1,721,076 research outputs found
Transversity
No full textTransversity distribution, together with the unpolarized and the helicity distributions, represents a basic piece of information on the internal structure of nucleons. Its peculiar property of being a chiral-odd quantity implies that it can be accessed only in combination with another
chiral-odd partner, making it much harder to measure. In this talk I will review its properties and describe the most relevant and feasible ways to access it. Recent phenomenological extractions,
their highlights and limitations, as well as perspectives are discussed
Transversity: theory and phenomenology
No full textThe distribution of transversely polarized quarks inside a transversely polarized nucleon, known as transversity, encodes a basic piece of information on the nucleon structure, sharing the same status with the more familiar unpolarized and helicity distributions. I will review its properties and discuss different ways to access it, with highlights and limitations. Recent phenomenological extractions and perspectives are also presented
TMDs: Universality and Factorization
Full text linkWe 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
Target fragmentation of the nucleon at high energies
No full textWe calculate target fragmentation in pp --> nX and gamma p --> nX reactions in the meson cloud picture of the nucleon. The pp --> nX reaction is used to fix the pn pi(+) form factor for two different models. We take into account the possible destruction of the residual neutron by the projectile. Using the form factor from the hadronic reaction we calculate photoproduction and small x(Bj) electroproduction of forward neutrons at HERA. In photoproduction we observe about the same amount of absorption as in the hadronic reaction. For deep inelastic events screening is found to be negligible. The signature of this color transparency on the nucleon is a shift of the dN/dE(n) distribution to higher neutron energies for photofragmentation
Target fragmentation in pp, ep and gamma p collisions at high energies
No full textWe calculate target fragmentation in pp --> nX and gamma p --> nX reactions in the meson cloud picture of the nucleon. The pp --> nX reaction is used to fix the pn pi(+) form factor for three different models. We take into account the possible destruction of the residual neutron by the projectile. Using the form factor from the hadronic reaction we calculate photoproduction and, small x(Bj) electroproduction of forward neutrons at HERA. Here the q-qbar dipoles in the photon can rescatter on the residual neutron. In photoproduction we observe slightly less absorption than in the hadronic reaction. For deep inelastic events (Q(2) > 10 GeV2) screening is weaker but still present at large Q2. The signature for this absorptive rescattering: is a shift of the d sigma/dE(n) distribution to higher neutron energies for photofragmentation
Azimuthal and single spin asymmetries in hard scattering processes
No full textIn this article we review the present understanding of azimuthal and single spin asymmetries for inclusive and semi-inclusive particle production in unpolarized and polarized hadronic collisions at high energy and moderately large transverse momentum. After summarizing the experimental information available, we discuss and compare the main theoretical approaches formulated in the framework of perturbative QCD. We then present in some detail a generalization of the parton model with inclusion of spin and intrinsic transverse momentum effects. In this context, we extensively discuss the phenomenology of azimuthal and single spin asymmetries for several processes in different kinematical configurations. A comparison with the predictions of other approaches, when available, is also given. We finally emphasize some relevant open points and challenges for future theoretical and experimental investigation
Parton intrinsic motion in inclusive particle production: unpolarized cross sections, single spin asymmetries, and the Sivers effect
No full textThe relevance of intrinsic (or primordial) transverse momentum of partons in the inclusive production of particles at high energy and moderately large p(T) has been known for a long time, beginning with Drell-Yan and diphoton processes, and continuing with photon and meson production in hadronic collisions. In view of its renewed interest in the context of polarized processes and single spin asymmetries we perform, in the framework of perturbative QCD with the inclusion of spin and k(perp)-effects, a detailed analysis of several such processes in different kinematical situations. We show that the inclusion of these effects leads, at the level of accuracy reachable in this approach, to an overall satisfactory agreement between theoretical predictions and experimental unpolarized data, thus giving support to the study of spin effects and single spin asymmetries within the same scheme. We present results for transverse single spin asymmetries, generated by the so-called Sivers effect, in inclusive pion and photon production in proton-proton collisions. We compare our results with the available experimental data and with previous results obtained using simplified versions of this approach
Soft and hard Pomeron in the structure function of the proton at low x and low Q(2)
No full textWe study inclusive electroproduction on the proton at low x and low Q(2) using a soft and a hard Pomeron. The contribution of the soft Pomeron is based on the Stochastic Vacuum Model; in which a nonperturbative dipole-dipole cross section can be calculated by means of a gauge invariant gluon field strength correlator. To model the hard Pomeron exchange we phenomenologically extend the leading order evolution of a power-behaved structure function, F2 proportional to x^(-lambda), proposed by Lopez and Yndurain. This extension allows to consider both the case Q(2) = 0 and the region of higher Q(2) on the basis of the same parametrization. A good simultaneous fit to the data on F2 and on the cross section sigma(gamma p) of real photoproduction is obtained for lambda = 0.37. With four parameters we achieve a chi(2)/d.o.f. = 0.98 for 222 data points. In addition, we use our model of the inclusive gamma*p interaction to compute the longitudinal structure function F(L)
Azimuthal distributions of pions inside jets at RHIC
No full textWe evaluate the azimuthal asymmetries for the distributions of leading pions inside a jet, produced in high-energy proton-proton collisions, in kinematic configurations currently under active investigation at RHIC. Adopting a transverse momentum dependent approach, which assumes the validity of a perturbative QCD factorization scheme and takes into account all the spin and intrinsic parton motion effects, we show how the main mechanisms underlying these asymmetries, namely the Sivers and the Collins effects, can be disentangled. Furthermore, we consider the impact of color-gauge invariant initial and final state interactions and suggest a method for testing the universality properties of the Sivers function for quarks
Azimuthal Distributions of Pions inside a Jet in Hadronic Collisions
No full textUsing a generalized parton model approach including spin and intrinsic parton motion effects, and assuming the validity of factorization for large pT jet production in hadronic collisions, we study the azimuthal distribution around the jet axis of leading pions, produced in the jet fragmentation process. We identify the observable leading-twist azimuthal asymmetries, which are generated by all the physically allowed combinations of transverse momentum dependent (TMD) parton distribution and fragmentation functions. In particular, we show how one can isolate the Collins and Sivers contributions, and suggest a test of the process dependence of the Sivers function by considering the effect of color-gauge invariant initial and final state interactions
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