arXiv subjects

Zoltan Ligeti

Publications and source records attributed to Zoltan Ligeti.

At least 37 records · Page 2Linked to original sources

arXiv2019

Combined analysis of semileptonic $B$ decays to $D$ and $D^$: $R(D^{()})$, $|V_{cb}|$, and new physics

The measured $\bar{B}\to D^{(*)} l \barν$ decay rates for light leptons ($l=e,μ$) constrain all $\bar{B}\to D^{(*)}$ semileptonic form factors, by including both the leading and ${\cal O}(Λ_{\text{QCD}}/m_{c,b})$ subleading Isgur-Wise functions in the heavy quark effective theory. We perform a novel combined fit to the $\bar{B}\to D^{(*)} l \barν$ decay distributions to predict the $\bar{B} \to D^{(*)} τ\barν$ rates and determine the CKM matrix element $|V_{cb}|$. Most theoretical and experimental papers have neglected uncertainties in the predictions for form factor ratios at order $Λ_{\text{QCD}}/m_{c,b}$, which we include. We also calculate ${\cal O}(Λ_{\text{QCD}}/m_{c,b})$ and ${\cal O}(α_s)$ contributions to semileptonic $\bar{B}\to D^{(*)} \ell \barν$ decays for all possible $b \to c$ currents. This result has not been available for the tensor current form factors, and for two of those, which are ${\cal O}(Λ_{\text{QCD}}/m_{c,b})$, the corrections are of the same order as approximations used in the literature. These results allow us to determine with improved precision how new physics may affect the $\bar{B}\to D^{(*)} τ\barν$ rates. Our predictions can be systematically improved with more data; they need not rely on lattice QCD results, although these can be incorporated.

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arXiv2018

Precise predictions for $Λ_b \to Λ_c$ semileptonic decays

We calculate the $Λ_b \to Λ_c \ell ν$ form factors and decay rates for all possible $b\to c \ell\barν$ four-Fermi interactions beyond the Standard Model, including nonzero charged lepton masses and terms up to order $α_s\, Λ_\text{QCD}/m_{c,b}$ and $Λ_\text{QCD}^2/m_c^2$ in the heavy quark effective theory. At this order, we obtain model independent predictions for semileptonic $Λ_b \to Λ_c$ decays in terms of only two unknown sub-subleading Isgur-Wise functions, which can be determined from fitting LHCb and lattice QCD data. We thus obtain model independent results for $Λ_b\to Λ_c\ell\barν$ decays, including predictions for the ratio $R(Λ_c) = {\cal B}(Λ_b\to Λ_c τ\barν) / {\cal B}(Λ_b\to Λ_c μ\barν)$ in the presence of new physics, that are more precise than prior results in the literature, and systematically improvable with better data on the decays with $μ$ (or $e$) in the final state. We also explore tests of factorization in $Λ_b \to Λ_cπ$ decays, and emphasize the importance of measuring at LHCb the double differential rate $d^2Γ(Λ_b\toΛ_c\ell\barν) / (d q^2\, d\cosθ)$, in addition to the $q^2$ spectrum.

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arXiv2018

New predictions for $Λ_b\toΛ_c$ semileptonic decays and tests of heavy quark symmetry

The heavy quark effective theory makes model independent predictions for semileptonic $Λ_b \to Λ_c$ decays in terms of a small set of parameters. No subleading Isgur-Wise function occurs at order $Λ_{\rm QCD}/m_{c,b}$, and only two sub-subleading functions enter at order $Λ_{\rm QCD}^2/m_c^2$. These features allow us to fit the form factors and decay rates calculated up to order $Λ_{\rm QCD}^2/m_c^2$ to LHCb data and lattice QCD calculations. We derive a significantly more precise standard model prediction for the ratio ${\cal B}(Λ_b\to Λ_c τ\barν) / {\cal B}(Λ_b\to Λ_c μ\barν)$ than prior results, and find the expansion in $Λ_{\rm QCD}/m_c$ well-behaved, addressing a long-standing question. Our results allow more precise and reliable calculations of $Λ_b\to Λ_c\ell\barν$ rates, and are systematically improvable with better data on the $μ$ (or $e$) modes.

