Transverse Momentum Distribution Of Partons Research Articles

Gradient Tomography of Jet Quenching in Heavy-Ion Collisions.

Transverse momentum broadening and energy loss of a propagating parton are dictated by the space-time profile of the jet transport coefficient q[over ^] in a dense QCD medium. The spatial gradient of q[over ^] perpendicular to the propagation direction can lead to a drift and asymmetry in parton transverse momentum distribution. Such an asymmetry depends on both the spatial position along the transverse gradient and path length of a propagating parton as shown by numerical solutions of the Boltzmann transport in the simplified form of a drift-diffusion equation. In high-energy heavy-ion collisions, this asymmetry with respect to a plane defined by the beam and trigger particle (photon, hadron, or jet) with a given orientation relative to the event plane is shown to be closely related to the transverse position of the initial jet production in full event-by-event simulations within the linear Boltzmann transport model. Such a gradient tomography can be used to localize the initial jet production position for more detailed study of jet quenching and properties of the quark-gluon plasma along a given propagation path in heavy-ion collisions.

Global extraction of partonic transverse momentum distributions from semi-inclusive deep inelastic scattering and Drell-Yan data

We discuss a first attempt at a global extraction of unpolarized partonic transverse momentum dependent (TMD) distributions and fragmentation functions from a simultaneous fit of data measured in semi-inclusive deep-inelastic scattering, Drell–Yan and Z boson production. This analysis is performed in the low transverse momentum region, at leading order in perturbative QCD. To connect data at different scales, we use TMD evolution at next-to-leading logarithmic accuracy.

Extraction of partonic transverse momentum distributions from semi-inclusive deep-inelastic scattering, Drell-Yan and Z-boson production

We present an extraction of unpolarized partonic transverse momentum distributions (TMDs) from a simultaneous fit of available data measured in semi-inclusive deep-inelastic scattering, Drell-Yan and Z boson production. To connect data at different scales, we use TMD evolution at next-to-leading logarithmic accuracy. The analysis is restricted to the low-transverse-momentum region, with no matching to fixed-order calculations at high transverse momentum. We introduce specific choices to deal with TMD evolution at low scales, of the order of 1 GeV2. This could be considered as a first attempt at a global fit of TMDs.

Polarized $$^{\varvec{3}}$$ 3 He Target and Final State Interactions in SiDIS

Jefferson Lab (JLab) is starting a wide experimental program aimed at studying the neutron's structure, with a great emphasis on the extraction of the parton transverse-momentum distributions (TMDs). To this end, Semi-inclusive deep-inelastic scattering (SiDIS) experiments on polarized $^3$He will be carried out, providing, together with proton and deuteron data, a sound flavor decomposition of the TMDs. Given the expected high statistical accuracy, it is crucial to disentangle nuclear and partonic degrees of freedom to get an accurate theoretical description of both initial and final states. In this contribution, a preliminary study of the Final State Interaction (FSI) in the standard SiDIS, where a pion (or a Kaon) is detected in the final state is presented, in view of constructing a realistic description of the nuclear initial and final states.

Process dependent nucleark⊥broadening effect

We study the process dependent nuclear $k_\perp$ broadening effect by employing the transverse momentum dependent(TMD) factorization approach in combination with the Mclerran-Venugopalan(MV) model. More specifically, we investigate how the parton transverse momentum distributions are affected by the process dependent gauge links in cold nuclear matter. In particular, our analysis also applies to the polarized cases including the nuclear quark Boer-Mulders function and the linearly polarized gluon distribution. Our main focus is on the nuclear TMDs at intermediate or large x.

Intrinsic transverse momentum and parton correlations from dynamical chiral symmetry breaking

The dynamical breaking of chiral symmetry in QCD is caused by nonperturbative interactions on a scale rho ~ 0.3 fm, much smaller than the hadronic size R ~ 1 fm. These short-distance interactions influence the intrinsic transverse momentum distributions of partons and their correlations at a low normalization point. We study this phenomenon in an effective description of low-energy dynamics based on chiral constituent quark degrees of freedom, which refers to the large-N_c limit of QCD. The nucleon is obtained as a system of constituent quarks and antiquarks moving in a self-consistent classical chiral field (chiral quark-soliton model). The calculated distributions of constituent quarks/antiquarks are matched with QCD partons at the scale rho^{-2}. The p_T distribution of valence quarks is localized at p_T^2 ~ R^{-2} and roughly of Gaussian shape. The sea quark distribution exhibits a would-be power-like tail ~1/p_T^2 extending up to the chiral symmetry-breaking scale. Such behavior is seen in the flavor-singlet unpolarized and nonsinglet polarized sea. The high-momentum tails are the result of short-range correlations between sea quarks in the nucleon's light-cone wave function, analogous to NN correlations in nuclei. The nucleon wave function contains correlated pairs of transverse size rho << R with sigma- and pi-like quantum numbers, whose internal wave functions become identical at p_T^2 ~ rho^{-2} (restoration of chiral symmetry). These features are model-independent and represent an effect of dynamical chiral symmetry breaking on the nucleon's partonic structure. Our results have numerous implications for the P_T distributions of particles produced in hard scattering processes. The nonperturbative parton correlations predicted here could be observed in particle correlations between the current and target fragmentation regions of DIS.

