arXiv subjects

Robert Fleischer

Publications and source records attributed to Robert Fleischer.

At least 55 records · Page 3Linked to original sources

arXiv2017

$B^0_s$-$\bar B^0_s$ Oscillations as a New Tool to Explore CP Violation in $D_s^\pm$ Decays

CP violation in $B^0_s$-$\bar B^0_s$ oscillations is expected at the $10^{-5}$ level in the Standard Model but could be enhanced by New Physics. Using $B^0_s\to D_s^-\ell^+ν_\ell$ decays, LHCb has recently reported the new result $(0.39\pm0.33)\times10^{-2}$ of the corresponding observable $a^s_{\rm sl}$. We point out that other current $B$ decay data imply $a^s_{\rm sl} = (0.004 \pm 0.075)\times10^{-2}$. In view of this strong constraint, we propose to use $B^0_s\to D_s^-\ell^+ν_\ell$ and similar flavor-specific decays as a new tool to determine both the production asymmetry between $B^0_s$ and $\bar B^0_s$ mesons, and the CP asymmetry in the subsequent $D^\pm_s$ decays. The former serves as input for analyses of CP violation in $B^0_s$ channels, with significant room for improvement, while the latter offers an exciting laboratory for New Physics.

hep-ph↗

arXiv2017

In Pursuit of New Physics with $B^0_{s,d}\to\ell^+\ell^-$

Leptonic rare decays of $B^0_{s,d}$ mesons offer a powerful tool to search for physics beyond the Standard Model. The $B^0_{s}\toμ^+μ^-$ decay has been observed at the Large Hadron Collider and the first measurement of the effective lifetime of this channel was presented, in accordance with the Standard Model. On the other hand, $B^0_{s}\toτ^+τ^-$ and $B^0_{s}\to e^+e^-$ have received considerably less attention: while LHCb has recently reported a first upper limit of $6.8\times10^{-3}$ (95% C.L.) for the $B^0_s\toτ^+τ^-$ branching ratio, the upper bound $2.8\times 10^{-7}$ (90% C.L.) for the branching ratio of $B^0_s\to e^+e^-$ was reported by CDF back in 2009. We discuss the current status of the interpretation of the measurement of $B^0_{s}\toμ^+μ^-$, and explore the space for New-Physics effects in the other $B^0_{s,d}\to\ell^+\ell^-$ decays in a scenario assuming flavour-universal Wilson coefficients of the relevant four-fermion operators. While the New-Physics effects are then strongly suppressed by the ratio $m_μ/m_τ$ of the lepton masses in $B^0_s\toτ^+τ^-$, they are hugely enhanced by $m_μ/m_e$ in $B^0_s\to e^+e^-$ and may result in a $B^0_s\to e^+e^-$ branching ratio as large as about 5 times the one of $B^0_{s}\toμ^+μ^-$, which is about a factor of 20 below the CDF bound; a similar feature arises in $B^0_{d}\to e^+e^-$. Consequently, it would be most interesting to search for the $B^0_{s,d}\to e^+e^-$ channels at the LHC and Belle II, which may result in an unambiguous signal for physics beyond the Standard Model.

hep-ph↗

arXiv2016

Towards New Frontiers in the Exploration of Charmless Non-Leptonic $B$ Decays

Non-leptonic $B$ decays into charmless final states offer an important laboratory to study CP violation and the dynamics of strong interactions. Particularly interesting are $B^0_s\to K^-K^+$ and $B^0_d\toπ^-π^+$ decays, which are related by the $U$-spin symmetry of strong interactions, and allow for the extraction of CP-violating phases and tests of the Standard Model. The theoretical precision is limited by $U$-spin-breaking corrections and innovative methods are needed in view of the impressive future experimental precision expected in the era of Belle II and the LHCb upgrade. We have recently proposed a novel method to determine the $B_s^0$-$\bar{B}_s^0$ mixing phase $ϕ_s$ from the $B_s^0\to K^-K^+$, $B_d^0\to π^-π^+$ system, where semileptonic $B^0_s\to K^-\ell^+ν_\ell$, $B^0_d\to π^-\ell^+ν_\ell$ decays are a new ingredient and the theoretical situation is very favourable. We discuss this strategy in detail, with a focus on penguin contributions as well as exchange and penguin-annihilation topologies which can be probed by a variety of non-leptonic $B$ decays into charmless final states. We show that a theoretical precision as high as ${\cal O}(0.5^\circ)$ for $ϕ_s$ can be attained in the future, thereby offering unprecedented prospects for the search for new sources of CP violation.

