CMS-EXO-19-006

CMS-EXO-19-006 ; CERN-EP-2024-002
Search for fractionally charged particles in proton-proton collisions at $\sqrt{s} =$ 13 TeV
CMS Collaboration
15 February 2024
Phys. Rev. Lett. 134 (2025) 131802
Abstract: A search is presented for fractionally charged particles with charge below 1 $e$ , using their small energy loss in the tracking detector as a key variable to observe a signal. The analyzed data set corresponds to an integrated luminosity of 138 fb $^{-1}$ of proton-proton collisions collected at $\sqrt{s} =$ 13 TeV in 2016-2018 at the CERN LHC. This is the first search at the LHC for new particles with a charge between $e$ /3 and 0.9 $e$ , including an extension of previous results at a charge of 2 $e$ /3. Masses up to 640 GeV and charges as low as $e$ /3 are excluded at 95% confidence level. These are the most stringent limits to date for the considered Drell-Yan-like production mode.
Links: e-print arXiv:2402.09932 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Feynman diagram of the kinetic mixing between the SM hypercharge field $B$ and a new U(1) gauge field $A'$ . In this work, FCPs are considered to be pair-produced through this process.
png pdf	Figure 2: The d $E$ /d $x$ distribution for hits on candidate tracks in the CR and SR in 2018 data. The last bin is an overflow bin. The vertical bars and the shaded area correspond to the statistical uncertainty in the SR and the CR, respectively.
png pdf	Figure 3: Distribution of $N_{\text{hits}}^{\text{low} $ e$/3}$ in the SR and the CR for the 2018 data set, as well as for an FCP signal at a mass of 100 GeV and different charge scenarios. The vertical bars and the shaded area correspond to the statistical uncertainty in the SR and the CR, respectively. The $p$ -value of the fit is 9%. The two lower panels show the ratio of the number of tracks observed in the CR (upper) and SR (lower), and the fit function. The vertical bars correspond to the uncertainty from statistical sources, while the shaded area shows the systematic uncertainty in the binomial fit (see text).
png pdf	Figure 4: Exclusion region (hatched) at 95% CL in the FCP charge-mass plane for the considered signal. The expected exclusion is shown with the associated 1 (green) and 2 (orange) standard deviations bands. Signal points at charges 0.9, 0.8, 2/3, 0.5, and 1 $/3e$ are connected by straight lines to guide the eye. This is a conservative interpolation. Previous exclusions from CMS [8,9] as well as OPAL [13] are given for comparison.
png pdf	Figure 5: Distribution of $N_{\text{hits}}^{\text{low}\ \mathrm{d}E/\mathrm{d}x }$ in the SR and the CR for the early 2016 data set, as well as for an FCP signal at a mass of 100 GeV and different charge scenarios. The vertical bars and the shaded area correspond to the statistical uncertainty in the SR and the CR, respectively. The $p$ -value of the fit is 6%. The two lower panels show the ratio of the number of tracks observed in the CR (upper) and SR (lower), and the fit function. The vertical bars correspond to the uncertainty from statistical sources, while the shaded area shows the systematic uncertainty in the fit due to the choice of the fitting function and the binomial fit range as explained in the main text. Comparing with respect to the binomial fit starting at $N_{\text{hits}}^{\text{low}\ \mathrm{d}E/\mathrm{d}x } =$ 2, and not $N_{\text{hits}}^{\text{low}\ \mathrm{d}E/\mathrm{d}x } =$ 1, is needed to account for the fact that early 2016 data is more strongly affected by instrumental effects that widen the $N_{\text{hits}}^{\text{low}\ \mathrm{d}E/\mathrm{d}x }$ distribution.
png pdf	Figure 6: Distribution of $N_{\text{hits}}^{\text{low}\ \mathrm{d}E/\mathrm{d}x }$ in the SR and the CR for the late 2016 data set, as well as for an FCP signal at a mass of 100 GeV and different charge scenarios. The vertical bars and the shaded area correspond to the statistical uncertainty in the SR and the CR, respectively. The $p$ -value of the fit is 78%. The two lower panels show the ratio of the number of tracks observed in the CR (upper) and SR (lower), and the fit function. The vertical bars correspond to the uncertainty from statistical sources, while the shaded area shows the systematic uncertainty in the fit due to the choice of the fitting function and the binomial fit range as explained in the main text.
png pdf	Figure 7: Distribution of $N_{\text{hits}}^{\text{low}\ \mathrm{d}E/\mathrm{d}x }$ in the SR and the CR for the 2017 data set, as well as for an FCP signal at a mass of 100 GeV and different charge scenarios. The vertical bars and the shaded area correspond to the statistical uncertainty on the SR and the CR, respectively. The $p$ -value of the fit is 65%. The two lower panels show the ratio of the number of tracks observed in the CR (upper) and SR (lower), and the fit function. The vertical bars correspond to the uncertainty from statistical sources, while the shaded area shows the systematic uncertainty in the fit due to the choice of the fitting function and the binomial fit range as explained in the main text.

