Nuclear Experiment

The proton--proton elastic differential cross section at very small four momentum transfer squared has been measured at three different incident proton momenta in the range of 2.5 to 3.2 GeV/c by detecting the recoil proton at polar angles close to 90 ∘ . The measurement was performed at COSY with the KOALA detector covering the Coulomb-nuclear interference region. The total cross section σ tot , which has been determined precisely, is consistent with previous measurements. The values of the slope parameter B and the relative real amplitude ratio ρ determined in this experiment alleviate the lack of data in the relevant energy region. This precise data on ρ might be an important check for a new dispersion analysis.

ALICE is designated to study the quark-gluon plasma (QGP), a state of matter where, due to high temperature and density, quarks and gluons are deconfined. One of the probes used to investigate this state of matter is quarkonium states, bound states of either a charm and anti-charm quark pair (charmonia) or a bottom and anti-bottom quark pair (bottomonia). The presence of the QGP is expected to modify the quarkonium production yields in a very specific way due to a balance between medium-induced suppression, and a recombination mechanism or a hadronization mechanism. To understand the the properties of the QGP in nucleus-nucleus collisions, it is essential to measure the quarkonium differential yields in proton-proton collisions, as it provides a reference and allows the investigation of quarkonium production mechanisms, as well as in proton-nucleus collisions to understand the cold nuclear matter effects that appear. In this contribution, the latest results for quarkonium production measured with the ALICE detector in pp collisions at different collision energies are reported. The measurements of the nuclear modification factor and anisotropic flow in Pb-Pb collisions at s NN ??????????=5.02 TeV and in p-Pb at s NN ??????????=8.16 TeV at mid- and forward rapidity are also reported. All measurements are compared to various theoretical predictions.

Jet quenching in relativistic heavy ion collisions can have multiple phenomenological consequences: jet energy loss, modification of jet substructure, and induced acoplanarity. In these proceedings, we report a measurement of the jet fragmentation function, which is one of the jet substructure observables, in peripheral Au+Au collisions at sqrt(s_NN) = 200 GeV by the STAR experiment at RHIC. In particular, we use a semi-inclusive population of jets recoiling from a high transverse momentum trigger hadron. The fragmentation function is constructed from the fraction of the transverse momentum of charged particles projected onto the jet axis over the transverse momentum of the jet. In a previous STAR publication of the semi-inclusive charged-jet spectra, the Mixed-Event technique was used along with the semi-inclusive approach to remove the uncorrelated background contributions, which enables the measurement of jet distributions at low transverse momentum and large jet radius (R) in heavy ion collisions. In this analysis we extend this approach to the measurement of jet fragmentation functions. The reported fragmentation functions are corrected for uncorrelated background and instrumental effects via unfolding. The results are compared to those in PYTHIA simulations for pp collisions.

The neutron capture cross section of 154 Gd was measured from 1 eV to 300 keV in the experimental area located 185 m from the CERN n\_TOF neutron spallation source, using a metallic sample of gadolinium, enriched to 67 % in 154 Gd. The capture measurement, performed with four C 6 D 6 scintillation detectors, has been complemented by a transmission measurement performed at the GELINA time-of-flight facility (JRC-Geel), thus minimising the uncertainty related to sample composition. An accurate Maxwellian averaged capture cross section (MACS) was deduced over the temperature range of interest for s process nucleosynthesis modeling. We report a value of 880(50) mb for the MACS at kT=30 keV, significantly lower compared to values available in literature. The new adopted 154 Gd(n, γ ) cross section reduces the discrepancy between observed and calculated solar s-only isotopic abundances predicted by s-process nucleosynthesis models.

We present a measurement of the low-energy (0--60 keV) γ ray spectrum produced in the α -decay of 233 U using a dedicated cryogenic magnetic micro-calorimeter. The energy resolution of ∼ 10 eV, together with exceptional gain linearity, allow us to measure the energy of the low-lying isomeric state in 229 Th using four complementary evaluation schemes. The most accurate scheme determines the 229 Th isomer energy to be 8.10(17) eV, corresponding to 153.1(37) nm, superseding in precision previous values based on γ spectroscopy, and agreeing with a recent measurement based on internal conversion electrons. We also measure branching ratios of the relevant excited states to be b 29 =9.3(6)% and b 42 =0.3(3)% .