hep-ph↗

arXiv2016

New Strategy to Explore CP Violation with $B^0_s\to K^-K^+$

The $U$-spin symmetry provides a powerful tool to extract the angle $γ$ of the Unitarity Triangle and the $B_s^0$-$\bar{B}_s^0$ mixing phase $ϕ_s$ from CP violation in the $B^0_s\to K^-K^+$, $B^0_d\toπ^-π^+$ system. LHCb has obtained first results with uncertainties at the $7^\circ$ level. Due to $U$-spin-breaking corrections, it will be challenging to reduce the uncertainty below ${\cal O}(5^\circ)$ at Belle II and the LHCb upgrade. We propose a new strategy, using $γ$ as input and utilizing $B^0_s\to K^-\ell^+ν_\ell$, $B^0_d\to π^-\ell^+ν_\ell$ decays, which allows an extraction of $ϕ_s$ with a future theoretical precision of up to ${\cal O}(0.5^\circ)$, thereby matching the experimental prospects. Since $B^0_s\to K^-K^+$ is dominated by penguin topologies, new sources of CP violation may be revealed.

hep-ph↗

arXiv2015

Theoretical Prospects for B Physics

The exploration of B-meson decays has reached an unprecedented level of sophistication, with a phase of even much higher precision ahead of us thanks to run 2 of the LHC and the future era of Belle II and the LHCb upgrade. For many processes, the theoretical challenge in the quest to reveal possible footprints of physics beyond the Standard Model will be the control of uncertainties from strong interactions. After a brief discussion of the global picture emerging from the LHC data, I will focus on the theoretical prospects and challenges for benchmark B decays to search for new sources of CP violation, and highlight future opportunities to probe the Standard Model with strongly suppressed rare B decays.

hep-ph↗

arXiv2015

Anatomy of $B\rightarrow D\bar{D}$ Decays

The decays $B_d^0\rightarrow D_d^-D_d^+$ and $B_s^0\rightarrow D_s^-D_s^+$ probe the CP-violating mixing phases $ϕ_d$ and $ϕ_s$, respectively. The theoretical uncertainty of the corresponding determinations is limited by contributions from penguin topologies, which can be included with the help of the $U$-spin symmetry of the strong interaction. We analyse the currently available data for $B^0_{d,s}\rightarrow D_{d,s}^-D_{d,s}^+$ decays and those with similar dynamics to constrain the involved non-perturbative parameters. Using further information from semileptonic $B_d^0\rightarrow D_d^-\ell^+ν_{\ell}$ decays, we perform a test of the factorisation approximation and take non-factorisable $SU(3)$-breaking corrections into account. The branching ratios of the $B_d^0\rightarrow D_d^-D_d^+$, $B_s^0\rightarrow D_s^-D_d^+$ and $B_s^0\rightarrow D_s^-D_s^+$, $B_d^0\rightarrow D_d^-D_s^+$ decays show an interesting pattern which can be accommodated through significantly enhanced exchange and penguin annihilation topologies. This feature is also supported by data for the $B_s^0\rightarrow D_d^-D_d^+$ channel. Moreover, there are indications of potentially enhanced penguin contributions in the $B_d^0\rightarrow D_d^-D_d^+$ and $B_s^0\rightarrow D_s^-D_s^+$ decays, which would make it mandatory to control these effects in the future measurements of $ϕ_d$ and $ϕ_s$. We discuss scenarios for high-precision measurements in the era of Belle II and the LHCb upgrade.

hep-ph↗

arXiv2015

A Roadmap to Control Penguin Effects in $B^0_d\to J/ψK_{\rm S}^0$ and $B^0_s\to J/ψϕ$

