Checking flavour models at neutrino facilities

In the recent years, the industry of model building has been the subject of the intense activity, especially after the measurement of a relatively large values of the reactor angle. Special attention has been devoted to the use of non-abelian discrete symmetries, thanks to their ability of reproducing some of the relevant features of the neutrino mixing matrix. In this Letter, we consider two special relations between the leptonic mixing angles, arising from models based on <math altimg="si1.gif" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mrow><mi>S</mi></mrow><mrow><mn>4</mn></mrow></msub></math> and <math altimg="si2.gif" xmlns="http://www.w3.org/1998/Math/MathML"><msub><mrow><mi>A</mi></mrow><mrow><mn>4</mn></mrow></msub></math> , and study whether, and to which extent, they can be distinguished at superbeam facilities, namely T2K, NO ν A and T2HK

Vacuum Insertion Approximation and the ΔI=1/2 rule: A lattice QCD test of the naïve factorization hypothesis for K , D , B and static mesons

Motivated by a recent paper by the RBC–UKQCD Collaboration, which observes large violations of the naïve factorization hypothesis in K→ππ decays, we study in this paper the accuracy of the Vacuum Insertion Approximation (VIA) for the matrix elements of the complete basis of four-fermion ΔF=2 operators. We perform a comparison between the matrix elements in QCD, evaluated on the lattice, and the VIA predictions. We also investigate the dependence on the external meson masses by computing matrix elements for K , Ds , Bs and static mesons. In commonly used renormalization schemes, we find large violations of the VIA in particular for one of the two relevant Wick contractions in the kaon sector. These deviations, however, decrease significantly as the meson mass increases and the VIA predictions turn out to be rather well verified for B -meson matrix elements and, even better, in the infinite mass limit

The m W − m Z interdependence in the Standard Model: a new scrutiny

The m W − m Z interdependence in the Standard Model is studied in the M S ¯ $$\overline{M\ S}$$ scheme at the two-loop level, including the known higher-order contributions. The relevant radiative parameters, Δ α ^ μ $$\varDelta \widehat{\alpha}\left(\mu \right)$$ , Δ r ^ W $$\varDelta {\widehat{r}}_W$$ , ρ ^ $$\widehat{\rho}$$ are computed at O ( α 2 ) taking into account higherorder QCD corrections and the resummation of the reducible contributions. We obtain m W = 80 . 357 ± 0 . 009 ± 0 . 003 GeV where the errors refer to the parametric and theoretical uncertainties, respectively. A comparison with the known result in the On-Shell scheme gives a difference of ≈ 6 MeV. As a byproduct of our calculation we also obtain the M S ¯ $$\overline{M\ S}$$ electromagnetic coupling and the weak mixing angle at the top mass scale, α ^ M t = 127.73 − 1 ± 0.0000003 $$\widehat{\alpha}\left({M}_t\right)={(127.73)}^{-1}\pm 0.0000003$$ and sin 2 θ ^ W M t = 0.23462 ± 0.00012 $${ \sin}^2{\widehat{\theta}}_W\left({M}_t\right)=0.23462\pm 0.00012$$

Lepton mixing from the interplay of the alternating group A 5 and CP

Assuming three generations of Majorana neutrinos, we study the different mixing patterns that arise from the non-trivial breaking of the flavor group A 5 and CP to the residual symmetries Z 3 , Z 5 or Z 2 × Z 2 in the charged lepton and to Z 2 × CP in the neutrino sector. All patterns contain only one free parameter θ and thus mixing angles as well as the Dirac and the two Majorana phases are strongly correlated. We perform an analytical and a numerical study of all possible mixing patterns. It turns out that only four patterns can describe the experimentally measured values of the mixing angles for a particular choice of θ well. All of them predict trivial Majorana phases, while the Dirac phase δ is maximal for two patterns and trivial for the two remaining ones. If δ is maximal, also the atmospheric mixing angle is fixed to be maximal

