arXiv subjects

Andrzej J. Buras

Publications and source records attributed to Andrzej J. Buras.

At least 199 records · Page 11Linked to original sources

arXiv1999

Operator Product Expansion, Renormalization Group and Weak Decays

A non-technical description of the Operator Product Expansion and Renormalization Group techniques as applied to weak decays of mesons is presented. We use this opportunity to summarize briefly the present status of the next-to-leading QCD corrections to weak decays and their implications for the unitarity triangle, the ratio epsilon prime/epsilon, the radiative decay B-> s gamma, and the rare decays K^+ -> pi^+ nu barnu and K_L to pi^0 nu barnu.

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arXiv1999

The Rare Decays $K\toπν\barν$, $B\to Xν\barν$ and $B\to l^+l^-$ -- An Update

We update the Standard Model predictions for the rare decays $K^+\toπ^+ν\barν$ and $K_L\toπ^0ν\barν$. In view of improved limits on $B_s$--$\bar B_s$ mixing we derive a stringent and theoretically clean Standard Model upper limit on $B(K^+\toπ^+ν\barν)$, which is based on the ratio of $B_d$--$\bar B_d$ to $B_s$--$\bar B_s$ mixing, $ΔM_d/ΔM_s$, alone. This method avoids the large hadronic uncertainties present in the usual analysis of the CKM matrix. We find $B(K^+\toπ^+ν\barν)< 1.67\cdot 10^{-10}$, which can be further improved in the future. In addition we consider the extraction of $|V_{td}|$ from a future measurement of $B(K^+\toπ^+ν\barν)$, discussing the various sources of uncertainties involved. We also investigate theoretically clean constraints on $B(K_L\toπ^0ν\barν)$. We take the opportunity to review the next-to-leading order (NLO) QCD corrections to $K\toπν\barν$, $K_L\toμ^+μ^-$, $B\to Xν\barν$ and $B\to l^+l^-$, including a small additional term that had been missed in the original publications. The phenomenological impact of this change is negligible, the corresponding numerical shift being essentially within the small perturbative uncertainties at the NLO level.

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arXiv1998

Non-Leptonic Two-Body B Decays Beyond Factorization

We present a general scheme and scale independent parameterization of two-body non-leptonic B decay amplitudes which includes perturbative QCD corrections as well as final state interactions in a consistent way. This parameterization is based on the Next-to-Leading effective Hamiltonian for non-leptonic B decays and on Wick contractions in the matrix elements of the local operators. Using this parameterization, and making no dynamical assumption, we present a classification of two-body B decay channels in terms of the parameters entering in the decay amplitudes. This classification can be considered as the starting point for a model-independent analysis of non-leptonic B decays and of CP violating asymmetries. We also propose, on the basis of the large N expansion, a possible hierarchy among the different effective parameters. We discuss the strategy to extract the most important effective parameters from the experimental data. Finally, we establish a connection between our parameterization and the diagrammatic approach which is widely used in the literature.

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arXiv1998

Upper Bounds on $K \to πν\bar ν$ and $K_L \to π^0 e^+ e^-$ from $ε^\prime/ε$ and $K_L \to μ^+ μ^-$

We analyze rare kaon decays in models in which the dominant new effect is an enhanced $\bar s d Z$ vertex $Z_{ds}$. We point out that in spite of large theoretical uncertainties the CP-violating ratio $ε^\prime/ε$ provides at present the strongest constraint on $\Im Z_{ds}$. Assuming $0 \le ε^\prime/ε\le 2 \cdot 10^{-3}$ and Standard Model values for the CKM parameters we obtain the bounds ${\rm BR}(K_L \to π^0 ν\bar ν) \le 2.4 \cdot 10^{-10}$ and ${\rm BR}(K_L \to π^0 e^+ e^-) \le 3.6 \cdot 10^{-11}$ (which are substantially stronger than the bounds found recently by Colangelo and Isidori, using $ε$ instead of $ε^\prime/ε$). We illustrate how these bounds can be improved with the help of the forthcoming data on $ε^\prime/ε$. Using the bound on $\Re Z_{ds}$ from $K_L \to μ^+ μ^-$ we find ${\rm BR}(K^+ \to π^+ ν\bar ν) \le 2.3 \cdot 10^{-10}$. In this context we derive an analytic upper bound on ${\rm BR}(K^+ \to π^+ ν\bar ν)$ as a function of ${\rm BR}(K_L \to π^0 ν\bar ν)$ and the short distance contribution to ${\rm BR}(K_L \to μ^+ μ^-)$. We also discuss new physics scenarios in which in addition to an enhanced $\bar s d Z$ vertex also neutral meson mixing receives important new contributions. In this case larger values of the branching ratios in question cannot be excluded.

