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Archive for December, 2010

This is the tenth (or so) post about Feynman diagrams, there’s an index to the entire series in the first post.

There is a famous quote by former Secretary of Defense Donald Rumsfeld that really applies to particle physicists:

There are known knowns.
These are things we know that we know.
There are known unknowns.
That is to say, there are things that we know we don’t know.
But there are also unknown unknowns.
There are things we don’t know we don’t know.

Ignoring originally intended context, this statement describes not only the current status of the Standard Model, but accurately captures all of our hopes and dreams about the LHC.

  • We have “known knowns” for which our theories have remarkable agreement with experiment. In this post I’d like to summarize some of these in the language of Feynman diagrams.
  • There are also “known unknowns” where our theories break down and we need something new. This is what most of my research focuses on and what I’d like to write about in the near future.
  • Finally, what’s most exciting for us is the chance to trek into unexplored territory and find something completely unexpected—“unknown unknowns.”

Today let’s focus on the “known knowns,” the things that we’re pretty sure we understand. There’s a very important caveat that we need to make regarding what we mean by “pretty sure,” but we’ll get to that at the bottom. The “known knowns” are what we call the Standard Model of particle physics*, a name that says much about its repeated experimental confirmations.

* — a small caveat: there’s actually one “known unknown” that is assumed to be part of the Standard Model, that’s the Higgs boson. The Higgs is currently one of the most famous yet-to-be-discovered particle and will be the focus of a future post. In the meanwhile, Burton managed to take a few charming photos of the elusive boson in his recent post.

First, let’s start by reviewing the matter particles of the Standard Model. These are called fermions and they are the “nouns” of our story.

Matter particles: the fermions

We can arrange these in a handy little chart, something like a periodic table for particle physics:

Let’s focus on only the highlighted first column. This contains all of the ‘normal’ matter particles that make up nearly all matter in the universe and whose interactions explain everything we need to know about chemistry (and arguably everything built on it).

The top two particles are the up and down quarks. These are the guys which make up the proton (uud) and neutron (udd). As indicated in the chart, both the up and down quarks come in three “colors.” These aren’t literally colors of the electromagnetic spectrum, but a handy mnemonic for different copies of the quarks.

Below the up and down we have the electron and the electron-neutrino (?e), these are collectively known as leptons.  The electron is the usual particle whose “cloud” surrounds an atom and whose interactions is largely responsible for most of chemistry. The electron-neutrino is the electron’s ghostly cousin; it only interacts very weakly and is nearly massless.

As we said, this first column (u, d, e, and ?e) is enough to explain just about all atomic phenomena. It’s something of a surprise, then, that we have two more columns of particles that have nearly identical properties as their horizontal neighbors. The only difference is that as you move to the right on the chart above, the particles become heavier. Thus the charm quark (c) is a copy of the up quark that turns out to be 500 times heavier. The top quark (t) is heavier still; weighing in at over 172 GeV, it is the heaviest known elementary particle. The siblings of the down quark are the strange (s) and bottom (b) quarks; these have historically played a key role in flavor physics, a field which will soon benefit from the LHCb experiment. Each of these quarks all come in three colors, for a total of 2 types x 3 colors x 3 columns = 18 fundamental quarks. Finally, the electrons and neutrinos come with copies named muon (?) and tau (?). It’s worth remarking that we don’t yet know if the muon and tau neutrinos are heavier than the electron-neutrino. (Neutrino physics has become one of Fermilab’s major research areas.)

So those are all of the particles. As we mentioned in our first post, we can draw these as solid lines with an arrow going through them.  You can see that there are two types of leptons (e.g. electron-like and neutrino) and two types of quarks (up-like and down-like), as well as several copies of these particles. In addition, each particle comes with an antiparticle of opposite electric charge. I won’t go into details about antimatter, but see this previous post for a very thorough (but hopefully still accessible) description.

