arXiv subjects

W. Melnitchouk

Publications and source records attributed to W. Melnitchouk.

At least 127 records · Page 7Linked to original sources

arXiv2012

Next-to leading order analysis of target mass corrections to structure functions and asymmetries

We perform a comprehensive analysis of target mass corrections (TMCs) to spin-averaged structure functions and asymmetries at next-to-leading order. Several different prescriptions for TMCs are considered, including the operator product expansion, and various approximations to it, collinear factorization, and xi-scaling. We assess the impact of each of these on a number of observables, such as the neutron to proton F_2 structure function ratio, and parity-violating electron scattering asymmetries for protons and deuterons which are sensitive to gamma-Z interference effects. The corrections from higher order radiative and nuclear effects on the parity-violating deuteron asymmetry are also quantified.

hep-ph↗

arXiv2011

How well do we know the neutron structure function?

We present a detailed analysis of the uncertainty in the neutron F2n structure function extracted from inclusive deuteron and proton deep-inelastic scattering data. The analysis includes experimental uncertainties as well as uncertainties associated with the deuteron wave function, nuclear smearing, and nucleon off-shell corrections. Consistently accounting for the Q^2 dependence of the data and calculations, and restricting the nuclear corrections to microscopic models of the deuteron, we find significantly smaller uncertainty in the extracted F2n/F2p ratio than in previous analyses. In addition to yielding an improved extraction of the neutron structure function, this analysis also provides an important baseline that will allow future, model-independent extractions of neutron structure to be used to examine nuclear medium effects in the the deuteron.

hep-ph↗

arXiv2011

Constraints on the large-x d/u ratio from electron-nucleus scattering at x>1

Recently the ratio of neutron to proton structure functions F_2n/F_2p was extracted from a phenomenological correlation between the strength of the nuclear EMC effect and inclusive electron-nucleus cross section ratios at x>1. Within conventional models of nuclear smearing, this "in-medium correction" (IMC) extraction constrains the size of nuclear effects in the deuteron structure functions, from which the neutron structure function F_2n is usually extracted. The IMC data determine the resulting proton d/u quark distribution ratio, extrapolated to x=1, to be 0.23 +- 0.09 with a 90% confidence level. This is well below the SU(6) symmetry limit of 1/2 and significantly above the scalar diquark dominance limit of 0.

hep-ph↗

arXiv2011

Review of two-photon exchange in electron scattering

We review the role of two-photon exchange (TPE) in electron-hadron scattering, focusing in particular on hadronic frameworks suitable for describing the low and moderate Q^2 region relevant to most experimental studies. We discuss the effects of TPE on the extraction of nucleon form factors and their role in the resolution of the proton electric to magnetic form factor ratio puzzle. The implications of TPE on various other observables, including neutron form factors, electroproduction of resonances and pions, and nuclear form factors, are summarized. Measurements seeking to directly identify TPE effects, such as through the angular dependence of polarization measurements, nonlinear epsilon contributions to the cross sections, and via e+ p to e- p cross section ratios, are also outlined. In the weak sector, we describe the role of TPE and gamma-Z interference in parity-violating electron scattering, and assess their impact on the extraction of the strange form factors of the nucleon and the weak charge of the proton.

nucl-th↗

arXiv2011

New formulation of gamma-Z box corrections to the weak charge of the proton

We present a new formulation of one of the major radiative corrections to the weak charge of the proton -- that arising from the axial-vector hadron part of the $γZ$ box diagram, $\Re{\rm e}\, \Box_{γZ}^{\rm A}$. This formulation, based on dispersion relations, relates the $γZ$ contributions to moments of the $F_3^{γZ}$ interference structure function. It has a clear connection to the pioneering work of Marciano and Sirlin, and enables a systematic approach to improved numerical precision. Using currently available data, the total correction from all intermediate states is $\Re{\rm e}\, \Box_{γZ}^{\rm A} = 0.0044(4)$ at zero energy, which shifts the theoretical estimate of the proton weak charge from $0.0713(8)$ to $0.0705(8)$. The energy dependence of this result, which is vital for interpreting the Q$_{\rm weak}$ experiment, is also determined.

