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K. Hafidi

Publications and source records attributed to K. Hafidi.

At least 19 recordsLinked to original sources
arXiv2026

First Measurement of Near-Threshold J/{\psi} Photoproduction on the Neutron

We report the first measurement of the total and t-differential cross sections for near-threshold J/$\psi$ photoproduction on the neutron, obtained with the CLAS12 detector at the Thomas Jefferson National Accelerator Facility. The measurements, performed using a liquid-deuterium target, also provide the incoherent total and t-differential cross sections on the bound proton, enabling the first direct comparison of near-threshold J/$\psi$ photoproduction on bound protons and neutrons. Interpreted within Vector Meson Dominance, holographic QCD, and GPD-based frameworks, the data allow for the investigation of the gluonic structure of bound nucleons. A comparison with free proton results reveals hints of modifications to the gluon structure of nucleons in the nuclear medium, within the assumptions of the extraction and on the underlying production mechanism. The comparison of bound neutron and bound proton cross sections provides new constraints on the near-threshold J/$\psi$ production mechanism, which is crucial in order to establish J/$\psi$ photoproduction as a probe of the nucleon's gluonic structure.

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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

First measurement of polarized spin-density matrix elements and differential cross sections d$\sigma$/d$t$ in $\omega$~photoproduction off the proton for $2.7 < E_\gamma < 5.2$ GeV using CLAS at Jefferson Lab

We report on the differential cross sections d$\sigma$/d$t$, the unpolarized spin-density matrix elements $\rho^0_{00}$, $\rho^0_{1-1}$, Re\,$\rho^0_{10}$, and the first extraction of the polarized elements Im\,$\rho^3_{10}$, Im\,$\rho^3_{1-1}$ for the reaction $\gamma p\to p\omega$ using the CLAS spectrometer at Jefferson Laboratory. The $\omega$~mesons were detected in their dominant charged decay mode, $\omega \to \pi^+\pi^-\pi^0$, and all $t$-dependent results are presented in a fine binning for incident photon energies between 2.73 and 5.16~GeV (corresponding to the center-of-mass energy range $W \in [\,2.45,3.25\,]$~GeV). All matrix elements are first measurements for $-t > 0.6$~GeV$^2$. Moreover, differential cross sections d$\sigma$/d(cos\,$\Theta_{\rm \,c.m.}^{\,\omega}$) and the corresponding angle-dependent unpolarized spin-density matrix elements in the Adair frame are presented for the incident photon energy range 1.56--3.80~GeV (corresponding to $W \in [\,1.95,2.83\,]$~GeV). These new $\omega$~photoproduction data are consistent with earlier CLAS results but extend the energy range well beyond the nucleon resonance region into the Regge regime. The comparison with Regge-theory-based model predictions shows that the new data impose more stringent constraints on our understanding of $\omega$~photoproduction.

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arXiv2026

Measurements of $\gamma_v p \to \pi^+ \pi^- p'$ Cross Sections with the CLAS Detector for $Q^{2}$ from 2.0--5.0~GeV$^{2}$ and $W$ from 1.400--2.125~GeV

Observables in the electroproduction of the $\pi^+\pi^-p$ reaction channel off the proton that are sensitive to the polarization of the virtual photon are presented for the first time in addition to the extraction of the nine single-differential and fully integrated cross sections within the kinematics area of 2.0~GeV$^2 < Q^2 < 5.0$~GeV$^2$ and 1.400~GeV $< W < 2.125$~GeV measured with the CLAS detector in Hall B at Jefferson Lab. The extraction of the unpolarized cross sections has been considerably improved in comparison with previously published results from this same dataset, offering finer binning over the five-dimensional hadronic reaction phase space, and including essential advances in the acceptance evaluation by implementing a new technique to stabilize the cross section extraction and minimize the systematic uncertainty associated with the simulation statistics. These improvements are of particular importance for the extraction of the $\gamma_v p N^*$ electrocouplings from these data.

