March 5, 2009
In late January, we held a meeting of our Physics Advisory Committee, PAC34 to be precise.
We had two primary goals for the PAC, one related to the currently operating program, the other related to future physics after completion of the 12 GeV Upgrade Project.
Of course, with its receipt of approval to enter the construction phase, we are treating the 12 GeV Project schedule as something to live by. The shutdowns and running of the machine derive from a combination of that schedule and the annual budget. This situation leads to tension between a desire to maximize the physics from the remaining 6 GeV running and the desire to make the upgrade happen. So, we laid out our planning again to the PAC and obtained its blessing on our approach. Of course, as things can happen quickly on the floor, we cannot ask PAC members about every decision we make.
The primary goal of this PAC was to examine the new proposals for running with a 12 GeV machine. We were keen to peer beyond those things explicitly addressed by the project and see what else might be exciting our user community. The result was extremely pleasing. We received 19 proposals and several letters of intent. The final drafting of reports and recommendations is still in train, but the spokespersons are all aware of the responses from the PAC.
For me, it is interesting to try to understand if there are trends, to try and pick out the big picture.
As I might have expected, some of the hot topics from 6 GeV, such as nucleon form factors were present. So also were nucleon-nucleon correlations, which were recently featured in a CERN Courier article.
It is clear that the extra energy at 12 GeV is encouraging lots of thoughts of further exploitation of semi-inclusive deep-inclusive scattering in which a single hadron is measured and sometimes identified. With emphasis on measurements of ratios and differences there is hope for improved sensitivity to some of the remaining subtleties associated with the parton distribution functions at high x Bjorken and, in particular, to the origins of the nucleon’s spin. The PAC took the opportunity to examine its own desires. Its members basically laid out what they expect. High statistics measurements are not enough, the experimental resolutions should be exploited to display the data in all their multi-dimensional splendor. Measurements that are complementary are encouraged. Indeed, the PAC recommended a higher degree of self-organization than was displayed by the range of proposals themselves.
Since the experimental program in Hall A had been left almost pristine, it was an obvious magnet. There were proposals to initiate a series of large-aperture measurements that would exploit a suite of apparatus in different configurations. The measurements would push for high luminosity and a concomitant reach in momentum transfer.
Finally there were two proposals for major steps in the parity violation program. A measurement of electron-electron (Møller) scattering would exploit the fact that the two electrons in the final state are identical and only one need be detected. The precision obtained on the electroweak mixing parameter would be top of the line if achieved. A sister experiment has ambitions for a broader set of parity violation measurements in deep inelastic scattering.
The resources requested range from the approval to use already approved apparatus with already approved beam time to, in the case of the parity experiments, very substantial new experimental equipment. Not all the experiments will make it through to publication of a result, but the breadth and depth of the spectrum of proposals was pleasing indeed. The Upgrade was developed with considerable attention to the needs and desires of the community. Nevertheless, had the laboratory employed a “build-it-and-they-will-come” attitude, we would know that “they have indeed arrived.”