Nuclear Experiment

The production of π ± , K ± , K 0 S , K∗(892 ) 0 , p , ϕ(1020) , Λ , Ξ − , Ω − , and their antiparticles was measured in inelastic proton-proton (pp) collisions at a center-of-mass energy of s √ = 13 TeV at midrapidity ( |y|<0.5 ) as a function of transverse momentum ( p T ) using the ALICE detector at the CERN LHC. Furthermore, the single-particle p T distributions of K 0 S , Λ , and Λ ¯ ¯ ¯ ¯ in inelastic pp collisions at s √ = 7 TeV are reported here for the first time. The p T distributions are studied at midrapidity within the transverse momentum range 0≤ p T ≤20 GeV/ c , depending on the particle species. The p T spectra, integrated yields, and particle yield ratios are discussed as a function of collision energy and compared with measurements at lower s √ and with results from various general-purpose QCD-inspired Monte Carlo models. A hardening of the spectra at high p T with increasing collision energy is observed, which is similar for all particle species under study. The transverse mass and x T ≡2 p T / s √ scaling properties of hadron production are also studied. As the collision energy increases from s √ = 7 to 13 TeV, the yields of non- and single-strange hadrons normalized to the pion yields remain approximately constant as a function of s √ , while ratios for multi-strange hadrons indicate enhancements. The p T -differential cross sections of π ± , K ± and p ( p ¯ ¯ ¯ ) are compared with next-to-leading order perturbative QCD calculations, which are found to overestimate the cross sections for π ± and p ( p ¯ ¯ ¯ ) at high p T .

The first measurement of the production of pions, kaons, (anti-)protons and ? mesons at midrapidity in Xe-Xe collisions at s NN ????????=5.44 TeV is presented. Transverse momentum ( p T ) spectra and p T -integrated yields are extracted in several centrality intervals bridging from p-Pb to mid-central Pb-Pb collisions in terms of final-state multiplicity. The study of Xe-Xe and Pb-Pb collisions allows systems at similar charged-particle multiplicities but with different initial geometrical eccentricities to be investigated. A detailed comparison of the spectral shapes in the two systems reveals an opposite behaviour for radial and elliptic flow. In particular, this study shows that the radial flow does not depend on the colliding system when compared at similar charged-particle multiplicity. In terms of hadron chemistry, the previously observed smooth evolution of particle ratios with multiplicity from small to large collision systems is also found to hold in Xe-Xe. In addition, our results confirm that two remarkable features of particle production at LHC energies are also valid in the collision of medium-sized nuclei: the lower proton-to-pion ratio with respect to the thermal model expectations and the increase of the ? -to-pion ratio with increasing final-state multiplicity.

The fusion and transfer induced fission reaction 9 Be( 238 U,~f) with 6.2 MeV/u beam energy, using a unique setup consisting of AGATA, VAMOS++ and EXOGAM detectors, was used to populate through the fission process and study the neutron-rich 119,121 In isotopes. This setup enabled the prompt-delayed γ -ray spectroscopy of isotopes in the time range of 100 ns−200 μs . In the odd- A 119,121 In isotopes, indications of a short half-life 19/ 2 − isomeric state, in addition to the previously known 25/ 2 + isomeric state, were observed from the present data. Further, new prompt transitions above the 25/ 2 + isomer in 121 In were identified along with reevaluation of its half-life. The experimental data were compared with the theoretical results obtained in the framework of large-scale shell-model calculations in a restricted model space. The ⟨π g 9/2 ν h 11/2 ;I⏐ H ^ ⏐π g 9/2 ν h 11/2 ;I⟩ two-body matrix elements of residual interaction were modified to explain the excitation energies and the B(E2) transition probabilities in the neutron-rich In isotopes. The (i) decreasing trend of E(29/ 2 + )−E(25/ 2 + ) in odd-In (with dominant configuration π g −1 9/2 ν h −2 11/2 and maximum aligned spin of 29/ 2 + ) and (ii) increasing trend of E(27/ 2 + )−E(23/ 2 + ) in odd-Sb (with dominant configuration π g +1 7/2 ν h −2 11/2 and maximum aligned spin of 27/ 2 + ) with increasing neutron number could be understood as a consequence of hole-hole and particle-hole interactions, respectively.

Mass-separated 187Ta in a high-spin isomeric state has been produced for the first time by multi-nucleon transfer reactions, employing an argon gas stopping cell and laser ionisation. Internal gamma rays revealed a 7.3 s isomer at 1778 keV, which decays through a rotational band with perturbations associated with the approach to a prolate-oblate shape transition. Model calculations show less influence from triaxiality compared to heavier elements in the same mass region. The isomer decay reduced E2 hindrance factor of 27 supports the interpretation that axial symmetry is approximately conserved.

