arXiv subjects

Andreas Crivellin

Publications and source records attributed to Andreas Crivellin.

At least 127 records · Page 7Linked to original sources

arXiv2018

QCD Improved Matching for Semi-Leptonic B Decays with Leptoquarks

Leptoquarks (LQs) provide very promising solutions to the tensions between the experimental measurements and the SM predictions of $b\to s\ell^+\ell^-$ and $b\to cτν$ processes. In this case the LQ masses are in general at the TeV scale and they can thus be produced at high energy colliders and dedicated LHC searches are ongoing. While for LQ production and decay the $O(α_s)$ corrections have been known for a long time, the $O(α_s)$ corrections to the matching on 2-quark-2-lepton operators have not been calculated, yet. In this article we close this gap by computing the QCD corrections to the matching of LQ models on the effective SM Lagrangian for both scalar and vector LQs. We find an enhancement of the Wilson coefficients of vector operators with respect to the tree-level results of around 8 % (13 %) if they originate from scalar (vector) LQs. This softens the LHC bounds and increases the allowed parameter space of LQ models addressing the flavour anomalies.

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arXiv2018

Matching of gauge invariant dimension 6 operators for $b\to s$ and $b\to c$ transitions

New physics realized above the electroweak scale can be encoded in a model independent way in the Wilson coefficients of higher dimensional operators which are invariant under the Standard Model gauge group. In this article, we study the matching of the $SU(3)_C \times SU(2)_L \times U(1)_Y$ gauge invariant dim-6 operators on the standard $B$ physics Hamiltonian relevant for $b \to s$ and $b\to c$ transitions. The matching is performed at the electroweak scale (after spontaneous symmetry breaking) by integrating out the top quark, $W$, $Z$ and the Higgs particle. We first carry out the matching of the dim-6 operators that give a contribution at tree level to the low energy Hamiltonian. In a second step, we identify those gauge invariant operators that do not enter $b \to s$ transitions already at tree level, but can give relevant one-loop matching effects.

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arXiv2018

$B$ Meson Anomalies in a Pati-Salam Model within the Randall-Sundrum Background

Lepton number as a fourth color is the intriguing theoretical idea of the famous Pati-Salam (PS) model. While in conventional PS models, the symmetry breaking scale and the mass of the resulting vector leptoquark are stringently constrained by $K_L\toμe$ and $K\toπμe$, the scale can be lowered to a few TeV by adding vector-like fermions. Furthermore, in this case, the intriguing hints for lepton flavour universality violation in $b\to sμ^+μ^-$ and $b\to cτν$ processes can be addressed. Such a setup is naturally achieved by implementing the PS gauge group in the five-dimensional Randall-Sundrum background. The PS symmetry is broken by boundary conditions on the fifth dimension and the resulting massive vector leptoquark automatically has the same mass scale as the vector-like fermions and all other resonances. We consider the phenomenology of this model in the context of the hints for lepton flavour universality violation in semileptonic $B$ decays. Assuming flavour alignment in the down sector we find that in $b\to s\ell^+\ell^-$ transitions the observed deviations from the SM predictions (including $R(K)$ and $R(K^*)$) can be explained with natural values for the free parameters of the model. Even though we find sizable effects in $R(D)$, $R(D^*)$ and $R(J/Ψ)$ one cannot account for the current central values in the constrained setup of our minimal model due to the stringent constraints from $D-\bar D$ mixing and $τ\to 3μ$.

