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Archive for October, 2010

Recently, I got my hands on a book entitled What’s Mine is Yours – the Rise of Collaborative Consumption by Rachel Botsman and Roo Rogers. I was intrigued by the concept.

After first taking a stab at the reigning consumer culture, the book discusses product service systems (say car-sharing or Netflix), redistribution networks (say e-bay or freecycle) and collaborative lifestyles.
Its main statement can be summarized as follows: the internet, thanks to its huge scale, can serve to match just about any offer to someone needing the offered item or service. Exchanging and sharing goes from being cumbersome to being convenient for everyone. So less resources need to be wasted.
If you have an odd item you’d like to get rid of, you don’t have to throw it away anymore. You can now easily find someone who wanted just that. One person’s thrash is another person’s treasure.

This book is not one of those pessimist “we are destroying the planet” kind of books, it actually has a quite uplifting tone.
I think to a scientist, the ideas described in the book are rather appealing. It talks a lot about collaborating and sharing, which is our daily bread. As a theorist, my work is based on collaboration and shared information. And it is facilitated immensely by the internet. Not only do I make extensive use of (free!) preprint archives to browse the existing literature (if I need them, my employer luckily also has the subscriptions to the electronic versions of the paid journals), also Wikipedia is often a great help. If I had to go to a library and leaf through hardcopies of old journals, hunting down information would take so much more time. I guess some of my projects would be delayed by weeks if not months! So I am very sympathetic to the idea of sharing information with everyone on the internet.

The book discusses many successful examples of collaborative consumption that many of us already use and love, such as e-bay, craigslist and freecycle (I managed to effortlessly sell all of my furniture via an internet market place prior to moving to Japan), but also less known services like car- and bicycle sharing, ride-sharing, even garden sharing and couch-surfing networks, time banks and bartering sites.
Young scientists who have to stay light since they have to move country every few years, are likely not to be adverse to the idea of paying to use something instead of paying to own it, let alone to buying and selling used items online prior to and after a move. Most of us have grown up in an interconnected world and are much more willing than our parents’ generation to buy and sell used goods online or take advantage of the zillions of other possibilities to get what we want via the internet. Maybe we’re also a little less materialistic.

What’s Mine is Yours was for me an interesting and fun read. I learned about the background of many internet services I already knew, but also about services I had not yet heard about which seem useful and worth giving a try. I guess it wasn’t hard to convince me since the book is largely in synch with the mindset I already had.
If taking advantage of the amazing possibilities provided by the internet is a way to save the planet, I’m all for it!


Tasting quark soup

Wednesday, October 27th, 2010

A lot of the articles in the news talk about the physics we’re trying to learn, but there’s not much discussion of how we do it.  I spend most of my day thinking about our detectors, how they work, and how to interpret the data.  Our detectors are the way we see, hear, feel, smell, and taste what goes on in proton-proton and lead-lead collisions.  Just like the Mars rover explores places that are uninhabitable to people, our detectors explore places that are too small for us.

ALICE is made up of several subsystems, each of which helps us sense collisions in a different way.  Below is a picture of ALICE with each subsystem labeled:

Some detectors are like our eyes – they help us see particles coming out of the collision.  These are our tracking detectors.  The main tracking detector is the Time Projection Chamber (TPC).  We also have an Inner Tracking System (ITS), comprising three different silicon detectors, the Silicon Strip Detector, the Silicon Drift Detector, and the Silicon Pixel Detector.  The ITS is a bit like our glasses – we can see particles with just the TPC, but with the ITS, the picture comes into sharper focus.  Tracking detectors tell us the momentum and spatial location of particles that go through our detector.

