arXiv subjects

Fabrizio Caola

Publications and source records attributed to Fabrizio Caola.

At least 55 records · Page 3Linked to original sources

arXiv2015

QCD corrections to ZZ production in gluon fusion at the LHC

We compute the next-to-leading order QCD corrections to the production of two Z-bosons in the annihilation of two gluons at the LHC. Being enhanced by a large gluon flux, these corrections provide distinct and, potentially, the dominant part of the N$^3$LO QCD contributions to Z-pair production in proton collisions. The $gg \to ZZ$ annihilation is a loop-induced process that receives the dominant contribution from loops of five light quarks, that are included in our computation in the massless approximation. We find that QCD corrections increase the $gg \to ZZ$ production cross section by ${\cal O}(50\%-100\%)$ depending on the values of the renormalization and factorization scales used in the leading order computation, and the collider energy. The large corrections to $gg \to ZZ$ channel increase the $pp \to ZZ$ cross section by about six to eight percent, exceeding the estimated theoretical uncertainty of the recent NNLO QCD calculation.

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arXiv2015

Fiducial cross sections for Higgs boson production in association with a jet at next-to-next-to-leading order in QCD

We extend the recent computation of Higgs boson production in association with a jet through next-to-next-to-leading order in perturbative QCD by including decays of the Higgs boson to electroweak vector bosons. This allows us to compute fiducial cross sections and kinematic distributions including realistic selection criteria for the Higgs boson decay products. As an illustration, we present results for $pp \to H + j \to γγ+ j$ closely following the ATLAS 8 TeV analysis and for $pp \to H+ j \to W W + j \to e^+ μ^- ν\bar ν+ j $ in a CMS-like 13 TeV setup.

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arXiv2015

Higgs boson production in association with a jet at next-to-next-to-leading order

We present precise predictions for Higgs boson production in association with a jet. Our calculation is accurate to next-to-next-to-leading order (NNLO) QCD in the Higgs Effective Field Theory and constitutes the first complete NNLO computation for Higgs production with a final-state jet in hadronic collisions. We include all relevant phenomenological channels and present fully-differential results as well as total cross sections for the LHC. Our NNLO predictions reduce the unphysical scale dependence by more than a factor of two and enhance the total rate by about twenty percent compared to NLO QCD predictions. Our results demonstrate for the first time satisfactory convergence of the perturbative series.

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arXiv2015

Two-loop helicity amplitudes for the production of two off-shell electroweak bosons in gluon fusion

We compute the part of the two-loop virtual amplitude for the process $gg \to V_1 V_2 \to (l_1 \bar l'_{1}) (l_2 \bar l'_2)$, where $V_{1,2}$ are arbitrary electroweak gauge bosons, that receives contributions from loops of massless quarks. Invariant masses of electroweak bosons are allowed to be different from each other. Our result provides an important ingredient for improving the description of gluon fusion contribution to the production of four-lepton final states at the LHC.

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arXiv2015

Two-loop helicity amplitudes for the production of two off-shell electroweak bosons in quark-antiquark collisions

Knowledge of two-loop QCD amplitudes for processes $q\bar q \to V_1 V_2 \to (l_1\bar l'_1)(l_2 \bar l'_2)$ is important for improving the theoretical description of four-lepton production in hadron collisions. In this paper we compute these helicity amplitudes for all intermediate vector bosons, $V_1 V_2 = γ^*γ^*, WW, ZZ, WZ, W γ^*$, including off-shell effects and decays to leptons.

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arXiv2015

Non-planar master integrals for the production of two off-shell vector bosons in collisions of massless partons

We present the calculation of all non-planar master integrals that are needed to describe production of two off-shell vector bosons in collisions of two massless partons through NNLO in perturbative QCD. The integrals are computed analytically using differential equations in external kinematic variables and expressed in terms of Goncharov polylogarithms. These results provide the last missing ingredient needed for the computation of two-loop amplitudes that describe the production of two gauge bosons with different invariant masses in hadron collisions.

