## Archive for June, 2010

### Highly contrived crisis: How physicist prisoners escaped the bullet (SPOILER ALERT)

Sunday, June 20th, 2010

Good morning. I am back on ALICE night shifts for the last time (*sniff*) and after a mad dash to make it here by midnight it has been a very slow and quiet night. Hopefully it will stay like this all week so I can write some thesis! 🙂

The response to last week’s problem was great, and many QD readers seem to have made light work of it. Well done. It is a sign of a logical mind. You are named and congratulated at the end. I’d be interested to know how many of you feel your interest in/knowledge of science/mathematics played a part in you getting it right. I am willing to bet that all of you do, despite the answer being very simple to compute once you know how (explanation at the bottom for those who want it). When I first started my undergraduate degree I was given problems like the “three children” story and whilst the actual maths involved was basic, I found myself infuriated by the seeming impossibility of it. This is the trick that scientists learn – be determined, defy your instinct and attack the problem logically (you can’t really give up on finding a way to fix your contaminated data sample because it seems like there is no way to do it). By the time I came round to the ALICE PhD “initiation problem” (again, no predetermined sciency/mathematical knowledge required) I was a dab hand – if only for my confidence that I would find a satisfactory answer. I’ll save that problem for another time because the solution – which I got after stopping in the middle of dinner and running to the nearest whiteboard for 4 hours – might be a little tricky to convey!

Now, if you don’t have the answer to the prisoner’s plight yet, I strongly recommend you don’t read on – you will kick yourself. Give your brain a little more thinking time, it will be worth it. If you think you have the answer, or you are ready to give up, here’s the solution:

Arvinder, the friend that gave me the problem, said that there was one word that could give the whole game away. The word he was referring to was “parity”. When I got the solution, I thought his word was “binary”. Some could say simply “operators”. But you don’t even need to know what any of those are to get this right.

Consider first of all, the man at the back, looking at the row of hats. He has to convey to the person in front what their hat colour is, and he can do that by either saying “black” or “white”. But “black” does not need to mean “your hat is black”. They can predefine a new definition for these words.

Now consider the information the second prisoner along receives, and what he does with it. Clearly he will determine his hat colour and be saved, but what then? That needs to be enough to allow prisoner three to save himself. How can this be possible?

So now, finally, consider the most information there is available to anyone – the prisoner at the back can see all hats except his own. Call this the whole system – a string of blacks and whites. The person in front of him sees the same system minus one hat. From there on, every prisoner will know, by the time it is his turn, the colour of all hats except his own. Here comes the binary/parity/charge/whatever you want to use (even symmetry of a wavefunction if you like!) All you need to consider is that what ever black does to the system, white does the opposite. The initial statement from the first prisoner needs to convey what state this whole system is in, in such a way that if only one of these hats was unknown, it would be clear from the state of the system what that one hat must be. If they were 1s and 0s and being multiplied, whether the result was 1 or 0 would determine what the one unknown hat was doing to the system. Similarly if they were -1 and +1, or if you were adding the 1s and 0s for an “odd” or “even” system. Yes, it’s as simple as adding up 1s and 0s.

Thanks so much for the responses. Well done to: Emlyn, Jacques Distler, Cavendish McKay, odd man out, M Kneebone, Sourabh and Nic. Your comments are all approved and you win the “Yes, I am a scientist at heart” prize! (It is not a real prize but it makes you feel nice).

### The Traveling Circus

Saturday, June 19th, 2010

The time has come again to leave home and IPMU for several weeks and temporarily embrace a more nomadic life form. Just as when I first started writing for the Quantum Diaries, I am “on tour” again, traveling from one conference or research institution to the next. As some put it, it’s the traveling circus.
Our first stop was the XVI. European Workshop on String Theory, held in the Real Jardin Botanico in Madrid. A beautiful location and an exciting city! Both I and my husband gave short talks. Unfortunately, the weather fell somewhat short of the expectations one has for Spain in June, but we had at least one nice day to take some pictures ;-). Overall, the meeting and the week in Madrid were a good start for our tour! It’s always good to meet colleagues again that I have not seen in months and hear about the newest developments. Next week, I’ll be back at CERN again.

