arXiv subjects

Zhong-Bo Kang

Publications and source records attributed to Zhong-Bo Kang.

At least 127 records · Page 7Linked to original sources

arXiv2016

Jet Quenching from QCD Evolution

Recent advances in soft-collinear effective theory with Glauber gluons have led to the development of a new method that gives a unified description of inclusive hadron production in reactions with nucleons and heavy nuclei. We show how this approach, based on the generalization of the DGLAP evolution equations to include final-state medium-induced parton showers, can be combined with initial-state effects for applications to jet quenching phenomenology. We demonstrate that the traditional parton energy loss calculations can be regarded as a special soft-gluon emission limit of the general QCD evolution framework. We present phenomenological comparison of the SCET$_{\rm G}$-based results on the suppression of inclusive charged hadron and neutral pion production in $\sqrt{s_{NN}}=2.76$ TeV lead-lead collisions at the Large Hadron Collider to experimental data. We also show theoretical predictions for the upcoming $\sqrt{s_{NN}} \simeq 5.1$ TeV Pb+Pb run at the LHC.

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arXiv2016

Spin asymmetries for vector boson production in polarized p+p collisions

We study the cross section for vector boson ($W^{\pm}/Z^0/γ^*$) production in polarized nucleon-nucleon collisions for low transverse momentum of the observed vector boson. For the case where one measures the transverse momentum and azimuthal angle of the vector bosons, we present the cross sections and the associated spin asymmetries in terms of transverse momentum dependent parton distribution functions (TMDs) at tree level within the TMD factorization formalism. To assess the feasibility of experimental measurements, we estimate the spin asymmetries for $W^{\pm}/Z^0$ boson production in polarized proton-proton collisions at the Relativistic Heavy Ion Collider (RHIC) by using current knowledge of the relevant TMDs. We find that some of these asymmetries can be sizable if the suppression effect from TMD evolution is not too strong. The $W$ program at RHIC can, thus, test and constrain spin theory by providing unique information on the universality properties of TMDs, TMD evolution, and the nucleon structure. For example, the single transverse spin asymmetries could be used to probe the well-known Sivers function $f_{1T}^{\perp q}$, as well as the transversal helicity distribution $g_{1T}^{q}$ via the parity-violating nature of $W$ production.

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arXiv2015

Next-to-leading order transverse momentum-weighted Sivers asymmetry in semi-inclusive deep inelastic scattering: the role of the three-gluon correlator

We study the Sivers asymmetry in semi-inclusive hadron production in deep inelastic scattering. We concentrate on the contribution from the photon-gluon fusion channel at ${\cal O}(α_{em}^2 α_s)$, where three-gluon correlation functions play a major role within the twist-3 collinear factorization formalism. We establish the correspondence between such a formalism with three gluon correlation functions and the usual transverse momentum dependent (TMD) factorization formalism at moderate hadron transverse momenta. We derive the coefficient functions used in the usual TMD evolution formalism related to the quark Sivers function expansion in terms of the three-gluon correlation functions. We further perform the next-to-leading order calculation for the transverse-momentum-weighted spin-dependent differential cross section, and identify the off-diagonal contribution from the three-gluon correlation functions to the QCD collinear evolution of the twist-3 Qiu-Sterman function.

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arXiv2015

A Study of Quasi-parton Distribution Functions in the Diquark Spectator Model

To facilitate lattice QCD calculations of nucleon structute, a set of quasi-parton distributions were recently introduced. These quasi-PDFs were shown to reduce to standard PDFs when the nucleon is boosted to high energies, $P_z\rightarrow \infty$. Since taking such limit is not feasible in lattice simulations, it is essential to provide guidance for what values of $P_z$ the quasi-PDFs are good approximations of standard PDFs. Within the framework of the spectator diquark model, we evaluate both the up and down quarks' quasi-PDFs and standard PDFs for all leading-twist distributions (unpolarized distribution $f_1$, helicity distribution $g_1$, and transversity distribution $h_1$). We find that, for intermediate parton momentum fractions $x$, quasi-PDFs are good approximations to standard PDFs (within $20-30\%$) when $P_z\gtrsim 1.5-2$ GeV. On the other hand, for large $x\sim 1$ much larger $P_z > 4$ GeV is necessary to obtain a satisfactory agreement between the two sets. We further find that the Soffer positivity bound does not hold in general for quasi-PDFs.

