Measuring the Invisible Higgs Width at the 7 and 8 TeV LHC

TL;DR

This work assesses the LHC's sensitivity to an invisibly decaying Higgs boson at $7$ and $8$ TeV, focusing on three direct-search channels: monojet, Higgsstrahlung ($Zh$), and weak boson fusion (WBF). It derives the invisible Higgs width in simple $Z_2$-protected models and shows that for $m_h$ in the $120$–$170$ GeV range the visible branching ratio can be suppressed, yielding sizable BR$_{\rm inv}$. A detector-level study identifies WBF as the most sensitive channel at $7$ TeV, with a projected exclusion of $BR_{\rm inv} \gtrsim 0.4$ at 95% CL with $20\,{\rm fb}^{-1}$, while the $8$ TeV reach is comparable. The work also integrates invisible searches with SM-visible Higgs channels, demonstrating enhanced constraints on the invisible fraction in the Higgs decays and outlining strategies to control systematics via data-driven methods. Overall, the results show that invisible Higgs decays are a powerful probe of Higgs portal new physics at the LHC, particularly when leveraging the WBF channel and channel combination.

Abstract

The LHC is well on track toward the discovery or exclusion of a light Standard Model (SM)-like Higgs boson. Such a Higgs has a very small SM width and can easily have large branching fractions to physics beyond the SM, making Higgs decays an excellent opportunity to observe new physics. Decays into collider-invisible particles are particularly interesting as they are theoretically well motivated and relatively clean experimentally. In this work we estimate the potential of the 7 and 8 TeV LHC to observe an invisible Higgs branching fraction. We analyze three channels that can be used to directly study the invisible Higgs branching ratio at the 7 TeV LHC: an invisible Higgs produced in association with (i) a hard jet; (ii) a leptonic Z; and (iii) forward tagging jets. We find that the last channel, where the Higgs is produced via weak boson fusion, is the most sensitive, allowing branching fractions as small as 40% to be probed at 20 inverse fb for masses in the range between 120 and 170 GeV, including in particular the interesting region around 125 GeV. We provide an estimate of the 8 TeV LHC sensitivity to an invisibly-decaying Higgs produced via weak boson fusion and find that the reach is comparable to but not better than the reach at the 7 TeV LHC. We further estimate the discovery potential at the 8 TeV LHC for cases where the Higgs has substantial branching fractions to both visible and invisible final states.

Figures (7)

• Figure 1: Left panel: the reduction of visible channel branching ratios after including a new invisible decay for the Higgs boson. The parameter $\mu_S$ is chosen to be zero, so the scalar $S$ has its mass proportional to the electroweak vacuum expectation value (VEV). Right: the same as the left panel but for a new fermion in the Higgs decay channel, with $m_\chi^0$ set to zero.
• Figure 2: Left panel: the solid line is the current 95% C.L. limit on the Higgs production times the invisible branching ratio from monojet searches at 1 fb$^{-1}$ with HighPt cuts. The dotted line is the projected limit at 20 fb$^{-1}$ assuming that the relative systematic error cannot be improved. The dashed line is the projected limit at 20 fb$^{-1}$ assuming that the relative systematic error can be improved and reduced alongside the statistical error by $1/\sqrt{\cal L}$. Right panel: A comparison of limits from three different cuts used in Ref. Atlasmonojet with 1 fb$^{-1}$ luminosity.
• Figure 3: Left: Comparison of limits on Higgs production times invisible branching ratio coming from different sets of cuts employed in ATLASHZZ2CMSHZZ, interpreted as limits on $p p \rightarrow Zh\rightarrow \ell^+ \ell^- +$ invisibles. Right: The most stringent of the current limits and future projections. The solid line is the current 95% C.L. limit from the CMS search at 4.6 fb$^{-1}$. The dotted line is the projected limit at 20 fb$^{-1}$ by assuming that the relative systematic error cannot be improved. The dashed line is the projected limit at 20 fb$^{-1}$ assuming that the relative systematic error can be improved and reduced alongside the statistical error by $1/\sqrt{\cal L}$.
• Figure 4: Left: Estimated 95% CL limit on the invisible branching fraction of the Higgs with 20 fb$^ {-1}$ of data at the 7 TeV LHC. The straight line at 1 denotes a Higgs produced with SM cross-sections decaying entirely into invisibles. Shaded bands show $\pm 1 \sigma$ with systematic uncertainties assigned as described in the text. Right: Dependence of the 95% CL limits on systematic uncertainties. The top row shows results for jet veto (1) as described in the text; the bottom row shows results for jet veto (2).
• Figure 5: Left: Estimated 95% CL limit on the invisible branching fraction of the Higgs with 20 fb$^ {-1}$ of data at the 8 TeV LHC. The straight line at 1 denotes a Higgs produced with SM cross-sections decaying entirely into invisibles. Shaded bands show $\pm 1 \sigma$ with systematic uncertainties assigned as described in the text. Right: Dependence of the 95% CL limits on systematic uncertainties.