Axel Drees, Stony Brook University, Lectures at Trento June 16-20, 2008

PHENIX at RHIC: The Challenge of High Energies

Axel Drees

RHIC Founding Fathers’ View before 1991

proposals for various experiments at RHICSTAR, TALES, SPARC, OASIS, DIMUON …except for STAR everything else is burned down

from the ashes rises PHENIXPioneering High Energy Nuclear Interaction eXperiment

1991: PHENIX “conceptual design report” philosophy

measure simultaneously as many observables relevant for QCD phase transitions as you can imagine

all but one: low-mass dielectrons why no dielectrons?

included in first TALES proposalconsidered to be “too difficult” for PHENIX

2005: A lot of work can make “impossible” things happen

Axel Drees

Signal to Background: S/B = 1 / 250

Low-Mass e+e- Pairs: The Problem

Electrons/event in PHENIX Most electrons from Dalitz decays and photon conversions PHENIX has now active rejection

= Ne = (dN/d)0 * (BR+CONV) * acc * f(pT>0.2GeV)

350 (0.012+0.02) 0.5*0.7 0.32 = 1.3

combinatorial background pairs/event (assume Poisson stat.) B = ½ P(2) = ½ ½ 2 e- = 0.1

expected signal pairs/event (m>0.2GeV, pT>0.2 GeV) S = 4.2*10-4

S/B signal/background as small as 1/ few hundred, depends on mass

Axel Drees

Why can one not reject Conversions/Dalitz Decays?

Typically only one “leg” of the pair is in the acceptance

out of acceptance “soft” tracks curl up in the

magnetic field Only option for rejection:

catch electrons before they are lost

need new detector and modification of magnetic field

HDB (hadron blind detector) upgradeplanned for 2009/2010 to solve issue

Till then subtract background with accuracy of << 0.5%

Axel Drees

First attempt from 2002 Au-Au Run failed S/B ~ 1/500 (!) for minimum bias events not enough statistics

Success with Au-Au data taken in 2004 minimum bias trigger 8 108 events recorded (100x stat.) Reduced material reduced background

Reference p-p data taken in 2005 Min. bias + single electron trigger (ERT)

xxx events sampled with ERT Xxx minimum bias events Low multiplicity significantly smaller

background

PHENIX Measures Dielectrons

BBC2

BBC1

Coincidence BBC1+BBC2+|z|<30cmAu-Au ~92% of cross sectionp-p ~

AuAu

PHENIX min. bias trigger

EMC

RICH

PHENIX single electron trigger

RHIC-EMC coincidencep>400 MeV

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PC1

PC3

DC

ee+

PHENIX: Tracking & Particle ID

Charged particle tracking Precision tracking

outside of B-field Extrapolate to vertex

to get momentum

Electron/Pion separation Signal in RICH EM shower Match of E/p

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PHENIX Electron Acceptancech

arg

e/p

T

min max

min max

206

309T

T

mrad GeV

p

mrad GeV

p

min

max

Acceptance not equal for + and – charged tracks!

Pairs will be recorded only if both tracks are within acceptance.

Different mass and pt distributions for like and unlike sign pairs!

Like sign pairs can not be used as estimate for combinatorial background!

Single track acceptance

=0

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Unphysical Background Rejection: “Pair Cut” Pions identified as electrons in presents of electron

RICH measures angle only and not position!!! Pion can be misidentified as electron Leads to correlated but unphysical pairs Not reproduced by mixed events Different probability and kinematics for like and unlike sign pairs

Remove by rejecting events with parallel tracks in RICH

e

UNLIKE

mass

pT

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Combinatorial Background: Event Mixing

Event with e+ (p)and/or e-(e)

Centrality iVertex j

pool (j,j)

e1e2e3e4..

en

p1p2p3p4..

pn

e0

example 1 track

eventcut

no pass

Axel Drees

Combinatorial Background: Event Mixing

Event with e+ (p)and/or e-(e)

Centrality iVertex j

pool (j,j)

e1e2e3e4..

en

p1p2p3p4..

pn

e0

example 1 track

mix with poolStore like sign pairsStore unlike sign pairs

eventcut

no pass

Axel Drees

Combinatorial Background: Event Mixing

Event with e+ (p)and/or e-(e)

Centrality iVertex j

pool (j,j)

e0e1e2e3..

en-1

p1p2p3p4..

pn

e0

example 1 track

mix with poolStore like sign pairsStore unlike sign pairs

eventcut

update pool

no pass

Axel Drees

Mixing Without Pair Cut

Large unphysical background!

