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Search for Higgs-like bosons decaying into long-lived exotic particles

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Abstract

A search is presented for massive long-lived particles, in the 20-60 GeV mass range with lifetimes between 5 and 100 ps. The dataset used corresponds to 0.62 1\fb of proton-proton collision data collected by the LHCb detector at sqrt(s)=7 TeV. The particles are assumed to be pair-produced by the decay of a Higgs-like boson with mass between 80 and 140 GeV. No excess above the background expectation is observed and limits are set on the production cross-section as a function of the long-lived particle mass and lifetime and of the Higgs-like boson mass.

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Data (black dots) and simulated distributions after preselection normalised to unit integral. There are two LLP candidates per event. The simulated $ b \overline b $ background is shown by the filled red histograms with error bars. The dashed (blue), dotted (purple) and solid (green) lines are distributions for fully simulated signal models. The subplots show (a) number of tracks used to reconstruct the LLP candidates, (b) LLP transverse momentum, (c) LLP invariant mass, (d) radial distance, $ R_{\rm xy}$ , (e) uncertainty of the radial position, $\sigma_{\rm R}$ , and (f) uncertainty of the longitudinal position, $\sigma_{\rm Z}$ , of the LLP vertex.

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Distributions for (a) the LLP distance of flight from the PV, and, (b), the radial distance of the LLP vertex, $ R_{\rm xy}$ . The fully simulated signal models are chosen with LLP lifetimes of 5, 10, and 50 $ {\rm \,ps}$ . Symbols are defined as in Fig. 1.

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Distributions for (a) the $p_{\rm T}$ of the Higgs-like candidate, and (b), its invariant mass. Symbols are defined as in Fig. 1.

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Results of the fit based on the model BV48 10ps mH114. In (a) log distribution and (b) linear scale with pull distribution. Dots with error bars are the data, the dotted (red) and the dashed (green) histograms show the fitted background and signal contributions, respectively. The purple histogram is the total fitted distribution.

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Results of the fit based on the model BV48 10ps mH114, for different combinations of signal and background selections, (a) signal from $\rm Sel_1$ and background from $\rm Bkg_2$ , (b) signal from $\rm Sel_2$ and background from $\rm Bkg_1$ . Dots with error bars are data, the dashed (green) line is the fitted signal and the dotted (red) line the background. In both cases the fitted signal is negative. The histogram (blue) is the total fitted function.

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Results of the fit to the data to which 10 signal events have been added randomly chosen following the signal model. For the theoretical model BV48 10ps mH100, in (a), the fitted signal is $11.1\pm7.0$ events; for BV48 10ps mH125, in (b), the result is $9.3\pm5.6$ events.

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Expected (open dots with 1$\sigma$ and 2$\sigma$ bands) and observed (full dots) upper limits at 95% confidence level, (a) -- (c) shown for different masses of the Higgs-like particle, (d) and (f) for different LLP lifetimes, and (e) as a function of the LLP mass. The values of the other parameters are indicated on the plots. Results inferred from the fast simulation.

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Tables and captions

Definition of the criteria used for the signal determination. Selections $\rm Sel_1$ and $\rm Bkg_1$ are the baseline selections used in the fit, $\rm Sel_2$ and $\rm Bkg_2$ are used for the determination of systematic effects. The material veto and the requirement $ R_{\rm xy} >0.4$ $\rm \,mm$ are applied to both LLP candidates. The last column gives the number of data events selected, for a di-LLP reconstructed mass above 19 $ {\mathrm{\,GeV\!/}c^2}$ .

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Values of the fitted signal and background events for the different fully simulated signal models. The signal/background combinations are defined in the first row.

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Contributions to the systematic uncertainty for fully simulated models. For the analysis based on the fast simulation the same total systematic uncertainty is adopted augmented by 5% to account for the relative imprecision of the fast and full simulations. The contributions from the signal and the data-driven background models used in the di-LLP mass fit are discussed in the text.

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Detection efficiency with total uncertainty, and upper limits at 95% CL on the cross-section times branching ratio for the process $ pp \rightarrow \mathrm{h}^0 X$, $\mathrm{h}^0 \rightarrow\tilde{\chi}^{0}_{1} \tilde{\chi}^{0}_{1} \rightarrow 6q$ for the fully simulated models.

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Parameters of the signal models generated by Pythia and fully simulated.

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Detection efficiency values in percent estimated by the fast simulation as a function of $ m_{\rm h^0}$ and $ m_{\rm{LLP}}$ . The LLP lifetime is 10 ps. The statistical uncertainty is 10% for $\epsilon \sim 0.02\%$, 5 \% for $\epsilon \sim 0.1\%$, 3\% for $\epsilon \sim 0.5\%$, and 2\% for $\epsilon \sim 1\%$.

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Detection efficiency in percent estimated by the fast simulation as a function of the $ m_{\rm{LLP}}$ and $\tau_{\rm{LLP}}$ , for $ m_{\rm h^0}$ =114 GeV/c$^2$. The statistical uncertainty is 10% for $\epsilon \sim 0.02\%$, 5 \% for $\epsilon \sim 0.1\%$, 3\% for $\epsilon \sim 0.5\%$, and 2\% for $\epsilon \sim 1\%$.

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Expected and observed 95% CL cross-section times branching ratio upper limits as a function of $ m_{\rm h^0}$ , with $ m_{\rm{LLP}} =35 {\mathrm{\,GeV\!/}c^2} $, and $\tau_{\rm{LLP}} =10 {\rm \,ps} $, estimated by the fast simulation.

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Expected and observed 95% CL cross-section times branching ratio upper limits as a function of $ m_{\rm h^0}$ , for LLP masses of 40, 48, 55, and 60 $ {\mathrm{\,GeV\!/}c^2}$ , $\tau_{\rm{LLP}} =10 {\rm \,ps} $, estimated by the fast simulation.

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Expected and observed 95% CL cross-section times branching ratio upper limits as a function of the LLP lifetime, for $ m_{\rm h^0} = 100 {\mathrm{\,GeV\!/}c^2} $ and $ m_{\rm{LLP}} =40 {\mathrm{\,GeV\!/}c^2} $, and for $ m_{\rm h^0} = 125 {\mathrm{\,GeV\!/}c^2} $ and $ m_{\rm{LLP}} =48 {\mathrm{\,GeV\!/}c^2} $, estimated by the fast simulation.

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Expected and observed 95% CL cross-section times branching ratio upper limits as a function of the LLP mass, with $ m_{\rm h^0} =125 {\mathrm{\,GeV\!/}c^2} $ and $\tau_{\rm{LLP}} =10 {\rm \,ps} $, estimated by the fast simulation.

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Created on 09 December 2018.Citation count from INSPIRE on 09 December 2018.