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Archive for September, 2010

Perks of the job

Friday, September 17th, 2010

Life as a high energy physicist is not without its perks.  I recently got back from my latest trip to CERN for the EMCal test beam.  I spent about a week on the midnight to 8 AM shift and then stayed a week to work with some of my collaborators in ALICE.  The hours are long and the work is hard but the company is good and there are many perks.

I’m an avid hiker so I took a day off to go hiking in the Juras.  My friend Daniel organized it and we ended up with a group of two physicists from ALICE, one from CMS, one from ATLAS, and one from a university in France.  We had one American, one Brit, one Spaniard, and two Mexicans.  A multicultural group in many ways.  Here you can see the view from the Juras:

Somewhere down there is CMS.  It was a nice hike but next time I’ll pack my good compass and get my own trail map.  We had some unintentional adventures.

After my trip to CERN I went to a conference in Sicily – which means I had to work on a talk while I was at CERN.  Of course Sicily is beautiful:

(This is the view from Taormina during the excursion.)

Then I packed up and left, first to Geneva and then back to the US.  Five flights and four countries in two days.  My luggage made it through Paris to Atlanta but then decided to take a vacation in Atlanta without me.  I’m now looking at a grueling travel schedule in the next four months.  Plans have changed and our detector, the electromagnetic calorimeter, is going in during the Christmas shutdown.  This is great news but it also defines my holiday schedule – November and part of January at CERN.  On top of that I have a few meetings and some personal travel.  I’ll be lucky if I manage to be home for two weeks in December.  However, I did not get much sympathy from my father the other day when I was complaining about how I might have to get extra pages in my passport because I’m running out of space.  Go figure.


The W mass from Fermilab

Thursday, September 16th, 2010

With the LHC running and experimentalists busy taking real data, one of the things left for theory grad students like me is to learn how to interpret the plots that we hope will be ready next summer.

A side remark: the LHC will keep taking physics into next year, but in 2012 will shut down for a year to make the adjustments necessary to ramp up to its full 7 TeV/beam energy. Optimistic theorists are hoping that the summer conferences before the shutdown will share plots that offer some clues about the new physics we hope to see in the subsequent years.

The most basic feature we can hope for is a resonance, as we described when we met the Z boson. The second most basic signal of a new particle is a little more subtle, it’s a bump in the transverse mass distribution. I was reminded of this last night because of a new conference proceeding paper on the arXiv last night (1009.2903) presenting the most recent fit to the W boson mass from the CDF and D0 collaborations at the Tevatron.

The result isn’t “earth shattering” by any extent. We’ve known that the W mass is around 80 GeV for quite some time. The combined results with the most recent data is really an update about the precision with which we measure the value because it is so important for determining other Standard Model relations.

Here’s the plot:

It’s not the prettiest looking plot, but let’s see what we can learn before going into any physics. In fact, we won’t go into very much physics in this post. The art of understanding plots can be subtle and is worth a discussion in itself.

  • On the x-axis is some quantity called mT. The plot tells us that it is measured in GeV, which is a unit of energy (and hence mass). So somehow the x axis is telling us about the mass of the W.
  • On the y-axis is “Events per 0.5 GeV.” This tells us how many events they measured with a given mT value.
  • What is the significance of the “per 0.5 GeV” on the y-axis? This means, for example, that they count the number of events between 70 GeV and 70.5 GeV and plot that on the graph. This is called “binning” because it sets how “bins” you divide your data set into. If you have very small bins then you end up with more data points on the x axis, but you have much fewer data points per bin (worse statistics per bin). On the other hand, if you have very large bins you end up with lots of data per bin, but less ability to determine the overall shape of the plot.
  • The label for the plot tells us that we are looking at events where a W boson is produced and decays into a muon and a neutrino. This means (I assume?) that the experimentalists have already subtracted off the “background” events that mimic the signature of a muon and a neutrino in the detector. (In general this is a highly non-trivial step.)
  • The blue crosses are data: the center of the cross is the measured value and the length of the bars gives the error.
  • The values under the plot give us the summary of the statistical fit: it tells us that the W mass is 80.35 GeV and that the χ2/dof is reasonable. This latter value is a measure of how consistent the data is with your theory. Any value near 1 is pretty good, so this is indeed a good fit.
  • The red line is the expected simulated data using the statistical fit parameters. We can see visually that the fit is very good. You might wonder why it is necessary to simulate data—can’t the clever theorists just do the calculation and give an explicit plot? In general it is necessary to simulate data because of QCD which leads to effects that are intractable to calculate from first principles, but this is a [very interesting] story for another time.

