arXiv subjects

Stefan Recksiegel

Publications and source records attributed to Stefan Recksiegel.

28 records · Page 2Linked to original sources

arXiv2005

Electroweak Penguin Hunting Through B -> pi pi, pi K and Rare K and B Decays

The $B\toπK$ decays with significant electroweak penguin contributions show a puzzling pattern. We explore this "$B\toπK$ puzzle" through a systematic strategy. The starting point, which is essentially unaffected by electroweak penguins, is the determination of the angle $γ$ of the unitarity triangle through the CP-violating $B^0_d\toπ^+π^-$, $B^0_d\toπ^-K^+$ asymmetries, yielding $γ=(73.9^{+5.8}_{-6.5})^\circ$, and the extraction of hadronic parameters through the measured $B\toππ$ branching ratios. Using arguments related to the SU(3) flavour symmetry, we convert the hadronic $B\toππ$ parameters into their $B\toπK$ counterparts, allowing us to predict the $B\toπK$ observables in the Standard Model. We find agreement with the data for those quantities that are only marginally affected by electroweak penguins, while this is not the case for the observables with sizeable electroweak penguin contributions. Since we may also perform a couple of internal consistency checks of our working assumptions, which are nicely satisfied for the current data, and find a small sensitivity of our results to large non-factorizable SU(3)-breaking corrections, the "$B\toπK$" puzzle may be due to new physics in the electroweak penguin sector. We show that it can indeed be resolved through such a kind of new physics with a large CP-violating phase. Further insights into the electroweak penguins are provided by the $B^+\toπ^0K^+$ and $B_d^0\toπ^0K_{\rm S}$ CP asymmetries, and in particular through correlations with various rare $K$ and B decays.

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arXiv2005

b -> d Penguins: CP Violation, General Lower Bounds on the Branching Ratios and Standard Model Tests

With the wealth of new data from the B-factories, b -> d penguin decays become available for study, in addition to their b -> s counterparts that have proven an indespensable tool for the exploration of new-physics effects in flavour physics. A prominent example of the b -> d penguin transitions is $\bar B^0_d \to K^0 \bar K^0$. We show that this decay can be charaterized in the Standard Model by a surface in the observable space of the direct and mixing-induced CP asymmetries and the branching ratio. The form of this surface, which is theoretically clean, implies a lower bound for the branching ratio that has recently been confirmed experimentally. If future measurements of the CP asymmetries yield a point away from the SM surface, this would be an interesting signal of new physics. We point out that the hadronic parameters in $\bar B^0_d \to K^0 \bar K^0$ that parameterize the position on the SM surface are related to hadronic parameters in the B -> pi K system. The fact that the branching ratio of $\bar B^0_d \to K^0 \bar K^0$ is very close to its lower bound yields interesting implications for B -> pi K even without knowledge of the CP asymmetries of $\bar B^0_d \to K^0 \bar K^0$. The mechanism that produces the lower bound for $\bar B^0_d \to K^0 \bar K^0$ is actually much more general; we derive lower bounds for various other b -> d penguin-induced processes, including B -> rho gamma and $B^\pm \to K^{(\ast)\pm} K^{(\ast)}$. Some of these theoretical lower bounds are very close to the current experimental upper bounds.

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arXiv2005

General Lower Bounds for b -> d Penguin Processes

For the exploration of flavour physics, b -> d penguin processes are an important aspect, with the prominent example of \bar B^0_d -> K^0 \bar K^0. We recently derived lower bounds for the CP-averaged branching ratio of this channel in the Standard Model; they were found to be very close to the corresponding experimental upper limits, thereby suggesting that \bar B^0_d -> K^0 \bar K^0 should soon be observed. In fact, the BaBar collaboration subsequently announced the first signals of this transition. Here we point out that it is also possible to derive lower bounds for \bar B -> ργdecays, which are again surprisingly close to the current experimental upper limits. We show that these bounds are realizations of a general bound that holds within the Standard Model for b -> d penguin processes, allowing further applications to decays of the kind B^\pm -> K^{(\ast)\pm} K^{(\ast)} and B^\pm -> π^\pm \ell^+ \ell^-, ρ^\pm \ell^+ \ell^-.

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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

Waiting for the Discovery of B^0_d -> K^0 \bar K^0

The CP asymmetries of the decay B_d^0 -> K^0 \bar K^0, which originates from \bar b -> \bar d s \bar s flavour-changing neutral-current processes, and its CP-averaged branching ratio BR(B_d -> K^0 \bar K^0) offer interesting avenues to explore flavour physics. We show that we may characterize this channel, within the Standard Model, in a theoretically clean manner through a surface in observable space. In order to extract the relevant information from BR(B_d -> K^0 \bar K^0), further information is required, which is provided by the B -> pi pi system and the SU(3) flavour symmetry, where we include the leading factorizable SU(3)-breaking corrections and discuss how experimental insights into non-factorizable effects can be obtained. We point out that the Standard Model implies a lower bound for BR(B_d -> K^0 \bar K^0), which is very close to its current experimental upper bound, thereby suggesting that this decay should soon be observed. Moreover, we explore the implications for ``colour suppression'' in the B -> pi pi system, and convert the data for these modes into a peculiar Standard-Model pattern for the CP-violating B_d^0 -> K^0 \bar K^0 observables.

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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

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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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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arXiv2000

Radiatively corrected shape function for inclusive heavy hadron decays

We discuss the non-perturbative and the radiative corrections to inclusive B decays from the point of view known from QED corrections to high energy e^+ e^- processes. Here the leading contributions can be implemented through the so called ``radiator function'' which corresponds to the shape function known in heavy hadron decays. In this way some new insight into the origin of the shape function is obtained. As a byproduct, a parameterization of the radiatively corrected shape function is suggested which can be implemented in Monte Carlo studies of inclusive heavy hadron decays.

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