Abstract: Metallic carbon nanotubes (CNTs) can provide ultra-dense, homogeneous plasma capable of sustaining resonant plasma waves-known as plasmons-with ultra-high field amplitudes. These waves can be efficiently driven by either high-intensity laser pulses or high-density relativistic charged particle beams. In this study, we use numerical simulations to propose that electrons and positrons can be accelerated in wakefields generated by the leaky electromagnetic field of surface plasmons. These plasmons are excited when a high-intensity optical laser pulse propagates paraxially through a cylindrical vacuum channel structured within a CNT forest. The wakefield is stably sustained by a non-evanescent longitudinal field with TV m-1-level amplitudes. This mechanism differs significantly from the plasma wakefield generation in uniform gaseous plasmas. Travelling at the speed of light in a vacuum, with phase-matched focusing fields, the wakefield acceleration is highly efficient for both electron and positron beams. We also examine two potential electron injection mechanisms: edge injection and self-injection. Both mechanisms are feasible with current laser facilities, paving the way for experimental realisation. Beyond presenting a novel method toward ultra-compact, high-energy solid-state plasma particle accelerators with ultra-high acceleration gradients, this work also expands the potential of high-energy plasmonics.

Abstract: A measurement of off-shell Higgs boson production in the H*-> ZZ -> 4l decay channel is presented. The measurement uses 140 fb-1 of proton-proton collisions at s=13 TeV collected by the ATLAS detector at the Large Hadron Collider and supersedes the previous result in this decay channel using the same dataset. The data analysis is performed using a neural simulation-based inference method, which builds per-event likelihood ratios using neural networks. The observed (expected) off-shell Higgs boson production signal strength in the ZZ -> 4l decay channel at 68% CL is 0.87-0.54+0.75 ( 1.00-0.95+1.04). The evidence for off-shell Higgs boson production using the ZZ -> 4l decay channel has an observed (expected) significance of 2.5 sigma (1.3 sigma). The expected result represents a significant improvement relative to that of the previous analysis of the same dataset, which obtained an expected significance of 0.5 sigma. When combined with the most recent ATLAS measurement in the ZZ -> 2l2 nu decay channel, the evidence for off-shell Higgs boson production has an observed (expected) significance of 3.7 sigma (2.4 sigma). The off-shell measurements are combined with the measurement of on-shell Higgs boson production to obtain constraints on the Higgs boson total width. The observed (expected) value of the Higgs boson width at 68% CL is 4.3-1.9+2.7 ( 4.1-3.4+3.5) MeV.

Abstract: Extended gamma-ray emission around isolated pulsars at TeV energies, also known as TeV halos, have been found around a handful of middle-aged pulsars. The halos are significantly more extended than their pulsar wind nebulae but much smaller than the particle diffusion length in the interstellar medium. The origin of TeV halos is unknown. Interpretations invoke either local effects related to the environment of a pulsar or generic particle transport behaviors. The latter scenario predicts that TeV halos would be a universal phenomena for all pulsars. We searched for extended gamma-ray emission around 36 isolated middle-aged pulsars identified by radio and gamma-ray facilities using 2321 days of data from the High-Altitude Water Cherenkov (HAWC) Observatory. Through a stacking analysis comparing TeV flux models against a background-only hypothesis, we identified TeV halolike emission at a significance level of 5.106. Our results imply that extended TeV gamma-ray halos may commonly exist around middle-aged pulsars. This reveals a previously unknown feature about pulsars and opens a new window to identify the pulsar population that is invisible to radio, x-ray, and GeV gamma-ray observations.