www.elsevier.com/locate/nimb

Nuclear Instruments and Methods in Physics Research B 261 (2007) 1063–1066

NIMBBeam Interactions

with Materials & Atoms

The STAR integrated tracking upgrade project

Frank Simon *, for the STAR Collaboration

Massachusetts Institute of Technology 26-402, 77 Massachusetts Avenue, Cambridge, MA 02139, USA

Available online 12 April 2007

Abstract

The STAR experiment at the Relativistic Heavy Ion Collider RHIC has a rich physics program both for the study of relativistic heavyion collisions and spin structure measurements with polarized protons. In order to improve the capabilities for heavy flavor measure-ments in heavy ion and polarized proton collisions and to allow the identification of the charge sign of W bosons produced in polarizedproton collisions at 500 GeV, an upgrade of the tracking system both in the central and the forward region is pursued. The integratedsystem providing high-resolution tracking and secondary vertex reconstruction capabilities will use silicon pixel, strip and GEMtechnology.� 2007 Elsevier B.V. All rights reserved.

PACS: 29.40.Gx; 29.40.Cs; 29.40.Wk

Keywords: Tracking detectors; Silicon; GEM

1. Introduction and current capabilities

The STAR experiment at RHIC studies the fundamentalproperties of the new state of strongly interacting matterproduced in relativistic heavy ion collisions and investi-gates the spin structure of the proton in polarized p + pcollisions. A variety of different measurements both inheavy ion collisions and polarized p + p collisions havealready been performed. A key step in these programs isthe ability for direct reconstruction of particles containingcharm and bottom quarks as well as flavor tagging of jetsto allow precise measurements of the spectra, yields andflow of open charm and bottom and to determine spindependent production asymmetries of heavy quarksconnected to the gluon polarization in the nucleon. The fla-vor dependence of the sea quark polarization will bedetermined by parity violating W production and decayin longitudinally polarized p + p collisions at

ps =

500 GeV.

0168-583X/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.nimb.2007.04.047

* Corresponding author. Tel.: +1 617 324 1830; fax: +1 617 258 5440.E-mail address: fsimon@mit.edu

STAR [1] is one of the two large detector systems atRHIC. Its main tracking detector is a large-volume timeprojection chamber (TPC) covering the pseudorapidityrange jgj < 1.2. Additional vertex resolution for the recon-struction of secondary decay vertices is obtained with thesilicon vertex tracker (SVT, jgj < 1), a three-layer silicondrift detector, and the one-layer silicon strip detector(SSD). Tracking in the forward region is provided by theforward TPCs (FTPCs, 2.5 < jgj < 4.0). The barrel(BEMC) and endcap (EEMC) electromagnetic calorimeterscover �1 < g < 1 and 1 < g < 2, respectively. Additionalsmall acceptance electromagnetic calorimetry at highrapidity is provided by the forward pion detector (FPD,3.1 < jgj < 4.2). The current tracking capabilities are insuf-ficient to address the future measurements outlined above.The integrated tracking upgrade project is designed to pro-vide the necessary vertex resolution to uniquely identifyopen charm and bottom and to provide precision trackingin the forward region to determine the charge sign of elec-trons from W+ and W� decays that are identified in theEEMC. Fig. 1 shows an overview of the planned trackingupgrades for STAR. The plans for the inner and forwardtracker together with brief outlines of the physics objectivesdriving them will be discussed.

Fig. 1. Side view of the STAR detector with planned tracking upgrades. The inner tracking region is shown enlarged. The inner tracking system coveringjgj < 1 consists of the Heavy Flavor Tracker HFT, the Hybrid Pixel Detector HPD, the Intermediate Silicon Tracker IST and the existing Silicon StripDetector SSD. The forward tracking system covering 1 < g < 2 consists of the Forward Silicon Tracker FST and the Forward GEM Tracker FGT. Inaddition to the barrel layout for the FGT shown here a disk option is also being investigated.

