On April 7, 2021, Fermi National Accelerator Laboratory (or Fermilab) had conducted an experiment on elementary particles (similar to electrons) known as ‘muons’. Fermilab, established in 1967, and located near Chicago (US), is a United States Department of Energy national laboratory that specialises in high-energy particle physics. Scientists in their recently published results of this experiment suggest at the possibility of new physics governing the laws of nature. The study has been published in the journal Physical Review Letters.
The particle physics experiment, known as the ‘muon g-2’ (g-minus two), was conducted in order to understand the properties of muons. The particle, muon, is similar to an electron but far heavier and is an integral element of the cosmos. The main objective of this experiment was to determine the anomalous dipole moment of the muon particle and compare the results with the anomalous dipole moment as calculated under the ‘Standard Model’ of particle physics.
The results of the experiment, which studied muon did not match the predictions of the ‘Standard Model’ on which all particle physics is based. Physicists say that there are forms of matter and energy vital to the nature and evolution of the cosmos that are not yet known to science. Instead, it reconfirmed a discrepancy that had been detected in an experiment which was conducted 20 years ago.
Standard Model of Particle Physics
All the matter that surrounds us is made up of fundamental (or elementary) particles and these are governed by four fundamental forces. These forces include the strong force (strong nuclear force), the weak force (weak nuclear force), the gravitational force, and the electromagnetic force. Standard Model covers the relation of these forces (excluding gravitational force) with the elementary particles.
- All fundamental particles exist in two groups, namely, ‘quarks’ and ‘leptons’. Both groups further comprise six particles, which occur in pairs (or generations).
- First generation includes particles, which are more stable and lighter. Second and third generations include particles that are heavier and relatively less stable.
- The Standard Model lays out rules for the six types of quarks, six leptons, the Higgs boson (elementary particle), three fundamental forces (strong force, weak force, and the electromagnetic force), and how the subatomic particles behave under the influence of electromagnetic forces.
- The muon is one of the leptons. It is similar to the electron, but 200 times larger, and much more unstable, surviving for a fraction of a second.
- According to the Standard Model, the three forces (except gravitational force) are formed as a result of exchange of ‘bosons’ (force-carrier particles). There exist bosons for each fundamental force:
* ‘W and Z bosons’ for the weak force;
* ‘photons’ carry the electromagnetic force;
* ‘gluons’ are responsible for the strong force; and
* ‘graviton’ is expected to be the boson for gravity; however, it is yet to be discovered.
Presently, even though the Standard Model provides the most accurate picture of the elementary particles, it still has a few drawbacks.
- It has only explained the nature of existence for three of the forces, excluding gravity.
- It fails to explain the existence of ‘dark matter’, and also, the existence of antimatter after the big bang.
- It is also unable to explain the differences in mass for the three generations of leptons and quarks.
About Muon Particles
Muon is an elementary particle which is similar to an electron. It has the same amount of electric charge as that of an electron (-1 e). However, it has a larger mass as compared to that of an electron (about 207 times heavier). These are highly unstable particles with a mean lifetime of 2.2 microseconds.
Muons classify as leptons. Lepton is an elementary particle, which possesses a half-integer spin and does have strong interactions. Leptons may or may not possess charge. Therefore, muons are categorised as charged leptons. Like all elementary particles, it has a corresponding antiparticle of opposite charge (+1e) whereas its mass and spin are equal. m represents a muon, and m+ represents an antimuon. They are not produced by radioactive decay due to greater mass and energy than the decay energy of radioactivity.
About muon g-2
- The muon possesses an internal magnet similar to a small bar magnet. It also possesses spin or angular momentum.
- The ‘g’ in g-2, denotes the gyromagnetic ratio, which is determined using the strength of the magnet and the rate of the magnet’s gyration.
- Since the value of ‘g’ for the muon showed a really minor deviation (by approximately 0.1 per cent) from the expected value of 2, this experiment was termed the ‘Muon g-2’.
- The mentioned anomaly which was studied with the help of this experiment, and was termed the anomalous magnetic moment of the muon.
The major reasons for conducting the g-2 experiment using muons is as follows:
- Muons do not have further composite parts.