hep-ph↗

arXiv2011

Uncertainties in determining parton distributions at large x

We critically examine uncertainties in parton distribution functions (PDFs) at large x arising from nuclear effects in deuterium F2 structure function data. Within a global PDF analysis, we assess the impact on the PDFs from uncertainties in the deuteron wave function at short distances and nucleon off-shell effects, the use of relativistic kinematics, as well as the use of less a restrictive parametrization of the d/u ratio. We find that in particular the d-quark and gluon PDFs vary significantly with the choice of nuclear model. We highlight the impact of these uncertainties on the determination of the neutron structure function, and on W boson production and parton luminosity at the Tevatron and the LHC. Finally, we discuss prospects for new measurements sensitive to the d-quark and gluon distributions but insensitive to nuclear corrections.

hep-ph↗

arXiv2011

Unpolarized structure functions at Jefferson Lab

Over the past decade measurements of unpolarized structure functions at Jefferson Lab with unprecedented precision have significantly advanced our knowledge of nucleon structure. These have for the first time allowed quantitative tests of the phenomenon of quark-hadron duality, and provided a deeper understanding of the transition from hadron to quark degrees of freedom in inclusive scattering. Dedicated Rosenbluth-separation experiments have yielded high-precision transverse and longitudinal structure functions in regions previously unexplored, and new techniques have enabled the first glimpses of the structure of the free neutron, without contamination from nuclear effects.

nucl-ex↗

arXiv2011

Observation of quark-hadron duality in gamma*-p helicity cross sections

Combining data on unpolarized and polarized inclusive proton structure functions, we perform the first detailed study of quark-hadron duality in individual helicity-1/2 and 3/2 virtual photoproduction cross sections. We find that duality is realized more clearly in the helicity-1/2 channel, with duality violating corrections < 10% over the entire nucleon resonance region, while larger, < 20% corrections are found in the helicity-3/2 sector. The results are in general agreement with quark model expectations, and suggest that data above the Delta resonance region may be used to constrain both spin-averaged and spin-dependent parton distributions.

nucl-th↗

arXiv2010

gamma-Z corrections to forward-angle parity-violating e-p scattering

We use dispersion relations to evaluate the gamma-Z box contribution to parity-violating electron scattering in the forward limit arising from the axial-vector coupling at the electron vertex. The calculation makes full use of the critical constraints from recent JLab data on electroproduction in the resonance region as well as high energy data from HERA. At the kinematics of the Qweak experiment, this gives a correction of 0.0047{+0.0011}{-0.0004}$ to the Standard Model value 0.0713(8) of the proton weak charge. While the magnitude of the correction is highly significant, the uncertainty is within the anticipated experimental uncertainty of +- 0.003.

hep-ph↗

arXiv2010

Nucleon structure at large x: nuclear effects in deuterium

I review quark momentum distributions in the nucleon at large momentum fractions x. Particular attention is paid to the impact of nuclear effects in deuterium on the d/u quark distribution ratio as x -> 1. A new global study of parton distributions, using less restrictive kinematic cuts in Q^2 and W^2, finds strong suppression of the d quark distribution once nuclear corrections are accounted for.

nucl-th↗

arXiv2010

Confirmation of quark-hadron duality in the neutron F_2 structure function

We apply a recently developed technique to extract for the first time the neutron F_2^n structure function from inclusive proton and deuteron data in the nucleon resonance region, and test the validity of quark-hadron duality in the neutron. We establish the accuracy of duality in the low-lying neutron resonance regions over a range of Q^2, and compare with the corresponding results on the proton and with theoretical expectations. The confirmation of duality in both the neutron and proton opens the possibility of using resonance region data to constrain parton distributions at large x.

