Author: Stephen Sekula

Steve Sekula is an Assistant Professor of Experimental Particle Physics at Southern Methodist University. He earned his Ph.D. from the University of Wisconsin-Madison in 2004, and currently spends most of his time either conducting research into the Higgs boson on the ATLAS Experiment or teaching. Oh, and he blogs. We all have flaws.
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The work begins anew

I’ve been thinking a lot about cosmology. This is primarily because Jodi is teaching an elective course  for undergraduates this semester. I’ve been sitting in on her lectures and doing the homework so that I can learn more about the subject. What’s been great about it is that I can connect cosmology directly to the particle physics that I love so much. For instance, in a recent homework problem we had to compute the equation of state for a cosmos dominated by a gas made of one kind of particle. Not only was this a fun exercise for thinking about the universe, it also taught me how to arrive at the “matter-dominated” or “radiation-dominated” universe.

A concordance of experimental astrophysics and cosmology has created a rather interesting picture of the real equation of state for our real universe. Observations of baryon acoustic oscillations (BAO), ratios of light elements (related to their production in big bang nucleosynthesis, or BBN), supernova observations, and measurements of the cosmic microwave background have revealed a universe that is dominated not by matter, not even by radiation, but instead by dark energy [1]. Whatever dark energy is, it appears to exert a negative pressure, accelerating the expansion of the universe.

The WMAP satellite [2], launched earlier in this decade, gave us the most stunning pictures to date of the cosmic microwave background. Its ability to render in fine detail the acoustic structure of the CMB allowed us to think about what kinds of players were at work in the early universe, just a few hundred thousand years after the big bang. Like a symphony tuning up and then frozen all at once in time, the tone and timbre of the instruments have been revealed. And yet, we do not know the nature of the instruments. We hear them in the CMB, yes, but we know nothing of their size, shape, number, or other properties.

In order to go deeper, a European experiment called Planck was launched early in the summer. Just this week, it reported its first data from scanning the CMB [3]. Five years of WMAP data were just made available in the last year, and here already we have another experiment out to measure the finer detail of the CMB. It’s refreshing to see one experiment entering its later years as a new experiment comes on line. This means that there won’t be a significant gap between the last and current generation of experiment, and the science can proceed. It also points to the strength of the European space and science programs.

So the work begins anew, and I hope that we soon will learn new things about the cosmic symphony frozen in the CMB. Will those discoveries point the way toward the nature of dark energy? Good news: we’ll know sooner than later.

[1] “Five-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Interpretation,” E. Komatsu, et al., 2009, ApJS, 180, 330-376

[2] http://map.gsfc.nasa.gov/

[3] http://news.bbc.co.uk/2/hi/science/nature/8260711.stm

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Accelerate for America

I was recently a participant in a welcome event for pre-med students at SMU. The event started with a panel discussion, including members of the science faculty at SMU. It ended with a social event outside the auditorium. Jodi and I participated in the social event (the panel discussion was packed and well underway by the time we showed up) and met a bunch of pre-med undergraduates. Several of them were interested in what they would get out of taking physics (one made the common remark about having a bad experience in high school physics). Jodi and I took a holistic approach – discuss the exciting reasons for doing physics, but emphasize the applications of physics across boundaries.

For instance, I enjoyed mentioning the role that subatomic particle accelerators play in modern cancer treatment. The statistic we used to bring to Washington D.C. was that 11,000 cancer patients every day are treated with medical accelerators. I mentioned the fact that subatomic charged particles are attractive because some of them can penetrate the body, doing little damage as they enter, and then dump all of their energy into the tumor. It’s like a guided missile for cancer.

There was definitely surprise from some of the students that physics plays this role. It was a pleasure to take this opportunity to start broadening them a little. Maybe they’ll take physics and hate it, or maybe they won’t take it at all. But the truth is that physics, like many other sciences, plays a central role in our lives. From medical imaging using antimatter (PET scans) to cancer therapies, physics is in all sorts of places. Let’s not forget the central role that classical mechanics plays in sports medicine (Newton, after all, can teach us a lot about bone breaking, shock and impulse on joints, and many other mechanical stresses on the body).

I learned today about www.acceleratorsamerica.org,  a website devoted to promoting a symposium about accelerators and their role in the future of the U.S. From manufacturing to medicine, particle accelerators are playing an increasing and important role in our world. How does the U.S. remain viable in the development of this technology? These and other questions linger over this enterprise.

