Scientific discoveries are different - the unexpected discovery of radioactivity or the long search for the predicted Higgs boson. But some discoveries are mixed when some hints in the data point to future measurements that can last for years. Now just a scientific study of the latter type is taking place, which can cause a great resonance in physics.
In February 2018, a collaboration of 190 scientists working at
the Fermi National Accelerator Laboratory in Illinois began to use a ring array of magnets with a diameter of 15 meters to carry out one of the most accurate measurements in history. In this study, called the
Ji experiment minus 2 (g-2), scientists will measure the
anomalous magnetic moment of a rare subatomic particle, a muon, a heavy relative of an electron. A muon alone can exist on the order of 2.2 millionths of a second.
The measurement of the magnetic moment, that is, the force of the magnet created by the muon, was carried out with an error of 10
-12 . It is the same as measuring the distance from the Earth to the Sun with an accuracy of one millimeter. At present, the calculated and measured values do not coincide, and this difference may be the first hint at physics beyond the Standard Model - the current theory describing the subatomic world.
This would be a resounding discovery, since physicists would gladly make a hole in the prevailing theory. It would lead to a new, improved scientific model, better coping with its current task. And given that the current theory is quite successful, it would really advance our knowledge.
Once in a magnetic field, muons begin to precess, that is, in a certain way to oscillate. In a magnetic field, we can measure the precession frequency. This measurement includes the particle charge and
the g factor , which is used to distinguish between certain theories. In the classical theory, g = 1, and in the nonrelativistic quantum theory, g = 2.
The measurements of the g factor for electrons, which began shortly after the Second World War, demonstrated a slight difference from the theoretical value of 2, and gave an experimental result of 2.00232. This difference is due to the effects described by the theory of
quantum electrodynamics , QED. Concentrating on the difference between theory and experiment, 0.00232, the researchers kind of deducted the two from the result, which is why the experiment was called (g-2).
In quantum electrodynamics, among other things, we study the existence of virtual particles, or what is sometimes called quantum foam. Virtual particles are a broth of particles of matter and antimatter, arising from non-existence for small fractions of a second, and then disappearing again, as if they were not. They appear everywhere, but are especially important when they appear next to subatomic particles.
From 1997 to 2001, researchers at Brookhaven National Laboratory measured the g-factor of a muon with an accuracy of 12 significant figures and compared this result with theoretical calculations of the same accuracy. The results did not match. To understand the importance of this discrepancy, it is necessary to understand their error. For example, if you wanted to know which of two people is higher, and the error of your measurements will be half a meter, then you are unlikely to come to any convincing conclusion.
The difference between the measured and calculated results, divided by the combined error (what scientists call sigma), is 3.5. In particle physics, a sigma of 3.0 is considered conclusive evidence, but a true discovery requires a value of 5.0.
Usually one would expect that experimenters at Brookhaven would improve their installation and collect more data, but insurmountable obstacles would stand in the way of the laboratory. Therefore, the researchers decided to transfer the ring g-2 to Fermilab, where there is an accelerator capable of producing more muons. The equipment was transported 5,000 km by barge along the East Coast and up the Mississippi River. In July 2013, it arrived at Fermilab.
Over the past years, the ring has been completely updated, improved detectors and electronics have been installed. The new installation has amazing features. By the way, residents of neighboring areas have a legend that the remains of a fallen flying saucer are stored in the laboratory. Say, somehow under the cover of night from the laboratory left the truck, accompanied by police, which was under a tarp was a 15-meter drive.
Collaboration Fermilab g-2 began its work. The installation will start and start recording data, which will last until the beginning of July.
What result can scientists get? If everything passes, as expected, and the value of g, measured at Fermilab, turns out to be the same as that measured in Brookhaven, then the data recorded in Fermilab will be 5 sigma. And that would mean discovery.
On the other hand, the Fermilab result may not be the same as in Brookhaven. A new dimension may coincide with the calculations, and then there will be no differences.
But what if g-2 makes a discovery? What will be the likely outcome? As I mentioned earlier, the anomalous magnetic moment of the muon is very sensitive to the existence of virtual particles nearby. These particles slightly change the magnetic moment of the muon. Moreover, the ultra-precise coincidence of measurements and calculations would not be possible if virtual particles did not exist.
However, quite obviously, only known virtual particles were used in the calculations. One possible explanation for the observed discrepancy may be the existence in the quantum foam of additional, as yet unknown, subatomic particles.
It is worth noting that discoveries in the field of subatomic particles have been managed by high-energy particle accelerators for decades. The famous Einstein equation E = mc
2 describes the identity of mass and energy. Therefore, it takes a lot of energy to open heavy particles. At present, the most powerful accelerator is the Large Hadron Collider at CERN.
However, the brute force method for making particles is not the only way to study the high-energy region. Heisenberg's uncertainty principle says that even energetically “impossible” events can occur if their time is short enough. Therefore, it is possible that a virtual particle, usually non-existent, may appear from non-existence for a time long enough to affect the magnetic moment of the muon. In this case, a very accurate measurement would reveal the existence of this particle. This is exactly the case when the scalpel is better than a sledgehammer, and perhaps in this case the g-2 experiment at Fermilab will be able to jump around the LHC.
But it is worth noting that the history of science is full of cases where discrepancies in 3 sigma disappeared after collecting additional data. Therefore, I do not advise you to bet on the result of this measurement. Discrepancies may turn out to be a statistical fluctuation. However, the measured value of g-2 in Brookhaven can still be the first sign of a paradigm-changing discovery. The data recorded this spring will be analyzed in the fall and the results may appear already this year. The results of the first run of the g-2 experiment can be expected with cautious optimism.