Accelerator Neutrinos

Abstract:

The current laws of physics do not explain the observed imbalance of matter and antimatter in the universe. Sakharov proposed that an explanation would require the violation of CP symmetry between matter and antimatter. The only CP violation observed so far is in the weak interactions of quarks, and it is too small to explain the matter-antimatter imbalance of the universe. It has been shown that CP violation in the lepton sector could generate the matter-antimatter disparity through the process called leptogenesis. The quantum mixing of neutrinos, the neutral leptons in the Standard Model, provides a potential source of CP violation through a complex phase dCP, which may have consequences for theoretical models of leptogenesis. This CP violation can be measured in muon neutrino to electron neutrino oscillations and the corresponding antineutrino oscillations, which are experimentally accessible with accelerator-produced beams as established by the T2K experiment. Until now, the value of dCP has not been significantly constrained by neutrino oscillation experiments. Here the T2K collaboration reports a measurement that favors large enhancement of the neutrino oscillation probability, excluding values of dCP which result in a large enhancement of the observed anti-neutrino oscillation probability at three standard deviations (3 sigma). The 3 sigma confidence level interval for dCP, which is cyclic and repeats every 2pi, is [-3.41,-0.03] for the so-called normal mass ordering, and [-2.54,-0.32] for the inverted mass ordering. Our results show an indication of CP violation in the lepton sector. Herein we establish methods for sensitive searches for matter-antimatter asymmetry in neutrino oscillations using accelerator-produced neutrino beams. Future measurements with larger data samples will determine whether the leptonic CP violation is larger than the quark sector CP violation.

Abstract:

This Letter reports on the first evidence of inclusive Higgs-boson production at high transverse momentum in the $b\overline{b}$ final state, reconstructed in a single large-radius jet. The results are based on proton-proton collision data recorded by the ATLAS detector at the Large Hadron Collider at a center-of-mass energies of 13 and 13.6 TeV, corresponding to a total integrated luminosity of $301{\mathrm{fb}}^{-1}$ . The study profits from the large background suppression provided by the use of a new transformer-based algorithm for jet identification and the sharper mass and transverse momentum, $p_{\mathrm{T}}$ , resolution from a dedicated regression model. The yield relative to the standard model prediction for Higgs bosons produced at $p_{\mathrm{T}}$ larger than 450 GeV is measured to be $1.53\pm 0.2{7)}_{-0.27}^{+0.33}\pm 0.17$ , where the first uncertainty is statistical, the second is experimental systematic, and the third accounts for theory systematic, corresponding to an observed (expected) significance of $3.8\sigma (2.5\sigma )$ relative to the background-only hypothesis. Results are also obtained in three Higgs boson $p_{\mathrm{T}}$ intervals and found to be compatible with standard model predictions, where the first uncertainty is statistical, the second is experimental systematic, and the third accounts for theory systematic.

Abstract:

A measurement of correction factors for the hadronic jet energy scale and resolution in the ATLAS detector is presented. These correction factors account for differences between simulated and observed data. They are obtained by analysing a selection of top quark events collected in proton–proton collisions by ATLAS between the years 2015 and 2018 at a centre-of-mass energy s=13$$\sqrt{s} = 13$$ TeV$$\text {T{eV}}$$ as well as in 2022 and 2023 at s=13.6$$\sqrt{s} = 13.6$$ TeV$$\text {T{eV}}$$. The impact of different jet energy scale or resolution corrections on the reconstructed mass of the hadronically decaying W boson from top-quark decays in simulation is quantified. The correction factors are extracted from a fit to the parameterised reconstructed W-boson mass distribution to data. The energy scale and resolution corrections are measured as a function of the jet transverse momentum between 20 and 200 GeV and absolute pseudorapidity less than 0.8. The uncertainties in the energy scale range from about 0.93% to about 1.7% for jets between 35 and 200 GeV$$\text {G{eV}}$$, while for the energy resolution the uncertainties range from about 14 to 28%. The method presented will be used in conjunction with other techniques to further improve ATLAS jet energy scale and resolution precision.

Abstract:

This paper presents the first search for non-resonant Higgs boson pair production in association with a top-quark pair (tt¯HH$$ t\overline{t} HH $$) using proton–proton collision data collected with the ATLAS detector at the Large Hadron Collider. The data sample corresponds to an integrated luminosity of 196 fb−1, comprising 140 fb−1 at a centre-of-mass energy of s=13$$ \sqrt{s}=13 $$ TeV and 56 fb−1 at 13.6 TeV. The search targets three distinct final states expected from tt¯HH$$ t\overline{t} HH $$ decays: (i) one lepton (electron or muon) with at least five b-quarks, (ii) at least two b-quarks accompanied by two leptons with the same electric charges or multiple leptons, and (iii) at least three b-quarks with two photons. The tt¯HH$$ t\overline{t} HH $$ production cross-section, relative to its Standard Model prediction, is measured to be μtt¯HH=−3−12+11$$ {\mu}_{t\overline{t} HH}=-{3}_{-12}^{+11} $$. This result corresponds to a 95% confidence-level upper limit of 20 times the Standard Model prediction for the tt¯HH$$ t\overline{t} HH $$ production cross-section, with an expected limit of 21. The Higgs effective field theory Wilson coefficient ctt¯HH$$ {c}_{t\overline{t} HH} $$ is also constrained at the same confidence level to the range of −3.9

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