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arXiv2018

Lepton flavour violation in the MSSM: exact diagonalization vs mass expansion

The forthcoming precision data on lepton flavour violating decays require precise and efficient calculations in New Physics models. In this article lepton flavour violating processes within the Minimal Supersymmetric Standard Model (MSSM) are calculated using the method based on the Flavour Expansion Theorem, a recently developed technique performing a purely algebraic mass-insertion expansion of the amplitudes. The expansion in both flavour-violating and flavour-conserving off-diagonal terms of sfermion and supersymmetric fermion mass matrices is considered. In this way the relevant processes are expressed directly in terms of the parameters of the MSSM Lagrangian. We also study the decoupling properties of the amplitudes. The results are compared to the corresponding calculations in the mass eigenbasis (i.e. using the exact diagonalization of the mass matrices). Using these methods, we consider the following processes: $\ell \to \ell' γ$, $\ell \to 3 \ell'$, $\ell \to 2\ell'\ell"$, $h \to \ell\ell'$ as well as $μ\to e$ conversion in nuclei. In the numerical analysis we update the bounds on the flavour changing parameters of the MSSM and examine the sensitivity to the forthcoming experimental results. We find that flavour violating muon decays provide the most stringent bounds on supersymmetric effects and will continue to do so in the future. Radiative $\ell\to\ell^\primeγ$ decays and leptonic three-body decays $\ell\to 3\ell^\prime$ show an interesting complementarity in eliminating "blind spots" in the parameter space. In our analysis we also include the effects of non-holomorphic $A$-terms which are important for the study of LFV Higgs decays.

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arXiv2018

A no-go theorem for non-standard explanations of the $τ\to K_Sπν_τ$ CP asymmetry

The CP asymmetry in $τ\to K_Sπν_τ$, as measured by the BaBar collaboration, differs from the Standard Model prediction by $2.8σ$. Most non-standard interactions do not allow for the required strong phase needed to produce a non-vanishing CP asymmetry, leaving only new tensor interactions as a possible mechanism. We demonstrate that, contrary to previous assumptions in the literature, the crucial interference between vector and tensor phases is suppressed by at least two orders of magnitude due to Watson's final-state-interaction theorem. Furthermore, we find that the strength of the relevant CP-violating tensor interaction is strongly constrained by bounds from the neutron electric dipole moment and $D$-$\bar{D}$ mixing. These observations together imply that it is extremely difficult to explain the current $τ\to K_Sπν_τ$ measurement in terms of physics beyond the Standard Model originating in the ultraviolet.

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arXiv2018

Large $h\to b s$ in generic two-Higgs-doublet models

We investigate the possible size of $h\to bs$ in two-Higgs-doublet models with generic Yukawa couplings. Even though the corresponding rates are in general expected to be small due to the indirect constraints from $B_s\toμ^{+}μ^{-}$ and $B_s$--$\overline{B}_s$ mixing, we find regions in parameter space where $h\to bs$ can have a sizable branching ratio well above 10\%. This requires a tuning of the neutral scalar masses and their couplings to muons, but then all additional constraints such as $B\to X_s γ$, $(g-2)_μ$, and $h\toμ^{+}μ^{-}$ are satisfied. In this case, $h\to bs$ can be a relevant background in $h\to b\bar{b}$ searches and vice versa due to the imperfect $b$-tagging purity. Furthermore, if $h\to bs$ is sizeable, one expects two more scalar resonances in the proximity of $m_h$. We briefly comment on other flavour violating Higgs decays and on the 95 GeV $γγ$ resonance within generic two-Higgs-doublet models.

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arXiv2018

Correlating Lepton Flavour (Universality) Violation in $B$ Decays with $μ\to eγ$ using Leptoquarks

Motivated by the measurements of $b\to s\ell^+\ell^-$ transitions, including $R(K)$ and $R(K^*)$, we examine lepton flavour (universality) violation in $B$ decays and its connections to $μ\to eγ$ in generic leptoquark models. Considering all 10 representations of scalar and vector leptoquarks under the Standard Model gauge group we compute the tree-level matching for semileptonic $b$-quark operators as well as their loop effects in $\ell\to\ell^\primeγ$. In our phenomenological analysis we correlate $R(K)$, $R(K^*)$ and the other $b\to sμ^+μ^-$ data to $μ\to eγ$ and $b\to sμe$ transitions for the three leptoquark representations that generate left-handed currents in $b\to s\ell^+\ell^-$ transitions and, therefore, provide a good fit to data. We find that while new physics contributions to muons are required by the global fit, also couplings to electrons can be sizeable without violating the stringent bounds from $μ\to eγ$. In fact, if the effect in electrons in $b\to s\ell^+\ell^-$ has opposite sign than the effect in muons the bound from $μ\to eγ$ can always be avoided. However, unavoidable effects in $b\to sμe$ transitions (i.e. $B_s\toμe$, $B\to Kμe$, etc.) appear which are within the reach of LHCb and BELLE II.

