Dihadron fragmentation
in vacuum and in matter

Abhijit Majumder

Nuclear Theory Group,

LBNL

in collaboration with

Xin-Nian Wang

Hard Probes 2004

Ericeira, Portugal

Nov. 4 - 10

HEAVY-ION COLLISIONS AND JETS

Select a leading particle 4<pt<6 GeV/c, |h |<0.75

Associate all other particles (0.15<pt<4 GeV/c, |h |<1.1) with the leading particle.

High pT particle

p+p

High pT particle

Au+Au

RESULTS FROM STAR, heavy-ion collisions

d+Au central very similar to p+p

Away side, central Au+Au suppressed

Near side, central Au+Au unchanged

Can explain the integral of the near side vs Npart.

RESULTS FROM HERMES, DIS on cold nuclei

Always measure a ratio of double to single production

Divide by same ratio in deuterium to remove detector systematics

P. Di Nezza @ QM2004

TWO POSSIBILITIES !

PARTONIC ENERGY LOSS :

High energy partons are created over the entire collision zone

Lose energy by partonic interaction, medium may be

hadronic or partonic

Emerge as partons and then fragment

p

Partonic energy loss models explain single inclusive suppression

pretty well !

(GLV, BDMPS, WGZ, SW)

To explain double inclusive spectra requires a new phenomenological object: Dihadron fragmentation function!

HADRONIC ENERGY LOSS:

Fragmentation occurs inside the hot medium

Hadrons become independent due to scattering

Each hadron suffers the same Energy Loss on average

Hadronic scattering models can explain mean single supp. !

C. Greiner et. al. @QM2004, V. Koch (unpublished!)

Probability of observing two hadrons factorizes P(1,2) = P(1)P(2)

Each probability is suppressed compared to p+p: P(h) = s p(h)

Thus the conditional probability is also suppressed

compared to p+p collisions

Hadronic absorption models cannot explain the double

inclusive spectrum

P(h) is for A+A; p(h) is for p+p

Quark evolution Gluon evolution

Evolving to a higher scale Q F solving DGLAP equations

Set of coupled differential equations containing the following processes:

D



z

1,

z

2



=

N

z

1



1

z

2



2



z

1



z

2





3



1

−

z

1





1



1

−

z

2





2



1

−

z

1

−

z

2





3

Results from Event generators: JETSET

insist z1 > z2 , fit a function to it !

Quark and Gluon evolution fits event generator data very well!

Thus we can understand evolution of FF from QCD.

the double to single ratio shows little change

Medium modification

The medium modification also has new set of diagrams!

Apply to DIS of Nuclei (HERMES expt. at DESY)

A parton in a nucleon is struck by EM probe

Parton scatters in medium and then exits & fragments

Fragmentation function is medium modified.

DIS followed by di-hadron fragmentation from a large nuclei may be generally expressed as

Luo, Qiu and Sterman PRD 50, 1951 (1994).

Wang and Guo NP A696, 788 (2001).

Quark and gluon can come from different nucleons

The theory curve is the number of pairs with one hadron at z1 = 0.5 and one at z2 .

The expt. curve is the number of events with a subleading hadron at z2,

and z1 > 0.5.

Gaussian approx. for nuclear density used!

Theory curve: (FF(2h)/FF(1h) in A) / (FF(2h)/FF(1h) in vac.)

Expt ratio =

Preliminary HERMES data

Dihadron results for hot medium

Very preliminary estimate for the same

side two body correlation

Results include the

effect of trigger bias.

Initiating parton in

a heavy-ion collision

has higher energy than that in p-p collision..

Summary & Conclusions!

We have defined a new phenomenological object in QCD: `` The Dihadron Fragmentation function ''

Demonstrated its factorization at LO in

Derived it evolution equation (has extra components)

Matched results with JETSET!!

Allowed a physical understanding of change with scale

Extended formalism to medium modification in DIS

Extended formalism to medium modification in heavy-ion collision

Back up

slides ...

Multiple higher twist diagrams need to be evaluated

Multiple scattering

from soft gluons lead to LPM interference

Assume a Gaussian

density distribution

for nucleons in a medium sized nucleus

= Formation time

= Nuclear size

= boost

Fragmentation functions come from the soft matrix elements

where

A. Mueller, PRD 18, 3705 (1978).

PowerPoint Presentation

1600-12-31T17:00:00