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Andreas Crivellin

Publications and source records attributed to Andreas Crivellin.

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Tauonic B decays in 2HDMs and the MSSM in the decoupling limit

In these proceedings we review two aspects of 2HDMs with generic Yukawa structures. The first part considers how recent deviations from the SM expectations in tauonic B decays (observed by BABAR) can be explained in a 2HDM of type III with sizable flavour violation in the up-sector. The second part discusses the matching of the MSSM on the 2HDM of type III. Here we focus on the recently calculated two-loop SQCD corrections to the Higgs-quark-quark couplings.

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Explaining B->Dτν, B->D*τνand B->τνin a 2HDM of type III

Recently, the BaBar collaboration reported first evidence for new physics in B->Dτνand B->D*τν. Combining both processes the significance is 3.4σ. This result cannot be explained in a two Higgs doublet model of type II. Furthermore, the CKMfitter group finds a 2.9σdiscrepancy between the Standard Model prediction for Br[B->τν] (using Vub from a global fit to the unitary triangle) and the measurements of the B factories. Altogether, these measurements are strong indications for physics beyond the SM in B meson decays to taus. We show that in a two Higgs doublet model of type III it is possible to simultaneously explain \bdtau and \bdstau using a single free parameter ε^u_32. Also Br[B->τν] can be brought into agreement with experiment using ε^u_31. Furthermore, for Higgs (A^0, H^0, H^+) masses around 500 GeV, as suggested by recent CMS results, all bounds from FCNC processes are satisfied and B->Dτν, B->D*τνand B->τνcan be explained without a significant degree of fine-tuning.

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Effective Higgs Vertices in the generic MSSM

In this article we consider chirally enhanced corrections to Higgs vertices in the most general MSSM. We include the contributions stemming from bilinear-terms, from the trilinear A-terms and their non-holomorphic analogues, the A'-terms, which couple squarks to the "wrong" Higgs field. We perform a consistent renormalization of the Higgs vertices beyond the decoupling limit M_SUSY->infinity, using a purely diagrammatic approach. The cancellation of the different contributions in and beyond the decoupling limit is discussed and the possible size of decoupling effects which occur if the SUSY particles are not much heavier than the electroweak-scale are examined. In the decoupling limit we recover the results obtained in the effective-field-theory approach. For the non-holomorphic A'-terms we find the well known $\tanβ$ enhancement in the down-sector similar to the one for terms proportional to mu. Due to the a priori generic flavor structure of these trilinear terms large flavour-changing neutral Higgs couplings can be induced. We also discover new tan(beta) enhanced contributions involving the usual holomorphic A-terms, which were not discussed before in the literature. These corrections occur only if also flavor-diagonal non-holomorphic corrections to the Higgs couplings are present. This reflects the fact that the A-terms, and also the chirality-changing self-energies, are physical quantities and cannot be absorbed into renormalization constants.

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Rare decays and MSSM phenomenology

In this article I review some aspects of flavour phenomenology in the MSSM. After an overview of various flavour observables I discuss the constraints on the off-diagonal elements of the squark mass matrices. In this context I present the Fortran code SUSY_FLAVOR which calculates these processes in the generic MSSM including the complete resummation of all chirally enhanced effects as a new feature of version 2. Than I discuss where large new physics effects in the MSSM are still possible. As an example of a model which can give large effects in flavour physics I review a model with "radiative flavour violation" (RFV) and update the results in the light of the recent LHCb measurement of Bs -> μμ. Finally, I recall that the MSSM can generate a sizable right-handed W-coupling which affects B -> τνand can solve the Vub problem.

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Flavor violation in the MSSM and implications for top and squark searches at colliders

In this article I review some connections between flavor physics and collider physics. The first part discusses the effect of right-handed charged currents on the determination of the CKM elements V_ub. It is shown that such an effective right-handed W-coupling can be generated in the MSSM which would lead to a sizable enhancement of single-top production at the LHC. The second part of this article focuses on the constraints on the mass splitting between left-handed squarks from Kaon and D mixing. Such a mass splitting has interesting consequences for squark decay chains at colliders.