Now for the relevant question: what exactly are we plotting? In order to answer this, we should start by thinking about the big picture. We smash together some particles, somehow produce a W boson, which decays into a muon and a neutrino. We would like to measure the mass of the W boson from the “final states” of the decay. The primary quantities we need to reconstruct the W mass are the energies and momenta of the muon and neutrino. Then we can use energy and momentum conservation to figure out the W‘s rest mass. (There’s some special relativity involved in here which I won’t get into.)

Homework: for those of you with some background in high school or college physics, think about how you would solve for the W mass if you had a measurement for the muon energy and momentum. For this “toy calculation” you don’t need special relativity, just use E = (rest mass energy) + (kinetic energy) and assume that the neutrino is massless. [The discussion below isn’t too technical, but it will help if you think about this problem a little before reading on.]

The first point is that we cannot measure the neutrino: it’s so weakly interacting that it just shoots out of our detector without any direct signals… like a ninja. That’s okay! Conservation of energy and momentum tells us that it is sufficient to determine the energy and momentum of the muon. We know that the neutrino momentum has to be ‘equal and opposite’ and from this we can reconstruct its energy (knowing that it has negligible mass).

… except that this too is a little simplified. This would be absolutely true if the W boson were produced at rest, such as at electron-positron colliders like LEP or SLAC. However, at the Tevatron we’re colliding protons and antiprotons…. which means we’re accelerating protons and antiprotons to equal and opposite energies, but the actual stuff that’s colliding are quarks, which each carry an unknown fraction of the proton energy and momentum! Thus the W boson could end up having some nonzero momentum along the axis of the beam and this spoils our ability to use a simple calculation based on energy/momentum conservation to determine the W mass.

This is where things get slick—but I’ll have to be heuristic because the kinematics involved would be more trouble than they’re worth. The idea is to ignore the momentum along the beam direction: it’s worthless information because we don’t know what the initial momentum in that direction was. We only look at the transverse momenta, which we know should be conserved and was initially zero.

If we use conservation of energy/momentum on only the transverse information, we can extract a “fake” mass. Let us call this the transverse mass, mT. (Technically this is not yet the “transverse mass,” but since we’re not giving rigorous mathematical definitions, it won’t matter.) This fake mass is exactly equal to the real mass when the W has no initial longitudinal momentum. This is a problem: we have no way to know the initial longitudinal momentum for any particular event… we just know that sometimes it is close to zero and other times its not.

The trick, then, is to take a bunch of events. Up to this point, in principle you didn’t need more than one event to determine the W mass as long as you knew that the one event had zero longitudinal momentum. Now that we don’t know this, we can plot a distribution of events. For the events where the longitudinal momentum of the W is zero, we expect that our transverse mass measurements are close to the true W mass. For the events with a non-negligible longitudinal momentum, part of the “energy” of the W goes into the longitudinal direction which we’re ignoring, and thus we end up measuring a transverse mass which is less (never greater!) than the true W mass.

Thus we have a strategy: if we can measure a bunch of events, we can look at the distribution and the largest possible value that we measure should represent those events with the smallest longitudinal momentum, and hence should give the correct W mass.

This is almost right. It turns out that there are a few quantum effects that spoil this. During the production of the W, nature can conspire to pollute even the transverse momentum data: the W might emit a photon that shifts its transverse momentum a little, or the quarks and gluons might produce some hadrons that also give the W some transverse momentum kick. This ends up smearing out the distribution. It turns out that these can be taken into account in a very clever—but essentially mathematical—way, and the result is the plot above. You can see that the distribution is still smeared out a little bit towards the tail, but that there is a sharp-ish edge at the true W boson mass. This is what experimentalists look for to fit their data to get extract the W mass. (For more discussion on the W mass and a CMS perspective, see this post by Tommaso a few months ago.)

I really like this story—there’s a lot of intuition and physics that goes into the actual calculations. It turns out, however, that for the LHC things can get a lot more complicated. Instead of single W bosons, we hope to produce pairs of exotic particles. These can each end up decaying into things that are visible and invisible, just like the muon–neutrino system that the W decays into. However, now that there are two such decays, the kinematics ends up becoming much trickier.

Recently some very clever theorists from Cambridge, Korea, and Florida have made lots of progress on this problem and have developed an industry for so-called “transverse mass” variables. For those interested in the technical details, there’s now an excellent review article (arXiv:1004.2732). [These sorts of analyses will probably not be very important until after the LHC 2012 shutdown when more data can be collected, but they offer a lot of promise for how we can connect models of new physics to data from experiments.]



