If you’re a particle physics enthusiast – and, I mean, who isn’t – you remember
2012 pretty well.
It was on July 4th of that year that two large experimental groups used data taken at the
Large Hadron Collider at the CERN laboratory to announce that they had discovered the Higgs
boson.
Before the Higgs boson was discovered, the Standard Model of particle physics was incomplete.
The discovery of the Higgs boson and, by extension, the Higgs field, completed the Standard Model.
This discovery led to British physicist Peter Higgs and Belgian physicist Francois Englert
to share the 2013 Nobel Prize in physics.
So that was a big deal, but the reality is that there were still some unknowns when the
particle was found.
Was it the Higgs boson predicted by Higgs and Englert back in the 1960s?
Or was it one of many?
Those are very important questions.
But one of those questions was particularly important question and needed to be answered.
You may not realize it, but the original Higgs idea was proposed to answer an important but
obscure question.
It was in the 1960s that theoretical physicists were able to show that they could unify the
weak nuclear force and electromagnetism.
Now that turned out to be a surprising thing for a couple of reasons.
The electromagnetic force behaves similar to gravity.
It is a force with an infinite range, which weakens as the square of the distance between
two charged objects.
That’s just mathematical fancy talk for saying that if you double the distance between
two objects, the force between them drops by two squared or four.
If you triple the distance between them, the force drops as three squared, or 9.
In contrast, the weak force doesn’t have an infinite range.
In fact, it seems to work for distances about one-one-thousandth that of a proton, but then
doesn’t have any real effect after that.
Yet scientists were saying that these two forces were the same thing.
That didn’t make any sense until the Higgs field was invented.
The Higgs field is kind of like a bandaid theory that was added on.
It gave mass to the particles that transmit the weak force and didn’t give mass to the
particle that transmits electromagnetism.
The name of the weak force particles are the W and Z bosons, while the name of the particle
that causes electromagnetism is the photon.
So that’s how the Higgs field originally fit into the theory- it gave mass to the W
and Z bosons and not to the photon, which is also a boson.
But there are other particles in the Standard Model, specifically the quarks and leptons
which are the particles that actually make up matter.
Quarks and leptons are fermions, not bosons.
The difference between a fermion and a boson is that fermions have a different amount of
spin compared to bosons.
The fermions have a spin of 1/2, 3/2, 5/2 and so on, while the bosons have a spin of
0, 1, 2, etc.
I’ll talk more about the significance of the differences of fermions and bosons in
a future video, but the bottom line is that they are different and the original Higgs
theory only gave masses to bosons.
But it would sure be economical if the Higgs field would also give mass to the fermions.
It doesn’t have to be that way- the massive fermions and heavy bosons could have gotten
their mass from different sources.
So a very important test was to see if the fermions also got their mass from the Higgs
field.
So how would you do that?
Well, to do that you have to remember a very crucial point.
You often hear people say that Higgs bosons interact more with heavy particles and that’s
true.
But there is a better way to say that.
The correct way to say it is that particles that interact more with the Higgs field and
boson get more mass.
This is very subtle point, so I’ll say it again.
It’s not that heavy particles interact more with the Higgs field.
It’s that particles that interact more with the Higgs field become heavy.
It’s the interaction with the field that comes first and the mass is the consequence.
So how can we test this?
Given this connection between the mass of particles and the degree to which the Higgs
field interacts with them, we can predict into which particles the Higgs boson prefers
to decay and which it doesn’t.
And remember that we can measure the mass of the particles without knowing anything
about the Higgs boson at all.
We’ve been doing that for decades.
We see here the mass of all of the known subatomic fermions.
The electron has a mass of 0.0005 billion electron volts, while the top quark is the
king of the subatomic world with a mass of 172 billion electron volts.
And the other particles are somewhere in between.
If we assume that the mass of the fermions and bosons are both caused by their interactions
with the Higgs field, we can use that theory to predict how often the Higgs bosons will
decay into those particles.
And the prediction is shown here on this graph.
If the Higgs theory is right for both fermions and bosons, the data for each particle should
appear exactly on this line.
So, let’s take a look, shall we?
Okay, so what happens when we add the data for the muon?
Well the black circle is the actual measurement, but the vertical line is the uncertainty on
the measurement and as long as the black line crosses the prediction or at least comes very
close, we can call that an agreement.
So the muon measurement agrees with theory, although the uncertainty is pretty big.
What about the tau lepton, which has a mass of 1.8 billion electron volts?
How do Higgs bosons decaying into tau leptons look?
We see that that measurement agrees with the prediction pretty well.
What about the bottom quark with a mass of about 4.2 billion electron volts?
We see pretty good agreement, although the error bar doesn’t quite cross the prediction.
But it’s close and this is real data, so that’s considered reasonable agreement.
And when we add the heavyweight top quark, with its mass of 172 billion electron volts,
we see again pretty good agreement.
So far, so good.
Now what about when we add the heavy bosons?
What sort of agreement do we see there?
The W boson has a mass of 80 billion electron volts and we see it plops right on the line.
And finally, we add the Z boson, with a mass of 91 billion electron volts.
We see that this also agrees with prediction.
So this is really extremely impressive support for the idea that the particle discovered
back in 2012 is really the Higgs boson predicted back in 1964.
On the other hand, that means that we aren’t exactly sure what to do next.
While a success like this needs to be celebrated, in some ways it would have been far more interesting
to see some real discrepancies between the prediction and the data.
But the universe isn’t obliged to give us surprising results.
We scientists must accept the truth, whatever that may be.
So we’ll keep looking into the data, hoping to find the next clue.
But, in the meantime, I’d like to propose a toast for all of my colleagues on the ATLAS
and the CMS experiments, as well as the amazing LHC operators for a measurement that is absolutely
astounding.
Cheers.