A New X17 Paper

I continue to be skeptical that an X17 particle exists. There is no sign of it at the LHC so far, and it has not been replicated at a different experiment from ATOMKI that has seen anomalies.

The ATOMKI anomalies motivate the possibility of a new particle, the X17, with a mass of 17 MeV and weak couplings to nuclei and electrons. Such a particle may be produced at the LHC, travel 480 m in the far-forward direction, and decay to e+e− in FASER. 

In this paper, previous FASER searches for dark photons and axion-like particles (ALPs) are combined and interpreted as searches for a candidate X17 particle, the protophobic vector boson. The searches use data collected from pp collisions at s√=13.6 TeV in LHC Run 3 from 2022 for the dark photon search and from 2022-2023 for the ALP search. 

No excess events are found above the expected background, and the combined null results exclude a previously-unconstrained "low-coupling window" with electron coupling 6 × 10^−5 < ϵe < 1.2 × 10^−4. A "high-coupling window" remains.

FASER Collaboration, "Constraints on a Long-Lived X17 Vector Boson with FASER" arXiv:2611.03904 (November 2, 2026).

Another preprint has more X17 constraint discussion here.

Another Dark Matter v. MOND Comparison Paper

MOND still looks respectable good compared to various dark matter particle models in the SPARC database of galaxies.

The observed rotation velocities of spiral galaxies at large scales exceed those predicted by their baryonic components, motivating the need for dark matter (DM) or Modified Newtonian Dynamics (MOND). We fit the rotation curves of 147 late-type galaxies from the SPARC database, using a suite of models including Cold Dark Matter (CDM) models with and without baryonic feedback, Warm DM, Fuzzy DM, and MOND. We also combine SPARC kinematics with weak-lensing data to extend the mean rotation curves to ~1 Mpc. On a galaxy-by-galaxy basis, the inner SPARC rotation curves marginally favour CDM models with baryonic feedback, although accounting for the uncertainties in distances and inclinations increases the degeneracy among DM-based models and MOND. All DM models predict host halo masses larger than abundance-matching expectations, with only the CDM model with baryonic feedback reducing the scatter around the mean luminosity-halo mass relation. MOND is able to reproduce the flatness of the mean galactic rotation curves on scales up to 1 Mpc, providing stellar mass-to-light ratios within observational uncertainties. In contrast, all DM models, without the contribution of mass external to the bound halo, struggle to explain the flat rotation curves. The inclusion of the current weak-lensing data tends to favour MOND over DM models, at least at the largest scales. Future observations from BIG-SPARC, the Square Kilometre Array, and most importantly, upcoming weak-lensing surveys such as EUCLID and Rubin will provide more stringent and independent tests of these competing frameworks.

Amy Smith, et al., "Dark Matter and MOND successes and degeneracies in reproducing SPARC rotation curves and weak lensing data" arXiv:2610.33157 (October 23, 2026) (Accepted at MNRAS).

Triton Station also has a new post comparing dark matter particle theories and MOND. The summary is as follows: