Did Dark Matter Kill the Dinosaurs?

So asks a new post summarizing a fairly recent geology paper. To give a quick summary: Almost certainly not. We already think we know what probably caused the mass extinction of the dinosaurs, so a dark matter proposal is really not needed.

Anyway, I’ve seen things like this (and related ones like “Did neutrinos kill the dinosaurs?,” but they are mostly fanciful thought experiments or even just April Fool’s jokes. I quickly glanced at the paper referenced in the post, and I am not at all convinced by it. The basic idea is that if a lot of dark matter is concentrated in the plane of the galaxy, then as the solar system oscillates in and out of the plane, Earth should feel effects from a changing density of dark matter possibly due to large clumps. Then, if dark matter annihilates to photons or charged particles, it can collect in the planet’s core and cause extra heating, which then can cause geological events leading to mass extinctions. It appears to me that the author found some old paper on the topic from the 90s and basically wrote a paper summarizing the earlier paper while also ignoring nearly all recent literature on the subject of dark matter. I suppose you can cook up a dark matter model that could cause problems on Earth, but it probably would not be allowed by current limits. This proposal requires very dense clumps of dark matter to exist in the galactic plane in order to explain the supposedly periodic (the plot isn’t very convincing) nature of mass extinctions.

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DoE WIPP Accident Report Released

The Department of Energy has released a report into its investigation into the minor radiation accident at the WIPP site near Carlsbad, NM last year. As had been suspected, the cause of the accident was the use of an improper packing/absorbing material in a waste container. While the people preparing the waste were supposed to use some sort of clay material (basically inorganic cat litter), they instead used an organic cat litter. While these two things might seem functionally similar for most uses, that was not the case. Material reacted with the organic cat litter, eventually causing a breach in the container that led to the accident.

The BBC on the LHC

The BBC recently published a post by one of its science editors entitled “What is the point of the Large Hadron Collider?” The author goes over several justifications for such large projects. There is the idealistic view: Big science projects are important for improving our understanding of the universe, which to many is important to our society in its own right. There is also the pragmatic view: We don’t know what will come out of the research but at the very least many important inventions and a lot of technological progress have been made as a result of such projects in the past.

I would say that both points of view are compelling. The latter is obviously important, but the former is also a good point to make. A great deal of what people devote resources to in modern society is largely extraneous. We already spend a great deal of money on things that we want but don’t actually need. I would say that large science projects are a way to devote a tiny fraction of our resources to projects that in some way advance human society. Science, like literature, music, visual arts, theater, architecture, cuisine, and many other things, is an important part of what makes our society and our culture what they are today.

Measurement of the Ultra High Energy Cosmic Ray Spectrum

The Pierre Auger Observatory has uploaded a new measurement of the energy spectrum of ultra high energy cosmic rays. They looks at events with energies greater than 1018 eV. Since most cosmic rays are protons, this is actually an incredibly large amount of energy for a single subatomic particle. In fact, these are particles and nuclei with macroscopic amounts of kinetic energy. The huge amount of energy means that these particles create huge air showers high in the atmosphere. Auger measures the light signal emitted as the shower progresses as well as signals from muons hitting a massive array of water Cherenkov tanks on the ground at the detector site in Argentina.

The spectrum of these cosmic rays is actually quite important in astrophysics. The cosmic ray spectrum generally follows a power law spectrum, but the shape is expected to change around 5×1019 eV. A knee-like feature should be seen around that energy as the spectrum changes from one power law to another.

The reason for this expected ship is the so-called GZK cutoff. The cosmic microwave background consists of a thermal distribution of microwaves (photons) with a temperature of 2.7 K, or an energy of around 225 μeV. It turns out that a proton can interact with a photon to create a short-lived Δ that then decays to a nucleon (proton or neutron) and a pion. The Δs decays via the strong force, so while they have a mass of around 1232 MeV, they also have widths of over 100 MeV. This means that the lifetime is very short and so we generally refer to a Δ “resonance” rather than a Δ “particle.” The GZK cutoff represents the approximate energy at which significant numbers of protons will interact with the CMB to create Δs.

