## Quantum Field Theory

### Gyromagnetic Ratio and Anomalous Magnetic Moment (+ 2021 Update)

The anomalous magnetic moment for the electron and muon illustrates the progress of our understanding in particle physics from classical mechanics to quantum theory, quantum field theory, the Standard Model, and beyond. A classical electron moving around a nucleus in a circular orbit has an orbital angular momentum, L=mevr, and a magnetic dipole moment, = -evr/2, where e, me, v, and r are the electron´s charge, mass, velocity, and radius, respectively. A classical electron of homogeneous mass and charge density rotating about a symmetry axis has an angular momentum, L=(3/5)meR2 , and a magnetic dipole moment, = -(3/10)eR2 , where R and are the electron´s classical radius and rotating frequency, respectively. The classical gyromagnetic ratio of an orbiting or a spinning electron is defined as the ratio

#### Figure 07 Classical g-ratio [view large image]

of the magnetic moment to the angular momentum. In both cases one finds cl = /L= -e/(2me). The minus sign indicates that is in the opposite direction to L (see Figure 07).

In quantum theory, the interaction between an electron and a magnetic field can be portrayed by the Feynman diagram in Figure 08a, which shows a photon from the magnetic field is absorbed by the electron and thus altered its trajectory. The gyromagnetic ratio derived from the Dirac equation takes the form: e = /L= -g e/(2me), where g = 2 is related to the fact that the spin of the electron equals to /2.  If the vertex correction as shown in Figure 08b is taken into account, then g = 2 ( 1 + /2 ), where = e2/(4  c) ~ 1/137.036 is the fine structure constant giving g - 2 = 0.002322814. The extra term arises from the electron self-interaction, in which it emits and reabsorbs a virtual photon, making a loop in the Feynman diagram as shown in Figure 08b. The same process also applies to the muon.

#### Figure 08b Vertex Cor-rection [view large image]

The more accurate calculation including higher loop diagrams up to the 4th order term yields the following expression:
g - 2 = 2 ( /2 - 0.328 ( /2 )2 + 1.181 ( /2 )3 - 1.510 ( /2 )4) = 0.0023193042800. The experimental value is: g - 2 = 0.0023193043768 (in agreement with the calculated value to ten significant figures).

In calculating the effects of the cloud of virtual particles, we need to include not just the effects of virtual photons and virtual electron-positron pairs, but also virtual quarks, virtual Higgs particles, and, in fact, all the particles of the Standard Model. It turns out, though, that because of the larger muon mass, any such heavy particles would affects the muon magnetic moment more than the electron magnetic moment. The muon g - 2 has been calculated with the Standard Model to a precision of 0.6 ppm (parts per million). The calculated value with the combined effect is g - 2 = 0.0023318360. A remarkable fact is that the muon g - 2 factor not only can be predicted to high precision, but also measured to equally high precision. The measurements at Brookhaven National Laboratory in 2001/2004 (Figure 09) yields an average value of g - 2 = 0.0023318416. Thus, the comparison of measurement and theory provides a sensitive test of the Standard Model. If there is physics not included in the current theory, and such new physics is of a nature that will affect the muon's spin, then the measurement would differ from the theory. This is what appears to have been observed, although there are several interpretations of the result that must be considered. One of the missing pieces in the theoretical calculation is the exotic particles predicted by the theory of supersymmetry. Although these particles are rare and unstable their mere existence in the vacuum would modify observable quantities such as the muon magnetic moment.

#### Figure 09 g-2 Experiment [view large image]

The modern version prefers to use a = (g - 2)/2 with the subscript to denote muon,
a (BNL) = 11659208x10-11 (from BNL Muon Experiment),
a (SM) = 11659180x10-11 (from SM theoretical calculation); a (BNL) - a (SM) = 28x10-11.
After a lapse of 20 years, the Muon g-2 collaboration at the Fermi National Accelerator Laboratory (FNAL) finally reports the latest measurements of updated muon g-2 on 7 April, 2021 (see "Measurement of the Positive Muon Anomalous Magnetic Moment to 0.46 ppm"). The new value of a (FNAL) = 11659204x10-11 (0.46 ppm); a (FNAL) - a (SM) = 24x10-11 confirming that there is a difference between theory and observations (both from BNL and FNAL, now the experimental value is averaged to a (Exp) = 11659206x10-11). Although the discrepancy seems to be rather small to the laymen, it is a big deal for theoretical physicists. For it reiterates the necessity to search for a new theory to resolve the "tension". Anyway, the confidence level of such discrepancy is claimed to be at 4.2 sigma, i.e., a near certainty but not enough to qualify as a discovery at 5 sigma, i.e., 1-in-3.5 million chance of a fluke.

