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G. Degrassi

Publications and source records attributed to G. Degrassi.

At least 37 records · Page 2Linked to original sources
arXiv2007

Scalar Particle Contribution to Higgs Production via Gluon Fusion at NLO

We consider the gluon fusion production cross section of a scalar Higgs boson in models where fermion and scalar massive colored particles are present. We report analytic expressions for the matrix elements of $gg\to Hg$, $q\bar{q}\to Hg$, and $qg\to Hq$ processes completing the calculation of the NLO QCD corrections in these extended scenarios. The formulas are written in a complete general case, allowing a flexible use for different theoretical models. Applications of our results to two different models are presented: i) a model in which the SM Higgs sector is augmented by a weak doublet scalar in the $SU(N_c)$ adjoint representation. ii) The MSSM, in the limit of neglecting the gluino contribution to the cross section.

hep-ph↗
arXiv2007

Analytic Results for Virtual QCD Corrections to Higgs Production and Decay

We consider the production of a Higgs boson via gluon-fusion and its decay into two photons. We compute the NLO virtual QCD corrections to these processes in a general framework in which the coupling of the Higgs boson to the external particles is mediated by a colored fermion and a colored scalar. We present compact analytic results for these two-loop corrections that are expressed in terms of Harmonic Polylogarithms. The expansion of these corrections in the low and high Higgs mass regimes, as well as the expression of the new Master Integrals which appear in the reduction of the two-loop amplitudes, are also provided. For the fermionic contribution, we provide an independent check of the results already present in the literature concerning the Higgs boson and the production and decay of a pseudoscalar particle.

hep-ph↗
arXiv2006

Two-loop electroweak corrections to Higgs production in proton-proton collisions

We study the impact of the two-loop electroweak corrections on the production of a Higgs boson via gluon-fusion in proton-proton collisions at LHC energies. We discuss the prescritpion to include the corrections to the hard scattering matrix element in the calculation of the hadronic cross-section sigma (p+p\to H+X). Under the hypothesis of factorization of the electroweak corrections with respect to the dominant soft and collinear QCD radiation, we observe an increase of the total cross-section from 4 to 8 %, for MH <=160 GeV. This increase is comparable with the present QCD uncertainties originating from hard scattering matrix elements.

hep-ph↗
arXiv2006

QCD Corrections to Radiative B Decays in the MSSM with Minimal Flavor Violation

We compute the complete supersymmetric QCD corrections to the Wilson coefficients of the magnetic and chromomagnetic operators, relevant in the calculation of b -> s gamma decays, in the MSSM with Minimal Flavor Violation. We investigate the numerical impact of the new results for different choices of the MSSM parameters and of the scale where the quark and squark mass matrices are assumed to be aligned. We find that the corrections can be important when the superpartners are relatively light, and that they depend sizeably on the scale of alignment. Finally, we discuss how our calculation can be employed when the scale of alignment is far from the weak scale.

hep-ph↗
arXiv2004

Two-loop light fermion contribution to Higgs production and decays

We compute the electroweak corrections due to the light fermions to the production cross section $σ(g g \to H)$ and to the partial decay widths $Γ(H \to γγ)$ and $Γ(H \to g g)$. We present analytic results for these corrections that are expressed in terms of Generalized Harmonic Polylogarithms. We find that for the gluon fusion production cross section and for the decay width $Γ(H \to g g)$ the corrections are large in the Higgs mass region below 160 GeV where they reach up to 9% of the lowest order term. For the decay width $Γ(H \to γγ)$ the corrections for Higgs mass above 160 GeV can reach $-$10% of the lowest order term.

hep-ph↗
arXiv2003

Towards High-Precision Predictions for the MSSM Higgs Sector

The status of the evaluation of the MSSM Higgs sector is reviewed. The phenomenological impact of recently obtained corrections is discussed. In particular it is shown that the upper bound on m_h within the MSSM is shifted upwards. Consequently, lower limits on tan beta obtained by confronting the upper bound as function of tan beta with the lower bound on m_h from Higgs searches are significantly weakened. Furthermore, the region in the M_A-tan beta-plane where the coupling of the lightest Higgs boson to down-type fermions is suppressed is modified. The presently not calculated higher-order corrections to the Higgs-boson mass matrix are estimated to shift the mass of the lightest Higgs boson by up to 3 GeV.

