arXiv subjects

Zoltan Ligeti

Publications and source records attributed to Zoltan Ligeti.

At least 19 recordsLinked to original sources

arXiv2026

CP asymmetries in charged meson decay to two pions

We study the CP asymmetries in $B^+$, $D^+$, and $K^+$ decays into $π^+π^0$. These asymmetries are commonly known to vanish in the isospin limit. We clarify what is meant by the term ``isospin limit" in this context. We provide a unified formalism to discuss these asymmetries, and develop an understanding of how various suppression factors distinguish between them qualitatively. We estimate the size of the asymmetries in the Standard Model: $A_{CP}(B^+\toπ^+π^0)\sim 3\times10^{-3}$, $A_{CP}(D^+\toπ^+π^0)\sim 10^{-5}$, and $A_{CP}(K^+\toπ^+π^0)\sim 10^{-6}$

hep-ph↗

arXiv2026

Lessons from LHCb and Belle II measurements of $B\to J/ψπ$ and $B\to J/ψK$ decays

The LHCb collaboration has recently measured the CP asymmetry in $B^+\to J/ψπ^+$ decay, while the Belle II collaboration has recently measured the CP asymmetries in $B^0\to J/ψπ^0$ decay. Within the Standard Model, and using flavor-$SU(3)$ relations including first-order breaking corrections, these measurements lead to new predictions with regard to CP violation in $B^+\to J/ψK^+$ and $B_s\to J/ψ\overline{K}{}^0$ decays, the difference between $S_{ψK_S}$ and $\sin2β$, and the rate and CP asymmetries in $B_s\to J/ψπ^0$ decay.

hep-ph↗

arXiv2025

Flavor Physics at the CEPC: a General Perspective

We discuss the landscape of flavor physics at the Circular Electron-Positron Collider (CEPC), based on the nominal luminosity outlined in its Technical Design Report. The CEPC is designed to operate in multiple modes to address a variety of tasks. At the $Z$ pole, the expected production of 4 Tera $Z$ bosons will provide unique and highly precise measurements of $Z$ boson couplings, while the substantial number of boosted heavy-flavored quarks and leptons produced in clean $Z$ decays will facilitate investigations into their flavor physics with unprecedented precision. We investigate the prospects of measuring various physics benchmarks and discuss their implications for particle theories and phenomenological models. Our studies indicate that, with its highlighted advantages and anticipated excellent detector performance, the CEPC can explore beauty and $τ$ physics in ways that are superior to or complementary with the Belle II and Large-Hadron-Collider-beauty experiments, potentially enabling the detection of new physics at energy scales of 10 TeV and above. This potential also extends to the observation of yet-to-be-discovered rare and exotic processes, as well as testing fundamental principles such as lepton flavor universality, lepton and baryon number conservation, etc., making the CEPC a vibrant platform for flavor physics research. The $WW$ threshold scan, Higgs-factory operation and top-pair productions of the CEPC further enhance its merits in this regard, especially for measuring the Cabibbo-Kobayashi-Maskawa matrix elements, and Flavor-Changing-Neutral-Current physics of Higgs boson and top quarks. We outline the requirements for detector performance and considerations for future development to achieve the anticipated scientific goals.

hep-ex↗

arXiv2025

Kaon Physics: A Cornerstone for Future Discoveries

The kaon physics programme, long heralded as a cutting-edge frontier by the European Strategy for Particle Physics, continues to stand at the intersection of discovery and innovation in high-energy physics (HEP). With its unparalleled capacity to explore new physics at the multi-TeV scale, kaon research is poised to unveil phenomena that could reshape our understanding of the Universe. This document highlights the compelling physics case, with emphasis on exciting new opportunities for advancing kaon physics not only in Europe but also on a global stage. As an important player in the future of HEP, the kaon programme promises to drive transformative breakthroughs, inviting exploration at the forefront of scientific discovery.

hep-ph↗

arXiv2024

Majorana phases beyond neutrinoless double beta decay

The $ν_M{\rm SM}$ is defined as the SM extended to include dimension-5 operators. In this model neutrino masses violate lepton number, and two parameters of the lepton mixing matrix, the Majorana phases, are yet to be constrained. One combination of these phases and the neutrino masses, often denoted by $m_{ee}$, is probed by neutrinoless double beta decays ($0νββ$). We explore what information may be obtained beyond $0νββ$, and how it depends on the lightest neutrino mass. We point out that with current central values of the mixing parameters, $ΔL_e=2$ and $ΔL_e=ΔL_μ= 1$ (or $ΔL_e=2$ and $ΔL_μ= 2$) processes cannot simultaneously vanish, providing a no-lose theorem, in principle, for excluding the $ν_M{\rm SM}$, even in the case of normal mass ordering.

