Quarkonium Measurements in AuAu Collisions at RHIC

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Quarkonium Measurements in AuAu Collisions at
RHIC
PhD Progress Report Nov. 12, 2008

• Graduate Student Matt Cervantes
• Advisor Prof. Saskia Mioduszewski

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Outline

• Relativistic Heavy Ion Collider (RHIC) and the
  Solenoidal Tracker at RHIC (STAR)
• Quark Gluon Plasma (QGP) and Quarkonium (QQ ?
  ee-)
• Electron identification methods available in STAR
• Barrel Electromagnetic Calorimeter (BEMC) and the
  Barrel Preshower (BPRS) in the STAR detector
• Electron identification study with BPRS detector
• Upsilon analysis with and without the BPRS
• Barrel Preshower Calibration
• Summary

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BRAHMS
PHOBOS
PHENIX
.
.
STAR

• RHIC ring collider of 3.834 km in circumference
• RHIC capable of energies up to 200 GeV/nucleon
  for Au ions

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RHIC and the Quark Gluon Plasma (QGP)

• In the Big Bang picture where does QGP fit in?
• QCD predicts hadronic to plasma phase transition
• Lattice QCD predicts a transition at T 170 MeV
  (1012K) corresponding to an energy density 1
  GeV/fm3
• RHIC collisions are expected to create energy
  densities 1-2 orders of magnitude larger

.
.
This is the universe at an age of 20-30 m
seconds after the Big Bang. Quarks and Gluons
are thought to be the only form of matter in
existence during this time.
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• Particles stream out from interaction region.
• STAR detector captures the remnants of the
  collision.
• What type of physics happens here?
• Is there a Quark Gluon Plasma created here?
• Quarkonium is a probe of QGP
• Medium effects on quarkonium
• - Suppression? 1 Matsui, T. and Satz, H.,
  Phys. Lett. B 178, 416 (1986).
• - Enhancement?

2 Braun-Munzinger, P. and Stachel, J., Phys.
Lett. B 490, 196 (2000)
Thews, R. L., Schroedter, M. and Rafelski, J.,
Phys. Rev. C 63, 054905 (2001) Grandchamp, L.
and Rapp, R., Nucl. Phys. A 709 415 (2002)

Andronic, A., Braun-Munzinger, P., Redlich, K.
and Stachel, J., Nucl. Phys. A 789 334 (2007).
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Heavy Quarkonia in a QGP

• Heavy Quarkonia are used to probe the QGP
• - J/y (charm anti-charm bound state)
• - U (bottom anti-bottom bound state)
• In a QGP the high energy density of the medium
  deconfines the J/Y and U bound states.
  Deconfinement could lead to suppression of
  measured J/y and U yields in data.
• At RHIC, J/Y may be regenerated (i.e.
  recombination of cc )
• At RHIC, the collision energy is NOT high enough
  to allow copious production of bottom anti-bottom
  quarks. U is not expected to undergo
  regeneration.
• Measurements of J/y and U at STAR will help us
  understand the properties of the QGP.
• Heavy quarkonia measurements via decay channel QQ
  ? e e-

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Physics motivation for electron ID

• Clean electron ID needed for electron-based
  physics analyses electrons do not interact with
  QGP
• Example J/Y ? e e- decay channel selected for
  an invariant mass reconstruction needs strong
  electron identification to reject hadronic
  background
• STAR has a large geometrical acceptance for data
  acquisition and subsequent event reconstruction
  
• -1 lt h lt 1 and DF
  2p

• Large acceptance increases ability to study
  heavier
• (larger opening angle) vector mesons such as
  the
• Y ? e e- decay channel (again, electrons used)

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Electron identification in STAR

• Time Projection Chamber (TPC)
• momentum charged track curvature
• dE/dx Ionization Energy loss of charged
  particles
• High momentum trackslead to convergence of
• dE/dx particle bands
• Project TPC tracks to
• BEMC and use the energy
• from BEMC to form ratio p/E
• Shower Maximum Detector (SMD)
• Energy, position and shower profile

BPRS
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Electromagnetic Calorimeter and Preshower in STAR

• Preliminary attempt at calibration of the BPRS
  channels gave us a cut to test
  effectiveness of BPRS to reject hadrons

• Layered Sampling Pb-scintillator (5-6 mm thick
  layer X0)
• Tower 21 X0 slightly
• less than 1 hadronic
• interaction length
• Interaction probability
• for hadrons (Pb only)
• is 3 before layer 1
• ( 6 before layer 2)
• BPRS 63 of electrons
• will shower before the
• scintillator layer 1
• (84 before layer 2)

The BPRS ADC spectrum with pedestal
location denoted by blue vertical line
15 Xo
20 Xo
Most hadron interactions with nuclear material
develop at depths beyond these first 2 layers!
Left panel Frequently the MIPs deposit low
amounts of energy in the BPRS. Bottom Less
often, electron candidates deposit greater energy
in the BPRS
BPRS
An ADC cut gt 65 is made for all 4800 BPRS
channels. The cut above 65 should keep us above
MIPs and position us well within the electron
rich region of the ADC spectrum.
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Electron Identification Study

