elena bruna, for the star collaboration yale university winter workshop on nuclear dynamics, big sky...
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Elena Bruna, for the STAR CollaborationYale University
Winter Workshop on Nuclear Dynamics, Big Sky Feb. 1-8 2009
Going from pp…Topological Jets in pp at RHIC energies
pT spectrum up to 50 GeVWhat we know:
Good agreement of jet spectrum with NLO/PYTHIA at RHIC energies
Jet spectra and fragmentation functions measured at Tevatron good agreement with theory
Perspectives in STAR:Measure jet Fragmentation Functions at RHIC energiesCompare conceptually different jet algorithms (kT, anti-kT,
cone)Energy resolution using PYTHIA simulationUnderstand the Trigger Bias
Goal: compare Fragmentation functions in pp and Au+Au 2Elena Bruna
...to Au+AuTopological jets measured in Au+Au in STAR
at RHIC energies!What we want to measure:
Jet pT spectrumJet Fragmentation Functions
What we expect with an unbiased jet population:pT spectrum Nbin scaling (jet production is a
hard process)Fragmentation functions modification
3Elena Bruna
Jets in high-energy collisionsHigh-pT partons produced in hard scatterings
a, xa b, xbσab
c, x
c
d, x
d
Full jet (spray of collimated hadrons) after parton fragmentation gives access to:
partonic kinematics jet cross sections
TOOL: Jet-Finding algorithms
4Elena Bruna
Jet Reconstruction
Seedless, not bound to a circular structure kT: starts from merging low pT particles close
in the phase-space Anti-kT: starts from merging high pT particles
close in the phase-space
Rcone
seed
tracks or towers
R=√(Δφ2+Δη2)
Seed Cone: ‘seed’ (E>Ethreshold) iterative approach
• Seedless Cone (SIS cone): all the particles used as
seeds Splitting/Merging
applied
Con
e
Alg
ori
thm
sR
ecom
bin
ati
on
A
lgori
thm
s
[Cacciari, Salam, Soyez, arXiv:0802.1189]
[Cacciari, Soyez, arXiv:0704.0292]
outgoing parton
fragmentation
seed
5Elena Bruna
• charged particle pT (TPC)
• neutral tower Et 0.05x0.05 (ηxϕ) (EMC)
• corrected for hadronic energy.
• Electron correction for double counting
• EMC provides fast trigger.
• Two trigger setups with the EMC:
• Jet Patch Trigger (JP):
• 1x1(ηxϕ), Et>8 GeV
• High Tower Trigger (HT):
•tower 0.05x0.05 (ηxϕ) Et> 5.4 GeV (pp)
• cluster 0.1x0.1 (ηxϕ) Et> 7.5 GeV (AuAu)
40 towers
: 120 towers in =0-2
6Elena Bruna
=-1
=0 =+1
EMC
Analyzed STAR data-sets:
• p+p (2006) High-Tower (HT) trigger (single tower Et>5.4 GeV)
• p+p (2006) Jet-Patch (JP) trigger (ηxϕ=1x1 with sum Et>8 GeV)
7Elena Bruna
Fragmentation functions for charged hadrons
Definition: =ln (Ejet/phadr)
We use: =ln (pT,jet/pT,hadr) : no assumptions on the particle mass ξξ ppT,hadr T,hadr (GeV/c)(GeV/c)
(p(pTjetTjet=30 GeV)=30 GeV)
11 1111
22 44
33 1.5 1.5
44 0.5 0.5
55 0.2 0.2
30< pT,jet<40 GeV
p+p √s=200 GeVJP trigger
Fragmentation function in qualitative agreement for all Jet-Finders•Statistical errors only
•Systematic studies ongoing 8Elena Bruna
STAR Preliminary
Jet-Patch vs High Tower triggers10< pT,jet<15 GeV
20< pT,jet<30 GeV
30< pT,jet<40 GeV
pT,jet>40 GeV
• Low pTjet: Multiplicity of charged particles influenced by Neutral Energy Fraction and high z fragmenting jets stronger trigger BIAS in the HT sample
