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Double parton scattering, diffraction and effective cross section

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 Added by Daniele Treleani
 Publication date 2007
  fields
and research's language is English




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The rates of multiparton collisions in high energy hadronic interactions provide information on the typical transverse distances between partons in the hadron structure. The different configurations of the hadron in transverse space are, on the other hand, at the origin of hadron diffraction. The relation between the two phenomena is exploted in an eikonal model of hadronic interactions.



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We present a calculation of the effective cross section $sigma_{eff}$, an important ingredient in the description of double parton scattering in proton-proton collisions. Our theoretical approach makes use of a Light-Front quark model as framework to calculate the double parton distribution functions at low-resolution scale. QCD evolution is implemented to reach the experimental scale. The obtained $sigma_{eff}$, when averaged over the longitudinal momentum fractions of the interacting partons, $x_i$, is consistent with the present experimental scenario. However the result of the complete calculation shows a dependence of $sigma_{eff}$ on $x_i$, a feature not easily seen in the available data, probably because of their low accuracy. Measurements of $sigma_{eff}$ in restricted $x_i$ regions are addressed to obtain indications on double parton correlations, a novel and interesting aspect of the three dimensional structure of the nucleon.
We examine present data for double parton scattering at LHC and discuss their energy dependence from its earliest measurements at the ISR. Different models for the effective cross-section are considered and their behavior studied for a variety of selected final states. We point out that data for pp ->4 jets or pp -> quarkonium pair indicate the effective cross-section to increase with energy. We compare this set of data with different models, including one inspired by our soft gluon resummation model for the impact parameter distribution of partons.
We study the $W^+W^-$ and $Z^0Z^0$ electroweak boson production in double parton scattering using QCD evolution equations for double parton distributions. In particular, we analyze the impact of splitting terms in the evolution equations on the double parton scattering cross sections. Unlike the standard terms, the splitting terms are not suppressed for large values of the relative momentum of two partons in the double parton scattering. Thus, they play an important role which we discuss in detail for the single splitting contribution to the cross sections under the study.
The observation of double parton collisions by CDF has provided the first direct information on the structure of the proton in transverse space. The actual quantity which has been measured is the `effective cross section $sigma_{eff}$, which is related to the transverse size of the region where hard interactions are localized. The actual value which has been measured is sizably smaller than naively expected and it is an indication of important correlation effects in the many-body parton distribution of the proton. We discuss the problem pointing out a possible source of correlations in the proton structure, which could have a significant effect on the value of $sigma_{eff}$.
We develop a factorization framework to compute the double differential cross section in soft drop groomed jet mass and groomed jet radius. We describe the effective theories in the large, intermediate, and small groomed jet radius regions defined by the interplay of the jet mass and the groomed jet radius measurement. As an application we present the NLL$$ results for the perturbative moments that are related to the coefficients $C_1$ and $C_2$ that specify the leading hadronization corrections up to three universal parameters. We compare our results with Monte Carlo simulations and a calculation using the coherent branching method.
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