A wonderfully incoherent press release came across my EurekAlert feeds yesterday, with the headline “Particle physics study finds new data for extra Z-bosons and potential fifth force of nature.” You can tell it’s going to make no sense at all from the very first sentence:
The Large Hadron Collider is an enormous particle accelerator whose 17-mile tunnel straddles the borders of France and Switzerland. A group of physicists at the University of Nevada, Reno has analyzed data from the accelerator that could ultimately prove or disprove the possibility of a fifth force of nature.
As the largest science instrument ever built, the LHC has the science community buzzing with excitement as it may help in understanding the inner workings of Nature.
Remarkably, some of the new physics that may be studied at this $6 billion facility can be probed using low-cost experiments fitting in a typical laboratory room.
In a forthcoming Physical Review Letter article, the University of Nevada, Reno physicists are reporting an analysis of an experiment on violation of mirror symmetry in atoms. Their refined analysis sets new limits on a hypothesized particle, the extra Z-boson, carving out the lower-energy part of the discovery reach of the LHC.
These paragraphs manage to be spectacularly wrong on several fronts, starting with the fact that the LHC is not in any way involved in the parity violation experiments. What’s even better, though, is that the actual work described is theoretical– the paper in question is most likely “Precision determination of electroweak coupling from atomic parity violation and implications for particle physics”, whose abstract is:
We carry out high-precision calculation of parity violation in cesium atom, reducing theoretical uncertainty by a factor of two compared to previous evaluations. We combine previous measurements with our calculations and extract the weak charge of the 133Cs nucleus, Q_W = -73.16(29)_exp(20)_th. The result is in agreement with the Standard Model (SM) of elementary particles. This is the most accurate to-date test of the low-energy electroweak sector of the SM. In combination with the results of high-energy collider experiments, we confirm the energy-dependence (or “running”) of the electroweak force over an energy range spanning four orders of magnitude (from ~10 MeV to ~100 GeV). Additionally, our result places constraints on a variety of new physics scenarios beyond the SM. In particular, we increase the lower limit on the masses of extra $Z$-bosons predicted by models of grand unification and string theories.
This is a very nice result, but it has nothing to do with the LHC. For one thing, the LHC hasn’t even been turned on all the way yet, so there’s no way it could have provided any data for this paper. More importantly, though, this is clearly described as a new theoretical analysis of existing experimental data, a fact that the press release manages to completely obscure.
What’s really going on here is a re-analysis of a precision measurement of transitions in cesium atoms, made several years ago by Carl Wieman’s group at JILA. The idea here is that there are pairs of states in cesium that are not connected by the normal transition mechanisms. A electron in one state can not move to the other state by absorbing or emitting a single photon, because that transition would require changing the symmetry of the atomic state in a way that’s not normally allowed.
Exotic theories of particle physics provide a way around this principle, though. If certain types of hypothetical particles exist, it would be possible to make these transitions, thanks to interactions between these new particles and the one we already know about. Thus, a measurement of the probability of these transitions gives you a way to detect these exotic particles without needing a multi-billion-dollar particle accelerator.
The tricky part of these experiments is that the physical situation is very complicated. In order to put a good limit on the properties of the hypothetical particles, you need to be able to completely rule out all other effects, which means that you need to understand the physics of the electron states extremely well. That’s very hard to do, and the theoretical uncertainty due to limited knowledge of the atomic states is one of the biggest limitations on these experiments. People like Tiku Majumder at Williams have made very good careers for themselves by measuring atomic properties that can be used to test these theoretical models, in order to be sure that these parity violation experiments are interpreted correctly.
The paper in question is a new theoretical analysis of the Wieman group’s data, which allows them to improve the limits on exotic particle properties due to those experiments. What they find is in complete agreement with the Standard Model, indicating that no new physics has yet been detected. This means that one of the hypothetical new particles that could lead to parity violation, the “extra” Z boson, if it exists, must have a higher mass than the previous limit would’ve predicted.
This connects to the LHC only in a prospective way. That is, this measurement further restricts the range of parameters that these particles could have, which tells people where to look whenever they finally get the accelerator working. That’s it.
The attempt to tie these results more closely to the overblown LHC hype led the writer to say things that are manifestly untrue, making this one of the worst press releases I’ve seen in some time.