STAR focus: STAR uses weak bosons to probe the spin structure of the proton

The STAR Collaboration has recently published, “Measurement of the longitudinal spin asymmetries for weak boson production in proton-proton collisions at √s = 510 GeV,” in Physical Review D 99, 051102(R).

This paper reports on measurements of the parity-violating asymmetry in the production of positrons and electrons from decays of weak bosons from collisions with one of the proton beams polarized longitudinally. In 510 GeV center-of-mass proton-proton collisions at RHIC, W+ bosons are produced primarily in the interactions of up quarks and down antiquarks, whereas W- bosons originate from down quarks and up antiquarks. The spin-asymmetry measurements of the decay positrons thus provide sensitivity to the up quark and down antiquark helicities in the proton, whereas the decay electrons do so for the down quark and up antiquark helicities. Combined, they make it possible to delineate the light quark and antiquark polarizations in the proton by flavor.

Left: Longitudinal single-spin asymmetries, AL, for W± production as a function of the positron or electron pseudorapidity, ηe, for the combined STAR 2011, 2012 and 2013 data samples for 25 < ET < 50GeV (points) in comparison to theory expectations (curves and bands). Right: The difference of the light sea-quark polarizations as a function of x at a scale of Q2 = 10(GeV/c)2. The green band shows the NNPDFpol1.1 results and the blue hatched band shows the corresponding distribution after the STAR 2013 W± data are included by reweighting.

These measurements provided one of the two initial motivations for the spin-physics program at RHIC and the data, shown in the left panel of the figure above, are the final data from STAR on this topic. Shown are the results from previously published data obtained in 2011 and 2012 combined with new results from the dedicated RHIC run in 2013. As seen from the right panel, the data have now reached a level of precision that makes it possible, for the first time, to conclude that the polarization of up antiquarks is larger than that of the down antiquarks.

Posted March 27, 2019

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STAR focus: Polarization of Λ (Λ) hyperons along the beam direction in Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 200 GeV

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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STAR focus: Measurements of inclusive $J/\psi$ suppression in Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 200 GeV through the dimuon channel

The STAR Collaboration has recently published "Measurements of inclusive $J/\psi$ suppression in Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 200 GeV through the dimuon channel at STAR" in Phys. Lett. B 797 (2019) 134917.

This publication reports the first measurement of inclusive $J/\psi$ suppression at mid-rapidity through the dimuon decay channel in Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 200 GeV. It is made possible thanks to the Muon Telescope Detector upgrade completed in 2014 at STAR. Among the various probes used to study the existence and properties of the Quark Gluon Plasma (QGP) created in relativistic heavy-ion collisions, quarkonia are considered a unique one. A fundamental feature of the QGP is that partons, instead of hadrons, are the relevant degrees of freedom of the system. Quarkonia are expected to exist as hadrons in the QGP until the Debye radius, which is inversely proportional to the medium temperature, becomes smaller than the quarkonium size. Under such conditions, the color-screening effect would lead to dissociation of the quarkonia in the medium. This can be observed experimentally through measurements of $J/\psi$ suppression in heavy-ion collisions with respect to that in p+p collisions. Such a suppression is believed to be unambiguous evidence of the QGP formation.

The figure below shows the $J/\psi$ RAA, used to quantify the suppression level, as a function of centrality for pT > 0.15 GeV/c and pT > 5 GeV/c in Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 200 GeV. At both low and high pT, the $J/\psi$ RAA is seen to decrease from peripheral to central collisions, consistent with increasing hot medium effects.

Figure: $J/\psi$ RAA as a function of centrality above 0.15 (left) and 5 (right) GeV/c in Au+Au collisions at $\sqrt{s_{\rm{NN}}}$ = 200 GeV, compared to those for Pb+Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 2.76 TeV.

For high-pT $J/\psi$, where the cold nuclear matter effects and the regeneration contribution are expected to be minimal, the $J/\psi$ production in 0-10% central collisions is suppressed by a factor of 3.1, providing strong evidence for the color-screening effect in a deconfined medium. Compared to similar measurements carried out for Pb+Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 2.76 TeV, the low-pT $J/\psi$'s are much more suppressed in central and semi-central collisions at RHIC than at the LHC, likely due to the smaller charm quark production cross-section and thus smaller regeneration contribution at RHIC. On the Other hand, there is a hint that the high-pT $J/\psi$ RAA is systematically higher at RHIC for semi-central bins. This could be because the temperature of the medium created at the LHC is higher than that at RHIC, leading to a higher dissociation rate. The new results presented in this publication could help constrain model calculations and deepen our understanding of the QGP properties.

Posted October 24, 2019

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STAR focus: Precise Measurement of the Mass Difference and the Binding Energy of the Hypertriton and Antihypertriton at STAR

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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