arXiv subjects

G. Niculescu

Publications and source records attributed to G. Niculescu.

At least 19 recordsLinked to original sources

arXiv2026

Multi-Differential DVCS Cross Section Measurement on the Proton in the Valence Region with CLAS12

We report the measurement of the four-fold differential cross section of deeply virtual Compton scattering (DVCS) on the proton using a longitudinally polarized 10.6 GeV CEBAF electron beam incident on a liquid-hydrogen target with the CLAS12 detector. The analysis provides 1312 data points, expanding the measured phase space in the valence quark region, 0.06 < $x_B$ < 0.58, 1.00 < $Q^2$ < 5.76 GeV$^2$, and 0.11 < $|t|$ < 1.00 GeV$^2$. DVCS cross section measurements directly constrain the real part of the DVCS amplitude through its interference with the Bethe-Heitler process, making them essential inputs to global extractions of the Compton Form Factors (CFFs), particularly the dominant CFF $H$. These data improve sensitivity to these observables and can contribute to future determinations of the proton's internal mechanical structure.

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arXiv2026

Charged kaon and proton multiplicities in semi-inclusive deep-inelastic scattering with 11 GeV electrons

Measurements of SIDIS multiplicities for charged kaons and protons from proton and deuteron targets are reported on a grid of hadron kinematic variables $0.3<z<0.7$ and $P_{t}<0.6$ GeV for leptonic variables $0.3<x<0.6$ and $3<Q^2<6$ GeV$^2$. Data were acquired in 2018-2019 at Jefferson Lab Hall C with 10.2 and 10.6~GeV electron beams impinging on 10-cm-long liquid hydrogen and deuterium targets. Electrons (hadrons) were detected in the HMS (SHMS) spectrometers. Multiplicities were fitted for each bin in $(x,~Q^2,~z,~P_{t})$ as $M_0[1+A\cos(\phi^*)+B\cos(2\phi^*)]$. The kaon kinematic range spans the regions where transverse-momentum-dependent factorization can be applied in SIDIS, and a `soft' central region where other processes are of critical importance. The kaon to pion ratios of $M_0$ are in reasonable agreement with predictions using the DSS fragmentation functions for $K^+$, but are mostly well below them for $K^-$. The kaon azimuthal modulations are consistent with zero. The kinematic range for protons is centered on the `soft' central region. The proton-to-pion multiplicity ratios are more than an order-of-magnitude larger than TMD predictions at the lowest value of $W^2$, decreasing to as little as a factor of two at the highest value of $W^2$. No significant difference is observed between proton and deuteron targets. These trends are consistent with Lund Monte Carlo predictions. The proton values of $A$ are consistently positive, with an average value of approximately 0.01, while $B$ is consistent with zero.

hep-ex↗

arXiv2026

Measurement of the near-threshold J$/\psi$ photoproduction cross section with the CLAS12 experiment

We present measurements of the total and differential cross sections for near-threshold J/$\psi$ photoproduction obtained with the CLAS12 detector at the Thomas Jefferson National Accelerator Facility. The results are based on data collected during the Fall 2018 and Spring 2019 running periods, using electron beams with energies of 10.6 and 10.2 GeV, respectively, scattered off a liquid-hydrogen target. Near-threshold J$/\psi$ photoproduction offers a unique sensitivity to the strong interaction in the non-perturbative regime of Quantum Chromodynamics (QCD). The energy dependence of the cross section constrains the underlying J$/\psi$ production mechanisms, including multi-gluon exchange and potential baryonic excitations. Additionally, the $t$-dependence of the differential cross section can be related to the transverse spatial distribution of gluons in the proton, providing critical input for theoretical descriptions of the gluonic structure of the proton. An interpretation of the results in terms of the gluon content of the proton is presented, providing new experimental constraints on QCD-inspired models of the proton structure and the role of gluonic degrees of freedom in hadronic mass generation.

