The news out of Geneva gives a tantalizing first taste of really discovering the Higgs Boson. Is this for real? In this week's "Ask a Physicist," we do a round-up of all things Higgs-related.
Top image credit: CERN
If you hang out in some of the nerdier websites (including this one), you probably couldn't help but get excited about yesterday's news regarding the Higgs Boson. If you somehow missed it, here's the breakdown.
As you probably know, one of the big goals –- arguably the big goal — of the LHC is to find the elusive Higgs Boson. The two biggest experiments in the Large Hadron Collider, ATLAS and CMS, announced their data updates yesterday. Both ATLAS and CMS have found a signal with relatively high significance suggesting that the Higgs Boson is real and that it has a mass of roughly 135 times the mass of a proton (or, to you experts, approximately 125 GeV). This is a big deal because 1) The Higgs Boson is the last undetected particle in the Standard Model of physics, and 2) The Higgs field is what gives other particles their mass.
I did a column a while ago about why discovering the Higgs would be so exciting, but surely you must have more questions. In fact, I know you do since we put out a call for questions yesterday and you guys posted andemailed dozens of questions about what it all means. Thanks to everyone who sent in questions.
I've consolidated some of the best and most probing questions about the Higgs for your reading pleasure, though I anticipate a spirited debate about a few of them in the comments section. Things can get a little technical, so besides the backgrounder on the Higgs, you may also want to take a look at Alasdair's field guide to subatomic particles.
Credit: ATLAS Collaboration.
Credit: CMS Collaboration.
Did they actually detect the Higgs?
Let me get this out of the way from the outset. By the standards of the particle physics community (and almost certainly by the standards of the Nobel prize committee) the Higgs has not officially been discovered. Why not? That takes a little explaining. This involves numbers, which means that the numerically squeamish may want to look away, and it also involves a few back-of-the envelope calculations so the mathematical experts may also want to look away.
I usually don't post data in my columns, but as you can see above, I've made an exception today. The two plots are from the ATLAS experiment and the CMS experiment, respectively. In each, the solid line is the data for different possible values of the Higgs boson mass (the x-axis). The dashed line is what you'd expect if there were no Higgs at a particular mass. The solid and dashed lines obviously don't match up. When you do an experiment, you don't expect to get exactly the null result (no Higgs) with every measurement. There's always a scatter.
If there were no Higgs, then approximately 95% of the time the solid line should be within the yellow curve. This is known, mathematically, as "2-sigma." You only have about a 2.5% of finding yourself 2-sigma above the null value by random chance. Both the ATLAS and CMS experiment have more than a 2-sigma result around 125 GeV (135 times the mass of the proton) in their data.
There are a few complications, however. For one thing, if you look at enough data you would expect to see a few random fluctuations. Even after correcting for that, the ATLAS result is about 2.3 sigma and the CMS result is about 1.9. Alone, neither is such a big deal BUT (and here's the important point) since the results are independent, you get to combine them giving you a 3-sigma result, or only about a 1 in 700 chance of seeing a signal like this anywhere in the experiments. Provided the teams have been careful about their error analysis (and there's no good reason to suppose they haven't) this is a fairly convincing result. Let me put it this way: criminal convictions, which require proof "beyond a reasonable doubt" almostnever rise to this level of certainty.
But particle physicists like to be more careful before they say something has been officially "discovered." They normally demand a 5-sigma detection — with less than a one in a million chance of being wrong. It makes sense to set a high bar, but for my part, I'm convinced that the Higgs is real and that we know its approximate mass. The remaining work will be to make it official.
Oh, and one more thing. These results are totally consistent with the range limit found by the Fermilab Tevatron. They only saw a 1-sigma result, but combining all of it, the odds come to something like 1 in 1100 against seeing all three measurements by pure chance.
Are the results consistent with what we expected?
Yup. This is very much in line with what was predicted by the standard model. In that respect, it's kind of boring to see what you expect.
However, this Higgs mass would be a bit above what is typically predicted by supersymmetry, giving one hint that while the standard model of particle physics is correct, supersymmetry (or at least certain variants of it) may not be.
Why do we have to "create" the Higgs in order to detect it? I thought it was all around us, giving particles their mass.
There are a lot of things called "Higgs" but the two most relevant are the Higgs particle and the Higgs field. It's true that the Higgs field is all around us; it is the field that gives at least some of the particles mass. That's very much like saying that there is an electromagnetic field all around. A photon can be thought of as something like a congealed lump of the electromagnetic field. Likewise, the Higgs boson could be thought of as a lump of the Higgs field. The tough part is slamming particles together with enough energy to rip out one of those lumps, even for a short period of time.
There's another important point here. Even under the best of circumstances, we still don't actually detect the Higgs boson itself. What happens is that a Higgs is created and (like so many massive particles do) it decays into lighter particles; two photons for example. By measuring the energies of the photons, you could get the mass of the Higgs, but decays into photons are quite rare. More commonly, the Higgs decays into a bottom quark and an anti-bottom (not the same as a top!), but this is really tough to measure. It's all very indirect, you see, which is why "discovering" a particle like the Higgs doesn't quite mean what most people think it means.
