Quantum Diaries

Tandis qu’un torrent de données inonde les expériences du LHC, le flux d’articles scientifiques exposant les mesures et résultats les plus récents ne tarit pas. Pour qui n’est pas versé en physique des particules, ces considérations pleines de sections transversales, de multiplicités et autres étouffement des jets, sont bien obscures. Si les nombres et les graphiques permettent au physicien d’y voir clair, les autres ont besoin de quelque chose de plus parlant, ce qui est sans doute l’une des raisons pour lesquelles les images d’événements sont si appréciées, notamment par les journalistes. Les images de traces de particules, aux couleurs vives, vous donnent au moins l’impression de voir l’invisible. Elles peuvent aussi être d’une beauté renversante.

En cette année où l’on célèbre plusieurs centaines de découvertes liées aux recherches menées au CERN – découverte de la supraconductivité, découverte du noyau atomique – il convient de rappeler le centenaire de la première observation de traces de particules, dans un appareil appelé « chambre à brouillard ». Le 9 juin 1911, la Royal Society of London publiait des photographies de Charles Wilson représentant de « fines volutes et traînées de brouillard » – en fait, des gouttelettes de condensation formées le long des trajectoires des particules alphas (noyaux d’hélium), qui avaient ionisé la vapeur sursaturée de la chambre. Au départ, Wilson avait mis au point sa chambre en vue d’étudier la formation de vrais nuages mais, constatant que l’appareil était sensible aux rayonnements, il l’a ensuite modifiée pour rendre visibles les traces des particules, procédant à ce qu’on appellerait aujourd’hui un transfert de technologie.

La chambre à brouillard de Wilson est l’ancêtre direct de bon nombre des détecteurs se trouvant au cœur des énormes expériences qui enregistrent aujourd’hui les traces des centaines de particules issues des collisions frontales se produisant dans le LHC. Le principe de base est le même : rendre visibles les traînées ionisées laissées par les particules lorsqu’elles traversent un gaz ; même s’il est vrai que les chambres modernes détectent l’ionisation de manière plus directe, sous forme d’impulsions électriques, plutôt qu’en la rendant visible à l’aide de traînées de gouttelettes. Faites jouer la magie de l’informatique pour colorer les traces, et les images que vous obtiendrez pourront être tout aussi séduisantes pour une génération qui connaît la télévision couleur et les jeux vidéos que l’étaient il y a un siècle les photographies de Wilson. Elles ne rendront peut-être pas plus compréhensible le monde étrange des particules, mais le rapprocheront certainement un peu de notre réalité.

Alors, la prochaine fois que vous apercevrez dans le ciel des traînées de condensation, ayez une pensée pour Wilson, et même, levez votre verre au centenaire de la première trace apparue dans sa chambre à brouillard.

Christine Sutton

I recently had an encounter with an unusual reporter. He had been roaming about CERN in search of a story, and by chance that brought him far afield to the test beam control room at CERN’s SPS North Area where I was spending the day on shift with a colleague. Because of technical difficulties (or was it a worker’s strike?), the beam was off, and our devices under test weren’t taking any data, so when the reporter came by and asked to speak with us, it was an emphatic “Yes, but I need coffee first.”

Now, to be fair, it wasn’t the reporter himself that was unusual, it was the story he sought: “The Sound of CERN.” Um, what? This gentleman had been taking field recordings all over in hopes of piecing together an aural representation of the world’s largest particle physics laboratory, and now he wished to speak with us. After seeing — hearing! — so much of CERN’s infrastructure and equipment, all of it humming whirring beeping pulsing clicking, he had decided to widen the scope of his project to include the scientists inhabiting this soundscape. Sounded reasonable to me.

After my colleague rather eloquently explained the work we were doing at the test beam and why it mattered in the grander scheme of things, our conversation moved on to the reporter’s work. Why sound? Well, it’s awfully difficult for people to relate to fundamental physics on a visceral level, and intellectually isn’t much easier — there’s a reason we spend so much time in grad school! But understanding is facilitated whenever you can associate a concrete bit of sensory information with an abstract concept. What sound does a Higgs boson make when it decays into two W bosons? What color is an electron? (No, I’m not talking quantum chromodynamics…) If you could reach out and touch a proton, would it be soft like a plushy, or hard like a billiard, or squishy like a hard-boiled egg?

