ExtreMe Matter Institute EMMI

GSI, November 22-24 2010: Introduction

Structure Functions, Geometric Scaling and Parton Saturation:
Assessment and Perspectives for the LHC

GSI, Darmstadt, November 22-24 2010

Description and Main topics

POSTER The main legacy of the HERA experiments are the great precision mesurements of the proton structure function F_2 in a broad range of the Bjorken variable x (down to 10^-5) and of the photon virtuality Q^2 (up to 10^5 GeV^2). They allowed for a precise determination of the parton distribution functions in a proton, using the QCD fits which are based on the linear Dokshitzer-Gribov-Lipatov-Alaterlli-Parisi (DGLAP) evolution equations in the (next)next-to-leading order approximation. However, in the region of small x (i.e. in the large energy limit) new phenomena are expected to occur, triggered by large values of the gluon distribution function. They are termed in general parton saturation and are expected to tame the rapid growth of the gluon distribution with decreasing Bjorken x. In more technical terms, a new phase of QCD is entered in which perturbative methods can still be applied but the resulting evolution equations become nonlinear because of high parton density states.

The most spectacular prediction of the new description is the appearence of a saturation scale and a new scaling law for the proton structure functions, called geometric scaling. Analyzing both inclusive and exclusive data from HERA, it seems that such a scaling has already been observed and saturation scale was determined. However, such a statement, especially in the context of the F_2 measurements, might be controversial since the DGLAP based fits are able to describe the same set of data. Thus, there is a strong need for a carefull examination of the issue of parton saturation looking from as broad as possible perspective. In particular, before the LHC starts to produce relevant data, it is a good time to discuss the present status of parton saturation by considering observables which have already been measured at HERA and RHIC, in order to plan future measurements at the LHC both in pp and AA scattering. In addition, the summary of the current state of the theoretical efforts in the description of this phenomenon, contrasted with the status of the description of relevant processes which uses conventional parton distribution function language, is of the utmost importance.

The main aim of the workshop is to bring together the key players in these developments in order to discuss the status and perspectives of the concept of parton saturation. It will also give young people a chance to get involved in this field of research concerning the basic properties of partonic matter.

It appears that there is not yet a generally accepted consensus on some important issues in the field of high density partonic states with saturation. Therefore, it is planned to reserve several hours for discussion sessions in order to find out how the LHC data can help to establish a more complete and coherent picture of this subject.


François Gelis    Krzysztof Golec-Biernat
IPhT    Institute of Nuclear Physics PAN
CEA/Saclay, France    Cracow, Poland