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The STAR Detector at RHIC

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The STAR Heavy Flavor Tracker

An Introduction and Brief Review of the Technical Design and Physics Goals at Mid-rapidity

Jim Thomas

Lawrence Berkeley National Laboratory

November 1st, 2007

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The Heavy Flavor Tracker

Mid-rapidity Pointing Devices: IST + SSDThe PXL: 2 layers of Si at mid rapidity

• The PXL is new technology– 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

– C & B Energy Loss to test pQCD in a hot and dense medium at RHIC

• The proposed Tracking Upgrades include

– PXL (2 layers)

– IST (1 or 2 layers)

– SSD (existing layer)

= PXL + IST + SSD

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~ 1 m

Inside the TPC– Goal: graded resolution

to achieve highly efficient tracking from the outside in

– TPC – SSD – IST – PXL

– TPC pointing resolution at the SSD is ~ 1 mm

– SSD pointing at the IST is ~ 300 m

– IST pointing at the HFT is ~ 250 m

– HFT pointing at the VTX is ~ 50 m

Summary of Si Detectors Inside the TPC

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The Heart of the Beast

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

See the talk by Howard Wieman for the latest developments (and photos)

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Direct Topological Identification of Open Charm

The STAR Heavy Flavor Tracker will identify daughters of the decay and do direct topological

reconstruction of 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

50-150 m

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“Heavy Flavor” is the Next 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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-mesons Flow: Partonic Flow

-mesons are very special:- they do not re-interact in hadronic environment- they show strong collective flow- they are formed via coalescence with thermal s-quarks

STAR Preliminary: QM06, S. Blyth Hwa and Yang, nucl-th/0602024; Chen et al., PRC73 (2006) 044903

STAR Preliminary

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Scaling as a Function of (mT – m0)

• The light quark sector scales beautifully with v2/nq .vs. (mT – m0)/nq

– Note that pT < 1 GeV always did scale !

• The strange quark sector also scales with <v2> and the scaling holds at all centralities

• Even the meson– See S. Blythe QM2006

V2 /

nq

Does it work in the Charm Sector?

A strong test of the theoryYuting Bai, QM 2006 for the STAR Collaboration

STAR Preliminary work by Yan Lu

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• pT distributions of electrons from semi-leptonic decay of heavy flavor mesons (left D-mesons, right B-mesons) as a function of parent pT. The inserted plots represent the projections to the corresponding heavy flavor distributions. The widths of the electron pT windows are indicated by dashed boxes.

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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 make it difficult to distinguish hard and soft physics below 3 GeV

– Also true for RAA measurements

• The decayed spectra are shown in black. The sensitivity to the original spectra are reduced.

• We heard this message many times at QM

S. Batsouli et al., Phys. Lett. B 557 (2003) 26.

We need direct topological identification of Charm

H. Liu et al., Phys. Lett. B 639 (2006) 441.

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Heavy Flavor Energy Loss … RAA for Charm

• Heavy Flavor energy loss is an unsolved problem

– Gluon density ~ 1000 expected from light quark data

– Better agreement with the addition of inelastic E loss

– Good agreement only if they ignore Beauty …

• Beauty dominates single electron spectra above 5 GeV

• We can separate the Charm and Beauty by the direct topological identification of Charm

Theory from Wicks et al. nucl-th/0512076v2

Where is the contribution from Beauty?

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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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The Properties of the Open Charm Hadrons

Particle Decay Channel c (m) Mass (GeV/c2)

D0 K + (3.8%) 123 1.8645

D+ K + + (9.5%) 312 1.8694

K+ K + (5.2%)+ + - (1.2%)

150 1.9683

p K + (5.0%) 59.9 2.2865

+

SD

+

C

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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 and Beauty sector

– Recombination and RAA in the Charm and Beauty sector

– Vector Mesons

– Charm Angular Correlations

– now accessible due to STAR’s large acceptance

– non-photonic electrons

– …

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Simulation results

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Finding the D0 … (whew, it works)

• The open circles show the primary vertex resolution in central Au+Au collisions. (absolute value of 3D distance)

• The solid circles show the D0 secondary decay-vertex resolution. The mean decay distance, c = 123 m for the D0, is shown to guide the eye.

• Each D0 decay length was scaled by the appropriate factor to provide a universal peak for the purposes of the illustration.