Some detectors are our taste buds – they help us determine the flavor of the particles we’re measuring.  A lot of different types of particles are produced in both a proton-proton collision and a lead-lead collision.  The Time-Of-Flight (TOF), the Transition Radiation Detector (TRD), and the High Momentum Particle Identification Detector (HMPID) are all designed to identify particles.  These detectors all work by measuring a particle’s velocity.  Momentum is velocity times mass, so if we know the velocity and the momentum of a particle (which we can get from the tracking detectors), we can determine its mass and therefore figure out what kind of particle it is.  The TOF measures the velocity of particles by measuring how long it takes for a particle to reach the TOF.  Since velocity is the change in distance over time and the distance traveled is known, this measures the velocity of the particle.  Here you can see one of our physics performance plots showing different particles identified by the TOF:

The x-axis is the momentum, as measured by the TPC and the ITS, and the y-axis is the ratio of the velocity to the speed of light in a vacuum.  Pions (π) are the lightest particle (140 MeV/c2) so at a given momentum, they have the highest velocity.  Protons (p) are the heaviest (938 MeV/c2) particle visible in the plot above so at a given momentum they have the lowest velocity.

The HMPID and the TRD both work on the same principle.  The speed of light in a vacuum is constant, but the speed of light in a medium can be lower.  For example, the speed of light is lower in water than in air – this is why images get distorted when you look through water.  If a fast particle moves through a medium faster than the speed of light in that medium, it will emit photons – called Cherenkov radiation – until it slows down to the speed of light in the medium.  At a given momentum, lighter particles go faster, so lighter particles will emit photons at a larger angle relative to their path.  The medium in the TRD is optimized so that only electrons (0.5 MeV/c2) will radiate photons, so the TRD can be used to distinguish electrons from everything else.  The HMPID is a ring imaging Cherenkov detector.  The photons emitted by a particle are emitted in a cone and the radius of that cone depends on the velocity of the particle.  The HMPID is optimized for distinguishing pions, kaons, and protons.  Here you can see the signal from the HMPID:

The x-axis is the momentum and the y-axis is the angle of the cone of light emitted by the particle.  At a given momentum, a pion is going faster than a kaon or a proton.  The radius of the cone of light emitted by the particle is larger the further the particle’s speed is from the speed of light in the medium, so at a given momentum the pion band is above the kaon band, which is above the proton band.

The tracking detectors, the TPC and the ITS, can also identify particles.  They work by measuring how much energy a particle loses as it travels through the detector.  A heavier particle will loss more energy than a light particle.  Think of one of those ball pits for kids.  If you threw a tennis ball in, it would knock some of the balls out of the way.  If you threw in a bowling ball, a lot of balls would get knocked around.  We know the bowling ball lost more energy than the tennis ball because the lighter balls got knocked around more.  We can distinguish between heavier particles and lighter particles like this.  If the TOF, the HMPID, and the TRD are the way we taste the particles created in the collision, the ITS and the TPC help us smell them.  Below you can see the signal from the TPC:

The x-axis is rigidity, which is the momentum over the charge.  Charge is in units of the electron charge.  All of the particles here have a charge of ±1.  Positively charged particles are on the right and negatively charged particles are on the left.  The y-axis is proportional to the energy lost by the particle in the TPC.  We see the same three particles we saw before – pions, kaons, and protons – but now we also see deuterons and tritons.  At a given momentum, heavier particles lose more energy, so as you go up the y-axis the mass of the particles increases.

My last post was on the Electromagnetic Calorimeter (EMCal).  A calorimeter is used to measure particles’ energy.  This is a way of feeling the collision – it’s like laying in the sun.  When you lay in the sun, you don’t feel photons hitting you but when photons hit you, they warm you up.  Particles hitting the calorimeter do the same thing – they hit the calorimeter and deposit their energy.  (Everything loses energy except muons – muons travel right through the calorimeters.)   We look at the energy deposited in the calorimeter to determine how much energy the particle had.  (See my post on the electromagnetic calorimeter for more details.)  We have two more calorimeters in ALICE,  the Photon Spectrometer (PHOS) and the Zero Degree Calorimeter (ZDC).  The PHOS is optimized to measure photons.  The ZDC is a calorimeter very close to the beam pipe far away from the interaction point, at an angle close to zero degrees from the beam pipe.  The ZDC is useful in lead-lead collisions for both measuring nucleons which did not participate in the collision.  These particles are called “spectators” and are not deflected by the magnetics that keep the beam in the beam pipe because the spectators do not have the same charge to mass ratio as lead nuclei.  We can figure out of the collision was head-on or just glancing using this information.