Parton transverse momentum distribution at the moment of hadronization at RHIC

We extract the transverse momentum distribution of effective partons using the spectra of Ω, Ξ, Λ and ϕ hadrons measured by the STAR Collaboration from Au + Au collisions at \( \sqrt {s_{NN} } \) = 200 GeV at RHIC. The extracted momentum distribution of strange quarks is flatter than that of up/down quarks consistent with hydrodynamics expansion in partonic phase prior to hadronization. Consistency in quark ratios derived from various hadron spectra gives clear evidence for hadron production as suggested by quark coalescence or recombination model.

Partonic transverse momenta in non-relativistic hyper-central quark potential models

We investigate the impact of three-body forces on the transverse-momentum distribution of partons inside the proton. This is achieved by considering the three-body problem in a class of hyper-central quark potential models. Solving the corresponding Schrödinger equation, we determine the quark wave function in the proton and with appropriate transformations and projections we find the transverse-momentum distribution of a single quark. In each case the parameters of the quark potentials are adjusted in order to sufficiently describe observable properties of the proton. Using a factorization ansatz, we incorporate the obtained transverse-momentum distribution in a perturbative QCD scheme for the calculation of the cross-section for prompt photon production in pp collisions. A large set of experimental data is fitted using as a single free parameter the mean partonic transverse momentum. The dependence of 〈k T〉 on the collision characteristics (initial energy and transverse momentum of the final photon) is much smoother when compared with similar results found in the literature using a Gaussian distribution for the partonic transverse momenta. Within the considered class of hyper-central quark potentials the one with the weaker dependence on the hyper-radius is preferred for the description of the data since it leads to the smoothest mean partonic transverse-momentum profile. We have repeated all the calculations using a two-body potential of the same form as the optimal (within the considered class) hyper-central potential in order to check if the presence of three-body forces is supported by the experimental data. Our analysis indicates that three-body forces influence significantly the form of the parton transverse-momentum distribution and consequently lead to an improved description of the considered data.

Deeply inelastic pions in the exclusive reactionp(e,e′π+)nabove the resonance region

A model for the $p(e,{e}^{\ensuremath{'}}{\ensuremath{\pi}}^{+})n$ reaction which combines an improved treatment of gauge invariant meson-exchange currents and hard deep-inelastic scattering (DIS) of virtual photons off nucleons is proposed. It is shown that DIS dominates and explains the transverse response at moderate and high photon virtualities ${Q}^{2}$, whereas the longitudinal response is dominated by hadronic degrees of freedom and the pion electromagnetic form factor. This leads to a combined description of the longitudinal and transverse components of the cross section in a wide range of photon virtuality ${Q}^{2}$ and momentum transfer to the target $t$ and solves the longstanding problem of the observed large transverse cross sections. The latter are shown to be sensitive to the intrinsic transverse momentum distribution of partons.

Parton distributions at hadronization from bulk dense matter produced inAu+Aucollisions atsNN=200GeV

We present an analysis of $\ensuremath{\Omega},\ensuremath{\Xi},\ensuremath{\Lambda}$, and $\ensuremath{\phi}$ spectra from $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{{s}_{\mathit{NN}}}=200$ GeV in terms of distributions of effective constituent quarks at hadronization. Consistency in quark ratios derived from various hadron spectra provides clear evidence for hadron formation dynamics as suggested by quark coalescence or recombination models. We argue that the constituent quark distribution reflects properties of the effective partonic degree of freedom at hadronization. Experimental data indicate that strange quarks have a transverse-momentum distribution flatter than that of up/down quarks consistent with hydrodynamic expansion in partonic phase prior to hadronization. Our extracted parton transverse-momentum distributions at the hadronization provide a unique constraint on the partonic evolution history. After the multiphase transport model is tuned to reproduce the strange and up/down quark distributions, the model can describe the measured spectra of hyperons and $\ensuremath{\phi}$ mesons very well where hadrons are formed through dynamical coalescence.

Di-hadron correlations and parton intrinsic transverse momentum

The transverse momentum distribution of partons in the proton is studied with the help of di-hadron correlations. A simple randomization model is compared to the data.

Decisive role of fragmentation functions in hard hadron production.

It is demonstrated that the fragmentation functions at large momentum fraction play a key role in hard hadron production from relativistic proton-proton collisions. We find that this region of the fragmentation functions is not strongly constrained by the electron-positron data. This freedom can be used (together with the transverse-momentum distribution of partons) to reproduce hard pion-to-proton ratio data in relativistic proton-proton collisions.

