Direct optimisation of the discovery significance when training neural networks to search for new physics in particle colliders

We introduce two new loss functions designed to directly optimise the statistical significance of the expected number of signal events when training neural networks to classify events as signal or background in the scenario of a search for new physics at a particle collider. The loss functions are designed to directly maximise commonly used estimates of the statistical significance, $s/\sqrt{s+b}$, and the Asimov estimate, $Z_A$. We consider their use in a toy SUSY search with 30~fb$^{-1}$ of 14~TeV data collected at the LHC. In the case that the search for the SUSY model is dominated by systematic uncertainties, it is found that the loss function based on $Z_A$ can outperform the binary cross entropy in defining an optimal search region

A search for supersymmetry in \sqrt{s}=13~\tev proton-proton collisions with the CMS detector at the LHC

An inclusive search for supersymmetry with jets and missing transverse energy is presented. Data from √s = 13 TeV pp-collisions with a total integrated luminosity of 12.9 fb^(−1) delivered by the LHC and collected by the CMS detector are analysed. The dominant quantum chromodynamic multijet background is strongly suppressed with several kinematic variables, which are also used to discriminate between Standard Model and supersymmetric processes. The observed events are found to be compatible with the expected contributions from Standard Model processes. This result is interpreted in the context of simplified supersymmetric models of gluino and third-generation squark production. The mass of the gluino, bottom squark and top squark are excluded to 1775, 1025 and 875 GeV respectively. In preparation for the collection of √s = 13 TeV data by CMS, the jet algorithm for the Level-1 trigger is upgraded. The new algorithm allows for dynamic pileup subtraction and takes advantage of hardware upgrades to the trigger. The performance of different types of pileup subtraction are evaluated and the most promising algorithm, chunky-donut subtraction, is chosen. The algorithm is found to give a significant performance improvement and has been used to collect data from 2016 onwards.Open Acces

Constraints on the χ_(c1) versus χ_(c2) polarizations in proton-proton collisions at √s = 8 TeV

The polarizations of promptly produced χ_(c1) and χ_(c2) mesons are studied using data collected by the CMS experiment at the LHC, in proton-proton collisions at √s=8  TeV. The χ_c states are reconstructed via their radiative decays χ_c → J/ψγ, with the photons being measured through conversions to e⁺e⁻, which allows the two states to be well resolved. The polarizations are measured in the helicity frame, through the analysis of the χ_(c2) to χ_(c1) yield ratio as a function of the polar or azimuthal angle of the positive muon emitted in the J/ψ → μ⁺μ⁻ decay, in three bins of J/ψ transverse momentum. While no differences are seen between the two states in terms of azimuthal decay angle distributions, they are observed to have significantly different polar anisotropies. The measurement favors a scenario where at least one of the two states is strongly polarized along the helicity quantization axis, in agreement with nonrelativistic quantum chromodynamics predictions. This is the first measurement of significantly polarized quarkonia produced at high transverse momentum

The s ---> d gamma decay in and beyond the Standard Model

The New Physics sensitivity of the s ---> d gamma transition and its accessibility through hadronic processes are thoroughly investigated. Firstly, the Standard Model predictions for the direct CP-violating observables in radiative K decays are systematically improved. Besides, the magnetic contribution to epsilon prime is estimated and found subleading, even in the presence of New Physics, and a new strategy to resolve its electroweak versus QCD penguin fraction is identified. Secondly, the signatures of a series of New Physics scenarios, characterized as model-independently as possible in terms of their underlying dynamics, are investigated by combining the information from all the FCNC transitions in the s ---> d sector.Comment: 54 pages, 14 eps figure