Almost a year ago, Stephanie Majewski, a post-doctoral researcher in Brookhaven National Laboratory's Physics Department, sold her car, her couch, and her TV, packed her bags, and moved to Switzerland, close to Geneva. There, she works on the ATLAS experiment and detector, part of CERN's Large Hadron Collider (LHC), the world's most powerful particle accelerator. (CERN is the Conseil Européen pour la Recherche Nucléaire, or the European Organization for Nuclear Research.)

The LHC is a massive engineering feat in and of itself, lying in a tunnel nearly 175 meters (or 570 ft) underground near the French border. It was designed to cause extremely high-energy, head-on particle collisions, creating sub-atomic particles never before seen. The ATLAS detector is supposed to find them. The generation of these subatomic particles is expected to enable scientists to learn more about the earliest workings of the universe. The agenda includes dark matter, which astronomers say make up a quarter of the universe; superparticles, a new category of matter thought to hold galaxies together; and the Higgs boson, a particle believed to imbue other particles with mass.

The LHC began operation last September. Unfortunately, it had problems with its electrical connections, and in the last year they have worked to repair the resulting damage and to add safety features. (On that day, Majewski wrote in her blog: "Don't worry, we have plenty to work on until then! I was bummed at first, but I'm sure this will make first collisions even sweeter!") The LHC is scheduled to be operational in November 2009.

Majewski's job is to ensure that the ATLAS liquid argon calorimeter - a central piece of the detector - is correctly measuring the particles produced from the LHC's extremely energetic proton-proton collisions.

Majewski received her Ph.D. in applied physics from Stanford University in 2007. She originally thought she wanted to study computer science or chemistry. But after introductory physics classes at the University of Illinois, she decided to study particle physics.

To follow Stephanie's adventures overseas, check out her excellent blog.

Q&A with Dr. Majewski

Hello, everyone! I am looking forward to answering your questions, whether they are about what got me into physics, what it's like to work at CERN, or about the science that we will do once the Large Hadron Collider starts up. Don't be shy! :)

It's really amazing to have you here. I read that your job is to ensure that the ATLAS liquid argon calorimeter works fine. I know that liquid Argon has been used in other experiments for detecting neutrinos. What kind of particles do you expect to find now? Do you believe that Higgs Boson really exists? Higgs Boson is the particle that gives to the matter the "property" of mass. If a Higgs Boson is detected in a collision and consequently extracted from the proton, then what will happen with the proton and the weak force?

Theorists have been working overtime the past few decades trying to come up with new models for physics beyond the "Standard Model" -- this is the theory that describes all of the particles we know about so far. There are some really exciting hints that we will find something new at the LHC.

One of those is as you mentioned, the Higgs boson. And you are exactly right that this particle gives particles mass. It is included in the Standard Model, but it's the only particle in that theory that we haven't seen yet, and we are hoping to be able to produce it through proton-proton collisions at the LHC. Each proton is made up of particles called quarks and gluons. When two protons collide head-on with enough energy to produce the Higgs boson, the protons break up and quarks and gluons go flying. It's kind of like colliding two blueberry pancakes; it makes a mess :) The experimenter's job is to dig through the mess of particles that came out of the collision and see what happened. One way to see if there was a Higgs produced is to look for 4 muons that came out of the collision. (A muon is similar to an electron, but ~200 times heavier.) We will dig through millions of collisions for ones that have 4 muons coming out, and then combine the energy and momentum of the 4 muons to see if they came from a Higgs boson.

Another hint that we might find something new at the LHC comes from astrophysics, and that is dark matter. We know it exists, and we have some clues about how it interacts, but we don't know what it is! So theorists have come up with elegant ideas that tie together the Standard Model and dark matter. One of the mainstream ideas is called supersymmetry (or SUSY for short). Experimenters on ATLAS hope that the LHC will produce new supersymmetric particles that could explain dark matter, too.