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arXiv2018

Patterns of New Physics in $b\to s\ell^+\ell^-$ transitions in the light of recent data

In the Standard Model (SM), the rare transitions where a bottom quark decays into a strange quark and a pair of light leptons exhibit a potential sensitivity to physics beyond the SM. In addition, the SM embeds Lepton Flavour Universality (LFU), leading to almost identical probabilities for muon and electron modes. The LHCb collaboration discovered a set of deviations from the SM expectations in decays to muons and also in ratios assessing LFU. Other experiments (Belle, ATLAS, CMS) found consistent measurements, albeit with large error bars. We perform a global fit to all available $b\to s\ell^+\ell^-$ data ($\ell=e,μ$) in a model-independent way allowing for different patterns of New Physics. For the first time, the NP hypothesis is preferred over the SM by $5\,σ$ in a general case when NP can enter SM-like operators and their chirally-flipped partners. LFU violation is favoured with respect to LFU at the 3-4$\,σ$ level. We discuss the impact of LFU-violating New Physics on the observable $P_5^\prime$ from $B \to K^*μ^+μ^-$ and we compare our estimate for long-distance charm contributions with an empirical model recently proposed by a group of LHCb experimentalists. Finally, we discuss NP models able to describe this consistent pattern of deviations.

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arXiv2017

Searching for New Physics with $b\to sτ^+τ^-$ processes

In recent years, intriguing hints for the violation of Lepton Flavour Universality (LFU) have been accumulated in semileptonic $B$ decays, both in the neutral-current transitions $b\to s\ell^+\ell^-$ (i.e., $R_K$ and $R_{K^*}$) and the charged-current transitions $b\to c\ell^-\barν_\ell$ (i.e., $R_D$, $R_{D^*}$ and $R_{J/ψ}$). LHCb has reported deviations from the Standard Model (SM) expectations in $b\to sμ^+μ^-$ processes as well as in the ratios $R_K$ and $R_{K^*}$, which together point at New Physics (NP) affecting muons with a high significance. Furthermore, hints for LFU violation in $R_{D^{(*)}}$ and $R_{J/ψ}$ point at large deviations from the SM in processes involving tau leptons. Together, these hints for NP motivate the possibility of huge LFU-violating effects in $b\to sτ^+τ^-$ transitions. In this article we predict the branching ratios of $B\to Kτ^+τ^-$, $B\to K^{*}τ^+τ^-$ and $B_s\to ϕτ^+τ^-$ taking into account NP effects in the Wilson coefficients $C_{9(')}^{ττ}$ and $C_{10(')}^{ττ}$. Assuming a common NP explanation of $R_{D^{}}$ , $R_{D^{(*)}}$ and $R_{J/ψ}$, we show that a very large enhancement of $b\to sτ^+τ^-$ processes, of around three orders of magnitude compared to the SM, can be expected under fairly general assumptions. We find that the branching ratios of $B_s\to τ^+τ^-$, $B_s\to ϕτ^+τ^-$ and $B\to K^{(*)}τ^+τ^-$ under these assumptions are in the observable range for LHCb and Belle II.

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arXiv2017

The MSSM without Gluinos; an Effective Field Theory for the Stop Sector

In this article we study the MSSM with stops and Higgs scalars much lighter than gluinos and squarks of the first two generations. In this setup, one should use an effective field theory with partial supersymmetry in which the gluino and heavy squarks are integrated out in order to connect SUSY parameters (given at a high scale) to observables in the stop sector. In the construction of this effective theory, valid below the gluino mass scale, we take into account $O(α_3)$ and $O(Y_{t,b}^2)$ effects and calculate the matching as well as the renormalization group evolution. As a result, the running of the parameters for the stop sector is modified with respect to the full MSSM and SUSY relations between parameters are broken. We show that for some couplings sizable numerical differences exist between the effective field theory approach and the naive calculation based on the MSSM running.

