arXiv subjects
Publications and source records attributed to Francesco Giacosa.
We calculate low-energy meson decay processes and pion-pion scattering lengths in a two-flavour linear sigma model with global chiral symmetry, exploring the scenario in which the scalar mesons $f_0$(600) and $a_0$(980) are assumed to be $\bar q q$ states.
We compute the two-point correlation of the energy density for the massless mode in deconfining SU(2) Yang-Mills thermodynamics and point towards a possible application for the physics of cold, dilute, and stable clouds of atomic hydrogen within the Milky Way.
We study the line shapes of radiative $ϕ$-decays involving virtual $f_{0}(980)$ and $a_{0}(980)$ mesons which decay, via derivative couplings, to $π^{0}π^{0}$ and $π^{0}η$ respectively. After developing the formalism for derivative interactions at one-loop level, we show that they can reproduce the measured peaked line shapes of $ϕ$-decays without including kaon loops.
In this Letter we investigate the possible emission of low-energy electron neutrinos and electron-positron pairs of anomalously large multiplicity in close-to-central $pp$ collisions at LHC. The scenario is based on confining SU(2) Yang-Mills dynamics of Hagedorn temperature $\sim m_e=511 $keV being responsible for the emergence of the lightest lepton family and the weak interactions of the Standard Model. Although cut off by LHC's detectors these electrons-positron bursts would be seen indirectly by a large defect energy and thus an anomalously strong decrease of events with interesting high-energy secondaries for increasing $\sqrt{s}$ . This is because the formation of superconducting (preconfining) SU(2) hot-spots `steals' a large fraction of $\sqrt{s}$ subsequently transferring it to a thermal spectrum of electron neutrinos, electrons, and positrons liberated through evaporation. We thus propose the detection of electrons and positrons of kinetic energy $\sim m_e$ and photons of energy $\sim 2 m_e$.
We calculate pion-pion scattering lengths and sigma, rho and a1 decay widths from a gauged linear sigma model with two flavours and its globally invariant generalisation.
The two-photon decay of a light scalar quarkonium is evaluated in a local and a nonlocal approaches. It is shown that the two-photon decay, driven by a triangle quark-loop diagram, is smaller than 1 keV for a mass below 0.7 GeV.
We shortly report on the two-photon decay width of the light $σ$-meson interpreted as a quarkonium state. Results are given in dependence on the $σ$-mass and the constituent mass of the light quark. The triangle quark-loop diagram, responsible for the two-photon transition, is carefully evaluated: a term in the transition amplitude, often omitted in literature, results in destructive interference with the leading term. As a result we show that the two-photon decay width of the $σ$ in the quarkonium picture is less than 1 keV for the physical range of parameters.
A model with N species of massless fermions interacting via (microscopic) gravitational torsion in de Sitter spacetime is investigated in the limit N->infinity. The U_V(N)*U_A(N) flavor symmetry is broken dynamically irrespective of the (positive) value of the induced four-fermion coupling. This model is equivalent to a theory with free but massive fermions fluctuating about the chiral condensate. When the fermions are integrated out in a way demonstrated long ago by Candelas and Raine, the associated gap equation together with the Friedmann equation predict that the Hubble parameter vanishes. Introducing a matter sector (subject to a finite gauge symmetry) as a source for subsequent cosmology, the neutral Goldstone field acquires mass by the chiral anomaly, resulting in a Planck-scale axion.
The tetraquark assignement for light scalar states below 1 GeV is discussed on the light of strong decays. The next-to-leading order in the large-N expansion for the strong decays is considered. Mixing with quarkonia states above 1 GeV is investigated within a chiral approach and the inclusion of finite-width effects is taken into account.
In the lattice work by Miller [1,2] and in the work by Zwanziger [3] a linear growth of the trace anomaly for high temperatures was found in pure SU(2) and SU(3) Yang-Mills theories. These results show the remarkable property that the corresponding systems are strong interacting even at high temperatures. We show that within an analytical approach to Yang-Mills thermodynamics this linear rise is obtained and is directly connected to the presence of a temperature-dependent ground state, which describes (part of) the nonperturbative nature of the Yang-Mills system. Our predictions are in approximate agreement with [1,2,3]
A scalar adjoint field is introduced as a spatial average over (anti)calorons in a thermalized SU(2) Yang-Mills theory. This field is associated with the thermal ground state in the deconfining phase and acts as a background for gauge fields of trivial topology. Without invoking detailed microscopic information we study the properties of the corresponding potential, and we discuss its thermodynamical implications. We also investigate the gluon condensate at finite temperature relating it to the adjoint scalar field.
