AION/Magis

Abstract:

The leading experimental determinations of the fine-structure constant 𝛼 currently rely on atomic photon-recoil measurements from Ramsey-Bordé atom interferometry with large-momentum transfer to provide an absolute mass measurement. We propose an experimental scheme for an intermediate-scale differential atom interferometer to measure the photon recoil of neutral atomic species with a single-photon optical clock transition. We calculate trajectories for our scheme that optimize the recoil phase while nullifying the undesired gravity-gradient phase by considering independently launching two clouds of ultracold atoms with the appropriate initial conditions. For Sr and Yb, we find an atom interferometer of height 3 m to be sufficient for an absolute mass measurement precision of 𝛥⁢𝑚/𝑚∼1×10−11 with current technology. Such a precise measurement would halve the current uncertainty in 𝛼 — an uncertainty that would no longer be limited by an absolute mass measurement. The removal of this limitation would allow the current uncertainty in 𝛼 to be reduced by a factor of 10 by corresponding improvements in relative mass measurements, thus paving the way for higher-precision tests of the standard model of particle physics.

Abstract:

A search for the resonant production of a heavy scalar X decaying into a Higgs boson and a new lighter scalar S, through the process X → S(→bb¯)H(→γγ), where the two photons are consistent with the Higgs boson decay, is performed. The search is conducted using an integrated luminosity of 140 fb−1 of proton-proton collision data at a centre-of-mass energy of 13 TeV recorded with the ATLAS detector at the Large Hadron Collider. The search is performed over the mass range 170 ≤ mX ≤ 1000 GeV and 15 ≤ mS ≤ 500 GeV. Parameterised neural networks are used to enhance the signal purity and to achieve continuous sensitivity in a domain of the (mX, mS) plane. No significant excess above the expected background is found and 95% CL upper limits are set on the cross section times branching ratio, ranging from 39 fb to 0.09 fb. The largest deviation from the background-only expectation occurs for (mX, mS) = (575, 200) GeV with a local (global) significance of 3.5 (2.0) standard deviations.

Abstract:

A search for neutral long-lived particles (LLPs) decaying in the ATLAS hadronic calorimeter using 140 fb−1 of proton-proton collisions at s = 13 TeV delivered by the LHC is presented. The analysis is composed of three channels. The first targets pair-produced LLPs, where at least one LLP is produced with sufficiently low boost that its decay products can be resolved as separate jets. The second and third channels target LLPs respectively produced in association with a W or Z boson that decays leptonically. In each channel, different search regions target different kinematic regimes, to cover a broad range of LLP mass hypotheses and models. No excesses of events relative to the background predictions are observed. Higgs boson branching fractions to pairs of hadronically decaying neutral LLPs larger than 1% are excluded at 95% confidence level for proper decay lengths in the range of 30 cm to 4.5 m depending on the LLP mass, a factor of three improvement on previous searches in the hadronic calorimeter. The production of long-lived dark photons in association with a Z boson with cross-sections above 0.1 pb is excluded for dark photon mean proper decay lengths in the range of 20 cm to 50 m, improving previous ATLAS results by an order of magnitude. Finally, long-lived photo-phobic axion-like particle models are probed for the first time by ATLAS, with production cross-sections above 0.1 pb excluded in the 0.1 mm to 10 m range.