“There are no physicists in the hottest parts of hell, because the existence of a ‘hottest part’ implies a temperature difference, and any marginally competent physicist would immediately use this to run a heat engine and make some other part of hell comfortably cool.” –Richard Davisson
The Large Hadron Collider is the most powerful particle accelerator in the world, capable of accelerating protons absurdly close to the speed of light: 99.9999991% of the way there! Within a 26-kilometer-long ring, largest in the world, protons are accelerated both clockwise and counterclockwise up to these incredible speeds, and smashed head-on into one another.
But these collisions don’t take place willy-nilly inside the ring. Instead, the protons are consolidated into tiny bunches, and strategically crossed at only two specific collision points: deep inside the two tremendous detectors designed to collect and analyze the debris from proton-proton collisions. These two detectors are ATLAS, which stands for A Toroidal LHC ApparatuS, and is huge!
ATLAS, at 44 meters (144 feet) long by 25 meters (82 feet) in diameter, is the largest detector ever built for an accelerator-collider, and weighs in at 7,000 tonnes. The other one is CMS, the Compact Muon Solenoid, and is the second largest detector ever built for these purposes. Although it’s “compact” in size only in comparison to ATLAS, it outweighs it by nearly a factor of two, coming in at 12,500 tonnes.
The two proton beams circle clockwise and counterclockwise — one atop the other — inside this 26 kilometer-long tunnel. Traveling in bunches spaced only 25 nanoseconds apart, they become highly focused and directed at one another, allowed to cross one another only inside each of these detectors.
Inside, a tiny fraction of the protons will collide with the ones going in the opposite direction, smashing the protons apart and producing highly relativistic debris. But these protons are moving incredibly fast, at 299,792,447 m/s (just 11 m/s slower than lightspeed), and traveling in bunches of literally trillions of particles.
These detectors face an amazingly difficult, twofold task: sifting through a firehose of collision data (an estimated 600 million collisions-per-second), while simultaneously detecting exactly which of the debris particles came from which collision (in which direction, with which energy) and from where. This is no small task when your raw “results” look like this.
But after sifting through (literally) hundreds of trillions of these ultra-high energy collisions, the ATLAS and CMS detector teams have released their preliminary results on the search for the Higgs boson, flooding the news media.
Before we plunge in to the results, like Tommaso, Clara and Peter have (admirably) done, let’s take a look at what we’re attempting to find, so that we won’t get swamped by speculations and misinterpretations.
These are the particles in the Standard Model of elementary particles. Quarks, coming in six flavors and three colors, leptons, of the charged (electron-like) and uncharged (neutrino-like) variety, as well as the force-carrying particles: the photon, the eight gluons, and the very heavy weak bosons (responsible for radioactive decay), the W± and the Z0. All told, these particles and the way they interplay with one another fundamentally and successfully explains every phenomenon ever observed, with the sole exception of gravitation.
Each of the quarks and leptons has an antiparticle, too, and over the years — mostly as a result of particle accelerators and colliders — we’ve actually detected and discovered all of them. All of them, that is, except the elusive Higgs boson, whose role is to give rest masses to all the other particles. And it’s no wonder we haven’t discovered it yet: it not only takes an incredible amount of energy to produce a Higgs, but the only theoretical processes that can do so even a fraction of the time are themselves incredibly rare!
Discovering the Higgs — and where (i.e., at what mass) we discover it — is incredibly important for telling us whether the Standard Model is likely the entire story, or whether there needs to be physics beyond it to explain what we see. For example, if we find the Higgs at energies of less than 120 GeV (where the mass of a proton is just under 1 GeV), there’s very likely supersymmetry in nature, a speculative but theoretically interesting idea, that would double the number of fundamental particles.
If, on the other hand, there’s no Higgs at all, anywhere then the Standard Model must necessarily need correcting, and the best idea we have is that quarks are actually not fundamental, but composite particles that can be broken apart, in a theory called Technicolor. Technicolor, of course, has its own technical predictions, and so if it exists, the LHC should see evidence of that, instead.
Of course, the Standard Model could also be right, and could also be the full story. What would it look like if there were a Standard Model Higgs and nothing else?
We’d expect that the Higgs would come in with an energy (convertible to a mass via E = mc2) of somewhere between 120 GeV and 140 GeV. But mass, in this context, doesn’t mean what you would expect. And that’s Heisenberg’s fault.
You almost certainly think about mass — which is equivalent to energy — as a fixed, constant property of an object. That if you make a million of a certain type of particle, they’ll all have the same exact mass. And this is true, kind of.
It’s exactly true if your particle is stable, and lives an arbitrarily long amount of time. But if your particle lives a short amount of time (Δt), the inherent uncertainty in its energy (ΔE) becomes large, thanks to Heisenberg’s uncertainty principle. For a long-lived particle, the uncertainty in energy (and hence in mass) is negligible. But for our purposes, what this means, theoretically, is that a very short-lived particle actually picks up an intrinsic uncertainty in its mass.
So if you make a bunch of them, they’re not all going be at exactly the same energy; they’re going to appear spread-out over a range of energies. What’s even worse about the experimental search for the Higgs is that, even if you produce a few of them, and even if your detector performs its herculean task perfectly, it’s only going to measure a finite number of events with a finite resolution. So the best you can hope for, with a realistic model of the Higgs, is for an experimental signature that looks like this.