More Than a Checklist

Thursday, September 16th, 2010

I’d like to make a few comments on Flip’s excellent post on advice for undergrads. I’m an argumentative sort, but none of this is disagreement as such; every time I wanted to argue, his entry anticipates it by giving caveats or suggesting sensible priorities. And it really is a great checklist, if you want to be a physicist some day. But I still want to caution you: college, like life, is more than a checklist. You can be a physicist some day even if you don’t do it all perfectly. I’m going to be a physicist, at least for a while yet, and here some confessions about my time in college:

  • I was well-rounded, sometimes in random ways. I took a course on the history and philosophy of science; even if it makes me a better physicist somehow, it won’t help my academic career. I took a course on Fairy Tales with some friends my senior year just because I was tired.
  • Talking to professors was hard. The ones I talked to were the ones I did research with, and one who reached out to classes to an unusual degree. (But yes, this makes doing research even more important.)
  • I didn’t learn LaTeX.
  • I didn’t go to very many talks. I read hardly any papers. In retrospect, I wish I had. But your time in college is yours, not an opportunity to make good on things I missed — and anyway I’ve read enough papers in grad school to make up for it.

My point isn’t that you should skip those things. My point is that you won’t do everything exactly the way you could have, or should have, or the way Flip or anyone else recommends. You do have to be pretty damn good to be a scientist, but you don’t have to be perfect. After all, the people you’ll be competing with are just bozos like me.


Meet the quarks

Tuesday, September 14th, 2010

One of the most important experiments in the history of physics was the Rutherford experiment where “alpha particles” were shot at a sheet of gold foil. The way that the particles scattered off the foil was a tell-tale signature that atoms contained a dense nucleus of positive charge. This is one of the guiding principles of high-energy experiments:

If you smash things together at high enough energies, you probe the substructure of those particles.

When people say that the LHC is a machine colliding protons at 14 TeV, what they really mean is that it’s a quark/gluon collider since these are the subnuclear particles which make up protons. In this post we’ll begin a discussion about what these subatomic particles are and why they’re so different from any of the other particles we’ve met.

(Regina mentioned QCD in her last post—I think “subtracting the effects of QCD,” loosely phrased, is one of the ‘problems’ that both theorists and experimentalists often struggle with.)

This post is part of a series introducing the Standard Model through Feynman diagrams. An index of these entries can be found on the original post. In this post we’ll just go over the matter particles in QCD. (I’m experimenting with more frequent—but shorter—posts.)

A (partial) periodic table for QCD

The theory that describes quarks and gluons is called Quantum Chromodynamics, or QCD for short. QCD is a part of the Standard Model, but for this post we’ll focus on just QCD by itself. Quarks are the fermions—the matter particles—of the theory. There are six quarks, which come in three “families” (columns in the table below):

The quarks have cute names: the up, down, charm, strange, top, and bottom. Historically the t and b quarks have also been called “truth” and “beauty,” but—for reasons I don’t quite understand—those names have fallen out of use, thus sparing what would have been an endless parade of puns in academic paper titles.

The top row (u,c,t) is composed of particles with +2/3 electric charge while the bottom row is composed of particles of -1/3 charge. These are funny values since we’re used to protons and electrons with charges +1 and -1 respectively. On the one hand this is a historical effect: if we measured the quark charges first we would have said that

  • the down quark has charge -1
  • the up quark has charge +2
  • the electron has charge -3
  • and the proton has charge +3

It’s just a definition of how much is one “unit” of charge. However, the fact that the quark and lepton charges have these particular ratios is a numerical curiosity, since it is suggestive (for reasons we won’t go into here) of something called grand unification. (It’s not really as “grand” as it sounds.)

One quark, two quark, red quark, blue quark?

I drew the above diagram very suggestively: there are actually three quarks for each letter above. We name these quarks according to colors: thus we can have a red up quark, a blue up quark, and a green up quark, and similarly with each of the five quarks. Let me stress that actual quarks are not actually “colored” in the conventional sense! These are just names that physicists use.

The ‘colors’ are really a kind of “chromodynamic” charge. What does this mean? Recall in QED (usual electromagnetism) that the electron’s electric charge means that it can couple to photons. In other words, you can draw Feynman diagrams where photons and electrons interact. This is precisely what we did in my first post on the subject. In QED we just had two kinds of charge: positive and negative. When you bring a positive and negative charge together, they become neutral. In QCD we generalize this notion by having three kinds of charge, and bringing all three charges together gives you something neutral. (Weird!)

The naming of different kinds of quarks according to colors is actually very clever and is based on the way that colored light mixes. In particular, we know that equal parts of red + green + blue = white. We interpret “white” as “color neutral,” meaning having no “color charge.”

There’s a second way to get something color neutral: you can add something of one color with it’s “anti-color.” (You can formalize these in color theory, but this would take us a bit off course.) For example, the “anti-color” of red is cyan. So we could have red + “anti-red” (cyan) = color neutral.

If we don’t see them, are quarks real?

