arXiv subjects

Andrzej J. Buras

Publications and source records attributed to Andrzej J. Buras.

At least 181 records · Page 10Linked to original sources

arXiv2003

Relations between ΔM_{s,d} and B_{s,d}\to μ\barμin Models with Minimal Flavour Violation

The predictions for the B_{s,d}-\bar B_{s,d} mixing mass differences ΔM_{s,d} and Br(B_{s,d}\toμ\barμ) within the Standard Model (SM) and its extensions suffer from considerable hadronic uncertainties present in the B_{s,d}-meson decay constants F_{B_{s,d}} that enter these quantities quadratically. We point out that in the restricted class of models with minimal flavour violation (MFV) in which only the SM low energy operators are relevant, the ratios Br(B_{q}\toμ\barμ)/ΔM_q (q=s,d) do not depend on F_{B_{q}} and the CKM matrix elements. They involve in addition to the short distance functions and B-meson lifetimes only the non-perturbative parameters \hat B_{s,d}. The latter are under much better control than F_{B_{s,d}}. Consequently in these models the predictions for Br(B_{q}\toμ\barμ) have only small hadronic uncertainties once ΔM_q are experimentally known. Of particular interest is also the relation Br(B_{s}\toμ\barμ)/Br(B_{d}\toμ\barμ)=\hat B_{d}/\hat B_{s} τ(B_{s})/τ(B_{d}) ΔM_{s}/ΔM_{d} that is practically free of theoretical uncertainties as \hat B_{s}/\hat B_{d}=1 up to small SU(3) breaking corrections. Using these ideas within the SM we find much more accurate predictions than those found in the literature: Br(B_{s}\toμ\barμ)=(3.4\pm 0.5)\cdot 10^{-9} and Br(B_{d}\toμ\barμ)=(1.00\pm 0.14)\cdot 10^{-10} were in the first case we assumed as an example ΔM_s=(18.0\pm 0.5)/ps.

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arXiv2003

The Impact of Universal Extra Dimensions on the Unitarity Triangle and Rare K and B Decays

We calculate the contributions of the Kaluza-Klein (KK) modes to the K_L-K_S mass difference ΔM_K, the parameter ε_K, the B^0_{d,s}-\bar B^0_{d,s} mixing mass differences ΔM_{d,s} and rare decays K^+\to π^+ν\barν, K_L\to π^0ν\barν, K_L\toμ\barμ, B\to X_{s,d}ν\barνand B_{s,d}\toμ\barμin the Appelquist, Cheng and Dobrescu (ACD) model with one universal extra dimension. For the compactification scale 1/R= 200 GeV the KK effects in these processes are governed by a 17% enhancement of the ΔF=2 box diagram function S(x_t,1/R) and by a 37% enhancement of the Z^0 penguin diagram function C(x_t/1/R) relative to their Standard Model (SM) values. This implies the suppressions of \vtd by 8%, of \barηby 11% and of the angle γin the unitarity triangle by 10^\circ. ΔM_s is increased by 17%. ΔM_K is essentially uneffected. All branching ratios considered in this paper are increased with a hierarchical structure of enhancements: K^+\to π^+ν\barν(16%), K_L\to π^0ν\barν(17%), B\to X_{d}ν\barν(22%), (K_L\toμ\barμ)_{\rm SD} (38%), B\to X_{s}ν\barν(44%), B_{d}\toμ\barμ(46%) and B_{s}\toμ\barμ(72%). For 1/R= 250 (300) GeV all these effects are decreased roughly by a factor of 1.5 (2.0). We give the relevant Feynman rules for the five dimensional ACD model. We also emphasize that a consistent calculation of branching ratios has to take into account the modifications in the values of the CKM parameters. As a byproduct we confirm the dominant \ord (g_2 G_F m_t^4 R^2) correction from the KK modes to the $Z^0 b\bar b$ vertex calculated recently in the large m_t limit.

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arXiv2002

The CKM Matrix and The Unitarity Triangle: Another Look

The unitarity triangle can be determined by means of two measurements of its sides or angles. Assuming the same relative errors on the angles $(α,β,γ)$ and the sides $(R_b,R_t)$, we find that the pairs $(γ,β)$ and $(γ,R_b)$ are most efficient in determining $(\bar\varrho,\barη)$ that describe the apex of the unitarity triangle. They are followed by $(α,β)$, $(α,R_b)$, $(R_t,β)$, $(R_t,R_b)$ and $(R_b,β)$. As the set $\vus$, $\vcb$, $R_t$ and $β$ appears to be the best candidate for the fundamental set of flavour violating parameters in the coming years, we show various constraints on the CKM matrix in the $(R_t,β)$ plane. Using the best available input we determine the universal unitarity triangle for models with minimal flavour violation (MFV) and compare it with the one in the Standard Model. We present allowed ranges for $\sin 2β$, $\sin 2α$, $γ$, $R_b$, $R_t$ and $ΔM_s$ within the Standard Model and MFV models. We also update the allowed range for the function $F_{tt}$ that parametrizes various MFV-models.

