Exaggerated nothing

The recent announcement for the search for dark matter (DM) at the Large Underground Xenon (LUX) experiment has prompted a number of very different responses! The New York Times online version of Oct 30, 2013 reported with this title “Dark Matter Experiment Has Detected Nothing, Researchers Say Proudly”. Pride in a non-detection! This is a turn-up for modern science and a potentially fatal for the ability to propose for more research dollars!

Phys.org is more conservative in its headline “First results from LUX dark matter detector rule out some candidates”.

So what exactly did the LUX experiment find, or not? A large tank containing 370 kg of liquid xenon is located 1500m (about 1 mile) beneath the Black Hills of South Dakota. It was designed to look for Weakly Interacting Massive Particles (WIMPs) by detecting their interactions with xenon nuclei. WIMPs are slow moving massive particles that have been proposed to make up DM. The LUX detections are made by looking for resultant photons (light) and ions in the tank due to the interaction between a WIMP and a xenon nuclei that would cause the xenon nuclei to recoil.

The 370 kg tank of liquid xenon. (Credit: Matt Kapust, Sanford Underground Research Facility)

The first paper by the LUX group  on this result (Akerib et al. 2013) gives information on the actual null result and the improvement in sensitivity over previous DM searches.

Plotting the WIMP mass versus WIMP cross section is  the best way to see the actual result constraints. The blue line is the confidence limit on the cross-section (equivalently the sensitivity to detection) of the LUX experiment against the proposed WIMP mass, showing the greatly improved sensitivity (compared to other DM WIMP experiments), especially between mass ranges 20-100 GeV/c2, with the best sensitivity at 33 GeV/c2.

WIMP mass versus WIMP cross section. The blue line shows the LUX 90% confidence limits for the WIMP cross section  (vertical axis) and the mass of the WIMP (horizontal axis).  (Credit: Akerib et al. 2013)

WIMP mass versus WIMP cross section. The blue line shows the LUX 90% confidence limits for the WIMP cross section (vertical axis) and the mass of the WIMP (horizontal axis). (Credit: Figure 5 from Akerib et al. 2013)

Some other DM experiments, such as CoGeNT and CDMS-II Si, have suggested DM signals, but the LUX result now rules these out. The LUX result taken as whole does not rule out WIMPs. It does however reduce the parameter space (specifically in mass mass) in which they might reside.

A comment by theoretical physicist Matt Strassler about the LUX result includes the following: “So why is the LUX experiment’s result so important? Well, it’s important, but not amazingly important, because indeed, (a) they didn’t find anything, and (b) it’s not like they ruled out a whole class of possibilities (e.g. WIMPs) all at once. But still, (i) they did rule out a possibility that several other experiments were hinting at, and that’s important, because it settles an outstanding scientific issue,  and (ii) their experiment works very, very well, which is also important, because it means they have a better chance at a discovery in their next round of measurements than they would have otherwise. In short: they deserve and will get a lot of praise and admiration for their work… but their result, unlike the discover of the Higgs particle by the LHC experiments, isn’t Nobel Prize-worthy. And indeed, it’s not getting a front-page spread in the New York Times, for good reason.”

The analogy (perhaps) with the LUX result was the recent search for the Higgs boson – a boson with apparent Higgs-like properties was eventually found at 125 GeV after initial exclusions across various mass ranges seemed likely to rule out its existence. LHC just kept looking.

LUX will do the same. The data set that LUX has just reported on was obtained over 85 days. Stay tuned as LUX is set to run across most of 2014 and 2015 with the hope of performing a 300 live-day experiment – that data set will be very interesting!

For more information, see:

[Glen Mackie & Sarah Maddison]
Note: original article title “The reports of nothing have been greatly exaggerated”, with apologies and acknowledgement to Mark Twain.

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