Influence of quantum conservation laws on particle production in hadron collisions
Małgorzata Anna Janik, Łukasz Kamil Graczykowski, Adam Kisiel
aa r X i v : . [ h e p - ph ] J a n Influence of quantum conservation laws on particle production in hadroncollisions
Małgorzata Anna Janik a , Łukasz Kamil Graczykowski a , Adam Kisiel a , a Warsaw University of Technology, Faculty of Physics, Koszykowa 75, PL-00-662 Warszawa, Poland
Abstract
Conservation laws strongly influence production of particles in high-energy particle collisions. E ff ects connectedto these mechanisms were studied in details using correlation techniques in e + e − collisions. At the time, modelswere tuned to correctly reproduce the measurements. Similar studies for hadron-hadron collisions have never beenperformed, until recent ALICE measurements. ALICE has reported on studies of untriggered two-particle angularcorrelations of identified particles ( π , K , and p) measured in pp collisions at center-of-mass energy of √ s = ff erent particle types and must be taken into accountwhile analysing the data. Moreover, they show that the contemporary models (PYTHIA, PHOJET) no longer repro-duce the experimental data well. Keywords: proton-proton collisions, ALICE experiment, LHC, angular correlations, conservation laws, baryoncorrelations
1. Introduction
One of the most important ingredients of the theoretical description of relativistic particle collisions is the particleproduction mechanism. Currently, there are few models describing such processes (e.g. string fragmentation model[1], also referred to as the ”Lund model”, incorporated in PYTHIA Monte Carlo generator [2]). However, the imple-mentation details of such theoretical calculations depend strongly on the experimental data which are used to constraintheir input parameters, especially in the non-perturbative regime at low transverse momenta of the produced particles.For example, some of the parameters in the current version of PYTHIA, particularly those describing production ofbaryons, have not been improved for many years (since e + e − collisions at much lower energies). This fact is reflectedin the comparison of the model predictions with the yet preliminary experimental results of angular correlations in ∆ η ∆ ϕ space (relative pseudorapidity and azimuthal angle) with the results from model predictions. Current modelsexplain correlations of mesons only qualitatively, but in the case of baryons they fail to describe experimental databoth qualitatively and quantitatively; anti-correlation is observed in experimental results in contrast to a correlationobserved in models. This newly discovered scientific problem has fundamental impact for the theoretical descriptionof particle production in high-energy physics.
2. Two-particle rapidity correlations in e + e − collisions Studies of the particle production mechanism in elementary collisions date back to the times of R. Feynman andR. Field, who proposed a simple mechanism describing the principles of the creation of the so-called jets (collimatedstreams of particles) in 1977 [3, 4]. They proposed rules on how the particles are produced, how the energy isdistributed and considered limitations connected with the conservation laws.Elements of the proposed scheme are used even today in the most popular fragmentation models. However, theimplementation details have to be connected with the experimental data. It is then a task for the experiment to providebasic information: how strong the correlations between the created hadrons should be? How does this correlationchange, when we create two or more baryons or strange particles in a single parton fragmentation? Answers to these
Preprint submitted to Elsevier September 5, 2018 nd other questions have been searched, so far, only in e + e − collisions, of much lower energies and on substantiallysmaller data samples [5–8].In Ref. [5], two-particle rapidity correlations are reported for e + e − collisions at 29 GeV. It was observed that pairsof particles with opposite baryon number (p-¯p, p- ¯ Λ , Λ - ¯ Λ ) produce significant correlation while pairs of particles withthe same baryon number (¯p-¯p, ¯p- ¯ Λ , ¯ Λ - ¯ Λ ) produce significant anti-correlations. Moreover, in the same article it wasreported that mechanisms ensuring that the baryon number is conserved not only globally, for the whole event, butalso locally for each parton fragmentation are crucial for reproducing the experimentally observed anti-correlation.Calculations employing the Lund model incorporating local compensation of baryon number described e + e − data(both correlation and anti-correlation) very well.
