Correlation in Jets

1
Correlation in Jets

• Rudolph C. Hwa
• University of Oregon

Workshop on Correlation and Fluctuation in
Multiparticle Production Hangzhou, China November
21-24, 2006
2
Two parts to this title
Correlation
Jets
and
Hard scattering is involved.
But that does not mean that the hard parton
fragments. Recombination has been found to be
important at intermediate pT, where most
correlation data exist.
3
colliding system
ee-
Au-Au
4
Correlation of pions in jets in HIC
Two-particle distribution
They do not contribute to C2(1,2)
5
Pion transverse momenta p1 and p2
Hwa Tan, PRC 72, 024908 (2005)
6
C2(1,2) treats 1 and 2 on equal footing.
Experimental data choose particle 1 as trigger,
and studies particle 2 as an associated particle.
(background subtraction)
Hard for medium modification of fragmentation
function to achieve, but not so hard for
recombination involving thermal partons.
7
Associated particle distributions in the
recombination model
Hwa Tan, PRC 72, 057902 (2005)
8
JetRidge on near side
Ridge is understood as enhanced thermal
background due to energy loss by hard parton to
the medium, and manifests through TT
recombination. Chiu Hwa, PRC 72 (05).
9

J. Bielcikova, HP06 --- at lower pt(assoc)

10
J/R 10 for 1ltpt(assoc)lt2 GeV/c suggests
dominance of soft partons that are not part of
the jet in the numerator.
Yet the ridge wouldnt be there without hard
parton, so it is a part of the jet in the broader
sense.
Phantom jet ridge only -- at low pt(assoc)
The existence of associated particles falsifies
our earlier prediction.
Phantom jet is the only way to understand the
problem.
11
Since shower s quark is suppressed in hard
scattering, ? is produced by recombination of
thermal partons, hence exponential in pT.
Normally, thermal partons have no associated
particles distinguishable from the background.
But if the s quarks that form the ? are from the
ridge, then ? can have associated particles above
the background, while having exponential pT
distribution.
The phantom jet is like a blind boy feeling the
leg of an elephant and doesnt know that it
belongs to an elephant. Low pt(trig) and low
pt(assoc) suppress the peak above the ridge, and
do not show the usual properties of a jet, yet
the jet is there, just as the phantom elephant is
to a short blind person.
12
A. Sickles (PHENIX)
Proton triggered events
Meson yield in jet is high. Meson yield in ridge
decreases exponentially with pT.
Ridge is developed in very central collisions.
13
Forward-backward asymmetry in dAu collisions
Expects more forward particles at high pT than
backward particles
F/B gt 1
B/F lt 1
14
Backward-forward ratio at intermediate pT
in dAu collisions (STAR)
B/F
15
B/F asymmetry taking into account TS
recombination (Hwa, Yang, Fries, PRC 05)
There are more thermal partons in B than in F.
16
Associated particles on the away side
Collective response of the medium Mach cone, etc.
Markovian parton scattering (MPS) Chiu Hwa (06)
Non-perturbative process
Markovian
17
Model input
18
Individual tracks may not be realistic, but (like
Feynmans path integral) the average over all
tracks may represent physical deflected jets.
(a) Exit tracks short, bend side-ways, large
??????
(b) Absorbed tracks longer, straighter, stay
in the medium until Eilt0.3 GeV.
19
Data from PHENIX (Jia)
1ltpT(assoc)lt2.5 GeV/c
??????
Chiu Hwa, nucl-th/0609038
PRC (to be published)
One deflected jet per trigger at most, unlike
two jets simultaneously, as in Mach cone, etc.
20
Extension to higher trigger momentum pT(trig)gt8
GeV/c, keeping model parameters fixed.
(a) 4ltpT(assoc)lt6 GeV/c
(b) pT(assoc)gt6 GeV/c
Physics not changed from low to high trigger
momentum.
21
Mid- and forward/backward-rapidity correlation
d-Au collision
Trigger 3ltpT(trig)lt10 GeV/c, ?(trig)lt1
(mid-rapidity)
Associated 0.2ltpT(assoc)lt2 GeV/c, (B)
-3.9lt?(assoc)lt-2.7 (backward) (F)
2.7lt?(assoc)lt3.9 (forward)
?? distributions of both (B) and (F) peak at
?, but the normalizations are very different.
22
STAR (F.Wang, Hard Probes 06)
23
higher yield
lower yield
Recombination of thermal and shower partons
24
Backward-forward ratio at intermediate pT
Inclusive single-particle distributions
in dAu collisions (STAR)
B/F
25
AuAu centrality variation
htriglt1, 2.7lthassoclt3.9 3ltpTtriglt10 GeV/c,
0.2ltpTassoclt 2 GeV/c
dN/dDf
Near side consistent with zero. Away-side
broad correlation in central collisions.
Df
Normalization fixed at Df1lt0.2. Systematic
uncertainty plotted for 10-0 data.
26
Au-Au collisions
At 2.7lt?lt3.9, the recoil parton is moving
almost as fast as the cylinder front. What is the
Mach cone effect?
No difference in F or B recoil
27
Two-jet recombination at LHC
Hwa Yang, PRL 97, 042301 (2006)
New feature at LHC density of hard partons is
high.
High pT jets may be so dense that neighboring jet
cones may overlap.
If so, then the shower partons in two nearby jets
may recombine.
28
The particle detected has some associated
partners.
There should be no observable jet structure
distinguishable from the background.
If this prediction is verified, one has to go to
pT(assoc)gtgt20 GeV/c to do jet tomography. What
happens to Mach cone, etc?
29
Conclusion
Many correlation phenomena related to associated
particles observed at moderate pT can be
understood in terms of recombination. However,
there remains a lot to be explained.
(a very conservative view)
Beyond what is known about jet quenching, not
much has been learned so far about the dense
medium from studies of correlation in jets.
More dramatic phenomena may show up at LHC, but
then the medium produced may be sufficiently
different to require sharper probes.
We have learned a lot from experiments at SPS,
RHIC, and soon from LHC. At each stage the
definition of a jet has changed from gt2 to gt8 to
gt20 GeV/c. What kind of correlation is
interesting will also change accordingly.