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arXiv2014

On the NNLO QCD corrections to single-top production at the LHC

We present a fully-differential calculation of the NNLO QCD corrections to the t-channel mechanism for producing single top quarks at the LHC. We work in the structure function approximation, computing QCD corrections to the light- and heavy-quark lines separately and neglecting the dynamical cross-talk between the two. The neglected contribution, which appears at NNLO for the first time, is color-suppressed and is expected to be sub-dominant. Within this approximation, we find that, for the total cross section, NNLO QCD corrections are in the few percent range and, therefore, are comparable to NLO QCD corrections. We also find that the scale independence of the theoretical prediction for single-top production improves significantly once NNLO QCD corrections are included. Furthermore, we show how these results change if a cut on the transverse momentum of the top quark is applied and derive the NNLO QCD prediction for the ratio of single top and single anti-top production cross sections at the 8 TeV LHC.

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arXiv2014

Muon decay spin asymmetry

We compute the spin asymmetry of the muon decay through O(alpha^2) in perturbative QED. These two-loop corrections are about a factor five (twenty) smaller than the current statistical (systematic) uncertainty of the most precise measurement, performed by the TWIST collaboration. We point out that at O(alpha^2) the asymmetry requires a careful definition due to multi-lepton final states and suggest to use familiar QCD techniques to define it in an infra-red safe way. We find that the TWIST measurement of the asymmetry is in excellent agreement with the Standard Model.

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arXiv2014

Constraining anomalous HVV interactions at proton and lepton colliders

In this paper, we study the extent to which CP parity of a Higgs boson, and more generally its anomalous couplings to gauge bosons, can be measured at the LHC and a future electron-positron collider. We consider several processes, including Higgs boson production in gluon and weak boson fusion and production of a Higgs boson in association with an electroweak gauge boson. We consider decays of a Higgs boson including $ZZ, WW, γγ$, and $Z γ$. Matrix element approach to three production and decay topologies is developed and applied in the analysis. A complete Monte Carlo simulation of the above processes at proton and $e^+e^-$ colliders is performed and verified by comparing it to an analytic calculation. Prospects for measuring various tensor couplings at existing and proposed facilities are compared.

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arXiv2013

Constraining the Higgs boson width with ZZ production at the LHC

We point out that existing measurements of $pp \to ZZ$ cross-section at the LHC in a broad range of ZZ invariant masses allow one to derive a model-independent upper bound on the Higgs boson width, thanks to strongly enhanced off-shell Higgs contribution. Using CMS data and considering events in the interval of ZZ invariant masses from 100 to 800 GeV, we find $Γ_H \le 38.8 \times Γ_H^{SM} \approx 163$ MeV, at the $95\%$ confidence level. Restricting ZZ invariant masses to $M_{ZZ} \ge 300$ GeV range, we estimate that this bound can be improved to $Γ_H \le 21 \times Γ_H^{SM} \approx 88$ MeV. Under the assumption that all couplings of the Higgs boson to Standard Model particles scale in a universal way, our result can be translated into an upper limit on the branching fraction of the Higgs boson decay to invisible final states. We obtain $Br(H \to inv) < 0.84 (0.78)$, depending on the range of ZZ invariant masses that are used to constrain the width. We believe that an analysis along these lines should be performed by experimental collaborations in the near future and also in the run II of the LHC. We estimate that such analyses can, eventually, be sensitive to the Higgs boson width as small as $Γ_H \sim 10 \times Γ_H^{SM}$.

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arXiv2013

Signal-background interference effects for $gg \to H \to W^+ W^-$ beyond leading order

We study the effect of QCD corrections to the $gg\to H\to W^+ W^-$ signal-background interference at the LHC for a heavy Higgs boson. We construct a soft-collinear approximation to the NLO and NNLO corrections for the background process, which is exactly known only at LO. We estimate its accuracy by constructing and comparing the same approximation to the exact result for the signal process, which is known up to NNLO, and we conclude that we can describe the signal-background interference to better than ten percent accuracy. We show that our result implies that, in practice, a fairly good approximation to higher-order QCD corrections to the interference may also be obtained by rescaling the known LO result by a K-factor computed using the signal process.

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arXiv2013

$O(α_s^2)$ corrections to fully-differential top quark decays

We describe a calculation of the fully-differential decay rate of a top quark to a massless $b$-quark and a lepton pair at next-to-next-to-leading order in perturbative QCD. Technical details of the calculation are discussed and selected results for kinematic distributions are shown.