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Phenomenological consequences of radiative flavor violation in the MSSM

In this article we investigate the consequences of radiative flavor violation (RFV) in the Minimal Supersymmetric Standard Model (MSSM). In this framework the small off-diagonal elements of the Cabibbo-Kobayashi-Maskawa (CKM) matrix and the small quark masses of the first two generations are generated from the trilinear supersymmetry-breaking terms. The impact of RFV on flavor-physics observables is studied in detail. We focus on the limiting cases in which the CKM matrix is either generated in the down-sector, i.e. by the soft SUSY-breaking mass insertions delta^dLR_i3 (i=1,2), or in the up-quark sector, i.e. by the mass insertions delta^uLR_i3. In the first case we find an enhancement of b->s gamma, which constrains the allowed range of sparticle masses (Fig. 3). In addition, neutral Higgs penguins significantly contribute to B_s,d->mu^+mu^- and, if also delta^dLR_32 is different from zero, these Higgs effects are capable of explaining the observed CP phase in the B_s system. If, on the other hand, the CKM generation takes place in the up-sector, |epsilon_K| receives additional positive contributions enforcing large squark and gluino masses (see Fig. 8). In this case also the rare decay K->pi nu nu receives sizable contributions. In conclusion we find that for SUSY masses around 1 TeV RFV is an interesting alternative to Minimal Flavor Violation (MFV).

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B -> X_d gamma and constraints on new physics

We combine recent progress in measuring the branching ratio of the decay B->X_d gamma$ with the discovery that hadronic uncertainties in the CP-averaged branching ratio drop out to a large extent. Implications of these improvements on the size of possible new physics effects are investigated. We find the updated SM prediction for the CP-averaged branching ratio to be Br[B->X_d gamma]^SM_E_gamma>1.6 GeV = 1.54^+0.26-0.31*10^-5, which should be compared with the experimental value of Br[B->X_d gamma]^exp_E_gamma>1.6GeV = (1.41+-0.57) 10^-5. After performing a model independent analysis, we consider different new physics models: the MSSM with generic sources of flavor violation, the two Higgs doublet model of type III and a model with right-handed charged currents. It is found that the constraints on the SUSY parameters delta^d_13 have improved and that the absolute value of the right-handed quark mixing matrix element |V^R_td| must be smaller than 1.5*10^-4.

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The MSSM with a softly broken U(2)^3 flavor symmetry

In this article we review the phenomenological consequences of radiative flavor-violation (RFV) in the MSSM. In the model under consideration the U(3)^3 flavor symmetry of the gauge sector is broken in a first step to U(2)^3 by the top and bottom Yukawa couplings of the superpotential (and possibly also by the bilinear SUSY-breaking terms). In a second step the remaining U(2)^3 flavor symmetry is softly broken by the trilinear A-terms in order to obtain the measured quark masses and the CKM matrix of the Standard Model (SM) at low energies. The phenomenological implications of this model depend on the actual choice of the SUSY breaking A-terms. If the CKM matrix is generated in the down sector (by A^d), Bs->mu^+mu^- receives non-decoupling contributions from Higgs penguins which become important already for moderate values of tan(beta). Also the Bs mixing amplitude can be significantly modified compared to the SM prediction including a potential induction of a new CP-violating phase (which is not possible in the MSSM with MFV).

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SUSY-breaking as the origin of flavour violation

In this article I review the consequences of radiative flavor violation (RFV) in the Minimal Supersymmetric Standard Model (MSSM). In this framework the trilinear supersymmetry (SUSY)-breaking terms are the only source of flavor violation. Constraints on the parameter space come from Kaon mixing and b->s+gamma and are satisfied for SUSY masses of the order of 1 TeV. In addition, neutral Higgs penguins significantly contribute to B_s->mu^+mu^- and are capable of explaining the observed CP phase in the B_s system.

