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Register a new userTMDs and SSAs in hadronic interactions
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E.C. Aschenauer
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We present an overview on the current experimental and phenomenological status of transverse single spin asymmetries (tSSAs) in proton-proton collisions. In particular, we focus on large-$p_T$ inclusive pion, photon, jet, pion-jet production and Drell-Yan processes. For all of them theoretical estimates are given in terms of a generalised parton model (GPM) based on a transverse momentum dependent (TMD) factorisation scheme. Comparisons with the corresponding results in a collinear twist-3 formalism and in a modified GPM approach are also made. On the experimental side, a selection of the most interesting and recent results from RHIC is presented.
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We present a short overview on transverse momentum dependent parton distribution and fragmentation functions, giving their partonic interpretation and ways to access them. We then discuss the issue of their universality and its connection to factorization in perturbative QCD.
HEP event generators aim to describe high-energy collisions in full exclusive detail. They combine perturbative matrix elements and parton showers with dynamical models of less well-understood phenomena such as hadronization, diffraction, and the so-called underlying event. We briefly summarise some of the main concepts relevant to the modelling of soft/inclusive hadron interactions in MC generators, in particular PYTHIA, with emphasis on questions recently highlighted by LHC data.
The NA61/SHINE collaboration has recently published high precision data on production of $pi^pm$ and $K^pm$ mesons, protons, antiprotons and $Lambda$ hyperons in ${rm pp}$ interactions at 20, 31, 40, 80 and 158 GeV/c, and in ${rm pC}$ interactions at 31 GeV/c. The collaboration also presented experimental data on production of particles - $pi^pm$, $K^pm$, $p^pm$, $rho^0$, $omega$ and $K^{*0}$ in $pi^-{rm C}$ collisions at 158 and 350 GeV/c. The collaboration has compared these data with various Monte Carlo model calculations: UrQMD, EPOS, GiBUU, and others. All of the models have various problems. The latest version of the FTF (Fritiof) model of Geant4 solves most of these problems. In the FTF model, we have improved the fragmentation of quark-gluon strings with small masses and introduced dependencies of probabilities of strange mesons and baryon-antibaryon pairs creation on string masses. Due to these changes, we describe the data of the NA61/SHINE collaboration on particle production in ${rm pp, pC}$, and $pi^-{rm C}$ interactions. The improved Geant4 FTF model also well reproduces experimental data on inclusive cross sections of $Lambda, bar{Lambda}$ and $K^{0}$ production in antiproton-proton interactions at various energies. The modified FTF model allows one to simulate realistic processes with two particle productions - $bar{p}p rightarrow Lambda bar{Lambda}$, $bar{p}p rightarrow K^{+} K^{-}$, $bar{p}p rightarrow Lambda bar{Sigma}$, and $bar{p}p rightarrowSigma bar{Sigma}$, which will be studied in the future by the PANDA experiment at FAIR (GSI, Germany).
We show that the large corrections due to final state interactions (FSI) in the D^+to pi^-pi^+pi^+, D^+_sto pi^-pi^+pi^+, and D^+to K^-pi^+pi^+ decays can be accounted for by invoking scattering amplitudes in agreement with those derived from phase shifts studies. In this way, broad/overlapping resonances in S-waves are properly treated and the phase motions of the transition amplitudes are driven by the corresponding scattering matrix elements determined in many other experiments. This is an important step forward in resolving the puzzle of the FSI in these decays. We also discuss why the sigma and kappa resonances, hardly visible in scattering experiments, are much more prominent and clearly visible in these decays without destroying the agreement with the experimental pipi and Kpi low energy S-wave phase shifts.
The QCD improved parton model is a very successful concept to treat processes in hadronic interactions, whenever large partonic transverse momenta are involved. However, cross sections diverge in the limit p_T -> 0, and the usual treatment is the definition of a lower cutoff p_T_min, such that processes with a smaller p_T -- so-called soft processes -- are simply ignored, which is certainly not correct for example at RHIC energies. A more consistent procedure amounts to introduce a technical parameter Q_0^2, referred to as soft virtuality scale, which is nothing but an artificial borderline between soft and hard physics. We will discuss such a formalism, which coincides with the improved parton model for high p_T processes and with the phenomenological treatment of soft scattering, when only small virtualities are involved. The most important aspect of our approach is that it allows to obtain a smooth transition between soft and hard scattering, and therefore no artificial dependence on a cutoff parameter should appear.
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