.
The existence of a new spin-zero particle with a mass below the electroweak scale is predicted by several theoretical models. Searches for resonant production of photon pairs at the LHC are able to probe these models. We present a search for a narrow resonance produced through gluon fusion that decays into a pair of photons with an invariant mass between 10 and 70 GeV, using a proton-proton collision dataset from the CMS experiment. This dataset, corresponding to an integrated luminosity of 54.4 fb − 1 , was recorded in 2018 at a center-of-mass energy of 13 TeV using a newly introduced diphoton trigger that enabled exploration of the low-mass diphoton spectrum. No significant excess above the expected background is observed. Upper limits are set on the product of the gluon fusion production cross section and the branching fraction of the diphoton decay of a narrow resonance. An interpretation of these limits within an effective field theory framework for axionlike particles is also provided.
A search for Higgs boson ( H ) production in association with two vector bosons ( V = W , Z ) via vector-boson scattering (VBS) is presented using proton-proton collision data collected at s = 13 TeV by the CMS experiment, corresponding to an integrated luminosity of 138 fb − 1 . Events containing two forward jets consistent with VBS, a large-radius jet from the decay of a boosted H to a pair of b quarks, and zero, one, or two charged leptons coming from V decays are selected. The process is excluded at 95% confidence level for observed (expected) values of the V V H H coupling modifier κ 2 V outside the interval 0.40 < κ 2 V < 1.60 ( 0.34 < κ 2 V < 1.66 ), assuming standard model values for all other couplings, thus establishing a novel probe of the V V H H interaction. Constraints are also set on the individual κ 2 W and κ 2 Z coupling modifiers and on the allowed region in the κ 2 W − κ 2 Z plane.
Plasma wakefield accelerators (PWFA) represent one of the promising new accelerator concepts that are now being developed intensively for future applications in high-energy physics and industry. Among the unresolved problems of practical implementation of PWFA there are maintaining the required quality (emittance, size, energy spread of bunches) and stable transportation of drive and accelerated (witness) bunches over long acceleration distances. For improving the bunch transport, we propose to fill the bunch transport channel with the background plasma, the density of which is lower than the main plasma density building up an accelerating wake wave. We call this waveguide structure a layered plasma waveguide (LPW). The wakefield excitation by a regular sequence of electron bunches in the LPW of cylindrical configuration has been explored both analytically and numerically. The layered plasma has been modeled as a combination of a tubular plasma and a plasma column of significantly different densities. The plasma column has a lower density. The dispersion dependencies of the TM-modes of the LPW was obtained and analyzed, and it was found that there was a single TM wave resonant with the electron bunch. Based on the obtained analytical expressions, the structures of the axial and radial wakefield amplitudes have been numerically investigated for the cases of a single drive bunch and a regular train of bunches. It is shown that for certain density ratios of the outer and inner plasmas, it is possible both to accelerate and focus simultaneously the drive and witness bunches. The 2.5D particle-in-cell code was used to simulate the witness acceleration of an electron bunch by a wakefield created by drive electron bunches in a two-layer plasma wakefield accelerator. The simulation showed good agreement with the results of analytical calculations.
Axisymmetric Alfvén eigenmodes have been observed at the plasma edge in deuterium-tritium (D-T) tokamak plasmas externally heated only by neutral beam injection in JET. The modes were detected only in D-T plasmas, not in pure D plasmas, indicating excitation by fusion-born α particles. The presence of the axisymmetric mode suggests that the modes were driven by positive energy gradients in the α particle distribution rather than radial gradients. The modes are driven by counter-current passing α particles with large orbit widths, allowing core-born α particles to interact with modes at the plasma edge. This reveals an excitation mechanism arising from positive energy gradients produced by the minimum energy required to confine particles at the edge, relevant to all burning plasmas.
A search for supersymmetric electroweakinos is performed using events with a low-momentum (soft) isolated track and large missing transverse momentum, targeting nearly mass-degenerate higgsinolike charginos and neutralinos. For mass splittings of 0.3–1 GeV, the chargino decays to the lightest neutralino and a low-momentum pion, which can produce a soft, potentially displaced track. A parametrized neural network separates signal from background using kinematic and impact parameter information. The analysis uses 138 fb − 1 of proton-proton collision data at a center-of-mass energy of 13 TeV recorded with the CMS detector. No significant excess above the standard model expectation is observed. At 95% confidence level, the considered higgsino model is excluded for mass splittings in the range 0.28–1.15 GeV and for chargino masses up to 185 GeV, setting stringent constraints on natural supersymmetry scenarios.
