arXiv subjects

Andrzej J. Buras

Publications and source records attributed to Andrzej J. Buras.

At least 109 records · Page 6Linked to original sources

arXiv2012

Complete NLO QCD Corrections for Tree Level Delta F = 2 FCNC Processes

Anticipating the important role of tree level FCNC processes in the indirect search for new physics at distance scales as short as 10^-19-10^-21 m, we present complete NLO QCD corrections to tree level Delta F=2 processes mediated by heavy colourless gauge bosons and scalars. Such contributions can be present at the fundamental level when GIM mechanism is absent as in numerous Z' models, gauged flavour models with new heavy neutral gauge bosons and Left-Right symmetric models with heavy neutral scalars. They can also be generated at one loop in models having GIM at the fundamental level and MFV of which Two-Higgs Doublet models with and without SUSY are the best known examples. In models containing vectorial heavy fermions that mix with the standard chiral quarks and models in which Z and SM neutral Higgs H mix with new heavy gauge bosons and scalars also tree-level Z and SM neutral Higgs contributions to Delta F=2 processes are possible. In all these extensions new local operators are generated having Wilson coefficients that are generally much stronger affected by RG QCD effects than it is the case of the SM operators. The new aspect of our work is the calculation of O(alpha_s) corrections to matching conditions for the Wilson coefficients of the contributing operators in the NDR-MS scheme that can be used in all models listed above. This allows to reduce certain unphysical scale and renormalization scheme dependences in the existing NLO calculations. We show explicitly how our results can be combined with the analytic formulae for the P_i^a QCD factors that include both hadronic matrix elements of contributing operators and RG evolution from high energy to low energy scales. For the masses of heavy gauge bosons and scalars O(1)TeV the remaining unphysical scale dependences for the mixing amplitudes are reduced typically from 10-25%, depending on the operator considered, down to 1-2%.

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arXiv2012

Phenomenology of a Gauged SU(3)^3 Flavour Model

We present an extensive analysis of Delta F=2 observables and of B -> X_s gamma in the framework of a specific Maximally Gauged Flavour (MGF) model of Grinstein et al. including all relevant contributions, in particular tree-level heavy gauge boson exchanges that are considered in the present paper for the first time. The model allows in principle for significant deviations from the Standard Model predictions for epsilon_K, Delta M_{B_{d,s}}, mixing induced CP-asymmetries S_{psi K_S} and S_{psi phi} and B -> X_s gamma decay. The tension between epsilon_K and S_{psi K_S} present in the SM can be removed by enhancing |epsilon_K| without modifying S_{psi K_S}. In this case, we find that in this model i) the results for S_{psi phi} and B -> X_s gamma turn out to be SM-like, ii) the exclusive determination of |Vub| is favoured and most importantly iii) the values of Delta M_{B_d} and Delta M_{B_s} being strongly correlated in this model with epsilon_K turn out to be much larger than the data for the central values of input parameters: Delta M_{B_d}~0.75/ps and Delta M_{B_s}~27/ps. Therefore, from the present perspective, the model suffers from a serious epsilon_K-Delta M_{B_{d,s}} tension. However, this tension can be softened considering theoretical and parametric uncertainties and in particular the decrease of the weak decay constants. On the other side, the model can be strongly constrained considering the theoretically cleaner ratios Delta M_{B_d}/Delta M_{B_s} and BR(B^+ -> tau^+ nu)/Delta M_{B_d} and we find that it is unable to remove simultaneously all the SM tensions on the data. Finally, we compare the pattern of flavour violation in MGF with selected extensions of the SM.

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arXiv2012

The Impact of Flavour Changing Neutral Gauge Bosons on B->X_s gamma

The branching ratio of the rare decay B->X_s gamma provides potentially strong constraints on models beyond the Standard Model. Considering a general scenario with new heavy neutral gauge bosons, present in particular in Z' and gauge flavour models, we point out two new contributions to the B->X_s gamma decay. The first one originates from one-loop diagrams mediated by gauge bosons and heavy exotic quarks with electric charge -1/3. The second contribution stems from the QCD mixing of neutral current-current operators generated by heavy neutral gauge bosons and the dipole operators responsible for the B->X_s gamma decay. The latter mixing is calculated here for the first time. We discuss general sum rules which have to be satisfied in any model of this type. We emphasise that the neutral gauge bosons in question could also significantly affect other fermion radiative decays as well as non-leptonic two-body B decays, epsilon'/epsilon, anomalous (g-2)_mu and electric dipole moments.

