SN0721 : STAR Collaboration Beam Use Request for Run-20 and Run-21

Author(s):STAR Collaboration
Date:May. 15, 2019
File(s): BUR2019_final_0.pdf

This Beam Use Request from the STAR collaboration for RHIC Run-20 and Run-21 is focused on the completion of the NSAC-endorsed second phase of the Beam Energy Scan (BES-II) program. This program started with Run-19 in which the collaboration aims to collect data from the two top collider energies of sqrt(sNN)=19.6 and 14.6 GeV as well as data from a subset of the fixed target (FXT) program. Collection of all the data for BES-II and the FXT program  is STAR’s highest scientific priority. Additionally, the STAR collaboration proposes a small system run to study the emergence of collectivity and the mechanism for early-time hydrodynamization in large collisions systems.

BES-II will dramatically enhance our understanding of the QCD phase diagram. The proposed program involves dedicated low beam energy running and high precision measurements of the observables which have been proposed as sensitive to the phase structure of QCD matter. In addition to the five lower collider energies that have been put forward in past BURs, STAR proposes a sixth collider beam energy at sqrt(sNN)= 16.7 GeV. These data will provide for a finer scan in a range where the energy dependence of the net-proton kurtosis and neutron density fluctuations appears to undergo a sudden change.

With Run-19, the collaboration will start its fixed-target (FXT) program which extends the reach of its BES-II program energy range down to lower center-of-mass energies.

Three detector upgrades have been proposed for BES-II and have been successfully installed for Run-19. The upgrades increase STAR’s acceptance both in rapidity and low transverse momentum, and extend its particle identification capabilities. The Event Plane Detector (EPD) was installed prior to Run-18. The inner Time Projection Chamber (iTPC) and the end-cap Time-of Flight (eTOF) commissioning have benefited from an extensive cosmic ray data taking campaign prior to Run-19. Following recommendations from the 2018 PAC and a very positive report from a BNL-convened cost and schedule review, the STAR collaboration has commenced preparations to significantly improve its forward detection capabilities. A Forward Calorimeter System (FCS) and Forward Tracking System (FTS) will provide superior detection capabilities in the forward region between 2.5 < η < 4.

STAR’s highest scientific priority for Run-20 is the continuation of the RHIC Beam Energy Scan II. The collaboration proposes to continue with the next two highest beam energies in collider mode (11.5 and 9.1 GeV), as well as the associated FXT energies (4.55 and 5.75 GeV) followed by the remaining FXT single-beam energies of 19.5, 13.5, and 3.85 GeV. Based on guidance from the Collider-Accelerator Department (C-AD), we allocate five cryo-weeks in Run-20 to the commissioning of Low-Energy RHIC electron Cooling (LEReC). As the commissioning efforts are still ongoing, the status of LEReC for the Run-20 11.5 GeV is not yet clear. Consequently, the proposed schedule will be somewhat fluid within the total budget of 28 cryo-weeks.

We also list our priorities and proposed sequence in the case the total budget for Run-20 is limited to 24 cryo-weeks. Top priority remains with the collider program of the Beam Energy Scan and the commissioning efforts of LEReC, bearing in mind its impact on the long 7.7 GeV run in the following year. With a combined cool-down and total set-up time for the various energies between two and three weeks, an optimistic projection for the non-cooled 11.5 GeV run, and an e-cooled 9.1 GeV run, we estimate that between three and four weeks of dedicated LEReC commissioning time can be set aside and at least 60% of the originally scheduled FXT program completed. The remaining two FXT runs could move to Run-21, as will the small system run.

STAR’s highest scientific priority for Run-21 is the completion of the RHIC Beam Energy Scan II. The bulk of the 20-cryoweeks budget will be devoted to Au+Au collisions at the lowest collider energy of the program, at sqrt(sNN)=7.7 GeV. We expect to refine our estimates of the projected run time for 7.7 GeV, currently 12 weeks, following some tests with C-AD towards the end of Run-19. The collaboration proposes to run the collider at sqrt(sNN)=16.7 GeV to allow collection of an important data point between 14.6 and 19.6 GeV as is pointed out earlier in this summary.

Depending on the availability of cryo-weeks in Run-20 and/or Run-21 the collaboration proposes to collect data set(s) in the context of a small system run using O+O collisions. These data would allow for a direct comparison with a similarly proposed higher-energy O+O run at the LHC around 2021-2022, and further motivate the case for a small system scan complementary to ongoing efforts by the NA61/SHINE collaboration at SPS energies, and other proposed light-ion species at the LHC. 

For FY22, we include a request for a dedicated 16-week pp run at sqrt(s) = 500 GeV. This run will take full advantage of STAR’s new forward detection capabilities and further capitalize on the recent BES-II detector upgrades. We motivate a program that will use RHIC’s unique ability to provide transverse and longitudinally polarized proton beams to exploit both an increased statistical power and kinematic reach from recent and planned detector upgrades.


Keywords:Beam Use Request, BUR, BES-II, Beam Energy Scan, Forward Upgrade