arXiv subjects

Xiaofei Zhang

Publications and source records attributed to Xiaofei Zhang.

At least 55 records · Page 3Linked to original sources

arXiv2004

The charged-hadron/pion ratio at the Relativistic Heavy Ion Collider

The hadron/pion ratio is calculated in 200 GeV AuAu collisions at midrapidity, applying pQCD and non-universal transverse-momentum broadening. Arguments are presented for such non-universality, and the idea is implemented in a model, which explains the enhancement of the hadron/pion ratio in central AuAu collisions. The model also describes the qualitative difference between the recently-measured dAu nuclear enhancement factors for pions and charged hadrons.

hep-ph↗

arXiv2004

Probing small-x gluons by low-mass Drell-Yan pairs at RHIC and LHC

We calculate the transverse momentum distribution of low-mass Drell-Yan pairs in QCD perturbation theory with all-order resummation. We demonstrate that the transverse momentum distribution of low-mass Drell-Yan pairs is an advantageous source of constraints on the gluon distribution and its nuclear dependence. With the reduction in background, we argue that low-mass Drell-Yan pairs in the forward region provide a good and clean probe of small-x gluons at RHIC and LHC.

hep-ph↗

arXiv2003

Full transverse-momentum spectra of low-mass Drell-Yan pairs at LHC energies

The transverse momentum distribution of low-mass Drell-Yan pairs is calculated in QCD perturbation theory with all-order resummation. We argue that at LHC energies the results should be reliable for the entire transverse momentum range. We demonstrate that the transverse momentum distribution of low-mass Drell-Yan pairs is an advantageous source of constraints on the gluon distribution and its nuclear dependence.

hep-ph↗

arXiv2002

$Z^0$ and W transverse momentum spectra at the LHC

We discuss the transverse momentum distribution of $Z^0$ and $W^\pm$ gauge bosons at the LHC with $\sqrt s=14$ TeV for pp collisions and with $\sqrt s=5.5$ TeV for Pb+Pb collisions. Effects of power corrections and shadowing are also studied.

hep-ph↗

arXiv2002

Z Production as a Test of Nuclear Effects at the LHC

We predict the Z transverse momentum distribution from proton-proton and nuclear collisions at the LHC. After demonstrating that higher-twist nuclear effects are very small, we propose $Z^0$ production as a precision test for leading-twist pQCD in the TeV energy region. We also point out that shadowing may result in unexpected phenomenology at the LHC.

hep-ph↗

arXiv2002

Role of nonperturbative input in QCD resummed heavy boson $Q_T$ distribution

We show that role of nonperturbative input in the $b$-space QCD resummation formalism for heavy boson transverse momentum ($Q_T$) distribution strongly depends on collision energy $\sqrt{S}$. At collider energies, the larger $\sqrt{S}$ is, the weaker role nonperturbative input plays, and better predictive power the $b$-space resummation formalism has.

hep-ph↗

arXiv2002

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.

hep-ph↗

arXiv2002

Nuclear Effects on Heavy Boson Production at RHIC and LHC

We predict W and Z transverse momentum distributions from proton-proton and nuclear collisions at RHIC and LHC. A resummation formalism with power corrections to the renormalization group equations is used. The dependence of the resummed QCD results on the non-perturbative input is very weak for the systems considered. Shadowing effects are discussed and found to be unimportant at RHIC, but important for LHC. We study the enhancement of power corrections due to multiple scattering in nuclear collisions and numerically illustrate the weak effects of the dependence on the nuclear mass.

hep-ph↗

arXiv2001

Strong nuclear enhancement in intermediate mass Drell-Yan production

We calculate nuclear effect in Drell-Yan massive lepton-pair production in terms of parton multiple scattering in Quantum Chromodynamics (QCD). We present the nuclear modification to inclusive Drell-Yan cross section $dσ/dQ^2$ in terms of multiparton correlation functions. By extracting the size of the correlation functions from measured Drell-Yan transverse momentum broadening in nuclear medium, we determine the nuclear modification at $O(α_s/Q^2)$. We find that the nuclear modification strongly enhances the inclusive Drell-Yan cross section in the intermediate mass region (IMR): $1.5 < Q < 2.5$ GeV. We argue that the Drell-Yan process is responsible for the majority of the observed dimuon enhancement in the IMR in the Pb-Pb collisions at CERN SPS.

hep-ph↗

arXiv2001

QCD Factorized Drell-Yan Cross Section at Large Transverse Momentum

We derive a new factorization formula in perturbative quantum chromodynamics for the Drell-Yan massive lepton-pair cross section as a function of the transverse momentum $Q_T$ of the pair. When $Q_T$ is much larger than the pair's invariant mass $Q$, this factorization formula systematically resums the logarithmic contributions of the type $α_s^m \ln^m(Q_T^2/Q^2)$ to all orders in the strong coupling $α_s$. When $Q_T\sim Q$, our formula yields the same Drell-Yan cross section as conventional fixed order QCD perturbation theory. We show that resummation is important when the collision energy $\sqrt{S}$ is large enough and $Q_T\gg Q$, and we argue that perturbative expansions are more stable and reliable in terms of the modified factorization formula.

hep-ph↗

arXiv2001

J/$ψ$ production and suppression in nuclear collisions

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.

nucl-th↗

arXiv2001

Role of the nonperturbative input in QCD resummed Drell-Yan $Q_T$-distributions

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.

hep-ph↗

arXiv2001

Drell-Yan Massive Lepton-Pair's Angular Distributions at Large $Q_T$

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.

hep-ph↗

arXiv2001

Virtual photon fragmentation functions

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.

hep-ph↗

arXiv2000

QCD prediction for heavy boson transverse momentum distributions

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 $Q_T \leq Q$.

hep-ph↗

arXiv2000

$x_F$ Dependence of Drell-Yan Transverse Moemtum Broadening

We analyze $x_F$ dependence of Drell-Yan transverse momentum broadening in hadron-nucleus collisions. In terms of generalized factorization theorem, we show that the $x_F$ dependence of transverse momentum broadening, $ Δ< q_T^2 > (x_F) $, can be calculated in perturbative QCD. We demonstrate that $Δ< q_T^2 > (x_F)$ is a good observable for studying the effects of initial-state multiple scattering and extracting quark-gluon correlation functions.

hep-ph↗

arXiv2000

Nuclear dependence coefficient $α(A,q_T)$ for the Drell-Yan and J/$ψ$ production

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 $q_T$.

hep-ph↗

arXiv2000

Universality in nuclear dependence coefficient $α(q_T)$

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.

hep-ph↗