The STAR Heavy Flavor Tracker
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The STAR Heavy Flavor Tracker Jim Thomas Lawrence Berkeley Laboratory 11 / 07 / 2006. “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

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The STAR Heavy Flavor Tracker

Jim Thomas

Lawrence Berkeley Laboratory

11 / 07 / 2006

Heavy flavor is the final frontier
“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

Flow probing thermalization of the medium

Semiperipheral collisions


Coordinate space:

initial asymmetry

Momentum space:

final asymmetry



Signals early equilibration (teq 0.6 fm/c)

Flow: Probing Thermalization of the Medium


Flow constituent quark number scaling
Flow: Constituent Quark Number Scaling

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

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

Where does charm come from
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)

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

  • pre-thermal: scattering between free streaming partons

  • thermal: assumes parton equilibration

  • Assume 3.5 GeV/fm3 at instant of equilibration

How many c c bar pairs per collision
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

Direct topological identification of open charm
Direct Topological Identification of Open Charm

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

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.

The HFT: 2 layers of Si at mid rapidity

The Heavy Flavor Tracker

  • A new detector

    • 30 mm silicon pixels

      to yield 10 mm space point resolution

  • Direct Topological reconstruction of Charm

    • Detect charm decays with small ct, 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

R&D is Driven by the Fabrication Schedule

Driven by the availability of CMOS Active Pixel Sensors

Build a full detector with each

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 )

Inside the IFC

~ 1 m

  • 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 mm

  • IST pointing at the HPD is ~ 150 mm

  • HPD pointing at the HFT is ~ 100 mm

  • HFT pointing at the VTX is ~ 50 mm

Andrew Rose, Sevil Salur, et al.

Keep the ssd it is a beautiful detector
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

The star inner tracking upgrade is unique at rhic
The STAR Inner Tracking Upgrade is Unique at RHIC

  • The Inner Tracking Upgrade will cover 2p in f azimuth

    • PHENIX Si covers 2p in f but the rest of the detector is 2 arms of p/2

  • The Inner Tracking Upgrade will cover ± 1 unit of h

    • PHENIX Si covers ± 1 unit but the rest of the detector covers 1/3 unit

  • The HFT uses 30x30 mm pixels for high resolution tracking

    • PHENIX uses 50x425 mm pixels (… strips …)

  • The HFT uses 50 mm thick Si in each of 2 layers

    • PHENIX uses 350 mm thick Si (sensor plus readout) in 2 layers and 1250 mm 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 mm pointing resolution

    • PHENIX will have 50 mm 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!


  • 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