Modification of the jet properties at the Relativistic Heavy Ion Collider

1
Modification of the jet properties at the
Relativistic Heavy Ion Collider

• Jan Rak
• Department of Physics and Astronomy
• Iowa State University

2
QCD in Heavy Ion collisions

• Two particles azimuthal correlations in pp, dAu
  and AuAu
• nuclear modification of jet properties
• Intrisic momentum kT
• Jet transverse fragmentation momentum jT
• Parton distribution function
• Fragmentation function

3
Hard scattering
4
Jet Fragmentation (width of the jet cone)
Partons have to materialize (fragment) in
colorless world
jT and kT are 2D vectors. We measure the mean
value of its projection into the transverse plane
?jTy? and ?kTy? .
?jTy? is an important jet parameter. Its
constant value independent on fragments pT is
characteristic of jet fragmentation
(jT-scaling). ?kTy? (intrinsic NLO radiative
corrections) carries the information on the
parton interaction with QCD medium.
5
Fragmentation Function (distribution of parton
momentum among fragments)
jet
In Principle
Fragmentation function
6
xE in pp collisions
PHENIX preliminary
1/?xE? ? -4 to 5
7
?z? extracted from pp data
xTtrigg2.pTtrigg/?s
We measured xE and
Only one unknown variable ?z? ? iterative
solution
8
pp and dAu correlation functions
pp h-
3.0ltpTlt6.0
1.0ltpTlt1.5
Fixed correlation both pTtrigg and pTassoc are
in the same range Assorted correlation pTtrigg
and pTassoc different
Away side peak
Near-side peak
5.0ltpTtrigg lt16.0 GeV/c
dAu ???h?
1.5ltpTlt2.0
dAu
Jet function assumed to be Gaussian
1.0ltpTlt1.5
Fit const Gauss(0)Gauss(??)
9
?N, ?A , ?jTy?, ?kTy? relations
Knowing ?N and ?A it is straightforward to
extract ?jTy? and ?ztrigg? ?kTy? In the
high-pT limit (pT gtgt?jTy? and pT gtgt?kTy? )
10
?N ,?A ??jTy?, ?kTy? in pp data
PHENIX preliminary
PHENIX preliminary ?jTy? 359?11
MeV/c ?kTy? 964?49 MeV/c
Both ?jTy? and ?kTy? in very good agreement
with previous measurements PLB97 (1980)163
PRD 59 (1999) 074007
?A
11
Di-jet fragmentation
Q2 gt Q2
zgtz
zltz
pTassoc
pTq
pTq
pTtrig
12
wave function collapse
For fixed pT correlation the parton distribution
function is almost unaffected by the condition of
having fragment on the opposite side.
13
kT-smearing
Associated parton distribution fa kT?ft and
the final formula for invariant cross section is
Correlation fcn between b2b parotns
gaussian for ktgt0 or delta function for kT0
14
Di-jet mean z
inclusive
conditional
Phase space effect
plateau
Deviation from power law
15
Simulation
Sorry, I can show only simulation the final
data Phys. Rev. C.
16
PHENIX/CCOR ptrigg slopes
trigg
17
Associated yields complementary method of
?kTy? extraction
PHENIX ?kTy?0.9 GeV/c
STAR ?kTy?1.2 GeV/c
We can describe all pTassoc distribution with
unique value of ?kTy?0.9 GeV/c, but not STAR
data.
18
AuAu ?jTy? and ?z? ?kTy? from CF
(2.5?pTtrigg?4.0)?(1.0?pTassoc?2.5)
19
AuAu associated yields
(2.5?pTtrigg?4.0)?(1.0?pTassoc?2.5) GeV/c
Note pT is rather low associated particle
yields increase with centrality
20
pT distributions on near and away side
Away side energy from initial parton seems to
be converted to lower pT particles
reminiscent of energy loss predictions
Near side Overall enhancement from pp to AA
Apparent modification of the fragmentation
function ?
21
Summary and conclusions

• Jet production and fragmentation
• Good agreement of the jet properties in pp
  collisions with other lower ?s experiments
• dAu jT and kT consistent with pp
• In AuAu significant broadening of effective kT
  - with centrality
• Yield of away side associated particles is
  suppressed at pTgt2GeV/c and shows rising trend
  with Npart below 2GeV/c. Remnant of high-pT jets
  - hint of jet-quenching balance ?

22
(No Transcript)