Category: Observation

Tom Wolfe, in his book “The Right Stuff”, asked “What is it that makes a man willing to sit up on top of an enormous Roman candle, such as a Redstone, Atlas, Titan or Saturn rocket, and wait for someone to light the fuse?” I look now at the giants ATLAS and CMS, and realize I wondered the same thing for a long time. Sitting on my little elephant, rich in a sea of data, I have long asked a similar question: “Who is willing to sit on top of ATLAS?”

This week, I attended my first ATLAS workshop. This is part of a process, one which will be very compressed for me (in some ways): orientation and familiarization with the ATLAS experiment, as I begin to think about a future rich in hadron collider physics. Moving from the flavor frontier to the energy frontier is, like all things in science, a process. For me, the ATLAS cosmic ray analysis jamboree at LBL was a very fun first step in that process.

I obviously won’t share the particulars of what I learned at the jamboree. I will say that I am already impressed by the care and methodology that went into designing the ATLAS tracking and alignment systems. Their advantages, and their mysteries, were on display at this workshop. The technical prowess needed to self-located all the parts of such a massive detector as ATLAS or CMS comes in many forms, and I was thoroughly impressed at the creativity of many of them. I see now one class of innovator willing to strap themselves atop the risky rocket to the energy frontier, and I’m glad for the fact that they did so. Ahead of colliding beam data, it’s clear that a tremendous amount of progress has been made in calibrating the detector using nature’s beam: cosmic rays.

While the alignment and calibration impressed me in the positive direction, the computing challenge facing the LHC experiments made me twitch a bit in the other. The sheer scale of this problem, facing all member states of ATLAS, will require a level of coordination and cooperation yet unseen in our field. But I also realized during the discussions of these issues that there is a tremendous willingness to solve the problem, though the path may not be entirely clear. The brainpower that has gone into trying to peer into the future of ATLAS computing leaves me wanting to know the future, just to see if they were right.

Computing, and demands on computing, are one of those “black swan” problems. All of your experience with past computing efforts is the guide to building a plan for the future; but, just because all past computing swans have been white is no guarantee that the next one you see won’t be black. A revolution in thinking in physics may put new, unexpected demands on the infrastructure and challenge the very assumptions of the computing model. How do you remain flexible and innovative in the face of the unknown? The black swan is alive and well.

There was also discussion of first physics analyses enabled by the cosmic ray data, and while I won’t get into the details I will say that the ingenuity and creativity of my new colleagues on ATLAS already impresses me.

My last comment on this new experience has to do with a sense of familiarity. For me, walking into a 2000 person collaboration could be the most daunting social experience of my life. It is made easier by the sheer number of familiar faces and names I encountered during this workshop. Old and recent BaBar colleagues, friends I’ve made on science advocacy trips to Washington D.C. – maybe 1/3 of the people at or presenting at this workshop were known to me. That gives a person comfort, to see old friends and know that making new ones won’t be that hard.

Light the fuse, I say.

2005 was the World Year of Physics, and a number of communications efforts materialized out of that year. It was a chance not just to highlight the past and future of the field, but the people engaged daily in the work of physics. Like all other sciences, physics is a people-driven enterprise that requires a diversity of ideas, talent, and backgrounds to make progress. Communicating the humanity of the field was one of the major outcomes of that year.

A leading effort on this front was the Quantum Diaries [1], a blog-based project to get active (and younger) members of the field speaking about themselves, their work, their lives, and their trials and tribulations. Many of us discovered new colleagues, or followed friends, during the year in which that project was active. Quantum Diaries closed at the end of 2005.

The project has been re-opened now, after a three year hiatus. A new generation of bloggers has been recruited by the communications team leading this project. I invite you to follow them as their threads spread through the community.

And can I make a personal recommendation? A friend of mine is one of the bloggers. A student in neutrino physics, she’s got a great perspective on the field. Check it out: Nicole’s Quantum Diary

[1] http://quantumdiaries.org/

Last year, when the rise of the bottomonium began, I found myself returning to basic quantum mechanics in order to make sure I understood the coming landscape. A primary goal of our effort to secure the Upsilon(3S) and Upsilon(2S) data was to discover the ground state of bottomonium. But, what does it mean to be bottomonium, and how do you know how to look for the ground state?

