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The STAR Heavy Flavor Tracker

Jim ThomasLawrence Berkeley Laboratory

11 / 07 / 2006

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“Heavy Flavor” is the Final Frontier

• The QGP is the universally accepted hypothesis at RHIC

• The next step in confirming this hypothesis is the proof of thermalization of the light quarks in RHIC collisions

• The key element in proving this assertion is to observe the flow of charm … because charm and beauty are unique in their mass structure

• If heavy quarks flow– frequent interactions among all quarks

– light quarks (u,d,s) likely to be thermalized

Current quark: a bare quark whose mass is due to electroweak symmetry breaking

Constituent quark: a bare quark that has been dressed by fluctuations in the QCD sea

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Flow: Probing Thermalization of the Medium

py

)(tan,2cos 12

x

y

p

pv

Coordinate space: initial asymmetry

Momentum space: final asymmetry

pxx

y

Semiperipheral collisions

Signals early equilibration (teq 0.6 fm/c)

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Flow: Constituent Quark Number Scaling

In the recombination regime, meson and baryon v2 can be obtained from the quark v2 :

2 2 2 2v22

v3

v3v Btt

q tM q tp ppp

Does it work in the Charm Sector? A strong test of the theory

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Where does Charm come from?

• Gluon Fusion and qq-bar annihilation dominate the production of charm at RHIC

– Initial state

• Thermal processes are important but not dominant

– Final state effects

– Instantaneously equilibrated QGP shown for reference

– In the real world, thermal distributions are less important due to the large mass of the c quark (not true in the strange quark sector)

– pre-thermal: scattering between free streaming partons

– thermal: assumes parton equilibration

– Assume 3.5 GeV/fm3 at instant of equilibration

Levai, Mueller, and Wang, PRC 51, 3326 (1995).

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How many c c-bar pairs per collision?

• Many ingredients are required to understand the formation of charmed hadrons at RHIC including the parton distribution functions for the projectile and target and the cross section for gluon fusion and qq-bar annihilation.

• The cross-sections can be calculated in NLO perturbative QCD

• The pdf’s come from e-p data

• Ramona Vogt updates these estimates every few years – R. Vogt, hep-ph/0203115, hep-ph/0203151

• The nucleon-nucleon cross sections are extrapolated to Au-Au by assuming ~1000 binary scatterings in a central collision

Theory: NN (c ) = 289 - 445 µb

Exp: NN (c ) = 900 - 1400 µb

20 - 30 c pairs per central Au+Au collision at √sNN = 200 GeV

Theory: NN (b) = 1.64 - 2.16 µb

Exp: NN (b) = ??

0.04 - 0.06 b pairs per central Au+Au collision at √sNN = 200 GeV

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Direct Topological Identification of Open Charm

The STAR Inner Tracking Upgrades will identify the daughters in the decay and do a direct topological

reconstruction of the open charm hadrons.

No Mixed events, no random background subtraction.

Goal: Put a high precision detector near the IP to extend the TPC tracks to small radius

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• A new detector– 30 m silicon pixels

to yield 10 m space point resolution

• Direct Topological reconstruction of Charm

– Detect charm decays with small c, including D0 K

• New physics– Charm collectivity and

flow to test thermalization at RHIC

– Charm Energy Loss to test pQCD in a hot and dense medium at RHIC

• R&D with HFT + SSD

• A proposal has been submitted and a TDR is in preparation

The Heavy Flavor Tracker

The HFT: 2 layers of Si at mid rapidity

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R&D is Driven by the Fabrication Schedule

Driven by the availability of CMOS Active Pixel Sensors

Fab-1999 Fab-2001 Fab-2003 Fab-2004 Fab-2005 Fab-2006 Fab-2007 Fab-2009

Mimosa-1 Mimosa-4 Mimosa-8 MimoSTAR-1 MimoSTAR-2 MimoSTAR-3 MimoSTAR-4 UltraSTAR

Build a full detector with each

10Jim Thomas - LBL Alexandre Shabetai & Xianming Sun

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Selected Parameters and Specifications

Min I efficiency 98%

Accidental rate < 100 /cm2

Position resolution < 10 m

Number of pixels 135,168,000

Pixel dimension 30 m 30 m

Detector chip active area 19.2 mm 19.2 mm

Detector chip pixel array 640 640

Number of ladders 33

Ladder active area 192 mm 19.2 mm

Number of barrels 2

Inner barrel (9 ladders) r = 2.5 cm

Outer barrel (24 ladders) r = 7.0 cm

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Surround the Vertex with Si

The HFT is a thin detector using 50 m Si to finesse the limitations imposed by MCS

Add the HPD, IST, and SSD to form the STAR Inner Tracking Upgrade ( ITUp )

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The Heavy Flavor Tracker

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~ 1 m

Inside the IFC– Goal: graded resolution from the outside – in

– TPC – IST – HPD – HFT

– TPC pointing resolution at the SSD is ~ 1 mm

– SSD pointing at the IST is ~ 300 m

– IST pointing at the HPD is ~ 150 m

– HPD pointing at the HFT is ~ 100 m

– HFT pointing at the VTX is ~ 50 m

Andrew Rose, Sevil Salur, et al.

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Keep the SSD, it is a beautiful detector!

• The SSD is thin– 1% - double sided Si

• The SSD lies at an ideal radius– 23 cm - midway between IP and IFC

• The SSD has excellent resolution – (rumor says better than design)

• The SSD is too large to be replaced– The money is better spent, elsewhere

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Hand Calculations of HFT + TPC Performance

Yan Lu & JT

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Hand Calculations

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The STAR Inner Tracking Upgrade is Unique at RHIC

• The Inner Tracking Upgrade will cover 2 in azimuth– PHENIX Si covers 2inbut the rest of the detector is 2 arms of /2

• The Inner Tracking Upgrade will cover ± 1 unit of – PHENIX Si covers ± 1 unit but the rest of the detector covers 1/3 unit

• The HFT uses 30x30 m pixels for high resolution tracking– PHENIX uses 50x425 m pixels (… strips …)

• The HFT uses 50 m thick Si in each of 2 layers– PHENIX uses 350 m thick Si (sensor plus readout) in 2 layers and 1250 m thick Si in 2 more

layers

• The HFT is 0.25% radiation lengths thick per ladder– PHENIX needs cooling … their first layer is 1.2% thick

• The HFT will have 10 m pointing resolution– PHENIX will have 50 m pointing resolution

• Our pT threshold for D0s will be ~700 MeV– PHENIX will have ~2 GeV ... we get 5 times the spectrum yield

• The large RHIC collaborations have similar physics goals– PHENIX does single electron spectra very well

– We will do this plus the direct topological reconstruction of open Charm!

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Summary

• The STAR Inner Tracking Upgrade will explore the Charm sector

• We will do direct-topological-reconstruction of open Charm

• Our measurements will be unique at RHIC

• The key measurements include– V2

– Energy Loss

– Charm Spectra, RAA & Rcp

– Vector mesons

– Angular Correlations

• The technology is available on an appropriate schedule