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Archive for August, 2010

Views of Fuji

Thursday, August 5th, 2010

Summer season is often synonymous to conference season. And in Japan, conference organizers know how to escape the summer heat for a few days. That’s why we find ourselves this week at the foot of Mt. Fuji. The seminar house where the Summer Institute 2010 on String Theory and Cosmology takes place, is called Fuji Calm, and calm it is. The balmy mountain air is only disturbed by sounds of insects and birds. The fact that the facilities are hovering on the brink of mild disrepair is amply made up by the fact that Mt. Fuji thrones right in front. The view is truly stunning. Yet Fuji is a bit of a diva and only shows itself in a good mood early in the morning and around sunset. The rest of the day it can only be identified by the heap of clouds under which it is hiding.


Tour a particle collider

Tuesday, August 3rd, 2010

This weekend I’ll be headed up to Long Island, where I’ll be one of the volunteers for the Brookhaven National Laboratory Summer Sundays public tours of the Relativistic Heavy Ion Collider.  It’s free and no reservations are required.  Details are available here.  I’d recommend it to anyone interested in particle accelerators.

The Relativistic Heavy Ion Collider (RHIC) is a little over a kilometer in diameter.  By comparison, the LHC is about 8.5 kilometers in diameter.  The top center of mass energy at RHIC is 500 GeV for proton-proton collisions and 200 GeV for heavy ion collisions, about 1/28th of the top LHC energies.  While the LHC can collide protons at the top energy in the world, RHIC is the only machine that can collide polarized protons.  Currently RHIC can collide heavy ions at the highest energy in the world – until this fall, when we expect our first heavy ion collisions at the LHC.  RHIC can produce collisions at center of mass energies as low as 7 GeV.  Additionally, RHIC can collide deuterons with gold.  With RHIC and the LHC combined, we can study different regions of the phase diagram of nuclear matter.

There are two main experiments still taking data at RHIC, STAR and PHENIX.  (I was on STAR as a PhD student; I am now a member of PHENIX.)  During the tours, you’ll be able to see part of the collider tunnel and both the STAR and PHENIX experiments.  You’ll be guided by physicists working on the collider and on STAR and PHENIX.  (I will be giving tours of the PHENIX experiment.)

If you’ve never seen an accelerator or a particle physics experiment and you’re in the area, I’d strongly recommend you make the trip out to Long Island.  Hope to see you on Sunday!


Busman’s (or miner’s) holiday

Tuesday, August 3rd, 2010

Shh — don’t tell anyone that I’m online! I am writing from a secure, undisclosed location in northern Minnesota, where I am taking a vacation between the end of the ICHEP rush and the start of the fall semester. But longtime readers will remember my penchant for physics tourism, and it turns out that I am within an hour’s drive of the Soudan Underground Mine State Park. So I had to go visit!

The Soudan mine is the oldest iron mine in Minnesota, with amazingly pure ore deposits. But by the 1960’s, it wasn’t cost-effective to operate, and it was turned into an historic park. In the late 1970’s, particle physicists at the University of Minnesota, led by Marvin Marshak, realized that it would be a great place for physics experiment. With a half mile of rock and iron overhead, the mine would have a very low flux of cosmic rays, and thus there would be low backgrounds for searches for very rare processes. The hot thing to look for at that time was proton decay, which was predicted by some simple extensions to the standard model to be observable at reasonable rates. The iron in the mine was full of protons, so “all” that had to be done was to place a detector in the mine and watch and wait. The first Soudan experiment set a lower limit on the proton lifetime of more than 10^30 years.

Since then, other experiments have operated in the mine, and there are two there right now. The MINOS experiment is searching for neutrino oscillations. Fermilab produces a beam that is mostly muon neutrinos, which is directed 500 miles northwest towards Soudan. The MINOS far detector in the mine looks for neutrino interactions and sees how often the neutrinos observed are muon neutrinos, or some other flavor. Meanwhile, the CDMS experiment is looking for dark matter. If we live within a cloud of dark matter, then a dark-matter particle might interact with the CDMS detector, a germanium crystal that is kept at temperatures very near absolute zero. Such an interaction will excite phonon vibrations in the crystal, which can be detected.

