Function Of Photon Virtuality Research Articles

Measurement of charged particle multiplicity distributions in DIS at HERA and its implication to entanglement entropy of partons

Charged particle multiplicity distributions in positron-proton deep inelastic scattering at a centre-of-mass energy sqrt{s}=319 GeV are measured. The data are collected with the H1 detector at HERA corresponding to an integrated luminosity of 136 pb^{-1}. Charged particle multiplicities are measured as a function of photon virtuality Q^2, inelasticity y and pseudorapidity eta in the laboratory and the hadronic centre-of-mass frames. Predictions from different Monte Carlo models are compared to the data. The first and second moments of the multiplicity distributions are determined and the KNO scaling behaviour is investigated. The multiplicity distributions as a function of Q^2 and the Bjorken variable x_{mathrm{bj}} are converted to the hadron entropy S_{mathrm{hadron}}, and predictions from a quantum entanglement model are tested.

Exclusive production of ϕ meson in the γ*p→ϕp reaction at large photon virtualities within kT -factorization approach

We apply the $k_T$-factorization approach to the production of $\phi$ meson in deep-inelastic scattering. The helicity-conserving $\gamma^*(T,L) \to \phi$ impact factor is calculated for longitudinal and transverse photon polarization using $\phi$ meson distribution amplitudes. Different unintegrated gluon distributions are used in the calculations. The formalism for massless quarks/antiquarks gives too large transverse and longitudinal cross sections for photon virtualities below $Q^2\sim 8 \, \rm{GeV}^2$. We suggest how to improve the description of the HERA data by introducing effective strange quark masses into the formalism. We derive the corresponding massive impact factor by comparing to the light-cone wave function representation used in previous $k_T$-factorization calculations and the color-dipole approaches. As a byproduct we present expressions for higher twist-amplitudes as weighted integrals over the light-cone wave function. The quark mass $m_q \approx 0.5$ GeV allows to improve the description of both longitudinal and transverse cross section down to $Q^2 \sim$ 4 GeV$^2$. We present also the polarized cross section ratio $\sigma_L/\sigma_T$ and the behavior of the total cross section $\sigma_{tot} = \sigma_{L} + \sigma_{T}$ as a function of photon virtuality.

Beam-target double-spin asymmetry in quasielastic electron scattering off the deuteron with CLAS

Quasi-elastic electron scattering on the deuteron is a benchmark reaction to test our understanding of deuteron structure and the properties and interactions of the two nucleons bound in the deuteron. The experimental data presented here can be used to test state-of-the-art models of the deuteron and the two-nucleon interaction in the final state after two-body breakup of the deuteron. Focusing on polarization degrees of freedom, we gain information on spin-momentum correlations in the deuteron ground state (due to the D-state admixture) and on the limits of the Impulse Approximation (IA) picture as it applies to measurements of spin-dependent observables like spin structure functions for bound nucleons. We measured the beam-target double spin asymmetry for quasi-elastic electron scattering off the deuteron at several beam energies using the CEBAF Large Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility. The deuterons were polarized along (or opposite to) the beam direction. The double spin asymmetries were measured as a function of photon virtuality , missing momentum, and the angle between the (inferred) "spectator" neutron and the momentum transfer direction. The results are compared with a recent model that includes Final State Interactions (FSI) using a complete parameterization of nucleon-nucleon scattering, as well as a simplified model using the Plane Wave Impulse Approximation (PWIA). We find overall good agreement with both the PWIA and FSI expectations at low to medium missing momenta, including the change of the asymmetry due to the contribution of the deuteron D-state at higher momenta. At the highest missing momenta, our data clearly agree better with the calculations including FSI.

Perturbative QCD calculation of real and virtual Compton scattering.

We present the predictions of perturbative QCD for Compton scattering as a function of photon virtuality. Individual helicity amplitudes are given, along with formulas to permit calculation of the $\mathrm{eN}\ensuremath{\rightarrow}{e}^{\ensuremath{'}}{N}^{\ensuremath{'}}\ensuremath{\gamma}$ amplitudes. We discover remarkable structure in some of the amplitudes as a function of photon virtuality. Since $\mathrm{eN}\ensuremath{\rightarrow}{e}^{\ensuremath{'}}{N}^{\ensuremath{'}}\ensuremath{\gamma}$ contains a contribution from Bethe-Heitler scattering, whose magnitude and phase are known, the phases of the virtual Compton amplitudes can in principle be measured by interference with it. It has been shown that the leading-twist behavior of high-energy, large-angle Compton scattering is not affected by soft physics; thus at sufficiently high energies these predictions provide a stringent test of QCD.