Direct photon production beyond leading order in QCD

The significance of the quark-quark scattering process (quark+quark→quark+quark+photon) for the production of large-q_T real photons is discussed in the framework of perturbative QCD. To extract the finite contribution of this process to the differential cross section dσ/dy d²q_T (hadron 1+hadron 2→photon+anything) we define the gluon distribution and the quark-to-photon fragmentation function beyond the leading approximation. The calculations are performed consistently in the dimensional regularization scheme. Our numerical estimates show the resulting finite qq→qqγ contribution to be a small (order α_s/2π) correction in comparison with the basic QCD subprocesses.     

Symmetry breaking in τ decay

In view of the recent attempts to detect theG-parity violating decay τ→πηv _τ which may have bearing on the presence of the second class current, we discuss the isospin symmetry breaking effects in the processes of τ→δv, Bv, and ηπv from a dynamical point of view. It is shown that all the symmetry breaking effects are proportional to theu?d quark mass difference. Depending on whether the current or the constituent quark masses are used, the decay τ→πηv _τ via δ is either of the same order or three orders of magnitude smaller than the upper limit set by the latest experiments. This is in agreement with the other theoretical estimates.     

n + p → d + γ in effective field theory

The radiative capture process n + p → d + γ provides clear evidence for meson exchange currents in nuclear physics. We compute this process at low energies using a recently developed power counting for the effective field theory that describes nucleon-nucleon interactions. The leading order contribution to this process comes from the photon coupling to the nucleon magnetic moments. At subleading order there are other contributions. Among these are graphs where the photon couples directly to pions, i.e. meson exchange currents. These diagrams are divergent and require the presence of a local four-nucleon-one-photon counterterm. The coefficient of this operator is determined by the measured cross section, σ^expt = 334.2±0.5 mb, for incident neutrons with speed |v| = 2200 m/s.     

Understanding of radial and elliptic expansion with quark number scaling and blast wave model in 200 GeV Au+Au at RHIC-PHENIX

Measurement of azimuthal anisotropy is one of the most important study because of its relation to the initial stage. Especially, the elliptical anisotropy which is measured as the second coefficient of Fourier expansion of particle azimuthal distribution is believed to carry the information about the initial geometrical anisotropy. It seems the identified hadron v ₂ depends on the number of quark contents of the hadron. The experimental result of quark number scaling of v ₂ suggests quark level collectivity in the hot dense matter and quark coalescence mechanism to form hadron from quark matter via quark-gluon phase transition. The measured v ₂ and p _T spectra are analyzed with various assumptions based on the blast wave model in order to understand the freeze-out temperature and collective flow after the system expansion.     

Finite corrections to quark fragmentation functions in perturbative QCD

The finite corrections of order α _s in perturbative QCD to the cross-sections for semi-inclusive hadron production from deep inelastic leptonhadron scattering and electron-positron annihilation are calculated. We define the effective quark fragmentation functions viae ⁺ e ^? → hadron +X including the finite terms in order to estimate these corrections for the reactions lepton + hadron → lepton + hadron + anything. Contrary to the leading term the next-to-leading order term does not factorize into parts depending on the target and the fragment, respectively. For the processese +p →e + π^± +X andv +p →μ ^? + π^± +X the finite corrections of order α _s turn out to be at most 20% in the range of momenta covered by present experiments.     

Some consequences of the quark parton model in inclusive electroproduction

Consequences of the quark parton model proposed by Feynman for inclusive electroproduction are studies. We show that the SLAC experiment γ_v+p(n) → h^±+anything which distinguishes the outcoming hdrons by their charges only gives the possibility to check sum rules with fractional quark quantum numbers, to determine the charge of the proton quark (neutron quark) and to measure the valence distribution function directly. We predict the multiplicity difference for the charged hadrons as the function of q² which agrees well with the experimental data published recently.     

QCD three-jet production in large-p⊥ processes

The contribution of the hard scattering subprocess quark + quark → quark + quark + gluon to two-particle inclusive distributions at large p_⊥ is studied. It is shown to be accessible experimentally, and agrees with recent data on azimuthal distributions in the adequate kinematical region.     

Some implications of the observed decay ψ → f + γ

Describing the ψ → f+γ decay via an intermediate one-photon state avoids difficulties encountered in strong OZI- violating transitions. The ψ → γ →f+γ transition is enhanced an order of magnitude because amplitudes for the u$\text{u}$ and d$\text{d}$ components of the f are proportional to squares of quark charges and add constructively. A rough dynamical estimate agrees with the experimental decay rate. Branching ratios for f+γ,A₂+γ and f′ + γ decays are shown to be very different for electromagnetic and strong transitions and can test models. Tests of the Han-Nambu model and contributions from the weak neutral current are discussed.     

Electroweak radiative corrections to the top quark decay

The top quark, once produced, should be an important window to the electroweak symmetry breaking sector. We compute electroweak radiative corrections to the decay processt→b+W ⁺ in order to extract information on the Higgs sector and to fix the background in searches for a possible new physics contribution. The large Yukawa coupling of the top quark induces a new form factor through vertex corrections and causes discrepancy from the tree-level longitudinalW-boson production fraction, but the effect is of order 1% or less form _H<1 TeV.     

Semileptonic decays of heavy quarks in quantum chromodynamics

We investigate the semileptonic decays of heavy quarks in the leading non-trivial order in quantum chromodynamics. Effects of gluon corrections and the initial quark Fermi motion on the semileptonic rates and decay distributions are calculated. The resulting lepton energy spectrum for the charm semileptonic decay is compared with data to extract the mass of the charm quark. This is combined with the semileptonic branching ratio to predict the charm-quark lifetime. We find the lepton energy spectrum very stable with respect to gluon corrections. Expected spectra from the semileptonic decays of bottom and top quarks are presented. We also study the semileptonic decay process Q → q?v_? + G, involving the emission of a single hard non-collinear gluon. This process should be observable with a branching ratio of a few percent in the decays of top (and heavier) quarks.     