You can think of them as nouns. We now want to give them  verbs to describe how they can interact with one another. To do this, we introduce force particles (bosons) and provide the Feynman rules to describe how the particles interact with one another. By stringing together various particle lines to the vertices describing interactions, we end up with a Feynman diagram that tells the story of a particle interaction. (This is the “sentence” formed from the fermion nouns and the boson verbs.)

We will refer to these forces by the ‘theories’ that describe them, but they are all part of the larger Standard Model framework.

Quantum Electrodynamics

The simplest force to describe is QED, the theory of electricity and magnetism as mediated by the photon. (Yes, this is just the “particle” of light!) Like all force particles, we draw the photon as a wiggly line. We drew the fundamental vertex describing the coupling of an electron to the photon in one of our earliest Feynman diagram posts,

For historical reasons, physicists often write the photons as a gamma, ?. Photons are massless, which means they can travel long distances and large numbers of them can set up macroscopic electromagnetic fields. As we described in our first post, you are free to move the endpoints of the vertex freely. At the end of the day, however, you must have one arrowed line coming into the vertex and one arrowed line coming out. This is just electric charge conservation.

In addition to the electron, however, all charged particles interact with the photon the same vertex. This means that all of the particles above, except the neutrinos, have this vertex. For example, we can have an “uu?” vertex where we just replace the e‘s above by u‘s.

QED is responsible for electricity and magnetism and all of the good stuff that comes along with it (like… electronics, computers, and the US LHC blog).

Quantum Flavordynamics

This is a somewhat antiquated name for the weak force which is responsible for radioactivity (among other things). There are two types of force particle associated with the weak force: the Z boson and the W boson. Z bosons are heavier copies of photons, so we can just take the Feynman rule above and change the ? to a Z. Unlike photons, however, the Z boson can also interact with neutrinos. The presence of the Z plays an important role in the mathematical consistency of the Standard Model, but for our present purposes they’re a little bit boring since they seem like chubby photon wanna-be’s.

On the other hand, the W boson is something different. The W carries electric charge and will connect particles of different types (in such a way that conserves overall charge at each vertex). We can draw the lepton vertices as:

We have written a curly-L to mean a charged lepton (e, ?, ?) and ?i to mean any neutrino (?e, ??, ??). An explicit set of rules can be found here. In addition to these, the quarks also couple to the W in precisely the same way: just replace the charged lepton and neutrino by an up-type quark and a down-type quark respectively. The different copies of the up, down, electron, and electron-neutrino are called flavors. The W boson is special because it mediates interactions between different particle flavors. Note that it does not mix quarks with leptons.

Because the W is charged, it also couples to photons:

It also couples to the Z, since the Z just wants to be a copy-cat photon:

Finally, the W also participates in some four-boson interactions (which will not be so important to us):

Quantum Chromodynamics

Finally, we arrive at QCD: the theory of “strong force.” QCD is responsible for binding quarks together into baryons (e.g. protons and neutrons) and mesons (quark–anti-quark pairs). The strong force is mediated by gluons, which we draw as curly lines. Gluons couple to particles with color, so they only interact with the quarks. The fundamental quark-gluon interaction takes the form

The quarks must be of the same flavor (e.g. the vertex may look like up-up-gluon but not up-down gluon) but may be of different colors. Just as the photon vertex had to be charge-neutral, the gluon vertex must also be color-neutral. Thus we say that the gluon carries a color and an anti-color; e.g. red/anti-blue. For reasons related to group theory, there are a total of eight gluons rather than the nine that one might expect. Further, because gluons carry color, they interact with themselves:

QCD—besides holding matter together and being a rich topic in itself—is responsible for all sorts of head aches from both theoretical and experimental particle physicists. On the experimental side it means that individual quarks and gluons appear as complicated hadronic jets in particle colliders (see, e.g. Jim’s latest post). On the theoretical side the issue of strong coupling (and the related idea of confinement) means that the usual ‘perturbative’ techniques to actually calculate the rate for a process quickly becomes messy and intractable. Fortunately, there are clever techniques on both fronts that we can use to make progress.