The point of all of these “color mixing” analogies is that [at low energies], QCD is a strongly coupled force. In fact, we often just call it the strong force. It’s responsible for holding together protons and neutrons. In fact, QCD is so strong that it forces all “color-charged” states to find each other and become color neutral. We’ll get into some details about this in follow up posts when we introduce the QCD force particles, the gluons. For now, you should believe (with a hint of scientific skepticism) that there is no such thing as a “free quark.” Nobody has ever picked up a quark and examined it to determine its properties. As far as you, me, the LHC, and everyone else is concerned, quarks are always tied up in bound states.

There are two kinds of bound states:

  • Bound states of 3 quarks: these are called baryons. You already know two: the proton and the neutron. The proton is a combination (uud) while the neutron is a combination (ddu). For homework, check that the electric charges add up to be +1 and 0. Because these have to be color neutral, we know that the quark colors have to sum according to red + green + blue.
  • Bound states of a quark and an anti-quark: these are called mesons. These are color-neutral because you have a color + it’s anti-color. The lightest mesons are called pions and are composed of up and down quarks. For example, the π+ meson looks something like (u anti-d).  (Check to make sure you agree that it has +1 electric charge.)

Collectively these bound states are called hadrons. In the real world (i.e. in our particle detectors) we only see hadrons because any free quarks automatically get paired up with either anti-quarks or two other quarks. (Where do these quarks come from? We’ll discuss that soon!)

This seems to lead to an almost philosophical question: if quarks are always tied up in hadrons, how do we know they really exist?

A neat historical fact: Murray Gell-Mann and Yuval Ne’eman, progenitors of the quark model, originally proposed quarks as a mathematical tool to understand the properties of hadrons; largely because we’d found lots of hadrons, but no isolated quarks. For a period in the 60s people would do calculations with quarks as abstract objects with no physical relevance.

Why we believe that quarks are real

This seems to lead to an almost philosophical question: if quarks are always tied up in hadrons, how do we know they really exist? Fortunately, we are physicists, not philosophers. Just as Rutherford first probed the structure of the atomic nucleus by smashing high energy alpha particles (which were themselves nuclei), the deep inelastic scattering experiments at the Stanford Linear Accelerator Center (joint with MIT and Caltech) in the 60s collided electrons into liquid hydrogen/deuterium targets and revealed the quark substructure of the proton.

A discussion of deep inelastic scattering could easily span several blog posts by itself. (Indeed, it could span several weeks in a graduate quantum field theory course!) I hope to get back to this in the future, since it was truly one of the important discoveries of the second half of the twentieth century. To whet your appetites, I’ll only draw the Feynman diagram for the process:

This is unlabeled, but by now you should see what’s going on. The particle on top is the electron that interacts with the proton, which is drawn as the three quark lines on the bottom left. The circle (technically called a “blob” in the literature) represents some QCD interactions between the three quarks (holding them together). The electron interacts with a quark through some kind of force particle, the wiggly line. For simplicity you can assume that it is a photon (for homework, think about what is different if it’s a W). We’ve drawn the quark that interacts as the isolated line coming out of the blob.

This quark is somewhat special because it’s the particle that the electron recoils against. This means that it gets a big kick in energy, which can knock it out of the proton. As I mentioned above, this quark is now “free” — but not for long! It has to hadronize into more complicated QCD objects, mesons or baryons. The spectrum of outgoing particles gives clues about what actually happened inside the diagram.

We’ve just glossed over the surface of this diagram: there is a lot of very deep (no pun intended) physics involved here. (These sorts of processes are also a notorious pain in the you-know-where to calculate the first time one meets them in graduate courses.)

(By the way: the typical interactions of interest at the LHC are similar to the diagram above, only with two protons interacting!)

A hint of group theory and unification

I would be negligent not to mention some of the symmetry of the matter content of the Standard Model. Let’s take a look at all of the fermions that we’ve met so far:

There are all sorts of fantastic patterns that one can glean from things that we’ve learned in these blog posts alone!

The top two rows are quarks (each with three different colors), while the bottom two rows are leptons. Each row has a different electric charge. Each column carries the same properties, except that each successive column is heavier than the previous one. We learned that the W boson mediates decays between the columns, and since heavy things decay into lighter things, most of our universe is made up of exclusively the first column.

There are other patterns we can see. For example:

  • When we first met QED, we only needed one type of particle, say the electron. We knew that electrons and anti-electrons (positrons) could interact with a photon.
  • When we met the weak force (the W boson), we needed to introduce another type or particle: the neutrino. An electron and an anti-neutrino could interact with a W boson.
  • Now we’ve met the strong force, QCD. In our next post we’ll meet the force particle, the gluon. What I’ve already told you, though, is that there are three kinds of particles that interact with QCD: red, green, and blue. In order to form something neutral, you need all three color charges to cancel.