Fragmentation of 7- to 9-GeV/c deuterons to cumulative kaons

Experimental results concerning the kaon yield in the reaction d + Be → K ^±(0°) + X for values of the cumulative variable x _c in the range between 0.88 and 1.37 are presented. Values in the region of x _c ≥ 1 correspond to short nucleon-nucleon distances (deuteron-core region) at which nucleon wave functions begin overlapping each other, forming a cluster (flucton) where the hadron-matter density is higher than the average matter density in the nucleus. The behavior of the K ⁺/K ^? yield ratio in this region may be interpreted within the hypothesis that the quark sea in nuclei becomes harder because of the presence of a flucton component in nuclear matter.     

Exclusive semileptonic heavy meson decays including lepton mass effects

We discuss the exclusive semi-leptonic (s.l.) bottom meson decaysB→D(D ^*)+l+v where we include non-zero lepton mass effects in the kinematics and dynamics. We develop the general formalism for the non-zero lepton mass case. We then look at how rates, spectra and angular correlations are affected by non-zero lepton masses in the context of a specific spectator quark model. Numerical results are presented for s.l. decays involving thee-, μ- and τ-leptons. We also discuss the s.l. decaysB→π(ρ),D→K(K ^*) and the free quark decay model.     

Inclusive production at large transverse momentum in QCD

We calculate the cross section for the process quark + quark → quark (with large transverse momentum) + anything, up to order g⁶ in the cross section and in the leading-logarithm approximation. The result is suggestive of a simple ansatz for the calculation of the asymptotic behaviour of cross sections for the production of particles or jets with large transverse momenta.     

Contribution of the triangle graphs to the neutron electric dipole moment

In the Kobayashi-Maskawa model we compute a new contribution to the neutron electric dipole moment induced byCP violating flavour-changing transitions of the typess→d+gluon+γ through heavy quark loops. This contribution, essentially given by the triangle anomaly graph, is by itself gauge invariant. We obtain from this mechanismD _n /e=3.10^?32 cm, one order of magnitude below our previous estimate from usual penguin diagram contributions.     

An estimate of J/ψ production in b decays

The branching ratio for the decay B → J/ψ + hadrons is estimated from a calculation of the quark decay b → J/ψ + s.     

Quark and strange quark matter in f(R) gravity for Bianchi type I and V space-times

Behaviors of quark matter and strange quark matter which exist in the first seconds of the early Universe in f(R) gravity are studied for Bianchi I and V universes. In this respect, we obtain exact solutions of the modified Einstein field equations by using anisotropy feature of Bianchi I and V space-times. In particular, we investigate exact f(R) functions for Bianchi I as the contribution of strange quark and quark matter. Also, we have concluded that quark matter may contribute to the early acceleration of the universe since quark matter behaves like phantom-type dark energy. Furthermore, obtained f(R) solutions represents early eras of the Universe since f(R) solutions for quark matter coincide with f(R) equations for inflation. From this point, we can reach the conclusion that quarks may be source of the early dark energy of the universe or source of little inflation due to their repulsive force.     

Polarization of gluon jets in photon-photon scattering

We show that the linear polarization of a gluon emerging from the reaction $\text{e}\text{+}\text{e}\text{→}\text{e}\text{+}\text{e}\text{+}\text{g}\text{+}\text{g}$ is large - on the order of 20%. This polarization arises from the process γ + γ → quark loop → g + g, which is expected to dominate the two gluon jet cross section. The polarization can be used to test quantum chromodynamics (QCD), even if the gluon jets cannot be distinguished clearly from the background of quark jets. If the QCD prediction is accepted, our result can be used to enhance the signal of gluon jets, thereby aiding in the study of their properties.     

π−Electroproduction off deuterium above the resonance region

The reactionse + d → e' + p_S + p + π^?ande + d → e' + n_S + n + π⁺ were measured detecting electron and pion in coincidence at an invariant hadronic mass of 2.19 GeV. The measurements were performed at electron four-momentum transfer squared of f² = 0.70 and 1.35 GeV² in the range of t = (γ_v?π)² between t_minand ?1.0 GeV². The cross section d²σ/dtdφ of the reaction e + n → e' + p + π^? was determined.     

Electromagnetic form factors of Λ_c in the space-like momentum region

We studied the electromagnetic form factors(EMFFs) of Λ_c and the contributions of the quark and diquark currents to the EMFFs of Λ_c in the space-like region in the Bethe–Salpeter equation approach with instantaneous approximation. In this picture, baryon Λ_c can be regarded as a two-body c(ud) system. We found that for different values of parameters the contribution of quark and diquark currents to the EMFFs of Λ_c is very different,while their total contribution to the EMFFs of Λ_c is similar. The EMFFs of Λ_c are similar to those of other baryons(proton, Ξ~-, and Σ~+) with a peak at ω = 1, where ω = v′·v is the velocity transfer between the initial state(with velocity v) and the final state(with velocity v′) of Λ_c.     

Flavor-changing electromagnetic vertex in a non-linear Rξ gauge

We calculate the general transition q_i → q_j + γ for arbitrary quark flavors in a non-linear R_ξ gauge. The reduced number of Feynman diagrams as well as simplified Ward identities greatly facilitates this calculation. A general result with intermediate quarks of arbitrary mass is presented and the special cases s → d + γ as well as s → d + gluon are discussed.