STAR: The STAR Collaboration

STAR focus: Measurement of Momentum and Angular Distributions of e⁺e^- pairs from Linearly Polarized Photons

Only a handful of fundamental interactions between light and matter are allowed by the theory of quantum electrodynamics, all of which but one have been observed in the 80 or so years since their prediction. The Breit-Wheeler process, the simplest mechanism for converting 'light quanta' into matter and antimatter, has eluded observation for decades, despite being hotly pursued.The idea that you can create matter from smashing together light is a striking demonstration of the physics immortalized in Einstein's famous E=mc² equation, which revealed that energy and matter are two sides of the same coin.

Recently, the STAR collaboration published "Measurement of Momentum and Angular Distributions of e⁺e^- pairs from Linearly Polarized Photons" in Physical Review Letters, in which presents observation of the Breit-Wheeler process in heavy-ion collisions for the first time. This discovery was made possible by a unique analysis which measured the quantum spin-momentum correlations of the produced e⁺e^- pair, revealing a striking 4th order angular modulation (See Figure). Virtual photons live only briefly as they mediate the electromagnetic force and carry a virtual mass. While virtual photons can be in the helicity 0 state, due to their virtual mass, real photons cannot, and instead must have +/-1 unit of helicity. This difference has a profound impact on the produced e⁺e^- pair, since the quantized spin of the colliding photons becomes encoded in the final momentum of the produced electron and positron, resulting in the observed modulated emission angle ($\Delta\phi$). As the figure shows, the STAR data agree with calculations of the Breit-Wheeler process (QED), which predicts a strong cos$4\Delta\phi$ modulation from the collision of linearly polarized photons.

The measured cos$4\Delta\phi$ modulation proves another tantalizing prediction from decades ago. Heavy-ion collisions have long been expected to produce the strongest magnetic fields in the Universe, of order 10¹⁵ Tesla. Physicists in the 1930's predicted that photons shooting through such strong magnetic fields can be "bent", despite the photon itself not being charged - and therefore not directly interacting with the electromagnetic field. However, in quantum mechanics, a real photon can briefly fluctuate into an e⁺e^- pair which can interact with the strong electromagnetic fields. The key prediction for this effect, called vacuum birefringence, is that the photon's path is split depending on the angle between the photon's polarization and the magnetic field direction. In the recent STAR measurement, the colliding photons result from the highly boosted electromagnetic fields of the heavy ions, so the photon's polarization direction is directly related to the classical electric and magnetic field direction. Therefore, the observed cos$4\Delta\phi$ modulation can be understood in terms of the absorption of light when the polarization of the photon (from one ion) is parallel vs. perpendicular to the magnetic field direction (produced by the other ion). This absorption effect is directly related to vacuum birefringence, and provides the first experimental verification that heavy-ion collisions really do produce ultra strong magnetic fields (approximately 10¹⁵ Tesla).

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Posted September 10, 2021

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STAR focus: Longitudinal Double-Spin Asymmetry for Inclusive Jet and Dijet Production in Polarized Proton Collisions at √s = 200 GeV

How is the spin of the proton distributed among its quark, anti-quark, and gluon constituents? The STAR experiment addressed this fundamental question using collisions of high-energy polarized protons. We recently published results on the “Longitudinal Double-Spin Asymmetry for Inclusive Jet and Dijet Production in Polarized Proton Collisions at √s = 200 GeV” in Phys. Rev. D 103, L091103 (2021) highlighted as PRD Editors' Suggestion. The data give insight in the gluon spin contribution to the proton spin. We measured the asymmetries in the differential production cross sections of inclusive jet and dijet probes for different longitudinal spin configurations of the colliding proton beams as a function of jet transverse momentum and dijet invariant mass. Since gluon-gluon and gluon-quark scattering contributions dominate the production of these probes in the STAR environment, the data provide sensitivity to the gluon spin contribution to the proton spin.
	Left Figure: Results of the double-spin asymmetry for dijets as a function of dijet invariant mass for two different event topologies. The topologies probe complementary gluon fractional momenta. The green square markers show the new results from data collected in 2015. The blue triangle markers show our prior results based on data collected in 2009 and are seen to be in good agreement. The curves show dijet asymmetry expectations from the DSSV14 and NNPDFpol1.1 theory collaborations and the band indicates the size of the uncertainty in the NNPDFpol1.1 expectation.
Our prior results provided first evidence for a positive polarization of the gluons in the polarized nucleon for gluon fractional momenta larger than 0.05 upon their inclusion in global analyses. Our new results have an approximately twice larger figure of merit, with improved systematic uncertainties, and thus considerably strengthen this evidence. Since we concluded our data taking with longitudinally polarized protons in 2015, these data are anticipated to provide the most precise insights in gluon polarization well into the future, likely until the future Electron-Ion Collider comes online.
Posted May 27, 2021 Previous STAR Focus Features