- Energy of a composite particle (for example, proton) is the sum of energies of its constituent particles. But a single muon only comprises its own energy.
- Further, the muon, being much heavier than the electron, makes it more responsive to new types of virtual particles.
- Fermilab made use of powerful particle accelerators that were capable of producing relatively more muons as compared to those produced in any other accelerator in the US. The accelerator is responsible for producing muons and circulating them almost at the speed of light.
- A muon produced as part of this process, exists for up to 64 microseconds before decaying into an electron and two neutrinos. This provides scientists with sufficient time to precisely measure its properties before the decay process takes place.
Significance of Muon g-2
- Scientists have discovered that vacuum is not exactly ‘empty’ space, but consists of virtual particles with real which keep coming in and out of existence.
- These particles also seem to interact with light and other particles, including photons, electrons, etc.
- Since the virtual particles have a really small period for existing in vacuum and could be really massive, the particle accelerators at Fermilab are currently incapable of producing or detecting them. Therefore, the scientists are using the vacuum as a medium to study about the elementary particles without directly producing the particles.
- Muons could be instantly generated inside the accelerators as a result of high-energy collisions. Hence, scientists are making use of vacuum to observe the properties of muons in a magnetic field.
- When placed in a magnetic field, the muon undergoes precession or change in orientation of its rotational axis.
- This is as a result of magnetic torque exerted by the magnetic field on its spinning magnetic moment.
- The g-value of the muon gets altered by the particles which keep coming in and out of existence inside the vacuum. The precession rate of the muon also gets changed by ‘g-2’.
- The difference found between these two values would indicate the existence of new particles which are yet to be discovered.
Working Concept behind Muon g-2
- A storage ring is used for placing a magnetic field of pre-determined parameters.
- A beam, consisting of muons with aligned spins, is directed inside this ring.
- Due to the magnetic field exertion, the muons undergo precession, causing a change in their spins.
- This precession rate is observed with high precision. The magnitude of this precession is directly related to the difference of g-2.
Generation of Muon Beam
- A beam of protons is initially created. The particle accelerators smash about 1012of these protons, every 12 times per second.
- This leads to the creation of different types of particles. Scientists have been intrigued by the creation of pions, as part of this process, whose spins point towards the same direction.
- These pions eventually decay into muons in a short span. Both pions and resultant muons are then directed into a triangular-shaped tunnel, with the use of magnets.
- This tunnel, termed as the Muon Delivery Ring, was previously used as Antiproton Source by Fermilab.
- The pions gradually decay into muons, while all the particles travel hundreds of yards inside this ring.
- Finally, the resultant beam, consisting of muons, is transferred into the precision storage ring, which is used for conducting the g-2 experiment.
Findings of the Muon g-2 Experiment
Fermilab scientists said that the results, while diverging from the Standard Model prediction, strongly agree with the Brookhaven results.
- The accepted theoretical values for the muon are:
g-factor: 2.00233183620
anomalous magnetic moment: 0.00116591810
- The new experimental results (combined from the Brookhaven and Fermilab results) announced on April 7, 2021 are:
g-factor: 2.00233184122
anomalous magnetic moment: 0.00116592061.
Significance of the Result
The earlier results from Brookhaven, and now from Fermilab, hint at the existence of unknown interactions between the muon and the magnetic field. These interactions could involve new particles or forces. It is, however, cannot be considered as the final finding as it has opened up avenues for more research in this field.
Comparison of Results between the Standard Model and g-2 Experiment
The predictions made for the muon g-2 using the Standard Model are accurate up to 400 parts per billion, whereas, those made by Fermilab’s muon g-2 are estimated to be accurate up to 140 parts per billion. In other words, to claim a discovery, scientists require results that diverge from the Standard Model by 5 standard deviations. Here, the combined results from Fermilab and Brookhaven diverge by 4.2 standard deviations.
The results of muon g-2 experiment would only be confirmed after the several-year run is completed. These results would then be compared with the Standard Model’s predictions (theoretical). The deviation of the experimental results from the theoretical predictions would indicate the possibility of undiscovered particles and further, new discoveries in the world of particle physics.
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