QCD Phenomenology and small x parton dynamics:

In electron-proton and proton-proton collisions the cross section is calculated as a convolution of the probablity to find a given parton (quark or gluon) in the proton with the partonic cross section for electron-parton or parton-parton scattering. When inclusive quantities like the total cross section at large Q2 or jet production at large enough transverse momenta are investigated, the parton inside the proton can be treated collinear with the proton. This means that the partons have negleglible transverse momentum compared to the transverse momenta of the jets or the large Q2 of the process. However, if differential cross sections are investigated in more detail, these transverse momenta of the incoming partons cannot be neglegted (see J. Collins, H. Jung: Need for fully unintegrated parton densities), and so-called unintegrated parton density functions (uPDFs) need to be considered.

At large enough energies in electron-proton or proton-proton collisions, the longitudinal momenta x of the interaction partons can become very small.  At small values of x the parton density becomes very large and eventually the partons can no longer be treated as independent but they can interact with each other. At these high parton densities, more than one parton can interact with the electron or with the other proton, leading to so-called multiple interactions. These multiple interactions are present in electron-proton processes as well as in proton-proton processes, and they can contribute significantly to jet production and/or also to Higgs production at the LHC. The detailed understanding of these multiple interaction is far from satisfactory. However, measurements at HERA provide the necessary informations: the observation that about 10% of the deep-inelastic cross section comes from diffractive processes  (which is essentially a 2-gluon exchange process, with the two gluons appearing in a color singlet state) gives a clear hint that multiple scattering and diffraction are different sides of the same medal and have to be understood in  a common framework. At the same time, gluon recombination g+g ->g (which is the reverse of gluon splitting g -> g+g) is also related to high parton densities and multiple interactions and diffraction.

Whereas it is important to investigate small x parton dynamics (as described above) the detailed calcualtion and simulation of QCD processes at large scales are also important, not only to have the possibility to observe deviations from the predictions.

Many investiagtions of small x parton dynamics, multiple interactions, and saturation can be performed with the measurements at HERA. These investigations are crucial for the physics reach at LHC. Activities have started for investigations of small x physics at the LHC and ideas are developed to improve the LHC detectors for a full angular coverage.

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The present activities are centered around the following topics:
Group members:
H. Jung
A. Cholewa (PhD):    uPDF determination
M. Deak (PhD):         Forward jets, W/Z production at the LHC
M. von den Driesch (PhD):      PDF4MC with charm
A. Grebenuk (PhD):      Charged particle spectra and uPDFs in CASCADE
A. Knutsson (Postdoc):   Jets at HERA and LHC
K. Kutak (Postdoc):    Saturation and kt-factorisation
Z. Staykova (PhD):   Small x parton dynamics in charm production
F. Samson-Himmelstierna (Diploma):    PDF4MC
N. Sen (PhD):    Forward Jets and multiparton interactions with CASTOR
T. Toll (PhD):            NLO calculations, parton showers and hadronisation (MC@NLO)



If you are interested in any of the above topics, interested for a diploma or PhD thesis, or just for discussion and information, please contact us.