Search for: All records

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 $10.4{\mathrm{GeV}}^2$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 ${\mathrm{GeV}}^2$. 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. 

We present the first threefold differential measurement for neutral-pion multiplicity ratios produced in semi-inclusive deep-inelastic electron scattering on carbon, iron, and lead nuclei normalized to deuterium from CLAS at Jefferson Lab. We found that the neutral-pion multiplicity ratio is maximally suppressed for the leading hadrons (energy fraction $z\to$1), suppression varying from 25% in carbon up to 75% in lead. An enhancement of the multiplicity ratio at low $z$and high $p_T^2$is observed, suggesting an interconnection between these two variables. This behavior is qualitatively similar to the previous twofold differential measurement of charged pions by the HERMES Collaboration and, recently, by CLAS Collaboration. The largest enhancement was observed at high $p_T^2$for heavier nuclei, namely, iron and lead, while the smallest enhancement was observed for the lightest nucleus, carbon. This behavior suggests a competition between partonic multiple scattering, which causes enhancement, and hadronic inelastic scattering, which causes suppression. 

The concentration and isotopic composition (δC; C/N) of sedimentary organic matter (SOM) in near-shore bays and offshore shelves and basins is impacted by organic matter source (e.g., marine algae, terrestrial plants, and agricultural and sewage runoff) and natural and anthropogenic processes such as pollution, terrestrial runoff, and climate change, which can expand oxygen minimum zones, leading to decreased bottom-water dissolved oxygen (DO) and enhanced organic matter preservation. The factors that affect the sources and concentrations of SOM have not been extensively investigatedin the California margin. The objective of this study was to determine how the SOM concentrations andstable isotopes (δC; C/N) vary between shallow urban bays, offshore shelves, and deep basins and with other factors (water depth, DO and grain size). On cruises in 2018, surface sediments were collected using multicores and van-veen grabs. Samples were collected from shelves (10-14km offshore; 100-300m) and basins (90-130km offshore; 618-997m)and for comparison, urban bays in San Diego. The dissolved oxygen (DO) concentrations of seafloor-water preserved in the multicores were measured with a hand-held DO meter. In the lab, SOM concentrations were determined by Loss on Ignition (5 hours, 550°C) and grain-size distributions were determined by scanning on a CILAS 1190 particle size analyzer. Select sediments were dissolved in HCl and filtered to remove inorganic carbonates and the δC and C/N measured at UC Davis. All sediments were organic rich (2-21%) with mean grain sizes of fi ne sand or silt with variable clay (3-12%). In general, the sands were lower in organic matter (< 5%) compared to silty samples withvariable concentrations (2-22%). The greatest organic matter was found in the deeper hypoxic basins where DO was less than 1.5 mg/L. The δC & C/N were consistent with mixed terrestrial and marine organic sources and there was not a difference in mean values between the bays, shelves and basins.However, the values were highly variable for the urban bay and shelf sediments suggesting heterogenous input. Organic matter in coastal sediments are an important component of the global carbon cycle and abetter understanding of controlling factors is important in the face of climate change and increased anthropogenic impacts. 

Measurements of the polarization observables $\mathrm{\Sigma },\mathbf{P},\mathbf{T},{\mathbf{O}}_{\mathbf{x}},{\mathbf{O}}_{\mathbf{z}}$for the reaction $\vec{\gamma }p\to K_S^0{\mathrm{\Sigma }}^{+}$using a linearly polarized photon beam of energy 1.1 to 2.1 GeV are reported. The measured data provide information on a channel that has not been studied extensively, but is required for a full coupled-channel analysis in the nucleon resonance region. Observables have been simultaneously extracted using likelihood sampling with a Markov-Chain Monte Carlo process. Angular distributions in bins of photon energy $E_{\gamma }$are produced for each polarization observable. $\mathbf{T},{\mathbf{O}}_{\mathbf{x}}$, and ${\mathbf{O}}_{\mathbf{z}}$are first time measurements of these observables in this reaction. The extraction of $\mathrm{\Sigma }$extends the energy range beyond a previous measurement. The measurement of $\mathbf{P}$, the recoil polarization, is consistent with previous measurements. The measured data are shown to be significant enough to affect the estimation of the nucleon resonance parameters when fitted within a coupled-channels model. Published by the American Physical Society2025 

Measuring deeply virtual Compton scattering (DVCS) 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 Compton form factors $\mathcal{H}$and $\mathcal{E}$. Published by the American Physical Society2024 

The ALICE Collaboration reports measurements of the large relative transverse momentum ( $k_T$) component of jet substructure in $pp$and Pb-Pb collisions at center-of-mass energy per nucleon pair $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$. Enhancement in the yield of such large- $k_{\mathrm{T}}$emissions in head-on Pb-Pb collisions is predicted to arise from partonic scattering with quasiparticles of the quark-gluon plasma. The analysis utilizes charged-particle jets reconstructed by the anti- $k_{\mathrm{T}}$algorithm with resolution parameter $R=0.2$in the transverse-momentum interval $60<p_{\mathrm{T},\mathrm{ch},\mathrm{jet}}<80\mathrm{GeV}/c$. The soft drop and dynamical grooming algorithms are used to identify high transverse momentum splittings in the jet shower. Comparison of measurements in Pb-Pb and $pp$collisions shows medium-induced narrowing, corresponding to yield suppression of high- $k_T$splittings, in contrast to the expectation of yield enhancement due to quasiparticle scattering. The measurements are compared to theoretical model calculations incorporating jet modification due to jet-medium interactions (“jet quenching”), both with and without quasiparticle scattering effects. These measurements provide new insight into the underlying mechanisms and theoretical modeling of jet quenching.