arXiv subjects

Andrzej J. Buras

Publications and source records attributed to Andrzej J. Buras.

At least 127 records · Page 7Linked to original sources

arXiv2010

FCNC Processes in the Littlest Higgs Model with T-Parity: an Update

We update our 2006-2007 results for FCNC processes in the Littlest Higgs model with T-parity (LHT). The removal of the logarithmic UV cutoff dependence in our previous results through a new contribution to the Z^0-penguin diagrams identified by Goto et al. and del Aguila et al., while making the deviations from the SM expectations in the quark sector less spectacular, still allows for sizable new physics effects in K -> pi nu anti-nu and K_L -> pi^0 l^+ l^- decays and in the CP-asymmetry S_{psi phi} with the latter unaffected by the new contribution. We extend our analysis by a study of the fine-tuning required to fit the data on epsilon_K and by the inclusion of the decay K_L -> mu^+ mu^-. A number of correlations can distinguish this model from the custodially protected Randall-Sundrum model analysed recently. We also reconsider lepton flavour violating decays, including now a discussion of fine-tuning. While the l_i -> l_j gamma decays are unaffected by the removal of the logarithmic cutoff dependence, the branching ratios for decays with three leptons in the final state, like mu -> 3 e are lowered by almost an order of magnitude. In spite of this, the pattern of lepton flavour violation in the LHT model can still be distinguished from the one in supersymmetric models.

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arXiv2010

A Lower Bound on hadronic EDMs from CP Violation in D0-D0bar mixing in SUSY Alignment Models

The SM predictions for CP violating effects in D meson systems are highly suppressed at the per mill level. Therefore, any experimental evidence for a sizable CP violation in D0-D0bar mixing would unambiguously point towards a New Physics (NP) signal. Within supersymmetric scenarios, the popular alignment models can naturally account for large, non-standard effects in D0-D0bar mixing. We demonstrate that, within alignment models, detectable CP violating effects in D0-D0bar mixing would unambiguously imply a lower bound for the electric dipole moment (EDM) of hadronic systems, like the neutron EDM and the mercury EDM, in the reach of future experimental sensitivities. The simultaneous evidence of CP violation in D meson systems together with non vanishing hadronic EDMs would strongly support the idea of SUSY alignment models and disfavour gauge-mediated SUSY breaking models, SUSY models with MFV and non-Abelian SUSY flavour models. As a byproduct of our analysis, we study the correlation between the time dependent CP asymmetry in decays to CP eigenstates S_f and the semileptonic asymmetry a_SL both model independently and in SUSY alignment models.

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arXiv2010

The Littlest Higgs Model with T-Parity Facing CP-Violation in B_s - anti-B_s Mixing

The non-minimal flavour violating interactions of mirror quarks and new heavy gauge bosons in the Littlest Higgs model with T-parity (LHT) give rise to naturally large CP-violating effects in the B_s system. In view of a large new CP phase in B_s - anti-B_s mixing hinted by the CDF and D0 data and the recent UTfit analysis, we update our 2006 analysis of particle-antiparticle mixing and rare K and B decays in the LHT model, using the most recent values of a number of input parameters and performing a more careful error analysis. We find that the CP-asymmetry S_{psi phi} can easily reach values ~ 0.15-0.20, compared to the SM value ~ 0.04, while higher values are rather unlikely though not excluded. Large enhancements are also possible in the branching ratios for K_L -> pi0 nu anti-nu, K+ -> pi+ nu anti-nu and K_L -> pi0 l+ l- with much more modest effects in B_{s,d} -> mu+ mu-. We perform a detailed study of correlations between the latter decays and S_{psi phi} as well as of the correlation between S_{psi phi} and S_{psi K_S}. We also point out that the possible tension between epsilon_K and the tree level CKM determination recently hinted by various analyses can easily be resolved in the LHT model.

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arXiv2010

Littlest Higgs Model with T-Parity Confronting the New Data on D^0-\bar D^0 Mixing

Motivated by the first experimental evidence of meson oscillations in the D system, we study D^0 - \bar D^0 mixing in the Littlest Higgs model with T-parity, we investigate its role in constraining the model parameters and its impact on the most interesting flavour observables. We find that the experimental data are potentially strongly constraining but at present limited by large theoretical uncertainties in the long-distance Standard Model contribution to D^0 - \bar D^0 mixing.

