The STAR Collaboration has recently published "Measurement of $D^{0}$-meson + hadron two-dimensional angular correlations in Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 200 GeV" in Physical Review C 102, 014905 (2020).

Open heavy flavor hadrons, such as the $D^{0}$-meson, provide unique probes of the medium produced in ultra-relativistic heavy-ion collisions. Due to their increased mass relative to light-flavor hadrons, long lifetime, and early production in hard-scattering interactions, they provide access to the full evolution of the Quark-Gluon Plasma (QGP). In previous studies from STAR, it has been established that charm quarks, which in-part comprise the $D$-mesons, experience strong interactions in the QGP in a way similar to that of light-flavor quarks. This study aims, for the first-time in heavy-ion collisions, to understand how a jet containing a charm quark is affected as it traverses the medium. This is acheived by computing two-dimensional angular correlations between the $D^{0}$-mesons and all other charged hadrons produced in the collisions. In general, it is expected that hadrons associated with the charm-jet would be nearby the $D^{0}$-meson in the angular phase space, and we therefore only focus on this portion of the correlation structure - the so-called "near side (NS)". These correlations, when projected onto angular coordinates, indicate that the jet containing the charm-quark is "broadened" as the system size becomes larger, while at the same time the number of charged hadrons associated with the jet increases by an order of magnitude from peripheral to central collisions (see the figure below; note the logarithmic scale). These observations indicate that the charm-jet produces more correlated particles at larger angles as it passes through more medium. When these correlations are compared to correlations at a similar transverse momentum ($p_{T}$), but with hadron triggers containing only light-flavor quarks in place of the $D^{0}$-meson, the resulting modifications to the jet-like correlation structure follow a very similar trend. These results, when examined in the context of previous STAR measurements of charm nuclear modification factor and collective flow, all point to the charm interacting strongly with the QGP, similar to what has been observed for light-flavor quarks.

Fig. 5: Correlated hadron yield per $D^0$ trigger in the near-side 2D Gaussian peak for the present data (stars), PYTHIA predictions (upright triangle), dihadron results for $\langle p_T \rangle = 2.56$ GeV/$c$ (upside down triangles), and dihadron results for $\langle p_T \rangle = 5.7$ GeV/$c$ (dots). Horizontal bars indicate the centrality ranges; vertical bars show the statistical errors, and cross bars show the systematic uncertainties. For more details on the analysis and discussion of the results see the full paper here.

Posted July 21, 2020

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The STAR Collaboration has recently published “Polarization of Λ (Λ) hyperons along the beam direction in Au+Au collisions at collisions at $\sqrt{s_{\rm{NN}}}$ = 200 GeV” in Physical Review Letter 123, 132301.

This paper reports on the first ever measurements of the Λ hyperon polarization along the beam direction in heavy-ion collisions. In a non-central heavy-ion collision, the system expands stronger in the reaction plane direction compared to that in the out-of-plane direction, a phenomenon known as elliptic flow. Such nontrivial velocity fields lead to a non-zero vorticity component along the beam direction dependent on the azimuthal angle of fluid elements relative to the reaction plane (see left figure), and therefore to particle spin polarization. Spin polarization can be experimentally measured via hyperons parity-violating weak decay.

Left: A sketch illustrating the system created in a non-central heavy-ion collision viewed in the transverse plane (x-y), showing stronger in-plane expansion (solid arrows) and expected vorticities (open arrows). In this figure the colliding beams are oriented along the z-axis and the x-z plane defines the reaction plane. Right: The second Fourier sine coefficient of the polarization of Λ and Λ along the beam direction as a function of centrality in Au+Au collisions at 200 GeV.

Results using Λ hyperons at 200 GeV exhibit Λ's emission angle dependence of the polarization along the beam direction, as expected from elliptic flow, indicating a quadrupole pattern of the vorticity z-component. Right figure presents a sine modulation of the polarization relative to the reaction plane angle as a function of collision centrality. The observed phase of the sine modulation is opposite to some theoretical predictions, e.g. a multi-phase transport model (AMPT) and viscous hydrodynamic model. In contrast, the blast-wave model calculations representing the kinematic vorticity reproduce the modulation phase better. These results together with the results of the global polarization (Nature 548, 62 (2017), PRC98, 014910 (2018)) may provide information on the relaxation time needed to convert the vorticity to particle polarization.

Posted October 24, 2019

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The STAR Collaboration has recently published in the Journal of High Energy Physics, JHEP 07 (2020) 178, a new paper titled "Measurement of the central exclusive production of charged particle pairs in proton-proton collisions at $\sqrt{s}=200$ GeV with the STAR detector at RHIC".

