Archive for WIMP

Has LZ seen a WIMP?

Posted in Bad Statistics, The Universe and Stuff with tags , , , , , , on September 1, 2026 by telescoper

One of the candidates for the dark matter that seems to pervade the cosmos is a Weakly Interacting Massive Particle (WIMP, for short). Such particles are hypothetical but, if they exist they could have been produced in large numbers in the early Universe. Among the possible WIMPs are supersymmetric particles, but there are alternative candidates. Whatever their nature it is possible that they could be directed directly in collisions with regular matter but, because they are weakly interacting, the cross-section for such events is low and potentially masked by background events caused by radioactive decays. So far these experiments have drawn a collective blank (and a number of false alarms).

One experiment trying to detect dark matter directly is LUX-ZEPLIN (LZ for short) which is based at the Sanford Underground Research Facility in South Dakota in the USA but is a collaboration of 30 institutions worldwide, including the the UK, Portugal and South Korea as well as the USA. This experiment tries to detect WIMPs using an ultra-sensitive detector made of 7 tonnes of liquid xenon to hunt for signals of their interactions with atomic nuclei.

Today there was a potentially exciting announcement that LZ may have seen a WIMP, in a preprint with the following abstract:

There is also a scientific presentation by Sam Eriksen from which I have extracted the crucial figure(s):

The event causing the excitement was detected over three years ago, on 16th June 2023. You can see it as the little black dot towards the upper right of the dashed square in the left hand panel. The LZ signal does appear unlikely to have been generated by known background events, but that does not necessarily mean that it is likely to be WIMP. The statistical significance quoted is not sufficiently high for it to be regarded as a clear detection; the implied odds are roughly the same as the odds that you, the reader, have a birthday on 16th June…

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Lux et Veritas

Posted in The Universe and Stuff with tags , , , , on October 31, 2013 by telescoper

There’s an important and interesting paper just out on the arxiv by the Lux Dark Matter Collaboration. Here is the abstract:

The Large Underground Xenon (LUX) experiment, a dual-phase xenon time-projection chamber operating at the Sanford Underground Research Facility (Lead, South Dakota), was cooled and filled in February 2013. We report results of the first WIMP search dataset, taken during the period April to August 2013, presenting the analysis of 85.3 live-days of data with a fiducial volume of 118 kg. A profile-likelihood analysis technique shows our data to be consistent with the background-only hypothesis, allowing 90% confidence limits to be set on spin-independent WIMP-nucleon elastic scattering with a minimum upper limit on the cross section of 7.6×10−46 cm2 at a WIMP mass of 33 GeV/c2. We find that the LUX data are in strong disagreement with low-mass WIMP signal interpretations of the results from several recent direct detection experiments.

For those of you not up with the lingo, a WIMP in this context is a Weakly Interacting Massive Particle, one of the preferred candidates for the dark matter that most cosmologists think pervades the Universe.

The most important thing about the LUX results is that they pretty much exclude results from previous experiments, especially DAMA/LIBRA, that have claimed evidence for dark matter particles at low mass (i.e. 6-10 GeV WIMPS): LUX had expected 1550 dark matter events if the other detections were valid, but could not claim any events that were not consistent with background. They also set new limits on higher mass dark matter, which is 20 times better than previous limits. These new limits are from 85 days of running the experiment; further results will be reported after an additional 300 days in 2014/2015, when the results will increase the sensitivity by a factor of five or so.

So the question is, if LUX is correct, what on Earth is going on at DAMA? Answers on a postcard, or through the comments box, please!

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