Robert G. Jacobsen

Professor
Particle Experiment

Research Interests

High energy physics, particularly from the experimental perspective, is my primary research interest. In the past 10 years the “Standard Model” of high energy physics has begun to yield precise predictions of various quantities. Experimental tests of these predictions are both the way to stress test the theory, hopefully to destruction, and to learn more about the remaining unknown parameters.

The LEP collider and detectors at CERN were until recently the best place to do these experiments. At LEP, it was possible to measure the mass and width of the neutral Z boson to a few parts in 105, allowing us to confront Standard Model expectations and predict the mass of the top quark with better precision than later measurements at Fermilab. To do this, many experimental problems needed to be addressed. In particular, the performance of an accelerator more than 27 kilometers in circumference had to be understood at a very detailed level, including measuring such subtle effects as the energy change due to tides and magnetic fields from trains passing miles away. Although this effort is now winding down, its a good illustration of a general principle: Doing careful precision experiments will lead you through all sorts of interesting questions before you get to the best possible accuracy on your result.

One part of the Standard Model associated with particle masses and coupling is the “CKM matrix,” which relates different types of quarks. Precision measurement of the more inaccessible elements of this matrix is the goal of the BaBar experiment constructed by Berkeley and other institutions and now in operation at the Stanford Linear Accelerator Center (SLAC). We are working to measure and understand the parameters that control the “CP violation” phenomena, related to the basic differences between matter and antimatter. This is a long-term program, which will produce its most important results around the middle of this decade. It will involve many problems in interesting areas of detector technology, accelerator physics, and theoretical physics, plus areas not yet anticipated. The final accuracy that will be achieved is not yet clear, but it is clear that there’s a lot of progress on the horizon.

Current Projects

The BaBar detector started taking data in the summer of 1999. Since then we’ve accumulated the world’s largest and best measured sample of B meson decays, along with a large samples of charmed mesons, tau leptons and every other low-mass particle. We’ve been concentrating on understanding and improving the operation and capabilities of both the BaBar detector and the PEP-II collider so we can get as much physics as possible from our data. Based on that work, we are now starting to address some of the important physics questions. For the next few years, effort will be focused on initial measurements of CP violation in the B system. These will form the basis of the precision physics program that is our eventual goal.

Selected Publications

R. Assmann, et al. (The LEP Energy Group), “The energy calibration of LEP in the 1993 scan,” Z. Phys. C 66, 567 (1995).

“LEP data confirm train timetables,” CERN Bulletin 48, 95 (27 November 1995).

The BaBar collaboration, The BaBar Physics Book: Physics at Asymmetric B Factory, SLAC-R-0504.

The BaBar collaboration, “The BaBar detector,” hep-ex/0105044, NIM (in press).