jet structure of baryons and mesons in nuclear collisions
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Jet Structure of Baryons and Mesons in Nuclear Collisions. Why jets in nuclear collisions? Initial state What happens in the nuclear medium? Medium modification of jet fragmentation. PHENIX Collaboration Barbara Jacak Stony Brook University Aug. 16, 2004. why jets in nuclear collisions?. - PowerPoint PPT PresentationTRANSCRIPT
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Jet Structure of Baryons and Mesons in Nuclear Collisions
Why jets in nuclear collisions? Initial state What happens in the nuclear medium? Medium modification of jet fragmentation
PHENIX CollaborationBarbara JacakStony Brook UniversityAug. 16, 2004
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pressure builds up
why jets in nuclear collisions?
PCM & clust. hadronization
NFD
NFD & hadronic TM
PCM & hadronic TM
CYM & LGT
string & hadronic TM
Kpnd,Reflect (thermal)
properties when collisions cease
Quark gluon plasma?use short wavelength probes. Fast q, g nearly ideal
Hard scattering, heavyquark production. Rate calculable from QCD + nuclear geometry.
System expands & cools
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Expected interaction with the medium
Hard scattering happens earlyaffected by initial state nucleus
Hard partons traverse the interesting stuff
Energy loss by induced gluon radiation
Modification of fragmentation outside the medium?? recombination with medium partons radiated gluons nearby!
coneRFragmentation:
z hadron
parton
pp
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Initial state: p+p collisions
p-p PRL 91 (2003) 241803
Good agreementwith NLO pQCD
2/ ( , )a Nf x Q
2/ ( , )ch aD z Q
Parton distribution functions Fragmentation functions
0
0 well described by pQCD and usual fragmentation functions
To generalize for nuclei:fa/N(xa,Q2,r) fa/N(xa,Q2) .
Sa/A(xa,r) .
tA(r)
Nuclear modification to structure function (shadowing, saturation, etc.)
Nuclear thickness function
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pQCD in Au+Au? direct photons
[w/ the real suppression]
( pQCD x Ncoll) / background Vogelsang/CTEQ6
[if there were no suppression]
( pQCD x Ncoll) / ( background x Ncoll)
Au+Au 200 GeV/A: 10% most central collisions
[]measured / []background = measured/background
Preliminary
Probe calculation works!
pT (GeV/c)
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Nuclear medium modifies initial state
Cronin effect for baryons > mesonsBut shouldn’t initial state scattering and fragmentation factorize?!
Probe response of cold nuclear matter with increased number of collisions.
Initial state multiple soft or semi-hard scatteringkT broadening
d+Au: Cronin Effect (RdA>1): Multiple Collisions broaden pT spectrum
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Turn to nuclear collisions: single particles
h/0 ratio shows baryons enhanced for pT < 5 GeV/c
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The baryons scale with Ncoll !
Greco, Ko, Levai: PRC 68 (2003)034904
But observed enhancement can be explained by recombination of thermal quarks from an expanding quark gluon plasma. NOT Jet-like!
Jet-likeNot suppressed!!?
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So, do jet analysis in Au+Au
Trigger:hadron with pT > 2.5 GeV/cIdentify as baryon or mesonBiased, low energy, high z jets!
Plot of associated partnersCount associated lower pT
particles for each trigger “conditional yield”Near side yield: number of jet associated particles from same jet in specified pT binAway side yield: jet fragments from opposing jet
trigger2 particle correlationsnear side < 90°Partner from same jet
away side > 90° opposing jet
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Subtract the underlying event
CARTOON
flow
flow+jet dNNtrig d
includes ALL triggers(even those with no
associated particles inthe event)
jetUnderlying event isbig! Collective flow causes another correlation in them:
B(1+2v2(pTtrig)v2(pT
assoc)cos(2))
associated particles with non-flow angular
correlations -> jets!
Treat as 2 Gaussians
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combinatorial background large in Au+Au!
