The 30 keV band empty, one flash at 248 keV
LUX-ZEPLIN, in South Dakota, ran its search for weakly interacting massive particles — WIMPs — below about 30 keV. That band is the one chosen under the simplest assumption, that the particle bounces off a single nucleon in the xenon nucleus. Nothing turned up there. LUX-ZEPLIN then reanalysed the same first 220 days of data at higher energies. What came back was one interaction, at about 248 keV. That energy sits well above the 30 keV window. The empty low-energy search and the single high-energy candidate belong to the same 220-day set. Those 220 days are the first slice accumulated by 7 ton of liquid xenon more than a kilometre underground; the high-energy candidate in that slice is a single event. The 30 keV window the WIMP search usually occupies and the 248 keV candidate sit side by side in the same tank, over the same span, as the output of two separate scans. There is no pile at low energy. There is no pile at high energy either; what stands is one flash.[1]
LUX-ZEPLIN uses a 7 ton tank of liquid xenon more than a kilometre underground. When an incoming particle strikes a xenon nucleus, it produces a flash of light that the surrounding detectors use to reconstruct the energy. That is the apparatus LZ spokesperson Richard Gaitskell describes: the tank reports an energy. 248 keV is the number built from that light. The reading called a WIMP candidate is an interpretation placed on that number. In 7 ton of xenon, more than a kilometre down, across 220 days, that energy appeared once. The flash of light carries the place and the size of the strike. A particle's identity is absent from that flash. The quantity LUX-ZEPLIN has put in public is this reconstructed energy. The tank in South Dakota is 7 ton of xenon placed more than a kilometre down to keep out other types of particles. The 220-day slice is the first look accumulated at that scale.[1]
2.6 sigma, short of the 5 sigma line
The signal at LUX-ZEPLIN sits at 2.6 sigma. That is a hint with roughly a 200 to 1 chance of being a background fluke, far short of the 5 sigma physicists require. LZ spokesperson Richard Gaitskell noted that a 100 to 1 event turns up fairly often across many experiments. The team presented the analysis at a conference. The paper is not yet out. 2.6 sigma in this form is a public conference result. The 5 sigma line has not been crossed. Gaitskell's 100 to 1 reminder is a plain account of how this class of event looks when many experiments are run. 200 to 1 and 100 to 1 sit in the same order of magnitude; 5 sigma stands as a threshold far beyond that order. The conference talk places a 2.6 sigma hint on the table. Until the paper is there, the quantity in public is this hint.[1]
An event at 248 keV in LUX-ZEPLIN would require coupling to the whole xenon nucleus and a particle more than 200 times the mass of a proton, if the candidate is a WIMP. That sentence is a WIMP reading of the energy. There is no second measurement. A background process that deposits about 248 keV in the same tank produces the same flash of light. What LUX-ZEPLIN reports is the energy reconstructed from that flash. WIMP mass and whole-nucleus coupling are a model placed on that energy. The empty scan below 30 keV sets the order of the move to higher energy. A WIMP identity for the 248 keV candidate does not follow from that empty scan. 200 times a proton is the mass scale of this reading. 2.6 sigma is where the statistical hint attached to that scale now sits.[1]
The remaining two-thirds looks at the same energy
The analysis covered about a third of the data the detector has already collected. The remaining two-thirds is the portion LUX-ZEPLIN already holds and has not yet shown in this high-energy search. Wick Haxton, who was not involved in the analysis, said a few more data points are needed before a discovery claim, and that the team now knows where to look. A few more events in the same energy range would be a pile that can move 2.6 sigma toward 5 sigma; no such pile is in hand. Other dark matter detectors around the world have their own data. The figure in public is LUX-ZEPLIN's single 248 keV event in the 220-day slice. A third and two-thirds are two pieces of the same tank's same accumulation; the second piece has not yet been published in this 248 keV window. Haxton's remark on where to look points at the region around 248 keV. The remaining slice LUX-ZEPLIN already holds is the place looking at that region.[1]
If the remaining two-thirds shows further events near 248 keV, 2.6 sigma can move. If that energy stays empty, the singleton sits in line with the 100 to 1 class Gaitskell named. Until the paper is out, the measurement is this: one event, one energy, a significance that has not crossed 5 sigma. The rest of the data LUX-ZEPLIN already holds is the place of the next look. The WIMP reading remains a hypothesis. 2.6 sigma stands short of 5. The single 248 keV candidate in the 220-day slice is what the remaining two-thirds will test. If further events pile up in the same region, 2.6 sigma shifts; if they do not, Gaitskell's 100 to 1 class stays on the table. That distinction belongs in a journal paper. That paper is not yet there. What LUX-ZEPLIN has given the public is a single 248 keV flash at 2.6 sigma.[1]