The name of the game
April 24, 2014
Over the past several months, we have had reasons to discuss the way we have handled difficulties, such as transitions from shutdown to commissioning, the potential for future physics, and a visit from the Secretary of Energy.
But there is one subject that perhaps surpasses all others in terms of what we are expected to deliver.
Physics!
When I came to the lab in 2008, it did not take me long to hear the name Q-weak. At the time, the experimental apparatus was in construction, and it clearly represented a large effort on the part of Hall C, the experimental physics division, and of the lab more broadly. If nothing else, the target was going to be a ferocious device, and the demands on the accelerator in terms of beam quality and intensity were daunting. For the two years of running leading up to May 2012, Q-weak was usually the primary experiment on the floor.
So, come September 2013, it was with some pleasure that we welcomed the first results of the primary measurement from Q-weak, that of the weak charge of the proton. This measurement, although of only 4% of the total data set, received the full analysis treatment, and from some points of view, it was actually a setup for the full analysis. But, for me, a completed result published in a refereed journal from some fraction of the data trumps a preliminary result with systematic errors still to be evaluated from a full data set. Further, even with 4% of the data, this was the best existing measurement. Physical Review Letters featured the article, and the laboratory reputation for good physics got a very nice boost.
Around that same time, one of the quieter members of our user community approached one or two of us in the directorate with a very early draft of a possible publication of a new result in Nature. Nature has a couple of features. First, if you are into numerical estimations of Impact Factors, Nature ranks higher than Physical Review Letters (it is one of the highest ranked journals). Second, its publication criteria are special; the article must be structured in a particular way with respect to both length and content. I had never participated in such a publication, so I suspect my advice was not very useful. Nevertheless, the authors worked the issue and kept the result under wraps until earlier this year, when a few months after the initial discussion, we heard that Nature had accepted the publication. The measurement was of the classic deep inelastic parity-violating scattering (PVDIS) from deuterium. We say classic because for some of us, the original measurement by a team led by Charlie Prescott at SLAC in the seventies is a Nobel prize quality measurement that could claim to have been the first to demonstrate the unification of electromagnetic and weak interactions. This year’s measurement was made in Hall A and was about a factor of five times more sensitive than its predecessor.
The final measurement I want to comment on falls into a slightly different category. An experimental physicist likes to think that he or his colleagues are ultimately responsible for all knowledge. But, as physicists, we are not perfect. Sometimes things that seem obvious become part of the lexicon without actually being fully checked. The electromagnetic form factors based on the Rosenbluth technique nearly fell into that trap before being rescued by the polarization measurements at Jefferson Lab. The measurement that I want to call out, made by the Hall B group of the properties of the Λ(1405), turned out less dramatically. For many years, this resonance has been a well-respected member of the Particle Data Group resonances. A very clear, literally textbook measurement and publication of the spin and parity of the resonance remedied the situation for the imposter.
These physics results are all important, as are the many that I have not mentioned here; let me take the opportunity to point out to our physicist readers that the Office of Nuclear Physics and the National Science Foundation program managers repeatedly stress the need for us to send one-page descriptions of results they can feature in their reports. These three results described above were all “passed on” and have also been featured on our own web site.