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Chirally enhanced corrections in the MSSM

In the general MSSM, chirally-enhanced corrections (to Yukawa couplings) are induced by gluino-squark, chargino-sfermion and neutralino-sfermion loops and can numerically compete with, or even dominate over, tree-level contributions, due to their chiral enhancement. In this article I review the resummation of these corrections to all orders in pertubation theory based on recent work done in collaboration with Lars Hofer and Janusz Rosiek (arXiv:1103.4272).

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Chiral Enhancement in the MSSM -- An Overview

In this article I review the origin and the effects of chirally enhanced loop-corrections in the MSSM based on Refs. [1-3]. Chiral enhancement is related to fermion-Higgs couplings (or self-energies when the Higgs field is replaced by its vev). I describe the resummation of these chirally-enhanced corrections to all orders in perturbation theory and the calculation of the effective fermion-Higgs and gaugino(higgsino)-fermion vertices. As an application a model with radiative flavor-violation is discussed which can solve the SUSY-CP and the SUSY-flavor problem while it is still capable of explaining the observed deviation from the SM in the Bs mixing phase.

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Yukawa coupling and anomalous magnetic moment of the muon: an update for the LHC era

We study the interplay between a soft muon Yukawa coupling generated radiatively with the trilinear A-terms of the minimal supersymmetric standard model (MSSM) and the anomalous magnetic moment of the muon. In the absence of a tree-level muon Yukawa coupling the lightest smuon mass is predicted to be in the range between 750 GeV and 2700 GeV at 2 sigma, if the bino mass M_1 is below 1 TeV. Therefore, a detection of a smuon (in conjunction with a sub-TeV bino) at the LHC would directly imply a non-zero muon Yukawa coupling in the MSSM superpotential. Inclusion of slepton flavor mixing could in principle lower the mass of one smuon-like slepton below 750 GeV. However, the experimental bounds on radiative lepton decays instead strengthen the lower mass bound, with larger effects for smaller M_1, We also extend the analysis to the electron case and find that a light selectron close to the current experimental search limit may prove the MSSM electron Yukawa coupling to be non-zero.

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Supersymmetric renormalisation of the CKM matrix and new constraints on the squark mass matrices

We compute the finite renormalisation of the Cabibbo-Kobayashi-Maskawa (CKM) matrix induced by gluino-squark diagrams in the MSSM with non-minimal sources of flavour violation. Subsequently we derive bounds on the flavour-off-diagonal elements of the squark mass matrices by requiring that the radiative corrections to the CKM elements do not exceed the experimental values. Our constraints on the associated dimensionless quantities delta^{d LR}_{ij}, j>i, are stronger than the bounds from flavour-changing neutral current (FCNC) processes if gluino and squarks are heavier than 500 GeV. Our bound on |delta^{u LR}_{12}| is stronger than the FCNC bound from D-D-bar mixing for superpartner masses above 900 GeV. We further find a useful bound on |delta_{13}^{u LR}|, for which no FCNC constraint is known. Our results imply that it is still possible to generate all observed flavour violation from the soft supersymmetry-breaking terms without conflicting with present-day data on FCNC processes. We suggest that a flavour symmetry renders the Yukawa sector flavour-diagonal and the trilinear supersymmetry-breaking terms are the spurion fields breaking this flavour symmetry. We further derive the dominant supersymmetric radiative corrections to the couplings of charged Higgs bosons and charginos to quarks and squarks.

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Chirally enhanced corrections to FCNC processes in the generic MSSM

Chirally enhanced supersymmetric QCD corrections to FCNC processes are investigated in the framework of the MSSM with generic sources of flavor violation. These corrections arise from flavor-changing self-energy diagrams and can be absorbed into a finite renormalization of the squark-quark-gluino vertex. In this way enhanced two-loop and even three-loop diagrams can be efficiently included into a leading-order (LO) calculation. Our corrections substantially change the values of the parameters delta^{d,LL}_{23}, delta^{d,LR}_{23}, delta^{d,RL}_{23}, and delta^{d,RR}_{23} extracted from Br[B->X_s gamma] if tan(beta) is large. We find stronger (weaker) constraints compared to the LO result for negative (positive) values of mu. The constraints on delta^{d,LR,RL}_{13} and delta^{d,LR,RL}_{23} from B_d mixing and B_s mixing change drastically if the third-generation squark masses differ from those of the first two generations. K mixing is more strongly affected by the chirally enhanced loop diagrams and even sub-percent deviations from degenerate down and strange squark masses lead to profoundly stronger constraints on delta^{d,LR,RL}_{12}.

