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PHENIX Single Non-Photonic Electron Spectra and v 2. Nathan Grau Journal Club April 12, 2006. Outline. What do single electrons tell us? Light quarks, heavy quarks, direct production Why is that interesting? Heavy quarks have a perturbative scale m Q Light vs. heavy quark differences

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phenix single non photonic electron spectra and v 2

PHENIX Single Non-Photonic Electron Spectra and v2

Nathan Grau

Journal Club

April 12, 2006

N. Grau, Journal Club

  • What do single electrons tell us?
    • Light quarks, heavy quarks, direct production
  • Why is that interesting?
    • Heavy quarks have a perturbative scale mQ
    • Light vs. heavy quark differences
  • How do we measure them?
    • Need to remove large backgrounds
  • What do we conclude?

N. Grau, Journal Club

sources of electrons
Physics sources of electrons

Light quarks/hadrons

fe+e-, w  e+e-

Kpen, etc.

Dalitz decay p0 ge+e-, etc.

Heavy quarks/hadrons

J/y e+e-, Y  e+e-

D Ken, etc.

Direct production

Other sources of electrons

Internal conversion of photons in material

Note: almost everything here is true about muons as well.

Sources of electrons

N. Grau, Journal Club

two definitions
Two definitions
  • Inclusive electrons are all of these sources
  • Non-photonic electrons are those not from light hadron decay and from internal conversions and virtual direct photon production
    • Primarily from heavy flavor decays and Drell-Yan
    • Drell-Yan is small component down by a factor of 100 because of aEM
    • New sources of electrons in A+A?
      • Enhancement of low mass dileptions?
      • Thermal radiation?

N. Grau, Journal Club

why not just measure heavy quarks directly
Why not just measure heavy quarks directly?
  • Typically charm and bottom are measured from their quarkonia spectra
    • PHENIX does this at least for J/y
  • Open charm and bottom are also typically measured from displaced vertices
    • ct ~ 100 mm for D and ~200 mm for B
    • PHENIX can’t do this yet
  • Measure open charm in the hadronic decay channel
    • DKp, Dppp
    • After three years still don’t see it (but STAR does)
  • Measuring electrons maximizes usage of statistics
    • Catch more of the branching ratio

N. Grau, Journal Club

interest in heavy flavors
Interest in Heavy Flavors
  • In HIC we would like a probe that is
    • Strongly interacting with the medium
      • Heavy quarks have color charge
    • Survive the hadronization process of the plasma
      • See the next couple of slides
  • Heavy flavors compared to jets
    • Can be calculated perturbatively: aS(mQ) << LQCD
    • Auto-generated in the interaction in similar processes.

N. Grau, Journal Club


But this is a long and complicated story that Tatia will probably fill us in on in a couple of weeks!

N. Grau, Journal Club

initial expectations for heavy quark energy loss
Initial Expectations for Heavy Quark Energy Loss
  • Heavy quarks from hard scattering traverse the medium and lose energy
    • Survives QGP hadronization.
  • “Dead cone” effect
    • Can someone please explain the dead cone effect to me. I really couldn’t find a clear explanation in the literature.

N. Grau, Journal Club

heavy to light comparison

Dokshitzer & Kharzeev PLB 519 199 (2001)


quark pT

Heavy-to-Light Comparison
  • Ratio of heavy quark RAA to light quark RAA.
  • 20% higher RAA predicted for heavy quarks at 5 GeV.

N. Grau, Journal Club

anisotropy of heavy quarks i
Anisotropy of Heavy Quarks (I)
  • Flow results from 2 sources
    • Pressure gradients in the overlap region of the nuclei
      • Low pT, hydrodynamics
    • Path length dependent energy loss
      • High pT
  • Question: Do heavy quarks couple as strongly to the medium as light quarks?
    • We should measure it!

N. Grau, Journal Club

anisotropy of heavy quarks ii
Anisotropy of Heavy Quarks (II)
  • Another question: Less energy loss for heavy quarks, but does that necessarily reduce the anisotropy?


(Good to <10% from Dokshitzer and Kharzeev)


We should measure it!

N. Grau, Journal Club

electrons in phenix
Electrons in PHENIX
  • Identification by
    • Charged track in DC/PC
      • Momentum, charge, position
    • Associated hit in RICH
      • Electrons only fire up to 3.5 GeV
      • Muons and pions then fire
        • Muons are rare
    • Associated EM cluster in calorimeter

N. Grau, Journal Club

final spectra
Final Spectra
  • Inclusive Electrons
  • Need to determine the photonic contribution




N. Grau, Journal Club

cocktail method
Cocktail Method
  • Parameterize the measured p0 spectrum as a function of centrality
  • Assume that all other light mesons mT scale, confirmed by h spectrum
  • Conversion photon spectrum determined from PISA simulation
  • Direct photons parameterized from NLO fit
  • Kaon spectrum parameterized from data
  • Run EXODUS which randomly picks from the given distribution and decays if necessary

N. Grau, Journal Club

non photonic spectrum i
Non-Photonic Spectrum (I)
  • Comparison of the minimum bias cocktail and converter spectra
    • Note that the cocktail is much more precise
  • Excellent agreement

N. Grau, Journal Club

non photonic spectrum ii
Non-Photonic Spectrum (II)
  • Published spectrum
    • The line indicates a fit to the p+p spectra
    • Note no centrality above 60%?
    • Suppression observed at high-pT in all centrality

N. Grau, Journal Club

  • A dramatic suppression is seen at high pT.
    • Comparable to suppression of p0
  • Is this misleading, shouldn’t we shift the electron spectrum to the left in order to compare heavy and light quark suppression?

