arXiv subjects
Publications and source records attributed to Jianwei Qiu.
In terms of a new QCD factorization formula for J/$ψ$ production, we calculate the J/$ψ$ suppression in nuclear collisions by including the multiple scattering between the pre-J/$ψ$ partonic states and the nuclear medium. We find agreement with all data on J/$ψ$ suppression in hadron-nucleus and nucleus-nucleus collisions, except a couple of points (the ``second drop'') at the highest $E_T$ bins of the new NA50 data.
We analyze the role of the nonperturbative input in the Collins, Soper, and Sterman (CSS)'s $b$-space QCD resummation formalism for Drell-Yan transverse momentum ($Q_T$) 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 $\sqrt{S}$ in addition to its well-known $Q^2$ dependence, and the $\sqrt{S}$ 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 $Q_T$ 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 $Q_T\le Q$. We also show that the $b$-space resummed $Q_T$ distributions provide a good description of Drell-Yan data at fixed target energies.
By measuring Drell-Yan massive lepton-pair's angular distributions, we can identify the polarization of the virtual photon of invariant mass $Q$ which decays immediately into the lepton-pair. In terms of a modified QCD factorization formula for Drell-Yan process, which is valid even if $Q_T\gg Q$, we calculate the massive lepton-pair's angular distributions at large $Q_T$. We find that the virtual photons produced at high $Q_T$ are more likely to be transversely polarized. We discuss the implications of this finding to the J/$ψ$ mesons' polarization measured recently at Fermilab.
We introduce operator definitions for virtual photon fragmentation functions, which are needed for reliable calculations of Drell-Yan transverse momentum ($Q_T$) distributions when $Q_T$ is much larger than the invariant mass $Q$. We derive the evolution equations for these fragmentation functions. We calculate the leading order evolution kernels for partons to fragment into a unpolarized as well as a polarized virtual photon. We find that fragmentation functions to a longitudinally polarized virtual photon are most important at small $z$, and the fragmentation functions to a transversely polarized virtual photon dominate the large $z$ region. We discuss the implications of this finding to the J/$ψ$ mesons' polarization at large transverse momentum.
We investigate the predictive power of Collins, Soper, and Sterman's $b$-space QCD resummation formalism for transverse momentum ($Q_T$) distributions of heavy boson production in hadronic collisions. We show that the predictive power of the resummation formalism has a strong dependence on the collision energy $\sqrt{S}$ in addition to its well-known $Q^2$ dependence, and the $\sqrt{S}$ dependence improves the predictive power at collider energies. We demonstrate that at Tevatron and the 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 $Q_T \leq Q$.
Define the nuclear dependence coefficient $α(A,q_T)$ in terms of ratio of transverse momentum spectrum in hadron-nucleus and in hadron-nucleon collisions: $\frac{dσ^{hA}}{dq_T^2}/ \frac{dσ^{hN}}{dq_T^2}\equiv A^{α(A,q_T)}$. We argue that in small $q_T$ region, the $α(A,q_T)$ for the Drell-Yan and J/$ψ$ production is given by a universal function:\ $a+b q_T^2$, where parameters a and b are completely determined by either calculable quantities or independently measurable physical observables. We demonstrate that this universal function $α(A,q_T)$ is insensitive to the A for normal nuclear targets. For a color deconfined nuclear medium, the $α(A,q_T)$ becomes strongly dependent on the A. We also show that our $α(A,q_T)$ for the Drell-Yan process is naturally linked to perturbatively calculated $α(A,q_T)$ at large $q_T$ without any free parameters, and the $α(A,q_T)$ is consistent with E772 data for all $q_T$.
We derive the nuclear dependence coefficient $α(q_T)$ for Drell-Yan and J/$ψ$ production. We show that at small $q_T$, the $α(q_T)$ is given by an universal functional form: $α(q_T)=a+b q_T^2$, and the parameters $a$ and $b$ are completely determined by either perturbatively calculable or independently measurable quantities. This universal functional form $α(q_T)$ is insensitive to the $A$, and is consistent with existing data.