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arXiv2012

Completing NLO QCD Corrections for Tree Level Non-Leptonic Delta F = 1 Decays Beyond the Standard Model

In various extensions of the Standard Model (SM) tree level non-leptonic decays of hadrons receive contributions from new heavy gauge bosons and scalars. Prominent examples are the right-handed W' bosons in left-right symmetric models and charged Higgs (H^\pm) particles in models with extended scalar sector like two Higgs doublet models and supersymmetric models. Even in the case of decays with four different quark flavours involved, to which penguin operators cannot contribute, twenty linearly independent operators, instead of two in the SM, have to be considered. Anticipating the important role of such decays at the LHCb, KEKB and Super-B in Rome and having in mind future improved lattice computations, we complete the existing NLO QCD formulae for these processes by calculating O(alpha_s) corrections to matching conditions for the Wilson coefficients of all contributing operators in the NDR-\bar{MS} scheme. This allows to reduce certain unphysical scale and renormalization scheme dependences in the existing NLO calculations. Our results can also be applied to models with tree-level heavy neutral gauge boson and scalar exchanges in Delta F = 1 transitions and constitute an important part of NLO analyses of those non-leptonic decays to which also penguin operators contribute.

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arXiv2011

Flavour Visions

This decade will allow to improve the resolution of the short distance scales by at least an order of magnitude, extending the picture of fundamental physics down to scales $5\cdot 10^{-20}$m with the help of the LHC. Further resolution down to scales as short as $10^{-21}$m should be possible with the help of high precision experiments in which flavour violating processes will play a prominent role. Will this increase in resolution allow us to see new particles (new animalcula) similarly to what Antoni van Leeuvenhoek saw by discovering bacteria in 1676? The basic question for particle physics is how these new animalcula will look like and which difficulties of the Standard Model (SM) they will help us to solve and which new puzzles and problems they will bring with them. I will describe what role flavour physics will play in these exciting times provided this new world is animalculated.

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arXiv2011

Symmetries and Asymmetries of B -> K* mu+ mu- Decays in the Standard Model and Beyond

The rare decay B -> K* (-> K pi) mu+ mu- is regarded as one of the crucial channels for B physics as the polarization of the K* allows a precise angular reconstruction resulting in many observables that offer new important tests of the Standard Model and its extensions. These angular observables can be expressed in terms of CP-conserving and CP-violating quantities which we study in terms of the full form factors calculated from QCD sum rules on the light-cone, including QCD factorization corrections. We investigate all observables in the context of the Standard Model and various New Physics models, in particular the Littlest Higgs model with T-parity and various MSSM scenarios, identifying those observables with small to moderate dependence on hadronic quantities and large impact of New Physics. One important result of our studies is that new CP-violating phases will produce clean signals in CP-violating asymmetries. We also identify a number of correlations between various observables which will allow a clear distinction between different New Physics scenarios.

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arXiv2011

FCNC Effects in a Minimal Theory of Fermion Masses

As a minimal theory of fermion masses we extend the SM by heavy vectorlike fermions, with flavor-anarchical Yukawa couplings, that mix with chiral fermions such that small SM Yukawa couplings arise from small mixing angles. This model can be regarded as an effective description of the fermionic sector of a large class of existing flavor models and thus might serve as a useful reference frame for a further understanding of flavor hierarchies in the SM. Already such a minimal framework gives rise to FCNC effects through exchange of massive SM bosons whose couplings to the light fermions get modified by the mixing. We derive general formulae for these corrections and discuss the bounds on the heavy fermion masses. Particularly stringent bounds, in a few TeV range, come from the corrections to the Z couplings.

