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The Charm of RHIC Electrons - Light Messengers from Heavy Quarks Outline introduction strongly interacting matter relativistic heavy-ion collisions probing the hot and dense medium electrons from heavy flavor at RHIC reference: p+p collisions cold nuclear matter: d+Au collisions

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outline
Outline
  • introduction
    • strongly interacting matter
    • relativistic heavy-ion collisions
    • probing the hot and dense medium
  • electrons from heavy flavor at RHIC
    • reference: p+p collisions
    • cold nuclear matter: d+Au collisions
    • hot matter: Au+Au collisions
  • going beyond single electrons: correlations
  • summary & outlook
nuclear matter as qcd laboratory
Nuclear matter as QCD laboratory
  • “ordinary” nuclear matter is made from nucleons
    • 3 (light) constituent quarks, carrying color charge
    • quarks interact via the exchange of gluons
    • gluons carry color charge (“charged photons”)!
  • key observations
    • isolated quarks are NEVER observed (“confinement“)
    • quark masses account for ~1% of the nucleon mass
  • properties of QCD (Quantum Chromo Dynamics), the theory of strong interaction
  • learn more → “extraordinary” nuclear matter
the qcd phase diagram
The QCD phase diagram
  • study fundamental properties of matter by
    • excitation to extreme temperature and/or density
    • phase transition from nuclear to “quark-gluon“ matter
  • unique approach: relativistic nuclear collisions
  • center-of-mass energy: where do you want to go today?
    • highest temperature at lowest baryon density  colliders: RHIC @ BNL and LHC @ CERN
    • moderate temperature at highest baryon density  fixed-target: FAIR @ GSI
rhic and its experiments

STAR

RHIC and its experiments
  • highest CMS energy currently available at
    • RHIC (Relativistic Heavy-Ion Collider) located at Brookhaven National Laboratory
    • p+p: √s ≤ 500 GeV (polarized beams!)
    • A+A: √sNN ≤ 200 GeV (per nucleon-nucleon pair)
  • experiments with specific focus
    • BRAHMS (until Run-6)
    • PHOBOS (until Run-5)
  • multi purpose experiments
    • PHENIX
    • STAR
the experimental challenge

STAR

PHENIX

The experimental challenge
  • ONE central Au+Au collision at max. energy
  • MANY secondary particles
  • how to look into the heart of matter?
a view behind the curtain

medium

g

g

A view behind the curtain
  • “tomography” with scattering experiments
    • Rutherford: a→ atom  discovery of the nucleus
    • SLAC: electron → proton  discovery of quarks
  • “tomography“ at RHIC
    • probe has to be “auto generated” in the collision
    • hard parton (quark, gluon) scattering, leading to
      • direct photons from quark-gluon Compton scattering
      • high pT jets
      • heavy quark-antiquark pairs
    • once calibrated for p+p collisions modifications observed in p(d)+A & A+A tell about the “medium”
  • calibration of hard probes
    • theoretically
      • perturbative QCD (pQCD)
    • experimentally
      • measurement in p+p
      • in-situ control: direct photons
direct photons at s nn 200 gev

Nbinary:

number of “binary” collisions, determined from the collision geometry (Glauber)

p-p

Direct photons at √sNN = 200 GeV
  • photons from quark-gluon Compton scattering

Au-Au

Medium produced in

Au+Au collisions is

transparent

for direct photons!

  • no strong final state interaction
  • direct photons are a calibrated probe
light hadrons at s nn 200 gev
(Light) hadrons at √sNN = 200 GeV
  • pQCD in reasonable agreement with p+p data
  • medium modifications in cold (hot) matter: d+Au (Au+Au)?
  • nuclear modification factor:

Quantitative assessment of medium parameters requires in-between (“grey”) probe

Medium produced in Au+Au collisions is opaque for light quark and gluon jets!