In this work we study the spectral functions of scalar mesons in one- and two-channel cases. When the propagators satisfy the Källen-Lehman representation a normalized spectral function is obtained, allowing to take into account finite-width effects in the evaluation of decay rates. In the one-channel case, suitable to the light sigma and k mesons, the spectral function can deviate consistently from a Breit-Wigner shape. In the two-channel case with one subthreshold channel the evaluated spectral function is well approximated by a Flatte' distribution; when applying the study to the a_0(980) and f_0(980) mesons the tree-level forbidden KK decay is analysed.
Based on the postulate that photon propagation is governed by a dynamically broken SU(2) gauge symmetry (scale $\sim 10^{-4} $eV) we make predictions for temperature offsets due to a low-temperature (a few times the present CMB temperature) spectral anomaly at low frequencies. Temperature offsets are extracted from least-square fits of the anomalous black-body spectra to their conventional counterparts. We discuss statistical errors, compare our results with those obtained from calibration data of the FIRAS instrument, and point out that our predicted offsets are screened by experimental errors given the frequency range used by FIRAS to perform their spectral fits. We also make contact with the WMAP observation by blueshifting their frequency bands. Although our results hint towards a strong dynamical component in the CMB dipole and an explanation of low-$l$ suppression it is important in view of its particle-physics implications that the above postulate be verified/falsified by an independent low-temperature black-body precision experiment.
Based on the trace anomaly for the energy-momentum tensor, an effective theory for the thermodynamics of the deconfining phase, and by assuming the asymptotic behavior to be determined by one-loop perturbation theory we compute the nonperturbative beta function for the fundamental coupling $g$ in SU(2) and SU(3) Yang-Mills theory. With increasing temperature we observe a very rapid approach to the perturbative running. The Landau pole is nonperturbatively screened.
We compute pion-nucleon scattering at tree-level within a gauged linear sigma model which contains the nucleon and its chiral partner. Such an investigation in principle allows to make definite predictions as to whether the main contribution to the nucleon mass comes from the chiral condensate or from the mixing with its chiral partner. We find that there seems to be no set of model parameters that allows for a simultaneous description of all experimentally measured scattering lengths and range parameters. This indicates the need to improve the dynamical ingredients of the model.
A chiral invariant Lagrangian describing the tetraquark-quarkonia interaction is considered at the leading and subleading order in the large-$N_{c}$ expansion. Spontaneous chiral symmetry breaking generates mixing of scalar tetraquark and quarkonia states and non-vanishing tetraquark condensates. In particular, the mixing strength is related to the decay strengths of tetraquark states into pseudoscalar mesons. The results show that scalar states below 1 GeV are mainly four-quark states and the scalars between 1 and 2 GeV quark-antiquark states, probably mixed with the scalar glueball in the isoscalar sector.
We postulate that the U(1)$_Y$ factor of the Standard Model is an effective manifestation of SU(2) gauge dynamics being dynamically broken by nonperturbative effects. The modified propagation properties of the photon at low temperatures and momenta are computed. As a result of strong screening, the presence of a sizable gap in the spectral power of a black body at temperatures $T=5... 20 $K and for low frequencies is predicted: A table-top experiment should be able to discover this gap. If the gap is observed then the Standard Model's mechanism for electroweak symmetry-breaking is endangered by a contradiction with Big-Bang nucleosynthesis. Based on our results, we propose an explanation for the stability of cold, old, dilute, and large clouds of atomic hydrogen in between spiral arms of the outer galaxy.
From the time of CMB decoupling onwards we investigate cosmological evolution subject to a strongly interacting SU(2) gauge theory of Yang-Mills scale $Λ\sim 10^{-4}$ eV (masquerading as the $U(1)_{Y}$ factor of the SM at present). The viability of this postulate is discussed in view of cosmological and (astro)particle physics bounds. The gauge theory is coupled to a spatially homogeneous and ultra-light (Planck-scale) axion field. As first pointed out by Frieman et al., such an axion is a viable candidate for quintessence, i.e. dynamical dark energy, being associated with today's cosmological acceleration. A prediction of an upper limit $Δt_{m_γ=0}$ for the duration of the epoch stretching from the present to the point where the photon starts to be Meissner massive is obtained: $Δt_{m_γ=0}\sim 2.2$ billion years.