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arXiv2010

Rare and CP-Violating K and B Decays in the Littlest Higgs Model with T-Parity

We calculate the most interesting rare and CP-violating K and B decays in the Littlest Higgs model with T-parity. We give a collection of Feynman rules including v^2/f^2 contributions that are presented here for the first time and could turn out to be useful also for applications outside flavour physics. We adopt a model-independent parameterization of rare decays in terms of the gauge independent functions X_i,Y_i,Z_i (i=K,d,s), which is in particular useful for the study of the breaking of the universality between K, B_d and B_s systems through non-MFV interactions. Performing the calculation in the unitary and 't Hooft-Feynman gauge, we find that the final result contains a divergence which signals some sensitivity to the ultraviolet completion of the theory. Including an estimate of this contribution, we calculate the branching ratios for the decays $K^+\toπ^+ν\barν$, $K_L\toπ^0ν\barν$, $B_{s,d}\to μ^+μ^-$, $B\to X_{s,d}ν\barν$, $K_L \toπ^0\ell^+\ell^-$ and $B\to X_{s,d}\ell^+\ell^-$, paying particular attention to non-MFV contributions present in the model. Imposing all available constraints we find that the decay rates for $B_{s,d} \to μ^+ μ^-$ and $B \to X_{s,d} ν\bar ν$ can be enhanced by at most 50% and 35% relative to the SM values, while $Br(K^+\toπ^+ν\barν)$ and $Br(K_L\toπ^0ν\barν)$ can be both as high as $5 \cdot 10^{-10}$. Significant enhancements of the decay rates $K_L \toπ^0\ell^+\ell^-$ are also possible. Simultaneously, the CP-asymmetries $S_{ψϕ}$ and $A^s_\text{SL}$ can be enhanced by an order of magnitude, while the electroweak penguin effects in $B \to πK$ turn out to be small, in agreement with the recent data.

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arXiv2010

Charged Lepton Flavour Violation and (g-2)_mu in the Littlest Higgs Model with T-Parity: a clear Distinction from Supersymmetry

We calculate the rates for the charged lepton flavour violating decays l_i -> l_j gamma, tau -> l pi, tau -> l eta, tau -> l eta', mu^- -> e^- e^+ e^-, the six three body leptonic decays tau^- -> l_i^- l_j^+ l_k^- and the rate for mu-e conversion in nuclei in the Littlest Higgs model with T-Parity (LHT). We also calculate the rates for K_{L,S} -> mu e, K_{L,S} -> pi^0 mu e and B_{d,s} -> l_i l_j. We find that the relative effects of mirror leptons in these transitions are by many orders of magnitude larger than analogous mirror quark effects in rare K and B decays analyzed recently. In particular, in order to suppress the mu -> e gamma and mu^- -> e^- e^+ e^- decay rates and the mu-e conversion rate below the experimental upper bounds, the relevant mixing matrix in the mirror lepton sector V_Hl must be rather hierarchical, unless the spectrum of mirror leptons is quasi-degenerate. We find that the pattern of the LFV branching ratios in the LHT model differs significantly from the one encountered in the MSSM, allowing in a transparent manner to distinguish these two models with the help of LFV processes. We also calculate (g-2)_mu and find the new contributions to a_mu below 10^{-10} and consequently negligible. We compare our results with those present in the literature.

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arXiv2010

Correlations between epsilon'/epsilon and Rare K Decays in the Littlest Higgs Model with T-Parity

