Diquark mass differences from unquenched lattice QCD

We calculate diquark correlation functions in the Landau gauge on the lattice using overlap valence quarks and 2+1-flavor domain wall fermion configurations. Quark masses are extracted from the scalar part of quark propagators in the Landau gauge. The scalar diquark quark mass difference and axial vector scalar diquark mass difference are obtained for diquarks composed of two light quarks and of a strange and a light quark. The light sea quark mass dependence of the results is examined. Two lattice spacings are used to check the discretization effects.The coarse and fine lattices are of sizes 24~3×64 and 32~3×64 with inverse spacings 1/a = 1.75(4) Ge V and 2.33(5) Ge V,respectively.     

Quark masses,the Dashen phase,and gauge field topology

The CP violating Dashen phase in QCD is predicted by chiral perturbation theory to occur when the up–down quark mass difference becomes sufficiently large at fixed down-quark mass. Before reaching this phase, all physical hadronic masses and scattering amplitudes are expected to behave smoothly with the up-quark mass, even as this mass passes through zero. In Euclidean space, the topological susceptibility of the gauge fields is positive at positive quark masses but diverges to negative infinity as the Dashen phase is approached. A zero in this susceptibility provides a tentative signal for the point where the mass of the up quark vanishes. I discuss potential ambiguities with this determination.     

On the light meson mass formulae from quantum chromodynamics

We reconsider the Gell-Mann-Okubo light meson mass formulae using the moment sum rules ratios and taking into account non-leading quark masses and vacuum condensate effects. We also extract the strange quark mass value from the ?-ρ mass difference. The poorly known masses of the (I=0,J ^PC =0⁺⁺) scalar mesons are discussed.     

Test of the heavy quark–light diquark approximation for baryons with a heavy quark

We check a commonly used approximation in which a baryon with a heavy quark is described as a heavy quark–light diquark system. The heavy quark influences the diquark internal motion reducing the average distance between the two light quarks. Besides, we show how the average distance between the heavy quark and any of the light quarks, and that between the heavy quark and the center of mass of the light diquark, are smaller than the distance between the two light quarks, which seems to contradict the heavy quark–light diquark picture. This latter result is in agreement with expectations from QCD sum rules and lattice QCD calculations. Our results also show that the diquark approximations produces larger masses than the ones obtained in a full calculation.     

Vector mesons in dense matter

We will summarize the progress in understanding the changes in the vector meson spectral density in nuclear medium using the constraint equations obtained from the Borel transformed dispersion relation and QCD Operator Product Expansion. We will discuss the results for the scalar mass shift and dispersion effects (three momentum dependence) for the light quark system (,ω), the strange quark system (Φ) and the heavy quark system (J/ψ) in nuclear medium. For the light quark system, a nontrivvial change in the mass and width is expected, while the dispersion effects are found to be small. Existing model calculations for the dispersion effects are compared to the constraint equation in detail. Very small, but accurate mass shifts are obtained for the heavy quark system.     

Determination of the b quark mass at the Z mass scale

In hadronic decays of Z bosons recorded with the OPAL detector at LEP, events containing b quarks were selected using the long lifetime of b flavoured hadrons. Comparing the 3-jet rate in b events with that in d,u,s and c quark events, a significant difference was observed. Using calculations for massive quarks, this difference was used to determine the b quark mass in the renormalisation scheme at the scale of the Z boson mass. By combining the results from seven different jet finders the running b quark mass was determined to be . Evolving this value to the b quark mass scale itself yields , consistent with results obtained at the b quark production threshold. This determination confirms the QCD expectation of a scale dependent quark mass. A constant mass is ruled out by 3.9 standard deviations. Received: 14 May 2001 / Published online: 17 August 2001     

A diquark model for baryons containing one heavy quark

We present a phenomenological ansatz for coupling a heavy quark with two light quarks to form a heavy baryon. The heavy quark is treated in the heavy mass limit, and the light quark dynamics is approximated by propagating scalar and axial vector ??diquarks??. The resulting effective lagrangian, which incorporates heavy quark and chiral symmetry, describes interactions of heavy baryons with Goldstone bosons in the low energy region. As an application, the Isgur-Wise form factors are estimated.     

