[Paper Review] Searches for Light Scalars, Pseudoscalars, and Gauge Bosons

A. Soffer|arXiv (Cornell University)|Jul 8, 2015

Dark Matter and Cosmic Phenomena40 references3 citations

TL;DR

This paper reviews recent experimental searches for light scalars, pseudoscalars, and gauge bosons—particularly dark photons and exotic Higgs-like particles—across $B$-factories, fixed-target experiments, and the LHC. It presents updated limits on kinetic mixing parameter $\epsilon$ and dark photon mass $m_{A^\prime}$, excluding previously favored parameter regions and setting stringent bounds via untagged $e^+e^-$ collisions and displaced lepton jet signatures.

ABSTRACT

In the past few years there has been a great deal of theoretical and experimental activity related to the search for low-mass scalars, pseudoscalars, and vectors in various scenarios of physics beyond the standard model. I review the current status of this topic, focusing on results obtained since FPCP 2014.

Motivation & Objective

• To review the current experimental status of searches for GeV-scale scalars, pseudoscalars, and gauge bosons in models beyond the Standard Model.
• To present updated constraints on the kinetic mixing parameter $\epsilon$ and mass $m_{A^\prime}$ of the dark photon from recent experiments.
• To evaluate the sensitivity of upcoming experiments and identify key improvements in analysis techniques such as untagged $e^+e^-$ searches.
• To test the viability of the $\pi^0$ impostor model via $e^+e^-\to\tau^+\tau^-\phi$ and $\phi\to\gamma\gamma$ decay channels.
• To assess the reach of displaced lepton jet signatures for long-lived dark photons in Higgs boson decays.

Proposed method

• Utilized untagged $e^+e^-\to\gamma\ell^+\ell^-$ events at BES-III to enhance acceptance for forward ISR photons, enabling a signal scan over the dilepton invariant mass.
• Applied a 4th-order polynomial fit to the dilepton invariant-mass spectrum to model background, with a moving signal peak to scan for resonances.
• Reinterpreted results from $\pi^0$, $\eta$, and $\phi$ decays to $\gamma\ell^+\ell^-$ across multiple experiments (BABAR, KLOE, WASA, PHENIX, etc.) to set combined limits on $\epsilon$.
• Used efficiency tables in terms of $L$ mass, lifetime, and transverse momentum to enable recasting of results for arbitrary models.
• Applied displaced lepton jet signatures in ATLAS to detect long-lived dark photons from Higgs decays, using isolation and angular separation cuts.
• Conducted a signal-peak scan in the $\gamma\gamma$ invariant mass spectrum to search for a $\pi^0$ impostor decaying to $\gamma\gamma$.

Experimental results

Research questions

• RQ1 What are the current experimental limits on the kinetic mixing parameter $\epsilon$ for dark photons across different mass ranges?
• RQ2 Can the observed $\tau^+\tau^-$ plus photon excess in $e^+e^-$ collisions be explained by a $\pi^0$ impostor scalar or pseudoscalar?
• RQ3 How do untagged $e^+e^-$ analyses improve sensitivity compared to tagged analyses in dark photon searches?
• RQ4 What is the sensitivity of the LHC and future fixed-target experiments to long-lived dark photons from Higgs decays?
• RQ5 To what extent do constraints from anomalous magnetic moment, beam-dump experiments, and supernova cooling rate shape the allowed parameter space for dark photons?

Key findings

• The BES-III untagged analysis excluded a previously favored region of $\epsilon$ vs. $m_{A^\prime}$ parameter space, improving sensitivity in the low-mass region.
• The Belle collaboration observed no signal in $e^+e^-\to A^{\prime*}\to A^\prime h^\prime$, $h^\prime\to A^\prime A^\prime$, setting limits on the $h^\prime$ coupling to dark photons.
• The BABAR search for a $\pi^0$ impostor in $e^+e^-\to\tau^+\tau^-\phi$ with $\phi\to\gamma\gamma$ set upper limits of $\sigma<73$ fb for pseudoscalars and $\sigma<370$ fb for scalars, ruling out the model of Ref. [55].
• ATLAS excluded a region of $\epsilon$ vs. $m_{A^\prime}$ parameter space in the context of a Higgs decay model with long-lived dark photons, based on displaced lepton jet signatures.
• The combined constraints from all experiments now fully exclude the region of $\epsilon$ and $m_{A^\prime}$ preferred by the muon anomalous magnetic moment discrepancy.
• Reinterpretations of older beam-dump experiments (e.g., E137, CHARM) exclude the very low-$\epsilon$, low-$m_{A^\prime}$ region, closing a long-standing loophole in the parameter space.