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arXiv2011

Minimal flavour violation and beyond: Towards a flavour code for short distance dynamics

This decade should provide the first definitive signals of New Physics (NP) beyond the Standard Model (SM) and the goal of these lectures is a review of flavour physics in various extensions of the SM that have been popular in the last ten years. After an overture, two pilot sections and a brief summary of the structure of flavour violation and CP violation in the SM, we will present the theoretical framework for weak decays that will allow us to distinguish between different NP scenarios. Subsequently we will present twelve concrete BSM models summarizing the patterns of flavour violation characteristic for each model. In addition to models with minimal flavour violation (MFV) accompanied by flavour blind phases we will discuss a number of extensions containing non-MFV sources of flavour and CP violation and, in particular, new local operators originating in right-handed charged currents and scalar currents. Next we will address various anomalies in the data as seen from the point of view of the SM that appear very natural in certain extensions of the SM. In this presentation selected superstars of this field will play a very important role. These are processes that are very sensitive to NP effects and which are theoretically clean. Particular emphasis will be put on correlations between various observables that could allow us to distinguish between various NP scenarios. Armed with this knowledge we will propose a coding system in a form of a 3*3 matrix which helps to distinguish between various extensions of the SM. Finding which flavour code is chosen by nature would be an important step towards the fundamental theory of flavour. We give several examples of flavour codes representing specific models.

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arXiv2010

Patterns of Flavour Violation in the Presence of a Fourth Generation of Quarks and Leptons

We calculate a number of observables related to particle-antiparticle mixing and the branching ratios for the most interesting rare and CP-violating K and B decays in the Standard Model (SM3) extended by a fourth generation of quarks and leptons (SM4). A model-independent parameterisation of these observables in terms of gauge-independent functions is adopted, which is useful for studying the breaking of the universality between K, Bd and Bs systems through non-minimal flavour violating interactions. We calculate first the mass differences Delta M_i in the neutral K and B system, the mixing-induced CP asymmetries S_{psi K_S}, S_{psi phi}, S_{phi K_S}, S_{eta' K_S} and epsilon_K. Subsequently, a detailed analysis of K^+ -> pi^+ nu nu, K_L -> pi^0 nu nu, B_{s,d} -> mu^+ mu^-, B -> X_{s,d} nu nu, K_L -> pi^0 l^+ l^-, B -> X_s gamma and B -> X_{s,d} l^+ l^- is presented, and also epsilon'/epsilon is considered. For some of these observables the departures from SM3 predictions can still be spectacular. We discuss how the new mixing parameters (3 angles, 2 CP phases) can be determined using the flavour observables in question. We identify the different hierarchical structures in the SM4 flavour mixing matrix, allowed by phenomenological and theoretical constraints, and define the corresponding generalised Wolfenstein expansion. Most importantly, we show how the characteristic patterns of correlations among the considered flavour observables allow to distinguish this New Physics scenario from supersymmetric flavour models, the Littlest Higgs model with T-parity and the Randall-Sundrum model with custodial protection.

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arXiv2010

Footprints of SUSY GUTs in Flavour Physics

Supersymmetric (SUSY) Grand Unified theories (GUTs) generally predict FCNC and CP violating processes to occur both in the leptonic and hadronic sectors. Assuming an underlying SU(5) group plus right-handed neutrinos (RN), we perform an extensive study of FCNC and CP violation, analyzing the correlations between leptonic and hadronic processes like mu-> e gamma and K-Kbar mixing, tau->mu gamma and b -> s transitions such as B_d -> phi K_s and Bs-Bsbar mixing. Moreover, we examine the impact of the considered scenario on the UT analyses, monitoring the low energy consequences implied by possible solutions to the various tensions in the present UT analyses. We compare the phenomenological implications of this NP scenario with the ones of supersymmetric flavour models finding a few striking differences that could allow to distinguish these different NP models.