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arXiv2002

Unitarity Triangle: 2002 and Beyond

We describe the present status of the Unitarity Triangle and we give an outlook for its future determinations. We discuss new sets of fundamental flavour parameters and comment briefly on new physics beyond the Standard Model.

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arXiv2002

B -> X_s gamma after completion of the NLO QCD calculations

Several years ago, Stefan Pokorski, Manfred Muenz and us outlined a program for calculation of the NLO QCD corrections to the weak radiative B meson decay B -> X_s gamma. Very recently, just before the 60th birthday of Stefan Pokorski, this program has been formally completed. In the present paper, we summarize the existing results and discuss perspectives for further improvement of the accuracy of the Standard Model prediction for BR[ B -> X_s gamma].

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arXiv2002

Completing the NLO QCD calculation of B -> Xs gamma

We evaluate two-loop b -> s gamma matrix elements of all the four-quark operators containing no derivatives. Contrary to previous calculations, no expansion in the mass ratio mc/mb is performed, and all the possible Dirac and flavor structures are included. Consequently, we are able to provide the last item in the NLO analysis of B -> Xs gamma that has been missing so far, namely the two-loop matrix elements of the QCD-penguin operators. Due to smallness of the Wilson coefficients of those operators in the Standard Model, their effect on the branching ratio is small: a reduction by roughly 1%. We find BR[ B -> Xs gamma]_{E_gamma > 1.6 GeV} = (3.57 +_ 0.30)*10^{-4}.

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arXiv2001

$ΔM_s/ΔM_d$, $\sin 2β$ and the angle $γ$ in the Presence of New $ΔF=2$ Operators

We present formulae for the mass differences $ΔM_d$ and $ΔM_s$ in the \BBds systems and for the CP violation parameter $ε$ which are valid in minimal flavour violation models giving rise to new four-fermion $ΔF=2$ operators. Short distance contributions to $ΔM_s$, $ΔM_d$ and $ε$ are parameterized by three {\it real} functions $F^s_{tt}$, $F^d_{tt}$ and $F^ε_{tt}$, respectively ($F^s_{tt} = F^d_{tt} = F^ε_{tt}$ holds only if the Standard Model $(V-A) \otimes (V-A)$ operators dominate). We present simple strategies involving the ratio $ΔM_s/ΔM_d$, $\sin2β$ and $γ$ that allow to search for the effects of the new operators. We point out that their sizable contributions to the ratio $ΔM_s/ΔM_d$ would in principle allow $γ$ to be larger than $90^\circ$. Constraints on the functions $F^i_{tt}$ imposed by the present (and future) experimental data are also discussed. As an example we show that for large $\tan\barβ\equiv v_2/v_1$ and $H^+$ not too heavy, $F^s_{tt}$ in the MSSM with heavy sparticles can be substantially smaller than in the SM due the charged Higgs box contributions and in particular due to the growing like $\tan^4\barβ$ contribution of the double penguin diagrams involving neutral Higgs boson exchanges. As a result the bounds on the function $F^s_{tt}$ can be violated which allows to exclude large mixing of stops. In this scenario the range of $\sin2β$ following from $ε$ and $ΔM_d$ is identical to the SM ones ($0.5<\sin 2β<0.8$). On the other hand $γ$ following from $ΔM_s/ΔM_d$ is lower.

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arXiv2001

Flavour Physics and CP Violation in the SM

We review the main aspects of Flavour Physics and CP Violation in the Standard Model. After presenting a grand view of the field including a Master Formula for weak decays we discuss i) Standard analysis of the unitarity triangle, ii) The ratio epsilon'/epsilon, iii) Rare decays K^+ -> pi^+ nu barnu and K_L -> pi^0 nu barnu, iv) CP violation in B decays and v) Models with Minimal Flavour Violation. Our review ends with 20 questions that hopefully will be answered in the coming years.