Charmonium dissociation cross sections and p– A collisions

Charmonium dissociations by nucleons are first studied from a quark–quark potential in combination with quark exchange mechanism. Wave functions of nucleons and charmed baryons involved in dissociation reactions result from a fit to mass splittings of baryons with spins 1 2 and 3 2 . Transverse momentum and x F distributions of prompt J/ ψ produced in 800 GeV/ c p–Be collisions are calculated by using NRQCD dσ/d t ̂ for parton–parton scatterings and taking into account initial transverse-momentum distributions of partons inside nucleons. We emphasize that the new dissociation cross sections of J/ ψ, ψ′ and χ cJ give rise to remarkable differences of J/ ψ, ψ′ and χ c suppressions at large negative x F in p–W collisions at s NN =38.8 and 41.6 GeV and p–Au collisions at s NN =130 GeV .

High-pTpion and kaon production in relativistic nuclear collisions

High-p_T pion and kaon production is studied in relativistic proton-proton, proton-nucleus, and nucleus-nucleus collisions in a wide energy range. Cross sections are calculated based on perturbative QCD, augmented by a phenomenological transverse momentum distribution of partons (``intrinsic k_T''). An energy dependent width of the transverse momentum distribution is extracted from pion and charged hadron production data in proton-proton/proton-antiproton collisions. Effects of multiscattering and shadowing in the strongly interacting medium are taken into account. Enhancement of the transverse momentum width is introduced and parameterized to explain the Cronin effect. In collisions between heavy nuclei, the model over-predicts central pion production cross sections (more significantly at higher energies), hinting at the presence of jet quenching. Predictions are made for proton-nucleus and nucleus-nucleus collisions at RHIC energies.

Effects of parton intrinsic transverse momentum on photon production in hard-scattering processes

We calculate the photon production cross section arising from the hard scattering of partons in nucleon-nucleon collisions by taking into account the intrinsic parton transverse momentum distribution and the next-to-leading-order contributions. As first pointed out by Owens, the inclusion of the intrinsic transverse momentum distribution of partons leads to an enhancement of photon production cross section in the region of photon transverse momenta of a few GeV/c for nucleon-nucleon collisions at a center-of-mass energy of a few tens of GeV. The enhancement increases as {radical} (s) decreases. Such an enhancement is an important consideration in the region of photon momenta under investigation in high-energy heavy-ion collisions. {copyright} {ital 1998} {ital The American Physical Society}

Space-time evolution and transverse- momentum structure of jets

We discuss models in which the temporal evolution of hadronic jets and the rapidity ordering of particles within jets are correlated. Observable effects on the particle average transverse momentum (energy and longitudinal momentum dependence) characteristic of such models are pointed out. In particular, models in which, within jets, slow particles are produced first and fast particles come out last should exhibit the well-known seagull effect, with 〈 p T〉 rising, for fixed x, proportionately to the square root of the mean particle multiplicity. If, by analogy, the transverse-momentum distributions of partons also exhibit such features, then we have a source of scaling violation in deep inelastic reactions that shows up at high energies rather than at low energies, and a source for an energy and Q 2 dependent 〈 q T〉 in lepton pair production.

Transverse momentum distribution of partons in QCD

The renormalization group approach is applied to the study of transverse momentum distribution of partons in QCD. We generalize the method of Altarelli and Parisi to obtain a formula for the mean squared average transverse momentum as a function of Q 2 and x, the virtual photon mass squared and the longitudinal momentum fraction of the parton. For large Q 2, it decreases with x in addition to the expected increase with Q 2. This x-dependence reduces substantially the transverse momentum of massive μ-pairs produced by hadronic collisions from its linear dependence on ( Q 2 ln Q 2 ) 1 2 .

Transverse-momentum distribution of the massiveμpair using dimensional counting

We discuss qualitatively and quantitatively the expectations for parton transverse-momentum distributions and their effect upon ..mu..-pair production.

The charge radius of the neutron and the quark-parton model

An attempt is made, within the framework of the quark parton model, to understand the negative sign of 〈 R 2〉 n , the mean square charge radius of the neutron. An hypothesis is made concerning the transverse momentum distribution of partons that is motivated by recent measurements on electroproduction. With this hypothesis, the result 〈 R 2〉 n < 0 implies the inequality σ υ N −σ υ N > 6 ∫ (F 2 ep −F 2 en ) dx ( σ's in units of G 2 ME π ) which appears to be in agreement with present data.

Connection between the Regge trajectory universal slope α′ and the transverse momentum distribution of partons in planar Feynman diagram model

We are estimating in an intuitive way the transverse momentum average of the virtual partons in our complicated planar Feynman diagram model with use of the assumption of Gaussian propagators. We find: ▪ is suprisingly good agreement with experimental hadron transverse momenta. It is suggested that this agreement is sensitive to the planarity of the diagrams and can be considered a test of the latter.