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Register a new userInterplay of soft and perturbative correlations in multiparton interactions at central rapidities
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We study the role of soft/nonperturbative correlations in the multi parton interactions in the central kinematics relevant for double parton scattering (DPS) and underlying event (UE) measurements at ATLAS and CMS. We show that the effect of soft correlations is negligible for DPS regime (typical transverse momenta larger than 10-20 GeV), but may be important for UE (several GeV scale). The characteristic scale where soft correlations become important increases with decrease of $x$ (energy increase) leading to approximately constant effs at small x.
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Photon-jet azimuthal correlations in proton-nucleus collisions are a promising tool for gaining information on the gluon distribution of the nucleus in the regime of non-linear color fields. We compute such correlations from the process $g+Ato q bar{q} gamma$ in the rapidity regime where both the projectile and target light-cone momentum fractions are small. By integrating over the phase space of the quark which emits the photon, subject to the restriction that the photon picks up most of the transverse momentum (to pass an isolation cut), we effectively obtain a $g+Ato q gamma$ process. For nearly back-to-back photon-jet configurations we find that it dominates over the leading order process $q+Ato q gamma$ by two less powers of $Q_perp / Q_S$, where $boldsymbol{Q}_perp$ and $Q_S$ denote the net photon-jet pair momentum and the saturation scale of the nucleus, respectively. We determine the transverse momentum dependent gluon distributions involved in $g+Ato q gamma$ and the scale where they are evaluated. Finally, we provide analytic expressions for $langlecos nphirangle$ moments, where $phi$ is the angle between $boldsymbol{Q}_perp$ and the average photon-jet transverse momentum $tilde{boldsymbol{P}}_perp$, and first qualitative estimates of their transverse momentum dependence.
We derive expressions for the cross section of the multiparton interactions based on the analysis of the relevant Feynman diagrams. We express the cross sections through the double (triple, ...) generalized parton distributions (GPDs). In the mean field approximation for the double GPDs the answer is expressed through the integral over two gluon form factor which was measured in the exclusive DIS vector meson production.We explain under what conditions the derived expressions correspond to an intuitive picture of hard interactions in the impact parameter representation. The mean field approximation in which correlations of the partons are neglected fail to explain the data, while pQCD induced correlation enhance large $p_perp$ and $ 0.001 < x < 0.1$ typically enhance the cross section by a factor of 1.5 -- 2 explaining the current data. We argue that in the small x kinematics ($10^{-4} le x le 10^{-3}$) where effects of perturbative correlations diminish, the nonperturbative mechanism kicks in and generates positive correlations comparable in magnitude with the perturbative ones. We explain how our technique can be used for calculations of MPI in the proton - nucleus scattering. The interplay of hard interactions and underlying event is discussed, as well as different geometric pictures for each of MPI mechanisms-pQCD, nonperturbative correlations and mean field. Predictions for value of effs for various processes and a wide range of kinematics are given. We show that together different MPI mechanisms give good description of experimental data, both at Tvatron, and LHC, including the central kinematics studied by ATLAS and CMS detectors, and forward (heavy flavors) kinematics studied by LHCb.
In this contribution we discuss the inclusive production of jets in central regions of rapidity in the context of $k_T$-factorization at next-to-leading order (NLO). We work in the Regge limit of QCD and use the NLO BFKL results. A jet cone definition is proposed together with a phase-space separation into multi-Regge and quasi-multi-Regge kinematics. We discuss scattering of highly virtual photons, with a symmetric energy scale to separate the impact factors from the gluon Greens function, and hadron-hadron collisions, with a non-symmetric scale choice.
Surprisingly enough, the ratio of elastic to inelastic cross sections of proton interactions increases with energy in the interval correspond- ing to ISR - LHC (i.e. from 10 GeV to 10 TeV). That leads to special features of their spatial interaction region at these and higher ener- gies. Within the framework of some phenomenological models, we show how the particular ranges of the transferred momenta measured in elastic scattering experiments expose the spatial features of the in- elastic interaction region according to the unitarity condition. The difference between their predictions at higher energies is discussed. The notion of central and peripheral collisions of hadrons is treated in terms of the impact parameters description. It is shown that the shape of the differential cross section in the diffraction cone is mostly determined by collisions with intermediate impact parameters. Elastic scattering at very small transferred momenta is sensitive to peripheral processes with large impact parameters. The role of central collisions in formation of the diffraction cone is less significant.
The transverse structure of the nucleon as probed in hard exclusive processes plays critical role in the understanding of the structure of the underlying event in hard collisions at the LHC, and multiparton interactions. We summarize results of our recent studies of manifestation of transverse nucleon structure in the hard collisions at the LHC, new generalized parton distributions involved in multiparton interactions, presence of parton fluctuations. The kinematic range where interaction of fast partons of the projectile with the target reach black disk regime (BDR) strength is estimated. We demonstrate that in the BDR postselection effect leads to effective fractional energy losses. This effect explains regularities of the single and double forward pion production in $ dAu$ collisions at RHIC and impacts on the forward physics in $pp$ collisions at the LHC.
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