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arXiv2017

Simultaneous Explanation of $R(D^{(*)})$ and $b\to sμ^+μ^-$: The Last Scalar Leptoquarks Standing

Over the past years, experiments accumulated intriguing hints for new physics (NP) in flavor observables, namely in the anomalous magnetic moment of the muon ($a_μ$), in $R(D^{(*)})={\rm Br}(B\to D^{(*)}τν)/{\rm Br}(B\to D^{(*)}\ellν)$ and in $b\to sμ^+μ^-$ transitions, which are all at the $3-4\,σ$ level. In this article we point out that one can explain the $R(D^{(*)})$ anomaly using two scalar leptoquarks (LQs) with the same mass and coupling to fermions related via a discrete symmetry: an $SU(2)$ singlet and an $SU(2)$ triplet, both with hypercharge $Y=-2/3$. In this way, potentially dangerous contributions to $b\to sνν$ are avoided and non-CKM suppressed effects in $R(D^{(*)})$ can be generated. This allows for smaller overall couplings to fermions weakening the direct LHC bounds. In our model, $R(D^{(*)})$ is directly correlated to $b\to sτ^+τ^-$ transitions where an enhancement by orders of magnitude compared to the standard model (SM) is predicted, such that these decay modes are in the reach of LHCb and BELLE II. Furthermore, one can also naturally explain the $b\to sμ^+μ^-$ anomalies (including $R(K)$) by a $C_9=-C_{10}$ like contribution without spoiling $μ-e$ universality in charged current decays. In this case sizable effects in $b\to sτμ$ transitions are predicted which are again well within the experimental reach. One can even address the longstanding anomaly in $a_μ$, generating a sizable decay rate for $τ\toμγ$. However, we find that out of the three anomalies $R(D^{(*)})$, $b\to sμ^+μ^-$ and $a_μ$ only two (but any two) can be explained simultaneously.

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arXiv2017

$K\to πν\overlineν$ in the MSSM in Light of the $ε^{\prime}_K/ε_K$ Anomaly

The Standard-Model (SM) prediction for the CP-violating quantity $ε_K^{\prime}/ε_K$ deviates from its measured value by 2.8 $σ$. It has been shown that this tension can be resolved within the Minimal Supersymmetric Standard Model (MSSM) through gluino-squark box diagrams, even if squarks and gluinos are much heavier than 1 TeV. The rare decays $K_L \to π^0ν\barν$ and $K^+ \to π^+ν\barν$ are similarly sensitive to very high mass scales and the first one also measures CP violation. In this article, we analyze the correlations between $ε^{\prime}_K/ε_K$ and $B(K_L \to π^0ν\barν)$ and $B(K^+ \to π^+ν\barν)$ within the MSSM aiming at an explanation of $ε_K^{\prime}/ε_K$ via gluino-squark box diagrams. The dominant MSSM contribution to the $K \to πν\barν$ branching fractions stems from box diagrams with squarks, sleptons, charginos, and neutralinos, and the pattern of the correlations is different from the widely studied $Z$-penguin scenarios. This is interesting in light of future precision measurements by KOTO and NA62 at J-PARC and CERN, respectively. We find $B(K_L \to π^0ν\barν)/B^{SM} (K_L \to π^0ν\barν)\lesssim 2\,(1.2)$ and $B(K^+ \to π^+ν\barν)/B^{SM}(K^+ \to π^+ν\barν) \lesssim 1.4\,(1.1)$, if all squark masses are above 1.5 TeV, gaugino masses obey GUT relations, and if one allows for a fine-tuning at the $1\%\,(10\%)$ level for the gluino mass. Larger values are possible for a tuned CP violating phase. Furthermore, the sign of the MSSM contribution to $ε_K^{\prime}$ imposes a strict correlation between $B(K_L \to π^0ν\barν)$ and the hierarchy between the masses $m_{\bar{U}}$, $m_{\bar{D}}$ of the right-handed up-squark and down-squark: sgn$[B(K_L \to π^0ν\barν)-B^{SM} (K_L \to π^0ν\barν)] = $sgn$(m_{\bar{U}}-m_{\bar{D}}) $.

