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Z. G. Si

Publications and source records attributed to Z. G. Si.

At least 19 recordsLinked to original sources
arXiv2015

Studying color connection effects of $e^+e^- \to c\bar{c}c\bar{c} \to Ξ_{cc}+X $ process within Quark Combination Model

The color connection of the parton system is an important bridge to connect the perturbative process and the hadronization one. The special color connection of four-heavy-quark system in $e^+e^-$ annihilation, which is the necessary one for the doubly heavy baryon and tetraquark productions is revisited. The hadronization effects, investigated with the help of the Quark Combination Model are compared with the corresponding results employing the Lund String Model in our previous work. The global properties related to a certain color connection are not sensitive to various hadronization models.

hep-ph↗
arXiv2007

Doubly Heavy Baryon Production at γγCollider

The inclusive production of doubly heavy baryons $Ξ_{cc}$ and $Ξ_{bb}$ at $γγ$ collider is investigated. It is found that the contribution from the heavy quark pair $QQ$ in color triplet and color sextet are important.

hep-ph↗
arXiv2006

NRQCD Factorization for Twist-2 Light-Cone Wave-Functions of Charmonia

We show that the twist-2 light-cone wave-functions of eta_c and J/ψcan be factorized with nonrelativistic QCD(NRQCD) at one-loop level, where the nonperturbative effects are represented by NRQCD matrix elements. The factorization is achieved by expanding the small velocity v, which the c- or \bar c- quark moves inside a rest quarkonium with. At leading order of v the twist-2 light-cone wave-functions of eta_c and J/ψcan be factorized as the product of a perturbative function and a NRQCD matrix element. The perturbative function is calculated at one-loop level and free from any soft divergence. Our results can be used for the production of J/ψη_c through e^+e^- -annihilation and of a charmonium in B-decays, which are studied in experiment of two B-factories.

hep-ph↗
arXiv2005

Mixed QCD and weak corrections to top quark pair production at hadron colliders

The order $α_s^2 α$ mixed QCD and weak corrections to top quark pair production by quark antiquark annihilation are computed, keeping the full dependence on the $t$ and $\bar t$ spins. We determine the contributions to the cross section and to single and double top spin asymmetries at the parton level. These results are necessary ingredients for precise standard model predictions of top quark observables, in particular of top spin-induced parity-violating angular correlations and asymmetries at hadron colliders.

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arXiv2005

NRQCD Factorization and Universality of NRQCD Matrix Elements

The approach of nonrelativistic QCD(NRQCD) factorization was proposed to study inclusive production of a quarkonium. It is widely used and successful. However, a recent study of gluon fragmentation into a quarkonium at two-loop level shows that the factorization is broken. It is suggested that the color-octet NRQCD matrix elements should be modified by adding a gauge link to restore the factorization. The modified matrix elements may have extra soft-divergences at one-loop level which the unmodified can not have, and this can lead to a violation of the universality of these matrix elements. In this letter, we examine in detail the NRQCD factorization for inclusive quarkonium production in $e^+ e^-$ annihilation at one-loop level. Our results show that the factorization can be made without the modification of NRQCD matrix elements and it can also be made for relativistic corrections. It turns out that the suggested gauge link will not lead to nonzero contributions to color-octet NRQCD matrix elements at one-loop level and at any order of $v$. Therefore the universality holds at least at one-loop level.

hep-ph↗
arXiv2005

Top quark pair production and decay at a polarized photon collider

Top quark pair production by (polarized) gamma gamma collisions offers an interesting testing ground of the Standard Model and its extensions. In this Letter we present results for differential cross sections of top quark pair production and decay including QCD radiative corrections. We take into account the full dependence on the top quark spins. We give analytic and numerical results for single and double differential angular distributions of t tbar decay products which are due to top quark polarizations and spin correlations in the intermediate state.

hep-ph↗
arXiv2004

Predictions for $e^+e^-\to J/ψη_c$ with Light-Cone Wave-Functions

Predictions for $e^+e^-\to J/ψη_c$ from previous studies are made by taking charmonia as a nonrelativistic bound state and by using nonrelativistic QCD(NRQCD) approach. The predicted cross-section is smaller by an order of magnitude than the experimentally observed. We study the process by taking charm quark as a light quark and use light-cone wave-functions to parameterize nonperturbative effects related to charmonia. The total cross section of $e^+e^-\to J/ψη_c$ can be predicted, if these wave-functions are known. Motivated by studies of light-cone wave-functions of light hadrons, we make a reasonable assumption of the forms of light-cone wave-functions. With these light-cone wave-functions we can obtain the cross section which is more closer to the experimentally observed than that from NRQCD approach. We also discuss in detail the difference between two approaches.

hep-ph↗
arXiv2004

Les Houches 'Physics at Tev Colliders 2003' QCD/SM Working Group: Summary Report

This report documents the results obtained by the Working Group on Quantum Chromodynamics and the Standard Model for the Workshop `Physics at TeV Colliders'', Les Houches, France, 26 May - 6 June 2003. After a Monte Guide description, the first contributions report on progress in describing multiple interactions, important for the LHC, and underlying events. An announcement of a Monte Carlo database, under construction, is then followed by a number of contributions improving parton shower descriptions. Subsequently, a large number of contributions address resummations in various forms, after which follow studies of QCD effects in pion pair, top quark pair and photon pair plus jet production. After a study of electroweak corrections to hadronic precision observables, the report ends by presenting recent progress in methods to compute finite order corrections at one-loop with many legs, and at two-loop.