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arXiv2001

Bounds on the Unitarity Triangle, $\sin 2β$ and $K\toπν\barν$ Decays in Models with Minimal Flavour Violation

We present a general discussion of the unitarity triangle from $ε_K$, $ΔM_{d,s}$ and $K \to πν\overlineν$ in models with minimal flavour violation (MFV), allowing for arbitrary signs of the generalized Inami--Lim functions $F_{tt}$ and $X$ relevant for $(ε_K,ΔM_{d,s})$ and $K \to πν\overlineν$, respectively. In the models in which $F_{tt}$ has a sign opposite to the one in the Standard Model, i.e. $F_{tt}<0$, the data for $(ε_K, ΔM_{d,s})$ imply an absolute lower bound on the $B_d\toψK_{S}$ CP asymmetry $a_{ψK_S}$ of 0.69, which is substantially stronger than 0.42 arising in the case of $F_{tt}>0$. An important finding of this paper is the observation that for given $Br(K^+\toπ^+ν\overlineν)$ and $a_{ψK_S}$ only two values for $Br(K_{L}\toπ^0ν\overlineν)$, corresponding to the two signs of $X$, are possible in the full class of MFV models, independently of any new parameters arising in these models. This provides a powerful test for this class of models. Moreover, we derive absolute lower and upper bounds on $Br(K_{L}\toπ^0ν\overlineν)$ as functions of $Br(K^+\toπ^+ν\overlineν)$. Using the present experimental upper bounds on $Br(K^+\toπ^+ν\overlineν)$ and $|V_{ub}/V_{cb}|$, we obtain the absolute upper bound $Br(K_{L}\toπ^0ν\overlineν)< 7.1 \cdot 10^{-10}$ (90% C.L.).

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arXiv2001

What is the (epsilon'/epsilon)_exp Telling Us ?

Nature might be kinder than previously thought as far as epsilon'/epsilon is concerned. We show that the recently obtained experimental value for epsilon'/epsilon does not require sizeable 1/N and isospin-breaking corrections. We propose to display the theoretical results for epsilon'/epsilon in a (P^{1/2}, P^{3/2}) plane in which the experimental result is represented by a (epsilon'/epsilon)_exp-path. This should allow to exhibit transparently the role of 1/N and isospin-breaking corrections in different calculations of epsilon'/epsilon. From now on theorists are allowed to walk only along this (epsilon'/epsilon)_exp-path.

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arXiv2001

Two-Loop Matrix Element of the Current-Current Operator in the Decay B -> X_s gamma

We evaluate the important two-loop matrix element of the operator (cbar gamma^mu P_L b) (sbar gamma_mu P_L b) contributing to the inclusive radiative decay B -> X_s gamma. The calculation is performed in the NDR scheme, by means of asymptotic expansions method. The result is given as a series in z = m_c^2/m_b^2 up to O(z^6). We confirm the result of Greub, Hurth and Wyler obtained by a different method up to 0(z^3). Higher-order terms are found to be numerically insignificant.

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arXiv2001

Master Formulae for Delta F=2 NLO-QCD Factors in the Standard Model and Beyond

We present analytic formulae for the QCD renormalization group factors relating the Wilson coefficients C_i(mu_t) and C_i(mu), with mu_t = O(m_t) and mu < mu_t, of the Delta F=2 dimension six four-quark operators Q_i in the Standard Model and in all of its extensions. Analogous analytic formulae for the QCD factors relating the matrix elements and with mu_K < 2GeV are also presented. The formulae are given in the NDR scheme. The strongest renormalization-group effects are found for the operators with the Dirac structures (1 - gamma_5)*(1 + gamma_5) and (1 - gamma_5)*(1 - gamma_5). We calculate the matrix elements <\bar K^0|Q_i|K^0> in the NDR scheme using the lattice results in the LRI scheme. We give expressions for the mass differences Delta M_K and Delta M_B and the CP-violating parameter epsilon_K in terms of the non-perturbative parameters B_i and the Wilson coefficients C_i(mu_t). The latter summarize the dependence on new physics contributions.

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arXiv2001

Flavour Dynamics: CP Violation and Rare Decays

These lectures give an up to date description of CP violation and rare decays of K and B mesons and consist of ten chapters: i) Grand view of the field including CKM matrix and the unitarity triangle, ii) General aspects of the theoretical framework based on effective weak Hamiltonians, the operator product expansion and the renormalization group, iii) Particle-antiparticle mixing and various types of CP violation, iv) Standard analysis of the unitarity triangle, v) The ratio epsilon'/epsilon, vi) Rare decays K^+ -> pi^+ nu barnu and K_L to pi^0 nu barnu, vii) Express review of other rare decays, viii) CP violation in B decays, ix) A brief look beyond the Standard Model discussing in particular the models with minimal flavour violation, x) Perspectives for the coming years.

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arXiv2000

Constraints on $γ$ and Strong Phases from $B\toπK$ Decays

As we pointed out recently, the neutral decays $B_d\toπ^\mp K^\pm$ and $B_d\toπ^0K$ may provide non-trivial bounds on the CKM angle $γ$. Here we reconsider this approach in the light of recent CLEO data, which look very interesting. In particular, the results for the corresponding CP-averaged branching ratios are in favour of strong constraints on $γ$, where the second quadrant is preferred. Such a situation would be in conflict with the standard analysis of the unitarity triangle. Moreover, constraints on a CP-conserving strong phase $δ_n$ are in favour of a negative value of $\cosδ_n$, which would be in conflict with the factorization expectation. In addition, there seems to be an interesting discrepancy with the bounds that are implied by the charged $B\toπK$ system: whereas these decays favour a range for $γ$ that is similar to that of the neutral modes, they point towards a positive value of $\cosδ_c$, which would be in conflict with the expectation of equal signs for $\cosδ_n$ and $\cosδ_c$.