There’s a very deep mathematical reason why we get this one-two-three kind of counting: it comes from the underlying “gauge symmetry” of the Standard Model. The mathematical field of group theory is (a rough definition) the study of how symmetries can manifest themselves. Each type of force in the Standard Model is associated with a particular “symmetry group.” Without knowing what these names mean, it should not surprise you if I told you that the symmetry group of the Standard Model is: U(1) SU(2) SU(3). There’s that one-two-three counting!

It turns out that this is also very suggestive of grand unification. The main thrust of the idea is that all three forces actually fit together in a nice way into a single force which is represented by a single “symmetry group,” say, SU(5). In such a scheme, each column in the “periodic table” above can actually be “derived” from the mathematical properties of the GUT (grand unified theory) group. So in the same way that QCD told us we needed three colors, the GUT group would tell us that matter must come in sets composed of quarks with three colors, a charged lepton, and a neutrino; all together!

By the way, while they sound similar, don’t confuse “grand unified theories” with a “theory of everything.” The former are theories of particle physics, while the latter try to unify particle physics with gravity (e.g. string theory). Grand unified theories are actually fairly mundane and I think most physicists suspect that whatever completes the Standard Model should somehow eventually unify (though there has been no direct experimental evidence yet). “Theories of everything” are far more speculative by comparison.

Where we’ll go from here?

I seem to have failed in my attempt to write shorter blog posts, but this has been a quick intro to QCD. Hopefully I can write up a few more posts describing gluons, confinement, and hadrons.

For all of you LHC fans out there: QCD is really important. (For all of you LHC scientists out there, you already know that the correct phrase is, “QCD is really annoying.”) When we say that SLAC/Brookhaven discovered the charm quark or that Fermilab discovered the top quark, nobody actually bottled up a quark and presented it to the Nobel Prize committee. Our detectors see hadrons, and the properties of particular processes like deep inelastic scattering allow us to learn somewhat indirectly about the substructure of these hadrons to learn about the existence of quarks. This, in general, is really, really, really hard—both experimentally and theoretically.

Thanks everyone,
Flip, US LHC Blogs

(By the way, if there are particle physics topics that people want to hear about, feel free to leave suggestions in the comments of the blog. I can’t promise that I’ll be able to discuss all of them, but I do appreciate feedback and suggestions. Don’t worry, I’ll get to the Higgs boson eventually… first I want to discuss the particles that we have discovered!)


A day in the life…

Monday, September 13th, 2010

It’s been a while since I’ve blogged and to my readers I apologize. I have been working on my leptoquark analysis, which is rolling right along. I’ll be sure to share that at a more appropriate time but I thought it would be fun to take note of what I do on an average work day. So why not today…

9:30-ish A.M. – arrive at Brookhaven National Lab. Usually I go to Stony Brook, but today I’m working with one of the post-docs at BNL on how to estimate QCD background.** Upon arriving, we get a quick coffee and get caught up on this weekend’s US Open tennis championships. Discussion of my analysis code ensues.

**ASIDE: QCD (Quantum ChromoDynamics) is the theory of strong interactions. Of the forces we study at the LHC, this is the least exactly understood. It relates to all quark/gluon (color charged) interactions. QCD has two peculiarities: confinement and asymptotic-freedom. In short, confinement means that as you move two quarks away from each other the force between them gets larger (like a rubber band). This explains why you never see single quarks, instead you see showers of quarks (called jets). As they move away from each other, more quarks pop out. Asymptotic-freedom means that high energy quarks and gluons interact less. At a hadron collider, the most prominent thing you get out are jets. They are a significant background that you have to remove to do an analysis. It’s also difficult to simulate just based on the sheer number of jets that come out. My advisor once told me that the least expensive way to simulate all the QCD background needed  is to build a collider and take data. **

10:30 A.M. – After getting some unexpected results in my electron analysis, the anonymous post-doc confirms that there is indeed a bug in my analysis code. This is what I spend the vast majority of my time doing. Writing and debugging code to perform an analysis on data and simulated data. He has written a modified version on my electron selection code, so I work to combine the two.

11:15 A.M. – We get into a discussion about what the electron/photon (called e/gamma) group has defined as an appropriate selection criteria for the electrons I want to include in my analysis. I want to use a standard selection criteria but the standards vary in the early days of data-taking. We decide that the best approach is to use the selection criteria of my greatest background: W bosons produced with jets.

12:15 P.M. – Lunch time. We join the other ATLAS scientists at BNL on the walk over to the cafeteria. The cafeteria contains your standard fare. I usually go for an egg-salad sandwich. The days I go to BNL, I don’t make my own lunch. The discussion flows from the fate of the Tevatron to the post-doc’s baby girl and back to the US Open (I’m a big tennis fan… go figure).