We hear the collision in the VZERO, a scintillator detector.  When a particle hits it, the scintillator emits photons and we know there was a collision when we see these photons.  Think of it as like a fire alarm – it’s what tells us there was a collision.

There’s a few detectors that don’t really fit into this metaphor but I want to mention them anyways.  The Photon Multiplicity Detector (PMD) measures the multiplicity of photons at angles close to the beam pipe.  The muon arm measures muons, the heavy cousin of the electron.  The ALICE Cosmic Ray Detector (ACORDE) is designed to trigger on cosmic rays so that the rest of ALICE can be used to study cosmic rays.  Cosmic rays were used to calibrate ALICE before the first collisions at the LHC.

Each of these detectors helps us understand proton-proton and lead-lead collisions in a different way.  When we put them all together, we have a sort of Quantum Chromodynamics rover that helps us explore exotic places – the insides of protons and nuclei – that are near us all the time.


PMT Testing

Wednesday, October 27th, 2010
Large PMTs

Me in our lab with the 10" and 20" PMTs.

Last week a group of LBNE collaborators, myself included, met by phone to discuss issues related to the photomultiplier tubes (PMT) we would use in the large water Cherenkov detector option.  Without getting into details, a PMT can be thought of as a single pixel in a camera, and we look at the brightness of light that each PMT sees to discern a signal.  We are lucky to have collaborators who are also on the Super-K experiment in Japan which is very similar to what we are talking about building.  One important difference is that Super-K uses much bigger PMTs.  The Super-K PMTs have a diameter of 20 inches (50cm) and one candidate we are considering for LBNE are half that size, 10 inches in diameter.  In general, the larger PMTs will see more light than a smaller one, but in our case, the smaller PMTs are actually more efficient, and so it is not so clear cut.

An important thing to know is how much better or worse the 10inch PMTs are compared to the 20 inch ones.  We can try to answer that question by compiling all of the data we have on each PMT and comparing, but there are a lot of uncertainties in that comparison.  During the discussion we decided that we should try to compare these PMTs in a real lab experiment with the same light source.  Coincidentally, there was only one collaborating institution that had one of each tube … us! So, I got volunteered to do this test, and we need the answer ASAP.

We had to make a number of modifications to our testing lab to accommodate the larger PMT, but we were able to make it work and are in the process of taking data.


Night Shift

Wednesday, October 27th, 2010


It’s late at night, and here I sit in the control room for the H6B beam line at CERN’s SPS North Area facility. The light is harsh fluorescent, the air is redolent of espresso, and the room is thrumming to the sound of heavy machinery. I am on a night shift.

(An aside: If you thought particle physics stopped for the night — and particle physicists for sleep, ha! — please recall that we have accelerators that create light, and calorimeters that eat light for breakfast. Besides, as a world-spanning collaboration, the sun never sets on our empire.)

There’s a certain charm to working the night shift, or at least a comforting familiarity. One generally follows a similar routine:

[00:00] Arrive a few minutes early and catch up on events from the previous shift(s). Scan through the e-log, then start an entry of your own. Don’t forget timestamps!

[00:15] Follow some sort of start-of-shift checklist. Ensure proper data-taking in spite of your presence. Find the most comfortable chair around and settle in for a long haul.

[01:00] Time for coffee.

[02:00] Continue checking up on the detector at regular intervals, updating the e-log as necessary. Engage yourself with work, or entertain yourself with Internet. Now is a good time to tick items off your to-do-but-not-urgent list.

[03:10] Snap out of a daze that lasted for five minutes.

[03:15] Time for coffee. And lunch.

[04:45] Check email repeatedly in case some industrious person over in the States is still working. Be productive regardless: Your advisor is visiting CERN, and he may appear without warning at any given moment.

[06:00] Enter the night shift doldrums, and despair.

[07:30] Note when the beam shuts off for a planned intervention. Take this opportunity to run a couple of scans on your sensors, if only to break the tedium.

[08:00] Look back on the past eight hours and take pride in the amount of data you’ve helped collect! Finish writing up the shift e-log, make sure the control room is in order, then head home for sleep. Mind that extraneous step in front of your apartment building. Definitely do not trip on it.

