Nuclear Experiment

We probe the N=82 nuclear shell closure by mass measurements of neutron-rich cadmium isotopes with the ISOLTRAP spectrometer at ISOLDE-CERN. The new mass of 132 Cd offers the first value of the N=82 , two-neutron shell gap below Z=50 and confirms the phenomenon of mutually enhanced magicity at 132 Sn. Using the recently implemented phase-imaging ion-cyclotron-resonance method, the ordering of the low-lying isomers in 129 Cd and their energies are determined. The new experimental findings are used to test large-scale shell-model, mean-field and beyond-mean-field calculations, as well as the ab initio valence-space in-medium similarity renormalization group.

We present the first direct measurement of the atomic mass of a superheavy nuclide. Atoms of 257 Db ( Z =105) were produced online at the RIKEN Nishina Center for Accelerator-Based Science using the fusion-evaporation reaction 208 Pb( 51 V, 2n) 257 Db. The gas-filled recoil ion separator GARIS-II was used to suppress both the unreacted primary beam and some transfer products, prior to delivering the energetic beam of 257 Db ions to a helium gas-filled ion stopping cell wherein they were thermalized. Thermalized 257 Db 3+ ions were then transferred to a multi-reflection time-of-flight mass spectrograph for mass analysis. An alpha particle detector embedded in the ion time-of-flight detector allowed disambiguation of the rare 257 Db 3+ time-of-flight detection events from background by means of correlation with characteristic α -decays. The extreme sensitivity of this technique allowed a precision atomic mass determination from 11 events. The mass excess was determined to be 100063(231 ) stat (132 ) sys ~keV/c 2 . Comparing to several mass models, we show the technique can be used to unambiguously determine the atomic number as Z =105 and should allow similar evaluations for heavier species in future work.

As one of the primordial radioactive isotopes, 232 Th mainly undergoes α -decay with a half-life of 1.402× 10 10 yr. However, it is also one of 35 double beta decay candidates in which the single β -decay is forbidden or strongly suppressed. 181 mg of thorium contained in a gas mantle was measured in an HPGe well-detector at the Gran Sasso Underground Laboratory (LNGS) with a total exposure of 3.25 g × d. We obtain half-life limits on all double beta decay modes of 232 Th to excited states of 232 U on the order of 10 11−15 yr. For the most likely transition into the 0 + 1 state we find a lower half-life limit of 6.3× 10 14 yr (90% credibility). These are the first constraints on double beta decay excited state transition in 232 Th.

The first measurement of the coherent photoproduction of ? 0 vector mesons in ultra-peripheral Xe-Xe collisions at s NN ????????=5.44 TeV is presented. This result, together with previous γ p and γ -Pb measurements, describes the atomic number ( A ) dependence of this process, which is particularly sensitive to nuclear shadowing effects and to the approach to the black-disc limit of QCD at a semi-hard scale. The cross section of the Xe+Xe??? 0 +Xe+Xe process, measured at midrapidity through the decay channel ? 0 ??? + ? ??, is found to be d?/dy=131.5±5.6 (stat.) +17.5 ??6.9 (syst.) mb. The ratio of the continuum to resonant contributions for the production of pion pairs is also measured. In addition, the fraction of events accompanied by electromagnetic dissociation of either one or both colliding nuclei is reported. The dependence on A of cross section for the coherent ? 0 photoproduction at a centre-of-mass energy per nucleon of the γA system of W γA,n =65 GeV is found to be consistent with a power-law behaviour ?(γA??? 0 A)??A α with a slope α=0.96±0.02(syst.) . This slope signals important shadowing effects, but it is still far from the behaviour expected in the black-disc limit.

We present the first measurement of the exclusive reaction γp→ a 2 (1320 ) 0 p in the photon energy range 3.5 - 5.5 GeV and four-momentum transfer squared 0.2<−t<2.0 GeV 2 . Data were collected with the CEBAF Large Acceptance Spectrometer at the Thomas Jefferson National Accelerator Facility. The neutral a 2 resonance was detected by measuring the reaction γp→ π 0 ηp and reconstructing the π 0 η invariant mass. The differential cross section dσ/dt was extracted at different beam energies in each −t bin. The most prominent feature of the differential cross section is a dip at −t≃0.55 GeV 2 . This can be well described in the framework of Regge phenomenology, where the exchange degeneracy hypothesis predicts a zero in the reaction amplitude for this value of the four-momentum transfer.

The polarization of inclusive J/ ψ and Υ(1S) produced in Pb-Pb collisions at s NN − − − √ =5.02 TeV at the LHC is measured with the ALICE detector. The study is carried out by reconstructing the quarkonium through its decay to muon pairs in the rapidity region 2.5<y<4 and measuring the polar and azimuthal angular distributions of the muons. The polarization parameters λ θ , λ ϕ and λ θϕ are measured in the helicity and Collins-Soper reference frames, in the transverse momentum interval 2< p T <10 GeV/ c and p T <15 GeV/ c for the J/ ψ and Υ(1S) , respectively. The polarization parameters for the J/ ψ are found to be compatible with zero, within a maximum of about two standard deviations at low p T , for both reference frames and over the whole p T range. The values are compared with the corresponding results obtained for pp collisions at s √ =7 and 8 TeV in a similar kinematic region by the ALICE and LHCb experiments. Although with much larger uncertainties, the polarization parameters for Υ(1S) production in Pb-Pb collisions are also consistent with zero.

The first measurement of the dependence on |t| , the square of the momentum transferred between the incoming and outgoing target nucleus, of coherent J/ ? photoproduction is presented. The data were measured with the ALICE detector in ultra-peripheral Pb-Pb collisions at a centre-of-mass energy per nucleon pair s NN ????????=5.02 TeV with the J/ ? produced in the central rapidity region |y|<0.8 , which corresponds to the small Bjorken- x range (0.3??.4)? 10 ?? . The measured |t| -dependence is not described by computations based only on the Pb nuclear form factor, while the photonuclear cross section is better reproduced by models including shadowing according to the leading-twist approximation, or gluon-saturation effects from the impact-parameter dependent Balitsky-Kovchegov equation. This new observable is therefore a valid tool to constrain the relevant model parameters and to investigate the transverse gluonic structure at very low Bjorken- x .

The first ever search for 2ε and ε β + decay of 174 Hf was realized using a high-pure sample of hafnium (with mass 179.8 g) and the ultra low-background HPGe-detector system located 225 m underground. After 75 days of data taking no indication of the double beta decay transitions could be detected but lower limits for the half-lives of the different channels and modes of the decays were set on the level of lim T 1/2 ∼ 10 16 − 10 18 a.

The nucleon form factors provide fundamental knowledge about the strong interaction. We review the flavor composition of the nucleon form factors and focus on an analysis of the possible impact of the s-quark contribution. A future experiment is presented to measure the strange form factor at large momentum transfer.

We present calculations of the chemical freeze-out temperature (T_ch) based on particle yields from STAR and ALICE measured at collision energies ranging from sqrt{s_{NN}} = 11.5 GeV to 5.02 TeV. Employing the Grand Canonical Ensemble approach using the Thermal-FIST Hadron Resonance Gas model package, we show evidence for a flavour-dependent chemical freeze-out in the crossover region of the QCD phase diagram. At a vanishing baryochemical potential, we calculate light and strange flavour freeze-out temperatures T_L = 150.2 +- 2.6 MeV and T_S = 165.1 +- 2.7 MeV, respectively.