NLO QCD corrections to Higgs pair production including dimension-6 operators

New Physics that becomes relevant at some high scale Λ beyond the experimental reach, can be described in the effective theory approach by adding higher-dimensional operators to the Standard Model (SM) Lagrangian. In Higgs pair production through gluon fusion, which gives access to the trilinear Higgs self-coupling, this leads not only to modifications of the SM couplings but also induces novel couplings not present in the SM. For a proper prediction of the cross section, higher order QCD corrections that are important for this process, have to be taken into account. The various higher-dimensional contributions are affected differently by the QCD corrections. In this paper, we provide the next-to-leading order (NLO) QCD corrections to Higgs pair production including dimension-6 operators in the limit of large top quark masses. Depending on the dimension-6 coefficients entering the Lagrangian, the new operators affect the relative NLO QCD corrections by several per cent, while modifying the cross section by up to an order of magnitude

Light stop decays: implications for LHC searches

We investigate the flavour-changing neutral current decay of the lightest stop into a charm quark and the lightest neutralino and its four-body decay into the lightest neutralino, a down-type quark and a fermion pair. These are the relevant stop search channels in the low-mass region. The SUSY-QCD corrections to the two-body decay have been calculated for the first time and turn out to be sizeable. In the four-body decay both the contributions from diagrams with flavour-changing neutral current couplings and the mass effects of final state bottom quarks and τ leptons have been taken into account, which are not available in the literature so far. The resulting branching ratios are investigated in detail. We find that in either of the decay channels the branching ratios can deviate significantly from 1 in large parts of the allowed parameter range. Taking this into account, the experimental exclusion limits on the stop, which are based on the assumption of branching ratios equal to 1, are considerably weakened. This should be taken into account in future searches for light stops at the next run of the LHC, where the probed low stop mass region will be extended

Accidental matter at the LHC

We classify weak-scale extensions of the Standard Model which automatically preserve its accidental and approximate symmetry structure at the renormalizable level and which are hence invisible to low-energy indirect probes. By requiring the consistency of the effective field theory up to scales of Λ eff ≈ 10 15 GeV and after applying cosmological constraints, we arrive at a finite set of possibilities that we analyze in detail. One of the most striking signatures of this framework is the presence of new charged and/or colored states which can be efficiently produced in high-energy particle colliders and which are stable on the scale of detectors

Two-loop QCD corrections to the MSSM Higgs masses beyond the effective-potential approximation

We compute the two-loop QCD corrections to the neutral Higgs-boson masses in the Minimal Supersymmetric Standard Model, including the effect of non-vanishing external momenta in the self-energies. We obtain corrections of O(αtαs) and O(ααs) , i.e., all two-loop corrections that involve the strong gauge coupling when the only non-vanishing Yukawa coupling is the top one. We adopt either the DR¯ renormalization scheme or a mixed on-shell (OS)– DR¯ scheme where the top/stop parameters are renormalized on-shell. We compare our results with those of earlier calculations, pointing out an inconsistency in a recent result obtained in the mixed OS– DR¯ scheme. The numerical impact of the new corrections on the prediction for the lightest-scalar mass is moderate, but already comparable to the accuracy of the Higgs-mass measurement at the Large Hadron Collider

Light stop decays into Wbχ˜10 near the kinematic threshold

We investigate the decays of the light stop in scenarios with the lightest neutralino χ˜10 being the lightest supersymmetric particle, including flavour-violating (FV) effects. We analyse the region where the three-body decay t˜1→Wbχ˜10 is kinematically allowed and provide a proper description of the transition region between the three-body decay and the four-body decay t˜1→χ˜10bff¯′ . The improved treatment has been implemented in the Fortran package SUSY-HIT and is used for the analysis of this region. A scan over the parameter range including all relevant experimental constraints reveals that the FV two-body decay into charm and χ˜10 can be as important as the three-, respectively, four-body decays if not dominant and therefore should be taken into account in order to complete the experimental searches for the light stop

Effects of intermediate scales on renormalization group running of fermion observables in an SO(10) model

In the context of non-supersymmetric SO(10) models, we analyze the renormalization group equations for the fermions (including neutrinos) from the GUT energy scale down to the electroweak energy scale, explicitly taking into account the effects of an intermediate energy scale induced by a Pati-Salam gauge group. To determine the renormalization group running, we use a numerical minimization procedure based on a nested sampling algorithm that randomly generates the values of 19 model parameters at the GUT scale, evolves them, and finally constructs the values of the physical observables and compares them to the existing experimental data at the electroweak scale. We show that the evolved fermion masses and mixings present sizable deviations from the values obtained without including the effects of the intermediate scale