• High pTjet: JP and HT do not show difference due to a smaller bias of different trigger selections JP and HT fragmentation functions similar for higher jet energy
STAR preliminary
STAR preliminary
STAR preliminary
STAR preliminary
kT
R=0.7Uncorrecte
d
10GeV jet
pThadr
(GeV/c)
20GeV jet pThadr
(GeV/c)
1 3.7 7.4
3 0.5 1
5 0.07 0.1
30GeV jet pThadr
(GeV/c)
40GeV jet pThadr
(GeV/c)
1 11 14.7
3 1.5 2
5 0.2 0.26
9
STAR Preliminary
STAR Preliminary
STAR Preliminary
STAR Preliminary
p+p √s=200 GeV
p+p √s=200 GeV
p+p √s=200 GeV
p+p √s=200 GeV
for different jet energies (1)
Elena Bruna
JP trigger
R=0.4R=0.4
10< pT,jet<15 GeV
20< pT,jet<30 GeV
30< pT,jet<40 GeV
pT,jet>40 GeV
10
• Uncorrected spectra • Different Jet Finders show similar performance for a given R
p+p √s=200 GeVJP trigger
p+p √s=200 GeVJP trigger
p+p √s=200 GeVJP trigger
p+p √s=200 GeVJP trigger
pT>=1 GeV/c
pT>=1 GeV/c
for different jet energies (2)10< pT,jet<15 GeV
20< pT,jet<30 GeV
30< pT,jet<40 GeV
pT,jet>40 GeVJP JP
triggertriggerR=0.7R=0.7
p+p √s=200 GeVJP trigger
p+p √s=200 GeVJP trigger
p+p √s=200 GeVJP trigger
p+p √s=200 GeVJP trigger
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pT>=1 GeV/c
pT>=1 GeV/c
pT>=1 GeV/c
pT>=1 GeV/c
pT>=1 GeV/c
pT>=1 GeV/c
• Conceptually different Jet Finders Similar performance for different cone radii and for different jet pT suggests no significant NLO effects at RHIC energies
p+p Fragmentation function vs. PYTHIA
IncreasingCone R
IncreasingJet EnergyIncreasingJet Energy
20<Ereco<30 GeV 30<Ereco<40 GeV 40<Ereco<50
R<0.4
R<0.7
Good agreement with PYTHIA especially at low R
JP triggerR=0.7 |ηjet|<0.3
12Elena Bruna
pp: reference for Au+AuJets in pp are a safe
baseline for Au+AuGood agreement with
NLO/PYTHIAFree choice of jet algorithms
in ppSystematic studies ongoing (e-
identification, hadronic shower, trigger bias)
Elena Bruna 13
Analyzed STAR data-sets:
• Au+Au (2007) High-Tower (HT) trigger (cluster ET>7.5 GeV)
• Au+Au (2007) Minimum-Bias (MB) trigger
Jet Finding in Heavy-Ion collisions
GOAL: Fully reconstruct the jet in high-multiplicity environment
How to suppress background:Reduce the jet areaApply a pTcut on tracks and towers
Elena Bruna 14
Jet energy fraction outside cone R=0.3
CDF preliminary
€
R = Δη 2 + Δφ2
• Background estimation:•Mean energy in out-of-cone areas
ϕη
pt p
er g
rid
cel
l [G
eV]
STAR preliminary~ 21 GeV
Reconstructed Jet
Out-of-cone area
Background in Au+Au 0-20%
<p t
,Bkg
> [
Gev
]
Reference multiplicity (~centrality)
Au+Au 0-20%Rc=0.4, no pt cut, out-of-cone area
Bac
kgro
und
flu
ctua
tions
[G
ev]
Rc
• Event-by-event basis:
• <pT,Bkg>=mean pT in out-of-cone area • Bkg for Fragmentation Functions = mean FF in out-of-cone area for a given pTjet
•Background energy in R=0.4 ~ 45 GeV (no pT cut)
• Substantial region-to-region background fluctuations
significantly reduced by applying a ⇒ pT cut
15Elena Bruna
STAR Preliminary
STAR Preliminary
Simulation1) Jet Finder on PYTHIA events
2) a. PYTHIA event embedded in Au+Au real event
b. Jet Finder on PYTHIA + AuAu
