Hadronization from color interactions

A quark coalescence model, based on semi-relativistic molecular dynamics with color interactions among quarks, is presented and applied to pp collisions. A phenomenological potential with two tunable parameters is introduced to describe the color interactions between quarks and antiquarks. The interactions drive the process of hadronization that finally results in different color neutral clusters, which can be identified as hadrons based on some criteria. A Monte Carlo generator PYTHIA is used to generate quarks in the initial state of hadronization, and different values of tunable parameters are used to study the final state distributions and correlations. Baryon-to-meson ratio, transverse momentum spectra, pseudorapidity distributions and forward-backward multiplicity correlations of hadrons produced in the hadronization process, obtained from this model with different parameters, are compared with those from PYTHIA.     

Some remarks on the particle multiplicities

Recent ALICE data for the multiplicity distributions in the central rapidity bins at LHC energies are compared with the results from two default versions of the PYTHIA 8 generator. We find that, contrary to the earlier versions of PYTHIA, the model overestimates the increase of average multiplicity with energy. Tuning two of the model parameters one obtains reasonable agreement with data. The dependence of the normalized moments of the distribution on the rapidity bin width and on energy is also qualitatively correct.     

Comparative study of the forward and backward methods for calculating jet properties in pp collisions at s~(1/2)=7 TeV

We propose a forward method based on PYTHIA6.4 to study the jet properties in ultra-relativistic pp collisions.In the forward method,the partonic initial states are first generated with PYTHIA6.4 and then hadronized in the Lund string fragmentation model,and finally the hadronic jets are constructed from the created hadrons.Jet properties calculated with the forward method for pp collisions at √s=7 TeV are comparable to those calculated with the usual anti-kt algorithm(backward method)in PYTHIA6.4.The comparison between the backward and forward methods may contribute to the understanding of the partonic origin of jets in the backward method.     

A simplified model for calculating RAA and RCP of cold nuclear matter effects

An incoherent binary nucleon-nucleon collision model of AA collisions is presented for simulating particle production in cold nuclear matter. With a simple phenomenological parameter, the mean nucleon energy loss fraction, this model yields pseudorapidity density distributions that are comparable to those of experiment, as well as those of HIJING. Particle production data for a given binary collision is extracted from the PYTHIA event generator. The nuclear geometry is described by the Glauber model. The preliminary R _AA and R _CP results are also presented and discussed, with a proposal that R _AA be redefined.     

ee0A GeV S+Au反应中光子产生的低横动量增强

用建立在LUND弦模型特别是PYTHIA事件产生器基础上的描写极端相对论性核一核碰撞的强子和弦级联模型—JPCIAE研究了入射能量为200A GeV的S+ Au中心碰撞中光子产生.模型同时考虑了部分子QCD散射过程、强子末态相互作用以及强子衰变等光子产生反应道并作了协调处理.JPCIAE模型计算结果很好再现了WA93实验数据所呈现的低横动量增强效应.     

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     

SANCnews: Sector 4f,charged current

In this paper we describe the implementation of the charged current decays of the type t→bl⁺ν_l(γ) in the framework of the SANC system. All calculations are done taking into account the one-loop electroweak correction in the standard model. The emphasis of this paper is on the presentation of numerical results. Various distributions are produced by means of a Monte Carlo integrator and event generator. Comparison with the results of the CompHEP and PYTHIA packages are presented for the Born and hard photon contributions. The validity of the cascade approximation at one-loop level is also studied. PACS 14.65.Ha; 12.15.-y; 12.15.Lk     

Femtoscopy application of the new EPOS model to the STAR experiment

The space-time structure at hadronization was studied within new EPOS model using femtoscopical methods. The results of the study was compared with the STAR HBT data for AuAu collision and first ALICE HBT data for pp collisions. The model predicted mT and centrality dependence of R _out, R _side and R _long femtoscopy parameters were found to be in accordance with the STAR data.     

Study of new boson W_1~± in the Minimal Higgsless Model at the LHC

The Minimal Higgsless Model predicts the existence of new vector gauge boson W1± . By the process PP → W1± qq → W±Z0qq, Z0 → l+l-, W±→ qq (l=e,μ; q is hadronized to be jets), we study the sensitivity of searching for this possible vector gauge boson in the level of generator events of signal and backgrounds, then give integrated luminosity required to discover 5σ signal as a function of W1± mass. The generator for the signal PP→W1± qq→W±Z0qq at tree level is developed with the Minimal Higgsless Model and then interfaced with PYTHIA for the parton showers and hadronization. The backgrounds are produced with MadGraph and PYTHIA.     

Transverse-momentum fluctuations in pi(+)p and K(+)p Collisions at 250 GeV/c

We report results on event-by-event fluctuations of transverse momentum, Phi(p(t)), in pi(+)p and K(+)p collisions at 250 GeV/c. For the first time, their dependence on rapidity region, transverse momentum acceptance, multiplicity, mean transverse momentum per event, and on the correlation between transverse momentum and multiplicity are systematically presented. The results are compared with those from the PYTHIA Monte Carlo generator. The fluctuations under the same acceptance cuts as used in current heavy-ion experiments are also presented.     