The missing piece: The Higgs Boson

Everything we’ve reviewed so far are known knowns, these are parts of our theory that have been tested and retested and give good agreement with all known experiments. There are a few unknown parameters such as the precise masses of the neutrinos, but these are essentially just numbers that have to be measured and plugged into the existing theory.

There’s one missing piece that we know must either show up, or something like it must show up: the Higgs boson. I’d like to dedicate an entire post to the Higgs later, so suffice it to say for now that the Higgs is an integral part of the Standard Model. In fact, it is intimately related to the weak sector. The importance of the Higgs boson is something called electroweak symmetry breaking. This is a process that explains why particles have the masses that they do and  why the W, Z, and photon should be so interwoven. More importantly, the entire structure of the Standard Model breaks down unless something like the Higgs boson exists to induce electroweak symmetry breaking: the mathematical machinery behind these diagrams end up giving nonsensical results like probabilities that are larger than 100%. Incidentally, this catastrophic nonsenical behavior begins at roughly the TeV scale—precisely the reason why this is the energy scale that the LHC is probing, and precisely the reason why we expect it to find something.

A fancy way of describing the Standard Model is that there are actually four Higgs bosons, but three of them are “eaten” by the W and Z bosons when they become massive. (This is called the Goldstone mechanism, but you can think of it as the Grimm’s Fairy Tale of particle physics.) This has led snarky physicists to say things like, “Higgs boson? We’ve already found three of them!”

Theories and Effective Theories

By specifying the above particles and stating how the Higgs induces electroweak symmetry breaking, one specifies everything about the theory up to particular numbers that just have to be measured. This is not actually that much information; the structure of quantum mechanics and special relativity fixes everything else: how to write down predictions for different kinds of processes between these particles.

But now something seems weird: we’ve been able to check and cross-check the Standard Model in several different ways. Now, however, I’m telling you that there’s this one last missing piece—the Higgs boson—which is really really important… but we haven’t found it yet. If that’s true, how the heck can we be so sure about our tests of the Standard Model? How can these be “known knowns” when we’re missing the most important part of the theory?

More generally, it should seem funny to say that we “know” anything with any certainty in science! After all, part of the excitement of the LHC is the hope that the data will contradict the Standard Model and force us to search for a more fundamental description of Nature. The basis of the scientific method is that a theory is only as good as the last experiment which checked it, and there are good reasons to believe that the Standard Model breaks down at some scale. If this is the case, then how can we actually “know” anything within the soon-to-be-overthrown Standard Model paradigm?

The key point here is that the Standard Model is something called an effective theory. It captures almost everything we need to know about physics below, say, 200 GeV, but doesn’t necessarily make any promises about what is above that scale. In fact, the sicknesses that the Standard Model suffers from when we remove the Higgs boson (or something like it) are just the theory’s way of telling us, “hey, I’m no longer valid here!”

This is not as weird as one might think. Consider the classical electromagnetic field of a point particle: it is a well known curiosity to any high school student that the potential at the exact location of the point source is infinity. Does that mean that an electron has infinite energy? No! In fact, this seemingly nonsensical prediction is classical electromagnetism telling us that something new has to fix it. That something new is quantum mechanics and the existence of antiparticles, as we previously discussed.

This doesn’t mean that the effective theory is no good, it only means that it breaks down above some region of validity. Despite the existence of quantum mechanics, the lessons we learn from high school physics were still enough for us to  navigate space probes to explore the solar system. We just shouldn’t expect to trust Newtonian mechanics when describing subatomic particles. There’s actually a rather precise sense in which a quantum field theory is “effective,” but that’s a technical matter that shouldn’t obfuscate the physical intuition presented here.

