Beyond the quark model of hadrons from lattice QCD

Lattice QCD can give direct information on OZI-violating contributions to mesons. Here we explore the contributions that split flavour singlet and non-singlet meson masses. I discuss in detail the spectrum and decays for scalar mesons (i.e. including glueball effects). I also review the status of hybrid mesons and their decays.Received: 30 September 2002, Published online: 22 October 2003PACS: 12.38.Gc Lattice QCD calculations - 12.39.Mk Glueball and nonstandard multi-quark/gluon states     

Interference of Nonstandard Interactions with Standard Model in $${B^0} \to {\pi ^0}\bar vv$$, $$B_c^ - \to {D^ - }\bar vv$$Bc−→D−v¯v and $$\bar B_s^0 \to {K^0}\bar vv$$B¯s0→K0v¯v Decays

We study the contributions of nonstandard neutrino interactions (NSI) to the rare decays of pseudoscalar mesons involving neutrinos in the final state \({B^0} \to {\pi ^0}\bar vv\), \(B_c^ - \to {D^ - }\bar vv\) and \(\bar B_s^0 \to {\bar K^0}\bar vv\), It is pointed that dominant contribution comes from the interference between standard model and nonstandard interaction We predict limits on NSIs free parameter ε ^uL _ττ and compare them with experimental data. We further compare our results with perturbative QCD (pQCD) and QCD results for these reactions.     

Mapping gluonic excitations and confinement

One of the outstanding and fundamental questions in physics is the quantitative understanding of the confinement of quarks and gluons in quantum chromodynamics (QCD). Confinement is a unique feature of QCD. Exotic hybrid mesons manifest gluonic degrees of freedom and their spectroscopy will provide the crucial data needed to test assumptions in lattice QCD and phenomenology leading to confinement. Photoproduction is expected to be particularly effective in producing exotic hybrids but data using photon probes are sparse. At Jefferson Lab, plans are underway to use the coherent bremsstrahlung technique to produce a linearly polarized photon beam. A solenoid-based hermetic detector will be used to collect data on meson production and decays with statistics that will exceed the current photoproduction data in hand by several orders of magnitude after the first year of running. In order to reach the ideal photon energy of 9 GeV/c for this mapping of the exotic spectra, the energy of the Jefferson Lab electron accelerator, CEBAF, will be doubled from its current maximum of 6 GeV to 12 GeV. The physics and project are described.Received: 30 September 2002, Published online: 22 October 2003PACS: 12.39.Mk Glueball and nonstandard multi-quark/gluon states - 13.60.Le Meson productionA.R. Dzierba: Present address: Department of Physics, Indiana University, Bloomington, IN 47405 USA     

Extraction of the x-dependence of the non-perturbative QCD b-quark fragmentation distribution component

Using recent measurements of the b-quark fragmentation distribution obtained in events, registered at the Z pole, the non-perturbative QCD component of the distribution has been extracted independently of any hadronic physics modelling. This distribution depends only on the way the perturbative QCD component has been denned. When the perturbative QCD component is taken from a parton shower Monte Carlo, the non-perturbative QCD component is rather similar with those obtained from the Lund or Bowler models. When the perturbative QCD component is the result of an analytic NLL computation, the non-perturbative QCD component has to be extended in a non-physical region and thus cannot be described by any hadronic modelling. In the two examples, used to characterize these two situations, which are studied at present, it happens that the extracted non-perturbative QCD distribution has the same shape, being simply translated to higher-x values in the second approach, illustrating the ability of the analytic perturbative QCD approach to account for softer gluon radiation than with a parton shower generator.     

AdS/QCD at finite density and temperature

We review some basics of AdS/QCD following a non-standard path and list a few results from AdS/QCD or holographic QCD. The non-standard path here is to use the analogy of the way one obtains an effective model of QCD like linear sigma model and the procedure to construct an AdS/QCD model based on the AdS/CFT dictionary.     

