Nuclear Experiment

We have performed an exclusive measurement of the K − + 3 He→Λpn reaction at an incident kaon momentum of 1 GeV/c .In the Λp invariant mass spectrum, a clear peak was observed below the mass threshold of K ¯ +N+N , as a signal of the kaonic nuclear bound state, K ¯ NN .The binding energy, decay width, and S -wave Gaussian reaction form-factor of this state were observed to be B K =42±3(stat. ) +3 −4 (syst.) MeV , Γ K =100±7(stat. ) +19 −9 (syst.) MeV , and Q K =383±11(stat. ) +4 −1 (syst.) MeV/c , respectively. The total production cross-section of K ¯ NN , determined by its Λp decay mode, was σ tot K ⋅B R Λp =9.3±0.8(stat. ) +1.4 −1.0 (syst.) μb .We estimated the branching ratio of the K ¯ NN state to the Λp and Σ 0 p decay modes as B R Λp /B R Σ 0 p ∼1.7 , by assuming that the physical processes leading to the ΣNN final states are analogous to those of Λpn .

Data on the beam asymmetry Σ in the photoproduction of η mesons off protons are reported for tagged photon energies from 1130 to 1790 MeV (mass range from W= 1748 MeV to W= 2045 MeV). The data cover the full solid angle that allows for a precise moment analysis. For the first time, a strong cusp effect in a polarization observable has been observed that is an effect of a branch-point singularity at the p η ′ threshold [ E γ = 1447 MeV ( W= 1896 MeV)]. The latest BnGa partial wave analysis includes the new beam asymmetry data and yields a strong indication for the N(1895) 1 2 − nucleon resonance, demonstrating the importance of including all singularities for a correct determination of partial waves and resonance parameters.

The Gamow-Teller strength distribution of the decay of 186 Hg into 186 Au has been determined for the first time using the total absorption gamma spectroscopy technique and has been compared with theoretical QRPA calculations using the SLy4 Skyrme force. The measured Gamow-Teller strength distribution and the half-life are described by mixing oblate and prolate configurations independently in the parent and daughter nuclei. The best description of the experimental beta strength is obtained with dominantly prolate components for both parent 186 Hg and daughter 186 Au. The approach also allowed us to determine an upper limit of the oblate component in the parent state. The complexity of the analysis required the development of a new approach in the analysis of the X-ray gated total absorption spectrum.

Isospin is an approximate symmetry in atomic nuclei, arising from the rather similar properties of protons and neutrons. Perhaps the clearest manifestation of isospin within nuclei is in the near-identical structure of excited states in mirror nuclei: nuclei with inverted numbers of protons and neutrons. Isospin symmetry, and therefore mirror-symmetry, is broken by electromagnetic interactions and the difference in the masses of the up and down quarks. A recent study by Hoff and collaborators presented evidence that the ground-state spin of 73 Sr is different from that of its mirror, 73 Br, due to an inversion of the ground- and first-excited states, separated by only 27 keV in the 73 Br system. In this brief note, we place this inversion within the necessary context of the past half-century of experimental and theoretical work, and show that it is entirely consistent with normal behaviour, and affords no new insight into isospin-symmetry breaking. The essential point is that isospin-breaking effects due to the Coulomb interaction frequently vary from level to level within a given medium-mass nucleus by as much as 200 keV. Any level splitting smaller than this is liable to manifest a level inversion in the mirror partner which, absent disagreement with an appropriate nuclear model, does not challenge our understanding. While we note the novelty of an inversion in nuclear ground states, we emphasize that in the context of isospin there is nothing specifically illuminating about the ground state, or a level inversion.

Heavy quarks are effective probes of the hot and dense nuclear matter, the quark-gluon plasma, produced in ultra-relativistic heavy-ion collisions. Due to the short time scale characterising their production, heavy quarks experience the whole evolution of the system. In particular, measurements of heavy-flavour hadron production in Pb-Pb collisions at LHC energies give insight into the mechanisms of heavy-quark transport in the deconfined matter. In small hadronic systems, pp and p-Pb collisions, heavy-flavour measurements provide the baseline for observations of hot-medium effects in heavy-ion collisions, as well as tests of perturbative quantum chromodynamic calculations and measurements of cold-nuclear-matter effects. In this contribution, recent ALICE results on open heavy-flavour hadron production in pp, p-Pb and Pb-Pb collisions at various energies are discussed. New measurements are presented both for fully-reconstructed charmed hadrons and for single electrons from heavy-flavour hadron decays, also investigating the beauty-quark production via the measurement of D mesons and electrons from beauty-hadron decays.