For physicists: the theory of the Standard Model without a Higgs is a type of non-linear sigma model (NL?M). This accurately describes a theory of massive vector bosons but suffers from a breakdown of unitarity. The Higgs is the linear completion of the NL?M that increases the theory’s cutoff. In fact, this makes the theory manifestly unitary, but does not address the hierarchy problem. For an excellent pedagogical discussion, see Nima Arkani-Hamed’s PiTP 2010 lectures.

Where we go from here

The particles and interactions we’ve described here (except the Higgs) are objects and processes that we have actually produced and observed in the lab. We have a theory that describes all of it in a nice and compact way, and that theory requires something like the Higgs boson to make sense at high energies.

That doesn’t mean that there aren’t lots of open questions. We said that the Higgs is related to something called “electroweak symmetry breaking.” It is still unknown why this happens. Further, we have good reason to expect the Higgs to appear in the 115 – 200 GeV range, but theoretically it takes a “natural” value at the Planck mass (1019 GeV!). Why should the Higgs be so much lighter than its “natural” value? What particle explains dark matter? Why is there more matter than anti-matter in the universe?

While the Higgs might be the last piece of the Standard Model, discovering the Higgs (or something like it!) is just the beginning of an even longer and more exciting story. This is at the heart of my own research interests, and involves really neat-sounding ideas like supersymmetry and extra dimensions.


Hello, again.  Thanks for all the excellent comments on my last post.

In my last post I explained that our current theory makes the assumption, which has not been experimentally verified, that quarks are indivisible, point-like particles (“elementary” particles).   I also discussed why it is reasonable to think that quarks might actually be made up of even smaller, currently unknown particles.   If this is the case, we would call quarks “composite” particles instead of “elementary” particles.

In this post I describe how we search for evidence that quarks are composite particles.  (Sometimes we phrase this as a “search for quark compositeness.”)   The distance scales in which we are interested (50,000 times smaller than the proton) are far too small to be probed with even a very powerful microscope, so we have to take a route that will probably seem indirect:

1. First we pick a physical quantity that we are able to measure with our particle detector and that is related to the composition of the quark – let’s call it Q.  Then we measure Q using our experiment.  Because of the inherent statistical nature of any experiment, there is some uncertainty on our measured value of Q. (This is the same type of “plus or minus” uncertainty that political pollsters quote when they predict that 50% plus or minus 3% of voters will choose Barack Obama, for instance, in an election.)

2. Then we also calculate Q making various assumptions about the composition of quarks.  We calculate Q assuming that our current theory of elementary quarks is correct, and we also calculate Q assuming that quarks are composite particles.

3. Finally, we compare the measured and calculated values of Q.  If we find that the measured Q is inconsistent with a certain calculated value of Q, we conclude that the assumptions used in that calculation are incorrect.  However, because of the statistical uncertainty on the measurement, our measured Q might be consistent with more than one of the calculated values of Q.  (If the election mentioned above ultimately showed that 47% or 50% or 52% of voters actually chose Obama, we would say that these three results are all consistent with the poll’s prediction of 50% plus or minus 3%.)  More on this possibility below …

So far this discussion has been very abstract.  To make things more concrete, I will describe the actual physical quantity Q that we use to search for quark compositeness.  Before I do, I need to explain what happens in a proton-proton collision in the Large Hadron Collider:  When two protons smash into each other at 99.999999% of the speed of light, a lot different things can happen, but a very frequent occurrence is a single quark from one proton colliding with a single quark from the other proton like this:

You can see that the quarks that collide exit the collision at a large angle, while the quarks that did not collide zip right past each other.  Because of the laws of physics governing the behavior of quarks, the quarks that collided instantly turn into showers of other particles such as pions, protons, neutrons, etc.  We call these showers of particles “jets.”   With our particle detector we measure the energy and direction of these jets.  Here is a picture of a two-jet collision event that was actually recorded by our detector:

Two jets (green) exiting the collision point (yellow dot). The measured energy of the jets is represented by the red and blue wedges. In this view, you are looking down the barrel of the detector; i.e., one proton would have been traveling into your computer screen and the other proton out of your screen.