N. Grau, Journal Club

what about 60 centrality
What about >60% Centrality?
  • We have spectra that compares well to the converter method
  • But RAA looks terrible! Was PHENIX just sneaky?
  • The paper claims “More peripheral collisions have insufficient electron statistics to reach pT = 5 GeV/c.”
  • The p0 spectra do not reach to the same pT in all centrality bins…

N. Grau, Journal Club

what can we say about heavy quark eloss
What can we say about heavy quark Eloss?
  • Comparison of data to theory
  • 1a-1c BDMPS (next weeks talk) calculation of charm only for
    • a: no medium, only Cronin
    • b:
    • c:
  • 2a-2b GLV calculation with charm and bottom, bottom pulls up the RAA because of dead cone.
    • a:
    • b:
  • Very extreme range of densities and opacities!

N. Grau, Journal Club

gluon contribution to spectrum
Gluon Contribution to Spectrum?
  • A hard gluon from a hard process could split (fragment?) to Q-Qbar and create two hard mesons
  • If the formation time for such a splitting is longer than say the lifetime of the plasma, the gluon would lose the energy and this would be reflected in the resulting charm hadrons.
    • Because the gluon is fast, gamma is large and there will be a time dilation in it’s “decay”
  • No calculation of this I have found
  • p+p spectrum errors leave room for this production
  • Is it implemented in pythia?

N. Grau, Journal Club

summary on spectra
Summary on Spectra
  • This is an open topic at the moment
  • No calculation can reproduce the observed spectra based on both charm and bottom contributions
  • On the face it seems that the charm and bottom loose as much energy as light quarks and gluons…
  • What about the coupling to the medium
    • i.e. do heavy quarks flow?

N. Grau, Journal Club

extracting inclusive electron v 2
Extracting Inclusive Electron v2
  • Measure the azimuthal angle wrt Y for both candidates and background
  • Subtract background from total to get signal and fit

N. Grau, Journal Club

inclusive electron v 2
Inclusive Electron v2

N. Grau, Journal Club

obtaining photonic v 2
Obtaining Photonic v2
  • Just use a cocktail similar to the singles spectra
  • EXODUS modified to produce a random RP and f distribution of the generated particles.
  • Study electron v2 given input v2 and spectra

p+/- and p0 as input

N. Grau, Journal Club

cocktail sources
Cocktail Sources
  • Cocktail sources (in order of importance)
    • p0 Dalitz(previous slide) and conversion (run through PISA)
      • Not suprisingly similar v2.
    • h Dalitz decay, assume v2 = kaon v2, spectrum mT scales
    • K decay, use measured v2 and spectra of K and STAR’s Ks0
  • Nothing else without further assuming about heavier particle v2 (r, w, f, J/y, etc.)

N. Grau, Journal Club

cocktail results
Cocktail Results
  • The resulting v2 for the different components
  • Relative contribution to the total is also known from the cocktail

e v2 from p0 Dalitz

e v2 from K

e v2 from h Dalitz

N. Grau, Journal Club

non photonic electron v 2 results
Non-photonic Electron v2 Results
  • The paper claims a 90% confidence level that non-photonic electron v2 !=0
    • Why does that seem too low?
    • All points except on are >0 at 1.5s?

N. Grau, Journal Club

but i m missing the point
But I’m Missing the Point
  • Non-zero non-photonic electron v2!
  • And it is consistent with charm flow!
  • Is recombination believable?

N. Grau, Journal Club

the summary
The Summary
  • PHENIX has measured single non-photonic electron spectra and v2 and found that
    • High-pT electrons are suppressed wrt binary scaled p+p collisions to the level of p0
    • There is a non-zero v2.
  • In RUN-4 these results have been extended to
    • Better the stats
    • Centrality binning
  • Other things that are necessary
    • Extending the pT reach of the electron spectra
      • Only reason stopping them at 5 GeV/c was pion turnon in RICH
      • Need to do this in p+p as well
    • Measure charmed hadrons and measure there v2
      • J/y v2 ongoing analysis (but Tatia will let us know if we can distriminate between partonic flow + recombination, etc. with the J/y)

N. Grau, Journal Club

backup slides

Backup Slides

N. Grau, Journal Club

electron id details
Electron ID details
  • Exactly the same cuts for both analyses
    • High quality tracks
      • Excellent p resolution, S/B?
    • 2s matching to EMCal
      • Cluster association, multiple scattering
    • n0>=3, n3>=1 (number of pmts with good timing fired)
      • ?
    • -2s < E/p < 3s

Overall S/B for 0.5-5 GeV/c is very good ~10/1

N. Grau, Journal Club

electron id background
Electron ID Background
  • Background is determined by the swap variables
    • z  -z of hits reassociate RICH and EMCal hits
    • Good for determining random association
  • Why is the background not the same shape as the tails?
  • Effect on the single particle spectrum and for the flow analysis
    • Just subtract off the background spectrum and dn/df shape from the measured spectrum and dn/df

N. Grau, Journal Club

acceptance and efficiency
Acceptance and Efficiency
  • Acceptance
    • Amount of dead area within the fiducial region
    • Study by PISA with detector response tuned to data
  • Efficiency
    • In active area probability for finding the electrons given the cuts in the analysis
    • Study by embedding single particles into real events



N. Grau, Journal Club

measuring the rp




Measuring the RP
  • wi are weights, could be n for number of particles in the ith bin, pT for pT flow correlations

N. Grau, Journal Club