Search for fractionally charged particles in proton-proton collisions at $\sqrt{s}$ = 13 TeV

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 charges 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.

• The paper establishes stringent exclusion limits for fractionally charged particles, excluding masses up to 640 GeV for charges as low as e/3.
• The paper utilizes 138 fb⁻¹ of 13 TeV collision data and the CMS tracker system to identify FCPs via reduced ionization energy loss.
• The paper applies a robust CLₛ statistical method to distinguish signal from background, enhancing our understanding of potential new physics phenomena.

Overview of the Search for Fractionally Charged Particles at the LHC

The research conducted by the CMS Collaboration on the search for fractionally charged particles (FCPs) at the Large Hadron Collider (LHC) represents a significant contribution to the field of particle physics. This study leverages proton-proton collision data at a center-of-mass energy of 13 TeV, collected over an integrated luminosity of 138 fb$^{-1}$ during 2016–2018. It aims to detect particles with electric charges below the elementary charge, specifically between $e/3$ and $e$, an endeavor that has not been previously investigated at the LHC.

Theoretical Background and Motivation

Fractionally charged particles, theoretically motivated by extensions to the Standard Model (SM), are hypothesized in scenarios invoking hidden symmetries and new gauge fields. Such theoretical frameworks, including the hypercharge portal and additional U(1) symmetries that are remnants of larger symmetry groups, propose particles with fractional charges. Although these FCPs have not been observed, there is no fundamental principle that forbids their existence. This research focuses on the kinetic mixing of a new U(1) gauge field with SM fields, leading to effective fractional electric charges in new Dirac fermions.

Experimental Methodology

The experimental strategy relies on the detection of FCPs through their unique interaction with the CMS detector. The CMS tracker system is instrumental in distinguishing FCPs from muons by their reduced ionization energy loss per unit length ($dE/dx$), which is proportional to the square of the charge. The high missing transverse energy and track timing are crucial components in isolating potential FCP events. Notably, since FCPs interact more weakly with matter compared to particles with full charge, the study exploits their small energy loss as a signature.

The research utilizes a signal simulation framework where events are generated for a hypothesized Drell-Yan-like production of FCP pairs. Background processes such as W and Z boson decays, along with top quark pair production and cosmic rays, are carefully modeled and subtracted to isolate the FCP signal.

Results

The analysis has yielded the most stringent constraints on FCPs across a range of masses and charges, with charges as low as $e/3$ being excluded up to masses of 640 GeV at the 95% confidence level. The statistical approach employs the CL$_s$ method to compute exclusion limits, thus offering robust evidence against the presence of FCPs within the parameters tested.

Implications and Future Directions

This study advances the experimental search for FCPs by setting new exclusion limits and enhancing our understanding of potential new physics phenomena. The results have important implications for the study of hidden-sector theories and motivate further experimental efforts to explore charges smaller than those probed in this research. Future collider experiments and upgrades may extend the sensitivity to even lower charge magnitudes and higher masses, thus continuing to probe the parameter space of hidden fundamental interactions.

In conclusion, the CMS Collaboration's search for fractionally charged particles at the LHC reflects a methodical and precise approach to exploring beyond the Standard Model physics. The findings contribute essential data to the ongoing discourse in particle physics, highlighting both the challenges and potential inherent in the search for new physics phenomena.