Theoretical summary talk of QCD 2002

This is a summary of the talks on QCD, not including QCD at finite temperature or density (which are discussed elsewhere) presented at the QCD 2002 meeting held at IIT, Kanpur. I have attempted to give only an overview of the talks since the details may be found in the individual contributions.     

A Determination of Nucleon Form Factor Ratios Using Perturbative QCD

Using perturbative QCD in conjunction with CVC and standard electroweak theory, it is shown that at large Q², F₁ⁿ/F₁^P = F₂ⁿ/F₁^P = -0.69. The comparison with other results on form factor ratios from QCD sum rules and/or PQCD is carried out.     

Scalar mesons and the search for the O + + glueball

The possibility that gluonic excitations of hadronic matter or of the QCD vacuum may exist is perhaps one of the most fascinating topics in hadron spectroscopy. Glueballs are predicted by many models; in particular, present-day lattice gauge calculations require their existence. All these models agree that the lightest glueball should have scalar quantum numbers and a mass around 1.6GeV, which corresponds to the mass region where the scalar -mesons are expected. Therefore, mixing effects can complicate the search for the glueball. Experiments indeed show an overpopulation of states, for which many different interpretations exist. This reflects the complexity of the situation. New data from various experiments on scalar states give hints toward an interpretation of the scalar states. But still many questions remain.Received: 30 September 2002, Published online: 22 October 2003PACS: 12.39.Mk Glueball and nonstandard multi-quark/gluon states - 12.38.-t Quantum chromodynamics - 14.70.Dj Gluons     

Asymptotic deconfinement in high-density QCD

We discuss QCD with two light flavors at large baryon chemical potential micro. Color superconductivity leads to partial breaking of the color SU(3) group. We show that the infrared physics is governed by the gluodynamics of the remaining SU(2) group with an exponentially soft confinement scale Lambda(')(QCD) approximately Deltaexp[-a(mu)/(gDelta)], where Delta< mu is the superconducting gap, g is the strong coupling, and a = 2sqrt[2] pi/11. We estimate that, at moderate baryon densities, Lambda(')(QCD) is O(10 MeV) or smaller. The confinement radius increases exponentially with density, leading to "asymptotic deconfinement." The velocity of the SU(2) gluons is small due to the large dielectric constant of the medium.     

核子(强子)结构和性质的QCD研究

核子 (强子 )是夸克、胶子的束缚态 ,由量子色动力学 QCD描述。由于 QCD的基本特性(高能标度下的渐近自由、低能标度下色禁闭及动力学手征对称性破缺 ) ,对核子 (强子 )结构和性质的 QCD图象是标度相关的 .在高能标度下描述强子的是与探测强子结构的硬过程相联系的QCD部分子模型 .强子的夸克、胶子结构信息通过 QCD部分子求和规则得到 .QCD微扰论是适用的理论 .在低能标度时 ,必须发展 QCD非微扰途径来描述核子 (强子 )物理 .这里简要地讨论各种非微扰途径 (格点 QCD、Dyson- Schwinger方程、有效场论、QCD求和规则 )的某些结果和进展 ,并指出 QCD真空结构在描述低能标度下强子物理中担任重要角色 . The nucleon (hadron) is the bound state of guarks and gluons, which is described by the quantum chromodynamics (QCD). Due to the basic properties of QCD (the asymptotic freedom at the high energy scale, the color confinement and the dynamical chiral symmetry breaking at the low energy scale), the QCD picture for the nucleon’s (hadron’s) structure and property is scale dependent. At high energy scale, the QCD parton model, which is relative to the hard process for testing the...     

