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 August 27, 2020

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The STAR Collaboration at the Relativistic Heavy-Ion Collider (RHIC) at BNL, has recently published a paper "Results on Total and Elastic Cross Sections in Proton--Proton Collisions at $\sqrt{s} = 200$ GeV" in Phys. Lett. B Phys. Lett. B Volume 808. The elastic differential cross section was measured in the squared four-momentum transfer $t$-range $0.045 \leq -t \leq 0.135$ GeV$^2$.

Elastic scattering plays an important role in proton-proton ($pp$) scattering at high energies, as evidenced by the fact that it contributes about 20% of the total cross section at the highest Large Hadron Collider (LHC) energies. The $pp$ elastic and total cross sections have been measured at colliders with center of mass energies $ 2.76 \le \sqrt{s} \le 13$ TeV at the LHC and at the Intersecting Storage Rings (ISR) at $\sqrt{s}=62.4$ GeV. RHIC has a unique capability to have those measurements in the energy gap between the ISR and the LHC to constrain the phenomenological models of the $pp$ cross sections since one still expects a difference between $pp$ and proton-antiproton ($p \bar p$), which were measured up to $\sqrt{s}=1.8$ TeV. Both the values of the cross sections and the difference between $pp$ and $p \bar p$ affect phenomenological models.

LEFT: Top panel: $pp$ elastic differential cross-section $d\sigma/dt$ fitted with exponential $A\exp{(Bt)}$; Bottom panel: Residuals (Data - Fit)/Fit. Uncertainties are statistical only. RIGHT: Comparison of STAR results on $\sigma_{tot}$, $\sigma_{inel}$ and $\sigma_{el}$ with the world data, COMPETE model prediction for $\sigma_{tot}$ is also shown. Dashed curves, drawn to guide the eye only, represent STAR fits to $\sigma_{inel}$ and $\sigma_{el}$ of the same function as used by COMPETE. STAR data points were not used in the fit.

In the left figure we show the measured $d\sigma/dt$ at STAR. The main quantity describing the forward peak of the elastic scattering is the exponential slope parameter $B$ of the elastic differential cross section $d\sigma/dt \sim e^{-Bt}$ in the measured $-t$ range, which was found to be $B = 14.32 \pm 0.09 (stat.)^{\scriptstyle +0.13}_{\scriptstyle -0.28} (syst.)$ GeV$^{-2}$. The total cross section $\sigma_{tot}$ was obtained using optical theorem by extrapolating the $d\sigma/dt$ to the optical point at $-t = 0$. The result on $\sigma_{tot}$ is: $\sigma_{tot} = 54.67 \pm 0.21 (stat.) ^{\scriptstyle +1.28}_{\scriptstyle -1.38} (syst.)$ mb.

The values of the elastic cross section $\sigma_{el} = 10.85 \pm 0.03 (stat.) ^{\scriptstyle +0.49}_{\scriptstyle -0.41}(syst.)$ mb, the elastic cross section integrated within the STAR $t$-range $\sigma^{det}_{el} = 4.05 \pm 0.01 (stat.) ^{\scriptstyle+0.18}_{\scriptstyle -0.17}(syst.$ mb, and the inelastic cross section $\sigma_{inel} = 43.82 \pm 0.21 (stat.) ^{\scriptstyle +1.37}_{\scriptstyle -1.44} (syst.)$ mb were also obtained.

We find that the obtained results are in good agreement with the world data as shown in the right figure. The $\sigma_{tot}$ agrees with the COMPETE prediction at $\sqrt s = 200$ GeV of 51.79 mb within about $2\sigma$ of the total uncertainty.

Posted August 27, 2020

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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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The STAR Collaboration has recently published "Strange hadron production in Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 7.7, 11.5, 19.6, 27, and 39 GeV" in Physical Review C 102, 034909 (2020) and it is also highlighted as PRC Editors' Suggestion.

Strange hadrons are an excellent probe for identifying the phase boundary and onset of deconfinement in the QCD phase diagram. The STAR Collaboration has performed precision measurements of strange hadron ($\mathrm{K}^{0}_{\mathrm S}$, $\Lambda$, $\overline{\Lambda}$, $\Xi^-$, $\overline{\Xi}^+$, $\Omega^-$, $\overline{\Omega}^+$, and $\phi$) production at mid-rapidity ($|y| < 0.5$) in Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 7.7, 11.5, 19.6, 27, and 39 GeV from the Beam Energy Scan Program at the Relativistic Heavy Ion Collider (RHIC). Transverse momentum spectra, averaged transverse mass, and the overall integrated yields of these strange hadrons have been extracted with high precision for all centralities and collision energies. Generally, the STAR BES data follow the trend of the previous measurements from AGS, SPS and RHIC. But the precision data also reveal new features, such as the deviation of $\overline{\Lambda}$ and $\Lambda$ $\left< m_{\rm T}\right>-m_0$ at lower energies and the non-monotonic energy dependence of $\Lambda$ and $\Xi^-$ $dN/dy$. The thermal model has been tested with the measured antibaryon-to-baryon ratios ($\overline{\Lambda}$/$\Lambda$, $\overline{\Xi}^+$/$\Xi^-$, $\overline{\Omega}^+$/$\Omega^-$), and then the temperature normalized strangeness and baryon chemical potentials at hadronic freeze-out ($\mu_{B}/T_{\rm ch}$ and $\mu_{S}/T_{\rm ch}$) are extracted for central collisions. The strange baryon-to-pion ratios are found to be consistent with the calculations of the statistical hadron gas model, and for $\Lambda$/$\pi$ ratio, consistent with hadronic transport models as well. The nuclear modification factors ($R_{\tiny{\textrm{CP}}}$) and antibaryon-to-meson ratios as a function of transverse momentum are presented for all collision energies, and they are shown in the left and right figures, respectively. The $\mathrm{K}^{0}_{\mathrm S}$ $R_{\tiny{\textrm{CP}}}$ shows no suppression for $p_{\rm T}$ up to 3.5 $\mbox{$\mathrm{GeV} / c$}$ at energies of 7.7 and 11.5 GeV. The $\overline{\Lambda}$/$\mathrm{K}^{0}_{\mathrm S}$ ratio also shows baryon-to-meson enhancement at intermediate $p_{\rm T}$ ($\approx$2.5 $\mbox{$\mathrm{GeV} / c$}$) in central collisions at energies above 19.6 GeV. Both observations suggest that there is likely a change of the underlying strange quark dynamics at collision energies below 19.6 GeV.

Left: The nuclear modifcation factor ($R_{\tiny{\textrm{CP}}}$) of strange hadrons at mid-rapidity ($|y|<0.5$) in Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = $7.7-39$ GeV. Right: $\overline{\Lambda}$/$\mathrm{K}^{0}_{\mathrm S}$ ratio versus $p_{\rm T}$ at mid-rapidity ($|y|<0.5$) in different centralities from Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = $7.7-39$ GeV.

The results in this paper significantly improve the experimental knowledge in the energy range where key features of the QCD phase diagram are nowadays being studied. For more details on the analysis and discussion of the results see the full paper here.

Posted October 5, 2020

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