Tours of the scientific facilities at Soudan are available twice a day during the summer, and it was encouraging to see how many people turned out for a physics lesson on the summer morning that I went. We all squeezed into a mine elevator (after it had been inspected for bats, who live in the mine), and headed downwards at a 78 degree angle to the lowest level of the mine; the trip takes about two and a half minutes. (I was expecting an open elevator car, but it was in fact totally enclosed, and thus less disconcerting than I had feared.) The mine is generally at a temperature of 50 F during all seasons, but the MINOS detector throws off enough heat from its electromagnet to make the experimental hall quite comfortable. Our tour guide was a high-school biology teacher from the area (I think Hibbing) who does this as a summer job; he said up front that he had to learn a lot to learn a lot to be able to give the tours. He did a fine job of explaining the physics behind the experiments. (I only caught one mistake, which was in a Fermilab-produced video that said that the Tevatron was the world’s highest-energy particle accelerator. True until last December.) CDMS requires a super-clean environment and thus it was off-limits to visitors, but we were able to get a good look at the MINOS detector, a long stack of iron plates instrumented with scintillating fibers.

I didn’t see any scientists on duty, but Fermilab is in the midst of a maintenance shutdown right now, and I also imagine that the the detector only operates with a skeleton crew anyway, as the site is quite remote, more than a four-hour drive from the Twin Cities. Any MINOS collaborators reading? How many of you have been up to Soudan for a visit? (And how many of you have a photo of yourself like the one of me below?)

So, dear readers, the next time you find yourself “up north” and want a break from the loons, be sure to stop by the mine to get a dose of particle physics!

KB and the MINOS far detector


I’ve been thinking of a good way to introduce flavor physics—a subject which can be surprisingly subtle—to a general audience. Here’s my best shot at it.

An invitation: the solar neutrino problem.

By the 1960s physicists thought they had a pretty good understanding of the nuclear reactions that caused the sun to shine. One of the many predictions of their model is the number of neutrinos emitted by the sun.

A scientific model is only as good as its the experimental verification of its predictions, so the next step was to actually count these solar neutrinos. Of course, our readers already know that neutrinos are very weakly interacting and this makes them very hard to detect.

Well, when a couple of enterprising astrophysicists set up such an experiment at the Homestake Mine in the 60s (which is still a template for modern neutrino detectors), they were shocked to find that they only counted only a third of the expected neutrino flux.

At this point a good scientist will go back and check their experiment, look for systematic errors, and then go back to check the assumptions of the underlying model. Let’s gloss over these rather important steps and just state that this discrepancy could not be explained by any known effect and would be referred to as the solar neutrino problem.

What gives??

From solar neutrinos to astronaut ice cream

Before answering this puzzle, let’s take a detour and fast forward many decades to my first visit to the National Air and Space Museum when I was about 10. I remember thinking that the big airplane displays were pretty cool… but nothing compared to a discovery I made in the gift shop: astronaut ice cream. (Sometimes I look back and wonder how I ever became a scientist.)

For those who aren’t familiar, astronaut ice cream is just freeze-dried ice cream that has a uniquely chalky texture. My favorite variety was Neapolitan, which was a combination of strawberry, chocolate, and vanilla. The bars looked something like this:

The Neapolitan astronaut ice cream bar will be a very useful analogy in what follows, so bear with me. Ordinarily one would expect the bars to come in a single flavor: strawberry, chocolate, or vanilla. Instead, a Neapolitan bar is a mixture of all three.

In fact, to properly set up the analogy, we should imagine that there are three types of Neapolitan bars so that if we took one of each bar, we would have the same amount of each flavor as we would if we had one of each single-flavor bar. Thus the three Neapolitan bars are just a mixture of the three single-flavor bars.

Now here’s the crux of the matter: even though the Neapolitan bar is packaged as a mix of three flavors, when you bite into it you only get to taste one flavor at a time.

Okay, maybe you can mix two flavors if you take a bite along the seam—but let’s forget about those cases because they break the careful analogy I’m trying to put together. 🙂

What this all has to do with neutrinos

Now let’s connect this to the solar neutrino problem. The incorrect assumption associated with the solar neutrino problem turned out to be that neutrinos are more like Neapolitan bars rather than single-flavor bars. The “flavor” in question is the identity of the neutrino as either electron-like, muon-like, or tau-like.

In other words, the “pre-packaged” neutrinos that propagate between the sun and Earth are a mixture of electron/muon/tau-like neutrinos. What we mean by this is that they are a quantum superposition of these three different flavors, in precisely the same way that Schrodinger’s cat is a superposition of different corporeal states.

Now here’s the neat part: even though the neutrinos propagate as Neapolitan bars, they only interact as definite flavors (electron, muon, or tau). In other words, when the neutrinos are produced in the sun, they are produced with a definite flavor. They are also detected on Earth with a definite flavor. But everywhere in between when they’re propagating on their own, they are a mixture of all three flavors.

Physicists will say that there is an “interaction basis” (electron, muon, tau neutrinos) and a “mass basis” (propagating superpositions).