But the most exciting prospect from the LHC is if it produced new particles that we weren't expecting! Then we would measure those new particles and figure out how they fit into our existing theories and what they tell us about the universe. That would be great fun :)

The fact that you are looking for 4 muons means that a higgs boson is made up by them? I have read about supersymmetry. Every single particle with integer spin has a partner with the same mass and half-integer spin. For every type of boson there exists a corresponding type of fermion. How can this help us to find the dark matter? Feel free to explain it in a more scientific language (if you want to).

The 4 muons come from the decay products of the HIggs boson; in this case the Higgs would decay first into 2 Z bosons and then each Z boson would decay into 2 muons. The Z boson is a heavy (its mass is ~90 GeV) mediator of the weak force with zero electric charge, and was originally discovered at CERN. Here is a link to a nice simulated image of what a Higgs to 4 muon event would look like in ATLAS: http://cdsweb.cern.ch/record/39448

One of the interesting features of some models in supersymmetry is that there might be a stable "LSP", which stands for lightest supersymmetric particle. This is exactly what it sounds like ... it is the lightest "super" partner to the particles in the Standard Model, and it is stable because it is not allowed to decay into any Standard Model particles (because of something called R-parity conservation). If the LSP was stable and had no electric charge, it would be a great candidate for dark matter.

How sure are we that quarks are the most basic particles of which everything (material, and that we know of) consist? Are there any hints suggesting more basic particles, particles of which quarks consist? It has been said that photons reaching us can have certain maximum energy, however photons with higher energies have been detected, could it be that general relativity may be missing something in this direction. Last question for now: Eenergy, is there a minimum limit in which energy can be "divided"? I know about Planks constant however this is directed to the minimum amount of energy difference between two photons ejected from an electron "inside" an atom (in short). Does this constant apply to any other system as well?Mmore macro systems maybe? For instance, the least amount of kinetic energy a "macro" object can gain is equal to the value of Planck's constant, or can any object gain any amount of energy in any form? In other words, does energy come in "quantum packets," is it always subdivided or is it "continuous" resulting in any object being able to gain any amount of energy in any form?

This is a great question because it seems very natural as you learn physics that things keep getting smaller and smaller.... from the atom to the nucleus to the quark, so why not even smaller? In fact, there is already a name for theoretical sub-quark particles: "preons" :) But we haven't found any evidence that quarks are "composite" -- made up of smaller particles -- just yet. When physicists talk about looking for compositeness, they usually describe it in terms of the energy of the interaction rather than a size. The Tevatron collider at Fermilab has looked for compositeness and has ruled it out for energies below 2.2 TeV. In comparison, the LHC should be able to search up to 10 TeV -- about 5 times higher than the Tevatron.

Hmm, your question about photons seems to be more of an astrophysics question, which is not really my specialty :) I know that there are a few experiments that are searching for high-energy photons from astrophysical sources such as supernovae and pulsars. Try this website about the H.E.S.S. experiment for some more reading.

Ah, quantization of energy! Yes, energy comes in quantum packets at the fundamental level, and Planck's constant is universal. As you stated, this is a minimum limit. It may be difficult to imagine since in macroscopic systems, the energies we measure seem to be continuous.

I haven't done very much physics, so this might be a wrong assumption, but aren't quarks and electron very similar? They both have electric charges, some kind of spin, and an extremely small mass.

This is a very good question! Quarks and electrons are similar in that they are very, very small (especially compared to us!) and as far as we know they are both "fundamental" -- meaning they aren't made up of anything smaller. However, we can measure their properties, just like the ones you mentioned (electric charge, spin, mass) plus the way they interact with other particles, and we find them to be quite different! Electrons have a charge of -1, a spin of 1/2, and a mass of 0.5 MeV (or "Mega electron Volts"). Quarks have charges of either +2/3 or -1/3, which is already quite different, they also have a spin of 1/2, and the lightest quark (called the "up") has a mass roughly 4 times larger than the electron. The heaviest quark, called the "top", has a mass of about 170 GeV (or "Giga electron Volts"), which is about 300,000 times larger than the mass of the electron!

Because of their properties and interactions, we put all of the quarks in one family (there are 6: up, down, charm, strange, top, and bottom) and we put the electrons into a different family called "leptons". Other kinds of particles in the lepton family are the muon, tau, and three kinds of neutrinos.

