Revealing timid pseudo-scalars with taus at the LHC

Abstract

A light pseudo-scalar that is copiously produced at the LHC may still be allowed by present searches. While masses above 65 GeV are effectively covered by di-photon searches, the lower mass window can be tested by a new search for boosted di-tau resonances. We test this strategy on a set of composite Higgs models with top partial compositeness, where most models can be probed with an integrated luminosity below 300 ${\text{fb}}^{-1}$ .

Fig. 1

Constraints on $f_a$ as a function of $m_a$ for the benchmark models M1 - M12, defined in Table 1. The bounds arise from di-muon searches [22, 23] in the low mass range, di-photon searches [3, 4] in the higher one, and from the BSM decay width of the Higgs [24] below 65 GeV. We have also indicated the current bounds obtained by adapting the results in [10] in the region between 20 and 65 GeV for the two models (M9 and M10) where they are the strongest

Fig. 2

Values of $f_a$ for models M1 - M12 for which $\mathcal{Z}\equiv S/\sqrt{B+{\delta }^2{B}^2}=3$ in the proposed di-tau search after an integrated Luminosity of 300 ${\text{fb}}^{-1}$. We assume a systematic error on the background $\delta =1\%$. Shown in grey are the current bounds as of Fig. 1

Fig. 3

Relative change ${\xi }_{f_a}\equiv f_a^{\delta ,\Delta R_{e\mu }}/f_a^{1\%,0}$ in the projected bounds on $f_a$ with 300 ${\text{fb}}^{-1}$ of data. We plot the relative change against the baseline presented in Fig. 2 for different values of systematic uncertainties $\delta =0,0.5,1,$ and $2\%$ (green, blue, black, and red) and choosing three different separation cuts $\Delta R_{e\mu }>0$ (solid) 0.1 (dashed) and 0.2 (dotted) respectively

Fig. 4

Angular separation ($\Delta R_{e\mu }$) between the two leptons for two signal (SG) masses (20 and 80 GeV) compared to the most relevant backgrounds (BG). Small separation angles can be a good discriminant particularly for low masses

Fig. 5

Top loop diagrams contributing to the effective coupling $h\to aa$

Fig. 6

Production cross section of a for LHC with $\sqrt{s}=7,8,13$ TeV, for $K_{g,\mathrm{eff}}/f_a=1/\text{TeV}$

Fig. 7

Main branching ratios of the TCP for the reference models M1-M12 (c.f. Table 1) as a function of $m_a$: $a\to {\tau }^{+}{\tau }^{-}$ (top-left), $a\to$ hadrons (top right), $a\to b\overline{b}$ (bottom-left) and $a\to \gamma \gamma$ (bottom-right)

Fig. 8

Projected bounds on $f_a$ from a boosted di-tau search with $\sqrt{s}=13$ TeV after an integrated Luminosity of 300 ${\text{fb}}^{-1}$ (red dashed, from Fig. 2) and from a boosted di-photon search with $\sqrt{s}=14$ TeV after an integrated Luminosity of 300 ${\text{fb}}^{-1}$ (blue solid, projections following Ref. [10])