Research is in full swing so I’ve been spending a lot of late nights in the office (and have been a bit slow to blog—sorry about that!) … here’s a photo out of my office window taken at the beginning of another long evening:

Yeah, those are some Feynman diagrams that I didn’t want to forget—I drew them on my window using a chalk marker. Actually, this picture is meant to be a bit of a joke: diagrams of this type are called Penguin diagrams, so the picture above is a bunch of flying penguins over Ithaca’s Cayuga Lake. (If you’re keeping up with my posts about Feynman diagrams I’ll eventually have a lot to say about penguins and why they’re so interesting.) Anyway, my calling in life is in physics and not poetry but—that being said—I think it’s cute.

I was reminded about the interplay between physics and poetry since I usually listen to something in the background while doing calculations; today it was This American Life. I should explain that after dinner time there’s two kinds of physics that I do:

1. The kind where I’m trying to figure out something that I didn’t understand properly during the day—in which case I’m usually listening to jazz or classical music to help me concentrate, or
2. The kind of where I’m just churning through a tedious calculation or typing up some code—in which case I usually listen to podcasts where I can half-listen to a narrative while doing something that’s otherwise kind of boring.

Tonight was a calculation night, and this week’s This American Life podcast was a rerun that I hadn’t heard in a while titled, “Family Physics.” The idea was that they’d tell stories whose overarching theme is the application of principles of physics to human interactions. I really enjoyed the episode, but as they mention in the introduction, physicists groan when popular writers do this (New Yorker, I’m looking at you).

In the 80s and 90s there were several popular books that tried to tie together themes in quantum physics with themes in eastern mysticism. Unfortunately for physicists, part of the effect of these books was to create this image that theoretical physics was somehow “mystical” and “philosophical” in a way that scientists tend to abhor. There’s nothing inherently wrong with identifying common themes between unrelated ideas—that’s poetry—but it’s important to note that physics is a science and is based on rigid scientific principles of rationalism and backed by the scientific method. I’m not saying the books weren’t any good—Fritjof’s Capra’s (a former theoretical physicist) Turning Point was adapted into a nice movie that became one of my favorites in high school—but they weren’t actually books about science.

Anyway, one offshoot of this were countless popular-level accounts of how Heisenberg’s uncertainty principle is supposed to tell us something deep about human existence. There’s something very charming and—indeed—poetic about this, since nature is something that is independent of humanity and so “truths” coming from nature must somehow be “deeper” than those written by people who don’t invoke fancy words like the cosmological principle.

Of course, this is wrong; using nature as an analogy for abstract human ideas makes them no more “true” than using human analogies to describe abstract ideas in nature. The analogies can be cute, they can even be insightful, but in the end the analogies themselves are not science. (Nor is quantum physics actually telling you how you should break up with your girlfriend, etc.)

The bottom line, of course, is that sometimes these analogies are so elegant that they become enjoyable and valuable in themselves. This is what I consider poetry. (Though I concede that more cultured readers may scoff at this as a simplistic definition! Like I said, I’m a scientist and not a poet.) Along these lines, there are three works—each in different media—that really stick out for me, and that I recently found myself thinking about while listening to This American Life. (I had to stop working on my calculation for a while. )

1. Godel, Escher, Bach, by Douglas Hofstadter, who is the son of Nobel Laureate physicist Robert Hofstadter. The book, however, is not about physics, but rather the common themes between Godel’s incompleteness theorem in mathematics, M.C. Escher’s graphic arts, and Bach’s music. The book is an absolute pleasure to read, though I admit that I have yet to finish it because it requires some attention to properly digest.
2. The photography of Naglaa Walker collected in on physics. These are more along the lines of the 90s New Yorker articles invoking the Uncertainty Principle in that they make superficial connections between physics ideas and her photographs, but it’s done without any pretense of depth and I enjoy Naglaa’s wit.
3. Finally, Hypermusic Prologue, an opera by Harvard theoretical physicist Lisa Randall that purports to draw from Randall’s seminal work on warped extra dimensions, something near and dear to my research. I haven’t seen the opera (I missed the adaptation at the Guggenheim in January), but am really intrigued by the idea. I think scientists need to have a facility with explaining their research to a broad audience, but the choice of medium here is—for many—just as esoteric as the physics behind it. Because of this I am curious to see what kind of interesting new analogies Randall and composer Hector Parra were able to develop.