If the Higgs field gives mass to other particles, how can it be so much more massive than a proton?
This goes back to the distinction between the Higgs particle and a Higgs boson. It's not like a Higgs boson is hiding inside a proton, for example. Rather, particles interact with the Higgs field, and the interaction creates energy. Just as E=mc^2, the effective mass created is m=E/c^2, where E is the energy of the interaction.
Energy and mass really are the same thing. If you don't believe me, consider this. Protons are made of quarks, but if you add up the masses of the individual quarks, they only add up to a per cent or so of the total proton mass. The rest is interaction energy. From the outside, you cannot tell the difference between "real mass" and mass with is really energy. There is no difference.
Why doesn't the Higgs field give mass to photons?
For one thing, it's not only the photon that gets off the hook, mass-wise, but also the gluon (the particle for the strong force), and (if it exists) the graviton. The real question might be, "Why do the W and Z bosons have mass, but not the other mediators?"
Once upon a time there were 4 particles carrying the "electroweak" force. Two of them were charged particles (which eventually became the W^+ and W^-), and two of them were neutral. In the very beginning, all four of them were massless. This was a long time ago, about the first trillionth of a second after the big bang. As the universe cooled, the interactions of these particles changed, and the Higgs field was created. In many respects, it's kind of like an ice crystal forming when water drops below a certain temperature. The whole structure of the electroweak particles changed.
Remember, the Higgs field creates an interaction energy, but one way of thinking about it is that the 4 electroweak particles got "mixed-up" with the photon being the massless combination of the two old neutral particles, and the Z being the massive combination. In the end, three of these particles (the W^+, W^-, and Z) interacted with the Higgs field, giving them mass, and one of them, the photon.
There will be those of you who will point out that all I've done here is say the photon doesn't interact with the Higgs because that's how the mathematical model works. Yup. That's exactly what I've done, but I'm afraid in this case, it doesn't get any simpler than that.
I should mention that while the Higgs mechanism does describe how the W and Z particles get their masses, it doesn't really explain how the quarks and electrons get their mass. We assume that it's due to the Higgs, but we really don't know, and we really can't say anything predictive. With the W and Z particles, on the other hand, we're able to compute the ratio of the masses to insane precision.
Would discovering the Higgs tells us anything about gravity?
No. The Higgs doesn't tell us anything about how gravity works, but since gravity responds to mass, interactions with the Higgs field produces gravitational fields, as does any form of mass or energy. Not only doesn't the Higgs tell us about gravity, there's nothing in the standard model at all that includes gravity. Our current best theory of gravity is general relativity, and one of the big goals of physics is to figure out a way to unify gravity with the other forces.
Does the Higgs have an antiparticle?
Nope. An antiHiggs and a Higgs are one and the same. Some particles are their own antiparticles, but they have to be neutral charge, because antimatter always has the opposite charge of regular matter. Photons, for example, are their own antiparticles. There are no "antiphotons." The Higgs boson is the same way.
What's the practical impact of discovering the Higgs?
Practical? Who knows? Simply knowing how mass is generated for some particles doesn't really allow us to manipulate masses any more than understanding how gravity works allows us to build black holes. Remember the warnings about black holes being generated in the LHC? Yeah. That didn't happen.
Personally, I think basic science is worth doing for its own sake. But the question of why people should care is a good enough one that the Imperial College, London, has a contest for students asking this very question.
Of course, this being io9, I'm sure people will come up with doomsday devices built around the Higgs particle. Remember, though, that even if you make a Higgs, you don't get to keep it for very long. They break down in less than a quintillionth of a second.
Would discovering the Higgs mean the end of Physics?
Not a chance. For one thing, the Higgs only tells us, at best, about ordinary matter in the universe. We still don't understand much about dark matter or dark energy, which combined comprise about 95% of the total energy. Certain models of supersymmetry, which might have shed light on dark matter actually seem less likely now, which means that we have a lot of work to do.
While the standard model does a great job describing the unification of electromagnetism and the weak force, we still don't have a "Grand Unified Theory" which also combines the strong force. We don't know how any of these forces combine with gravity in a way that would allow us to describe the beginning of time or the centers of black holes in a useful way. We don't know why the parameters of the universe are what they are or why there are 3 sets of quarks, 3 sets of neutrinos, and 3 sets of charged leptons or even why neutrinos really have mass or why they are specifically left-handed. We just know, in all of these cases, that they are, and not why they are.
I could go on and on, but the point should be clear. If the Higgs mechanism is right then this simply means that we have found one piece of the puzzle — a useful corner piece, to be sure, but still just a piece. There's still lots to do.
Dave Goldberg is the author, with Jeff Blomquist, of "A User's Guide to the Universe." (follow us on twitter, facebook, twitter or our blog.) He is an Associate Professor of Physics at Drexel University and is currently working on "The Universe in the Rearview Mirror," a new book all about symmetry that will be published by Dutton in 2013. Please send email to firstname.lastname@example.org with any questions about the universe.