You have one (1) dark matter particle detection

There is a press release from the LZ collaboration claiming to have detected one WIMP.

LZ is an amazing experiment. They’ve amassed ten metric tons of super-pure liquid xenon to be the target nuclei for WIMP detection. That’s $\sim5 \times 10^{28}$ xenon nuclei waiting for a WIMP to scatter off them. LZ is super-cooled to near absolute zero and shielded by multiple layers of material to exclude non-WIMPs from entering the active part of the detector. The biggest shield is the Earth itself, as the experiment is conducted over a kilometer deep in the Sanford mine. The technology is amazing. The results, not so much.

Here is the apparent detection from Fig. 4 of their just-released preprint⁺.

LZ is an incredibly sensitive detector, so a huge part of the program is to reject uninteresting events. Many can be shielded, but there’s always something that gets through, including radioactive isotopes of xenon* in the detector itself. Hence there are many events that fall into an uninteresting part of parameters space (those that track the blue lines): these are expected backgrounds. To give a flavor for all the many things that still show up, here is their Table 1 listing them all:

All the fitted events are nicely consisted with those expected. Then, there at the bottom, is the one ($1^{+1.4}_{-0.7}$) event consistent with being a WIMP.

Do I find this convincing?

I mean, come on. One event? I don’t know what statistics they’re smoking to come up with their error bars, but simple counting statistics makes it the square root of the number counted. You can’t beat that. So they have one ($1 \pm 1$) detection. A $1\,\sigma$ result. Hardly the $5\,\sigma$ that is the usual standard to claim a detection. That’d take 25 WIMPs, even if we believe all the background rejections.

We’ve been down this road before. Back in 2008-2009, there were months of speculation surrounding the planned unblinding of the CDMS experiment. People were sure we’d finally detect WIMPs. Excitement mounted until the day of the announcement. And lo and behold! On that occasion, there were two ($2\pm1.4$) WIMPs. WIMPs with properties that have not been reproduced, so that $1.4\,\sigma$ result went away, as low significance results usually do. I expect this “detection” will follow the same trajectory. That’s what experience teaches.

It is customary to acknowledge grants that support one’s research, so I recount here part of what they list as going into this:

Funding for this work is supported by the U.S. Department of Energy, Office of Science, Office of High Energy Physics under Contract Numbers DE-AC02-05CH11231, DE-AC02-76SF00515, DE-AC52-07NA27344, DE-SC0008475, DE-SC0010004, DE-SC0010010, DE-SC0010072, DE-SC0011702, DE-SC0012447, DE-SC0012704, DE-SC0014223, DE-SC0015535, DE-SC0015910, DE-SC0018982, DE-SC0019066, DE-SC0019193, DE-SC0020216, DE-SC0025446, DE-SC0025629, DE-SC0026014, DE-SC0026544. This research was also supported by U.S. National Science Foundation (NSF) ; the UKRI’s Science UKRI2842, UKRI2843, UKRI2844, UKRI2847, UKRI2851, UKRI2852; the Portuguese Foundation for Science and Technology (FCT) under award number PTDC/FIS-PAR/2831/2020; the Institute for Basic Science, Korea (budget number IBS-R016-D1); the Swiss National Science Foundation (SNSF) under award number 10001549; the European Union’s research and innovation programme Horizon Europe under the Marie Sklodowska-Curie grant agreement No 101209429. This research was supported by the Australian Government through the Australian Research Council Centre of Excellence for Dark Matter Particle Physics under award number CE200100008. 

That’s a lot of grants.

The list continues for a while beyond this, but the acknowledgements trend towards more general entities beyond just LZ, like support for Fermilab. You get the idea: this is a large, well-funded, international collaboration. That’s great; WIMPs were an obvious thing to look for. But these experiments have run their course; they’ve succeeded in failing. WIMPS are not there. At what point do we admit that and stop throwing^$ good money after bad?

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⁺Just-released probably means unrefereed, but I see no reason to doubt what they say – just what it means.

*Xenon is one of my favorite nuclei, and not just because it was the subject of my first science paper. It has seven stable isotopes, so its sort of a noble nucleus as well as a noble gas. ¹²⁴Xe and ¹³⁶Xe are not stable but practically so, with half-lives in excess of 10²¹ years. That’s 10¹¹ Hubble times. And yet, LZ is so sensitive it saw over a hundred decays by these isotopes during its active run. Amazing. If only that xenon were used for good instead of nothing.

(My early work on Na-Xe coupling, which I did not enjoy, was one tiny step among many on the path that made its use in medical imaging possible. LZ by itself uses about a quarter of global annual Xe production, and it’s not the only such experiment. I’ve heard discussions of plans to scale up dark matter detection experiments to use practically all of the xenon in the world.)

^$We’re close to hitting the neutrino background, so if funding for WIMP searches sours, look for these experiments to seek to continue operating by rebranding themselves as neutrino observatories.