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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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arXiv2025

Measurement of Beam-Recoil Observables $C_x$ and $C_z$ for $K^+\Lambda$ Photoproduction

Exclusive photoproduction of $K^+ \Lambda$ final states off a proton target has been an important component in the search for missing nucleon resonances and our understanding of the production of final states containing strange quarks. Polarization observables have been instrumental in this effort. The current work is an extension of previously published CLAS results on the beam-recoil transferred polarization observables $C_x$ and $C_z$. We extend the kinematic range up to invariant mass $W=3.33$~GeV from the previous limit of $W=2.5$~GeV with significantly improved statistical precision in the region of overlap. These data will provide for tighter constraints on the reaction models used to unravel the spectrum of nucleon resonances and their properties by not only improving the statistical precision of the data within the resonance region, but also by constraining $t$-channel processes that dominate at higher $W$ but extend into the resonance region.

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arXiv2025

Proton Transparency and Neutrino Physics: New Methods and Modeling

Extracting accurate results from neutrino oscillation and cross section experiments requires accurate simulation of the neutrino-nucleus interaction. The rescattering of outgoing hadrons (final state interactions) by the rest of the nucleus is an important component of these interactions. We present a new measurement of proton transparency (defined as the fraction of outgoing protons that emerge without significant rescattering) using electron-nucleus scattering data recorded by the CLAS detector at Jefferson Laboratory on helium, carbon, and iron targets. This analysis by the Electrons for Neutrinos ($e4\nu$) collaboration uses a new data-driven method to extract the transparency. It defines transparency as the ratio of electron-scattering events with a detected proton to quasi-elastic electron-scattering events where a proton should have been knocked out. Our results are consistent with previous measurements that determined the transparency from the ratio of measured events to theoretically predicted events. We find that the GENIE event generator, which is widely used by oscillation experiments to simulate neutrino-nucleus interactions, needs to better describe both the nuclear ground state and proton rescattering in order to reproduce our measured transparency ratios, especially at lower proton momenta.

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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).

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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.

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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 $E$.

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arXiv2023

Demonstration of Event Position Reconstruction based on Diffusion in the NEXT-White Detector

Noble element time projection chambers are a leading technology for rare event detection in physics, such as for dark matter and neutrinoless double beta decay searches. Time projection chambers typically assign event position in the drift direction using the relative timing of prompt scintillation and delayed charge collection signals, allowing for reconstruction of an absolute position in the drift direction. In this paper, alternate methods for assigning event drift distance via quantification of electron diffusion in a pure high pressure xenon gas time projection chamber are explored. Data from the NEXT-White detector demonstrate the ability to achieve good position assignment accuracy for both high- and low-energy events. Using point-like energy deposits from $^{83\mathrm{m}}$Kr calibration electron captures ($E\sim45$keV), the position of origin of low-energy events is determined to $2~$cm precision with bias $< 1$mm. A convolutional neural network approach is then used to quantify diffusion for longer tracks (E$\geq$1.5MeV), yielding a precision of 3cm on the event barycenter. The precision achieved with these methods indicates the feasibility energy calibrations of better than 1% FWHM at Q$_{\beta\beta}$ in pure xenon, as well as the potential for event fiducialization in large future detectors using an alternate method that does not rely on primary scintillation.

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arXiv2023

Design, characterization and installation of the NEXT-100 cathode and electroluminescence regions

NEXT-100 is currently being constructed at the Laboratorio Subterr\'aneo de Canfranc in the Spanish Pyrenees and will search for neutrinoless double beta decay using a high-pressure gaseous time projection chamber (TPC) with 100 kg of xenon. Charge amplification is carried out via electroluminescence (EL) which is the process of accelerating electrons in a high electric field region causing secondary scintillation of the medium proportional to the initial charge. The NEXT-100 EL and cathode regions are made from tensioned hexagonal meshes of 1 m diameter. This paper describes the design, characterization, and installation of these parts for NEXT-100. Simulations of the electric field are performed to model the drift and amplification of ionization electrons produced in the detector under various EL region alignments and rotations. Measurements of the electrostatic breakdown voltage in air characterize performance under high voltage conditions and identify breakdown points. The electrostatic deflection of the mesh is quantified and fit to a first-principles mechanical model. Measurements were performed with both a standalone test EL region and with the NEXT-100 EL region before its installation in the detector. Finally, we describe the parts as installed in NEXT-100, following their deployment in Summer 2023.

