arXiv subjects

Thomas Mannel

Publications and source records attributed to Thomas Mannel.

At least 37 records · Page 2Linked to original sources

arXiv2019

The Heavy Quark Expansion for Inclusive Semileptonic Charm Decays Revisited

The Heavy Quark Expansion (HQE) has become an extremely powerful tool in flavor physics. For charm decays, where the expansion parameters $α_s(m_c)$ and $Λ_{\rm QCD}/m_c$ are bigger than for bottom decays, it remains to be seen if the HQE can be applied with similar success. Nevertheless, to make optimal use of the plethora of data already available and coming in the near future, a better understanding of HQE for charm decays is crucial. This paper discusses in detail how the HQE for charm decays is set up, what is the role of four-quark (weak annihilation) operators and how this compares to the well understood bottom decays. Subtleties concerning radiative corrections and the charm mass scheme are briefly discussed. An experimental study of the relevant HQE hadronic matrix elements will then show if the HQE expansion for charm converges well enough. Besides serving as an important cross check for inclusive $B$ decays, in the end, this study might open the road for inclusive $|V_{cs}|$ and $|V_{cd}|$ extractions.

hep-ph↗

arXiv2019

Chiral condensates in massless QCD and the $U(1)_A$ boson mass

The $U(1)_A$ boson mass is calculated through the phenomenological characteristics of the vacuum related to spontaneous breaking of chiral symmetry in QCD. The mass is determined by the mixed quark-gluon condensate $\langle \bar q G q \rangle$ that emerges in an appropriate correlation function due to triangle anomaly. For three flavor QCD in chiral limit we find the numerical value for the $U(1)_A$ boson mass to be $M_0=310\pm 50~{\rm MeV}$.

hep-ph↗

arXiv2019

QCD corrections to inclusive heavy hadron weak decays at $Λ_{\rm QCD}^3 /m_Q^3$

We present an analytical calculation of the $α_s$ corrections for the coefficient of $ρ_D/m_Q^3$ term in the heavy quark expansion for the inclusive semileptonic decays of heavy hadrons, such as $B \to X_c \ell \barν$. The full dependence of the coefficient on the final-state quark mass is taken into account. Our result leads to further improvement of the theoretical predictions for the precision determination of CKM matrix element $|V_{cb}|$.

hep-ph↗

arXiv2019

$V_{cb}$ determination from inclusive $b \to c$ decays: an alternative method

The determination of $V_{cb}$ relies on the Heavy-Quark Expansion and the extraction of the non-perturbative matrix elements from inclusive $b\to c$ decays. The proliferation of these matrix elements complicates their extraction at $1/m_b^4$ and higher, thereby limiting the $V_{cb}$ extraction. Reparametrization invariance links different operators in the Heavy-Quark expansion thus reducing the number of independent operators at $1/m_b^4$ to eight for the total rate. We show that this reduction also holds for spectral moments as long as they are defined by reparametrization invariant weight-functions. This is valid in particular for the leptonic invariant mass spectrum ($q^2$), i.e. the differential rate and its moments. Currently, $V_{cb}$ is determined by fitting the energy and hadronic mass moments, which do not manifest this parameter reduction and depend on the full set of 13 matrix elements up to $1/m_b^4$. In light of this, we propose an experimental analysis of the $q^2$ moments to open the possibility of a model-independent $V_{cb}$ extraction from semileptonic decays including the $1/m_b^4$ terms in a fully data-driven way.

hep-ph↗

arXiv2019

Linking lepton number violation with $B$ anomalies

Hints of violation of lepton flavor universality in semileptonic $B$ decays have prompted a renewed interest in leptoquarks at the low TeV scale. Among the different scenarios suggested, some happen to violate also lepton number, yet not much attention has been paid to the expected size of the associated lepton number violating processes. In this note we examine this issue. We find that there is a single leptoquark scenario compatible with the current size of the anomalies which also violates lepton number. In this scenario (Majorana) neutrino masses are radiatively generated. With the leptoquark parameters extracted from fitting the flavor anomalies, one actually gets the right order of magnitude for neutrino masses. We examine the associated effective field theories both at the electroweak scale and at the hadronic scale and estimate the size of the most relevant lepton number violating processes.

