arXiv subjects
Publications and source records attributed to Francesco Murgia.
We give a short overview of the phenomenology of azimuthal and transverse single spin asymmetries in (un)polarized high-energy hadronic collisions. We briefly summarize a transverse momentum dependent, generalized parton model approach to these polarization phenomena, and discuss some of its applications. Finally, open points and future developments will be outlined.
The 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 transverse momentum 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.
A phenomenological description of single transverse spin effects in hadron-hadron inclusive processes is proposed, assuming a generalized factorization scheme and pQCD hard interactions. The transverse momentum, k_T, of the quarks inside the hadrons and of the hadrons relatively to the fragmenting quark, is taken into account in distribution and fragmentation functions, and leads to possible non zero single spin asymmetries. The role of k_T and spin dependent quark fragmentations -- the so-called Collins effect -- is investigated in details in p(transv. polarized)-p -> pion + X processes: it is shown how the experimental data could be described, obtaining an explicit expression for the spin asymmetry of a polarized fragmenting quark, on which some comments are made. Predictions for other processes, possible further applications and experimental tests are discussed.
We consider several single spin asymmetries in inclusive, transversely polarized proton(antiproton) - proton processes as higher twist QCD contributions, taking into account spin and intrinsic transverse momentum effects in the quark distribution functions. This approach has been previously applied to the description of the single spin asymmetries observed in transversely polarized proton - proton -> pion + X reactions and all its parameters fixed: we give here predictions for new processes, which agree with experiments for which data are available, and suggest further possible measurements.
We discuss possible measurements and origins of single spin asymmetries in DIS and of some unusual spin properties of vector mesons produced in lepton- nucleon, photon-nucleon and photon-photon interactions. Such effects have already been observed in other processes.
Final state quark-antiquark interactions may give origin to non zero values of the off-diagonal element rho_{1,-1} of the helicity density matrix of vector mesons produced in e+e- annihilations, as confirmed by recent OPAL data on phi and D^*'s. Predictions are given for rho_{1,-1} of several mesons produced at large z and small p_T, i.e. collinear with the parent jet; the values obtained for phi and D^* are in agreement with data.
We study a possible origin of single spin asymmetries in the large $p_T$ inclusive production of hadrons in the scattering of unpolarized protons or leptons on transversely polarized nucleons. Such asymmetry is related to the single-spin asymmetry for the process $\p^\uparrow \to q+X$ or, equivalently, to the off-diagonal matrix element of the quark density operator $\bar ψγ^+ ψ$; this quantity need not be zero if transverse momentum and spin-isospin interactions are taken into account. The different signs for the large single spin asymmetries observed in $\p^\uparrow p \to π^+ X$ and $\p^\uparrow p \to π^- X$ can be explained as a consequence of chiral symmetry. Crucial tests of the model are suggested.
Stimulated by the experimental observation, made by the E760 Collaboration at Fermilab, of the $^1\!P_1$ state of charmonium resonantly formed in $p\,\bar p$ annihilation, we perform a calculation of the decay width for the $^1\!P_1\to p\,\bar p$ process. To this end, we employ a phenomenological model which adds constituent quark mass corrections to the usual massless QCD models for exclusive processes. For massless models, in fact, the process under consideration is forbidden by the so-called helicity selection rules, while it is allowed in our extended model. We find $Γ(^1\!P_1\to p\,\bar p)$ to be in the range $1-10$ eV. We also compare our results with previous, indirect estimates, based on QCD multipole expansion models.
The general formalism to describe single spin asymmetries in hadron-hadron high energy and large $p_T$ inclusive production within the QCD-improved parton model and assuming the factorization theorem to hold for higher twist contributions is discussed. Non zero values of the single spin asymmetries originate from and reveal non perturbative universal properties of quarks: the quark distribution and fragmentation analysing powers, which need not be zero, provided the quark intrinsic motion is taken into account. A simple model is constructed which reproduces the main features of the data on the single spin asymmetries observed in inclusive pion production in $pp$ collisions.
Using a non--relativistic gluon bound--state model for glueballs (G), we compute the subprocess $q\,\bar q\, \to\, G\,π$, and we therefrom derive the yield of the overall reaction $p\,\bar p\, \to\, G\,πX$, assuming the glueball and the pion to be emitted with their transverse momenta large, opposite and approximately equal. Numerical results are presented in the form of $p_T$ spectra for various glueball candidates and their possible quantum states, assuming those particles to be produced, in the type of reactions here considered, at high--energy $p\,\bar p$ colliders such as the CERN SppS.
We compute the helicity density matrix of $ρ$ vector mesons produced in the two-body decays of polarized $χ_{c2}$'s in the framework of massless perturbative QCD. The $χ_{c2}$'s are either exclusively or inclusively produced in $p \, \bar p$ or $p \, p$ interactions, via quark-antiquark annihilation or gluon fusion. Our results show unambiguous significant differences depending on the choice of the $ρ$ distribution amplitudes and allow to discriminate between different proposed models.
We consider mass correction effects on the polar angular distribution of a baryon--antibaryon pair created in the chain decay process $e^-e^+ \to J/ψ\to B\bar B$, generalizing a previous analysis of Carimalo. We show the relevance of the features of the baryon distribution amplitudes and estimate the electromagnetic corrections to the QCD results.