A search for resonances decaying into a Higgs boson and a new particle $X$ in the $XH \to qqbb$ final state with the ATLAS detector

TL;DR

This paper reports a search for heavy resonances decaying into a Higgs boson and a new particle X, using ATLAS data, setting upper limits on production cross-sections for various mass ranges in a fully hadronic final state.

Contribution

It introduces a novel search strategy for high-mass resonances decaying into Higgs and a new particle X in a fully hadronic final state with boosted objects.

Findings

No significant excess observed over Standard Model background.

Set 95% CL upper limits on production cross-sections for resonance masses between 1 and 4 TeV.

Excluded certain mass ranges for the new particle X at high resonance masses.

Abstract

A search for heavy resonances decaying into a Higgs boson ($H$) and a new particle ($X$) is reported, utilizing 36.1 fb$^{-1}$ of proton-proton collision data at $\sqrt{s} =$ 13 TeV collected during 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider. The particle $X$ is assumed to decay to a pair of light quarks, and the fully hadronic final state $XH \rightarrow q\bar q'b\bar b$ is analysed. The search considers the regime of high $XH$ resonance masses, where the $X$ and $H$ bosons are both highly Lorentz-boosted and are each reconstructed using a single jet with large radius parameter. A two-dimensional phase space of $XH$ mass versus $X$ mass is scanned for evidence of a signal, over a range of $XH$ resonance mass values between 1 TeV and 4 TeV, and for $X$ particles with masses from 50 GeV to 1000 GeV. All search results are consistent with the expectations for the background due to Standard Model processes, and 95% CL upper limits are set, as a function of $XH$ and $X$ masses, on the production cross-section of the $XH\rightarrow q\bar q'b\bar b$ resonance.