A search is presented for longlived particles with a mass between 25 and 50 GeV$/c^2$ and a lifetime between 1 and 200 ps in a sample of protonproton collisions at a centreofmass energy of $\sqrt{s}=7$ TeV, corresponding to an integrated luminosity of 0.62 fb$^{1}$, collected by the LHCb detector. The particles are assumed to be pairproduced by the decay of a Standard Modellike Higgs boson. The experimental signature of the longlived particle is a displaced vertex with two associated jets. No excess above the background is observed and limits are set on the production crosssection as a function of the longlived particle mass and lifetime.
Invariant mass (a) and $p_{\rm T}$ {} distribution (b) for dijet candidates in data and in hidden valley models with \num{25}, \num{35} and 50 $ {\mathrm{\,GeV\!/}c^2}$ $ {\pi_v}$ masses and 10 $ {\rm \,ps}$ lifetime. For visibility, the simulated signal is scaled to 0.62 $\,fb^{1}$ assuming a Higgs crosssection of 10 $\rm \,nb$ and branching fractions of \SI{100}{%} for ${\cal B}(H\rightarrow {\pi_v} {\pi_v} )$ and ${\cal B}( {\pi_v} \rightarrow b \overline b )$. 
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Distribution of the distance of the displaced vertex to the interaction region in the transverse plane for data and for a hidden valley model with $m_{ {\pi_v} } = 35 {\mathrm{\,GeV\!/}c^2} $ and $\tau_{ {\pi_v} } =10 {\rm \,ps} $ after the full selection. For visibility, the simulated signal is scaled to 0.62 $\,fb^{1}$ assuming a Higgs crosssection of 10 $\rm \,nb$ and branching fractions of \SI{100}{%} for ${\cal B}(H\rightarrow {\pi_v} {\pi_v} )$ and ${\cal B}( {\pi_v} \rightarrow b \overline b )$. The boundaries of the intervals used in the fit are indicated by the dotted lines. The generated $ R_{xy}$ {} distribution is approximately exponential with an average of about 2 $\rm \,mm$ . 
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Dijet invariant mass distributions for each of the five $ R_{xy}$ bins, superimposed with the fits for a hidden valley model with $m_{ {\pi_v} } = 35 {\mathrm{\,GeV\!/}c^2} $ and $\tau_{ {\pi_v} } =10 {\rm \,ps} $. The blue line indicates the result of the total fit to the data. The black shortdashed line is the backgroundonly contribution, and the red longdashed line is the fitted signal contribution. For illustration, the green dashdotted line shows the signal scaled to a crosssection of 17 $\rm \,pb$ , which corresponds to the SM Higgs production crosssection at 7 TeV \protec [35]. 
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Observed \SI{95}{%} CL crosssection upper limits on a hidden valley model [6] for various $ {\pi_v}$ masses, as a function of $ {\pi_v}$ {} lifetime. Both $ {\pi_v}$ particles are assumed to decay into $ b \overline b $ , unless specified otherwise. 
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Animated gif made out of all figures. 
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Average number of selected candidates per event (efficiency) in %{} for the main stages of the offline selection for simulated $H^0 \rightarrow {\pi_v} {\pi_v} $ events with $ {\pi_v} \rightarrow b \overline b $, $m_{H^0} =120 {\mathrm{\,GeV\!/}c^2} $, $m_{ {\pi_v} } = 35 {\mathrm{\,GeV\!/}c^2} $ and $\tau_{ {\pi_v} } =10 {\rm \,ps} $. The preselection consists of the acceptance, trigger and offline vertex reconstruction. It represents the first stage in which the candidate yield on the total data sample, shown in the right column, can be counted. The reported uncertainty on the efficiency is only the statistical uncertainty from the finite sample size. 
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Average number of selected candidates per event (efficiency) in %{} for different $ {\pi_v}$ masses, lifetimes and decay modes. The reported uncertainty is only the statistical uncertainty from the finite sample size. No simulated samples were generated for the 100 $ {\rm \,ps}$ decay to light quarks. 
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Systematic uncertainties on the selection efficiency and luminosity for simulated hidden valley events with a lifetime of 10 $ {\rm \,ps}$ and various $ {\pi_v}$ masses. 
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Observed \SI{95}{%} CL crosssection upper limits on $\sigma(H)\times {\cal B}(H\rightarrow {\pi_v} {\pi_v} )$ (in { $\rm \,pb$ }) on a hidden valley [6] model for various $ {\pi_v}$ masses and lifetimes. Both $ {\pi_v}$ particles are assumed to decay into $ b \overline b $ , unless specified otherwise. 
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Created on 09 December 2018.Citation count from INSPIRE on 09 December 2018.