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Finally, physics

My subtitle for this blog is "Musings of a physicist, a witch, and a gamer." I've posted on wicca and gaming, but I've never said much about physics until now.

I have been reticent to talk about physics because what I physically do is sit in front of a computer terminal and type (and occasionally think) all day. But evidently someone thinks that the work I do is interesting:



This image (© CBS) is from an episode of Big Bang Theory, "The Speckerman Recurrence."

In the background you can see the results of a recent measurement by the Double Chooz experiment:

sin2(2θ13) = 0.085 ± 0.029(stat) ± 0.042(syst) at 68% CL



In spoken scientific English, this is: sine squared of two theta one-three equals zero point zero eight five plus-or-minus zero point zero two nine (statistical error) plus-or-minus zero point zero four two (systematic error) at sixty-eight per-cent confidence level.

I know this sounds like gobbledygook to you. That's OK. Unless you're a physicist, a particle physicist in particular, you're not likely to understand what this measurement means or its significance. I doubt the equation means much to the actors in the picture; it was probably put there by a member of the BBT production team to indicate that they're on top of the cutting edge in physics research.

I'm not going to explain the measurement, at least not directly. Instead, I'm going to give two reasons why I find the measurement interesting.

The first is that I share an office with the guys who did a good part of the analysis work to get that measurement.

I was not part of that analysis effort. My only role in the measurement was to help provide computer resources for the group. My direct physics work is for an experiment called MicroBooNE. That experiment is a cousin of Double Chooz, in that it also measures properties of the neutrino. (Huh? What's a neutrino? Give me a moment.)

The second reason why this measurement is interesting to me is that it relates to the general area of particle-physics research of which I'm a part. It's not as headline-worthy as the recent announcements about the Higgs particle, but it's no less fundamental. My research is concerned with an important question:

Why are we here?

There many ways to interpret and answer this question. I'm going to pick a particular one: Why is the universe made out of matter?

This may seem like a silly question. What else could the universe be "made" out of if not "matter"? Isn't that what "matter" is?

But I ask this question as a physicist. To a particle physicist, there's not only matter, but anti-matter. That is, for every known particle with some kind of charge, there exists an anti-particle with the opposite charge. As anyone who's watched Star Trek knows, matter plus anti-matter equals energy.

Now let's look at earliest moments of the universe, in the first fractions of a millisecond after the Big Bang, the same one after which the TV series is named. During that first 10-32 seconds after the Big Bang (that number means 0.000...0001, where instead of writing "..." you write 32 zeroes between the decimal point and the "1") the universe went through a phase called inflation.

If you follow those links to the Wikipedia articles, you'll learn about cosmology and the early history of the universe. One point that might not be clear: During cosmic inflation, the "stuff" of the universe was thoroughly mixed together.

If the universe started as a big "blob" of energy, wouldn't it have produced matter and anti-matter in equal amounts? The matter and anti-matter would have annihilated each other in equal amounts as well. That means the universe would now consist solely of energy (photons, particle of light).

Of course, that's not what we see today. The universe is dominated by "matter": electrons, protons, neutrons. Astrophysicists have searched for regions of the universe that consist of anti-matter (anti-protons, anti-neutrons, positrons); so far they haven't found any. We live in a matter universe.

That's great, but the next question is why? The answer appears to be CP violation. To understand that, you've got to understand CP; then you can understand how it can be violated. "C" means "charge" and "P" means "parity," which in turn relates to the direction a particle spins.

Consider some particle interaction, say an electron "hits" a proton. In the anti-matter version of the interaction, an anti-electron (positron) hits an anti-proton. Are the matter and anti-matter reactions identical, except for the electric charge? No, because it turns out that anti-particles spin in the opposite direction of the matter partners. So if you want to make the matter and anti-matter reactions look the same, you have to "reflect" both the charge and spin of the particles. That's CP symmetry: switch both charge and parity, and particle reactions look identical.

Well, almost identical. If you look carefully, you can find reactions where reflecting both charge and parity does not produce identical reactions. This was first proposed by Lee and Yang in 1956, and experimentally verified by C. S. Wu. (I'm name-dropping here; Lee and Wu were both at Columbia University, and I attended their lectures.)

Matter and anti-matter are not identical. If they're not, that means at the moments immediately after the Big Bang, while most of the matter and anti-matter would have annihilated, there would have been some small excess of matter over anti-matter. Everything we see in the universe is made of that small excess allowed by CP violation.

So we're done, right? Not yet.

Since the discovery of CP violation, particle physicists have measured it in every reaction they could. They added up the amount of CP violation they observed... and it's not enough to explain the amount of matter we see in the universe. There must be some source of CP violation that we haven't seen yet. Where could it be?

