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  1. Abstract

    This paper presents the observation of four-top-quark ($$t\bar{t}t\bar{t}$$tt¯tt¯) production in proton-proton collisions at the LHC. The analysis is performed using an integrated luminosity of 140 $$\hbox {fb}^{-1}$$fb-1at a centre-of-mass energy of 13 TeV collected using the ATLAS detector. Events containing two leptons with the same electric charge or at least three leptons (electrons or muons) are selected. Event kinematics are used to separate signal from background through a multivariate discriminant, and dedicated control regions are used to constrain the dominant backgrounds. The observed (expected) significance of the measured$$t\bar{t}t\bar{t}$$tt¯tt¯signal with respect to the standard model (SM) background-only hypothesis is 6.1 (4.3) standard deviations. The$$t\bar{t}t\bar{t}$$tt¯tt¯production cross section is measured to be$$22.5^{+6.6}_{-5.5}$$22.5-5.5+6.6 fb, consistent with the SM prediction of$$12.0 \pm 2.4$$12.0±2.4fb within 1.8 standard deviations. Data are also used to set limits on the three-top-quark production cross section, being an irreducible background not measured previously, and to constrain the top-Higgs Yukawa coupling and effective field theory operator coefficients that affect$$t\bar{t}t\bar{t}$$tt¯tt¯production.

     
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    Free, publicly-accessible full text available June 1, 2024
  2. Abstract

    In this paper we report the first close, high‐resolution observations of downward‐directed terrestrial gamma‐ray flashes (TGFs) detected by the large‐area Telescope Array cosmic ray observatory, obtained in conjunction with broadband VHF interferometer and fast electric field change measurements of the parent discharge. The results show that the TGFs occur during strong initial breakdown pulses (IBPs) in the first few milliseconds of negative cloud‐to‐ground and low‐altitude intracloud flashes and that the IBPs are produced by a newly identified streamer‐based discharge process called fast negative breakdown. The observations indicate the relativistic runaway electron avalanches (RREAs) responsible for producing the TGFs are initiated by embedded spark‐like transient conducting events (TCEs) within the fast streamer system and potentially also by individual fast streamers themselves. The TCEs are inferred to be the cause of impulsive sub‐pulses that are characteristic features of classic IBP sferics. Additional development of the avalanches would be facilitated by the enhanced electric field ahead of the advancing front of the fast negative breakdown. In addition to showing the nature of IBPs and their enigmatic sub‐pulses, the observations also provide a possible explanation for the unsolved question of how the streamer to leader transition occurs during the initial negative breakdown, namely, as a result of strong currents flowing in the final stage of successive IBPs, extending backward through both the IBP itself and the negative streamer breakdown preceding the IBP.

     
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