According to physicists, CMS cooperation This is the first time this process has been observed in proton-proton collisions at CERN's Large Hadron Collider (LHC). This is also the most accurate measurement of tau's anomalous magnetic moment and provides a new way to constrain the existence of new physics.

of TauIt is a special particle of the lepton family, also called tauon.

In general, leptons, together with quarks, constitute the matter content of the Standard Model.

Tau was first discovered in the 1970s, and its associated neutrino (tau neutrino) was discovered by Fermilab's DONUT collaboration in 2000 to complete the tangible matter part.

However, tau has a very short lifetime and can remain stable for only 290*10 hours, making it quite difficult to study it accurately.^-15 seconds.

Two other charged leptons, electrons and muons, are fairly well studied.

Much is also known about their magnetic moments and their associated anomalous magnetic moments.

The former can be understood as the strength and direction of a virtual bar magnet within the particle.

However, this measurable quantity requires correction at the quantum level resulting from the virtual particles pulling on the magnetic moment and deviates from the predicted value.

The quantum correction, called the anomalous magnetic moment, is about 0.1%.

If the theoretical and experimental results do not agree, this anomalous magnetic moment_Iopens the door to physics beyond the Standard Model.

The anomalous magnetic moment of the electron is one of the most accurately known quantities in particle physics and is in perfect agreement with models.

Its muon counterpart, on the other hand, is one of the most studied, and research is ongoing.

So far, theory and experiment are largely in agreement, but recent results raise tensions that require further investigation.

But for Tau, the race is still on. Its anomalous magnetic moment is particularly difficult to measure._τThis is because tau has a short lifespan.

The first attempt was_τ After the discovery of tau, there was an uncertainty 30 times higher than the size of the quantum correction.

Experimental efforts at CERN improved the constraints and reduced the uncertainty to 20 times the size of the quantum correction.

In collisions, physicists look for special processes. That is, two photons interact to produce two tau leptons (also called a ditau pair), which then decay into muons, electrons, or charged pions, and neutrinos.

So far, both ATLAS and CMS collaborations have observed this in ultraperipheral lead-to-lead collisions.

Now, CMS physicists report: first observation The same process occurs during proton-proton collisions.

These collisions provide greater sensitivity to physics over the standard model, as new physical effects increase with collision energy.

Taking advantage of the superior tracking capabilities of the CMS detector, the collaboration will isolate this particular process from other processes by selecting events that produce a tau with no other tracking within a distance of just 1 mm. I was able to separate it.

“This remarkable achievement in detecting proton-proton collisions in the super-periphery sets the stage for many breakthrough measurements of this kind from CMS experiments,” said Dr. Michael Pitt, a member of the CMS team. said.

This new method provided a new way to constrain tau's anomalous magnetic moments, and the CMS Collaboration quickly put it to the test.

Future driving data will likely improve the significance, but their new measurements impose the tightest constraints to date, with greater precision than ever before.

This reduces the prediction uncertainty to just three times the size of the quantum correction.

“We're really excited to finally be able to narrow down some of the fundamental properties of the elusive tau lepton,” said CMS team member Dr. Isaac Neutelings.

“This analysis introduces a new approach to investigating tau g-2 and revitalizes a measurement that has been stagnant for more than 20 years,” said CMS team member Dr. Xuelong Qin.