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arXiv2023

Beam Spin Asymmetry Measurements of Deeply Virtual $\pi^0$ Production with CLAS12

The new experimental measurements of beam spin asymmetry were performed for the deeply virtual exclusive $\pi^0$ production in a wide kinematic region with the photon virtualities $Q^2$ up to 8 GeV$^2$ and the Bjorken scaling variable $x_B$ in the valence regime. The data were collected by the CEBAF Large Acceptance Spectrometer (CLAS12) at Jefferson Lab with longitudinally polarized 10.6 GeV electrons scattered on an unpolarized liquid-hydrogen target. Sizable asymmetry values indicate a substantial contribution from transverse virtual photon amplitudes to the polarized structure functions.The interpretation of these measurements in terms of the Generalized Parton Distributions (GPDs) demonstrates their sensitivity to the chiral-odd GPD $\bar E_T$, which contains information on quark transverse spin densities in unpolarized and polarized nucleons and provides access to the proton's transverse anomalous magnetic moment. Additionally, the data were compared to a theoretical model based on a Regge formalism that was extended to the high photon virtualities.

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arXiv2023

Demonstration of neutrinoless double beta decay searches in gaseous xenon with NEXT

The NEXT experiment aims at the sensitive search of the neutrinoless double beta decay in $^{136}$Xe, using high-pressure gas electroluminescent time projection chambers. The NEXT-White detector is the first radiopure demonstrator of this technology, operated in the Laboratorio Subterr\'aneo de Canfranc. Achieving an energy resolution of 1% FWHM at 2.6 MeV and further background rejection by means of the topology of the reconstructed tracks, NEXT-White has been exploited beyond its original goals in order to perform a neutrinoless double beta decay search. The analysis considers the combination of 271.6 days of $^{136}$Xe-enriched data and 208.9 days of $^{136}$Xe-depleted data. A detailed background modeling and measurement has been developed, ensuring the time stability of the radiogenic and cosmogenic contributions across both data samples. Limits to the neutrinoless mode are obtained in two alternative analyses: a background-model-dependent approach and a novel direct background-subtraction technique, offering results with small dependence on the background model assumptions. With a fiducial mass of only 3.50$\pm$0.01 kg of $^{136}$Xe-enriched xenon, 90% C.L. lower limits to the neutrinoless double beta decay are found in the T$_{1/2}^{0\nu}>5.5\times10^{23}-1.3\times10^{24}$ yr range, depending on the method. The presented techniques stand as a proof-of-concept for the searches to be implemented with larger NEXT detectors.

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arXiv2023

NEXT-CRAB-0: A High Pressure Gaseous Xenon Time Projection Chamber with a Direct VUV Camera Based Readout

The search for neutrinoless double beta decay ($0\nu\beta\beta$) remains one of the most compelling experimental avenues for the discovery in the neutrino sector. Electroluminescent gas-phase time projection chambers are well suited to $0\nu\beta\beta$ searches due to their intrinsically precise energy resolution and topological event identification capabilities. Scalability to ton- and multi-ton masses requires readout of large-area electroluminescent regions with fine spatial resolution, low radiogenic backgrounds, and a scalable data acquisition system. This paper presents a detector prototype that records event topology in an electroluminescent xenon gas TPC via VUV image-intensified cameras. This enables an extendable readout of large tracking planes with commercial devices that reside almost entirely outside of the active medium.Following further development in intermediate scale demonstrators, this technique may represent a novel and enlargeable method for topological event imaging in $0\nu\beta\beta$.

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arXiv2023

First Measurement of $Λ$ Electroproduction off Nuclei in the Current and Target Fragmentation Regions

We report results of $Λ$ hyperon production in semi-inclusive deep-inelastic scattering off deuterium, carbon, iron, and lead targets obtained with the CLAS detector and the Continuous Electron Beam Accelerator Facility 5.014~GeV electron beam. These results represent the first measurements of the $Λ$ multiplicity ratio and transverse momentum broadening as a function of the energy fraction~($z$) in the current and target fragmentation regions. The multiplicity ratio exhibits a strong suppression at high~$z$~and~an enhancement at~low~$z$. The measured transverse momentum broadening is an order of magnitude greater than that seen for light mesons. This indicates that the propagating entity interacts very strongly with the nuclear medium, which suggests that propagation of diquark configurations in the nuclear medium takes place at least part of the time, even at high~$z$. The trends of these results are qualitatively described by the Giessen Boltzmann-Uehling-Uhlenbeck transport model, particularly for the multiplicity ratios. These observations will potentially open a new era of studies of the structure of the nucleon as well as of strange baryons.

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