Parton Model Research Articles

Surface versus volume emissions in photon–hadron correlations

High-p T photon–hadron correlations are studied within the next-to-leading order (NLO) perturbative QCD parton model with modified parton-jet fragmentation functions due to jet quenching in high-energy A+A collisions. In central A+A collisions, the away-side hadrons with large z T=p T /p T are controlled mainly by the surface emission of the gamma-jet events, while a small z T region will be volume emission bias. In other words, gamma jets for a small-z T region probe the dense matter deeper than those gamma jets for a large-z T region, so the small-z T gamma jets are found to be slightly more sensitive to the properties of the dense matter than the large-z T gamma jets.

Azimuthal asymmetry of pions inppandpAcollisions

We investigate the azimuthal asymmetry v{sub 2} of produced pions in pp and pA collisions at both the Relativistic Heavy Ion Collider and CERN SPS energies. In our approach, based on the perturbative QCD parton model and the light-cone QCD-dipole formalism, the azimuthal asymmetry results from a correlation between the color-dipole orientation and the impact parameter of the collision. We introduce the color-dipole orientation within an improved Born approximation and the saturation model which satisfies available deep inelastic scattering data, showing that the azimuthal asymmetry of partons and pions is very sensitive to the choice of the model, and that it is reduced in the saturation model. We find that v{sub 2} of quarks and gluons in parton-nucleus collisions have very different patterns. The azimuthal asymmetry of gluons in gluon-nucleus collisions can be negative at small transverse momentum, and can change sign and become positive at high transverse momentum. The azimuthal asymmetry of quarks in quark-nucleus collisions is positive at all values of transverse momentum. We find that the azimuthal anisotropy v{sub 2} of produced pions in both pp and pA collisions is positive, albeit rather small.

Electromagnetic form factors from the fifth dimension

We analyse various U(1)EM form factors of mesons at strong coupling in an = 2 flavored version of = 4 SYM which becomes conformal in the UV. The quark mass breaks the conformal symmetry in the IR and generates a mass gap. In the appropriate limit, the gravity dual is described in terms of probe D7-branes in AdS5 × S5. By studying the D7 fluctuations we find the suitable terms in a ``meson effective theory which allow us to compute the desired form factors, namely the γπρ and γf0ρ transition form factors. At large q2 we find perfect agreement with the naive parton model counting, which is a consequence of the conformal nature of our model in the UV. By using the same tools, we can compute the γ*γ*π form factor. However this channel is more subtle and comparisons to the QCD result are more involved.

Where does the energy loss lose strength?

Nuclear modification factors for pion production in AuAu and CuCu collisions are analyzed at very high transverse momenta. At pT ≳ 10 GeV/c, the RAA(pT) is determined mostly by the initial-state nuclear modifications (e.g. the EMC effect) and the non-Abelian jet energy loss in the final state. At high momenta these effects together are strong enough to suppress RAA(pT) to below 1 at RHIC energies. We display results using HKN shadowing in our pQCD-improved parton model. Result of a similar calculation at LHC energies for PbPb collisions is also displayed. Based on estimates, a larger opacity value, L/λg ≈ 10 ± 2, is used for the produced partonic matter in central collisions at the LHC.

Punch-through jets in A + A collisions at RHIC/LHC

High pT single and dihadron production is studied within a NLO pQCD parton model with jet quenching in high energy A + A collisions at RHIC/LHC energy. A simultaneous χ2-fit to both single and dihadron spectra can be achieved within a narrow range of the energy loss parameter. Punch-through jets are found to result in a dihadron suppression factor slightly more sensitive to medium properties than the single hadron suppression factor at RHIC. Such jets at LHC are found to dominate high pT dihadron production and the resulting dihadron spectra are more sensitive to the initial parton distribution functions than the single hadron spectra.

STAR's measurement of long-range forward–backward multiplicity correlations as the signature of ‘dense partonic matter’ in heavy-ion collisions at

Forward–backward multiplicity correlations have been measured with the STAR detector for Au + Au, Cu + Cu and p + p collisions at . A strong, long-range correlation is observed for central heavy-ion collisions that vanishes in semi-peripheral events and p + p collisions. Both the dual parton model and the color glass condensate indicate that long-range correlations are due to multiple parton interactions. This suggests that dense partonic matter might have been created in mid-central and central Au + Au collisions at .

Azimuthal angle dependence of dijet production in unpolarized hadron scattering

We study the azimuthal angular dependence of back-to-back dijet production in unpolarized hadron scattering H{sub A}+H{sub B}{yields}J{sub 1}+J{sub 2}+X, arising from the product of two Boer-Mulders functions, which describe the transverse spin distribution of quarks inside an unpolarized hadron. We find that when the dijet is of two identical quarks (J{sub q}+J{sub q}) or a quark-antiquark pair (J{sub q}+J{sub q}), there is a cos{delta}{phi} angular dependence of the dijet, with {delta}{phi}={phi}{sub 1}-{phi}{sub 2}, and {phi}{sub 1} and {phi}{sub 2} are the azimuthal angles of the two individual jets. In the case of J{sub q}+J{sub q} production, we find that there is a color factor enhancement in the gluonic cross section, compared with the result from the standard generalized parton model. We estimate the cos{delta}{phi} asymmetry of dijet production at RHIC, showing that the color factor enhancement in the angular dependence of J{sub q}+J{sub q} production will reverse the sign of the asymmetry.

