CMS-HIG-15-001

CMS-HIG-15-001 ; CERN-EP-2016-039
Search for neutral resonances decaying into a Z boson and a pair of b jets or $\tau$ leptons
CMS Collaboration
9 March 2016
Phys. Lett. B 759 (2016) 369
Abstract: A search is performed for a new resonance decaying into a lighter resonance and a Z boson. Two channels are studied, targeting the decay of the lighter resonance into either a pair of oppositely charged $\tau$ leptons or a $\mathrm{ b \bar{b} }$ pair. The Z boson is identified via its decays to electrons or muons. The search exploits data collected by the CMS experiment at a centre-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 19.8 fb$^{-1}$. No significant deviations are observed from the standard model expectation and limits are set on production cross sections and parameters of two-Higgs-doublet models.
Links: e-print arXiv:1603.02991 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Observed 95% CL upper limits on $\sigma _{\mathrm { {\mathrm {H}} / {\mathrm {A}} } \to {\mathrm {Z}} \mathrm { {\mathrm {A}} / {\mathrm {H}} } \to \ell \ell {\mathrm {b}} {\mathrm {b}} }$ as a function of $m_{ {\mathrm {A}} }$ and $m_{ {\mathrm {H}} }$.
png pdf	Figure 2-a: (a) The $m_{ {\mathrm {b}} {\mathrm {b}} }$ spectrum for events selected in the 222 $ < m_{\ell \ell {\mathrm {b}} {\mathrm {b}} }< $ 350 GeV region for data and simulation and the relative ratio. (b) The $m_{\ell \ell {\mathrm {b}} {\mathrm {b}} }$ spectrum for events selected inside the region 72 $ < m_{ {\mathrm {b}} {\mathrm {b}} } < $ 114 GeV region for data and simulation and the relative ratio. The signal corresponding to $m_{ {\mathrm {H}} }= $ 270 GeV and $m_{ {\mathrm {A}} }= $ 104 GeV, normalized to the NNLO SusHi cross section, is superimposed for $\tan \beta = $ 1.5 and $\cos(\beta -\alpha )= $ 0.01 in the 2HDM type-II scenario. The overall systematic uncertainties in the simulation are reported as a hatched band.
png pdf	Figure 2-b: (a) The $m_{ {\mathrm {b}} {\mathrm {b}} }$ spectrum for events selected in the 222 $ < m_{\ell \ell {\mathrm {b}} {\mathrm {b}} }< $ 350 GeV region for data and simulation and the relative ratio. (b) The $m_{\ell \ell {\mathrm {b}} {\mathrm {b}} }$ spectrum for events selected inside the region 72 $ < m_{ {\mathrm {b}} {\mathrm {b}} } < $ 114 GeV region for data and simulation and the relative ratio. The signal corresponding to $m_{ {\mathrm {H}} }= $ 270 GeV and $m_{ {\mathrm {A}} }= $ 104 GeV, normalized to the NNLO SusHi cross section, is superimposed for $\tan \beta = $ 1.5 and $\cos(\beta -\alpha )= $ 0.01 in the 2HDM type-II scenario. The overall systematic uncertainties in the simulation are reported as a hatched band.
png pdf	Figure 3: svfit mass distributions for different final states of the $ {\mathrm {H}} \to {\mathrm {Z}} {\mathrm {A}} \to \ell \ell \tau \tau $ process, where the $ {\mathrm {Z}} $ boson decays to $ {\mathrm {e}} {\mathrm {e}}$ (right column) and $\mu \mu $ (left column). The expected signal corresponding to $m_{ {\mathrm {H}} }= $ 350 GeV and $m_{ {\mathrm {A}} }= $ 90 GeV, whose cross section times branching fraction is normalised to the NNLO SusHi prediction, is superimposed for $\tan \beta = $ 1.5 and $\cos(\beta -\alpha )= $ 0.01 in the 2HDM type-II scenario. Only statistical uncertainties are reported as a hatched band.
png pdf	Figure 4: Observed 95% CL upper limits on $\sigma _{\mathrm { {\mathrm {H}} / {\mathrm {A}} }\to \mathrm { {\mathrm {Z}} {\mathrm {A}} / {\mathrm {H}} } \to \ell \ell \tau \tau }$ as a function of $m_{ {\mathrm {A}} }$ and $m_{ {\mathrm {H}} }$.
png pdf	Figure 5: Observed limits on the signal strength $ \mu = \sigma _{95\%} / \sigma _{\mathrm {th}} $ for the 2HDM benchmark, after combining results from $\ell \ell {\mathrm {b}} {\mathrm {b}} $ and $\ell \ell {\tau } {\tau } $ final states. The cross sections are normalised to the NNLO SusHi prediction, for a 2HDM type-II scenario with $\tan \beta = $ 1.5 and $\cos(\beta -\alpha )= $ 0.01. The dashed contour shows the region expected to be excluded. The solid contour shows the region excluded by the data.
png pdf	Figure 6: Observed limits on the signal strength $\mu =\sigma _{95\%}/\sigma _{\mathrm {th}}$ for the 2HDM benchmark after combining results from $\ell \ell {\mathrm {b}} {\mathrm {b}} $ and $\ell \ell {\tau } {\tau } $ final states. The cross sections are normalised to the NNLO SusHi prediction. Limits are shown in the 2HDM parameters $\cos(\beta -\alpha )$ and ${\tan \beta }$ for the signal masses of $m_{ {\mathrm {H}} }= $ 378 GeV and $m_{ {\mathrm {A}} }= $ 188 GeV. The dashed contour shows the region expected to be excluded. The solid contour shows the region excluded by the data.