For a fun way to get to know all of the particles, check out the Particle Zoo!

Thanks for your answers. They really helped, but I was now wondering, at what point (if any) is it that energy "transforms" into matter, so when the energy is high enough to take up properties of matter. I know about photons, which are energy, can "transform" into electrons and then back again, is this how it happens, the energy of a "particle" becomes higher and higher until it just cannot go back from its matter state (electron for instance) to its energy "state (this would result in the matter state then not being an electron but something of higher energy), but is there a limit where a single mass cannot be "transformed" back into a single "energy particle.” I was also wondering, when a photon changes from photon to electron, where does it get the charge and the spin, etc.

I can tell you're really thinking about this energy-matter duality, which is great. I hope I can help clear some things up. When particles "transform" (as you put it) to energy and back, they actually have to obey certain rules. Some of these include conservation of charge and conservation of angular momentum (which includes spin). So a photon can't convert into a single electron, because that wouldn't conserve charge! So instead, a single photon will convert into a pair of particles, an electron and an anti-electron (which is called a positron). A positron is the antimatter equivalent of an electron, so it has the opposite electric charge, +1. Therefore the net charge of a photon (charge = 0) turning into an electron (charge=-1) and positron (charge=+1) is still zero.

One more point... energy also has to be conserved. So the process of a photon turning into an electron-positron pair and then back into a photon can't create energy! In this case, the electron and positron are actually virtual particles.

I realized as I'm trying to explain this in words, that this is much better explained with pictures! And Feynman thought so too... he developed diagrams (which we now call "Feynman diagrams") to help keep track of allowed particle interactions. For example, this is the diagram of the photon --> electron + positron --> photon: http://upload.wikimedia.org/wikipedia/commons/9/99/Vacuum_polarization.svg. I hope I've included enough tantalizing terms to start you reading about Feynman diagrams and virtual particles. If you can, I would suggest heading to your local library for more resources :)

Oh, I just got another question, virtual photons represent the electromagnetic force, now they are said to be like a person on a slippery surface throwing a ball. Because of Newton’s law (each action has an equal and opposite reaction) the thrower will move backwards when throwing the ball, the catcher (other electron for instance - as long as they are equally charged) catches the ball (virtual photon) and also moves backwards, this is also represented through Feynman's diagrams, now this makes sense. But what happens when a positive and a negative exchange virtual photons? I know the ball throw thing was just to get a picture in your head, but it cannot explain how opposites attract. they say for the attraction that the two people which originally threw balls threw a rope and then pulled each other. Now how does the virtual photon act as a rope for a proton and as a ball for an electron. This is now using the "picture" that they give beginners to understand the virtual particle electromagnet interaction concept, but for me it hasn't really helped much. (Don't be afraid to explain it scientifically rather than to use "pictures/stories" like the one above.)

Perhaps some math would help! The "picture" that you describe is trying to help you gain intuition, but the language of physics is actually math :) The electromagnetic force is related to charge 1 x charge 2 / distance^2. The signs of the charges tell you if two particles are attracted or repelled by each other (since distances are only positive!). If charge 1 = -1 and charge 2 = -1, multiplying them together gives you +1, which tells you that the force is positive, and therefore they are repelled. If charge 1 = +1 and charge 2 = -1, their product is -1, which tells you that the force is negative, and they are attracted. This might not help you gain intuition, but the math doesn't lie! I hope that gives you a more quantitative perspective on electromagnetism.

I have read a bit about supersymmetry a while ago. What if it is completely wrong though? Is it possible at all, and what could be potential consequences of it? String theory requires the existence of super-heavy "sparticles." If such particles exist and are not found by the LHC, will that mean that string theory is wrong? Why does the LHC use protons and not electrons? Won't electrons attain higher speeds and create a bigger impact? Are electrons harder to control?

Great questions! Supersymmetry could be completely wrong! That's the exciting part about doing experiments -- theorists come up with beautiful models to try to explain and describe nature, and experimentalists come along and measure what's actually there (and then theorists try to explain it...). As I mentioned in a previous post, there are some exciting hints that there is some new physics waiting for us at the LHC, but it may or may not be supersymmetry. We hope that even if there's no supersymmetry, that there's something new and exciting to discover!