We review applicability of QCD factorization theorem to multiple scattering in deeply inelastic lepton-nucleus scattering. We show why A^{1/3}-type nuclear enhancement can be calculated consistently in perturbative QCD. We derive the transverse momentum broadening of the leading pions in deeply inelastic lepton-nucleus collisions. We argue that the measurement of such transverse momentum broadening can provide direct information on the multiple-parton correlation functions inside a nucleus. We also estimate the numerical values of the broadening at different values of x_B and Q^2.
We show that when Bjorken $x_B$ is large, the leading order twist-four contributions to the structure functions can be expressed in terms of the derivatives of the normal twist-two parton distributions. Our analytical results are not only consistent with the parameterized power corrections extracted from the data, but also predict the flavor dependence of the power corrections. We also discuss the impact of our analytical results on the extraction and the flavor separation of the parton distributions at large $x$.
We propose a model for the production and suppression of J/ψmesons in high-energy hadronic collisions. We factorized the process into a production of the $c\bar{c}$ pairs of relative momentum $k=k_c-k_{\bar{c}}$ convoluted with a transition probability from the produced $c\bar{c}$ pairs into the observed J/$ψ$ mesons. As the produced $c\bar{c}$ pairs exit the nuclear matter, the multiple scattering between the colored $c$ and $\bar{c}$ and the nuclear medium increases the square of the relative momentum between the $c$ and $\bar{c}, q^2=-k^2$, such that some of the $c\bar{c}$ pairs gain enough invariant mass to transmute into open charm states. With only one parameter, the amount of energy gained by the produced $c\bar{c}$ pair per unit length in the nuclear medium, our model can fit all observed J/ψsuppression data including recent NA50 data from Pb-Pb collisions.
In this talk, I explore the possibilities of probing the multiparton correlation functions at CEBAF at its current energy and the energies with its future upgrades.
We analyze single transverse-spin asymmetries for hadronic pion production at large transverse momenta using QCD factorization. In the large $x_F$ region, leading contributions to the asymmetries are naturally produced by twist-3 parton correlation functions that couple quark fields and gluon field strengths. With a simple model for these matrix elements, leading-order asymmetries calculated from QCD are consistent with data on pion production from Fermilab, and can be used to predict single-spin asymmetries at RHIC. We argue that our perturbative calculation for the asymmetries is relevant to pion transverse momenta as low as a few GeV.
We show that the conventionally defined partonic cross section for the production of isolated prompt photons is not an infrared safe quantity. We work out the case of $e^+e^- \to γ+ X$ in detail, and we discuss implications for hadron reactions such as $p \bar{p} \to γ+ X$.
We show that the conventionally defined partonic cross section for the production of isolated prompt photons is not an infrared safe quantity. We work out the case of $e^+e^-\toγ+ X$, and discuss implications for hadron reactions.
We review the extension of the factorization formalism for perturbative QCD to soft initial- and final-state scattering associated with hard processes in nuclei.
We provide complete analytic expressions for the isolated prompt photon production cross section in $e^+e^-$ annihilation reactions through one-loop order in quantum chromodynamics (QCD) perturbation theory. Functional dependences on the isolation cone size $δ$ and isolation energy parameter $ε$ are derived. The energy dependence as well as the full angular dependence of the cross section on $θ_γ$ are displayed, where $θ_γ$ specifies the direction of the photon with respect to the $e^+e^-$ collision axis. We point out that conventional perturbative QCD factorization breaks down for isolated photon production in $e^+e^-$ annihilation reactions in a specific region of phase space. We discuss the implications of this breakdown for the extraction of fragmentation functions from $e^+e^-$ annihilation data and for computations of prompt photon production in hadron-hadron reactions.
Using $e^+e^-\rightarrowγ+ X$ as an example, we show that the conventional factorization theorem in perturbative quantum chromodynamics breaks down for isolated photon cross sections in a well defined part of phase space. Implications and physical consequences are discussed.
We calculate the nuclear dependence of direct photon production in hadron-nucleus collisions. In terms of a multiple scattering picture, we factorize the cross section for direct photon production into calculable short-distance partonic parts times multiparton correlation functions in nuclei. We present the hadron-nucleus cross section as $A^α$ times the hadron-nucleon cross section. Using information on the multiparton correlation functions extracted from photon-nucleus experiments, we compute the value of $α$ as a function of transverse momentum of the direct photon. We also compare our results with recent data from Fermilab experiment E706.