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arXiv2013

Higgs boson production in association with a jet at next-to-next-to-leading order in perturbative QCD

We report on a calculation of the cross-section for Higgs boson production in gluon fusion in association with a hadronic jet at next-to-next-to-leading order (NNLO) in perturbative QCD. The computational technique is discussed in detail. We show explicitly how to employ known soft and collinear limits of scattering amplitudes to construct subtraction terms for NNLO computations. Cancellation of singularities is demonstrated numerically for the collinearly-subtracted $gg \to H + j $ cross-section through NNLO and the finite $σ_{gg \to Hj}$ cross-section is computed through ${\cal O}(α_s^5)$ as a function of the center-of-mass collision energy. We present numerical results for the gluon-fusion contribution to Higgs production in association with a jet at the LHC. The NNLO QCD corrections significantly reduce the residual scale dependence of the cross-section. The computational method that we describe in this paper is applicable to the calculation of NNLO QCD corrections to any other $ 2 \to 2$ process at a hadron collider

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arXiv2013

On the $O(α_s^2)$ corrections to $b \to X_u e \bar ν$ inclusive decays

We present $O(α_s^2)$ QCD corrections to the fully-differential decay rate of a $b$-quark into inclusive semileptonic charmless final states. Our calculation provides genuine two-loop QCD corrections, beyond the Brodsky-Lepage-Mackenzie (BLM) approximation, to any infra-red safe partonic observable that can be probed in $b \to X_u e \bar ν$ decays. Kinematic cuts that closely match those used in experiments can be fully accounted for. To illustrate these points, we compute the non-BLM corrections to moments of the hadronic invariant mass and the hadronic energy with cuts on the lepton energy and the hadronic invariant mass. Our results remove one of the sources of theoretical uncertainty that affect the extraction of the CKM matrix element $|V_{ub}|$ from charmless inclusive B-decays.

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arXiv2013

A complete next-to-leading order QCD description of resonant $Z'$ production and decay into $t\bar t$ final states

We discuss QCD radiative corrections to the production of a heavy neutral resonance $Z'$ at the LHC assuming that it decays into a $ t\bar t$ final state. Compared to previous studies, our computation includes top quark decays as well as interference between the $Z'$ signal process and the QCD $t \bar t$ background. The interference contribution appears for the first time at next-to-leading order (NLO) QCD and requires new one-loop amplitudes that are not present when signal and background are treated separately. We describe some examples of how QCD radiative corrections may influence both the exclusion limits and studies of properties of the new resonance, once it is discovered.

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arXiv2012

Tree amplitudes and color decomposition in broken SU(2)

We propose a color decomposition for general tree amplitudes in a SU(2) gauge theory which is spontaneously broken via the Higgs mechanism. Working in the unitary gauge, we construct color-ordered amplitudes by explicitly presenting a set of color-ordered Feynman rules. Those primitive amplitudes are gauge-invariant, and they preserve perturbative unitarity in the high-energy limit. Serving as building blocks of color-dressed tree amplitudes, they allow for efficient evaluation of tree-level scattering amplitudes involving gauge bosons and the Higgs boson via the Berends-Giele recursion relations for color-ordered currents. We demonstrate the efficiency of this computational scheme by calculating on-shell amplitudes for scattering of five, six and nine W-bosons in the limit of vanishing Weinberg angle.

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arXiv2011

High energy resummation for rapidity distributions

We discuss the generalisation of high-energy resummation to rapidity distributions to leading logarithmic accuracy. We test our procedure applying it to Higgs production in gluon-gluon fusion both with finite top mass and in the infinite mass limit. We check that they reproduce the known results at fixed order and we estimate finite top mass corrections to the NLO distribution.

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arXiv2011

Finite fermion mass effects in pseudoscalar Higgs production via gluon-gluon fusion

We compute the leading logarithmic behaviour of the cross-section for the production of a pseudoscalar Higgs boson in gluon-gluon fusion to all-orders in perturbation theory, in the limit of large partonic centre of mass energy. We also calculate the Higgs rapidity distribution to the same accuracy. We include the contributions of top and bottom quarks, together with their interference. Our results are given in terms of single and double integrals, evaluated explicitly up to next-to next-to leading order (NNLO). We use our results to improve the known NNLO inclusive cross-section computed in the effective theory where the fermions in the loop are integrated out. The size of finite fermion mass effects on the inclusive cross-section is found to be small, reaching a few percent only for large values of the pseudoscalar mass.

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