In fact, this is the reason why physicists (or maybe it’s just me?) often get so annoyed when people make very glib or uninspired analogies to “deep ideas” in science: it’s because there’s so much more that one can make out of these analogies!

A final remark: one of my favorite magazines, Symmetry Magazine, is an excellent particle physics outreach publication and has a regular section where they feature science-inspired artists. There’s been a lot of fun and interesting graphic art over the past year which I encourage you to check out in their back-issues. I should explain that I view these as being rather different from analogies based on physics; instead, they are inspired by the aesthetics of physics itself (a common example is the shape of the ATLAS detector). This week’s artist, Kate Nichols, takes a more active role in the science of her work.

Anyway, maybe the conclusion is that I’m better off doing physics than being an art critic. [Stick to your day job, Flip!]

Cheers,
Flip

[Some of you have said that you're waiting for more Feynman diagram posts---there are a few that I'm working on, I promise!]

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It’s 5:38 AM.  Do you know where we physicists are?

Right now I’m on a test beam shift for the ALICE electromagnetic calorimeter (EMCal).  The test beam delivers particles at a fixed momentum – right now a mixture of 60% electrons and 40% pions at 10 GeV/c.  We have a miniature version of our EMCal, 64 towers (8×8) complete with read out electronics.  It’s positioned in front of the beam line so that we can measure the response of the EMCal to these particles.  We move the beam around on the detector so that we can see the response of each tower to the beam.  We also try different momenta.

We have about a week to use the test beam and we want to make the most of our time, so we take shifts around the clock.  This is where I am right now:

The building to the right – the barracks – is where we sit when we take data.  Our little detector is to the left, behind the large cement blocks.  The cement blocks are there to shield people in the hall from radiation from the beam.  The beam comes from the far end of the hall.  The cables take data from our detector to the barracks.

And we are not alone – there are several other groups using data from the test beam and doing other experiments right now.  The lab that never sleeps.  Our test beam comes from the Super Proton Synchrotron – once the highest energy accelerator in the world and now both the injection source for the LHC and the beam source for multiple ongoing experiments.

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This post is meant to have a positive tone. Really.

The LHC experiments all rely heavily on some form of teleconferencing to get their work done. As experimental collaborators number in the thousands, we can’t get by without conversing with each other. And with collaborators all over the world, we can’t expect people to physically appear at every single meeting. This could work fifteen or twenty years ago, when people typically participated in experiments on the regional or national scale. I know a whole fleet of professors who used to drive a car or take a plane to Fermilab once every two weeks, or even every week, so that they could be in the room for some particular meeting. Now that we are spread over so many miles, it seems too much to ask. But teleconferencing has allowed us to move past that era. It is absolutely not as good as being there in person, but given the monetary costs of moving people around, and the amount of people’s time that can be wasted in transit, not to mention the wear and tear on all of us when we are away from home, it makes sense to take advantage of teleconferencing technology.

The good news in all this is that we have reached a point in teleconferencing technology where anyone who has a computer with a microphone, speaker and network connection can take part, from any office that they might be sitting in, making teleconferences much more convenient than ever before. The bad news, of course, is that we have reached a point in teleconferencing technology where anyone who has a computer with a microphone, speaker and network connection can take part, from any office that they might be sitting in. Not all microphones are of such high quality. Some microphones tend to be rather close to computer speakers. Some connections are unreliable and have limited bandwidth.

