non-photonic leptons and charm production at rhic an experimental overview alexandre suaide...

Non-photonic leptons and charm production

at RHICan experimental overview

Non-photonic leptons and charm production

at RHICan experimental overview

Alexandre Suaide

University of São Paulo – Brazil

2Alexandre SuaideUniversity of São Paulo, Brazil

Quark Matter 2006Shanghai, China

RHIC white papers: many new things…

RHIC has produced matter that behaves differently from anything we have seen previously... Can we fully describe it? Can we see the phase transition/critical point?

Lower energies, different system sizes? ... is dense (many times cold nuclear matter

density)... ... is dissipative... ... exhibits strong collective behavior...

Does dissipation and collective behavior both occur at the partonic stage? How partons interact with matter?

... and seems to be thermally equilibrated Is it?

... and still many questions.



How heavy flavors can help in this search? Heavy quarks are ideal probes for medium created at RHIC

Two ways of doing that Quarkonium investigation

Deconfinement Medium thermometer

Open heavy flavor Production mechanisms thermalization Interaction with the medium tomography

B. Mueller, nucl-th/0404015

D mesons

, ’,



Open heavy flavors Useful tool to probe the

medium Yield, spectra, correlations, jets…

How do we do it? Hadronic reconstruction

Clean probe, but difficult in high multiplicity environments

Semi-leptonic decays Easier, but depends on ‘magic’

to disentangle flavors

(or



How do we measure it?• Designed for leptonic

measurements• Low radiation length• Open heavy flavors

• Electron measurements and muons

• Quarkonia states

• Large acceptance and efficiency

• Good particle identification• dE/dx, EMC and ToF

• Open heavy flavors• hadronic

reconstruction, muons and electrons

• Quarkonia states depend on special triggers



How do we measure it? Phenix

Electrons Electromagnetic calorimeter

and RICH at mid rapidity Muons

Muon arms at forward rapidities

STAR Hadronic reconstruction of D-

mesons Muon identification with

TPC/ToF Electrons

ToF + TPC for low momentum (pT<4 GeV/c)

EMC + TPC for high momentum (pT>1.5 GeV/c)



Not all electrons come from heavy flavor Most of electrons are originated from

sources other than heavy flavors e+ + e- (small for Phenix) 0 + e+ + e-

, etc.

Phenix is almost material free, so their background is highly reduced when compared to STAR

Phenix applies two different methods with very good consistency between them Converter method Cocktail method

STAR has the advantage of being capable of measuring the background despite the amount of material

PHENIX

So, keep in mind that electronsgo through a lot of plastic surgery

Mass (GeV/c2)



Knowing that experiments are capable of measuring heavy

flavors at RHIC, lets go through some findings.

3 main topics to discuss

1. Total charm cross section 2. Interactions of heavy flavors with the medium 3. Separation of charm from bottom at RHIC



Production mechanisms Charm quarks are

believed to be produced at early stage by initial gluon fusions. (M. Gyulassy & Z. Lin,

PRC 51 (1995) 2177) Sensitive to initial

gluon distribution Nuclear and medium

effects in the initial state



Baseline – production in p+p collisions

M. C

acciari et al., P

RL 95:122

001,2005

Heavy Quark production is a “hard” process pQCD Calculation on NLO depends on:

Quark mass mc, mb

Factorization scale F (typically F = mT or 2mT)

Renormalization scale R (typically R = F)

Parton density functions (PDF)

Fragmentation functions (FF) – plays important role

Fixed-Order plus Next-to-Leading-Log (FONLL) designed to cure large logs

for pT >> mq where mass is not relevant



Charm cross section from STAR Use all possible signals

D mesons Electrons Muons

Charm cross section is well constrained 95% of the total cross

section Direct measurement D-mesons and muons

constrain the low-pT region

Y. Z

hang (ST

AR

), Hard P

robes 2006



Charm cross section from PHENIX Many different

datasets Non-photonic

electron spectra Improving statistics

over time Reducing pT cut

Reduces extrapolation uncertainties

hep-ex/0609010



Charm production at RHIC: total cross section FONLL as baseline

Large uncertainties due to quark masses, factorization and renormalization scale

Phenix about a factor of 2 higher but consistent within errors Only electrons but less

background

STAR data about a factor of 5 higher More material but it is the

only direct measurement of D-mesons 95% of the total cross

section is measured



Charm production at RHIC: total cross section Data from both

experiments independently indicate total cross section follow Nbin scaling Charm is produced by

initial collisions No room for thermal

production in the sQGP



Charm production at RHIC: spectra shape Does FONLL describe

the spectral shape despite of any normalization discrepancy?