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arXiv2026

Measurements of Beam Spin Asymmetries of $\pi^\pm\pi^0$ dihadrons at CLAS12

A first measurement of beam spin asymmetries for $\pi^+\pi^0$ and $\pi^-\pi^0$ pairs in semi-inclusive deep inelastic scattering is reported. The asymmetries in the dihadron angular distributions were measured from the scattering of a 10.6 GeV longitudinally polarized electron beam off a proton target, using the CLAS12 detector at Jefferson Lab. A photon classifier using a Gradient Boosted Trees (GBTs) architecture was trained with Monte Carlo simulations to reduce the amount of false combinatorial background $\pi^0$s, increasing statistics by up to five-fold compared to previous CLAS12 $\pi^0$ analyses. A nonzero $\sin\phi_{R_\perp}$ asymmetry is observed. This measurement is sensitive to the underexplored collinear twist-3 PDF $e(x)$, which encodes quark-gluon correlations in the proton, and presents a new avenue for its point-by-point extraction. The asymmetries also provide the first experimental evidence for the isospin-dependence of the helicity-dependent dihadron fragmentation function $G_1^\perp$, revealed by a sign-difference between the $\pi^+\pi^0$ and $\pi^-\pi^0$ channels in the $\sin(\phi_h-\phi_{R_\perp})$ modulation. In contrast, a large, same-sign enhancement near the $\rho$ mass for the $\sin(2\phi_h-2\phi_{R_\perp})$ modulation is observed, matching spectator model predictions in $\pi^+\pi^-$ pairs.

hep-ex↗

arXiv2026

Frascati 22 GeV Workshop Summary

This document summarizes the outcomes of the "Science at the Luminosity Frontier: Jefferson Lab at 22 GeV" workshop, held at the INFN Laboratori Nazionali di Frascati in December 2024. The primary goal of the workshop was to critically assess and refine the scientific case for a proposed energy upgrade of the Continuous Electron Beam Accelerator Facility (CEBAF) to 22 GeV. This document intends to capture the progress on developing the scientific case since the publication of a lengthy "White Paper" in summer 2024 signed by about 450 authors.

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arXiv2025

First Study of the Nuclear Response to Fast Hadrons via Angular Correlations between Pions and Slow Protons in Electron-Nucleus Scattering

We report on the first measurement of angular correlations between high-energy pions and slow protons in electron-nucleus ($eA$) scattering, providing a new probe of how a nucleus responds to a fast-moving quark. The experiment employed the CLAS detector with a 5-GeV electron beam incident on deuterium, carbon, iron, and lead targets. For heavier nuclei, the pion-proton correlation function is more spread-out in azimuth than for lighter ones, and this effect is more pronounced in the $\pi p$ channel than in earlier $\pi\pi$ studies. The proton-to-pion yield ratio likewise rises with nuclear mass, although the increase appears to saturate for the heaviest targets. These trends are qualitatively reproduced by state-of-the-art $eA$ event generators, including BeAGLE, eHIJING, and GiBUU, indicating that current descriptions of target fragmentation rest on sound theoretical footing. At the same time, the precision of our data exposes model-dependent discrepancies, delineating a clear path for future improvements in the treatment of cold-nuclear matter effects in $eA$ scattering.

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arXiv2025

Probing hard/soft factorization via beam-spin asymmetry in exclusive pion electroproduction from the proton

Deep exclusive meson production (DEMP) reactions, such as $p(\vec{e},e'\pi^+)n$, provide opportunities to study the three-dimensional structure of the nucleon through differential cross section and beam- and target-spin asymmetry measurements. This work aims to probe the onset of the hard/soft factorization regime through the exclusive $p(\vec{e},e'\pi^+)n$ reaction, as measured in the KaonLT experiment at Jefferson Lab Hall C. A 10.6 GeV longitudinally polarized electron beam was incident on an unpolarized liquid hydrogen target, and the scattered electron and produced meson were detected in two magnetic focusing spectrometers, enabling precision cross section measurements. The cross section ratio $\sigma_{LT'}/\sigma_0$ was extracted from the beam-spin asymmetry $A_{LU}$. The $t$-dependence of $\sigma_{LT'}/\sigma_0$ was determined at fixed $Q^2$ and $x_B$ over a range of kinematics from $2 2$ GeV). Furthermore, these data are combined with recent results from CLAS/CLAS12 to determine the $Q^2$-dependence of $\sigma_{LT'}/\sigma_0$ at two ($x_B$, $t$) settings. This was fairly flat, with $Q^2$ not having a measurable effect on the value of $\sigma_{LT'}/\sigma_0$ in the range explored. Results are compared to predictions from the generalized parton distribution (GPD) formalism, which relies explicitly on hard/soft factorization, and Regge formalism. The Regge models better predict $\sigma_{LT'}/\sigma_0$, which suggests that the factorization regime is not yet reached.

