The STAR experiment at RHIC at Brookhaven National Laboratory completed the installation of an endcap time-of-flight subsystem (eTOF) in February 2019. The eTOF subsystem provided essential mid-rapidity particle identification (PID) for the fixed-target (FXT) portion of phase II of the beam energy scan (BES II). The FXT program allowed BES II to include center-of-mass energies from $\sqrt{s_{_{NN}}} = 3.0$ GeV to $\sqrt{s_{_{NN}}} = 7.7$ GeV, not accessible by colliding beams. The eTOF detectors and readout electronics were designed for the CBM experiment at FAIR and adapted for use at STAR. In this paper, we describe the details of the system in terms of geometrical layout, acceptance, calibration, hit reconstruction, and particle identification. The system achieved a time resolution of about 70 ps and a PID efficiency of about 70\%, meeting the design goals of the project.
Studying the phase diagram of nuclear matter under extreme conditions is the main focus of the beam energy scan (BES) at the Relativistic Heavy-Ion Collider (RHIC) at Brookhaven National Laboratory. RHIC collided heavy nuclei ( 197 Au) at a range of center-of-mass energies to study the properties of nuclear matter at different temperatures and baryon chemical potentials (µ B ). Achieving low-energy collisions required injecting beams into the RHIC synchrotrons at energies below the design injection energies (9.8 GeV for each ring, √ s NN = 19.6
GeV). Circulating beams below the design energy of the synchrotons challenged the ability of RHIC to focus the ion beams.
The lowest collision energy achieved in the collider configuration was √ s NN = 7.7 GeV (corresponding to µ B ≈ 420 MeV [1]). In order to more extensively scan the quantum chromodynamic phase diagram, it was possible to further lower the center of mass energies at RHIC by implementing a fixedtarget (FXT) configuration. The FXT configuration was implemented by STAR ahead of BES phase II (BES-II) by installing a gold foil in the beam pipe near the west end of the detector and circulating beam in only one of the two RHIC synchroton rings. The FXT configuration allowed for collisions as low as √ s NN = 3.0 GeV (µ B ≈ 720 MeV). STAR recorded data at 12 collision energies in the FXT configuration ( √ s NN = 3.0, 3.2, 3.5, 3.9, 4.5, 5.2, 6.2, 7.2, 7.7, 9.2, 11.5, 13.7 * Corresponding author. Email: mlabonte@ucdavis.edu GeV), with the four highest energies overlapping with data taken in the collider mode allowing for validation of the new experimental geometry.
The FXT program comes with its own challenges for a collider detector like STAR [2]. Midrapidity moves outside of the acceptance of the barrel time-of-flight detector (bTOF) [3] for collision energies higher than √ s NN = 3.9 GeV (0 < η bTOF < 1.5 in FXT mode). To expand the available phase-space and recover midrapidity for the higher FXT energies, an endcap timeof-flight detector (eTOF) [4] was installed in 2019 for BES-II. eTOF extended the pseudorapidity coverage by 0.7 units (1.55 < η eTOF < 2.17) in FXT mode, and allowed for midrapidity measurements for collision energies up to √ s NN = 7.7 GeV.
Figure 1 shows a cartoon schematic of the STAR central barrel, with eTOF located at z = -303 to -270 cm. The other main detectors in the STAR central barrel are also shown, the time projection chamber (TPC) [5] and bTOF. The TPC provides tracking capability as well as low-momentum particle identification. eTOF and bTOF are similar time-of-flight detectors and provide high-momentum PID in conjunction with a momentum measurement from the TPC.
In this paper, the performance and capabilities of eTOF during BES-II will be discussed as well as its utility for physics analysis at STAR. The TPC provides the momentum measurement necessary for TOF PID. The TPC is a cylinder with radius of 200 cm and a length of 200 cm for each half. The iTPC [6,7] is also an upgrade for BES-II, which increased the low-p T coverage and tracking resolution [8]. eTOF is shown in green with different shades corresponding to the the three layers of the eTOF wheel. The inner and outermost radii of eTOF are 112 cm and 222 cm, respectively. The central high voltage cathode of the STAR TPC is seen at Z = 0 cm. Red lines indicate pseudorapidity (η) values in FXT mode at key points in the STAR geometry.
The endcap-time-of-flight detector is composed of 108 multigap resistive plate chambers (MRPCs [9]) arranged in 36 modules. Each module has an active area of 92 cm × 27 cm. There are two, 2 cm overlap regions of the 3 MRPCs in a module. The total size of the gas-tight module box is 120 cm × 49 cm × 11 cm, including the crate for the readout electronics at one end. Three modules form an eTOF sector. There are 12 sectors numbered 13 to 24 with a rotation angle of 30 • per sector that form a wheel-like structure matching sectors 13 to 24 of the east side of the TPC. Due to the overlap of the modules within one sector, 3 layers are obtained with distances of 277, 290, and 303 cm, respectively, from the center of the TPC. The radial extension of the active area ranges from 112 to 222 cm from the center of the beam. Installed eTOF modules are shown on the left side of Fig. 2 and Fig. 3.
The 3 MRPCs within one module are tilted toward the beam axis by 10 • (cf. right side of Fig. 2) and overlap with their neighboring detectors by almost 2 readout strips. The MR-PCs themselves have a double-stack configuration and encase a gas volume subdivided by multiple resistive plates between the high-voltage electrodes and read-out strips (see Fig. 4). Two types of MRPCs originally designed for two regions with different rate exposure within the CBM time-of-flight wall [10,11] are used. An MRPC (called MRPC3a) developed by Tsinghua University in Beijing (THU) can handle charged particle fluxes of up to several tens of kHz/cm 2 . They use low-resistive
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