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arXiv2018

Model independent analysis of semileptonic $B$ decays to $D^{**}$ for arbitrary new physics

We explore semileptonic $B$ decays to the four lightest excited charm mesons, $D^{**} = \{D_0^*,\, D_1^*,$ $D_1,\, D_2^*\}$, for nonzero charged lepton mass and for all $b\to c \ell\barν$ four-Fermi interactions, including calculation of the ${\cal O}(Λ_\text{QCD}/m_{c,b})$ and ${\cal O}(α_s)$ corrections to the heavy quark limit for all form factors. In the heavy quark limit some form factors are suppressed at zero recoil, therefore, the ${\cal O}(Λ_\text{QCD}/m_{c,b})$ corrections can be very important. The $D^{**}$ rates exhibit sensitivities to new physics in $b\to cτ\barν$ mediated decays complementary to the $D$ and $D^*$ modes. Since they are also important backgrounds to $B\to D^{(*)}τ\barν$, the correct interpretation of future semitauonic $B\to D^{(*)}$ rate measurements requires consistent treatment of both the $D^{**}$ backgrounds and the signals. Our results allow more precise and more reliable calculations of these $B\to D^{**}\ell\barν$ decays, and are systematically improvable by better data on the $e$ and $μ$ modes. As an example, we show that the $D^{**}$ rates are more sensitive to a new $\bar c\, σ_{μν} b$ tensor interaction than the $D^{(*)}$ rates.

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arXiv2017

Flavor models for $\bar{B} \to D^{(*)} τ\barν$

The ratio of the measured $\bar B\to D^{(*)}\ell\barν$ decay rates for $\ell = τ$ vs. $e,μ$ deviate from the Standard Model (SM) by about $4σ$. We show that the data are in tension with the SM, independent of form factor calculations, and we update the SM prediction for $\mathcal{B}(B\to X_cτ\barν) / \mathcal{B}(B\to X_c \ell\barν)$. We classify the operators that can accommodate the measured central values, as well as their UV completions. We identify models with leptoquark mediators that are minimally flavor violating in the quark sector, and are minimally flavor violating or $τ$-aligned in the lepton sector. We explore experimental signatures of these scenarios, which are observable in the future at ATLAS/CMS, LHCb, or Belle II.

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arXiv2017

Tensions and correlations in $|V_{cb}|$ determinations

Recently several papers extracted $|V_{cb}|$ using the Belle measurement of the exclusive $\bar B \to D^* \ell\barν$ unfolded differential decay rates, available for the first time. Depending on the theoretical inputs, some of the fits yield higher $|V_{cb}|$ values, compatible with those from inclusive semileptonic $B$ decays. Since these four fits use mostly the same data, if their correlations were close to 100\%, the tension between them would be over $5σ$. We determine the correlations, find that the tension between the results is less than $3σ$, and explore what might lead to improving the consistency of the fits. We find that fits that yield the higher values of $|V_{cb}|$, also suggest large violations of heavy quark symmetry. These fits are also in tension with preliminary lattice QCD data on the form factors. Without additional experimental data or lattice QCD input, there are no set of assumptions under which the tension between exclusive and inclusive determinations of $|V_{cb}|$ can be considered resolved.

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arXiv2017

The Role of Flavor in 2016

This talk explores the role of flavor physics to constrain beyond standard model phenomena and future prospects, from a theoretical point of view. Possible implications of some experimental results in tension with the standard model are discussed, such as the 4 sigma deviation in the B -> D(*) tau nu decay rates. We use the examples of constraining new physics contributions to neutral meson mixing and the search for possible vector-like fermions to illustrate the expected progress over the next decade to increase the sensitivity to new physics at shorter distance scales. We also speculate about the ultimate limitations of (quark) flavor physics probes of new physics.

