arXiv subjects
Publications and source records attributed to F. Murgia.
We consider possible mechanisms for single spin asymmetries in inclusive Deep Inelastic Scattering (DIS) processes with unpolarized leptons and transversely polarized nucleons. Tests for the effects of non-zero $\bfk_\perp$, for the properties of spin dependent quark fragmentations and for quark helicity conservation are suggested.
We present an approach that generalizes in a natural way the perturbative QCD formalism developed by Brodsky and Lepage for the study of exclusive hadronic processes to the case of $L\neq 0$ mesons. As an application of our approach we consider here the production of meson pairs, involving tensor and pseudotensor mesons, in photon-photon collisions.
Starting from a bound-state model of weakly bound quarks for ($q \bar{q}$) mesons, we derive a formalism for computing the production or decay of such mesons, whatever the value of their internal orbital angular momentum L. That approach appears as a natural generalization of the Brodsky-Lepage formalism (valid only for L=0) that has been widely used in recent years for the computation of exclusive processes in perturbative QCD. We here apply it to the production, in photon-photon collisions, of: i) tensor-meson pairs; ii) pseudotensor-meson pairs; iii) hybrid pairs made of a pion and a pseudotensor meson. The numerical results we obtain allow for some hope of experimentally identifying such pairs, in the charged channels, at high-energy e^+e^- colliders of the next generation, provided integrated luminosities as high as $\approx 10^{40} cm^{-2}$ can be reached.
We present a formalism for studying the exclusive production or decay of mesons with any value of the internal orbital angular momentum L. As an application, we discuss the production of meson pairs (involving tensor and pseudotensor mesons) in photon-photon collisions.
We consider a general formalism, previously developed to describe single spin asymmetries (SSA) in inclusive particle production at moderately large $p_T$ in hadron-hadron high-energy processes. In this approach SSA can originate from non perturbative, universal, quark distribution analyzing powers, provided the quark intrinsic motion is taken into account. A fit of the available data on $p^\uparrow p \to πX$ fixes simplified phenomenological expressions of these quark analyzing powers for the dominant (at large $x_F$) $u$ and $d$ quark contributions. We use the results of the fit as input to provide a possible scenario for SSA in a number of interesting processes (e.g. $p p \to γX$, $p p \to K X$) for which experimental results are or will soon be available.
The production of hadrons in polarized lepton-nucleon deep inelastic scattering is discussed. The helicity density matrix of the hadron is computed within the QCD hard scattering formalism and its elements are shown to yield information on the spin structure of the nucleon and the spin dependence of the quark fragmentation process. The case of $ρ$ vector mesons is considered in more detail and estimates are given.
The physics of polarized nucleon--nucleon collisions originating from an internal polarized target in the HERA proton beam is investigated. Based on 240 pb^-1 integrated luminosity at 40 GeV c.m. energy, statistical sensitivities are given over a wide (x_F, p_T) range for a variety of inclusive and exclusive final states. By measuring single spin asymmetries unique information can be obtained on higher twist contributions and their p_T-dependence. From double spin asymmetries in both photon and J/psi production it appears possible to measure the polarized gluon distribution in the range 0.1<x_gluon<0.4 with a good statistical accuracy.
We discuss the production of polarized hadrons in polarized lepton nucleon interactions and show that the helicity density matrix of the hadron, when measurable, can give information on the spin structure of the nucleon and the spin dependence of the quark fragmentation process. Single spin asymmetries in the $\ell N^\uparrow \to hX$ process are also briefly discussed.
We discuss the production of hadrons in polarized lepton nucleon interactions and in the current jet fragmentation region; using the QCD hard scattering formalism we compute the helicity density matrix of the hadron and show how its elements, when measurable, can give information on the spin structure of the nucleon and the spin dependence of the quark fragmentation process. The cases of $ρ$ vector mesons and $Λ$ baryons are considered in more details and, within simplifying assumptions, some estimates are given.
Within the QCD-improved parton model and assuming the factorization theorem to hold in the helicity basis and for higher twist contributions, we show how non zero single spin asymmetries in hadron-hadron high energy and moderately large $p_T$ inclusive processes can be obtained, even in massless perturbative QCD, 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 asymmetry observed in inclusive pion production in $p\,p$ collisions.
It is shown how the single spin asymmetry observed in inclusive pion production is related, in the helicity basis, to the imaginary part of the product of two different distribution amplitudes, rather than to the usual quark and gluon distribution functions; there is then no reason why it should be zero even in massless perturbative QCD, provided the quark intrinsic motion is taken into account. A simple model is constructed which reproduces the main features of the data.
Massless perturbative QCD forbids, at leading order, the exclusive annihilation of proton-antiproton into some charmonium states, which, however, have been observed in the $p\bar p$ channel, indicating the significance of higher order and non perturbative effects in the few GeV energy region. The most well known cases are those of the $^1S_0$ ($η_c$) and the $^1P_1$. The case of the $^1D_2$ is considered here and a way of detecting such a state through its typical angular distribution in the radiative decay $^1D_2 \to$ $^1P_1 γ$ is suggested. Estimates of the branching ratio $BR(^1D_2 \to p\bar p)$, as given by a quark-diquark model of the nucleon, mass corrections and an instanton induced process are presented.
We compute the helicity density matrix of $ρ$ vector mesons produced in the two-body decays of $χ_{c2}$'s in the framework of perturbative QCD, allowing for mass corrections. The $χ_{c2}$'s are inclusively produced in hadronic interactions via gluon fusion and are shown to be strongly polarized. Both the case in which the light quarks inside the final mesons are treated as massless current quarks and the case in which they are seen as constituent massive ones are considered. Our results show significant differences between the two cases; the measurement of the $ρ$ helicity density matrix should allow a test of the importance of these mass effects.
We calculate the spin density matrix of the hadron $h$ created via quark fragmentation in the process $e^-e^+ \to q\bar q \to h + X$. In the case of $h=Λ$ the experimental data could possibly elucidate the problem of $s$-quark contribution to the spin of the $Λ$-hyperon, to be compared with the case of the proton (``spin crisis''). Generally we find that for hadrons with spin 1/2 the parton description, using only probabilities, works much better than for vector particles, since in the former case the non diagonal matrix elements of the hadron spin density matrix are suppressed by a small factor $p_T/(z\sqrt s)$, where $p_T$ is the hadron transverse momentum inside the jet.