That’s all the background you need for me to tell you about the physical quantity Q that we use to determine whether quarks are composite particles.   The quantity Q is the direction of these jets.   As I mentioned above, for the jet direction to be useful in determining quark composition, the direction of the jets must depend on quark composition, and indeed, according to calculations, if quarks really are made up of smaller particles, the angle at which the colliding quarks exit the collision will tend to be larger than if quarks are really elementary particles.

Now let’s go back to the three step method for searching for quark compositeness that I described above:

1. The first step was to measure the quantity Q:   Well, to measure the average direction the jets, we simply count the number of proton-proton collisions that produce jets in the center of our detector (red-shaded region in the diagram below).    (We actually measure the ratio of the number of central jets to the number of non-central events, but that is just a detail.)

Diagram of CMS particle detector as viewed from the side. The protons enter from the left and right and collide in the center. The blue shapes represent the detector components that measure the energy and direction of jets. The red-shaded region denotes the central region of the detector.

2. The second step was to calculate Q under several assumptions:   I already mentioned that, if quarks are composite particles, colliding quarks will exit the collision at larger angles.  For this reason, the number of central jets calculated assuming  composite quarks is larger than the number calculated assuming elementary quarks.

3. The last step was to compare the measured and calculated values of Q:   We take our measured number and compare it with our calculations.  If we were to find too many central jets,  we could conclude that our measurement is inconsistent with our calculations performed under the assumption that quarks are elementary particles.  This would be evidence that quarks are made up of smaller, unknown particles.  (It could also just be evidence that our calculation is wrong, and so this is something that we cross check thoroughly.)

The actual results:  Unfortunately, the new LHC measurements of jet directions from the CMS and ATLAS experiments do not find evidence that quarks are composite particles.   Just as in the above example of the political poll results that are consistent with several election outcomes, the LHC results are consistent with both elementary quarks and quarks made up of particles interacting at distances as large as 1/20,000 of the proton radius.  This means that, while we confirm the current theory of elementary quarks to be correct at distances 10,000 times smaller than the proton, we cannot conclude that quarks are certainly elementary particles, because our measurements are also consistent with quarks made up of particles that interact at distances 20,000 (or 50k or 100k)  times smaller than the proton.

Fortunately, as we record and analyze more data, the statistical uncertainty on our measurement of the jet directions will get smaller, and we will be able to probe distance scales even smaller than 1/20,000 of the proton radius.  Stayed tuned for more results from the LHC and more blog posts from me!


Working at heights

Thursday, December 2nd, 2010

I spent Tuesday at working at heights training.  Here you can see me in the ALICE hall on the crane track:

And here’s another picture of me hanging off the electromagnetic calorimeter insertion tool, the device used to physically install the electromagnetic calorimeter:

I need this training for when we install the rest of the electromagnetic calorimeter supermodules.  I won’t actually be working on physically installing the supermodules, but I’ll be working on installing the electronics and the cables to read out the data.  This means I may have to spend some time working where our detector is, well above the ground.  So on Tuesday I learned the proper way to use a harness, the proper way to climb a (very tall) ladder, how to secure ourselves to solid structures, and all of the rules and laws that apply at CERN.  And then we had practical exercises.  Yes this is supposed to be work, but it’s also fun!


Holiday Peaks

Wednesday, December 1st, 2010

Holidays at CERN continually take me by surprise.

