Fermi's interaction explained

Fermi's interaction should not be confused with Fermi contact interaction.

In particle physics, Fermi's interaction (also the Fermi theory of beta decay or the Fermi four-fermion interaction) is an explanation of the beta decay, proposed by Enrico Fermi in 1933.^[1] The theory posits four fermions directly interacting with one another (at one vertex of the associated Feynman diagram). This interaction explains beta decay of a neutron by direct coupling of a neutron with an electron, a neutrino (later determined to be an antineutrino) and a proton.^[2]

Fermi first introduced this coupling in his description of beta decay in 1933.^[3] The Fermi interaction was the precursor to the theory for the weak interaction where the interaction between the proton–neutron and electron–antineutrino is mediated by a virtual W⁻ boson, of which the Fermi theory is the low-energy effective field theory.

According to Eugene Wigner, who together with Jordan introduced the Jordan–Wigner transformation, Fermi's paper on beta decay was his main contribution to the history of physics.^[4]

History of initial rejection and later publication

Fermi first submitted his "tentative" theory of beta decay to the prestigious science journal Nature, which rejected it "because it contained speculations too remote from reality to be of interest to the reader."^{[5] [6]} It has been argued that Nature later admitted the rejection to be one of the great editorial blunders in its history, but Fermi's biographer David N. Schwartz has objected that this is both unproven and unlikely.^[7] Fermi then submitted revised versions of the paper to Italian and German publications, which accepted and published them in those languages in 1933 and 1934.^{[8] [9] [10] [11]} The paper did not appear at the time in a primary publication in English.^[5] An English translation of the seminal paper was published in the American Journal of Physics in 1968.^[11]

Fermi found the initial rejection of the paper so troubling that he decided to take some time off from theoretical physics, and do only experimental physics. This would lead shortly to his famous work with activation of nuclei with slow neutrons.

The "tentativo"

Definitions

The theory deals with three types of particles presumed to be in direct interaction: initially a “heavy particle” in the “neutron state” (

\rho =+1 ), which then transitions into its “proton state” (\rho = -1 ) with the emission of an electron and a neutrino.

Electron state

\psi = \sum _{s} \psi _{s} a _{s,}

where

\psi is the single-electron wavefunction, \psi _{s} are its stationary states.a _{s} is the operator which annihilates an electron in state s which acts on the Fock space asa _{s} \Psi(N _{1,} N _{2,} \ldots, N _{s,} \ldots) = ( -1 ) ^{N _{1} + N _{2} + … + N _{s} -1} ( 1 - N _{s)} \Psi(N _{1,} N _{2,} \ldots, 1 - N _{s,} \ldots). a ^{*} _{s} is the creation operator for electron state s: a ^{*} _{s} \Psi(N _{1,} N _{2,} \ldots, N _{s,} \ldots) = ( -1 ) ^{N _{1} + N _{2} + … + N _{s} -1} N _{s} \Psi(N _{1,} N _{2,} \ldots, 1 - N _{s,} \ldots).

Neutrino state

Similarly,

\phi = \sum _{\sigma} \phi _{\sigma} b _{\sigma,}

where

\phi is the single-neutrino wavefunction, and \phi _{\sigma} are its stationary states.b _{\sigma} is the operator which annihilates a neutrino in state \sigma which acts on the Fock space asb _{\sigma} \Phi(M _{1,} M _{2,} \ldots, M _{\sigma,} \ldots) = ( -1 ) ^{M _{1} + M _{2} + … + M _{\sigma} -1} ( 1 - M _{\sigma)} \Phi(M _{1,} M _{2,} \ldots, 1 - M _{\sigma,} \ldots). b ^{*} _{\sigma} is the creation operator for neutrino state \sigma .

Heavy particle state

\rho is the operator introduced by Heisenberg (later generalized into isospin) that acts on a heavy particle state, which has eigenvalue +1 when the particle is a neutron, and −1 if the particle is a proton. Therefore, heavy particle states will be represented by two-row column vectors, where\begin{pmatrix} 1 \\ 0 \end{pmatrix}

represents a neutron, and

\begin{pmatrix} 0 \\ 1 \end{pmatrix}

represents a proton (in the representation where

\rho is the usual \sigma _{z} spin matrix).

