Higgs Boson Part III: How to Discover a Particle

Uploaded by minutephysics on 26.07.2012

Suppose you want to discover a particle. First, you need to-
Of course! Thanks for walking us through that point, John. If we're honest, we should say
that the mathematical model for the Higgs was discovered in the 1960s, but the particle
itself wasn't dis- wasn't confirmed until 2012. In fact, the Higgs boson ISN'T even
the first new particle to be... "uncovered" at the Large Hadron Collider: the Xi-b particle,
basically a heavy version of the neutron, was actually found several months earlier.
You probably didn't hear much about it because the Xi-b is just a combination of quarks that
we already know existÐ so it's not really that exciting. I mean, if you know about cheese
and you know about crackers, then the discovery of "cheese and crackers," as delightful as
it is, isn't likely to upend your universe.
But the Standard Model of particle physics also predicts something beyond cheese and
crackers - that is, about one out of every bajillion collisions should produce a Higgs
boson, which then decays into everyday stuff like electrons and photons, which are the
same crumbs we catch in the detector all the time.
This battle between the tiny chance for a collision to have produced a Higgs-like particle
versus all the trizillion other collisions that produce similar crumbs is part of why
we need a big machine like the Large Hadron Collider at all. There were earlier accelerators
that had enough energy to create Higgs bosons in principle - but they couldn't actually
do enough collisions to be confident they were actually seeing a Higgs boson and not
just an assortment of crumbs that looks by chance like it's from a Higgs Boson.
It's kind of like trying to find out if a 20-sided die is rigged. Maybe you suspect
it's twice as likely to land on a three than on any of the other numbers. But how can you
check? Well, that sounds easy enough - just roll the die a few times and if you see extra
threes, it's rigged, right?
Not so fast. For example, if you roll the die ten times, there's a pretty good chance
that you won't get any threes at all! That's because even though rolling a three is twice
as likely as all the other numbers, there are still a lot of other numbers you could
roll. So random chance and big numbers can be surprisingly deceptive - even if you roll
the die a hundred times and DO get an excess of threes, there's still a one in fifty chance
that the die IS fair and you just got this number by accident. How much are you willing
to bet that you actually have evidence for a new particle if there's a one in fifty chance
your results are simply a random fluctuation and the particle doesn't actually exist? What
if a Nobel Prize is on the line - how sure do you want to be? One in a thousand? One
in ten thousand?
Actually, physicists are even more stringent they won't say they've "discovered" a particle
unless the odds that they might get the same results even if the particle DOESN'T exist
are less than one in a million… so if you want to convince a particle physicist that
you've discovered an unfair die, you'll need to roll over five hundred and fifty times
to satisfy them! And that's just to check if a twenty-sided die is rigged – there
are far more than twenty possible outcomes of a high-energy particle collision, so in
order to be confident about announcing evidence for a new particle at the LHC, you need around
600 million collisions… every second… for two years. Only then can you uncork the
wine to go with your cheese and crackers, and claim a successful discov– I mean, successful
scientific fact-checking.