Another Fermi-LAT Dead End

The Fermi-LAT collaboration looks for signals of dark matter annihilations. All of its previous potential signals have turned out to have other more plausible explanations than dark matter annihilation. The latest blip in its data set is no different.

A narrow gamma-ray line would be a distinctive signature of dark matter, but its interpretation depends as much on its spatial distribution as on its energy. 

We reanalyze the reported 43 GeV feature in Fermi-LAT observations of Virgo, Fornax, and Ophiuchus. The excess is reproduced in the same public data and survives event-type partitions, a test using data withheld from the original selection, and detector-coordinate permutation tests. Realistic hadronic and inverse-Compton emission is too broad to explain it. A spatial--spectral likelihood analysis restricted to Good Time Intervals and including diffuse backgrounds and catalogued sources finds a broad line-like component. Uniform surface brightness and substructure-enhanced annihilation are preferred, whereas smooth NFW annihilation and a central point source are unlikely explanations. An independent annular analysis agrees: the events associated with the excess extend through much of the virial region. Calibration over 20--70 GeV and five morphologies gives a conditional global significance of approximately 2.7σ, peaking at 44.5 GeV for uniform brightness. This result is conditional on the preselected three-cluster sample. 

The broad morphology is the main obstacle to a dark-matter interpretation. Such an interpretation would require photons to be a leading annihilation channel, annihilation to remain efficient in cold subhalos, and subhalos to supply nearly all the cluster luminosity. Yet a boost consistent with ΛCDM drives the required two-photon cross section into the range most tightly constrained by Galactic-halo line limits. The most likely explanation is a chance fluctuation amplified by analysis and target-selection effects, possibly compounded by residual Galactic diffuse or unresolved-source mismodeling; if celestial, its morphology makes a simple dark-matter interpretation unlikely.

Stefano Profumo, "Right Energy, Wrong Profile: Why the 43 GeV Cluster Line Is Unlikely to Be Dark Matter" arXiv:2609.16425 (September 14, 2026).

Partially Muonic Helium

Muonic Helium is an atom made up of a helium atom nucleus with one electron and one muon, rather than two electrons. It isn't stable (since muons have a half-life of about 10^-6 seconds), but they are long lived enough to study rigorously. Their hyperfine splitting has been measured to a precision of about 0.02 M Hz in a recent experiment and the result was about 0.5 M Hz different from the previous state of the art calculation of the Standard Model expectation for its value (which also had an uncertainty much larger than the experimental value which was on the order of 0.5 M Hz).

A new and more precise calculation was made in a new paper:

Using perturbation theory for the fine-structure constant α and the electron-to-muon mass ratio, we calculated new contributions to the hyperfine structure of the ground state of the muon-electron helium atom. Compared to our previous results, we calculated new corrections in second-order perturbation theory. This calculation reduces the discrepancy between the theoretical value and the new experimental hyperfine structure measurement.

F. A. Martynenko, K. A. Seredina, A. P. Martynenko "Improved calculation of the hyperfine structure of muonic helium" arXiv:2609.17454 (September 15, 2026).

The calculation in the new paper which considers a variety of mostly small refinements to the previous state of the art calculation comes up with a result that is consistent with the old calculation (not hard due to its great uncertainties) which is about 0.05 M HZ away from the experimental result and has an uncertainty on the order of 0.07 M Hz which is a huge improvement on the old calculation. 

The uncertainty in the new experimental result, however, is still several times greater than the uncertainty in the experimental measurement, which is unusual in a calculation that is predominantly a quantum electrodynamics (QED) calculation, a part of particle physics which tends to be ultra-precise. The more complicated make up of a muonic hydrogen atom compared to many other ultra-precise QED calculations is the main source of these uncertainties. 

Glueballs

A first installment on the non-vanilla hadron blogging project that I mentioned in a previous post notes a new review article on the topic. 

Glueballs are always bosons, which means that they can blend with other bosons with the same quantum numbers. As the article below explains:

If glueballs are easily studied in the quenched approximation, they have proved remarkably elusive in practice. This is because, in the real world, quarks are light and dynamical. Because the lowest-lying glueballs carry J^PC = 0++, 2++ and 0−+, all of which are quantum numbers that ordinary isoscalar q¯q mesons also carry, nothing forbids mixing. As a result, any physical resonance in these channels is a superposition of the possible bare resonances[.]

This is one of several reasons that it is hard to precisely predict the mass of glueball resonances even though, naively, it should be simpler than other hadron mass calculations because the only experimentally measured physical constant that enters into the calculation at leading order is the strong force coupling constant. Gluons have no electromagnetic charge, don't decay via the weak force, are themselves massless (although their energy gives rise to an emergent mass for glueballs), and don't need to take into account quark masses at leading order.

The theoretical calculations put essentially all of the potential glueball states in a narrow mass range of about 1.3-5.0 GeV, which is a mass range that is also crowded with a background of all sorts of more conventional hadron resonances, which further complicates the process of determining whether a resonance has a significant glueball component.

But despite these challenges, some experimentally observed resonances have been identified with a probable high scalar glueball or pseudoscalar glueball content, validating a key prediction of quantum chromodynamics (QCD). There are a couple of other possible glueball types that are harder to match to experimentally observed resonances.