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arXiv2015

Quenching of inclusive and tagged b-jets at the LHC

We present theoretical predictions for the nuclear-induced attenuation of the differential cross sections for inclusive and tagged b-jet production in heavy ion collisions at the LHC. We find that for inclusive b-jet production at high transverse momentum the mass effects are negligible, and that the attenuation is comparable to the one observed for light jets. On the other hand, for isolated-photon and B-meson-tagged b-jets the sample of events with heavy quarks produced at the early stages of the collision is greatly enhanced. Thus, these tagged b-jet final-states have a much more direct connection to the physics of b-quark energy loss. We present theoretical predictions for the quenching of such tagged b-jet events at the LHC and the QGP-induced modification of the related momentum imbalance and asymmetry. We demonstrate that these tagged processes can be used to accurately study the physics of heavy quark production and propagation in dense QCD matter.

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arXiv2015

Hadronic production of $W$ and $Z$ bosons at large transverse momentum

We introduce a modified factorization formalism in quantum chromodynamics for hadronic production of $W$ and $Z$ bosons at large transverse momentum $p_T$. When $p_T$ is much larger than the invariant mass $Q$ of the vector boson, this new factorization formalism systematically resums the large fragmentation logarithms, $α_s^m\ln^m(p_T^2/Q^2)$, to all orders in the strong coupling $α_s$. Using our modified factorization formalism, we calculate the next-to-leading order (NLO) predictions for $W$ and $Z$ boson production at high $p_T$ at the CERN Large Hadron Collider and at a future 100 TeV proton-proton collider. Our NLO results are about $5\%$ larger in normalization, and they show improved convergence and moderate reduction of the scale variation compared to the NLO predictions derived in a conventional fixed-order perturbative expansion.

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arXiv2015

Extraction of Quark Transversity Distribution and Collins Fragmentation Functions with QCD Evolution

We study the transverse momentum dependent (TMD) evolution of the Collins azimuthal asymmetries in $e^+e^-$ annihilations and semi-inclusive hadron production in deep inelastic scattering (SIDIS) processes. All the relevant coefficients are calculated up to the next-to-leading logarithmic (NLL) order accuracy. By applying the TMD evolution at the approximate NLL order in the Collins-Soper-Sterman (CSS) formalism, we extract transversity distributions for $u$ and $d$ quarks and Collins fragmentation functions from current experimental data by a global analysis of the Collins asymmetries in back-to-back di-hadron productions in $e^+e^-$ annihilations measured by BELLE and BABAR Collaborations and SIDIS data from HERMES, COMPASS, and JLab HALL A experiments. The impact of the evolution effects and the relevant theoretical uncertainties are discussed. We further discuss the TMD interpretation for our results, and illustrate the unpolarized quark distribution, transversity distribution, unpolarized quark fragmentation and Collins fragmentation functions depending on the transverse momentum and the hard momentum scale. We make detailed predictions for future experiments and discuss their impact.

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arXiv2015

Nucleon tensor charge from Collins azimuthal asymmetry measurements

We investigate the nucleon tensor charge from current experiments by a combined analysis of the Collins asymmetries in two hadron production in $e^+e^-$ annihilations and semi-inclusive hadron production in deep inelastic scattering processes. The transverse momentum dependent evolution is taken into account, for the first time, in the global fit of the Collins fragmentation functions and the quark transversity distributions at the approximate next-to-leading logarithmic order. We obtain the nucleon tensor charge contribution from up and down quarks as $δu=+0.30^{+0.12}_{-0.08}$ and $δd=-0.20_{-0.11}^{+0.28}$ at 90\% confidence level for momentum fraction $0.0065 \le x_B \le 0.35$ and $Q^2=$ 10 GeV$^2$.

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arXiv2015

The 1-Jettiness DIS Spectrum: Factorization, Resummation, and Jet Algorithm Dependence