The pion in the final state after the Δ decays carries a sizable fraction of the initial proton energy. So, the proton will continue to lose energy until the energy is too low for this process to continue. As a result, the proton spectrum at energies higher than the GZK cutoff should be suppressed compared to the spectrum at energies lower than the cutoff. There is a mean free path for this process that is still quite large, so some protons might be expected at energies above the cutoff, but they should originate in some region in the vicinity (on cosmological scales) of Earth.

New Combined ATLAS/CMS Higgs Mass Result

The ATLAS and CMS collaborations have finally released a joint result on their Higgs analyses. The paper uses the two channels where the final state invariant mass is most easily calculated and where backgrounds are expected to be small (Higgs to two gammas or Higgs to four charged leptons). These two channels don’t have have very many signal events, which limits the ability of the two experiments to make a precise measurement.

Combining results from different experiments is always tricky, since the analyzers have to be sure that systematic uncertainties are treated properly. Some systematics will be completely separate for each experiment, while others may be correlated. Regardless, the new result is mH = 125.09±0.24 GeV. The measurement is still statistics-limited, but is getting closer and closer to being limited by systematics. This is quite good, but still nowhere near the kind of precision to which the Z and W masses are known. To do that at the LHC, the experiments would need to improve systematics by more than an order of magnitude as well as collect many times more data. If we realistically hope to know the Higgs mass as precisely as for the W and Z bosons, we’ll need an e+e Higgs factor collider (i.e. the ILC or something similar).

Dark Matter Candidates: Axions

In my last dark matter post, I described WIMPs. While WIMPs are a very attractive dark matter candidate, they are by no means the only one. Axions are a much more difficult particle to describe than WIMPs. As an experimental particle physicist, even I don’t understand a lot about them (though I admittedly have never studies them in detail). As usual, you can look at the PDG axion review for more detailed info (that’s where a lot of the information I’m using here is from).

Axions were first proposed by Peccei and Quinn to solve a very different problem in particle physics: the strong CP problem. It turns out that in a generic theory of quantum chromodynamics (QCD), we would expect there to a be a CP-violating term ΘGμνGμν where Θ determines the amount of CP violation. CP violation in strong interactions can be found by looking for things like the existence of the neutron electric dipole moment. If CP is preserved, this must be exactly 0. Experimental bounds have found no evidence for the existence of the neutron EDM, so the parameter Θ must be incredibly small. Such a coincidence would seem very strange to physicists, so this suggests that maybe there is something that forces this to happen. The axion is the particle arising from a scalar/pseudoscalar field added (using a new U(1) symmetry) to force QCD to preserve CP symmetry.

The axion is expected to interact with photons through an interaction term that looks like gφFμνFμν where the coupling constant g is related to the axion mass. This in particular, allows for the axion to be converted into a photon in the presence of a strong electromagnetic field. Most experimental searches try to find axions using strong magnetic fields in resonating cavities.

In the early universe, while axions can be formed through the usual thermal processes, these will lead to hot (relativistic) axions that probably won’t be a suitable dark matter candidate. However, a condensate of axions can be created during spontaneous symmetry breaking, generating a large number of non-relativistic axions. It turns out that for masses around 10 μeV the correct relic density is achieved. So, dark matter axions will have incredibly tiny masses, likely even less than neutrinos and, as for WIMPs, could be related to the solution for several open questions about the Standard Model.

Is Top Gear Over?

The BBC announced that it is not renewing Jeremy Clarkson’s contract for Top Gear. Given the popularity of the show, this really means that he’s being fired. While there have been many controversies in recent years about his comments and behavior, the proverbial straw that broke the camel’s back was an incident where he reportedly assaulted a producer. The other hosts have more or less said that they won’t do the show without Clarkson, so it seems that Top Gear may be facing an ignominious end. While this is sad for fans of the show and probably bad for the BBC’s brand, I can’t say that I blame them. They have much more of an obligation to protect their regular employees from abuse than to protect their stars/personalities from themselves. While many people are understandably upset, they really should focus their anger on Clarkson and not on the BBC, which is doing what any responsible organization would do when faced with responding to an incident of workplace violence.