The operators that change a heavy particle from a proton into a neutron and vice versa are respectively represented by

Q = \sigma _{x} - i \sigma _{y} = \begin{pmatrix} 0 & 1 \ 0 & 0 \end{pmatrix}

and

Q ^{*} = \sigma _{x} + i \sigma _{y} = \begin{pmatrix} 0 & 0 \ 1 & 0 \end{pmatrix}. u _{n} resp. v _{n} is an eigenfunction for a neutron resp. proton in the state n .

Hamiltonian

The Hamiltonian is composed of three parts:

H _{h.p.} , representing the energy of the free heavy particles, H _{l.p.} , representing the energy of the free light particles, and a part giving the interaction H _{int.} .H _{h.p.} = (1)/(2) ( 1 + \rho)N + (1)/(2) ( 1 - \rho)P,

where

N and P are the energy operators of the neutron and proton respectively, so that if \rho = 1 , H _{h.p.} = N , and if \rho = -1 , H _{h.p.} = P .H _{l.p.} = \sum _{s} H _{s} N _{s} + \sum _{\sigma} K _{\sigma} M _{\sigma,}

where

H _{s} is the energy of the electron in the s ^{th} state in the nucleus's Coulomb field, and N _{s} is the number of electrons in that state; M _{\sigma} is the number of neutrinos in the \sigma ^{th} state, and K _{\sigma} energy of each such neutrino (assumed to be in a free, plane wave state).

The interaction part must contain a term representing the transformation of a proton into a neutron along with the emission of an electron and a neutrino (now known to be an antineutrino), as well as a term for the inverse process; the Coulomb force between the electron and proton is ignored as irrelevant to the

\beta -decay process.

Fermi proposes two possible values for

H _{int.} : first, a non-relativistic version which ignores spin:H _{int.} = g \left[ Q \psi(x) \phi(x) + Q ^{*} \psi ^{*(x)} \phi ^{*(x)} \right],

and subsequently a version assuming that the light particles are four-component Dirac spinors, but that speed of the heavy particles is small relative to

c and that the interaction terms analogous to the electromagnetic vector potential can be ignored:H _{int.} = g \left[ Q \tilde{\psi} ^{*} \delta \phi + Q ^{*} \tilde{\psi} \delta \phi ^{*} \right],

where

\psi and \phi are now four-component Dirac spinors, \tilde{\psi} represents the Hermitian conjugate of \psi , and \delta is a matrix\begin{pmatrix} 0 & -1 & 0 & 0 \\ 1 & 0 & 0 & 0 \\ 0 & 0 & 0 & 1 \\ 0 & 0 & -1 & 0 \end{pmatrix}.

Matrix elements

\rho, n, N _{1,} N _{2,} \ldots, M _{1,} M _{2,} \ldots, where \rho = \pm 1 specifies whether the heavy particle is a neutron or proton, n is the quantum state of the heavy particle, N _{s} is the number of electrons in state s and M _{\sigma} is the number of neutrinos in state \sigma .

Using the relativistic version of

H _{int.} , Fermi gives the matrix element between the state with a neutron in state n and no electrons resp. neutrinos present in state s resp. \sigma , and the state with a proton in state m and an electron and a neutrino present in states s and \sigma asH ^{\rho =1 , n, N _{s=0,} M _{\sigma=0}} _{\rho =-1 ,m,N _{s=1,M} _{\sigma=1}} = \pm g \int v ^{*} _{m} u _{n} \tilde{\psi} _{s} \delta \phi ^{*} _{\sigma} d\tau,

where the integral is taken over the entire configuration space of the heavy particles (except for

\rho ). The \pm is determined by whether the total number of light particles is odd (−) or even (+).

Transition probability

See also: Fermi's golden rule.

To calculate the lifetime of a neutron in a state

n according to the usual quantum perturbation theory, the above matrix elements must be summed over all unoccupied electron and neutrino states. This is simplified by assuming that the electron and neutrino eigenfunctions \psi _{s} and \phi _{\sigma} are constant within the nucleus (i.e., their Compton wavelength is much larger than the size of the nucleus). This leads toH ^{\rho =1 , n, N _{s=0,} M _{\sigma=0}} _{\rho =-1 ,m,N _{s=1,M} _{\sigma=1}} = \pm g \tilde{\psi} _{s} \delta \phi ^{*} _{\sigma} \int v ^{*} _{m} u _{n} d\tau,

where

\psi _{s} and \phi _{\sigma} are now evaluated at the position of the nucleus.

