On the robustness of the indirect determination of the width of the detected Higgs boson

Abstract

The indirect determination of the total width of the detected Higgs boson that is carried out by the experimental collaborations at the LHC relies on the assumption that the coupling modifiers for the on-shell and off-shell couplings are the same. However, physics beyond the Standard Model affecting the on-shell and off-shell regions differently could invalidate this assumption, so that the actual width of the detected Higgs boson could be larger than the bounds obtained under this assumption. Relaxing the assumption and investigating different types of extensions of the Standard Model, we analyse under which conditions a larger total width of the detected Higgs boson is compatible with all experimental and theoretical constraints. For the considered scenarios of scalar extensions with an additional state contributing as a resonance or at the loop level, we find that the indirect bounds obtained by ATLAS and CMS remain valid over large parts of the parameter space, with the exception of parameter regions where the additional particles have relatively small masses. We discuss the potential of experimental searches for new particles to further constrain such scenarios. Based on the existing experimental and theoretical constraints we conclude that relaxing the assumption of equal on-shell and off-shell coupling modifiers that is used in the experimental analyses at the LHC yields an upper bound on the total width of the detected Higgs boson in realistic extensions of the Standard Model that is only weakened by up to a factor of about two compared to the case where this assumption is valid.

Figures (13)

• Figure 1: The blue region indicates the flat direction of the on-shell signal strength measurements for a universal Higgs coupling modifier, $\kappa \equiv \kappa_{i,\text{on}}$, using the on-shell $95\%$ CL ATLAS results on the inclusive signal strength, $\mu_ {\text{on}} = 1.01^{+0.23} _ {-0.20}$, from the four-lepton channel ATLAS:2020rej and a varying branching ratio to (potentially undetectable) new-physics final states, $\text{BR}_{\text{BSM}}$. The red solid, dashed and dotted lines indicate different values for the total Higgs width, and the black dot-dashed line is the off-shell limit $\mu_\text{off} \simeq \kappa^4 < 2.4$ under the assumption $\kappa_{i,\text{on}} = \kappa_{i,\text{off}} = \kappa$.
• Figure 2: Absolute value squared of the matrix element factor expressing the modification of $gg \rightarrow ZZ$ caused by the presence of an additional scalar $S$. The black dotted and solid lines show the SM value and an enhancement from $\kappa_t \kappa_Z = 1.6$, respectively. The coloured lines show the combined effects of the enhanced coupling modifiers together with the contribution of the additional scalar, $gg \rightarrow S \rightarrow ZZ$, for two mass values of $S$.
• Figure 3: Histograms for $gg \rightarrow H,S \rightarrow ZZ$ for two values of $m_S$, without including contributions from the square of the SM box diagrams (the interference contributions involving the box diagrams are included). A peak is present where $S$ goes on-shell at $m_{ZZ} = m_S$, while for higher values of $m_{ZZ}$ a decrease of events occurs as compared to the SM bin heights.
• Figure 4: The allowed regions for the product of coupling modifiers $\kappa_t \kappa_Z$ from the off-shell Higgs contributions to $gg \to ZZ$ are indicated by the black dashed contours for the case where an additional scalar $S$ is present with $\Gamma_S / m_S = 10\%$ ($\Gamma_S / m_S = 20\%$) on the left (right). In the region between the two black dashed contours the bound $R(\kappa_t \kappa_Z, C_{Stt} C_{SZZ}) < R^\text{up}$ with the imposed sum-rule $\kappa_t \kappa_Z + C_{Stt} C_{SZZ} = 1$ holds, while the hatched region is theoretically inaccessible for the assumed value of $\Gamma_S / m_S$. The red dashed line shows the limit of $\kappa_t^2 \kappa_Z^2 = 2.4$ that would be obtained for the case where the additional scalar is absent. The HiggsBounds limits from searches for additional scalars are indicated by the blue contours, and the blue area corresponds to the region that is allowed by these limits.
• Figure 5: Upper limits on the Higgs width ratio $\Gamma^H / \Gamma^H_\text{SM}$ as a function of $m_S$ for the case where an additional propagating scalar $S$ contributes in the off-shell region (but is assumed not to affect the on-shell region). The upper limits that are obtained from the combination of the on-shell and off-shell results are indicated by the dashed lines for two values of $\Gamma_S/m_S$. The upper bounds on $\Gamma^H / \Gamma^H_\text{SM}$ arising from the limits from direct Higgs searches implemented via HiggsBounds are shown by the solid lines, where the on-shell signal strength is used to infer an upper bound on the total width from the bound on the coupling modifiers.