Spin Physics with the PHENIX Silicon Vertex Tracker Junji Tojo RIKEN for the PHENIX Collaboration Advanced Studies Institutes - Symmetries and Spin July 27 - August 3, 2005, Prague

Silicon Vertex Tracker (VTX) Upgrade for PHENIX The PHENIX baseline detector was completed since RHIC-Run3 (2003). The detector upgrade program has been developed to enhance the physics capabilities. → M. Grosse Perdekamp’s talk The VTX upgrade in the PHENIX Central Arm Spectrometers has been proposed for : –Precise measurement of heavy-quark (charm & bottom) production w/ displaced decay vertices, –Large acceptance tracking especially for jet production. The VTX has to be operated is in high luminosity and high multiplicity environment for polarized pp, p(d)A, and AA collisions in the future RHIC running.

Spin Physics with the VTX Detector Gluon polarization meas. w/ longitudinal double- spin asymmetry A LL for –heavy-quark production –photon-jet production Precise determination by extending x-range in different channels beyond the baseline

Gluon Polarization with Heavy-Quark Production Dominated by gluon-gluon fusion : Clean process with wide x g -range Limitation in the baseline detector –Indirect meas. w/ inclusive electrons –Huge background from π 0 Dalitz decay and photon conversion Precise vertexing w/ VTX allows direct meas. by detecting displaced decay vertices thr. distance of closest approach (DCA). Parton level asymmetry from pQCD calculation g g m (GeV) c  μm  D D ± B B ± D, B e Primary event vertex Displaced vertex DCA

Gluon Polarization with Photon-Jet Production Dominated by gluon Compton process : Golden channel for precise measurement Limitation in the baseline detector –Recoil jet reconstruction is desirable for better parton kinematics constraint, but difficult due to limited acceptance. Large acceptance tracking w/ VTX makes it possible to determine recoil jet axis and thus improve x g determination. Parton level asymmetry from pQCD calculation g γ q q Measured in pol DIS

Specifications –4 layers with large acceptance (  &  < 1.2) –Displaced vertex measurement :  < 40  m –Charged particle tracking :  p /p ~ 5% p at high p T –Working detector for both of heavy ion and pp collisions Technology Choice –Hybrid pixel detectors in 2 inner layers –Stripixel sensors w/ SVX4 readout chip in 2 outer layers The VTX Detector Pixel layers r=5.0 cm, Δz~±10 cm r=2.5 cm, Δz~±10 cm Strip layers r=10.0 cm, Δz~±16 cm r=14.0 cm, Δz~±19 cm Beam pipe

The VTX Detector in PHENIX VTX |η| < 1.2

Hybrid Silicon Pixel Detector Solder bump ~ 15 μ m Hybrid pixel detector –Technology developed by CERN/ALICE –Bump-bonding btw R/O chip and sensor R/O chip : ALICE1/LHCB chip –0.25 μ m process, rad hard ~ 30Mrad –Matrix of 32 x 256=8,192 cells –Pixel size 50 x 425  m 2 –Active area 12.8 x 13.6 mm 2 –150μm thickness –Operation at 10 MHz –Power consumption ~ 1 W/chip Pixel sensor –p+/n/n+ structure –Same pixel size as R/O chip –Bump-bonded to 4 R/O chip –200μm thickness Sensor R/O Chip Pixel sensor

Hybrid Silicon Pixel Detector Half-module –2 bump-bonded ladder –High density Al/Kapton bus 70μm pitch & 5 layers R&D on-going –1 pilot module : R/O & control 2 half-modules makes 1 ladder Bus 2 bump-bonded ladder Bus Support+cooling Production of bump-bonded ladder and ladder test w/ 90 Sr are on-going

Silicon Strip Detector Strip sensor –BNL’s new “stripixel” concept : single-sided sensor w/ 2-D position sensitivity –Charge sharing by 2 spirals in one pixel (80 μm ×1000 μm) & projective x/u-strip readout –Pre-production sensor (Hamamatsu) p+/n/n+ structure 3.5×6.4 cm 2 625/500 μ m thickness Spiral : 5/3μm line/gap Pixels : 384×30×2=23,040 Strips : 384×2×2=1,536 –Probing tests are on-going for evaluation Z. Li, NIMA518, 738 (2004) x3’ u1’u2’ x2’ x1’u1’ u2 u3x3 u1 x1 x2 CV IV

Readout chip –FNAL/LBNL’s SVX4 chip 0.25 μ TSMC process, radiation hard > 20 Mrad 128ch pipelined mixed signal chip 128ch parallel 8-bit ADC “Dead timeless operation” Multi-event buffering (4 events) Selectable on-chip 0-suppression Power consumption ~ 0.4W/chip –12 SVX4s and control chip (RCC) per sensor R&D is on-going –SVX4 3-chip hybrids –Readout control board for 4 hybrids –Sensor+hybrid test will start soon. Silicon Strip Detector Readout Control BoardHybrid Sensor

Detector Integration in the VTX Region ± 40 cm ± 34 cm 42 cm Magnet Calorimeter Magnet Calorimeter VTX Endcap extension VTX Endcap extension

Expected Performance :Occupancy Layer-by-layer occupancy calculation –Maximum occupancy estimated from simulated central Au-Au collisions at 200 GeV –Au-Au collisions generated with HIJING event generator –The VTX Detector implemented in a GEANT-based PHENIX detector simulation (PISA) The VTX Detector will work for both heavy ion and polarized pp collisions.

Expected Performance : Heavy-Quark Detection w/ Displaced Vertices Displaced vertex meas. using the distance of closest approach (DCA) DCA resolution of 36μm simulated w/ two inner-most pixel layers & 2 GeV/c pions : consistent w/ r φ pitch (50 μm) and detector geometry DCA (  m) 200μm DCA cut Electron DCA distribution from - π 0 Dalitz decay - Charm semi-leptonic decay - Bottom semi-leptonic decay Charm S/N improvement w/ VTX Bottom S/N improvement w/ VTX Significant S/N improvement

Expected Performance : Jet axis Determination w/ Large Acc. Tracking Reconstruction of recoil jet axis (η jet, φ jet ) in direct photon event Simple cone algorism : p T -weighted ave. of charged tracks’ (ηi, φi) w/ > 1 GeV/c within VTX & corn radius < 0.5 Photon p T = GeV/c η-distribution of - Recoil quark - Recoil quark within VTX -η(recons. jet)-η(quark) Strong constraint of jet axis No jet info. w/ jet axis reconst. Improved parton kinematics determination

Summary Silicon vertex tracker (VTX) upgrade was proposed to enhance the physics capability of the PHENIX detector. Intensive R&Ds and production are on going. Expected performance showed significant improvement especially for spin physics capabilities w/ heavy-quark & photon+jet production. Installation and operation of the VTX is planned in RHIC-Run8 (2007/2008).