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Andrzej J. Buras

Publications and source records attributed to Andrzej J. Buras.

At least 163 records · Page 9Linked to original sources
arXiv2005

Particle-Antiparticle Mixing, ε_K and the Unitarity Triangle in the Littlest Higgs Model

We calculate the K^{0}-\bar{K}^{0}, B_{d,s}^{0}-\bar{B}_{d,s}^{0} mixing mass differences ΔM_K, ΔM_{d,s} and the CP-violating parameter ε_{K} in the Littlest Higgs (LH) model. For f/v as low as 5 and the Yukawa parameter x_L<0.8, the enhancement of ΔM_{d} amounts to at most 20%. Similar comments apply to ΔM_s and ε_{K}. The correction to ΔM_{K} is negligible. The dominant new contribution in this parameter range, calculated here for the first time, comes from the box diagrams with (W_L^\pm,W_H^\pm) exchanges and ordinary quarks that are only suppressed by the mass of W_H^\pm but do not involve explicit O(v^2/f^2) factors. This contribution is strictly positive. The explicit O(v^2/f^2) corrections to the SM diagrams with ordinary quarks and two W_L^\pm exchanges have to be combined with the box diagrams with a single heavy T quark exchange for the GIM mechanism to work. These O(v^2/f^2) corrections are found to be of the same order of magnitude as the (W_L^\pm,W_H^\pm) contribution but only for x_L approaching 0.8 they can compete with it. We point out that for x_L>0.85 box diagrams with two T exchanges have to be included. Although formally O(v^4/f^4), this contribution is dominant for x_L \approx 1 due to non-decoupling of T that becomes fully effective only at this order. We emphasize, that the concept of the unitarity triangle is still useful in the LH model, in spite of the O(v^2/f^2) corrections to the CKM unitarity involving only ordinary quarks. We demonstrate the cancellation of the divergences in box diagrams that appear when one uses the unitary gauge for W_L^\pm and W_H^\pm.

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arXiv2005

Non-Decoupling Effects of the Heavy T in the B^0_{d,s}-\bar B^0_{d,s} Mixing and Rare K and B Decays

We point out that in the case of a heavy top quark T, present in the Littlest Higgs model (LH), and the t-T mixing parameter x_L > 0.90 the contribution to B^0_{d,s}-\bar B^0_{d,s} mixing from box diagrams with two T exchanges cannot be neglected. Although formally O(v^4/f^4) with v = 246 GeV and f > 1 TeV, this contribution increases linearly with x_T = m_T^2/M^2_W and with x_T = O(f^2/v^2) constitutes effectively an O(v^2/f^2) correction. For x_L ~ 1, this contribution turns out to be more important than the genuine O(v^2/f^2) corrections. In particular it is larger than the recently calculated O(v^2/f^2) contribution of box diagrams with a single T exchange that increases only logarithmically with x_T. For x_L = 0.95 and f/v = 5,10,15, the short distance function S governing the B^0_{d,s}-\bar B^0_{d,s} mixing mass differences ΔM_{d,s} receives 56%, 15% and 7% enhancements relative to its Standard Model (SM) value, implying a suppression of the CKM element V_td and an enhancement of ΔM_s. The short distance functions X and Y, relevant for rare K and B decays, increase only logarithmically with x_T. With the suppressed V_td, K->πν\barνand B_d->μ^+μ^- decays are only insignificantly modified with respect to the SM, while the branching ratio Br(B_s->μ^+μ^-) receives 66%, 19% and 9% enhancements for x_L = 0.95 and f/v = 5,10,15, respectively. Similar enhancement is found for Br(B_s->μ\barμ)/Br(B_d->μ\barμ).

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arXiv2004

CP Asymmetry A_{ψK_S}, \sin 2β, the Sign of ΔM_B and the Physics Beyond the Standard Model

We demonstrate that the sign of the B^0-\bar B^0 mass difference ΔM_B is irrelevant for the extraction of the angle β=ϕ_1 of the unitarity triangle from the CP asymmetry A_{ψK_S}. Only the weak phase in the mixing amplitude M_{12} matters. Consequently the extraction of \sin 2βdone by BaBar and Belle can only be affected by new weak phases in M_{12} in the extensions of the SM and possibly but unlikely by new physics contributions to the decay amplitude.

