Search for a Light Pseudoscalar Higgs Boson in the Dimuon Decay Channel in $pp$ Collisions at $\sqrt{s}\mathbf{=}7\mathrm{TeV}$

The dimuon invariant mass spectrum is searched in the range between 5.5 and 14 GeV for a light pseudoscalar Higgs boson $a$, predicted in a number of new physics models, including the next-to-minimal supersymmetric standard model. The data sample used in the search corresponds to an integrated luminosity of $1.3{\mathrm{fb}}^{-1}$ collected in $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$ with the CMS detector at the LHC. No excess is observed above the background predictions and upper limits are set on the cross section times branching fraction $\sigma \times \mathcal{B}(pp\to a\to {\mu }^{+}{\mu }^{-})$ in the range of 1.5–7.5 pb. These results improve on existing bounds on the $ab\overline{b}$ coupling for $m_a<m_{{\rm Y}(1S)}$ and are the first significant limits for $m_a>m_{{\rm Y}(3S)}$. Constraints on the supersymmetric parameter space are presented in the context of the next-to-minimal model.

Figure 1

Dimuon invariant mass distribution for the barrel (upper) and end caps (lower) after the event selection. The invariant mass distributions are fitted accounting for the three ${\rm Y}$ resonances and QCD continuum. Hypothetical signals from pseudoscalar Higgs bosons $a$ at 7 and 12 GeV are shown.

Figure 2

Upper limits at 95% C.L. on $\sigma \times \mathcal{B}(pp\to a\to {\mu }^{+}{\mu }^{-})$ in mass range 1 (upper panel) and mass range 2 (lower panel) including systematic uncertainties. The dotted lines correspond to the expected limits, and the bands correspond to 1- and 2-$\sigma$ level uncertainties on the expected limits.

Figure 3

Upper limits on the NMSSM parameter $|\cos {\theta }_A|$ as a function of $m_{a_1}$ in the two mass ranges. The solid curves correspond to different $\tan \beta$ values: from top to bottom, $\tan \beta =1$, $\tan \beta =2$, $\tan \beta =3$, $\tan \beta =10$, $\tan \beta =30$, and $\tan \beta =50$. For each $\tan \beta$ value in mass range 1, the second, dotted curve shows the limits from the BABAR ${\rm Y}$ analysis. There are no BABAR limits for $\tan \beta =1$ in mass range 1, or for any $\tan \beta$ in mass range 2. The line at $|\cos {\theta }_A{|}^{\max }=1$ is equivalent to no limit. Results from CDF are not shown as they are less stringent than the BABAR limits.