Heavy flavor measurements at RHIC

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Heavy flavor measurements at RHIC

• Alexandre Suaide
• University of São Paulo Brazil

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Motivation the original thoughts

• Heavy quarks are ideal probes for medium created
  at RHIC
• Two ways of doing that
• Quarkonium investigation
• Deconfinement
• Medium thermometer
• Open heavy flavor
• Production mechanisms
• thermalization
• Interaction with the medium
• tomography

B. Mueller, nucl-th/0404015
D mesons
, Y, c
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Production mechanisms

• Charm quarks are believed to be produced at early
  stage by initial gluon fusions.
• (M. Gyulassy Z. Lin, PRC 51 (1995) 2177)
• Sensitive to initial gluon distribution
• Nuclear and medium effects in the initial state

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Baseline production in pp collisions

• Heavy Quark production is a hard process pQCD
  Calculations on NLO
• depend on
• Quark mass mc, mb
• Factorization scale mF (typically mF mT or 2mT)
  
• Renormalization scale mR (typically mR mF)
• Parton density functions (PDF)
• Fragmentation functions (FF) plays important
  role
• Fixed-Order plus Next-to-Leading-Log (FONLL)
• designed to cure large logs for pT gtgt mq where
  mass is not relevant

M. Cacciari et al., PRL 95122001,2005
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Open heavy flavors

• Useful tool to probe the medium
• Yield, spectra, correlations, jets
• How do we do it?
• Hadronic reconstruction
• Clean probe but difficult in high multiplicity
  environments
• Semi-leptonic decays
• Easier but depends on magic to disentangle
  flavors

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Energy loss in the medium

• Light quarks
• High pT suppression / quenching of away-side jet
  for light quark hadrons
• What happens to heavy quarks?

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Open Heavy Flavors Energy Loss in Medium

• In vacuum, gluon radiation suppressed at q lt
  mQ/EQ
• dead cone effect implies lower energy loss
  (Dokshitzer-Kharzeev, 01)
• energy distribution w dI/dw of radiated gluons
  suppressed by angle-dependent factor
• Collisional E-loss qg ? qg, qq ? qq
• dE/dx ? ln p - small?
• Various models in the market

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Open Heavy Flavors Elliptic Flow

• Observed large elliptic flow of light/s quark
  mesons at RHIC
• Strong evidence for thermalization
• What about charm?
• Naïve kinematical argument need mq/T 7 times
  more collisions to thermalize
• v2 of charm closely related to RAA

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Melting quarkonia statesCharmonia J/y, Y, cc
Bottomonia ?(1S), ?(2S), ?(3S)

• Original idea by Matsui and Satz
• Color screening
• Suppression depends on T/Tc and binding energy
• Tdiss(Y) lt Tdiss(?(3S)) lt Tdiss(J/Y) ?
  Tdiss(?(2S)) lt Tdiss(?(1S))
• But life is not that simple
• Need to understand many different aspects

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Quarkonia in pp and pA

• Baseline
• p p ? production baseline
• d A ? cold matter effects (absorption,
  shadowing)
• p p
• Color Evaporation Model (CEM)
• Quarkonium production treated as fraction of
  all?QQ pairs below?HH threshold
• CEM taken to NLO (Gavai et al., G. Schuler and
  R.Vogt)
• Parameters adjusted to existing data

hep-ph/0412158
J/y
? ? ?
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Quarkonia in pA

• Nuclear Absorption
• Breakup of quarkonia in the final state
• Depends if produced as color singlet or octet
• Shadowing
• Modification of PDFs in the nucleus w.r.t. free
  nucleon
• y distributions are more sensitive

R. Vogt, RHIC-II Science Workshop
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Quarkonia Effects in A A

• Feed down
• Large from cc states (30-40 ?)
• Not well measured in hadronic collisions
• Unknown at RHIC energies
• Other sources of quarkonia production
• Statistical coalescense (thermal production)
• too small at RHIC larger at LHC ?
• Dynamic coalescence
• coalescence?cc ?? J/y
• recombination J/y??cc ? J/y
• narrower y and softer pT distributions
• Quenching at high-pT
• Comover absorption
• J/y p (r) ??DD (negligible for ?)

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How do we measure it?

