/ Maggie Koerth-Baker / 4 am Thu, Dec 15 2011
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  • 3 Things the Higgs Boson can teach you about physics

    3 Things the Higgs Boson can teach you about physics

    Last Tuesday, particle physicists at CERN did not announce that they had found the Higgs Boson particle. Nor did they announce that they had not found the Higgs Boson. Instead, what we got was an update on the state of the research. But it's a really tantalizing update.

    The Higgs Boson is a popular, but confusing, bit of physics. You know that reality is like a Lego model, it's made up of smaller parts. We are pieced together out of atoms. Atoms are made from protons, neutrons, and electrons. Protons and neutrons are made of quarks. (Quarks and electrons, as far as we know, are elementary particles, with nothing smaller inside.) When you're talking about the Higgs Boson, you're talking about the mass of these particles. Here's an imperfect analogy: A top quark, the most massive particle we know of, is like an elephant. An electron, on the other hand, is more like a mouse. And nobody knows for certain why those differences exist.

    There is a theory, though. Back in the 1960s, a guy named Peter Higgs came up with the idea that all these particles exist in a field, and their mass is a reflection of how much they interact with that field. Heavy particles have a lot of interaction. Lighter particles are relatively standoffish. If this field exists, the Higgs Boson is the tiny thing it's made of. Fermilab physicist Don Lincoln has a really great video explaining this, where he compares the Higgs field to water, and Higgs Bosons to the molecules that make up water. Everything that exists swims in an ocean of Higgses.

    Tuesday morning, we learned a little more about the hunt for the Higgs Boson. But the point of the presentation wasn't really to say, "Yes, we found it" or "No, we haven't." In fact, if all you're paying attention to is that simple yes-or-no answer, you're going to miss a lot of interesting information—information that can help you better understand how science works and why the Higgs Boson is so important.

    1: "Pretty sure" isn't good enough.

    In the presentations, CERN researchers told us two big things:

    First: Looking for the Higgs Boson used to be a lot harder because nobody knew its mass. Think of it like trying to find a single Lego piece, in a giant box of Legos, when you don't know what the piece you're looking for looks like. That's changed. Researchers now believe that the Higgs Boson, if it exists, probably has a mass somewhere between 115 and 131 gigaelectronvolts.

    Second: Two different detectors on the Large Hadron Collider have found signals, consistent with what you'd expect to see from a Higgs Boson particle, within that mass range—at 126 and 124 gigaelectronvolts.

    So why was the un-announcement so non-committal? Simple: Physicists don't like to be wrong.

    You can't actually see a Higgs Boson. This isn't like sitting in the jungle and waiting for a rare species of panther to come along so you can photograph it. Instead, scientists are looking for the particles they've predicted that a Higgs Boson would leave behind as it decays. Say you have a hypothesis that a new species of panther exists, but it's invisible as long as it's alive. The only way to figure out whether or not it's actually there is to look for panther poop, or maybe some bits of bone and fur. Trouble is, there are lots of things in the jungle that could leave behind poop, bone, and fur. How do you know what you've found is actually evidence for the existence of the hypothetical panther?

    That's essentially the problem physicists are faced with. Those intriguing signals could be decaying Higgs Bosons. They could also be normal things you'd expect to find in the aftermath of proton collisions. The only way to tell the difference is to look for an excess of those signals in the mass range where you'd expect the Higgs Boson to be. But, even if you see that, it could still be a coincidence. This is especially true of the hunt for the Higgs Boson, because it started out looking at a huge range of masses, says Greg Landsberg, professor of physics at Brown University.

    "When you look at many, many places what is unlikely in a given place becomes more likely in one of many, many places," he says. "Ask an astronomer what is the probability of a particular star having a planet orbiting it. He’d say the probability is extremely small. However if you asked that differently, 'What the probability of any star having a planet,' the probability would be much closer to 1."

    To make sure that the promising signals they're seeing aren't just flukes, the physicists at CERN will need to run their experiments, in that much-more-specific mass range, many more times. Right now, says Don Lincoln, there's a 1 in a 1000 chance that what they're seeing is a coincidence. But, in the past, particle physicists have found that 1-in-1000 chances aren't a very good bet. "That's equivalent to tossing a coin 10 times and having it always come up heads. To be comfortable saying we've found something we'd have to have the equivalent of tossing a coin 20 times and having that all come up heads—a 1 in a million chance," he says. The more coin tosses, the more you rule out coincidence. The physicists I spoke with said we're likely to have enough data to do that by next summer.

    2: The mass of the Higgs Boson matters.

    If the Higgs Boson is actually there, around 124 or 126 gigaelectronvolts, that means it's a lighter particle than many people had guessed. In fact, originally, people were looking for the Higgs Boson at masses as high as 600 gigaelectronvolts. This is interesting for a couple of reasons.

    First, there's the "d'oh" factor. A big selling point on the Large Hadron Collider was the fact that it had the power necessary to study very high energies. That's what makes it different from particle accelerators that have come before, like the recently closed Tevatron. But 124-126 gigaelectronvolts is well within the range of what the Tevatron could study.

