Description
The first phase of Beam Energy Scan (BES) program of the Relativistic
Heavy Ion Collider (RHIC) was an exploration of the QCD phase diagram.
The second phase is an exploration for criticality and phase transition
signals. For the Solenoidal Tracker at RHIC (STAR) a quantitative
understanding of these signals requires an increase in statistics in for
7, 11, 14, and 19GeV AuAu collisions as well as dedicated hardware
upgrades. The Event Plane Detector (EPD) is a
proposed high $\eta$ hit detector that would replace the STAR Beam Beam
Counter (BBC – a 32 channel hit detector 3.3 < $\eta$ < 5.0 used for
BES triggering and first order event plane reconstruction) for BES II,
which is scheduled to begin in 2019. The EPD would provide improved
triggering, increased detector coverage in jet-like $\eta$-$\phi$
correlation measurements, improved resolution for event plane
determination independent of the STAR Time Projection Chamber (TPC – a
charged particle tracker $| \eta |$ < 1), and provide a TPC independent
centrality definition. Divorcing the determination of the event plane
positions as well as collision centrality from the TPC via a forward
detector is crucial for correlation measurements performed at
mid-rapidity.
The EPD design consists of two scintillator discs at $\pm$ 3.75m, each
is separated into ~500 tiles. A tile has embedded wavelength shifting
fiber coupled to clear fiber outside of the tile which is, in turn,
coupled to a silicon photomultiplier (SiPM) – an inexpensive and
magnetic field insensitive replacement for traditional phototubes. A
pre-prototype of the detector, featuring scintillator with embedded
fiber coupled to SiPMs was integrated into STAR during the 2015 run.
Currently tile designs varying geometry and detector specifications are
being fabricated and tested along with latest generation SiPMs.
Additionally simulations have been performed to optimize tile
$\eta$/$\phi$ segmentation, size, and shape. A newly machined prototype
featuring the anticipated geometry of the EPD will be put in place
during the 2016 STAR run.