Or, in this reporter’s case, What does CERN sound like? Well, I will give you some hints — in words, until I acquire the means to record the audio myself:

• like a room full of computers, fans blowing hard while merciless physicsts bang on their keyboards
• like a 1.6 Tesla Morpugo magnet thrumming softly while a beam of 180 GeV pions passes through it at close to the speed of light
• like a flock of sheep grazing in a field 100 m directly above the low-energy end of CERN’s accelerator complex, the bells on their necks chiming pleasantly
• like the coffee machines in R1 dispensing liquid energy to tired grad students
• like the electronic tone emanating from television screens once every forty seconds, indicating when the SPS is spilling beam your way
• like water cascading down the inside of the ATLAS cooling towers at Point 1 on the LHC
• like two people chatting excitedly as you pass them by, “… saw good agreement in the high-pT tails, but something strange …”
• like this, maybe

Until next time — keep your eyes open and your ears tuned!

Burton

Physicists have got a great sense of humor when it comes to software bugs. A “bug” is simply something which stops the software from running when we want it to continue. When a bug is known, but seemingly unavoidable, it can be called a “feature”, in an attempt to make it sound a little less ominous. There’s usually a little chuckle in a meeting when someone announces that there’s “an interesting feature in this code”. When I discovered the terminology behind the quantum bug I was almost in hysterics. Here’s a selection of them:

Heisenbug

Hey you! Get out of my code!

Named after the Heisenberg uncertainty principle, this kind of bug disappears when you try to study it! The Heisenberg uncertainty principle usually reads \(\Delta p_x \Delta x \ge \hbar /2\), where \(\Delta p_x\) is the uncertainty in the momentum (in the \(x\) direction, say) and \(\Delta x\) is the uncertainty in the \(x\) position. When we know one of these quantities with a given precision, we lose precision in the other quantity. For a Heisenbug we have:

\[
\Delta K_c \Delta K_b \ge I
\]
This time \(\Delta K_c\) is the uncertainty in our knowledge of the conditions that caused the bug, and \(\Delta K_b\) is the uncertainty in our knowledge of the behavior of the bug, and \(I\) is some (unknown) amount of information we can get about the bug. So we know what caused the bug, we turn on the debugger, or try to get some other information about the bug to try to “catch it in in the act”, and in doing so we lose information about what caused the bug, and the bug disappears! Weird stuff!

Schroedingbug

This kind of bug is usually found in code that never gets executed. When it does get executed the outcome is unpredictable. For example, think about this:

if(a<0 && a>0) {
  a = a/b ;
}

Since a cannot be both negative and positive, the code inside the curly braces would never get executed. Some clever programmer comes along and changes things:

if(a<0 || a>0) {
  a = a/b ;
}

Now the code executes if a is positive or negative (ie not zero.) But in fixing this line of code the programmer has introduced another bug. When happen when b is zero? The programmer has introduced a Shroedinbug, with a wavefunction:

\[
\psi_{code} = P(b \ne 0)\psi_{succeed} + P(b=0)\psi_{fail}
\]

Good luck to whoever has to debug that!

Cosmic bug

Also know known as the alpha particle bug, this happens when a small part of the data set used is corrupt in some way. In a lot of cases this would be the same as having normal data, and then a cosmic ray comes in from space, hits a part of the memory or hard disk, and some 1s and 0s get all messed up, leading to corrupt data. Finding these bugs is usually quite easy, but it can also be frustrating. After all, your code can run perfectly fine for several hours, processing millions of events, only to have it fail in a single event. Wishing away corrupt data is just like wishing away cosmic rays. It can’t be done. And very often, nobody knows where they come from.

Bonus material: How to use the cosmic bug your advantage!