We measured two-neutrino double beta decay of 130 Te using an exposure of 300.7 kg ??yr accumulated with the CUORE detector. Using a Bayesian analysis to fit simulated spectra to experimental data, it was possible to disentangle all the major background sources and precisely measure the two-neutrino contribution. The half-life is in agreement with past measurements with a strongly reduced uncertainty: T 2ν 1/2 = 7.71 +0.08 ??.06 (stat.) +0.12 ??.15 (syst.)? 10 20 yr. This measurement is the most precise determination of the 130 Te 2 νββ decay half-life to date.

The E12-14-012 experiment, performed in Jefferson Lab Hall A, has collected exclusive electron-scattering data (e,e ??p) in parallel kinematics using natural argon and natural titanium targets. Here, we report the first results of the analysis of the data set corresponding to beam energy of 2,222 MeV, electron scattering angle 21.5 deg, and proton emission angle -50 deg. The differential cross sections, measured with ??4% uncertainty, have been studied as a function of missing energy and missing momentum, and compared to the results of Monte Carlo simulations, obtained from a model based on the Distorted Wave Impulse Approximation.

We report the first measurement of the average of the electron-proton and positron-proton elastic scattering cross sections. This lepton charge-averaged cross section is insensitive to the leading effects of hard two-photon exchange, giving more robust access to the proton's electromagnetic form factors. The cross section was extracted from data taken by the OLYMPUS experiment at DESY, in which alternating stored electron and positron beams were scattered from a windowless gaseous hydrogen target. Elastic scattering events were identified from the coincident detection of the scattered lepton and recoil proton in a large-acceptance toroidal spectrometer. The luminosity was determined from the rates of Møller, Bhabha and elastic scattering in forward electromagnetic calorimeters. The data provide some selectivity between existing form factor global fits and will provide valuable constraints to future fits.

Background: Generalized polarizabilities (GPs) are important observables to describe the nucleon structure, and measurements of these observables are still scarce. Purpose: This paper presents details of a virtual Compton scattering (VCS) experiment, performed at the A1 setup at the Mainz Microtron by studying the ep→epγ reaction. The article focuses on selected aspects of the analysis. Method: The experiment extracted the P LL − P TT /ϵ and P LT structure functions, as well as the electric and magnetic GPs of the proton, at three new values of the four-momentum transfer squared Q 2 : 0.10, 0.20 and 0.45 GeV 2 . Results: We emphasize the importance of the calibration of experimental parameters. The behavior of the measured ep→epγ cross section is presented and compared to the theory. A detailed investigation of the polarizability fits reveals part of their complexity, in connection with the higher-order terms of the low-energy expansion. Conclusions: The presented aspects are elements which contribute to minimize the systematic uncertainties and improve the precision of the physics results.

We report measurements of the photon beam asymmetry Σ for the reaction γ ⃗ p→ K + Σ 0 (1193) using the GlueX spectrometer in Hall D at Jefferson Lab. Data were collected using a linearly polarized photon beam in the energy range of 8.2-8.8 GeV incident on a liquid hydrogen target. The beam asymmetry Σ was measured as a function of the Mandelstam variable t , and a single value of Σ was extracted for events produced in the u -channel. These are the first exclusive measurements of the photon beam asymmetry Σ for the reaction in this energy range. For the t -channel, the measured beam asymmetry is close to unity over the t -range studied, −t=(0.1−1.4) (GeV/ c ) 2 , with an average value of Σ=1.00±0.05 . This agrees with theoretical models that describe the reaction via the natural-parity exchange of the K ∗ (892) Regge trajectory. A value of Σ=0.41±0.09 is obtained for the u -channel integrated up to −u=2.0 ~(GeV/ c ) 2 .