#### Figure 10 Muon g-2, 2021 Update [view large image]

Figure 10 summarizes the theoretical, experimental processes and the prospect for new physics. The followings provides a little bit more details : • In the Standard Model of elementary particles (SM), it is the quantum fields that are ubiquitous in the formalation. As shown in Figure 11, the various quantum fields permeate throughout the Universe at ground state with virtual particles popping out incessantly according to the Uncertainty Principle. The particle becomes real when enough excitation energy is injected into the field. It is the virtual particles from these quantum fields that contribute to the anomalous magnetic dipole moment of muon (actually it is the correction to the charge or/and mass). The second order corrections from the various boson fields as shown in Figure 10,a include :
• #### Figure 11 Quantum Fields [view large image]

1. The vertex correction (to the electric charge) in QED (Quantum ElectroDynamics) is the dominant contribution to :
a(QED) = /2 = 0.0011617,
where = 1/137 is the fine structure constant. The original derivation was published 74 years ago in 1947 by J. Schwinger, Phys. Rev. 73, 416. The divergences in the expression for very high and very low 4-momentum |k2| of the virtual particles have been removed by mathematical tricks such as the "Regularization Schemes". The a(QED) = 0.00116584719 in Figure 10,a should be more accurate by including more virtual quantum fields such as the leptons.

2. The vertex correction in EW (Electro-Weak Interaction) could be large since EW = 1/30, i.e., about 4.5 times larger than . But the over all value is very small with a(EW) = 0.00000000153 because it is short range with EW = 0 for distance large than 10-17 cm from the particle, and the virtual particles in this case are the Z, W, and Higgs bosons which have mass ~ 100 Gev, i.e., more 100 times that of the proton; while the virtual particle (the photon) for QED has no rest mass.

3. 4. Contribution from hadron introduces some uncertainty because the calculation cannot empoly perturbation method. The a(Had) in Figure 10,a is estimated from the R-ratio methodology which asserts that cross-section in hadronic process is proportional to the similar QED process. The ratio is obtained by experimental measurements. Theoretical values are computed by "Lattice Theory". The numerical calculations consume enormous amount of computer time, but the results improve the agreement between a (SM) and a (Exp). Figure 12 shows the effect of the 2 different methods on the need for "new physics". Further theoretical and
5. #### Figure 12 Muon g-2, QCD experimental works on muon g-2 are still in progress. So the dis-agressment between theory and experiment is not a "done deal".
See "Why You Should Doubt ‘New Physics’ From The Latest Muon g-2 Results".
Update of May 5, 2021 : the new QCD lattice computation obtains a = (g-2)/2 = 116591950x10-11 bringing the theoretical value closer to the experimental 11659206x10-11 a (EXP) - a (SM) = 11x10-11. See "Prediction for magnetic moment of the muon informs a test of the standard model of particle physics", and "Leading hadronic contribution to the muon magnetic moment from lattice QCD".

• The experimental work involves simpler mathematics. The efforts are devoted to keep the magnetic field uniform, the alignment of the muon beams, ... (see "How does Muon g-2 work?"). The followings provides a little bit more details on the basic of experimental measurement :

1. Frequency of Larmor Precession - When a muon with spin S = /2 is placed in a magnetic field B, it experiences a torque which can be expressed in the form : = SB,
for which the direction of spin is assumed to be perpendicular to B and = ge/2m is the gyromagnetic ratio. The torque is also related to S by another formula (see Figure 13,a): = S (d /dt),
combining these 2 formulas yields : S = 2 /T = B = (ge/2m)B,
where T is the period of the precession.

2. Cyclotron Frequency - The cause of this effect is different from the case above. It is derived from the balance between the Lorentz and centripetal forces in the form (see Figure 13,b) : C = (e/m)B.