Fortunately, this was a smooth and trouble-free night shift during which boredom was the biggest problem faced, but you can imagine how some shifts are made truly terrible: no (stable) beam, sensors failing for mysterious reasons, repeated software crashes, too many emails from industrious people over in the States, the dilemma of whether or not to call an expert at 5am for something that may incite their ire… Clearly, I lucked out tonight.

Oh, one last thing! Be sure to check your work from last night before submitting it for other people’s approval. (Really, “eat light for breakfast”??? It’s staying in, but only as an instructive example.)

— Burton


Keeping it friendly

Monday, October 25th, 2010

I join you fresh from a weekend of two majestic parties at two ends of the spectrum of graduate student experience. Although that ship sailed for me more years ago than I care to admit, I do still have the odd young friend who has yet to weigh anchor and head for the island of postdoc-dom. And some who have just undertaken that voyage.
The first of whom we partied for this weekend was a man who recently graduated. He made the bold move, about one and half years ago to jump ship from ATLAS and go “back” to work on D0 at the TeVatron, whereupon he finished an analysis on real data in time to return back to ATLAS to triumphantly accept his new job. We celebrated his graduation and his upcoming post. While the party was ongoing we received news of another close friend successfully defending his thesis and, having already secured a postdoc job, graduating from student life with aplomb. He had also moved to the TeVatron from ATLAS to experience some real data and get a good thesis out. For these two chaps that move in “the wrong direction” has paid dividends.

Our Saturday night was spent celebrating the company one last time of another dear friend who is moving back from CERN to his institution in the US in order to finish up his thesis and proceed on with his own move to bigger and better. Less a going away party than a moment to take stock of the transient nature of the field and how friends and colleagues move on before our eyes.

Friendships at CERN are strange things. Thrown together, as we are, into the mix of work, the lines between social life and working life can become quite blurred. Often there are friends of convenience instead of true comradeship. But every now and then we make, through our work connections friends for life, not just for collisions.
One friend leaving and we wish him well, hoping for his safe return quite soon.

One other thing to report is the pending release of the ATLAS cd. It’s been about a year since it was supposed to be coming out and now we hear that they are doing the final mastering. I’ll report back with more news and some links when it’s out.

An early snippet of the band I’m involved with can be seen here. Enjoy!


CALICE goes Digital

Sunday, October 24th, 2010

This weekend, CALICE has opened a new chapter of calorimeter testing: Our first full physics prototype of a digital hadron calorimeter saw its first pions at Fermilab! So, what is this about?

One of the first 32 GeV pions seen in the CALICE Digital Hadron Calorimeter at the Fermilab Meson Test Beam Facility: Amazingly detailed pictures of the shower structure!

One of the first 32 GeV pions seen in the CALICE Digital Hadron Calorimeter at the Fermilab Meson Test Beam Facility: Amazingly detailed pictures of the shower structure!

As you know from many previous posts here, the CALICE collaboration develops calorimeter technologies for future experiments based on particle flow event reconstruction. On the hadron calorimeter side, so far we’ve been looking at a highly granular analog hadron calorimeter: Many small plastic scintillator tiles, each read out with a tiny silicon-based photon sensor, and all that sandwiched between layers of steel absorbers, 38 of them. The “analog” in the name means that each of these cells gives out a signal that is (approximately) proportional to the energy deposited in it by throughgoing particles. By summing up the energy of all cells that saw something, we get the energy of the particle that hit the calorimeter. Of course there are very sophisticated techniques how the “summing up” is done precisely, which results in better measurements. In total, this device has close to 8000 electronics channels, all packed into one cubic meter.

Now, a digital calorimeter does things a bit differently: Instead of an energy measurement for each cell, you only get a “0” or a “1”, depending on if the cell was hit by at least one particle or not. The total energy you now get by just counting the number of “1” in your detector, again with the possibility for more sophisticated techniques. At first glance this might look like a bad deal, since you loose information. However, with the digital readout you can use simpler detectors and electronics, and thus get many more readout channels. The current 1 cubic meter prototype, that uses exactly the same steel as our analog calorimeter, has 350 000 readout channels, quite a bit more than the analog detector. And it uses so-called Resistive Plate Chambers (RPCs), simple gas-based detectors.  So you get a more precise spatial image of the particle showers, for the price of sacrificing the cell-by-cell energy measurement.

