Performance of the Endcap Time-of-Flight detector in the STAR beam-energy scan

TL;DR

This study presents the Endcap Time-of-Flight (eTOF) detector for STAR, deployed to restore midrapidity PID in the fixed-target BES-II beam-energy scan, enabling measurements down to $\sqrt{s_{NN}}=3.0$ GeV. The eTOF, built from CBM-inspired MRPC technology, achieves a time resolution near $70$ ps and a PID efficiency around $70\%$, with an acceptance extension to $1.55<\eta<2.17$ and coverage of the FXT rapidity region. The paper details the detector geometry, hit reconstruction, calibration, alignment, and error handling, and quantifies performance in geometric acceptance, system resolution, efficiency/purity, $1/ (\beta\gamma)^2$ cuts, and acceptance stability. The physics implications include enabling critical-point searches via net-proton fluctuations, improving kaon/pion/proton spectra and $dN/dy$ distributions, and providing overlap with collider energies for cross-checks and systematic studies. Overall, eTOF enhances STAR’s FXT program by delivering robust PID at midrapidity, bridging collider and fixed-target data, and offering a valuable platform for testing CBM MRPC technology in a large-scale heavy-ion experiment.

Abstract

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.

Figures (19)

• Figure 1: Schematic of the STAR central barrel. 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 BESupgradesiTPCRelated is also an upgrade for BES-II, which increased the low-$p_T$ coverage and tracking resolution star2015technical. 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 ($\eta$) values in FXT mode at key points in the STAR geometry.
• Figure 2: Left: Photograph of the eTOF wheel. Modules are arranged in three layers. Right: open modules in the clean room equipped with 3 MRPCs each.
• Figure 3: Schematic layout of the eTOF detector. (a): full wheel structure composed of 12 sectors. (b): single module containing 3 MRPC's.
• Figure 4: Scematic drawing of the MRPC3b used at eTOF.
• Figure 5: Schematic depiction of the eTOF readout chain and its components.