3) Compare (1) and (2)detector effects neglected 16Elena Bruna
STAR preliminary
17
SimulationSimulation
STAR Preliminary
Simulation: ξ distribution for 30 GeV (pt,cut>2 GeV)
Charged particle FF:Rc(FF)=0.7 and pt>0 GeV
• ξ background subtraction method and jet energy resolution in Au+Au 0-20% causes deviations < 10-20% for ξ<2-2.5 using using Pythia fragmentationPythia fragmentation
• Systematic deviations in the ξ shape ratio at low ξ are caused by jet-energy resolution
• Only statistical errors
STAR preliminary
30 GeV mono-jet embedded in 0-20% central Au+Au STAR event
18
STAR preliminarySTAR Preliminary
Data: ξ distribution for jet energies > 30 GeV in Au+Au
Cone and kt algorithm give similar fragmentation-function measurements
for ptcut > 2 GeV and reconstructed jet pt above 30 GeV in Au+Au 0-
10%
LOCone
FastJet kt
STAR preliminarystat. errors only
pthadron~10 GeV
Au+Au HT Et>7.5 GeV
19Elena Bruna
Fragmentation Function in Au+Au 0-20% and p+p for 30 GeV jets
1) pt,jetrec.(pp) > 30 GeV
2) pt,jetrec.(Au+Au)>31 GeV for pt
cut>2 GeV3) pt,jet
rec.(Au+Au)>35 GeV for ptcut>1 GeV
No apparent modification in the fragmentation
function with respect to p+p
20Elena Bruna
STAR Preliminary
dominated by uncertaintiesdue to backgroundsubtraction for pt
hadron<2 GeV
STAR Preliminary
AuAu jet energies should correspond to 30 GeV pp jets
Statistical errors only
Au+Au results:Simulation jet Finding works in high multiplicity
environment! background reasonably under controlData:
SPECTRA (MinBias data): Binary scaling Binary scaling observed with no pT cut (S. Salur talk)
PYTHIA fragmentation assumed
FRAGMENTATION FUNCTIONS (HT trigger data):
-- no apparent modification w.r.t. pp Corrections for energy resolution (bkg fluctuations) Corrections based on PYTHIA fragmentation
-- we would expect a modification (high-pT hadron suppression)
Measurements consistent: why? 21
What’s happening!
22
Effect A:Effect A: Biased sample of jets due to the High-Tower Trigger: the HT trigger favors “surface” jets that are not modified by the
medium Ejet (AuAu) = Ejet (pp) FF unmodified If this is true HT jets should not binary scale even without pTcut!
Effect BEffect B: Biased sample of jets due to energy loss and pTcut
The jet softens in the medium Its energy is not recovered with pTcut AND
assuming PYTHIA fragmentation Its energy is UNDERESTIMATED ξ=ln(pt
jet/pt) should be larger If this is true Quenching models could address this issue
dN/d
ξ
ξ
Au+Aup+p
dN/d
ξ
ξ
Au+Aup+p
ConclusionsFull jet reconstruction feasible at RHICJet Fragmentation Functions:
Measured and under control in ppPowerful tool to study medium effects in
Au+Au
Elena Bruna 23
We do not see modification of the Fragmentation Functions
Surface effect Surface effect on HT events or SofteningSoftening of jets in the medium or both both?
What’s nextHow to address this issue:
High-Tower jet-pT spectrumdi-jets in Min Bias and HT events Quenching models will helpSystematic studies (detector effect,…) to have more
control on our understanding of jet finder in Au+Au
Jets are a precious tool to explore the medium: exciting physics is coming!
Elena Bruna 24
Thank you !