Fig. 2. Cross-section view of the inner tracking system with the HFT,HPD, IST and the existing SSD.

1064 F. Simon / Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 1063–1066

2. Inner tracker

Heavy quarks are good probes for the properties of thematter created in relativistic heavy ion collisions [2]. Due totheir high intrinsic mass, frequent interactions are neededto bring c and b quarks into equilibrium with the surround-ing matter. Collective flow of heavy quarks is thus a strongindication of thermalization in the early stages of the reac-tion. Flavor tagged jets will provide information on theenergy loss of light versus heavy quarks in the created med-ium. In p + p collisions the production of heavy quarks isdominated by gluon–gluon fusion, gg! cc; bb. The doublelongitudinal spin asymmetry in this process thus providesdirect access to the gluon polarization in the proton andis largely independent of the quark helicity distributions[3]. Since cs 120 lm for D0 and cs � 460 lm for B0, excel-lent vertex resolution is needed to directly identify theseparticles. The planned upgrade for the STAR inner trackeris designed to achieve this in both the high multiplicity ofheavy ion collisions and in the high rate environment ofpolarized p + p collisions. A thin beryllium beam pipe witha radius of 2 cm will be used to give the detectors closeaccess to the collision point and to limit multiple scattering.

The inner tracker consists of three devices, all coveringjgj < 1.0. The Heavy Flavor Tracker HFT [4] is a light-weight two layer detector based on Active Pixel Sensors(APS) with 30 lm · 30 lm pixels consisting of siliconthinned down to 50 lm, limiting the material budget to0.3% X0 per layer. The inner sensor layer sits at a radiusof 2.5 cm, the staggered outer layer sits at 6.5 cm and7.5 cm radius. The radius of the layers has been increasedwith respect to the original proposal due to an increaseof the beam pipe radius in current designs. This device isdesigned provide a spatial resolution better than 10 lm atthe inner layer. A fast intermediate tracker is needed toact as a pointing device from the TPC to the HFT to con-nect the precision points in the HFT to TPC tracks and toprovide the time resolution necessary for high luminosity

running. Outside the HFT a single layer hybrid pixel detec-tor (HPD), using readout cells of 50 lm · 425 lm with theweaker resolution along the beam axis, is planned. Thisdetector sits at a radius of 9.1 cm. It is based on the ALICEsilicon pixel detector [5], using the same chips and mechan-ical structure, albeit with slightly longer ladders to providefor the larger radius and thus increased length along thebeam direction. The gap to the existing SSD at 23 cm willbe bridged by the intermediate silicon tracker IST, consist-ing of two layers of conventional back-to-back silicon stripsensors at 12 cm and at 17 cm radius. The material budgetfor this fast device is estimated to be 1.5% X0 per layer. Theintermediate tracker will replace the existing SVT, whichdoes not have sufficient rate capability for future collider

Fig. 3. Typical 55Fe X-ray spectrum taken with a triple GEM test detectorusing CERN GEM foils. The spectrum is fitted with the sum of twoGaussians and a linear background. The energy resolution (FWHM of thephoto peak divided by the mean) is �20%.

Fig. 4. 55Fe spectrum recorded with a triple GEM test detector usingTechEtch GEM foils. The energy resolution is �23%, comparable to thatobserved with a detector based on GEMs manufactured at CERN.

F. Simon / Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 1063–1066 1065

luminosities and is incompatible with future high-rateupgrades of the STAR data acquisition system. The preciselayout, the number of layers and the technology choices forthe intermediate tracker are currently being studied in sim-ulations aimed at an optimized design for the inner tracker.Fig. 2 shows a cross section view of the current design ofthe inner tracking system for STAR.

