Nuclear Experiment

The CNO cycle is the main energy source in stars more massive than our sun, it defines the energy production and the cycle time that lead to the lifetime of massive stars, and it is an important tool for the determination of the age of globular clusters. One of the largest uncertainties in the CNO chain of reactions comes from the uncertainty in the 14 N (p,γ ) 15 O reaction rate. This uncertainty arises predominantly from the uncertainty in the lifetime of the sub-threshold state in 15 O at E x = 6792 keV. Previous measurements of this state's lifetime are significantly discrepant. Here, we report on a new lifetime measurement of this state, as well as the excited states in 15 O at E x = 5181 keV and E x = 6172 keV, via the 14 N (p,γ ) 15 O reaction at proton energies of E p =1020 keV and E p =1570 keV. The lifetimes have been determined with the Doppler-Shift Attenuation Method (DSAM) with three separate, nitrogen-implanted targets with Mo, Ta, and W backing. We obtained lifetimes from the weighted average of the three measurements, allowing us to account for systematic differences between the backing materials. For the 6792 keV state, we obtained a ?=0.6±0.4 fs. To provide cross-validation of our method, we measured the known lifetimes of the states at 5181 keV and 6172 keV to be ?=7.5±3.0 and ?=0.7±0.5 fs, respectively, which are in good agreement with previous measurements.

In high-energy nuclear collisions, light nuclei can be regarded as a cluster of baryons and their yields are sensitive to the baryon density fluctuations. Thus, the production of light nuclei can be used to study the QCD phase transition, at which the baryon density fluctuation will be enhanced. The yield ratio of light nuclei, defined as N(t) × N(p) / N 2 (d) , is predicted to be sensitive observable to search for the 1st-order phase transition and/or QCD critical point in heavy-ion collisions. In this paper, we present the energy and centrality dependence of (anti)deuteron and triton production in Au+Au collisions at s NN − − − √ = 7.7, 11.5, 14.5, 19.6, 27, 39, 54.4, 62.4, and 200 GeV measured by the STAR experiment at RHIC. We show beam-energy dependence for the coalescence parameter, B 2 (d) and B 3 (t) , particle ratios, d/p , t/p , and t/d , and the yield ratio of N(t) × N(p) / N 2 (d) . More importantly, non-monotonic energy dependence is observed for the yield ratio, N(t) × N(p) / N 2 (d) , in 0-10\% central Au+Au collisions with a peak around 20-30 GeV. Their physics implications on QCD critical point search and change of the equation of state will be discussed.

Light neutral meson differential invariant cross section and nuclear modification factor measurements have been carried out with the ALICE detector at the CERN LHC in pp collisions at s √ =8 TeV and p--Pb collisions at s NN − − − √ =8.16 TeV. The analysis combines results from several partially independent reconstruction techniques where the π 0 and η meson decay photons were detected with the electromagnetic calorimeter, EMCal, the photon spectrometer, PHOS, or via reconstruction of e + e − pairs from conversions in the ALICE detector material using the central tracking system. The neutral pion measurement reaching a p T of 200 GeV/ c poses as the highest measured identified particle spectrum to date while the η meson is measured to an unprecedented p T of 50 GeV/ c . The spectra are found to be generally overestimated by NLO pQCD calculations. The nuclear modification factors of both mesons exhibit a suppression for p T <10 GeV/ c which is stronger compared to previous measurements at s NN − − − √ =5.02 TeV and consistent with CGC and cold nuclear matter energy loss calculations. For p T >10 GeV/ c , R pPb is consistent with unity and theory predictions.

The production of light neutral mesons in AA collisions probes the physics of the Quark-Gluon Plasma (QGP), which is formed in heavy-ion collisions at the LHC. More specifically, the centrality dependent neutral meson spectra in AA collisions compared to its spectra in minimum-bias pp collisions, scaled with the number of hard collisions, provides information on the energy loss of partons traversing the QGP. The measurement allows to test with high precision the predictions of theoretical model calculations. In addition, the decay of the π 0 and η mesons are the dominant backgrounds for all direct photon measurements. Therefore, pushing the limits of the precision of neutral meson production is key to learning about the temperature and space-time evolution of the QGP. In the ALICE experiment neutral mesons can be detected via their decay into two photons. The latter can be reconstructed using the two calorimeters EMCal and PHOS or via conversions in the detector material. The excellent momentum resolution of the conversion photons down to very low p T and the high reconstruction efficiency and triggering capability of calorimeters at high p T , allow us to measure the p T dependent invariant yield of light neutral mesons over a wide kinematic range. Combining state-of-the-art reconstruction techniques with the high statistics delivered by the LHC in Run 2 gives us the opportunity to enhance the precision of our measurements. In these proceedings, new ALICE run 2 preliminary results for neutral meson production in pp and Pb--Pb collisions at LHC energies are presented.