Up until now, all the measurements of CP violation have involved a class of particles called baryons. This class includes protons and neutrons; basically, when particle physicist talk about "baryonic matter," they mean anything made of quarks. Apart from protons and neutrons, these kinds of particles are only found in high-energy cosmic rays or in particle accelerators; they rapidly decay (sometimes violating CP in the process, but not often enough!) into the particles found in the universe today: electrons, protons, neutrons, and neutrinos.

There's the neutrino again. What is it? Maybe you've already looked up the Wikipedia entry, but here's my take: The neutrino is a particle that has no electric charge, and very little mass. It's produced in radioactive decay, and in every other form of the weak nuclear interaction.

You don't hear much about the neutrino. That's because it only interacts via the weak nuclear interaction, and the weak interaction is really weak. Here's an example: During my thesis experiment, for every billion high-energy neutrinos that passed through a 700-ton chunk of steel, perhaps only one would interact. A typical neutrino produced by the sun can pass through 50 light-years of lead before it would have a 50% chance to interact.

Several trillion neutrinos from the sun are passing through your body every second. You don't notice because the weak interaction is that weak. You're getting more radioactive exposure from the metals in your computer than you're getting from solar neutrinos.

That's enough of the "how weak is the weak interaction" game. What does this have to do with CP violation?

Neutrinos are light particles; their exact masses aren't yet known, but it's less than one two-millionth the mass of an electron. However, it may be that there are heavy versions of neutrinos, the "massive neutrinos." These particles would still interact only weakly, but they would have to have a mass greater than 46 times the mass of the proton; physicists have already excluded any mass lower than that. In particle physics, that's pretty heavy.

Here's the connection: If massive neutrinos exist, there's may be CP-violating, and by an amount that's enough to explain the matter in the universe.

So why don't we make some massive neutrinos and measure their CP-violation? The answer is money. The highest-energy particle accelerator and detectors in the world, at CERN, can't see them. To produce them would require particle accelerators more powerful than we know how to build.

We're not quite stuck. We can't look at massive neutrinos, so we look at the neutrinos we can detect. If we can spot CP-violation in those neutrinos, it wouldn't explain the matter excess in the universe; neutrinos are too light for that. It would be a "naturalness argument": If the light neutrinos are CP-violating, it would be reasonable to assume that their heavier versions are as well... if they exist.

It's a long chain of reasoning, isn't it? Big Bang -> matter excess -> CP violation -> massive neutrinos -> CP violation in light neutrinos. As we go down the chain, the reasoning becomes more speculative. But it's all we got.

So can we go and measure CP-violation in neutrinos? If only it were that simple. CP violation is hard enough to observe in baryonic matter. It's much harder to observe in neutrinos, which interact only through the weak interaction. It turns out that one experiment is not enough to measure CP-violation. You have to combine the results of several different experiments to understand neutrino properties well enough to see it.

The Double Chooz experiment is measuring sin2(2θ13). No, this isn't CP-violation; it's a number you have to measure before you can plan an experiment to measure CP-violation. It's one of several experiments going on right now to measure this number; you want more than one experiment to make sure you've got the most accurate value.

MicroBooNE, the experiment I'm working on, isn't measuring CP-violation either. The physics purpose of the experiment is check an anomalous result from an earlier experiment, but the research purpose is to test a new type of detector (a liquid-argon-based time-projection chamber) that would be required to accurately measure CP-violation in the future. I'm one member of a team working on a computer simulation of the performance of the MicroBooNE detector before it's built.

With hard work, good luck, and continued funding of follow-up experiments, physicists may have a measurement of neutrino CP-violation by 2025. It will be one more step to answering the question: "Why are we here?"

Whew! After all that, it will probably be a long time before I write about physics again. It's easier to write a game review.

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Comments

( 2 comments — Leave a comment )
shades_of_nyx
Dec. 17th, 2011 11:15 pm (UTC)
Okay, I followed all of that, even though my specialty is Newtonian physics, and if things are that small, I don't tend to care about them.

So, what -is- 2-Theta-sub13? Are we discussing an angular measurement, a velocity, an energy state?

Enquiring geeks want to know.
wgseligman
Dec. 18th, 2011 12:20 am (UTC)
Theta13 is a mixing angle. It's difficult to describe in Newtonian terms, since it's purely quantum-mechanical.

Neutrinos oscillate between three flavors: electron-neutrino, muon-neutrino, tau-neutrino. They also oscillate between three tiny "mass eigenstates", denoted m1, m2, and m3. Theta13 relates to the probability that neutrino in mass state m1 will oscillate to to state m3.

The other two mass mixing angles, Theta12 and theta23, are relatively large and have already been measured. Theta13 is small, and might be close to zero; if it's too close to zero, no existing detector technology will be able to measure CP violation. That why physicists have their fingers crossed that it will be measured to be substantially greater than zero.

The measurement from Double Chooz is large enough for physicists to be optimistic, but the errors are so large that it's hard to tell. Double Chooz will take more data, and other experiments (RENO and Daya-Bay) will make the same measurement, and hope that it all forms a consistent picture.
( 2 comments — Leave a comment )