Measurements of CP violation in $B^0_d\to J/ψK_{\rm S}^0$ and $B^0_s\to J/ψϕ$ decays play key roles in testing the quark-flavour sector of the Standard Model. The theoretical interpretation of the corresponding observables is limited by uncertainties from doubly Cabibbo-suppressed penguin topologies. With continuously increasing experimental precision, it is mandatory to get a handle on these contributions, which cannot be calculated reliably in QCD. In the case of the measurement of $\sin2β$ from $B^0_d\to J/ψK_{\rm S}^0$, the $U$-spin-related decay $B^0_s\to J/ψK_{\rm S}^0$ offers a tool to control the penguin effects. As the required measurements are not yet available, we use data for decays with similar dynamics and the $SU(3)$ flavour symmetry to constrain the size of the expected penguin corrections. We predict the CP asymmetries of $B^0_s\to J/ψK_{\rm S}^0$ and present a scenario to fully exploit the physics potential of this decay, emphasising also the determination of hadronic parameters and their comparison with theory. In the case of the benchmark mode $B^0_s\to J/ψϕ$ used to determine the $B^0_s$-$\bar B^0_s$ mixing phase $ϕ_s$ the penguin effects can be controlled through $B^0_d\to J/ψρ^0$ and $B^0_s\to J/ψ\bar{K}^{*0}$ decays. The LHCb collaboration has recently presented pioneering results on this topic. We analyse their implications and present a roadmap for controlling the penguin effects.

hep-ph↗

arXiv2014

Towards New Frontiers in CP Violation in B Decays

CP-violating effects in decays of B mesons offer a wide spectrum of probes for testing the phase structure of the quark-flavour sector of the Standard Model. After a brief discussion of the picture emerging from the current LHC data, the focus will be put on two specific topics: hadronic uncertainties from penguin topologies on measurements of the $B^0_q$-$\bar B^0_q$ mixing phases ($q\in\{d,s\}$), and the U-spin-related decays $B^0_s\to K^+K^-$ and $B^0_d\toπ^+π^-$. Valuable new insights are expected from future studies of CP violation in B decays. For the detection of possible new sources of CP violation, it will be crucial to match the experimental and theoretical precisions and to have a careful look at the underlying assumptions.

hep-ph↗

arXiv2014

Probing New Physics with $B^0_s\to μ^+μ^-$: Status and Perspectives

The rare decay $B^0_s \to μ^+μ^-$ plays a key role for the testing of the Standard Model. It is pointed out that the sizable decay width difference $ΔΓ_s$ of the $B_s$-meson system affects this channel in a subtle way. As a consequence, its calculated Standard Model branching ratio has to be upscaled by about 10%. Moreover, the sizable $ΔΓ_s$ makes a new observable through the effective $B^0_s\to μ^+μ^-$ lifetime accessible, which probes New Physics in a way complementary to the branching ratio and adds an exciting new topic to the agenda for the high-luminosity upgrade of the LHC. Further probes of New Physics are offered by a CP-violating rate asymmetry. Correlations between these observables and the $B^0_s \to μ^+μ^-$ branching ratio are illustrated for specific models of New Physics.

hep-ph↗

arXiv2013

Probing New Physics with the B_s to μ+ μ- Time-Dependent Rate

The B_s to mu+ mu- decay plays an outstanding role in tests of the Standard Model and physics beyond it. The LHCb collaboration has recently reported the first evidence for this decay at the 3.5 sigma level, with a branching ratio in the ballpark of the Standard Model prediction. Thanks to the recently established sizable decay width difference of the B_s system, another observable, A^mumu_DeltaGamma, is available, which can be extracted from the time-dependent untagged B_s to mu+ mu- rate. If tagging information is available, a CP-violating asymmetry, S_mumu, can also be determined. These two observables exhibit sensitivity to New Physics that is complementary to the branching ratio. We define and analyse scenarios in which these quantities allow us to discriminate between model-independent effective operators and their CP-violating phases. In this context we classify a selection of popular New Physics models into the considered scenarios. Furthermore, we consider specific models with tree-level FCNCs mediated by a heavy neutral gauge boson, pseudoscalar or scalar, finding striking differences in the predictions of these scenarios for the observables considered and the correlations among them. We update the Standard Model prediction for the time-integrated branching ratio taking the subtle decay width difference effects into account. We find (3.56 +/- 0.18) x 10^-9, and discuss the error budget.

hep-ph↗

arXiv2013

Rare Decays and CP Violation in the $B_s$ System

CP violating phenomena and rare decays of $B_s$ mesons offer interesting probes to test the quark-flavor sector of the Standard Model. In view of plenty of data reported in particular from the Large Hadron Collider, this topic has received a lot of attention in 2012. We give an overview of the the most recent experimental results, new theoretical developments, and discuss the prospects for the future exploration of the $B_s$-meson system.