What will work better? That is exactly what we are trying to find out. The first CALICE Digital HCAL, constructed in the US under the leadership of Argonne National Lab, has now started to take data at Fermilabs Meson Test Beam Facility. With those data, compared to the ones we already have for the analog calorimeter, we can hopefully make a significant step in the direction of answering that question. The first event images show amazing detail, quite a bit beyond what we had previously (for a shower picture from the analog HCAL take a look here).

I, for one, am very excited to see what the results from this new detector will look like!


Linear Colliders in Geneva

Friday, October 22nd, 2010
Early morning start on the parking lot outside of CERN: Sunrise, and a fantastic alpine panorama.

Early morning start on the parking lot outside of CERN: Sunrise, and a fantastic alpine panorama.

Finally some down-time – in seat 3F on the late evening flight from Geneva to Munich. All week has been exceptionally busy: I attended the International Workshop on Linear Colliders (IWLC) in Geneva. This workshop, for the first time, truly combined the CLIC in ILC communities, both on the accelerator side and on the physics and detector side in one single meeting. Not surprisingly, it was a very big meeting, beyond what could be hosted at CERN. That is why we spent most of the week in a conference center of Geneva. A very nice facility, but with one down-side: To keep my travel budget from a complete disaster this year, I was staying in the CERN hostel, which meant leaving for the conference center with a bus at 7:45 each morning… That translates to getting up at 6:30, not exactly my favorite time. Especially since conferences always mean very late nights, due to drinks with colleagues and work on talks.

The CERN DG, consulting a "crystal ball" during the conference dinner.

The CERN DG, consulting a "crystal ball" during the conference dinner.

This time, things were even more tough than usual: I gave the summary talk of the Top/QCD, Electroweak and Alternative Physics session. Such a task means that the last night of the conference is completely spent preparing the talk for the next (final) morning… And often, that is the evening of the conference dinner. Not so this time, the (admittedly fantastic) dinner was on Wednesday, much more summary-speaker friendly.

Stress, lack of sleep and all, the meeting was absolutely worth it. A lot of interesting talks, good discussions, and a highlight: Rolf Heuer, General Director of CERN, looking into his Crystal Ball. Apparently he saw 2025 as the physics start date for a Linear Collider – but he did not want to assign a confidence level to this, I guess the crystal ball still needs some calibration…


QCD and Confinement

Friday, October 22nd, 2010

Now that we’ve met quarks and gluons, what I should do is describe how they interact with the other sectors of the Standard Model: how do they talk to the leptons and gauge bosons (photon, W, Z) that we met in the rest of this series on Feynman diagrams. I’ll have to put this off a little bit longer, since there’s still quite a lot to be said about the “fundamental problem” of QCD:

The high energy degrees of freedom (quarks and gluons) are not what we see at low energies (hadrons).

Colliders like the LHC smash protons together at high energies so that the point-like interactions are between quarks and gluons. By the time these quarks and gluons scatter into the LHC detectors, however, they have now “dressed” themselves into hadronic bound states. This is the phenomenon of confinement.

As a very rough starting point, we can think about how protons and electrons are bound into the hydrogen atom. Here the electric potential attracts the proton and electron to one another. We can draw the electric field lines something like this:

These are just like the patterns of iron filings near a bar magnet. The field lines are, of course, just a macroscopic effect set up by lots and lots of photons, but we’re in a regime where we’re justified in taking a “semi-classical” approximation. In fact, we could have drawn the same field lines for gravity. They are all a manifestation of the radially symmetric 1/r potential. We can try to extend this analogy to QCD. Instead of a proton and electron attracted by the electric force, let’s draw an up quark and a down quark attracted by the color (chromodynamic) force.

This looks exactly the same as the electric picture above, but instead of photons setting up a classical field, we imagine a macroscopic configuration of gluons. But wait a second! There’s no such thing as a macroscopic configuration of gluons! We never talk about long range classical chromodynamic forces.

