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You probably heard the big news in 2012.
Titles like “Physicists find elusive particle seen as key to universe“ or “God particle
found”.
It was a big day in particle physics and one of the biggest scientific triumphs in decades.
The last piece of the standard model was discovered, the Higgs boson, a unique particle responsible
for giving fundamental particles their mass.
With all the hoopla, you would assume the scientists must have seen or somehow measured
the particle.
But no such thing happened.
It was never seen nor measured.
What?
How is that possible?
Yes, it’s true.
But if scientists can declare the discovery of a particle without ever having seen or
measured it, then we have to ask the question, what is a measurement anyway?
In this video I will tell you the truth about particle discoveries and how we can’t really
see most the particles of the standard model, but we know they exist.
How can we be so sure?
I’m going to explain that coming up right now…
The chart that you are looking at represents the Standard Model of particle physics.
It lists all the fundamental particles that we know of.
Everything in the universe that we can see is made up of them.
This chart along with all the equations representing how all these particles interact, represents
the best theory we have about how the universe works.
But here is a hard truth about these particles.
We have never actually seen or measured most of them.
The standard model is based on something called quantum field theory.
According to this theory, a particle is nothing but an excitation, or a kind of wave in a
quantum field that permeates all of spacetime.
Each particle is an excitation or a quanta of energy in its own field.
So for example, an electron would be quanta of energy equal to 0.511 Mega Electron Volts, or MeV, the rest mass
of an electron, in the electron field.
Similarly, an Up Quark would be an excitation of 2.2 MeV in the Up Quark field.
If you have a field and add a quanta of energy, you get a particle.
If you add another quanta, you get a two particles in the field, and so on.
Since the Higgs particle has a mass of 125 Giga Electron Volts, or GeV, you must add 125 GeV worth of energy
in the Higgs Field to form a Higgs particle.
Note that this a relatively very high energy level, equivalent to the rest mass of about
244,000 electrons.
So, based on this you might logically conclude that all we need to do is add 125 GeV to the
Higgs field, and boom, we can discover the Higgs particle, right?
In principle, yes.
That’s all you need to do.
In practice this is not easy, because the Higgs particle is extremely heavy compared
to other particle of the standard model. In fact, only the top quark is heavier at 173
GeV.
The issue is that most heavy particles of the standard model are not stable, because
they decay to lower mass particles.
This is also the case for the Higgs particle.
Why?
Because everything in the universe is intrinsically lazy and wants to go to its minimum state
of energy.
I have a video on why the universe is this way, and how entropy plays a role up here,
if you want to know the details.
The upshot of this is that is, as I said, heavy particles will be unstable and tend
to decay into lighter particles.
So the rules of particle physics is pretty simple.
If a particle can decay into something lighter, it will do so eventually.
And the greater the mass difference, which is also the energy difference, the faster
the particle will decay.
For example, the heavier cousin of the electron, the tau particle has a lifetime of around
2.9x10^-13 seconds.
The slightly lighter cousin of the electron, the muon particle, has a lifetime of 2.2x10^-6
seconds.
But the electron is stable simply because it’s the lightest of its type.
There are some details to this that we are ignoring right now, but in short, there is
no lighter particle which the electron can turn into, so it is stable.
Now the issue is that since there are many particles of the standard model that are lighter
than the Higgs particle, there are many energy paths that lead to more stable particles if
you start out with the Higgs particle.
Consider for example the electron.
Its mass is almost 250,000 times less than the Higgs mass.
In protons, which are stable, we have up and down quarks which have masses of around 2
and 5 MeV.
This is 5 and 10 times heavier than the electron, but still nowhere close to the Higgs mass.
The other heavier quarks are all less stable, and are thus not readily available in the
universe.
And neutrinos are even lighter and almost never interact with any other particle, so
they are completely hopeless to work with in this context.
At this point you might think, how the heck can we ever even hope to make such a heavy
particle as the Higgs, if the only stable things we have to work with are particles
thousands of times lighter?
Well, here we are rescued by the mass-energy equivalence principle.
Recall that mass is only part of the energy
of the particle.
So if you want to create a Higgs particle from electrons, you would need to give it
a lot of momentum, or kinetic energy, such that you add the equivalent of about 250,000
times its rest mass, such that its total energy is 125 GeV.
Now, this is not the full story.
You can’t just turn an electron into Higgs particle, but the general idea is that we
can form heavy particles from lighter particles by accelerating them and smashing them into
each other and by combining potential energy from mass and kinetic energy from movement,
so that we have enough energy to make something heavier.
This is the principle behind particle accelerators like the Large Hadron Colliders at CERN in
Geneva, the world’s largest and most powerful collider.
In principle, you could make the Higgs by smashing an electron and an anti-electron.
But you’d have to pump a lot of energy to do this.
The LHC actually accelerates protons, not electrons, to do this because it’s a bit easier since a proton
is much heavier at around 1 GeV, so it needs less kinetic energy to create the Higgs particle.
But now, the question is how do you detect the Higgs once it is made?
This is the tricky part, because you cannot detect the particle itself directly.
