phenix at rhic: the challenge of high energies
DESCRIPTION
PHENIX at RHIC: The Challenge of High Energies. RHIC Founding Fathers ’ View. before 1991 proposals for various experiments at RHIC STAR, TALES, SPARC, OASIS, DIMUON … except for STAR everything else is burned down from the ashes rises PHENIX - PowerPoint PPT PresentationTRANSCRIPT
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
Axel Drees
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
Axel Drees
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
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
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!
Axel Drees
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
Axel Drees
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%
Axel Drees
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+
Axel Drees
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
Axel Drees
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
Axel Drees
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
Axel Drees
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
Axel Drees
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
Axel Drees
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!!!
Axel Drees
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
Axel Drees
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
Axel Drees
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