Initial-State Temperature of Light Meson Emission Source From Squared Momentum Transfer Spectra in High-Energy Collisions

The squared momentum transfer spectra of light mesons, π0, π+, η, and ρ0, produced in high-energy virtual photon-proton (γ*p) → meson + nucleon process in electron-proton (ep) collisions measured by the CLAS Collaboration are analyzed by the Monte Carlo calculations, where the transfer undergoes from the incident γ* to emitted meson or equivalently from the target proton to emitted nucleon. In the calculations, the Erlang distribution from a multi-source thermal model is used to describe the transverse momentum spectra of emitted particles. Our results show that the average transverse momentum (⟨pT⟩) and the initial-state temperature (Ti) increase from lower squared photon virtuality (Q2) and Bjorken variable (xB) to higher one. This renders that the excitation degree of emission source, which is described by ⟨pT⟩ and Ti, increases with increasing of Q2 and xB.

Low-x improved TMD approach to the lepto- and hadroproduction of a heavy-quark pair

We study the lepto- and hadroproduction of a heavy-quark pair in the ITMD factorization framework for dilute-dense collisions. Due to the presence of a nonzero quark mass and/or nonzero photon virtuality, new contributions appear compared to the cases of photo- and hadroproduction of dijets, for which the ITMD framework was originally derived. These extra terms are sensitive to gluons that are not fully linearly polarized. At small x, those gluons emerge only when all saturation effects are carefully taken into account. Therefore, the resulting contributions are absent in linear small-x frameworks, where gluons are fully linearly polarized. We show, however, that even for large gluon transverse momentum, these contributions are not always negligible, due to the behavior of the off-shell hard factors.

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 $$\sqrt{s}=319$$ s = 319 GeV are measured. The data are collected with the H1 detector at HERA corresponding to an integrated luminosity of 136 pb$$^{-1}$$ - 1 . Charged particle multiplicities are measured as a function of photon virtuality $$Q^2$$ Q 2 , inelasticity y and pseudorapidity $$\eta $$ η 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 $$Q^2$$ Q 2 and the Bjorken variable $$x_{\mathrm{bj}}$$ x bj are converted to the hadron entropy $$S_{\mathrm{hadron}}$$ S hadron , and predictions from a quantum entanglement model are tested.

MEASUREMENT OF DIFFERENTIAL CROSS SECTIONS OF p(e, e′π+)n FOR HIGH-LYING RESONANCES AT Q2 < 5 GeV2

The exclusive electroproduction process ep → e′nπ+ was measured in the range of the invariant mass for nπ+ system 1.6 GeV ≤ W ≤ 2.0 GeV, and the photon virtuality 1.8 GeV2 ≤ Q2 ≤ 4.0 GeV2 using CLAS. For the first time, these kinematics are probed in exclusive π+ production from the protons with nearly full coverage in the azimuthal and polar angles of the nπ+ center-of-mass system. In this experiment, approximately 39,000 differential cross-section data points were measured. In this conference proceeding, preliminary results of our latest analysis work are presented on differential cross sections and structure functions as well as Legendre Moments.

On the Interplay between Q2 and t Dependences in Exclusive Diffractive Production of Real Photons and Vector Mesons in ep Collisions

We show how the familiar phenomenological way of combining the Q2 (photon virtuality) and t (squared momentum transfer) dependences of the scattering amplitude in Deeply Virtual Compton Scattering (DVCS) [1, 2] and Vector Meson Production (VMP) [2] processes can be understood in an off-mass-shell generalization of dual amplitudes with Mandelstam analyticity [3]. By comparingdifferent approaches, we managed also to constrain the numerical values of the free parameters.

DEEPLY VIRTUAL AND EXCLUSIVE PRODUCTION OF THE OMEGA MESON

Exclusive ω electroproduction off the proton was measured at the highest possible four-momentum transfer with the (close to) 6 GeV beam now available at CEBAF. Cross sections are presented, together with an analysis of the ω spin density matrix elements. Indications are that π0 exchange in the t-channel (or rather the exchange of the corresponding saturating Regge trajectory) seems to dominate the process γ*p→ωp, even for photon virtuality Q2 as large as 5 GeV2. Contributions of the handbag type, related to Generalized Parton Distributions in the nucleon, are therefore difficult to extract from this particular process.

HIGHLIGHTS OF RECENT RESULTS WITH CLAS

Recent results on the study of the electromagnetic structure of nucleon resonances, the spin structure of protons and neutrons at small and intermediate photon virtuality, and the search for exotic pentaquark baryons are presented.

VECTOR MESON PRODUCTION AT HERA

The diffractive production of vector mesons ep→eVMY, with VM=ρ0, ω, ϕ, J/ψ, ψ′ or ϒ and with Y being either the scattered proton or a low mass hadronic system, has been extensively investigated at HERA. HERA offers a unique opportunity to study the dependences of diffractive processes on different scales: the mass of the vector meson, mVM, the centre-of-mass energy of the γp system, W, the photon virtuality, Q2 and the four-momentum transfer squared at the proton vertex, |t|. Strong interactions can be investigated in the transition from the hard to the soft regime, where the confinement of quarks and gluons occurs.