3. Two-particle ∆ η ∆ ϕ correlations in pp collisions ALICE has reported the results of the angular correlation measurements from pp collisions in Ref. [9–11]. Inthis article we would like to focus on the analysis of identified particles ( π , K , p); see ALICE preliminary resultsfrom Ref. [12]. These results show that the magnitude of the near-side peak (positive correlation structure centeredat ( ∆ η, ∆ ϕ ) = (0 , ff erent particle species. The observed qualitative di ff erences can be explainedwith conservation laws applied locally for each single fragmentation: energy, momentum, charge, strangeness, andbaryon number.Most importantly, it should be noted that ALICE results also show an anti-correlation for like-sign protons, sim-ilarly as in e + e − collisions. This finding is surprising, since at LHC energies we expect jet production to stronglysuppress anti-correlation e ff ects (production of correlated protons should be much more abundant). Such expectationswere also supported by the models (see Fig. 1 for three examples): it can be seen that in all theoretical simulationssignificant correlation, the near-side peak, is seen for like-sign protons. Similar discrepancy can be seen for corre-lation functions of unlike-sign protons; the magnitude of the near-side peak in theoretical models is nearly twice aslarge as in experimental data. This comparison suggests that in models significantly more correlated protons are beingproduced than in the experiment.In summary, in contrast to e + e − collisions at √ s =
29 GeV, at LHC energies the two-particle correlations mea-sured by ALICE in pp collisions are not described by the Monte Carlo models. Moreover, these results suggest thatproduction of baryon-baryon (or antibaryon-antibaryon) pairs in a single jet / minijet is suppressed.
4. ConservAtion Laws Model
As a basic step towards understanding the role of conservation laws and their influence on the analyzed observ-ables, we studied events in which energy and momentum are conserved and no other physics mechanisms are involved.For such a case a simple Monte Carlo model was developed in order to explore the impact of the conservation lawson the correlation functions – CALM (ConservAtion Laws Model). CALM allows us to generate events in whichonly energy, momentum and quantum numbers local to the emission from boosted clusters are conserved and no otherprocesses are involved . The model also reproduces the usual jet / minijet correlation shape with the near-side peak andthe away-side ridge. In Fig. 3 the correlation function presenting neutral pions distributed isotropically in the wholephase-space, with momentum conservation as the only constraint, is shown. Such a simple description reproducesqualitatively structures observed for like-sign protons in ALICE data presented in Ref. [12].
5. Summary
We expect that as a result of the analysis of two-particle angular correlations in ALICE novel information onparticle production, both mesons ( π , K ) and baryons (p), can be obtained. Preliminary results of the experimental dataanalysis (in pp collisions) exhibit significant qualitative di ff erences with respect to theoretical predictions calculatedfrom models – they are especially visible for baryons. We are able to reproduce the interesting structure observed inALICE with a new model, CALM. Further analysis of the results may be of a key significance for the development ofthe theoretical description of particle collisions and may trigger a need for major modifications of particle productionmodels. N-body Monte Carlo event generators are used for computing the phase space distribution of particles produced in the collision [13, 14] ∆ η ∆ − ) η ∆ , ϕ ∆ ( C (a) proton like-sign pairs ϕ ∆ η ∆ − ) η ∆ , ϕ ∆ ( C = 7 TeV s PYTHIA 6.4 Perugia-2011, pp (b) kaon like-sign pairs ϕ ∆ η ∆ − ) η ∆ , ϕ ∆ ( C (c) pion like-sign pairs ϕ ∆ η ∆ − ) η ∆ , ϕ ∆ ( C (a) proton like-sign pairs ϕ ∆ η ∆ − ) η ∆ , ϕ ∆ ( C = 7 TeV s PYTHIA 8.210 Monash tune, pp (b) kaon like-sign pairs ϕ ∆ η ∆ − ) η ∆ , ϕ ∆ ( C (c) pion like-sign pairs ϕ ∆ η ∆ − ) η ∆ , ϕ ∆ ( C (a) proton like-sign pairs ϕ ∆ η ∆ − ) η ∆ , ϕ ∆ ( C = 7 TeV s PHOJET 1.12, pp (b) kaon like-sign pairs ϕ ∆ η ∆ − ) η ∆ , ϕ ∆ ( C (c) pion like-sign pairs Figure 1: Theoretical correlation functions for like-sign pairs of (a) protons, (b) kaons, (c) pions for pp collisions at 7 TeV from top: PYTHIAPerugia-2011, middle: PYTHIA 8 Monash tune, bottom: PHOJET 1.12 generators; they all show the near-side peak for like-sign protons, incontrast to the data, which shown an anti-correlation; see Ref. [12].
