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The STAR Heavy Flavor TrackerAn Introduction and Brief Review of the Physics Goals

Jim Thomas

Lawrence Berkeley National Laboratory

March 15th, 2006

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The STAR Detector at RHIC

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Direct Topological Identification of Open Charm

The STAR HFT 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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Surround the vertex with Si

A thin detector using 50 m Si to finesse the limitations imposed by MCS

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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

• Desirable to have it in time for the next long Au-Au run

• Proposal moving forward

The Heavy Flavor Tracker

The HFT: 2 layers of Si at mid rapidity

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The Light Quark Program at RHIC is Compelling …

Lattice results

Its hot

Its dense

and it flowsat the partonic scale

and , too!

Spectra

Vn

Jets & Rcp

Now we can make these measurements in the charm sector

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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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Elliptic Flow with Charm

M. Kaneta (PHENIX), J. Phys. G: Nucl. Part. Phys. 30, S1217 (2004).

• D e +X

Single electron spectra from PHENIX show hints of elliptic flow

• The HFT will cut out large photonic backgrounds: e+e-

and reduce other large statistical and systematic uncertainties

• STAR can make this measurement with 50 M Au+Au events in the HFT

• Smoking gun for thermalization at RHIC!

Better if we can do direct topological identification of Charm

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Single Electron Spectra … are not sufficient

• Hydro and Pythia are extreme models on opposite ends of the model spectrum

– Charm in red, Beauty in Blue … Hydro is the solid line, Pythia is dashed

• Single electron spectra are not sufficient to distinguish hard and soft physics below 3 GeV

– We will also see this in the RAA measurements

• The decayed spectra are shown in black and are nearly indistinguishable

• We heard this message many times at QM2005S. Batsouli et al., Phys. Lett. B 557 (2003) 26.

We need direct topological identification of Charm

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RAA and RCP for D0s… Critical measurements

• Two models for RAA of D mesons

– scaled light quark data

– hydro

• These measurements cannot be done with single electron spectra

– the decayed curves are indistinguishable

We need direct topological identification of Charm

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Heavy Flavor Energy Loss … RAA for Charm

• Heavy Flavor energy loss is uncertain

– Gluon densities up to 3500 are insufficient to describe the data

– ~ 1000 from light quark data

• Beauty dominates single electron spectra above 5 GeV and makes the model worse

M. Djordjevic, et. al. nucl-th/0507019

Current energy loss mechanisms can only account for part of the strong suppression of RAA for electrons

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Baryons vs. mesons

• Coalescence and fragmentation conspire at intermediate pT to give constituent quark number scaling and Baryon-Meson differences.

• Coalescence and fragmentation of charm quarks is different than for light quarks … so it is a strong test of the theory

• Coalescence of light quarks implies deconfinement and thermalization prior to hadronization

• How do baryons and mesons behave in the Charm sector?

• The Λc will be a fascinating test … and we might be able to do it with the HFT via Λc / D

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Coalescence of Charm Quarks?

• Charm fragments into a variety of hadrons

• If Charm quarks equilibrate with the surrounding medium then they might coalesce with the light quarks

– which would imply they travel a large distance in the thermalized medium

• Coalescence Increases the yield of Λc and J/ by 80% and a factor of 10, respectively

• Systematic errors cancel in Ds

+/D+ due to similar decay channels

e-p and e+-e average Pythia

Statistical coalescence

(c D + ) 0.232 0.162 0.21

(c D 0 ) 0.549 0.639 0.483

(c Ds+ ) 0.101 0.125 0.182

(c c+ ) 0.076 0.066 0.080

(c J/ ) 0.006 0.057

Table 1: Charm quark fragmentation functions. The D+ and D0 yields include feed-down from D*+ and D*0 decays.

Andronic et al.,. Phys. Lett. B571, 36 (2003).