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D0 Reconstruction Efficiency

• The yield of reconstructed D0s divided by the simulated D0 yield

• The red squares show the maximum possible efficiency for reconstructing a D0 based upon the single track efficiencies for the daughter particles.

• The black circles show the efficiency for actually finding these D0s after applying the topological cuts.

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Significance of the D0 measurement

• The expected significance of the D0 measurement with 100 M central events as a function of pT. For pT above 5 GeV/c, the upper limit on the signal significance is shown because we do not have sufficient statistics to properly estimate the background.

• UPC electrons and pileup of minimum-bias events during the integration time of the PIXEL detectors are not included in this simulation

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c Reconstruction with the HFT

c reconstruction is difficult due to 3 body final state and the short decay length, however

• a c spectrum can be measured in 50 M central Au-Au events

• an RAA measurement with 20% error bars can be made out to 4 GeV with 500 M events

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Charm Meson Flow

• The green line describes the expected magnitude of the v2

parameter if only the light quarks in the D0 exhibit flow.

• The red line illustrates how the flow parameter can increase if the charmed quarks also flow.

• The blue error bars shown at the top of the figure show the anticipated errors of measurement in 100 M Au+Au minimum-bias events.

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Rates Estimation - v2

(a) dN/dpT distributions for D-

mesons.

Scaled by <Nbin > = 290, corresponds to the minimum bias Au + Au collisions at RHIC.

(b) Assumed v2 distributions for D-

mesons.

---- PLB 595, 202 (2004)

Error bars shown are from 15% systematic errors

(c) D0 meson v2 rates from minimum bias Au + Au collisions at 200 GeV.

The small and large error bars are for 15% and 30% systematic errors, respectively. For the v2 analysis, 12 bins in are used.

pT(GeV/c) ∆pT(GeV/c) # eventsqC = qu,d,s

# eventsqC = 0

0.6 0.2 260x106 525x106

1.0 0.5 70x106 140x106

2.0 0.5 53x106 125x106

3.0 1.0 390x106 270x106

5.0 1.0 520x106 880x106

500 M Events is one RHIC Month with DAQ1000

0 1 2 3 4 5

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Hints of Elliptic Flow with Charm

• D e +X

Single electron spectra from PHENIX show hints of elliptic flow

Is it charm or beauty?

• The HFT will cut out large photonic backgrounds:

e+e-

and reduce other large stat. and systematic uncertainties

• STAR can make this measurement with 100M Au+Au events in the HFT

• Smoking gun for thermalization at RHIC!

Better if we can do direct topological identification of Charm

Shingo Sakai, QM 2006 PRL 98, 172301 (2007)

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PHENIX PRL 98, 172301 (2007)

• RAA of heavy-flavor electrons in 0%–10% central collisions compared with 0 data and model calculations

• V2 of heavy-flavor electrons in minimum bias collisions compared with 0 data and the same models.

• Conclusion is that heavy flavor flow corresponds to /s at the conjectured QM lower bound

However, Hydro doesn’t know about Quark Coalescence

0 0

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Charm Meson Flow – coalescence view

• The green line describes the expected magnitude of the v2

parameter if only the light quarks in the D0 exhibit flow.

• The red line illustrates how the flow parameter can increase if the charmed quarks also flow.

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Fridays PRL 99, 172301 (2007) … new insights

• Romatschke2 perform relativistic viscous hydrodynamics calculations

• Data on the integrated elliptic flow coefficient v2 are consistent with a ratio of viscosity over entropy density up to /s 0.16

• But data on minimum bias v2 seem to favor a much smaller viscosity over entropy ratio, below the bound from the anti–de Sitter conformal field theory conjecture

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A possible interpretation

• Assume the PHENIX data are correct, then they may have seen charmed meson flow

• A couple of assumptions …

• The PHENIX data do not show charmed quarks flowing … rather they see the light quarks flowing ( ½ the full value for the flow of the light hadrons)

• This would be consistent with the /s ratio fit by Teaney et al.

• It would also be consistent with the even lower value of the /s ratio found by Romatschke & Romatschke for the light hadrons

• Conclusion: charmed quarks don’t flow and the conjectured QM lower bound has been violated

Future data sets and further discussion will be very interesting!

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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.

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Summary

• The STAR HFT will explore the Charm and Beauty sectors

• 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