Wednesday, 13 June 2012

The Hunt For The Higgs - Why So Long?

[EDIT: The Higgs was "discovered" with two independent observations averaging 4.9 Sigma around a week after I posted this.  4.9 ....pah.[/EDIT] 

The Large Hadron Collider's search for the Higgs boson continues.  It's been running for a few years now, gathering an unprecedentedly big amount of data, and everything seems to be pointing at a discovery being announced towards the end of this year (2012).

So the obvious question is, if they're expecting to discover the Higgs in November, why not just skip to whatever's on the LHC calendar for November 1st and save a whole bunch of time and money?

It's all down to the way the experiment works.  A hadron is simply a particle which is made of quarks, such as a proton or a neutron - the LHC uses protons because they carry an electrical charge, meaning you can use magnets to accelerate them and focus the beam.  Two "packets" of protons are sent whizzing round the LHC in opposite directions and smashed into each other in any one of several detectors, which are simply huge digital cameras with a few specialist add-ons.  When they smash into each other the protons are broken apart and a whole bunch of "debris" comes flying out and is picked up by the detectors.  The whole setup is essentially based on Einstein's famous equation, "E=mc2" and something called a "conservation law" which simply says that the accounts have to match up, you always get exactly the same amount of mass/energy coming out as you put in.

Now the Higgs boson, if it exists, is far heavier than a couple of protons, but there's a trick that can be used here.  If any object travels quickly enough then it becomes more massive, some of the energy of the motion is converted into mass, which is precisely what "E=mc2" actually means.  So 2 protons can equal a Higgs, as long as they're moving quickly enough.

So far so good, take two protons, bang them together hard enough, and out pops a Higgs.  Except it's never that simple is it?

The problem is that two protons can also equal a lot of other things.  There's a whole zoo of other particles, muons and pions, W+ and Z- particles, gluons, electrons and the mysterious neutrino, a particle that comes very close to not existing at all.  The list goes on, and you can get any combination of particles from any collision, as long as the accounts match up at the end of the day.  It's impossible to intentionally create a specific type of particle from any given collision.

A common analogy is to take a few thousand grand pianos and blow them all up with a few kilos of Semtex.  The job of the physicists at the LHC is to take all of the wreckage and work out how many pianos there were and exactly how a piano works in the first place.

Imagine you've just got a job at the LHC - you are now in charge of working out how many keys there are on a grand piano.  So on day 1 you blow up a piano and manage to piece together 85 keys, but there's a few bits left over, so it might be 86, 87 or 88.  The next day you blow up another piano and from the various bits you conclude there's 91 keys, but you think a few of them might have landed in a nearby river, so it might be fewer.  The evidence from the third piano suggests somewher between 87 keys and 90.  Carry on like this and eventually you'll find there's just one number that is common to all experiments, 88 keys, which as it turns out is the right answer.

So even though each individual experiment produces wildly different results and is difficult to get an answer from, on average they point to the right result.  This is what's happening with the search for the Higgs.  There are plenty of "candidate signals" where a physicist can point at a spike on a graph and say "that looks like a Higgs", but they're never sure, there are simply too many possible errors or alternative explanations.  As they gather more and more data, however, they get a better overall picture.  The more data, the more reliable the result is considered.

This reliability is measured in "sigmas", a statistical term.  One sigma means you can be approximately 68% sure about the measurement, two sigma is 95% accurate, three sigma is 99.7%.  If you have a three sigma measurement you're officially allowed to upgrade it to an "observation".

For a scientific "discovery" you need an overall confidence of five sigma, you need to be 99.999999% sure that it's not an error.

So this is what's keeping the Big Announcement on hold for the moment - many scientists at the LHC will quietly admit that they've "probably" seen the Higgs boson already, but "probably" isn't good enough for science.  They need a five sigma signal overall, and the only way to get that is to gather more data.  And to do that they need another five months or so.