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Register a new userYield ratio of hypertriton to light nuclei in heavy-ion collisions from $rm sqrt{s_{NN}}$ = 4.9 GeV to 2.76 TeV
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We resolve the difference in the yield ratio $rm S_3$ = $rm frac{N_{^3_{Lambda}H}/N_Lambda}{N_{^3He}/N_p}$ measured in Au+Au collisions at $rm sqrt{s_{NN}}$ = 200 GeV and in Pb-Pb collisions at $rm sqrt{s_{NN}}$ = 2.76 TeV by adopting a different treatment of the weak decay contribution to the proton yield in Au+Au collisions at $rm sqrt{s_{NN}}$ = 200 GeV. We then use the coalescence model to extract information on the $Lambda$ and nucleon density fluctuations at the kinetic freeze-out of heavy ion collisions. We also show from available experimental data that the yield ratio $rm S_2$ = $rm frac{N_{^3_{Lambda}H}}{N_Lambda N_ d}$ is a more promising observable than $rm S_3$ for probing the local baryon-strangeness correlation in the produced medium.
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The nuclear modification factors ($R_{AA}$) of $pi^{pm}, p(bar p)$, and $d(bar d)$ with $|y|<0.5, p_T<20.0$~GeV/c in peripheral (40-60%) and central (0-5%) Pb-Pb collisions at $sqrt {s_{NN}}=2.76$ TeV have been studied using the parton and hadron cascade ({footnotesize PACIAE}) model plus the dynamically constrained phase space coalescence ({footnotesize DCPC}) model. It is found that the $R_{AA}$ of light (anti)nuclei ($d, bar d$) is similar to that of hadrons ($pi^pm, p, bar p$), and the $R_{AA}$ of antiparticles is the same as that of particles. The suppression of $R_{AA}$ at high-$p_T$ strongly depends on event centrality and mass of the particles, i.e., the central collision is more suppressed than the peripheral collision. Besides, the yield ratios and double ratios for different particle species in $pp$ and Pb-Pb collisions are discussed, respectively. It is observed that the yield ratios and double ratios of $d$ to $p$ and $p$ to $pi$ are similar to those of their anti-particles in three different collision systems, suggesting that the suppressions of matter ($pi^{+}, p, d$) and the corresponding antimatter ($pi^{-},bar{p},bar{d}$) are around the same level.
Dihadron azimuthal correlations containing a high transverse momentum ($p_T$) trigger particle are sensitive to the properties of the nuclear medium created at RHIC through the strong interactions occurring between the traversing parton and the medium, i.e. jet-quenching. Previous measurements revealed a strong modification to dihadron azimuthal correlations in Au+Au collisions with respect to p+p and d+Au collisions. The modification increases with the collision centrality, suggesting a path-length or energy density dependence to the jet-quenching effect. This paper reports STAR measurements of dihadron azimuthal correlations in mid-central (20-60%) Au+Au collisions at $sqrt{s_{_{rm NN}}}=200$ GeV as a function of the trigger particles azimuthal angle relative to the event plane, $phi_s=|phi_t-psi_{rm EP}|$. The azimuthal correlation is studied as a function of both the trigger and associated particle $p_T$. The subtractions of the combinatorial background and anisotropic flow, assuming Zero Yield At Minimum (ZYAM), are described. The correlation results are first discussed with subtraction of the even harmonic (elliptic and quadrangular) flow backgrounds. The away-side correlation is strongly modified, and the modification varies with $phi_s$, with a double-peak structure for out-of-plane trigger particles. The near-side ridge (long range pseudo-rapidity $Deltaeta$ correlation) appears to drop with increasing $phi_s$ while the jet-like component remains approximately constant. The correlation functions are further studied with subtraction of odd harmonic triangular flow background arising from fluctuations. It is found that the triangular flow, while responsible for the majority of the amplitudes, is not sufficient to explain the $phi_s$-dependence of the ridge or the away-side double-peak structure. ...
Electromagnetic dissociation of heavy nuclei in ultra-peripheral interactions at high energies can be used to monitor the beam luminosity at colliders. In ALICE neutrons emitted by the excited nuclei close to beam rapidity are detected by the Zero Degree Calorimeters (ZDCs), providing a precise measurement of the event rate. During the 2010 Pb run, a dedicated data taking was performed triggering on electromagnetic processes with the ZDCs. These data, combined with the results from a Van der Meer scan, allowed to measure the electromagnetic dissociation cross-section of Pb nuclei at $sqrt{s_{rm NN}}$~=~2.76~TeV. Experimental results on various cross-sections are presented together with a comparison to the available predictions.
Dihadron angular correlations in $d$+Au collisions at $sqrt{s_{rm NN}}=200$ GeV are reported as a function of the measured zero-degree calorimeter neutral energy and the forward charged hadron multiplicity in the Au-beam direction. A finite correlated yield is observed at large relative pseudorapidity ($Deltaeta$) on the near side (i.e. relative azimuth $Deltaphisim0$). This correlated yield as a function of $Deltaeta$ appears to scale with the dominant, primarily jet-related, away-side ($Deltaphisimpi$) yield. The Fourier coefficients of the $Deltaphi$ correlation, $V_{n}=langlecos nDeltaphirangle$, have a strong $Deltaeta$ dependence. In addition, it is found that $V_{1}$ is approximately inversely proportional to the mid-rapidity event multiplicity, while $V_{2}$ is independent of it with similar magnitude in the forward ($d$-going) and backward (Au-going) directions.
Measurements of the fractional momentum loss ($S_{rm loss}equiv{delta}p_T/p_T$) of high-transverse-momentum-identified hadrons in heavy ion collisions are presented. Using $pi^0$ in Au$+$Au and Cu$+$Cu collisions at $sqrt{s_{_{NN}}}=62.4$ and 200 GeV measured by the PHENIX experiment at the Relativistic Heavy Ion Collider and and charged hadrons in Pb$+$Pb collisions measured by the ALICE experiment at the Large Hadron Collider, we studied the scaling properties of $S_{rm loss}$ as a function of a number of variables: the number of participants, $N_{rm part}$, the number of quark participants, $N_{rm qp}$, the charged-particle density, $dN_{rm ch}/deta$, and the Bjorken energy density times the equilibration time, $varepsilon_{rm Bj}tau_{0}$. We find that the $p_T$ where $S_{rm loss}$ has its maximum, varies both with centrality and collision energy. Above the maximum, $S_{rm loss}$ tends to follow a power-law function with all four scaling variables. The data at $sqrt{s_{_{NN}}}$=200 GeV and 2.76 TeV, for sufficiently high particle densities, have a common scaling of $S_{rm loss}$ with $dN_{rm ch}/deta$ and $varepsilon_{rm Bj}tau_{0}$, lending insight on the physics of parton energy loss.
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