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2 particle correlations
Select particles with pT= 2.5-4.0GeV/c
Identify them as mesons or baryons viaTime-of-flight
Find second particle with pT = 1.7-2.5GeV/c
Plot distribution of the pair opening angles
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jet partner equally likely for trigger baryons & mesons
Same side: slight decrease with centrality for baryonsLarger partner probability than pp, dAu
Away side: partner rate as in p+p confirms jet source of baryons!“disappearance” of away-side jet for both baryons and mesons
intermediate pT baryons ARE from jets
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What’s going on?
Thermal quark recombination
Radiated gluons are collinear (inside jet cone)
Increases partner yield
Fries, Bass & Muellernucl-th/0407102
Meson triggerbaryon
Dilutes jet partner yield
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Jet partner distribution on trigger side
Corrected to full jet yieldPartner spectrum flatter, indicates jet sourcePartners soften in most central collisions
Fragmentation fn. modified!
Jet partners
Inclusive
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Baryon formation is not outside medium
formation time from hadron size, Rh and mass, mh
In laboratory frame: f ~ Rh (Eh /mh)consider 2.5 GeV pT hadrons f, ~ 9-18 fm/c (Rh~0.5-1 fm); f,p~ 2.7 fm/c (Rh~1 fm)
So, could expect hard-soft recombination (C.M. Ko)
Partner spectrumHwa & Yangnucl-th/0407081
Soft-hard recomb.can also explainbaryon Cronineffect!
(4-6 GeV/c)
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Conclusions
Baryon excess has a significant jet componentDilution becoming visible in most central collisions
Jet fragmentation is modified by the mediumAu+Au jets richer in soft hadrons than p+p or d+Au
effect of induced radiation?Au+Au jets baryon yield increases with medium volume
effect of prevalent soft quarks?
See some evidence that jet fragments are beginning to thermalize in the medium, even on near side
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Jets in PHENIX
Large multiplicity of charged particles--solution: find jets in a statistical manner using angular correlations of particles
mixed events give combinatorial background 2 x 90 degree acceptance in phi and ||<0.35
--solution: correct for azimuthal acceptance,but not for acceptance
Elliptic flow correlations --solutions: use published strength values and subtract (could integrate over 90° to integrate all even harmonics to zero)
PHENIX PRL 91 (2003) 182301
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Compare p+p and d+Au to PYTHIA
d+Au
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Hydro. expansion at low pT + jet quenching at high pT.
Coalesce (recombine) boosted quarks hadrons enhances mid pT hadrons baryons especially
pQCD spectrum shifted by 2.2 GeV
Teff = 350 MeV
R. Fries, et al
Are extras from the (soft) underlying event?
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Phase space filled with partons:coalesce into hadrons
ReCo of hadrons: convolution of Wigner functions
Where does ReCo win?
),()2
,2
;2
,2
()2()2( 3
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,3
3
3 qrqPrRqPrRWrqddRdPd
dNMab
ba
M Wab(1;2) = wa(1)wb(2)
fragmenting parton:ph = z p, z<1
recombining partons:p1+p2=ph
Power law:
Exponential: TpTAew /~
DAeDwN TzPT /frag ~
TPTeAwwN /2reco ~
Tpw ~
TPN ~frag
2reco ~
TPN
Use lowest Fock state, i.e. valence quarks
R. Fries
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Coalescence Model results
Fries et al: Phys.Rev. C68 (2003) 044902Greco, Ko, Levai: PRC 68 (2003)034904
•particle ratios and spectra OK
•intermediate pT hadrons from coalescence of flowing partons NOT from jets, so no jet-like associated particles
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kT, jT at RHIC from p+p Data
J. Rak, Wed.
J. Rak, DNP03
s=200 GeV
di-hadronStatistical Errors Only
near-side away-side
nearfar
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Pions in 3 detectors in PHENIX
Charged pions from TOF
Neutral pions from EMCAL
Charged pions from RICH+EMCAL
Cronin effect gone at pT ~ 8 GeV/c
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Does Cronin enhancement saturate? A different approach:
Intrinsic momentum broadening in the excited projectile proton:
hpA: average number of collisions:
X.N.Wang, Phys.Rev.C 61 (2000): no upper limit.Zhang, Fai, Papp, Barnafoldi & Levai, Phys.Rev.C 65 (2002): n=4 due to proton d dissociation.