So today I found myself on yet another conference in which we had to remind people to mute because we were hearing other speakers echo through their sound pickup, and had to work our way through some parties becoming inaudible or distorted at times, and had to listen to the occasional background conversation, and had to ask people to repeat themselves, a little louder please. It is, honestly a bit of a drag. I’ll admit that I pine for the days when you really just could sit around the table with a couple of co-workers and point at the plots in your notebook and be done with it.

But this post has a positive tone, really. I just try to keep in mind that yes, we are able to work with people who are scattered all around the globe, and actually get things done, thanks to this technology, even though it gives me fits.

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I’m off to Geneva for a couple of weeks. While I’m there I’ll work on the test beam for the ALICE electromagnetic calorimeter. I’ll tell you more about that in the next posts. But I thought I’d share with you the contents of my long trip survival kit:

A travel pillow, a bandana (which serves both as an eye mask and a lazy hair style), an outlet adapter, a netbook and mini-optical mouse, ear plugs, an mp3 player with a 30 hour battery, a hair brush and extra hair bands, two change purses (one for Euros, the other for Swiss Francs) and little mini-toothbrushes with toothpaste already on them.  I don’t deal with sleep deprivation very well so these flights are never very fun – but they’re easier to take than flights between the US and Asia.

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Workers in France are guaranteed at least 5 weeks paid vacation time each year.^[1,2] Many people take that time off in the month of August.  I don’t know how August became the vacation month, but that’s the way it is.  Hours for many stores become even more limited or simply close – for the month!

Even in my hometown of Madison, WI there is a French bakery owned by a french family and they close up shop for most of August.

The disappearing of French workers also happens at CERN – professors, scientists, etc, many of them are gone.  That leaves the rest of us with the chance to either get ahead in our work, or relax and take it easy as well.

(Oh, and did I mention that the French also have a 35-hour work week?^[3])

Don’t worry though, the LHC is still on and they’re trying to reach higher beam luminosities.  At the moment they’re working on some cryo problems:

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One of the nights at ICHEP, I ended up by myself and wandered around the middle of Paris a bit. At last I was hungry, and decided I wanted something easy and affordable if at all possible. The best solution for this, in Paris, is one of the touristy restaurants. So what I ate is below. Some aspects of it are typically French, but there is nothing unfamiliar to an American about it except the concept of an omelet at dinner.

You can also see what I was reading: a book of papers on the “multiverse hypothesis” adapted from some conference lectures. Among some theoretical physicists trying to build a fundamental theory of life, the universe, and everything, there is serious research and debate on this subject — but to me as an experimentalist, it’s crazy far-out philosophy. But it’s also amusing dinner reading, and the university publishers who had booths set up at ICHEP were the only source of English-language books I knew of in Paris.

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We toss around the term “TeV” – a teraelectron volt, 10¹² electron volts (eV). But how much energy is it really?

An electron volt is the energy an electron gains when it is accelerated through a potential difference of one volt. An electron volt is defined as a unit of energy. (Various prefixes are defined here.)

Let’s put this in terms we can all understand. A Lindt 70% cocoa chocolate bar has 194 Calories. To convert this to electron volts:

194 Calories × 1000 calories/Calorie ×4.2J/calorie / 1.6 × 10^-19 J/eV = 5 × 10²⁴eV

(Note that the dietary unit, a Calorie, is 1000 calories, the amount of energy needed to raise the temperature of one mL of water by one degree Celsius.) So it would take a hundred billion (10¹¹) proton-proton collisions at top energy (14 TeV in the center of mass) to get the same amount of energy as in a chocolate bar.

The difference is how much space we pack that energy into. A proton has a volume of roughly 1 fm³, or about 10^-39 cm³. A Lindt chocolate bar is about 10 cm x 1/2 cm x 20 cm = 100 cm³. A chocolate bar then has an energy density of about 194 Cal/100 cm³, or around 2 Cal/cm³. A proton-proton collision at 14 TeV has an energy density around 14 x 10¹² eV/10^-39 cm³ x 1.6 × 10^-19 J/eV *1000 Calorie/4.2J = 5 x 10³⁵ Cal/cm³. So our proton-proton collisions have an energy density about 10³⁵ times a chocolate bar.

