More Evidence That The Standard Model Still Works

The ATLAS Paper

Once again, a search for beyond the Standard Model particles comes up empty and places strict limits on the parameter space of such particles. Also, the author list for this 27 page long paper is 17 pages long.

A model-independent search for low-mass resonances decaying into pairs of oppositely charged muons is presented. The analysis uses proton-proton collision data corresponding to an integrated luminosity of 140 fb−1, recorded by the ATLAS detector at the Large Hadron Collider between 2015 and 2018.

The search targets hypothetical dimuon resonances in the invariant mass range from 35 GeV to 75 GeV. The modelling of this mass region is particularly challenging for conventional analytic background parameterisations. To address this, a Gaussian process regression technique is used to model the background. 

The dimuon mass spectrum is analysed for potential signals, and no statistically significant excess is observed. Upper limits at the 95% confidence level are set on the fiducial production cross-section of new resonances decaying promptly into muons, ranging from 20 fb to 110 fb, depending on the resonance mass. These results are further interpreted in the context of dark-photon and dark-matter-mediator models, leading to new constraints on their parameter spaces.

ATLAS Collaboration, "Search for dimuon resonance in the 35 to 75 GeV mass range using 140 fb−1 of 13 TeV pp collisions with the ATLAS detector" arXiv:2601.21361 (February 2, 2026) (44 pages in total, author list starting page 27, 9 figures, 3 tables, submitted to JHEP).

The introduction to the body text of the paper notes that:

Searches for low-mass dimuon resonances have been performed by the CMS and LHCb Collaborations, covering mass ranges of 1.1–7.9 GeV, 11.5–75 GeV and 110–200 GeV for CMS, and 30.214–70 GeV for LHCb.

Those searches also came up empty. 

The trickiest mass range to study of those already studied is the 1.1-7.9 GeV mass range which has lots of different hadron resonances that decay in a great many different ways, with each decay having its own probably of occurring, generating substantial background noise, even though the backgrounds are well understood.

The range from 11.5-75 GeV has very little background noise, because it exceeds all but the heaviest hadron resonance masses (with most hadrons predicted to have masses above 11.5 GeV having never been definitively observed even in the numerous and extremely high energy collisions of the LHC), but it is comfortably less than the W boson mass (roughly 80.4 GeV), the Z boson mass (roughly 91.2 GeV), or the Standard Model Higgs boson mass (roughly 125.1 GeV).

In the 110-200 GeV mass range, the only significant backgrounds that can have dimuon decays are single Standard Model Higgs bosons (roughly 125.1 GeV), W boson pairs (roughly 160.8 GeV), and Z boson pairs (roughly 182.4 GeV). 

So, the total observations of dimuon resonances should have three very precisely predictable bumps and can often be confirmed to be background events because additional decay products in addition to the dimuons are observed. The decays of these background processes to particles other than dimuon pairs can also be used to calibrate the expected number of background dimuons for each of these three resonances.

For example, you can estimate the total Higgs boson production from the number of b quark pair decays that are observed by using that to determine how many dimuon decays from Higgs bosons should be expected, since the ratio of the b quark pair branching fraction of Higgs boson decays to the dimuon branching fraction of Higgs boson decays can be theoretically predicted to high precision. And, when you know how many background dimuon events you expect to see from Higgs boson decays, you can subtract that background from the observed number of dimuon events to determine if there is any beyond the Standard Model particle decay signal in the vicinity of the 125.1 GeV Higgs boson mass. 

You can do something similar for W boson pair decays and Z boson pair decays.

The 110-200 GeV mass range range is far more massive than any predicted Standard Model hadrons or any single W or Z boson, however. But, it is also far less than than the combined mass of a Higgs boson pair (roughly 250.2 GeV) or a top quark-antitop quark pair (which is roughly 345 GeV) or a Toponium meson (which is just a bit more massive that an unbound pair of oppositely charged top quarks).

Charged leptons, like those found in dimuon decays (which also only decay in turn to electrons quite slowly compared to other conceivable decay products with their roughly one microsecond mean lifetime, that turns out to be longer from an outside observer's perspective due to special relativity) are easy for the detectors at the LHC to see, so there are few false negatives. 

The rest mass of a dimuon pair is about 0.21 GeV, so even with an invariant mass of 1.1 GeV, the special relativistic kinetic energy of the dimuon pair is about four times its rest mass, so the pair of charged leptons will be traveling at very close to the speed of light, and the closer to the speed of light that the muons are traveling at is, the slower time passes in their rest frame relative to the rest frame of an outside observer. So, from the perspective of an outside observer, the dimuon pair takes much more than the microsecond of a muon at rest to decay.

Furthermore, a lot of false positive dimuon decays would be accompanied by additional detectible decay products that could distinguish those events from the pure dimuon decay signal that the experimenters were looking for in this paper and its companion papers over other invariant mass ranges.

So, these measurements can be quite precise, and can rule out even quite small beyond the Standard Model signals in these mass ranges.

The CMS Paper

In another recent paper, not only does the data on W boson pair production at the LHC confirm the Standard Model, it also strongly suggests that both the experimental and theoretical uncertainties are highly conservative estimates that greatly overstate the true uncertainty (something that is commonly seen in measurements of electroweak phenomena in high energy physics experiments). 

The first measurement is 0.08 sigma away from the predicted value, and the second is 0.10 sigma away from the predicted value. If a prediction and experiment were repeatedly conducted at random and had those uncertainties, the difference would average 1 sigma. So, the results are 10-12 times closer to each other than would be predicted by random chance given the stated uncertainties. It is very unlikely that this particular experiment is such a statistical fluke. 

This particularly unlikely given that this unexpected closeness between the experimentally measured value and the predicted value is seen in a large share of all high energy physics experiments involving electroweak phenomena but not the strong force, which prevents this seemingly fluke result being due to look elsewhere effects that would undermine their global statistical significance.

So, while the stated experimental uncertainties in the total production cross-section are on the order of ± 12%, the actual uncertainties are closer to being on the order of ± 1%. And, while the stated experimental uncertainties in the fiducial production cross-section are on the order of 20%, the actual uncertainties are closer to being on the order of ± 2%.

This analysis presents an observation of the photon-fusion production of W boson pairs using the CMS detector at the LHC. The total cross section of the W+W− production in photon fusion is measured using proton-proton collision data with an integrated luminosity of 138 fb−1 collected with the CMS detector in 2016−2018 at a center-of-mass energy of s√ = 13 TeV. Events are selected in the final state with one isolated electron and one isolated muon, and no additional tracks associated with the electron-muon production vertex. 

The total and fiducial production cross sections are 643 +82 −78 fb and 3.96 +0.53 −0.51 fb, respectively, in agreement with the standard model predictions of 631 ± 126 fb and 3.87 ± 0.77 fb. 

This agreement enables stringent constraints to be imposed on anomalous quartic gauge couplings within a dimension-8 effective field theory framework.

CMS Collaboration, "Measurement and effective field theory interpretation of the photon-fusion production cross section of a pair of W bosons in proton-proton collisions at s√ = 13 TeV" arXiv:2601.21574 (January 29, 2026).