[Paper Review] Search for low-mass Higgs and dark photons at BESIII

V. Prasad, Haibo Li+1 more|arXiv (Cornell University)|Aug 31, 2015

Particle physics theoretical and experimental studies1 references3 citations

TL;DR

This paper presents BESIII's search for low-mass Higgs and dark photons via radiative decays of $J/\psi$ and $\psi(3770)$, using initial-state radiation and di-lepton final states. No signal was observed; the study sets new, stringent 90% C.L. upper limits on the product branching fraction $\mathcal{B}(J/\psi\to\gamma A^0)\times\mathcal{B}(A^0\to\mu^+\mu^-)$ in the range $(2.8{-}386.5)\times10^{-8}$ for $m_{A^0}$ from 0.212 to 3.0 GeV/$c^2$, improving previous limits by an order of magnitude.

ABSTRACT

Many extensions of the Standard Model introduce a new type of weak-interacting degrees of freedom. These models are motivated by the results of recent experimental anomalies. Typical models, such as Next-to-Minimal Supersymmetric Standard Model and Light Hidden Dark-sector Model, introduce the possibilities of low-mass Higgs and dark bosons. The masses of such particles are expected to be few GeV and thus making them accessible at BESIII experiment, an $e^+e^-$ collider experiment running at tau-charm region. This report summarizes the recent results of low-mass Higgs and dark bosons searches at BESIII.

Motivation & Objective

• To search for low-mass Higgs bosons ($A^0$) and dark photons ($\gamma'$) in $e^+e^-$ collisions at BESIII, motivated by extensions of the Standard Model such as NMSSM and hidden dark-sector models.
• To probe weakly interacting, light new physics particles with masses below a few GeV, which are accessible at the $J/\psi$ and $\psi(3770)$ resonances.
• To improve existing exclusion limits on the coupling of these new particles to Standard Model fermions, particularly in the $\mu^+\mu^-$ final state.
• To constrain parameter spaces in models like the NMSSM and light hidden dark-sector models, where such particles could explain recent experimental anomalies.

Proposed method

• Utilizes initial-state radiation (ISR) in $e^+e^-$ collisions at $\sqrt{s} \approx m_{J/\psi}$ and $m_{\psi(3770)}$, producing a photon and a virtual $\gamma'$ or $A^0$.
• Applies an untagged ISR photon method, requiring the ISR photon to be outside the EMC acceptance to suppress non-ISR backgrounds.
• Performs a one-constraint (1C) kinematic fit assuming zero mass for the missing track to improve invariant mass resolution of the di-lepton system.
• Employs a cocktail Monte Carlo sample including $J/\psi\to\mu^+\mu^-$, $\rho$, $f_2(1270)$, $f_0(1710)$, and $f_4(2050)$ resonances to model backgrounds.
• Uses a negative log-likelihood (NLL) scan with systematic uncertainties convolved via Gaussian smearing to set upper limits at 90% confidence level.
• Applies particle identification (PID) cuts and spatial constraints (within 10 cm along beam and 1 cm transverse) to suppress beam-related backgrounds.

Experimental results

Research questions

• RQ1 What are the exclusion limits on the product branching fraction $\mathcal{B}(J/\psi\to\gamma A^0)\times\mathcal{B}(A^0\to\mu^+\mu^-)$ for a low-mass CP-odd Higgs boson in the $0.212{-}3.0$ GeV/$c^2$ mass range?
• RQ2 How do the BESIII results improve upon previous limits from BABAR, CLEO, and CMS in the search for low-mass Higgs and dark photons?
• RQ3 What is the sensitivity of the BESIII experiment to dark photon production via $e^+e^-\to\gamma_{\rm ISR}\gamma'$ followed by $\gamma'\to\mu^+\mu^-$ in the $\psi(3770)$ data?
• RQ4 To what extent can the observed limits constrain the parameter space of the NMSSM and light hidden dark-sector models?
• RQ5 How do systematic uncertainties—particularly from reconstruction efficiency, resolution, and PID—affect the final upper limit determination?

Key findings

• No evidence of a narrow resonance was observed in the di-muon invariant mass spectrum for either the low-mass Higgs or dark photon search.
• The 90% C.L. upper limits on $\mathcal{B}(J/\psi\to\gamma A^0)\times\mathcal{B}(A^0\to\mu^+\mu^-)$ range from $2.8\times10^{-8}$ to $386.5\times10^{-8}$ across $m_{A^0}$ from 0.212 to 3.0 GeV/$c^2$, with the highest sensitivity near 1.0 GeV/$c^2$.
• The new BESIII limits improve upon previous measurements by approximately one order of magnitude, particularly in the $1{-}2$ GeV/$c^2$ region.
• Systematic uncertainties were carefully evaluated, with additive uncertainties ranging from 0.502 to 0.767 events and multiplicative uncertainties from 5.95% to 8.96%, depending on $m_{A^0}$.
• The muon PID uncertainty was estimated at $4.0{-}5.73\%$, and the photon detection efficiency uncertainty was measured to be less than $1\%$ using a control sample.
• The results are preliminary and based on a cocktail Monte Carlo sample matching the expected full data sample, with unblinding of the full $J/\psi$ dataset pending.