Experimental Determination

Oct 13, 2025 by wbrum.org 130 views

Introduction

In the realm of particle physics, Fermi's constant plays a crucial role in describing the weak interaction, a fundamental force of nature responsible for certain types of radioactive decay. The concept of Fermi's constant is deeply rooted in the work of physicist Enrico Fermi, who proposed a theory to explain the weak interaction in the 1930s. In this article, we will delve into the definition of Fermi's constant in the context of 4-fermion interaction, exploring its significance and the underlying mathematical framework.

Fermi's Theory of Weak Interaction

When discussing Fermi's theory of the weak interaction from a historical perspective, one first mentions Fermi's attempt of directly coupling two vector currents, as in $$H_V = G_F (\overline{\psi}_1 \gamma^\mu \psi_2)(\overline{\psi}3 \gamma\mu \psi_4)$$, where $\psi_i$ represents fermion fields, $\gamma^\mu$ are the Dirac matrices, and $G_F$ is the Fermi coupling constant. However, this approach was later found to be inconsistent with the principles of quantum field theory.

The 4-Fermion Interaction

In the 1950s, physicists such as Feynman and Gell-Mann proposed an alternative approach to describe the weak interaction, known as the 4-fermion interaction. This theory postulates that the weak interaction can be mediated by a scalar boson, which couples to four fermion fields. The Lagrangian density for the 4-fermion interaction is given by $$\mathcal{L} = -G_F (\overline{\psi}_1 \gamma^\mu \psi_2)(\overline{\psi}3 \gamma\mu \psi_4) + \text{h.c.}$$, where "h.c." denotes the Hermitian conjugate.

Fermi's Constant

Fermi's constant, denoted by $G_F$ , is a dimensionless quantity that characterizes the strength of the 4-fermion interaction. It is a measure of the probability of a weak interaction occurring between two fermion fields. In the context of the 4-fermion interaction, Fermi's constant is related to the coupling constant of the scalar boson that mediates the interaction.

Mathematical Framework

The mathematical framework underlying the 4-fermion interaction is based on the principles of quantum field theory. The Lagrangian density for the 4-fermion interaction is a relativistic invariant, which means that it remains unchanged under Lorentz transformations. This property ensures that the theory is consistent with the principles of special relativity.

Physical Significance

Fermi's constant plays a crucial role in determining the strength of the weak interaction. It is a measure of the probability of a weak interaction occurring between two fermion fields. In the context of beta decay, for example, Fermi's constant determines the rate at which a neutron decays into a proton, an electron, and an antineutrino.

Experimental Determination

Fermi's constant has been experimentally determined through a variety of measurements, including those of beta decay rates and neutrino scattering cross-sections. The most precise determination of Fermi's constant comes from measurements of the muon decay rate, which has been performed with high precision at particle accelerators.

Conclusion

In conclusion, Fermi's constant plays a crucial role in describing the weak interaction, a fundamental force of nature responsible for certain types of radioactive decay. The concept of Fermi's constant is deeply rooted in the work of physicist Enrico Fermi, who proposed a theory to explain the weak interaction in the 1930s. Through the mathematical framework of the 4-fermion interaction, we have seen how Fermi's constant is related to the coupling constant of the scalar boson that mediates the interaction. The physical significance of Fermi's constant is evident in its role in determining the strength of the weak interaction, and its experimental determination has been performed with high precision through a variety of measurements.

References

Feynman, R. P., & Gell-Mann, M. (1958). Theory of the Fermi interaction. Physical Review, 109(5), 1937-1947.
Fermi, E. (1934). Versuch einer Theorie der beta-Strahlung. Zeitschrift für Physik, 88(3-4), 161-177.
Particle Data Group. (2020). Review of particle physics. Physical Review D, 102(1), 010001.

Q: What is Fermi's constant?

A: Fermi's constant, denoted by $G_F$ , is a dimensionless quantity that characterizes the strength of the 4-fermion interaction. It is a measure of the probability of a weak interaction occurring between two fermion fields.

Q: What is the 4-fermion interaction?

A: The 4-fermion interaction is a theory that postulates that the weak interaction can be mediated by a scalar boson, which couples to four fermion fields. This theory was proposed in the 1950s by physicists such as Feynman and Gell-Mann.

Q: What is the significance of Fermi's constant in the context of beta decay?

A: In the context of beta decay, Fermi's constant determines the rate at which a neutron decays into a proton, an electron, and an antineutrino. It is a measure of the probability of this process occurring.

Q: How is Fermi's constant experimentally determined?

A: Fermi's constant has been experimentally determined through a variety of measurements, including those of beta decay rates and neutrino scattering cross-sections. The most precise determination of Fermi's constant comes from measurements of the muon decay rate, which has been performed with high precision at particle accelerators.

Q: What is the relationship between Fermi's constant and the coupling constant of the scalar boson?

A: In the context of the 4-fermion interaction, Fermi's constant is related to the coupling constant of the scalar boson that mediates the interaction. The exact relationship between the two constants depends on the specific theory being considered.

Q: What are some of the key implications of Fermi's constant in particle physics?

A: Fermi's constant has a number of important implications in particle physics. For example, it determines the rate at which certain types of radioactive decay occur, and it is a key ingredient in the Standard Model of particle physics.

Q: What are some of the challenges associated with measuring Fermi's constant?

A: Measuring Fermi's constant is a challenging task due to the small size of the effect being measured. Additionally, the measurement must be performed with high precision in order to obtain a reliable value for the constant.

Q: What are some of the current research directions in the study of Fermi's constant?

A: There are a number of current research directions in the study of Fermi's constant, including the development of new experimental techniques for measuring the constant and the study of the implications of Fermi's constant for our understanding of the Standard Model of particle physics.

Q: What are some of the potential applications of Fermi's constant in other fields?

A: Fermi's constant has a number of potential applications in other fields, including nuclear physics and materials science. For example, it can be used to study the properties of certain types of materials and to understand the behavior of particles in high-energy collisions.

Q: What is the current value of Fermi's constant?

A: The current value of Fermi's constant is $G_F = 1.166 \times 10^{-5} \text{ GeV}^{-2}$ .

Q: What is the uncertainty associated with the current value of Fermi's constant?

A: The uncertainty associated with the current value of Fermi's constant is on the order of $10^{-8} \text{ GeV}^{-2}$ .

Q: What are some of the future research directions in the study of Fermi's constant?

A: There are a number of future research directions in the study of Fermi's constant, including the development of new experimental techniques for measuring the constant and the study of the implications of Fermi's constant for our understanding of the Standard Model of particle physics.

References

Feynman, R. P., & Gell-Mann, M. (1958). Theory of the Fermi interaction. Physical Review, 109(5), 1937-1947.
Fermi, E. (1934). Versuch einer Theorie der beta-Strahlung. Zeitschrift für Physik, 88(3-4), 161-177.
Particle Data Group. (2020). Review of particle physics. Physical Review D, 102(1), 010001.