We also use an electron volt as a unit of temperature. An atom in a monatomic (helium, argon, etc.) ideal gas has a kinetic energy of 3/2k_BT where k_B is the Boltzmann constant. The factor in front (3/2) is different for different systems. For instance, it’s 5/2 for a diatomic gas, such as hydrogen (H₂), oxygen (O₂), or nitrogen (N₂). But the energy is usually k_BT times some factor between 1-10. So to convert an electron volt into a unit of temperature, we use eV=k_BT and T=eV/k_B=11604 Kelvin.

So how hot is a cup of coffee in electron volts? When I worked at a coffee shop in high school, we made our cappuccinos and lattes at 160°F (71°C). This works out to be 344K, or 0.03 eV. So a proton moving at 7 TeV is about 100,000,000,000,000 (10¹⁴⁾ times more energetic than the average molecule in a cup of coffee.

The Quark Gluon Plasma created at the Relativistic Heavy Ion Collider is at a temperature of about 170 MeV. (Note this is the temperature of the medium produced, not the energy of the incoming beam.) The fluid we’ll create at the LHC will be hotter – over ten billion (10¹⁰) times hotter than a cup of coffee.

These collisions are hot stuff!

[Note these are all what we call “back-of-the-envelope” calculations. The goal is to figure out the right order of magnitude for various quantities, not to do a detailed, precise calculation.]

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Over the weekend the LHC was able to deliver our first pb^-1 of data! Milestones keep rolling on by and the data keeps rolling in. This is a big first step in getting what will hopefully be lots and lots of data. I’ve included a link to the ATLAS luminosity plot for your viewing pleasure. (CMS has one too… but I’m on ATLAS )

To anyone who isn’t a particle physicist an inverse picobarn (pb^-1) is a pretty bizarre unit. I’ll start out with the base unit: the barn (b). It’s a measurement of area, proportional to m^2 or cm^2. The barn unit comes from when nuclear physics was in its infancy and refers to a uranium nucleus which is as big as a barn (1 barn = 10^-24 cm^2). (I still think physicists should hire writers to come up with this stuff… anywho, back to the post).

An inverse barn (or b^-1) in the particle physics world is a measure of collision events in an area of a barn. Throw in a metric prefix (pico which is 10^-12*base unit) and now you’re all caught up to speed. But what does that mean really? Fermilab has over an inverse femtobarn (fb^-1, which means 1000x an inverse picobarn) of data but of course they’ve been running their collider for over a decade. We’ll still need much more data to do searches for things like the Higgs, but very early searches are definitely underway – not to mention all the Standard Model physics and calibration that’s going on too.

So cheers to the first pb-1 of data… I can’t wait to start analyzing.

-Regina

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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!

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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!

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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.

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.

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!

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.

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

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I’m on pixel shift in the ATLAS control room at the moment. We’ve had a very successful run, with stable LHC beams for about the past 12 hours. The luminosity of the detector is another step forward, because the past two nights have seen more bunches per beam than ever before. The lifetime of the beam is also excellent. You can see below that the luminosity of the beam has only been decaying very slowly for the past 9 or 10 hours.

The LHC reports that, after the dump, they will fill the LHC for another physics run. I’m actually not sure why they’re dumping so soon, given that it will likely take a few hours to dump and refill, and the beam still has a lot of oomph in it. Some ideas off the top of my head: maybe the LHC experts think it’s best to do the dump and refill during the day shift; maybe they think that the luminosity in the next run can be made even better; or maybe they want to get more practice and collect more data about the start of physics fills in this configuration. But the bottom line, for me and for ATLAS, is that when they dump the beam, we reset our detector and get ready for the next physics run.