    In fact, the Tevatron looked at that range. Unfortunately, it doesn't yet have enough data to make any definitive statements. That could change. Don Lincoln says that when the Tevatron researchers are finished analyzing their data, they might be able to back up the CERN findings. That's because the Tevatron noticed a small signal around 125 gigaelectronvolts. However, Lincoln also says it's a smaller signal than you'd expect if the CERN results really were correct. This could mean the Tevatron saw the Higgs Boson signal first, but couldn't verify its results before CERN got better data. It could also mean the signal from CERN is just a coincidence. Right now, it's too early to tell.*

    Second, if the Higgs Boson exists and if it is light in mass, that opens up a way more awesome world for future physics research than would exist with a heavy Higgs. Physics operates on the Standard Model—a mathematical theory aimed at explaining the forces at work in the Universe and how particles interact with one another. The Standard Model requires the existence of a Higgs Boson, but a light Higgs Boson would mean that we'd have to make some changes to the way the Model works, possibly incorporating ideas like supersymmetry, says Frank Close, particle physicist at the University of Oxford.

    A light Higgs also means the Large Hadron Collider has enough power to find lots of other previously unseen particles. "Strategically, if this thing turns out to be real, the fact that it is at low mass end is good news," Close says. "If it was at the high end you’d have that fear that the interesting physics was out of the LHC's reach. This suggests lots of interesting things are still available for us to find using the LHC."

    3: This story will actually be a lot more exciting if it turns out that the Higgs Boson doesn't exist.

    Everybody is all excited about the prospect of finding the Higgs Boson particle. This is kind of the wrong way of thinking about it.

    I told you before that the existence of the Higgs Boson is part of the Standard Model—finding it is a key part of verifying that the Universe works the way we think it does.

    "If we don’t find it, it’s extremely weird," says Jeremiah Mans, associate professor of physics at the University of Minnesota. "We know from other measurements that properties of regular particles aren’t quite right unless there’s a Higgs around to pull them just a bit. If you take it out, then the theory, that otherwise works well, breaks down and doesn’t make the right predictions. That points to something being there."

    Verifying that your big, important theory is correct is a big deal. But that outcome is also just a little boring. It would be a lot more exciting if everything we thought we knew turned out to be wrong.

    And there is more than one way that the Higgs Boson could throw off the Standard Model. It could, of course, flagrantly refuse to exist. But it could also be quite a bit weirder. We could find the Higgs Boson, and it could turn out to be different than what we've been predicting. See, all this time, physicists have only really been looking for what's known as "the minimal Higgs." You can think of it as the simple version. In the minimal Higgs theory, there's only one type of Higgs Boson and it has no electrical charge, among other characteristics.

    What happens if the Higgs we find turns out to have a charge? What happens if the Higgs turns out to actually be five different types of Higgses, as some versions of supersymmetry predict? If that's the case, the Standard Model could end up having to dramatically change, just like if the Higgs Boson didn't exist at all.

    If, in six months, the physicists at CERN are able to say definitively that they've found the Higgs Boson, particle physicists will be gratified. They will understand the Universe in a way they didn't before and they will be able to work on some new questions. But if CERN is wrong, particle physicists won't hang their heads in shame. Instead, it could well be the most exciting day of their lives.

    "If we don't find it, that would be a huge discovery," says Fermilab's Don Lincoln. "The Standard Model does work very well. We’re talking on a phone, after all, and that’s done through electricity, and how that works is part of the Standard Model. A new theory would make have to make some similar predictions, but it would rewrite textbooks."

    • • • •

    For more information on the Higgs Boson and particle physics, I recommend checking out a couple of books:

    Frank Close has a book called The Infinity Puzzle, about the history of Peter Higgs, the Higgs field, and the hunt for the Higgs Boson.

    Don Lincoln, who, over the last couple days, has become one of my favorite explainers of physics, has a book called The Quantum Frontier. It's about the Large Hadron Collider—how it works, what we can do with it, and what it might teach us about the Universe.

    *This part of the story has been changed from its original version. The previous version was incorrect and read, "First, there's the "d'oh" factor. A big selling point on the Large Hadron Collider was the fact that it had the power necessary to study very high energies. That's what makes it different from particle accelerators that have come before, like the recently closed Tevatron. But 124-126 gigaelectronvolts is well within the range of what the Tevatron could study. In fact, the Tevatron looked at that range. It didn't see anything. This could mean that the signal CERN saw is just a coincidence. It could also mean that the Tevatron was in the right place, but missed seeing something really important." Thanks to Don Lincoln for helping me get this fixed, and to Joe Haley and Peter for pointing out the problem.

    Image: A rendering of one of the events, captured by the Large Hadron Collider's CMS detector, that could be evidence of a decaying Higgs Boson particle. Or, it could just be stuff that happens when protons collide. Either way, it's kind of pretty.

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