Cuautle, E

Paic, G

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PPEUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH European Laboratory for Particle Physics     Internal Note/ ALICE reference number ALICE-INT-2005-027 version 1.0Institute reference number [-]Date of last change  Instituto de Ciencias Nucleares UNAM, A.P. 70-543, México D.F. 04510, México 31.10.05   Study of the pion and proton production in pp collisions at 14 TeV    Authors:  E. Cuautle*, G. Paic† Abstract: The hadronization into baryons is still a theoretically ill defined process. The event generator Pythia 6.3 includes several mechanisms for the production of baryons in its code. We have simulated events at the nominal pp collision energy of LHC 14 TeV including in the simulations three mechanisms, Simple Popcorn, Advanced popcorn, and Baryon Junction. The results are discussed in view of their sensitivity to experimentally accessible observables namely multiplicity distribution and ratios of p /p and p/π.   1   Introduction In heavy ion collisions at RHIC two phenomena remain up to now without a valid explanation: one is the p/π ratios which exhibits a departure from the same ratio, measured in pp collisions at transverse momenta between 2.0 and the maximum reach of identification at RHIC ~4.5 GeV/c[1], the second deals with the ratio p /p which has been measured to be virtually independent of the pt of the particles [2], and is smaller than expected if classical mechanisms of baryon stopping only are invoked. Hence it is legitimate to say that, as of today, all the details of hadronization into baryons are not well understood neither in nucleon nor in nucleus nucleus collisions. Therefore it is of the highest interest to use the possibilities of the ALICE tracking and PID detectors in the central rapidity to study in details the proton production and compare with the existing models. We have mapped the results we might expect in pp collisions at 14 TeV using as a guideline the different baryon production mechanisms incorporated in the latest version of Pythia [3].   ∗ ecuautle@nucleares.unam.mx †guypaic@nucleares.unam.mx 2 Pion and proton productionProduction of mesons and baryons depends on the fragmentation processes. Whilethe meson production is represented as a simple piece of the string between two q− q¯endpoints the extension to the case of baryons is not straightforward. Actually thereare diﬀerent scenarios like diquark, Simple Popcorn, Advanced Popcorn and BaryonJunction mechanism that we will shortly review. A general description of these modelscan be found in the Pythia Manual [3] and references there in.2.1 Diquark pictureIn most string models particle production in a pp collision occurs mainly in the formof two diquark-quark strings. There is strong experimental evidence that with a largeprobability the diquark acts as a single entity and fragments directly into a leadingbaryon. Since the diquark is fast in average, taking a large fraction of the incomingproton momentum, the produced baryon will be in the proton fragmentation region.The corresponding baryon spectrum at rapidity y ∼ 0 decreases, quite fast withenergy - roughly as s−1 [4].In Pythia, the probability to produce various diquarks pairs is parametrized with:the probability to pick a q¯q diquark rather than a q, a suppression factor associ-ated with diquarks containing a strange quark, the suppression of spin 1 relative tospin 0. One important constraint is the requirement of a symmetric state of threequarks. When a diquark and quark are joined to form a baryon, the combination istherefore weighted with the probability that they form a symmetric three quark state.Figure 1: The system conﬁgurations of Simple (left) and Advanced (right) Popcorn.The left show the BMB¯ system. The right show an example of two pairs of quarksproduced to form a B¯MMB system.22.2 Simple PopcornIn this picture diquarks as such are never produced, but rather baryons appear fromthe successive production of several qiq¯i pairs. It assumes that the original q is red rand the q¯ is r¯. The produced pairs (q1q¯1), are such that q¯1 is pulled towards q and viceversa, and two separate colour-singlet systems qq¯1 and q1q¯ are formed. Usually thepairs are such that there is net colour charge acting on either q1 or q¯1. The sequenceof production is the same for the other pair q2q¯2, where q2 now is pulled towardsqq1 and q¯2 towards ¯qq1 with not net colour ﬁeld between q2 and q¯2. When all thesehappens, the baryon B will be made up out of q1, q2 and some q4 produced between qand q1, and B¯ of q¯1, q¯2 and some q¯5. Hence, the B and B¯ will be