arXiv subjects

Francesco Tramontano

Publications and source records attributed to Francesco Tramontano.

At least 37 records · Page 2Linked to original sources

arXiv2018

Associated production of a Higgs boson decaying into bottom quarks at the LHC in full NNLO QCD

We consider the production of a Standard Model Higgs boson decaying to bottom quarks in association with a vector boson W/Z in hadron collisions. We present a fully exclusive calculation of QCD radiative corrections both for the production cross section and for the Higgs boson decay rate up to next-to-next-to-leading order (NNLO) accuracy. Our calculation also includes the leptonic decay of the vector boson with finite-width effects and spin correlations. We consider typical kinematical cuts applied in the experimental analyses at the Large Hadron Collider (LHC) and we find that the full NNLO QCD corrections significantly decrease the accepted cross section and have a substantial impact on the shape of distributions. We point out that these additional effects are essential to obtain precise theoretical predictions to be compared with the LHC data.

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arXiv2017

Electroweak corrections to diphoton plus jets

We calculate the next-to-leading order electroweak corrections to the production of a photon pair in association with zero, one and two jets at the LHC. We use GoSam and Sherpa to obtain the results in a fully automated way. For a typical set of fiducial cuts the electroweak corrections lead to a modification of the total cross section of up to 3 per cent, depending on the jet multiplicity. We find substantial contributions in differential distributions, leading to tens of per cent corrections for phase space regions within the reach of the LHC. Furthermore we investigate the importance of photon induced processes as well as subleading contributions. Photon induced processes are found to be negligible, subleading contributions can have a sizeable impact however they can be removed by appropriate phase space cuts.

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arXiv2017

Single top theory

I breafly discuss very recent progress in the theoretical description of Standard Model single top production at hadron colliders.

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arXiv2016

Automation of electroweak corrections for LHC processes

For the Run 2 of the LHC next-to-leading order electroweak corrections will play an important role. Even though they are typically moderate at the level of total cross sections they can lead to substantial deviations in the shapes of distributions. In particular for new physics searches but also for a precise determination of Standard Model observables their inclusion in the theoretical predictions is mandatory for a reliable estimation of the Standard Model contribution. In this article we review the status and recent developments in electroweak calculations and their automation for LHC processes. We discuss general issues and properties of NLO electroweak corrections and present some examples, including the full calculation of the NLO corrections to the production of a W boson in association with two jets computed using GoSaM interfaced to MadDipole.

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arXiv2015

Boosted Horizon of a Boosted Space-Time Geometry

We apply the ultrarelativistic boosting procedure to map the metric of Schwarzschild-de Sitter spacetime into a metric describing de Sitter spacetime plus a shock-wave singularity located on a null hypersurface, by exploiting the picture of the embedding of an hyperboloid in a five-dimensional Minkowski spacetime. After reverting to the usual four-dimensional formalism, we also solve the geodesic equation and evaluate the Riemann curvature tensor of the boosted Schwarzschild-de Sitter metric by means of numerical calculations, which make it possible to reach the ultrarelativistic regime gradually by letting the boost velocity approach the speed of light. Eventually, the analysis of the Kretschmann invariant (and of the geodesic equation) shows the global structure of space- time, as we demonstrate the presence of a "scalar curvature singularity" within a 3-sphere and find that it is also possible to define what we have called "boosted horizon", a sort of elastic wall where all particles are surprisingly pushed away. This seems to suggest that such "boosted geometries" are ruled by a sort of "antigravity effect" since all geodesics seem to refuse entering the "boosted horizon" and are "reflected" by it, even though their initial conditions are aimed at driving the particles towards the "boosted horizon" itself.

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arXiv2015

Riemann curvature of a boosted spacetime geometry

The ultrarelativistic boosting procedure had been applied in the literature to map the metric of Schwarzschild-de Sitter spacetime into a metric describing de Sitter spacetime plus a shock-wave singularity located on a null hypersurface. This paper evaluates the Riemann curvature tensor of the boosted Schwarzschild-de Sitter metric by means of numerical calculations, which make it possible to reach the ultrarelativistic regime gradually by letting the boost velocity approach the speed of light. Thus, for the first time in the literature, the singular limit of curvature through Dirac's delta distribution and its derivatives is numerically evaluated for this class of spacetimes. Eventually, the analysis of the Kretschmann invariant and the geodesic equation show that the spacetime possesses a scalar curvature singularity within a 3-sphere and it is possible to define what we here call boosted horizon, a sort of elastic wall where all particles are surprisingly pushed away, as numerical analysis demonstrates. This seems to suggest that boosted geometries are ruled by a sort of antigravity effect since all geodesics seem to refuse to enter the boosted horizon, even though their initial conditions are aimed at driving the particles towards the boosted horizon.