STAR focus: Observation of D_s/D⁰ in Au+Au collisions at √s_NN = 200 GeV

The STAR Collaboration has recently published an article titled "Observation of D_s/D⁰ in Au+Au collisions at √s_NN = 200 GeV” in Phys. Rev. Lett. 127, 092301. Charm quarks are produced on timescales shorter than the Quark-Gluon Plasma (QGP) formation in heavy-ion collisions and they subsequently experience the whole evolution of the QGP matter, making them an excellent probe to study the transport properties of the QGP. In the QGP medium, one expects a different hadronization mechanism from p+p collisions through the recombination of charm quarks and light/strange quarks (namely coalescence hadronization) to dominate at low p_T (< 5 GeV/c) and fragmentation hadronization to dominate at higher p_T. Due to the enhanced strange-quark abundance in the QGP, an increased D_s production in heavy-ion collisions relative to p+p collisions has been predicted in case of hadronization via quark recombination. Comparing the D_s/D⁰ yield ratio in heavy-ion collisions with that in p+p therefore helps us understand the QGP effects on charm-quark hadronization. In this paper we present the first measurement of D_s production and D_s/D⁰ yield ratio as a function of p_T for different collision centralities at midrapidity (\|y\| < 1) in Au+Au collisions at √s_NN = 200 GeV. A clear enhancement of the D_s/D⁰ yield ratio is found compared to PYTHIA simulations of p+p events at the same collision energy. For the D_s/D⁰ ratios integrated over 1.5 < p_T < 5 GeV/c in the 10%–60% centrality range, the significance of this observation is more than 5 standard deviations. The p_T-integrated D_s/D⁰ ratio is compatible with predictions from a statistical hadronization model. The enhancement, and its p_T dependence, can be qualitatively described by model calculations incorporating thermal abundance of strange quarks in the QGP and coalescence hadronization that includes charm quarks. These results suggest that recombination of charm quarks with strange quarks in the QGP plays an important role in D_s-meson production in heavy-ion collisions.

Figure: (a) D_s/D⁰ yield ratio as a function of p_T compared to various model calculations from He/Rapp (0%–20%), Tsinghua, Catania, and Cao-Ko in 0%–10% centrality interval of Au+Au collisions, and PYTHIA prediction in p+p collisions at √s_NN = 200 GeV. (b) D_s/D⁰ yield ratio as a function of p_T compared to model calculations from Tsinghua in 20%–40% (solid circles) and 40%–80% (open circles) centrality intervals of Au+Au collisions at √s_NN = 200 GeV. Vertical bars and brackets on data points represent statistical and systematic uncertainties, respectively.
Posted August 30, 2021 Previous STAR Focus Features

STAR focus: Invariant Jet Mass Measurements in pp Collisions at √s = 200 GeV at RHIC

In a new paper published in Phys. Rev. D 104, 052007 (2021), the STAR Collaboration measured for the first time the invariant mass, M, of inclusive jets in pp collisions at √s = 200 GeV with data taken in 2012. The anti-k_T algorithm from the FastJet package was used to cluster the collimated hadrons that result from parton showers into collective objects called jets. The invariant mass of a jet is defined as the magnitude of the four-momentum sum of all of the constituents falling within the jet radius, R. By comparing jet substructure measurements — with jet mass as one such substructure observable — with calculations or event generators, we can improve our understanding of both perturbative QCD (parton production, shower) and non-perturbative QCD (hadronization), and further tune our models to describe the data. To make this comparison possible the data had to be corrected for detector effects, in this case using a two-dimensional iterative Bayesian deconvolution method in the RooUnfold package, which brought the jet mass and transverse momentum, p_T, to particle-level simultaneously. Additionally, STAR presented the fully corrected mass of SoftDrop "groomed" jets, which have soft radiation reduced, allowing for better comparison to results from perturbative calculations and event generators at parton-level.

Figure: Comparison between hadron-level fully corrected STAR data and a calculation at next-to-leading-log accuracy at parton-level as well as parton-level Monte Carlo, for jet mass, M, and SoftDrop-groomed jet mass, M_g, of anti-k_T jets in pp collisions at √s = 200 GeV.

The jet mass results were presented for three selections of the jet radius to tune the amount of typically soft radiation unassociated with the initial partonic scattering contaminating the jet, and the relative importance of radiation leaving the jet, or exiting and then re-entering via a subsequent splitting. Three jet p_T selections are shown as well, to study the dependence of the jet mass on its momentum. We find, as expected, that the jet mass increases both with increasing R and p_T, due to the increased phase space for radiation. For each R and jet p_T, the jet mass is compared to leading-order Monte Carlo event generators PYTHIA-6 (tuned to STAR), PYTHIA-8 (LHC tuned), and HERWIG-7 (LHC tuned). We find that PYTHIA-6 describes data accurately in almost all cases, while HERWIG-7 and PYTHIA-8 in general predict smaller and larger jet mass, respectively, than what is observed. This demonstrates the need for further tuning of these event generators at RHIC.

Finally, a comparison (see figure) was also made to parton-level, with both parton-level PYTHIA-8 and a calculation from Lee and Ringer. This comparison demonstrated the large impact of hadronization on the jet mass as well as the efficacy of SoftDrop grooming in suppressing the non-perturbative radiation in the jets to reduce the effect of hadronization. These new data can be used to extract a non-perturbative shape function for future calculations performed at RHIC energies.

Posted October 4, 2021

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