Acknowledgements
This research has been financed by the Polish National Science Centre in Poland, based on the decisions no.DEC-2014 / / B / ST2 / / / M / ST2 / References [1] B. Andersson, The Lund model, Camb.Monogr.Part.Phys.Nucl.Phys.Cosmol. 7 (1997) 1–471.[2] T. Sjstrand, S. Mrenna, P. Z. Skands, PYTHIA 6.4 Physics and Manual, JHEP 0605 (2006) 026. arXiv:hep-ph/0603175 .[3] R. Field, R. Feynman, A Parametrization of the Properties of Quark Jets, Nucl.Phys. B136 (1978) 1.[4] R. Field, Quark elastic scattering as a source of high transverse momentum mesons, Int. J. Mod. Phys. A30 (01) (2015) 1530005.[5] H. Aihara, et al., Study of baryon correlations in e + e − annihilation at 29 GeV, Phys.Rev.Lett. 57 (1986) 3140.[6] M. Altho ff , et al., Evidence for Local Compensation of Baryon Number in e + e − Annihilation, Phys.Lett. B139 (1984) 126.[7] P. Abreu, et al., Rapidity correlations in Lambda baryon and proton production in hadronic Z0 decays, Phys.Lett. B416 (1998) 247–256. doi:10.1016/S0370-2693(97)01380-4 . ∆ η ∆ − ) η ∆ , ϕ ∆ ( C (a) proton unlike-sign pairs ϕ ∆ η ∆ − ) η ∆ , ϕ ∆ ( C = 7 TeV s PYTHIA 8.210 Monash tune, pp (b) kaon unlike-sign pairs ϕ ∆ η ∆ − ) η ∆ , ϕ ∆ ( C (c) pion unlike-sign pairs Figure 2: Theoretical correlation functions for unlike-sign pairs of (a) protons, (b) kaons, (c) pions for pp collisions at 7 TeV from PYTHIA 8Monash tune. Models suggest that significantly more correlated protons are being produced than it is suggested by data; compare with Ref. [12]. ϕ ∆ η ∆ − ) ϕ ∆ , η ∆ C ( Figure 3: Correlation function of charged particles obtained from CALM model including only conservation laws.[8] P. Acton, et al., Evidence for chain-like production of strange baryon pairs in jets, Phys.Lett. B305 (1993) 415–427.[9] Ł. K. Graczykowski, M. A. Janik, Angular correlations measured in pp collisions by ALICE at the LHC, Nucl.Phys. A926 (2014) 205–212. arXiv:1401.4306 , doi:10.1016/j.nuclphysa.2014.03.004 .[10] M. A. Janik, Two-particle angular correlations in pp collisions recorded with the ALICE detector at the LHC, EPJ Web Conf. 71 (2014)00058. arXiv:1402.3988 .[11] M. A. Janik, ∆ η ∆ φ angular correlations in pp collisions at the LHC registered by the ALICE experiment, PoS WPCF2011 (2011) 026. arXiv:1203.2844 .[12] M. Janik, L. Graczykowski, A. Kisiel, Influence of quantum conservation laws on particle production in hadron collisions (2015). QM2015,Kobe,slides .URL https://indico.cern.ch/event/355454/session/59/contribution/753 [13] F. E. James, Monte Carlo phase space, CERN, Geneva, 1968, p. 41.[14] M. Meres, I. Melo, B. Tomasik, V. Balek, V. Cerny, Generating heavy particles with energy and momentum conservation, Comput. Phys.Commun. 182 (2011) 2561–2566. arXiv:1101.3339 , doi:10.1016/j.cpc.2011.06.015 ..