Physicists, who dedicate their lives to studying the subject, do not really like physics very much because they always hope that it is broken. But we have to forgive them; The finding that the slightest use of theory is not possible is that the experimental results are an indication that we probably need a new theory, which would excite any physicist.
In recent years, one of the most broken things is a simple measurement: the charge radius of a proton, which is a measure of its physical size. Measurements made with hydrogen atoms, in which a single electron is orbiting a proton, gave us an answer. The measurement in which the electron was replaced by a heavy particle, called a muon, gave us a different answer — and the two results were inconsistent. Much effort has been made to correct this discrepancy, and it has shortened, but it has not gone away.
This is theistic saliva. There is no space for such a difference between electrons and muons in the standard model, so could this be an indication that the standard model is incorrect? The team behind some of the earlier measurements is now back with a new one, it monitors the behavior of a muon orbiting a helium nucleus. The results are consistent with other measurements of the charge radius of helium, suggesting that there is nothing ridiculous about muons. Therefore the standard model can breathe a sigh of relief.
The measurement involved is, just to put it, too crazy. The moons are essentially heavier versions of electrons, so substitution for one atom in another is relatively straightforward. And the mass of a muon provides some advantages for such measurements. The mass ensures that the orbitals of the muon are so compact that its wave function overlaps with the wave function of the nucleus. As a result, the behavior of the muon is very sensitive to the charge radius of the nucleus when orbiting the nucleus.
All of this would be great if it were not for the fact that muons are unstable and typically decay under two microseconds. Orbiting a helium around a nucleus adds complications, since helium usually has two electrons in orbit, and they can interact with each other. The expected three-way interaction of a nucleus – muon – electron is beyond our ability to compute current, meaning that we would have no idea if the actual behavior was different from theory.
So the researchers solved this problem by creating a positively charged ion composed of a helium nucleus and a single muon orbit. One of these – or, more correctly, making hundreds of them – is where the madness begins.
The researchers had access to a beam of muons produced by a particle collider, and decided to direct the beam to some helium gas. In this process, as the muons enter, they have too much energy to orbit around a helium nucleus, so they bounce around, losing energy with each bump. Once the muon slows down enough, they can enter a helium atom in a high-energy orbit, colliding one of their electrons in the process. But the second electron is still around, messing up any possible measurements.
But due to its mass, the muon has much higher momentum, and the transfer of energy within an atom is faster than losing energy to the environment. So as the muon transfers some of its energy to the electron, the small mass of the electron ensures that it is sufficient to boot the electron out of the atom, and we are left with a muonic helium ion. Fortunately, all of this happens quickly enough that the muons did not get a chance to decay.
Let the madness begin
By this point, the muon is typically in an orbital which is low energy, but has higher energy than the ground state. Researchers set up triggers sensitive to the presence of muons in the experiment. After a delay in allowing the muons to boot two electrons, the trigger causes the laser to hit the sample with the correct amount of energy to boost the muon from the 2S orbital to the 2S orbital. From there, it will decay to the ground state, releasing an X-ray in the process.
Many muons will not be in the 2S orbital, and the laser will have no effect on them. Researchers were willing to sacrifice more of the muonic helium that they had in the right condition. Their presence was indicated by the identification of X-rays with the correct energy. To further ensure that they are seeing the right thing, researchers have only taken data that were associated with a high-energy electron created by the decay of the muon.
And remember, this all took place within a millisecond time window to take place quite quickly.
The first step involves placing the laser used for the correct frequency to boost the muon in the 2P orbit, as this is the value we need to measure. This was done by adjusting a tunable laser in a frequency range until helium started producing X-rays. Once the frequency was identified, the researchers took the data for 10 days, which was sufficient for accurate measurement of the frequency. During this time, researchers observed 582 muonic helium ions.
Based on calculations using the laser frequency, the researchers found that the charge radius of the helium nucleus is 1.6782 femtometers. Measurements made by bouncing electrons off the nucleus indicate that it is 1.681. These two values are within experimental errors, so they are in strong agreement.
We’re sorry it’s not broken
At the simplest level, the fact that muon measurements agree with measurements taken independently indicates that there is nothing special about muons. Consequently, the standard model, which says the same thing, remains intact to a much lower extent than allowed by experimental errors here. (This is not to say that it is not broken in any other way, certainly.) So theorists everywhere will be disappointed.
As a gripping aside, researchers compared their value to those generated decades ago in particle accelerators at CERN. It turns out that this value is the same, but only by accident, because earlier work had two offset errors. “Their quoted charge radius is not far from our value,” the researchers noted, “but it can be traced back to a strange coincidence of an incorrect experiment combined with an incomplete 2P-2S theory prediction , Incidentally, it is not so – wrong value. ” So in this case, two mistakes are almost corrected.
In any case, this work will focus on trying to regain the researchers’ attention that separate experiments with protons produce results that don’t quite agree, because we can’t blame things when they’re weird Huh. In the meantime, we can all appreciate how amazing it is that we can manage to do this with the muons within the tiny fraction of a second in which they exist.
Nature, 2021. DOI: 10.1038 / s41586-021-03183-1 (About DOI).