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arXiv2004

B -> pi pi, New Physics in B -> pi K and Rare K and B Decays

We summarize a recent strategy for a global analysis of the B -> pi pi, pi K systems and rare decays. We find that the present B -> pi pi and B -> pi K data cannot be simultaneously described in the Standard Model. In a simple extension in which new physics enters dominantly through Z^0 penguins with a CP-violating phase, only certain B -> pi K modes are affected by new physics. The B -> pi pi data can then be described entirely within the Standard Model but with values of hadronic parameters that reflect large non-factorizable contributions. Using the SU(3) flavour symmetry and plausible dynamical assumptions, we can then use the B -> pi pi decays to fix the hadronic part of the B -> pi K system and make predictions for various observables in the B_d -> pi^-+ K^+- and B^+- -> pi^+- K decays that are practically unaffected by electroweak penguins. The data on the B^+- -> pi^0 K^+- and B_d -> pi^0 K modes allow us then to determine the electroweak penguin component which differs from the Standard Model one, in particular through a large additional CP-violating phase. The implications for rare K and B decays are spectacular. In particular, the rate for K_L -> pi^0 nu \bar nu is enhanced by one order of magnitude, the branching ratios for B_{d,s} -> mu^+ mu^- by a factor of five, and BR(K_L -> pi^0 e^+ e^-, pi^0 mu^+ mu^-) by factors of three.

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arXiv2004

The B -> pi pi,pi K Puzzles in the Light of New Data: Implications for the Standard Model, New Physics and Rare Decays

Recently, we developed a strategy to analyse the B -> pi pi,pi K data. We found that the B -> pi pi measurements can be accommodated in the Standard Model (SM) through large non-factorizable effects. On the other hand, our analysis of the ratios R_c and R_n of the CP-averaged branching ratios of the charged and neutral B -> pi K modes, respectively, suggested new physics (NP) in the electroweak penguin sector, which may have a powerful interplay with rare decays. In this paper, we confront our strategy with recent experimental developments, addressing also the direct CP violation in B_d -> pi^-+ K^+-, which is now an established effect, the relation to its counterpart in B^+- -> pi^0 K^+-, and the first results for the direct CP asymmetry of B_d -> pi^0 pi^0 that turn out to be in agreement with our prediction. We obtain hadronic B -> pi pi,pi K parameters which are almost unchanged and arrive at an allowed region for the unitarity triangle in perfect accordance with the SM. The ``B -> pi K puzzle'' persists, and can still be explained through NP, as in our previous analysis. In fact, the recently observed shifts in the experimental values of R_c and R_n have been predicted in our framework on the basis of constraints from rare decays. Conversely, we obtain a moderate deviation of the ratio R of the CP-averaged B_d -> pi^-+ K^+- and B^+- -> pi^+- K rates from the current experimental value. However, using the emerging signals for B^+- -> K^+- K modes, this effect can be attributed to certain hadronic effects, which have a minor impact on R_c and do not at all affect R_n. Our results for rare decays remain unchanged.

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arXiv2004

B->X_s l^+l^- in the MSSM at NNLO

We present the results of the calculation of QCD corrections to the matching conditions for the Wilson coefficients of operators mediating the transition b->s l^+l^- in the context of the MSSM. Within a scenario with decoupled heavy gluino the calculated contributions together with those present already in the literature allow for the first time a complete NNLO analysis of B->X_s l^+l^-. We study the impact of the QCD corrections and the reduction of renormalization scale dependencies for the dilepton invariant mass distribution and the forward-backward asymmetry in the inclusive decay B->X_s l^+l^- restricting the analysis to the "low-s" region and small values of tan(beta). The NNLO calculation allows to decrease the theoretical uncertainties related to the renormalization scale dependence below the size of supersymmetric effects in B->X_s l^+l^- depending on their magnitude. While it will be difficult to distinguish the MSSM expectations for the branching ratio from the Standard Model ones, this can become possible in the dilepton invariant mass distribution depending on the MSSM parameters and \hat{s}. In this respect the position of the zero of the forward-backward asymmetry s_0 is even more promising.

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arXiv2004

K^+ -> pi^+ nu nu-bar and K_L -> pi^0 nu nu-bar Decays in the General MSSM

We reanalyze the rare decays K^+ -> pi^+ nu nu-bar and K_L -> pi^0 nu nu-bar in a general MSSM with conserved R-parity. Working in the mass eigenstate basis and performing adaptive scanning of a large space of supersymmetric parameters, 16 parameters in the constrained scan and 63 in the extended scan, we find that large departures from the Standard Model expectations are possible while satisfying all existing constraints. Both branching ratios can be as large as a few times 10^{-10} with Br(K_L -> pi^0 nu nu-bar) often larger than Br(K^+ -> pi^+ nu nu-bar) and close to its model independent upper bound. We give examples of supersymmetric parameters for which large departures from the SM expectations can be found and emphasize that the present 90% C.L. experimental upper bound on Br(K^+ -> pi^+ nu nu-bar) gives a non trivial constraint on the MSSM parameter space. Unlike previous analyses, we find that chargino box diagrams can give, already for moderately light charged sleptons, a significant contribution. As a byproduct we find that the ranges for the angles beta and gamma in the unitarity triangle are relaxed due to the presence of new CP-violating phases in K^0 - K^0-bar and B^0_d - B^0_d-bar mixing to 12 degrees <= beta <= 27 degrees and 20 degrees <= gamma <= 110 degrees.