• Large acceptance and efficiency
• Good particle identification
• de/dx, EMC and ToF
• Open heavy flavors
• hadronic reconstruction, muons and electrons
• Quarkonia states depend on special triggers

• Designed for leptonic measurements
• Low radiation lenght
• Open heavy flavors
• Electron measurements
• Quarkonia states

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RHIC results charm cross section

• Use all possible signals
• D mesons
• Electrons
• Muons
• Charm cross section is well constrained
• A factor of 5 higher than FONLL calculations
• Follow a Nbin scale from pp to AA collisions
• Produced in the initial stages
• No room for thermal production

Y. Zhang (STAR), Hard Probes 2006
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Heavy flavor energy loss at RHIC

• Use of non-photonic electron spectra as proxy for
  energy loss study
• Shapes at high-pT agree with FONLL but need to
  scale up
• RAA plots show increasing suppression from
  peripheral to central AuAu
• First evidence of heavy quark EL
• Seems to suggest elastic EL
• Do not favor the relative charm and bottom from
  FONLL calculations

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Large suppression requires extreme conditions

• In order to get close to the observed RAA we need
  extreme conditions
• dNg/dy 3500 (Gyulassy et al)
• qhat 14 GeV/fm (Armesto et al)

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But charm looses much more energy than bottom

• In both radioactive and collisional mechanisms
• Maybe we are overestimating bottom?

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The RAA puzzle (?)

• NPE are highly suppressed in central AuAu
  collisions
• Charm alone seems to explain
• Where is bottom?
• Need direct measurement of Ds via hadronic
  decays and Bs via J/y or displaced vertex!
• Other creative methods?

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Do heavy quarks flow?

• Study of non-photonic single electrons (from
  semileptonic D decays)
• First hint of strong charm elliptic flow for pTlt2
  GeV/c
• Seems to decrease at higher-pT
• Does the suppression of charm makes bottom
  evident in this region in AuAu?
• Many issues
• Statistics limited
• Uncertainties due to photonic background
• Large sys errors
• Cannot deconvolute contributions from charm and
  bottom
• Need direct measurement of D mesons (via K p) v2

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How to disentangle charm and bottom?

• Understand charm and bottom production is a key
  point to understand suppression and flow
• Direct measurement is very complicated
• Displaced vertex reconstruction
• Need very precise vertex detector
• One possible idea electron-hadron correlations
• We can study suppression if we look at the away
  side distribution, just like light hadron
  correlations
• But the near side peak may have different
  structure if the electron comes from charm or
  bottom decays.

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Electron-hadron correlations in pp
Data suggest that D mesons dominate Non-photonic
electrons up, at least, pT 5-6 GeV/c
X. Lin (STAR), Hard Probes 2006
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RHIC Results J/y Suppression

• Study of J/y ? ee and mm in AuAu and CuCu
• Yield is suppressed compared to that in pp
  collisions
• Suppression is larger for more central
  collisions.
• Suppression beyond that of cold nuclear matter
  for most central collisions even if sabs 3 mb.
• Cold matter effects under predict the suppression

V. Ciancolo, PANIC05

• Issues
• Lack of statistics
• Only J/y measurement so far
• Need more statistics and data on Y, cc, and ?
  states

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RHIC Results J/y Suppression

• Recombination predicts narrow pT and rapidity
  distribution
• ?pT2? vs. Ncollisions
• Predictions of recombination model match better.
• RAA vs. Rapidity
• No significant change in rapidity shape compared
  to pp result.
• Recombination compensates suppression?
• Issues
• Charm rapidity distributions at RHIC are open
  questions
• Require more data on vs, A dependence

A. Bickley, HP06
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RHIC results other quarkonia states

• J/y is not the only state but may be the easiest
  one ?
• PHENIX Upsilon signal
• PHENIX y
• Hungry for statistics!

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Quarkonium at STAR the first steps

• Large acceptance at mid-rapidity
• Electron ID-capabilities
• Triggering on Barrel EMC
• Suitable for electrons
• Suitable for di-electrons?
• J/y, ? are rare,
• triggering where possible
• STAR developed a special trigger, using topology
  and mass reconstruction in real time to enhance
  the number of events with quarkonium decays.

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What we have so far

• J/y in AuAu collisions
• Trigger is not possible due to the high rate of
  combinatorial events
• Need to rely on large datasets and time consuming
  analysis
• J/y in pp collisions
• Trigger is available
• Low statistics in 2004/2005
• 10x more data in 2006

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Final comments

• Heavy flavor physics is an important tool to
  understand HI physics at RHIC
• Heavy Flavor Physics at RHIC is just at the
  beginning
• First RHIC results are interesting and
  challenging
• Why x-section is too high, compared to FONLL?
• Charm and bottom relative production. Where
  bottom starts dominating?
• Why so much suppression at high-pT?
• Do heavy flavors flow?
• Quarkonia is in the first steps
• Suppression vs. recombination? Feed down issues