What is lepton universality?
Now there aren't many different types of particles that make up matter.
There are six types of quarks.
Up. Down. Strange. Charm. Top and bottom.
And six types of leptons.
Three of these leptons are charged. Three of them are neutral.
Electrons are the most common charged lepton.
But there are two others - the muon and the tau lepton
that behave in a similar way to the electron
but they're just progressively more massive.
Now our theory of particle physics, the standard model,
assumes that the behaviour of electrons, muons and tau leptons
are identical to each.
And that means that if a quark decays to a lepton,
which it can do sometimes under one of the fundamental forces,
then all things being equal it should decay
into electrons, muons and taus equally often.
And this assumption is the one that goes by the grand title - lepton universality.
Now it is just an assumption.
And because it's an assumption, it's really important to test this.
Because if the assumption is wrong, then so is the standard model,
and so is our understanding of particle physics.
So we really have to make sure about it.
Now one way that lepton universality can be tested
is to look how often particles called B mesons,
these are particles that contain bottom quarks,
decay to kaons and either an electron and anti-electron pair,
or a muon, anti-muon pair.
If lepton universality holds we should see B meson
decaying to electron pairs as often as B mesons decay to muon pairs.
If they don't, then something weird is happening.
LHCb can make measurements of lepton universality.
And that's because when the large hadron collider
collides its beams 40 million times a second inside the experiment
it generates huge data sets of quarks and anti quarks.
Amongst which are bottom quraks and anti-bottom quarks
that form the B mesons we want to study.
By analysing all LHC collision data,
we can record collisions that contain
B mesons decaying to a kaon and a pair of electrons or a pair of muons.
And then, simply, we count how many of each type we have.
And compare one to the other.
And see whether we have any differences or not.
Now LHCb first made a measurement of this back in 2014,
a really preliminary measurement. However, a measurement that was interesting.
Because it seemed to show that muons were
being produced 25% less often than electrons.
However, it was a really preliminary measurement
and it definitely wasn't significant enough to really get excited about.
Since that time we've been analysing all of our data.
And what we've measured now with higher precision
is that muons seem to be produced in B meson decays
15% less often than electrons are produced.
This is a tantalising result because it's not yet definitive.
There's still a chance, a small probability, 1:100,
that what we're seeing could just be a fluctuation
from B mesons decaying to electrons and muons at the same rate.
So in order to be absolutely sure, we're going to have to analyse more data.
To get that final significance.
But the reason why this is so interesting and thrilling
is that besides being unexpected,
this could genuinely be the first sight of something new.
Because if, when we've analysed more data,
this difference between muons and electrons stays,
and lepton universality seems to be violated.
Well, what that means is, there's something wrong with the standard model.
We found the crack, finally, in our understanding of particle physics.
There's absolutely no way that we can explain
this observation with our current understanding.
In fact the only way we think we can explain what's going on
is if there are new particles in the universe
associated with new physics processes beyond those that we understand.
And that these new particles are interfering with the B mesons as they decay.
And suppressing the decay to muons compared to electrons.
Now if that turns out to be the case that will truly be momentous.
Forget the Higgs, this will be the first sign of physics beyond the standard model.
And it's what we've built the LHC to find out.