One of the ways to overcome the discrepancies between dark matter particle theories and what we observe is to attribute the discrepancies to baryonic feedback effects that are not terribly well understood. An ambitious new paper with many co-authors examines feedback effects in multiple cosmology simulations. The trouble is that the feedback seems to aggravate the discrepancies between what of observed and what simulations predict, rather than resolving them.
Galaxy cores behave more or less like galaxies without dark matter phenomena, while the dynamics of galactic fringes are dominated by dark matter phenomena. And, more massive galaxies are less proportionately dark matter phenomena driven than less massive galaxies. Yet, these are just the opposite of the effects of baryonic feedback in the simulations considered.
Baryonic processes such as radiative cooling and feedback from massive stars and active galactic nuclei (AGN) directly redistribute baryons in the Universe but also indirectly redistribute dark matter due to changes in the gravitational potential. In this work, we investigate this "back-reaction" of baryons on dark matter using thousands of cosmological hydrodynamic simulations from the Cosmology and Astrophysics with MachinE Learning Simulations (CAMELS) project, including parameter variations in the SIMBA, IllustrisTNG, ASTRID, and Swift-EAGLE galaxy formation models.
Matching haloes to corresponding N-body (dark matter-only) simulations, we find that virial masses decrease owing to the ejection of baryons by feedback. Relative to N-body simulations, halo profiles show an increased dark matter density in the center (due to radiative cooling) and a decrease in density farther out (due to feedback), with both effects being strongest in SIMBA (> 450% increase at r < 0.01 Rvir). The clustering of dark matter strongly responds to changes in baryonic physics, with dark matter power spectra in some simulations from each model showing as much as 20% suppression or increase in power at k ~ 10 h/Mpc relative to N-body simulations.
We find that the dark matter back-reaction depends intrinsically on cosmology (Omega_m and sigma_8) at fixed baryonic physics, and varies strongly with the details of the feedback implementation. These results emphasize the need for marginalizing over uncertainties in baryonic physics to extract cosmological information from weak lensing surveys as well as their potential to constrain feedback models in galaxy evolution.
Recent DESI baryon acoustic oscillation data reveal deviations from ΛCDM cosmology, conventionally attributed to dynamical dark energy (DE). We demonstrate that these deviations are equally, if not better, explained by interactions between dark matter and dark energy (IDE), without requiring a time-varying DE equation of state. Using a unified framework, we analyze two IDE models--coupled quintessence and coupled fluid--against the latest CMB (Planck, ACT, SPT), DESI BAO, and SN (including DES-Dovekie recalibrated) data. Both IDE scenarios show robust evidence for non-vanishing interactions at the 3-5σ level, with marginalized constraints significantly deviating from the ΛCDM limit. This preference persists even under DES-Dovekie SN recalibration, which weakens dynamical DE evidence. Crucially, for the same number of free parameters, IDE models provide fits to low- and high-redshift data that match or exceed the performance of the CPL dynamical DE parametrization. Our results establish IDE as a physically motivated alternative to dynamical DE, highlighting the necessity of future cosmological perturbation measurements (e.g., weak lensing, galaxy clustering) to distinguish between these paradigms.
5 comments:
Updated (g − 2)μ, (g − 2)e and PADME-Favored Couplings
Narrowly Compatible with the Preferred Region of ATOMKI X17,
Given a Protophobic Vector Interpretation
Emrys Peets∗1,2
1Department of Physics, Stanford University, Stanford, CA 94305, USA
2Fundamental Physics Directorate, SLAC National Accelerator Laboratory, Menlo Park,
CA 94025, USA
January 12, 2026
Abstract
We re-evaluate the viability of a kinetically mixed dark photon (A′) as a solution to the muon
anomalous magnetic moment (g − 2)μ discrepancy and the ATOMKI nuclear anomalies near 17 MeV,
using the final FNAL measurement and the latest theory predictions (BMW21, WP25). For mA′ =
17 MeV, the allowed kinetic mixing parameter narrows to εμ = 7.03(58) × 10−4 (WP25). We then
directly compare the allowed region for the muon and X17 bands to those preferred by the electron
magnetic moment measurements. For the electron, we obtain εe = 1.19(15) × 10−3 (Cs, 2018) and εe =
0.69(15) × 10−3 (Rb, 2020), based on two recent measurements of the fine structure constant compared
to the most recent experimental value determined using a one-electron quantum cyclotron. While a mild
tension persists, we identify a narrow overlapping region, 3.4 × 10−4 ≲ ε ≲ 5.6 × 10−4, between recent
PADME results and NA64 exclusions, compatible with a protophobic gauge boson interpretation. These
results provide well-defined targets for future experimental searches and motivate further theoretical
refinements, both of which will play a decisive role in assessing the validity of the ATOMKI anomaly
claims.
1 Introduction
Using the latest results from the (g-2) experiment [1], and considering BMW lattice QCD corrections to
the gyromagnetic ratio to the muon [2], we report updated allowed parameter space for dark sector heavy
photons between masses of 5 and 500 MeV that could couple to muons. We include the first comparison of
the (g − 2)μ allowed ε and the preferred coupling given the ATOMKI measurements [3, 4, 5]. We illustrate
how the theoretical prediction of the magnetic moment has changed over time by comparing to the 2020 G-2
white paper, and the 2021 BMW correction including lattice QCD corrections [6, 2].
Additionally, we include a comparison with the allowable coupling of a heavy photon calculated from
(g − 2)e using precision measurements of α from Cs and Rb measurements, noting small exclusions of the
It's the kind of garbage paper that I routine scorn.
A better X17 paper. https://arxiv.org/abs/2601.08567
um
Emrys Peets∗1,2
1Department of Physics, Stanford University, Stanford, CA 94305, USA
2Fundamental Physics Directorate, SLAC National Accelerator Laboratory, Menlo Park,
CA 94025, USA
works at SLAC National Accelerator Laboratory. Former Internship at Los Alamos National Laboratory. Studies Physics PhD at Stanford University.
Rising Stars in Experimental Physics - Emrys Peets
IFIC seminars: "Casting light on the X17 particle with the NA64 experiment at CERN"
by Dr. Luca Marsicano (INFN Genova)
Tuesday 10 Feb 2026, 12:00 → 13:30 Europe/Madrid
1001-Primera-1-1-1 - Paterna. Seminario (Universe)
Description
The anomaly recently observed by the ATOMKI collaboration in the angular correlations of electron–positron pairs emitted in nuclear transitions of 8Be, 4He and 12C has attracted significant attention in the particle physics community. This effect can be interpreted as the emission of a bosonic particle with a mass of 17 MeV, commonly referred to as the X17 particle. If confirmed, such a particle would constitute clear evidence of physics beyond the Standard Model. After an introduction to the ATOMKI anomaly, including a summary of the experimental observations and theoretical interpretations, this seminar will cover the current status of the X17 searches at particle accelerators, highlighting results from several experiments, including MEG, PADME and NA64, emphasizing the complementary experimental strategies employed in this search. I will then focus on the NA64 experiment, which uses high-intensity electron and muon beams to search for feebly interacting particles at CERN SPS. Existing NA64 results in the X17 search will be discussed, followed by prospects for possible future measurements, aimed at providing a decisive test of the X17 explanation of the ATOMKI anomaly.
Organized by
IFIC seminars
https://indico.ific.uv.es/event/8386/
CERN is going all in the experiment with NA64 for x17
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