We calculate the CP-violating ratio epsilon'/epsilon in the Littlest Higgs model with T-parity (LHT) and investigate its correlations with the branching ratios for K_L -> pi^0 nu {bar nu}, K_L -> pi^0 l^+ l^- and K^+ -> pi^+ nu {bar nu}. The resulting correlations are rather strong in the case of K_L decays, but less pronounced in the case of K^+ -> pi^+ nu {bar nu}. Unfortunately, they are subject to large hadronic uncertainties present in epsilon'/epsilon, whose theoretical prediction in the Standard Model (SM) is reviewed and updated here. With the matrix elements of Q_6 (gluon penguin) and Q_8 (electroweak penguin) evaluated in the large-N limit and m_s^MS(2 GeV) = 100 MeV from lattice QCD, (epsilon'/epsilon)_SM turns out to be close to the data so that significant departures of Br(K_L -> pi^0 nu {bar nu}) and Br(K_L -> pi^0 l^+ l^-) from the SM expectations are unlikely, while Br(K^+ -> pi^+ nu {bar nu}) can be enhanced even by a factor 5. On the other hand, modest departures of the relevant hadronic matrix elements from their large-N values allow for a consistent description of epsilon'/epsilon within the LHT model accompanied by large enhancements of Br(K_L -> pi^0 nu {bar nu}) and Br(K_L -> pi^0 l^+ l^-), but only modest enhancements of Br(K^+ -> pi^+ nu {bar nu}).

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arXiv2009

The Impact of Kaluza-Klein Fermions on Standard Model Fermion Couplings in a RS Model with Custodial Protection

We reconsider the impact of heavy vector-like fermions on the couplings of standard model (SM) quarks to the SM gauge bosons W and Z and to the SM Higgs boson H. Integrating out these fermions at tree level we derive general formulae that can be used in any model containing such particles. We apply these formulae to the case of the lightest Kaluza-Klein (KK) fermions in a Randall-Sundrum (RS) model with a custodial protection of flavour conserving $Zd_L^i d_L^i$ and flavour violating $Zd_L^i d_L^j$ couplings. We point out that in this model also the couplings of $Zu_R^i u_R^i$ and $Zu_R^i u_R^j$ are protected. In particular we demonstrate explicitly that this protection is not spoiled by the mixing of the SM quarks with the KK fermions, which is guaranteed by the underlying P_LR symmetry. We find that the impact of KK fermions on the Z-couplings, while not negligible, is significantly smaller than the one coming from the mixing of Z with the heavy KK gauge bosons Z_H and Z'.The situation is similar for the W couplings with the only exception of the tb coupling where the impact of KK fermions can in principle be larger than the effects that are induced by gauge boson mixing. We also show explicitly that at O(v^2/M_{KK}^2) the fermion-Higgs couplings of a Higgs placed on the infrared (IR) brane is not affected by the KK contributions up to strongly chirally suppressed contributions. The corrections to the CKM matrix, in particular the breakdown of its unitarity, are found to be small. We also investigate the right-handed couplings of W that are generated through the mixing with KK-fermions and introduce a 3x3 matrix that describes the pattern of flavour violation in these new interactions of the standard W.

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arXiv2009

FCNC Processes Waiting for the Next Decade

FCNC processes are expected to offer us a deep insight into the physics at very short distance scales. We present a list of 20 goals in quark and lepton flavour physics that could be reached already in the next decade. This list includes also flavour conserving observables like electric dipole moments of the neutron and leptons and (g-2)_μ. Subsequently we will present some aspects of these goals by concentrating on supersymmetric flavour models. A much more extensive presentation of this material can be found in my recent EPS09 talk.

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arXiv2009

Flavour Theory: 2009

After an overture and a non-technical exposition of the relevant theoretical framework including a brief discussion of some of the most popular extensions of the Standard Model, we will compile a list of 20 goals in flavour physics that could be reached already in the next decade. In addition to K, D and B_{s,d} decays and lepton flavour violation also flavour conserving observables like electric dipole moments of the neutron and leptons and (g-2)_μare included in this list. Flavour violation in high energy processes is also one of these goals. Subsequently we will discuss in more detail the most urgent issues for the coming years in the context of several extensions of the Standard Model like models with Minimal Flavour Violation, the general MSSM, the Littlest Higgs Model with T parity, Randall-Sundrum models and supersymmetric flavour models. This presentation is not meant to be a comprehensive review of flavour physics but rather a personal view on this fascinating field and an attempt to collect those routes that with the help of upcoming experiments should allow us to reach a much deeper understanding of physics, in particular flavour physics, at very short distance scales.