Something is wrong with this picture. We could guess that maybe the chromodynamic force law takes a different form than the usual V(r) ~ 1/r potential for electricity and gravity. This is indeed a step in the right direction. In fact, the chomodynamic potential is linear: V(r)~ r. But what does this all mean?

By the way, the form of the potential is often referred to as the phase of the theory. The “usual” 1/r potential that we’re used to in classical physics is known as the Coulomb phase. Here we’ll explain what it means that QCD is in the confining phase. Just for fun, let me mention another type of phase called the Higgs phase, which describes the weak force and is related to the generation of fermion masses.

Okay, so I’ve just alluded to a bunch of physics jargon. We can do better. The main question we want to answer is: how is QCD different from the electric force? Well, thing about electricity is that I can pull an electron off of its proton. Similarly, a satellite orbiting Earth can turn on its thrusters and escape out of the solar system. This is the key difference between electricity (and gravity) and QCD. As we pull the electron far away from the proton, then the field lines near the proton “forget” about the electron altogether. (Eventually, the field lines all reach the electron, but they’re weak.)

QCD is different. The as we pull apart the quarks, the force is that pulls them back together becomes stronger energy stored in the gluon field gets larger. The potential difference gets larger and it takes more energy to keep those quarks separated, something like a spring. So we can imagine pulling the quarks apart further and further. You should imagine the look of anguish on my face as I’m putting all of my strength into trying to pull these two quarks apart—every centimeter I pull they want to spring back towards one another with even more force

… stores more and more energy in the gluon field. (This is the opposite of QED, where the energy decreases as I pull the electron from the proton! Errata: 10/23, this statement is incorrect! See the comments below. Thanks to readers Josh, Leon, Tim, and Heisenberg for pointing this out!) Think of those springy “expander” chest exercise machines. Sometimes we call this long, narrow set of field lines a flux tube. If we continued this way and kept pulling, then classical physics would tell us that we can get generate arbitrarily large energy! Something has to give. Classically cannot pull two quarks apart.

Errata (10/22): Many thanks to Andreas Kronfeld for pointing out an embarrassing error: as I pull the quarks apart the force doesn’t increase—since the potential is linear V(r) ~ r, the force is constant, F(r) ~ -V'(r) ~ constant. Physicists often make this mistake when speaking to the public because in the back of their minds they’re thinking of a quantum mechanical property of QCD called asymptotic freedom in which the coupling “constant” of QCD actually increases as one goes to large distances (so it’s not much of a constant). As Andreas notes, this phenomenon isn’t the relevant physics in the confining phase so we’ll leave it for another time, since a proper explanation would require another post entirely. I’ve corrected my incorrect sentences above. Thanks, Andreas!

What actually happens is that quantum mechanics steps in. At some point, as I’m pulling these quarks apart, the energy in the gluon field becomes larger than the mass energy of a quark anti-quark pair. Thus it is energetically favorable for the gluons to produce a quark–anti-quark pair:

From the sketch above, this pair production reduces the energy in the gluon field. In other words, we turned one long flux tube into two shorter flux tubes. Yet another way to say this is to think of the virtual (quantum mechanical) quark/anti-quark pairs popping in and out of the vacuum, spontaneously appearing and then annihilating. When the energy in the gluon field gets very large, though, the gluons are able to pull apart the quark/anti-quark pair before they can annihilate, thus making the virtual quarks physical.

This is remarkably different behavior from QED, where we could just pull off an electron and send it far away. In QCD, you can start with a meson (quark–anti-quark pair) and try to pull apart its constituents. Instead of being able to do this, however, you inadvertently break the meson not into two quarks, but into two mesons. Because of this, at low energies one cannot observe individual quarks, they immediately confine (or hadronize) into hadronic bound states.

Some context

This idea of confinement is what made the quark model so hard to swallow when it was first proposed: what is the use of such a model if one of the predictions is that we can’t observe the constituents? Indeed, for a long time people thought of the quark model as just a mathematical trick to determine relations between hadrons—but that “quarks” themselves were not physical.

