Flavor Dependence Research Articles

Prepotentials in N = 2 SU(2) supersymmetric Yang-Mills theory with massless hypermultiplets

We calculate the prepotential of the low-energy effective action for N = 2 SU(2) supersymmetric Yang-Mills theory with N f massless hypermultiplets ( N f = 1, 2, 3). The precise evaluation of the instanton corrections is performed by making use of the Picard-Fuchs equations associated with elliptic curves. The flavor dependence of the instanton effect is determined explicitly both in the weak- and strong-coupling regimes.

Anomalous structure of weak intermediate bosons.

The unpolarized and polarized quark and gluon content of the weak intermediate bosons of the standard model is investigated at an energy scale much higher than their masses. The evolution equations for polarized quark and gluon densities are constructed and solved asymptotically. The structure of the Z and W bosons turns out to be much richer than that of the photon. Strong flavor and spin dependence is observed.

Diffractive production of dijets at HERA

We investigate the production of a quark-antiquark pair in diffractive photon-proton scattering, approximating soft pomeron exchange by the exchange of two nonperturbative gluons. In deep inelastic scattering at HERA, events with two jets and the scattered proton in the final state are predicted to be observable, with an important contribution from charm production. For photoproduction of light quark jets with high transverse momentum we find that both exchanged gluons must have a large invariant mass, so that the cross section is very small, whereas for charm quarks it is quite appreciable. From our calculation we also extract the quark structure function of the pomeron for the scaling variablez no too close to 0 or 1, finding a strong flavour dependence and a behaviour somewhat harder thanz(1−z) for light quarks.

Flavour dependence of form factors in heavy meson decays

Recently we have shown that due account of Wigner spin rotations is needed to match the ISGW model consistently onto HQET. We now discuss the flavour dependence of this procedure. We find that for finite mass “heavy” quark the universal function ξ( y = 1) ≈ 1 but the slope is proportional to the energy of the spectator quarks, and ratios of form factors are corrected in a non-trivial way. Predictions for heavy quarks, such as in B c → ψ( η c ) and to mixed systems, such as D → K and B → π show a systematic mass dependence that may be confronted with data.

A Study of Structure Functions for the Bag Beyond Leading Order

There has recently been surprising progress in understanding the spin and flavor dependence of deep inelastic structure functions in terms of the same physics needed in the simple quark models used for hadronic spectroscopy. However, the corresponding scale is usually very low, casting doubt on the use of leading order $QCD$ evolution. We show that the conclusions are not significantly altered if one goes to next-to-leading order. In particular, the excellent agreement with unpolarized and polarized valence quark distributions is retained.

Parton distributions of the proton.

To obtain improved parton densities of the proton, we present a new global analysis of deep inelastic and related data including, in particular, the recent measurements of $F_2$ at HERA, of the asymmetry of the rapidity distributions of $W^\pm$ production at the FNAL $pp$ collider and of the asymmetry in Drell-Yan production in $pp$ and $pn$ collisions. We also incorporate data to determine the flavour dependence of the quark sea distributions. We find that the behaviour of the partons at small $x$ is consistent with the precocious onset of BFKL leading $\log(1/x)$ dynamics. We discuss the ambiguities remaining in the gluon distribution. We present improved predictions for $W$ boson (and $t$ quark) production at the FNAL $\pp$ collider.

The Levin-Frankfurt additive quark model revisited — new insight into QCD?

The simple AQM prediction σ tot (π -p)/σ tot (pp) ≈ 2 3 still fits experimental data to better than 10% suggesti an underlying dynamics describing the meson-baryon difference primarily in terms of the number of constituent quarks. Equally valid fits to the energy dependence of total cross sections from two models predicting very different asymptotic behaviour indicate that present data cannot distinguish between different pomeron models. Meson-nucleon and hyperon-nucleon data at higher energies could clarify finer details of flavor and baryon number dependence supporting a hierarchy of contributions inspired by large N c QCD: (1) multigluon exchange, (2) planar quark diagrams, (3) nonplanar quark diagrams.

