Unified description of transverse momentum spectrums contributed by soft and hard processes in high-energy nuclear collisions

The transverse momentum spectrums of π ^?, π ⁺, K ^?, K ⁺, \(\bar p\) , and p produced in p-Pb collisions at √s _NN = 502 TeV measured by the CMS Collaboration and in Pb-Pb collisions at √s _NN = 2.76 TeV measured by the ALICE Collaboration are described by a two-component Erlang distribution. The first component corresponds to “soft” excitation process and contributes in the low transverse momentum region, which is contributed by 2–5 partons (sea quarks and gluons) with strong interactions. The second component corresponds to “hard” scattering process and contributes in the high transverse momentum region, which is contributed by 2 partons (valent quarks) with violent head-on collision. Each parton source contributes an exponential transverse momentum spectrum. Both the soft and hard processes result in an Erlang distribution. The transverse momentum spectrums of final-state charged particles are then described by the two-component Erlang distribution. The contribution ratio (30–40%) of the hard process extracted from nuclear collisions at the Large Hadron Collider is consistent with that (17–46%) obtained from other methods.     

Fluctuations and transverse momentum distributions in the color clustering approach

We present our results on transverse momentum fluctuations, multiplicity fluctuations and transverse momentum distributions for baryons and mesons in the framework of the clustering of color sources. We determine under what conditions the initial state configurations can lead to color connection, and more specifically, if variations of the initial state can lead to a transition from disconnected to connected color clusters, modifying the number of effective sources. We find that beyond a critical point, one has a condensate, containing interacting and hence color-connected sources. This point thus specifies the onset of color deconfinement. We show that the transverse momentum and multiplicity distributions are related to each other in a defined way. We obtain a non-monotonic dependence of the p _T and multiplicity fluctuations with the number of participants. We present our results for the fluctuations and the transverse momentum distributions at RHIC energies compared to the existing experimental data and our predictions for LHC energies.     

A method of experimentally probing transverse momentum dependent distributions

We calculate the double spin asymmetry A_LL$A_{LL}$ for the π⁰${\pi }^0$ production in semi-inclusive deep inelastic lepton–proton scattering with a spectator model of power-law and a model based on the factorization ansatz. We also calculate the double spin asymmetry for the integration over parts of the kinematic range for the setups of the experiments of COMPASS, HERMES, and JLab. We find that the results are characteristically dependent on the model used. Therefore, we suggest that the measurements of the double spin asymmetry provides a method of experimentally probing the transverse momentum dependent distributions.     

Unintegrated CCFM parton distributions and transverse momentum of gauge bosons

Transverse momentum distribution of gauge bosons W^± and Z⁰ is calculated using unintegrated parton distributions obtained by solving leading order CCFM equation(s) in the impact parameter space. For illustration we compare the results of the fixed-order collinear, soft-gluon resummation and CCFM approaches. A parameter of a nonperturbative form factor is adjusted to the D0 data. In contrast to the collinear approach, the leading order calculation based on the unintegrated CCFM parton distributions generates transverse momentum distributions of gauge bosons, which are almost consistent with experimental data provided the parameter of the nonperturbative form factor is appropriately adjusted. The W⁺–W⁻ asymmetry is calculated as a function of the boson rapidity and transverse momentum. Predictions are given for RHIC and LHC.     

Large transverse momentum dilepton and photon production by photoproduction processes

We calculate the production of large transverse momentum dileptons and photons by using direct and resolved photoproduction processes in relativistic heavy ion collisions. Considering the central collisions of heavy nuclei at Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC) energies, we find that the photoproduction processes modify the dilepton and photon production in the large transverse momentum region.     

Universality of the transverse momentum distributions in the framework of percolation of strings

In the framework of percolation of color sources, the transverse momentum distribution in heavy ion and p + p collisions at all centralities and energies are shown to follow a universal behavior. The width of the distribution depends on the fluctuations of the number of color sources per cluster. At low densities, there are only independent single color sources, no fluctuations occur and the distribution is described by a single exponential. At very high densities, only one cluster of many color sources appears and therefore there are not fluctuations either and the hardness of the distribution is suppressed. The Cronin effect in this framework is due to a maximum of the fluctuations which decreases as the density increases. We obtain a good agreement with experimental data including the low p _T behavior and the spectra for different particles. We show that the transverse momentum and multiplicity distributions are related to each other in a defined way. This point is satisfied by the experimental data on p + p collisions at different energies.Received: 15 July 2004, Revised: 17 December 2004, Published online: 15 February 2005     

Quark fragmentation and trigger side momentum distributions in high-p T processes

It has been widely argued that the experimental evidence concerning the momentum accompanying highp T triggers is a grave problem for models which take the trigger hadron to be a quark fragment. It is claimed that the trigger hadron takes much too large a fraction (z _c ) of the jet momentum for the trigger side jet to be a quark. The jet momentum is not directly measured, but deduced from the derivative of the momentum (p _x ) accompanying the trigger with respect to the trigger transverse momentum-p _T ^t . This argument is shown to be unsafe. Using both an approximate analytic approach to illustrate the physics and subsequently a full numerical computation it is proved that the deduction of the fractional momentum accompanying the trigger, 1/z _c ?1, fromdp _x /dp _T ^t is not correct. Further we show that models—specifically that of Feynman and Field—which do take the trigger to be a quark fragment are essentially in agreement with the data on trigger side momentum distributions. A surprising prediction of our analysis is thatp _x should be approximately constant forp _T ^t ≧6GeV/c.     

