Charged particle production in proton-, deuteron-, oxygen- and sulphur-nucleus collisions at 200 GeV per nucleon

The transverse momentum and rapidity distributions of net protons and negatively charged hadrons have been measured for minimum bias proton–nucleus and deuteron–gold interactions, as well as central oxygen–gold and sulphur–nucleus collisions at 200 GeV per nucleon. The rapidity density of net protons at midrapidity in central nucleus–nucleus collisions increases both with target mass for sulphur projectiles and with the projectile mass for a gold target. The shape of the rapidity distributions of net protons forward of midrapidity for d+Au and central S+Au collisions is similar. The average rapidity loss is larger than 2 units of rapidity for reactions with the gold target. The transverse momentum spectra of net protons for all reactions can be described by a thermal distribution with ‘temperatures’ between MeV (p+S interactions) and MeV (central S+Au collisions). The multiplicity of negatively charged hadrons increases with the mass of the colliding system. The shape of the transverse momentum spectra of negatively charged hadrons changes from minimum bias p+p and p+S interactions to p+Au and central nucleus-nucleus collisions. The mean transverse momentum is almost constant in the vicinity of midrapidity and shows little variation with the target and projectile masses. The average number of produced negatively charged hadrons per participant baryon increases slightly from p+p, p+A to central S+S,Ag collisions. Received: 28 October 1997 / Published online: 10 March 1998     

Confronting models of cosmic ray interactions with particle physics at LHC energies

Inelastic pp collisions are dominated by soft (low momentum transfer) physics, to which perturbative QCD cannot be fully applied. A deep understanding of both soft and semi-hard processes is crucial for predictions of minimum bias and underlying events of the pp large hadron collider (LHC) now coming on line. Moreover, the interaction of cosmic ray particles entering in the atmosphere is extremely sensitive to these soft processes and consequently cannot be formulated from first principles. Because of this, air shower analyses strongly rely on hadronic interaction models, which extrapolate collider data by several orders of magnitude. A comparative study of Monte Carlo simulations of pp collisions (at the LHC center-of-mass energy ≃14 TeV) using the most popular hadronic interaction models for ultrahigh energy cosmic ray (SIBYLL and QGSJET) and for collider physics (the PYTHIA multiparton model) is presented. The most relevant distributions are studied including the observables from diffractive events with the aim of discriminating between the different models. PACS 13.85.-t; 96.40.-z     

Study on‘soft＇ and ‘hard＇ interactions in ^-pp （pp） collisions using HIJING and PYTHIA

This paper presents a study on ＇hard＇ and ＇soft＇ interactions in ^-pp （pp） collisions using a phenomenological model of HIJING, the jet-cone reconstruction method is employed to select the ＇hard＇ and ＇soft＇ event sub-samples from minimum bias events. It is found that the HIJING model can reproduce the energy scaling behaviour of mean transverse momentum （〈PT〉） distributions of charged hadrons versus multiplicity （Nch） in ＇soft＇ events. From the PYTHIA simulation comparing with the HIJING model, the enhancement of the kaon and proton yields from ＇hard＇ interactions comparing with ＇soft＇ interactions is observed to be due to the mini-jets effect. These mechanisms responsible for the increase of charged hadron＇s （PT） are different in ＇soft＇ and ＇hard＇ interactions.     

Limiting fragmentation in hadron–hadron collisions at high energies

Limiting fragmentation in proton–proton, deuteron–nucleus and nucleus–nucleus collisions is analyzed in the framework of the Balitsky–Kovchegov equation in high energy QCD. Good agreement with experimental data is obtained for a wide range of energies. Further detailed tests of limiting fragmentation at RHIC and the LHC will provide insight into the evolution equations for high energy QCD.     

