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"Door to unknown physics wider open" : Potential contradiction discovered in the Standard Model of Particle Physics
The standard model of particle physics is a kind of family tree for the nanoworld. It describes which particles there are - from electrons and protons to quarks and the Higgs boson. It also describes how the disintegration of heavier particles creates lighter ones and what forces act between them.
This model has been further developed by physicists over decades and it is quite robust. It has been confirmed again and again in numerous experiments.
It is all the more exciting when contradictions arise and other values are measured in experiments than the theory predicts. Because these weak points could be the way to a “new physics” that ultimately explains phenomena that the Standard Model cannot explain: dark matter, for example, or the mass of neutrinos.
An international research team at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, USA has now discovered such a contradiction.
"This is a very special day"
In an online seminar and several publications, the researchers reported on Wednesday about their measurements of the anomalous magnetic moment of a particle called the muon. The measured value turned out a little larger than it should have been according to the standard model. “This is a very special day,” says Martin Fertl from the University of Mainz, who was involved in the experiment from the start. "The result opened the door to previously unknown physics even further."
Muons are also known as the “big brothers” of electrons. They are much heavier and disintegrate after a few fractions of a second. You can imagine muons like tiny bar magnets that rotate around their axis and make an additional circular movement like a wobbly humming top. In this way they create a magnetic field.
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If you take a closer look, it gets a little more complicated: The space around each muon is not empty, but according to theory, new particles are constantly forming and disappearing again. All of these affect the muon's magnetic field. If this is taken into account, the magnetic moment of the muon is calculated to be around 0.1 percent greater than without the virtual particles.
An experiment at Brookhaven National Laboratory on Long Island 20 years ago suggested, however, that the deviation is actually even greater. Even then, the physicists were very excited. But the values were not reliable enough. It could also have been an accidental deviation. Therefore, the researchers devised another experiment that should work even more precisely.
It is on the Fermilab and is simply called "g-2" by the experts. The "g" stands for "gyromagnetic factor": the size of the measured magnetic moment divided by the size of the magnetic moment that would theoretically be expected according to classical physics. In the case of muons, the g-factor is close to 2, but not quite, but slightly larger. It is important to clarify this difference.
The system at Fermilab went into operation in 2018. It creates muons and chases them into a ring that is surrounded by magnets and is 15 meters in diameter. Using sensitive measuring devices, the rotation frequency of the “inner compass needle” of the muons in the magnetic field is determined and the magnetic field itself and from it the anomalous magnetic moment are calculated.
Data from further measurement rounds are evaluated
So far, Fertl and his colleagues have only evaluated the data from the first of a total of four measurement rounds. And they proceeded very carefully so as not to unconsciously achieve a “desired result”. Up to six teams were formed to evaluate the measurements independently of one another. In addition, as in medical studies, the data was partially “blinded” so that the analysts had no clues as to the direction in which their results would go.
It was not until the end of February that previously determined and previously secret correction factors were announced - only then could the physicists, who apparently consider themselves potentially biased, really see the end result. It showed another deviation: According to the theory, the g-factor should have a value of 2.00233183620. However, the experimenters determine 2.00233184122.
The deviation therefore only begins at the eighth position after the decimal point.
An accident? Fertl is "very convinced" that the deviation is real. “Although our experiment was completely rebuilt with many newly developed components, it confirms the results from Brookhaven.” According to the researchers, the probability of a chance find is lower than in the experiment from 2001 and is now only 0.0025 percent. The physicists say “4.2 standard deviations”. However, for any experiment, 5 standard deviations must be achieved (error probability of less than 0.00005 percent) before a solid result can be spoken of. According to Fertl, this criterion could be achieved with the next data analysis, which is expected for the next year. It will also include the measurements from the second and third campaigns, which have already been completed.
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If the contradiction to the theory is confirmed, then the theory simply cannot be correct. There are a number of approaches to get out of the dilemma, from supersymmetry and leptoquarks to other Higgs particles, says Dominik Stöckinger from the TU Dresden, who is involved in the collaboration as a theorist. He does not expect a major revolution in which the standard model will be completely overturned. “It is far too robust for that and has proven itself in many tests.” He estimates that it should be sufficient to adapt the existing theory.
“Depending on which value the g-2 measurement delivers in the end, certain concepts are ruled out or prove to be good approaches.” If the number levels off at the current value, a model could be considered, for example, the additional Higgs bosons contains and not just the one known so far. “According to theory, these other Higgs particles should have rather strong interactions.” This also increased the chance of finding these postulated particles with the LHC at the CERN nuclear research center in and near Geneva - or another accelerator.
The further one discusses in this direction, the more conjunctives appear - and particles that no one has seen before. Stöckinger also points out this and urges caution: "In the past it happened more often that experiments indicated something that was not confirmed in the end." The reasons were, for example, fundamental errors in the measurements - or a supposed signal simply disappeared again, the more data was collected.
Results have yet to be confirmed by other teams
Even if the results from the Fermilab are robust, they have to be confirmed by other teams. This could be done at the Japan Proton Accelerator Research Complex (J-PARC), where a new measurement method for the magnetic moment of the muons is currently being developed.
Despite all the skepticism, one notices Stöckinger's hope that the physicists may have finally found a weak point in the standard model. Because it cannot be the last word in wisdom. Its biggest drawback: It cannot explain gravity, an interaction that is extremely weak at the particle level, but which makes the well-known interplay of stars, planets and black holes possible at the macro level.
One of the great goals of physics is to combine the two, to find the macro and the mini world and a principle that explains both equally. The Dresden researcher is skeptical whether this can happen so soon. "We can develop many theories, but there is no input from experiments - but it is necessary to develop the ideas further and ultimately to come to a result."
His Mainz colleague Fertl also points out that the “g-2” experiment at most marks the beginning of a path and that changing the theory is not an easy thing: “If you try to add it to the real measured variables at one end, the tensions at the other end are all the greater. ”Theorists and experimenters still need a lot of progress before the standard model is improved or even replaced.
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