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: Precise Measurements of the Open Charm Meson Prodution at STAR

The STAR Collaboration has recently published “Centrality and transverse momentum dependence of D0-meson production at mid-rapidity in Au + Au collisions at √sNN = 200 GeV,” in Physical Review C 99, 034908 (2019).

This paper presents a new measurement of D0 meson production enabled by the Heavy Flavor Tracker (HFT) high-resolution silicon detector system in Au + Au collisions at √sNN = 200 GeV. Compared to the previous measurement with the TPC only, the new data contain greatly improved precision which allows us to systematically investigate the D0 meson production in wide centrality and transverse momentum regions. The new improved data are expected to further constrain our understanding of the charm medium interaction as well as to better determine the medium transport parameters together with the previous publication on D0 elliptic flow measurement.

Left: D0 RCP for different centrality classes with the 40–60% spectrum as the reference compared to that of other light and strange mesons (π, Κs, and φ) as well as the model calculations. Right: Integrated D0 cross section per nucleon-nucleon collision at mid-rapidity for pT > 0 (a) and pT > 4 GeV/c (b) as a function of centrality Npart. The green boxes on the data points depict the overall normalization uncertainties in p + p and Au + Au data respectively.

The nuclear modification factors RCP of D0 meson are shown in the left figure, and show significantly suppression at high pT and the suppression level is comparable to that of light hadrons at pT > 5 GeV/c, indicates that charm quarks lose significant energy when traversing through the hot QCD medium. Right figure shows the pT-integrated D0 production cross section per nucleon-nucleon collisions in Au + Au collisions together with the measurement in p + p. The full-pT integrated cross section in Au + Au collisions seem to be smaller than that in p + p collisions by 1.5σ, indicating that cold nuclear matter effects (CNM) and/or hadronization through quark coalescence may play an important role in Au + Au collisions. While for pT > 4GeV/c, it shows a clear decreasing trend from peripheral to mid-central and central collisions.

Posted March 27, 2019

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STAR focus: Collision Energy Dependence of pt Correlations in Au+Au Collisions at RHIC

The STAR Collaboration recently has had the paper “Collision-energy dependence of pt correlations in Au+Au collisions at energies available at the BNL Relativistic Heavy Ion Collider” has published in Phys. Rev. C 99, 044918.

The study of event-by-event correlations and fluctuations in global quantities can provide insight into the properties of the hot and dense matter created in Au+Au collisions at RHIC. Correlations of transverse momentum, pt, have been proposed as a measure of thermalization and as a probe for the critical point of QCD. A detailed study of the of dependence of two-particle pt correlations on collision energy and centrality may demonstrate the effects of thermalization. If the matter produced in collisions at RHIC passes through the QMD critical point, the fluctuations are predicted to increase with respect to a baseline of uncorrelated emission. A possible signature of the critical point could be non-monotonic behavior of the two-particle correlations as a function of the collision energy in central collisions.

This paper reports two-particle transverse-momentum correlations from Au+Au collisions taken during the RHIC Beam Energy Scan at center of mass energies ranging from 7.7 GeV to 200 GeV. These measurements are compared to previous measurements from the CERES Collaboration at the Super Proton Synchrotron and from ALICE at the Large Hadron Collider. The data are compared with UrQMD model calculations and with a model based on a Boltzmann-Langevin approach incorporating effects from thermalization.

Left: The relative dynamical correlation for 7.7 GeV and 200 GeV Au+Au collisions compare with similar results fron 2.76 TeV Pb+Pb collision. The dashed line represents a fit to the data at 200 GeV given by 22.3%/(Npart)1/2. Statistical and systematic errors are shown. Right: The relative dynamical correlation for Au+Au collisions as a function of collision energy for the 0-5% centrality bin along with results for Pb+Pb collisions from ALICE along with UrQMD calculations and results from Boltzmann-Langevin model calculations. The solid line is drawn to guide the eye. Statistical and systematic errors are shown.

Posted April 29, 2018

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April 29, 2019
Congratulations to Alex Jentsch who has just successfully defended his Ph.D. thesis at the University of Austin, Texas. His thesis was entitled "Studying heavy-flavor quark jets using D0+hadron two-dimensional angular correlations in Au+Au collisions at √sNN = 200 GeV".

March 25, 2019
Congratulations to Tetsuro Sugiura from the University of Tsukuba who has successfully defended his Ph.D. thesis on "Exploring the QCD phase diagram measured by cumulants of net-charge distributions in Au+Au collisions at the STAR experiment".

March 25, 2019
Congratulations to Ryo Aoyama who has just successfully defended his Ph.D. thesis at the University of Tsukuba. His these was entitled "Collision geometry and event plane dependence of two particle correlations in √sNN = 200 GeV Au+Au collisions at RHIC-STAR".

March 12, 2019
Congratulations to Arghya Chatterjee who successfully defended his PhD thesis at VECC, Kolkata on March 9, 2019. His thesis title is: "Study of conserved quantities and their correlations in the RHIC Beam Energy Scan Program".

March 7, 2019
The February 2019 STAR Newsletter focuses mainly on the startup of the experiment for Run 19, and a thoughtful look at cosmic events in the upgraded TPC, plus some public outreach.

February 19, 2019
Congratulations to Dr. Lang He who has successfully defended his Ph.D. thesis at Purdue. His thesis was titled: “Measurement of D0 directed flow and elliptic flow in Au+Au Collisions at √sNN=200 GeV".

January 2, 2019
The December 2018 edition of the STAR Newsletter recaps the past year's progress, plus some recent STAR meetings, and our spokespersons catch us up on schedules and personnel for the upcoming Run 19.

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