Ds+/ D0, Ds

+/ D+ and J/ / D0 are sensitive probes of thermal charm production & history

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Backgrounds can be reduced with the HFT

• Direct topological reconstruction of charm avoids the single electron background problem

• But the HFT can also reduce the conversion electron backgrounds by judicious cuts in the TPC & HFT

• The HFT enables better single electron measurements

Figure: Electron pT spectra from conversions

reconstructed by requiring TPC tracking or TPC hits and 2 HFT hits. The rejection factor is about 16:1

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Working with the rest of STAR … ( )n

Combining the power of the STAR TOF barrel to identify electrons withthe ability of the TPC+HFT to identify and eliminate conversion electrons means we can execute a vigorous single electron and di-electron program of measurements

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Measuring Vector Mesons with Dileptons

• Dileptons are a valuable probe of the early stages in a HI collision

• However, the signals are relatively rare and the backgrounds are high (Signal in Black)

• Red Curve shows the sum or all backgrounds into the 2004 detector (including the SVT)

• Grey curve is e+e- spectrum after rejection by the HFT

• Blue dashed curve is Dalitz decays after rejection by the TPC

Pulling the Signal out of the Background

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An example: The and

The large reduction in photonic background will enable us to observed short lived vector meson decays

– The conversion background is reduced by requiring a hit in the HFT

– Charm semi-leptonic decay background filtered by DCA

– Reject and 0 Dalitz decays by measuring both electrons, of a pair, in the TPC

Detectors Comments

TPC+TOF+SSD+HFT 22 K 6 K Pairs per RHIC year

Table 3: The number of vector meson pairs which can be recorded by STAR in one RHIC year. We assume 200 M central Au + Au events will be recorded per RHIC year when DAQ 1000 is operational.

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A Rich Physics Program

• There is a rich physics program when all of the STAR physics detectors are working together

– Flow in the Charm sector

– dE/dx in the Charm sector

– Recombination and RAA in the Charm sector

– Vector Mesons

– Charm Angular Correlations

– non-photonic electrons

– …

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Technological Realization of the Program

• Technologies to explore– The Silicon Chips

– Further refinement of on-chip electronics

– Readout Electronics

– speed, heat dissipation, compatibility with STAR DAQ

– The Mechanical Arms to insert the detector

– Alignment and stability

– Calibration, Tracking & Software

– New levels of precision

– The beam pipe

– Smaller than ever before … Operation and robustness

• The R&D profile allows us to complete the development of the MimoSTAR chips and to readout data with a 4 msec frame rate

– Mount them in STAR – explore the background environment

– Use the real beam pipe – detectors at 1.5 cm from the vertex

– Use the real mechanical insertion device – explore calibration and alignment

• The Construction Profile allows us to complete the development of the Mimosa-8 style chips and readout with 200 sec frame rate

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Upcoming HFT Talks

• Introduction to the HFT and its Physics Program– Jim Thomas

• Simulations for the HFT – Andrew Rose

• HFT Technology and Mechanical Design– Howard Wieman

• HFT Readout and Ladder Tests– Leo Greiner

• CMOS Detectors for Particle Detection– Marc Winter

• HFT Cost and Schedule– Jim Thomas

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The Role of the Intermediate Si Tracker

• The IST is a highly segmented tracking detector that will lie between the HFT and the SSD

– High rate detector for the Heavy Flavor and Spin program in pp

– High accuracy pointing at the HFT for track finding

• The IST completes the physics program

– The RAA measurements need reference spectra in p-p or d-Au

– The Flow measurements are good to 60% peripherality with the SSD alone, but going from 60% to 90% is possible with the IST

– The p-p and SPIN program depends on the IST for its high rate capabilities

– All of the low multiplicity physics measurements use the IST

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Summary

• The HFT & IST will explore the Charm sector at RHIC

• We will do direct topological reconstruction of 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

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Inner & Forward Tracking Upgrades