This paper reports on a high statistics measurement of a process called "central exclusive production". In this process the beam particles (protons) remain intact after an interaction, while a small fraction of their initial energy is transformed into mass of the centrally-produced system. The forward-scattered protons leave the interaction point at very small angles with respect to the beamline, which makes their detection possible in special devices called Roman Pots. The STAR experiment, with its diverse physics programme, is one of only a few experiments capable of tagging intact protons.

The process is particularly interesting because it proceeds mainly through the exchange of two Pomerons between interacting protons, that fuse and form the central state (Double Pomeron Exchange). The simplest QCD picture of the Pomeron - a pair of gluons - makes the process suitable for production of exotic hadrons called glueballs (gluon bound states), whose observation in nature has not yet been undeniably confirmed.

In the paper, the differential cross sections are reported for the exclusive production of $\pi^+\pi^-$, $K^+K^-$ and $p\bar{p}$ pairs, measured in the fiducial region of high geometrical acceptance of the STAR detector. It is currently the highest center-of-mass energy measurement, in which this process has been measured with direct detection of the forward-scattered protons. The systematic uncertainties of the cross sections are several times better compared to previous measurements of this kind, which should significantly constrain parameters of the phenomenological models used to describe the process.

In the figure the differential cross section for central exclusive production of $\pi^+\pi^-$ pairs is presented, for two ranges of the azimuthal angle between protons measured in Roman Pots provided in the plot. The cross section was successfully fitted with a model containing four quantum-interfering components, among which one is consistent with $f_0(1500)$ meson, considered as a potential glueball candidate. A significant dependence of the production cross section for the mesonic states was observed on the angle between forward-scattered protons (equal to an angle between interacting Pomerons).

Posted July 28, 2020

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In March 2020, the STAR Collaboration published "Measurement of the mass difference and the binding energy of the hypertriton and antihypertriton" in Nature Physics.

In this paper, we present two measurements from gold-gold collisions at a center-of-mass energy per nucleon pair of $\sqrt{s_{NN}} = 200$ GeV: the relative mass difference between $\rm^3_\Lambda H$ (the hypertriton) and $\rm^3_{\bar{\Lambda}}\overline{H}$ (the antihypertriton) (see Fig. 1), as well as the $\Lambda$ hyperon binding energy for $\rm^3_\Lambda H$ and $\rm^3_{\bar{\Lambda}}\overline{H}$ (see Fig. 2). The hypernucleus $\rm^3_{\Lambda}H$ is reconstructed through its mesonic decay channels $\rm^3_{\Lambda}H \rightarrow {^3}He + \pi^-$ (2-body decay) and $\rm^3_{\Lambda}H \rightarrow$ $ d + p + \pi^-$ (3-body decay). The significance $S/ \sqrt{S+B}$, where $S$ is signal counts and $B$ is background counts in the invariant mass window $2.986 - 2.996$ GeV$/c^{2}$, is 11.4 for $^3_\Lambda$H and 6.4 for $\rm^3_{\bar{\Lambda}}\overline{H}$. The signal counts from 2-body/3-body decay channels are about 121/35 for $^3_\Lambda$H and 36/21 for $\rm^3_{\bar{\Lambda}}\overline{H}$, respectively.

According to the CPT theorem, which states that the combined operation of charge conjugation, parity transformation and time reversal must be conserved, particles and their antiparticles should have the same mass and lifetime but opposite charge and magnetic moment. Here, we test CPT symmetry in a nucleus containing a strange quark, more specifically in the hypertriton. A comparison of the masses of the hypertriton and the antihypertriton allows us to test CPT symmetry in a nucleus with strangeness for the first time, and we observe no deviation from the expected exact symmetry with precision of 10$^{-4}$.

This hypernucleus is the lightest one yet discovered and consists of a proton, a neutron, and a $\Lambda$ hyperon. We measure the $\Lambda$ hyperon binding energy $B_{\Lambda}$ for the hypertriton, and find that it differs from the widely used value and from predictions, where the hypertriton is treated as a weakly bound system. Our results place stringent constraints on the hyperon-nucleon interaction, and have implications for understanding neutron star interiors, where strange matter may be present.

Fig 1: Measurements of the relative mass-to-charge ratio differences between nuclei and antinuclei. The current measurement of the relative mass difference ${\Delta m}/m$ between $^3_\Lambda$H and $\rm^3_{\bar{\Lambda}}\overline{H}$ constrained by the existing experimental limits for decay daughters is shown by the red star marker. The green point is the new $^{3}$He result after applying the constraint provided by the present $^{3}_{\Lambda}$H result. The differences between $d$ and $\bar{d}$ and between $^3$He and $\rm^3\overline{He}$ measured by the ALICE collaboration are also shown. The two $^3$He - $\rm^3\overline{He}$ points are staggered vertically for visibility. The dotted vertical line at zero is the expectation from CPT invariance. The horizontal error bars represent the sum in quadrature of statistical and systematic uncertainties.

Posted March 10, 2020

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