Particle Physics

Is the Standard Model falling apart?

In the past few weeks, not one, but two scientific papers have hit the headlines, hinting at serious changes to the Standard Model. But is this a sign the equations just need to be tweaked, or is it time to throw the model out all together?

What is the Standard Model?

At its core, the standard model is a series of equations that attempts to describe everything in the universe- not too tricky then!?

But if you don’t have a degree level understanding of maths and physics (like me), it usually makes more sense shown as a table. 

And it’s a really neat way of describing what we see around us! 

If you already know about the standard model, or just want to know about the problems, skip to here

It’s split into 2 sections: the fermions and the bosons. Fermions are probably the easiest to wrap your head around:

In school you are first taught that the smallest unit of matter is the atom- then you learn the atom is made up of protons, neutrons, and electrons- then you learn that the protons and neutrons are made up of even smaller particles called quarks! Perhaps quarks are made up of something even more fundamental, but for today we shall be ending here and calling up quarks, down quarks and electrons the fundamental units of matter.

But they aren’t alone! Each type of fermion has 3 ‘generations’- aka 3 particles that have identical properties other than their mass- and the particles making up the atom are just the lightest of each. There are 12 fermions, split into quarks and leptons (which include electrons and neutrinos). These fermions can combine to make all sorts of combinations, but few are actually stable.

The bosons are a little trickier- they are the particles that ‘carry’ the force. One you may know of is the photon! The discovery that light can be modelled as both a wave and a particle was one of the first breakthroughs in quantum physics, which eventually led to today’s standard model. The photon is the force-carrier for light, or more generally, for electromagnetism. This means that every particle interaction that involves electromagnetism, is due to an exchange, release or absorption of a photon. This is also the case for the other 3 bosons:

  • The gluon mediates the strong force (which overcomes electromagnetic repulsion over short distances to hold the nucleus together) and only affects quarks, as they have a property called ‘colour’.
  • The W bosons and the Z boson mediate the weak force, which is involved in radioactive decay.

The Higgs Boson is the most recently discovered particle, and is responsible for giving the other particles mass. The more a particle interacts with the higgs field, the more mass they have.

All particles are just vibrations in a field of the same name.

What’s the problem?

The glaring problem is that it doesn’t describe the universe’s most easily seen fundamental force: gravity! Gravity is described really well by the other big theory in physics- Einstein’s relativity, but we are yet to find how it fits into the standard model. It’s been theorised that gravity is carried by the graviton, but the particle has proved elusive, and even when tested theoretically, it fails to describe reality as accurately as relativity does.

New measurements of the solar spectrum verify Einstein's theory of General  Relativity
model of gravity as a deformed ‘sheet’ of spacetime,
via, credit: Gabriel Pérez Díaz, SMM (IAC)

It also doesn’t describe dark matter or dark energy, at least not to the full extent. It’s possible that dark matter could be (either entirely or partially) described by MACHOS (massive compact halo objects- basically just dense clumps of normal matter that don’t emit light like brown dwarfs (failed stars) or the leftover matter after a star stops nuclear fusion (black dwarfs or old neutron stars)) but this is slowly being discounted, and the idea of dark matter being some kind of undiscovered particle becoming favoured. 

There are lots of other discrepancies in the model that can be ‘fixed’ with new particles.

And the new discoveries?

Muon G-2

Dianna at The Physics Girl youtube channel explained it really well in her latest video. When you spin a spinning top in a gravitational field (ie. on earth), you’ll notice that it precesses- the axis that its rotating around also moves in a circle, similar to how the earth moves as well!

Precession of the equinoxes | astronomy | Britannica
via Encyclopedia Britannica

Fundamental particles also have a property called spin, and if they are charged then they also wobble, but when in a *magnetic* field. This is called the magnetic moment, and the standard model predicts a specific value for how much the particle wobbles- for the muon this value (called the gyromagnetic factor) is exactly 2. Except it isn’t exactly 2 because it is affected by virtual particles. Virtual particles are a result of the uncertainty principle and ‘probabilistic nature’ of quantum mechanics and space-time, and randomly appear and disappear over very short timescales in what otherwise might be a vacuum! The effect of these interactions has been very carefully calculated, and results in the value for the magnetic moment of a muon being 2.00233183620- a very precise number, only just published last year!

But to prove a theory, it has to be backed up by experimentation, and so scientists across the world have been conducting experiments to test every part of the Standard Model. Recently, scientists at Fermilab (the national particle accelerator of the USA), have been investigating this very property of muons. They had decided on this experiment because it had already been done by Brookhaven, another US national lab, in 2001, which had hinted at an interesting value, but wasn’t precise enough to know for sure. This new experiment is much more precise, and it found the value to be 2.00233184122. 

Only the last 4 decimal places are different, but in a field that deals with very small numbers, this small change is enough to hint that there’s something else at play that we haven’t predicted, as the discrepancy could be caused by forces- and therefore particles- beyond the standard model!


Remember what I said about only the first generation of matter being stable? Well that means that the larger 2 decay into other more stable particles. There are actually many different pathways for that decay, and lots of different combinations of particles produced. The bottom quark (also known as the beauty quark) is the 3rd generation negatively charged quark. It usually decays into either electrons or muons, and the standard model predicts these will occur with the same probability- so the ratio of decays into muons to decays into electrons should be very close to one, but by analysing data from decays in the LHC-b experiment from 2011-2018, scientists have found the observed ratio is actually 0.846, so decays into electrons are more frequent than decays into muons. This new evidence goes against the standard model, which treats all leptons as identical other than their masses. 

The newest result is LHCb 9- as you can see it is the most precise yet and the error bars don’t include the expected ratio of 1

This could mean that there is an unknown force at play, which scientists have dubbed the ‘leptoquark’, a hypothetical boson which would interact with muons and electrons differently, thus causing the difference. 

So, is it time to throw out the Standard Model?

Not yet! Although the list of discrepancies in the model is growing, there is a lot the standard model correctly describes, a recent highlight being the prediction and subsequent discovery of the Higgs boson. Additionally, neither of the experiments have reached the gold standard 5 sigma significance level required for them to be called a real discovery- at the moment they simply offer evidence of a potential change, as the LHCb experiment still has a 1 in 1000 chance of being a statistical fluke (the g-2 experiment is slightly better at 1 in 40,000, but still not low enough). However, it adds to the growing bank of evidence that hints at least one new force we’ve been missing. It could be just one, or there could be a whole host of different particles yet to be discovered! Only time will tell as more data is crunched to reach that 5 sigma significance level, and more and more experiments are conducted across the field!

What do you think? Let me know in the comments!



3 thoughts on “Is the Standard Model falling apart?”

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