• STAR data from RHIC collisions (Run 7 AuAu Data)
• Electron candidates satisfy Etower gt 2 GeV and pT
  gt 2 GeV/c
• Method of Electron identification for this study
• TPC dE/dx
• TPC tracks projected to BEMC p/E
• SMD shower profile
• BPRS preshower
• The electron rich region is used to study the
• BPRS effect on electron identification by
  requiring the cut ADCBPRS gt 65

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dE/dx plots from AuAu data

• The data provides the dE/dx vs. p distribution

• Projection onto the dE/dx with pT gt 2 cut
  applied

Tight cut used to remove hadronic contributions

• Standard STAR tight cut of 3.4 x 10-6 lt dE/dx lt
  5.0 x 10-6

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p/E plots from AuAu data

• The p/E distribution with pT gt 2 and tight
  electron cut

• The p/E distribution with pT gt 2 cut applied

3.4 x 10-6 lt dE/dx lt 5.0 x 10-6
Note p/E lt 2 cut will provide a small effect on
data (tight electron cut does most of the work
in removing hadrons)

• Standard STAR cut of p/E lt 2 GeV/c is chosen

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SMD electron ID (pp data)

• SMD distribution for hadrons and electrons in pp
  data

• The SMD distribution for hadrons has a more
  narrow profile than that of the distribution
  for the electrons

• Standard STAR electron cut in the number of
  strips hit is h gt 2 and f gt 2

• What is the effect of a SMD cut on dE/dx in pp
  data?

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Effect of the SMD cut on pp dE/dx

• We explore the effect of an SMD cut on our dE/dx
  distribution at pT gt2 by forming the ratio
• (dE/dx) SMD(on) / (dE/dx) SMD(off)
• i.e.

45 reduction in hadron region
30 rejection difference
15 reduction in (tight) electron region

• SMD effect in hadron vs. electron regions in dE/dx

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SMD in AuAu vs. pp events

• SMD distributions for the AuAu vs. pp data

The SMD distributions in the AuAu data for the
electron and hadron regions do not look to be as
discriminated
SMD appears to be less effective in
high multiplicity events!
SMD hadrons
SMD electrons
The SMD distributions in the pp data for the
electron and hadron regions does appear to be
much more discriminated

• SMD may not be a straight forward cut in AuAu data

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Effect of the SMD cut on AuAu dE/dx
I.e. Relative to the low multiplicity pp
collisions, the effectiveness of the SMD cut
appears to be dropping for high multiplicity AuAu
collisions in STAR. We should look at what the
BPRS does in the high multiplicity AuAu events.

• We explore the effect of an SMD cut on our dE/dx
  distribution at pT gt2 by forming the ratio
• (dE/dx) SMD(on) / (dE/dx) SMD(off)
• i.e.

50 reduction in hadron region
Discrimination w/ the SMD in AuAu might not help
us very much
15 rejection difference
35 reduction in (tight) electron region
region suffers an extra 5 hit relative to pp
data
region suffers an extra 20 hit relative to pp
data

• SMD effect in hadron vs. electron regions in dE/dx

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Effect of BPRS cut on AuAu dE/dx

• We explore the effect of a BPRS cut on our dE/dx
  distribution at pT gt2 by forming the ratio
• (dE/dx) BPRS(on) / (dE/dx) BPRS(off)
• i.e.

I.e. The BPRS cut relative to the SMD cut rejects
only an extra 5 of electron candidates in the
tight electron region, while rejecting an extra
20 of the hadron candidates in the hadron
region in the AuAu data.
70 reduction in hadron region
30 rejection difference
40 reduction in (tight) electron region

• So what is BPRS effect in hadron vs. electron
  region?

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Effect of BPRS on dE/dx with all cuts

• We explore the effect of a BPRS cut on our dE/dx
  distribution at pT gt2 by forming the ratio
• all cuts (dE/dx) BPRS(on) / all cuts
  (dE/dx) BPRS(off)
• i.e.

By no difference, we mean that the BPRS cut
relative to the (SMD p/E) cuts appears to be
the strongest cut of the three
SMD p/E cuts
70 reduction in hadron region (no difference)
40 reduction in (tight) electron region (n
o difference)

• Final comments on BPRS effect on dE/dx here

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Summary so far

• The RHIC experiment at BNL looks like a promising
  place for the production and study of a Quark
  Gluon Plasma (QGP)
• Currently STAR has very good methods in place for
  electron identification
• Additional Hadron discrimination using the Barrel
  Preshower (BPRS) is a real possibility
• Barrel Preshower (BPRS) appears to be a stronger
  hadronic rejection tool than the currently
  available cuts of Shower Maximum Detector and/or
  p/E lt 2
• Shower Maximum Detector performance in high
  multiplicity events (AuAu collisions) needs
  further study, but we take note of the strong
  indications of the possibility to improve the
  electron identification in STAR by using the
  Barrel Preshower detector (BPRS)
• With the BPRS proof of principle established, we
  then performed a Heavy Quarkonia analysis with
  electron rich data