Parque del Buen Retiro - Madrid

### Millions of Simulations

Thursday, June 17th, 2010

Proton-Proton collision simulation "jobs" for the CMS detector running on the grid.

To compare with the data we record from our detector (CMS), we need to run a few simulations…well more like billions of simulations.

Each “job” in the plot above is actually a program running on a computer at a university.  Each program typically simulates a few hundred, or a few thousand, proton-proton collisions.  Each individual “collision simulation” calculates what a certain kind of collision would look like in our 12,500-ton detector.

And I don’t mean they just make pretty pictures.  A single simulation really consists of: some particles within each proton interact with some probability, they produce other particles with some probability, those particles decay to other particles with some probability, and so on…  Eventually, stable particles are made and the passage of those particles through the detector are also simulated.

As you can imagine, this requires a lot of random numbers.  One mistake that happens sometimes is that different jobs have the same initial ‘seed’ for the random numbers, and this results in duplication of simulations.  Not only is that a waste of CPU-cycles, but it also means a fuller range of collision possibilities doesn’t get simulated.

My job at times is to herd thousands of simulation jobs at a time to various places and monitor them, make sure they don’t crash, and finish in a timely fashion to return the needed data.

By the way, when I wrote the job monitoring script that makes plots like the one above (written in Python and using matplotlib), I tried using their school colors when I could, but sometimes that resulted in colors that were too similar or confusing.

### Why physicists would survive a highly contrived crisis

Thursday, June 17th, 2010

I want to thank my fellow PhD student Arvinder Palaha for helping to distract me at ALICE Physics week in Paris last week, when I was horrendously ill. He also gave me a fresh reminder of what makes scientists so strong.

It was a very busy week of deadlines and talks and fancy meals, and I was hit with a wave of nausea at the most incredible restaurant during our group dinner. As each course was placed in front of me, all I could think about was the smell. I needed a distraction or I might be sick. Arvinder knew that there are only two surefire ways to distract me. One was chocolate, and that would probably not have the desired effect at this point. The other was a good juicy problem – when I hear one I can’t think about anything else until I have solved it. This one was excellent because despite being deceptively simple, it epitomizes what I love about physics – mathematics and logic help you through when instinct fails completely.

I warn you – if you read on, and if you are anything like me, you won’t be very sociable for a while. Good luck. I will post the answer in a few days. If you think you have the answer, comment. If you get it right or come close then I will temporarily block your comment because of spoilers, but I will credit you later. 🙂

There are 100 prisoners. They are told by a prison guard that in 15 mins time they will be blindfolded, and then each given a cap. Some will be black, some will be white. The amount of each will be arbitrary (in fact there may not be any white caps at all, for example). They will not be allowed to talk or make any contact with each other. They will be lined up, each one behind the next, so that all but the one in front are facing the back of another person’s head. Following so far?

Their blindfolds will then be taken off. They will still not be allowed to communicate in any way. They will all be able to see every head in front of them (a line of black and white caps in some random order) but they will not be allowed to look back/around. The prison guard will then, starting at the back (with the prisoner who can see the most heads), ask each prisoner in turn what their hat colour is. If they get it wrong, they get shot. They will not be able to say anything other than the words “black” or “white”, and they can’t risk making a code with intonations in their voice or something similar because he might pick up on it, and if anything fishy goes on AT ALL he will shoot them all.

After telling them all this, the prison guard leaves and the timer starts. So, obeying the prison guard’s rules, how can they (in the 15 minutes they have to discuss their predicament before being blindfolded) come up with a plan to save as many of them as possible?

Just to show you why I love this problem so much, I can tell you that they do come up with a plan that guarantees the safety of 99 of the prisoners. The other one has a 50:50 chance of survival.

### World Cup!

Tuesday, June 15th, 2010

I’m dumb about soccer, almost as dumb as I am about wide-area networking. I don’t follow it during non-World Cup years, and besides that I couldn’t tell you where the last World Cup was held, or who won it. (Brazil is a good guess, right?) If I were at home in the US, I almost surely wouldn’t be paying attention.