The production of light neutral mesons in different collision systems is interesting for a variety of reasons: In nucleus-nucleus (AA) collisions the measurements provide important information on the energy loss of partons traversing the Quark-Gluon Plasma (QGP) which is formed in heavy-ion collisions at the LHC. In proton--proton (pp) collisions, neutral mesons allow us to test with high precision the predictions of perturbative QCD and other model calculations, and also serve as a reference for pA and AA collisions. In pA collisions, cold nuclear matter effects are studied. In the ALICE experiment, which is dedicated to the study of the QGP, neutral mesons can be detected via their decay to two photons. The latter can be reconstructed using the two calorimeters EMCal and PHOS or via conversions in the detector material. Combining state-of-the-art reconstruction techniques with the large data sample delivered by the LHC in Run 2 gives us the opportunity to enhance the precision of our measurements. In these proceedings, an overview of neutral meson production in pp, p--Pb and Pb--Pb collisions at LHC energies, as measured with the ALICE detector is presented.

The higher order harmonic flow observables v n ( n>3 ) and their non-linear responses to the initial state anisotropy have the strong potential to constrain shear and bulk viscosity to entropy ratios because of different sensitivities for various stages of heavy-ion collisions. The measurements of the flow coefficients and the non-linear coefficients up to the ninth and fifth harmonic, respectively, are presented in Pb-Pb collisions at s NN − − − √ =5.02TeV for charged hadrons. In addition, the results of p T -differential non-linear flow modes for π ± , K ± , p+ p ¯ , K 0 s , Λ+ Λ ¯ 0 s and ϕ are presented. The results are compared to the same measurements at 2.76 TeV and calculations from state of the art hydrodynamic models.

Two-particle angular correlations are measured in high-multiplicity proton-proton collisions at s ??=13 TeV by the ALICE Collaboration. The yields of particle pairs at short-( ?η ??0) and long-range ( 1.6<|?η|<1.8 ) in pseudorapidity are extracted on the near-side ( ?? ??0). They are reported as a function of transverse momentum ( p T ) in the range 1< p T <4 GeV/ c . Furthermore, the event-scale dependence is studied for the first time by requiring the presence of high- p T leading particles and jets for varying p T thresholds. The results demonstrate that the long-range "ridge" yield, possibly related to the collective behavior of the system, is present in events with high- p T processes. The magnitudes of the short- and long-range yields are found to grow with the event scale. The results are compared to EPOS LHC and PYTHIA 8 calculations, with and without string-shoving interactions. It is found that while both models describe the qualitative trends in the data, calculations from EPOS LHC show a better quantitative agreement, in particular for the p T and event-scale dependencies.

New STAR differential and integral v{2,3} measurements that explicitly account for non-flow contributions are reported for p/d/3He+Au, collisions at Roots=200 GeV. The measurements, which leverage the two-particle correlators for p/d/3He+Au and minimum-bias p+p collisions in tandem with three well-established methods of non-flow subtraction, are observed to be method-independent. For comparable multiplicities, they further indicate system-independent v2{2} and v3{2} values that are consistent with the critical role of both size (N{ch}) and the subnucleonic-fluctuations-driven eccentricities e{2,3} but are inconsistent with the notion of shape engineering in p/d/3He+Au collisions. The scaling function derived from the measurements, confirm the important role of final-state effects across a broad spectrum of collision-system sizes and energies and suggests an increase in eta/s(T,muB) for small collision-systems.

The first measurement of longitudinal decorrelations of harmonic flow amplitudes v n for n=2 , 3 and 4 in Xe+Xe collisions at s NN − − − √ =5.44 TeV is obtained using 3 μ b −1 of data with the ATLAS detector at the LHC. The decorrelation signal for v 3 and v 4 is found to be nearly independent of collision centrality and transverse momentum ( p T ) requirements on final-state particles, but for v 2 a strong centrality and p T dependence is seen. When compared with the results from Pb+Pb collisions at s NN − − − √ =5.02 TeV, the longitudinal decorrelation signal in mid-central Xe+Xe collisions is found to be larger for v 2 , but smaller for v 3 . Current hydrodynamic models reproduce the ratios of the v n measured in Xe+Xe collisions to those in Pb+Pb collisions but fail to describe the magnitudes and trends of the ratios of longitudinal flow decorrelations between Xe+Xe and Pb+Pb. The results on the system-size dependence provide new insights and an important lever-arm to separate effects of the longitudinal structure of the initial state from other early-time and late-time effects in heavy-ion collisions.

Lighter heavy elements beyond iron and up to around silver can form in neutrino-driven ejecta in core-collapse supernovae and neutron star mergers. Slightly neutron-rich conditions favour a weak r-process that follows a path close to stability. Therefore, the beta decays are slow compared to the expansion time scales, and ( α ,n) reactions become critical to move matter towards heavier nuclei. The rates of these reactions are calculated with the statistical model and their main uncertainty, at energies relevant for the weak r-process, is the α +nucleus optical potential. There are several sets of parameters to calculate the α +nucleus optical potential leading to large deviations for the reaction rates, exceeding even one order of magnitude. Recently the 96 Zr( α ,n) 99 Mo reaction has been identified as a key reaction that impacts the production of elements from Ru to Cd. Here, we present the first cross section measurement of this reaction at energies (6.22 MeV ≤ E c.m. ≤ 12.47 MeV) relevant for the weak r-process. The new data provide a stringent test of various model predictions which is necessary to improve the precision of the weak r-process network calculations. The strongly reduced reaction rate uncertainty leads to very well-constrained nucleosynthesis yields for Z=44−48 isotopes under different neutrino-driven wind conditions.