  • limiting factor, preventing RAA to drop even further: surface (“Corona”) emission
heavy quarks to the rescue

D mesons

, Y’, c

Heavy quarks to the rescue?
  • heavy quarks (cc, bb): mu,d ~ MeV, mc~1.25 GeV, mb~4.5 GeV
    • hard process (mq >> LQCD)
      • production at leading order (LO)
        • mainly gluon fusion
  • naïve expectation
    • large mass → small energy loss
    • confirmed in (most) models
    • (quantitative) details depend on energy loss mechanism
      • example: energy loss via gluon radiation
        • larger parton mass implies less energy loss in forward direction (“dead cone” effect) (Dokshitzer, Kharzeev: PLB 519(2001)199)
        • partially compensated by medium induced gluon radiation (Armesto, Salgado, Wiedemann: PRD 69(2003)114003)
  • systematic experimental study
    • heavy quarks from p+p, p(d)+A, and A+A collisions
  • in addition: bound states, quarkonia (J/y, U)
heavy quark data pre rhic
Heavy quark data pre-RHIC
  • total cross section measurements at lower √s
    • recent review:C. Lourenco & H. Woehri: Phys. Rep. 433 (2006) 127
      • charm and bottom cross sections measured at the same √s can differ by more than a factor 10!
  • differential cross section at higher √s (1.8 TeV) (PRL 91, 241804 (2003))
  • CDF: PRL 91, 241804 (2003)
    • reconstruction of charmed mesons for pT > 5 GeV/c only!
charm measurements at rhic

K+

p-

Charm measurements at RHIC
  • ideal (but very challenging in HI environment)
    • direct reconstruction of charm decays(e.g. )
    • STAR (for pT < 3 GeV/c)
    • helpful to constrain charm cross section

D0 K+p-

PRL 94, 062301 (2005)

  • alternative (but indirect, and still challenging)
    • contribution of semileptonic decays to lepton spectra
    • PHENIX & STAR
    • only systematic study: electron spectra at y~0
e from heavy flavor problem i

charged pions: (p+ + p-)/2

neutral pions: p0

(e+ + e-)/2 from

heavy flavor

electrons: (e+ + e-)/2

e± from heavy flavor: problem I
  • how to measure a clean spectrum of inclusive e±?
  • electrons are RARE!
phenix star at rhic

2 central electron/photon/hadron

spectrometer arms: |h|  0.35 p  0.2 GeV/c

2 forward muon spectrometers:

1.2 < |h| < 2.4 p  2 GeV/c

PHENIX & STAR at RHIC

3 detectors for event characterization:

vertex, centrality, reaction plane

STAR

optimized for hadrons, but can do leptons

PHENIX

optimized for leptons, but can do hadrons

  • charged particles
    • |h| < 1
    • pT > 0.15 GeV/c
    • charged particle ID:
      • TPC (dE/dx)
      • Time-of-Flight detector
    • additional electron ID: EMC
  • muons
    • 1.2 < |h| < 2.4
    • p > 2 GeV/c
    • tracking
    • muon ID:
      • “absorber”
  • electrons
    • |h| < 0.35
    • pT > 0.2 GeV/c
    • tracking
    • electron ID:
      • RICH + EMC
e from heavy flavor problem ii

PHOTONIC

NON

PHOTONIC

e± from heavy flavor: problem II
  • there are MANY electrons sources
    • Dalitz decay of light neutral mesons
      • most important p0→ g e+e-
      • but also: h, w, h’, f
    • conversion of photons in material
      • main photon source: p0→ gg
      • in material: g → e+e-
    • weak kaon decays
      • Ke3, e.g.: K± → p0 e±ne
    • dielectron decays of vector mesons
      • r, w, f → e+e-
    • direct radiation
      • conversion of direct photons in material
      • virtual photons: g* → e+e-
    • thermal radiation
    • heavy flavor decays
  • how to extract e± from heavy flavor decays from the inclusive spectrum?
extracting e from heavy flavor
Extracting e± from heavy flavor
  • PHENIX
    • cocktail subtraction method
      • ALL relevant background sources are measured
      • background subtraction  e± from semileptonic heavy quark decays
    • converter subtraction method
      • converter of known thickness added for part of the run
      • converter multiplies photonic background by KNOWN factor

PRL 96(2006)032001

p+p @ √s = 200 GeV

  • STAR
    • large acceptance
      • direct measurement of ~60% of photonic background
      • rest: extrapolation + cocktail
how well does this work for phenix
How well does this work for PHENIX?
  • test case: p+p at √s = 200 GeV (PRL 97, 252002 (2006))
  • how well is the e± background determined?
    • comparison of two methods
      • converter measurement
      • cocktail calculation
    • excellent agreement
  • how large is the ratio of signal to background?
    • S/B > 1 for pT > 2.5 GeV/c
    • only Dalitz decays and photon conversions are important
    • PHENIX: conversion ~ 0.5 x Dalitz
    • STAR: conversion ~ 5 x Dalitz
p p @ s 200 gev the reference
p+p @ √s = 200 GeV: the reference