Fermi gas bug

Okay, I made this name up and it just a fancy word for “memory leak”, but it goes hand in hand with the cosmic bug. Suppose you think you have a cosmic bug and you suspect that event number 24601 is corrupt. How do you test this? Well you exclude the first 24600 events and take a closer look at what is happening. (This just skips about 24,000 events, saving you lots of time.) Everything works fine until you get to about event 50,000, then things crash again. So you start from event 50,000, and find that things are okay until you get to event 75,000. It seems that these three events are simultaneously cosmic events and not cosmic events! In fact, it’s a Fermi gas bug. The software uses more and more memory until eventually it runs out of memory and crashes. That’s a bit like how electrons act in a Fermi gas. You can only fit so many electrons in to the available states, and the bits of memory act like electrons. Once you’ve filled the states, you’re out of luck!

Mandelbug

The original Mandelbug?

Named after Benoit B Mandelbrot, this kind of bug is very very complicated. (By the way, the B in Benoit B Mandelbrot stands for Benoit B Mandelbrot…) The conditions that cause the bug are so amazingly complicated that they’re almost impossible to reproduce, and may even be chaotic. Even worse, they can be the product of lots of other much smaller bugs. The conditions are still deterministic, but good luck reproducing them! It’s a bit like making a fractal or predicting the weather. The answer is out there, but until you let the system go through the motions you never know what the outcome will be. Failed fits are a great example of this kind of bug. Changing the initial values of the parameters by a tiny amount can be the difference between success and failure.

Bohr bug

A Bohr bug is the simplest bug of all. Just like the semiclassical quantum mechanics of the early 20th century, the Bohr bug is in a definite state and you can measure all of its properties. These come up all the time and usually easy to spot and fix. But just like in physics, we’d be fooling ourselves if we thought that everything was this simple.

Generic quantum bug

So what was my bug? Well it didn’t seem to fit into any of these categories. Using the same conditions sometimes it would work and sometimes it wouldn’t. If I had to write a wavefunction for it might look something like this:

\[
\psi_{code} = a\psi_{succeed} + b\psi_{fail}
\]

Unfortunately I don’t know the size of \(a\) or \(b\)! So I’d better work out how to fix this.

I suppose we could add a few more bugs to the list:

Bose bug

Like two bosons occupying one state, two variables can use the same memory (for example, two pointers to the same object). When the state of one changes, so does the state of the other! Hours of hilarious debugging!

Quantum tunnel bug

Suppose you declare an unsigned int (a positive integer) and assign it a negative value. What happens? Well usually you get a huge positive number instead. The value of the variable tunneled from 0 (a sensible value) to 4294967295, which is unphysically huge!

Nuclear chain reaction bug

Suppose you have a bug, which causes two more (identical) bugs, which cause four more identical bugs… You’ve got a chain reaction which isn’t going to end any time soon! We all have these from time to time and they can be nearly impossible to see in advance. For example, let’s say you apply a weight of 0 to a sample. Anything weight that gets multiplied by this weight will also be 0. Hours later you find your code worked and you got an output. But everything is 0.

Any more suggestions? Leave them in the comments!

Last month had the unique pleasure of making my first trip to CERN (more on that in a later post). I made a point to stop by the CERN gift shop to pick up a snazzy mug to show off to my colleagues back in the US, and am now the proud owner of a new vessel for my tea:

My brand new "Standard Model Lagrangian" mug from CERN.

The equation above is the Standard Model Lagrangian, which you can think of as the origin of all of the Feynman rules that I keep writing about. Each term on the right-hand side of the above equation actually encodes several Feynman rules. Roughly speaking, terms with an F or a D contain gauge fields (photon, W, Z, gluon), terms with a ψ include fermions, and terms with a ϕ include the Higgs boson. Some representative diagrams coming from each of the terms are depicted below:

Representative Feynman rules coming from each term in the Lagrangian.

But alas, there’s a bit of a problem with the design. It appears that there’s an extra term which isn’t included in the usual parametrization of the Standard Model:

This term really shouldn't be here. It's not necessarily "wrong," but it is misleading and doesn't match what is written in textbooks. Technically, it is not `canonically normalized.'

I won’t go so far as to call this a mistake because technically it’s not wrong, but I suspect that whoever designed the mug didn’t mean to write this term. Let me put it this way: if I had written the above expression, my adviser would pretend he didn’t know me. The “h.c.” means Hermitian conjugate, which is a generalization of the complex conjugate of a complex number. In terms of Feynman diagrams, this “+h.c.” term means “the same diagram with antiparticles.”

