arXiv subjects

Robert Fleischer

Publications and source records attributed to Robert Fleischer.

At least 109 records · Page 6Linked to original sources

arXiv2004

Flavour Physics and CP Violation

The starting point of these lectures is an introduction to the weak interactions of quarks and the Standard-Model description of CP violation, where the key element is the Cabibbo--Kobayashi--Maskawa matrix and the corresponding unitarity triangles. Since the B-meson system will govern the stage of (quark) flavour physics and CP violation in this decade, it will be -- after a brief look at the kaon system -- our main focus. We shall classify B-meson decays, introduce the theoretical tools to deal with them, explore the requirements for non-vanishing CP-violating asymmetries, and discuss B^0_q--B^0_q_bar mixing (q={d,s}). We will then turn to B-factory benchmark modes, discuss the physics potential of B^0_s mesons, which is particularly promising for B-decay experiments at hadron colliders, and emphasize the importance of studies of rare decays, which are absent at the tree level in the Standard Model, complement nicely the studies of CP violation, and provide interesting probes for new physics.

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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

Theoretical Review of CP Violation

The focus of our considerations is the B-meson system, which will provide stringent tests of the Kobayashi-Maskawa mechanism of CP violation in this decade. After a classification of the main possible strategies to achieve this goal, we discuss the status of the B-factory benchmark modes in view of the current experimental data. We shall then turn to the "El Dorado" for B-decay studies at hadron colliders, the Bs-meson system, where we will also address new, theoretically clean strategies to explore CP violation.

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arXiv2003

New, Efficient and Clean Strategies to Explore CP Violation Through Neutral B Decays

We point out that decays of the kind $B_d\to D_\pm K_{S(L)}$ and $B_s\to D_\pmη^{(')},D_\pmϕ, ...$, where $D_+$ and $D_-$ denote the CP-even and CP-odd eigenstates of the neutral D-meson system, respectively, provide very efficient, theoretically clean determinations of the angle $γ$ of the unitarity triangle. In this new strategy, we use the $B^0_q$--$\bar{B^0_q}$ ($q\in\{d,s\}$) mixing phase $ϕ_q$ as an input, and employ only ``untagged'' and mixing-induced CP-violating observables, which satisfy a very simple relation, allowing us to determine $\tanγ$. Using a plausible dynamical assumption, $γ$ can be fixed in an essentially unambiguous manner. The corresponding formalism can also be applied to $B_d\to D_\pmπ^0,D_\pmρ^0, >...$ and $B_s\to D_\pm K_{S(L)}$ decays. Although these modes appear less attractive for the extraction of $γ$, they provide interesting determinations of $\sinϕ_q$. In comparison with the conventional $B_d\to J/ψK_{S(L)}$ and $B_s\to J/ψϕ$ methods, these extractions do not suffer from any penguin uncertainties, and are theoretically cleaner by one order of magnitude.

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arXiv2003

A Closer Look at $B_{d,s}\to D f_r$ Decays and Novel Avenues to Determine $γ$

Decays of neutral B mesons originating from $b\to D r$ $(r\in\{s,d\})$ processes, i.e. $B_d\to DK_{S(L)}$, $B_s\to Dη^{(')}, Dϕ, ...$ $(r=s)$ and $B_s\to DK_{S(L)}$, $B_d\to Dπ^0, Dρ^0, ...$ $(r=d)$ modes, offer valuable insights into CP violation. Here the neutral $D$ mesons may be observed through $D\to f_\pm$ or $D\to f_{NE}$, where $f_\pm$ and $f_{NE}$ are CP eigenstates and CP non-eigenstates, respectively. Recently, we pointed out that ``untagged'' and mixing-induced CP-violating $B_{d,s}\to D_\pm f_s$ observables provide a very efficient, theoretically clean determination of the angle $γ$ of the unitarity triangle. Here we perform a more detailed analysis of the $B_{d,s}\to D[\to f_\pm]f_s$ channels, and focus on $B_{d,s}\to D[\to f_{NE}] f_s$, where interference effects between Cabibbo-favoured and doubly Cabibbo-suppressed D-meson decays lead to complications. We show that $γ$ can nevertheless be determined in an elegant and essentially unambiguous manner from the $B_{d,s}\to D[\to f_{NE}]f_s$ observables with the help of the corresponding ``untagged'' $B_{d,s}\to D[\to f_\pm]f_s$ measurements. Moreover, we may also extract hadronic B- and D-decay parameters. Several interesting features of decays of the kind $B_{d,s}\to D f_d$ are also pointed out.

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arXiv2003

New Strategies to Extract Weak Phases from Neutral B Decays

We discuss new, theoretically clean strategies to determine the angle $γ$ of the unitarity triangle from $B_d\to DK_{S(L)}$, $B_s\to Dη^{(')}, Dϕ, ...$ decays, and point out that the $B_s\to DK_{S(L)}$ and $B_d\to Dπ^0, Dρ^0, ...$ modes allow very interesting determinations of the $B^0_q$--$\bar{B^0_q}$ mixing phases $ϕ_s$ and $ϕ_d$, respectively. Their colour-allowed counterparts $B_s\to D_s^{(*)\pm} K^\mp, ...$ and $B_d\to D^{(*)\pm} π^\mp, ...$ also offer new methods to probe $γ$.