But here I am on a visit to CERN (again!), and the World Cup is all that anyone is talking about. (Or at least all that anyone is talking about beside the experiments and air travel.) This is not entirely surprising, given the broad representation of different nations working on the LHC experiments. On CMS we have institutions from 38 different countries, plus people from other countries who work for those institutions. In my own research group of about twenty people, I count seven or maybe eight different nationalities. It’s part of the fun of working in particle physics — it is a world-wide effort, and you get to learn about what’s going on in lots of different places.

I’m doing my best to pay attention. I’ve been checking on which matches are being played each day, so I am ready to make the appropriate small talk with colleagues. On Saturday night, a colleague invited me to join him in downtown Geneva (near Plainpalais) for “Festifoot,” an outdoor event with large TV screens for watching the game, lots of food vendors, and live music after the days’ matches are over. This happened to be the evening that the US played England, but I was the only person in the crowd that obviously stuck out to me as American (although a few people did cheer when the US scored their one goal.) And all of the TV broadcasts of the games are being projected onto a wall in CERN’s Restaurant 1; there was a very big crowd in there this evening, while Brazil was playing.

My secret weapon in my soccer discussions comes from Nate Silver and those amazing folks at FiveThirtyEight.com, who totally nailed their prediction of the 2008 presidential election. They are now running Monte Carlo simulations of the World Cup, based on a detailed ranking scheme of the teams. Based on the rankings, they can calculate the probability of a given outcome of every matchup, and on the basis of that run a large number of simulated World Cups and predict the probability of any of the 32 teams winning in the final. It’s a totally particle physics way of thinking about the game! Brazil is favored, but is only given a ~21% chance of winning; that will probably go down after including tonight’s data, but not by much. (Don’t take the bookies’ odds on Spain.)

Honduras v. Chile tomorrow!

### Nu2010第一个报告提到张文裕

Monday, June 14th, 2010

### Room for promotion?

Saturday, June 12th, 2010

Yesterday about a dozen or so people from our university research group were asked to sit down in a room here at CERN and talk with a professor who is the DOE reviewer for our main grant.

This fall our 3-year grant is up for review, and he’ll help decide our fate, basically.

Our group had about 9 graduate students there and he asked questions to figure out what problems we were experiencing either within our group, within particle physics, or living in Europe.

Towards the end he also asked us about what we all wanted to do after we graduate.  He then led us through a somewhat sad “back of the envelope” calculation:

“Lets say the average professor’s tenure at a university is 30
at any time, and the average time for completion is 6 years.
So, the typical professor produces a total of about 10 PhD’s.
Well, they only need 1 to replicate themselves, and 1 more to
replicate positions available at national labs.  And that’s it, that’s
all there is room for in academia, typically, 2 out of 10.”

It’s an over-simplified example, but I think not too far off the mark.  About 1,000-some physics PhD’s are awarded in the US every year(AIP), but the number of vacant positions at universities each year is only a fraction of that(AIP Chart).

Update June 13:  I began searching for the names of my advisor’s former students and happened upon an on-topic article from the American Physical Society, Sean Mattingly, PhD High-Energy Particle Physics, Dedicated Client Support, Bank of America.  Sean is quoted as saying “I think every student should be thinking about a job outside physics.”  And that “in grad school we all think that we’re on the academic path, but you’re not – there’s a lot of competition for the few jobs available and most of you are going to have to leave the field.”

### Neutrinos

Sunday, June 6th, 2010

Time for another dose of particles for the people (eh, working title). In previous installments (Part 1, Part 2, Part 3, Part 4) we started a basic theory (QED; electrons and photons) and added on muons, taus, and the Z boson. Now we’re going to add on a set of particles that have recently made some news, the neutrino.

Here’s the Particle Zoo‘s depiction of an electron-neutrino:

There are, in fact, three types of neutrino: one to pair with each of our electron-like particles. Thus in addition to the electron-neutrino, we also have the muon-neutrino and the tau-neutrino. As their name suggests, neutrinos are neutral and have no electric charge. Further, they’re extremely light.