The problem is that the term above,

already includes its Hermitian conjugate. In physics-speak, we say that the kinetic term is self-conjugate (or Hermitian, or self-adjoint). This just means that there is no additional “+h.c.” necessary. In fact, including the “+h.c.” means that you are writing the same term twice and the equation is no longer “canonically normalized.” This just means that you ought to rescale some of your variables.

I was mulling over this not-quite-correct term on my mug while looking over photos from CERN when I discovered the same ‘error’ in a chalkboard display in the “Universe of Particles” exhibit:

Display at the "Universe of Particles" exhibit in The Globe of Science and Innovation at CERN.

The “+h.c.” on the top right is the same ‘error’ as printed on the CERN mug. I wonder who wrote this?

To be clear: this expression does summarize the basic structure of the Standard Model in the sense that it does give all of the correct Feynman rules. However, the extra “+h.c.” introduces a factor of two that needs to be accounted for by weird conventions elsewhere (that would not match any of the usual literature or textbooks).

Anyone who has done tedious physics calculations is familiar with the frequent agony of being off by a factor of 2. Now when people make remarks about this ‘error’ on my mug, I’m quick to tell them that the factor of 2 mistake just makes it more authentic.

——

It is more than two months since my first blogging. There were several events that could be good topics for blogging, such as the Chinese traditional dragon boat festival, the launching of the AMS detector, the recent indication of a large theta13 by the T2K accelerator neutrino experiment, etc. However, I was fully occupied and even not have a chance to write one word. This is not so good as a blogger. I need write something down. Let me start with a recent interesting event—ISSP.

——

When walking on the small path of Erice (a beautiful town in Sicily, Italy) and breathing the mixture atmosphere from Mediterranean and this old town, I believe I’m back to here again. This is my second time to be Erice to participate in the International School of Subnuclear Physics (ISSP). This year is the first of the three devoted to the celebrations of the 50^th Anniversary of the Subnuclear Physics School. Why the celebration will be during three year? The school was first started in 1961 by Prof. Antonino Zichichi and John Bell at CERN and formally established in 1962 with Bell, Blackett, Weisskopf and Rabi in Geneva (CERN); the first Course at Erice being in June 1963. So they have three celebrations.

 ISSP is a high level school, under <ETTORE MAJORANA> Foundation and Centre for Scientific Culture (EMFCSC). Experimental physicists like to talk about numbers. Here are some about the EMFCSC activities since 1963:

123 schools, 1,497 courses, 103,484 participants (124 of which NOBEL Laureates) coming from 932 universities and laboratories of 140 nations

 ISSP is also very interesting. At the end of the School twenty-three Diplomas will be awarded to the Best New Talents by a Committee composed by the Lecturers and the Invited Scientists. Each Deploma is named after a late physicist, to stimulate the young talents. It is really like a ‘school’, rather than the summer school I’ve participated or heard.

It will be a great enjoyment to discuss physics with the  top physicists, S. Ting, A. Zichichi, G. ‘t Hooft, H. Fritzsch, P. Minkowski, M. J. Tannenbaum, etc.

Particle physics is hard work. I don’t just mean that the theory is difficult, I mean that the day to day work is difficult even if you know what you’re doing. The only thing more complicated than working with people is working with computers, and at CERN we have to do a lot of both. It’s even more difficult when you don’t know what you’re doing it, and since we work in research there are plenty of things that nobody will know! In that kind of scenario, having more than a passing knowledge of some area of hardware or computing can give you the title “expert”. The past two weeks have been a non-stop barrage of really hard things happening, and having to make tough choices about how to deal with them. The humidity and heat hasn’t helped, and only made it harder for everyone to concentrate! But today, the skies opened up, the rain poured down and the work finally became manageable again!

In the corridors of CERN, no one can hear you scream!