hep-ph↗

arXiv2024

Imprints of supersymmetry at a future Z factory

We study the discovery potential of Z branching ratios due to contributions induced by the MSSM electroweak sector, assuming that the squarks and gluinos are heavy. Precision measurements at a future Z factory would yield sensitivity to MSSM that is complementary to direct searches at the LHC, provided that the systematic uncertainties can be reduced to a level comparable to the expected statistical uncertainties.

hep-ph↗

arXiv2024

On magnitudes of some CKM matrix elements

This writeup follows the presentation at the Symposium, with emphasis on topics and ideas discussed there. It is purposefully informal, not a review of the field, and neither does it include a complete list of references. However, I hope that readers might find some comments useful or amusing, and may appreciate the challenges and reasons for excitement about recent progress and future opportunities in flavor physics.

hep-ph↗

arXiv2024

Focus topics for the ECFA study on Higgs / Top / EW factories

In order to stimulate new engagement and trigger some concrete studies in areas where further work would be beneficial towards fully understanding the physics potential of an $e^+e^-$ Higgs / Top / Electroweak factory, we propose to define a set of focus topics. The general reasoning and the proposed topics are described in this document.

hep-ph↗

arXiv2023

Novel approaches to determine $B^\pm$ and $B^0$ meson production fractions

We propose novel methods to determine the $Υ(4S)\to B^+B^-$ and $Υ(4S)\to B^0\bar B^0$ decay rates. The precision to which they and their ratio are known yields at present a limiting uncertainty around $2\%$ in measurements of absolute $B$ decay rates, and thus in a variety of applications, such as precision determinations of elements of the Cabibbo-Kobayashi-Maskawa matrix and flavor symmetry relations. The new methods we propose are based in one case on exploiting the $Υ(5S)$ data sets, in the other case on the different average number of charged tracks in $B^\pm$ and $B^0$ decays. We estimate future sensitivities using these methods and discuss possible measurements of $f_d / f_u$ at the (HL-)LHC.

hep-ph↗

arXiv2023

Exploring the $τ$ polarization in $B\to Xτ\barν$ along different axes

The $τ$ polarization in semileptonic $B$ decays provides probes of new physics complementary to decay rate distributions of the three-body final state. Prior calculations for inclusive decays used a definition for the polarization axis that is different from the choice used in calculations (and the only measurement) for exclusive channels. To compare inclusive and exclusive predictions, we calculate the $τ$ polarization in inclusive $B\to Xτ\barν$ using the same choice as in the exclusive decays, and construct a sum rule relating the inclusive $τ$ polarization to a weighted sum of exclusive decay polarizations. We use this relation, experimental data, and theoretical predictions for the decays to the lightest charm or up-type hadrons to make predictions for excited channels.

hep-ph↗

arXiv2022

Theory Techniques for Precision Physics -- Snowmass 2021 TF06 Topical Group Report

The wealth of experimental data collected at laboratory experiments suggests that there is some scale separation between the Standard Model (SM) and phenomena beyond the SM (BSM). New phenomena can manifest itself as small corrections to SM predictions, or as signals in processes where the SM predictions vanish or are exceedingly small. This makes precise calculations of the SM expectations essential, in order to maximize the sensitivity of current and forthcoming experiments to BSM physics. This topical group report highlights some past and forthcoming theory developments critical for maximizing the sensitivity of the experimental program to understanding Nature at the shortest distances.

hep-ph↗

arXiv2022

Constrained second-order power corrections in HQET: $R(D^{(*)})$, $|V_{cb}|$, and new physics

We postulate a supplemental power counting within the heavy quark effective theory, that results in a small, highly-constrained set of second-order power corrections, compared to the standard approach. We determine all $\bar{B} \to D^{(*)}$ form factors, both within and beyond the standard model to $\mathcal{O}(α_s/m_{c,b}, 1/m_{c,b}^2)$, under truncation by this power counting. We show that the second-order power corrections to the zero-recoil normalization of the $\bar{B} \to D^{(*)} l ν$ matrix elements ($l = e$, $μ$, $τ$) are fully determined by hadron mass parameters, and are in good agreement with lattice QCD (LQCD) predictions. We develop a parametrization of these form factors under the postulated truncation, that achieves excellent fits to the available LQCD predictions and experimental data, and we provide precise updated predictions for the $\bar{B} \to D^{(*)} τ\barν$ decay rates, lepton flavor universality violation ratios $R(D^{(*)})$, and the CKM matrix element $|V_{cb}|$. We point out some apparent errors in prior literature concerning the $\mathcal{O}(1/m_cm_b)$ corrections, and note a tension between commonly-used simplified dispersive bounds and current data.

hep-ph↗

arXiv2022

Interpreting LHCb's $Λ_b\to Λ_cτ\barν$ measurement and puzzles in semileptonic $Λ_b$ decays