Drell–Yan diffraction: breakdown of QCD factorization

We consider the diffractive Drell–Yan process in proton–(anti)proton collisions at high energies in the color dipole approach. The calculations are performed at forward rapidities of the leptonic pair. The effect of eikonalization of the universal “bare” dipole–target elastic amplitude in the saturation regime takes into account the principal part of the gap survival probability. We present predictions for the total and differential cross sections of the single-diffractive lepton-pair production at RHIC and LHC energies. We analyze implications of the QCD factorization breakdown in the diffractive Drell–Yan process, which is caused by a specific interplay of the soft and hard interactions, resulting in rather unusual properties of the corresponding observables.     

Relating multihadron production in hadronic and nuclear collisions

The energy-dependence of charged particle mean multiplicity and pseudorapidity density at midrapidity measured in nucleus–nucleus and (anti)proton–proton collisions are studied in the entire available energy range. The study is performed using a model, which considers the multiparticle production process according to the dissipating energy of the participants and their types, namely a combination of the constituent quark picture together with Landau relativistic hydrodynamics. The model reveals interrelations between the variables under study measured in nucleus–nucleus and nucleon–nucleon collisions. Measurements in nuclear reactions are shown to be well reproduced by the measurements in pp/[`(p)]p{\bar{\mathrm{p}}}{\mathrm{p}} interactions and the corresponding fits are presented. Different observations in other types of collisions are discussed in the framework of the proposed model. Predictions are made for measurements at the forthcoming LHC energies.     

Soft diffraction at the LHC: a partonic interpretation

We present a ‘new generation’ model for high energy proton–proton ‘soft’ interactions. It allows for a full set of multipomeron vertices as well as for including multichannel eikonal scattering. It describes the behaviour of the proton–proton total, σ_tot, and elastic, dσ_el/dt, cross sections together with those for low- and high-mass proton dissociation. Although the model contains a comprehensive set of multipomeron diagrams, it has a simple partonic interpretation. Including the more complicated multipomeron vertices reduces the absorptive effects as compared to the predictions in which only the triple-pomeron vertex is considered. Tuning the model to describe the available ‘soft’ data in the CERN ISR–tevatron energy range, we predict the total, elastic, single- and double-diffractive dissociation cross sections at the LHC energy. An inescapable consequence of including multichannel eikonal and multipomeron effects is that the total cross section is expected to be lower than before: indeed, we find σ_tot≃ 90 mb at the LHC energy. We also present differential forms of the cross sections. In addition, we calculate soft diffractive central production.     

On lepton pair production in proton-antiproton collisions at intermediate energies

The lepton pair production via the quark-antiquark annihilation subprocess in collisions of beam antiproton with the proton target at E _beam = 14 GeV is studied on the basis of the event sample simulated by PYTHIA6 generator. Different kinematical variables which may be useful for the design of the muon system and the electromagnetic calorimeter of the detector of PANDA experiment at FAIR, as well as for the study of proton structure functions in the available x-Q ² kinematical region, are considered. It is also argued that the measurement of the total transverse momentum of a lepton-antilepton system may provide important information about the intrinsic transverse momentum 〈k _T 〉 that appears due to the Fermi motion of quarks inside the nucleon. Another interesting possibility is the measurement of the production rate of two or three lepton pairs in one event that can give the information about the rate of multiple quarks interactions and the proton space structure. The problems due to the presence of fake leptons that appear from meson decays, as well as due to the contribution of background QCD processes and minimum bias events, are also discussed. The set of cuts which allows one to separate the events with the signal lepton pairs from different kind of background events is proposed. The article is published in the original.     

φ meson production in NA60

NA60 is a fixed-target experiment at the CERN SPS which measured dimuon production in nucleus–nucleus and proton–nucleus collisions. The experiment collected muon pair samples of unprecedented quality in heavy-ion experiments. This paper presents a high quality measurement of the p_T distribution of the φ meson, covering a broad p_T window. The data were collected in 2003 in In-In collisions at 158 GeV per nucleon. The results, presented as a function of centrality, were studied against several possible sources of systematic effects and proved to be fairly stable. We show that the inverse m_T slope measured in In-In collisions, in the φ→μμ decay channel, depends significantly on the range used to perform the fit. When the fit is performed at low transverse momentum, the effective inverse slope increases from peripheral to central collisions, as measured by other experiments. We finally show that our measurement for In-In is compatible with the overall systematics of T slope versus mass, measured in different collision systems by the NA49 experiment PACS 25.75.Nq; 25.75.-q; 25.75.Dw; 14.40.Cs; 12.38.Mh     

How important are next-to-leading order models in predicting strange particle spectra in p+p collisions at STAR?

