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Hunting for Higgses

Another version of this post appears on Cosmic Variance, where night owls will also be able to follow Sean Carroll's liveblogging of the 3 am ET July 4 announcement from CERN.

Greetings from Geneva, where I'm visiting CERN to attend the much-anticipated Higgs update seminars on Wednesday, July 4. We're all wondering whether the physicists from the Large Hadron Collider will say the magic words "We've discovered the Higgs," but there's more detailed information to watch out for. I've been hard at work on a book on the subject, entitled The Particle at the End of the Universe, so I'm hoping for some big and exciting news, but not so big that I have to rewrite the whole thing. (Note that I'm a theoretical physicist, so I personally am not hunting for Higgses, any more than someone who orders catfish at a seafood restaurant has "gone fishing." The real hunters are the experimenters, and this is their moment to shine.)

Lecture fall
If at all possible, I'll try to live-blog here at CV during the seminars. They will start at 9am Geneva time, a slot chosen to enable a simulcast in Melbourne for people attending the ICHEP Conference. For folks in the U.S., not so convenient: it's 3 am Eastern time, Midnight (July 3/4) Pacific time. Here is the seminar announcement, and of course CERN will have a live webcast. Or try to, anyway; last time something like this was arranged, back in December, the live feed collapsed pretty quickly under the load. I'm sure I won't be the only one live-blogging: here's Aidan Randle-Conde and Tommaso Dorigo.

So what are we looking for?

The Higgs boson is the final piece of the Standard Model of particle physics, an amazing theoretical edifice that has stood the test of time for more than three decades now. The Standard Model features two kinds of particles. There are "fermions," the particles of matter, which take up space (you can't pile them on top of each other). Then there are "bosons," the force particles, which can pile up. The four forces include gravity, electromagnetism, the strong nuclear force, and the weak nuclear force. That last one-- the weak force--is the most messy and complicated of the forces. And the Higgs boson (or really the Higgs field from which it arises) is the reason why.

For the other forces (gravity, electromagnetism, strong), the force carrying particles all have precisely zero mass. That's not an accident; according to modern physics, the very existence of these forces reflects a deep symmetry of Nature's laws, and this symmetry demands that the force-carrying particles remain massless. For the weak force, the force-carrying particles are called the W and Z bosons, and they have quite substantial masses. The reason why (we believe) is that the underlying symmetry has been broken by the Higgs field.

The Higgs field breaks this hidden symmetry because it fills space--it has a nonzero value absolutely everywhere. You move through it as you go through your day, and you would still be moving through it if you were flying through interstellar space. It's invisible and hard to interact with, but it's there. And it doesn't only give mass to the W and Z bosons; it's also responsible for giving mass to all of the fermions that make up matter. Electrons, muons, quarks--all these particles get mass because of the Higgs. It's kind of a big deal.

We can't see the ambient Higgs field that is all around us; it's literally undetectable. What we can do is to create ripples in that field, vibrations that propagate through the universe. When we look very closely at ripples moving through a field, what we actually see are individual particles--that's a consequence of quantum mechanics you'll have to take my word for right now. When the Higgs field ripples, the particles we see are Higgs bosons. That's what they're looking for here at CERN, at the experiments of the Large Hadron Collider. They're smashing protons together at enormous energies, trying to get the Higgs field to jiggling, producing a boson that will revolutionize our understanding of the universe.

You don't find the Higgs by looking at an individual event in a detector and going "Yes! There it is!" For one thing, you don't see the Higgs at all; it decays too quickly. For another, there's nothing that the Higgs can decay into that can't be produced in other ways as well. So what the experimentalists look for is a tiny bump--an enhancement of the rate of some process at some specific energy. That's an indication that there's a particle with a mass equal to that energy, whose decays have led to the extra events. Here are some of the results from last year--see the bump at 126 GeV? (Particle physicists measure mass as well as energy in units of billions of electron volts, or GeV, approximately equal to the mass of a proton.)

Atlas data

We need statistics to tell whether any given feature is likely to be real or not, of course. We speak in "sigmas"--the number of standard deviations away from the expected value we're looking at. In other words, how likely is it that the data we collected would be produced as a random fluctuation, rather than because we're seeing a real effect? A three-sigma fluctuation happens less than 0.3% of the time, and that's considered "evidence for" something--that's what we got in December. A five-sigma result is less than one in a million; that qualifies as "discovery of" something, and that's what everyone is waiting for.

It's more complicated than that, of course. We have two experiments collecting data, CMS and ATLAS. Both are giant multipurpose detectors; the LHC smashes protons together inside them, and they detect sprays of electrons, photons, muons, and all sorts of hadrons. If the Higgs is really there, both experiments should have comparable sensitivity to looking for it--so we'd love to see similar signals in both detectors. If both see a signal in exactly the same place with high statistical significance but not quite five sigma, it will be impossible to resist mentally combining them and concluding the total evidence for the Higgs is better than five sigma. (Although there are reasons to resist...)

Besides comparing different experiments, we want to compare different channels within each experiment, and we want to compare experiment to theory. In the Standard Model, once you fix the Higgs mass, there are no more free parameters to mess with; you make precise predictions for the rate of production of any particles the Higgs might decay into. Deviating from those predictions implies that you haven't precisely found the Standard Model Higgs--but maybe a close relative thereof?

If the Higgs really is lurking at 125 GeV, Nature is giving us a very nice opportunity, because the Higgs should (if it's the simple Standard-Model version) decay into a variety of different particles, and we can study each one separately. Every experimental possibility is a different "channel." Here are the predictions for the simple Higgs at this mass. (Note that in some cases these particles quickly decay themselves.)

Higgs decay
To make sure that what you've really found is the Higgs, you'd like to check that it decays into all the various channels with the right percentages. Not all final states are equally easy to find, however. When the Higgs decays into quarks or gluons, it releases a spray of jets that tend to get lost in all the background noise of other processes. The same is true if it decays into W's or Z's or taus and then those decay into quarks and gluons, which happens a lot. Our favorite processes, then, are when the Higgs decays all the way to leptons and photons. Indeed, a lot of the excitement from last December (including the ATLAS plot above) came from looking at two-photon events, even though those are expected to happen less than one percent of the time. Decays into four charged leptons (electrons or muons), which can happen when the Higgs goes to two Z's and each Z decays into charged leptons, happen something like 0.01% of the time, but they're so easy to spot that they're also a favorite channel. With more data, they're certainly hoping to do a better job in comparing with other channels as well.

Promising decays

So that's what we're looking for. Are there excess events at some particular energy, enough to declare a discovery? Are they consistent between the two experiments? Is the overall rate consistent with the predictions of the Standard Model? Are the rates in different channels compatible with each other?

The Higgs boson is not the end of a road; it's a bridge from one world to another. It's the last particle we need to make the Standard Model complete, but it also gives us a way to travel to what's beyond, whether that might be dark matter, supersymmetry, extra dimensions, or what have you. Sadly we're not in possession of a reliable map; we just have to cross the bridge and see where it takes us.

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