Available online at www.sciencedirect.com
Nuclear Physics A 904–905 (2013) 921c–924c www.elsevier.com/locate/nuclphysa
STAR Upgrade Plan for the Coming Decade Huan Zhong Huang(for the STAR Collaboration)1 Department of Physics and Astronomy, University of California, Los Angeles, CA 90095-1547, USA
Abstract The STAR Collaboration will complete the Heavy Flavor Tracker (HFT) and the Muon Telescope Detector (MTD) upgrades by 2014. STAR has also embarked on an upgrade plan to extend the capabilities for measuring jets, electron/photon and leading particles in the forward rapidity region in the coming decade. Planned detector upgrades include tracking detectors for charged particles, electro-magnetic and hadronic calorimeters and particle identification detector in the forward direction. We will present physics motivations, status of detector R&D and design considerations for the forward measurements focusing on p+p/p+A and polarized p+p collisions. 1. Introduction RHIC is a dedicated QCD facility exploring new horizons in QCD phase diagram, properties of the partonic matter, vacuum excitation, hadron structure and exotics. The STAR collaboration has been developing a plan for our scientific program and detector upgrades in the coming decade [1]. We divide the upgrades into two categories according to the schedule: Near Term for those within 2-3 years; Intermediate Term for beyond the 3 years in the late half of the decade. In this contribution we will review the status of our major detector upgrades and the key physics motivations for each upgrade. 2. Near Term Upgrades – HFT and MTD The HFT is made of three sub-systems: a pixel detector with two layers of CMOS Monolithic Active Pixel Sensor (MAPS) located at radii of 2.5 cm and 8.0 cm, respectively; an Intermediate Silicon Tracker (IST) at 14.0 cm radius; and a layer of existing STAR Silicon Strip Detector (SSD) at a radius of 22.0 cm. The MAPS is an integrated sensor with very thin silicon wafer, 0.4%X0 , and an expected pixel resolution of 12 μm. The IST is based on conventional silicon strip technology with strips along the beam direction. The existing SSD is a single layer silicon with strips on both sides. Notable for the STAR pixel detector design include a carbon fiber sector tube structure for ladder support and an insertion mechanism to allow for easy detector removal/insertion and accurate position reproducibility. A prototype of 3 out of a total of 10 sectors of pixel MAPS will be instrumented for an engineering run in 2013. The full HFT including the pixel, the IST and the SSD will be available for RHIC 2014 run. Figure 1 shows a drawing of the HFT inside the STAR detector system. 1A
list of members of the STAR Collaboration and Acknowledgements can be found at the end of this issue.
0375-9474/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nuclphysa.2013.02.165
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Figure 1: Schematic layout of the HFT inside the STAR TPC and the carbon fiber sector tube structure for ladder support.
The major physics program for the HFT upgrade includes the measurement of charm mesons through hadronic decay channels extending to the low transverse momentum (pT ) region and the separation of electrons from charm and bottom semi-leptonic decays. The measurement of elliptic flow v2 for charmed meson at low pT is particularly interesting because the charm quark hydrodynamic flow, not accessible in the lepton measurement from heavy quark decays due to uncertainties from decay kinematics, uniquely probes the heavy quark collectivity and thermal properties of the QCD medium [2]. Measurements of D0 , D s and Λc are possible with the STAR HFT upgrade based on our simulations for Au+Au collisions at RHIC. High pT electrons from heavy quark decays also provide unique measurement for parton energy loss in the QCD medium. Existing measurements of nuclear modification factor and elliptic flow for non-photonic electrons show considerable parton energy loss. But we cannot separate the charm and bottom quark energy loss experimentally with the current measurement, which significantly hindered our understanding of the parton energy loss mechanism. The HFT will allow us to statistically separate electrons of charm decays from those of bottom decays via the distance of closest approach (DCA) to the primary vertex. The MTD was designed to measure muons at mid-rapidity in STAR. The essential detector technology was based on the multi-gap resistive plate chamber (MRPC) used for the STAR Timeof-Flight (TOF) detector upgrade. We have installed approximately 10% of the MTD modules for run 2012. We expect to install about 50% MTD super-modules for run 2013 and complete the full MTD for run 2014. Figure 2 shows the structure of MRPC module and the installed MTD super-modules outside the magnet iron yoke.
Figure 2: Mechanical structure of the MRPC and the installed MTD super-module outside the magnet yoke.