According to Fermi's golden rule, the probability of this transition is

\begin{align} \left|a ^{\rho =1 , n, N _{s=0,} M _{\sigma=0}} _{\rho =-1 ,m,N _{s=1,M} _{\sigma=1}} \right| ^{2} & = \left|H ^{\rho =1 , n, N _{s=0,} M _{\sigma=0}} _{\rho =-1 ,m,N _{s=1,M} _{\sigma=1}} x (\exp{ (2 \pi i)/(( - W + H _{s} + K _{\sigma)} t h))/(-) 1 }{ - W + H _{s} + K ^{2} _{\sigma}\right|} \\ & = 4 \left|H ^{\rho =1 , n, N _{s=0,} M _{\sigma=0}} _{\rho =-1 ,m,N _{s=1,M} _{\sigma=1}} \right| ^{2} x (\sin ^{2 \left( (\pi t)/(h)} ( - W + H _{s} + K _{\sigma)\right)})/(( - W + H _{s} + K ^{2} _{\sigma)}) , \end{align}

where

W is the difference in the energy of the proton and neutron states.

Averaging over all positive-energy neutrino spin / momentum directions (where

\Omega ^{-1} is the density of neutrino states, eventually taken to infinity), we obtain\left\langle \left|H ^{\rho =1 , n, N _{s=0,} M _{\sigma=0}} _{\rho =-1 ,m,N _{s=1,M} _{\sigma=1}} \right| ^{2} \right \rangle _{avg} = (g ^{2})/(4 \Omega) \left|\int v ^{*} _{m} u _{n} d\tau\right| ^{2} \left( \tilde{\psi} _{s} \psi _{s} - (\mu c ^{2})/(K _{\sigma}) \tilde{\psi} _{s} \beta \psi _{s\right),}

where

\mu is the rest mass of the neutrino and \beta is the Dirac matrix.

Noting that the transition probability has a sharp maximum for values of

p _{\sigma} for which - W + H _{s} + K _{\sigma} = 0 , this simplifies to t (8 \pi ^{3} g ^{2})/(h ^{4}) x \left| \int v ^{*} _{m} u _{n} d\tau \right| ^{2} (p ^{2} _{\sigma})/(v _{\sigma}) \left(\tilde{\psi} _{s} \psi _{s} - (\mu c ^{2})/(K _{\sigma}) \tilde{\psi} _{s} \beta \psi _{s\right),}

where

p _{\sigma} and K _{\sigma} is the values for which - W + H _{s} + K _{\sigma} = 0 .

Fermi makes three remarks about this function:

• Since the neutrino states are considered to be free,

K _{\sigma} > \mu c ^{2} and thus the upper limit on the continuous \beta -spectrum is H _{s} \leq W - \mu c ^{2} .

• Since for the electrons

H _{s} > mc ^{2} , in order for \beta -decay to occur, the proton–neutron energy difference must be W \geq (m + \mu)c ^{2}

• The factor

Q ^{*} _{mn} = \int v ^{*} _{m} u _{n} d\tau

in the transition probability is normally of magnitude 1, but in special circumstances it vanishes; this leads to (approximate) selection rules for

\beta -decay.

Forbidden transitions

See main article: article and Forbidden transition.

As noted above, when the inner product

Q ^{*} _{mn} between the heavy particle states u _{n} and v _{m} vanishes, the associated transition is "forbidden" (or, rather, much less likely than in cases where it is closer to 1).

If the description of the nucleus in terms of the individual quantum states of the protons and neutrons is accurate to a good approximation,

Q ^{*} _{mn} vanishes unless the neutron state u _{n} and the proton state v _{m} have the same angular momentum; otherwise, the total angular momentum of the entire nucleus before and after the decay must be used.