3. Forward tracker

From polarized deep inelastic scattering experiments itis known that the flavor-integrated contribution of quarksto the proton spin is surprisingly small. A flavor separatedstudy of quark and anti-quark polarizations can help eluci-date that question by providing a measurement of thepolarization of the quark sea [3]. At RHIC flavor separatedmeasurements will be carried out via the maximally parityviolating production of W bosons in u �d!Wþ andd �u!W� reactions. These reactions are ideal to accessthe quark polarizations since the W boson couples onlyto left-handed quarks and right-handed anti-quarks. ForW production away from mid-rapidity, the quark is mostlikely a valence quark from the proton traveling in thesame direction as the produced W, while the anti-quarkcomes from the sea of the other proton. That way the spinstate of the proton is cleanly linked to the partons involvedin the reaction. At STAR, produced Ws will be detected viatheir leptonic decays into an electron and a neutrino,W+! e+ me and W� ! e� me. The energy of the forwardgoing lepton will be measured in the EEMC, providing aclean signature for a W decay. It is crucial to distinguishbetween W+ and W� since this carries the informationon the flavor of the colliding quarks. This is achieved byidentifying the charge sign of the high momentum leptonfrom the W decay. This needs high resolution tracking inthe acceptance of the EEMC from 1 to 2 in g, a region cur-rently not covered by trackers in STAR. The necessary res-olution will be provided by two detector systems. TheForward Silicon Tracker FST is based on the same technol-ogy as the IST. It will consist of up to four silicon disksusing conventional back-to-back silicon strip detectorsclose to the interaction point. The Forward GEM TrackerFGT will provide additional space points with a largerlever arm. Two geometries are currently being evaluatedfor this device, namely a two layer barrel (each layer pro-viding a space point) or multiple discs that provide at leasttwo points on a track for 1 < g < 2. The FGT will be basedon GEM technology [6], using a triple GEM configurationsimilar to the one successfully applied by the COMPASSexperiment [7]. The front-end electronics for this detectorwill be based on the APV25-S1 chip [8], which will alsobe used for the FST and the IST, significantly reducingdevelopment costs for the readout and data acquisition sys-tem of the inner and forward tracking system. For thedevelopment and construction of such a large area GEMtracker the commercial availability of GEM foils is neces-sary. A collaboration with TechEtch Inc. of Plymouth,

MA, USA has been established to develop the productionprocess for these foils. Fig. 3 shows typical 55Fe X-ray spec-tra (main line at 5.9 keV) recorded with triple GEM testdetectors using CERN made foils, while fig. 4 shows theresults obtained with a detector using TechEtch made foils.The test detectors are read out via standard preamplifierand amplifier setups, collecting the full charge in a singlechannel. The energy resolution and signal quality is compa-rable for CERN and TechEtch made foils and similar tothe resolution observed with the COMPASS GEM detec-tors in similar tests [7]. Some issues with gain stability overtime still exist with the detector using TechEtch foils. Theinfluence of different production steps on the operationalcharacteristics of the foils is currently being studiedwith the goal of determining an optimized productionprocess.

1066 F. Simon / Nucl. Instr. and Meth. in Phys. Res. B 261 (2007) 1063–1066

4. Summary

The STAR collaboration is preparing a challengingtracking upgrade program to further investigate the prop-erties of the new state of strongly interacting matter pro-duced in relativistic heavy ion collisions and to providefundamental studies of the nucleon spin structure in high-energy polarized proton–proton collisions. The design ofthe integrated tracker is driven by the requirements fordirect reconstruction of charm and bottom decays andthe need to determine the charge sign of electrons producedin W decays. The mid-rapidity inner tracker includes high-resolution active silicon pixel sensors, hybrid pixels andstandard single sided silicon strip detectors. The forward

tracker is based on discs of silicon strip detectors and largearea triple-GEM trackers.

References

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50 (2000) 525.[4] Z. Xu et al., LBNL-PUB-5509 (2006).[5] P. Riedler et al., Nucl. Instr. and Meth. A 549 (2005) 65.[6] F. Sauli, Nucl. Instr. and Meth. A 386 (1997) 531.[7] M.C. Altunbas et al., Nucl. Instr. and Meth. A 490 (2002) 177.[8] M.J. French et al., Nucl. Instr. and Meth. A 466 (2001) 359.