hep-ph↗
arXiv2004

Top quark pair production and decay at hadron colliders

In ongoing and upcoming hadron collider experiments, top quark physics will play an important role in testing the Standard Model and its possible extensions. In this work we present analytic results for the differential cross sections of top quark pair production in hadronic collisions at next-to-leading order in the QCD coupling, keeping the full dependence on the spins of the top quarks. These results are combined with the corresponding next-to-leading order results for the decay of polarized top quarks into dilepton, lepton plus jets, and all jets final states. As an application we predict double differential angular distributions which are due to the QCD-induced top quark spin correlations in the intermediate state. In addition to the analytic results, we give numerical results in terms of fit functions that can easily be used in an experimental analysis.

hep-ph↗
arXiv2003

Nonperturbative Corrections with Nonlocal Operators to Lifetime Ratios of Beauty Hadrons

The motion of spectator quarks in decay of a beauty hadron is a nonperturbative effect which can usually be neglected. We find that the motion in some decay channels, which contribute total decay widths of beauty hadrons, can not be neglected.The contributions from these decay channels to decay widths are proportional to certain averages of the squared inverse of the momentum carried by a spectator quark. This fact results in that these contributions, suppressed by $1/m_b^3$ formally, are effectively suppressed by $1/m_b$. We find these contributions can be factorized into products of perturbative coefficients and nonperturbative parameters. We calculate these coefficients and define these nonperturbative parameters in terms of HQET matrix elements.Since these parameters are unknown, we are unable to give numerical predictions in detail. But with a simple model it can be shown that these contributions can be large.

hep-ph↗
arXiv2003

Factorization Approach for Inclusive Production of Doubly Heavy Baryon

We study inclusive production of doubly heavy baryon at a $e^+e^-$ collider and at hadron colliders through fragmentation. We study the production by factorizing nonpertubative- and perturbative effects. In our approach the production can be thought as a two-step process: A pair of heavy quarks can be produced perturbatively and then the pair is transformed into the baryon. The transformation is nonperturbative. Since a heavy quark moves with a small velocity in the baryon in its rest frame, we can use NRQCD to describe the transformation and perform a systematic expansion in the small velocity. At the leading order we find that the baryon can be formed from two states of the heavy-quark pair, one state is with the pair in $^3S_1$ state and in color ${\bf \bar 3}$, another is with the pair in $^1S_0$ state and in color ${\bf 6}$. Two matrix elements are defined for the transformation from the two states, their perturbative coefficients in the contribution to the cross-section at a $e^+e^-$ collider and to the function of heavy quark fragmentation are calculated. Our approach is different than previous approaches where only the pair in $^3S_1$ state and in color ${\bf \bar 3}$ is taken into account. Numerical results for $e^+e^-$ colliders at the two $B$-factories and for hadronic colliders LHC and Tevatron are given.

hep-ph↗
arXiv2003

Spin properties of top quark pairs produced at hadron colliders

We discuss the spin properties of top quark pairs produced at hadron colliders at next-to-leading order in the coupling constant alpha_s of the strong interaction. Specifically we present, for some decay channels, results for differential angular distributions that are sensitive to t tbar spin correlations.

hep-ph↗
arXiv2002

QCD-corrected spin analysing power of jets in decays of polarized top quarks

We present results for the differential distributions of jets from non-leptonic decays of polarized top quarks within the Standard Model, including QCD radiative corrections. Our work extends existing results which are only available for semileptonic top quark decays at the parton level. For $t(\uparrow)$ -> b-jet + 2 light jets we compute in particular the QCD-corrected top-spin analysing power of the b-quark jet and the least energetic light jet. The dependence of the results on the choice of the jet recombination scheme is found to be small. In addition we compute the spin analysing power of the thrust axis. Our results constitute a so far missing ingredient to analyse top quark production and subsequent non-leptonic decay at next-to-leading order in alpha_s, keeping the full information on the top quark polarization.

hep-ph↗
arXiv2001

Top Quark Pair Production and Decay including Spin Effects at Hadron Colliders: Predictions at NLO QCD

Top quark-antiquark ($t\bar t$) pairs will be produced copiously at the Tevatron collider and in huge numbers at the LHC. This will make possible detailed investigations of the properties and interactions of this quark flavor. The analysis and interpretation of future data requires precise predictions of the hadronic production of $t\bar t$ pairs and of their subsequent decays. In this talk the reactions $p {\bar p}, p p \to t{\bar t} + X \to l^+ l'^- + X$ are considered and results are presented of our calculation of the dilepton angular distribution at next-to-leading order QCD, keeping the full dependence on the spins of the intermediate $t\bar{t}$ state. The angular distribution is determined for different choices of reference axes that can be identified with the $t$ and $\bar t$ spin axes. While the QCD corrections to the leading-order distribution turn out to be small in the case of the LHC, we find them to be sizeable in the case of the Tevatron and find, moreover, the angular distribution to be sensitive to the parton content of the proton.

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