The 1-Jettiness (tau_1) event shape for Deep Inelastic Scattering (DIS), allows for a quantitative and global description of the pattern of QCD radiation for single jet (J) production in electron-nucleus (N_A) collisions e^- + N_A \to e^- + J + X. It allows for precision studies of QCD and is a sensitive probe of nuclear structure and dynamics. The large transverse momentum (P_{J_T}) of the final state jet $J$, characterizes the hard scale in the problem. The region of phase space where tau_1 << P_{J_T}, corresponds to configurations where energetic radiation (E~ P_{J_T}) is only along either the single jet direction or the beam direction with only soft radiation (E ~ τ_1 <<P_{J_T}) in between. Thus, the restriction tau_1 << P_{J_T} corresponds to a veto on additional jets and leads to large Sudakov logarithms of τ_1/P_{J_T} that must be resummed. Based on a factorization framework, derived using the Soft Collinear Effective Theory (SCET), we provide resummation results at the NNLL level of accuracy and match them onto the NLO result in fixed order perturbation theory, appropriate in the τ_1 \sim P_{J_T} region where additional jets and hard radiation are allowed. The τ_1-distribution depends on the jet algorithm used to find the leading jet in the region tau_1 ~ P_{J_T}, unlike the resummation region where this dependence is power suppressed in tau_1/P_{J_T} << 1. We give results for the entire tau_1 spectrum, with a smooth matching between the resummation region and fixed-order region, where we make use of the anti-kt jet algorithm. The 1-Jettiness event shape can be a powerful probe of nuclear and QCD dynamics at future electron-ion colliders and by analyzing existing HERA data.

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arXiv2015

Quasi-parton distribution functions: a study in the diquark spectator model

A set of quasi-parton distribution functions (quasi-PDFs) have been recently proposed by Ji. Defined as the matrix elements of equal-time spatial correlations, they can be computed on the lattice and should reduce to the standard PDFs when the proton momentum $P_z$ is very large. Since taking the $P_z\to \infty$ limit is not feasible in lattice simulations, it is essential to provide guidance for what values of $P_z$ the quasi-PDFs are good approximations of standard PDFs. Within the framework of the spectator diquark model, we evaluate both the up and down quarks' quasi-PDFs and standard PDFs for all leading-twist distributions (unpolarized distribution $f_1$, helicity distribution $g_1$, and transversity distribution $h_1$). We find that, for intermediate parton momentum fractions $x$, quasi-PDFs are good approximations to standard PDFs (within $20-30%$) when $P_z\gtrsim 1.5-2$ GeV. On the other hand, for large $x\sim 1$ much larger $P_z > 4$ GeV is necessary to obtain a satisfactory agreement between the two sets. We further test the Soffer positivity bound, and find that it does not hold in general for quasi-PDFs.

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arXiv2014

Multiple scattering effects on heavy meson production in p+A collisions at backward rapidity

We study the incoherent multiple scattering effects on heavy meson production in the backward rapidity region of p+A collisions within the generalized high-twist factorization formalism. We calculate explicitly the double scattering contributions to the heavy meson differential cross sections by taking into account both initial-state and final-state interactions, and find that these corrections are positive. We further evaluate the nuclear modification factor for muons that come form the semi-leptonic decays of heavy flavor mesons. Phenomenological applications in d+Au collisions at a center-of-mass energy $\sqrt{s}=200$ GeV at RHIC and in p+Pb collisions at $\sqrt{s}=5.02$ TeV at the LHC are presented. We find that incoherent multiple scattering can describe rather well the observed nuclear enhancement in the intermediate $p_T$ region for such reactions.

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arXiv2014

Sivers Asymmetry with QCD Evolution

We analyze the Sivers asymmetry in both Drell-Yan (DY) production and semi-inclusive deep inelastic scattering (SIDIS), while considering properly defined transverse momentum dependent parton distribution and fragmentation functions and their QCD evolution. After finding a universal non-perturbative spin-independent Sudakov factor that can describe reasonably well the world's data of SIDIS, DY lepton pair and $W/Z$ production in unpolarized scatterings, we perform a global fitting of all the experimental data on the Sivers asymmetry in SIDIS from HERMES, COMPASS and Jefferson Lab. Then we make predictions for the asymmetry in DY lepton pair and $W$ boson production, which could be compared to the future experimental data in order to test the sign change of the Sivers function.

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arXiv2014

Heavy Quarkonium Production at Collider Energies: Factorization and Evolution

We present a factorization formalism for inclusive production of heavy quarkonia of large transverse momentum, $p_T$ at collider energies, including both leading power (LP) and next-to-leading power (NLP) behavior in $p_T$. We demonstrate that both LP and NLP contributions can be factorized in terms of perturbatively calculable short-distance partonic coefficient functions and universal non-perturbative fragmentation functions, and derive the evolution equations that are implied by the factorization. We identify projection operators for all channels of the factorized LP and NLP infrared safe short-distance partonic hard parts, and corresponding operator definitions of fragmentation functions. For the NLP, we focus on the contributions involving the production of a heavy quark pair, a necessary condition for producing a heavy quarkonium. We evaluate the first non-trivial order of evolution kernels for all relevant fragmentation functions, and discuss the role of NLP contributions.