H.Z. Huang / Nuclear Physics A 904–905 (2013) 921c–924c
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The MTD upgrade will allow STAR to separate Upsilon 1S from 2S/3S states from Au+Au collisions. We will be able to experimentally test the proposed sequential melting scenario for Upsilon states, which is considered sensitive to the temperature of the QGP [3]. The yield of di-leptons in the intermediate mass region of 1.0 to 3.0 GeV/c2 has significant contributions from charmed meson decays. The exact shape of the mass distribution depends on the final state azimuthal angular correlations between charmed and anti-charmed mesons. Such angular correlation can vary dramatically if heavy quarks suffer considerable parton energy loss while traversing the QCD medium. We will measure the electron-muon correlations from heavy quark decays, which will allow us to experimentally determine the di-lepton mass distribution from charmed meson decays at the intermediate mass region. 3. Intermediate Term Upgrades – TPC Inner Sector and Forward Instrumentation Our intermediate term upgrade projects beyond 2015 include the Time-Projection Chamber (TPC) inner sector upgrade and the forward instrumentation upgrade. The STAR TPC outer and inner sectors have very different geometries. The outer sector has 32 pad rows covering the full area, with each pad of 6.2mm×19.5mm. The inner sector has only 13 pad rows with each pad of 2.85mm×11.5mm. The active pads only cover a small area of the inner sector while most areas are covered by a ground plane. The design was mostly motivated by a very conservative approach to deal with possible large particle densities from nucleus-nucleus collisions at RHIC when there was no reliable theoretical predictions to guide the experiment. The compactness of readout electronics also limited the density of read-out channels. As a result the TPC inner sectors in STAR only provide sparse hit measurement from charged tracks limiting the detection capability for low pT and forward tracks which may miss the outer sectors. Recent advances in integrated readout electronics make it possible for much higher read-out densities and the pad layout design for the STAR TPC can also be further optimized for the expected particle density in central Au+Au collisions at RHIC. The STAR collaboration will upgrade the TPC inner sectors including the multi-wire proportional chambers, the pad layout and the readout electronics, extending the STAR tracking capability to higher pseudo-rapidity ( |η| < 1.7) and lower pT region. Preliminary data from the STAR phase I beam energy scan program have yielded many exciting results, among which is the intriguing observation that there may be a distinct transition between 19.6 and 11.5 GeV beam energies from partonic matter to matter whose dynamics are dominated by hadron degrees of freedom [4, 5]. The result on higher moments of net protons [6] is limited by statistics. The STAR collaboration will have a phase II beam energy scan program with at least an order of magnitude increase in the data sample for the low energy collisions and finer scan steps in the interesting energy region. In order to achieve these goals electron cooling devices for the RHIC heavy ion beams are needed, which will take several years to develop and manufacture. Our planned iTPC upgrade will match the schedule for the electron cooling installation. The phase II beam energy scan program is expected to commence after 2016. The STAR collaboration pioneered a forward physics program at RHIC with the first measurement of single spin asymmetry AN for π0 at forward rapidity from transversely polarized p+p collisions [7]. In addition, results on π0 -π0 correlations from d+Au collisions at forward rapidity revealed intriguing centrality dependence, consistent with expectations from Color Glass Condensate (CGC) picture for low x gluon saturation in the gold nuclei. Future advances in the forward physics program requires a full azimuthal large acceptance detector capable of measurements of photons, neutral π, charged hadrons, Drell-Yan electrons, jets and their correlations.
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Figure 3: Schematic drawing for the planned Forward Instrumentation Upgrade on the west side of the STAR TPC.
Figure 3 shows a schematic drawing for the planned STAR Forward Instrumentation Upgrade. Several GEM based trackers will provide tracking information for charged particles covering the pseudo-rapidity range of 2.5-4.0. The Cerenkov detector is needed to provide proton and meson (Kaon or pion) separation in the forward direction. The detector technology, RICH versus Threshold Cerenkov, has not been decided. We envision that the PID detector will be included in the phase II project of the Forward Instrumentation Upgrade. The Forward Calorimeter System (FCS) consists of a front Electromagnetic Calorimeter (EMCal) followed by hadron calorimeters. The EMCal will be built using Tungsten-Powder and scintillating fibers, a new calorimeter construction technology developed by a team from UCLA, PSU and TAMU under the support of an EIC generic detector R&D program [8]. Compact EMC modules have been built with W-powder and scintillating fibers, and tested with beams at FNAL. Excellent energy resolutions and detector uniformity have been achieved with Spaghetti Calorimeter (SPACAL) geometry and they have met our specifications for STAR and EIC. The forward hadron calorimeters will be able to contain most of the hadronic showers. We plan to re-use the E864 SPACAL modules for partial coverage of the acceptance. New calorimeter modules made of lead and scintillator plates will be constructed to cover the rest of the forward acceptance. The FCS detector is expected to be a compensated calorimeter for electron and hadron responses. Preliminary Monte Carlo simulations indicated that we can achieve an electron and hadron discrimination factor suitable for a Drell-Yan physics program. A full scale prototype of the hadron calorimeter and beam testing of the FCS prototype will be needed to evaluate the performance, which will be used for physics simulations in the coming years. We aim at a schedule such that the construction will begin in 2015, and installation and commissioning of the detectors will start two years after. References [1] [2] [3] [4] [5] [6] [7] [8]
C.A. Gagliardi, for the STAR collaboration, J. Phys. G:Nucl. Part. Phys. 38 (2011) 124130. M. He, R.J. Fries and R. Rapp, Phys.Rev. C86 (2012) 014903. A. Mocsy and P. Petreczky, Phys. Rev. Lett. 99 (2007) 211602. X. Dong, for the STAR Collaboration, these proceedings. X. Zhang, for the STAR Collaboration, these proceedings. X. Luo, for the STAR Collaboration, these proceedings. J. Adams et al. (STAR Collaboration), Phys. Rev. Lett. 92 (2004)171801. O. Tsai at al., Presentation at XVth Int. Conf. on Calorimetry in High Energy Physics, June 4-8, 2012.