STAR has measured a variety of strange particle species in p+p collisions at  =200 GeV. These high statistics data are ideal for comparing to existing leading- and next-to-leading order perturbative QCD (pQCD) models. Next-to-leading (NLO) models have been successful in describing inclusive hadron production using parameterized fragmentation functions (FF) for quarks and gluons. However, in order to describe identified strange particle spectra at NLO, knowledge of flavor separated FF is essential. Such FF have recently been parameterized using data by the OPAL experiment and allow for the first time to perform NLO calculation for strange baryons. In fact, comparing the STAR Λ data with these calculations allow to put a constraint on the gluon fragmentation function. We show that the leading-order (LO) event generator PYTHIA has to be tuned significantly to reproduce the STAR identified strange particle data. In particular, it fails to describe the observed enhancement of baryon-to-meson ratio at intermediate p_T (2–6 GeV/c). In heavy-ion (HI) collisions this observable has been extensively compared with models and shows a strong dependency on collision centrality or parton density. In the HI context the observed enhancement has been explained by recent approaches in terms of parton coalescense and recombination models. PACS 25.75.Dw; 25.40.Ep; 24.10.Lx     

Monte-Carlo simulation of heavy-ion collisions

We present Monte-Carlo simulations for heavy-ion collisions combining PYTHIA and the McGill-AMY formalism to describe the evolution of hard partons in a soft background, modelled using hydrodynamic simulations. MARTINI generates full event configurations in the high p_T region that take into account thermal QCD and QED effects as well as effects of the evolving medium. This way it is possible to perform detailed quantitative comparisons with experimental observables.     

Determining the inelastic proton–proton cross section at the large hadron collider using minimum bias events

In this paper a new method is described for determining the non-diffractive part of the inelastic proton–proton cross section, at the LHC centre of mass energy of 14 TeV. The method is based on counting the number of inelastic proton–proton interactions in the collision regions. According to a preliminary investigation, this measurement will be best suited for the initial low luminosity phase of the LHC. Knowledge of the proton–proton luminosity is likely to dominate the measurement uncertainty. PACS 13.85.Hd     

Saturation physics and angular correlations at RHIC and LHC

We investigate the angular correlation between pions and photons produced in deuteron–gold collisions at RHIC and proton–lead collisions at LHC using the color glass condensate formalism. We make predictions for the dependence of the production cross section on the angle between the pion and the photon at different rapidities and transverse momenta. Measuring this dependence would shed further light on the role of the high gluon-density effects and saturation dynamics at RHIC and LHC.     

<Emphasis Type="Italic">γ</Emphasis>–hadron correlation measurements to study jets fragmentation with ALICE at LHC

The possibility to study the fragmentation function of jets with energies below 50 GeV, which cannot be reconstructed in the heavy-ion environment, is explored. We propose to measure the imbalance distribution between prompt photon and charged hadrons with the ALICE experiment at LHC. In the present article, the case of proton–proton collisions at  TeV is examined with Monte-Carlo simulations. Performances for prompt photon identification and rejection of the π ⁰ decay photons are presented. We find that the measurement in proton–proton is meaningful with respect to statistical and systematic errors for photon energies larger than 20 GeV.     

