Nuclear Experiment

The STAR Collaboration reports measurements of differential jet shapes and semi-inclusive jet fragmentation functions in Au+Au collisions at s NN − − − √ = 200 GeV with the STAR detector at RHIC. Jet shapes, which represent the radial distribution of momentum carried by constituents, are measured differentially for (1) the charged particles transverse momentum and (2) the jet azimuthal angle relative to the second-order event plane. Based on the semi-inclusive population of jets recoiling from a high transverse momentum trigger hadron, jet fragmentation functions in 40-60% central heavy-ion collisions are measured, and compared to those in PYTHIA simulations for pp collisions.

Recent ALICE measurements of jet splittings in Pb--Pb and pp collisions at s NN − − − √ = 5.02 TeV are reported. These measurements scan the phase space of jet emissions in search of medium-induced signals which are expected to emerge at different scales. These include effects such as multiple soft-radiation, single hard emissions, and color coherence. The Lund plane diagram is shown, including projections onto distributions of the splitting scale k T in intervals of the splitting angle R g . Soft Drop grooming is applied to access hard splittings within the jet, enabling measurements of groomed substructure variables. These include the shared momentum fraction z g between the two hardest subjets and the number of Soft Drop splittings n SD . The results in Pb--Pb collisions are compared to PYTHIA events embedded into a Pb--Pb background to separate out background from in-medium effects. Measurements of z g and the normalized splitting angle θ g will also be shown in pp collisions at s √ = 5.02 TeV for different grooming settings.

In order to attribute the partonic energy loss experienced by jets (jet quenching) observed in A+A collisions to the traversal of partons through the hot QCD medium, it is necessary to examine the cold nuclear matter (CNM) effects on the corresponding jets. Such an examination has historically been done using p +A collisions. We present fully corrected measurements of the jet mass and SoftDrop groomed jet mass in p + p and p +Au collisions at STAR at s NN − − − √ =200 GeV as a function of the event activity (EA) to increase or decrease the magnitude of CNM effects. EA is determined in the backward (Au-going) rapidity ( −5.0<η<−3.3 ) by the STAR Beam-Beam Counter to minimize auto-correlation with jets measured at mid-rapidity. Comparison of the jet mass distribution in p +Au collisions to that in p + p collisions allows for isolation of CNM effects in anticipation of an upcoming jet mass measurement in Au+Au collisions.

We report jet substructure measurements in pp and Pb-Pb collisions at s NN − − − √ =5.02 TeV with the ALICE detector. Charged-particle jets were reconstructed at midrapidity with the ALICE tracking detectors using the anti- k T algorithm with resolution parameters R=0.2 and R=0.4 . In pp collisions, the groomed jet momentum fraction, z g , and the groomed jet radius, θ g ≡ R g /R , are measured for the first time using the Dynamical Grooming method. Additionally, new systematic measurements of the infrared and collinear (IRC) safe ungroomed jet angularities are presented. In heavy-ion collisions, we measure z g and θ g with the Soft Drop grooming algorithm. The large underlying event in heavy-ion collisions poses a challenge for the reconstruction of groomed jet observables, since fluctuations in the background can cause groomed splittings to be misidentified. By using strong grooming conditions to reduce this background, we report these observables fully corrected for detector effects and background fluctuations for the first time, and compare them to several theoretical models.

Magnetic Johnson-Nyquist noise (JNN) originating from metal electrodes, used to create a static electric field in neutron electric-dipole-moment (nEDM) experiments, may limit the sensitivity of measurements. In this study, we derive surface and volume-averaged root-mean-square normal noise amplitudes at a certain frequency bandwidth for a cylindrical geometry. In addition, we model the source of noise as a finite number of current dipoles and demonstrate a method to simulate temporal and three-dimensional spatial dependencies of JNN. The calculations are applied to estimate the impact of JNN on measurements with the new apparatus, n2EDM, at the Paul Scherrer Institute. We demonstrate that the performances of the optically pumped 133 Cs magnetometers and 199 Hg co-magnetometers, which will be used in the apparatus, are not limited by JNN. Further, we find that in measurements deploying a co-magnetometer system, the impact of JNN is negligible for nEDM searches down to a sensitivity of 4? 10 ??8 e?�cm in a single measurement; therefore, the use of economically and mechanically favored solid aluminum electrodes is possible.

