Search for fractionally charged particles in $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$

A search is presented for free heavy long-lived fractionally charged particles produced in $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$. The data sample was recorded by the CMS detector at the LHC and corresponds to an integrated luminosity of $5.0{\mathrm{fb}}^{-1}$. Candidate fractionally charged particles are identified by selecting tracks with associated low charge measurements in the silicon tracking detector. Observations are found to be consistent with expectations for background processes. The results of the search are used to set upper limits on the cross section for pair production of fractionally charged, massive spin-$1/2$ particles that are neutral under $SU(3{)}_C$ and $SU(2{)}_L$. We exclude at 95% confidence level such particles with electric charge $\pm 2e/3$ with masses below 310 GeV, and those with charge $\pm e/3$ with masses below 140 GeV.

Figure 1

Distribution of $dE/dx$ measurements associated with tracks passing the preselection in the search sample and the $Z$-peak control sample. Simulated $L_q{\overline{L}}_q$ signal samples for a mass of 100 GeV are shown for $q=2/3$, $1/3$. The distributions are normalized to the area of the search sample. The magenta vertical line at $dE/dx=2\mathrm{MeV}/\mathrm{cm}$ indicates the upper limit of the range of measurements considered to be low ionizing. The inset is an enlargement of the region of low $dE/dx$, plotted on a semilogarithmic scale.

Figure 2

Number of events with at least one track with the given number of low $dE/dx$ measurements, for search data and the scaled $Z$-peak control sample background estimate. The binomial function fit to the control sample is shown, with the band representing its uncertainty. The ratio of the data to the binomial fit is also presented. No tracks in the search sample have five or more low $dE/dx$ measurements.

Figure 3

Expected and observed limits on the cross section of $L_q{\overline{L}}_q$ for $q=2/3$ and $1/3$. The theoretical prediction of modified Drell-Yan production with $t_{3L}=0$ is shown. The lines for the expected and observed limits are overlapping.