neighbours in rankand will share two quarks pairs. With two production vertex q2q¯2 and q3q¯3, a centralmeson q¯2q3 may be formed. The conﬁguration is then BMB¯ (see Fig 1 (left)).The relative probability for a BMB¯ systems given by the uncertainty relation suppres-sion of having a q1 and q2 suﬃciently far apart so that a meson may be formed inbe-tween. The suppression of the BMB¯ system is estimated by | ΔF |2≈ exp(−2μ⊥M⊥/κ),where μ⊥ and M⊥ are the transverse masses of q1 and the meson, respectively. Themodel depends on some parameters, like quark and diquarks masses. The default pa-rameter values are based on a combination of experimental observation and internalmodel predictions. A complete description of the model is given by Anderson [5].2.3 Advanced PopcornA new version of the popcorn is given in [6]. This new version takes better accountof the detailed kinematics of the process. For instance, it has implemented twoparameters which govern both the diquarks and the intermediate meson production.Additionally conﬁgurations like BMMB¯ are considered in a natural way (see Fig. 1(right)).3 String FragmentationIn the string picture, the various points where the string breaks by the productionof qq¯ pairs are causally disconnected. One may therefore make the convenient choiceof starting an iteration process at the ends of string and proceeding towards themiddle. The string fragmentation is rather complicated for a generic multipartonstate, however, we only describe brieﬂy the main ideas in order to get the baryonjunction model incorporated in the production of hadrons.3.1 JunctionWhen a quark or several quarks are kicked out of an incoming proton, diﬀerent kindsof string topology can be produced. The main is Y shape string, with a quark at eachend and a junction where the strings meet. As quarks are pulled out, the junction willalso move, so as to minimize the total string energy. In the rest frame the opening3angle between any pairs of quarks is 1200, so that the forces acting on the junctioncancel.Each of the three strings fragment as ordinary strings, in back to back qq¯ pairs ofjets. And iterative procedure can be used by which the q is combined with a newlyproduced q1, to form a meson and leave behind a jet q1. Eventually, when the energyis not anymore suﬃcient to produce pairs, the three remaining quarks qiqjqk form asingle baryon, which thus has a reasonably small momentum in the rest frame of thejunction. In this sense, the junction is the carrier of the net baryon number of thesystem. This process results in a usual slower decrease of the baryon contributionsat y ∼ 0 and the energy (∼ s1/4) [4]. The algorithm to implement this in a MonteCarlo is not easy. An approximation consists in taking two of the strings, preferablythe ones with the lowest energy, and fragment them until their remaining energy isbelow some cut oﬀ value. The two remaining ﬂavours are combined into one eﬀectivediquark which is assigned all the remaining energy and momentum. The resultingstring piece, between this diquarks and the third diquark, can now be considered asdescribed for simple qq¯ string.3.2 Underlying events and baryon junctionPythia 6.3 has incorporated a new mechanism for baryon production. Baryon junc-tion [7, 4] together with underlying events (UE) gives interesting results. The majorcharacteristics are as follow: We have multiple interaction [8] between two or morepartons instead of one as before. The basic scheme is based on 2 → 2 QCD matrixelements convoluted with standard parton densities, and a sequence of p⊥ orderedinteractions are generated. The sequence is stopped at some lower cut oﬀ scale p⊥min .An impact parameter dependence of the probability to form a junction is also intro-duced, considering a hadronic matter distribution for the hadron-hadron interactions.The hadronic matter distribution is parameterized in diﬀerent ways: Gaussian, dou-ble Gaussian or exp(−bd), where b is the impact parameter. These distributions aﬀectdirectly the multiplicity distributions. The ﬂavour content of the beam remnant hasbeen taken into account, and as a consequence, the parton distribution function suf-fer some modiﬁcations. All of these elements were considered to simulate events inPythia 6.3.4 Proton production in pp collisions at 14 TeVWe present simulations with the above mentioned mechanisms in order to investigatetheir inﬂuences in some experimental observables. The simulation at this time do nottake into account the feed down of weaks decays on protons and pions. The collisionenergies were 200 GeV to compare to experimental results and 14 TeV to LHC. Foreach case 106 events were generated.411010 210 310 40 1 2 3 4 5 6 7 8 9 10pt(GeV/c)dN/dp t/(100 MeV)0.70.80.911.11.21.31.40 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5pt (GeV/c)p- /pFigure 2: Pt spectrum (a) and ratio for anti proton to proton (b) for three diﬀerentmodels. Advanced Popcorn, Simple Popcorn and Baryon Junction.4.1 p¯/p ratioFig. 