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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

New Strategies to Obtain Insights into CP Violation Through $B_s\to D_s^{\pm}K^\mp, D_s^{\ast\pm}K^\mp, ...$ and $B_d\to D^{\pm}π^\mp, D^{\ast\pm}π^\mp, ...$ Decays

Decays of the kind $B_s\to D_s^{\pm}K^\mp, D_s^{\ast\pm}K^\mp, ...$ and $B_d\to D^{\pm}π^\mp, D^{\ast\pm}π^\mp, ...$ allow us to probe $ϕ_s+γ$ and $ϕ_d+γ$, respectively, involving the angle $γ$ of the unitarity triangle and the $B^0_q$--$\bar B^0_q$ mixing phases $ϕ_q$ ($q\in\{d,s\}$). Analysing these modes in a phase-convention-independent way, we find that their mixing-induced observables are affected by a subtle $(-1)^L$ factor, where $L$ denotes the angular momentum of the $B_q$ decay products, and derive bounds on $ϕ_q+γ$. Moreover, we emphasize that ``untagged'' rates are an important ingredient for efficient determinations of weak phases, not only in the presence of a sizeable width difference $ΔΓ_q$; should $ΔΓ_s$ be sizeable, the combination of untagged with tagged $B_s\to D_s^{\pm}K^\mp,D_s^{\ast\pm}K^\mp, ...$ observables provides an elegant and unambiguous extraction of $\tan(ϕ_s+γ)$, whereas the conventional determination of $ϕ_s+γ$ is affected by an eightfold discrete ambiguity. Finally, we propose a combined analysis of $B_s\to D_s^{\pm}K^\mp, D_s^{\ast\pm}K^\mp, ...$ and $B_d\to D^{\pm}π^\mp, D^{\ast\pm}π^\mp, ...$ modes, which has important advantages, offering various interesting new strategies to extract $γ$ in an essentially unambiguous manner.

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arXiv2003

CP Violation and New Physics in $B_s$ Decays

The $B_s$-meson system is a key element in the $B$-physics programme of hadron colliders, offering various avenues to explore CP violation and to search for new physics. One of the most prominent decays is $B_s\to J/ψϕ$, the counterpart of $B_d\to J/ψK_S$, providing a powerful tool to search for new-physics contributions to $B^0_s$--$\bar{B^0_s}$ mixing. Another benchmark mode is $B_s\to K^+K^-$, which complements $B_d\toπ^+π^-$, thereby allowing an extraction of the angle $γ$ of the unitarity triangle that is sensitive to new-physics effects in the QCD penguin sector. Finally, we discuss new methods to constrain and determine $γ$ with the help of $B_s\to D_s^{(\ast)\pm} K^\mp$ decays, which complement $B_d\to D^{(\ast)\pm}π^\mp$ modes. Since these strategies involve ``tree'' decays, the values of $γ$ thus extracted exhibit a small sensitivity on new physics.

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arXiv2003

Flavour Symmetry and Clean Strategies to Extract $γ$

One of the key elements in the testing of the Standard-Model description of CP violation through $B$-meson decays is the direct determination of the angle $γ$ of the unitarity triangle in a variety of ways. We give a brief overview of the implications of the current $B$-factory data for flavour-symmetry strategies, and discuss new, theoretically clean methods employing pure tree decays of neutral $B_{d,s}$ mesons.

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arXiv2003

Exploring CP Violation through B Decays

The B-meson system provides many strategies to perform stringent tests of the Standard-Model description of CP violation. In this brief review, we discuss implications of the currently available B-factory data on the angles alpha, beta and gamma of the unitarity triangle, emphasize the importance of Bs studies at hadronic B experiments, and discuss new, theoretically clean strategies to determine gamma.

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arXiv2003

Shedding Light on the "Dark Side" of $B^0_d$--$\bar B^0_d$ Mixing through $B_d\toπ^+π^-$, $K\toπν\barν$ and $B_{d,s}\toμ^+μ^-$

In a wide class of NP models, which can be motivated through generic arguments and within SUSY, we obtain large contributions to $B^0_d$--$\bar B^0_d$ mixing, but not to $ΔB=1$ processes. If we assume such a scenario, the solutions $ϕ_d\sim 47^\circ\lor 133^\circ$ for the $B^0_d$--$\bar B^0_d$ mixing phase implied by $A_{CP}^{mix}(B_d\to J/ψK_S)$ cannot be converted directly into a constraint in the $ρ$--$η$ plane. However, we may complement $ϕ_d$ with $|V_{ub}/V_{cb}|$ and the recently measured CP asymmetries in $B_d\toπ^+ π^-$ to determine the unitarity triangle, with its angles $α$, $β$ and $γ$. To this end, we have also to control penguin effects, which we do by means of the $B_d\toπ^\mp K^\pm$ branching ratio. Interestingly, the present data show a perfectly consistent picture not only for the ``standard'' solution of $ϕ_d\sim 47^\circ$, but also for $ϕ_d\sim 133^\circ$. In the latter case, the preferred region for the apex of the unitarity triangle is in the second quadrant, allowing us to accommodate conveniently $γ>90^\circ$, which is also favoured by other non-leptonic B decays such as $B\toπK$. Moreover, also the prediction for BR$(K^+\toπ^+ν\barν})$ can be brought to better agreement with experiment. Further strategies to explore this scenario with the help of $B_{d,s}\toμ^+μ^-$ decays are discussed as well.