A model of jet quenching in ultrarelativistic heavy ion collisionsand high- p T hadron spectra at RHIC

The method to simulate the rescattering and energy loss of hard partons in ultrarelativistic heavy ion collisions has been developed. The model is a fast Monte Carlo tool introduced to modify a standard PYTHIA jet event. The full heavy ion event is obtained as a superposition of a soft hydro-type state and hard multi-jets. The model is applied to the analysis of the jet quenching pattern at RHIC. Received: 20 June 2005, Revised: 11 October 2005, Published online: 24 November 2005     

PHENIX J/Ψ measurement at \sqrt {s_{NN} } = 200 GeV

In 2002, PHENIX had the first measurements on J/Ψ → e⁺ e^? and J/Ψ→μ ⁺ μ ^? in AuAu and pp collisions at $\sqrt {s_{NN} } = 200$ GeV. The preliminary results are presented and discussed. The ongoing analysis status on year 2003 dAu collision data are presented.     

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.     

Z0 Boson measurement with the ALICE central barrel in pp collisions at 14TeV

The possibility to detect the Z⁰ in the ALICE central barrel is studied via the electronic decay channel Z⁰ → e ⁺ e ⁻. The signal and the background are simulated with the leading order event generator PYTHIA 6. The total cross-sections are taken from NLO calculations. Based on test beam data, the electron identification performance of the Transition Radiation Detector is extrapolated to high momenta. The expected yields for minimumbias pp collisions at 14TeV are presented. An isolation cut on the single electron, together with a minimum transverse momentum cut, allows to obtain a clear signal. The expected background is of the order of 1% with the main contribution coming from misidentified pions from jets.     

Λ Production and Λ/p Ratio in Relativistic Heavy Ion Collisions

The Λ multiplicity and Λ/p ratio are studied by hadron transportation-string fragmentation model in relativistic heavy ion collisions. Firstly, the dependence of Λ multiplicity and Λ/p ratio on the system size and the collision centrality is studied. It shows that the Λ and p multiplicities go up as the increase of system size and the increase of collision centrality. However, their ratio keeps almost a constant. The effect of Λ annihilation cross section to Λ multiplicity and Λ/p ratio is also studied. It is found that this effect is weak: Λ multiplicity and Λ/p ratio have a little amount of increase by the decrease of Λ annihilation cross section. Even the cross section is down to zero, Λ/p ratio is only 1.2 in 200A GeV AuAu head on collision. The Λ/p ratio is obtained to be 0.28 in pp collision, lying in the range of experimental data:0.2—0.3. It is also obtained that the ratio in AA collisions is 3—5 times of that in pp collision.     

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．     

Inclusive photoproduction of neutral pions in the photon hemisphere at HERA

The inclusive cross section for the photoproduction of neutral pions has been measured as a function of the transverse momentum, rapidity, and Feynman x of the mesons at an average photon–proton centre-of-mass energy of 208 GeV and for photon virtualities below GeV. The measurement extends the range covered by previous charged particle measurements at HERA by two units of rapidity in the photon direction down to a value of in the centre-of-mass frame. The transverse momentum distribution is well described over the whole measured range by a power law ansatz, while an exponential fit falls below the data at transverse momentum values above 1.5 GeV/c. Good agreement with the predictions of the Monte Carlo models PYTHIA and PHOJET is found. In the context of the PYTHIA model the data are inconsistent with large intrinsic transverse momentum values in the photon. Received: 14 June 2000 / Published online: 27 November 2000     

Two-particle angular correlations in pp and heavy-ion collisions

This paper presents a selected review of recent experimental results on two-particle angular correlations in AuAu and pp collisions. Two-particle correlations in AuAu collisions exhibit a rich structure as a function of hadron transverse momentum, carrying information about the expansion of the medium produced in nuclear collisions, the energy loss of high-p_T partons in the medium and the medium response to these high-p_T partons or other initial state inhomogeneities. Evidence on the interplay between initial state fluctuations and the final state correlation structure is discussed.     

Simulation Study on the Splitting Effect of Transverse Mass Spectra in p+p Collisions at √S=200GeV Using PYTHIA

In proton-proton (p+p) collisions at √S=200GeV, it is found that the previously observed universal shape of transverse mass spectra of hadron production seems to break down into two species of baryons and mesons at higher transverse mass region. In order to understand the underlying physics mechanism, a Monte Carlo study is done using the PYTHIA event generator. The simulation results demonstrate that this difference exists not only within string fragmentation scheme but also within independent fragmentation scheme, and comes primarily from gluon jets within string fragmentation scheme at RHIC energy. The new introduced physics mechanisms in PYTHIA version6.3 indicate that the complicated string junction may contribute to this splitting effect between mesons and baryons.