hep-ph↗

arXiv2018

$B^0$-$\bar{B}^0$ mixing: matching to HQET at NNLO

We compute perturbative matching coefficients to the Heavy Quark Effective Theory representation for the QCD effective local $ΔB=2$ Hamiltonian that determines the mass difference in $B^0-\bar{B}^0$ system of states. We report on the results at NNLO in the strong coupling constant for matching coefficients of two physical operators in HQET. Our results provide the firm confirmation that the recent NLO sum rules analysis of the bag parameter $B_q$ is stable with regard of inclusion of higher order radiative corrections. As a by-product of our calculation we give a fully analytical solution for the one loop QCD-to-HQET matching problem: we present the explicit formulas for the renormalization of four quark operators of the full bases in both QCD and HQET and the expressions for matching coefficients in a closed form.

hep-ph↗

arXiv2018

On the decays $B \to K^{(*)} + $ leptonium

We determine the rates of the $B$ meson decays into a $K^{(*)}$ and an $\ell^+ \ell^-$ bound state, the leptonium, where $\ell = e,μ,τ$. The two spin states of the leptonium, the spin singlet and the spin triplet, couple to the axial current and to the vector current, respectively, thus probing different helicity structures of the underlying $b\to s \ell^+\ell^-$ effective Hamiltonian. Since ortho- and para-leptonia have different decay modes, a distinction between the two is relatively easy and these decays may become a cross check for the results of lepton-flavour-violation searches obtained with free leptons. We find that some of the decays involving muon and tau have a branching ratio of the order of $10^{-13}$ and they may become accessible at the LHCb with 50 fb$^{-1}$ of integrated luminosity. In addition, since the tau-pair threshold lies right between the $J/ψ$ and the $ψ(2S)$ resonances, we estimate the charm-loop contribution to the decays $B \to K^{(*)} + $tauonium.

hep-ph↗

arXiv2018

Reparametrization Invariance and Partial Re-Summations of the Heavy Quark Expansion

We extend existing work on reparametrization invariance (RPI) of the heavy-quark expansion. We discuss the total rates of inclusive processes and obtain results which have a manifest RPI and can be expressed through matrix elements of operators and states defined in full QCD. This approach leads to a partial re-summation of higher-order terms in the heavy-quark expansion and has the advantage that the number of independent parameters is reduced.

hep-ph↗

arXiv2017

Towards a Next-to-Next-to-Leading Order analysis of matching in $B^0$-$\bar{B}^0$ mixing

We compute perturbative corrections to matching coefficients of QCD to HQET for the matrix element of the $ΔB=2$ operator that determines the mass difference in $B^0-\bar{B}^0$ system of states. This involves the technical point of choosing the operator basis in HQET, separating physical operators from evanescent ones. We obtain an analytical result for some of the two-loop corrections at order $α_s^2$.

hep-ph↗

arXiv2017

CP Violation in Multibody $B$ Decays from QCD Factorization

We test a data-driven approach based on QCD factorization for charmless three-body $B$-decays by confronting it to measurements of CP violation in $B^- \to π^- π^+ π^-$. While some of the needed non-perturbative objects can be directly extracted from data, some others can, so far, only be modelled. Although this approach is currently model dependent, we comment on the perspectives to reduce this model dependence. While our model naturally accommodates the gross features of the Dalitz distribution, it cannot quantitatively explain the details seen in the current experimental data on local CP asymmetries. We comment on possible refinements of our simple model and conclude by briefly discussing a possible extension of the model to large invariant masses, where large local CP asymmetries have been measured.

hep-ph↗

arXiv2017

Inclusive Semitauonic $B$ Decays to Order ${\cal O} (Λ_{QCD}^3/m_b^3)$

We calculate the decay width and the $τ$-lepton energy distribution as well as relevant moments for inclusive $\bar B \to X_c τ\bar ν_τ$ process including power corrections up to order $Λ_{QCD}^3/m_b^3$ and QCD corrections to the partonic level. We compare the result with the sum of the standard-model predictions of the branching fractions of the exclusive semileptonic $\bar B \to (D, D^*, D^{**}) τ\bar ν_τ$ decays as well as with the relevant experimental data. Our prediction is in agreement with the LEP measurement and is consistent with the standard-model calculation of the exclusive modes. We discuss the impact from physics beyond the Standard Model.