Influence

Shortly after Fermi's paper appeared, Werner Heisenberg noted in a letter to Wolfgang Pauli^[12] that the emission and absorption of neutrinos and electrons in the nucleus should, at the second order of perturbation theory, lead to an attraction between protons and neutrons, analogously to how the emission and absorption of photons leads to the electromagnetic force. He found that the force would be of the form

(Const.)/(r ^{5}) , but that contemporary experimental data led to a value that was too small by a factor of a million.^[13]

The following year, Hideki Yukawa picked up on this idea,^[14] but in his theory the neutrinos and electrons were replaced by a new hypothetical particle with a rest mass approximately 200 times heavier than the electron.^[15]

Later developments

Fermi's four-fermion theory describes the weak interaction remarkably well. Unfortunately, the calculated cross-section, or probability of interaction, grows as the square of the energy

\sigma ≈ G ^{2} _{\rm F} E ^{2} . Since this cross section grows without bound, the theory is not valid at energies much higher than about 100 GeV. Here is the Fermi constant, which denotes the strength of the interaction. This eventually led to the replacement of the four-fermion contact interaction by a more complete theory (UV completion)—an exchange of a W or Z boson as explained in the electroweak theory.

The interaction could also explain muon decay via a coupling of a muon, electron-antineutrino, muon-neutrino and electron, with the same fundamental strength of the interaction. This hypothesis was put forward by Gershtein and Zeldovich and is known as the Vector Current Conservation hypothesis.^[16]

In the original theory, Fermi assumed that the form of interaction is a contact coupling of two vector currents. Subsequently, it was pointed out by Lee and Yang that nothing prevented the appearance of an axial, parity violating current, and this was confirmed by experiments carried out by Chien-Shiung Wu.^{[17] [18]}

The inclusion of parity violation in Fermi's interaction was done by George Gamow and Edward Teller in the so-called Gamow–Teller transitions which described Fermi's interaction in terms of parity-violating "allowed" decays and parity-conserving "superallowed" decays in terms of anti-parallel and parallel electron and neutrino spin states respectively. Before the advent of the electroweak theory and the Standard Model, George Sudarshan and Robert Marshak, and also independently Richard Feynman and Murray Gell-Mann, were able to determine the correct tensor structure (vector minus axial vector,) of the four-fermion interaction.^{[19] [20]}

Fermi constant

The most precise experimental determination of the Fermi constant comes from measurements of the muon lifetime, which is inversely proportional to the square of (when neglecting the muon mass against the mass of the W boson).^[21] In modern terms, the "reduced Fermi constant", that is, the constant in natural units is^[22]

G ^{0= (G _{\rm})/((\hbar c) ^{3})} _{\rm F} = (\sqrt{ 2)/() }\frac=1.1663787(6)\times10^ \; \textrm^ \approx 4.5437957\times10^ \; \textrm^\ .Here, is the coupling constant of the weak interaction, and is the mass of the W boson, which mediates the decay in question.v = \left(\sqrt{ 2 } G ^{0 \right)} _{\rm F} ^{-1/2} \simeq 246 . 22 rm{GeV} .^[23]

More directly, approximately (tree level for the standard model),

G ^{0 \simeq} _{\rm F} (\pi \alpha)/(\sqrt{ 2) ~ M ^{2} _{\rm W} ( 1- M ^{2} _{\rm W} /M ^{2} _{\rm Z} )}.

This can be further simplified in terms of the Weinberg angle using the relation between the W and Z bosons with

M _{Z = (M _{W})/(\cos\theta _{W})} , so thatG ^{0 \simeq} _{\rm F} (\pi \alpha)/(\sqrt{ 2) ~ M ^{2} _{\rm Z} \cos ^{2} \theta _{\rm} \sin ^{2} \theta _{\rm} }.