And finally, just as there are multiple pathways for making a Higgs, there are multiple pathways for the Higgs to decay, which it will always do in incredibly (like, yoctosecond-scale) short order. So if you want to find the Higgs, you need to look for its decays into all sorts of things: photons, W-bosons, quark-antiquark pairs, gluons, leptons, etc.
Based on the number of Higgs particles you think you should be producing, how many of these events can you reconstruct into supposed Higgs? That is, you’re also going to produce photons, W-bosons, quark-antiquark pairs, gluons, leptons, etc., whether there’s a Higgs or not. So you calculate your expected background without a Higgs, do your experiment and measure what you actually see, and then ask yourself: do I see something interesting, and if so, how likely is it that it’s real, rather than just a statistical fluke.
Because we’ve “fluked” ourselves good once before.
Back in 1976, there were only four quarks that had been discovered, but suspicions were incredibly strong that there were actually six. (There are, in fact, six.) If you look at the above graph, the dotted line represents the expected background, while the solid line represents the signal published here from a E288 Collaboration’s famous Fermilab experiment. Looking at it, you would very likely suspect that you’re seeing a new particle right at that 6.0 GeV peak, where there ought to be no background. Statistically, you can analyze the data yourself and find that you’d be 98% likely to have found a new particle, rather than have a fluke. In fact, the particle was named (the Upsilon), but when they looked to confirm its existence… nothing!
In other words, it was a statistical fluke, now known as the Oops-Leon (after Leon Lederman, one of the collaboration’s leaders). The real Upsilon was found the next year, and you shouldn’t feel too bad for Leon; he was awarded the Nobel Prize in 1988.
But the lesson was learned. It takes a 99.99995% certainty in order to call something a discovery these days. So with all of this in mind, what did the CMS and ATLAS collaborations find, and did they discover anything?
Above is the results from the CMS team’s search for the Higgs in the high mass range: up to about 600 GeV. What you’re looking at shows a dotted line from what the “expected background” is supposed to be. Anything that falls within the green range is just where it’s supposed to be, anything that falls within the yellow range is perhaps suggestive of something interesting, but not at all compelling. What we’re really hoping for is whether anything rises up and above that yellow region (into the white), which could be construed as evidence (if not yet a discovery) of something new and interesting. Barring that, anything that sinks down below the red line — Biggest Loser-style — is ruled out as being the Higgs.
As you can see, CMS sees nothing in the high-mass range that’s at all interesting. Well, what about in the low-mass range? Remember, that’s the one we expect to be more interesting!
Ahh! Now, this is not a discovery, and it’s not even particularly strong evidence, but if I were a detective, I would have just uncovered a set of fingerprints looking at this!
First off, the interesting part of the “low-mass” range just got a lot smaller; anything for the Higgs of over about 127 GeV is looking pretty unlikely, thanks to CMS. But looking just under 127 GeV, there is an excess. It isn’t a huge excess, but there’s definitely something interesting there. The next step would be to look at the different decay channels. Because if this is evidence of a Higgs, I should see an excess in each of the channels, and of roughly the same magnitude. What do we get when we break it down?
This is so hard. I look at this and I know, immediately, that I need more data to know whether there’s something interesting, or whether I’m just seeing a fluctuation. If all I had was the CMS data, I’d be well within my rights to publicize what we’ve ruled out, and to speculate that if there was anything interesting, it would be in the low-mass range.
But we are not restricted to CMS results, there’s a whole other collaboration doing these experiments! So let’s find out: what did ATLAS see?
Taking a look at the high-mass range, it’s almost the same story as CMS. Heavy exclusions, mostly, with hardly anything appearing even mildly interesting. (Maybe that little bump at ~250 GeV, but even that’s still in the yellow.)
But — here’s the deal — based on what I’ve already seen from CMS, I’d be really interested if ATLAS, working completely independently, saw something in that same below 127 GeV energy range. So what do they see at low energies?
You’re kidding, right?
Right there, at 126 GeV, is a bump in ATLAS’s data?! As others note, this is really interesting. Officially, the significance and strength of this signal — if there is a Higgs right at 126 GeV — is perfectly consistent with a Standard Model Higgs boson, and nothing more. Furthermore, independently of CMS, anything over 131 GeV is ruled out, all the way up to that bump at ~250 GeV.
But, as you may have guessed, this is not significant enough to claim a discovery. It’s estimated that the Large Hadron Collider will need to take about 3-4 times as much data in order to know at the desired confidence levels whether or not there’s a Higgs at the low-end of our mass range. Although it’s extremely suggestive that both CMS and ATLAS see something there, the evidence isn’t nearly as strong as we’d like. That said, if you combine all the data from the different ATLAS channels, what do you get?
The odds that you’d get a combined fluctuation like this are something like 1-in-50, if you consider the entire range of where we looked. However, if you restrict yourself to looking in the same place that CMS saw their fluctuation, the odds of that happening are now something appallingly small, like 1-in-2000. It still isn’t good enough to claim discovery, but it’s striking enough that if you asked me to bet, I’d bet you even money that they confirm a Higgs somewhere around 126 GeV by the end of 2013. And that’s what I’m hoping for, too.
Some of the best physics takes on this that I haven’t yet linked to can be found here, here and here, and if you need a stream of constant, up-to-the-minute LHC news, I’ve got you covered. So now, you know what’s been found at the LHC and what it means for the Higgs; there’s nothing left to do but enjoy the findings, wait, and hope! Maybe I’ll see you back on the news sometime soon!