The fact that neutrinos don’t have any charge means that they don’t couple to photons, i.e. there are no Feynman rules for neutrinos to interact with photons. In fact, the only particle we’ve met so far that does interact with the neutrino is the Z boson, with the following Feynman rules:

Exercise: Consider a theory with only neutrinos and Z bosons so that we only have the Feynman rules above. Check that this looks just like three copies of QED (a theory of electrons and photons).

Question: How is the theory of only neutrinos and Z bosons different from “three copies of QED?”
Answer: Unlike the photon, the Z boson has mass! This means that the Z boson doesn’t produce a long-range force like electromagnetism. We’ll discuss this soon when we introduce the W boson and explain that the W and Z together mediate the so-called “weak nuclear force.”

Exercise: Draw the Feynman diagrams for an electron and positron annihilating into a neutrino and anti-neutrino. What are the possible final states? (e.g. can you have a muon-neutrino and anti-muon neutrino? Can you have an electron neutrino and an anti-tau neutrino?) Given that neutrinos don’t interact electrically and that the Z boson interacts very weakly, what do you think this would look like in a particle detector? (Consider the significance of the phrase “missing energy.”)

This should start to sound very boring!

If you’re starting to get bored because we keep writing down the same QED-like theory, then you’re keeping up. So far we’ve introduced all of the basic players in the game, but we haven’t told them how to interact with each other in exciting ways: don’t worry! We’ll get to this in the next post on the W boson.

Let’s recap how boring we have been:

• We started with a theory of electrons and photons called QED.
• We then “doubled” the theory by adding muons which were heavier electrons that coupled in the same way to photons. Then we “tripled” the theory by adding taus, which are yet another heavy copy of electrons.
• Next we added a new force particle, the Z. This is a heavy version of the photon (with a weaker interaction strength), but otherwise our Feynman rules again seemed like a doubling of the rules in the previous step. (We now have 6 “copies” of QED.)
• Now we’ve added three neutrinos, which only interact with the Z in a way that looks just like QED. We now have 9 “copies” of QED.

I promise things will get a lot more exciting very soon. First, here’s a pop quiz to make sure you’ve been paying attention:

Question: Can you draw a diagram where an electron decays into any number of neutrinos? Why not?

Some properties of neutrinos

We don’t quite have the full story of neutrinos yet, but here’s a glimpse of what’s to come:

• Those familiar with chemistry will know that neutrinos are produced in beta-decay processes.
• There is a neutrino for each electron-like particle. This is not a coincidence.
• One of the great experimental discoveries in the past 15 years was that neutrinos have [a very tiny] mass. It turns out that this is related to another remarkable property: neutrinos change identity! An electron neutrino can spontaneously turn into a muon or a tau neutrino. What’s even more remarkable is that this turns out to have a very deep connection to the difference between matter and antimatter. This is something we’ll have a lot to say about very soon.
• Because neutrinos are so light they played a key role in the early universe. As the universe cooled down from the big bang, heavy particles could no longer be produced by the ambient thermal energy. This left only neutrinos and photons buzzing around to redistribute energy. This turned out to play an important role in the formation of galaxies from quantum fluctuations.

In the interests of getting to the electroweak model of leptons, I will not do justice to the rich and fascinating history of neutrino physics. Here are a few highlights that I’ve found interesting.

