arXiv subjects

Christoph Langenbruch

Publications and source records attributed to Christoph Langenbruch.

10 recordsLinked to original sources

arXiv2026

Flavour Changing Neutral Current decays at LHCb

Flavour Changing Neutral Current (FCNC) decays are forbidden at lowest perturbative order in the Standard Model (SM) and only allowed via quantum loops. These transitions are therefore heavily suppressed in the SM, and New Physics (NP) can give significant contributions through virtual corrections. Of particular interest are semileptonic $b\to s(d)\ell^+\ell^-$ and radiative $b\to s(d)γ$ decays that allow to not only to search for the presence of NP, but also probe its potential operator structure through a multitude of observables. These observables include measurements of branching fractions, angular observables, CP-asymmetries and tests of lepton flavour universality. Long-standing tensions of data with SM predictions have been observed in $b\to sμ^+μ^-$ decays, consistently in measurements of branching fractions and angular analyses. However, the significance of these tensions depends on assumptions on hadronic uncertainties in the SM prediction. These proceedings summarise the most recent results on FCNC decays from the LHCb experiment, including the legacy measurement of the key decay $B^0\to K^{*0}μ^+μ^-$, using $8.4\mathrm{fb}^{-1}$ of data from the LHC Run 1 and 2.

hep-ex↗

arXiv2026

MoreFit: A More Optimised, Rapid and Efficient Fit

Parameter estimation via unbinned maximum likelihood fits is a central technique in particle physics. This article introduces MoreFit, which aims to provide a more optimised, rapid and efficient fitting solution for unbinned maximum likelihood fits. MoreFit is developed with a focus on parallelism and relies on computation graphs that are compiled just-in-time. Several novel automatic optimisation techniques are employed on the computation graphs that significantly increase performance compared to conventional approaches. MoreFit can make efficient use of a wide range of heterogeneous platforms through its compute backends that rely on open standards. It provides an OpenCL backend for execution on GPUs of all major vendors, and a backend based on LLVM and Clang for single- or multithreaded execution on CPUs, which in addition allows for SIMD vectorisation. MoreFit is benchmarked against several other fitting frameworks and shows very promising performance, illustrating the power of the approach.

physics.data-an↗

arXiv2023

$b\to s\ell\ell$ and Lepton Universality at LHCb

Lepton universality, meaning the equal coupling of the electroweak gauge bosons to the different lepton flavours, is a central property of the Standard Model (SM). Tests of lepton universality in ratios of rates of $b\to s\ell^+\ell^-$ decays involving electron and muon final states profit from precise SM predictions, free from hadronic uncertainties. They therefore constitute particularly powerful probes for New Physics (NP) scenarios. These proceedings summarise the latest and most precise tests of lepton universality in rare $b\to s\ell^+\ell^-$ transitions by the LHCb collaboration.

hep-ex↗

arXiv2022

WG3 Summary -- Rare $B$, $D$ and $K$ decays

We summarize the presentations made within Working Group 3 of the CKM2021 workshop. This working group is devoted to rare $B$, $D$ and $K$ decays, radiative and electroweak-penguin decays, including constraints on $V_{\rm td}/V_{\rm ts}$ and $ε^\prime / ε$. The working group has thus a very broad scope, and includes very topical subjects such as the coherent array of discrepancies in semi-leptonic $B$ decays. Each contribution is here summarized very succinctly with the aim of providing an overview of the main results. The reader interested in fuller details is referred to the individual contributions.

hep-ph↗

arXiv2022

Parameter uncertainties in weighted unbinned maximum likelihood fits

Parameter estimation via unbinned maximum likelihood fits is central for many analyses performed in high energy physics. Unbinned maximum likelihood fits using event weights, for example to statistically subtract background contributions via the sPlot formalism, or to correct for acceptance effects, have recently seen increasing use in the community. However, it is well known that the naive approach to the estimation of parameter uncertainties via the second derivative of the logarithmic likelihood does not yield confidence intervals with the correct coverage in the presence of event weights. This paper derives the asymptotically correct expressions and compares them with several commonly used approaches for the determination of parameter uncertainties, some of which are shown to not generally be asymptotically correct. In addition, the effect of uncertainties on event weights is discussed, including uncertainties that can arise from the presence of nuisance parameters in the determination of sWeights.