Parton content of the real photon: astrophysical implications

The parton model provides a relatively explicit and transparent technique for the description of high energy particle interactions. Thus, cross sections of two processes, which may have non-trivial astrophysical implications, are calculated by using the parton language. The first one is inclusive on-shell W+ boson production in the νeγ reaction. The second one is deep inelastic neutrino–photon scattering.

New target transverse spin dependent azimuthal asymmetries from COMPASS experiment

In general, eight target transverse spin-dependent azimuthal modulations are allowed in semi inclusive deep inelastic scattering of polarized leptons on a transversely polarized target. In the QCD parton model four of these asymmetries can be interpreted within the leading order approach. Two of them, namely Collins and Sivers effects were already measured by HERMES and COMPASS experiments. Other two leading twist and remaining four azimuthal asymmetries which can be interpreted as twist-three contributions have been measured for the first time in COMPASS using a 160 GeV/c longitudinally polarized ($P_{beam}\simeq -0.8$) muon beam and a transversely polarized $^6LiD$ target. We present here the preliminary results from the 2002-2004 data.

On the role of heavy flavor parton distributions at high energy colliders

We compare ‘fixed flavor number scheme’ (FFNS) and ‘variable flavor number scheme’ (VFNS) parton model predictions at high energy colliders. Based on our recent LO- and NLO-FFNS dynamical parton distributions, we generate radiatively two sets of VFNS parton distributions where also the heavy quark flavors h=c,b,t are considered as massless partons within the nucleon. By studying the role of these distributions in the production of heavy particles (hh¯, tb¯, hW±, Higgs–bosons, etc.) at high energy ep, pp¯ and pp colliders, we show that the VFNS predictions are compatible with the FFNS ones (to within about 10–20% at LHC, depending on the process) when the invariant mass of the produced system far exceeds the mass of the participating heavy quark flavor.

Dynamical next-to-next-to-leading order parton distributions and the perturbative stability ofFL(x,Q2)

Recent measurements for ${F}_{2}(x,{Q}^{2})$ have been analyzed in terms of the ``dynamical'' and ``standard'' parton model approach at next-to-leading order (NLO) and next-to-next-to-leading order (NNLO) of perturbative QCD. Having fixed the relevant NLO and NNLO parton distributions, we present the implications and predictions for the longitudinal structure function ${F}_{L}(x,{Q}^{2})$. It is shown that the previously noted extreme perturbative NNLO/NLO instability of ${F}_{L}(x,{Q}^{2})$ is an artifact of the commonly utilized ``standard'' gluon distributions. In particular it is demonstrated that using the appropriate---dynamically generated---parton distributions at NLO and NNLO, ${F}_{L}(x,{Q}^{2})$ turns out to be perturbatively rather stable already for ${Q}^{2}\ensuremath{\ge}\mathcal{O}(2--3\text{ }\text{ }{\mathrm{GeV}}^{2})$.

Predictions of the QCD Parton Model

The QCD parton model allows to establish approximate relations among transverse momentum dependent quark distribution functions and, simultaneously, to fix an energy scale, involved in the parametrization of the correlator, which determines the Q 2 dependence of the azimuthal asymmetries in inclusive reactions, like SIDIS and Drell-Yan. Some of the present data support such model predictions. We suggest further measurements of asymmetries and an alternative way of determining (approximately) the proton transversity.

Sivers Single-Spin Asymmetry in Photon-Jet Production

We study a weighted asymmetry in the azimuthal distribution of photon-jet pairs produced in the process pp-->gamma jet X with a transversely polarized proton. We focus on the contribution of the Sivers effect only, considering experimental configurations accessible at the Relativistic Heavy Ion Collider. We show that predictions for the asymmetry, obtained in terms of gluonic-pole cross sections calculable in perturbative QCD, can be tested and clearly discriminated from those based on a generalized parton model, involving standard partonic cross sections. Experimental measurements of the asymmetry will therefore test our present understanding of single-spin asymmetries.

