Non photonic leptons and charm production at rhic an experimental overview
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Non-photonic leptons and charm production at RHIC an experimental overview. Alexandre Suaide University of São Paulo – Brazil. RHIC white papers: many new things…. ... and still many questions. RHIC has produced matter that behaves differently from anything we have seen previously...

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Non photonic leptons and charm production at rhic an experimental overview l.jpg

Non-photonic leptons and charm production at RHICan experimental overview

Alexandre Suaide

University of São Paulo – Brazil


Rhic white papers many new things l.jpg
RHIC white papers: many new things…

... and still many questions.

  • 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?


How heavy flavors can help in this search l.jpg
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

, Y’, c


Open heavy flavors l.jpg

(or m)

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


How do we measure it l.jpg
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 it6 l.jpg

p

m

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 l.jpg

PHENIX

Not all electrons come from heavy flavor

  • Most of electrons are originated from sources other than heavy flavors

    • g e+ + e- (small for Phenix)

    • p0  g + e+ + e-

    • h, w, f, 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

So, keep in mind that electrons

go through a lot of plastic surgery

Mass (GeV/c2)


Slide8 l.jpg

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 l.jpg
Production mechanisms flavors at RHIC, lets go through some findings.

  • 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 l.jpg
Baseline – production in p+p collisions flavors at RHIC, lets go through some findings.

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

    • depends on:

      • Quark mass mc, mb

      • Factorization scale mF (typically mF = mT or 2mT)

      • Renormalization scale mR (typically mR = mF)

      • 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

M. Cacciariet al., PRL 95:122001,2005


Charm cross section from star l.jpg

p flavors at RHIC, lets go through some findings.

m

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. Zhang (STAR), Hard Probes 2006


Charm cross section from phenix l.jpg

hep-ex/0609010 flavors at RHIC, lets go through some findings.

Charm cross section from PHENIX

  • Many different datasets

    • Non-photonic electron spectra

    • Improving statistics over time

    • Reducing pT cut

      • Reduces extrapolation uncertainties


Charm production at rhic total cross section l.jpg
Charm production at RHIC: total cross section flavors at RHIC, lets go through some findings.

  • 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 section14 l.jpg
Charm production at RHIC: total cross section flavors at RHIC, lets go through some findings.

  • 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 l.jpg
Charm production at RHIC: spectra shape flavors at RHIC, lets go through some findings.

  • 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 shape16 l.jpg
Charm production at RHIC: spectra shape flavors at RHIC, lets go through some findings.

  • 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-pTm

  • Is this shown only at high-pT?


Charm cross section the issue l.jpg
Charm cross section: the issue flavors at RHIC, lets go through some findings.

  • 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 on

the understanding of the interaction between

heavy quarks and the medium created at RHIC?


Energy loss in the medium l.jpg

Pedestal&flow subtracted flavors at RHIC, lets go through some findings.

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?


Energy loss in the medium19 l.jpg

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

Increasing density

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?


Open heavy flavors energy loss in medium l.jpg

Q Nucl. Phys. A747 (2005) 511

light

(M.DjordjevicPRL 94 (2004))

Open Heavy Flavors – Energy Loss in Medium

  • In vacuum, gluon radiation suppressed at q < mQ/EQ

    • “dead cone” effect implies lower energy loss (Dokshitzer-Kharzeev, ‘01)

    • energy distribution w dI/dw 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?


Collisional el for heavy quarks l.jpg
Collisional EL for heavy quarks Nucl. Phys. A747 (2005) 511

  • 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

M. Djordjevic, nucl-th/0603066


High p t electrons and energy loss l.jpg

STAR Nucl. Phys. A747 (2005) 511

High-pT electrons and energy loss

PHENIX nucl-ex/0611018

STAR nucl-ex/0607012 (*)

(*) updated data


Electron r aa from d au to central au au l.jpg
Electron R Nucl. Phys. A747 (2005) 511AA from d+Au to central Au+Au

PHENIX nucl-ex/0611018

STAR nucl-ex/0607012

  • 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?


Understanding npe suppression l.jpg
Understanding NPE suppression Nucl. Phys. A747 (2005) 511

PHENIX nucl-ex/0611018

STAR nucl-ex/0607012

  • 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 suppression25 l.jpg
Understanding NPE suppression Nucl. Phys. A747 (2005) 511

PHENIX nucl-ex/0611018

STAR nucl-ex/0607012

  • 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 suppression26 l.jpg
Understanding NPE suppression Nucl. Phys. A747 (2005) 511

PHENIX nucl-ex/0611018

STAR nucl-ex/0607012

  • Charm alone seems to describe better the suppression at high-pT

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


Understanding npe suppression27 l.jpg
Understanding NPE suppression Nucl. Phys. A747 (2005) 511

PHENIX nucl-ex/0611018

STAR nucl-ex/0607012

  • 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


Open heavy flavors elliptic flow l.jpg

V.Greco, C.M. Nucl. Phys. A747 (2005) 511Ko

nucl-th/0405040

solid: STAR

open: PHENIX

PRL91(03)

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

Open Heavy Flavors – Elliptic Flow

  • 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


Do heavy quarks flow l.jpg
Do heavy quarks flow? Nucl. Phys. A747 (2005) 511

PHENIX nucl-ex/0611018

  • Study of non-photonic single electrons (from semileptonic D decays)

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

    • Compatible with v2charm = v2light-q

  • Seems to decrease at higher-pT (????)

    • Does the suppression of charm makes bottom evident in this region in Au+Au?

Increase statistics


Many questions l.jpg
Many questions… Nucl. Phys. A747 (2005) 511

  • 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 l.jpg
e-h correlations in p+p: bottom vs. charm Nucl. Phys. A747 (2005) 511

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


E h correlations in p p bottom vs charm32 l.jpg
e-h correlations in p+p: bottom vs. charm Nucl. Phys. A747 (2005) 511

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


Some considerations l.jpg
Some considerations… Nucl. Phys. A747 (2005) 511

  • 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 l.jpg
We are just in the beginning… Nucl. Phys. A747 (2005) 511

  • 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 l.jpg
Extras Nucl. Phys. A747 (2005) 511



How to do it l.jpg
How to do it? Nucl. Phys. A747 (2005) 511

  • 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 shape38 l.jpg
Charm production at RHIC: spectra shape Nucl. Phys. A747 (2005) 511

FONLL describe the shape well, despite normalization


Systematic of charm cross section data l.jpg

pp Nucl. Phys. A747 (2005) 511

pp

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

Theory 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 l.jpg
Where bottom become significant? Nucl. Phys. A747 (2005) 511

  • Large uncertainties in FONLL prediction on the relative b/c yield

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


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