15703012
Non-standard time reversal for particle multiplets and the spin-flavor structure of hadrons
Abstract We show that a system of quarks interacting with chiral fields provides a physical representation of a “non-standard” time reversal for particle multiplets proposed by Weinberg. As an application, we argue that, if the internal structure of hadrons is described by a chiral lagrangian, the so-called time-reversal-odd quark distribution functions might not be forbidden by time-reversal invariance.
18925545
Flavor Asymmetry of the Polarized Light Sea: Models vs. Data
The flavor asymmetry of the polarized light sea, ∆ū − ∆d̄, discriminates between different model calculations of helicity densities. We show that the chiral chromodielectric model, differently from models based on a 1/Nc expansion, predicts a small value for this asymmetry, what seems in agreement with preliminary HERMES data. 1. Following the discovery that the quark contribution to the spin of the nucleon is surprisingly small [1], a considerable experimental effort was made to elucidate the details of the helicity densities of valence, sea and glue (for reviews on longitudinal spin physics, see [2]). On the other theoretical side, many models have been studied and most of them reproduce the gross features of the spin content of the nucleon, namely the g1 structure function and the singlet distribution ∆Σ = ∑ f(∆f+∆f̄). In order to discriminate between the models, one has to look at the quark and antiquark helicity densities for each separate flavor. This has been a lacking piece of information until last year, when the HERMES Collaboration at DESY, measuring semi-inclusive deep inelastic scattering, succeeded in extracting the polarizations of u, ū, d, d̄, s+ s̄ [3]. Thus, HERMES experiment opened for the first time the possibility to test model results against data. To this purpose, non-singlet distributions are especially interesting because their evolution does not involve the polarized gluon density: as a consequence, they can be predicted in a more reliable way, without any extra assumption on the constituent polarized glue. In what follows, our attention will be directed to the isoscalar and isovector combinations of antiquark densities, f̄+(x) = ū(x) + d̄(x) , f̄−(x) = ū(x) − d̄(x) , (1) ∆f̄+(x) = ∆ū(x) + ∆d̄(x) , ∆f̄−(x) = ∆ū(x) − ∆d̄(x) . (2) The two classes of models most widely used for computing quark distributions, i.e. the chiral quark soliton model (CQSM, based on a 1/Nc expansion) and the bag-like confinement models – including the chiral chromodielectric model (CDM) –, make very different predictions for the relative weight of ∆f̄− and ∆f̄+, and of f̄+ and f̄−. In particular, in the 1/Nc expansion, ∆f̄− is a leading quantity compared to ∆f̄+ and f̄−, hence it is expected to be large (in absolute value) and to satisfy the inequalities (which may be, as a matter of fact, strong inequalities) |∆ū− ∆d̄| > |ū− d̄| , (3) |∆ū− ∆d̄| > |∆ū+ ∆d̄| . (4)
46109082
Softening of the equation of state of matter at large densities and temperatures: chiral-symmetry restoration versus quark deconfinement.
We discuss two models in which a softening of the equation of state takes place due to the appearance of new degrees of freedom. The first is a hadronic model in which the softening is due to chiral symmetry restoration. In the second model the softening is associated with the formation of clusters of quarks in the mixed phase. We show that in the first case the bulk modulus is mainly dependent on the density, while in the mixed-phase model the bulk modulus strongly depends on the temperature and it is not vanishing due to the presence of two conserved charges, the baryon and the isospin one.
119481339
Transverse polarisation of quarks in hadrons
Abstract We review the present state of knowledge regarding the transverse polarisation (or transversity) distributions of quarks. After some generalities on transverse polarisation, we formally define the transversity distributions within the framework of a classification of all leading-twist distribution functions. We describe the QCD evolution of transversity at leading and next-to-leading order. A comprehensive treatment of non-perturbative calculations of transversity distributions (within the framework of quark models, lattice QCD and QCD sum rules) is presented. The phenomenology of transversity (in particular, in Drell–Yan processes and semi-inclusive leptoproduction) is discussed in some detail. Finally, the prospects for future measurements are outlined.
17601537
Gravitational wave bursts induced by r-mode spin-down of hybrid stars
We show that sudden variations in the composition and structure of a hybrid star can be triggered by its rapid spin-down, induced by r-mode instabilities. The discontinuity of this process is due to the surface tension between hadronic and quark matter and in particular to the overpressure needed to nucleate new structures of quark matter in the mixed phase. The consequent mini-collapses in the star can produce highly energetic gravitational wave bursts. The possible connection between the predictions of this model and the burst signal found by EXPLORER and NAUTILUS detectors during 2001 is also investigated.
120683407
Structure of compact stars with color superconductivity quark phase
We study the effects of color superconductivity on the structure of compact stars, and we show that it is possible to satisfy most of recent observational boundaries on masses and radii if a diquark condensate forms in a hybrid or in a quark star. Furthermore, we find that the constraints on masses and radii of the compact stellar objects put rather stringent limits on the parameter values of the nuclear equation of state.
119190599
Synchrotron oscillation damping by beam-beam collisions in DAFNE
In DA{\Phi}NE, the Frascati e+/e- collider, the crab waist collision scheme has been successfully implemented in 2008 and 2009. During the collision operations for Siddharta experiment, an unusual synchrotron damping effect has been observed. Indeed, with the longitudinal feedback switched off, the positron beam becomes unstable with beam currents in the order of 200-300 mA. The longitudinal instability is damped by bringing the positron beam in collision with a high current electron beam (~2A). Besides, we have observed a shift of \approx 600Hz in the residual synchrotron sidebands. Precise measurements have been performed by using both a commercial spectrum analyzer and the diagnostics capabilities of the DA{\Phi}NE longitudinal bunch-by-bunch feedback. This damping effect has been observed in DA{\Phi}NE for the first time during collisions with the crab waist scheme. Our explanation is that beam collisions with a large crossing angle produce a longitudinal tune shift and a longitudinal tune spread, providing Landau damping of synchrotron oscillations.