Rapidity and species dependence of particle production at large transverse momentum ford+Aucollisions atsNN=200GeV

We determine rapidity asymmetry in the production of charged pions, protons and anti-protons for large transverse momentum (pT) for d+Au collisions at \sqrt s_NN = 200 GeV. The identified hadrons are measured in the rapidity regions |y| < 0.5 and 0.5 < |y| < 1.0 for the pT range 2.5 < pT < 10 GeV/c. We observe significant rapidity asymmetry for charged pion and proton+anti-proton production in both rapidity regions. The asymmetry is larger for 0.5 < |y| < 1.0 than for |y|< 0.5 and is almost independent of particle type. The measurements are compared to various model predictions employing multiple scattering, energy loss, nuclear shadowing, saturation effects, and recombination, and also to a phenomenological parton model. We find that asymmetries are sensitive to model parameters and show model-preference. The rapidity dependence of \pi^{-}/\pi^{+} and \bar{p}/p ratios in peripheral d+Au and forward neutron-tagged events are used to study the contributions of valence quarks and gluons to particle production at high pT. The results are compared to calculations based on NLO pQCD and other measurements of quark fragmentation functions.

A NLO ANALYSIS ON AZIMUTHAL ANISOTROPY OF HIGH pT HADRON IN HEAVY-ION COLLISIONS

The azimuthal anisotropy of high pT hadron in high energy A + A collisions is studied within the next-to-leading order (NLO) perturbative QCD parton model. The effect of jet quenching is incorporated via a model for modified jet fragmentation functions due to radiative parton energy loss in dense medium. Because NLO contributions behave with stronger quenching effect than LO contributions, the NLO elliptic flow parameter is found to be larger than the LO in the medium pT region.

FORWARD-BACKWARD MULTIPLICITY CORRELATIONS IN Au+Au COLLISIONS AT $\sqrt{s_{NN}} = 200\, {\rm GeV}$

The study of correlations among particles produced in different rapidity regions may provide understanding of the mechanisms of particle production. Correlations that extend over a longer range are observed in hadron-hadron interactions only at higher energies. Results for short and long-range multiplicity correlations (Forward-Backward) are presented for Au + Au collisions at [Formula: see text]. The growth of long range correlations are observed as a function of the pseudorapidity gap in central Au + Au collisions. The Dual Parton model and Color Glass Condensate phenomenology have been explored to understand the origin of long range correlations.

Drell-Yan production and Lam-Tung relation in the color glass condensate formalism

We study the Drell-Yan production cross section and structure functions in proton (deuteron)-nucleus collisions using the color glass condensate formalism. The nucleus is treated in the color glass condensate framework which includes both higher twist effects due to the inclusion of multiple scatterings and leading twist pQCD shadowing due to the small $x$ resummation, while the proton (or deuteron) is treated within the DGLAP improved parton model. In particular, the Drell-Yan structure functions are used in order to investigate the Lam-Tung relation at small $x$, which is known to be identically zero at leading twist up to next-to-leading order, and is thus a good playground for studying higher twist effects. In agreement with this, we find that violations of this relation are more important for low momentum and invariant mass of the Drell-Yan pair, and also in the region of rapidity that corresponds to smaller values of $x$ in the nucleus.

HIGH ENERGY SCATTERING IN QUANTUM CHROMODYNAMICS

In this series of three lectures, we discuss several aspects of high energy scattering among hadrons in Quantum Chromodynamics. The first lecture is devoted to a description of the parton model, Bjorken scaling and the scaling violations due to the evolution of parton distributions with the transverse resolution scale. The second lecture describes parton evolution at small momentum fraction x, the phenomenon of gluon saturation and the Color Glass Condensate (CGC). In the third lecture, we present the application of the CGC to the study of high energy hadronic collisions, with emphasis on nucleus-nucleus collisions. In particular, we provide the outline of a proof of high energy factorization for inclusive gluon production.

Heavy quarkonium production in the regge limit of quantum chromodynamics

Heavy quarkonium \((c\bar c,b\bar b)\) hadronic production at Tevatron (run I and run II) and LHC energies is considered in the framework of nonrelativistic quantum chromodynamics in the leading order with respect to α s and v using the quasi-multi-Regge kinematics approach. Fitting of p T spectra of different S and P wave heavy quarkonium states at Tevatron (run I and run II) energies is carried out. The obtained set of octet nonperturbative matrix elements is used for prediction of heavy quarkonium production at LHC energies. The results obtained in the framework of quasi-multi-Regge kinematics are compared with predictions of the collinear parton model.

THE QCD PARTON MODEL: A USEFUL APPROXIMATION

Approximate relations among transverse momentum dependent quark distribution functions are established in the framework of the QCD parton model. The validity of such results survives QCD evolution effects, owing to the Politzer theorem on equations of motion. Furthermore the model fixes an energy scale, involved in the parametrization of the correlator, which determines the Q2 dependence of the azimuthal asymmetries in inclusive reactions. Some of the present data — in particular the sin 2ϕ single spin asymmetry in semi-inclusive deep inelastic scattering (SIDIS) and the cos ϕ asymmetry in unpolarized SIDIS — support model predictions. Further measurements of SIDIS and Drell–Yan asymmetries are suggested, in particular the SIDIS double spin asymmetry, which allows to determine approximately the proton transversity.