The recent paper by Chiara et al. provided the first experimental evidence of nuclear excitation by electron capture (NEEC), responding a long-standing theoretical prediction. NEEC was inferred to be the main channel to excite an isomer in Molybdenum-93 to a higher state, leading to a rapid release of full isomer energy (isomer depletion). The deduced large excitation probability P exc =0.010(3) for this mechanism implied strong influence on the survival of nuclei in stellar environments. However, the excitation probability is much higher than the estimated NEEC probability P NEEC according to a following theoretical work by approximately 9 orders of magnitude. Nevertheless, the reported P exc is predicted to be due to other unknown mechanism causing isomer depletion, which is expected to open up a new era of the storage and release of nuclear energy. Here we report an analysis of the reported experimental results, showing that the observed isomer depletion is significantly overestimated due to the contamination.

The existence and location of the QCD critical point is an object of vivid experimental and theoretical studies. Rich and beautiful data recorded by experiments at SPS and RHIC allow for a systematic search for the critical point - the search for a non-monotonic dependence of various correlation and fluctuation observables on collision energy and size of colliding nuclei.

One of the ultimate goals of nuclear collision experiments at high energy is to map the phase diagram of strongly interacting matter. A very challenging task is the determination of the QCD phase structure including the search for critical behavior and verification of the possible existence of a critical end point of a first order phase transition line. A promising tool to probe the presence of critical behavior is the study of fluctuations and correlations of conserved charges since, in a thermal system, these fluctuations are directly related to the equation of state (EoS) of the system under the study. In this report an overview is given of several experimental measurements on net-proton multiplicity distributions such as cumulants and multi-particle correlation functions.

ALICE is devoted to the study of the properties of the Quark-Gluon Plasma (QGP). This state of matter is created in ultra-relativistic heavy-ion collisions at the LHC. Heavy quarks are considered effective probes of the QGP since, due to their large masses, they are produced in hard scattering processes and experience the full evolution of the hot and dense medium while interacting with its constituents. The heavy-quark measurements provide insights on processes like in-medium energy loss and hadronization. Measurements in proton-proton collisions provide the baseline for interpreting heavy-ion collision results and constitute an excellent test of pQCD calculations. In addition, proton-nucleus collisions allow separating cold nuclear matter effects from those due to the deconfined strongly interacting matter created in heavy-ion collisions. In this contribution, an overview of recent ALICE results for open heavy flavours, quarkonia, and heavy-flavour jets is presented.

The PRad experiment has credibly demonstrated the advantages of the calorimetric method in e-p scattering experiments to measure the proton root-mean-square (RMS) charge radius with high accuracy. The PRad result, within its experimental uncertainties, is in agreement with the small radius measured in muonic hydrogen spectroscopy experiments and it was a critical input in the recent revision of the CODATA recommendation for the proton charge radius. Consequently, the PRad result is in direct conflict with all modern electron scattering experiments. Most importantly, it is 5.8% smaller than the value from the most precise electron scattering experiment to date, and this difference is about three standard deviations given the precision of the PRad experiment. As the first experiment of its kind, PRad did not reach the highest precision allowed by the calorimetric technique. Here we propose a new (and) upgraded experiment -- PRad-II, which will reduce the overall experimental uncertainties by a factor of 3.8 compared to PRad and address this as yet unsettled controversy in subatomic physics. In addition, PRad-II will be the first lepton scattering experiment to reach the Q^2 range of 10^{-5} GeV^2 allowing a more accurate and robust extraction of the proton charge radius. The muonic hydrogen result with its unprecedented precision (~0.05%) determines the CODATA value of the proton charge radius, hence, it is critical to evaluate possible systematic uncertainties of those experiments, such as the laser frequency calibration that was raised in recent review articles. The PRad-II experiment with its projected total uncertainty of 0.43% could demonstrate whether there is any systematic difference between e−p scattering and muonic hydrogen results. PRad-II will establish a new precision frontier in electron scattering and open doors for future physics opportunities.