I got email this afternoon from Andrei Derevianko, the leader of the research project badly described by the press release mentioned in the previous post. He sounds a little surprised by the whole thing (though not much more surprised than I am that my griping on the Internet got brought to anybody's attention), and explains what happened:
The original story (that I have went through with a writer) is posted here: www.unr.edu.
Unfortunately, the release writers have added introductory paragraph and the title without consulting with me (I was travelling giving talk about our result).
Now the story is being pulled out and will be re-released with corrections. Sorry for the confusion.
The story at that link does appear to have the problems fixed, so hooray for that. And, as a special bonus, he also sent some additional comments on the science, which is the important part:
With respect to the hard numbers: our limit on Z' mass is 1.3 TeV/c^2. This raises the previous upper mass limit from Tevatron collider.
LHC: there is a dedicated hunt for Z'. After all, Z' is "the second-best motivated" extension to the Standard Model.
The projected discovery reach at the LHC is 2 TeV/c^2 initially; and 5 TeV/c^2 at full luminosity.
Overall, I think it is remarkable that the low-cost atomic precision experiments/theory are capable of constraining new physics at the level competitive to colliders.
This is the "David and Goliath"-ian aspect to the story. More generally, the results from atomic parity violation are both unique and complimentary to colliders.
The price we pay is that it took us almost eight years to design sufficiently accurate computational scheme that can be run on modern computers. Roughly there is a 1,000-fold increase in computational complexity over previous computations.
I hope that clears things up, and puts the focus back on the physics here, which really is wonderful stuff. Many thanks to Andrei for the email, and for getting the story corrected.
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To be slightly pedantic: Z' means something like "any heavy neutral gauge boson beyond the Standard Model," without reference to how it couples to existing matter. So a Z' is a whole family of possible particles, not a particular one. Some Z' particles could contribute to atomic parity violation, while others might respect parity. For the particular set of couplings that they study, they find a better limit than direct searches; but, in general, this search is complementary to the direct searches and sets no limit at all on some scenarios that are constrained by colliders. Still, it's interesting.
At second glance, if I understand the paper correctly, I think this doesn't actually improve on collider physics bounds for a Z' at all. What is constrained by this analysis is what particle physicists call the "S parameter", which is some higher-dimension operator in the electroweak chiral Lagrangian (that could get contributions from a Z', or from plenty of other things). Their bound is |S| < 0.45 at 1 sigma, whereas the last time I checked the latest analysis of the LEP precision data implied |S| <~ 0.2 at 1 sigma. So it's a nice result, and it's good to see consistency between low-energy and high-energy experiments, but there is no new constraint on new physics.
Forgot to use the right HTML for "less than". Should say:
Their bound is |S| < 0.45 at 1 sigma, whereas the last time I checked the latest analysis of the LEP precision data implied |S| < 0.2 at 1 sigma. So precision collider data is still the stronger constraint. Still, it's a nice result, and good to see agreement between low- and high-energy experiments.
@onymous
#1: Numbers quoted are for Z' predicted by SO(10) GUT , as clearly stated in the introductory paragraph of the paper.
#2 & #3: The bound on the Z' mass comes from the lowest order ("tree"-level) diagram. This is to be contrasted with the Peskin-Takeuchi parameter S that characterizes new physics in higher orders (loops). Your statement "this doesn't actually improve on collider physics bounds for a Z' at all" is incorrect as the Z' bound is NOT derived from the S parameter. Please read the paper for details.
From the quoted text: Overall, I think it is remarkable that the low-cost atomic precision experiments/theory are capable of constraining new physics at the level competitive to colliders.
I'm not. There are lots of cases where low energy experiments (such as weak decays in nuclear physics or rare transitions in atomic physics) have constrained parameters of the standard model. You can probe those energy ranges by going up in energy or by going out in intensity.
Although it is an example from within high energy physics, the observation of weak mixing clearly predicted a fairly specific Z mass without being anywhere near the energy needed to make an actual Z. From that perspective, the Z was not discovered at CERN, it was confirmed.
#2 & #3: The bound on the Z' mass comes from the lowest order ("tree"-level) diagram. This is to be contrasted with the Peskin-Takeuchi parameter S that characterizes new physics in higher orders (loops).
The Peskin-Takeuchi parameter S characterizes any new physics contributing to the operator Tr(W^3_{mu nu}H^\dagger B^{mu nu}H) in the Standard Model. Integrating out a Z' at tree level is one possible such contribution.
On further thought: the Z' constraints from S come from scenarios where the Z' is mixing at tree level with the usual Z boson, so this tree-level S from a Z' differs from the tree-level Z' corrections you're considering. My apologies. There are other indirect collider constraints on four-fermion operators (sometimes referred to as "compositeness bounds") that are complementary to your bounds and to the direct Z' search; have you checked whether these are also less stringent?
Let me put a link to our e-print here again, so this thread is hopefully referenced by the arxiv.
@CCPhysicist
"There are lots of cases where low energy experiments (such as weak decays in nuclear physics or rare transitions in atomic physics) have constrained parameters of the standard model"
Agree. For the electroweak sector that we are testing there are a handful of low-energy experiments. This is usually referred to as the ``physics below Z-poleââ. Among these experiments, our probe is
(i)The least energetic (Q ~30 MeV);
(ii)The most accurate at the moment. Our error bars on the sin^2 of the Weinberg angle are somewhat tighter than those of the SLAC E158 Moller scattering experiment.
(iii)I think they are the least expensive (âphotons are cheapâ + in a typical experiment one could use as much as a kilogram of Cs atoms). I am a theorist -- please correct me if you think otherwise.
Having said that, I should emphasize that different probes are complimentary. For example, Qweak experiment planned at JLab will probe couplings to protons, while atomic parity violation probes couplings to neutrons (i.e., different linear combinations of up and down quark couplings).
As long as we are on the subject of comparison of various experiments, now we confirmed the SM running of the electroweak coupling at the least energetic reference point. So in combination with high-energy experiments, the predicted running is confirmed over 4 orders of magnitude in energy.
P.S.: Thanks for the support on the press-release thread.
@onymous.
"My apologies."
I am glad that we have sorted this out.
"There are other indirect collider constraints on four-fermion operators (sometimes referred to as "compositeness bounds") that are complementary to your bounds and to the direct Z' search; have you checked whether these are also less stringent?"
No, we have not. I will have to read about these.