Neutrinoless Quadruple Beta Decay

While detection of neutrinoless double beta decay is a sign that neutrinos are Majorana particles, neutrinoless quadruple beta decay in the absence of neutrinoless double beta decay is a sign that neutrinos have Dirac mass. So far, neither form of decay has been observed.

The observation of neutrinoless quadruple beta decay (0ν4β) in the absence of neutrinoless double beta decay (0ν2β) has been argued to provide a strong indication that neutrinos are Dirac particles. We report a search for 0ν4β decay of 136Xe using a total 136Xe exposure of 148.4 kg⋅yr, collected during the commissioning and the first science runs of the PandaX-4T experiment. No significant excess of events over the background is observed. A lower limit on the 0ν4β decay half-life of 136Xe is set at 6.01 x 10^24 yr at the 90% confidence level. This result establishes the most stringent constraint on this process in xenon, demonstrating the unique capability of the PandaX-4T detector in probing lepton number violation and shedding light on the fundamental nature of neutrinos.

PandaX Collaboration, "Search for neutrinoless quadruple beta decay of 136Xe in PandaX-4T detector" arXiv:2609.10048 (September 9, 2026).

A Heart Warming Math Story

A 92 year old retired math professor (a woman), and a 15 year old girl, who both live in the same neighborhood in Manhattan, solve an important unsolved problem in the topology of knot theory together (via Not Even Wrong which links to the New York Times, the article that they published about their discovery, and Scientific American). 

Read the whole thing, it is not very amenable to being summarized.

Hadron Physics To Do

One of my long-standing to dos for a blog post, which keeps getting put off because it is a pretty big project, is to survey the current state of the literature regarding hadron and/or hadron molecule resonances that aren't simple pseudo-scalar valance quark-antiquark, and simple three valance quark baryons, with u, d, s, c, and b valance quarks.

These include scalar mesons, axial-vector mesons, tetraquarks, pentaquarks, hexaquarks (if any), quarkonia, toponium, glueballs, mixed/blended meson resonances, glueball-quark hybrids, hadron molecules, excited hadron resonances, and other XYZ resonances. 

There are also "leptonic atoms" which substitute positively charged leptons for protons in an atomic nucleus and are bound by quantum electrodynamics (i.e. by electromagnetism) rather than by the strong force, that probably belong in the same discussion (and generally have a mass of less than 4 GeV).

As a prelude, the big bottom line is that there is not a global solution, really, even to any large group of unclassified resonances. Each resonance has to be figured out on its own. It is sometimes quite an epic effort to discriminate between plausible explanations of their structure. 

But there is also no BSM physics. QCD can explain it, but you have to be open to more involved hadron and hadron molecule structures than the vanilla mesons and baryons display. Thus, we are slowly and painfully, but inexorably, reaching a point where essentially all resonances have a Standard Model explanation.

Also, except for toponium, this highly sophisticated analysis and classification of hadron resonances, while it requires lots of data points, doesn't require the extreme high energies of the 13-14 TeV LHC (Large Hadron Collider). 

Generally speaking, all hadron resonances are somewhere between 135 MeV (the lightest pion) and about 30 GeV (a hypothetical six b quark hexaquark), and the lower middle part of this range is very crowded with all sorts of resonances. This is comfortably below the energy scale of even a W or Z or Higgs boson, and is also below the energy scale of a top quark-antitop quark pair. 

Maybe a post just spelling out the possibilities would be a good prelude to a post putting forth the leading theories about which resonances are most likely matches to which possibilities.

Where Do Grammatical Inflections Come From?

The maxim "today's morphology is yesterday's syntax," coined by linguist Thomas Givon, means that grammatical word endings and affixes (morphology) evolve over time from separate, independent words once used together in phrases and sentences (syntax).

Via Language Log. 

Vaguely related: Some karaoke machines display words sung with almost no gap between them as a single word (German style), which indeed does help in getting the phrasing of the words in a song that you don't know very well right.