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arXiv2004

Early SU(4)_PS x SU(2)_L x SU(2)_R x SU(2)_H Unification of Quarks and Leptons

We discuss various aspects of the early petite unification (PUT) of quarks and leptons based on the gauge group G_PUT=SU(4)_PS x SU(2)_L x SU(2)_R x SU(2)_H. This unification takes place at the scale M= O(1-2 TeV) and gives the correct value of sin^2 theta_W(M_Z^2) without the violation of the upper bound on the K_L -> mu e rate and the limits on FCNC processes. These properties require the existence of three new generations of unconventional quarks and leptons with charges up to 4/3 (for quarks) and 2 (for leptons) and masses O(250 GeV) in addition to the standard three generations of quarks and leptons. The horizontal group SU(2)_H connects the standard fermions with the unconventional ones. We work out the spontaneous symmetry breaking (SSB) of the gauge group G_PUT down to the SM gauge group, generalize the existing one-loop renormalization group (RG) analysis to the two-loop level including the contributions of Higgs scalars and Yukawa couplings, and demonstrate that the presence of three new generations of heavy unconventional quarks and leptons with masses O(250 GeV) is consistent with astrophysical constraints. The NLO and Higgs contributions to the RG analysis are significant while the Yukawa contributions can be neglected.

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arXiv2004

B -> pi pi, New Physics in B -> pi K and Implications for Rare K and B Decays

The measured B -> pi pi, pi K branching ratios exhibit puzzling patterns. We point out that the B -> pi pi hierarchy can be nicely accommodated in the Standard Model (SM) through non-factorizable hadronic interference effects, whereas the B -> pi K system may indicate new physics (NP) in the electroweak (EW) penguin sector. Using the B -> pi pi data and the SU(3) flavour symmetry, we may fix the hadronic B -> pi K parameters, which allows us to show that any currently observed feature of the B -> pi K system can be easily explained through enhanced EW penguins with a large CP-violating NP phase. Restricting ourselves to a specific scenario, where NP enters only through Z^0 penguins, we derive links to rare K and B decays, where an enhancement of the K_L-> pi^0 nu nu_bar rate by one order of magnitude, with BR(K_L -> pi^0 nu nu_bar) > BR(K^+ -> pi^+ nu nu_bar), BR(K_L -> pi^0 e^+ e^-)=O(10^{-10}), (\sin2β)_{pi nu nu_bar}<0, and a large forward-backward CP asymmetry in B_d -> K* mu^+ mu^-, are the most spectacular effects. We address also other rare K and B decays, epsilon'/epsilon and B_d -> phi K_S.

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arXiv2004

Waiting for Clear Signals of New Physics in B and K Decays

We classify the extensions of the Standard Model (SM) according to the structure of local operators in the weak effective Hamiltonian and the presence or absence of new flavour and CP-violating interactions beyond those represented by the CKM matrix. In particular we review characteristic properties of models with minimal flavour violation (MFV), models with significant contributions from Higgs penguins and models with enhanced Z^0 penguins carrying a large new CP-violating phase. Within the latter models, the anomalous behaviour of certain B\toπK observables implies large departures from the SM predictions for rare and CP-violating K and B decays. Most spectacular is the enhancement of Br(K_L->pi^0 nu nubar) by one order of magnitude and a strong violation of the MFV relation (\sin2β)_{πν\barν}=(\sin2β)_{ψK_S}. On the other hand our prediction for (\sin2β)_{ϕK_S}\approx 0.9 differs from the Belle result by the sign but is consistent with the BaBar value. We give a personal shopping list for the coming years.