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arXiv2002

Impact of Bottom-Quark Measurements on our Knowledge of the Standard Model

In this decade, B-decay experiments will allow stringent tests of the Standard-Model description of CP violation. After a classification of the main strategies and a discussion of the most recent results on benchmark decay modes of $B^\pm$ and $B_d$ mesons, we focus on the "El Dorado" for hadron colliders, the $B_s$-meson system, discussing the differences between the $B_d$ and $B_s$ systems, as well as prominent $B_s$ modes. First access to these decays is already provided by run II of the Tevatron. In the LHC era, it will be possible to fully exploit their physics potential, in particular at LHCb and BTeV.

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arXiv2002

Heavy Quarks on the Lattice

I review the basic ideas behind lattice QCD calculations that involve charm and bottom quarks. I report on the progress in getting the correct hyperfine splitting in charmonium from lattice QCD. Some of the basic technology behind numerical lattice QCD calculations is explained by studying some specific examples: computation of the charm quark mass, and the calculation of fB.

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arXiv2002

Status and Prospects of CKM Phase Determinations

We give an overview of direct determinations of the angles of the unitarity triangle of the CKM matrix, using CP-violating effects in B-meson decays. After a discussion of B --> pi K modes, which can be described efficiently through allowed regions in observable space and play an important role to determine gamma, we turn to extractions of the B^0_d -- bar{B^0_d} mixing phase phi_d, which equals 2 beta in the Standard Model, from B_d --> J/psi K_S, and emphasize that it is important to determine this phase unambiguously. Finally, we focus on B_d --> pi^+ pi^-, where recent B-factory data point towards large penguin contributions. The question arises now how the CP-violating observables of this mode can be transformed into information on the angles of the unitarity triangle. A promising tool to achieve this goal is offered by B_s --> K^+K^-, which is very accessible at hadronic B experiments, and allows a determination of phi_d and gamma. A variant for the e^+e^- B-factories is provided by B_d --> pi^{+/-} K^{+/-}, where data are already available, pointing to an exciting picture and a highly constrained allowed region in B_s --> K^+K^- observable space.

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arXiv2002

Extracting gamma Through Flavour-Symmetry Strategies

A brief overview of flavour-symmetry strategies to extract the angle gamma of the unitarity triangle is given, focusing on B --> pi K modes and the B_d --> pi^+ pi^-, B_s --> K^+ K^- system. We discuss also a variant of the latter approach for the e^+ e^- B-factories, where B_s --> K^+ K^- is replaced by B_d --> pi^{+/-} K^{+/-}.

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arXiv2002

Exploring CP Violation through Correlations in B --> pi K, B_d --> pi^+pi^-, B_s --> K^+K^- Observable Space

We investigate allowed regions in observable space of B --> pi K, B_d --> pi^+pi^- and B_s --> K^+K^- decays, characterizing these modes in the Standard Model. After a discussion of a new kind of contour plots for the $B\toπK$ system, we focus on the mixing- induced and direct CP asymmetries of the decays B_d --> pi^+pi^- and B_s--> K^+K^-. Using experimental information on the CP-averaged B_d --> pi^{+/-}K^{+/-} and B_d --> pi^+pi^- branching ratios, the relevant hadronic penguin parameters can be constrained,implying certain allowed regions in observable space. In the case of B_d --> pi^+pi^-, an interesting situation arises now in view of the recent B-factory measurements of CP violation in this channel, allowing us to obtain new constraints on the CKM angle gamma as a function of the B^0_d--\bar{B^0_d} mixing phase phi_d=2beta, which is fixed through A_{CP}^{mix}(B_d --> J/psi K_S) up to a twofold ambiguity. If we assume that A_{CP}^{mix}(B_d --> pi^+pi^-) is positive, as indicated by recent Belle data, and that phi_d is in agreement with the ``indirect'' fits of the unitarity triangle, also the corresponding values for gamma around 60 degrees can be accommodated. On the other hand, for the second solution of phi_d, we obtain a gap around gamma ~ 60 degrees. The allowed region in the space of A_{CP}^{mix}(B_s --> K^+K^-) and A_{CP}^{dir}(B_s --> K^+K^-) is very constrained in the Standard Model, thereby providing a narrow target range for run II of the Tevatron and the experiments of the LHC era.

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