The STAR Detector at RHIC

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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
PXL IST SSD

• The PXL is new technology
• 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
• 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)

The PXL 2 layers of Si at mid rapidity
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Summary of Si Detectors 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 mm
• IST pointing at the HFT is 250 mm
• HFT pointing at the VTX is 50 mm

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

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Scaling as a Function of (mT m0)
STAR Preliminary work by Yan Lu

• The light quark sector scales beautifully with
  v2/nq .vs. (mT m0)/nq
• Note that pT
• The strange quark sector also scales with
  and the scaling holds at all centralities
• Even the ? meson
• See S. Blythe QM2006

Does it work in the Charm Sector? A strong test
of the theory
Yuting Bai, QM 2006 for the STAR Collaboration
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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.
H. Liu et al., Phys. Lett. B 639 (2006) 441.
We need direct topological identification of Charm
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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 ?c will be a fascinating test and we might
  be able to do it with the HFT via ?c / D

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The Properties of the Open Charm Hadrons
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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 STARs 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 AuAu 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 AuAu minimum-bias events.

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Rates Estimation - v2

• dN/dpT distributions for D-mesons.
• Scaled by 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.

500 M Events is one RHIC Month with DAQ1000
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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
  
• g ? ee-
• and reduce other large stat. and systematic
  uncertainties
• STAR can make this measurement with 100M AuAu
  events in the HFT
• Smoking gun for thermalization at RHIC!

Better if we can do direct topological
identification of Charm
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PHENIX PRL 98, 172301 (2007)

• RAA of heavy-flavor electrons in 010 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

?0
?0
However, Hydro doesnt know about Quark
Coalescence
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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 antide 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 dont 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