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arXiv2010

Flavour Theory and the LHC Era

This decade should make a significant progress towards the Theory of Flavour and the main goal of this talk is to transfer this believe to my colleagues in the particle physics community. Indeed a significant part of this decade could turn out to be the Flavour Era with participation of the LHC, Belle II, Super-Flavour-Facility and dedicated Kaon and lepton flavour violation experiments. Selected superstars of flavour physics listed below will play a prominent role in these events. In this writeup the leading role is played by the prima donna of 2010: CP violation in B_s system.

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arXiv2010

Higgs-mediated FCNCs: Natural Flavour Conservation vs. Minimal Flavour Violation

We compare the effectiveness of two hypotheses, Natural Flavour Conservation (NFC) and Minimal Flavour Violation (MFV), in suppressing the strength of flavour-changing neutral-currents (FCNCs) in models with more than one Higgs doublet. We show that the MFV hypothesis, in its general formulation, is more stable in suppressing FCNCs than the hypothesis of NFC alone when quantum corrections are taken into account. The phenomenological implications of the two scenarios are discussed analysing meson-antimeson mixing observables and the rare decays B -> mu+ mu-. We demonstrate that, introducing flavour-blind CP phases, two-Higgs doublet models respecting the MFV hypothesis can accommodate a large CP-violating phase in Bs mixing, as hinted by CDF and D0 data and, without extra free parameters, soften significantly in a correlated manner the observed anomaly in the relation between epsilon_K and S_psi_K.

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arXiv2010

EDMs vs. CPV in B_{s,d} mixing in two Higgs doublet models with MFV

We analyze the correlations between electric dipole moments (EDMs) of the neutron and heavy atoms and CP violation in B_{s,d} mixing in two Higgs doublet models respecting the Minimal Flavour Violation hypothesis, with flavour-blind CP-violating (CPV) phases. In particular, we consider the case of flavour-blind CPV phases from i) the Yukawa interactions and ii) the Higgs potential. We show that in both cases the upper bounds on the above EDMs do not forbid sizable non-standard CPV effects in B_s mixing. However, if a large CPV phase in B_s mixing will be confirmed, this will imply EDMs very close to their present experimental bounds, within the reach of the next generation of experiments, as well as BR(B_{s,d}-> mu^+ mu^-) typically largely enhanced over its SM expectation. The two flavour-blind CPV mechanisms can be distinguished through the correlation between S_psi K_S and S_psi phi that is strikingly different if only one of them is relevant. Which of these two CPV mechanisms dominates depends on the precise values of S_psi phi and S_psi K_S, as well as on the CKM phase (as determined by tree-level processes). Current data seems to show a mild preference for a hybrid scenario where both these mechanisms are at work.

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arXiv2010

Lepton Flavour Violation in the Presence of a Fourth Generation of Quarks and Leptons

We calculate the rates for the charged lepton flavour violating (LFV) decays l_i -> l_j gamma, tau -> l pi, 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 Standard Model (SM3) extended by a fourth generation of quarks and leptons (SM4), assuming that neutrinos are Dirac particles. We also calculate branching ratios for K_L -> mu e, K_L -> pi^0 mu e, B_{d,s} -> mu e, B_{d,s} -> tau e and B_{d,s} -> tau mu. We find that the pattern of the LFV branching ratios in the SM4 differs significantly from the one encountered in the MSSM, allowing to distinguish these two models with the help of LFV processes in a transparent manner. Also differences with respect to the Littlest Higgs model with T-parity are found. Most importantly the branching ratios for l_i -> l_j gamma, tau -> l pi, tau -> l eta('), mu^- -> e^-e^+e^-, tau^- -> e^-e^+e^-, tau^- -> mu^- mu^+ mu^-, tau^- -> e^- mu^+ mu^- and tau^- -> mu^- e^+ e^- can all still be as large as the present experimental upper bounds but not necessarily simultaneously. Also the rate for mu-e conversion in nuclei can reach the corresponding upper bound.

