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Christoph Bobeth

Publications and source records attributed to Christoph Bobeth.

At least 37 records · Page 2Linked to original sources
arXiv2015

Anomalous triple gauge couplings from $B$-meson and kaon observables

We consider the three CP-conserving dimension-6 operators that encode the leading new-physics effects in the triple gauge couplings. The contributions to the standard-model electromagnetic dipole and semi-leptonic vector and axial-vector interactions that arise from the insertions of these operators are calculated. We show that radiative and rare $B$-meson decays provide, under certain assumptions, constraints on two out of the three anomalous couplings that are competitive with the restrictions obtained from LEP II, Tevatron and LHC data. The constraints arising from the $Z \to b \bar b$ electroweak pseudo observables, $K \to πν\bar ν$ and $ε^\prime/ε$ are also studied.

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arXiv2015

Constraints on tensor and scalar couplings from $B\to K\barμμ$ and $B_s\to \barμμ$

The angular distribution of $B\to K\bar\ell\ell$ ($\ell = e,\,μ,\,τ$) depends on two parameters, the lepton forward-backward asymmetry, $A_{\rm FB}^\ell$, and the flat term, $F_H^\ell$. Both are strongly suppressed in the standard model and constitute sensitive probes of tensor and scalar contributions. We use the latest experimental results for $\ell = μ$ in combination with the branching ratio of $B_s\to \barμμ$ to derive the strongest model-independent bounds on tensor and scalar effective couplings to date. The measurement of $F_H^μ$ provides a complementary constraint to that of the branching ratio of $B_s\to \barμμ$ and allows us---for the first time---to constrain all complex-valued (pseudo-)scalar couplings and their chirality-flipped counterparts in one fit. Based on Bayesian fits of various scenarios, we find that our bounds even become tighter when vector couplings are allowed to deviate from the standard model and that specific combinations of angular observables in $B \to K^*$ are still allowed to be up to two orders of magnitude larger than in the standard model, which would place them in the region of LHCb's sensitivity.

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arXiv2015

On new physics in $ΔΓ_d$

Motivated by the recent measurement of the dimuon asymmetry by the DØ collaboration, which could be interpreted as an enhanced decay rate difference in the neutral $B_d$-meson system, we investigate the possible size of new-physics contributions to $ΔΓ_d$. In particular, we perform model-independent studies of non-standard effects associated to the dimension-six current-current operators $(\bar{d} p)(\bar p^{\hspace{0.25mm}\prime} b)$ with $p,p^\prime= u,c$ as well as $(\bar{d}b) (\barττ)$. In both cases we find that for certain flavour or Lorentz structures of the operators sizable deviations of $ΔΓ_d$ away from the Standard Model expectation cannot be excluded in a model-independent fashion.

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arXiv2014

Comprehensive Bayesian Analysis of Rare (Semi)leptonic and Radiative B Decays

The available data on $|ΔB| = |ΔS| = 1$ decays are in good agreement with the Standard Model when permitting subleading power corrections of about 15% at large hadronic recoil. Constraining new-physics effects in $\mathcal{C}_7$, $\mathcal{C}_9$, $\mathcal{C}_{10}$, the data still demand the same size of power corrections as in the Standard Model. In the presence of chirality-flipped operators, all but one of the power corrections reduce substantially. The Bayes factors are in favor of the Standard Model. Using new lattice inputs for $B\to K^*$ form factors, the evidence of models with chirality-flipped operators increases, but does not outperform the Standard Model. We use the data to further constrain the hadronic form factors in $B\to K$ and $B\to K^*$ transitions.

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arXiv2014

Weak annihilation and new physics in charmless $B \to M M$ decays

We use currently available data of nonleptonic charmless 2-body $B\to MM$ decays ($MM = PP, PV, VV$) that are mediated by $b\to (d, s)$ QCD- and QED-penguin operators to study weak annihilation and new-physics effects in the framework of QCD factorization. In particular we introduce one weak annihilation parameter for decays related by ($u \leftrightarrow d$) quark interchange and test this universality assumption. Within the standard model, the data supports this assumption with the only exceptions in the $B\to K π$ system, which exhibits the well-known "$ΔA_{CP}$ puzzle", and some tensions in $B \to K^* ϕ$. Beyond the standard model, we simultaneously determine weak-annihilation and new-physics parameters from data, employing model-independent scenarios that address the "$ΔA_{CP}$ puzzle", such as QED-penguins and $b\to s \bar{u}u$ current-current operators. We discuss also possibilities that allow further tests of our assumption once improved measurements from LHCb and Belle II become available.