Usually when theorists talk about string theory and the LHC, they are combining string theory with supersymmetry or other theoretical models. I'm not a string theorist, but as far as I know, we won't be able to prove or disprove string theory at the LHC (although we may be able to constrain it).

The LHC uses protons because they are able to achieve higher energies with protons than electrons. Electrons are actually easier to control, and the collisions are a lot cleaner, but it's harder (and more expensive) to accelerate them to the same energy as protons. The issue is that electrons are lighter and therefore lose more energy to something called "synchrotron radiation" when they are bent in a ring. Therefore the strategy is that we would be able to discover some new particles with proton-proton collisions (even though they are messy!), and then build a large linear electron collider (like the proposed International Linear Collider) to more precisely study the new particles we discovered.

Why doesn't the LHC use proton + anti-proton collisions instead of proton + proton collisions? Wouldn't the protons repel each other and make it much harder for them to hit each other?

If the LHC finds little or nothing at all, that might pose a difficult scenario for particle physics. If we don't find the Higgs boson for example, I think it would be very interesting, because it would mean that the Standard Model of particle physics is incomplete. But, if we don't find the Higgs or anything else, I think it would be difficult to justify building another huge international accelerator. Then we might have to get more creative and rely on cosmology and astro-particle physics to point us in the right direction and explain why there's nothing there!

Luckily, we don't have to worry about the protons repelling each other in the collisions. This is because the protons have very high energy (7 TeV each) when they collide, and the strong force (Quantum Chromodynamics) is much, well, stronger! than the electromagnetic force in that interaction. The Tevatron accelerator at Fermilab collides antiprotons with protons. But it's much more difficult to create an intense beam of antiprotons than a beam of protons and so the LHC decided to stick to proton + proton collisions at a higher energy. I found a nice site about how antiprotons are created if you are interested.

Oops, I completely forgot about the strong force... But don't gluons only operate under extremely short distances? Would the combined electric charge of all the protons in a single beam be able to "nudge" the other beam or spread itself out? Can I ask a question more about the workings of a particle accelerator than particle physics in general? How do the magnets of the LHC or any other accelerator act in synchrony to pull particles one way, when "right next door" the magnets have to pull another beam of charged particles the other way? Will the magnets interfere with each other? Doesn't the LHC produce proton collisions with energies large enough to create tiny black holes? What would they look like?

Gluons do only operate under short distances, which is why you need the two proton "pancakes" to hit each other to produce high-energy particles. You are right about the combined electric charge of a bunch of protons interacting with the other bunch of protons. In accelerator physics terms, this is called "beam-beam interactions" and has to be taken into account when steering and focusing the beam.

Your question about the nearby magnetic fields is a very good one, and had to be taken into account when designing the LHC magnets. I found a diagram of an LHC dipole on p.24 of this PDF file. This shows the arrangement of the superconducting cables and the magnetic field lines in a dipole magnet, taking into account the 2 beamlines going in opposite directions.

As for the black hole question, I'll post the standard CERN explanation:

According to the well-established properties of gravity, described by Einstein’s relativity, it is impossible for microscopic black holes to be produced at the LHC. There are, however, some speculative theories that predict the production of such particles at the LHC. All these theories predict that these particles would disintegrate immediately. Black holes, therefore, would have no time to start accreting matter and to cause macroscopic effects.

Although theory predicts that microscopic black holes decay rapidly, even hypothetical stable black holes can be shown to be harmless by studying the consequences of their production by cosmic rays. Whilst collisions at the LHC differ from cosmic-ray collisions with astronomical bodies like the Earth in that new particles produced in LHC collisions tend to move more slowly than those produced by cosmic rays, one can still demonstrate their safety. The specific reasons for this depend whether the black holes are electrically charged, or neutral. Many stable black holes would be expected to be electrically charged, since they are created by charged particles. In this case they would interact with ordinary matter and be stopped while traversing the Earth or Sun, whether produced by cosmic rays or the LHC. The fact that the Earth and Sun are still here rules out the possibility that cosmic rays or the LHC could produce dangerous charged microscopic black holes. If stable microscopic black holes had no electric charge, their interactions with the Earth would be very weak. Those produced by cosmic rays would pass harmlessly through the Earth into space, whereas those produced by the LHC could remain on Earth. However, there are much larger and denser astronomical bodies than the Earth in the Universe. Black holes produced in cosmic-ray collisions with bodies such as neutron stars and white dwarf stars would be brought to rest. The continued existence of such dense bodies, as well as the Earth, rules out the possibility of the LHC producing any dangerous black holes.