We present an analysis of proton number fluctuations in s NN − − − − √ = 2.4 GeV Au+Au collisions measured with the High-Acceptance DiElectron Spectrometer (HADES) at GSI. With the help of extensive detector simulations done with IQMD transport model events including nuclear clusters, various nuisance effects influencing the observed proton cumulants have been investigated. Acceptance and efficiency corrections have been applied as a function of fine grained rapidity and transverse momentum bins, as well as considering local track density dependencies. Next, the effects of volume changes within particular centrality selections have been considered and beyond-leading-order corrections have been applied to the data. The efficiency and volume corrected proton number moments and cumulants Kn of orders n = 1, . . . , 4 have been obtained as a function of centrality and phase-space bin, as well as the corresponding correlators C_n . We find that the observed correlators show a power-law scaling with the mean number of protons, i.e. C n ∝<N > n , indicative of mostly long-range multi-particle correlations in momentum space. We also present a comparison of our results with Au+Au collision data obtained at RHIC at similar centralities, but higher s NN − − − − √ .

Collinear laser spectroscopy of the N=Z=21 self-conjugate nucleus 42 Sc has been performed at the JYFL IGISOL IV facility in order to determine the change in nuclear mean-square charge radius between the I ? = 0 + ground state and the I ? = 7 + isomer via the measurement of the 42g,42m Sc isomer shift. New multi-configurational Dirac-Fock calculations for the atomic mass shift and field shift factors have enabled a recalibration of the charge radii of the 42??6 Sc isotopes which were measured previously. While consistent with the treatment of proton-neutron, proton-proton and neutron-neutron pairing on an equal footing, the reduction in size for the isomer is observed to be of a significantly larger magnitude than that expected from both shell model and ab-initio calculations.

The pseudorapidity dependence of flow coefficients, v n , is presented over a large range in pseudorapidity ( −3.5≤η≤5 ) in Pb − Pb collisions at 5.02 TeV. The flow coefficients are measured with 2- and 4-particle cumulants with |Δη|>0 and |Δη|>2 . The difference between v 2 {2,|Δη|>0} and v 2 {2,|Δη|>2} implies changes in either the de-correlation of flow vectors or non-flow effects between the two methods. The AMPT model shows a qualitative agreement with the measurements.

The multiplicity dependence of the pseudorapidity density of charged particles in proton-proton (pp) collisions at centre-of-mass energies s √ = 5.02, 7 and 13 TeV measured by ALICE is reported. The analysis relies on track segments measured in the midrapidity range ( |η|<1.5 ). Results are presented for inelastic events having at least one charged particle produced in the pseudorapidity interval |η|<1 ( INEL >0 ). The multiplicity dependence of the pseudorapidy density of charged particles is measured with mid and forward rapidity multiplicity estimators, the latter being less affected by autocorrelations. A detailed comparison with predictions from the PYTHIA 8 and EPOS LHC event generators is also presented. Both generators provide a good description of the data.

We report the latest results on the search for the QCD critical point in the QCD phase diagram through high energy heavy-ion collisions. The measurements discussed are based on the higher moments of the net-proton multiplicity distributions in heavy-ion collisions. A non-monotonic variation in the product of kurtosis times the variance of the net-proton distribution is observed as a function of the collision energy with 3 ? significance. We also discuss the results of the thermal model in explaining the measured particle yield ratios in heavy-ion collisions and comparison of the different variants of hardon resonance gas model calculation to the data on higher moments of net-proton distributions. We end with a note that the upcoming programs in high baryon density regime at various experimental facilities will complete the search for the QCD critical point through heavy-ion collisions.

The Multi-Phase Transport model, AMPT, and the Anomalous Viscous Fluid Dynamics model, AVFD, are used to assess a possible chiral-magnetically-driven charge separation ( ΔS ) recently measured with the R Ψ 2 (ΔS) correlator in Au+Au collisions at s NN − − − √ =200 GeV. The Comparison of the experimental and simulated R Ψ 2 (ΔS) distributions indicates that background-driven charge separation is insufficient to account for the measurements. The AVFD model calculations, which explicitly account for CME-driven anomalous transport in the presence of background, indicate a CME signal quantified by the P -odd Fourier dipole coefficient a ′ 1 ≈0.5% in mid-central collisions. A similar evaluation for the Δγ correlator suggests that only a small fraction of this signal ( f CME =Δ γ CME /Δγ≈25% ) is measurable with this correlator in the same collisions. The related prediction for signal detection in isobaric collisions of Ru+Ru and Zr+Zr are also presented.