PRL 97, 252002 (2006)

  • e± from heavy flavor decays
  • comparison with FONLL calculation: Fixed Order Next-to-Leading Log pQCD (M. Cacciari, P. Nason, R. Vogt PRL95,122001 (2005))
  • theory has uncertainties / parameters too
  • data are at “upper edge” of theory band
  • does this look familiar?
  • total cross section
    • scc= 567±57(stat)±224(sys) mb
star s e from p p collisions
STAR’s e± from p+p collisions
  • ratio of e± from heavy flavor decays to FONLL pQCD expectation
    • STAR (scaled down by 25% compared to original preprint)
    • earlier STAR publication
    • PHENIX: PRL 97, 252002 (2006)
  • PHENIX & STAR e± spectra exhibit the SAME shape as predicted by FONLL!!
  • scale difference: factor ~2
  • PHENIX: superior electron measurement
  • STAR: D meson measurement should help constrain scc
star d mesons versus e

STAR data

STAR: D mesons versus e±
  • D meson and electron measurements at “low” pT
    • consistent within (large) uncertainties
  • e± from heavy flavor decays
    • who is right/wrong?
  • pro PHENIX: e± data
  • pro STAR: D data
  • how to resolve this issue?
    • PHENIX: D measurement
      • difficulty: K identification
    • STAR: reduce material
      • difficulty: Silicon Vertex Tracker
  • is this a “show stopper”?
cold nuclear matter phenix

PHENIX PRELIMINARY

1/TABEdN/dp3 [mb GeV-2]

Cold nuclear matter: PHENIX
  • e± spectrum from heavy flavor decays in d+Au at 200 GeV
    • d+Au data scaled down assuming binary collision scaling
    • scaled d+Au data are consistent with fit to p+p reference
    • agreement holds for various d+Au centrality classes
  • no indication for large cold effects on heavy flavor production at y = 0.
cold nuclear matter star

STAR e± data

Cold nuclear matter: STAR
  • nuclear modification factor RdA for e± from heavy quark decay
    • RdA is consistent with binary scaling
    • indication for “Cronin” enhancement (initial state scattering, pT broadening)
    • consistent with PHENIX
  • PHENIX & STAR
    • conclude the SAME regarding cold nuclear matter effects on e± from heavy flavor decays!
  • comparison of PHENIX/STAR d+Au and p+p data
    • normalization discrepancy cancels in ratio (d+Au)/(p+p)!
hot matter e yield in au au

PHENIX: PRL 94, 082301 (2005)

extrapolation to

full phase space

  • charm cross section per NN collision: 622 ± 57 ± 160 mb
  • STAR: 1.4 ± 0.2 ± 0.4 mb (d+Au)
  • central Au+Au collision: ~20 cc pairs!

total yield for pT > 0.8 GeV/c

Hot matter: e± yield in Au+Au
  • total yield in Au+Au follows binary collision scaling (as expected for hard probe)!
  • spectra of e± from heavy flavor decays for different centralities
binary scaling of charm yield at rhic
Binary scaling of “charm” yield at RHIC
  • PHENIX and STAR measure heavy quark production in various systems
  • determine sccper binary collision
  • experiments are self consistent but not consistent with each other
  • spectral shapes measured by PHENIX & STAR agree in p+p and d+Au → what about Au+Au?
discovery of heavy quark energy loss

PRL 96, 032301 (2006)

PRL 96, 032301 (2006)