hep-ph↗

arXiv2012

Theory News on $B_{s(d)} \to μ^+μ^-$ Decays

The rare decays $B_{s(d)} \to μ^+μ^-$ play a key role for the testing of the Standard Model. An overview of the most recent theoretical predictions of the corresponding branching ratios is given, emphasizing that the sizable decay width difference $ΔΓ_s$ of the $B_s$-meson system affects the $B_s\to μ^+μ^-$ channel. As a consequence, the calculated Standard Model branching ratio has to be upscaled by about 10% to BR$(B_s\to μ^+μ^-)=(3.54\pm0.30)\times 10^{-9}$. This prediction is the reference value for the comparison with the time-integrated experimental branching ratio, where LHCb has recently reported $(3.2^{+1.5}_{-1.2})\times10^{-9}$ corresponding to the first evidence for $B_s\to μ^+μ^-$. The $ΔΓ_s$ effects have also to be included in the constraints on the parameter space of New-Physics models following from the experimental data. Furthermore, $ΔΓ_s$ makes a new observable through the effective $B_s\to μ^+μ^-$ lifetime accessible, which probes New Physics in a way complementary to the branching ratio and adds an exciting new topic to the agenda for the high-luminosity upgrade of the LHC.

hep-ph↗

arXiv2012

Exploring $B_s \to D_s^{(*)\pm} K^\mp$ Decays in the Presence of a Sizable Width Difference $ΔΓ_s$

The $B_s \to D_s^{(*)\pm} K^\mp$ decays allow a theoretically clean determination of $ϕ_s+γ$, where $ϕ_s$ is the $B^0_s$-$\bar B^0_s$ mixing phase and $γ$ the usual angle of the unitarity triangle. A sizable $B_s$ decay width difference $ΔΓ_s$ was recently established, which leads to subtleties in analyses of the $B_s \to D_s^{(*)\pm} K^\mp$ branching ratios but also offers new "untagged" observables, which do not require a distinction between initially present $B^0_s$ or $\bar B^0_s$ mesons. We clarify these effects and address recent measurements of the ratio of the $B_s\to D_s^\pm K^\mp$, $B_s\to D_s^\pmπ^\mp$ branching ratios. In anticipation of future LHCb analyses, we apply the SU(3) flavour symmetry of strong interactions to convert the $B$-factory data for $B_d\to D^{(*)\pm}π^\mp$, $B_d\to D_s^{\pm}π^\mp$ decays into predictions of the $B_s \to D_s^{(*)\pm} K^\mp$ observables, and discuss strategies for the extraction of $ϕ_s+γ$, with a special focus on untagged observables and the resolution of discrete ambiguities. Using our theoretical predictions as a guideline, we make simulations to estimate experimental sensitivities, and extrapolate to the end of the planned LHCb upgrade. We find that the interplay between the untagged observables, which are accessible thanks to the sizable $ΔΓ_s$, and the mixing-induced CP asymmetries, which require tagging, will play the key role for the experimental determination of $ϕ_s+γ$.

hep-ph↗

arXiv2012

Penguin Effects in $ϕ_{d,s}$ Determinations

The theoretical precision of the measurement of the $B^0_{d,s}$-$\bar B^0_{d,s}$ mixing phases $ϕ_{d,s}$ through the benchmark decays $B^0_d\to J/ψK_S$, $B^0_s\to J/ψϕ$ and $B^0_s\to J/ψf_0(980)$ is limited by doubly Cabibbo-suppressed penguin topologies which are usually neglected. However, the search for New-Physics effects in the quark-flavor sector has entered a territory where these effects have to be taken into account, which will be particularly relevant for the LHCb upgrade era. Thanks to their non-perturbative nature, the penguin corrections cannot be calculated but have rather to be controlled through experimental data. An overview of the picture of the penguin parameters originating from the current data for $B_{(s)}\to J/ψπ, J/ψK$ decays and the physics potential of the control channels $B^0_s\to J/ψK_{\rm S}$, $B^0_s\to J/ψ\bar K^{*0}$ and $B_d^0\to J/ψf_0(980)$ is given, emphasizing also the usefulness of effective $B_s$ decay lifetimes.

