The Social Higgs

TL;DR

The study investigates whether the observed $m_{ ilde{h}}=125.5$ GeV Higgs could mix with a neutral scalar via a Higgs portal, introducing a Higgs friend (mixing angle $\\theta$) and a Higgs accomplice (additional coupling $\\alpha$ to photons). It develops a likelihood framework to combine Higgs search channels and accounts for potential signal overlap between two scalars, then fits $m_{ ilde{s}}$, $\\theta$, $\\alpha$, and an invisible width parameter to ATLAS/CMS/Tevatron data. Results show only mild improvements over the SM for most parameter regions; a notable exception is an accomplice near $m_{ ilde{s}}\\approx210$ GeV, where the data modestly favor the extended scenario, though not with strong significance. Overall, large mixing is disfavored and there is no compelling evidence for a social Higgs, underscoring the need for more data to clarify potential BSM mixing in the Higgs sector.

Abstract

Using published Higgs search data we investigate whether any evidence supports the possibility that the Higgs may be mixed with other neutral scalars. We combine the positive evidence for the Higgs at 125.5 GeV with search constraints at other masses to explore the viability of two simple models. The first Higgs 'friend' model is simply a neutral scalar mixed with the Higgs. In the second Higgs 'accomplice' model the new scalar has an enhanced coupling to photons due to couplings to additional charged fields. We find that the latter scenario allows improvement in fitting the data by accommodating enhanced diphoton rates and suppression in other channels for a Higgs mass of 125.5 GeV. Small excesses at other masses allow the additional scalar to further improve the fit to the data, particularly if it has mass in the vicinity of 210 GeV. Due to observed event rates at 125.5 GeV and strong limits in high mass Higgs searches, mixing angles greater than pi/4 are typically disfavored at the 95% confidence level, depending on the mass of the scalar.

Figures (4)

• Figure 1: The best fit mixing angle as a function of the Higgs friend mass, $m_{\tilde{s}}$, for the combined likelihood with signal overlap included (black) and omitted (red). $95\%$ confidence bands are also shown. Above $m_{\tilde{s}} \sim210$ GeV the difference between both methods becomes negligible, demonstrating that above this mass the simple product of individual likelihoods can be trusted. Below this mass the overlap of signal becomes important, suggesting that the simple individual likelihood products lose accuracy.
• Figure 2: The best fit mixing angle as a function of the scalar mass, $m_{\tilde{s}}$, at high masses. The limit of vanishing $\tilde{s}\rightarrow 2 \tilde{h}$ branching ratio is shown in black and for $\kappa = 0.5$ (see Eq. (\ref{['eq:BR']})) in blue. $95\%$ confidence contours are also shown (dashed) in corresponding colors. On the left panel we set $\alpha=0$ and allow only mixing with the Higgs friend, with $95\%$ confidence contours determined via $\Delta (-2 \log\lambda) = 3.84$. On the right panel we allow for enhanced $\tilde{h} \rightarrow \gamma \gamma$ decays in the Higgs accomplice scenario and find the best fit values of $\theta$ and $\alpha$. In this case we find $95\%$ confidence contours by finding the maximum value of $\theta$ for which $\Delta (-2 \log\lambda) = 5.99$. On both panels it is clear that due to strong limits in the region $380$ GeV $\lesssim m_{\tilde{s}}\lesssim 450$ GeV the SM is preferred over both scenarios. However, due to mild excesses or weak limits at other masses both scenarios can slightly improve the fit to the data in comparison to the SM. Due to a small excess in $h\rightarrow ZZ$ events in ATLAS and CMS, around $m_{\tilde{s}} \approx 210$ GeV the accomplice scenario becomes marginally preferred over the SM at $95\%$, while for the friend scenario the SM lies on the $95\%$ confidence contour. Following the discussion of Sec. \ref{['sec:estmate']} the reader should keep in mind that in the region below $m_{\tilde{s}}\approx 210$ GeV some error is introduced by neglecting signal overlap.
• Figure 3: Best fit points and $68\%$ and $95\%$ confidence contours, corresponding to $\Delta (-2 \log\lambda) = 2.28, 5.99$ for the specific scenario of a Higgs accomplice beyond collider reach (left panel) and a Higgs accomplice at $210$ GeV (right panel). The left panel shows the fit for the Higgs to solely the $125.5$ GeV data since the accomplice is decoupled. Due to enhancement of $\tilde{h} \rightarrow \gamma \gamma$ and suppression of other channels, non-zero mixing angles are preferred, alongside $\mathcal{O} (1)$ values of $\alpha$. The SM, $\sin^2(\theta)=0$, is within the $95\%$ confidence contour. The change in $-2 \log\lambda$, which can be interpreted as the change in $\chi^2$, from the best fit point to the SM is also shown, and is marginally greater than the number of extra parameters introduced. When signal from the Higgs accomplice is included at $210$ GeV, and the likelihoods for both scalars are combined (right panel), the overall fit is improved significantly and the SM becomes marginally disfavored at greater than $95\%$.
• Figure 4: Contours of $\Delta S = S(\tilde{h},\tilde{s},\theta) - S (h)$ and $\Delta T = T(\tilde{h},\tilde{s},\theta) - T (h)$, for the simple Higgs friend model. We have set $m_{\tilde{h}} = m_h = 125.5$ GeV. For the majority of parameter space this model is consistent with electroweak precision data at $1 \sigma$.