading quarks. When the flavor decomposition is not dominated bythe final state interactions, this leading-quark-less component can be enhanced byconsidering strange baryon ratios.One motivation for looking for a baryon stopping component with a flattish ra-pidity distribution comes from the simple color evaporation concepts. If soft gluonscan arbitrarily arrange colors, a configuration can appear in which the baryoniccharge ends up moved to the center. The actual transport is just an effect of theorientation of the color-compensation during the soft hadronisation.There are many approaches to heavy-ion physics, and baryon transport canappear in many scenarios. It is generally agreed that there is an initial phase andFIZIKA B (Zagreb) 17 (2008) 1, 159–166 159Bopp et al.: antibaryon to baryon production ratios in pb-pb and . . .there are subsequent phases in an interaction. The initial phase contains hard par-tonic interactions. In the models like ours it also contains softer, sometimes multi-peripheral interactions leading to the production of color singlet strings. The sub-sequent phases always contain hadronisation of singlets formed in the initial phase.In many models, they also contain a statistical partonic and hadronic multiple-scattering phase. The concepts are not easy to distinguish as there is an intrinsicrelation between the multi-peripheral propagator and the statistical partition func-tion leading e.g. to a similar m⊥ =√p2⊥ + m2 dependence.These are controversial aspects which we want to avoid by considering baryons.Also, for the baryon stopping, alternative ideas were proposed, but most people willagree that fast baryon stopping, as part of the initial process, is a valid attractiveoption.Baryon stopping is not a new idea. The phenomenology was developed 30 yearsago in “Dual” models in a “Topological” framework [2]. Critical are various baryo-nium Regge interceptsα0Barionium, α1Barionium, or α2Barioniumof processes in which the exchanged baryonia, respectively, contain 0, 1 or 2 quarkpairs transporting 0, 1 or 2 valence quarks. The idea is that α2Barioniumis dominantin the leading region. As it has a low intercept, it will not reach very far and thenext baryonium will take over in a more central region and so on. Eventually aflattish α0Barioniumcontribution will survive. The intercepts were estimated usingthe energy dependence of annihilation cross sections and the inclusive baryoniccharge distributions. Some ambiguity remains for the long-range component anda confirmation of the flattish distribution indicated by HERA and RHIC data atLHC would be useful.Today such baryon processes are still of fundamental interest. Many people areconvinced that, under specified conditions, very high-energy hadronic scattering canbe understood with BFKL Pomeron exchanges described by ladders of dispersiongraphs. In these graphs, soft effects are thought to be contained in effective gluonexchanges calculated in a self-consistent way. In principle, these soft effects includethe color compensating mechanism usually modeled as two strings neutralizingtriplet colors.In string phenomenology, it is assumed that these predictions somehow ap-ply to minimum bias physics. The idea is that BFKL QCD results extrapolatesmoothly into the minimum bias region and offer in this way a stringent guidancefor modelling the non-perturbative region. Also, the relevant scale of minimum biasinteraction is not determined by decay momenta. For high energies, the minimumbias events involve string ends with respectable O(1− 2GeV) transverse momentaclosely neighboring the perturbative range. The separation of soft and hard pro-cesses is often exaggerated.However, there is a problem. Beside the Pomeron, this theory predicts a threegluon Odderon exchange [3]. It is a necessary ingredient of the approach. In com-160 FIZIKA B (Zagreb) 17 (2008) 1, 159–166Bopp et al.: antibaryon to baryon production ratios in pb-pb and . . .parison to the Pomeron, it has a similar but somewhat lower intercept. Parameter-izations of available cross sections require a much smaller coupling to nucleons.8x8−> +(8) (8)+ . . . .