1:00 P.M.: Head back from lunch, and start the discussion of QCD estimation. Unfortunately the bug in my code and discussion of selection criteria for electrons took up the morning, but that leaves the afternoon for some strong force fun. **So how do we do this ** The discussion takes a while, but once it’s done….

**Another Aside!: QCD estimation is done a couple of different ways. It gets pretty detailed, but here is a brief overview. You look at two different regions: an area that you believe to be signal poor and QCD background rich, and the other signal rich and QCD background poor. You can then pick selection variables that are uncorrelated: like a lepton ID variable, and a Missing Energy variable (from neutrinos). You can then plot the values of the events in terms of the two variables and come with four regions: an area that is signal for both variables, an area that passes signal cuts for one variable but not the other (2 of these), and an area that fails the signal cuts for both variables. From these numbers and ratios you can predict the amount of QCD you should get. –I’ll note again, this is simplified, but the general idea is there.**

2:45 P.M.: I now get to write the code that makes these plots 🙂 And spend the rest of the afternoon doing so.

6:45 P.M.: I get home in time to see the US Open men’s championship get rain delayed, so I pop open my laptop and start coding again. I have a farewell party for a friend who graduated and is moving to Germany tonight, so I can’t work too much longer.


Tevatron Accelerator

Tevatron Accelerator

There has been a great discussion raging at Fermilab surrounding the recent report given by the Physics Advisory Committee on August 31st. In this report the committee considered the impact of extending the life of the Tevatron through 2014 in what is being called around the lab Run III.

Basically what has been outlined is trying to answer the difficult question of whether or not the immediate physics payout of extending the life of the experiments and most likely doubling the data sets out weighs the potential impact on the future experiments at Fermilab. In addition, the performance of the experiments (CDF & D0) in terms of hardware, man power, and analysis reach have to be considered when viewed in light of the draw for many scientists to move onto other interesting experiments happening at Fermilab (NuMI, Project X, etc…) and elsewhere (CMS, ATLAS, and the like…)

What faces the lab, the director of Fermilab (Pier Oddone), and the scientists that work in the world of particle physics is a really difficult one. What they have to do is look into their crystal balls and ask the questions:

1) With the LHC going into a 15 month shutdown at the end of 2011, what will the data the is already on tape look like and what kind of physics reach will it provide us?

2) With the ever improving performance of the Tevatron and the experiments at Fermilab what is the likelihood of having a discovery with a larger data set (read: Find the Higgs or exclude the Standard Model flavor in the low mass ranges)

3) What does the funding question look like for the other interests of the lab in light of the extended running of the Tevatron? Not to mention the timeline / manpower / and resource availability!

These are just some of the big issues….there are clearly 100’s more that me as a lowly graduate student am probably not even aware of! But from my own perspective I see the PAC report as a great sign! Their conclusion was simple:

The Committee strongly endorses the extension of the Tevatron run for three years during 2011–2014 under either funding scenario presented in the charge. The Committee is aware that the development of the future programs might be severely affected and projects delayed by the Collider run. The Committee recommends that efforts be made to mitigate the effects. While the Tevatron run extension would take advantage of a compelling opportunity, the long-term plans of the Laboratory and of the field, as outlined by the P5 report, should be pursued vigorously.

I was really excited to hear this! As a young researcher on the verge of graduating I saw this recommendation as an opportunity to continue my with a post-doc at Fermilab working at a time in particle physics where the chance of a real discovery (Higgs/SUSY/Beyond Standard Model) is a real possibility and to be able to contribute to the American thrust of physics in the global arena during the shutdown/upgrade of the LHC.

There is no question what the future of high energy physics will be, and that is the Large Hadron Collider at CERN for many years. There is also no denying that Fermilab is looking to the future with the intensity frontier in such experiments like NuMI and Project X. However, we are at a time where the physics possibilities are so great, the timing too perfect, and the reach of our experiments so close, that I think it would be a shame not to extend the run and take this chance to make a major discovery!

Higgs Exclusion that could be extender (or discovery made) in the low mass region still to be explored

Higgs Exclusion that could be extended (or discovery made) in the low mass region still to be explored

I encourage everyone to read the PAC Report and get excited for the potential reach of the Tevatron! Coming back out of our summer shutdown we are already colliding with inital luminosities near 250 nb-1 and delivering 5000 nb-1 per store. There are so many exciting hints and clues in the analysis in the pipeline at CDF that adding more of this quickly accumulating data will help shed light on all the great mysteries.