Both STAR and PHENIX recently submitted electron spectra up to about 10 GeV/c

How do they compare to FONLL?



Charm production at RHIC: spectra shape FONLL describes the

shape well

Experiments do not agree to each other Low material in Phenix

Less electron background to subtract

Direct measurement of D-mesons at STAR and low-pT

Is this shown only at high-pT? 0,0 0,5 1,0 1,5 2,0 2,5 3,0

10-5

10-4

10-3

10-2

10-1

100

1/(2N

evp T

)d2 N

/dp T

d y [(G

eV/c

)-2]

pT [GeV/c]

STAR Combined fit MB , electrons and D-mesons

Phenix MB Au+Au data



Charm cross section: the issue STAR and PHENIX reported charm cross

section in different collision configurations Data are self-consistent within experiments

Both cross section and spectral shapes Both suggest Nbin scaling in the cross section

But experiments do not agree to each other PHENIX is a factor of ~2 lower than STAR

D-mesons/muons/electrons measurement vs. Lower electron background

Very important issue to be addressed in the next months Low material run at STAR and more precise D-mesons

measurements are needed

Are the discrepancies show stoppers onthe understanding of the interaction between

heavy quarks and the medium created at RHIC?



Energy loss in the medium Light quarks

High pT suppression / quenching of away-side jet for light quark hadrons

Can we learn something about the medium?

Pedestal&flow subtracted



Energy loss in the medium Strong suppression observed for light quarks creates bias towards surface emission

Medium is extremely opaque for light quarks

What about heavy quarks?

K.J. Eskola, H. Honkanken, C.A. Salgado, U.A. Wiedemann, Nucl. Phys. A747 (2005) 511

Increasing density



Open Heavy Flavors – Energy Loss in Medium In vacuum, gluon radiation

suppressed at < mQ/EQ “dead cone” effect implies

lower energy loss (Dokshitzer-Kharzeev, ‘01)

energy distribution d/d of radiated gluons suppressed by angle-dependent factor

Smaller energy loss would probe inside the medium

Collisional E-loss: qg qg, qq qq

dE/dx ln p - small?

light

(M.Djordjevic PRL 94 (2004))

Q



Collisional EL for heavy quarks

M. Djordjevic, nucl-th/0603066

Collisional and radiative energy losses are comparable! M.G.Mustafa,Phys.Rev.C72:014905 A. K. Dutt-Mazumder et al.,Phys.Rev.D71:094016,2005

Should strongly affect heavy quark RAA



High-pT electrons and energy loss

STAR

STAR nucl-ex/0607012 (*)PHENIX nucl-ex/0611018(*) updated data



Electron RAA from d+Au to central Au+Au Use of non-photonic

electron spectra as proxy for energy loss study

RAA show increasing suppression from peripheral to central Au+Au First evidence of heavy

quark EL Differences between

STAR and PHENIX disappear in RAA

Is it smaller than for light-quark hadrons?

PHENIX nucl-ex/0611018STAR nucl-ex/0607012



Understanding NPE suppression Radiative EL with

reasonable gluon densities do not explain the observed suppression Djordjevic, Phys. Lett. B632

81 (2006)

Even extreme conditions with high transport coefficient do not account for the observed suppression Armesto, Phys. Lett. B637 362

(2006)

Other EL mechanisms?




Understanding NPE suppression Collisional EL may

be significant for heavy quarks Wicks,

nucl-th/0512076 van Hess, Phys.

Rev. C73 034913 (2006)

Still marginal at high-pT




Understanding NPE suppression Charm alone

seems to describe better the suppression at high-pT

Dead cone more significant for bottom quark larger collisional (relative) EL




Understanding NPE suppression Other effects may

contribute to the observed suppression What if heavy quarks

fragment inside the medium and are suppressed by dissociation? Adil and Vitev,

hep-ph/0611109 Similar suppression

for B and D at high-pT




solid: STARopen: PHENIX PRL91(03)

Open Heavy Flavors – Elliptic Flow

Van Hees & Rapp, PRC 71, 034907: resonant heavy-light quark scattering via scalar, pseudoscalar, vector, and axial vector D-like-mesons

Observed large elliptic flow of light/s quark mesons at RHIC

Strong evidence for thermalization What about charm?