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arXiv2025

Flavor, transverse momentum, and azimuthal dependence of charged pion multiplicities in SIDIS with 10.6 GeV electrons

Measurements of SIDIS multiplicities for $\pi^+$ and $\pi^-$ from proton and deuteron targets are reported on a grid of hadron kinematic variables $z$, $P_{T}$, and $\phi^{*}$ for leptonic kinematic variables in the range $0.3<x<0.6$ and $3<Q^2<5$ GeV$^2$. Data were acquired in 2018-2019 at Jefferson Lab Hall C with a 10.6~GeV electron beam impinging on 10-cm-long liquid hydrogen and deuterium targets. Scattered electrons and charged pions were detected in the HMS and SHMS spectrometers, respectively. The multiplicities were fitted for each bin in $(x,~Q^2,~z,~P_{t})$ to extract the $\phi^{*}$ independent $M_0$ and the azimuthal modulations $\langle \cos(\phi^{*}) \rangle$ and $\langle \cos(2\phi^{*}) \rangle$. The $P_t$-dependence of the $M_0$ results was found to be remarkably consistent for the four cases studied: $ep\rightarrow e \pi^+ X$, $ep\rightarrow e \pi^- X$, $ed\rightarrow e \pi^+ X$, $ed\rightarrow e \pi^- X$ over the range $0<P_t<0.4$ GeV, as were the multiplicities evaluated near $\phi^* = 180^\circ$ over the extended range $0<P_t<0.7$ GeV. The Gaussian widths of the $P_t$-dependence exhibit a quadratic increase with $z$. The $\cos(\phi^{*})$ modulations were found to be consistent with zero for $\pi^+$, in agreement with previous world data, while the $\pi^-$ moments were, in many cases, significantly greater than zero. The $\cos(2\phi^{*})$ modulations were found to be consistent with zero. The higher statistical precision of this dataset compared to previously published data should allow improved determinations of quark transverse momentum distributions and higher twist contributions.

hep-ex↗

arXiv2025

Recoil Polarization in $K^+Y$ Electroproduction in the Nucleon Resonance Region with CLAS12

Hyperon recoil polarization measurements for the exclusive electroproduction of $K^+\Lambda$ and $K^+\Sigma^0$ final states from an unpolarized proton target have been carried out using the CLAS12 spectrometer at Jefferson Laboratory. The measurements at beam energies of 6.535~GeV and 7.546~GeV span the range of four-momentum transfer $Q^2$ from 0.3 to 4.5~GeV$^2$ and invariant mass $W$ from 1.6 to 2.4~GeV, while covering the full center-of-mass angular range of the $K^+$. These new $\Lambda$ polarization observables extend the existing data in a similar kinematic range but from a significantly larger dataset. However, they represent the first electroproduction measurements of this observable for the $\Sigma^0$. These data will allow for better exploration of the reaction mechanism in strangeness production, for further understanding of the spectrum and structure of excited nucleon states that couple to $KY$, and for improved insight into the strong interaction in the non-perturbative domain.

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arXiv2025

Measurement of single- and double-polarization observables in the photoproduction of $\pi^+\pi^-$~meson pairs off the proton using CLAS at Jefferson Laboratory

The photoproduction of $\pi^+\pi^-$ meson pairs off the proton has been studied in the reaction $\gamma p\to p\,\pi^+\pi^-$ using the CEBAF Large Acceptance Spectrometer (CLAS) and the frozen-spin target (FROST) in Hall B at the Thomas Jefferson National Accelerator Facility. For the first time, the beam and target asymmetries, $I^{s,c}$ and $P_{x,y}$, have been measured along with the beam-target double-polarization observables, $P^{s,c}_{x,y}$, using a transversely polarized target with center-of-mass energies ranging from 1.51 GeV up to 2.04 GeV. These data and additional $\pi\pi$ photoproduction observables from CLAS and experiments elsewhere were included in a partial-wave analysis within the Bonn-Gatchina framework. Significant contributions from $s$-channel resonance production are observed in addition to $t$-channel exchange processes. The data indicate significant contributions from $N^\ast$ and $\Delta^\ast$ resonances in the third and fourth resonance regions.