• The IST will add high quality space points to tracks in the TPC

– Si at = 0

• The FST and FGT add high quality space points at forward η

– 1 < η < 2

• High Rate tracking for heavy flavor physics and W production

• The HFT, IST and forward tracking upgrades are complementary

– The goal is for these detectors to be fully compatible at all stages of engineering design, and data taking.3 layers of Si + 2 layers of GEM at forward rapidity

3 layers of Si at mid rapidity

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Si Pixel Developments in Strasbourg

• Mimosa – 1– 4k array of 20 m pixels

with thick epi layer

• Mimosa – 4– Introduce Forward

Biased Diode

• Mimosa – 5 – 1M array of pixels, 17

m pixels using AMS 0.6 process

– 4 msec readout scan rate

• Mimosa – 8– Fast parallel column

readout with internal data sparsification

– 200 sec readout scan rate

– MimoSTAR – 1 128x128 pixels using TSMC 0.25

– MimoSTAR – 2 128x128 pixels using AMS 0.35– Duct tape these to the STAR Beam Pipe for 07 run

– MimoSTAR – 3 320x640 pixels using AMS 0.35

– MimoSTAR – 4 640x640 pixels production run

– Ultra – 1

– Ultra – 2

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The HFT is Unique at RHIC

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

• The HFT 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

• Both collaborations have similar physics goals– PHENIX does single electron spectra very well

– We will do this as well as the direct topological reconstruction of Charm!

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BNL Mid-Term Plan

• The BNL Mid-Term Plan creates a new opportunityThe BNL FWP and LBL FWP include the following schedule

HFT Funding Profile

06 07 08 09 10

300K 1M 800K+300K 2.5M 2.5M

R&D R&D R&D+Const Const Const

IST+FST+FGT Funding Profile

06 07 08 09 10 11

200K 200K 500K 1.5M 4.0M 3.0M

R&D R&D R&D Const Const Const

• The financially driven schedule makes Mimosa-8 technology available in time to complete the project

– We propose to extend the scope of the project

– Do extensive R&D and risk analysis with MimoSTAR-4 chips

– The final detector will be based on MimoSTAR-XXX chips

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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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Charm Yields

• Open charm yields have been measured at RHIC by STAR and PHENIX

• Single electron spectra and direct toplogical reconstruction of open charm

– D0 K± + ±

– D e ± + X

• Yields appear to exceed Pythia and NLO pQCD expectations

The blue dashed line depicts a PYTHIA calculation. The red dot-dashed line depicts a NLO pQCD calculation with mc = 1.2 GeV/c2, µF = 2mc, µR = 2mc

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High pT Suppression for Charm and Beauty

Figure 8: The ratio of suppression factors in hot matter for charm (H) and light (L) quarks. The solid line represents results from calculations with unrestricted gluon radiation, while the dashed line is based on calculations with a cut on gluon energies > 0.5 GeV. The size of the static medium traversed by the fast quark is assumed to be 5 fm. From Dokshitzer and Kharzeev.

• A major contributor to light and heavy quark energy loss is Gluon Bremsstrahlung

• The radiation is determined by multiple scattering in the medium and is suppressed when the energy of the radiation is so high that the gluon formation time exceeds the finite size of the medium

• The radiation comes out in a cone and is suppressed when

< M/E

“The dead cone effect”

Heavy Quark energy loss should be lower than for light quarks.

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Charm Angular Correlations

• Correlations between charmed hadrons are a way to separate charm and beauty physics at RHIC

• Heavy quark production requires a large momentum transfer

– back to back topology for quarks

• The amount of broadening of this correlation is a measure of energy loss in the medium

Figure 10: D-meson correlation functions for 200 GeV p+p collisions. Default parameters in the Pythia model were used in these calculations. A clear back-to-back correlation in the angular distribution of charmed mesons is observed (shown by the open circles). The Solid-line and the diamonds represent the results with angular smearing for = /4 and /2, respectively.

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Direct Topological Identification of Open Charm

The STAR HFT will identify the daughters in the decay and do a direct topological reconstruction of the open charm hadron.

No mixed events, no random background subtraction.