+ + . . . .D F−>8x8x8                                (0)                                       (0)exchangedobjects:gggg ggvvvvvvvortex−line        (3) − strings  Fig. 1 (left). Structure of Odderon. Fig. 2. Exchange contribution.We now consider the Odderon more closely to the leading 1/Nc order in Fig. 1.Two gluons can couple into an octet in an even or odd way (line 1). With a thirdgluon they can then couple to a C even or C odd singlet (line 2). The exchangedtopological object is a cylinder or a baryonium (line 3). In inelastic collision, thisobject can be cut in the way indicated by the dashed line. The central observationis that a vortex line can remain on each side. In inelastic exchanges, Odderons willtherefore contribute to the baryonic charge exchange with three strings as shownin Fig. 2.The first experimental indication for a flat component came from preliminaryH1 data at HERA [4]. As RHIC runs pp or heavy ions instead of pp̄, the centralasymmetry allows to address this question much better than before, and the dataseem to require a flat contribution as seen in Fig. 3.To understand the data, we turn to the Dual-Parton/Quark-Gluon-Stringmodel. In a factorizing inclusive approximation of the model (QGS-calculation)[5], the best fit to RHIC BRAHMS pp data at√s = 200 GeV [6] required di-quarks with a probability of ǫ = 0.024 to involve a quark-less baryonium-exchangewith an intercept α{SJ} = 0.9 (solid line). The dotted line corresponds to no suchcontribution. At LHC the rapidity extends two units more, allowing considerableclarification.In the full microscopic generator of the model, DPMJET III [7], there are severalcomponents affecting the position of the net baryon charge:FIZIKA B (Zagreb) 17 (2008) 1, 159–166 161Bopp et al.: antibaryon to baryon production ratios in pb-pb and . . .initial diquark structure functionduring the string decay: qq →meson+qqduring the string decay: qq →baryonium+qqstring fusion effectsThe baryonium (line 3 of the table) is just a wide two meson state like the f0(600)of Krupa [8]. The transport mechanisms from these contributions are not sufficient.Fig. 3 (left). RHIC BRAHMS pp data at√s = 200 GeV compared with the QGS-calculations (lines). The dotted line corresponds to no SJ .Fig. 4 The flipped configuration.To obtain a long-range baryon transport, we introduced a new string interaction,reshuffling the initial strings configuration in a certain way indicated in Fig. 4. Twocases are thereby considered:string rearrangements with Glauber sea quarksstring rearrangements with unitary sea quarksIt introduces an exchange with a conservative intercept of α{SJ} = 0.5.Again, a good fit is obtained for the BRAHMS data on the ratio p̄/p as functionof ycm (Fig. 5 left). However, as shown, at this energy for nucleons, the contributionactually mainly comes from the non-flipping effects. Fig. 5 (right) shows there isno visible p⊥ dependence in the considered soft range as found in BRAHMS andSTAR[9] collaborations data. The same applies to the centrality dependence.For strange baryons, Fig. 6 (left) shows the net Λ’s distribution compared withthe data from the STAR collaboration.The Ω-asymmetry measured by the E798162 FIZIKA B (Zagreb) 17 (2008) 1, 159–166Bopp et al.: antibaryon to baryon production ratios in pb-pb and . . .collaboration [19] in πp scattering is shown in Fig. 6 (right). The inclusion of thebaryonium production moves the result from the (blue) circles to the (blue) crosses 0 0.2 0.4 0.6 0.8 1 1.2 0  1  2  3  4  5 − p / p  yC.M.S.   p − p     200 GeV  BRAHMS (with syst. errors) DPMJET   DPMJET (nostop)   0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 0  0.5  1  1.5  2  2.5  3 − p / p  p⊥  [GeV]   d − Au    200 GeV PHOBOS       STAR          DPMJET    Fig. 5 . RHIC data compared with the DPMJET results. 0 0.05 0.1 0.15 0.2-6 -4 -2  0  2  4  6 dN( Λ - −− Λ ) / dyC.M.S.  yC.M.S.  d − Au   200 GeVDPMJET   STAR       -0.2 0 0.2 0.4 0.6 0.8 1-0.4 -0.2  0  0.2  0.4 A(Ω / −− Ω) xF  π−p 500 GeVmodified DPMJET original DPMJET  BRAHMS (with syst. errors)Fig. 6. Net strange baryon distribution.to reproduce the undashed data. Not shown is the Ξ asymmetry, which is alsoavailable [19]. It indicates a backward peak which is not reproduced.Now turning to LHC predictions, the DPMJET III prediction for the pseudorapidity of p, p̄, and p – p̄ is shown in Fig. 7. The new baryon stopping processis now also in pp scattering a 40% effect. Of course, with the effective interceptof 0.5, the present implementation of the baryon stopping is a rather conservativeestimate. For an intercept of 1.0, the value at η = 0 would roughly correspond tothe present value of η = 4.For heavy ion collisions, the baryon stopping gets stronger. The rapidity andpseudorapidity proton distributions in PbPb scattering are given in Fig. 8.FIZIKA B (Zagreb) 17 (2008) 1, 159–166 163Bopp et al.: antibaryon to baryon production ratios in pb-pb and . . .