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Search for massive long-lived particles decaying semileptonically in the LHCb detector

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Abstract

A search is presented for massive long-lived particles decaying into a muon and two quarks. The dataset consists of proton-proton interactions at centre-of-mass energies of 7 and 8 TeV, corresponding to integrated luminosities of 1 and 2 1/fb, respectively. The analysis is performed assuming a set of production mechanisms with simple topologies, including the production of a Higgs-like particle decaying into two long-lived particles. The mass range from 20 to 80 GeV and lifetimes from 5 to 100 ps are explored. Results are also interpreted in terms of neutralino production in different supersymmetric models, with masses in the 23-198 GeV range. No excess above the background expectation is observed and upper limits are set on the production cross-section for various points in the parameter space of theoretical models.

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Four topologies considered as representative LLP production mechanisms: $ P\!A$ non-resonant direct double LLP production, $ P\!B$ single LLP production, $ P\!\,C$ double LLP production from the decay of a Higgs-like boson, $ P\!D$ double LLP indirect production via squarks.

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Distributions for the 7 TeV dataset (black histogram) compared to simulated $ b \overline b $ events (blue squares with error bars), showing a) transverse momentum and b) isolation of the muon, c) number of tracks of the displaced vertex, d) reconstructed mass, e) radial position of the vertex, f) and g) vertex fit uncertainties in the radial and z directions. The fully simulated signal distributions for LV38 10 $ {\mathrm{ \,ps}}$ are shown (red dashed histograms), as well as LV38 50 $ {\mathrm{ \,ps}}$ (green dotted histogram) in e). The distributions from simulation are normalised to the number of data entries.

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Reconstructed mass of the LLP candidate from the 8 TeV dataset. The top plots correspond to events with candidates selected from the background region of the muon isolation variable. They are fitted with the sum of two exponential functions. In the bottom row the candidates from the signal region are fitted including a specific signal shape, added to the background component. Subfigures a) and c) correspond to the analysis which assumes the LV38 5 $ {\mathrm{ \,ps}}$ signal model, b) and d) are for LV98 10 $ {\mathrm{ \,ps}}$ .

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Expected (open dots with 1$\sigma$ and 2$\sigma$ bands) and observed (full dots) cross-section times branching fraction upper limits at 95% confidence level, as a function of the LLP mass from the 8 TeV dataset. The theoretical models assume the full set of SUSY production processes available in Pythia 6 with default parameter settings, unless otherwise specified. The gluino mass is 2000 $ {\mathrm{\,GeV\!/}c^2}$ .

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Expected (open dots and 1$\sigma$ and 2$\sigma$ bands) and observed (full dots) cross-section times branching fraction upper limits (95% CL) for the processes indicated in the bottom left corner of each plot, $\tau_{\rm{LLP}}$ is always 10 $ {\mathrm{ \,ps}}$ . The results correspond to the 8 TeV dataset. a) upper limits as a function of the LLP mass for process $ P\!A$ ; b) as a function of the LLP mass for process $ P\!B$ , with $ m_{\rm ``\tilde{g}"} =100$ $ {\mathrm{\,GeV\!/}c^2}$ ; c) as a function of $ m_{\rm h^0}$ for process $ P\!\,C$ for $ m_{\rm{LLP}}$ of 20, 40, and 60 $ {\mathrm{\,GeV\!/}c^2}$ , from top to bottom (the single point at 130 $ {\mathrm{\,GeV\!/}c^2}$ with $ m_{\rm{LLP}} =60 {\mathrm{\,GeV\!/}c^2} $ has been shifted to the right for visualisation); d) upper limits as a function of the LLP mass for process $ P\!D$ with $ m_{\tilde{\rm q}} =60$ $ {\mathrm{\,GeV\!/}c^2}$ .

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Expected (open dots with 1$\sigma$ and 2$\sigma$ bands) and observed (full dots) cross-section times branching fraction upper limits (95% CL) for the processes $ P\!\,C$ as a function of the LLP mass; the LLP lifetime $\tau_{\rm{LLP}}$ is indicated in each plot, $ m_{\rm h^0} =125 {\mathrm{\,GeV\!/}c^2} .$ The results correspond to the 8 TeV dataset.

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Tables and captions

Total signal detection efficiency $\epsilon$, including the geometrical acceptance, and numbers of fitted signal and background events, $N_{\rm s}$ and $N_{\rm b}$, for the different signal hypotheses. The last column gives the value of $\chi^2/\mathrm{ndf}$ from the fit. The signal models are from the full simulation. Uncertainties are explained in Sect. 6.

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Summary of the contributions to the relative systematic uncertainties, corresponding to the 8 TeV dataset, (the sub-total for the 7 TeV dataset is also given). The indicated ranges cover the fully simulated LV models. The detection efficiency is affected by the parton luminosity model and depends upon the production process, with a maximum uncertainty of 7% for the gluon-gluon fusion process $ P\!\,C$ . For the fast simulation based analysis there is an additional contribution of 5%. The systematic effects associated with the signal and background models used in the LLP mass fit are not shown in the table.

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Upper limits (95% CL) on the production cross-section times branching fraction (pb) for the $\rm 7\,TeV$ and $\rm 8\,TeV$ datasets, based on the fully simulated LV signal samples.

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Parameters for the generation of the nine fully simulated signal models. The LLP is the lightest neutralino, $\tilde{\chi}^{0}_{1} $ with $ m_{\tilde{\chi}^{0}_{1} } = m_{\rm{LLP}} $; $M_1$ and $M_2$ are the Pythia parameters RMSS(1) and RMSS(2), $m_{\tilde{\rm g}}$ is RMSS(3), $\mu$ is RMSS(4), $\tan{\beta}$ RMSS(5) and $m_{\tilde{\rm q}}$ is RMSS(8-12). Samples with lifetime of 5, 10 and 50 $ {\mathrm{ \,ps}}$ have been produced for each mass.

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Supplementary Material [file]

Supplementary material full pdf

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supplementary.pdf : tables of cross-section times branching fraction upper limits. Complementary plots for the H(125)->LLP LLP decay. tables-LV-7TeV.dat, tables-LV-8TeV.dat : idem, flat format, for LV processes tables-PX-7TeV.dat, tables-PX-8TeV.dat : idem, processes PA, PB, PC and PD PAPER-2016-047_sup1 Limit as a function of the LLP lifetime and mass PAPER-2016-047_sup2 Excluded regions in the LLP lifetime-mass plane

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Created on 09 December 2018.Citation count from INSPIRE on 18 December 2018.