Xenon Looks for Modulating WIMP Signal in Electron Recoils

Xenon has a new paper out looking for an annual modulation in the spectrum of electron recoils in their detector. This is a potentially interesting work because it tests an alternative explanation of the DAMA signal. If instead of interacting with nuclei, WIMP dark matter interacts with electrons, DAMA might still see a signal while most direct detection searches, which look exclusively for nuclear recoils, won’t see anything. Electron analyses are difficult with most low-background dark matter detectors because things like beta decay products will contaminate the signal. Their result clearly seems to exclude the electron recoil hypothesis for the DAMA result, although they also appear to exclude 0-amplitude at over a 99% confidence level. The best phase, however, is quite far from the expected phase of a dark matter signal, so this may not be anything interesting in the end.

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Boston Drops Olympics Bid

Boston has dropped its bid for the 2024 Summer Olympics. The plan was never particularly popular with the people who actually live there, and the mayor refused to sign the host contract due to concerns about possible cost overruns. Given Boston’s experience with the Big Dig, this was probably a financially prudent decision. Now it looks like Los Angeles is going to try to take over the US bid for 2024.

Super-Kamiokande to Be Upgraded

The Guardian has reported that some upgrades are coming for Super-K, the water Cherenkov neutrino detector in the Kamioka mine in the mountains of Japan. In particular, a small amount of gadolinium will be added to the water. Gadolinium is notable in particle and nuclear physics for its huge thermal neutron capture cross section (Gd-157 has a capture cross section of 2.5×105 barns for thermal neutrons according to ENDF tabulated data). Adding gadolinium to a detector allows it to tag neutron captures on gadolinium using gamma rays emitted in the capture & de-excitation process.

In antineutrino charged current interactions on nuclei, one of the signals of interest to Super-K measurements, protons are converted into neutrons while the antineutrino is converted to a charged lepton. If only the lepton is seen, then Super-K will typically not be able to distinguish neutrinos from antineutrinos since it can’t reconstruct the sign of a charged particle. Neutron tagging with gadolinium will allow Super-K to select an antineutrino-enhanced sample of events. Neutrons can still be found in neutrino events so a pure sample is probably not really possible, but I also haven’t read all the papers to see exactly how much this will enhance the analysis.