Optimisation of variables for studying dilepton transverse momentum distributions at hadron colliders

In future measurements of the dilepton (Z/γ ^∗) transverse momentum, Q _T , at both the Tevatron and LHC, the achievable bin widths and the ultimate precision of the measurements will be limited by experimental resolution rather than by the available event statistics. In a recent paper the variable a _T , which corresponds to the component of Q _T that is transverse to the dilepton thrust axis, has been studied in this regard. In the region, Q _T < 30 GeV, a _T has been shown to be less susceptible to experimental resolution and efficiency effects than the Q _T . Extending over all Q _T , we now demonstrate that dividing a _T (or Q _T ) by the measured dilepton invariant mass further improves the resolution. In addition, we propose a new variable, f_h^*\phi _{\rm \eta }^{*}, that is determined exclusively from the measured lepton directions; this is even more precisely determined experimentally than the above variables and is similarly sensitive to the Q _T . The greater precision achievable using such variables will enable more stringent tests of QCD and tighter constraints on Monte Carlo event generator tunes.     

Large transverse momentum dilepton Production in heavy ion collisions with two-photon processes

The cold component of large transverse momentum dilepton production via semi-coherent two-photon interaction is calculated.The cold contribution is essential to the dilepton spectra in the soft region for different mass bins.The results are compared with the PHENIX experimental data at RHIC,and we find that the modification of semi-coherent two-photon processes is more evident with the rising dilepton mass bins.     

Large transverse momentum dilepton production in heavy ion collisions with two-photon processes

The cold component of large transverse momentum dilepton production via semi-coherent two-photon interaction is calculated. The cold contribution is essential to the dilepton spectra in the soft region for different mass bins. The results are compared with the PHENIX experimental data at RHIC, and we find that the modification of semi-coherent two-photon processes is more evident with the rising dilepton mass bins.     

Rapidity and transverse momentum distributions of heavy quarks produced by double pomeron exchange

Zeitschrift für Physik C Particles and Fields - Rapidity and transverse momentum distributions of heavy quarks produced by double exchange of Donnachie-Landshoff pomeron are presented and...     

The transverse momentum of partons in large pT processes

An approximate method is used to investigate the effects of parton transverse momentum in large p_T particle production within the framework of hard scattering models. We derive an approximate expression for the mean bias towards the trigger of each of the two participating partons and find that event one of the partons is biased more than the other, even with a 90° trigger. We treat the transverse momentum of partons and their closely related off-mass-shell behaviour as a perturbation in the equation for the single-particle inclusive cross section, and then expand in a Taylor series. We calculate the first non-zero correction term and find that to this order, the cross section is increased by parton transverse momentum effects by typically a factor of 1.7 for p_T = 3 GeV/c with 〈k_T〉 = 500 MeV/c, or 1.25 for p_T = 3 GeV/c with 〈k_T〉 = 300 MeV/c, and that the correction decreases rapidly with increasing p_T.     

Transverse momentum and transverse mass distributions of charged hadrons produced in Au--Au collisions at high energies

The transverse momentum distribution and the transverse mass distribution of charged hadrons produced in nucleus nucleus collisions at high energies are described by using a two-cylinder model. The results calculated by the model are compared and found to be in agreement with the experimental data of the STAR and E895 Collaborations, measured in A~Au collisions at the relativistic heavy ion collider （RHIC） and alternating-gradient synchrotron （AGS） energies, respectively. In the energy range concerned, the excitation degree of emission source close to the central axis of cylinders increases obviously with the collision centrality and incident energy increasing, but it does not show any obvious change with the increase of the （pseudo）rapidity in central collisions. The excitation degree of emission source close to the side-surface of cylinders does not show any obvious change with the collision centrality, the （pseudo）rapidity, and the incident energy increasing.     

High transverse momentum correlations in STAR

Azimuthal correlations of charged hadrons with transverse momenta above 2 GeV/c are measured relative to ≥ GeV/c trigger particle in p+p and Au+Au at $\sqrt {s_{NN} } = 200$ GeV in STAR. The correlations exhibit small relative azimuthal angle correlations characteristic of jets. The p+p and peripheral Au+Au data exhibit a peak on the opposite side of the jet indicating the presence of hard scattering in the form of di-jets. The away-side jet disappears for the most central Au+Au collisions, suggesting jet quenching in a dense medium.     

The transverse momentum of partons

It is shown that when the parton model is formulated in proper, covariant fashion the average transverse momentum of the partons in a hadron varies with their fractional longitudinal momentum x. It is small near x = 0, but could rise to a large as x → 1.     

Universal behavior of transverse momentum distributions of baryons and mesons in the framework of percolation of strings

In the framework of percolation of strings, the transverse momentum distributions in AA and hh collisions at all centralities and energies show a universal behavior. The width of these distributions is related to the width of the distribution of the size of the clusters formed by the overlapping of the strings produced. The difference between the distributions for baryons and mesons originates in the fragmentation of clusters of several strings, which enhances the particles with a higher number of constituents. The results agree with SPS and RHIC data. The predictions for LHC show differences for baryons compared with RHIC. At LHC energies we obtain also a high p_T suppression for pp high multiplicity events compared with the pp minimum bias. PACS 25.75.Nq; 12.38.Mh; 24.85.+p     

Effect of the jet production on pseudorapidity,transverse momentum and transverse mass distributions of charged particles produced in pp-collisions at Tevatron energy

We investigate the effects of jet production on the following parameters: pseudorapidity, transverse momentum and transverse mass distributions of secondary charged particles produced in pp-collisions at 1.8 Te V,using the HIJING code. These distributions are analyzed for the whole range and for six selected regions of the polar angle as a function of the different number of jets. The obtained simulation results for these parameters are interpreted and discussed in connection to the increase observed in the multiplicity of secondary charged particles as a result of its multi-jet dependence, and are also discussed in comparison with the experimental results from the CDF Collaboration.