Glueballs are colour-singlet bound states built from gluons alone. They are an unavoidable consequence of the non-Abelian structure of Quantum Chromodynamics (QCD), and, in the pure Yang--Mills limit, they are the only physical excitations of the theory. The present article reviews what is known about them. 

After establishing which spin, parity and charge-conjugation quantum numbers two- and three-gluon states can carry, the pure-gauge spectrum is surveyed. Each of the main theoretical approaches and their respective conclusions are briefly presented. They include lattice QCD, constituent-gluon and Coulomb-gauge models, functional Dyson--Schwinger and Bethe--Salpeter equations, holographic models, and QCD sum rules. Particular attention is paid to the scale-setting ambiguity that limits how precisely a quenched glueball mass can be converted into physical units. 

The discussion then turns to full QCD. After discussing the meaning of a glueball assignment in this context, unquenching effects and questions related to glueball--qq¯ mixing are addressed. The large-N(c) counting that underpins the mixing picture, as well as mass-matrix and effective-Lagrangian treatments of mixing are presented. The selection rules and dynamical mechanisms that shape glueball decays are also discussed. 

The gluon-rich production mechanisms used experimentally are finally reviewed. The candidates are assessed sector by sector: f(0)(1370)/f(0)(1500)/f(0)(1710) and the competing interpretations of the scalar sector, η(1405)/η(1475) and X(2370) in the pseudoscalar sector, the crowded tensor region, and the essentially unexplored C=−1 sector and its connection to the Odderon. 

Outlooks on the programmes that could help in validating some of these candidates or identifying others are reviewed as a conclusion.

Cyrille Chevalier, Vincent Mathieu, "Glueballs: hadrons without quarks" arXiv:2609.16790 (September 15, 2026) (Submission to Encyclopedia of Nuclear Physics (Elsevier)).

Inferred Dark Matter Cores Are Indifferent To Baryonic Physics

Lots of dark matter models of galaxies rely on baryonic feedback (i.e. influence from ordinary matter) to account for its dark matter halo distributions. But a new paper states that the inferred core of a dark matter distribution over many galaxies is indifferent to the distribution of baryonic matter, which undermines this approach.

We present the first dark matter density profile derived directly from James Webb Space Telescope JADES Data Release~3 NIRSpec observations of N=587 galaxies spanning the cosmic noon epoch 1.5 ≤ z ≤ 3.5. From each NIRSpec G235M/G395M spectrum we extract the Hα emission-line velocity dispersion σha, stack galaxies in four redshift bins of Δz=0.5, and reconstruct representative group rotation curves within the General Theory of Relativity. Fitting the four-parameter modified exponential model and deriving the exact GTR energy density, we reduce the profile to a compact [2/3] Padé approximant with all coefficients determined in closed form from JWST observations. The profile is cusp-free, fully analytic, and redshift-dependent. Energy conditions, causality, and orbital stability are all satisfied. Most strikingly, the central density ρ(0) varies by less than 15% across z=1.5--3.5 despite a 35% decline in the asymptotic rotation velocity, revealing a universal dark matter core saturation density at cosmic noon decoupled from baryonic evolution.

Aritra Sanyal, Farook Rahaman "The New Dark Matter Density Profile from JWST JADES Galaxies" arXiv:2609.14071 (September 12, 2026).

Another paper also looks at inferred dark matter halo distributions, and, yet again (confirming perhaps dozens, if not hundreds, of previous papers), the distributions are not a good fit to the theoretically mandated NFW distribution for collisionless dark matter which is widely used by astrophysicists nonetheless.

We fit the circular velocity data, derived from weak lensing observations of isolated galaxies selected from the KiDS survey in four baryonic mass bins, using three different dark matter profiles. These include NFW, Burkert, and pseudo-isothermal profile. We find that the NFW and Burkert profiles cannot adequately fit the circular velocity data for all the four baryonic mass bins, whereas only the pseudo-isothermal profile can fit the data for all the four bins.

Sri Devaki Meduri, Shantanu Desai, "A comprehensive assessment of weak-lensing inferred circular velocity profiles of isolated galaxies" arXiv:2609.14085 (September 12, 2026).

See also, again undermining the baryonic feedback solution to the core-cusp problem of cold dark matter particle theories, this paper:

Baryonic feedback in hydrodynamical simulations is typically invoked to alleviate the core--cusp problem in dwarf galaxies. Yet baryonic processes also induce adiabatic contraction of dark matter, producing overly steep density profiles and excessively high dark matter fractions in the inner regions of massive galaxies. The dark matter distribution of galaxies across a wide stellar-mass range is therefore a critical test for such simulations, but a comprehensive benchmark has remained absent. 

Here, we consistently measure the dark matter distribution from galaxy centres out to radii of 20--50 kpc for 136 nearby galaxies that together span the local mass--size relation over the stellar mass interval 10^9 -10^11.5 M⊙. We identify central regions with lower dark matter densities relative to ΛCDM simulation expectations---whose extent grows from about 10 kpc to >50 kpc as stellar mass increases from 10^10M⊙ to 10^11.5M⊙. Although their physical origin remains unclear, these low--dark matter regions are clearly indicated by the data. Our results provide an important observational benchmark for future hydrodynamical simulations that explore alternative dark matter models and feedback processes.

Yu Lei, et al., "Lower central dark matter densities in nearby galaxies than predicted by simulations" arXiv:2609.16740 (September 15, 2026) (Accepted in principle by Nature Astronomy).