What they would "look like" in our detector is a bunch of junk! (jets, leptons, etc) that is uniformly sprayed at our detector and doesn't respect conservation laws like baryon number. (aside: baryon number conservation just means that the number of baryons, particles with 3 quarks, has to be conserved in any reaction. So if you start with 0 baryons, you need to end with a baryon + an anti-baryon to respect this conservation law.)

Thanks for the questions!

I have a question that I hope doesn't sound too ridiculous. What is energy? I mean, I've red stuff about physics and they all say stuff like "loops/strings of energy" and things like that. But what really is energy?

This is not a ridiculous question at all! It's actually a very difficult question, and one that I think different physicists would answer quite differently. If you look up a "definition", you get something like this: the property of matter and radiation that is manifest as a capacity to perform work. You can certainly read about potential energy and kinetic energy, about how energy is conserved and quantized. All of those concepts are certainly correct, but they don't really answer your question... what really is energy?

The "loops/strings" of energy that you mention are one idea of energy in the context of string theory. It is a way to visualize what is going on at a fundamental level, by picturing vibrating "strings", where a high frequency corresponds to a massive particle and a low frequency corresponds to a lighter particle (perhaps you've seen diagrams similar to these. But I should make the caveat that even though string theory is fascinating, we don't know yet if it is a valid picture (and it's very difficult to prove or disprove!).

To spin your head around a bit more ;) you can also start to think about "dark energy", which makes up 70% of the universe, and yet we know almost nothing about it (the NASA website basically says the same thing). I'm very glad you asked the question, because energy is a word that is tossed around very casually, but it's important to know what we don't understand yet :) Perhaps you will design an experiment one day that gives us some answers!

I was curious about how n-categories are useful in mathematical physics nowadays, and also what areas of mathematics in general tend to be most relevant. Thanks for your help.

I don't know much about n-categories myself, but the nice thing about physics (and academics in general) is that many of your colleagues have varying interests and areas of expertise. I posed your question to a more hard-core math-oriented colleague of mine, and this is his very nicely explained response about using n-categories in topological quantum field theories:

In topology you often stretch and bend and twist things into something else. Which is cool as long as you don't break anything or tape two things together. And the way you twist the things is the map from the original object to the new object. So if I give you an object and the rules for bending it, I've given you a category. But if I give you that and the rules for bending the resulting object, I've given you a 2-category, and so forth. So, in normal, everyday quantum field theories we've got normal, everyday invariants, like s. (I should mention that invariants are just things that don't change no matter what. Speed changes depending on how fast the observer is moving, but invariants don't.) So physicists like invariants, because you calculate them once and then you're done. But topological quantum field theories are used to compute topological invariants. So, for example, if I make two blue marks on a rubber band, and then a red mark inside them, I could compress or stretch the rubber band so the red mark is closer or farther from one of the blue marks, but nothing I could do would put it outside of the two blues. That's a kind of crappy invariant, but topological invariants get pretty heinous pretty quickly.

(Only slightly paraphrased from Marc Weinberg, University of Wisconsin graduate student on CMS)

Experimental particle physicists tend to use statistics most regularly, but theorists span many areas of mathematics, and these vary a lot depending on the type of physics they do. A general rule for doing research is that physicists try to have a broad background in mathematics and problem-solving, and then learn (or develop!) the math that they need to know to solve a particular problem.

Thanks for the question! I learned something too :)

Thank you for participating in this discussion. You have made what might otherwise be inaccessible material to those without the expertise not just accessible but really compelling. We will be watching the LHC's progress! Touche de bois!