On Large-Angle Bhabha Scattering at LEP

The theoretical accuracy of the program TOPAZ0 in the large-angle Bhabha channel is estimated. The physical error associated with the full Bhabha cross section and its forward and backward components separately is given for some event selections and several energy points of interest for LEP1 physics, both for the $s$ and non-$s$ contributions to the cross section.Comment: 8 pages, LaTeX, six tables, no figure

Asking for an extra photon in Higgs production at the LHC and beyond

We study the inclusive production of a Higgs boson in association with a high-$p_T$ photon at the LHC, detailing the leading-order features of the main processes contributing to the $H\gamma$ final state. Requiring an extra hard photon in Higgs production upsets the cross-section hierarchy for the dominant channels. The $H\gamma$ inclusive production comes mainly from photons radiated in vector-boson fusion (VBF), which accounts for about 2/3 of the total rate, for $p_T^{\gamma,j} >30$ GeV, at leading order. On the other hand, radiating a high-$p_T$ photon in the main top-loop Higgs channel implies an extra parton in the final state, which suppresses the production rate by a further $\alpha_S$ power. As a result, the $H\gamma$ production via top loops at the LHC has rates comparable with the ones arising from either the $H t\bar t$ production or the $HW(Z)\gamma$ associated production. Then, in order of decreasing cross section, comes the single-top-plus-Higgs channel, followed in turn by the heavy-flavor fusion processes $b\bar b \to H\gamma$ and $c\bar c \to H\gamma$. The $H\gamma$ production via electroweak loops has just a minor role. At larger c.m. energies, the $H t\bar t\gamma$ channel surpasses the total contribution of top-loop processes. In particular, requiring $p_T^{\gamma,j} >30$ GeV at $\sqrt S \simeq 100$ TeV, $H t\bar t\gamma$ accounts for about $1/4$ of the inclusive $H\gamma$ production at leading order, about half of the total being due to VBF production.Comment: 20 pages, 13 figures, two comments added; one typo corrected; version published in JHE

Diquark-antidiquark states with hidden or open charm

Some features and predictions of a recently proposed model based on diquark-antidiquark bound states are illustrated. Its ability in accomodating newly discovered charmed resonances around 4 GeV is discussed.Comment: To appear in the proceedings of the HEP2005 International Europhysics Conference on High Energy Physics, July 21st-27th 2005, Lisboa, Portuga

ISR corrections to associated HZHZ production at future Higgs factories

We evaluate the QED corrections due to initial state radiation (ISR) to associated Higgs boson production in electron-positron ($e^+ e^-$) annihilation at typical energies of interest for the measurement of the Higgs properties at future $e^+ e^-$ colliders, such as CEPC and FCC-ee. We apply the QED Structure Function approach to the four-fermion production process $e^+ e^- \to \mu^+ \mu^- b \bar b$, including both signal and background contributions. We emphasize the relevance of the ISR corrections particularly near threshold and show that finite third order collinear contributions are mandatory to meet the expected experimental accuracy. We analyze in turn the r\^ole played by a full four-fermion calculation and beam energy spread in precision calculations for Higgs physics at future $e^+ e^-$ colliders.Comment: 11 pages, 3 figure

Higgs boson decay into four leptons at NLOPS electroweak accuracy

In view of precision studies of the Higgs sector at the Run II of the LHC, the improvement of the accuracy of the theoretical prediction is becoming a pressing issue. In this framework, we detail a calculation of the full Next-to-Leading Order (NLO) electroweak corrections to Higgs boson decay into four charged leptons, by considering the gold-plated channel H -> Z(*) Z(*) -> 2l 2l', l,l' = e, mu. We match the NLO corrections with a QED Parton Shower (PS), in order to simulate exclusive multiple photon emission and provide novel results at NLOPS electroweak accuracy. We compare our NLO predictions to those of the program Prophecy4f and present NLOPS phenomenological results relevant for Higgs physics studies, with particular attention to precision measurements of the Higgs boson mass, spin-parity assignment and tests of the Standard Model. Our calculation is implemented in a new code, Hto4l, which can be easily interfaced to any generator describing Higgs boson production. As an example, we provide illustrative results for Higgs production and decay in the process gg -> H -> 4l using POWHEG with NLOPS accuracy in the production mode.Comment: 27 pages, 2 tables, 9 figures. New numerical results and plots for dressed leptons. Conclusions unchanged. Version to appear in JHE

