Search for dimuon resonance in the 35 to 75 GeV mass range using 140 fb$^{-1}$ of 13 TeV $pp$ collisions with the ATLAS detector

TL;DR

This ATLAS study searches for low-mass dimuon resonances in the 35–75 GeV window using 140 fb$^{-1}$ of 13 TeV $pp$ data, addressing challenging backgrounds with Gaussian process regression. The analysis models the background nonparametrically, performs a data-driven, simultaneous signal-plus-background fit, and derives upper limits on the fiducial cross-section times branching ratio for hypothetical resonances decaying to $oldsymbol{μμ}$. The results show no statistically significant excess, and the observed limits translate into constraints on benchmark models including a Z$'$ mediator and a kinetically mixed dark photon Z$_ ext{D}$, thereby probing dark-matter and dark-sector scenarios. The methodology, particularly the GPR background treatment, enhances sensitivity and can inform similar resonance searches with unprescaled muon triggers in this mass range.

Abstract

A model-independent search for low-mass resonances decaying into pairs of oppositely charged muons is presented. The analysis uses proton--proton collision data corresponding to an integrated luminosity of 140 fb$^{-1}$, recorded by the ATLAS detector at the Large Hadron Collider between 2015 and 2018. The search targets hypothetical dimuon resonances in the invariant mass range from 35 GeV to 75 GeV. The modelling of this mass region is particularly challenging for conventional analytic background parameterisations. To address this, a Gaussian process regression technique is used to model the background. The dimuon mass spectrum is analysed for potential signals, and no statistically significant excess is observed. Upper limits at the 95% confidence level are set on the fiducial production cross-section of new resonances decaying promptly into muons, ranging from 20 fb to 110 fb, depending on the resonance mass. These results are further interpreted in the context of dark-photon and dark-matter-mediator models, leading to new constraints on their parameter spaces.

Figures (9)

• Figure 1: Feynman diagrams for Drell--Yan production of (a) a generic resonance $Z'$ and (b) a kinetically mixed dark photon $Z_\text{D}$.
• Figure 2: Parameterisation of the acceptance and efficiency for the new resonance. (a) The product of acceptance and efficiency ($\mathcal{A} \times \varepsilon$) as a function of the resonance mass, fitted with a second-order polynomial. (b) The detector correction factor (efficiency, $\varepsilon$) as a function of the resonance mass, fitted with a third-order polynomial.
• Figure 3: (a) Width of the DSCB Gaussian core as a function of the new resonance mass (solid markers), with a linear fit overlaid. (b) Simulated dimuon mass distribution for a hypothetical $Z^\prime$ resonance with a mass of 55 $\text{Ge V}$ and a width $\Gamma$ = 0.21 $\text{Ge V}$ (solid markers), overlaid with the signal model, defined as a Breit–Wigner distribution convolved with a DSCB function and obtained from a parametric fit to the signal template. The lower panel displays the relative difference between the MC simulation and the signal model.
• Figure 4: The dimuon mass, $m_{\mu\mu}$, distribution in the range 30--80 $\text{Ge V}$, after applying the selections described in Section \ref{['sec:eventselection']}.
• Figure 5: Spurious signal yield as a function of the new resonance mass, obtained by fitting signal-plus-background models to background-only templates. The points represent the fitted signal yields, with vertical error bars indicating the corresponding statistical uncertainties. The thick curves denote the symmetrised envelope, which defines the systematic uncertainty associated with the background modelling. Discontinuities in the spurious signal reflect transitions between different Gaussian process hyper-parameter configurations.