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U ? ee- inv. mass reconstruction

• STAR data from RHIC collisions (Run 7 AuAu Data)
• Event sample 4.9 Million triggered upsilon
  events
• Candidate ee- pairs are sorted into signal and
  background
• Candidate ee- pairs form the signal while
  pairs of ee and e-e- make up our background.
• Method of Electron identification for inv. mass
  study
• TPC 3.4 x 10-6 lt dE/dx lt 5.0 x 10-6
• TPC tracks projected to BEMC p/E lt 2
• BPRS preshower (ADCBPRS gt 65)

Preshower OFF
Preshower ON
Mass GeV/c2
Mass GeV/c2
Due to the inefficiency of BPRS detector and the
harshness of the BPRS selection cut, we have lost
much of the upsilon mass signal as well as the
background distribution.
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U ? ee- inv. mass reconstruction II

• STAR data from RHIC collisions (Run 7 AuAu Data)
• Event sample 4.9 Million triggered upsilon
  events
• Candidate ee- pairs are sorted into signal and
  background
• Candidate signal ee- pairs are made within
  same events while background pairs are made by
  ee e-e- across events.
• Method of Electron identification for inv. mass
  study
• TPC 3.4 x 10-6 lt dE/dx lt 5.0 x 10-6
• TPC tracks projected to BEMC p/E lt 2
• SMD shower profile (SMD cut not used in this
  analysis)
• BPRS preshower (ADCBPRS gt 65)

Preshower OFF
Preshower ON
Consider a mixing across events method to
reconstruct our invariant mass signal. The
background distribution has a smoother shape
via increased statistics (reduced errors) but
jet correlations are thought to be inherent
with this method, and further investigations
would be required to move forward.
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Current Calibration Status of the Preshower

• Recall that in the proof of principle we used a
  harsh cut of ADCBPRS gt 65 to stay within the
  electron rich region.
• The BEMC has 4800 independent towers (each with a
  BPRS readout). Each uncalibrated BPRS detector
  will have a different response. In order to
  calibrate the BPRS detector , we need to tune
  each channel to have a uniform response .
• A rough calibration for the U invariant mass
  reconstruction used U triggered data.
  Triggered events are inherently biased, but it
  was the only data set available at the time.
• To do a proper job of calibrating the preshower
  we need to use Minimum Bias data.
• We will use full ADC spectra with no tracking
  requirement to fit slopes.

ADC
Ring number
After all slopes are calibrated we can correct
our ADC distribution as ADC ADC C , where C
is determined by slope of individual tower ADC
divided by the average ADC slope from a Ring. An
invariant mass analysis can be attempted with the
calibrated preshower. Next we will also apply a
BPRS selection cut to a high background signal
such as the J/y.
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Summary

• Current work Calibration of the BPRS.
• Future work Continue invariant mass analysis of
  Upsilon (U) in AuAu as well as a J/Y analysis in
  dAu and AuAu.
• Perhaps the BPRS will be more helpful in a J/Y
  analysis.

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QGP event reconstruction

• Direct photons SPS (WA98, NA45, NA50)
• g escapes strong medium unaffected, but 95 of g
  from background processes gt 5 free quark g
  production!
• PT distribution of secondary hadrons Quark
  energy losses while traversing through a medium
  at high PT. Heavy quarks were predicted to be
  less afflicted by medium, but RHIC data showed
  strong heavy quark interactions within the
  medium!
• Relative production rates of strange particles
  Use of known strange formation yields in the
  initial hadron processes vs. the final measured
  strange abundances gt Quark soup !?!
• J/Y dilepton events Dilpton productions include
  qqbar Drell-Yan annihilation, thermal production
  in a hot QGP, and hadronic formation.

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J/Y enhancement at RHIC

• Charm cross sections at RHIC are greater than SPS
  due to increased run energy.
• Increased initial charm production and an
  increased chance to thermalize at RHIC creates
  more charmonium events!
• i.e. J/Y production still suppressed via
  deconfinement, while a
• thermal enhancement mechanism drives up J/Y
  formation!
• Thermal J/Y models take enhancement and
  suppression to conspire and possibly negate
  observable effects in J/Y data!
• SPS data with enhancement inspired models fit the
  measured J/Y yields very well! How about fitting
  the RHIC data?

• How does this thermal enhancement modeling fit
  the SPS data?

• How does this thermal enhancement modeling fit
  the SPS data?

(Grandchamp, Rapp)
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Upsilon at RHIC

• How about quarks heavier than charm?
• Can RHIC produce the bottom quark? Yes!
• Smaller cross section but RHIC can produce!
• Hundreds of Upsilon to electron decays/year!
• Can STAR detector read them out? Yes!
• Detector upgrades record few hundred/year!
• How will the suppression or enhancement mechanism
  behave for bottom in a QGP?
• Theory predicts suppression mechanism again!
  Enhancement via thermalzation suppressed due to
  heaviness of the bottom quarks!

(Grandchamp, Rapp, Lumpkins, van Hees, Sun)