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Do squarks have to be degenerate? Constraining the mass splitting with Kaon and D mixing

We study the constraints on the mass-splitting of the first two generations of left-handed squarks obtained from Delta M_K, epsilon_K and D mixing. The different contributions from gluino, neutralino and chargino diagrams are examined in detail, concluding that it is not justified to neglect electroweak gaugino diagrams if the squark mass matrices contain flavor non-diagonal LL elements. We find that the constraints on the mass-splitting are very strong for light gluino masses. However, if the gluino is heavier than the squarks the constraints on the mass-splitting are much weaker. There are even large regions in parameter space where the different NP contributions cancel each other, leaving the mass-splitting nearly unconstrained.

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Constraining the MSSM sfermion mass matrices with light fermion masses

We study the finite supersymmetric loop corrections to fermion masses and mixing matrices in the generic MSSM. In this context the effects of non-decoupling chirally-enhanced self-energies are studied beyond leading order in perturbation theory. These NLO corrections are not only necessary for the renormalization of the CKM matrix to be unitary, they are also numerically important for the light fermion masses. Focusing on the tri-linear A-terms with generic flavor-structure we derive very strong bounds on the chirality-changing mass insertions delta^{f\,LR,RL}_{IJ} by applying 't Hooft's naturalness criterion. In particular, the NLO corrections to the up quark mass allow us to constrain the unbounded element delta^{u\,RL}_{13} if at the same time $δ^{u\,LR}_{13}$ is unequal to zero. Our result is important for single-top production at the LHC.

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Effects of right-handed charged currents on the determinations of |V_ub| and |V_cb|

We study the effect of a right-handed coupling of quarks to the W-boson on the measurements of |V_ub| and |V_cb|. It is shown that such a coupling can remove the discrepancies between the determinations of |V_ub| from B\toπ\ellν, B^+\toτ^+ νand B\to X_u\ellν. Further the measurements of |V_cb| from B\to D^*\ellν, B\to D\ellνand inclusive B\to X_c\ellνdecays can be brought into better agreement. We demonstrate that a right-handed coupling can be generated within the MSSM by a finite gluino-squark loop. The effect involves the parameters δ^{u RL}_{23} and δ^{u RL}_{13} of the squark mass matrices, which are poorly constrained from other processes. On the other hand, all gluino-squark corrections to the regular left-handed coupling of the W-boson are found to be too small to be relevant.

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CKM Elements from Squark Gluino Loops

I present results for the finite renormalization of the Cabibbo-Kobayashi-Maskawa (CKM) matrix induced by gluino--squark diagrams in the MSSM with non-minimal sources of flavour violation. Subsequently I derive bounds on the flavour--off--diagonal elements of the squark mass matrices by requiring that the radiative corrections to the CKM elements do not exceed the experimental values. The constraints on the associated dimensionless quantities $δ^{d LR}_{ij}$, $j>i$, are stronger than the bounds from flavour-changing neutral current (FCNC) processes if gluino and squarks are heavier than 500$ \gev$. The results imply that it is still possible to generate all observed flavour violation from the soft supersymmetry-breaking terms without conflicting with present-day data on FCNC processes. Therefore we reappraise the idea that only $Y^q_{33}$ is different from zero at tree-level and all other Yukawa couplings are generated radiatively. This model solves the SUSY flavour as well as the SUSY CP problem. The presented results are published in \cite{Crivellin:2008mq}.

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