Decay-mode independent searches for new scalar bosons with the OPAL detector at LEP

TL;DR

<3-5 sentence high-level summary>The OPAL collaboration conducts decay-mode independent searches for neutral scalar bosons $S^0$ produced with $Z^0$ in $e^+e^-$ collisions, using recoil-mass spectra from leptonic $Z^0$ decays and a $Z^0\to\nu\bar{\nu}$ channel to cover all possible $S^0$ decays, including long-lived and invisible states. They interpret results both for a single narrow scalar and for continuous-mass scenarios—the Uniform Higgs and Stealthy Higgs models—through a coupling-density function $\tilde{K}(m)$ and a width-enhanced Higgs, respectively, enabling decay-mode independent constraints. No excess over SM backgrounds is found, and 95% CL upper limits on the cross section are set via a scale factor $k$ relative to the SM Higgs-strahlung cross section, with $k^{95}_{obs}<0.1$ for $1\,\mathrm{keV}<m_{S^0}<19$ GeV and $k^{95}_{obs}<1$ up to $m_{S^0}\lesssim81$ GeV. The results also provide continuum limits on $\tilde{K}(m)$, exclusions in the $m_A$–$m_B$ plane for the Uniform Higgs scenario, bin-by-bin $\tilde{K}(m)$ measurements, and Stealthy Higgs constraints in the $\omega$–$m_H$ plane, thus extending sensitivity to broad Higgs sectors at LEP energies.

Abstract

This paper describes topological searches for neutral scalar bosons S0 produced in association with a Z0 boson via the Bjorken process e+e- -> S0Z0 at centre-of-mass energies of 91 GeV and 183-209 GeV. These searches are based on studies of the recoil mass spectrum of Z0 -> e+e- and mu+mu- events on a search for S0Z0 with Z0 -> nunu bar and S0 -> e+e- or photons. They cover the decays of the S0 into an arbitrary combination of hadrons, leptons, photons and invisible particles as well as the possibility that it might be stable. No indication for a signal is found in the data and upper limits on the cross section of the Bjorken process are calculated. Cross-section limits are given in terms of a scale factor k with respect to the Standrad Model cross section for the Higgs-strahlung process e+e- -> H0smZ0. These results can be interpreted in general scenarios independently of the decay modes of the S0. The examples considered here are the production of a single new scalar particle with a decay width smaller than the detector mass resolution, and for the first time, two scenarios with continuous mass distributions, due to a single very broad state or several states close in mass.

Figures (14)

• Figure 1: Cut variables for $\mathrm{Z}^{0}\xspace \to \mathrm{e}^+\mathrm{e}^-\xspace$ at $\sqrt{s}$ = 91.2 GeV. The OPAL data are indicated by dots with error bars (statistical error), the four-fermion background by the light grey histograms and the two-fermion background by the medium grey histograms. The signal distributions from a 1.2 $\hbox{GeV}$$\mathrm{S}^0$ are plotted as dashed lines and those from a 30 $\hbox{GeV}$$\mathrm{S}^0$ as dotted lines, respectively. The signal histograms are normalised corresponding to 0.1 and 1.5 times of the Standard Model Higgs-strahlung cross section and show the decay channel $\mathrm{S}^0\xspace \to \mathrm{gg}$. Each variable is shown with the cuts applied before the cut on this variable is done, respecting the order of cuts in Table \ref{['t:cutflow_LEP1']}. The arrows indicate the accepted regions. The histograms in Figure a) have non-constant bin widths.
• Figure 2: Cut variables for $\mathrm{Z}^{0}\xspace \to \mu^+\mu^-\xspace$ at $\sqrt{s}$ = 91.2 GeV. The OPAL data are indicated by dots with error bars (statistical error), the four-fermion background by the light grey histograms and the two-fermion background by the medium grey histograms. The signal distributions from a 1.2 $\hbox{GeV}$$\mathrm{S}^0$ are plotted as dashed lines and those from a 30 $\hbox{GeV}$$\mathrm{S}^0$ as dotted lines, respectively. The signal histograms are normalised corresponding to 0.1 and 1.5 times of the Standard Model Higgs-strahlung cross section and show the decay channel $\mathrm{S}^0\xspace \to \mathrm{gg}$. Each variable is shown with the cuts applied before the cut on this variable is done, respecting the order of cuts in Table \ref{['t:cutflow_LEP1']}. The arrows indicate the accepted regions. The histograms in Figure a) have non-constant bin widths.
• Figure 3: The efficiency versus the $\mathrm{S}^0$ mass at $\sqrt{s}=91.2$ GeV for a subset of decay modes of $\mathrm{S}^0$: a)+b) $\mathrm{Z}^{0}\xspace\to\mathrm{e}^+\mathrm{e}^-\xspace$ in linear and logarithmic mass scale; c)+d) $\mathrm{Z}^{0}\xspace\to\mu^+\mu^-\xspace$ in linear and logarithmic mass scale. The minimum efficiencies which are used in the limits are also shown. In the low mass region, below the threshold for the decays of the $\mathrm{S}^0$ into a pair of SM fermions, only the decays into photons or invisible particles are possible. For $m_{\mathrm{S}^0\xspace}\xspace \lesssim \Gamma_{\mathrm{Z}^{0}\xspace}$ the efficiency is almost flat. This is indicated by the dashed line which marks the average efficiency for $m_{\mathrm{S}^0\xspace}\xspace \le 1$$\hbox{GeV}$. The shaded bands show the typical error on the efficiencies in this region.
• Figure 4: The recoil mass spectra from $\sqrt{s}=$91.2 GeV a) for the decays $\mathrm{Z}^{0}\xspace \to \mathrm{e}^+\mathrm{e}^-\xspace$ and b) for $\mathrm{Z}^{0}\xspace \to \mu^+\mu^-\xspace$. OPAL data are indicated by dots with error bars (statistical error), the four-fermion background by the light grey histograms and the two-fermion background by the dark grey histograms. The dashed lines for the signal distributions are plotted on top of the background distributions with normalisation corresponding to the cross section excluded at 95% confidence level from the combination of both channels.
• Figure 5: Cut variables for $\mathrm{Z}^{0}\xspace \to \mathrm{e}^+\mathrm{e}^-\xspace$ at $\sqrt{s} = 183$--$209~\hbox{GeV}\xspace$. The OPAL data are indicated by dots with error bars (statistical error), the four-fermion background by the light grey histograms and the two-fermion background by the medium grey histograms. The signal distributions from a 30 GeV $\mathrm{S}^0$ are plotted as dashed lines and those from a 100 GeV $\mathrm{S}^0$ as dotted lines, respectively. The signal histograms are normalised to 10 and 100 times of the Standard Model Higgs-strahlung cross section, respectively, and show the decays $\mathrm{S}^0\xspace\to\mathrm{gg}$. Each variable is shown with the cuts applied before the cut on this variable is done, respecting the order of cuts in Table \ref{['t:cutflow_LEP2']}. The arrows indicate the accepted regions.