2 shows the pt and the ratio antiproton to proton from Popcorn Advanced com-pared to Simple Popcorn and Baryon Junction with new Multiple Interaction models.Qualitatively we observe from Fig. 2a that Simple Popcorn produces more protonscompared to the Advanced popcorn, in all the pt range. This behaviour comes fromthe model, because Advanced Popcorn has higher probability to produce B¯MMBsystems, consequently there are more mesons and less baryons compared to SimplePopcorn. The spectrum at low pt from Baryon Junction is higher than for the otherat low pt. This behaviour is because the mechanism itself favour the production of lowmomentum protons. Although the Baryon Junction does produce distinct pt spectrawith a speciﬁc behavior at high pt, the p¯/p is the same for three models, see Fig. 2b.Recent experimental data of p¯/p at 200 GeV [2] show a completely diﬀerent behaviourcompared to Baryon Junction at two energies, as we can see in Fig. 3.50.20.30.40.50.60.70.80.911.11.20 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5pt (GeV/c)p- /pFigure 3: Antiproton to proton ratio versus pt for one production mechanism: BaryonJunction. The solid line is the central value and dotted are errors bar from BRAHMSat 200 GeV4.2 p/π ratioThe production rates of baryons and mesons are best exempliﬁed with the ratios ofproton versus pions. Fig. 4 shows the ratios obtained with the three models. Indeedin this ratio there is important diﬀerences observed both in the whole pseudorapidityand in the central rapidity. Since the multiplicity distributions are diﬀerent dependingon the mechanism see Fig. 5, we have also extracted the predictions for diﬀerentmultiplicity bins. The results are shown in Fig. 6a and Fig. 6b for the multipleinteraction plus baryon junction and for the Simple Popcorn mechanism. From thesimulations it is impossible to determine a clear trend with multiplicity althoughsigniﬁcant diﬀerences are observed.The case of the proton to kaon production ratio in function of pt is qualitativelysimilar to the case of the p/π ratio.5 ConclusionThe particle identiﬁcation of ALICE in pp collisions will allow interesting insight onthe mechanism of baryon production. We have demonstrated here that the p/π ratioin the existing Pythia mechanisms allow for a large spread. Although the p¯/p ratiosare very stable and do not depend on the mechanism (at 14 TeV) it is useful toremind that Pythia does not reproduce the existing data measured in pp collisionsat RHIC [2] as is showed in Fig. 3. We see that, while Pythia at 200 GeV clearly6indicate a slope stemming from the increasing contribution of valence quarks withpt, the experimental data are ﬂat and are much below the predictions. The Pythia00.10.20.30.40.50.60.70.80.910 1 2 3 4 5 6 7pt (GeV/c) p/π+00.10.20.30.40.50.60.70.80.910 1 2 3 4 5 6 7pt (GeV/c) p/π+Figure 4: Proton to pion ratio as function of pt, comparing the Advanced, SimplePopcorn and Baryon Junction mechanism of production.10-410-310-20 100 200 300 400 500 600 700 multiplicityP(n ch)Figure 5: Charge multiplicity distributions for three diﬀerent baryon productionmechanisms700.10.20.30.40.50.60 1 2 3 4 5 6pt (GeV/c)p/π+00.10.20.30.40.50.60 1 2 3 4 5 6pt (GeV/c)p/π+Figure 6: Proton to pion ratio versus multiplicity, m, for popcorn (a) and baryonjunction (b) production mechanism8predictions for other models give essentially the same result. Therefore it is notexcluded that the p¯/p ratio at LHC be also in contradiction to the presently availablemechanisms.6 AcknowledgmentsWe acknowledge the help of T. Sjo¨strand and P. Skands in setting the Pythia mech-anismsReferences[1] K. Adcox et al., [PHENIX Collaboration], Phys. Rev. C69 (2004) 024904.[2] I.G. Bearden, et. al., (BRAHMS Collaboration), Phys. Lett. B607 (2005) 42.[3] T. Sjo¨strand et. al., Pythia 6.3 Physics and Manual, (hep-ph/0308153).[4] A. Capella and B. Z. Kopeliovich, Phys. Lett. B381 (1996) 325.[5] B. Andersson, G. Gustafson and T. Sjo¨strand, Physica Scripta 32 (1985) 574.[6] P. Ede´n and G. Gustafson, Z. Phys. C75 (1997) 41;P. Ede´n, LUTP 96–29 (hep-ph/9610246).[7] T. Sjo¨strand and P.Z. Skands Nucl. Phys. B659 (2003)243.[8] T. Sjo¨strand and P.Z. Skands JHEP 0403 (2004) 053.9

Study of the pion and proton production in pp collisions at 14 TeV

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