Search potential for direct slepton pair production at the CEPC with $\sqrt{s}$ = 360 GeV

TL;DR

This study evaluates direct slepton pair production at CEPC-360GeV using full MC simulations with $1.0\ \mathrm{ab}^{-1}$ and a $5\%$ systematic, focusing on staus and smuons decaying to a lepton plus the LSP. Three signal regions, tuned to different mass splittings $\Delta M$, optimize sensitivity via an $N$-dimensional scan of kinematic variables and a $Z_n$ significance measure. The projected discovery reaches are $M_{\tilde{\tau}} \approx 170$ GeV (combined LH+RH), $169$ GeV (LH), $162$ GeV (RH) for staus and $M_{\tilde{\mu}} \approx 178$ GeV for smuons, representing substantial improvements over LEP and filling gaps in the compressed region that challenge LHC searches. These results underscore the CEPC-360GeV's potential to probe sleptons and provide a benchmark for future $e^+e^-$ collider programs like FCC-ee and ILC.

Abstract

The Circular Electron Positron Collider (CEPC) is designed to operate at the key center-of-mass energies: 91.2 GeV as a Z factory for precision Z boson studies,$\approx$ 160 GeV at the threshold for W boson pair production, and 240 GeV as a Higgs factory for copious Higgs boson production. It can be upgraded to 360 GeV (CEPC-360GeV) to enable top quark-antiquark ($t\bar{t}$) pair production. Beyond enabling high-precision measurements of the Standard Model (SM), CEPC-360GeV is uniquely position to perform searches for new physics beyond the SM (BSM) physics, serving as a valuable complement to hadron colliders. This paper presents a sensitivity study on the direct pair production of staus and smuons at CEPC with $\sqrt{s}$ = 360 GeV, conducted via full Monte Carlo (MC) simulation. Under the assumptions of integrated luminosity 1.0 ab^{-1} and a flat 5% systematic uncertainty, CEPC-360GeV could potentially discover the combined production of left-handed and right-handed staus up to a mass of 170 GeV (if they exist), or up to 169 GeV for pure left-handed staus and 162 GeV for pure right-handed staus. For direct smuon production, the discovery potential reaches up to 178 GeV under the same conditions.

Figures (8)

• Figure 1: A representative diagram illustrating the pair production of charged staus (smuons) and their subsequent decay into a di-tau (di-muon) final state.
• Figure 2: The kinematic distributions for direct stau pair production after the two OS tau selection with $E_{\tau}$ larger than 2.5 GeV. The stacked histograms show the expected SM background. To illustrate, the distributions of three SUSY reference points are shown as dashed lines.
• Figure 3: "N-1" distributions after signal region requirements for the direct stau pair production. All signal region requirements are applied except for the variable shown. The stacked histograms show the expected SM backgrounds. To illustrate, the distributions from SUSY reference points are shown as dashed lines. The lower pad is the sensitivity Zn calculated with a statistical uncertainty and a 5% flat systematic uncertainty.
• Figure 4: The projected exclusion (2$\sigma$) and discovery ($5\sigma$) contours for direct $\tilde{\tau}\xspace$ production at the CEPC.
• Figure 5: The kinematic distributions for direct smuon pair production after applying the preselection. The stacked histograms show the expected SM background. To illustrate, the distributions of three SUSY reference points are shown as dashed lines.