ZIMANYI-SCHOOL11, Budapest, 01/12/2011 A. Ster, RMKI, Hungary 1

Correlations in double parton distributions at small x

MTA KFKI RMKI, Budapest, Hungary Dept. of Astronomy and Theoretical Physics, Lund University, Sweden

András Ster

ZIMANYI-SCHOOL11, Budapest, 01/12/2011 A. Ster, RMKI, Hungary 2

Content

• Introduction

• Double Parton Scattering & Double Parton Distributions

• The Lund Dipole Cascade Model

• Application to Double Parton Distributions

• Results

• Summary

ZIMANYI-SCHOOL11, Budapest, 01/12/2011 A. Ster, RMKI, Hungary 3

Introduction - Motivation

Multiple hard parton (q,g) collisions are common in high energy pp collisions

For example:

• 4 jets in 63 AGeV events @ CERN ISR (AFS)

• 4 jets and 3 jets + in 1.8 TeV events @ Tevatron (CDF, D0)

? @ LHC

Larger probability found than expected for uncorrelated hard subcollisions

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Literature

Connection between correlations in momentum and impact parameter (b) space have not yet been studied

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Correlation studies

Our solution: detailed dynamical model for parton evolution (Lund Dipole Cascade Model)

Earlier Sjöstrand and van Zilj assumed that the dependence of double-parton density on kinematic variables (x, Q2) and on the separation in impact parameter space (b) factorizes.

Implemented in PYTHYA and HERWIG event generators

Problem: how to extrapolate to higher energies (LHC)

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Correlation studies

Lund Dipole Cascade Model

Based on BFKL and Müller’s dipole cascade model

Correlation sources in the model: 1) from fluctuations (event by event) 2) “hot spots” makes the profile narrower

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Double Parton Scattering & Double Parton Distributions - Experimental results

Measure of correlation is defined by eff according to

D(A,B) is the cross-section of the two hard processes

SA and S

B are the single inclusive cross-sections

If hard interactions were uncorrelated, eff would be equal to the total non-diffractive corss-section

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Experimental results

CERN ISR (AFS) results:

eff 15 mb

Tevatron (CDF, D0) results:

eff 5 mb

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The formalism:

Stirling and Gaunt:

Essential formulae

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The Lund Dipole Cascade Model

We define:

With the constraint:

ZIMANYI-SCHOOL11, Budapest, 01/12/2011 A. Ster, RMKI, Hungary 11

The Lund Dipole Cascade Model

Dipole cascades:

ZIMANYI-SCHOOL11, Budapest, 01/12/2011 A. Ster, RMKI, Hungary 12

The Lund Dipole Cascade Model

We define:

With the constraint:

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Results

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Results

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Results

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Results

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Results

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Results

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Results

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Summary

In our analysis we have found that the two-parton correlation function F(b) in a non-trivial way on all kinematic variables x1,x2,Q2

1,Q22

Lund Dipole Cascade Model offers unique possibility to study gluon evolution inside hadrons at small x

Our results show a non-flat energy dependence within errors compatible with experiments in their energy range.