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 that have been found to be sensitive 

to the phase structure of QCD matter. In addition to the four lower energies from BES-I, STAR's plan is to run a fifth beam energy at √s_NN=9.1 GeV. This energy will bridge the large gap in chemical potential between the 7.7 and 11.5 GeV energies. The collaboration proposes to extend its energy range further down to lower center-of-mass energies by means of a fixed-target (FXT) program. 

 

Three upgrades were proposed for 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; these increase the rapidity and low transverse momentum acceptance of STAR, and extend our particle identification capabilities. The event plane detector (EPD) is currently being commissioned as it was completed and installed prior to Run 18.

 

STAR's highest scientific priority for Run 19 is the commencement of the RHIC Beam Energy Scan II. The collaboration proposes to start with the two highest beam energies in collider mode (19.6 and 14.5 GeV), as well as the associated FXT energies (4.5 and 3.9 GeV). Next, FXT energies starting at √s_NN =7.7 GeV should follow. Access to FXT data at √s_NN=$7.7 GeV will provide for an important cross-check with the collider-mode data at that same energy.
 

STAR's highest scientific priority for Run 20 is the continuation of the RHIC Beam Energy Scan II. The collaboration proposes to start with the highest beam energies in collider mode, as well as the fixed-target energies that are associated with the single-beam energies for those collider-mode energies. 

 

The request for the start of the BES-II program considers the following scenarios:

With guidance from the Collider-Accelerator Department, each scenario has cryo-weeks assigned to commissioning of Low-Energy RHIC electron Cooling (LEReC): six weeks in Run 19 and five weeks in Run 20. For all scenarios we have to assume a third year of RHIC running to follow, in order to allow the completion of the BES-II physics mission. Specifically, the request of twelve weeks for √s_NN =7.7 GeV will need to be collected in a third year of BES-II. Moreover, some scenarios necessitate this third run to address parts of the requests for the √s_NN = 9.1 GeV. Run 21 would thus combine the remainder of BES-II with the start of STAR's forward physics program which would see a √s =500GeV polarized pp run that year as proposed in STAR Notes SN0669 and SN0648.

 

In this  BUR document  we report highlights  from analyses completed since the last PAC meeting, with a focus on data from Runs 14-17.  We  discuss the Collaboration’s BES-II and Fixed-Target physics programs  which motivate the Run 19 and Run 20 beam use request. Next we outline the status of the upgrades intended for both programs.