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arXiv2014

B_{s,d} -> l+ l- in the Standard Model with Reduced Theoretical Uncertainty

We combine our new results for the O(alpha_em) and O(alpha_s^2) corrections to B_{s,d} -> l^+ l^-, and present updated branching ratio predictions for these decays in the standard model. Inclusion of the new corrections removes major theoretical uncertainties of perturbative origin that have just begun to dominate over the parametric ones. For the recently observed muonic decay of the B_s meson, our calculation gives BR(B_s -> mu^+ mu^-) = (3.65 +_ 0.23) * 10^(-9).

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arXiv2013

New Physics in Gamma_12^s: (sbar b) (taubar tau) Operators

Measurements performed at the Tevatron of both the like-sign dimuon charge asymmetry in B_d,s-meson samples and the mixing-induced CP asymmetry in B_s -> J/psi phi depart from their standard model (SM) predictions. This could be an indication for new CP phases in Delta B = 2 transitions, preferentially in B_s-Bbar_s mixing. The experimental situation, however, remained inconclusive, as it favored values of the element Gamma_12^s of the decay matrix in the B_s-meson system that are notably different from the SM expectation, suggesting the presence of new physics in the Delta B = 1 sector as well. The very recent LHCb measurements of B_s -> J/psi phi and B_s -> J/psi f_0, which do not find any evidence for a new-physics phase in the element M_12^s of the mass matrix, point into this direction as well. In this article, we explore the potential size of non-standard effects in Gamma_12^s stemming from dimension-six operators with flavor content (sbar b) (taubar tau). We show that since the existing constraints imposed by tree- and loop-level mediated B_d,s-meson decays are quite loose, absorptive new physics of this type can in part explain the current data. Our model-independent conclusions are finally contrasted with explicit analyses of the new-physics effects in B_s-Bbar_s mixing that can arise from leptoquarks or Z' bosons.

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arXiv2013

Electroweak Corrections to $B_{s,d} \to \ell^+ \ell^-$

We calculate the full two-loop electroweak matching corrections to the operator governing the decay B_q --> l^+ l^- in the Standard Model. Their inclusion removes an electroweak scheme and scale uncertainty of about 7% of the branching ratio. Using different renormalization schemes of the involved electroweak parameters, we estimate residual perturbative electroweak and QED uncertainties to be less than 1% at the level of the branching ratio.

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arXiv2013

Implications of the experimental results on rare b --> s + (gamma, l^+ l^-) decays

The experimental measurements of flavor-changing neutral-current B-meson decays governed by b --> s + (gamma, l^+ l^-) transitions have entered a new level of precision. Recent results by Belle, CDF, Babar, and LHCb on B -->K^(*) l^+ l^- and B_s --> mu^+ mu^- decays are used in model-(in)dependent analyses to test the Standard Model predictions and to derive stronger constraints on nonstandard contributions. While in agreement with the Standard Model, they still leave sizable room for new physics.

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arXiv2012

Theory status of $b\to s \ell^+\ell^-$ decays and their combined analysis

The experimental information on b --> s l^+ l^- decays has grown enormously during the last couple of years due to BaBar, Belle, CDF and LHCb. Especially, exclusive modes B --> K^(*) l^+ l^-, which have the largest rate and are easily accessible experimentally, provide a variety of observables which constrain non-standard interactions that would affect these flavour-changing neutral current decays beyond the Standard Model. Nowadays, theoretical predictions focus on low- and high dilepton invariant mass regions, where expansions in Lambda_QCD/m_b and form factor symmetries provide means to identify optimised observables. The first experimental results of non-optimised observables have stimulated first global analysis of b --> s l^+ l^- decays in combination with b --> s photon and B_s --> mu^+ mu^-. Such global analysis are now ready to be applied to include high-statistics results from LHCb and Super-Flavour factories which are about to come within the next years.

