arXiv subjects

Florian U. Bernlochner

Publications and source records attributed to Florian U. Bernlochner.

At least 37 records · Page 2Linked to original sources

arXiv2022

Cover Your Bases: Asymptotic Distributions of the Profile Likelihood Ratio When Constraining Effective Field Theories in High-Energy Physics

We investigate the asymptotic distribution of the profile likelihood ratio (PLR) when constraining effective field theories (EFTs) and show that Wilks' theorem is often violated, meaning that we should not assume the PLR to follow a $χ^2$-distribution. We derive the correct asymptotic distributions when either one or two real EFT couplings modulate observable cross sections with a purely linear or quadratic dependence. We then discover that when both the linear and quadratic terms contribute, the PLR distribution does not have a simple form. In this case we provide a partly-numerical solution for the one-parameter case. Using a novel approach, we find that the constants which define our asymptotic distributions may be obtained experimentally using a profile of the Asimov likelihood contour. Our results may be immediately used to obtain the correct coverage when deriving real-world EFT constraints using the PLR as a test-statistic.

physics.data-an↗

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↗

arXiv2021

Publishing statistical models: Getting the most out of particle physics experiments

The statistical models used to derive the results of experimental analyses are of incredible scientific value and are essential information for analysis preservation and reuse. In this paper, we make the scientific case for systematically publishing the full statistical models and discuss the technical developments that make this practical. By means of a variety of physics cases -- including parton distribution functions, Higgs boson measurements, effective field theory interpretations, direct searches for new physics, heavy flavor physics, direct dark matter detection, world averages, and beyond the Standard Model global fits -- we illustrate how detailed information on the statistical modelling can enhance the short- and long-term impact of experimental results.

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↗

arXiv2019

Angles on CP-violation in Higgs boson interactions

CP-violation in the Higgs sector remains a possible source of the baryon asymmetry of the universe. Recent differential measurements of signed angular distributions in Higgs boson production provide a general experimental probe of the CP structure of Higgs boson interactions. We interpret these measurements using the Standard Model Effective Field Theory and show that they do not distinguish the various CP-violating operators that couple the Higgs and gauge fields. However, the constraints can be sharpened by measuring additional CP-sensitive observables and exploiting phase-space-dependent effects. Using these observables, we demonstrate that perturbatively meaningful constraints on CP-violating operators can be obtained at the LHC with luminosities of ${\cal{O}}$(100/fb). Our results provide a roadmap to a global Higgs boson coupling analysis that includes CP-violating effects.

hep-ph↗

arXiv2019

The Full Event Interpretation -- An exclusive tagging algorithm for the Belle II experiment

The Full Event Interpretation is presented: a new exclusive tagging algorithm used by the high-energy physics experiment Belle II. The experimental setup of Belle II allows the precise measurement of otherwise inaccessible $B$ meson decay-modes. The Full Event Interpretation algorithm enables many of these measurements. The algorithm relies on machine learning to automatically identify plausible $B$ meson decay chains based on the data recorded by the detector. Compared to similar algorithms employed by previous experiments, the Full Event Interpretation provides a greater efficiency, yielding a larger effective sample size usable in the measurement.

hep-ex↗

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.

hep-ph↗

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.

hep-ph↗

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.

hep-ph↗

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.

hep-ph↗

arXiv2017

FlavBit: A GAMBIT module for computing flavour observables and likelihoods

Flavour physics observables are excellent probes of new physics up to very high energy scales. Here we present FlavBit, the dedicated flavour physics module of the global-fitting package GAMBIT. FlavBit includes custom implementations of various likelihood routines for a wide range of flavour observables, including detailed uncertainties and correlations associated with LHCb measurements of rare, leptonic and semileptonic decays of B and D mesons, kaons and pions. It provides a generalised interface to external theory codes such as SuperIso, allowing users to calculate flavour observables in and beyond the Standard Model, and then test them in detail against all relevant experimental data. We describe FlavBit and its constituent physics in some detail, then give examples from supersymmetry and effective field theory illustrating how it can be used both as a standalone library for flavour physics, and within GAMBIT.

hep-ph↗

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.

hep-ph↗

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.

hep-ph↗

arXiv2015

The B -> pi tau nu decay in the context of the 2HDM type II

In this manuscript the B -> pi tau nu decay is investigated in the context of the 2HDM type II extension of the SM. In particular, predictions for the ratio of semileptonic branching fractions from l = tau and l = electron, muon are produced and an exclusion based on the MSSM parameters tan beta and mH+ from a preliminary Belle measurement are reported. An alternative variable to the ratio of branching fractions that could offer increased sensitivity is discussed that could result in increased sensitivity to rule out contributions from additional scalar mediators.

hep-ph↗

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.

hep-ph↗

arXiv2014

Constraints on exclusive branching fractions $BF_i(B^+\to X_c^il^+ν)$ from moment measurements in inclusive $B\to X_clν$ decays

As an alternative to direct measurements, we extract exclusive branching fractions of semileptonic B-meson decays to charmed mesons, $BF_i(B\to X_c^ilν)$ with $X_c^i = D, D^*, D_0, D'_1, D_1, D_2, D', D'^*$ and non-resonant final states $(D^{(*)}π)_{nr}$, from a fit to electron energy, hadronic mass and combined hadronic mass-energy moments measured in inclusive $B\to X_clν$ decays. The fit is performed by constraining the sum of exclusive branching fractions to the measured value of $BF(B\to X_clν)$, and with different sets of additional constraining terms for the directly measured branching fractions. There is no fit scenario in which a single branching fraction alone is enhanced to close the gap between $BF(B\to X_clν)$ and the sum of known branching fractions $BF_i(B\to X_c^ilν)$. The fitted $BF(B\to D^*lν)$ is 5% to 10% larger than the direct measurement depending on whether or not $BF(B\to D^*lν)$ is constrained to its direct measurement. $BF(B\to Dlν)$ values are in good agreement with the direct measurement unless $BF(B\to D^*lν)$ is constrained, which results in a higher $BF(B\to Dlν)$ value. Within large uncertainties, $BF(B\to D'_1lν)$ agrees with direct measurements. Depending on the fit scenario, $BF(B\to D_0lν)$ is consistent with or larger than its direct measurement. The fit is not able to easily disentangle $B\to D_1lν$ and $B\to D_2lν$, and tends to increase the sum of these two branching fractions. $BF(B\to (D^{(*)}π)_{nr}lν)$ with non-resonant $(D^{(*)}π)_{nr}$ final states is found to be 0.2-0.4%, consistent with $B\to D^{(*)}πlν$ and $B\to D^{**}(D^{(*)}π)lν$ measurements. No indication is found for significant contributions from so far unmeasured $B\to D'^{(*)}lν$ decays assuming that the $D'$ and $D'^*$ can be identified with the observed $D(2550)$, respectively, $D^*(2600)$ state.

hep-ph↗