Naïve kinematical argument: need mq/T ~ 7 times more collisions to thermalize

v2 of charm closely related to RAA

V.Greco, C.M. Ko nucl-th/0405040



Do heavy quarks flow? Study of non-photonic

single electrons (from semileptonic D decays)

First hint of strong charm v2 for pT<2 GeV/c Compatible with v2

charm = v2

light-q

Seems to decrease at higher-pT (????) Does the suppression

of charm makes bottom evident in this region in Au+Au?

Increase statistics

PHENIX nucl-ex/0611018



Many questions… The NPE RAA and v2 shows

interesting results Suppression is very large when

compared to the expectation from radiative energy loss that seems to work well for light quark hadrons

Other possible mechanisms? Collisional EL, resonances,

in medium fragmentation… Need to investigate in detail

different aspects of the suppression Centrality dependence,

system size, …

But, very important, need to disentangle charm from bottom! PHENIX nucl-ex/0611018

STAR nucl-ex/0607012



e-h correlations in p+p: bottom vs. charm See Xiaoyan Lin’s talk for STAR

Understand charm and bottom production is a key point to understand suppression and flow

Direct measurement is very complicated

One possible idea: electron-hadron correlations Near side peak dominated by

decay kinematics

Preliminary e-h correlations from p+p collisions in STAR Extract relative bottom contribution

for different electrons pT

exp 1B CR R



See Xiaoyan Lin’s talk for STAR

FONLL has large uncertainties in the b/(c+b) ratio Could the data nail it

down?

First measurement of open-bottom at RHIC Non-zero

contribution of bottom

Very close to FONLL predictions

e-h correlations in p+p: bottom vs. charm



Some considerations… Heavy flavor is an important tool to understand HI

physics at RHIC

First RHIC results are interesting and challenging Large differences in cross section between Phenix and

STAR Why so much suppression at high-pT? Do heavy flavors flow? Charm and bottom relative production. Where bottom

starts dominating? First attempts from STAR indicates a non-zero contribution of bottom

to the NPE spectra Very first step on the understanding of heavy quark EL



We are just in the beginning… Heavy flavor is challenging

Measurements are complicated and hungry for statistics

The future is promising… STAR and PHENIX upgrades visioning heavy

flavor measurements RHIC II upgrades will provide more luminosity



Extras



Open Heavy Flavor – Goals and RequirementsPhysics Motivation Probes Studies Requirements

Baseline D/B mesons, non-photonic electrons

Rapidity y(xF) and pT spectra in AA, pA as a function of A, √s

High Luminosity

High resolution vertex detectors (c(D) ~ 100-300 m)

High-pT PID (DK)

Thermalization,

Transport properties of the medium

D mesons, B?

non-photonic electrons (D+B)

Elliptic flow v2

pT spectra

as above

Properties of the medium

Initial conditions

D, B (B J/ + X) mesons, non-photonic electrons

RAA(pT), RCP of D , B as a function of pT for various √s

as above

Properties of the medium

Heavy Flavor Production

D mesons, non-photonic electrons

Correlations: charm-charm charm-hadron J/-hadron

HIGH luminosity (eff2 !)

Large coverage

Trigger ?



How to do it? RHIC-II: increased luminosity (RHIC-II ≈ 40 × RHIC)

collision diamond s = 20 cm at RHIC and s = 10 cm at RHIC II gain in usable luminosity is larger than “nominal” increase

PHENIX & STAR: more powerful upgraded detectors crucial to the Heavy Flavor physics program - completed in mid/near term ~5 years. STAR:

DAQ upgrade increases rate to 1 KHz, triggered data has ~ 0 dead time. Silicon tracking upgrade for heavy flavor, jet physics, spin physics. Barrel TOF for hadron PID, heavy flavor decay electron PID. EMCAL + TOF J/y trigger useful in Au+Au collisions. Forward Meson Detector

PHENIX: Silicon tracker for heavy flavor, jet physics, spin physics. Forward muon trigger for high rate pp + improved pattern recognition. Nose cone calorimeter for heavy flavor measurements. Aerogel + new MRP TOF detectors for hadron PID. Hadron-blind detector for light vector meson e+e- measurements.



Charm production at RHIC: spectra shape

FONLL describe the shape well, despite normalization



Systematic of charm cross section data Exp. discrepancy is not a

new event

Discrepancy with theory has also a long history Only recently data and

theory touched the bases

pp

pTheory has to deal with many choices of parameters

Experiments need to deal with many corrections on data if measuring NPE

Knowledge evolves in both sides with time!



Where bottom become significant? Large uncertainties

in FONLL prediction on the relative b/c yield

It is important to reduce the uncertainties by measuring the relative contribution

non-photonic leptons and charm production at rhic an experimental overview alexandre suaide...

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