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{ Ge V /}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 ${\cal B}( {\pi_v} \rightarrow b \overline b )$. 
Fig1a.pdf [24 KiB] HiDef png [787 KiB] Thumbnail [365 KiB] *.C file 

Fig1b.pdf [28 KiB] HiDef png [946 KiB] Thumbnail [416 KiB] *.C file 

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{ Ge V /}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 $\rm mm$ . 
Fig2.pdf [22 KiB] HiDef png [488 KiB] Thumbnail [259 KiB] *.C file 

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{ Ge V /}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]. 
Fig3.pdf [28 KiB] HiDef png [721 KiB] Thumbnail [554 KiB] *.C file 

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. 
Fig4.pdf [14 KiB] HiDef png [334 KiB] Thumbnail [218 KiB] *.C file 

Animated gif made out of all figures. 
PAPER2014062.gif Thumbnail 
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{ Ge V /}c^2} $, $m_{ {\pi_v} } = 35 {\mathrm{ Ge V /}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. 
Table_1.pdf [31 KiB] HiDef png [48 KiB] Thumbnail [23 KiB] tex code 

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. 
Table_2.pdf [53 KiB] HiDef png [53 KiB] Thumbnail [26 KiB] tex code 

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. 
Table_3.pdf [40 KiB] HiDef png [89 KiB] Thumbnail [41 KiB] tex code 

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. 
Table_4.pdf [43 KiB] HiDef png [71 KiB] Thumbnail [34 KiB] tex code 
Supplementary material full pdf 
supple[..].pdf [255 KiB] 

This ZIP file contains supplemetary material for the publication LHCbPAPER2014062. The files are: supplementary.pdf : An overview of the extra figures *.pdf, *.png, *.eps : The figures in variuous formats 
Fig1aS.pdf [10 KiB] HiDef png [382 KiB] Thumbnail [300 KiB] *C file 

Fig1bS.pdf [10 KiB] HiDef png [399 KiB] Thumbnail [301 KiB] *C file 

Fig1cS.pdf [10 KiB] HiDef png [390 KiB] Thumbnail [295 KiB] *C file 

Fig1dS.pdf [10 KiB] HiDef png [374 KiB] Thumbnail [284 KiB] *C file 

Fig2S.pdf [13 KiB] HiDef png [188 KiB] Thumbnail [64 KiB] *C file 
Created on 19 October 2019.