Accelerator Neutrinos

Abstract:

Jets with different radius parameters R are an important tool for probing quantum chromodynamics processes at different angular scales. Jets with small R=0.2$$R=0.2$$ are instrumental in measurements of the substructure of large-R jets resulting from collimated hadronic decays of energetic W,  Z,  and Higgs bosons, top quarks, and of potential new resonances. This paper presents measurements of the energy scale, resolution, and associated uncertainties of jets with radius parameters R=0.2$$R=0.2$$ and 0.6,  obtained using the ATLAS detector. The results are based on 37fb-1$$37~\text {fb}^{-1}$$ of proton–proton collision data from the Large Hadron Collider at a centre-of-mass energy of s=13$$\sqrt{s}=13$$ TeV. A new in situ method for measuring jet energy scale differences between data and Monte Carlo simulations is presented. The systematic uncertainties in the jet energy scale for central jets (|η|<1.2)$$(|\eta | < 1.2)$$ typically vary from 1% to about 5% as a function of |η|$$|\eta |$$ at very low transverse momentum, pT,$$p_{\text {T}},$$ of around 20 GeV for both R=0.2$$R=0.2$$ and 0.6 jets. The relative energy resolution ranges from (35±6)%$$(35 \pm 6)\%$$ at pT=20$$p_{\text {T}} = 20$$ GeV to (6±0.5)%$$(6 \pm 0.5)\%$$ at pT=300$$p_{\text {T}} = 300$$ GeV for central R=0.2$$R=0.2$$ jets, and is found to be slightly worse for R=0.6$$R=0.6$$ jets. Finally, the effect of close-by hadronic activity on the jet energy scale is investigated and is found to be well modelled by the ATLAS Monte Carlo simulations.

Abstract:

A search is performed for long-lived heavy neutral leptons (HNLs), produced through the decay of a W boson along with a muon or electron. Two channels are explored: a leptonic channel, in which the HNL decays into two leptons and a neutrino, and a semi-leptonic channel, in which the HNL decays into a lepton and a charged pion. The search is performed with 140 fb−1 of s = 13 TeV proton-proton collision data collected by ATLAS during Run 2 of the Large Hadron Collider. No excess of events is observed; Dirac-like and Majorana-like HNLs with masses below 14.5 GeV and mixing coefficients as small as 10−7 are excluded at the 95% confidence level. The results are interpreted under different assumptions on the flavour of the leptons from the HNL decays.

Abstract:

This paper presents a search for massive, charged, long-lived particles with the ATLAS detector at the Large Hadron Collider using an integrated luminosity of 140 fb−1 of proton-proton collisions at s = 13 TeV. These particles are expected to move significantly slower than the speed of light. In this paper, two signal regions provide complementary sensitivity. In one region, events are selected with at least one charged-particle track with high transverse momentum, large specific ionisation measured in the pixel detector, and time of flight to the hadronic calorimeter inconsistent with the speed of light. In the other region, events are selected with at least two tracks of opposite charge which both have a high transverse momentum and an anomalously large specific ionisation. The search is sensitive to particles with lifetimes greater than about 3 ns with masses ranging from 200 GeV to 3 TeV. The results are interpreted to set constraints on the supersymmetric pair production of long-lived R-hadrons, charginos and staus, with mass limits extending beyond those from previous searches in broad ranges of lifetime.