hep-ph↗
arXiv2002

On the two-loop sbottom corrections to the neutral Higgs boson masses in the MSSM

We compute the O(ab*as) two-loop corrections to the neutral Higgs boson masses in the Minimal Supersymmetric Standard Model, using the effective potential approach. Such corrections can be important in the region of parameter space corresponding to tan(beta)>>1 and sizeable mu. In spite of the formal analogy with the O(at*as) corrections, there are important differences, since the dominant effects are controlled by the sbottom-Higgs scalar couplings. We propose a convenient renormalization scheme that avoids unphysically large threshold effects associated with the bottom mass, and absorbs the bulk of the O(ab*as + ab*at) corrections into the one-loop expression. We give general explicit formulae for the O(ab*as) corrections to the neutral Higgs boson mass matrix. We also discuss the importance of the O(ab^2) corrections and derive a formula for their contribution to mh in a simple limiting case.

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arXiv2001

Where is the Higgs?

I discuss the theoretical uncertainties in the indirect Higgs mass determination. I present the probability density function for the Higgs mass obtained combining together the information from precision measurements with the results from the direct search experiments carried out at LEP. The probability that the Higgs weights less than 116 GeV comes out to be around 35 % while the 95 % upper limit is located around 210-230 GeV.

hep-ph↗
arXiv2000

B -> X_s gamma in supersymmetry: large contributions beyond the leading order

We discuss possible large contributions to B -> X_s gamma, which can occur at the next-to-leading order in supersymmetric models. They can originate from terms enhanced by tan(beta) factors, when the ratio between the two Higgs vacuum expectation values is large, or by logarithm of M_{susy}/M_W, when the supersymmetric particles are considerably heavier than the W boson. We give compact formulae which include all potentially large higher-order contributions. We find that tan(beta) terms at the next-to-leading order do not only appear from the Hall-Rattazzi-Sarid effect (the modified relation between the bottom mass and Yukawa coupling), but also from an analogous effect in the top-quark Yukawa coupling. Finally, we show how next-to-leading order corrections, in the large tan(beta) region, can significantly reduce the limit on the charged-Higgs mass, even if supersymmetric particles are very heavy.

hep-ph↗
arXiv2000

Next-to-Leading QCD Corrections to B\to X_s γin Supersymmetry

We compute the QCD next-to-leading order matching conditions of the (chromo)-magnetic operators relevant for B\to X_s γin supersymmetric models with minimal flavour violation. The calculation is performed under the assumption that the charginos and one stop are lighter than all other squarks and the gluino. In the parameter region where a light charged Higgs boson is consistent with measurements of BR(B\to X_s γ), we find sizeable corrections to the Wilson coefficients. As a consequence, there is a significant reduction of the stop-chargino mass region where the supersymmetric contribution has a large destructive interference with the charged-Higgs boson contribution.

hep-ph↗
arXiv2000

Constraining the Higgs boson mass through the combination of direct search and precision measurement results

We show that the likelihood ratio of Higgs search experiments is a form to report the experimental results suitable to be combined with the information from precision measurements to obtain a joint constraint on the Higgs mass. We update our previous combined analysis using the new results on direct searches and recent precision measurements, including also the Z0 leptonic partial width result. The method is also improved to take into account small non linearity effects in the theoretical formulae. We find an expected value for the Higgs mass around 160-170 GeV with an expectation uncertainty, quantified by the standard deviation of the distribution, of about 50-60 GeV. The 95% probability upper limit comes out to be around 260-290 GeV.