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arXiv2014

Heavy Quarkonium Production at Collider Energies: Partonic Cross Section and Polarization

We calculate the ${\cal O}(α_s^3)$ short-distance, QCD collinear-factorized coefficient functions for all partonic channels that include the production of a heavy quark pair at short distances. This provides the first power correction to the collinear-factorized inclusive hadronic production of heavy quarkonia at large transverse momentum, $p_T$, including the full leading-order perturbative contributions to the production of heavy quark pairs in all color and spin states employed in NRQCD treatments of this process. We discuss the role of the first power correction in the production rates and the polarizations of heavy quarkonia in high energy hadronic collisions. The consistency of QCD collinear factorization and non-relativistic QCD factorization applied to heavy quarkonium production is also discussed.

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arXiv2014

Pre-Town Meeting on Spin Physics at an Electron-Ion Collider

A polarized $ep/eA$ collider (Electron--Ion Collider, or EIC), with polarized proton and light-ion beams and unpolarized heavy-ion beams with a variable center--of--mass energy $\sqrt{s} \sim 20$ to $\sim100$~GeV (upgradable to $\sim 150$ GeV) and a luminosity up to $\sim 10^{34} \, \textrm{cm}^{-2} \textrm{s}^{-1}$, would be uniquely suited to address several outstanding questions of Quantum Chromodynamics, and thereby lead to new qualitative and quantitative information on the microscopic structure of hadrons and nuclei. During this meeting at Jefferson Lab we addressed recent theoretical and experimental developments in the spin and the three--dimensional structure of the nucleon (sea quark and gluon spatial distributions, orbital motion, polarization, and their correlations). This mini--review contains a short update on progress in these areas since the EIC White paper~\cite{Accardi:2012qut}.

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arXiv2014

Left-right spin asymmetry in $\ell N^\uparrow \to h X$

We consider the inclusive production of hadrons in lepton-nucleon scattering. For a transversely polarized nucleon this reaction shows a left-right azimuthal asymmetry, which we compute in twist-3 collinear factorization at leading order in perturbation theory. All non-perturbative parton correlators of the calculation are fixed through information from other hard processes. Our results for the left-right asymmetry agree in sign with recent data for charged pion production from the HERMES Collaboration and from Jefferson Lab. However, the magnitude of the computed asymmetries tends to be larger than the data. Potential reasons for this outcome are identified. We also give predictions for future experiments and highlight in particular the unique opportunities at an Electron Ion Collider.

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arXiv2014

Transverse momentum broadening at NLO and QCD evolution of $\hat q$

Within the framework of a high-twist approach, we show the first complete next-to-leading order calculation of transverse momentum broadening in semi-inclusive deeply inelastic $e+A$ scattering and Drell-Yan dilepton production in $p+A$ collisions. We demonstrate at one-loop level how QCD factorization holds for multiple scattering in nuclear medium, and the university of the associated quark-gluon correlation function. Our calculation also identifies QCD evolution equation for the quark-gluon correlation function, which determines the QCD factorization scale and energy dependence of jet transport parameter $\hat q$.

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arXiv2014

Probing nuclear dynamics in jet production with a global event shape

We study single jet production in electron-nucleus collisions e^- + N_A -> J + X, using the 1-jettiness (τ_1) global event shape. It inclusively quantifies the pattern of radiation in the final state, gives enhanced sensitivity to soft radiation at wide angles from the nuclear beam and final-state jet, and facilitates the resummation of large Sudakov logarithms associated with the veto on additional jets. Through their effect on the observed pattern of radiation, 1-jettiness can be a useful probe of nuclear PDFs and power corrections from dynamical effects in the nuclear medium. This formalism allows for the standard jet shape analysis while simultaneously providing sensitivity to soft radiation at wide angles from the jet. We use a factorization framework for cross-sections differential in $τ_1$ and the transverse momentum (P_{J_T}) and rapidity (y) of the jet, in the region τ_1<< P_{J_T}. The restriction $τ_1 << P_{J_T}$ allows only soft radiation between the nuclear beam and jet directions, thereby acting as a veto on additional jets. This region is also insensitive to the details of the jet algorithm, allowing for better theoretical control over resummation, while providing enhanced sensitivity to nuclear medium effects. We give numerical results at leading twist, with resummation at the next-to-next-to-leading logarithmic (NNLL) level of accuracy, for a variety of nuclear targets. Such studies would be ideal for the EIC and the LHeC proposals for a future electron-ion collider, where a range of nuclear targets are planned.

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