3. Anomalous Frequency - These 2 kinds of frequencies are the same in quantum theory with g = 2. However, when virtual particles are included in quantum field theory with g > 2, the difference becomes the anomalous frequency :   a = S - C = a(e/m)B,
where a = (g-2)/2.
The a can be measured as the spin direction is off a little bit from the orbital path. When the muon decays into positron, such product moves in the direction of the original muon spin and can be captured by the 24 detectors lining inside the storage ring (see Figure 14).

#### Figure 14 g-2 Experiment

• In the experiment, million muons with prefect alignment in spin direction are injected into the storage ring. The number would be gradually reduced as the muons decay into positrons and neutrinos with half-life a = 1.56 s. However, the detectors lining along the inside of the storage ring will experience cycle of maximum and minimum as the positrons point toward and then away from them with frequency a. The result is recorded in a graph such as the one in Figure 15. The corresponding formula is :
N(t) = N0e-(t/ a)[1 + cos( at + )]. A DIY evaluation of a = (g-2)/2 is straight forward by reading off the period Ta = 4.4x10-6 sec from Figure 15. It follows : a = 2 /Ta = 1.43x106 sec-1.
The rest of the input parameters includes B = 1.45 T = 1.45 kg/sec2-A, and
(m/e) = 1.176x10-9 kg/sec-A, finally

#### Figure 15 g-2 Calculation (DIY) [view large image]

a = (g-2)/2 = ( a/B)(m/e) ~ 0.00116.

• • As for new particles, the constituent for dark matter is still elusive after extensive searchings since 1980s. The LHC has failed to discover new particles including those SUSY superpartners so far. Another possibility is the 12 very heavy gauge bosons ( > 1015 Gev) from "Grand Unified Theory (GUT)", and the "Axion" etc. Anyway, even if the g-2 experiment has confirmed the requirement of "new particle", physicists would still have a hard time to find such particle to fit into the grand scheme of things.
• #### Figure 16 The 24 Quantum Fields

Figure 16 lists the 24 elementary particles (or quantum fields) in the Standard Model (should add the Higgs hoson discovered in 2012).

• Since the introduction of the Standard Model (SM) in early 1970s, it is found to be incomplete. It is "guilty" of at least 10 counts for failing to explain :
1. The cosmological constant or vacuum energy.
2. Dark energy.
3. The inflaton in the first fraction of a second of the Big Bang.
4. Matter-antimatter asymmetry.
5. Cold dark matter.
6. The form of the Higgs field.
7. Hierarchy problem - huge Higgs boson mass implies huge mass for all elementary particles.
8. Gravity.
9. Masses of the quarks and leptons.
10. Three generations of elementary particles.
Half of the list above from 1 to 5 is related to cosmology and astronomy. It emphasizes that understanding of the largest and the smallest phenomena must come together. Supersymmetry can address items 1, 3 - 7, while superstring theory may be able to explain items 8, 9, 10. Thus item 2 about dark energy remains to be the most enigmatic subject in physics and astronomy. The Superstring Theory (Figure 17) became the most studied since SM. It posits that the fundamental element is the one dimensional string open or closed. This particular choice is to satisfy with the demand of Lorentz invariance (i.e., the string formalism is unchanged under the Lorentz transformation, other shapes won't do), which guarantees all inertial observers using the same form of the theory. The theory requires the adoption of the not yet proven supersymmetry, adding extra-dimenstions, ... (see a list of other problems). It becomes obsolete over the years as there is no test to support the claims.

#### Figure 17 Superstring Theory Another approach is to keep the notion of composite "Point" particles with consecutive layers :

1. A "point" is always Lorentz invariant.
2. 3. A "point" avoids the invention of certain shape.
4. All quantum systems from molecule, to atom, to nuclei are composite system with the assumption of "point" constituents. The size of each one is getting smaller while the energy scale becomes bigger according to such sequence (see Figure 18).
5. The elementary particles in SM could be similar with the next level at even smaller size and requires higher energy (beyond the current capacity by LHC) to probe.
6. As mentioned in the "Introduction" 20 years ago, systems in the universe are somewhat similar to the Russian Doll.
7. #### Figure 18 GUT [view large image]

The Grand Unified Theory (GUT) just adds more gauge bosons to the list of elementary particles. The problem is the prediction of proton decay with half life of about 1032 years. The theory is abandoned for failure to detect the decay after years of experiments since 1980s.
It fails to provide further insight into the more "fundamental structure" in "elementary particles" such as electron and quarks.

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