Spin identification of the mono-Z$^{\prime}$ resonance in muon-pair production at the ILC with simulated electron-positron collisions at $\sqrt{s}$ = 500 GeV

TL;DR

The study investigates spin identification for a light Z$^{\prime}$ mediator in a mono-$Z^{\prime}$ portal to dark matter at the ILC with $\sqrt{s}=500$ GeV and $L=4$ ab$^{-1}$ using the Collins-Soper angular variable $\cos\theta_{CS}$. Through MC simulations with WHIZARD/MG5, ISR via Pythia, and fast detector modeling (Ilcgen/Delphes), the authors demonstrate that the spin-1 nature of a low-mass $Z^{\prime}$ can be distinguished from alternatives by the $\cos\theta_{CS}$ distribution after stringent event selection and background suppression. They show discovery potential for $M_{Z'}=50$ GeV with $M_{\chi_1}=1$ GeV at as little as $\sim$300 fb$^{-1}$, while heavier DM masses demand more luminosity; they also derive 95% CL upper limits on $\sigma\times BR(Z'\to\mu\mu)$ and map the excluded regions in the $(M_{Z'},M_{\chi_1})$ plane. The results underscore the ILC’s sensitivity to light neutral gauge bosons and leptophilic DM portals beyond current hadron-collider reach, leveraging spin-structure information encoded in $\cos\theta_{CS}$.

Abstract

In this analysis, we examine the angular distribution of low-mass dimuon pairs produced in simulated electron-positron collisions at the proposed International Linear Collider (ILC), which operates at a center-of-mass energy of 500 GeV and has an integrated luminosity of 4 ab\(^{-1}\). Our focus is on the cos\(θ_{\text{CS}}\) variable, which is defined in the Collins-Soper frame. In the Standard Model, the production of low-mass dimuon pairs is primarily driven by the Drell-Yan process, which exhibits a notable forward-backward asymmetry. However, many scenarios beyond the Standard Model predict different shapes for the cos\(θ_{\text{CS}}\) distribution. This angular distribution can be valuable for distinguishing between these models, especially in the event of observing excesses beyond the Standard Model expectations. We utilized the mono-Z\(^{\prime}\) model to interpret the simulated data. In the absence of any discoveries of new physics, we establish upper limits at the 95\% confidence level on the masses of various particles in the model, which includes the spin-1 \(Z^{\prime}\) boson and fermionic dark matter.

Figures (12)

• Figure 1: The Feynman diagram for the Light Vector (LV) scenario illustrating the production of a neutral gauge boson (Z$^{\prime}$) alongside a dark matter pair ($\chi_{1}$) R1.
• Figure 2: Normalized cos$\theta_{CS}$ distributions for a resonant model in the LV scenario with a $Z^{\prime}$ mass of 20 GeV, analyzing WW and ZZ($2\mu 2\nu$) events at $\sqrt{s} = 500$ GeV in \ref{['cos1']}, and for the LV signal, only a polynomial 3 function fit is applied as shown in \ref{['cos2']}. Events must meet pre-selection criteria from Table \ref{['cuts']} and have a reconstructed invariant mass between 18 - 45 GeV. Histograms are normalized to unity to emphasize qualitative features.
• Figure 3: The dimuon invariant mass spectrum, after pre-selection (see Table \ref{['pre-cuts']}), for estimated SM backgrounds and various neutral gauge boson (Z$^{\prime}$) masses based on the LV scenario, with dark matter mass ($M_{\chi_{1}} = 1$ GeV).
• Figure 4: The distributions of cos$\theta_{CS}$ are presented for events that pass the pre-selection criteria listed in Table \ref{['pre-cuts']}. The histograms show the standard model expectations, while the signal samples corresponding to the light vector model with different mass values $M_{A^{\prime}}$ ranging from 20 to 60 GeV are also superimposed. The analysis focuses on several dimuon mass windows, specifically: 18 $< M_{\mu^+\mu^-} <$ 45 GeV \ref{['bin20']}, 27 $< M_{\mu^+\mu^-} <$ 55 GeV \ref{['bin30']}, 36 $< M_{\mu^+\mu^-} <$ 65 GeV \ref{['bin40']}, 45 $< M_{\mu^+\mu^-} <$ 75 GeV \ref{['bin50']}, 54 $< M_{\mu^+\mu^-} <$ 85 GeV \ref{['bin60']}.
• Figure 5: The invariant mass spectrum for $e^+\mu^-$ events in \ref{['mass-emu']}, and for dimuon events after subtraction of $e^+\mu^-$ events in \ref{['mass-residual']} that pass the pre-selection listed in Table \ref{['pre-cuts']}. For the estimated SM backgrounds and various neutral gauge boson (Z$^{\prime}$) masses based on the LV scenario, with dark matter mass ($M_{\chi_{1}} = 1$ GeV).