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arXiv2012

Bayesian Fit of Exclusive $b \to s \bar\ell\ell$ Decays: The Standard Model Operator Basis

We perform a model-independent fit of the short-distance couplings $C_{7,9,10}$ within the Standard Model set of $b\to sγ$ and $b\to s\bar\ell\ell$ operators. Our analysis of $B \to K^* γ$, $B \to K^{(*)} \bar\ell\ell$ and $B_s \to \barμμ$ decays is the first to harness the full power of the Bayesian approach: all major sources of theory uncertainty explicitly enter as nuisance parameters. Exploiting the latest measurements, the fit reveals a flipped-sign solution in addition to a Standard-Model-like solution for the couplings $C_i$. Each solution contains about half of the posterior probability, and both have nearly equal goodness of fit. The Standard Model prediction is close to the best-fit point. No New Physics contributions are necessary to describe the current data. Benefitting from the improved posterior knowledge of the nuisance parameters, we predict ranges for currently unmeasured, optimized observables in the angular distributions of $B\to K^*(\to Kπ)\,\bar\ell\ell$.

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arXiv2011

Towards global analysis of b --> s l^+ l^-

Flavour changing neutral current decays mediated by b --> s l^+ l^- were under experimental investigation at B-factories and the Tevatron during the last decade and the final analysis are expected soon. Moreover, new data has been released this summer and more is currently taken at the LHC mainly by LHCb. The theoretical methods for both inclusive and exclusive decays have been also refined and a global analysis of these decays becomes more and more feasible. First analysis of combined b --> s l^+ l^- data towards a global analysis provide constraints on the involved short distance couplings. Recently, specially designed observables, which can be determined in the angular distribution of B --> K^*(--> K pi) l^+ l^-, became of particular interest since they are subject of reduced hadronic uncertainties and currently their potential role in a global analysis is under investigation.

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arXiv2011

More Benefits of Semileptonic Rare B Decays at Low Recoil: CP Violation

We present a systematic analysis of the angular distribution of Bbar -> Kbar^\ast (-> Kbar pi) l^+ l^- decays with l = e, mu in the low recoil region (i.e. at high dilepton invariant masses of the order of the mass of the b-quark) to account model-independently for CP violation beyond the Standard Model, working to next-to-leading order QCD. From the employed heavy quark effective theory framework we identify the key CP observables with reduced hadronic uncertainties. Since some of the CP asymmetries are CP-odd they can be measured without B-flavour tagging. This is particularly beneficial for Bbar_s,B_s -> phi(-> K^+ K^-) l^+ l^- decays, which are not self-tagging, and we work out the corresponding time-integrated CP asymmetries. Presently available experimental constraints allow the proposed CP asymmetries to be sizeable, up to values of the order ~ 0.2, while the corresponding Standard Model values receive a strong parametric suppression at the level of O(10^-4). Furthermore, we work out the allowed ranges of the short-distance (Wilson) coefficients C_9,C_10 in the presence of CP violation beyond the Standard Model but no further Dirac structures. We find the Bbar_s -> mu^+ mu^- branching ratio to be below 9*10^-9 (at 95% CL). Possibilities to check the performance of the theoretical low recoil framework are pointed out.

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arXiv2011

The Decay B --> K l^+ l^- at Low Hadronic Recoil and Model-Independent Delta B = 1 Constraints