Normalizing the recent LHCb measurement of $Λ_b \to Λ_c τ\barν$ to the standard model (SM) prediction for the $Λ_b \to Λ_c μ\barν$ rate, instead of a LEP measurement, provides a more consistent comparison with the SM prediction for the lepton flavor universality ratio $R(Λ_c)$. This modestly increases $R(Λ_c)$ compared to the quoted LHCb result, such that it no longer hints at a suppression compared to the SM, which would be hard to accommodate in new physics scenarios that enhance $R(D^{(*)})$. We point out that the fraction of excited states in inclusive semileptonic $Λ_b$ decay may be significantly greater than the corresponding fraction in $B$ decays. Possible implications are speculated upon.

hep-ph↗

arXiv2022

Theoretical predictions for inclusive $B\to X_u τ\barν$ decay

With the expected large increase in data sets, previously not measured decays will be studied at Belle II. We derive standard model predictions for the $B\to X_u τ\barν$ decay rate and distributions. The region in the lepton energy spectrum where higher-dimension operators in the local OPE need to be resummed into the $b$-quark light-cone distribution function is a significantly greater fraction of the phase space than for massless leptons. The finite $τ$ mass has the novel effect of shifting and squeezing how the distribution function enters the lepton energy spectrum. We also derive new predictions for the $τ$ polarization.

hep-ph↗

arXiv2021

Theoretical challenges for flavor physics

We discuss some highlights of the FCC-ee flavor physics program. It will help to explore various aspects of flavor physics: to test precision calculations, to probe nonperturbative QCD methods, and to increase the sensitivity to physics beyond the standard model. In some areas, FCC-ee will do much better than current and near-future experiments. We briefly discuss several probes that can be relevant for maximizing the gain from the FCC-ee flavor program.

hep-ph↗

arXiv2020

Das ist der HAMMER: Consistent new physics interpretations of semileptonic decays

Precise measurements of $b\to cτ\barν$ decays require large resource-intensive Monte Carlo (MC) samples, which incorporate detailed simulations of detector responses and physics backgrounds. Extracted parameters may be highly sensitive to the underlying theoretical models used in the MC generation. Because new physics (NP) can alter decay distributions and acceptances, the standard practice of fitting NP Wilson coefficients to SM-based measurements of the $R(D^{(*)})$ ratios can be biased. The newly developed HAMMER software tool enables efficient reweighting of MC samples to arbitrary NP scenarios or to any hadronic matrix elements. We demonstrate how HAMMER allows avoidance of biases through self-consistent fits directly to the NP Wilson coefficients. We also present example analyses that demonstrate the sizeable biases that can otherwise occur from naive NP interpretations of SM-based measurements. The HAMMER library is presently interfaced with several existing experimental analysis frameworks and we provide an overview of its structure.

hep-ph↗

arXiv2020

Precision Global Determination of the $B\to X_sγ$ Decay Rate

We perform the first global fit to inclusive $B\to X_sγ$ measurements using a model-independent treatment of the nonperturbative $b$-quark distribution function, with next-to-next-to-leading logarithmic resummation and $\mathcal{O}(α_s^2)$ fixed-order contributions. The normalization of the $B\to X_sγ$ decay rate, given by $\lvert C_7^{\rm incl} V_{tb} V_{ts}^*\rvert^2$, is sensitive to physics beyond the Standard Model (SM). We determine $\lvert C_7^{\rm incl} V_{tb} V_{ts}^* \rvert = (14.77 \pm 0.51_{\rm fit} \pm 0.59_{\rm theory} \pm 0.08_{\rm param})\times 10^{-3}$, in good agreement with the SM prediction, and the $b$-quark mass $m_b^{1S} = (4.750 \pm 0.027_{\rm fit} \pm 0.033_{\rm theory} \pm 0.003_{\rm param})\,\mathrm{GeV}$. Our results suggest that the uncertainties in the extracted $B\to X_sγ$ rate have been underestimated by up to a factor of two, leaving more room for beyond-SM contributions.

hep-ph↗

arXiv2019

N = 5, 6, 7, 8: Nested hypothesis tests and truncation dependence of $|V_{cb}|$

The determination of $|V_{cb}|$ from exclusive semileptonic $B\to D^*\ellν$ decays is sensitive to the choice of form factor parametrization. Larger $|V_{cb}|$ values are obtained fitting the BGL versus the CLN parametrization to recent Belle measurements. For the BGL parametrization, published fits use different numbers of parameters. We propose a method based on nested hypothesis tests to determine the optimal number of BGL parameters to fit the data, and find that six parameters are optimal to fit the Belle tagged and unfolded measurement. We further explore the differences between fits that use different numbers of parameters. The fits which yield $|V_{cb}|$ values in better agreement with determinations from inclusive semileptonic decays, tend to exhibit tensions with heavy quark symmetry expectations. These have to be resolved before the determinations of $|V_{cb}|$ from exclusive and inclusive decays can be considered understood.

hep-ph↗