Recent result from D0 showing hints of new physics to still be understood in the asymmetry of matter and anti-matter

Recent result from D0 showing hints of new physics to still be understood in the asymmetry of matter and anti-matter

So, when asked: “To run or not to run?” The answer is TO RUN! At least in this humble blogger’s opinion


It’s the beginning of another academic year, which means another generation of young people entering universities across the US. As a grad student one feels a sense of nostalgia when realizing that some fraction of these students will be following in your footsteps to become your future colleagues at the scientific frontier. Of course, the nostalgia fades away when you realize that those students are hidden among the other hundred or so that are just taking the class you’re teaching only because it’s a requirement for their major and just want to figure out how to pass the final exam. 🙂

With slightly more seriousness, a warm “welcome to college” to all of the freshmen out there, and a special ‘hello’ to all of the future scientists among you. I have a strong belief that one of the roles of the graduate student community is to provide mentors for undergraduates, especially those who are interested in pursuing academic careers. To that extent, there are a few things that I always thought I would have appreciated knowing when I was a freshman and that I’d like to share with the blogosphere.

(Random factoid: the first mention of my name on the blogosphere came from a Cosmic Variance post about applying to grad schools. I really appreciated that post and write this in the same spirit.)

These tend to be physics/science-oriented, but I hope at least some of it is broadly-applicable:

1. Figure out what you want. There’s no single definition for success in college; this all depends on what you want to get out of your undergraduate years. Depending on whether you pursue an academic career, go into industry, professional school, politics, start the next great rock band—whatever it is you end up following, you will be judged on different criteria. College is a time of self discovery—and that can take time—but it helps if you discover yourself sooner rather than later since that gives you more time to prepare for the next step. Once you find what it is you want to do, dive into it with enthusiasm.

2. Find mentors. No matter what you end up doing (but especially if you want to pursue scientific research), find people who can provide guidance and inspiration. For scientists this usually means faculty and grad students. They’ve been where you are and they’re bursting with advice about how they navigated their path through academia. Make a point to talk to them! Go to office hours and ask about their research. Invite them to lunch. Be pro-active about this.

3. Do research. Undergraduate research experience is practically an unstated prerequisite for a strong grad school application. Research is where your coursework comes to life and you find out what it’s like to work on open-ended questions. It’s also a chance to try out different fields: not sure if you’d enjoy particle physics? Spend a summer (or better: a year) as an undergrad research assistant!

Actually doing research will help you figure out what you really want to pursue (theory or experiment? condensed matter or high energy?). Even if you end up deciding that you never want to set foot into a condensed matter lab ever again in your life, at the very least you’ve learned something new and valuable about yourself; maybe you’ll find you are more drawn to the nuances of developing theoretical models rather than the ingenuity required to construct experiments. That’s great! And if you get a few publications and a nice letter of recommendation from a respected professor out of it as well—then all the better.

Keep an eye out for undergraduate research opportunities in your department. Talking to faculty is really important here. If there aren’t many options at your university, look for summer research opportunities at other universities or national labs.

4. Be lopsided. Forget the idea of a “well rounded” university student, be “well lopsided!” Find the things that you are really passionate about and devote yourself to them. You don’t have to join every single club, be on every single committee, and juggle three majors. Your passions don’t have to be all academic things, either: even if all you do is party and research (with a reasonable division between the two), then you’ll still be a better researcher than someone who is split ten ways between several extracurricular (even academic) activities.

As a caveat: while you’re being lopsided, try to keep your balance. You might want to do nothing more than eat, breathe, and physics. This isn’t enough. Make time to socialize, exercise, and otherwise challenge yourself in ways that you wouldn’t be able to outside of college.

5. Do your problem sets. I cannot over-emphasize how important it is to practice your science. This is especially true in mathematics and physics. Problem sets are more than just ways to make sure you do your reading; they force your brain to apply what you’ve learned to new problems. This is—at a very fundamental level—what scientific research is all about. I will go so far as to say that you only learn something meaningfully when you’ve used it to generate new (at least to you) ideas, such as when you solve a hard homework problem. (In grad school this becomes: “… when you’ve used it to write an academic paper.”)

While we’re on the topic: do your problem sets with friends. You should always be able to write up a solution on your own, but it is good to discuss with others to learn how to generate solutions. Again: this is how real science is done, though collaboration and communication. Anyway, it’s always good to find friends with similar goals: it forms a kind of “support group” to encourage one another. ((It’s also important to make friends with people who are doing completely different things from you!))

6. Learn to communicate. This holds universally no matter what you want to do, but somehow this ends up being understated for scientists. A big part of science is communicating your work to a broad range of people. Whether it’s a colleague whom you are working with on a particular problem, a funding agency that needs to be convinced that your line of research is a good investment, or the general public (whose tax dollars fund research, from whom future scientists emerge, and who really do want to learn about the frontiers of human knowledge), you need to be able to explain your work. Be comfortable discussing, presenting, and writing about ideas in your field.