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arXiv2015

Wbbj production at NLO with POWHEG+MiNLO

We present a next-to-leading order plus parton-shower event generator for the production of a W boson plus two bottom quarks and a jet at hadron colliders, implemented in the POWHEG BOX framework. Bottom-mass effects and spin correlations of the decay products of the W boson are fully taken into account. The code has been automatically generated using the two available interfaces to MadGraph4 and GoSam, the last one updated to a new version. We have applied the MiNLO prescription to our Wbbj calculation, obtaining a finite differential cross section also in the limit of vanishing jet transverse momentum. Furthermore, we have compared several key distributions for Wbbj production with those generated with a next-to-leading order plus parton-shower event generator for Wbb production, and studied their factorization- and renormalization-scale dependence. Finally, we have compared our results with recent experimental data from the ATLAS and CMS Collaborations.

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arXiv2015

A facility to Search for Hidden Particles at the CERN SPS: the SHiP physics case

This paper describes the physics case for a new fixed target facility at CERN SPS. The SHiP (Search for Hidden Particles) experiment is intended to hunt for new physics in the largely unexplored domain of very weakly interacting particles with masses below the Fermi scale, inaccessible to the LHC experiments, and to study tau neutrino physics. The same proton beam setup can be used later to look for decays of tau-leptons with lepton flavour number non-conservation, $τ\to 3μ$ and to search for weakly-interacting sub-GeV dark matter candidates. We discuss the evidence for physics beyond the Standard Model and describe interactions between new particles and four different portals - scalars, vectors, fermions or axion-like particles. We discuss motivations for different models, manifesting themselves via these interactions, and how they can be probed with the SHiP experiment and present several case studies. The prospects to search for relatively light SUSY and composite particles at SHiP are also discussed. We demonstrate that the SHiP experiment has a unique potential to discover new physics and can directly probe a number of solutions of beyond the Standard Model puzzles, such as neutrino masses, baryon asymmetry of the Universe, dark matter, and inflation

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arXiv2015

Higgs boson decay into b-quarks at NNLO accuracy

We compute the fully differential decay rate of the Standard Model Higgs boson into b-quarks at next-to-next-to-leading order (NNLO) accuracy in alpha_S. We employ a general subtraction scheme developed for the calculation of higher order perturbative corrections to QCD jet cross sections, which is based on the universal infrared factorization properties of QCD squared matrix elements. We show that the subtractions render the various contributions to the NNLO correction finite. In particular, we demonstrate analytically that the sum of integrated subtraction terms correctly reproduces the infrared poles of the two-loop double virtual contribution to this process. We present illustrative differential distributions obtained by implementing the method in a parton level Monte Carlo program. The basic ingredients of our subtraction scheme, used here for the first time to compute a physical observable, are universal and can be employed for the computation of more involved processes.

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arXiv2014

Associated ZH production at hadron colliders: the fully differential NNLO QCD calculation

We consider Standard Model Higgs boson production in association with a Z boson in hadron collisions. We present a fully exclusive computation of QCD radiative corrections up to next-to-next-to-leading order (NNLO). Our calculation includes the Higgs boson decay to bottom quarks (b) in next-to-leading order QCD and the leptonic decay of the Z boson with finite-width effects and spin correlations. The computation is implemented in a parton level Monte Carlo program that makes possible to consider arbitrary kinematical cuts on the final-state leptons, the b jets and the associated QCD radiation, and to compute the corresponding distributions in the form of bin histograms. We assess the impact of QCD radiative effects in the boosted kinematics at the LHC and show that the inclusion of the NNLO corrections is crucial to control the pT spectrum of the Higgs boson candidate.

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arXiv2014

Higher-order QCD effects for associated WH production and decay at the LHC

We consider Standard Model Higgs boson production in association with a W boson in hadron collisions. We supplement the fully exclusive perturbative computation of QCD radiative effects up to next-to-next-to-leading order (NNLO) with the computation of the decay of the Higgs boson into a bb pair at next-to-leading order (NLO). We consider the selection cuts that are typically applied in the LHC experimental analysis, and we compare our fixed-order predictions with the results obtained with the MC@NLO event generator. We find that NLO corrections to the H -> bb decay can be important to obtain a reliable pT spectrum of the Higgs boson, but that, in the cases of interest, their effect is well accounted for by the parton shower Monte Carlo. NNLO corrections to the production process typically decrease the cross section by an amount which depends on the detail of the applied cuts, but they have a mild effect on the shape of the Higgs pT spectrum. We also discuss the effect of QCD radiative corrections on the invariant mass distribution of the Higgs candidate.