-0.05 0 0.05 0.1 0.15 0.2 0.25 0.3-15 -10 -5  0  5  10  15  ρ (η) p, − p, (p-− p) η C.M.S. p p  14TeVDPMJET p  −p   p - −p -0.1-0.05 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0  1  2  3  4  5  6  7  A (η) η C.M.S. pp     14 TeV DPMJET (p-−p) / (p+−p)Fig. 7. pp – LHC predictions for p and p̄. 0 5 10 15 20 25 30 35 40 0  2  4  6  8  10  12 ρ( η C.M.S. )  η C.M.S. PbPb   5.5TeV p  −p   p - −p  (p + −p) / 2 0 5 10 15 20 25 30 35 40 0  2  4  6  8  10 ρ( yC.M.S. )  yC.M.S. PbPb   5.5 TeVDPMJET  p  −p   p - −p  (p + −p) / 2Fig. 8. PbPb – LHC predictions for p and p̄. 0 20 40 60 80 100 0  2  4  6  8  10 ρ( η C.M.S. )  η C.M.S. PbPb   5.5 TeVDPMJET   p  −p   p - −p  (p + −p) / 2 0 20 40 60 80 100 0  2  4  6  8  10 ρ(η)  ηC.M.S. PbPb  5.5TeVDPMJET  Λ  −Λ   Λ - −Λ   (Λ + −Λ)/2 Fig. 9. PbPb – central LHC predictions for p and p̄, resp. Λ and Λ̄.164 FIZIKA B (Zagreb) 17 (2008) 1, 159–166Bopp et al.: antibaryon to baryon production ratios in pb-pb and . . .The DPMJET III prediction for the especially interesting most central 10% ofthe events for the pseudorapidity proton and Λ distributions are shown in Fig. 9.Of course these most central results are somewhat uncertain, as the model is so farnot well tested in this region.The asymmetry for the 10% most central events is plotted in Fig. 10. The linedrawn corresponds to an α0Barionium= 0.5 with an arbitrary normalisation. It is asafe, quite conservative estimate and it could be considerably flatter. 0 0.1 0.2 0.3 0.4 0.5 0  1  2  3  4  5  6  7 A( η C.M.S. ) η C.M.S. PbPb   5.5 TeV  DPMJET    p / −p   Λ / −Λ 0.021 cosh(x/2)Fig. 10. PbPb – central LHC predictions for p, resp. Λ asymmetry.To conclude there is a strong evidence for the baryon stopping component. Itmoderately depends on the density. There is still some uncertainty how flat theintercept has to be.References[1] F. W. Bopp, R. Engel, J. Ranft and S. Roesler, “Inclusive distributions at the LHC aspredicted from the DPMJET-III model with chain fusion,” arXiv:0706.3875 [hep-ph].[2] G. Veneziano, Nucl. Phys. B 117 (1976) 519; G.C. Rossi and G. Veneziano, Nucl.Phys. B 123 (1977) 507; Phys. Rept. 63 (1980) 153; P. Aurenche and F. W. Bopp,Nucl. Phys. B 119 (1977) 157.[3] J. Bartels, L. N. Lipatov and G. P. Vacca, “A new odderon solution in perturbativeQCD,” Phys. Lett. B 477 (2000) 178; [hep-ph/9912423].[4] C. Adloff et al. [H1 Collaboration], Measurement of dijet cross-sections at low Q**2and the extraction of an effective parton density for the virtual photon; hep-ex/9812024.[5] F. Bopp and M. Shabelski, Eur. Phys. J. A 28 (2006) 237; [arXiv:hep-ph/0603193].[6] I. G. Bearden et al., BRAHMS Collaboration, Phys. Rev. Lett. 94 (2005) 032301;[nucl-ex/0401025].[7] S. Roesler, R. Engel and J. Ranft, hep–ph/0012252; Proc. Monte Carlo 2000, Lisboa,Oct. 2000, Springer, p.1033.[8] D. Krupa, Fizika B (Zagreb) 17 (2008) 319.FIZIKA B (Zagreb) 17 (2008) 1, 159–166 165Bopp et al.: antibaryon to baryon production ratios in pb-pb and . . .[9] J. Adams et al., STAR Collaboration, Phys. Lett. B 637 (2006) 161; [nucl-ex/0601033];F. Simon, STAR Colaboration, Acta Phys. Hung. A 27 (2006) 287; [nucl-ex/0412031].[19] E. M. Aitala et al., E791 Collaboration, Phys. Lett. B 469 (2000) 9; [hep-ex/0009016].[11] F. W. Bopp, J. Ranft, R. Engel and S. Roesler, Antiparticle to particle production ratiosin d Au and p p collisions in the DPMJET-III Monte Carlo, arXiv:hep-ph/0505035v2;F. W. Bopp, R. Engel, J. Ranft and S. Roesler, Antibaryon to Baryon ProductionRatios in Pb-Pb and p-p collision at LHC energies of the DPMJET-III Monte Carlo,arXiv:0706.3870 [hep-ph].OMJER TVORBE ANTIBARIONA I BARIONA U SUDARIMA Pb-Pb I p-pNA ENERGIJAMA LHC PROGRAMOM DPMJET-III MOMTE CARLOPredvid–amo poveću komponentu zaustavljenih bariona u pp i PbPb u LHC mjeren-jima. Predstavljamo LHC predvid–anja za pp i PbPb sudare dana nasim vǐse-niznimMonte Carlo DPMJET-III simulacijama zasnovanim na analizama RHIC podataka.166 FIZIKA B (Zagreb) 17 (2008) 1, 159–166

Omjer tvorbe antibariona i bariona u sudarima Pb-Pb i p-p na energijama LHC programom DPMJET-III Monte Carlo

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