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arXiv2004

Anatomy of Prominent B and K Decays and Signatures of CP-Violating New Physics in the Electroweak Penguin Sector

The recent observation of B_d -> pi^0 pi^0 at the B factories with a surprisingly large branching ratio represents a challenge for theory, and complements the amazingly small B_d -> pi^+ pi^- rate. We point out that all puzzling B -> pi pi features can be accommodated in the Standard Model (SM) through non-factorizable hadronic interference effects, extract the relevant parameters, and predict the CP asymmetries of B_d -> pi^0 pi^0. Using then SU(3) flavour-symmetry and plausible dynamical assumptions, we fix the hadronic B -> pi K parameters through their B -> pi pi counterparts, and determine the CKM angle gamma, with a result in remarkable accordance with the usual fits for the unitarity triangle. We may then analyse the B -> pi K system in the SM, where we find agreement with the experimental picture, with the exception of those observables that are significantly affected by electroweak (EW) penguins, thereby suggesting new physics (NP) in this sector. Indeed, a moderate enhancement of these topologies and a large CP-violating NP phase allow us to describe any currently observed feature of the B -> pi K modes, and to predict the CP-violating B_d -> pi^0 K_S observables. If we then restrict ourselves to a specific scenario where NP enters only through Z^0 penguins, we obtain a link to rare K and B decays, where the most spectacular NP effects are an enhancement of the K_L -> pi^0 nu nu_bar rate by one order of magnitude with BR(K_L -> pi^0 nu nu_bar) ~ 4 BR(K^+ -> pi^+ nu nu_bar), BR(K_L -> pi^0 e^+ e^-) = O(10^-10), (sin 2beta)_{pi nu nu_bar} < 0, and a large forward-backward CP asymmetry in B_d -> K^* mu^+ mu^-. We address also epsilon'/epsilon and other prominent decays, including B -> phi K and B -> J/psi K modes.

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arXiv2004

epsilon'/epsilon at the NLO: 10 Years Later

During the last four years several parameters relevant for the analysis of the CP-violating ratio epsilon'/epsilon improved and/or changed significantly. In particular, the experimental value of epsilon'/epsilon and the strange quark mass decreased, the uncertainty in the CKM factor has been reduced, and for a value of the hadronic matrix element of the dominant electroweak penguin operator Q_8, some consensus has been reached among several theory groups. In view of this situation, ten years after the first analyses of epsilon'/epsilon at the next-to-leading order, we reconsider the analysis of epsilon'/epsilon within the SM and investigate what can be said about the hadronic Q_6 matrix element of the dominant QCD penguin operator on the basis of the present experimental value of epsilon'/epsilon and todays values of all other parameters. Employing a conservative range for the reduced electroweak penguin matrix element R_8=1.0+-0.2 from lattice QCD, and present values for all other input parameters, on the basis of the current world average for epsilon'/epsilon, we obtain the reduced hadronic matrix element of the dominant QCD penguin operator R_6=1.23+-0.16 implying _0^NDR(m_c) ~ -0.8 _2^NDR(m_c). We compare these results with those obtained in large-N_c approaches in which generally R_6 ~ R_8 and _0^NDR(m_c) is chirally suppressed relatively to _2^NDR(m_c). We present the correlation between R_6 and R_8 that is implied by the data on epsilon'/epsilon provided new physics contributions to epsilon'/epsilon can be neglected.

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arXiv2003

Minimal Flavour Violation

These lectures give a description of models with minimal flavour violation (MFV) that can be tested in $B$ and $K$ meson decays. This class of models can be formulated to a very good approximation in terms of 11 parameters: 4 parameters of the CKM matrix and 7 values of the {\it universal} master functions $F_r$ that parametrize the short distance contributions. In a given MFV model, $F_r$ can be calculated in perturbation theory and are generally correlated with each other but in a model independent analysis they must be considered as free parameters. We conjecture that only 5 or even only 4 of these functions receive significant new physics contributions. We summarize the status of the CKM matrix, outline strategies for the determination of the values of $F_r$ and present a number of relations between physical observables that do not depend on $F_r$ at all. We emphasize that the formulation of MFV in terms of master functions allows to study transparently correlations between $B$ and $K$ decays which is very difficult if Wilson coefficients normalized at low energy scales are used instead. We discuss briefly a specific MFV model: the Standard Model with one universal large extra dimension.

