arXiv subjects

Christoph Niehoff

Publications and source records attributed to Christoph Niehoff.

7 recordsLinked to original sources

arXiv2017

Status of the $B\to K^*μ^+μ^-$ anomaly after Moriond 2017

Motivated by recent results by the ATLAS and CMS collaborations on the angular distribution of the $B \to K^* μ^+μ^-$ decay, we perform a state-of-the-art analysis of rare $B$ meson decays based on the $b \to s μμ$ transition. Using standard estimates of hadronic uncertainties, we confirm the presence of a sizable discrepancy between data and SM predictions. We do not find evidence for a $q^2$ or helicity dependence of the discrepancy. The data can be consistently described by new physics in the form of a four-fermion contact interaction $(\bar s γ_αP_L b)(\bar μγ^αμ)$. Assuming that the new physics affects decays with muons but not with electrons, we make predictions for a variety of theoretically clean observables sensitive to violation of lepton flavour universality.

hep-ph↗

arXiv2017

Electroweak symmetry breaking and collider signatures in the next-to-minimal composite Higgs model

We conduct a detailed numerical analysis of the composite pseudo-Nambu-Goldstone Higgs model based on the next-to-minimal coset $\text{SO}(6)/\text{SO}(5)\cong\text{SU}(4)/\text{Sp}(4)$, featuring an additional SM singlet scalar in the spectrum, which we allow to mix with the Higgs boson. We identify regions in parameter space compatible with all current experimental constraints, including radiative electroweak symmetry breaking, flavour physics, and direct searches at colliders. We find the additional scalar, with a mass predicted to be below a TeV, to be virtually unconstrained by current LHC data, but potentially in reach of run 2 searches. Promising indirect searches include rare semi-leptonic $B$ decays, CP violation in $B_s$ mixing, and the electric dipole moment of the neutron.

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arXiv2017

$B_s\toμ^+μ^-$ as current and future probe of new physics

The rare flavour-changing neutral current decay $B_s\toμ^+μ^-$ is among the most important indirect probes of new physics at the LHC, as it is strongly suppressed in the Standard Model, very sensitive to new physics effects, and theoretically exceptionally clean. We present a thorough state-of-the-art analysis of the constraints on new physics from present and future measurements of this decay, focusing on scalar operators. We show model-independently and in concrete new physics models, namely the MSSM and two leptoquark scenarios, that a future precise measurement of the mass-eigenstate rate asymmetry in $B_s\toμ^+μ^-$ would allow to disentangle new physics scenarios that would be indistinguishable based on measurements of the branching ratio alone. We also highlight the complementarity between $B_s\toμ^+μ^-$ and direct searches in both model classes. Our numerics is based on the open source code flavio and is made publicly available.

hep-ph↗

arXiv2015

Direct and indirect signals of natural composite Higgs models

We present a comprehensive numerical analysis of a four-dimensional model with the Higgs as a composite pseudo-Nambu-Goldstone boson that features a calculable Higgs potential and protective custodial and flavour symmetries to reduce electroweak fine-tuning. We employ a novel numerical technique that allows us for the first time to study constraints from radiative electroweak symmetry breaking, Higgs physics, electroweak precision tests, flavour physics, and direct LHC bounds on fermion and vector boson resonances in a single framework. We consider four different flavour symmetries in the composite sector, one of which we show to not be viable anymore in view of strong precision constraints. In the other cases, all constraints can be passed with a sub-percent electroweak fine-tuning. The models can explain the excesses recently observed in $WW$, $WZ$, $Wh$ and $\ell^+\ell^-$ resonance searches by ATLAS and CMS and the anomalies in angular observables and branching ratios of rare semi-leptonic $B$ decays observed by LHCb. Solving the $B$ physics anomalies predicts the presence of a dijet or $t\bar t$ resonance around 1 TeV just below the sensitivity of LHC run 1. We discuss the prospects to probe the models at run 2 of the LHC. As a side product, we identify several gaps in the searches for vector-like quarks at hadron colliders, that could be closed by reanalyzing existing LHC data.

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arXiv2015

On $B\to K^{(*)}\bar νν$ decays in and beyond the Standard Model

In this talk an analysis of the rare exclusive $B$ decays $B\to Kν\barν$ and $B\to K^{*}ν\barν$ within and beyond the Standard Model is presented. Combining new form factor determinations from lattice QCD with light-cone sum rule results and including complete two-loop electroweak corrections to the SM Wilson coefficient, precise Standard Model predictions will be given. Beyond the SM, we make use of an effective theory with dimension-six operators invariant under the SM gauge symmetries to relate NP effects in $b\to s\barνν$ transitions to $b\to s\ell^+\ell^-$ transitions and use the wealth of experimental data on $B\to K^{(*)}\ell^+\ell^-$ and related modes to constrain NP effects in $B\to K^{(*)}ν\barν$. We then consider leptoquark models as a showcase for models where in principle large effects are possible.

hep-ph↗

arXiv2015

Violation of lepton flavour universality in composite Higgs models

We investigate whether the $2.6σ$ deviation from lepton flavour universality in $B^+\to K^+\ell^+\ell^-$ decays recently observed at the LHCb experiment can be explained in minimal composite Higgs models. We show that a visible departure from universality is indeed possible if left-handed muons have a sizable degree of compositeness. Constraints from $Z$-pole observables are avoided by a custodial protection of the muon coupling. The deficit in the invisible $Z$ width at LEP is explained in the same region of parameters.

hep-ph↗

arXiv2014

$B\to K^{(*)}ν\barν$ decays in the Standard Model and beyond

We present an analysis of the rare exclusive $B$ decays $B\to Kν\barν$ and $B\to K^{*}ν\barν$ within the Standard Model (SM), in a model-independent manner, and in a number of new physics (NP) models. Combining new form factor determinations from lattice QCD with light-cone sum rule results and including complete two-loop electroweak corrections to the SM Wilson coefficient, we obtain the SM predictions $\text{BR}(B^+\to K^+ν\barν) = (4.0 \pm 0.5) \times 10^{-6}$ and $\text{BR}(B^0\to K^{* 0}ν\barν) = (9.2\pm1.0) \times 10^{-6}$, more precise and more robust than previous estimates. Beyond the SM, we make use of an effective theory with dimension-six operators invariant under the SM gauge symmetries to relate NP effects in $b\to sν\barν$ transitions to $b\to s\ell^+\ell^-$ transitions and use the wealth of experimental data on $B\to K^{(*)}\ell^+\ell^-$ and related modes to constrain NP effects in $B\to K^{(*)}ν\barν$. We then consider several specific NP models, including $Z'$ models, the MSSM, models with partial compositeness, and leptoquark models, demonstrating that the correlations between $b\to sν\barν$ observables among themselves and with $B_s\toμ^+μ^-$ and $b\to s\ell^+\ell^-$ transitions offer powerful tests of NP with new right-handed couplings and non-MFV interactions.

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