The dihadron fragmentation function
&
hadron hadron correlations

Abhijit Majumder

Nuclear Theory Group,

LBNL

in collaboration with

Xin-Nian Wang

Hot Quarks 2004,

Taos, NM

OUTLINE

HEAVY-ION COLLISIONS, MOTIVATIONS

HIGH PT JETS, TWO PARTICLE CORRELATIONS

VARIOUS APPROACHES: PARTONIC INTERACTION!

FACTORIZATION: NAILING IT DOWN IN

DGLAP EVOLUTION: NON-SINGLET, SINGLET,GLUONS

COMPARISONS WITH JETSET !

IN-MEDIUM MODIFICATIONS PRELIMINARY !!

RESULTS FROM STAR @ QM2004

d+Au central very similar to p+p

Away side, central Au+Au suppressed

Near side, central Au+Au unchanged

Why?  simple pictures!

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

Partonic energy loss models explain single inclusive suppression

pretty well !

(Gyulassy Wang, Gyulassy Levei Vitev, Wang Guo Zhang,

Wiedemann Salgado, BDMPS )

All models depend require a high density of scattering centers

High density seen as evidence of QGP.

To explain double inclusive spectra requires a new

phenomenological object: Dihadron fragmentation function!

p

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 @QM2004, V. Koch (unpublished!)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

Important to A+A, d+A, DIS and experiments.

To date observations in A+A d+Au and DIS

How does partonic interaction effect dihadrons?

Wish List!

Definition and factorization of dihadron fragmentation function

Calculate the effect of medium modification

Requires the evaluation of 23 twist-4 diagrams, very difficult!

But medium modification similar to vacuum evolution

Calculate and check vacuum evolution first (simpler!)...

Measure the function at the scale m, can be done in 2 ways

Can be factorized from hard process if

Quark evolution Gluon evolution

Evolving to a higher scale Q F solving DGLAP equations

Set of coupled differential equations containing the following processes:

Evolution of Non-singlet quarks

Simpler: contribution from gluon fragmentation cancels out

Note: ratio double/single shows little change at intermediate z. Why ?

Ratio is the number of associated particles for given trigger !

Regular evolution softens the spectrum: as for single hadrons

Single gluon fragmentation increases multiplicity!

D



z

1,

z

2



=

N

z

1

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1

z

2



2



z

1



z

2





3



1

−

z

1





1



1

−

z

2





2



1

−

z

1

−

z

2



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3

Results from Event generators:

a bit ragged (Monte Carlo), fit a function to it !

Quark and Gluon evolution fits event generator data very well!

Thus we can understand evolution of FF from QCD.

Once again the double to single ratio shows little change

Medium modification

The medium modification equation looks very similar

to the vacuum evolution equation...

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

A parton in a nucleon is struck by EM probe

Parton traverses cold medium and then fragments

Fragmentation function is medium modified.

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

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

Calculation in progress, preliminary results encouraging!

SURFACE EMISSION PICTURE

Suppose the matter produced is very opaque

Hence only hard collisions on the surface will produce

observable jets

Inconsistent with an RAA between participant and binary scaling

Inconsistent with all energy loss models which require bulk

emission and fit single inclusive data!

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

PowerPoint Presentation

1600-12-31T17:00:00