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arXiv2025

Multidimensional Measurements of Beam Single Spin Asymmetries in Semi-inclusive Deep-inelastic Charged Kaon Electroproduction off Protons in the Valence Region

Measurements of beam single spin asymmetries in semi-inclusive deep inelastic electron scattering (SIDIS) with positively charged kaons off protons have been performed with 10.6 and 10.2 GeV incident electron beams using the CLAS12 spectrometer at Jefferson Lab. We report an analysis of the electroproduction of positively charged kaons over a large kinematic range of fractional energy, Bjorken $x$, transverse momentum, and photon virtualities $Q^2$ ranging from 1 GeV$^2$ up to 6 GeV$^2$. This is the first published multi-dimensionally binned CLAS12 measurement of a kaon SIDIS single spin asymmetry in the valence quark regime. The data provide constraints on the structure function ratio $F_{LU}^{\sin{\phi}}/F_{UU}$, where $F_{LU}^{\sin{\phi}}$ is a quantity with a leading twist of twist-3 that can reveal novel aspects of the quark-gluon correlations within the nucleon. The impact of the data on understanding the underlying reaction mechanisms and their kinematic variation is explored using theoretical models for the different contributing twist-3 parton distribution functions (PDFs) and fragmentation functions (FFs).

hep-ex↗

arXiv2025

The SHMS 11 GeV/c Spectrometer in Hall C at Jefferson Lab

The Super High Momentum Spectrometer (SHMS) has been built for Hall C at the Thomas Jefferson National Accelerator Facility (Jefferson Lab). With a momentum capability reaching 11 GeV/c, the SHMS provides measurements of charged particles produced in electron-scattering experiments using the maximum available beam energy from the upgraded Jefferson Lab accelerator. The SHMS is an ion-optics magnetic spectrometer comprised of a series of new superconducting magnets which transport charged particles through an array of triggering, tracking, and particle-identification detectors that measure momentum, energy, angle and position in order to allow kinematic reconstruction of the events back to their origin at the scattering target. The detector system is protected from background radiation by a sophisticated shielding enclosure. The entire spectrometer is mounted on a rotating support structure which permits measurements to be taken with a large acceptance over laboratory scattering angles from 5.5 to 40 degrees, thus allowing a wide range of low cross-section experiments to be conducted. These experiments complement and extend the previous Hall C research program to higher energies.

physics.ins-det↗

arXiv2025

Inclusive Electron Scattering in the Resonance Region off a Hydrogen Target with CLAS12

Inclusive electron scattering cross sections off a hydrogen target at a beam energy of 10.6 GeV have been measured with data collected from the CLAS12 spectrometer at Jefferson Laboratory. These first absolute cross sections from CLAS12 cover a wide kinematic area in invariant mass W of the final state hadrons from the pion threshold up to 2.5 GeV for each bin in virtual photon four-momentum transfer squared $Q^2$ from 2.55 to 10.4~GeV$^2$ owing to the large scattering angle acceptance of the CLAS12 detector. Comparison of the cross sections with the resonant contributions computed from the CLAS results on the nucleon resonance electroexcitation amplitudes has demonstrated a promising opportunity to extend the information on their $Q^2$ evolution up to 10 GeV$^2$. Together these results from CLAS and CLAS12 offer good prospects for probing the nucleon parton distributions at large fractional parton momenta $x$ for $W$ < 2.5 GeV, while covering the range of distances where the transition from the strongly coupled to the perturbative regimes is expected.

hep-ex↗

arXiv2025

Investigation of Medium Modifications to $^{12}$C Structure Functions in the Resonance Region