Discovery of heavy quark energy loss
  • cocktail analysis of PHENIX Run-2 Au+Au data set
  • strong modification of heavy quark e± spectra at high pT (similar to p0)
  • uncertainties too large for stronger conclusions!
dramatic progress run 2 run 4
Dramatic progress: Run-2 → Run-4
  • Run-4 Au+Au data sample: ~109 MB events (~40 x Run-2)
  • PHENIX: nucl-ex/0611018
  • electron measurement extended beyond RICH Cerenkov threshold for pions (pT > 5 GeV/c)
    • stringent Cerenkov ring selection
    • “shower shape” cuts in the electromagnetic calorimeter
dramatic progress run 2 run 427
Dramatic progress: Run-2 → Run-4
  • stronger high pT suppression in central collisions
  • strikingly similar to suppression of light hadrons except for
    • intermediate pT
    • highest pT?
    • careful: decay kinematics!
  • bottom???
  • Run-4 Au+Au data sample: ~109 MB events (~40 x Run-2)

nucl-ex/0611018

  • indication for light vs. heavy quark mass hierarchy in energy loss at intermediate pT
heavy flavor e r aa phenix vs star
Heavy flavor e± RAA: PHENIX vs. STAR
  • is the disagreement between PHENIX & STAR a normalization issue “only”?
  • use RAA of e± from heavy flavor decays as test case
  • for d+Au collisions PHENIX & STAR agree in RdA
  • the same is true for the Au+Au system in
  • peripheral
  • mid-central
  • central collisions
  • differences between PHENIX & STAR “disappear” in RAA!
heavy flavor e r aa data vs theory
Heavy flavor e± RAA: data vs. theory
  • calculations invoking heavy quark energy loss by gluon radiation
  • describing the measured suppression is difficult
    • radiative energy loss of charm and bottom quarks is not enough with typical gluon densities of the produced medium in Au+Au collisions (Djordjevic et al., PLB 632(2006)81)
    • models involving extreme conditions, implemented via a large transport coefficient q (Armesto et al., PLB 637(2006)362)
      • agree better with e± data
      • very “opaque” medium
      • problems with entropy conservation
  • there must be something else
heavy flavor e r aa data vs theory30
Heavy flavor e± RAA: data vs. theory
  • the return of collisional energy loss
  • collisional energy loss can be important for heavy quarks
    • the original idea is old (1982):
      • J.D. Bjorken (Fermilab-Pub-82/59-THY)
  • implement collisional energy loss into models
    • agreement with data gets better, but isn’t perfect yet
      • collisional + radiative energy loss: Wicks et al., nucl-th/0512076
      • additional resonant elastic scattering: van Hees & Rapp, PRC73 (2006) 034913
heavy flavor e r aa data vs theory31
Heavy flavor e± RAA: data vs. theory
  • and now for something completely different: collisional dissociation
  • let’s take heavy quark dynamics serious
  • what if heavy quarks
    • fragment inside the medium
    • form D/B mesons, which then dissociate
      • Adil & Vitev, hep-ph/0611109
  • strong suppression for charm AND bottom at high pT
  • open questions
    • how do heavy quarks interact in detail with the medium produced in Au+Au collisions at RHIC
    • where does bottom decay become important?
  • need more information
electrons are not born alone

K+

p-

Electrons are not “born alone”
  • e± (m±) from semileptonic heavy quark decays are correlated with products from the original cc pair
    • hadrons originating from the same parent D/B meson decay
      • eh correlations (“near side”) → bottom/charm
    • hadrons originating from the decay of the associated D/B meson
      • eh correlations (“away side”): too insensitive
    • leptons from the decay of the associated D/B meson
      • ee correlations: → energy loss / thermalization
      • em correlations: → “intermediate” rapidity
      • mm correlations: → “forward” rapidity
eh correlations in p p b vs c
eh correlations in p+p: b vs. c
  • azimuthal angle correlation of e± from heavy flavor decay with hadrons
    • “near side” correlation is dominated by decays kinematics
      • bottom is “wider” than charm due to the larger parent meson mass
    • assumptions
      • decays are described properly in PYTHIA
      • background of (jet) correlations of photonic electrons with hadrons is subtracted properly
    • ratio of bottom/charm can be determined from line shape analysis
      • preliminary STAR result agrees with FONLL within large (model dependent) uncertainties
  • alternative (more direct) approach
    • invariant mass of eh pairs
      • pairs with meh>mD ARE from B decays
dielectrons in au au i

PHENIX Preliminary

Systematic and

Normalization Error

Dielectrons in Au+Au (I)
  • invariant mass analysis of e+e- pairs (~870x106 MB events)
    • problem: HUGE combinatorial background
      • subtracted via event mixing (sys. error in BG normalization ~0.25 %)
  • finally
    • a spectrum with familiar features (J/y)
  • what else?
    • where are correlated charm decays?
    • how does the interaction of charm with the medium manifest itself?
dielectrons in au au ii