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arXiv2017

New Physics in the Visible Final States of $B\to D^{(*)}τν$

We derive compact expressions for the helicity amplitudes of the many-body $B \to D^{(*)}(\to DY)τ(\to Xν)ν$ decays, specifically for $X = \ell ν$ or $π$ and $Y = π$ or $γ$. We include contributions from all ten possible new physics four-Fermi operators with arbitrary couplings. Our results capture interference effects in the full phase space of the visible $τ$ and $D^*$ decay products which are missed in analyses that treat the $τ$ or $D^*$ or both as stable. The $τ$ interference effects are sizable, formally of order $m_τ/m_B$ for the standard model, and may be of order unity in the presence of new physics. Treating interference correctly is essential when considering kinematic distributions of the $τ$ or $D^*$ decay products, and when including experimentally unavoidable phase space cuts. Our amplitude-level results also allow for efficient exploration of new physics effects in the fully differential phase space, by enabling experiments to perform such studies on fully simulated Monte Carlo datasets via efficient event reweighing. As an example, we explore a class of new physics interactions that can fit the observed $R(D^{(*)})$ ratios, and show that analyses including more differential kinematic information can provide greater discriminating power for new physics, than single kinematic variables alone.

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arXiv2017

Semileptonic $B_{(s)}$ decays to excited charmed mesons with $e,μ,τ$ and searching for new physics with $R(D^{**})$

Semileptonic $B$ meson decays into the four lightest excited charmed meson states ($D_0^*$, $D_1^*$, $D_1$, and $D_2^*$) and their counterparts with $s$ quarks are investigated, including the full lepton mass dependence. We derive the standard model predictions for the differential branching fractions, as well as predictions for the ratios of the semi-tauonic and light lepton semileptonic branching fractions. These can be systematically improved using future measurements of the total or differential semileptonic rates to $e$ and $μ$, as well as the two-body hadronic branching fractions with a pion, related by factorization to the semileptonic rate at maximal recoil. To illustrate the different sensitivities to new physics, we explore the dependence of the ratio of semi-tauonic and light-lepton branching fractions on the type-II and type-III two-Higgs-doublet model parameters, $\tanβ$ and $m_{H}^\pm$, for all four states.

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arXiv2017

A new look at the theory uncertainty of epsilon_K

The observable epsilon_K is sensitive to flavor violation at some of the highest scales. While its experimental uncertainty is at the half percent level, the theoretical one is in the ballpark of 15%. We explore the nontrivial dependence of the theory prediction and uncertainty on various conventions, like the phase of the kaon fields. In particular, we show how such a rephasing allows to make the short-distance contribution of the box diagram with two charm quarks, eta_cc, purely real. Our results allow to slightly reduce the total theoretical uncertainty of epsilon_K, while increasing the relative impact of the imaginary part of the long distance contribution, underlining the need to compute it reliably. We also give updated bounds on the new physics operators that contribute to epsilon_K.

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arXiv2016

Flavor Constraints on New Physics

This talk highlights, from a theoretical point of view, some recent exciting results in flavor physics, as well as future prospects. We discuss possible implications of a subset of the experimental results in tension with the standard model, such as the $4σ$ deviation in the $B\to D^{(*)}τ\barν$ decay rates, and recent improvements in the constraints on axion portal dark matter models. We use the examples of constraining new physics contributions to neutral meson mixing and the search for possible vector-like fermions to illustrate the expected progress over the next decade to increase the sensitivity to new physics at shorter distance scales. We also speculate about the ultimate limitations of (quark) flavor physics probes of new physics.

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arXiv2015

Towards more precise determinations of the CKM phase $β$

We derive a new flavor symmetry relation for the determination of the weak phase $β=ϕ_1$ from time-dependent CP asymmetries and $B \to J/ψP$ decay rates. In this relation, the contributions to $\sin2β$ proportional to $V_{ub}$ are parametrically suppressed compared to the contributions in the $B \to J/ψK^0$ time-dependent CP asymmetry alone. This relation uses only $SU(3)$ flavor symmetry, and does not require further diagrammatic assumptions. The current data either fluctuate at the $2σ$ level from expectations, or may hint at effects of unexpected magnitude from contributions proportional to $V_{ub}$ or from isospin breaking.