Quantum theory, without which we cannot understand the subatomic, is needed to begin answering these questions. Let’s keep things simple. Let us imagine a universe in which there are just two fermions, attracted to one another by some force. For now, let us not concern ourselves with the nature of that force – only that it is attractive. Fermions are particles with half-integer spin – 1/2, 3/2, etc. For the sake of this matter, let us assume that, like the bottom quark, these fermions have 1/2 spin.

Spin is intrinsic angular momentum; that is, even if the particles are not in motion, they carry an irreducible internal quantum number which behaves like an angular momentum. The particle is not spinning; in fact, you can do some math and show that in order to get this behavior we call “spin” an electron would have to be spinning at a speed in excess of light. The word “spin” is a guide; it helps human to relate to intrinsic momentum, but it’s a misnomer.

There is also the angular momentum related to real motion – orbital angular momentum. Together, spin and orbital angular momentum tell us the total angular momentum of a pair of particles. The rule is pretty simple (although deriving it is, well, a long lesson). The total angular momentum, denoted by “j”, allowed for a given spin (“s”) and orbital (“l”) angular momentum state, is given by:

| l – s |  <= j <= | l + s |

What are the allowed spin states of a pair of spin 1/2 fermions? The spin is either 0 (spins in an anti-symmetric state) and 1 (spins in a symmetric state). So, s = 0 or 1.

What are the allowed orbital angular momentum states? Well, the pair could have l = 0, 1, 2, . . . With this knowledge, we can begin to map out the structure of our two-particle universe.

The bound states of the pair can have s = 0,1 and l = 0, 1, 2, . . . So, what is the total angular momentum of all of these possibilities?

The first is s = 0 and l = 0, giving us j = 0. This is the least angular momentum the pair can have, and this defines the ground state of the system. There is no lower-energy state, because this combination of total spin and total orbital angular momentum is as low as it goes. Can’t have less than zero!

What about s = 1, l = 0? Here, j = 1, and the two particles are now above the ground state – but only just. You can get here by flipping the spin of one of the particles in the pair, for instance, from the ground state.

This is fun! Let’s keep going. OK. Let’s try s = 0, l = 1. Now, again, j = 1, but here we have a state like the ground state in spin, but with orbital momentum in the pair.

Let’s do one more, our first big one: s = 1, l = 1. Now, j gets complicated. The total angular momentum can be either of j = 0, 1, 2. Refer to our formula above. | l – s | = | 1 – 1 | = 0. | l + s | = | 1 + 1 |  = 2. That mean that 1, in between 0 and 2, is also allowed, giving us three angular momentum configurations for s = 1, l = 1. A rich structure begins to emerge.

Going from our generic example of a pair of spin 1/2 particles, we can get back to our universe for a moment. Let’s pretend that the two particles are a bottom quark and a bottom anti-quark. (s,l) = (0,0) is the ground state – the eta_b [1].(s,l) = (1,0) is the Upsilon, the lowest energy vector state of the system. (0,1) is the undiscovered h_b, an analog to the eta_b but with orbital momentum. The (1,1) system are the three chi_bJ states – chi_b0, chi_b1, and chi_b2.

You can play this game with an electron and a positron, and many other particles. What you learn is that it doesn’t matter what the particles are – you know a lot just by knowing their spin, and that they bind.

Now if you want to know the MASSES of the states, things get more complicated. Mass is a function not just of the masses of the two particles, but also the force between them. Now is when things get complicated, and you have to know more about how the force binds them – how strong it is, for instance.

A lot can be learned about the world from so humble a beginning. It makes you realize just how simple knowledge can teach you deep truths about the cosmos.

[1] http://steve.cooleysekula.net/goingupalleys/2008/07/07/behold-the-elusive-ground-state-of-bottomonium/