The first patient to receive radiation therapy from the medical linear accelerator at Stanford.
The first patient to receive radiation therapy from the medical linear accelerator at Stanford.

I like to look back at a haunting but uplifting photograph that a good friend of mine showed me many years ago. In it, a small boy is seen sitting, almost casually, under the watchful eye of a particle accelerator [1]. The black and white photo is of the first person ever treated by a particle accelerator in the Western Hemisphere. The article referenced here tells us,

[Henry] Kaplan and [Edward] Ginzton, PhD, professor of electrical engineering and of physics, developed the first medical linear accelerator in the Western Hemisphere, installed at Stanford-Lane Hospital in San Francisco. In January 1956 the machine was ready to be used on their first patient, a boy with retinoblastoma in his one remaining eye after surgeons had removed the tumor in the other eye. Destroying the tumor while sparing the eye would have been impossible with earlier, less-focused radiation sources.

“I will never forget the puzzled look on the face of the garage owner down on Fillmore Street when I asked him to borrow a heavy-duty automobile jack,” recalled Kaplan in the book. “I explained that it was to carry a large block of lead with a pinhole in it, to enable us to position that pinhole day after day for six weeks directly opposite the tumor in the boy’s eye while missing the lens and cornea.”

As in all things in learning, don’t forget that learning in a narrow lane won’t give you the creativity you need to do great things. Expanding yourself a little, challenging your educational comfort zone, leads to greatness. If physics kept to physics, biology to biology, chemistry to chemistry, how many important medical procedures and treatments would have been missed? How many lives lost?

A little physics can go a long way. Heck, it might save a life.

[1] http://news-service.stanford.edu/news/2007/april18/med-accelerator-041807.html

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Extraordinary Claims

In the spring of 2008, just months after the last electron-positron collisions at PEP-II, the BaBar collaboration announced discovery of the long-sought ground-state of bottomonium. The evidence seemed overwhelming – a “big peak” at 9389 MeV/c^2 in the mass spectrum recoiling against a photon. The strategy was not new; the idea of looking for the ground state, the eta_b, using Y(3S) → photon + eta_b had been around a long time. What was new was the unprecedented data set available to perform the search and the innovation of the analysts leading the search. I get to say the latter; I had the pleasure of coordinating and reviewing their work [1].

Not half-a-year later, BaBar announced confirmation of the discovery using the related process Y(2S) → photon + eta_b. While it seemed that this was the last word, the discovery that sealed the case for discovery of the ground state, the pure scientist in me wanted one more thing. I wanted to see independent confirmation from a different experiment. Back-of-the-envelope calculations suggested that either Belle or CLEO could achieve this with data they had already taken, albeit with lower significance then that achievable at BaBar.

Extraordinary claims require extraordinary evidence. The claim by my own experiment is not exempt from this basic tenet of science. A conversation last fall with a Cleon (a member of the CLEO collaboration) colleage suggested CLEO was hot on the trail; it would only be a matter of time before they had something to say.

Well, they are speaking! Just days ago, Kamal Seth represented the CLEO collaboration at the BEAUTY 2009 conference and unveiled the results of their search for the eta_b (paper forthcoming) [2]. CLEO took their own clever path on this analysis, just as we took a path that varied from previous searches. They measured the mass of the eta_b to be (9391.8 ± 6.6 ± 2.0) MeV/c^2. They used the process Y(3S) → photon + eta_b, and they found the rate at which this occurs to be (7.1 ± 1.8 ± 1.1) × 10^−4. BaBar and CLEO appear to agree very well on how often this happens, and on the mass of the eta_b.

The saga of the eta_b continues to be rewarding. Computations of hadron properties are being improved by the knowledge of the eta_b mass [3]. There are still questions about the nature of the eta_b, since nobody has yet measured any of its decay modes. What will the future hold? The drive to map out the bottomonium system continues. This program is accompanied by a parallel program of searches for physics beyond the Standard Model. Who knows what we’ll discover?

[1] http://www.symmetrymagazine.org/cms/?pid=1000655

[2] http://beauty2009.physi.uni-heidelberg.de/Programme/talks/thursday-session1/seth.pdf

[3] Search for papers citing the original BaBar eta_b discovery