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arXiv2000

A Lower Bound on sin 2βfrom Minimal Flavour Violation

We point out that there exists an absolute lower bound on sin 2βin all models with minimal flavour violation (MFV), that do not have any new operators beyond those present in the Standard Model and in which all flavour changing transitions are governed by the CKM matrix with no new phases beyond the KM phase. This bound depends only on V_{cb}, V_{ub} and the hadronic parameters \hat B_K, F_{B_d}\sqrt{\hat B_d} and ξrelevant for the CP-violating parameter \varepsilon and the B^0_{d,s}-\bar B^0_{d,s} mixings. Performing a simple scanning over the present ranges of these parameters we find sin 2β\ge 0.34. We illustrate how this bound could become stronger when our knowledge of the parameters in question improves and when the upper bound on the B^0_{s}-\bar B^0_{s} mixing ((ΔM)_s) will be experimentally known. Provided the future accurate measurements of sin 2βthrough the CP asymmetry in B_d^0(\bar B_d^0)\to ψK_S will confirm the low values recently reported by BaBar and Belle, there is a likely possibility that this class of models will be excluded. This would firmly imply the necessity of new CP-violating phases and/or new effective operators in the weak effective Hamiltonians for K^0-\bar K^0 and B^0_{d,s}-\bar B^0_{d,s} mixings. We also point out that within the MFV models there exists also an absolute lower bound on the angle γ. We find \sinγ\ge 0.24. This lower bound could become stronger in the future.

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arXiv2000

Constraints on the CKM Angle $γ$ and Strong Phases from $B\toπK$ Decays

As we pointed out recently, the neutral decays $B_d\toπ^\mp K^\pm$ and $B_d\toπ^0K$ may provide non-trivial bounds on the CKM angle $γ$. In this paper, we reconsider this approach in the light of recent CLEO data, which look very interesting. In particular, the results for the corresponding CP-averaged branching ratios are in favour of strong constraints on $γ$, where the second quadrant is preferred. Such a situation would be in conflict with the standard analysis of the unitarity triangle. Moreover, constraints on a CP-conserving strong phase $δ_n$ are in favour of a negative value of $\cosδ_n$, which would be in conflict with the factorization expectation. In addition, there seems to be an interesting discrepancy with the bounds that are implied by the charged $B\toπK$ system: whereas these decays favour a range for $γ$ that is similar to that of the neutral modes, they point towards a positive value of $\cosδ_c$, which would be in conflict with the expectation of equal signs for $\cosδ_n$ and $\cosδ_c$. If future data should confirm this ``puzzle'', it may be an indication for new-physics contributions to the electroweak penguin sector, or a manifestation of large non-factorizable SU(3)-breaking effects.

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arXiv1999

Theoretical Status of epsilon'/epsilon

We review the present theoretical status of the CP violating ratio epsilon'/epsilon in the Standard Model (SM) and confront its estimates with the most recent data. In particular we review the present status of the most important parameters m_s, B_6^(1/2), B_8^(3/2), Λ_MSbar and Omega_{η+η'}. While the sign and the order of magnitude of SM estimates for epsilon'/epsilon agree with the data, for central values of these parameters most estimates of epsilon'/epsilon in the SM are substantially below the grand experimental average (21.2\pm 4.6)\cdot 10^{-4}. Only in a small corner of the parameter space can epsilon'/epsilon in SM be made consistent with experimental results. In view of very large theoretical uncertainties, it is impossible to conclude at present that the data on epsilon'/epsilon indicate new physics. A brief discussion of epsilon'/epsilon beyond the Standard Model is presented.

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arXiv1999

CP Violation and Rare Decays of K and B Mesons

These lectures describe CP violation and rare decays of K and B mesons and consist of ten chapters: i) Grand view of the field including CKM matrix and the unitarity triangle, ii) General aspects of the theoretical framework, iii) Particle-antiparticle mixing and CP violation, iv) Standard analysis of the unitarity triangle, v) The ratio epsilon'/epsilon including most recent developments, vi) Rare decays K^+ -> pi^+ nu barnu and K_L -> pi^0 nu barnu, vii) Express review of other rare decays, viii) Express review of CP violation in B decays, ix) A brief look beyond the Standard Model including connections between epsilon'/epsilon and CP violating rare K decays, x) Final messages.

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