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The Charm of RHIC Electrons - Light Messengers from Heavy Quarks PowerPoint PPT Presentation

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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The Charm of RHIC Electrons - Light Messengers from Heavy Quarks

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The charm of rhic electrons light messengers from heavy quarks l.jpg

The Charm of RHICElectrons - Light Messengers from Heavy Quarks


Outline l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

(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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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 l.jpg

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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