We present results from a high precision experimental study of the nuclear modification of the longitudinal ($F_L$) to transverse ($F_1$) structure function ratio for bound nucleons in the resonance region. The inclusive electron scattering cross sections were measured in Jefferson Lab Experimental Hall C on carbon and deuterium nuclei for a large range of kinematics, allowing for separations of the longitudinal and transverse structure functions to be performed at a range of four-momentum transfer values $0.5 \le Q^2 \le$ 3.75 GeV$^2$. In contrast to the significant body of measurements of the nuclear modification of the $F_2$ structure function in the deep inelastic scattering region, there is very little on $F_L$ and $R = F_L / 2xF_1$ in the region of the nucleon resonances. In this paper we present measurements of the nuclear effect on $R$ for $^{12}$C ($R_C$) relative to deuterium ($R_D$). These results indicate regions in which in $R_C>R_D$, requiring that the nuclear modifications be different in all three structure functions, $F_2$, $F_1$ and $F_L$.

nucl-ex↗

arXiv2025

Studying the Strangeness $D$-Term in Hall C via Exclusive $\phi$ Electroproduction

We propose a measurement of exclusive electroproduction of $\phi$ mesons near threshold in Hall C. We will measure the |t|-dependence of the exclusive $\phi$ electroproduction cross section, which has recently been proposed as an observable sensitive to the strangeness $D$-term. The contribution of strangeness to the total $D$-term is presently unknown, with different arguments favoring $D_s$ being large, being small, or even having opposite sign from the total $D$-term. Our exploratory measurement is designed to distinguish between these hypotheses. If $D_s$ turns out to be small, $\phi$ electroproduction can be used to study the gluon $D$-term. In addition, this dataset will allow us to perform measurements of other exclusive meson final states, including the first measurement of $\eta'$ electroproduction and multi-differential measurements of $\eta$ and $\omega$ electroproduction.

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arXiv2024

Quasielastic $\overrightarrow{^{3}\mathrm{He}}(\overrightarrow{e},{e'})$ Asymmetry in the Threshold Region

A measurement of the double-spin asymmetry from electron-$^{3}$He scattering in the threshold region of two- and three-body breakup of $^{3}$He was performed at Jefferson Lab, for Q$^{2}$ values of 0.1 and 0.2 (GeV/$c$)$^{2}$. The results of this measurement serve as a stringent test of our understanding of few-body systems. When compared with calculations from plane wave impulse approximation and Faddeev theory, we found that the Faddeev calculations, which use modern nuclear potentials and prescriptions for meson-exchange currents, demonstrate an overall good agreement with data.

nucl-ex↗

arXiv2024

Flavor Dependence of Charged Pion Fragmentation Functions

We have measured the flavor dependence of multiplicities for pi^+ and pi^- production in semi-inclusive deep-inelastic scattering (SIDIS) on proton and deuteron targets to explore a possible charge symmetry violation in fragmentation functions. The experiment used an electron beam with energies of 10.2 and 10.6 GeV at Jefferson Lab and the Hall-C spectrometers. The electron kinematics spanned the range 0.3<x<0.6, 2<Q^2<5.5 GeV^2, and 4<W^2<11 GeV^2. The pion fractional momentum range was 0.3< z <0.7, and the transverse momentum range was 0<p_T<0.25 GeV/c. Assuming factorization at low p_T and allowing for isospin breaking, we find that the results can be described by two "favored" and two "un-favored" effective low $p_T$ fragmentation functions that are flavor-dependent. However, they converge to a common flavor-independent value at the lowest x or highest W of this experiment.

nucl-ex↗

arXiv2024

First Measurement of Deeply Virtual Compton Scattering on the Neutron with Detection of the Active Neutron

Measuring Deeply Virtual Compton Scattering on the neutron is one of the necessary steps to understand the structure of the nucleon in terms of Generalized Parton Distributions (GPDs). Neutron targets play a complementary role to transversely polarized proton targets in the determination of the GPD $E$. This poorly known and poorly constrained GPD is essential to obtain the contribution of the quarks' angular momentum to the spin of the nucleon. DVCS on the neutron was measured for the first time selecting the exclusive final state by detecting the neutron, using the Jefferson Lab longitudinally polarized electron beam, with energies up to 10.6 GeV, and the CLAS12 detector. The extracted beam-spin asymmetries, combined with DVCS observables measured on the proton, allow a clean quark-flavor separation of the imaginary parts of the GPDs $H$ and $E$.

hep-ex↗