hep-ph↗

arXiv2012

On Branching Ratios of $B_s$ Decays and the Search for New Physics in $B^0_s\to μ^+μ^-$

The LHCb experiment has recently established a sizable width difference between the mass eigenstates of the $B_s$-meson system. This phenomenon leads to a subtle difference at the 10% level between the experimental branching ratios of $B_s$ decays extracted from time-integrated, untagged data samples and their theoretical counterparts. Measuring the corresponding effective $B_s$-decay lifetimes, both branching ratio concepts can be converted into each other. The rare decay $B^0_s\to μ^+μ^-$ and the search for New Physics through this channel is also affected by this effect, which enhances the Standard-Model reference value of the branching ratio by O(10%), while the effective lifetime offers a new observable to search for physics beyond the Standard Model that is complementary to the branching ratio.

hep-ph↗

arXiv2012

Branching Ratio Measurements of $B_s$ Decays

We have just entered an era of precision measurements for $B_s$-decay observables. A characteristic feature of the $B_s$-meson system is $B^0_s$--$\bar B^0_s$ mixing, which exhibits a sizable decay width difference. The latter feature leads to a subtle complication for the extraction of branching ratios of $B_s$ decays from untagged data samples, leading to systematic biases as large as O(10%) that depend on the dynamics of the considered decay. We point out that this effect can only be corrected for using information from a time-dependent analysis and suggest the use of the effective $B_s$ decay lifetime, which can already be extracted from the untagged data sample, for this purpose. We also address several experimental issues that can play a role in the extraction of effective lifetimes at a hadron collider, and advocate the use of the $B_s$ branching ratios, as presented in this note, for consistent comparisons of theoretical calculations and experimental measurements in particle listings.

hep-ph↗

arXiv2012

Probing New Physics via the $B^0_s\to μ^+μ^-$ Effective Lifetime

We have recently seen new upper bounds for $B^0_s\to μ^+μ^-$, a key decay to search for physics beyond the Standard Model. Furthermore a non-vanishing decay width difference $ΔΓ_s$ of the $B_s$ system has been measured. We show that $ΔΓ_s$ affects the extraction of the $B^0_s\to μ^+μ^-$ branching ratio and the resulting constraints on the New Physics parameter space, and give formulae for including this effect. Moreover, we point out that $ΔΓ_s$ provides a new observable, the effective $B^0_s\to μ^+μ^-$ lifetime $τ_{μ^+μ^-}$, which offers a theoretically clean probe for New Physics searches that is complementary to the branching ratio. Should the $B^0_s\to μ^+μ^-$ branching ratio agree with the Standard Model, the measurement of $τ_{μ^+μ^-}$, which appears feasible at upgrades of the LHC experiments, may still reveal large New Physics effects.

hep-ph↗

arXiv2011

Exploring CP Violation and $η$-$η'$ Mixing with the $B^0_{s,d} \to J/ψη^{(\prime)}$ Systems

The $B^0_{s,d} \to J/ψη^{(\prime)}$ decays provide new terrain for exploring CP violation. After briefly discussing $η$-$η'$ mixing, we analyse the effective lifetimes and CP-violating observables of the $B_s$ channels, which allow us to probe New-Physics effects in $B^0_s$-$\bar B^0_s$ mixing. We have a critical look at these observables and show how hadronic corrections can be controlled by means of the $B_d$ decays. Using measurements of the $B^0_{s,d}\to J/ψη^{(\prime)}$ branching ratios by the Belle collaboration, we discuss tests of the $SU(3)_F$ flavour symmetry of strong interactions, obtain the first constraints on the hadronic parameters entering the $B^0_{s,d} \to J/ψη$ system, and predict the $B^0_d\to J/ψη'$ branching ratio at the $5\times10^{-6}$ level. Furthermore, we present strategies for the determination of the $η$-$η'$ mixing parameters from the $B^0_{s,d} \to J/ψη^{(\prime)}$ observables. We also observe that the $B^0_{s,d} \to J/ψη$ and $B^0_{s,d} \to J/ψη'$ decays are - from a formal point of view - analogous to the quark-antiquark and tetraquark descriptions of the $f_0(980)$ in the $B^0_{s,d} \to J/ψf_0(980)$ channels, respectively.

hep-ph↗