On the other hand, imagine how bizarre this confinement phenomenon must have seemed without the quark model. As you try to pull apart a meson, instead of observing “smaller” objects, you end up pulling out two versions of the same type of object! How could it have been that inside one meson is two mesons? This would be like a Russian matryoshka doll where the smaller dolls are the same size as the larger ones—how can they fit? (Part of the failure here is classical intuition.) This sort of confusion led to the Smatrix or “bootstrap” program in the 60s where people thought to replace quantum field theory with something where the distinction “composite” versus “elementary” particles was dropped. The rise of QCD showed that this was the wrong direction for the problem and that the “conservative” approach of keeping quantum theory was able to give a very accurate description of the underlying physics.

In some sense the S-matrix program is a famous “red herring” in the history of particle physics. However, it is a curious historical note—and more and more so a curious scientific note—that this ‘red herring’ ended up planting some of the seeds for the development of string theory, which was originally developed to try to explain hadrons! The “flux tubes” above were associated with the “strings” in this proto-string theory. With the advent of QCD, people realized that string theory doesn’t describe the strong force, but seemed to have some of the ingredients for one of the “holy grails” of theoretical physics, a theory of quantum gravity.

These days string theory as a “theory of everything” is still up in the air, as it turns out that there are some deep and difficult-to-answer questions about string theory’s predictions. On the other hand, the theory has made some very remarkable progress in directions other than the “fundamental theory of everything.” In particular, one idea called the AdS/CFT correspondence has had profound impacts on the structure of quantum field theories independent of whether or not string theory is the “final theory.” (We won’t describe what the AdS/CFT correspondence is in this post, but part of it has to do with the distinction between elementary and composite states.) One of the things we hope to extract from the AdS/CFT idea is a way to describe theories which are strongly coupled, which is a fancy phrase for confining. In this way, some branches of stringy research is finding its way back to its hadronic origins.

Even more remarkable, there has been a return to ideas similar to the S-matrix program in recent research directions involving the calculation of scattering amplitudes. While the original aim of this research was to solve problems within quantum field theory—namely calculations in QCD—some people have started to think about it again as a framework beyond quantum field theory.

High scale, low scale, and something in-between

This is an issue of energy scales. At high energies, we are probing short distance physics so that the actual “hard collisions” at the LHC aren’t between protons, but quarks and gluons. On the other hand, at low energies these “fundamental” particles always confine into “composite” particles like mesons and these are the stable states. Indeed, we can smash quarks and gluons together  at high energies, but the QCD stuff that reaches the outer parts of the experimental detectors are things like mesons.

In fact, there’s an intermediate energy scale that is even more important. What is happening between the picture of the “high energy” quark and the “low energy meson?” The quark barrels through the inner parts of the detector, it can radiate energy by emitting gluons.

… These gluons can produce quark/anti-quark pairs
… which themselves can produce gluons
… etc., etc.

At each step, the energy of the quarks and gluons decrease, but the number of particles increases. Eventually the energy is such that the “free quarks” cannot prevent the inevitable and they must hadronize. Because there are so many, however, there are a lot of mesons barreling through the detector. The detector is essentially a block of dense material which can measure the energy deposited into it, and what it ‘sees’ is a “shower” of energy in a particular direction. This is what we call a jet, and it is the signature of a high energy quark or gluon that shot off in a particular direction and eventually hadronizes. Here’s a picture that I borrowed from a CDF talk:

Read the picture from the bottom up:

  1. First two protons collide… by which we really mean the quarks and gluons inside the proton interact.
  2. High energy quarks and gluons spit off other quark/gluons and increase in number
  3. Doing this reduces their energy so that eventually the quarks and gluons must confine (hadronize) into mesons
  4. … which eventually deposit most of their energy into the detector (calorimeter)

Jets are important signatures at high energy colliders and are a primary handle for understanding the high energy interactions that we seek to better understand at the LHC. In order to measure the energy and momentum of the initial high energy quark, for example, one must be able to measure all of the energy and momentum from the spray of particles in the jet, while taking into account the small cracks between detecting materials as well as any sneaky mesons which may have escaped the detector. (This is the hadronic analog of the electromagnetic calorimeter that Christine recently described.)