hep-ph↗
arXiv1999

Two-loop heavy top corrections to the Z boson partial widths

We present the evaluation of the two-loop O(g^4 mt^2) effects in the partial widths of the Z boson in the MSbar scheme and in two different implementations of the on-shell scheme. We observe a clear reduction of the scheme dependence of the predictions. The renormalization procedure and the Heavy Top Expansion employed in the O(g^4 mt^2) calculations are illustrated in some detail and intermediate results are provided. We discuss the implication of our results on the constraints for the Higgs mass making use of simple interpolating formulas. We find that precision data give mh < 285 GeV at 95% C.L. taking into account the theory uncertainty. Including also the information from direct search experiments we obtain a 95% upper bound mh < 345 GeV.

hep-ph↗
arXiv1999

Constraints on the Higgs Boson Mass from Direct Searches and Precision Measurements

We combine, within the framework of the Standard Model, the results of Higgs search experiments with the information coming from accurate theoretical calculation and precision measurements to provide a probability density function for the Higgs mass, from which all numbers of interest can be derived. The expected value is 170 GeV, with an expectation uncertainty, quantified by the standard deviation of the distribution, of about 80 GeV. The median of the distribution is 150 GeV, while 75 % of the probability is concentrated in the region $M_H \leq 200$ GeV. The 95 % probability upper limit comes out to be around 300 GeV.

hep-ph↗
arXiv1998

QED Logarithms in the Electroweak Corrections to the Muon Anomalous Magnetic Moment

We employ an effective Lagrangian approach to derive the leading-logarithm two-loop electroweak contributions to the muon anomalous magnetic moment, a_mu. We show that these corrections can be obtained using known results on the anomalous dimensions of composite operators. We confirm the result of Czarnecki et al. for the bosonic part and present the complete sin^2 θ_W dependence of the fermionic contribution. The approach is then used to compute the leading-logarithm three-loop electroweak contribution to a_mu. Finally we derive, in a fairly model-independent way, the QED improvement of new-physics contributions to a_mu and to the electric dipole moment (EDM) of the electron. We find that the QED corrections reduce the effect of new physics at the electroweak scale by 6% (for a_mu) and by 11% (for the electron EDM).

hep-ph↗
arXiv1998

Next-to-Leading QCD Corrections to B -> X_s gamma: Standard Model and Two-Higgs Doublet Model

We present the QCD corrections to the matching conditions of the Delta B =1 magnetic and chromo-magnetic operators in the Standard Model and in two-Higgs doublet models. We use an off-shell matching procedure which allows us to perform the computation using Taylor series in the external momenta, instead of asymptotic expansions. In the Standard Model case, we confirm previous results derived on-shell and we obtain BR(B -> X_s gamma)=(3.62\pm 0.33) 10^{-4}. In the case of the usual two-Higgs doublet model, we show that going from the leading to the next-to-leading order result improves the CLEO bound on the charged-Higgs mass from 260 GeV to 380 GeV. This limit is very sensitive to the definition of errors and we carefully discuss the theoretical uncertainties. Finally, in the case of the two-Higgs doublet model in which both up- and down-type quarks couple to the same Higgs field, the theoretical prediction for BR(B -> X_s gamma) can be reduced by at most 20% with respect to the Standard Model value.

hep-ph↗
arXiv1997

The Role of M(W) in Precision Studies of the Standard Model

Recent calculations have significantly decreased the scheme and residual scale dependence of basic radiative corrections of the Standard Electroweak Model. This leads to a theoretically accurate prediction of the W-boson mass M(W), as well as a reduced upper bound for the Higgs boson mass M(H). The implications of a precise M(W) measurement on the M(H) estimate are emphasized.

hep-ph↗
arXiv1996

Precise calculation of MW, sin^2 theta_MSbar, and sin^2 theta_eff

The two-loop O(g^4 mt^2/mw^2) corrections are incorporated in the theoretical calculation of MW, sin^2 theta_MSbar(MZ), and sin^2 theta_eff, as functions of MH. The analysis is carried out in a previously proposed MSbar formulation and two novel on-shell resummation schemes. It is found that the inclusion of the new effects sharply decreases the scheme and residual scale dependence of the calculations. QCD corrections are incorporated in two different approaches. Comparison with the world average of sin^2 theta_eff leads to MH= 127 +143 -71 GeV and MW= 80.367 +/- 0.048 GeV, with small variations among the six calculations.

hep-ph↗