Enhanced nonperturbative effects in jet distributions

We consider the triple differential distribution d Gamma/dE(J)dm(2)(J)d Omega(J) for two-jet events at center of mass energy M, smeared over the end-point region m(2)(J)相似文献   

Cross sections and transverse single-spin asymmetries in forward neutral-pion production from proton collisions at sqrt[s]=200 GeV

Measurements of the production of forward high-energy pi(0) mesons from transversely polarized proton collisions at sqrt[s]=200 GeV are reported. The cross section is generally consistent with next-to-leading order perturbative QCD calculations. The analyzing power is small at x(F) below about 0.3, and becomes positive and large at higher x(F), similar to the trend in data at sqrt[s]< or =20 GeV. The analyzing power is in qualitative agreement with perturbative QCD model expectations. This is the first significant spin result seen for particles produced with p(T)>1 GeV/c at a polarized proton collider.     

A measurement of the QCD colour factors using event shape distributions at \sqrt{s}=14 to 189 GeV

Measurements of the QCD colour factors and and of the number of active quark flavours in the process at high energy are presented. They are based on fits of +NLLA QCD calculations to distributions of the event shape observables and measured at centre-of-mass energies from 14 GeV to 189 GeV. Hadronisation effects are approximated with power correction calculations which also depend on the QCD gauge structure. In this approach potential biases from hadronisation models are reduced. Our results for individually measured quantities obtained from and C are in good agreement with QCD based on the SU(3) symmetry group where for the energies considered here, and . From simultaneous fits of and with and C we find which is also in good agreement with the QCD expectation. Received: 3 May 2001 / Revised version: 22 June 2001 / Published online: 3 August 2001     

QCD Correction to σ(e+e-→ZH→Zbb)

We study the QCD corrections to the calculated values of σ(e⁺e^-→ZH→Zbb), and find that for M_H<100 GeV, the process can be measured at the LEP energy to extract information of Higgs and the QCD corrections are negligible, namely the tree-level calculation is sufficiently aciurate for the energy range, while for M_H > 100 GeV, the Higgs-involved subprocess can only be investigated at NLC, and then the QCD correction becomes as large as 45%. For M_H > 2mt, considering the subprocess e⁺e^-→ZH→Ztt, the QCD corrections are also important and must be taken into account for the cross section evaluation.     

Precision Calculations for the T-Odd Quark Pair Production at the CLIC e+e- Linear Collider

We perform the precision calculations for the e⁺e^-→q_q_(q_q_=u_u_, c_c_, d_d_,s_s_) processes up to the QCD next-to-leading order (NLO) including full weak decays for the final T-odd mirror quarks in the littlest Higgs model with T-parity (LHT) at the Compact Linear Collider (CLIC). We show the dependence of the leading order (LO) and NLO QCD corrected cross sections on the colliding energy √s, and provide the LO and QCD NLO kinematic distributions of final particles. The results show that the LO cross section can be enhanced by the NLO QCD correction and the K-factor increases obviously when the threshold of the on-shell q_q_-pair production approaches the colliding energy √s. The K-factor value varies in the range of 1.04 ～ 1.41 in our chosen parameter space. We find that a simple approximation of multiplying the LO kinematic distribution with the integrated K-factor is not appropriate for precision study of the e⁺e^-→q_q_(q_q_=u_u_, c_c_, d_d_,s_s_) processes, since the NLO QCD corrections are phase space dependent. It is necessary to calculate the differential cross sections including full NLO QCD corrections to get reliable results.     

Formation and decay of scalar Leptoquarks/Squarks in ep collisions

The cross sections for the formation of scalar resonances, leptoquarks or squarks, in electron/positron-proton collisions at HERA are presented, including next-to-leading order QCD corrections. Depending mildly on the mass of the resonances, the K-factors increase the production cross sections by up to 30% if the target quarks are valence quarks. The QCD corrections to the partial decay widths of leptoquarks/ squarks to leptons and quarks are small. The electron spectrum in the decays is nevertheless softened by perturbative gluon radiation at a level of 2 GeV for a leptoquark/ squark mass of 200 GeV.     

