SN0670 : STAR Collaboration Beam Use Request for run 18 and run 19

Author(s):the STAR Collaboration
Date:May. 15, 2017
File(s): star-beam-request-2017.pdf
Abstract:This Beam Use Request from the STAR collaboration for RHIC Runs 18 and 19 is focused on two compelling programs that are key to the completion of the RHIC mission. In Run 18 Isobaric collisions will provide enhanced clarity as to the role of the magnetic field in the charge separation measurements. While for Run 19 STAR proposes to embark on the NSAC endorsed Beam Energy Scan II (BES-II) program which will dramatically enhance our understanding of the QCD phase diagram.

STAR’s highest scientific priority for Run 18 is the successful realization of the isobaric collision program. We are requesting two 3.5 week runs with collisions of Ruthenium-96 (Ru+Ru) and Zirconium-96 (Zr+Zr) at psNN = 200 GeV. Comparison of results from these events will help clarify the interpretation of measurements related to the chiral magnetic effect. Since Ru nuclei have an atomic charge of 44 compared to 40 for Zr, Ru+Ru collisions will generate a magnetic field approximately 10% larger than Zr+Zr collisions while all else remains essentially constant. Comparison of charge separation results will aid in determining the fraction of those measurements which are related to the chiral magnetic effect by isolating the magnetic field dependence. Our understanding of the chiral magnetic effect will thereby be greatly advanced and have a fundamental impact beyond the field of high-temperature QCD.

STAR’s second highest priority for Run 18 is Au+Au collisions at psNN = 27 GeV to allow for statistically significant Lambda and antiLambda global polarization measurements. These measurements will take advantage of the newly installed Event Plane Detector’s improved first-order reaction plane resolution. STAR’s third highest priorities for Run 18 are Au+Au collisions at psNN = 3.2 GeV in “fixed target” mode. psNN = 3.2 GeV running will allow for a statistically significant measurement of Kurtosis at a collision energy between the HADES measurement at 2.4 GeV and STAR’s lowest energy point at 7.7 GeV. This energy is accessible, and being pursued, by other international experiments, BM@N in 2019 and CBM in 2025, making a timely measurement imperative.

STAR’s highest scientific priority for Run 19 is the commencement of the RHIC Beam Energy Scan II. Three upgrades are planned for completion prior to the BES-II. Both the inner Time Projection Chamber (iTPC) and the endcap Time of Flight (eTOF) are on schedule for full installation in Run 19; increasing the rapidity and low transverse momentum acceptance of STAR, and extending our particle identification capabilities. The event plane detector (EPD) will be fully installed for Run 18 after completing a commissioning run this year. The EPD provides enhanced event plane resolution and forward centrality measurements.
Table 1 summarizes our request for Run 18 assuming 15 cryo-weeks. Table 2 and Table 3 summarize the request for the start of the BES-II program considering two scenarios: 
• Scenario 1: 24 cryo-weeks in 2019 
• Scenario 2: 20 cryo-weeks from a combined run during 2019-2020 
For both scenarios we assume another RHIC run occurs to allow the completion of the BES-II. Table 4 outlines the dataset STAR currently plans to collect at that time. Collection of these data will allow us to achieve our high impact physics goals on a timescale consistent with the current intense international interest while utilizing RHIC beams effectively and taking full advantage of intended improvements in machine and detector capabilities.
Keywords:Beam Use Request, Beam Energy Scan, Critical Point, isobar, chiral magnetic effect