Meson Structure Functions Research Articles

Resummed rate forB→Xsγ

In this paper we investigate the effect of the resummation of threshold logs on the rate for B -> X_s gamma. We calculate the differential rate dGamma/dE_gamma including the infinite set of terms of the form alpha_s^n log^{n+1}(1-x) and alpha_s^n log^n(1-x) in the Sudakov exponent. The resummation is potentially important since these logs turn into log(2E_{cut}/m_b), when the rate is integrated from the lower cut x=2E_{cut}/m_b to 1. The resummed rate is then convolved with models for the structure function to study whether or not the logs will be enhanced due to the fermi motion of the heavy quark. A detailed discussion of the accuracy of the calculation with and without the inclusion of the non-perturbative effects dictated by the B meson structure function is given. We also investigate the first moment with respect to (1-x), which can be used to measure \bar\Lambda and lambda_1. It is shown that there are some two loop corrections which are just as large as the alpha_s^2 beta_0 term, which are usually expected to dominate. We conclude that, for the present energy cut, the threshold logs do not form a dominant sub-series and therefore their resummation is unnecessary. Thus, the prospects for predicting the rate for B -> X_s gamma accurately, given the present energy cut, are promising.

ExtractingVubwithout recourse to structure functions

We present a closed form expression for |V_{ub}|^2/ |V_{tb} V_{ts}^*|^2 in terms of the endpoint photon and lepton spectra from the inclusive decays B -> X_s\gamma and B -> X_u\ell\nu, respectively, which includes the resummation of the endpoint logs at next to leading order and is completely independent of the B meson structure function. The use of this expression for extracting V_{ub} would eliminate the large systematic errors usually incurred due to the modeling of the heavy quarks' Fermi motion.

Using Drell-Yan processes to probe nucleon and meson structure functions

We investigate how Drell-Yan processes can be used to measure the magnitude of flavor symmetry violation in the proton sea. We examine the utility of the following beams: protons, charged pions, and charged kaons. In each case we present an approximate expression for the Drell-Yan asymmetry. Using currently available parton distributions, we locate those kinematic regions which provide the greatest information on the quantity $\bar{d}^p(x) - \bar{u}^p(x)$. If sufficiently intense $K^+$ beams were available, they could provide an efficient measurement of this quantity. Finally we present and discuss sets of sum rules for the Drell-Yan processes.

Use of quark wave functions in the calculation of structure functions of mesons.

In the description of deep-inelastic scattering the most extensively used model is the parton model. However, there has been an ongoing effort to understand structure functions of nucleons and mesons in terms of wave functions of quarks. Calculations made using the constituent quark model usually do not fit the experimental data. It has been argued that such calculations refer to some low-momentum scale parameterized by {ital Q}{sup 2}{sub 0}. The calculated distribution is then evolved up to a large scale, {ital Q}{sup 2}, by perturbative QCD evolution. The change needed if the distribution is to fit the data is quite large and, therefore, the scale {ital Q}{sup 2}{sub 0} is taken to be quite small. The large changes required for the distribution makes this procedure suspect. In this work we introduce a class of wave functions that are meant to apply at the scale {ital Q}{sup 2}, so that evolution is not required. These wave functions describe the motion of a current quark, or parton, relative to a residual system. (In order to calculate a cross section the residual system is placed on mass shell in the calculation.) Our procedure is inherently asymmetric in the description of the struck particlemore » and the residual system. We show how we are able to fit the (valence) structure function of the pion at {ital Q}{sup 2}=20 GeV{sup 2} (without QCD evolution) using our scheme. {copyright} {ital 1996 The American Physical Society.}« less

Meson structure in deep inelastic scattering

We study the deep inelastic structure functions of mesons within the Nambu and Jona-Lasinio model. We calculate the valence-quark distributions in π-, K- and ϱ-mesons at the low-energy model scale, which are evoluted to the experimental momentum scale in terms of the Altarelli-Parisi equation. The resulting distribution functions show reasonable agreements with experiment. We also discuss the semi-inclusive lepton-nucleon scattering process with aslow nucleon in coincidence in the final state, which reveals the off-shell structure of the pion.