Higgs Boson Production in Association with a Photon in Vector Boson Fusion at the LHC

Higgs boson production in association with two forward jets and a central photon at the CERN Large Hadron Collider is analyzed, for the Higgs boson decaying into a b bbar pair in the m_H <= 140 GeV mass region. We study both irreducible and main reducible backgrounds at parton level. Compared to the Higgs production via vector-boson fusion, the request of a further photon at moderate rapidities dramatically enhances the signal/background ratio. Inclusive cross sections for p_T^\gamma >= 20 GeV can reach a few tens of fb's. After a suitable choice of kinematical cuts, the cross-section ratio for signal and irreducible-background can be enhanced up to >= ~1/10, with a signal cross section of the order of a few fb's, for m_H ~ 120 GeV. The request of a central photon radiation also enhances the relative signal sensitivity to the WWH coupling with respect to the ZZH coupling. Hence, a determination of the cross section for the associated production of a Higgs boson decaying into a b bbar pair plus a central photon in vector-boson fusion could help in constraining the b bbar H coupling, and the WWH coupling as well. A preliminary study of QCD showering effects points to a further significant improvement of the signal detectability over the background.Comment: 30 pages, 8 figures, 8 tables; minor corrections to the text; version appeared in Nuclear Physics

Neutral current Drell-Yan with combined QCD and electroweak corrections in the POWHEG BOX

Following recent work on the combination of electroweak and strong radiative corrections to single W-boson hadroproduction in the POWHEG BOX framework, we generalize the above treatment to cover the neutral current Drell-Yan process. According to the POWHEG method, we combine both the next-to-leading order (NLO) electroweak and QED multiple photon corrections with the native NLO and Parton Shower QCD contributions. We show comparisons with the predictions of the electroweak generator HORACE, to validate the reliability and accuracy of the approach. We also present phenomenological results obtained with the new tool for physics studies at the LHC.Comment: 10 pages, 14 figures, 1 table. One new figure, some plots with updated results, added text. Final version to appear in EPJ

W gamma production in hadronic collisions using the POWHEG+MiNLO method

We detail a calculation of W gamma production in hadronic collision, at Next-to-Leading Order (NLO) QCD interfaced to a shower generator according to the POWHEG prescription supplemented with the MiNLO procedure. The fixed order result is matched to an interleaved QCD+QED parton shower, in such a way that the contribution arising from hadron fragmentation into photons is fully modeled. In general, our calculation illustrates a new approach to the fully exclusive simulation of prompt photon production processes accurate at the NLO level in QCD. We compare our predictions to those of the NLO program MCFM, which treats the fragmentation contribution in terms of photon fragmentation functions. We also perform comparisons to available LHC data at 7 TeV, for which we observe good agreement, and provide phenomenological results for physics studies of the W gamma production process at the Run II of the LHC. The new tool, which includes W leptonic decays and the contribution of anomalous gauge couplings, allows a fully exclusive, hadron-level description of the W gamma process, and is publicly available at the repository of the POWHEG BOX. Our approach can be easily adapted to deal with other relevant isolated photon production processes in hadronic collisions.Comment: 38 pages, 5 Tables, 9 Figures. Final version published in JHEP. Acknowledgments to Galileo Galilei Institute for Theoretical Physics and to INFN adde

Electroweak Sudakov Corrections to New Physics Searches at the CERN LHC

We compute the one-loop electroweak Sudakov corrections to the production process Z (nu bar{nu}) + n jets, with n = 1,2,3, in p p collisions at the LHC. It represents the main irreducible background to new physics searches at the energy frontier. The results are obtained at the leading and next-to-leading logarithmic accuracy by implementing the general algorithm of Denner-Pozzorini in the event generator for multiparton processes ALPGEN. For the standard selection cuts used by ATLAS and CMS collaborations, we show that the Sudakov corrections to the relevant observables can grow up to - 40% at sqrt{s} = 14 TeV. We also include the contribution due to undetected real radiation of massive gauge bosons, to show to what extent the partial cancellation with the large negative virtual corrections takes place in realistic event selections.Comment: 5 pages, 3 figures, 1 table. Extended discussion on real radiation processes. Added references. Results unchanged. Version to appear in Phys. Rev. Let