The level structure of the very neutron rich and unbound 9He nucleus has been the subject of significant experimental and theoretical study. Many recent works have claimed that the two lowest energy ...9He states exist with spins Jπ=1/2+ and Jπ=1/2− and widths on the order of 100–200 keV. These findings cannot be reconciled with our contemporary understanding of nuclear structure. The present work is the first high-resolution study with low statistical uncertainty of the relevant excitation energy range in the 8He+n system, performed via a search for the T=5/2 isobaric analog states in 9Li populated through 8He+p elastic scattering. The present data show no indication of any narrow structures. Instead, we find evidence for a broad Jπ=1/2+ state in 9He located approximately 3 MeV above the neutron decay threshold.
Elliptic flow measurements from two-, four-, and six-particle correlations are used to investigate flow fluctuations in collisions of U+U at sqrts_{NN}=193 GeV, Cu+Au at sqrts_{NN}=200 GeV and ...Au+Au spanning the range sqrts_{NN}=11.5-200 GeV. The measurements show a strong dependence of the flow fluctuations on collision centrality, a modest dependence on system size, and very little if any, dependence on particle species and beam energy. The results, when compared to similar LHC measurements, viscous hydrodynamic calculations, and trento model eccentricities, indicate that initial-state-driven fluctuations predominate the flow fluctuations generated in the collisions studied.
The STAR collaboration reports a measurement of the transverse single-spin asymmetries, AN , for neutral pions produced in polarized proton collisions with protons ( pp ), with aluminum nuclei ( pAl ...) and with gold nuclei ( pAu ) at a nucleon-nucleon center-of-mass energy of 200 GeV. Neutral pions are observed in the forward direction relative to the transversely polarized proton beam, in the pseudorapidity region 2.7<η<3.8 . Results are presented for π0 s observed in the STAR forward meson spectrometer electromagnetic calorimeter in narrow Feynman x ( xF ) and transverse momentum ( pT ) bins, spanning the range 0.17<xF<0.81 and 1.7<pT<6.0 GeV/c . For fixed xF<0.47 , the asymmetries are found to rise with increasing transverse momentum. For larger xF , the asymmetry flattens or falls as pT increases. Parametrizing the ratio r ( A ) ≡AN ( pA ) / AN ( pp ) =AP over the kinematic range, the ratio r ( A ) is found to depend only weakly on A , with ( P ) =-0.027±0.005 . No significant difference in P is observed between the low- pT region, pT<2.5 GeV/c , where gluon saturation effects may play a role, and the high- pT region, pT>2.5 GeV/c . It is further observed that the value of AN is significantly larger for events with a large- pT isolated π0 than for events with a nonisolated π0 accompanied by additional jetlike fragments. The nuclear dependence r ( A ) is similar for isolated and nonisolated π0 events.
The TWIST Collaboration has measured the Michel parameter rho in normal muon decay, mu(+)--> e(+)nu(e)nu (mu). In the standard model, rho = 3/4. Deviations from this value imply mixing of left- and ...right-handed muon and electron couplings. We find rho=0.750 80+/-0.000 32(stat) +/- 0.000 97(syst) +/- 0.000 23, where the last uncertainty represents the dependence of rho on the Michel parameter eta. This result sets new limits on the W(L)-W(R) mixing angle in left-right symmetric models.
A MICROMEGAS detection amplifier has been incorporated into the design of the TAMU-MDM focal plane detector with the purpose of improving the energy resolution and thus, the particle identification. ...Furthermore, beam tests showed a factor of 2 improvement over the original design, from 10%–12% to 4%–6%, for ions with A ≤ 40 at E/A ~ 10–20 MeV.
The elliptic (v2) and triangular (v3) azimuthal anisotropy coefficients in central 3He + Au, d + Au, and p + Au collisions at $\sqrt{S_{NN}}$ = 200 GeV are measured as a function of transverse ...momentum (PT) at midrapidity (|n| < 0.9) via the azimuthal angular correlation between two particles both at |n| < 0.9 while the v2(PT) values depend on the colliding systems, the v3(PT) values are system independent within the uncertainties, suggesting an influence on eccentricity from subnucleonic fluctuations in these small-sized systems. Furthermore, these results also provide stringent constraints for the hydrodynamic modeling of these systems.
