Ralf Averbeck Department of Physics & Astronomy

Seminar at

January 24, 2007

The Charm of RHICElectrons - Light Messengers from Heavy Quarks

R. Averbeck, 2 , 1/24/2007

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

R. Averbeck, 3 , 1/24/2007

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

R. Averbeck, 4 , 1/24/2007

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

R. Averbeck, 5 , 1/24/2007

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)

STARSTAR

experiments with specific focus BRAHMS

(until Run-6) PHOBOS

(until Run-5)

multi purpose experiments PHENIX STAR

R. Averbeck, 6 , 1/24/2007

The experimental challengeSTAR ONE central

Au+Au collision at max. energy

MANY secondary particleshow to look into the heart

of matter?

PHENIX

R. Averbeck, 7 , 1/24/2007

g

g

medium

A view behind the curtain “tomography” with scattering experiments

Rutherford: → atom discovery of the nucleus SLAC: electron → proton discovery of quarks

calibration of hard probes theoretically

– perturbative QCD (pQCD) experimentally

– measurement in p+p– in-situ control: direct photons

“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”

R. Averbeck, 8 , 1/24/2007

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

p-p

Au-Au

direct photons are a calibrated probe

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

no strong final state interaction

Medium produced in

Au+Au collisions is

transparent

for dire

ct photons!

R. Averbeck, 9 , 1/24/2007

(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:

ppinYieldN

AAinYieldR

binaryAA

limiting factor, preventing RAA to drop even further: surface (“Corona”) emission

Medium produced in

Au+Au collisions is

opaque for li

ght quark

and gluon jets! Quantitativ

e

assessment of

medium parameters

requires in

-between

(“grey”) probe

R. Averbeck, 10 , 1/24/2007

heavy quarks (cc, bb): mu,d ~ MeV, mc~1.25 GeV, mb~4.5 GeV

hard process (mq >> QCD)– 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,

Heavy quarks to the rescue?

D mesons

, ’,

R. Averbeck, 11 , 1/24/2007

total cross section measurements at lower √srecent 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!

Heavy quark data pre-RHIC

CDF: PRL 91, 241804 (2003) – reconstruction of

charmed mesons for pT > 5 GeV/c only!

differential cross section at higher √s (1.8 TeV) (PRL 91, 241804 (2003))

R. Averbeck, 12 , 1/24/2007

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

Charm measurements at RHIC

D0 K+ -

alternative (but indirect, and still challenging) contribution of semileptonic

decays to lepton spectra PHENIX & STAR only systematic study:

electron spectra at y~0

c c

0DK

0D

K+

-

PRL 94, 062301 (2005)

R. Averbeck, 13 , 1/24/2007

e± from heavy flavor: problem I electrons are RARE!

charged pions: (+ + -)/2

neutral pions: 0

electrons: (e+ + e-)/2

(e+ + e-)/2 fromheavy flavor

how to measure a clean spectrum of inclusive e±?

R. Averbeck, 14 , 1/24/2007

3 detectors for event characterization:vertex, centrality, reaction plane

PHENIX & STAR at RHIC

muons 1.2 < || < 2.4 p > 2 GeV/c tracking muon ID:

“absorber”

electrons || < 0.35 pT > 0.2 GeV/c tracking electron ID:

RICH + EMC

2 central electron/photon/hadron spectrometer arms: 0.35 p 0.2 GeV/c

charged particles || < 1 pT > 0.15 GeV/c charged particle ID:

TPC (dE/dx) Time-of-Flight detector

additional electron ID: EMC

2 forward muon spectrometers:1.2 < || < 2.4 p 2 GeV/c

PHENIX

optimized fo

r leptons,

but can do hadrons STAR

optimized fo

r hadrons,

but can do leptons

R. Averbeck, 15 , 1/24/2007

e± from heavy flavor: problem II there are MANY electrons sources

Dalitz decay of light neutral mesons– most important → e+e-

– but also: ’ conversion of photons in material

– main photon source: → – in material: → e+e-

weak kaon decays– Ke3, e.g.: K± → e± e

dielectron decays of vector mesons– → e+e-

direct radiation– conversion of direct photons in material– virtual photons: * → e+e-

thermal radiation heavy flavor decays

how to extract e± from heavy flavor decays from the inclusive spectrum?

PHOTONICNON

PHOTONIC

R. Averbeck, 16 , 1/24/2007

Extracting e± from heavy flavorPHENIX

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

R. Averbeck, 17 , 1/24/2007

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 well does this work for PHENIX?

how large is the ratio of signal to background?S/B > 1 for pT > 2.5 GeV/conly Dalitz decays and photon

conversions are importantPHENIX: conversion ~ 0.5 x DalitzSTAR: conversion ~ 5 x Dalitz

R. Averbeck, 18 , 1/24/2007

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

p+p @ √s = 200 GeV: the reference

total cross section cc= 56757(stat)±224(sys) b

PRL 97, 252002 (2006)

does this look familiar?