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arXiv1998

A General Analysis of $γ$ Determinations from $B\toπK$ Decays

We present a general parametrization of $B^\pm\toπ^\pm K, π^0K^\pm$ and $B_d\toπ^0 K, π^\mp K^\pm$ decays, taking into account both electroweak penguin and rescattering effects. This formalism allows -- among other things -- an improved implementation of the strategies that were recently proposed by Neubert and Rosner to probe the CKM angle $γ$ with the help of $B^\pm\toπ^\pm K, π^0K^\pm$ decays. In particular, it allows us to investigate the sensitivity of the extracted value of $γ$ to the basic assumptions of their approach. We find that certain SU(3)-breaking effects may have an important impact and emphasize that additional hadronic uncertainties are due to rescattering processes. The latter can be controlled by using SU(3) flavour symmetry arguments and additional experimental information provided by $B^\pm\to K^\pm K$ modes. We propose a new strategy to probe the angle $γ$ with the help of the neutral decays $B_d\toπ^0 K, π^\mp K^\pm$, which is theoretically cleaner than the $B^\pm\toπ^\pm K, π^0K^\pm$ approach. Here rescattering processes can be taken into account by just measuring the CP-violating observables of the decay $B_d\toπ^0K_{\rm S}$. Finally, we point out that an experimental analysis of $B_s\to K^+K^-$ modes would also be very useful to probe the CKM angle $γ$, as well as electroweak penguins, and we critically compare the virtues and weaknesses of the various approaches discussed in this paper. As a by-product, we point out a strategy to include the electroweak penguins in the determination of the CKM angle $α$ from $B\toππ$ decays.

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arXiv1998

Weak Hamiltonian, CP Violation and Rare Decays

These lectures describe in detail the effective Hamiltonians for weak decays of mesons constructed by means of the operator product expansion and the renormalization group method. We calculate Wilson coeffcients of local operators, discuss mixing of operators under renormalization, the anomalous dimensions of operators and anomalous dimension matrices. We elaborate on the renormalzation scheme and renormalization scale dependences and their cancellations in physical amplitudes. In particular we discuss the issue of gamma-5 in D-dimensions and the role of evanescent operators in the calculation of two-loop anomalous dimensions. We present an explicit calculation of the 6 times 6 one-loop anomalous dimension matrix involving current-current and QCD-penguin operators and we give some hints how to properly calculate two-loop anomalous dimensions of these operators. In the phenonomenological part of these lectures we discuss in detail: CKM matrix, the unitarity triangle and its determination, two-body non-leptonic B-decays and the generalized factorization, the ratio epsilonprime/epsilon, B to X_s gamma, K^+ to pi^+ nu barnu, K_L to pi^0 nu barnu, B to X_s nu barnu, B_s to mu bar mu and some aspects of CP violation in B-decays.

hep-ph↗

arXiv1998

Penguin Topologies, Rescattering Effects and Penguin Hunting with $B_{u,d}\to K\bar{K}$ and $B^\pm\toπ^\pm K$

In the recent literature, constraints on the CKM angle $γ$ arising from the branching ratios for $B^\pm\toπ^\pm K$ and $B_d\toπ^\mp K^\pm$ decays received a lot of attention. An important theoretical limitation of the accuracy of these bounds is due to rescattering effects, such as $B^+\to\{π^0K^+\}\toπ^+K^0$. We point out that these processes are related to penguin topologies with internal up quark exchanges and derive SU(2) isospin relations among the $B^+\toπ^+K^0$ and $B_d^0\toπ^-K^+$ decay amplitudes by defining ``tree'' and ``penguin'' amplitudes in a proper way, allowing the derivation of generalized bounds on the CKM angle $γ$. We propose strategies to obtain insights into the dynamics of penguin processes with the help of the decays $B_{u,d}\to K\bar{K}$ and $B^\pm\toπ^\pm K$, derive a relation among the direct CP-violating asymmetries arising in these modes, and emphasize that rescattering effects can be included in the generalized bounds on $γ$ completely this way. Moreover, we have a brief look at the impact of new physics.

