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In this article, we review some of the complexities of jet algorithms and of the resultant comparisons of data to theory. We review the extensive experience with jet measurements at the Tevatron, the extrapolation of this acquired wisdom to the LHC and the differences between the Tevatron and LHC environments. We also describe a framework (SpartyJet) for the convenient comparison of results using different jet algorithms.
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In hadron interactions at the LHC energies, the reflective scattering mode starts to play a role which is expected to be even a more significant beyond the energies of the LHC. This new but still arguable phenomenon implies a peripheral dependence of the inelastic probability distribution in the impact parameter space and asymptotically evolving to the black ring. As a consequence, the straightforward extension to hadrons of the centrality definition adopted for nuclei needs to be modified.
Hadron production in single and central diffraction dissociation is studied in a model which includes soft hadron interaction as controlled by a supercritical pomeron parametrization and hard diffraction. Within this model, particle production in collisions with pomerons exhibit properties like multiple soft interactions and multiple minijets, quite similar to hadron production in non-diffractive hadronic collisions at high energies. However, important differences occur in transverse momentum jet and hadron distributions. It is shown that the model is able to describe data from the CERN-SPS collider and from the HERA collider. Model predictions are presented for single and central diffraction at TEVATRON.
We investigate lepton-pair production in hard exclusive hadron-hadron collisions. We consider a double handbag (DH) mechanism in which the process amplitude factorizes in hard subprocesses, qq -> qq gamma* and qg -> qg gamma*, and in soft hadron matrix elements parameterized as generalized parton distributions (GPDs). Employing GPDs extracted from exclusive meson electroproduction, we present predictions for the lepton-pair cross section at kinematics typical for the LHC, NICA and FAIR. It turns out from our numerical studies that the quark-gluon subprocess dominates by far, the quark-quark (antiquark) subprocesses are almost negligible.
Parton distribution functions (PDFs) describe the structure of hadrons as composed of quarks and gluons. They are needed to make predictions for short-distance processes in high-energy collisions and are determined by fitting to cross section data. We review definitions of the PDFs and their relations to high-energy cross sections. We focus on the PDFs in protons, but also discuss PDFs in nuclei. We review in some detail the standard statistical treatment needed to fit the PDFs to data using the Hessian method. We discuss tests that can be used to critically examine whether the assumptions are indeed valid. We also present some ideas of what one can do in the case that the tests indicate that the assumptions fail.
We calculate inclusive hadron productions in pA collisions in the small-x saturation formalism at one-loop order. The differential cross section is written into a factorization form in the coordinate space at the next-to-leading order, while the naive form of the convolution in the transverse momentum space does not hold. The rapidity divergence with small-x dipole gluon distribution of the nucleus is factorized into the energy evolution of the dipole gluon distribution function, which is known as the Balitsky-Kovchegov equation. Furthermore, the collinear divergences associated with the incoming parton distribution of the nucleon and the outgoing fragmentation function of the final state hadron are factorized into the splittings of the associated parton distribution and fragmentation functions, which allows us to reproduce the well-known DGLAP equation. The hard coefficient function, which is finite and free of divergence of any kind, is evaluated at one-loop order.
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