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Combinatorial Background: Like Sign Pairs

--- Foreground: same evt N++--- Background: mixed evt B++

Shape from mixed events Excellent agreements for like sign pairs

Normalization of mixed pairs Small correlated background at low masses from double conversion or Dalitz+conversion normalize B++ and B to N++ and N for m > 0.7 GeV Normalize mixed pairs to

Subtract correlated BG

Systematic uncertainties statistics of N++ and N--: 0.12 % different pair cuts in like and unlike sign: 0.2 %

TOTAL SYSTEMATIC ERROR = 0.25%

2N N N

Au-Au

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Background Description of Function of pT

Good agreement

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e+ e

Unlike: data - mixedLike: data - mixedMonte Carlo:Cross LikeCross Unlike

0*

X

unlikecross likecross unlike4-body

yield in 4

yield in acceptance

Subtraction of “Cross” Pairs

e+ e

Include also decay

Axel Drees

submitted to Phys. Rev. Lett

arXiv:0706.3034

Raw unlike-sign mass spectrum

Mixed unlike sign pairs normalized to:

2N N N

Unlike sign pairs data

signal/signal = BG/BG * BG/signal

large!!!0.25%

Systematic errors from background subtraction:

up to 50% near 500 MeV

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Cross Check with Converter Method

Increase background by increasing radiation length in experiment

Add brass sheet around beam pipe (1.7% X0/X)

Number of electrons increases by factor ~1.6

Combinatorial background increases by factor 2.5 ~ (1.6)2

If “signal” really not subtracted

background signal must be larger

with converter!Photon Converter (Brass: 1.7% X0)

Axel Drees

submitted to Phys. Rev. Lett

arXiv:0706.3034

Raw unlike-sign mass spectrum

Mixed unlike sign pairs normalized to:

2N N N

Unlike sign pairs data

Independent check of background normalization ~ 0.1%

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Background Subtraction in pp

What works in Au-Au does not work in p-p …

some months later …

Mixed unlike sign pairs normalized to:

2N N N

Unlike sign pairs data

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• Near side located at small mass and high pT

• Away side at low pT and large mass

• In between exists a region that can be described by mixed events

Observe difference from mixed events at near- and away-side

In like and unlike sign Background normalized to yield in Δφ = (π/2±

π/10) rad

The Background in p+p

Could jet correlations show up as signal? Would produce like and unlike sign pairs Generated p+p events with PYTHIA

compare same event spectra with mixed events

π0

π0

e+

e-

e+

e-

γ

γ

π0

e-

γ

e+

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Correlated Background Data & MC: ppCross pairs

Simulate cross pairs with decay generator Normalize to like sign data for small mass

Jet pairs Simulate with PYTHIA Normalize to like sign data

Unlike sign pairs Use same simulations Use normalization from like sign pairs

Alternative methode Correct like sign correlated background with

mixed pairs

( , ) 22

T T

BGFG m p FG FG

BG BG

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Comparison of BG Subtraction Methods

22

Monte Carlo methodLike sign method (with some variations)give consistent results over the full invariant mass range

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Correlated Background Data & MC: Au-AuCross pairs

Simulate cross pairs with decay generator

Normalize to like sign data for small mass

Jet pairs Simulate with PYTHIA Normalize to like sign data No away side jet contribution!

Complicated method to measure jet quenching!

Background subtraction ok within systematic errors

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Efficiency Correction

Analysis requires that electron and positron are in the detector acceptance, but we correct for detector and analysis artifacts:

Correct for losses due to dead detector areas Correct for losses due to analysis cuts, e.g. electron ID Correct for losses due to pair cut

Single track efficiency:including cuts and dead areas

~40% at higher pT

pT (Gev/c) m (Gev/c2)

pair efficiency: 10-18%

hadron decay generator

For pp collisions trigger efficiency is corrected in similar way

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Cocktail Tuning (p+p)• Start from the π0 , assumption: π0 = (π+ + π-)/2

• parameterize PHENIX pion data:

n0T2TT

3

3

pp)bpapexp(

A

pd

σdE

Other mesons well measured in electronic and hadronic channelOther mesons are fit with:

mT scaling of π0 parameterization pT→√(pT

2+mmeson2-mπ

2) fit the normalization constantAll mesons mT scale!!!

PHENIX Preliminary

arXiv: 0802.0050

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p+p Cocktail Comparison

submitted to Phys. Lett.B

arXiv: 0802.0050

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Determine Charm and Bottom Cross Sections

Charm: integration after cocktail subtraction c=544 ± 39 (stat) ± 142 (sys) ± 200 (model) b

Simultaneous fit of charm and bottom: c=518 ± 47 (stat) ± 135 (sys) ± 190 (model) b b= 3.9 ± 2.4 (stat) +3/-2 (sys) b

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Charm and bottom cross sections

CHARM BOTTOM

Dilepton measurement in agreement with single electron, single muon, and with FONLL (upper end)

Dilepton measurement in agreement with measurement from e-h correlation and with FONLL (upper end)

First measurements of bottom cross section at RHIC energies!!!