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arXiv1998

On the Scale Uncertainties in the $B \to X_s γ$ Decay

We analyze the theoretical uncertainties in $Br(B\to X_sγ)$ due to the choice of the high energy matching scale $μ_W=\ord(\mw)$ and the scale $μ_t$ at which the running top quark mass is defined: $\mtb(μ_t)$. To this end we have repeated the calculation of the initial conditions confirming the final results of Adel and Yao and Greub and Hurth and generalizing them to include the dependences on $μ_t$ and $μ_W$ with $μ_t\not=μ_W$. In the leading order the $μ_W$ and $μ_t$ uncertainties in $Br(B\to X_sγ)$ turn out to be $\pm 13%$ and $\pm 3%$ respectively. We show analytically how these uncertainties are reduced after including next-to-leading QCD corrections. They amount to $\pm 1.1%$ and $\pm 0.4%$ respectively. Reanalyzing the uncertainties due to the scale $μ_b=\ord(m_b)$ we find that after the inclusion of NLO effects they amount to $\pm 4.3%$ which is a factor 2/3 smaller than claimed in the literature. Including the uncertainties due to input parameters as well as the non-perturbative $1/m_b^2$ and $1/m_c^2$ corrections we find $Br(B{\to}X_s γ) = (3.60 \pm 0.33) \times 10^{-4}$ where the error is dominated by uncertainties in the input parameters. This should be compared with $(3.28 \pm 0.33) \times 10^{-4}$ found by Chetyrkin et al. where the error is shared evenly between the scale and parametric uncertainties.

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arXiv1997

CKM Matrix: Present and Future

We review the present status of the CKM matrix and we offer some visions of its future. After a brief presentation of the theoretical framework for weak decays we discuss the following topics: i) CKM matrix from tree level decays, ii) Standard analysis of the unitarity triangle, iii) CKM matrix from rare and CP violating K- and B-decays, iv) CKM matrix from CP violation in two-body B-decays. In particular we compare the CKM potentials of the standard analysis of the unitarity triangle, of the very clean K --> πν\barνdecays and of CP asymmetries in B-decays.

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arXiv1997

Two-Loop Large-$m_t$ Electroweak Corrections to $K\toπν\barν$ for Arbitrary Higgs Boson Mass

We consider for the first time the leading large top mass corrections, arising at higher order in electroweak interactions, to the rare decays $K\toπν\barν$ and the related modes $B\to X_sν\barν$ and $B\to l^+l^-$. Higher order effects of similar type have previously been calculated in the large-$m_t$ limit for key observables of precision electroweak physics at Z-factories. Here we obtain the corresponding corrections of order ${\cal O}(G^2_F m^4_t)$ at the amplitude level for short-distance dominated rare meson decays. This allows us to quantify the importance of higher order electroweak effects for these processes, which can be reliably computed and have very small uncertainties from strong interactions. Simultaneously it becomes possible to remove, to some extent, ambiguities in the definition of electroweak parameters describing the strength of FCNC interactions. The corrections we discuss are at the level of a few percent.

hep-ph↗

arXiv1997

Quark mixing, CP violation and rare decays after the top quark discovery

We review the highlights of quark mixing, particle-antiparticle mixing, CP violation and rare K- and B-decays in the Standard Model. The top quark discovery, the precise measurement of its mass, the improved knowledge of the couplings $V_{cb}$ and $V_{ub}$, and the calculations of NLO short distance QCD corrections improved considerably the predictions for various decay rates, the determination of the couplings $V_{td}$ and $V_{ts}$ and of the complex phase in the Cabibbo-Kobayashi-Maskawa matrix. After presenting the general theoretical framework for weak decays, we discuss the following topics in detail: i) the CKM matrix, its most convenient parametrizations and the unitarity triangle, ii) the CP-violating parameter $ε_K$ and $B^0_{d,s}-\bar B^0_{d,s}$ mixings, iii) the ratio $ε'/ε$, iv) the rare K-decays $K_L\toπ^0e^+e^-$, $K^+\toπ^+ν\barν$, $K_L\toπ^0ν\barν$ and $K_L\toμ^+μ^-$, v) the radiative decays $B\to X_sγ$ and $B\to X_s l^+l^-$, vi) the rare B-decays $B\to X_{s,d}ν\barν$ and $B_{d,s}\to l^+l^-$, vii) CP violation in neutral and charged B-decays putting emphasis on clean determinations of the angles of the unitarity triangle, and viii) the role of electroweak penguins in B-decays. We present several future visions demonstrating very clearly the great potential of CP asymmetries in B-decays and of clean K-decays such as $K^+\toπ^+ν\barν$ and $K_L\toπ^0ν\barν$ in the determination of the CKM parameters and in decisive testing of the Standard Model. An outlook for the coming years ends our review.