Large-angle production of charged pions by 3-12.9?GeV/c protons on beryllium, aluminium and lead targets

Measurements of the double-differential π^± production cross-section in the range of momentum 100 MeV/c≤p< 800 MeV/c and angle 0.35 rad ≤θ<  2.15 rad in proton–beryllium, proton–aluminium and proton–lead collisions are presented. The data were taken with the HARP detector in the T9 beam line of the CERN PS. The pions were produced by proton beams in a momentum range from 3 GeV/c to 12.9 GeV/c hitting a target with a thickness of 5% of a nuclear interaction length. The tracking and identification of the produced particles was performed using a small-radius cylindrical time projection chamber (TPC) placed inside a solenoidal magnet. Incident particles were identified by an elaborate system of beam detectors. Results are obtained for the double-differential cross-sections d²σ/dpdθ at six incident proton beam momenta (3 GeV/c, 5 GeV/c, 8 GeV/c, 8.9 GeV/c (Be only), 12 GeV/c and 12.9 GeV/c (Al only)) and compared to previously available data.     

Large-angle production of charged pions by 3?GeV/c–12?GeV/c protons on carbon, copper and tin targets

A measurement of the double-differential π^± production cross-section in proton–carbon, proton–copper and proton–tin collisions in the range of pion momentum 100 MeV/c≤p<800 MeV/c and angle 0.35 rad≤θ<2.15 rad is presented. The data were taken with the HARP detector in the T9 beam line of the CERN PS. The pions were produced by proton beams in a momentum range from 3 GeV/c to 12 GeV/c hitting a target with a thickness of 5% of a nuclear interaction length. The tracking and identification of the produced particles was done using a small-radius cylindrical time projection chamber (TPC) placed in a solenoidal magnet. An elaborate system of detectors in the beam line ensured the identification of the incident particles. Results are shown for the double-differential cross-sections d²σ/dpdθ at four incident proton beam momenta (3 GeV/c, 5 GeV/c, 8 GeV/c and 12 GeV/c). PACS 13.75.Cs; 13.85.Ni     

On transverse momentum event–by–event fluctuations in string hadronic models

Transverse momentum event–by–event fluctuations are studied within the string–hadronic model of high energy nuclear collisions, LUCIAE. Data on non–statistical fluctuations in p+p interactions are reproduced. Fluctuations of similar magnitude are predicted for nucleus–nucleus collisions, in contradiction to the preliminary NA49 results. The introduction of a string clustering mechanism (Firecracker Model) leads to a further, significant increase of fluctuations for nucleus–nucleus collisions. Secondary hadronic interactions, as implemented in LUCIAE, cause only a small reduction of fluctuations. Received: 23 October 1998 / Published online: 11 March 1999     

How to catch a ‘fat’ proton

We argue that high-multiplicity events in proton–proton or proton–nucleus collisions originate from large-size fluctuations of the nucleon shape. We discuss a pair of simple models of such proton shape fluctuations. A “fat” proton with a size of 3 fm occurs with observable frequency. In light of this result, collective flow behavior in the ensuing nuclear interaction seems feasible. We discuss the influence of these models on the parton structure of the proton.     

Fluctuations of multiplicities in rapidity windows in sulphur-sulphur collisions at 200A GeV

Multiplicity distributions and their second moments fornegatively charged particles produced in³²S-S central and minimum bias interactions at 200A GeV are studied in various rapidity intervals. Fritiof and Venus models mostly describe the dependence of second moments on rapidity intervals in minimum bias interactions, but not in central collisions. For central collisions the behaviour of second moments might indicate enhanced multiplicity fluctuations.     

S~(1/2)=200GeV质子对撞中横质量谱劈裂效应的PYTHIA模拟研究(英文)

在质心系能量为200GeV的质子-质子对撞中,高横质量区域产生的强子横质量谱分裂成两类——重子和介子．应用PYTHIA产生器进行Monte Carlo分析其内在的物理机制．模拟结果表明,这种劈裂效应不仅在弦碎裂模型中出现,而且独立碎裂模型中也有,并且在RHIC能区(200GeV)下主要来源于胶子的贡献．在PYTHIA6．3版本中引入的新的物理机制表明复杂的弦纠缠(string junction)形式可能是这种重子一介子差异的主要原因．