hep-ph↗

arXiv2016

$B^0-\bar{B}^0$ Mixing at Next-to-Leading Order

We compute the perturbative corrections to the HQET sum rules for the matrix element of the ΔB=2 operator that determines the mass shift of $B^0$, $\bar{B}^0$ states. Technically, we obtain analytically the non-factorizable contributions at order $α_s$ to the bag parameter that first appear at the three-loop level. Together with the known non-perturbative corrections due to vacuum condensates and $1/m_b$ corrections, the full next-to-leading order result is now available. We present a numerical value for the renormalization group invariant bag parameter that is phenomenologically relevant and discuss comparison with recent lattice determinations.

hep-ph↗

arXiv2016

Revisiting Uraltsev's BPS limit for Heavy Quarks

Motivated by the recent experimental data of the values of the Heavy Quark Expansion parameters, in particular the spin-orbit and Darwin terms, we argue that nature actually may be close to a limit of QCD which has been suggested by Uraltsev more than ten years ago. Assuming that this limit is not accidental, we derive the relations among the Heavy Quark Expansion parameters that occur up to the order 1/mb^5.

hep-ph↗

arXiv2016

Generalized Effective Operator Formalism for Decaying Systems

Systems of neutral kaons can be used to observe entanglement and the violation of Bell inequalities. The decay of these particles poses some problems, however, and recently an effective formalism for treating such systems has been derived. We generalize this formalism and make it applicable to other quantum systems that can be made to behave in a similar manner. As examples, we discuss two possible implementations of the generalized formalism using trapped ions such as $^{171}$Yb or $^{172}$Yb, which may be used to simulate kaonic behavior in a quantum optical system.

quant-ph↗

arXiv2015

Direct probes of flavor-changing neutral currents in $e^+e^-$ collisions

We propose a novel method to study flavor-changing neutral currents in the $e^+e^-\to D^{*0}$ and $e^+e^-\to B_{s}^* $ transitions, tuning the energy of $e^+e^-$- collisions to the mass of the narrow vector resonance $D^{*0}$ or $ B_{s}^*$. We present a thorough study of both short-distance and long-distance contributions to $e^+e^-\to D^{*0}$ in the Standard Model and investigate possible contributions of new physics in the charm sector. This process, albeit very rare, has clear advantages with respect to the $D^0 \to e^+e^-$ decay: the helicity suppression is absent, and a richer set of effective operators can be probed. Implications of the same proposal for $B_{s}^*$ are also discussed.

hep-ph↗

arXiv2015

Three-Body Non-Leptonic B Decays and QCD Factorization

We extend the framework of QCD factorization to non-leptonic $B$ decays into three light mesons, taking as an example the decay $B^+ \to π^+ π^+ π^-$. We discuss the factorization properties of this decay in different regions of phase space. We argue that, in the limit of very large $b$-quark mass, the central region of the Dalitz plot can be described in terms of the $B \to π$ form factor and the $B$ and $π$ light-cone distribution amplitudes. The edges of the Dalitz plot, on the other hand, require different non-perturbative input: the $B \to ππ$ form factor and the two-pion distribution amplitude. We present the set-up for both regions to leading order in both $α_s$ and $Λ_{\rm QCD} / m_b$ and discuss how well the two descriptions merge. We argue that for realistic $B$-meson masses there is no perturbative center in the Dalitz plot, but that a systematic description might be possible in the context of two-pion states. As an example, we estimate the $B\toρπ$ branching fraction beyond the quasi-particle approximation. We also discuss the prospects for studies of three-body and quasi-two-body non-leptonic $B$ decays from QCD.

hep-ph↗

arXiv2015

Light-Quark Decays in Heavy Hadrons

We consider weak decays of heavy hadrons (bottom and charmed) where the heavy quark acts as a spectator. Theses decays are heavily phase-space suppressed but may become experimentally accessible in the near future. These decays are interesting as a QCD laboratory to study the behaviour of the light quarks in the colour-background field of the heavy spectator.

hep-ph↗

arXiv2015

Renormalization Group Evolution of Flavour Invariants

The fermion spectrum in the Standard Model (SM) exhibits hierarchical structures between the eigenvalues of the Yukawa matrices which determine the fermion masses, as well as certain hierarchical patterns in the mixing matrix that describes weak transitions between different fermion generations. A basis-independent description of the SM flavour structure can be given in terms of a complete set of flavour invariants. In this paper, we construct a convenient set of such invariants, and discuss the general form of the renormalization-group equations. We also discuss the simplifications that arise from exploiting hierarchies in Yukwawa couplings and mixings which are present in the SM or its minimal-flavour violating extensions.

hep-ph↗