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arXiv2017

New Physics in Flavour Observables

LHCb found hints for physics beyond the Standard Model (SM) in $B\to K^*μ^+μ^-$, $R(K)$ and $B_s\toϕμ^+μ^-$. These intriguing hints for NP have recently been confirmed by the LHCb measurement of $R(K^*)$ giving a combined significance for NP above the $5\,σ$ level. In addition, the BABAR, BELLE and LHCb results for $B\to D^{(*)}τν$ also point towards lepton flavour universality (LFU) violating new physics (NP). Furthermore, there is the long-standing discrepancy between the measurement and the theory prediction of the anomalous magnetic moment of the muon ($a_μ$) at the $3\,σ$ level. Concerning NP effects, $b\to sμ^+μ^-$ data can be naturally explained with a new neutral gauge bosons, i.e. a $Z^\prime$ but also with heavy new scalars and fermions contributing via box diagrams. Another promising solution to $b\to sμ^+μ^-$, which can also explain $B\to D^{(*)}τν$, are leptoquarks. Interestingly, leptoquarks provide also a viable explanation of $a_μ$ which can be tested via correlated effects in $Z\toμ^+μ^-$ at future colliders. Considering leptoquark models, we show that an explanation of $B\to D^{(*)}τν$ predicts an enhancement of $b\to sτ^+τ^-$ processes by around three orders of magnitude compared to the SM. In case of a simultaneous explanation of $B\to D^{(*)}τν$ and $b\to sμ^+μ^-$ data, sizable effects in $b\to sτμ$ processes are predicted.

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arXiv2017

Renormalisation-group improved analysis of $μ\to e$ processes in a systematic effective-field-theory approach

In this article, a complete analysis of the three muonic lepton-flavour violating processes $μ\to e γ$, $μ\to 3e$ and coherent nuclear $μ\to e$ conversion is performed in the framework of an effective theory with dimension six operators defined below the electroweak symmetry breaking scale $m_W$. The renormalisation-group evolution of the Wilson coefficients between $m_W$ and the experimental scale is fully taken into account at the leading order in QCD and QED, and explicit analytic and numerical evolution matrices are given. As a result, muonic decay and conversion rates are interpreted as functions of the Wilson coefficients at any scale up to $m_W$. Taking the experimental limits on these processes as input, the phenomenology of the mixing effects is investigated. It is found that a considerable set of Wilson coefficients unbounded in the simplistic tree-level approach are instead severely constrained. In addition, correlations among operators are studied both in the light of current data and future experimental prospects.

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arXiv2017

Loop effects of heavy new scalars and fermions in $b\to sμ^+μ^-$

Recent measurements of $b\to sμ^+μ^-$ processes at LHCb and BELLE have revealed tensions at the $2-3\,σ$ level between the Standard Model (SM) prediction and the experimental results in the channels $B\to K^*μ^+μ^-$ and $B_s\toϕμ^+μ^-$, as well as in the lepton-flavor universality violating observable $R_K={\rm Br}(B\to Kμ^+μ^-)/{\rm Br}(B\to Ke^+e^-$). Combined global fits to the available $b\to sμ^+μ^-$ data suggest that these tensions might have their common origin in New Physics (NP) beyond the SM because some NP scenarios turn out to be preferred over the SM by $4-5\,σ$. The fact that all these anomalies are related to muons further suggests a connection (and a common NP explanation) with the long-standing anomaly in the anomalous magnetic moment of the muon, $a_μ$. In this article, we study the impact of a generic class of NP models featuring new heavy scalars and fermions that couple to the SM fermions via Yukawa-like interactions. We consider two different scenarios, introducing either one additional fermion and two scalars or two additional fermions and one scalar, and examine all possible representations of the new particles under the SM gauge group with dimension up to the adjoint one. The models induce one-loop contributions to $b\to sμ^+μ^-$ and $a_μ$ which are capable of solving the respective anomalies at the $2σ$ level, albeit a relatively large coupling of the new particles to muons is required. In the case of $b\to sμ^+μ^-$, stringent constraints from $B_s-\overline{B}_s$ mixing arise which can be relaxed if the new fermion is a Majorana particle.