Since the discovery of the Λ(1405) , it remains poorly described by conventional constituent quark models, and it is a candidate for having an "exotic" meson-baryon or "penta-quark" structure, similar to states recently reported in the hidden charm sector. The Λ(1405) can be produced in the reaction γp→ K + Λ(1405) . The pure I=0 decay mode into Σ 0 π 0 is prohibited for the mass-overlapping Σ(1385) . Combining a large aperture forward magnetic spectrometer and a central BGO crystal calorimeter, the BGO-OD experiment is ideally suited to measure this decay with the K + in the forward direction. Preliminary results are presented. *Supported by DFG (PN 388979758, 405882627).

The measurement of K ∗ (892 ) 0 resonance production via its K + π − decay mode in inelastic p+p collisions at beam momentum 158 GeV/ c ( s NN − − − − √ =17.3 GeV) is presented. The data were recorded by the NA61/SHINE hadron spectrometer at the CERN Super Proton Synchrotron. The \textit{template} method was used to extract the K ∗ (892 ) 0 signal and double differential transverse momentum and rapidity spectra were obtained. The full phase-space mean multiplicity of K ∗ (892 ) 0 mesons was found to be (78.44±0.38(stat)±6.0(sys))⋅ 10 −3 . The NA61/SHINE results are compared with the EPOS1.99 and Hadron Resonance Gas models as well as with world data from p+p and nucleus-nucleus collisions.

Quasielastic scattering excitation function at large backward angle has been measured for the weakly bound system, 7 Li+ 159 Tb at energies around the Coulomb barrier. The corresponding quasielastic barrier distribution has been derived from the excitation function, both including and excluding the α -particles produced in the reaction. The centroid of the barrier distribution obtained after inclusion of α -particles was found to be shifted higher in energy, compared to the distribution excluding the α -particles. The quasielastic data, excluding the α -particles, have been analyzed in the framework of continuum discretized coupled channel calculations. The quasielastic barrier distribution for 7 Li+ 159 Tb, has also been compared with the fusion barrier distribution for the system.

The mean-square charge radii of 207,208 Hg ( Z=80,N=127,128 ) have been studied for the first time and those of 202,203,206 Hg ( N=122,123,126 ) remeasured by the application of in-source resonance-ionization laser spectroscopy at ISOLDE (CERN). The characteristic \textit{kink} in the charge radii at the N=126 neutron shell closure has been revealed, providing the first information on its behavior below the Z=82 proton shell closure. A theoretical analysis has been performed within relativistic Hartree-Bogoliubov and non-relativistic Hartree-Fock-Bogoliubov approaches, considering both the new mercury results and existing lead data. Contrary to previous interpretations, it is demonstrated that both the kink at N=126 and the odd-even staggering (OES) in its vicinity can be described predominately at the mean-field level, and that pairing does not need to play a crucial role in their origin. A new OES mechanism is suggested, related to the staggering in the occupation of the different neutron orbitals in odd- and even- A nuclei, facilitated by particle-vibration coupling for odd- A nuclei.

The heaviest N=Z doubly-magic nucleus, 100 Sn, and the neighboring nuclei offer unique opportunities to investigate the properties of nuclear interaction in extreme conditions. In particular, the Cd isotopes are expected to present features similar to those found in the Sn isotopic chain, since they have only two proton holes in the Z=50 shell. In this manuscript, the lifetime measurements of low-lying states in the even-mass 102??08 Cd is presented. Thanks to the powerful detection capabilities of AGATA array and VAMOS++ spectrometer, the unusual employment of multi-nucleon transfer reactions permitted to investigate the first 2 + and 4 + states in all these nuclei, together with various deformed bands in 106 Cd. The results were interpreted in the context of new state-of-the-art beyond-mean-field calculations, using the symmetry-conserving configuration-mixing approach. Despite the similarities in the electromagnetic properties of the low-lying states, there is a fundamental structural difference between the ground-state bands in the Z=48 and Z=50 isotopes. The comparison between experimental and theoretical results revealed a rotational character of the Cd nuclei, which have prolate-deformed ground states with β 2 ??.2 . At this deformation Z=48 becomes a closed-shell configuration, which is favored with respect to the spherical one.