We study the decay B --> K l^+ l^- for l = e,mu,tau with a softly recoiling kaon, that is, for high dilepton invariant masses sqrt{q^2} of the order of the b-quark mass. This kinematic region can be treated within an operator product expansion and simplified using heavy quark symmetry, leading to systematic predictions for heavy-to-light processes such as B --> K^(*) l^+ l^-. We show that the decay rates of both B --> K^* l^+ l^- and B --> K l^+ l^- decays into light leptons depend on a common combination of short-distance coefficients. The corresponding CP-asymmetries are hence identical. Furthermore we present low recoil predictions for B --> K l^+ l^- observables, including the flat term in the angular distribution which becomes sizable for taus. We work out model-independently the constraints on Delta B = 1 operators using the most recent data from the experiments BaBar, Belle, CDF and LHCb. For constructive interference with the standard model, generic new physics is pushed up to scales above 44 TeV at 95% CL. Assuming none or small CP-violation we obtain a lower bound on the position of the zero of the forward-backward asymmetry of B^0 --> K^*0 l^+ l^- decays as q_0^2 > 1.7 GeV^2, which improves to q_0^2 > 2.6 GeV^2 for a standard model-like sign b --> s gamma amplitude.

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arXiv2011

Complete NNLO QCD Analysis of B -> X_s l^+ l^- and Higher Order Electroweak Effects

We complete the next-to-next-to-leading order QCD calculation of the branching ratio for B -> X_s l^+ l^- including recent results for the three-loop anomalous dimension matrix and two-loop matrix elements. These new contributions modify the branching ratio in the low-q^2 region, BR_ll, by about +1% and -4%, respectively. We furthermore discuss the appropriate normalization of the electromagnetic coupling alpha and calculate the dominant higher order electroweak effects, showing that, due to accidental cancellations, they change BR_ll by only -1.5% if alpha(mu) is normalized at mu = O(m_b), while they shift it by about -8.5% if one uses a high scale normalization mu = O(M_W). The position of the zero of the forward-backward asymmetry, q_0^2, is changed by around +2%. After introducing a few additional improvements in order to reduce the theoretical error, we perform a comprehensive study of the uncertainty. We obtain BR_ll(1 GeV^2 <= q^2 <= 6 GeV^2) = (1.57 +- 0.16) x 10^-6 and q_0^2 = (3.76 +- 0.33) GeV^2 and note that the part of the uncertainty due to the b-quark mass can be easily reduced.

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arXiv2011

Angular analysis of B --> V (--> P_1 P_2) l^+ l^- decays

The angular analysis of exclusive rare B-meson decays via intermediate vector mesons V into 4-body final states of two pseudo-scalars P_1, P_2 and a pair of light leptons l = e, mu offers a large set of observables. They can be used to test the electroweak short-distance couplings in the Standard Model and to search for New Physics. The two kinematic regions of low and high dilepton mass depend on short-distance physics in complementary ways and can be expanded in powers of Lambda_QCD/m_b. These expansions guide towards suitable combinations of observables allowing to i) reduce the hadronic uncertainties in the extraction of the short-distance couplings or ii) test the lattice QCD (B -> V) form factors in short-distance independent combinations. Several such possibilities of CP-averaged and CP-asymmetric (T-even and T-odd) quantities are presented for B_d^0 --> K^*0 (-> K pi) l^+ l^- and time-integrated CP-asymmetries without tagging for (anti-B_s, B_s) --> phi (--> K^- K^+) l^+ l^- decays in view of the latest B-factory and CDF results and the forthcoming LHCb measurements.

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arXiv2010

The Benefits of B ---> K* l+ l- Decays at Low Recoil

Using the heavy quark effective theory framework put forward by Grinstein and Pirjol we work out predictions for B -> K* l+ l-, l = (e, mu), decays for a softly recoiling K*, i.e., for large dilepton masses sqrt{q^2} of the order of the b-quark mass m_b. We work to lowest order in Lambda/Q, where Q = (m_b, sqrt{q^2}) and include the next-to-leading order corrections from the charm quark mass m_c and the strong coupling at O(m_c^2/Q^2, alpha_s). The leading Lambda/m_b corrections are parametrically suppressed. The improved Isgur-Wise form factor relations correlate the B -> K* l+ l- transversity amplitudes, which simplifies the description of the various decay observables and provides opportunities for the extraction of the electroweak short distance couplings. We propose new angular observables which have very small hadronic uncertainties. We exploit existing data on B -> K* l+ l- distributions and show that the low recoil region provides powerful additional information to the large recoil one. We find disjoint best-fit solutions, which include the Standard Model, but also beyond-the-Standard Model ones. This ambiguity can be accessed with future precision measurements.

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