In the past couple of weeks I’ve had an awkward dealing with the IT admin. I have a series of computing tasks which must work each night for the Trigger Rates group, and if they don’t work, we run into problems with the monitoring of the trigger rates. Blips happen and jobs fail from time to time, but when it happens for more than a few days people get unhappy. Unfortunately these jobs required large resources, and as the software migrated from one version to another this lead to huge problems with allocations of resources. The IT people noticed this and contacted me immediately! What could I do? If I continued to run the jobs the problems with resources would continue, and I’d get into some serious trouble. If I stopped the jobs we’d have problems with the rates monitoring system. That’s not a nice position to be in! After speaking the experts it looks like there’s a solution and today I got an E-mail telling me how to get the jobs to work without annoying anyone. That’s one crisis averted!

If things were looking bad for my service work, what about my analysis? That was looking a bit shaky as well. I needed to perform a study on the data to see how often we see QCD events (These are events that produce huge numbers of quarks and gluons and they dominate the data. Our poor simulations just aren’t vast enough to model these backgrounds properly!) The problem involved using two pieces of software which were written by different people. That’s usually okay, I’d just E-mail the software experts for help and arrange an informal meeting. Unfortunately neither of these experts were available for various reasons, and when they became available I was still handling the trigger rates problem. So I was submitting analysis jobs to the GRID (the huge internationally distributed network of computers designed for handling analysis jobs) and hoping they would work. At around 2am last night, after failing to get things to work for the fifth time I gave up and went to sleep. Then I awoke suddenly at 7am with a revelation! All I had to do was a simple search function to find out where the problem came from, change a few lines and it should work fine. I resubmitted the jobs and went back to bed. Then this afternoon, with a few precious hours left before the deadline I had to get the output, rescale it properly and perform a fit to count the QCD events. This part had never worked for me in the past, and I’d already failed to show results of the study twice. With 10 minutes left before the start of the meeting I changed a line of code, pressed enter and it worked perfectly! After more than two weeks of struggling with GRID jobs, ROOT fits and late nights it all fell into place very quickly.

Science should always be this much fun!

Now spending that time on a study of the QCD events meant that I couldn’t spend it on other areas of the analysis. I would have liked to see if I could get a better efficiency with a different choice of trigger (picking out events with b-jets instead of missing energy.) I was speaking to a colleague and he was under the impression that this was an easy change to make and would give me better results for very little effort. In a sense he was right, I’d just change one line of code and get more events. But the problem comes back to working with other people. My group is just one small part of a larger collaboration and most of our time is spent coordinating our efforts. We perform cross check upon cross check to make sure that we agree on various definitions and values. It’s necessarily tedious, but it makes sure that we get it right. So if I was to change the choice of trigger I’d have to then convince the other groups that this is a good idea, perform all kinds of studies and then perform cross checks with other people. That means weeks of work and the deadlines for the European Physical Society (EPS) are rapidly approaching! On the one hand I want to see our work presented at EPS, but on the other hand I want to spend time improving the analysis. There just isn’t enough time to do both, so I had to make the decision to pursue EPS. It’s frustrating all on its own, but when a colleague doesn’t see the dilemma it’s made all the more worse. After all, physics is fun! We all love to play around with different ideas and see how we can best improve our results. But bureaucracy is a necessary part of getting a respectable publication, so we must practice some restraint and spend time getting it right. It’s a bit like going on a road trip, only to get stuck in traffic! We’re moving forward slowly and safely, but I want to speed up and see the exciting parts.

So now that these stumbling blocks have passed can I relax and enjoy the fruits of my work? Well I can for one evening. Tomorrow I need to get started on the next project- a high profile talk to the Trigger experts on another part of my service work. That involves all the things that make this work more complicated. I need to coordinate responses from lots of people, each with their own agenda and niche knowledge, and then running jobs on computers (without getting in trouble with the IT admin!) Don’t get me wrong, I enjoy working with people and I enjoy it when the computers work properly, but it can quickly lead to information overload, and when it’s for a high profile talk it can also be stressful. On the other hand it indicates that this work is coming to an end and I can focus on something else such as control room shifts (and more blogging!)

It’s hard and at times it’s frustrating, but when it works properly physics is amazing. Days like today make the weeks that precede them worth while. I just hope I get more days like today and less stressful ones this summer! With so many interesting results around the corner I want to be able to enjoy them all and have time to make the most of one of the most exciting years in the history of high energy physics.