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arXiv2003

The B -> pi K Puzzle and its Relation to Rare B and K Decays

The Standard-Model interpretation of the ratios of charged and neutral B-> pi K rates, R_c and R_n, respectively, points towards a puzzling picture. Since these observables are affected significantly by colour-allowed electroweak (EW) penguins, this ``B -> pi K puzzle'' could be a manifestation of new physics in the EW penguin sector. Performing the analysis in the R_n-R_c plane, which is very suitable for monitoring various effects, we demonstrate that we may, in fact, move straightforwardly to the experimental region in this plane through an enhancement of the relevant EW penguin parameter q. We derive analytical bounds for q in terms of a quantity L, that measures the violation of the Lipkin sum rule, and point out that strong phases around 90 deg are favoured by the data, in contrast to QCD factorisation. The B -> pi K modes imply a correlation between q and the angle gamma that in the limit of negligible rescattering effects and colour suppressed EW penguins depends only on the value of L. Concentrating on a minimal flavour-violating new-physics scenario with enhanced Z^0 penguins, we find that the current experimental values on B -> X_s mu^+ mu^- require roughly L <= 1.8. As the B -> pi K data give L = 5.7 +- 2.4, L has either to move to smaller values once the B -> pi K data improve or new sources of flavour and CP violation are needed. In turn, the enhanced values of L seen in the B -> pi K data could be accompanied by enhanced branching ratios for rare decays. Most interesting turns out to be the correlation between the B -> pi K modes and BR(K^+ -> pi^+ nu nu), with the latter depending approximately on a single ``scaling'' variable \bar L= L (|V_{ub}/V_{cb}|/0.086)^2.3.

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arXiv2003

Neutrino Physics - Theory

We discuss recent developments in neutrino physics and focus, in particular, on neutrino oscillations and matter effects of three light active neutrinos. Moreover, we discuss the difference between Dirac and Majorana neutrinos, neutrinoless $ββ$-decay, absolute neutrino masses and electromagnetic moments. Basic mechanisms and a few models for neutrino masses and mixing are also presented.

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arXiv2003

The Impact of Universal Extra Dimensions on B -> X_s gamma, B -> X_s gluon, B -> X_s mu^+ mu^-, K_L -> pi^0 e^ +e^-, and epsilon'/epsilon

We calculate the contributions of the Kaluza-Klein (KK) modes to the gamma-penguins, gluon-penguins, gamma-magnetic penguins and chromomagnetic penguins in the Appelquist, Cheng and Dobrescu (ACD) model with one universal extra dimension. Together with our previous calculation of Z^0 penguin diagrams (See http://arxiv.org/abs/hep-ph/0212143) this allows to study the impact of the KK modes on the decays B -> X_s gamma, B -> X_s gluon, B -> X_s mu^+ mu^- and K_L -> pi^0 e^+ e^- and on the CP-violating ratio epsilon'/epsilon. For the compactification scale 1/R= 300 GeV the perturbative part of the branching ratio for B -> X_s mu^+ mu^- is enhanced by 12 % while the zero in the A_FB asymmetry is shifted from \hat s_0=0.162 to \hat s_0=0.142. The sizable suppressions of Br(B -> X_s gamma) ~ 20 % and Br(B -> X_s gluon) ~ 40 % could have interesting phenomenological implications on the lower bound on 1/R provided the experimental and theoretical uncertainties will be decreased. Similar comments apply to epsilon'/epsilon that is suppressed relative to the Standard Model expectations with the size of the suppression depending sensitively on the hadronic matrix elements. The impact on K_L -> pi^0 e^+ e^- is below 10 %. We point out a correlation between the zero \hat s_0 in the A_FB asymmetry and Br(B -> X_s gamma) that should be valid in most models with minimal flavour violation.

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arXiv2003

Petite Unification of Quarks and Leptons: Twenty Two Years After

A recent surge of interest in the {\em early} unification of quarks and leptons, has prompted us to revive a paper \cite{HBB} written twenty two years ago. We provided there a general discussion of quark-lepton unification characterized by the gauge group $G_{S} \otimes G_{W}$ with two couplings $g_S$ and $g_W$ and by the unification mass scales $10\tev-1000\tev$. The constraint from $\sin^2 θ_{W}$ restricts the choices for $G_W$ and our favorite model was chosen to be $SU(4)_{\rm PS}\otimes SU(2)^4$. We review the main results of \cite{HBB}, update the original $SU(4)_{\rm PS}\otimes SU(2)^4$ model and propose two new models based on the groups $SU(4)_{\rm PS}\otimes SU(2)^3$ and $SU(4)_{\rm PS}\otimes SU(3)^2$ for which the consistency with the measured value of $\sin^{2}θ_{W}(M_{Z}^{2})$ determines the unification scale to be roughly $1 {\rm TeV}$ and $3-10 {\rm TeV}$, respectively. The implications of this very early unification is the existence of new quarks and leptons with charges up to 4/3 (for quarks) and 2 (for leptons) and masses $\ord(250\gev)$. Interestingly, in these models the rare decay $K_L\to μe$ is automatically absent at tree level and the one-loop contributions are consistent with the experimental upper bound for this decay.

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