Accelerator Neutrinos

Abstract:

A search for exotic decays of the Higgs boson into a pair of low-mass scalars that subsequently decay into τ -leptons, H → a a → τ + τ − τ + τ − , is presented. In models with Yukawa-like couplings, the decay to τ -leptons is favoured for light a -bosons, with mass in the range of 2 m τ < m a < 2 m b . Results are presented in the range of 4 GeV < m a < 15 GeV using the 140 fb − 1 of proton–proton collisions at s = 13 TeV recorded with the ATLAS detector during Run 2 of the Large Hadron Collider. This search focuses on the scenario where, for both di- τ pairs, one of the τ -leptons decays to hadrons and neutrinos, while the other decays to a muon and neutrinos. In this mass range, the a → τ + τ − is Lorentz-boosted and a dedicated muon removal technique is used to reconstruct the di- τ pairs. No significant excess above the Standard Model background prediction is observed. Upper limits on ( σ ( H ) / σ SM ( H ) ) × B ( H → a a → 4 τ ) at 95 % confidence level are provided, ranging from 0.03 to 0.10 depending on the a -boson mass.

Abstract:

This Letter presents an investigation of low-energy electron-neutrino interactions in the Fermilab Booster Neutrino Beam by the MicroBooNE experiment, motivated by the excess of electron-neutrino-like events observed by the MiniBooNE experiment. This is the first measurement to use data from all five years of operation of the MicroBooNE experiment, corresponding to an exposure of $1.11\times {10}^{21}$ protons on target, a 70% increase on past results. Two samples of electron neutrino interactions without visible pions are used, one with visible protons and one without any visible protons. The MicroBooNE data show reasonable agreement with the nominal prediction, with $p$ values $\ge 26.7\%$ when the two ${\nu }_e$ samples are combined, though the prediction exceeds the data in limited regions of phase space. The data are further compared to two empirical models that modify the predicted rate of electron-neutrino interactions in different variables in the simulation to match the unfolded MiniBooNE low energy excess. In the first model, this unfolding is performed as a function of electron neutrino energy, while the second model aims to match the observed shower energy and angle distributions of the MiniBooNE excess. This measurement excludes an electronlike interpretation of the MiniBooNE excess based on these models at $>99\%{\mathrm{CL}}_{\mathrm{s}}$ in all kinematic variables.

Abstract:

Measurements of W+W− → e±νμ∓ν production cross-sections are presented, providing a test of the predictions of perturbative quantum chromodynamics and the electroweak theory. The measurements are based on data from pp collisions at s = 13 TeV recorded by the ATLAS detector at the Large Hadron Collider in 2015–2018, corresponding to an integrated luminosity of 140 fb−1. The number of events due to top-quark pair production, the largest background, is reduced by rejecting events containing jets with b-hadron decays. An improved methodology for estimating the remaining top-quark background enables a precise measurement of W+W− cross-sections with no additional requirements on jets. The fiducial W+W− cross-section is determined in a maximum-likelihood fit with an uncertainty of 3.1%. The measurement is extrapolated to the full phase space, resulting in a total W+W− cross-section of 127 ± 4 pb. Differential cross-sections are measured as a function of twelve observables that comprehensively describe the kinematics of W+W− events. The measurements are compared with state-of-the-art theory calculations and excellent agreement with predictions is observed. A charge asymmetry in the lepton rapidity is observed as a function of the dilepton invariant mass, in agreement with the Standard Model expectation. A CP-odd observable is measured to be consistent with no CP violation. Limits on Standard Model effective field theory Wilson coefficients in the Warsaw basis are obtained from the differential cross-sections.