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arXiv2017

Lepton Flavor Non-Universality in B decays from Dynamical Yukawas

The basic features of quark and lepton mass matrices can be successfully explained by natural minima of a generic potential with dynamical Yukawa fields invariant under the $[\mathrm{SU(3)}]^5\times \mathcal{O}(3)$ flavor symmetry. If this symmetry is gauged, in order to avoid potentially dangerous Goldstone bosons, and small perturbations are added to exactly fit the observed pattern of fermion masses, the spectrum of massive flavor gauge bosons can naturally explain the hints for new physics in $b\to s \ell^+\ell^-$ transitions, including $R_K$. In particular, the desired pattern of the Standard Model Yukawa couplings is compatible with a gauged $\mathrm{U(1)}_q$ in the quark sector, and $\mathrm{U(1)}_{μ-τ}$ in the lepton sector spontaneously broken around the TeV scale. In order to explain the aforementioned experimental hints, the corresponding neutral gauge bosons are required to mix, yielding to potentially observable signals in dimuon resonance searches at the LHC.

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arXiv2016

$(g-2)_μ$, Lepton Flavour Violation and $Z$ Decays with Leptoquarks: Correlations and Future Prospects

The long-standing anomaly in the anomalous magnetic moment of the muon indicates the presence of chirality violating new physics contributions. A possible solution involves scalar leptoquarks with left- and right-handed couplings to the top quark. Two such representations of scalar leptoquarks exist for which the contribution to $(g-2)_μ$ can possess an $m_t/m_μ$ enhancement compared to the Standard Model. The leptoquarks also induce loop corrections to $Z$ couplings to muons which probe as well new physics contributions which possess sources of $SU(2)$ symmetry breaking and we find that this effect should be observable at future experiments as GigaZ or TLEP. Furthermore, once interactions of the leptoquark with tau leptons and electrons are present, additional correlated effects in anomalous magnetic moments, $Z\to\ell\ell^\prime$ and $\ell\to\ell^\primeγ$ arise, which can be used to test the model and to determine the flavour structure of the couplings. We find that the two representations of leptoquarks can be distinguished also from low energy experiments: one representation predicts constructive interference with the Standard Model in $Z$ couplings to leptons and effects in $B\to K^{(\star)}ν\barν$, while the other representation interferes destructively with the Standard Model in $Z$ couplings to leptons and gives a $C_9=C_{10}$-like contribution to $b\to s\ell^+\ell^-$ processes.

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arXiv2016

Probing lepton flavour (universality) violation at NA62 and future kaon experiments

Recent results from the LHC's first run have revealed intriguing departures from lepton flavour universality in the semi-leptonic decays of $B$-mesons. We discuss the complementary role that rare kaon decays can provide in testing new physics explanations of these flavour anomalies. In the framework of minimal flavour violation, we relate the chiral low-energy constants involved in $K\toπ\ell\ell'$ and $K\to\ell\ell'$ ($\ell = μ\mbox{ or } e$) with the new physics Wilson coefficients of the $b\to s$ effective Hamiltonian. We comment on the determination of these low-energy constants at NA62 and future kaon experiments, as well as the required improvements in sensitivity necessary to test the $B$-physics anomalies in the kaon sector.

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