arXiv subjects

Giancarlo Ferrera

Publications and source records attributed to Giancarlo Ferrera.

44 records · Page 3Linked to original sources

arXiv2007

Modelling non-perturbative corrections to bottom-quark fragmentation

We describe B-hadron production in e+e- annihilation at the Z pole by means of a model including non-perturbative corrections to b-quark fragmentation as originating, via multiple soft emissions, from an effective QCD coupling constant, which does not exhibit the Landau pole any longer and includes absorbitive effects due to parton branching. We work in the framework of perturbative fragmentation functions at NLO, with NLL DGLAP evolution and NNLL large-x resummation in both coefficient function and initial condition of the perturbative fragmentation function. We include hadronization corrections via the effective coupling constant in the NNLO approximation and do not add any further non-perturbative fragmentation function. As part of our model, we perform the Mellin transforms of our resummed expressions exactly. We present results on the energy distribution of b-flavoured hadrons, which we compare with LEP and SLD data, in both x- and N-spaces. We find that, within the theoretical uncertainties on our calculation, our model is able to reasonably reproduce the data at x<0.92 and the first five moments of the B cross section.

hep-ph↗

arXiv2007

Threshold Resummation in Semi-Inclusive B decays

We discuss threshold resummation in radiative and charmless semileptonic B decays. To deal with the large non perturbative effects, we introduce a model for NNLL resummed form factors based on the analytic QCD coupling. By means of this model we can reproduce with good accuracy the experimental data. Finally we briefly present an improved threshold resummed formula to deal with jets initiated by massive quarks as in the case of semileptonic charmed decays.

hep-ph↗

arXiv2006

Semi-Inclusive B Decays and a Model for Soft-Gluon Effects

We compare experimental spectra of radiative and semileptonic B decays with the predictions of a model based on soft-gluon resummation to next-to-next-to leading order and on a ghost-less time-like coupling. We find a good agreement with photon spectra in the radiative decay and with hadron mass distributions in the semileptonic one: the extracted values for alpha_S(m_Z) are in agreement with the current PDG average within at most two standard deviations. The agreement is instead less good for the electron spectra measured by BaBar and Belle in semileptonic decays for small electron energies (< 2.2 GeV): our spectrum is harder. We also show that, in general, the inclusion of next-to-next-to-leading order effects is crucial for bringing the model closer to the data and that the non-power expansion introduced in the framework of analytic coupling studies does not accurately describe soft-gluon effects.

hep-ph↗

arXiv2006

Threshold resummed spectra in B ->Xu l nu decays in NLO (III)

We resum to next-to-leading order (NLO) the distribution in the light-cone momentum p+ = EX - |pX| and the spectrum in the electron energy, in the semileptonic decays B-> Xu l nu, where EX and pX are the total energy and three-momentum of the final hadron state Xu respectively. By expanding our formulas, we obtain the coefficients of all the infrared logarithms alpha^n L^k at order alpha^2 and at order alpha^3, with the exception of the alpha^3 L coefficient. We explicitly show that the relation between these semileptonic spectra and the hadron mass distribution in the radiative decay B -> Xs gamma is not a purely short distance one. By expanding our result for the p+ distribution and comparing it with the recent O(alpha^2 n_f) computation, we obtain an explicit and non trivial verification of our resummation scheme at the two-loop level. We also discuss the validity of the so-called Brodsky-Lepage-Mackenzie (BLM) scheme in the evaluation of the semileptonic spectra by comparing it with our results, finding that it is a reasonable approximation for the leading terms only. Finally, we show that long-distance phenomena, such as Fermi motion, are expected to have an effect on the electron spectrum smaller than on the hadron mass distribution.

hep-ph↗

arXiv2005

Threshold Resummed Spectra in B -> Xu l nu Decays in NLO (I)

We evaluate thresholds resummed spectra in B -> Xu l nu decays in next-to-leading order. We present results for the distribution in E_X and in m_X^2/E_X^2, for the distribution in E_X and E_l and for the distribution in E_X, where E_X and m_X are the energy and the invariant mass of the final hadronic state Xu respectively and E_l is the energy of the charged lepton. We explicitly show that all these spectra (where there is no integration over the hadronic energy) can be directly related to the photon spectrum in B -> Xs gamma via short-distance coefficient functions.

hep-ph↗

arXiv2005

Threshold resummed spectra in B -> Xu l nu decays in NLO (II)

We resum to next-to-leading order the distribution in the ratio of the invariant hadron mass mX to the total hadron energy EX and the distribution in mX in the semileptonic decays B -> Xu l nu. By expanding our formulas, we obtain the coefficients of all the infrared logarithms at O(alphaS^2) and of the leading ones at O(alphaS^3). We explicitly show that the relation between these semileptonic spectra and the photon spectrum in the radiative decay B -> Xs gamma is not a purely short-distance one. There are long-distance effects in the semileptonic spectra which are not completely factorized by the structure function as measured in the radiative decay and have to be modelled in some way.

hep-ph↗

arXiv2005

Associated production of a light Higgs boson and a chargino pair in the MSSM at linear colliders

In the Minimal Supersymmetric Standard Model (MSSM), we study the light Higgs-boson radiation off a light-chargino pair in the process e+e- -> h chi^+ chi^- at linear colliders with \sqs=500 GeV. We analyze cross sections in the regions of the MSSM parameter space where the process can not proceed via on-shell production and subsequent decay of either heavier charginos or the pseudoscalar Higgs boson A. Cross sections up to a few fb's are allowed, according to present experimental limits on the Higgs-boson, chargino and sneutrino masses. We also show how a measurement of the process production rate could provide a determination of the Higgs-boson couplings to charginos.

hep-ph↗

arXiv2004

Higgs-boson coupling to charginos in the MSSM at linear colliders

We discuss the associated production of a light Higgs boson and a light chargino pair in the process e+e- -> h chi+ chi- in the Minimal Supersymmetric Standard Model (MSSM) at linear colliders (LC) with \sqrt s=500 GeV. This process gives direct informations about the Higgs-boson coupling to light charginos, that cannot be analyzed in decay processes due to phase-space restrictions. We compute total cross sections in the regions of the MSSM parameter space where the process cannot proceed via on-shell production and subsequent decay of either heavier charginos or pseudoscalar Higgs bosons A. Cross sections up to a few 0.1 fb are allowed, making this process potentially detectable at high-luminosity linear colliders. We also compute analytically the final h momentum distributions in the limit of very heavy electron-sneutrino masses.

hep-ph↗