Jian-Wei Qiu

Global QCD analysis and the CTEQ parton distributions

The CTEQ program for the determination of parton distributions through a global QCD analysis of data for various hard scattering processes is fully described. A new set of distributions, CTEQ3, incorporating several new types of data is reported and compared to the two previous sets of CTEQ distributions. A comparison with current data is discussed in some detail. The remaining uncertainties in the parton distributions and methods to further reduce them are assessed. Comparisons with the results of other global analyses are also presented.

CTEQ parton distributions and flavor dependence of sea quarks

Abstract This paper describes salient features of new sets of parton distributions obtained by the CTEQ Collaboration based on a comprehensive QCD global analysis of all available data. The accuracy of the new data on deep inelastic scattering structure functions obtained by the very high statistics NMC and CCFR experiments provides unprecedented sensitivity to the flavor dependence of the sea-quark distributions. In addition to much better determination of the small-x dependence of all parton distributions, we found: (i) the strange quark distribution is much softer than the non-strange sea quarks and rises above the latter at small-x; and (ii) the difference d - u changes sign as a function of x. A few alternative sets of viable distributions with conventional assumptions are also discussed.

Single transverse spin asymmetries in direct photon production

We derive single transverse spin asymmetries for direct photon production cross sections in perturbative QCD, relating them to partonic matrix elements that couple the quark and gluon fields. Our method may be generalized to asymmetries for other reactions.

Proton-nucleus collisions at the LHC: Scientific opportunities and requirements

Proton–nucleus (p+A) collisions have long been recognized as a crucial component of the physics program with nuclear beams at high energies, in particular for their reference role to interpret and understand nucleus–nucleus data as well as for their potential to elucidate the partonic structure of matter at low parton fractional momenta (small-x). Here, we summarize the main motivations that make a proton–nucleus run a decisive ingredient for a successful heavy-ion program at the Large Hadron Collider (LHC) and we present unique scientific opportunities arising from these collisions. We also review the status of ongoing discussions about operation plans for the p+A mode at the LHC.

QCD evolution of the Sivers function

We extend the Collins-Soper-Sterman (CSS) formalism to apply it to the spin-dependence governed by the Sivers function. We use it to give a correct numerical QCD evolution of existing fixed-scale fits of the Sivers function. With the aid of approximations useful for the non-perturbative region, we present the results as parametrizations of a Gaussian form in transverse momentum space, rather than in the Fourier conjugate transverse coordinate space normally used in the CSS formalism. They are specifically valid at small transverse momentum. Since evolution has been applied, our results can be used to make predictions for Drell-Yan and semi-inclusive deep inelastic scattering at energies different from those where the original fits were made. Our evolved functions are of a form that they can be used in the same parton model factorization formulas as used in the original fits, but now with a predicted scale dependence in the fit parameters. We also present a method by which our evolved functions can be corrected to allow for twist-3 contributions at large parton transverse momentum.

Unified Picture for Single Transverse-Spin Asymmetries in Hard-Scattering Processes

Using lepton-pair production in hadron-hadron collisions as an example, we explore the relation between two well-known mechanisms for single-transverse-spin asymmetries in hard processes: twist-three quark-gluon correlations when the pairs transverse momentum is large, q perpendicular >> Lambda QCD, and time-reversal-odd and transverse-momentum-dependent parton distributions when q perpendicular is much less than the pairs mass. We find that, although the two mechanisms each have their own domain of validity, they describe the same physics in the kinematic region where they overlap. This unifies the two mechanisms and imposes an important constraint on phenomenological studies.

Role of the nonperturbative input in QCD resummed Drell-Yan

We analyze the role of the nonperturbative input in the Collins-Soper-Sterman (CSS) b-space QCD resummation formalism for Drell-Yan transverse momentum (Q{sub T}) distributions, and investigate the predictive power of the CSS formalism. We find that the predictive power of the CSS formalism has a strong dependence on the collision energy S in addition to its well-known Q{sup 2} dependence, and the S dependence improves the predictive power at collider energies. We show that a reliable extrapolation from perturbatively resummed b-space distributions to the nonperturbative large b region is necessary to ensure the correct Q{sub T} distributions. By adding power corrections to the renormalization group equations in the CSS formalism, we derive a new extrapolation formalism. We demonstrate that at collider energies the CSS resummation formalism plus our extrapolation has an excellent predictive power for W and Z production at all transverse momenta Q{sub T}{<=}Q. We also show that the b-space resummed Q{sub T} distributions provide a good description of Drell-Yan data at fixed target energies.

Q_{T}

We construct two sets of twist-3 correlation functions that are responsible for generating the novel single transverse-spin asymmetry in the QCD collinear factorization approach. We derive evolution equations for these universal three-parton correlation functions. We calculate evolution kernels relevant to the gluonic pole contributions to the asymmetry at the order of {alpha}{sub s}. We find that all evolution kernels are infrared safe as they should be and have a lot in common with the Dokshitzer-Gribov-Lipatov-Alterelli-Parisi evolution kernels of unpolarized parton distributions. By solving the evolution equations, we explicitly demonstrate the factorization scale dependence of these twist-3 correlation functions.

distributions

Abstract We argue that high energy proton–nucleus (p + A) collisions provide an excellent laboratory for studying nuclear size enhanced parton multiple scattering where power corrections to the leading twist perturbative QCD factorization approach can be systematically computed. We identify and resum these corrections and calculate the centrality- and rapidity-dependent nuclear suppression of single and double inclusive hadron production at moderate transverse momenta. We demonstrate that both spectra and dihadron correlations in p + A reactions are sensitive measures of such dynamical nuclear attenuation effects.

Evolution of twist-3 multiparton correlation functions relevant to single transverse-spin asymmetry

We investigate the predictive power of the Collins, Soper, and Sterman b-space QCD resummation formalism for transverse momentum ( Q(T)) distributions of heavy boson production in hadronic collisions. We show that the predictive power has a strong dependence on the collision energy sqrt[S] in addition to its well known Q2 dependence, and the sqrt[S] dependence improves the predictive power at collider energies. We demonstrate that, at the Fermilab Tevatron and the CERN LHC energies, the Q(T) distributions derived from b-space resummation are not sensitive to the nonperturbative input at large b, and give good descriptions of the Q(T) distributions of heavy boson production at all transverse momenta Q(T)< or =Q.