SN0657 : The STAR Beam Use Requests for run 17 and run 18 (2016)

Author(s):the STAR Collaboration
Date:May. 30, 2016
File(s): STAR_BUR_Run1718_v21.pdf
Abstract:In this Beam Use Request the STAR Collaboration presents four compelling and prioritized scientific programs for the 2017 and 2018 RHIC runs.

STAR's highest scientific priority is the first significant measurement of the sign change of the Sivers function, as compared to the value measured in semi-inclusive deep inelastic scattering experiments, through measurements of single spin asymmetries in W+/-, Z, direct photon and Drell-Yan production in transversely polarized √s = 500 GeV p+p collisions. This measurement will also shed light on the size and nature of the evolution of these transverse momentum dependent distributions. The sign change measurement is a fundamental test of QCD and is being pursued by other experiments, making a timely measurement imperative. We therefore request 13 weeks of 500 GeV p+p running in Run17.

STAR’s second scientific priority is to clarify the interpretation of measurements related to the chiral magnetic effect, chiral magnetic wave and chiral vortical effect. We therefore request two 3.5 week runs in Run 18 with collisions of isobaric nuclei, Ruthenium-96 (Ru+Ru) and Zirconium-96 (Zr+Zr). Ru nuclei have an atomic charge of 44 compared to 40 for Zr. Ru+Ru collisions will therefore generate a magnetic field approximately 10% larger than Zr+Zr collisions while all else remains virtually fixed. Comparisons of charge separation in Ru+Ru and Zr+Zr collisions will isolate the magnetic field dependence of the observed charge separation thereby determining what fraction of those measurements are related to the chiral magnetic effect. These results will greatly advance our understanding of the chiral magnetic effect and have fundamental impact beyond the field of high-temperature QCD.

Our next scientific priorities involve taking data at two beam energies that are lower than the nominal energies, but are not part of STAR’s proposed Beam-Energy- Scan Phase-II (BES-II) program in 2019-2020.

The first of these requests is driven by theoretical calculations suggesting that the partonic coupling to the medium is stronger when temperatures are close to the critical value. Results from Run 10 at 62.4 GeV provide tantalizing evidence that this is true, but the data are statistically limited. We therefore request a 4 weeks 62.4 GeV Au+Au run in Run 17 to significantly enhance the statistical precision of these results. Precise measurements of non-photonic electron v2 will confirm that this enhanced coupling results in the partial thermalization of charmed quarks even at this low energy, while our semi-inclusive recoil jet studies will probe possible differences in the jet energy redistribution. Finally, measurements of direct (virtual) photons will improve our understanding of thermal photon production and their medium coupling.

The second request is derived from STAR’s recent report of the observation of the global polarization of hyperons (GPH) in non-central Au+Au collisions at energies below 40GeV. This is the first direct probe of the plasma vorticity, a fundamental characteristic of any fluid that must be quantified in ordrer to understand the physics in detail. It also provides a measure of the magnetic field present over the evolution of the QGP. In addition to being of fundamental interest on their own accounts, these measurements provide critical context for recent high-profile studies of exotic phenomenon. In particular, the Chiral Vortical Effect (CVE) and Chiral Magnetic Effect (CME) are among the RHIC program’s most exciting and visible topics today. However, they are nontrivial and require several conditions to be simultaneously met; “extraordinary claims require extraordinary evidence.” GPH has nothing “chiral” or exotic about it. In a theoretically well-grounded way, it measures the average vorticity (ω) and field (B). Hence, GPH must be understood first, to put more “extraordinary” claims on firmer ground. A solid understanding of GPH and the magnetic field, however, requires the increased statistics available with a 2-week run at 27 GeV. While similar high-statistics measurements of GHP will eventually be performed in BES-II, the scientific impact of getting firm measures of vorticity and B much sooner is high, given the current intensity of research and attention on CME and CVE.

Table 1.1 and Table 1.2 summarize the above requests, along with our scientific priorities and proposed running sequence. We have considered two scenarios: Scenario 1: 19 cryo-weeks for Run 17 and 13 cryo-weeks for Run 18 (Table 1-1)
Scenario 2: 24 cryo-weeks for Run 17 and 13 cryo-weeks for Run 18 (Table 1-2)

In this document we first present highlights from papers published since the last PAC meeting and on-going analyses. Next we summarize the analysis status of data from STAR’s two recent upgrades, the Heavy Flavor Tracker (HFT) and the Muon Telescope Detector (MTD), and discuss the detector’s performance in Run 16. In Sections 4 and 5, we detail the STAR Collaboration’s compelling physics program driving our Run 17 and Run 18 beam-use request. Collection of these data will allow us to achieve our spin and relativistic heavy ion physics goals on a timescale consistent with the current intense international interest while utilizing RHIC beams effectively and taking full advantage of recent improvements in machine and detector capability. Finally, we outline the planned upgrades for Run 17 and 18 and the BES-II.
Keywords:BUR, run 17, run 18, spin, isobar