Is the Standard Model of Particle Physics Really Kaput?

A recent muon experiment at Fermilab is raising questions about the Standard Model of Particle Physics.
Marcia Wendorf
G-2 experiment at FermilabReidar Hahn/Fermilab

Lately, the internet has been awash with stories about the Standard Model of Particle Physics having been broken.

These stories arise from a recent experiment conducted by physicists at the Fermilab in Illinois, where a group of scientists discovered that muons "twerk." Well, sort of. Don't worry if don't know what a muon is, you'll find out in a moment, and if you don't know what twerking is, try googling Miley Cyrus.

Muons are electrically charged particles, which means that when they're placed within a magnetic field, they start to spin. Their frequency of rotation is determined by the muon's interactions with other particles and forces – this is called its g-factor.

Just like the Earth wobbles on its axis as it rotates so too does the spin axis of a muon also wobble. Twenty years ago, scientists at the Brookhaven National Laboratory first measured the g-factor and wobble of muons, and they came up with values that didn't match predictions made by the Standard Model. Brookhaven's data came in at 3-sigma, or three standard deviations.

Last week, Fermilab's G-2 experiment, which is still ongoing, concluded that the muons zipping around their magnetized ring wobbled more originally theorized. The group's findings rose to the level of 4.2-sigma, which is very close to the magic 5-sigma which corresponds to a 1-in-3.5 million chance that the data is a statistical fluke. Physicists consider 5-sigma to be irrefutable evidence of a discovery.

The question then is: "What is giving the muons that extra push that causes them to wobble?" One explanation is that they are being shoved by virtual particles that pop into and out of existence due to quantum fluctuations.

Virtual particles pop into existence in pairs — one of matter and one of antimatter. An example is an electron and its antimatter counterpart, a positron. If the muons are being shoved by virtual pairs of particles that are part of the Standard Model, well and good, but what if the muons are being affected by a pair of virtual particles that are unknown? This question is what is keeping physicists up at night.

What is the Standard Model?

The Standard Model of Particle Physics is the set of equations that describe all 17 of the known elementary particles. Elementary particles are particles that are not composed of other particles.

Before they were discovered, the Standard Model predicted the existence of, and properties of, the W and Z bosons, the gluon, and the top and charm quarks. The Standard Model also predicted the existence of the Higgs boson, which we'll get to meet in a minute.

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The Standard Model first began taking shape in 1897, when the English physicist J.J. Thomson discovered the electron, and it wasn't considered complete until 2012 when scientists at the Large Hadron Collider at CERN discovered the Higgs boson.

The chart below displays the particles comprising the Standard Model. They are divided into fermions and bosons, with the 12 fermions divided into six quarks and six antiquarks, and six leptons and antileptons. 

The Standard Model of Particle Physics
The Standard Model of Particle Physics Source: Wikimedia Commons/Marcia Wendorf


What distinguishes quarks is that they have something called color charge, which makes them interact via the strong force. Quarks can combine in either of two ways:

1.  A quark and an antiquark, called a meson.
2.  Three quarks, called a baryon. The lightest baryons are the proton and neutron.

Quarks also have an electric charge and weak isospin, which means that they can interact with one another via electromagnetism and the weak interaction.

Standard Model of Elementary Particles
Standard Model of Elementary Particles Source: MissMJ, Cush/Wikimedia Commons


Leptons don't carry color charge, so they don't respond to the strong force. Three of the leptons, the electron, muon, and tau, carry an electric charge and thus interact with other particles electromagnetically. Three of the leptons, the neutrinos, don't carry an electric charge, which means that they only respond to the weak force. This makes them very hard to detect.

Generations of Fermions

Just like the generations of people are comprised of grandparents, parents, and children, the fermions also come in generations, with every two members of a subsequent generation having greater mass than an earlier generation.

In the chart above, the first generation of quarks is comprised of the up and down quarks, the second generation is comprised of the charm and strange quarks, and the third generation is comprised of the top and bottom quarks.

First-generation charged particles don't decay, which is a good thing since protons and neutrons are comprised of up and down quarks, which are first-generation quarks. Second- and third-generation fermions do decay, which means that they have very short half-lives. A half life is the time it takes for one half of a sample to decay.

Later-generation fermions can only be observed in very high-energy environments, such as the Large Hadron Collider. Neutrinos pervade our universe, and all three generations don't decay. However, neutrinos are very hard to detect because they hardly ever interact with matter.

Gauge bosons

Our universe has four fundamental forces: electromagnetism, the strong force, the weak force, and gravity. Now for some bad news, the Standard Model cannot account for gravity, so for now, we're going to ignore it.

The Standard Model explains the other three forces as resulting from particles exchanging other particles, with the effect being that the force influences both particles. This is why the gauge bosons are called force mediating particles.

The electromagnetic force is transmitted between electrically charged particles by the photon, which has no mass. The weak force is transmitted between quarks and leptons by the W+, W−, and Z gauge bosons. These are massive particles, with the Z boson being more massive than the W±.

Now get ready for your head to hurt: W± bosons act on either left-handed particles or right-handed antiparticles, while the electrically neutral Z boson interacts with both left-handed particles and antiparticles.

The W± bosons carry an electric charge of +1 and −1, and they couple to the electromagnetic interaction, so when grouped with photons, they collectively mediate what is called the electroweak interaction. 

There are eight gluons that transmit the strong force amongst the six quarks. Gluons are massless, and because they themselves have a color charge, they can interact with one another.

The Higgs boson

The video of 83-year-old Peter Higgs taking out his handkerchief and wiping his eyes at the July 4, 2012 announcement at CERN that, at long last, the Higgs boson had been found is truly moving. Higgs had theorized the particle back in 1964.

The Higgs boson generates the masses of the leptons, the electron, muon, and tau, and the masses of the quarks. It does not generate mass for the photon and the gluon, and because the Higgs boson is itself massive, that means that it must interact with itself.

Not only is the Higgs boson massive, having a mass of around 125 GeV/c2, or about 133 proton masses, but it decays almost immediately once created. That means that the Higgs can only be created and observed in a very high-energy particle accelerator. Before it was observed at CERN, scientists at Fermilab had been searching for the Higgs.

A year after the discovery of the Higgs boson, in 2013, Peter Higgs was at last honored with a Nobel Prize in Physics, along with François Englert. The day of the announcement, Higgs wanted to avoid media attention, so he went out. He didn't own a cell phone so he only found out that he had won the Nobel when he bumped into a neighbor.

Is the Standard Model really kaput?

Just last month, New Scientist reported that scientists at the Large Hadron Collider have found a deviation from the predicted rates at which particles containing the bottom quark decay into an electron and a muon. While the production of electrons and muons should be equal, it isn't.

Other issues not explained by the Standard Model include:

  • Does the Higgs boson also give mass to neutrinos?
  • Around 95 percent of the universe is not made of ordinary matter but consists of dark energy and dark matter which do not fit into the Standard Model.
  • Gluons that convey the force of gravity have never been found.
  • Baryon asymmetry.
  • Neutrino oscillations and non-zero masses.
  • Why is the universe expanding ever faster?
  • Why is the universe comprised of more matter than antimatter?

The next couple of years will determine whether the Standard Model is still a correct representation of our universe, or whether it will need to be modified, or scrapped altogether. Whatever happens, it's going to be one heck of a ride.

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