PHENIX Preliminary

Dielectrons in Au+Au (II)
  • intermediate mass region of e+e- continuum (from f to J/y)
    • expected to be dominated by charm decays
    • contribution from thermal radiation (not shown) is possible
  • charm interaction with medium
    • energy loss
    • loss of angular correlation
  • p+p reference is unavailable
  • RCP: the poor man’s RAA
  • charm quarks interact strongly with the medium: thermalization?
does charm thermalize

Y

High pressure

X

Low pressure

Z

Reaction plane: Z-X plane

pY

asymmetric pressure gradients (early, self quenching)

pX

Does charm thermalize?
  • RAA/RCP << 1 → strong interaction with the medium
  • large charm mass implies long thermalization time scale
  • unless interaction with the medium is very strong
  • collective motion of the medium produced in Au+Au collisions at RHIC
  • elliptic flow
  • spatial anisotropy in initial stage
  • momentum anisotropy in final stage
  • elliptic flow strength
interaction of charm with the medium

G. Moore and D. Teaney: PRC 71, 064904 (2005)

Interaction of charm with the medium
  • do charm quarks participate in collective motion?
  • elliptic flow parameter v2
    • momentum aniso- tropy w.r.t. reaction plane orientation
  • viscous 3-d hydrodynamics calculation
    • RAA and v2 go hand in hand!
    • decreasing diffusion coefficient D of charm quarks in the medium
      • RAA of charm quarks gets smaller at high pT
      • v2 of charm quarks gets larger
    • this should still be visible in the e± from semi leptonic decays
  • where there is energy loss there should be elliptic flow!
does charm flow

χ2 minimum result

D->e

Does charm flow?
  • strong elliptic flow of electrons from D meson decays → v2D > 0
  • v2c of charm quarks?
  • recombination Ansatz: (Lin & Molnar, PRC 68 (2003) 044901)
  • universal v2(pT) for all quarks
  • simultaneous fit to p, K, e v2(pT)

a = 1

b = 0.96

c2/ndf: 21.85/27

  • within recombination model: charm flows as light quarks!
combining r aa and v 2
Combining RAA and v2
  • large suppression and v2 of electrons → charm thermalization
  • transport models suggest
    • small heavy quark relaxation time
    • small diffusion coefficient DHQ x (2pT) ~ 4-6
    • this value constrains the ratio viscosity/entropy
      • h/s ~ (1.5 – 3) / 4p
      • within a factor 2-3 of conjectured lower quantum bound
      • consistent with
        • light hadron v2 analysis (R. Lacey et al., nucl-ex/0609025)
        • pT fluctuation analysis (S. Gavin & M. Abdel-Aziz, nucl-th/0606061)
  • while this conclusion is MODEL DEPENDENT it motivates the term “perfect fluid” for the medium produced in Au+Au collisions at RHIC

nucl-ex/0611018

summary heavy quarks at rhic
Summary: heavy quarks at RHIC
  • first systematic and comprehensive “heavy quark” measurements in hadronic collisions
  • heavy quarks are a COMPLEMENTARY hard probe
    • unique and powerful observables
    • agreement between PHENIX & STAR is not perfect
    • many surprising results
    • challenges for the current theoretical understanding
  • much more to expect with increasing luminosity and detector upgrades available at RHIC
the future is bright for heavy quark physics
The future is bright for heavy quark physics
  • at RHIC
    • detector upgrades
      • helpful for electron measurements (in particular for low to intermediate-mass e+e- pairs)
        • Dalitz and conversion rejection for single e± and e+e- pairs
        • hadron blind detector (“HBD”) available in PHENIX by Fall 2006!
      • helpful for improved reaction plane measurements
        • PHENIX reaction plane detector
      • needed for optimum heavy quark measurements
        • measurement of displaced heavy quark decay vertices
        • silicon vertex trackers are THE cornerstones in the upgrade programs of both PHENIX and STAR
    • RHIC-II (40 x design luminosity of RHIC)
      • luminosity matters:
        • J/y and U spectroscopy and high statistics c & b data
  • and elsewhere
    • ALICE / CMS / ATLAS @ LHC: √sLHC ~ 30 x √sRHIC
    • CBM @ FAIR: “terra incognita”

RHIC is CHARMING and the future looks BEAUTIFUL!

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