Ok, gotta go, it’s time for the beam dump!

LHC Status from July 31, 2010 at 11:49 AM

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As we speak there are 25 bunches of protons in both proton beams in the LHC.  See all those steps in the graph (red and blue lines)?  Each little step is one bunch being added, and each “big” step is 4 bunches being added.  So if you count the steps yourself you should get a total bunch count of 25 in each beam.  The red and blue lines correspond to the left-hand y-axis showing “Intensity”.

The energy of the proton beams is in black and goes with the right-hand y-axis, “Energy (GeV)”.  As I write this the protons are around 500-some GeV and being ramped up to 3500 GeV which should take about half an hour.

Once both beams are at 3500 GeV and they declare stable beams, it’s time to record some data with the most bunches in the LHC to date!

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Yesterday was, I suppose, the biggest and most formal day here at the 35th International Conference of High Energy Physics. I wore my suit, and took some ribbing from some of my colleagues for dressing up so much, but I’ve worn it for far less excuse and am not sorry in the slightest. It’s not every day, or even every ICHEP, that one hears an address from the President of the French Republic!

Mr. Sarkozy’s speech was great to hear. He is a very emotive, enthusiastic, and informal speaker, which made him relatively easy to understand for those in the audience (like me) with limited French. He didn’t claim to know the details of our work, and seemed to think we’re all a little weird, but spoke mostly about the importance of fundamental questions about the universe and his support for basic research. This was very well received indeed. He also talked a bit about the contributions of the French labs to the LHC and other projects, which is fair: France plays one of the central roles at CERN, and the French particle physics community has made significant contributions throughout the field. This is reflected in the excellent and informative conference that they’re hosting here in Paris.

If there was one thing I would ask to be improved for the next ICHEP, however, I wish there had been a bit more food at the banquet! You can see the banquet, which was held in the National Natural History Museum, at right. It was a very impressive museum — the main hall reminds me of the ATLAS cavern, and seems to be just about the same size — and the food was certainly varied and interesting. But the lines were long to get even a little of it, and we had to go out to dinner afterwards to be full.

Yesterday was also the final talk on the combined Tevatron Higgs results. Fermilab sent out the press release while we were still on coffee break, so I saw the excluded mass range on Twitter before going into the talk. (I overcame the temptation to shout out the answer at the start.) It was still a very entertaining talk, and obviously the details of how the measurement was done were as important to see as the final numbers. Of course, we all wish there had been more to see than mass limits in the expected range and a few possible tantalizing hints. We also had talks from the CERN’s Director for Accelerators and Technology on the LHC and from the spokespeople of the LHC experiments. Although we haven’t seen anything really new at the LHC yet, it’s clear that the accelerator and experiments are all making great progress in getting ready to make the exciting discoveries that we’ll see at future conferences!

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Lots of us bloggers have been talking about ICHEP which is going on this week. I’m not attending the conference, although some of the work I’ve been doing is .
Now I’ve been turning my attention back to my physics analysis.  As of about a week ago we have 200 nb-1 (now closer to 300 nb-1) of data – which is about 1/50th of what I hope to get for an analysis.

I briefly mentioned that I’ll be doing a search in a previous post. Now I’d like to share a bit what this particle beast is. A leptoquark carries quantum numbers for both quarks and leptons. It would decay by generation such that it mixes families of quarks and leptons. So why do we think it exists? In a word: Symmetry.

Physicists love symmetry (and symmetry breaking ). Symmetry in forces (like electricity and magnetism), symmetry in families and generations of particles, symmetries everywhere. Since the quarks and leptons in the Standard Model have the same family structure it seems like there should be something that ties them together, like leptoquarks.

Granted this is a bit of an oversimplification, theorists have put in lots of work into understanding how these particles work. And now I’m going to be looking for them.

I’ll be giving updates over the next few months explaining more about these particles over the coming weeks.

-Regina

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