It’s that time of year – lists of the best and the worst of 2005 are popping up everywhere. One of my favorites is published by the American Institute of Physics with their annual compilation of the Top Physics Stories of the year. The list contains 25 physics events which occured throughout 2005, ranging from the arrival of Cassini at Saturn to various exciting physics results to the announcement of the Nobel Prize. It’s a great read and a great way to catch up on the hot news from various physics subfields. It’s a standard reference for anyone who is on a departmental colloquium committee.
According to the AIP, this year’s most interesting results ranged from the development of lasing in silicon, the biggest burst ever recorded from a soft-gamma repeater, to the observation of geoneutrinos. The latter is cool: KamLAND (the Kamioka Liquid scintillator AntiNeutrino Detector in Japan) has observed the appearance of electron neutrinos orginating from radioactive decays inside the Earth, presumably from U-238 or Th-232.
The top story of the year was the quest to observe quark-gluon plasma at the Relativistic Heavy Ion Collider at Brookhaven National Lab. Theorists say that quarks and gluons become unconfined at high temperatures and form a plasma-like sea of free particles, consistent with conditions in the early universe. The search for this quark-gluon plasma has been long and frought with peril (CERN miraculously announced its discovery just before the turnon of RHIC – an announcement which later proved to be premature and a stretching of the truth). But now, after years of collecting and analyzing data, all 4 RHIC experiments have formed a concensus on the observation of a quark-gluon liquid. That’s right, not a plasma at all, but a liquid! So much for the theorists! When gold ions collide at RHIC, the detectors see a dense liquid which flows with very little viscosity. In fact, it flows so freely that it approximates a perfect liquid, the kind governed by the standard laws of hydrodynamics. Having discovered the surprising liquid nature of the free quark-gluon sea, the experiments next want to probe its properties, such as its heat capacity and its reaction to shock waves. However, RHIC has been subject to extensive budget cuts this year (run time is reduced by 61% fewer hours) and the collider may be terminated in the near future.
Lastly, one of the top 25 is from my homeground, SLAC! It is billed as the best measurement yet of the weak nuclear force. The experiment, known as E-158 (we sometimes have the custom in high energy physics of naming our experiments simply by the numbers, i.e., this is the 158th experiment to be conducted in the End Station at SLAC), carried out the most precise measurement of the weak mixing angle at low momentum transfer. The weak mixing angle relates the stength of the electromagnetic coupling to that of the weak coupling, and provides a sensitive test of our Standard Model of particle physics. This mixing angle has been measured extremely precisely at energies corresponding to the Z-boson resonance, but a full test of our Standard Model requires precision measurements of this quantity at different energies as well. Using the polarized 90 GeV electron beam from the SLAC linac, E-158 measured Moller scattering, which is electrons scattering off of electrons to form electrons + more electrons. It may sound a bit silly, but it is a very clean process from which we can precisely determine the properties of the particles being exchanged in this scattering (the photon, Z boson, and possible new heavy bosons). The accuracy of E-158’s measurement is roughly 0.6% and reaching this level of precision was an experimental tour de force. The End Station at SLAC has since been shut-down for HEP experiment due to budget constraints.
The bottom line from this “best of” list is that there is lots of exciting physics being produced!