Now you can at least heuristically see why this information can be so hard to extract. First the actual particles that are interacting at high energies are different from the particles that exist at low energies. Secondly, even individual high-energy quarks and gluons lead to a big messy experimental signature that require careful analysis to extract even “basic” information about the original particle.



One of the things that brings me much joy during the often difficult and trying times of finishing a PhD in particle physics is the overwhelming exposure I have to some really great intellectual minds and discussions. Being stationed at Fermilab and taking part in an experiment like CDF (which has a long history and much expertise) makes this even more possible!

Image from talk by Prof Sundrum

Image from talk by Prof Sundrum

In fact just a few weeks ago I got to see a lecture from one of my physics icons Prof Raman Sundrun on Warped Extra Dimensions. (See his talk here). This was really a great talk and the images used were simple but conveyed a really understanding of an incredibly difficult subject (only on this blog would working out the mathematics of 11 dimensions be considered “standard operating procedure”)

Along these same lines I was pleased when I stumbled across the website http://fora.tv. This website complies talks by many different experts and academics from all over the US and puts it all in one place (mostly) for free.

I’ve already watched some great lectures by Prof Steven Levitt (Author of Freakonomics and Prof of Economy and University of Chicago) NASA scientists David Morrison on the end of the world and 2012 myth…and I’m just getting started.

These types of intellectual discussions are great stimulus during those long coding sessions and paper editing nights. Where the intellectual work is already done and now you just need to bear down hard and turn through the work. So I thought this would be a great thing to share with our readers here.

Enjoy the talks!



Thursday, October 21st, 2010

Hi, all!

This is my first post as a US LHC blogger. Please allow me to introduce myself in standard CERN style:

Hi, I’m Burton. And you are…? [response] Nice to meet you. [reciprocation, follow-up question] Ah, I’m a graduate student at Stony Brook University — you know it? [indication of familiarity, next question] I work on ATLAS, doing Pixels and an exotics search. You? [elaboration of details, physics]

And that’s it! Conversation ensues. Note the three items that form the basis of a standard CERN introduction: Name, Institution, Experiment. It’s a surprisingly solid conversational foundation. If you’ve worked hard and gotten your name out there, the other person might have already heard of you (this is ideal); if not, chances are you have friends and/or colleagues in common. The world of particle physics is, as its name suggests, quite small. Now, institutions have reputations and spheres of influence, and your association with one carries a certain weight. This weight will vary from person to person. If nothing else, it serves to indicate where you’re coming from and, occasionally, the focus of your research. Since there are six primary experiments at the LHC (though this number depends on whom you ask), and many others based at CERN, the experiment you work on is crucial information: It attaches you to a point on the LHC ring (or not), as well as a set of working groups, reconstruction algorithms, results, publications, and life choices. The tone and topics of your conversation may depend on it!

Okay, I am exaggerating — just a little. Not every CERN introduction follows the N/I/E format, and the character of an individual can not, in any way, be conveyed by such broad strokes. It’s a safe and sure starting point, yes, but hardly sufficient. Besides, second-order corrections tend to be more interesting:

I’m a photographer. Viewing the world through a lens helps me put things in perspective, and it brings everyday beauty into sharper focus (yes, that was two photography puns in quick succession — it’s a passion!). My photo archives also serve as a memory backup, should something (e.g. senescence) ever happen to me. I am an avid consumer of music; I listen to it constantly. Right now, for instance, I’m approaching the end of the new Sufjan Stevens album “The Age of Adz,” and it’s putting me in a chaotic but accomplished mood. This post will probably end soon. I love language. I used to be fluent in Spanish, but I’ve found that the more French I learn, the less Spanish I remember. Apparently there’s only room in my head for two full languages… I also love (in no particular order) cats, video games, the Python programming language, tortilla española, playful banter, and “The Daily Show with Jon Stewart.”

“I got into physics by way of consecutive childhood obsessions: dinosaurs -> astronauts -> Star Wars -> computer-generated physics simulations. The jump to the LHC came about naturally.” [1]

I look forward to our next conversation. Here is a picture. 🙂