Low-energy QCD and ultraviolet renormalons

We discuss the contribution of ultraviolet (UV) renormalons in QCD to two-point functions of quark current operators. This explicitly includes effects due to the exchange of one renormalon chain as well as two chains. It is shown that, when the external Euclidean momentum of the two-point functions becomes smaller than the scale AL associated with the Landau singularity of the QCD one-loop running coupling constant, the positions of the UV renormalons in the Borel plane become true singularities in the integration range of the Bore] transform. This introduces ambiguities in the evaluation of the corresponding two-point functions. The ambiguities associated with the leading UV renormalon singularity are of the same type as the contribution due to the inclusion of dimension d = 6 local operators in a low-energy effective Lagrangian valid at scales smaller than AL. We then discuss the inclusion of an infinite number of renormalon chains and argue that the previous ambiguity hints at a plausible approximation scheme for low-energy QCD, resulting in an effective Lagrangian similar to the one of the extended Nambu-Jona-Lasinio (ENJL) model of QCD at large N_c.     

SUSY QCD Impact on Top-Pair Production Associated with a Z0-Boson at a Photon-Photon Collider

The top-pair production in association with a Z⁰-boson at a photon-photon collider is an important process in probing the coupling between top-quarks and vector boson and discovering the signature of possible new physics. We describe the impact of the complete supersymmetric QCD (SQCD) next-to-leading order (NLO)radiative corrections on this process at a polarized or unpolarized photon collider, and make a comparison between the effects of the SQCD and the standard model (SM) QCD. We investigate the dependence of the lowest-order (LO) and QCD NLO corrected cross sections in both the SM and minimal supersymmetric standard model (MSSM) on colliding energy s^1/2 in different polarized photon collision modes. The LO, SM NLO, and SQCD NLO corrected distributions of the invariantmass of t\bar{t}-pair and the transverse momenta of final Z⁰-boson are presented. Our numerical results show that the pure SQCD effects in γγ→t\bar{t}z⁰ process can be more significant in the + + polarized photon collision mode than in other collision modes, and the relative SQCD radiative correction in unpolarized photon collision mode varies from 32.09\% to -1.89\% when s^1/2 goes up from 500 GeV to 1.5 TeV.     

Single slepton production associated with a top quark at LHC in NLO QCD

Single slepton production in association with a top quark at the CERN Large Hadron Collider (LHC) is one of the important processes in probing the R-parity violation couplings. We calculate the QCD next-to-leading order (NLO) corrections to the $pp \to t\tilde{\ell}^{-}(\bar{t}\tilde{\ell}^{+})+X$ process at the LHC and discuss the impacts of the QCD corrections on kinematic distributions. We investigate the dependence of the leading order (LO) and the NLO QCD corrected integrated cross section on the factorization/renormalization energy scale, slepton, stop-quark and gluino masses. We find that the uncertainty of the LO cross section due to the energy scale is obviously improved by the NLO QCD corrections, and the exclusive jet event selection scheme keeps the convergence of the perturbative series better than the inclusive scheme. The results show that the polarization asymmetry of the top-quark will be reduced by the NLO QCD corrections, and the QCD corrections generally increase with the increment of the $\tilde{t}_{1}$ or $\tilde {g}$ mass value.     

Gluon polarization in nucleons

In QCD the gauge-invariant gluon polarization in a nucleon can be defined either in a non-local way as the integral over the Ioffe-time distribution of polarized gluons, or in light-cone gauge as the forward matrix element of the local topological current. We have investigated both possibilities within the framework of QCD sum rules. Although the topological current is built from local fields, we have found that its matrix element retains sensitivity to large longitudinal distances. Because QCD sum rules produce artificial oscillations of the Ioffe-time distribution of polarized glue at moderate and large light-like distances, the calculation of the matrix element of the topological current results in a small value of . In a more consistent approach QCD sum rules are used to describe the polarized gluon distribution only at small light-like distances. Assuming that significant contributions to arise only from longitudinal length scales not larger than the nucleon size leads to . Received: 19 August 1997 / Published online: 20 February 1998