Science1 publisher3 min readPublished
One LZ xenon recoil sends theorists back to the higgsino and supersymmetry
LZ's September 1 report of one high-energy xenon recoil has drawn dozens of arXiv papers, with much of the interest on the higgsino. A confirmed higgsino would be supersymmetry's first discovery, though LZ says the event could still be a blip from known physics.
The Scientist · Science desk
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What happened
- Several arXiv papers proposing the higgsino as the explanation appeared within a day of LZ's announcement.
- On September 2, Harikrishnan Ramani and a colleague reported that higgsinos trapped in the sun should have produced neutrinos at the IceCube detector, and none have been seen.
- On September 3, Tracy Slatyer and colleagues reported that a higher-energy plot in LZ's appendix lacks the extra events a higgsino at the reported energy would predict.
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Why it matters
- contradiction Slatyer's team reads the higher-energy appendix as evidence against a higgsino while LZ declines to stand behind that region, so the sharpest early objection depends on how far uncalibrated data can be trusted.
- constraint Any higgsino model that keeps the LZ fit has to account for the missing solar neutrinos, because the IceCube bound follows from supersymmetry's own prediction about annihilation in the sun.
- constraint One event with known physics still a possible source cannot establish dark matter, so the dozens of arXiv papers are fitting a single data point that may yet turn out to be background.
The case for the higgsino rests on what LZ did not see. Its detector, a large tank of liquid xenon at the Sanford Underground Research Facility in Lead, South Dakota, waits for a dark matter particle to strike a xenon nucleus and knock it loose [3]. In the reported event the nucleus recoiled with particularly high energy [5]. Most dark matter candidates would also produce low-energy recoils, and LZ saw none [5]. A higgsino would come in two mass states. It could only scatter when a collision carried enough energy to lift it from the lighter state to the heavier one, so low-energy hits would never happen [6]. "It comes pretty close to working," said Tracy Slatyer, a theoretical physicist at MIT [7].
The weak point is the denominator, which is one. Dark matter interacts so weakly with ordinary matter that hits like this are expected to be rare [4], and LZ's own description allows that its event could still be a blip from known physics processes [1]. The Science News account does not give the event's statistical significance or how many such recoils known backgrounds would be expected to produce at that energy.
Theorists moved anyway, and fast. "I got pretty excited," said Katherine Freese of the University of Texas at Austin, a coauthor of a higgsino paper submitted on September 1, adding, "First paper I ever wrote in 24 hours." [8] The objections came nearly as quickly. The IceCube argument rests on supersymmetry's own prediction that higgsinos captured by the sun's gravity would annihilate one another into neutrinos [10]. The appendix argument uses LZ's own data, but in an energy range the detector was not calibrated for [15][16]. "We didn't make any formal claims about detection or even nondetection in that event region," LZ spokesperson Rick Gaitskell, a physicist at Brown University, wrote in a text message [17].
The stakes explain the rush. Supersymmetry gives every known particle a heavier partner, and the higgsino is the partner of the Higgs boson, discovered in 2012 [11]. The idea's popularity has waned as particle colliders failed to find evidence for it [12]. "If we discover it, it would be the first discovery of supersymmetry," said Harikrishnan Ramani, a theoretical physicist at the University of Delaware [13]. Slatyer pointed to the candidate's history: "If you asked people in the year 2000, 'What's your favorite dark matter candidate?' probably a lot of people would already have said the higgsino then." [14]
I think the higgsino is a sensible hypothesis to test and a poor one to bank on. It accounts for one feature of one event [5][6]. Both checks published since cut against it, one of them on data LZ does not stand behind, and the collaboration has not ruled out known physics as the source [1][10][15][17].
What to watch
- An LZ estimate of how often known physics produces a nuclear recoil at this energy; that sets how surprising one event is.
- Further high-energy recoils in LZ or another xenon detector without accompanying low-energy hits.
- A calibrated LZ analysis of the higher-energy region in its appendix, which would decide whether Slatyer's objection holds.