A diquark model for baryons containing one heavy quark

We present a phenomenological ansatz for coupling a heavy quark with two light quarks to form a heavy baryon. The heavy quark is treated in the heavy mass limit, and the light quark dynamics is approximated by propagating scalar and axial vector ‘diquarks’. The resulting effective lagrangian, which incorporates heavy quark and chiral symmetry, describes interactions of heavy baryons with Goldstone bosons in the low energy region. As an application, the Isgur-Wise form factors are estimated.     

A study of top quark production at HERA using parton shower simulation of DIS events

A study of the hadronic properties of top quark events at HERA is made in the hope that some characteristics of such events may emerge that distinguish them from light flavour QCD events. Electronproton collisions are simulated using a parton shower Monte Carlo event generator which includes both initial and final gluon radiation and also incorporates certain gluon interference effects. It is found that applying a multiplicity cut of 35 together with considering only charged current events gives a top to back-ground ratio of about 1 and a total of 40 top events at an integrated luminosity of 500 pb^?1 for a top quark mass of 60 GeV. Unfortunately no clear peak in the mass distribution can be obtained as the background and top quark peaks coincide after application of the multiplicity cut. If the top quark mass is 100 GeV there will be only about 6 top quark events under these conditions.     

粲夸克辐射的死区效应

基于轻夸克辐射能量谱, 考虑夸克质量对部分子辐射能量损失的影响, 利用死区因子把轻部分子辐射能量损失谱推广到重部分子辐射能量损失的情形. 研究结果表明, 利用与夸克质量相关的死区因子推广轻夸克辐射能量损失谱得到的重夸克辐射能量损失谱, 与直接利用微扰QCD计算所得到的Djordjevic-Gyulassy(D-G)谱的结果一致, 且极大简化了重夸克辐射能量损失的计算.     

A numerical study of an expanding plasma of quarks in a chiral model

We numerically solve the transport equations for a quark gas described by the the Nambu-Jona-Lasinio model. The mean field equations of motion, which consist of the Vlasov equation for the density and the gap equation for the mean field, are discussed, and energy and momentum conservation are proven. Numerical solutions of the partial differential equations are obtained by applying finite difference methods. For an expanding fireball the light quark mass evolves from small values initially to the value of 350 MeV. This leads to a depletion of the high energy part of the quark spectrum and an enhancement at low momenta. When collisions are included one obtains an equation of the Boltzmann type, where the transition amplitudes depend on the properties of the medium. These equations are given for flavor SU(3), i.e. including strangeness. They are solved numerically in the relaxation time approximation and the time evolution of various observables is given. Medium effects in the relaxation times do not significantly influence the shape of the spectra. The mass of the strange quark changes little during the expansion. The strangeness yield and the slope temperatures of the final spectra are studied as a function of the size of the initial fireball.     

夸克和弦模型中非奇异轻介子的类雷吉谱(英文)

本工作重新考察了相对论弦模型和准相对论夸克模型中轻介子的类雷吉谱。基于辅助场技术,提出了非奇异轻介子的一个解析质量公式,由此推导出了一个准线性的Regge-Chew-Frautschi轨迹关系,并将此关系用相应介子的实验数据（PDG）进行了检验。检验结果表明,和实验观测数据比较,夸克模型对介子质量的预言与弦模型的相符,但在角动量较大时,夸克模型的预言稍微优于弦模型。We revisit the Regge-like spectra of light mesons in the relativistic string model and the simirelativistic quark model. An analytical mass formula is proposed for light mesons based on the auxiliary field technique, and a quasi-linear Regge-Chew-Frautschi plot with flavor dependence is derived and verified with the experimental data(PDG) for light mesons.The results show that the quark model predictions for meson masses agree with those of the string model, but are slightly better when the angular momentum is relatively large as compared with the observed data.     