We can now work out the resolution of the solar neutrino problem. The nuclear reactions in the sun involve electrons (not muons or taus) and so produce electron-neutrinos. Similarly, the detectors on Earth only detect electron-neutrinos since are composed molecules made up of electrons. In between, however, the neutrinos travel a long enough distance that they get all mixed up into Neapolitan admixtures of all three flavors. Thus when the solar neutrinos reach the detectors, only one third of them are detectable, explaining the deficit of neutrino counts!

Actually, this explanation for the factor of 1/3 is a big fat lie… it’s just a cute numerical coincidence. The point is that mixing causes one to only observe a fraction of the total neutrinos, but the specific fraction depends on many things. We’ll discuss this below.

Neapolitan Neutrinos and their relation to mass

Of course, this resolution came from decades of progress in theory and experiment, including many red-herring directions which we won’t discuss (but is a key part of doing real science!). One important a fact that from our understanding of quantum field theory is particularly important:

Particles which propagate through any appreciable distance are states of definite mass.

For more advanced readers, the reason for this is that the mass term is part of the quadratic part of the action which can be expilcitly solved and about which we perform perturbation theory.

The reason why neutrinos propagate as Neapolitan mixtures is that those are the mixtures that have definite mass. A purely electron-flavored neutrino turns out not to have a definite mass, but rather a ‘quantum superposition’ of masses. Conservation of energy requires that only a single mass state should be allowed to travel over long (i.e. non-quantum) distances.

Thus the discovery of neutrino mixing (and hence the resolution of the solar neutrino problem) only came hand-in-hand with the discovery that neutrinos have tiny but non-zero masses in 1998. This discovery, at the joint US/Japan Super-Kamiokande detector in Japan, is a great science story for another day.

Update (3 Aug 2010): as a commenter pointed out, the definitive solution to the solar neutrino problem actually only came with data from the joint US/UK/Canada Solar Neutrino Observatory (SNO) in Ontario. In 2001, SNO detected a 1/3 of the expected solar neutrinos while Super-K detected 1/2. The difference between the two experiments is that SNO is sensitive only to electron-neutrinos, while Super-K also has some sensitivity to muon- and tau-neutrinos. By combining the information from the two experiments, SNO researchers were able to extrapolate the total number of neutrinos (of all flavors) and found that this number matched the total neutrino flux expected from the sun. These solar neutrinos were all produced as electron-neutrinos, but “oscillated” into other flavors while propagating as mass-states. For a more detailed but accessible account of this story written by one of its heroes, see John Bachall’s contribution to the Nobel eMuseum.

Revised Feynman Rules

Recall that the W boson mediates flavor-changing effects. In that previous post, readers mori and Stephen correctly point out that I was being a little misleading about the W interactions. This was a deliberate choice to avoid this “flavor vs. mass” state issue. Now that we’re familiar with the difference between neutrino flavor states (electron, muon, tau) versus neutrino mass states (Neapolitan mixtures which we’ll just call 1, 2, and 3), however, we can revise our W boson Feynman rules to be more accurate.

Let’s start in the flavor basis. For clarity I will associate electron-neutrinos with strawberry ice cream. These single-flavor states are the actual states that interact with other particles. In particular, electrons will only interact with electron-neutrinos. In terms of these interacting-states, the Feynman rules are simple:

We’re only drawing the electron interactions. There are also interactions with muons which only interact with muon-neutrinos (chocolate flavored), and similarly for taus (vanilla). However, although the Feynman rules are simple, the flavor basis isn’t so useful since these states only exist at the instant of interaction. The moment the neutrino flies off, it settles into one of three mass states, which we will call neutrino-1, neutrino-2, and neutrino-3. We’ll represent these as Neapolitan ice cream bars.

Let us draw the Feynman rules in terms of these mass states. In other words, we’re drawing the Feynman rules with the assumption that the particles are given a chance to travel some distance. Now an electron can interact with any of the three mass states:

The reason for this is that the electron only interacts with electron-neutrinos, i.e. strawberry flavor; but each of the three mass states (ν1, ν2, ν3) contain some electron (strawberry). This is where flavor mixing really shows up in the W interactions: the e doesn’t only interact with ν1, but all of the mass eigenstate neutrinos.

How much mixing?

There’s no reason to believe that the mass-state neutrinos all have an equal amount of each flavor. In fact, the particular mixtures look something more like this:

These ratios are set by the particular values of the neutrino masses.

  • ν1 is about 2/3 electron-neutrino and 1/6 each of muon/tau-neutrino
  • ν2 is about and equal mixture of all three
  • ν3 is mostly an even split between muon and tau neutrinos

Note that this may lead you to wonder why it was that the original Homestake experiment detected 1/3 of the expected neutrinos, since this is the value we would expect if each mass state had an equal fraction of each flavor. The answer: this is a coincidence!