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arXiv1996

Flavour Changing Neutral Current Processes

We review the status of the flavour-changing neutral-current processes (FCNC). In particular we discuss: i) Main targets of the field, ii) The theoretical framework for FCNC, iii) Standard analysis of the unitarity triangle, iv) $\varepsilon'/\varepsilon$, v) Radiative, rare and CP-violating decays, vi) CP-asymmetries in B-decays, vii) Comparision of the potentials of $K \to πν\barν$ and CP-B asymmetries, vii) Some aspects of the physics beyond the Standard Model.

hep-ph↗

arXiv1996

Theoretical review of K physics

We review several aspects of K-Physics: i) Main targets of the field, ii) The theoretical framework for K-decays, iii) Standard analysis of the unitarity triangle, iv) epsilon'/epsilon, v) Rare and CP violating K-decays, vi) Comparision of the potentials of K --> pi nu nu-bar and CP-B asymmetries. vii) Some aspects of the physics beyond the Standard Model.

hep-ph↗

arXiv1996

$K\toπν\barν$ and High Precision Determinations of the CKM Matrix

We investigate the future determination of the CKM matrix using theoretically clean quantities, like $B(K^+\toπ^+ν\barν)$, $B(K_L\toπ^0ν\barν)$ or $\sin 2β$, $\sin 2α$ as extracted from CP violation studies in $B$ physics. The theoretical status of $K\toπν\barν$ is briefly reviewed and their phenomenological potential is compared with that of CP asymmetries in $B$ decays. We stress the unique opportunities provided by measuring the CP violating rare decay $K_L\toπ^0ν\barν$. It is pointed out that this mode is likely to offer the most precise determination of ${\rm Im}V^*_{ts}V_{td}$ and the Jarlskog parameter $J_{CP}$, the invariant measure of CP violation in the Standard Model.

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arXiv1995

Weak Decays Beyond Leading Logarithms

We review the present status of QCD corrections to weak decays beyond the leading logarithmic approximation including particle-antiparticle mixing and rare and CP violating decays. After presenting the basic formalism for these calculations we discuss in detail the effective hamiltonians for all decays for which the next-to-leading corrections are known. Subsequently, we present the phenomenological implications of these calculations. In particular we update the values of various parameters and we incorporate new information on m_t in view of the recent top quark discovery. One of the central issues in our review are the theoretical uncertainties related to renormalization scale ambiguities which are substantially reduced by including next-to-leading order corrections. The impact of this theoretical improvement on the determination of the Cabibbo-Kobayashi-Maskawa matrix is then illustrated in various cases.

hep-ph↗

arXiv1995

Theoretical Review of B-Physics

We review several aspects of B-Physics. In particular we discuss: i) The theoretical framework for B-decays, ii) Weak decays beyond leading logarithms, iii) Standard analysis of the unitarity triangle, iv) Rare B-decays, v) CP violation including the issue of electroweak penguins vi) Future visions of this field.

hep-ph↗

arXiv1995

A Determination of the CKM-angle $α$ using Mixing-induced CP Violation in the Decays $B_d\toπ^+π^-$ and $B_d\to K^0 \bar K^0$

We present a method of determining the CKM-angle $α$ by performing simultaneous measurements of the mixing-induced CP asymmetries of the decays $B_d\toπ^+π^-$ and $B_d\to K^0\bar K^0$. The accuracy of our approach is limited by $SU(3)$-breaking effects originating from $\bar b\to \bar ds\bar s$ QCD-penguin diagrams. Using plausible power-counting arguments we show that these uncertainties are expected to be of the same order as those arising through electroweak penguins in the standard Gronau-London-method in which $α$ is extracted by means of isospin relations among $B\toππ$ decay amplitudes. Therefore our approach, which does not involve the experimentally difficult mode $B_d\toπ^0π^0$ and is essentially unaffected by electroweak penguins, may be an interesting alternative to determine $α$.

hep-ph↗

arXiv1995

Towards the Control over Electroweak Penguins in Nonleptonic B-Decays

We present strategies for determining electroweak penguins from experimental data. Using the CKM-angle $γ$ as one of our central inputs and making some reasonable approximations, we show that the $\bar b\to\bar s$ electroweak penguin amplitude can be determined in a two-step procedure involving i) BR$(B^+\toπ^0K^+)$, BR$(B^-\toπ^0K^-)$, BR$(B^+\toπ^+K^0)$ and ii) either BR$(B^0_d\toπ^-K^+)$, BR$(\bar B^0_d\toπ^+K^-)$ or $\acp(t)$ of the mode $B_s\to K^+K^-$. The determination employing the $B\toπK$ transitions is not affected by $SU(3)$-breaking effects. Relating the $\bar b\to\bar s$ electroweak penguin amplitude to the $\bar b\to\bar d$ case through $SU(3)$ symmetry arguments, we are in a position to estimate the electroweak penguin uncertainty affecting the extraction of the CKM-angle $α$ by using isospin relations among $B\toππ$ decays. Our results allow in principle the determination of CKM-phases in a variety of $B$-decays.

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