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arXiv2013

HW/HZ + 0 and 1 jet at NLO with the POWHEG BOX interfaced to GoSam and their merging within MiNLO

We present a generator for the production of a Higgs boson H in association with a vector boson V=W or Z (including subsequent V decay) plus zero and one jet, that can be used in conjunction with general-purpose shower Monte Carlo generators, according to the POWHEG method, as implemented within the POWHEG BOX framework. We have computed the virtual corrections using GoSam, a program for the automatic construction of virtual amplitudes. In order to do so, we have built a general interface of the POWHEG BOX to the GoSam package. With this addition, the construction of a POWHEG generator within the POWHEG BOX is now fully automatized, except for the construction of the Born phase space. Our HV + 1 jet generators can be run with the recently proposed MiNLO method for the choice of scales and the inclusion of Sudakov form factors. Since the HV production is very similar to V production, we were able to apply an improved MiNLO procedure, that was recently used in H and V production, also in the present case. This procedure is such that the resulting generator achieves NLO accuracy not only for inclusive distributions in HV + 1 jet production but also in HV production, i.e. when the associated jet is not resolved, yielding a further example of matched calculation with no matching scale.

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arXiv2013

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.

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arXiv2013

Electroweak Corrections at High Energies

We present a survey of the most abundant processes at the LHC for sensitivity to electroweak corrections at \sqrt{s} = 8, 14, 33, and 100 TeV proton-proton collision energies. The processes studied are pp -> dijet, inclusive W and Z, W/Z+jets, and WW. In each case we compare the experimental uncertainty in the highest kinematic regions of interest with the relative magnitude of electroweak corrections and fixed-order α_S calculations.

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arXiv2012

Automated One-Loop Calculations with GoSam

We present the program package GoSam which is designed for the automated calculation of one-loop amplitudes for multi-particle processes in renormalisable quantum field theories. The amplitudes, which are generated in terms of Feynman diagrams, can be reduced using either D-dimensional integrand-level decomposition or tensor reduction. GoSam can be used to calculate one-loop QCD and/or electroweak corrections to Standard Model processes and offers the flexibility to link model files for theories Beyond the Standard Model. A standard interface to programs calculating real radiation is also implemented. We demonstrate the flexibility of the program by presenting examples of processes with up to six external legs attached to the loop.

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arXiv2011

Automation of One-Loop Calculations with GoSam: Present Status and Future Outlook

In this presentation, we describe the GoSam (Golem/Samurai) framework for the automated computation of multi-particle scattering amplitudes at the one-loop level. The amplitudes are generated analytically in terms of Feynman diagrams, and can be evaluated using either D-dimensional integrand reduction or tensor decomposition. GoSam can be used to compute one-loop corrections to Standard Model (QCD and EW) processes, and it is ready to link generic model files for theories Beyond SM. We show the main features of GoSam through its application to several examples of different complexity.

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arXiv2010

Modern Feynman Diagrammatic One-Loop Calculations

In this talk we present techniques for calculating one-loop amplitudes for multi-leg processes using Feynman diagrammatic methods in a semi-algebraic context. Our approach combines the advantages of the different methods allowing for a fast evaluation of the amplitude while monitoring the numerical stability of the calculation. In phase space regions close to singular kinematics we use a method avoiding spurious Gram determinants in the calculation. As an application of our approach we report on the status of the calculation of the amplitude for the process $pp\to b\bar{b}b\bar{b}+X$.

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arXiv2009

Hints for the existence of hexaquark states in the baryon-antibaryon sector

The discovery of some baryon-antibaryon resonances has led us to consider 3q~3\bar{q} systems as possible candidates. We predict their spectrum in the framework of a constituent model, where the chromo-magnetic interaction plays the main role. The relevant parameters are fixed by the present knowledge on tetraquarks. The emerging scenario complies well with experiment. Besides the description of the baryon-antibaryon resonances, we find evidence for new tetraquark states, namely the a0(Y) in the hidden strangeness sector and, in the cs\bar{c}\bar{s} sector, the Y(4140) and the X(4350). A detailed account of the spectra and the decay channels is provided for future comparisons with data.

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