physics.data-an↗

arXiv2022

Custom Orthogonal Weight functions (COWs) for Event Classification

A common problem in data analysis is the separation of signal and background. We revisit and generalise the so-called $sWeights$ method, which allows one to calculate an empirical estimate of the signal density of a control variable using a fit of a mixed signal and background model to a discriminating variable. We show that $sWeights$ are a special case of a larger class of Custom Orthogonal Weight functions (COWs), which can be applied to a more general class of problems in which the discriminating and control variables are not necessarily independent and still achieve close to optimal performance. We also investigate the properties of parameters estimated from fits of statistical models to $sWeights$ and provide closed formulas for the asymptotic covariance matrix of the fitted parameters. To illustrate our findings, we discuss several practical applications of these techniques.

stat.ME↗

arXiv2021

Flavor Anomalies in Heavy Quark Decays

Recent measurements of $b$-hadron decays show a pattern of consistent tensions with the respective Standard Model (SM) predictions. These tensions appear both in the sector of rare flavour-changing neutral currents and in tree-level semileptonic $b$-hadron decays. Flavour-changing neutral-current decays are loop-suppressed in the SM and are thus very susceptible to contributions from new heavy particles and/or new interactions beyond the SM. In rare semileptonic decays tensions are observed in measurements of branching fractions and angular observables, as well as in lepton flavour universality tests. Lepton flavour universality is also tested by comparing tree-level $b\to c\ell^-\barν_τ$ processes involving third generation leptons $\ell=τ$ to semileptonic decays with light leptons $\ell=e,μ$ in the final state. These tests also show tensions between measurements and the SM prediction. Taken together, these tensions constitute the so-called flavour anomalies in $b$-hadron decays, and could be first signs of New Physics (NP) beyond the SM, if established beyond any reasonable doubt. This article reviews both the current experimental status of the flavour anomalies and developments for the relevant theoretical predictions. The review concludes with a discussion of future prospects for the field.

hep-ex↗

arXiv2018

Lepton Flavour Universality tests in $B$ decays as a probe for New Physics

In the Standard Model (SM), the coupling of the electroweak gauge bosons to the leptons is lepton flavour universal. Tests of this property constitute sensitive probes for new physics models that violate lepton flavour universality. Recent tests of lepton universality in rare $b\to s\ell\ell$ decays and semileptonic $b\to cτ\barν_τ$ transitions have shown some tensions with the precise SM predictions. These proceedings summarise the latest results on lepton flavour universality from the LHCb experiment.

hep-ex↗

arXiv2016

Rare decays at LHCb

Rare decays are flavour changing neutral current processes that are loop-suppressed in the Standard Model (SM). New particles in SM extensions can therefore give significant contributions, modifying branching fractions and angular distributions. Consequently, rare decays are particularly sensitive probes for New Physics (NP). These proceedings summarize the latest results from the LHCb experiment on rare decays. While most results are in good agreement with SM predictions, some tensions that recently appeared in rare semileptonic $b\to s\ell^+\ell^-$ decays are also discussed.

hep-ex↗

arXiv2015

Latest results on rare decays from LHCb

Rare flavour changing neutral current decays are sensitive indirect probes for new effects beyond the Standard Model (SM). In the SM, these decays are forbidden at tree level and are therefore loop-suppressed. In SM extensions, new, heavy particles can significantly contribute and affect both their branching fractions as well as their angular distributions. The rare decay $B^0\to K^{*0}(\to K^+π^-)μ^+μ^-$ is of particular interest, since it gives access to many angular observables, allowing to model-independently test the operator structure of the decay. A previous analysis of the angular distributions of the final state particles showed interesting tensions with SM predictions using the data sample taken by the LHCb detector during 2011. These proceedings will summarize latest results on rare decays from the LHCb experiment with emphasis on the angular analysis of the decay $B^0\to K^{*0}μ^+μ^-$, using the full Run I data sample of the LHCb experiment.

hep-ex↗