Diquark correlations in the nucleon structure function

The nucleon structure functions are studied with a special emphasis on diquark correlations inside the nucleon. Assuming a phenomenological diquark-quark model of the nucleon at the low-energy hadronic scale, we calculate the leading-twist contributions to the nucleon structure functions. We take the Nambu and Jona-Lasinio model for the calculation of the quark distributions in the diquarks, and use the convolution method for the diquark scattering process. The resulting quark distributions are evolved to the experimental momentum scale using the perturbative QCD. We discuss the flavor dependence of the nucleon structure functions and the importance of the quark-quark correlations for the understanding of the nucleon structure.

Constituent quark model analysis of weak mesonic decays of charm baryons.

In view of recent experimental trends we investigate the weak nonleptonic decays of charm baryons within the framework of the constituent quark model. Branching ratios and asymmetry parameters for all ${\mathrm{\textonehalf{}}}^{+}$ \ensuremath{\rightarrow} ${\mathrm{\textonehalf{}}}^{+}$ + ${0}^{\ensuremath{-}}$ charm-changing modes are calculated with appropriate QCD corrections. The effect of flavor dependence on the scale is found to be quite significant.

Structure functions of W and Z bosons

The strong content of the weak intermediate bosons is investigated at energies much higher than their masses. The evolution equations of quark and gluon densities are constructed and solved asymptotically. Strong flavour and spin dependence is observed. The special role of longitudinally polarized bosons is discussed.

Heavy meson hyperfine splittings. A puzzle for heavy quark chiral perturbation theory

We show that there is a large discrepancy between the expected light flavor dependence of the heavy pseudoscalar-vector mass splittings and the measured values. We demonstrate that the contribution from the one-loop calculation does not agree with the measured value. We show that because agreement with experiment requires the leading dependence on SU(3) symmetry breaking to be nearly cancelled, the heavy quark mass dependence is unknown and the expected dependence on the light quark mass is not realized.

CTEQ parton distributions and flavor dependence of sea quarks

This paper describes salient features of new sets of parton distributions obtained by the CTEQ Collaboration based on a comprehensive QCD global analysis of all available data. The accuracy of the new data on deep inelastic scattering structure functions obtained by the very high statistics NMC and CCFR experiments provides unprecedented sensitivity to the flavor dependence of the sea-quark distributions. In addition to much better determination of the small- x dependence of all parton distributions, we found: (i) the strange quark distribution is much softer than the non-strange sea quarks and rises above the latter at small- x; and (ii) the difference d- u changes sign as a function of x. A few alternative sets of viable distributions with conventional assumptions are also discussed.

New parton distributions

Recent measurements of the structure functions for deep inelastic scattering made by the NMC and the CCFR collaboration provide new information on parton distributions, particularly in the 0.01 < x < 0.1 interval. We describe (next-to-leading order) determinations of parton distributions whih incorporate these new data together with other deep-inelastic and related data, and which allow the possibility of flavour dependence of the light quark sea (i.e. u ≠ d ) as implied by the Gottfried sum rule. We discuss extrapolations to small x.

Transverse flow due to minijets in [formula omitted] collisions at √ s=1.8 TeV

The multiplicity and flavor dependence of average transverse momentum of charged particles produced in high energy pp and p p collisions is studied in a PQCD inspired model. The experimentally observed transverse flow effect at √ s=1.8 TeV is naturally explained in the context of the fragmentation of multiple minijets which dominate the hadronic interaction mechanism at high collider energies.

Flavor dependence of the chiral phase transition in strong-coupling QCD.

We have studied the chiral phase transition in SU(3) lattice gauge theory with {ital f} species of staggered fermions at nonzero chemical potential and temperature. The results at infinite coupling are improved by adding systematic 1/{ital g}{sup 2} corrections. At zero chemical potential we find a flavor dependence of the chiral transition which is in qualitative agreement with Monte Carlo data. At zero temperature, however, the chiral transition turns out to be flavor independent. The thermodynamics of the chiral-symmetric and broken phases is discussed in detail.