Tables
png pdf	Table 1: Number of tracks observed and expected from the background in the $N_{\text{hits}}^{\text{low}\ \mathrm{d}E/\mathrm{d}x }$ bin with highest signal significance for early 2016 ( $\geq$ 9 hits), late 2016 ( $\geq$ 7 hits), 2017 ( $\geq$ 7 hits), and 2018 data ( $\geq$ 7 hits). The corresponding systematic uncertainties in the background estimation are also listed.
png pdf	Table 2: Number of tracks observed and expected for background and signal events, in the $N_{\text{hits}}^{\text{low}\ \mathrm{d}E/\mathrm{d}x }$ bin with highest signal significance ( $\geq$ 9), for early 2016 data. The corresponding systematic uncertainties are also listed. They are considered to follow a log-normal distribution. The results are shown for FCP scenarios at a mass of 100 GeV with a charge of 2 $e$ /3 (the most sensitive scenario) and $e$ /3 (the lowest considered charge).
png pdf	Table 3: Number of tracks observed and expected for background and signal events, in the $N_{\text{hits}}^{\text{low}\ \mathrm{d}E/\mathrm{d}x }$ bin with highest signal significance ( $\geq$ 7), for late 2016 data. The corresponding systematic uncertainties are also listed. They are considered to follow a log-normal distribution. The results are shown for FCP scenarios at a mass of 100 GeV with a charge of 2 $e$ /3 (the most sensitive scenario) and $e$ /3 (the lowest considered charge).
png pdf	Table 4: Number of tracks observed and expected for background and signal events, in the $N_{\text{hits}}^{\text{low}\ \mathrm{d}E/\mathrm{d}x }$ bin with highest signal significance ( $\geq$ 7), for 2017 data. The corresponding systematic uncertainties are also listed. They are considered to follow a log-normal distribution. The results are shown for FCP scenarios at a mass of 100 GeV with a charge of 2 $e$ /3 (the most sensitive scenario) and $e$ /3 (the lowest considered charge).
png pdf	Table 5: Number of tracks observed and expected for background and signal events, in the $N_{\text{hits}}^{\text{low}\ \mathrm{d}E/\mathrm{d}x }$ bin with highest signal significance ( $\geq$ 7), for 2018 data. The corresponding systematic uncertainties are also listed. They are considered to follow a log-normal distribution. The results are shown for FCP scenarios at a mass of 100 GeV with a charge of 2 $e$ /3 (the most sensitive scenario) and $e$ /3 (the lowest considered charge).

Summary

In summary, we presented a search for fractionally charged particles using proton-proton collisions corresponding to an integrated luminosity of 138 fb

$^{-1}$ , collected at

$\sqrt{s} =$ 13 TeV with the CMS detector. The key feature of low ionization energy in the CMS tracker detector is used to discriminate a potential signal from the background. No significant deviation is observed in data with respect to the expected background. The search presents for the first time a scan of the new particle charge between

$e$ /3 and

$e$ . The existence of fractionally charged particles arising from the considered DY-like pair production is excluded in the mass range between 50 and 640 (60) GeV for a signal of charge

$Q =$ 2

$e$ /3 (

$e$ /3). These are the most stringent limits to date for this production mode.

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