Notes and References

1. Yang . C. N. . 2012 . Fermi's β-decay Theory . Asia Pacific Physics Newsletter . 1 . 1. 27–30 . 10.1142/s2251158x12000045.
2. Book: Feynman, R.P. . Theory of Fundamental Processes. 1962. W. A. Benjamin. Chapters 6 & 7.
3. Book: Griffiths, D. . 2009 . Introduction to Elementary Particles . 2nd . 314–315 . 978-3-527-40601-2.
4. Book: Fermi, Enrico . Fermi Remembered . University of Chicago Press . 241-244 . 2004 . 0226121119 . Edited by James W. Cronin.
5. Book: Close, Frank . Neutrino . Frank Close . Oxford University Press . 23 February 2012 . 24 . 978-0199695997 .
6. Book: Pais. Abraham. Inward Bound. registration. 1986. Oxford University Press. Oxford. 0-19-851997-4. 418.
7. Book: Schwartz, David N. . The Last Man Who Knew Everything. The Life and Times of Enrico Fermi, Father of the Nuclear Age . Basic Books . 2017 . 978-0465093120 . Part II, Section 8, notes 60, 61, 63. According to Schwartz, it is not proven that there was a retraction by the magazine, since the archives relating to those years were lost during a move. He argues that it is even unlikely that Fermi seriously requested publication from the journal, since at that time Nature only published short notes on such articles, and was not suitable for the publication of even a new physical theory. More suitable, if anything, would have been the Proceedings of the Royal Society.
8. Fermi . E. . 1933 . Tentativo di una teoria dei raggi β . . 2 . 12 . it.
9. Fermi . E. . 1934 . Tentativo di una teoria dei raggi β . . 11 . 1 . 1–19 . 1934NCim...11....1F . 10.1007/BF02959820 . 123342095 . it.
10. Fermi . E. . 1934 . Versuch einer Theorie der beta-Strahlen. I. . 88 . 3–4 . 161 . 1934ZPhy...88..161F . 10.1007/BF01351864 . 125763380 . de.
11. Wilson . F. L. . 1968 . Fermi's Theory of Beta Decay . . 36 . 12 . 1150–1160 . 1968AmJPh..36.1150W . 10.1119/1.1974382. Includes complete English translation of Fermi's 1934 paper in German
12. Book: Pauli, Wolfgang . Scientific Correspondence with Bohr, Einstein, Heisenberg a.o. Volume II:1930–1939. 1985. Springer-Verlag Berlin Heidelberg GmbH. p. 250, letter #341, Heisenberg to Pauli, January 18th 1934.
13. Book: Brown, Laurie M. The Origin of the Concept of Nuclear Forces. registration. 1996. Institute of Physics Publishing. 978-0-7503-0373-6 . Section 3.3.
14. Yukawa. H. . 1935 . On the interaction of elementary particles. I.. Proceedings of the Physico-Mathematical Society of Japan . 17 . 1 . en.
15. Book: Mehra, Jagdish . The Historical Development of Quantum Theory, Volume 6 Part 2 (1932–1941). 2001. Springer. p. 832.
16. Gerstein . S. S. . Zeldovich . Ya. B. . Meson corrections in the theory of beta decay . 1955 . . 698–699.
17. Lee . T. D. . Yang . C. N. . 1956 . Question of Parity Conservation in Weak Interactions . . 104 . 1 . 254–258 . 1956PhRv..104..254L . 10.1103/PhysRev.104.254. free .
18. Wu . C. S. . Ambler . E . Hayward . R. W. . Hoppes . D. D. . Hudson . R. P. . 1957 . Experimental Test of Parity Conservation in Beta Decay . . 105 . 4 . 1413–1415 . 1957PhRv..105.1413W . 10.1103/PhysRev.105.1413. free .
19. Feynman . R. P. . Gell-Mann . M. . 1958 . Theory of the Fermi interaction . Physical Review . 109 . 1. 193 . 10.1103/physrev.109.193. 1958PhRv..109..193F .
20. Sudarshan . E. C. . Marshak . R. E. . 1958 . Chirality invariance and the universal Fermi interaction . Physical Review . 109 . 5. 1860 . 10.1103/physrev.109.1860.2. 1958PhRv..109.1860S .
21. Chitwood . D. B. . MuLan Collaboration . MuLan . 2007 . Improved Measurement of the Positive-Muon Lifetime and Determination of the Fermi Constant . . 99 . 3. 032001 . 0704.1981 . 2007PhRvL..99c2001C . 10.1103/PhysRevLett.99.032001. etal . 17678280. 3255120 .
22. Web site: CODATA Value: Fermi coupling constant . June 2015 . The NIST Reference on Constants, Units, and Uncertainty . . 2016-10-31.
23. Plehn . T. . Rauch . M. . 2005 . Quartic Higgs coupling at hadron colliders . . 72 . 5 . 053008 . hep-ph/0507321 . 2005PhRvD..72e3008P. 10.1103/PhysRevD.72.053008 . 10737764 .