Pushing Higgs Effective Theory over the Edge

Based on a vector triplet model we study a possible failure of dimension-6 operators in describing LHC Higgs kinematics. First, we illustrate that including dimension-6 contributions squared can significantly improve the agreement between the full model and the dimension-6 approximation, both in associated Higgs production and in weak-boson-fusion Higgs production. Second, we test how a simplified model with an additional heavy scalar could improve the agreement in critical LHC observables. In weak boson fusion we find an improvement for virtuality-related observables at large energies, but at the cost of sizeable deviations in interference patterns and angular correlations.Comment: 19 pages. v2: references added. v3: minor corrections, more references added, matches published versio

Mining gold from implicit models to improve likelihood-free inference

Simulators often provide the best description of real-world phenomena. However, they also lead to challenging inverse problems because the density they implicitly define is often intractable. We present a new suite of simulation-based inference techniques that go beyond the traditional Approximate Bayesian Computation approach, which struggles in a high-dimensional setting, and extend methods that use surrogate models based on neural networks. We show that additional information, such as the joint likelihood ratio and the joint score, can often be extracted from simulators and used to augment the training data for these surrogate models. Finally, we demonstrate that these new techniques are more sample efficient and provide higher-fidelity inference than traditional methods.Comment: Code available at https://github.com/johannbrehmer/simulator-mining-example . v2: Fixed typos. v3: Expanded discussion, added Lotka-Volterra example. v4: Improved clarit

Better Higgs-CP Tests Through Information Geometry

Measuring the CP symmetry in the Higgs sector is one of the key tasks of the LHC and a crucial ingredient for precision studies, for example in the language of effective Lagrangians. We systematically analyze which LHC signatures offer dedicated CP measurements in the Higgs-gauge sector, and discuss the nature of the information they provide. Based on the Fisher information measure, we compare the maximal reach for CP-violating effects in weak boson fusion, associated ZH production, and Higgs decays into four leptons. We find a subtle balance between more theory-independent approaches and more powerful analysis channels, indicating that rigorous evidence for CP violation in the Higgs-gauge sector will likely require a multi-step process.Comment: 27 pages, 8 figure

Symmetry Restored in Dibosons at the LHC?

A number of LHC resonance search channels display an excess in the invariant mass region of 1.8 - 2.0 TeV. Among them is a $3.4\,\sigma$ excess in the fully hadronic decay of a pair of Standard Model electroweak gauge bosons, in addition to potential signals in the $HW$ and dijet final states. We perform a model-independent cross-section fit to the results of all ATLAS and CMS searches sensitive to these final states. We then interpret these results in the context of the Left-Right Symmetric Model, based on the extended gauge group $SU(2)_L\times SU(2)_R\times U(1)'$, and show that a heavy right-handed gauge boson $W_R$ can naturally explain the current measurements with just a single coupling $g_R \sim 0.4$. In addition, we discuss a possible connection to dark matter.Comment: 25 pages, 12 figures, V2: references added, extended discussion of Minimal Left-Right Dark Matter, small correction to decay width - conclusions unchanged, V3: expanded discussion of input parameters and statistical procedure, V4: matches published versio

New Ideas for Effective Higgs Measurements

An effective field theory provides a model-independent and phenomenologically powerful parametrisation of new physics in the Higgs sector. We analyse two aspects of this framework that are relevant for measurements of the Higgs properties during Run 2 of the LHC. First, the limited precision of the LHC analyses cannot guarantee a clear hierarchy between the experimental momentum transfer and the probed new physics scales, casting doubt on the validity of the effective model. By comparing a range of new physics scenarios to their dimension-six approximation, we analyse if an effective description of the Higgs sector is useful, where it breaks down, and how its validity can be improved. Second, we use information geometry to understand and optimise Higgs measurements at the LHC. Our novel approach is based on the Fisher information, which encodes the maximum precision with which theory parameters can be measured in an experiment. We develop an algorithm to calculate the Fisher information in LHC processes and compute the information on dimension-six operators in different Higgs signatures. We demonstrate how information geometry lets us improve event selections, determine the most powerful observables, and compare the power of modern multivariate techniques to that of traditional histogram-based analyses