R. Averbeck, 19 , 1/24/2007

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 measurementSTAR: D meson measurement should help constrain cc

R. Averbeck, 20 , 1/24/2007

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”?

R. Averbeck, 21 , 1/24/2007

PHENIX PRELIMINARY

1/T

ABE

dN/d

p3 [m

b G

eV-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.

R. Averbeck, 22 , 1/24/2007

STAR e± data

ppinYieldN

AudinYieldR

binarydAu

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)!

R. Averbeck, 23 , 1/24/2007

PHENIX: PRL 94, 082301 (2005)Hot matter: e± yield in Au+Au

spectra of e± from heavy flavor decays for different centralities

total yield in Au+Au follows binary collision scaling (as expected for hard probe)!

total yield for pT > 0.8 GeV/c

charm cross section per NN collision: 622 ± 57 ± 160 b

STAR: 1.4 ± 0.2 ± 0.4 mb (d+Au) central Au+Au collision:

~20 cc pairs!

extrapolation to

full phase space

R. Averbeck, 24 , 1/24/2007

Binary scaling of “charm” yield at RHIC PHENIX and STAR

measure heavy quark production in various systems

determine cc per

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?

R. Averbeck, 25 , 1/24/2007

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 ) uncertainties too

large for stronger conclusions!

R. Averbeck, 26 , 1/24/2007

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

R. Averbeck, 27 , 1/24/2007

Run-4 Au+Au data sample: ~109 MB events (~40 x Run-2)

stronger high pT suppression in central collisions

strikingly similar to suppression of light hadrons except for

intermediate pT

highest pT? careful: decay

kinematics!

bottom???

indication for light vs. heavy quark mass hierarchy in energy loss at intermediate pT

Dramatic progress: Run-2 → Run-4

nucl-ex/0611018

R. Averbeck, 28 , 1/24/2007

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!

R. Averbeck, 29 , 1/24/2007

calculations invoking heavy quark energy loss by gluon radiation

Heavy flavor e± RAA: data vs. theory

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

R. Averbeck, 30 , 1/24/2007

the return of collisional energy loss

Heavy flavor e± RAA: data vs. theory

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

R. Averbeck, 31 , 1/24/2007

and now for something completely different: collisional dissociation

Heavy flavor e± RAA: data vs. theory

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

R. Averbeck, 32 , 1/24/2007

e± (±) 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– e correlations: → “intermediate” rapidity– correlations: → “forward” rapidity

Electrons are not “born alone”

c c

0DK

0D

K+

-

R. Averbeck, 33 , 1/24/2007

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

R. Averbeck, 34 , 1/24/2007

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 %)

Dielectrons in Au+Au (I)

PHENIX Preliminary

Systematic and Normalization

Error

finally a spectrum

with familiar features (J/)

what else? where are

correlated charm decays?

how does the interaction of charm with the medium manifest itself?

R. Averbeck, 35 , 1/24/2007

intermediate mass region of e+e- continuum (from to J/) expected to be dominated by charm decays contribution from thermal radiation (not shown) is possible

Dielectrons in Au+Au (II)

PHENIX Preliminary

charm interaction with medium energy loss loss of angular correlation

p+p reference is unavailable RCP: the poor

man’s RAA

PeripheralColl

CentralCollCP NYield

NYieldR

)/(

)/(

charm quarks interact strongly with the medium: thermalization?

R. Averbeck, 36 , 1/24/2007

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

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

2 Rcos 2v

pY

pX

Y

X

ZReaction plane: Z-X plane

High pressure

Low pressureasymmetric pressure gradients (early, self quenching)

3 3

R30T T

2 cosnn

d N d NE v nd p p d dp dy

R. Averbeck, 37 , 1/24/2007

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!

R. Averbeck, 38 , 1/24/2007

χ2 minimum resultD->e

2σ

4σ

1σ

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 , K, e v2(pT)

eT

D

cqT

D

uqT

D vpm

mbvp

m

mavpv 2222 )()()(

a = 1

b = 0.96

2/ndf: 21.85/27

within recombination model: charm flows as light quarks!

R. Averbeck, 39 , 1/24/2007

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 (2T) ~ 4-6 this value constrains the ratio

viscosity/entropy– /s ~ (1.5 – 3) / 4– 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

R. Averbeck, 40 , 1/24/2007

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

R. Averbeck, 41 , 1/24/2007

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/ and spectroscopy and high statistics c & b data

and elsewhere ALICE / CMS / ATLAS @ LHC: √sLHC ~ 30 x √sRHIC CBM @ FAIR: “terra incognita”

RHIC is

CHARMIN

G

and the fu

ture

looks

BEAUTIFUL!