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arXiv2015

New Vector-Like Fermions and Flavor Physics

We study renormalizable extensions of the standard model that contain vector-like fermions in a (single) complex representation of the standard model gauge group. There are 11 models where the vector-like fermions Yukawa couple to the standard model fermions via the Higgs field. These models do not introduce additional fine- tunings. They can lead to, and are constrained by, a number of different flavor- changing processes involving leptons and quarks, as well as direct searches. An interesting feature of the models with strongly interacting vector-like fermions is that constraints from neutral meson mixings (apart from CP violation in neutral kaon mixing) are not sensitive to higher scales than other flavor-changing neutral-current processes. We identify order 1/(4 pi M)^2 (where M is the vector-like fermion mass) one-loop contributions to the coefficients of the four-quark operators for meson mixing, that are not suppressed by standard model quark masses and/or mixing angles.

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arXiv2015

TASI Lectures on Flavor Physics

These notes overlap with lectures given at the TASI summer schools in 2014 and 2011, as well as at the European School of High Energy Physics in 2013. This is primarily an attempt at transcribing my hand-written notes, with emphasis on topics and ideas discussed in the lectures. It is not a comprehensive introduction or review of the field, nor does it include a complete list of references. I hope, however, that some may find it useful to better understand the reasons for excitement about recent progress and future opportunities in flavor physics.

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arXiv2014

Precise predictions for B -> Xc tau nu decay distributions

We derive precise standard model predictions for the dilepton invariant mass and the tau energy distributions in inclusive B -> Xc tau nu decay. We include Lambda_QCD^2/m_b^2 and alpha_s corrections using the 1S short-distance mass scheme, and estimate shape function effects near maximal tau energy. These results can improve the sensitivity of b -> c tau nu related observables to beyond standard model physics.

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arXiv2014

New ways to search for right-handed current in B -> rho ell nu decay

An interesting possibility to ease the tension between various determinations of |V_{ub}| is to allow a small right-handed contribution to the standard model weak current. The present bounds on such a contribution are fairly weak. We propose new ways to search for such a beyond standard model contribution in semileptonic B -> rho ell nu decay. Generalized asymmetries in one, two, or three angular variables are introduced as discriminators, which do not require an unbinned analysis of the fully differential distribution, and a detailed study of the corresponding theoretical uncertainties is performed. A discussion on how binned measurements can access all the angular information follows, which may be useful in both B -> rho ell nu and B -> K^* ell^+ ell^-, and possibly essential in the former decay due to backgrounds. The achievable sensitivity from the available BaBar and Belle data sets is explored, as well as from the anticipated 50 ab^{-1} Belle II data.

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arXiv2014

Future sensitivity to new physics in B_d, B_s and K mixings

We estimate, in a large class of scenarios, the sensitivity to new physics in B_d and B_s mixings achievable with 50 ab-1 of Belle II and 50 fb-1 of LHCb data. We find that current limits on new physics contributions in both B_{d,s} systems can be improved by a factor of ~5 for all values of the CP violating phases, corresponding to over a factor of 2 increase in the scale of new physics probed. Assuming the same suppressions by CKM matrix elements as those of the standard model box diagrams, the scale probed will be about 20 TeV for tree-level new physics contributions, and about 2 TeV for new physics arising at one-loop. We also explore the future sensitivity to new physics in K mixing. Implications for generic new physics and for various specific scenarios, such as minimal flavor violation, light third-generation dominated flavor violation, or U(2) flavor models are studied.

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arXiv2013

More Flavor SU(3) Tests for New Physics in CP Violating B Decays

The recent LHCb measurements of the $B_s \to K^-π^+$ and $B_s \to K^+K^-$ rates and CP asymmetries are in agreement with U-spin expectations from $B_d \to K^+π^-$ and $B_d \to π^+π^-$ results. We derive the complete set of isospin, U-spin, and SU(3) relations among the CP asymmetries in two-body charmless $B \to PP$ and $B \to PV$ decays, some of which are novel. To go beyond the unbroken SU(3) limit, we present relations which are properly defined and normalized to allow incorporation of SU(3) breaking in the simplest manner. We show that there are no CP relations beyond first order in SU(3) and isospin breaking. We also consider the corresponding relations for charm decays. Comparing parametrizations of the leading order sum rules with data can shed light on the applicability and limitations of both the flavor symmetry and factorization-based descriptions of SU(3) breaking. Two factorization relations can already be tested, and we show they agree with current data.

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