There are two reasons for this.
First, the protons collide with the same energy, but in opposite directions.
The combined momentum is roughly zero.
This means that if you create a Higgs boson in the accelerator, it will be roughly stationary
in the particle beam.
It’s difficult to detect something that doesn’t move because the detectors only
pick up particles that fly away from the collision.
The detector is built around the beam.
It cannot be built within the beam because the energy of the beam would destroy the detector.
So anything formed within the beam is not really detected. Only things that fly away.
Furthermore, because the Higgs is really heavy, it’s not stable.
Its lifetime is incredibly short at around 1.5x10^-22 seconds.
So it decays effectively instantly.
These two things make the Higgs boson practically impossible to detect.
It should be no surprise then that we have never actually detected it.
But on top of that, the Higgs is not a charged particle.
And since we generally rely on some electromagnetic interaction to physically detect a particle,
it’s not clear exactly how you would detect it even if it could reach the detector.
If all that is true, you have to ask the question then, what’s all the fuss about regarding
its so called “discovery” of the Higgs?
As it turns out, you don’t need to measure it to know that it’s there.
Think of the dinosaurs.
We don’t see them today, yet by looking at fossils, we can learn a lot about them.
And we can certainly conclude that they once existed.
We can do something similar with the Higgs, by looking at its decay products.
The idea is something like this: if you smash two protons together and get an event where
the sum of the decay products adds up to the mass of the Higgs, then we can reasonably
conclude that the event likely created a Higgs particle.
But you might ask, what if the event created random interactions which just happened to
yield a decay products equal to the Higgs mass.
Yes, that could happen.
And this is called background noise.
But the trick is, that if you many multiple measurements over a long period of time, then
you can eliminate the possibility of just random interactions that just happen to add
up to the same mass.
And in this case, you can make a reasonable conclusion that the consistent signal you
are detecting, in the form of an unexplainable spike, represents in fact, the mass of the Higgs particle,
and is not just random noise..
And in the case of the 2012 announcement,
this spike achieved 5 sigma significance, which is the gold standard in particle physics,
for determining that a new particle was detected.
So the guys working at the LHC declared the discovery of the Higgs boson not because they
had actually measured the Higgs particle itself, but because they measured events that could
only make sense if the Higgs particle had been there.
It is thus as statistically significant discovery.
And given that it is 5 sigma, the chance of this being just a statistical fluke is 1 in
3.5 million, which is pretty low.
Today in fact the measurement has achieved a higher than 6 sigma significance and so
there is no longer any doubt that the particle is there.
And it turns out that in there are many other particles, that we also never actually directly
measures, because of similar limitations.
For example, the quarks and gluons that make up protons and neutrons, cannot because of
the nature of the strong force, ever be directly detected.
Yet, scientists still claim we discovered them.
They can make this claim because the procedure of their discovery is similar to that of the
Higgs.
We smash things together and end up with a result, and from looking at the decay products
of the process, we declare that it statistically only makes sense if these particles where
in fact present as part of the process we measured.
This next part of the video is going to be the most interesting part to some of you,
who love details.
I am going to talk about exactly how the Higgs particle was created at the LHC and how it
was detected.
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Now, back to our program.
While there are several ways it can be made, the most prominent process used at the Large
hadron collider is the so-called gluon fusion process.
What happens is that in some high energy proton-proton collisions, two high energy gluons can be
produced.
These can, in some cases, due to strong force interactions, turn into top quarks, and fuse
together via a triangle loop.
This loop represents top quark, and anti-top quark creation and annihilation.
The energy of this annihilation can create a Higgs boson.
In general, when particles decay, they tend to decay into the next highest mass particle.
Because the top quark has the highest mass of all elementary particles, its annihilation
is most likely to result in a Higgs boson which has the second highest mass of all the
elementary particles.
This Higgs particle of course, as I stated earlier, almost instantly decays.
So, what does it decay into?
A very interesting fact about the Higgs decay that led to its initial discovery is that
it came from the detection of photons.
Now this might seem really confusing to you, because I said earlier that the Higgs doesn’t
interact directly with massless particles like photons.
The key word there is “directly.”
The Higgs decays to form very heavy bottom/anti-bottom quarks, which are strongly coupled with the
Higgs.
And these quarks do interact with photons because they are electrically charged.
So what you get as a result, is a Bottom/anti-Bottom loop which annihilates into two high energy
photons.
And the energy of these photons adds up to the mass of the Higgs.
The photon is what we actually detect, not the HIggs.
I might make it sound simple, but this is actually a rather rare event.
But because the signal is very clean it is easy to detect, and was the way the Higgs
was first discovered.
Today, it has been found via many decay processes precisely as the standard model predicts.
Now, you might say, wait a minute, this is all bullshit, we need to see a particle to
conclude that it actually exists.
To this, I can’t completely disagree with you.
But I think you should keep in mind that physics is does not guarantee the truth, but only
the most reasonable explanation for the observations we make.
The hard truth is that evidence for the nature of reality is really nothing more than a statistically
significant result.
If you learned something, leave a like and a comment.
I will see you in the next video my friend.