hep-ph↗

arXiv2010

New parton distributions from large-x and low-Q^2 data

We report results of a new global next-to-leading order fit of parton distribution functions in which cuts on W and Q are relaxed, thereby including more data at high values of x. Effects of target mass corrections (TMCs), higher twist contributions, and nuclear corrections for deuterium data are significant in the large-x region. The leading twist parton distributions are found to be stable to TMC model variations as long as higher twist contributions are also included. The behavior of the d quark as x-->1 is particularly sensitive to the deuterium corrections, and using realistic nuclear smearing models the d-quark distribution at large x is found to be softer than in previous fits performed with more restrictive cuts.

hep-ph↗

arXiv2009

Two-photon exchange corrections to the pion form factor

We compute two-photon exchange corrections to the electromagnetic form factor of the pion, taking into account the finite size of the pion. Compared to the soft-photon approximation for the infrared divergent contribution which neglects hadron structure effects, the corrections are found to be < 1% for small Q^2 (Q^2 < 0.1 GeV^2), but increase to several percent for Q^2 > 1 GeV^2 at extreme backward angles.

nucl-th↗

arXiv2009

Hadron mass corrections in semi-inclusive deep inelastic scattering

We derive mass corrections for semi-inclusive deep inelastic scattering of leptons from nucleons using a collinear factorization framework which incorporates the initial state mass of the target nucleon and the final state mass of the produced hadron. The formalism is constructed specifically to ensure that physical kinematic thresholds for the semi-inclusive process are explicitly respected. A systematic study of the kinematic dependencies of the mass corrections to semi-inclusive cross sections reveals that these are even larger than for inclusive structure functions, especially at very small and very large hadron momentum fractions. The hadron mass corrections compete with the experimental uncertainties at kinematics typical of current facilities, and will be important to efforts at extracting parton distributions or fragmentation functions from semi-inclusive processes at intermediate energies.

hep-ph↗

arXiv2009

What can break the Wandzura--Wilczek relation?

We analyze the breaking of the Wandzura-Wilczek relation for the g_2 structure function, emphasizing its connection with transverse momentum dependent parton distribution functions. We find that the relation is broken by two distinct twist-3 terms, and clarify how these can be separated in measurements of double-spin asymmetries in semi-inclusive deep inelastic scattering. Through a quantitative analysis of available g_2 data we also show that the breaking of the Wandzura-Wilczek relation can be as large as 15-30% of the size of g_2.

hep-ph↗

arXiv2009

Detailed analysis of two-boson exchange in parity-violating e-p scattering

We present a comprehensive study of two-boson exchange (TBE) corrections in parity-violating electron-proton elastic scattering. Within a hadronic framework, we compute contributions from box (and crossed box) diagrams in which the intermediate states are described by nucleons and Delta baryons. The Delta contribution is found to be much smaller than the nucleon one at backward angles (small epsilon), but becomes dominant in the forward scattering limit (epsilon -> 1), where the nucleon contribution vanishes. The dependence of the corrections on the input hadronic form factors is small for Q^2 < 1 GeV^2, but becomes significant at larger Q^2. We compute the nucleon and Delta TBE corrections relevant for recent and planned parity-violating experiments, with the total corrections ranging from -1% for forward angles to 1-2% at backward kinematics.

nucl-th↗

arXiv2009

Duality in semi-inclusive pion electroproduction

We explore quark-hadron duality in semi-inclusive pion electroproduction on proton and neutron targets. Using the spin-flavor symmetric quark model, we compute ratios of pi+ and pi- cross sections for both unpolarized and polarized scattering, and discuss realizations of duality in several symmetry breaking scenarios. The model calculations allow one to understand some of the key features of recent data on semi-inclusive pion production at low energies.

nucl-th↗

arXiv2009

New method for extracting neutron structure functions from nuclear data

We propose a new method for extracting neutron structure functions from inclusive structure functions of nuclei. Unlike earlier approaches, the new method is applicable to both spin-averaged and spin-dependent structure functions. We test the reliability of the method on unpolarized F_2 and polarized g_1 structure functions of the deuteron in both the nucleon resonance and deep inelastic regions. The new method is able to reproduce known input functions of almost arbitrary shape to very good accuracy with only several iterations.

nucl-th↗