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Cocktail Tuning (Au+Au)• Start from the π0 , assumption: π0 = (π+ + π-)/2

• parameterize PHENIX pion data:

n0T2TT

3

3

pp)bpapexp(

A

pd

σdE

Other mesons well measured in electronic and hadronic channelOther mesons are fit with:

mT scaling of π0 parameterization pT→√(pT

2+mmeson2-mπ

2) fit the normalization constantAll mesons mT scale!!!

Axel Drees

Au+Au Cocktail Comparison

submitted to Phys. Rev. Lett

arXiv:0706.3034

Low-mass continuum: enhancement 150 <mee<750 MeV: 3.4±0.2(stat.) ±1.3(syst.)±0.7(model)

Axel Drees

Au+Au Cocktail Comparison

Charm from PYTHIAfiltered by acceptancec= Ncoll x 567±57±193b

Intermediate-mass Continuum: consistent with PYTHIAif charm is modified room for thermal radiation

Charm “thermalized” filtered by acceptancec= Ncoll x 567±57±193b

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Comparison to Theoretical Models

Freeze-out Cocktail + “random” charm + spectral function

Low mass M>0.4GeV/c2:

some calculations OK M<0.4GeV/c2:

not reproduced

Intermediate mass Random charm + thermal

partonic may work

Axel Drees

Yield in Different Mass Ranges

0-100 MeV: 0 dominated; scales approximately with Npart

150-750 MeV: continuum;scaling?

1.2-2.8 GeV: charm dominated; scales with Ncoll

study centrality dependence of yields in these regions

Axel Drees

Centrality Dependence 0 production scales approximately

with Npart

expectation for low-mass continuum if in-medium enhancement is related

to or qq annihilation

yield should scale faster than Npart (and it does)

charm is a hard probe total yield follows binary scaling

(known from single e±) intermediate mass yield shows the

same scaling

Axel Drees

pT Dependence

p+p: follows the cocktail Au+Au: enhancement concentrated at low pT

0<pT<0.7 GeV/c

0.7<pT<1.5 GeV/c 1.5<pT<8 GeV/c

0<pT<8.0 GeV/c

p+pAu+Au

arXiv: 0802.0050arXiv: 0706.3034

Axel Drees

Acceptance for Virtual PhotonsData presented as e+ and e- in acceptance, this is not the same as virtual photon in acceptance! Physical distribution requires that virtual photon is in acceptance!

detector

*

e

e

Virtual photon and electron and positron in the acceptance

B-field

*

e

e

Virtual photon in acceptanceelectron and/or positron NOT in the acceptance

detector

B-field

Case A

Case B

Case APair acceptance =

Case A + Case B

Acceptance depends on pair dynamics!

Axel DreespT

0<m<100 100<m<200 200<m<300 300<m<400

800<m<900 900<m<1000 1000<m<1050 2900<m<3300

400<m<500 500<m<600 600<m<700 700<m<800

37

Acceptance as function of pT and mass

pT=5 GeV 10%

20%

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pT dependence II

p+p: follows the cocktail for all the mass binsAu+Au: significantly deviate at low pT

p+p Au+Au

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Understanding the pT Dependence

Comparison with cocktail Single exponential fit:

Low-pT: 0<mT<1 GeV High-pT: 1<mT<2 GeV

2-components fits 2exponentials mT-scaling of 0 +

exponential

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Yields and Slopes

Intermediate pT: inverse slope increase with mass,

consistent with radial flow

Low pT: inverse slope of ~ 120MeV

accounts for most of the yield

SLOPES YIELDS

Total yield (DATA)

2expo fitmT-scaling +expo

fit

Low-pT yield

Axel Drees

Theory Comparison IICalculations from•R.Rapp & H.vanHees•K.Dusling & I.Zahed•E.Bratovskaja & W.Cassing (in 4)

Models fail to describe datain particular low pT raise!

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Summary First measurements of dielectron continuum at RHIC

p+pLow mass Excellent agreement with cocktail

Intermediate mass Extract charm and bottom

c = 544 ± 39 (stat) ± 142 (sys) ± 200 (model) b

b= 3.9 ± 2.4 (stat) +3/-2 (sys) b

Au+AuLow mass Enhancement above the cocktail

expectations: 3.4±0.2(stat.) ±1.3(syst.)±0.7(model)

Centrality dependency: increase faster than Npart

pT dependency: enhancement concentrated at low pT

Intermediate mass Agreement with PYTHIA:

coincidence?

Theory models fail to describes data

Huge enhancement Very soft component