Phenix single non photonic electron spectra and v 2
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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

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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

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

  • 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?

  • 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

  • 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

  • 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

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)

  • 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)

  • 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

  • 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

  • Inclusive Electrons

  • Need to determine the photonic contribution




N. Grau, Journal Club

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)

  • 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)

  • 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?

  • 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?

  • 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?

  • 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

  • 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 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 v2

N. Grau, Journal Club

Inclusive electron v2 is a weighted average of the components. True for any v2!

Obtaining Non-photonic electron v2

N. Grau, Journal Club

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 (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

  • 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 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

  • Non-zero non-photonic electron v2!

  • And it is consistent with charm flow!

  • Is recombination believable?

N. Grau, Journal Club

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

N. Grau, Journal Club

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

  • 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

    • 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

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

N. Grau, Journal Club

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