Pion density in nuclei and deep-inelastic lepton scattering

Recent data indicate that quark densities in a high-$A$ nucleus differ significantly from those measured in the free-nucleon case. We examine whether this effect can be explained in terms of deepinelastic scattering from mesons responsible for nuclear binding. Although features of the data are reproduced qualitatively, we are unable to approach a quantitative description of the magnitude of the experimental effect at small $x$ unless we postulate an enhanced pion density of about 0.4 extra pions per nucleon (roughly three times that provided by conventional nuclear models), and/or make assumptions about the small-$x$ behavior of the meson structure function which are inconsistent with available data. These conclusions are insensitive to the manner in which we impose momentum conservation. The slope of the ratio of structure functions vs $x$ is reproduced easily in the range $0.05<x<0.5$. We speculate that a quantitative fit to the data requires an "intrinsic" enhancement of the nucleon structure function due to the nuclear medium as well as scattering from the mesons associated with binding.

Experimental determination of the π meson structure functions by the Drell-Yan mechanism

We have studied high statistics samples of dimuon events (∼35,000) produced from π± on platinum target in the mass interval 4.2≦Mμμ≦8.5 GeV at 150, 200 and 280 GeV/c. The π structure function is obtained by a fit ofd2σ/dx1dx2 to π+ and π− data. At 200 GeV, the simultaneous use of π+ and π− data allows a separate determination of the valence and sea structure functions of the π. Furthermore, the 150 and 280 GeV data allow an accurate determination of the shape of the valence structure function and give an estimate of its evolution betweenQ2=25 and 50 GeV2.

On a covariant quark parton model with confinement

We examine the standard-parton-model unitarity relations in the context of a covariant quark parton model which incorporates confinement in a very naive way. We also make a deeper analysis than was made in recent papers by the same authors about the sensitivity of the baryonic structure functions to the confinement mechanism. This new analysis shows that, contrarily to what was stated in those papers, one can expect, because of the confinement, changes in the behaviour of the baryonic structure functions similar to those already found for the meson structure functions. However, because of the different quark content of mesons and baryons, we have not found any connection between the size of these changes for mesons and baryons.

Exclusive Processes and Hadron Dynamics at Short Distances

The predictions of perturbative QCD for a number of areas of hadron dynamics are discussed, including exclusive processes at large momentum transfer, the endpoint behavior of hadronic structure functions, and the Fock state structure of hadron wavefunctions-especially their behavior at short-distance. New results for exclusive two-photon processes, the normalization of higher twist contributions to the meson structure function, and the calculation of the valence Fock state probability of the pion are presented. We also review the contrasting features of QCD and parton model dynamics.

Meson Structure Functions from High-Transverse-Momentum Hadron Interactions

Data on large--transverse-momentum ..pi../sup 0/ production by proton, pion, and kaon beams is used to extract the large-x behavior of meson structure functions. Good agreement is found with the proton and pion quark distribution functions obtained from deep inelastic and dimuon production experiments. This study provides the first look at the kaon structure function. A significantly faster falloff is found as x approaches unity than for that of pions.

Meson structure function in the context of the covariant parton model

If in the context of the covariant parton model one takes into account the differences the quark model establishes between baryons and mesons, one can obtain, in turn, essential differences between baryonic and mesonic structure functions. In particular, if the sum of the free masses of the valence quarks in a meson is less than the meson mass, M q +M q ′ <M m , the structure functions do not vanish at ω −1=1, while for baryons our analysis does not add anything new to the well-known results on their structure functions. Also, if M m >M q +M q ′ , the Callan-Gross relation is violated to an extent dependent on the parametrization of the residue functions of the pole contributions to the quark-meson amplitudes. Our formalism prevents a meson creating a quark-antiquark pair on-mass-shell when M m>M q+M q ′ .

Quark Structure Functions of Mesons and the Drell-Yan Process

For massive-lepton pair production in meson-induced reactions, we use quantum chromodynamics perturbation theory to predict that the decay angular distribution in the pair rest frame will change from predominantly $1+{cos}^{2}\ensuremath{\theta}$ to ${sin}^{2}\ensuremath{\theta}$ as the longitudinal-momentum fraction of the pair ${x}_{\mathrm{F}}\ensuremath{\rightarrow}+1$. The two angular distributions are associated respectively with ${(1\ensuremath{-}x)}^{2}$ and ${Q}^{\ensuremath{-}2}{(1\ensuremath{-}x)}^{0}$ components of the valence-quark structure function of the meson.