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arXiv2009

Patterns of Flavour Violation in the RSc Model, the LHT Model and Supersymmetric Flavour Models

We summarize the results on patterns of flavour violation in a Randall-Sundrum model with custodial protection (RSc) and compare them with those identified in the Littlest Higgs Model with T--parity (LHT) and in a number of SUSY Flavour Models. While K decays play in this presentation a prominent role, the inclusion of B physics and lepton flavour violation is crucial in the distinction between these three popular extensions of the Standard Model (SM) by means of flavour physics.

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arXiv2009

Electroweak and Flavour Structure of a Warped Extra Dimension with Custodial Protection

We present the electroweak and flavour structure of a model with a warped extra dimension and the bulk gauge group SU(3) x SU(2)_L x SU(2)_R x P_LR x U(1)_X. The presence of SU(2)_R implies an unbroken custodial symmetry in the Higgs system allowing to eliminate large contributions to the T parameter, whereas the P_LR symmetry and the enlarged fermion representations provide a custodial symmetry for flavour diagonal and flavour changing couplings of the SM Z boson to left-handed down-type quarks. We diagonalise analytically the mass matrices of charged and neutral gauge bosons including the first KK modes. We present the mass matrices for quarks including heavy KK modes and discuss the neutral and charged currents involving light and heavy fields. We give the corresponding complete set of Feynman rules in the unitary gauge.

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arXiv2009

CP Violation in D0 - anti-D0 Oscillations: General Considerations and Applications to the Littlest Higgs Model with T-Parity

The observed D0 - anti-D0 oscillations provide a new stage in our search for New Physics in heavy flavour dynamics. The theoretical verdict on the observed values of x_D and y_D remains ambiguous: while they could be totally generated by Standard Model dynamics, they could also contain a sizable or even leading contribution from New Physics. Those oscillations are likely to enhance the observability of CP violation as clear manifestations of New Physics. We present general formulae for D0 - anti-D0 oscillations, concentrating on the case of negligible direct CP violation. In particular we derive a general formula for the time-dependent mixing-induced CP asymmetry in decays to a CP eigenstate and its correlation with the semileptonic CP asymmetry a_SL(D0) in D0(t) -> l nu K. We apply our formalism to the Littlest Higgs model with T-parity, using the time-dependent CP asymmetry in D -> K_S phi as an example. We find observable effects at a level well beyond anything possible with CKM dynamics. Comparisons with CP violation in the K and B systems offer an excellent test of this scenario and reveal the specific pattern of flavour and CP violation in the D0 - anti-D0 system predicted by this model. We discuss a number of charm decays that could potentially offer an insight in the dynamics of CP violation in D decays. We also apply our formalism to B_s - anti-B_s mixing.

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arXiv2009

Testing the CKM Picture of Flavour and CP Violation in Rare K and B Decays and Particle-Antiparticle Mixing

We summarize briefly the CKM picture of flavour and CP violation that governs the models with minimal flavour violation (MFV). We then describe how this framework can be efficiently tested through particle-antiparticle mixing and rare $K$ and B decays. In particular we provide a list of theoretically clean tests that the simplest version of the MFV framework, the constrained MFV hypothesis, has to face in the coming years. Finally we offer a brief look at the most popular SM extensions that go beyond the CKM framework like the general MSSM, Little Higgs model with T-parity and Randall-Sundrum models with bulk fermions.

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arXiv2009

Rare K and B Decays in a Warped Extra Dimension with Custodial Protection

We present a complete study of rare K and B meson decays in a warped extra dimensional model with a custodial protection of (both diagonal and non-diagonal) Z d_L^i \bar d_L^j couplings, including K^+ -> pi^+ nu anti-nu, K_L -> pi^0 nu anti-nu, K_L -> pi^0 l^+ l^-, K_L -> mu^+ mu^-, B_{s,d} -> mu^+ mu^-, B -> K nu anti-nu, B -> K^* nu anti-nu and B -> X_{s,d} nu anti-nu. In this model in addition to Standard Model one loop contributions these processes receive tree level contributions from the Z boson and the new heavy electroweak gauge bosons. We analyse all these contributions that turn out to be dominated by tree level Z boson exchanges governed by right-handed couplings to down-type quarks. Imposing all existing constraints from Delta F=2 transitions analysed by us recently and fitting all quark masses and CKM mixing parameters we find that a number of branching ratios for rare K decays can differ significantly from the SM predictions, while the corresponding effects in rare B decays are modest, dominantly due to the custodial protection being more effective in B decays than in K decays. In order to reduce the parameter dependence we study correlations between various observables within the K system, within the B system and in particular between K and B systems, and also between Delta F=2 and Delta F=1 observables. These correlations allow for a clear distinction between this new physics scenario and models with minimal flavour violation or the Littlest Higgs Model with T-parity, and could give an opportunity to future experiments to confirm or rule out the model. We show how our results would change if the custodial protection of Z d_L^i bar d^j_L couplings was absent. In the case of rare B decays the modifications are spectacular.