We report here the first observation of directed flow ($v_1$) of the hypernuclei $^3_Λ\text{H}$ and $^4_Λ\text{H}$ in mid-central Au + Au collisions at $\sqrt{s_{\text{NN}}}$ = 3 GeV at RHIC. These ...data are taken as part of the beam energy scan program carried out by the STAR experiment. From 165 × 106 events in 5%–40% centrality, about 8400 $^3_Λ\text{H}$ and 5200 $^4_Λ\text{H}$ candidates are reconstructed through two- and three-body decay channels. In this work, we observe that these hypernuclei exhibit significant directed flow. Comparing to that of light nuclei, it is found that the midrapidity $v_1$ slopes of $^3_Λ\text{H}$ and $^4_Λ\text{H}$ follow baryon number scaling, implying that the coalescence is the dominant mechanism for these hypernuclei production in the 3 GeV Au + Au collisions.
We report the beam energy and collision centrality dependence of fifth and sixth order cumulants (C5, C6) and factorial cumulants (κ5, κ6) of net-proton and proton distributions, from ...$\sqrt{s_{NN}}$=3-200 GeV Au+Au collisions at RHIC. The net-proton cumulant ratios generally follow the hierarchy expected from QCD thermodynamics, except for the case of collisions at $\sqrt{s_{NN}}$ = 3 GeV. C6/C2 for 0-40\% centrality collisions is increasingly negative with decreasing $\sqrt{s_{NN}}$, while it is positive for the lowest $\sqrt{s_{NN}}$ studied. These observed negative signs are consistent with QCD calculations (at baryon chemical potential, μB≤ 110 MeV) that include a crossover quark-hadron transition. In addition, for $\sqrt{s_{NN}}$≥ 11.5 GeV, the measured proton κn, within uncertainties, does not support the two-component shape of proton distributions that would be expected from a first-order phase transition. Taken in combination, the hyper-order proton number fluctuations suggest that the structure of QCD matter at high baryon density, μB~750 MeV ($\sqrt{s_{NN}}$ = 3 GeV) is starkly different from those at vanishing μB~20MeV ($\sqrt{s_{NN}}$ = 200 GeV and higher).
We report the triton (t) production in midrapidity (|y| < 0.5) Au + Au collisions at $\sqrt{s_{NN}} = 7.7-200$ GeV measured by the STAR experiment from the first phase of the beam energy scan at the ...Relativistic Heavy Ion Collider. The nuclear compound yield ratio ($N_t \times N_p / N^2_d$), which is predicted to be sensitive to the fluctuation of local neutron density, is observed to decrease monotonically with increasing charged-particle multiplicity ($dN_{ch}/dη$) and follows a scaling behavior. The $dN_{ch}/dη$ dependence of the yield ratio is compared to calculations from coalescence and thermal models. Enhancements in the yield ratios relative to the coalescence baseline are observed in the 0%-10% most central collisions at 19.6 and 27 GeV, with a significance of 2.3σ and 3.4σ, respectively, giving a combined significance of 4.1σ. The enhancements are not observed in peripheral collisions or model calculations without critical fluctuation, and decreases with a smaller $p_T$ acceptance. The physics implications of these results on the QCD phase structure and the production mechanism of light nuclei in heavy-ion collisions are discussed.
We report the triton (t) production in midrapidity (|y| < 0.5) Au + Au collisions at $\sqrt{s_{NN}} = 7.7-200$ GeV measured by the STAR experiment from the first phase of the beam energy scan at the ...Relativistic Heavy Ion Collider. The nuclear compound yield ratio ($N_t \times N_p / N^2_d$), which is predicted to be sensitive to the fluctuation of local neutron density, is observed to decrease monotonically with increasing charged-particle multiplicity ($dN_{ch}/dη$) and follows a scaling behavior. The $dN_{ch}/dη$ dependence of the yield ratio is compared to calculations from coalescence and thermal models. Enhancements in the yield ratios relative to the coalescence baseline are observed in the 0%-10% most central collisions at 19.6 and 27 GeV, with a significance of 2.3σ and 3.4σ, respectively, giving a combined significance of 4.1σ. The enhancements are not observed in peripheral collisions or model calculations without critical fluctuation, and decreases with a smaller $p_T$ acceptance. The physics implications of these results on the QCD phase structure and the production mechanism of light nuclei in heavy-ion collisions are discussed.
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