Combination of Searches for Invisible Higgs Boson Decays with the ATLAS Experiment

Abstract

The ATLAS CollaborationDark matter particles, if sufficiently light, may be produced in decays of the Higgs boson. This Letter presents a statistical combination of searches for H → invisible decays where H is produced according to the Standard Model via vector boson fusion, Z( )H, and W/Z(had)H, all performed with the ATLAS detector using 36.1 fb −1 of pp collisions at a center-of-mass energy of √ s = 13 TeV at the LHC. In combination with the results at √ s = 7 and 8 TeV, an exclusion limit on the H → invisible branching ratio of 0.26 (0.17 +0.07 −0.05 ) at 95% confidence level is observed (expected). 1 ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point (IP) in the center of the detector and the z-axis along the beam pipe. The x-axis points to the center of the LHC ring, and the y-axis points upward. Cylindrical coordinates (r, φ) are used in the transverse plane, φ being the azimuthal angle around the z-axis. The pseudorapidity is defined in terms of the polar angle θ as η = − ln tan(θ/2). The distance between two objects in η-φ space is ∆R = (∆η) 2 + (∆φ) 2 . Transverse momentum is defined by p T = p sin θ.

The Higgs-portal for vector dark matter and the effective field theory approach: A reappraisal

“…with the couplingg fixed such that invisible branching ratios from the kinematically allowed decay H 1 → V V of BR(H 1 → inv) = 25% and 2.5% are obtained. While the first value corresponds to the present experimental sensitivity [23,26], 2.5 % represents the projected limit from the high-luminosity upgrade of the LHC [37]. The obtained model points shown in red have been compared with the exclusion bound from XENON1T, depicted by the blue region above the dashed blue line, as well as with the projected sensitivities from the XENONnT [38] (a similar sensitivity is expected for the LZ experiment [39]) and DARWIN experiments [40], the dashed magenta and solid purple lines, respectively, and finally with the contour corresponding to invisible Higgs branching ratios of BR(H → inv) = 25% (left) and 2.5% (right panel), shown by the thick solid black lines, obtained in the effective vector Higgs-portal scenario.…”

“…In Fig. 1, we further include the constraints on the [M V , λ HV V ] parameter space from the most recent LHC measurements of the invisible Higgs branching ratio [23,26] which exclude the grey area and from the direct detection of DM states by the XENON1T experiment [33] which excludes the blue area. As can be seen, the latter constraints are, in any case, much more stringent than the perturbative unitarity bound.…”

Search for invisible decays of the Higgs boson at the ILC

“…By combining all runs, an upper limit of 0.26% can be achieved at the ILC. This result can be compared with the current model dependent LHC results of 26% and 19% by ATLAS [7] and CMS [8], respectively, and 3.8% at HL-LHC with 3 ab −1 [9]. The ILC will give two orders (one order) of magnitude better result than current LHC (future HL-LHC) experiments.…”

Electroweak breaking and Higgs boson profile in the simplest linear seesaw model

“…For example, it implies the existence of a new invisible Higgs decay channel with majoron emission [8]. This has phenomenomenological implications for collider experiments [17][18][19][20][21][22][23][24][25][26][27] and has indeed been searched by LEP and LHC collaborations [28,29].…”