Pomeron physics in diffractive deep inelastic scattering at large 1/x

I review recent progress in the perturbative QCD approach to pomeron physics in deep inelastic scattering (DIS) at large1/x.(refs.1–4). I discuss the crucial role the color transparency plays in a diffractioe DIS, and how it can be quantified in terms of the wave function (WF) of (partonic) hadronic fluctuations of photons and universal, flavour independent, qqpair interaction cross section. Different DIS processes probe this universal cross section at different sizes. The emerging WF formalism greatly simplifies the involved calculations. I present quantitative predictions for the nuclear shadowing and the diffraction dissociation (DD) rate in DIS, and the valence and sea quark distributions in a QCD Pomeron. I point out a dramatic difference of sea densities in a nucleon as probed by muons and neutrinos and flavour dependence of the pomeron couplings in DD and conclude that perturbative QCD pomeron does not factorize. I emphasize a numerical importance of shadowing in deuterium for tests of the Gottfried sum rule.

Pomeron physics in diffractive deep inelastic scattering at large

I review recent progress in the perturbative QCD approach to pomeron physics in deep inelastic scattering (DIS) at large 1 x .(refs.1–4). I discuss the crucial role the color transparency plays in a diffractive DIS, and how it can be quantified in terms of the wave function (WF) of (partonic) hadronic fluctuations of photons and universal, flavour independent, q q pair interaction cross section. Different DIS processes probe this universal cross section at different sizes. The emerging WF formalism greatly simplifies the involved calculations. I present quantitative predictions for the nuclear shadowing and the diffraction dissociation (DD) rate in DIS, and the valence and sea quark distributions in a QCD Pomeron. I point out a dramatic difference of sea densities in a nucleon as probed by muons and neutrinos and flavour dependence of the pomeron couplings in DD and conclude that perturbative QCD pomeron does not factorize. I emphasize a numerical importance of shadowing in deuterium for tests of the Gottfried sum rule.

The chiral phase transition in QCD: Critical exponents and their flavour dependence

Abstract We discuss the strong flavour dependence of the critical temperature as well as the order of the phase transition found in Monte Carlo simulations of QCD. Analytical and numerical studies of strong coupling QCD show that the lattice U ( f ) × U ( f ) symmetry may lead to modifications of the order of the phase transition on the lattice, which in the continuum limit is expected to be controlled by the global SU ( n f ) × SU ( n f ) chiral symmetry.

Pomeron structure function and diffraction dissociation of virtual photons in perturbative QCD

We study scaling properties of the diffraction dissociation of virtual photons in a deep inelastic scattering. We concentrate on the total diffraction dissociation rate, diffraction excitation mass spectrum and the pomeron structure function to the lowest order in perturbative QCD. We calculate the valence structure function and the strangeness and charm content of the pomeron and estimate the ocean structure function using the pomeron factorization property. We find that quarks carry ≈ 10% of pomeron's momentum. Differential cross section of the (virtual) photon-pomeron scattering is found to exhibit features typical of the hadronic two-body reactions, supporting a treatment of the Pomeron as a particle, whereas the flavor dependence of structure functions does not support the particle treatment of the pomeron. Diffraction dissociation of photons is predicted to make ≈ 15% of the total deep inelastic scattering rate at smallx and largeQ2. Detailed predictions for the mass spectrum and angular distribution of jets produced on the valence component of the pomeron are presented.

The flavor dependence of the QCD finite temperature phase transition

We present finite temperature full QCD simulations on 16 3 × N t obtained with the 256-node Columbia Parallel processor during the last year. We see a first order transition with four flavors at N t = 4 for m q a = 0.01, 0.025, 0.0375, and 0.05, and at N t = 6 for m q a = 0.01 but not for m q a = 0.025, suggesting that the transition weakens as we move closer to the continuum limit. With two flavors at N t = 4 we do not see a transition for m q a = 0.01 and 0.025. With three flavors at N t = 4 we see a first order transition for m q a = 0.025; fixing m u, d a = 0.025 and increasing the strange quark mass to m s a = 0.1 we find no evidence of a transition and that m K / m p = 0.46(1), raising the possibility that in nature QCD may not undergo a finite temperature phase transition.