1,927 research outputs found

    Rotation-invariant relations in vector meson decays into fermion pairs

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    The rotational properties of angular momentum eigenstates imply the existence of a frame-independent relation among the parameters of the decay distribution of vector mesons into fermions. This relation is a generalization of the Lam-Tung identity, a result specific to Drell-Yan production in perturbative QCD, here shown to be equivalent to the dynamical condition that the dilepton always originates from a transversely polarized photon

    A new approach to quarkonium polarization studies

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    Significant progress in understanding quarkonium production requires improved polarization measurements, fully considering the intrinsic multidimensionality of the problem. We propose a frame-invariant formalism which minimizes the dependence of the measured result on the experimental acceptance, facilitates the comparison with theoretical calculations, and provides a much needed control over systematic effects due to detector limitations and analysis biases. This formalism is a direct and generic consequence of the rotational invariance of the dilepton decay distribution and is independent of any assumptions specific to particular models of quarkonium production

    The long way to Ithaca. The reconfiguration of the urban political spaces in Southern Europe

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    This text frames a reflection upon the recent evolution on the sociopolitical and governance landscapes in urban Southern Europe. A global analysis of urban/metropolitan geographical and political tendencies is here made, from the most recent economic crisis and austerity politics years, towards expected future perspectives. The example of Lisbon metropolitan Area is exposed as a concrete case to show the pressures and the induced (and deduced) changes presently at stake. The analysis is set out on three different parts: a general reflection upon urban paradigm shifts and it’s still quite relevant unbalances and misunderstandings; an overall southern European geographical and political urban analytical perspective; and a proposed systematization of consequent political as well as research development needs. The text concludes with a systematized proposal of main diagnosis vectors, as well of possible main drivers and focuses for present and future southern European urban sociopolitical analysis

    Minimal physical constraints on the angular distributions of two-body boson decays

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    The angular distribution of the two-body decay of a boson of unknown properties is strongly constrained by angular momentum conservation and rotation invariance, as well as by the nature of the detected decay particles and of the colliding ones. Knowing the border between the "physical" and "unphysical" parameter domains defined by these "minimal constraints" (excluding specific hypotheses on what is still subject of measurement) is a useful ingredient in the experimental determinations of angular distributions and can provide model-independent criteria for spin characterizations. In particular, analysing the angular decay distribution with the general parametrization for the J = 2 case can provide a model-independent discrimination between the J = 0 and J = 2 hypotheses for a particle produced by two real gluons and decaying into two real photons

    Rotation-invariant observables in parity-violating decays of vector particles to fermion pairs

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    The di-fermion angular distribution observed in decays of inclusively produced vector particles is characterized by two frame-independent observables, reflecting the average spin-alignment of the produced particle and the magnitude of parity violation in the decay. The existence of these observables derives from the rotational properties of angular momentum eigenstates and is a completely general result, valid for any J=1 state and independent of the production process. Rotation-invariant formulations of polarization and of the decay parity-asymmetry can provide more significant measurements than the commonly used frame-dependent definitions, also improving the quality of the comparisons between the measurements and the theoretical calculations.Comment: To be published in Phys. Rev.

    Quarkonium production in the LHC era: a polarized perspective

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    Polarization measurements are usually considered as the most difficult challenge for the QCD description of quarkonium production. In fact, global data fits for the determination of the non-perturbative parameters of bound-state formation traditionally exclude polarization observables and use them as a posteriori verifications of the predictions, with perplexing results. With a change of perspective, we move polarization data to the centre of the study, advocating that they actually provide the strongest fundamental indications about the production mechanisms, even before we explicitly consider perturbative calculations. Considering psi(2S) and Y(3S) measurements from LHC experiments and state-of-the-art NLO short-distance calculations in the framework of non-relativistic QCD factorization (NRQCD), we perform a search for a kinematic domain where the polarizations can be correctly reproduced together with the cross sections, by systematically scanning the phase space and accurately treating the experimental uncertainties. This strategy provides a straightforward solution to the "quarkonium polarization puzzle" and reassuring signs that the theoretical framework is reliable. At the same time, the results expose unexpected hierarchies in the non-perturbative NRQCD parameters, that open new paths towards the understanding of bound-state formation in QCD.Comment: Submitted to Phys. Lett.

    From identical S- and P-wave pT/M spectra to maximally distinct polarizations: probing NRQCD with chi states

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    A global analysis of ATLAS and CMS measurements reveals that, at mid-rapidity, the directly-produced χc1\chi_{c1}, χc2\chi_{c2} and J/ψ\psi mesons have differential cross sections of seemingly identical shapes, when presented as a function of the mass-rescaled transverse momentum, pT/Mp_{\rm T}/M. This identity of kinematic behaviours among S- and P-wave quarkonia is certainly not a natural expectation of non-relativistic QCD (NRQCD), where each quarkonium state is supposed to reflect a specific family of elementary production processes, of significantly different pTp_{\rm T}-differential cross sections. Remarkably, accurate kinematic cancellations among the variegated NRQCD terms (colour singlets and octets) of its factorization expansion can lead to a surprisingly good description of the data. This peculiar tuning of the NRQCD mixtures leads to a clear prediction regarding the χc1\chi_{c1} and χc2\chi_{c2} polarizations, the only observables not yet measured: they should be almost maximally different from one another, and from the J/ψ\psi polarization, a striking exception in the global panorama of quarkonium production. Measurements of the difference between the χc1\chi_{c1}, χc2\chi_{c2} and J/ψ\psi polarizations, complementing the observed identity of momentum dependences, represent a decisive probe of NRQCD.Comment: Submitted to European Physical Journal

    J/psi polarization from fixed-target to collider energies

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    The determination of the magnitude and "sign" of the J/psi polarization crucially depends on the reference frame used in the analysis of the data and a full understanding of the polarization phenomenon requires measurements reported in two "orthogonal" frames, such as the Collins-Soper and helicity frames. Moreover, the azimuthal anisotropy can be, in certain frames, as significant as the polar one. The seemingly contradictory J/psi polarization results reported by E866, HERA-B and CDF can be consistently described assuming that the most suitable axis for the measurement is along the direction of the relative motion of the colliding partons, and that directly produced J/psi's are longitudinally polarized at low momentum and transversely polarized at high momentum. We make specific predictions that can be tested on existing CDF data and by LHC measurements, which should show a full transverse polarization for direct J/psi mesons of pT > 25 GeV/c
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