Static and dynamic properties of heavy light mesons in infinite mass limit

We summarize the consequences of the infinite limit of heavy quark mass in the results of form factors, charge radii and decay constants of heavy light mesons within a QCD inspired quark model recently reported.     

Heavy quark energy loss in a nuclear medium

Multiple scattering, modified fragmentation functions, and radiative energy loss of a heavy-quark propagating in a nuclear medium are investigated in perturbative QCD. Because of the quark mass dependence of the gluon formation time, the medium size dependence of heavy-quark energy loss is found to change from a linear to a quadratic form when the initial energy and momentum scale are increased relative to the quark mass. The radiative energy loss is also significantly suppressed relative to a light quark due to the suppression of collinear gluon emission by a heavy quark.     

Light quark masses from scalar sum rules

In this work, the mass of the strange quark is calculated from QCD sum rules for the divergence of the strangeness-changing vector current. The phenomenological scalar spectral function which enters the sum rule is determined from our previous work on strangeness-changing scalar form factors [1]. For the running strange mass in the scheme, we find . Making use of this result and the light quark mass ratios obtained from chiral perturbation theory, we are also able to extract the masses of the lighter quarks and . We then obtain and . In addition, we present an updated value for the light quark condensate. Received: 18 October 2001 / Revised version: 22 January 2002 / Published online: 12 April 2002     

Mass spectra of light and heavy mesons in the-Dirac equation with power-law potential

The mass spectra of both light and heavy mesons are studied in the Dirac equation with an equally mixed 4-vector and scalar power-law potential model. This potential provides an excellent fit not only to the mass spectra ofp°, ϕ, Ψ and υ families but also to those ofD, F andB mesons. The light quark masses inp° and ϕ as well as in atom-like mesons are very close to the current quark masses.     

Do electroweak precision data and Higgs-mass constraints rule out a scalar bottom quark with mass of order 5 GeV?

We study the implications of a scalar bottom quark, with a mass of O (5 GeV), within the minimal supersymmetric standard model. Light sbottoms may naturally appear for large tan(beta) and, depending on the decay modes, may have escaped experimental detection. We show that a light sbottom cannot be ruled out by electroweak precision data and the bound on the lightest CP-even Higgs-boson mass. We infer that a light b scenario requires a relatively light scalar top quark whose mass is typically about the top-quark mass. In this scenario the lightest Higgs boson decays predominantly into b pairs and obeys the mass bound m(h) less, similar 123 GeV.     

The top quark mass and flavour mixing in a seesaw model of quark masses

The top quark mass and the flavour mixing are studied in the context of a seesaw model of quark masses based on the gauge group SU(2)_L × SU(2)_R × U(1). Six isosinglet quarks are introduced to give rise to the mass hierarchy of ordinary quarks. In this scheme, we reexamine a mechanism for the generation of the top quark mass. It is shown that, in order to prevent the seesaw mechanism to act for the top quark, the mass parameter of its isosinglet partner must be much smaller than the breaking scale of SU(2)_R. As a result the fourth lightest up quark must have a mass of the order of the breaking scale of SU(2)_R, and a large mixing between the right-handed top quark and its singlet partner occurs. We also show that this mechanism is compatible with the mass spectrum of light quarks and their flavour mixing.     

Spectra of baryons containing two heavy quarks

Doubly heavy baryons, i.e., the baryons containing two heavy quarks are treated in the adiabatic approximation, considering the motion of the light quark as a relativistic motion. The binding energy and mass spectra of doubly heavy baryons are calculated solving the two-center Dirac equation the one-centre Schr?dinger equation for Coulomb plus linear potential. The binding energy of the light quark as a function of the distance between heavy quarks is found. Received: 16 January 2002 / Accepted: 7 February 2002