Tables
png pdf	Table 1: Summary of systematic uncertainties for both $\ell \ell \tau \tau $ and $\ell \ell {\mathrm {b}} {\mathrm {b}} $ final states.

Summary

The paper describes the first CMS search for a new resonance decaying into a lighter resonance and a Z boson. Two searches have been performed, targeting the decay of the lighter resonance into either a pair of oppositely charged $\tau$ leptons or a $\mathrm{ b \bar{b} }$ pair. The Z boson is identified via its decays to electrons or muons. The search is based on data corresponding to an integrated luminosity of 19.8 fb$^{-1}$ in proton-proton collisions at $\sqrt{s}=$ 8 TeV. Deviations from the SM expectations are observed with a global significance of less than 2 SD and upper limits on the product of cross section and branching fraction are set. The search excludes $\sigma \,\mathcal{B}$ as low as 5 fb and 1 fb for the $\ell\ell\mathrm{ b }\mathrm{ b }$ and $\ell\ell\tau\tau$ final states, respectively, depending on the light and heavy resonance mass values. Limits are also set on the mass parameters of the type-II 2HDM model that predicts the processes $\mathrm{ H }\to\mathrm{ Z }\mathrm{A}$ and $\mathrm{A}\to\mathrm{ Z }\mathrm{ H }$, where H and A are CP-even and CP-odd scalar bosons, respectively. Combining the $\ell\ell\mathrm{ b }\mathrm{ b }$ and $\ell\ell\tau\tau$ final states, the specific model corresponding to the parameter choice $\cos(\beta -\alpha)=$ 0.01 and $\tan{\beta}=$1.5 is excluded for $m_{\mathrm{ H }}$ in the range 200-700 GeV and $m_{\mathrm{A}}$ in the range 20-270 GeV with $m_{\mathrm{ H }}>m_{\mathrm{A}}$, or alternatively for $m_{\mathrm{A}}$ in the range 200-700 GeV and $m_{\mathrm{ H }}$ in the range 120-270 GeV with $m_{\mathrm{A}}>m_{\mathrm{ H }}$. Limits on the signal cross section modifier are also derived as a function of $\tan{\beta}$ and $\cos(\beta-\alpha)$ parameters. As a result, for specific $m_{\mathrm{ H }}$-$m_{\mathrm{A}}$ mass values, a fairly large region in the parameter space $\tan{\beta}$ vs $\cos(\beta-\alpha)$ is excluded. This covers a region unexplored so far, that cannot be probed by studying production and decay modes of the SM-like Higgs boson. In particular, for $m_{\mathrm{ H }}=$ 378 GeV and $m_{\mathrm{A}}=$ 188 GeV, a range where $\tan{\beta}$ lies between 0.5 and 2.3 and $\cos(\beta-\alpha)$ between $-0.7$ and $0.3$ is excluded, after the combination of the $\ell\ell\mathrm{ b }\mathrm{ b }$ and $\ell\ell\tau\tau$ final states.

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