Between ever-pressing to-do lists, meeting schedules, deadlines, and emails, they just don’t demand the same anticipation — and participation — as they used to. Are there no holiday peaks above my day-to-day background?! Halloween was a 2-sigma deviation in an otherwise unremarkable work-week; Labor Day was observed only in theory; Father’s Day got swamped out by a 3D pixel test beam (sorry, Dad!). I mean, if you look up from your code one Friday and realize you still don’t have a costume for or ticket to the [email protected] Halloween party, and you attempt to print out a Mark Zuckerberg mask to pair with that grungy old hoodie you used to wear, thinking maybe you could crash the party anyway, but the printer on your floor is out of toner, and the IT people are already gone for the weekend, and you could just go to that other party in Geneva where costumes aren’t even mentioned, all you have to do is pay a cover and buy a drink — I mean, physics never sleeps, and it rarely takes a holiday.

Also, for no particular reason, I keep missing holidays by traveling away from the places where they are observed. I spent Memorial Day in Hamburg for a beam telescope workshop at DESY; 4th of July in Paris to visit an old friend; Bastille Day in Cambridge for a conference on advances in radiation detector technology; la Fête de Genève in San Francisco for a summer school on neutrinos; and, most recently, Thanksgiving in Rome to visit another old friend. I didn’t find much in the way of holiday cheer, though I did keep finding the Higgs boson.

Plus, as an American living in Europe, I run into lots of holidays I wouldn’t normally celebrate — or know about in advance. A visit to Paris on May 1st coincided unfortunately with France’s Labor Day, on which even the Louvre was closed. November 5th was Guy Fawkes Day in Britain, which my friends and I celebrated in international company at a British store located in France, complete with fire and fireworks. An upcoming weekend is l’Escalade, a holiday unique to Geneva that commemorates a night four hundred years ago when a hard-working and quick-thinking housewife poured hot soup on an attacking French army and thereby saved the city. Seriously. If the snow clears up (Did I mention that the area has had record-breaking snowfall this week? Winter arrives with a vengeance!), I’ll probably celebrate in Old Town with hot soup and spiced wine. So good.

This post began like a complaint, but it wasn’t meant to be! I fault my opening: I should have written “Holidays at CERN continually surprise me.” It’s true! And without the negative implications and ensuing negative paragraph. (Amazing how that happens…) Yes, some holidays are spent at work, some are spent away, and some aren’t spent at all. Many are spent in unexpected or unconventional ways compared to holidays back in the States. Regardless, sometimes you just have to cut hard on the day-to-day background and celebrate a beautiful holiday peak. Happy Holidays, All!

— Burton 🙂


The conga line

Wednesday, December 1st, 2010

During the debate over health insurance reform in the House of Representatives this past March, I was taken by the description of a couple hundred representatives delaying the passage of the bill as a “conga line.” We’re seeing a similar conga line develop in CMS right now, but it is more about getting results out than delaying them. With the close of the 2010 dataset, the effort is on to complete measurements that can be done with this data. Within the collaboration there are hundreds of people working on tens of analyses across a wide range of physics topics. For all of these results to be released by the collaboration and subsequently published in journals, they have to go through a multi-layered approval process. It’s a huge effort on the part of the people doing the analyses and the people who do internal reviews of the analyses, but that’s how we ensure that our public results are of the highest quality. (Just how high is the quality? I intend to comment on this in a future posting.)

As part of the review process, an analysis has to be presented several times in different forums in the experiment, to allow for a wide range of people to engage in discussion with the analysis proponents. But there are only so many hours in a day and so many of those hours that you can fill with meetings, and lots of people who want to talk at those meetings. Hence the conga lines; meeting agendas are filling up quickly as people race to complete their analyses and get them out as quickly as possible.

I’m currently serving on CMS’s Publications Committee, which is the last stop for a paper that gets green-lighted for submission to a journal. Needless to say, the workload is increasing and it doesn’t look to be getting smaller anytime soon. This is ultimately good news, because we’re going to be learning so much physics…unless you are on the committee. Our committee chair recently observed to me, “The wave is about to become a tsunami.” Let’s hope the conga line doesn’t get washed away by it.

(Gee, can we let the entry end with an oddly mixed metaphor?)