The particular fraction of the total number of detected neutrinos depends on a lot of factors in a rather involved equation. These factors include:

  • The differences between the neutrino masses
  • The distance between the Earth and the sun
  • The energy (or rather the energy spectrum) of neutrinos emitted by the sun
  • How the neutrinos interact within the sun
  • The range of energies to which our neutrino detectors are sensitive

Different solar neutrino detection experiments have found a range of different values for the number of detected neutrinos, but once these effects are taken into account, they are all consistent and shed light on the fundamental parameters that govern the neutrino sector.

Analogy to quark mixing

I haven’t yet properly introduced the Feynman rules quarks, but it turns out that you can obtain the interactions of the quarks with the photon, W, and Z by simply taking our lepton Feynman rules and replacing charged leptons with up-type quarks and neutrinos with down-type quarks.

In particular, there are three up/down-type flavor pairs:

  • up quark and down quark
  • charm quark and strange quark
  • top quark and bottom quark

The W boson again causes mixing between these families, while all other interactions only stay within an up/down pair. It turns out that the mixing between quarks is not as dramatic as that between leptons, but because of hadronic effects (i.e. the strong force) measurements of quark flavor can be notoriously difficult. (For experts: See this post at Resonaances for an update on a recent interesting quark flavor storyline at the D0 detector in Fermilab and this post from ICHEP by the same author for a broader status report.)

Closing Remarks

  • This pattern of neutrino mixing has a fancy name, tri-bimaximal mixing, and one interesting line of research is to understand where this structure comes from. (It seems to be related to the symmetries of the tetrahedron.)
  • Because the amount of detected mixing depends on so many experimental parameters, there are many different neutrino experiments that differ by baseline (distance between source and observer). Since we can’t change the distance between the sun and the Earth, a good alternative is to detect neutrinos coming from nuclear reactors by setting up detectors at fixed distances.
  • Yet another source of neutrinos come from the atmosphere, when cosmic rays interact with molecules in the upper atmosphere (some at LHC energies!) and send a shower of particles down to Earth.
  • Here’s a really, really good question that may even stump a few physicists: why is it that the neutrinos mix while the charged leptons don’t? (Alternately, why do down quarks mix but not up quarks?) Shouldn’t they somehow behave similarly? The answer turns out to be somewhat technical, but the punchline is that they do, but the time scales involved make the effect irrelevant. I refer those with a technical background to arXiv:0706.1216.

That’s all for now!
Flip, US/LHC


I contributed to an interesting project yesterday – the3six5 is a diary of 2010 written worldwide by a new person each day who also submits a photo that encompasses the feeling. My day was relatively relaxing after a busy week on Birmingham’s Entrepreneurship and Enterprise summer school. The spirit of what I wrote can be contained in the fact that I did the course in the first place though – I am about to be thrown into the employment pool and I will need to adapt to stand out in the right places. A role-playing game with friends reminded me of the fact. Yet, with Phil by my side it feels less terrifying to be facing it together. The entry went like this:


August is finally here. The wave of summer weddings is dying down. I’ll be 25 in a few weeks. Only two more months of my PhD left. Only two more months before Phil and I really need to be moving out, starting new jobs, beginning the rest of our lives together. No jobs lined up yet but we’ll work something out.

It’s been a lazy day: I went with friends to Owen’s to play a fantasy roleplaying game he’s been pushing for us to try. We armed ourselves as best we could for surviving in a modern but crazed world – my character was my usual mischievous thief, one of us was an ex- veteran, one was a martial-arts monk and one even specialised in biology and was carrying a flesh-eating virus just in case it came in handy. Yet, the game transported us to a new world, leaving us almost entirely helpless. The time came when we had to abandon hope of returning home and instead make our goal to kill off immediate zombie-like dangers and carve out a home in this inhospitable place.

It got me thinking: The world feels very scary these days – nearly enough everyone I know has been at least a little bit affected by the economic crisis. New government is clearly not making any exceptions in cuts, and a lot of people are being made redundant all over the place. It’s already fairly unavoidable that to stay in this country my early career in heavy- ion physics will have to change direction. I can’t say I feel so confident about finding my way outside academia right now.

To get through the next few years, we are going to have to adapt quickly. Many of us will have to adapt our dreams too, to fit with what is possible. I got home quite late, feeling a little wired and fed only on snacks and coffee. Phil was waiting, with the table set and a candle lit, risotto and wine at the ready. I love him so much. Despite lack of physical evidence, it feels like we are turning a corner today. The future is coming and I think we can handle it.