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Search for long-lived heavy charged particles using a ring imaging Cherenkov technique at LHCb

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Abstract

A search is performed for heavy long-lived charged particles using 3.0 fb$^{-1}$ of pp collisions collected at $\sqrt{s}$= 7 and 8 TeV with the LHCb detector. The search is mainly based on the response of the ring imaging Cherenkovdetectors to distinguish the heavy, slow-moving particles from muons. No evidence is found for the production of such long-lived states. The results are expressed as limits on the Drell-Yan production of pairs of long-lived particles, with both particles in the LHCb pseudorapidity acceptance, $1.8 < \eta < 4.9$. The mass-dependent cross-section upper limits are in the range 2-4 fb (at 95% CL) for masses between 124 and 309 GeV/c$^2$.

Figures and captions

CMSP velocity spectrum for the CMSP masses of 124 $ {\mathrm{\,GeV\!/}c^2}$ and 309 $ {\mathrm{\,GeV\!/}c^2}$ . The proton-proton centre-of-mass energy is 7 TeV. The dots with error bars show the efficiency to detect tracks as a function of the $\beta$ of the particle (right scale).

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Number of CMSP candidates, as a function of the four variables used as inputs to the ANN. There are two CMSP candidates per event. The black dots with error bars show the 2012 data. The dashed red histogram is the expected shape for 124 $ {\mathrm{\,GeV\!/}c^2}$ CMSPs and the blue histogram shows the background from $ Z /\gamma^{\star}$ decays into muons. The energy in the VELO is given in units of minimum ionising particle (MIP) deposition. The first bin of the histogram for $\Delta$E in the ECAL has been multiplied by a factor 0.25.

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Number of CMSP pairs, $N$, as a function of the pair significance. The left and right figures correspond to the CMSP masses of 124 and 309 $ {\mathrm{\,GeV\!/}c^2}$ , respectively. The black points with their statistical uncertainty show the 7 TeV (top) and 8 TeV (bottom) data sets. The red dashed histogram is the expected shape from CMSP pairs and the blue histogram the background; both are normalised to the number of events. The arrows indicate the chosen selection criteria.

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(a) DLLx distributions from muons selected from $Z$ decays in 7 TeV data (black points) and simulation (blue histogram). The expected shape from 124 $ {\mathrm{\,GeV\!/}c^2}$ and 309 $ {\mathrm{\,GeV\!/}c^2}$ CMSPs are also shown. (b) The simulated DLLx values are shifted in such a way that data and simulation have the same fraction of entries with $\mathrm{DLLx}>-5$.

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The DLLx variable for: (a) protons selected above the Cherenkov threshold, $ p > 30$ $ {\mathrm{\,GeV\!/}c}$ , in 7 TeV data (black line) and simulation (blue points); (b) and (c) below-threshold protons, $ p < 3.8 {\mathrm{\,GeV\!/}c} $, from data and simulation, respectively. The results from the fit of a Gaussian function plus polynomial to the below-threshold proton distributions are shown by the red curves.

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DLLx for protons with $ p < 3.8 {\mathrm{\,GeV\!/}c} $. In black the result with nominal simulation parameters are shown. (a) The red, dashed, and green, dash-dot, plots are for a change by +20% and -20% of the photon yield; (b) is for a change by +40% (red, dashed) and -40% (green, dash-dot) of the random noise probability.

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Upper limits at 95% CL on the cross-sections for the pair production of CMSPs in the LHCb acceptance (points) and the corresponding predictions assuming the Drell-Yan production of $\widetilde{\tau_1}$ (bands representing $\pm 1 \sigma$ uncertainty) with SPS7 parameters, for proton-proton collisions as a function of the CMSP mass at $\sqrt{s}$ =7 and 8 TeV.

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Tables and captions

Values of the mGMSB $\Lambda$ parameters in the SPS7 scenario used in this study, the corresponding masses of the $\widetilde{\tau_1}$ , $m_{\widetilde{\tau}}$, and the cross-section of the pair production at next-to-leading order. The last two columns give the detector acceptance $A$.

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Number of events with both CMSPs candidates satisfying $\rm DLLx>-5$, and the final efficiency, $\epsilon$, after the multivariate analysis selection, given for each mass hypothesis.

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Refractive indices and Cherenkov $\beta$ thresholds for the three radiators. The momentum threshold is given for muons, protons, and 124 and 309 $ {\mathrm{\,GeV\!/}c^2}$ CMSPs.

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Nominal values of the parameters used in the fast simulation.

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Systematic uncertainties (in %) for the selection of the signal of CMSP pairs and on the background retention. Where relevant, the lower value corresponds to the CMSP mass of 124 $ {\mathrm{\,GeV\!/}c^2}$ and the higher one to 309 $ {\mathrm{\,GeV\!/}c^2}$ .

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Cross-section upper limits at 95% CL for CMSP pair production in the LHCb acceptance in the 7 and 8 TeV.

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