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arXiv2010

Quark flavour mixing with right-handed currents: an effective theory approach

The impact of right-handed currents in both charged- and neutral-current flavour-violating processes is analysed by means of an effective theory approach. More explicitly, we analyse the structure of dimension-six operators assuming a left-right symmetric flavour group, commuting with an underlying $SU(2)_L \times SU(2)_R \times U(1)_{B-L}$ global symmetry, broken only by two Yukawa couplings. The model contains a new unitary matrix controlling flavour-mixing in the right-handed sector. We determine the structure of this matrix by charged-current data, where the tension between inclusive and exclusive determinations of $|V_{ub}|$ can be solved. Having determined the size and the flavour structure of right-handed currents, we investigate how they would manifest themselves in neutral current processes, including particle-antiparticle mixing, $Z\to b \bar b$, $B_{s,d}\to μ^+μ^-$, $B\to {Xs,K,K*} ν\barν$, and $K\to πν\barν$ decays. The possibility to explain a non-standard CP-violating phase in $B_s$ mixing in this context, and the comparison with other predictive new-physics frameworks addressing the same problem, is also discussed. While a large $S_{ψϕ}$ asymmetry can easily be accommodated, we point out a tension in this framework between $|V_{ub}|$ and $S_{ψK}$.

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arXiv2010

The Impact of a 4th Generation on Mixing and CP Violation in the Charm System

We study D0-D0 mixing in the presence of a fourth generation of quarks. In particular, we calculate the size of the allowed CP violation which is found at the observable level well beyond anything possible with CKM dynamics. We calculate the semileptonic asymmetry a_SL and the mixing induced CP asymmetry eta_fS_f which are correlated with each other. We also investigate the correlation of eta_fS_f with a number of prominent observables in other mesonic systems like epsilon'/epsilon, Br(K_L -> pi0 nu nu), Br(K+ -> pi+ nu nu), Br(B_s ->mu+ mu-), Br(B_d -> mu+ mu-) and finally S_psi phi in the B_s system. We identify a clear pattern of flavour and CP violation predicted by the SM4 model: While simultaneous large 4G effects in the K and D systems are possible, accompanying large NP effects in the B_d system are disfavoured. However this behaviour is not as pronounced as found for the LHT and RSc models. In contrast to this, sizeable CP violating effects in the B_s system are possible unless extreme effects in eta_fS_f are found, and Br(B_s ->mu+ mu-) can be strongly enhanced regardless of the situation in the D system. We find that, on the other hand, S_psi phi > 0.2 combined with the measured epsilon'/epsilon significantly diminishes 4G effects within the D system.

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arXiv2010

Slepton mass-splittings as a signal of LFV at the LHC

Precise measurements of slepton mass-splittings might represent a powerful tool to probe supersymmetric (SUSY) lepton flavour violation (LFV) at the LHC. We point out that mass-splittings of the first two generations of sleptons are especially sensitive to LFV effects involving $τ-μ$ transitions. If these mass-splittings are LFV induced, high-energy LFV processes like the neutralino decay ${\nt}_2\to\nt_1τ^{\pm}μ^{\mp}$ as well as low-energy LFV processes like $τ\toμγ$ are unavoidable. We show that precise slepton mass-splitting measurements and LFV processes both at the high- and low-energy scales are highly complementary in the attempt to (partially) reconstruct the flavour sector of the SUSY model at work. The present study represents another proof of the synergy and interplay existing between the LHC, i.e. the {\em high-energy frontier}, and high-precision low-energy experiments, i.e. the {\em high-intensity frontier}.

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arXiv2010

On epsilon_K beyond lowest order in the Operator Product Expansion

We analyse the structure of long distance (LD) contributions to the CP-violating parameter epsilon_K, that generally affect both the absorptive (Gamma_12) and the dispersive (M_12) parts of the K0 -- K0bar mixing amplitude. We point out that, in a consistent framework, in addition to LD contributions to Im(Gamma_12), estimated recently by two of us, also LD contributions to Im(M_12) have to be taken into account. Estimating the latter contributions the impact of LD effects on epsilon_K is significantly reduced (from -6.0 % to -3.6 %). The overall effect of LD corrections and of the superweak phase being different from 45 degrees is summarised by the multiplicative factor kappa_epsilon = 0.94 +/- 0.02.

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