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arXiv2009

New strategies for New Physics search in B -> K* nu anti-nu, B -> K nu anti-nu and B -> X(s) nu anti-nu decays

The rare decay B -> K* nu anti-nu allows a transparent study of Z penguin and other electroweak penguin effects in New Physics (NP) scenarios in the absence of dipole operator contributions and Higgs (scalar) penguin contributions that are often more important than Z contributions in B -> K* l+l- and B(s) -> l+l- decays. We present a new analysis of B -> K* nu anti-nu with improved form factors and of the decays B -> K nu anti-nu and B -> X(s) nu anti-nu in the SM and in a number of NP scenarios like the general MSSM, general scenarios with modified Z/Z' penguins and in a singlet scalar extension of the SM. We also summarize the results in the Littlest Higgs model with T-parity and a Randall-Sundrum (RS) model with custodial protection of left-handed Z-di-dj couplings. Our SM prediction BR(B -> K* nu anti-nu)=(6.8^+1.0_-1.1) x 10^-6 turns out to be significantly lower than the ones present in the literature. Our improved calculation BR(B -> X(s) nu anti-nu)=(2.7+-0.2) x 10^-5 in the SM avoids the normalization to the BR(B -> X(c) e anti-nu(e)) and, with less than 10% total uncertainty, is the most accurate to date. The results for the SM and NP scenarios can be transparently summarized in a (epsilon,eta) plane with a non-vanishing eta signalling the presence of new right-handed down-quark flavour violating couplings which can be ideally probed by the decays in question. Measuring the three branching ratios and one additional polarization observable in B -> K* nu anti-nu allows to overconstrain the resulting point in the (epsilon,eta) plane with (epsilon,eta)=(1,0) corresponding to the SM. The correlations of these three channels with the rare decays K+ -> pi+ nu anti-nu, KL -> pi0 nu anti-nu, B -> X(s) l+ l- and B(s) -> mu+ mu- offer powerful tests of New Physics with new right-handed couplings and non-MFV interactions.

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arXiv2009

Messages on Flavour Physics Beyond the Standard Model

I present a brief summary of the main results on flavour physics beyond the Standard Model that have been obtained in 2008 by my collaborators and myself at the TUM. In particular I list main messages coming from our analyses of flavour and CP-violating processes in Supersymmetry, the Littlest Higgs model with T-Parity and a warped extra dimension model with custodial protection for flavour violating $Z$ boson couplings.

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arXiv2009

On the consistency between the observed amount of CP violation in the K- and Bd-systems within minimal flavor violation

We reappraise the question whether the Standard Model, and Minimal Flavor Violating (MFV) models at large, can simultaneously describe the observed CP violation in the K- and Bd-systems. We find that CP violation in the Bd-system, measured most precisely through (sin 2 beta)_{J/psi Ks}, implies |epsilon_K^{SM}| = 1.78(25) x 10^{-3} for the parameter epsilon_K, measuring indirect CP violation in the K-system, to be compared with the experimental value |epsilon_K^{exp}| = 2.23(1) x 10^{-3}. To bring this prediction to 1 sigma agreement with experiment, we explore then the simplest new-physics possibility not involving new phases, namely that of MFV scenarios with no new effective operators besides the Standard Model ones. We emphasize the crucial input and/or measurements to be improved in order to probe this case. In particular we point out that this tension could be removed in this framework, with interesting signatures, e.g. correlated suppression patterns for rare K decay branching ratios. On the other hand, MFV contributions from new operators appear, in the calculable case of the MSSM, to worsen the situation. We finally explore some well-motivated new-physics scenarios beyond MFV, like those involving generic new contributions in Z-penguins.

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