7. Learn to think. This is a little more abstract, but I think it’s important in a very general way. This generation of college students grew up with Wikipedia at their fingertips. Information is cheap and readily available. You don’t need to spend tens of thousands of dollars in tuition to learn facts. The value of being at a university is to learn how to generate and use those facts. This is the “transformative” nature of education; you need to be able to parse information and generate meaning.The professors giving your lectures aren’t trying to make you memorize facts from their textbook; they want you to interact with those facts: question them, generalize them to principles, apply them elsewhere, cross-reference against accepted dogma, etc.

8. Develop tools. The other thing you should get out of your classes are a set of tools that will be valuable in the future. If you’re going to be a physicist, then you will certainly need to be well versed in quantum mechanics, for example. One often under-appreciated skill: programming. Also, for those who will be working in physics and mathematics, learn how to use the LaTeX typesetting system. (For particle theorists in particular: differential geometry, complex analysis, and group theory!)

9. Go to academic talks and read academic papers. You don’t magically learn how to read papers and listen to talks when you become a grad student. These are skills that you have to develop. Challenge yourself—even if you only understand the first five minutes of a talk, you’ll at least begin to familiarize yourself with words and ideas. Start with what’s accessible: departments usually have colloquia which are meant to be accessible to a broad audience within the department, and look for “review articles” which are meant to be pedagogical introductions to current research. If you’re just starting out, read the American Journal of Physics (which has lots of undergraduate-level discussions) or Physics Today. (Everyone who reads this blog should follow Symmetry Magazine.) As you learn more, start attending seminars in the fields that you’re interested in and start to peruse current research on the arXiv.

10. Have fun. This is an amazing time in your life where you have professors who will teach all sorts of things to you, a vibrant community of young people around you, and no responsibilities other than to make the most of your time. Do it!


Blackboard, illuminated by prof. Hirosi Ooguri

Blackboard, illuminated by prof. Hirosi Ooguri

昨日は大栗さんに来ていただき、4+1次元Maxwell-CSに電場を入れたときの不安定性とホログラフィーについて、セミナーをしていただいた。大栗さんの、相変わらずの非常に分かりやすいお話。不安定性のend pointが見つかるということにも驚き。少し勉強しよう。講演のあとでは自分の話も少し聞いていただき、議論させてもらう。









Theoretical Physics and Frustration

Wednesday, September 8th, 2010

Theoretical physics is known to be an extremely frustrating endeavor. Already as an undergrad, I was warned against that. But why is this the case? There are several reasons, I believe.

  • Too high expectations.
    Many students who decide to do theoretical physics have in mind people like Albert Einstein or similarly imposing role models. And let’s face it, even among the most successful in our trade, having an impact like this is rare. So even those who eventually succeed in getting a tenured position are likely to have fallen short of their original hopes.
    I guess I am at peace with the fact that I’m not the new Einstein, but I can’t shake the feeling that I should be doing better than I am.
  • It’s hard.
    What we do is hard. Before we can even start to do our own bit of research, we have to assimilate an enormous amount of knowledge, consisting of very difficult physical and mathematical concepts. For each new project, we have no acquire new knowledge first. Not knowing is my daily bread. And since as a researcher, I have to constantly push the boundary of what’s known and what I personally am able to do, there is little I can do about it. It’s in the nature of the job. And yet, feeling ignorant all the time really gets to me at times. I never know enough. I seem to constantly be aiming one step ahead of myself.
  • Very little gratification.
    More the opposite. It keeps happening that you follow a lead that ultimately does not bring you further. Sometimes it’s a few days wasted, sometimes a few weeks, and once in a while even a few months.
    The moments when you have a new idea or reach a new result are few. Often, the process is so gradual that you hardly have a feeling of satisfaction at all. You spend months working on a project, but in the end the resulting paper goes largely unremarked by the rest of the world.

The result of the high levels of frustration is that many people give up at some point along the road. It’s true that the job market is not good and that there are far less positions than applicants. But I’ve seen many people leave even though they did not have to.
Others eventually get tenure, but become bitter in the process. Many (even, from my point of view, successful) colleagues seem to have an inexhaustible reservoir of complaints about all the times their work did not get the attention they thought it deserved, when they did not get invited to a conference they feel they would have been the perfect speaker for, not to mention all the jobs they should have gotten instead of someone else who was obviously less qualified.

Like everyone, I suffer from time to time from the accumulated frustration. But I try to hold on as well as I can because I believe that one of the ingredients of succeeding as a theoretical physicist is to be able to keep going in the face of these adversities.
But I try to avoid like hell becoming bitter. Of course thoughts like “How come X gets invited to speak at this conference and not I?” sometimes cross my mind. But I try to fight them. We all have our frustrations to battle with, but I don’t want to make my life miserable by harboring all these extra resentments.


Physics by Poets

Monday, September 6th, 2010

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!]


[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!]