The following items have been supplied relating to Rb:
Latest results and re-analysis of data from LEP experiments show revised values of Rb and Rc which have moved closer to the Standard Model predictions. However, these new figures (which were presented at the ICHEP96 conference in Warsaw) are still consistent with the alternative supersymmetric models.
Watch this space for the latest developments and measurements from LEP II (including superparticle searches)...
The latest set of numbers for Rb from Spring 1996 continue to show an intriguing discrepancy from the predictions of the standard model (SM). The Rc problem may be less severe, but various sources are not so sure that the Rb problem will be resolved easily: While a null result for sparticle searches at LEP-2 will kill off most supersymmetric attempts to explain Rb, other ad-hoc alternative (non SM) models may not be so easily dismissed. As for time scales, it seems unlikely that any new data will be ready in time for the Warsaw meeting; this all depends on how reliable the machine (LEP) is at the beginning of the new run.
Some of the latest LEP data has been presented at the spring conferences, including the latest on Rb and Rc. Last summer, the measured value of Rb was 3.7 standard deviations above the theoretical Standard Model prediction, while Rc was 2.5 standard deviations below theory. The latest numbers have brought Rc a little closer to the prediction, and the deviation is now just 1.8 st. devs. Rb also came down slightly, although it still lies significantly above theoretical expectations. If one assumes that the measured experimental value of Rc is a little off, and take it at its Standard Model value, then the experimentally measured value of Rb is only 2.9 st. devs. above the Standard Model prediction. This is still a little high...and perhaps a signal for new physics.
[N.B. The measured value of Rb depends on Rc, since charm creates a background for b-quarks, and charm subtraction is made (based on an assumed value for Rc) in obtaining the experimentally measured value for Rc.]
The reduction in the value for Rb could have implications for the strong coupling constant: If new physics increases Rb slightly, then the LEP determination of the strong coupling constant would come down. It's present value is in the vicinity of 0.123. If Rb is as large as is presently measured, the strong coupling constant would come down to about 0.105, too low compared to other measurements. On the other hand, a modestly enhanced Rb would put the strong coupling constant between 0.11 and 0.12, perhaps more in line with other measurements (although there is still a vigorous debate in the particle physics community whether the discrepancy in the data for the strong coupling should be taken seriously).
Regarding possible theoretical interpretations the standard supersymmetric models can account for an enhanced Rb, but it is very difficult to get as large an enhancement as the current data seem to imply. prediction. So, for supersymmetry enthusiasts, the observed Rb is too high. Other far less motivated theoretical models have been proposed to enhance Rb. They may not be attractive, but they shouldn't be dismissed entirely.
It could still turn out that the systematic errors of the experimental measurements have been underestimated, a possibility that only time and more investigation will resolve. Meanwhile, future measurements from the SLC could add useful information. In addition, ALEPH at LEP still has a fair bit of Rb data to process which could have an impact.
The bottom line is the LEP-2 running beginning shortly. By running at higher energy, we will see if any new particles show up (which could be contributing "virtually" to the enhancement of Rb).
Watch this space to follow the latest developments...
[News item supplied by Howard Haber - June 1996]
DELPHI have now published their preliminary result for Rb (CERN preprint ppe-96-015) but it is almost unchanged.
On Rc, however, there has been some movement with the LEP average at 1.8 sigma below the SM instead of 2.5 sigma of last summer. Rb is now 2.9 sigma high instead of 3.1 sigma of last summer (if Rc is fixed to the SM expectation): so essentially no change on Rb (since almost no new results). In fact the SM prediction moved almost as much as the average (because CDF/D0 have updated their top mass averages, moving it down from 180+-12 to 175+-9).
Probably there will be new Rb results including a significantly larger data sample for the summer conferences, Warsaw this year. The newer analyses should have significantly improved precision, so it might be four sigma this summer...
[News item received April 1996]
So far, the so-called "Standard Model" has proven a most successful tool for high energy particle physicists. The model predicts, to astonishing levels of accuracy, how nature behaves in terms of the known elementary particles (e.g. quarks and leptons) and the forces which influence them (e.g. the strong and electroweak interactions). But new results from the 27 km long underground LEP particle accelerator at CERN in Geneva show intriguing signs of a discrepancy with one of the quantities predicted by this hitherto robust model.
Speaking at the European Physical Society's conference held in Brussels early this August, Dave Charlton of Birmingham University described the latest measurements of Rb, the fraction of b quark pairs produced by Z0 particle decays in high energy electron-positron collisions. The preliminary results give Rb = 0.2219 +- 0.0039, whereas the Standard Model predicts a value of 0.2155 +- 0.0005. While this difference (which is equivalent to four standard deviations) could be a statistical "fluke" or due to some yet-to-be-accounted-for systematic effect, the possibility of exciting new physics cannot be ruled out.
One explanation put forward by theorists is that the effect could be the first experimental indications of supersymmetry, an as-yet unproven theoretical construct in which each known particle has an (as yet undiscovered) "superparticle" partner. Supersymmetric theories are attractive because they give a more complete and self-consistent description of the universe which could lead (eventually) to an all-encompassing "theory of everything". But Howard Haber, a University of California (Santa Cruz) theorist who reviewed the implications at the Brussels meeting, emphasised the need for caution: "One has to be very careful with the present measurements. There are different points of view on possible causes, but the reason for the excitement is that in principle Rb gives an indirect probe of any new effects which may show up: If supersymmetry is responsible, then we might expect to see the first superparticles being produced for real when LEP's collision energy is upgraded, perhaps even as early as the initial run this fall."
Naturally, other physicists at the conference were also cautious: "When you do so many (experimental) precision tests of the Standard Model, something's bound to give a wrong result somewhere" commented one observer. But some experimentalists were less pessimistic about tantalising hints of new physics: "Every time the statistics improve, the value of Rb hardly seems to shift, but the statistical error is reduced. The more this happens, the less likely it is that a random effect is responsible. If the discrepancy gets any more pronounced, and if no (systematic) explanation has been found, people will have to start asking whether the effect is something more important..."
Only time (and some more results) will tell.
R.W. Poultney, August 1995 [News item submitted to "New Scientist"]