• The Super Kamiokande detector in Japan was originally built to look for signals of proton decay that is predicted by many models of grand unification. These proton decay signals were never found (and are still being searched for), but in 1998 Super-K made a breakthrough observation of neutrino oscillation.
• Neutrino oscillation solved the solar neutrino problem.
• More recently, last month the OPERA experiment at the Gran Sasso Laboratory in Italy found further evidence for neutrino oscillation by directly observing a tau-neutrino coming from a beam of muon neutrinos which had traveled 730 km from CERN.
• One of the great theorists of the 1900s, Wolfgang Pauli, postulated the existence of a neutral, light particle to explain apparent violations to energy conservation coming from nuclear decays. He called the proposed particle a “neutron,” but also noted that it would be extremely difficult to detect directly. Later Chadwick discovered the neutron (what we call the neutron) but it was clearly too heavy to be Pauli’s “neutron,” so Fermi renamed the latter to be the neutrino (“little neutral one”). Here’s a nice Logbook article in Symmetry Magazine about Pauli’s original postulate that such a particle should exist.
• Neutrino physics has become one of the focus points of Fermilab’s research program into the ‘intensity frontier.’ The general idea is to generate a beam of high-energy neutrinos (using the Tevatron’s proton beam) and shoot it towards targets at different distances (up to 450 miles away in Minnesota!). Because neutrinos are so weakly interacting, they pass harmlessly through the earth at a slight downward angle until a small number of them interact with large underground detectors at the target site.
• There are lots of neat proposals about interesting things one can do with neutrinos. To the best of my knowledge, most of these are still in the “interesting idea” phase, but it’s a nice example of potential spin-off technologies from fundamental research. Some examples include
1. Probing geological activity deep underground, or even forecasting earthquakes.
2. One-way communication with deep ocean submarines.
3. Non-intrusive nuclear reactor inspection to check if nuclear reactors were being used to produce weapons-grade plutonium.
4. Even more dramatically, neutralization of nuclear weapons.

Coming soon

Make sure you’re thoroughly familiar with the different particles we’ve introduced so far and how we’ve allowed them to interact. Next time we’re going to spice things up a lot by introducing the W boson and some of the remarkable things it does for us. By then we’ll have nearly all of the pieces necessary to describe the electroweak theory of leptons and we can discuss neutrino oscillations, CP violation, and the Higgs boson. After this we’ll move on to the quark sector, which we’ll see is partly a “copy” of everything we’ll have done with the leptons.

### Wrapping up another owl shift

Saturday, June 5th, 2010

It’s 5:45am, and my shift doesn’t end for another hour at 7am.

We were so close to getting to record some more collision data during this shift, only for the beam to be dumped due to some problems.  I’ll have to ask experts to see what a “1/3 resonance” means.

Hopefully they’ll have more luck during the morning and afternoon shift so that by the time I’m on shift again at 11pm I’ll get to watch more data being taken in person.

### The crossroads of particle physics

Saturday, June 5th, 2010

Particle physicists come from all over the world to eat at CERN’s Restaurant 1. Actually it is more accurate to say that they come from all over the world and eat at R1, but that doesn’t change the fact that at any mealtime there, it seems like you can run into just about anyone in particle physics. (This is why some people prefer to eat at Restaurant 2.) When I’ve visited CERN, I’ve enjoyed happening upon all sorts of friends and acquaintances from my twenty years as a working particle physicist. Some I collaborated with years ago and haven’t seen in a long time; others are fellow CMS members who I just didn’t know were also going to be in town that week.

That being said, I never expected to run into my thesis adviser there, as I did two weeks ago. She is not a collaborator on an LHC experiment; her interests started heading towards astrophysics some time ago, and her current administrative duties largely preclude her from doing day-to-day research. However, she had been at DESY earlier that week for an event, and that gave her an opportunity to visit SLAC’s ATLAS collaborators at CERN. It was just dumb luck that I was still lingering at R1 when she came through to get some dinner for herself.

We quickly scheduled breakfast together for the next morning. While we’ve managed to exchange some email and talk on the phone a couple of times, it was the first time in nearly six years that I had seen her in person. In that time my life has changed almost completely, so it was very good to catch up. It was good to hear her perspective on where our field is and where it’s going, and to discuss the challenges of building a career as a university professor while being a parent of young children. I still don’t really know how she did it, and our conversation reminded me once again that I’m not as smart as her, nor do I have as broad an understanding of particle physics as she does. Now that I’m the same age she was when I was a graduate student, it troubles me a bit. But what can you do — we can’t all be our advisers.

Since we get to see each other so rarely, I thought that a souvenir photo was in order, but it was hard to get her to pose for it. “Lab directors don’t smile!” she said. The experimental evidence is to the contrary.

I’m off to CERN again this week — we’ll see who shows up in R1 this time!

p.s. As I write, the LHC is circulating beam, and the beam energy has been ramped to 3.5 TeV in preparation for a physics run. As we would say back at CESR, “Beams are at energy, not yet colliding!”