In a tank holding three tons of liquid xenon, buried deep underground, something stirred one day. Not the xenon itself, but one of its atomic nuclei: it received a kick with an energy of 248 thousand electronvolts and recoiled like a billiard ball struck by an invisible cue ball. The LUX-ZEPLIN detector, LZ for short, recorded that kick, and since then physicists have been scratching their heads: who was that invisible player?
The traces don't match any known background process, neither cosmic rays nor radioactivity from materials. So a group of theorists proposed a candidate: a particle named the higgsino. In supersymmetry, every known particle has a heavier partner, and the higgsino is the partner of the Higgs boson. This particle is invisible and barely interacts with ordinary matter, but occasionally it can strike an atomic nucleus and make it recoil.
For such a kick to happen, the higgsino must have the right mass and a particular internal configuration. Calculations show that if the higgsino mass is around 1 TeV (about a thousand times heavier than a proton) and the energy difference between its two states is 350 keV, then the interaction strength with the nucleus falls right into the corridor of values that LZ derived from observations. In other words, a higgsino with these parameters could leave exactly the trace seen in the detector.
There is a catch, though. A more precise calculation of the higgsino mass, based on how much dark matter has remained in the universe since the Big Bang, gives about 1.1 TeV. With that mass, the higgsino prediction falls just below the lower edge of the LZ corridor. The authors admit that their estimate does not account for how the shape of the recoil spectrum changes, and a full analysis of the data is needed for a final conclusion. If the mass splitting is increased slightly, the prediction falls back into the corridor. For now, this is not a discovery but a tantalizing lead.
The invisible guest left a fingerprint, but identification is not yet complete. A single event is too weak a signal to confidently say it is the higgsino. More data, more kicks in the xenon, are needed to distinguish a rare fluke from a pattern. If the trace is confirmed, it would be the first direct hint of supersymmetry and would simultaneously pin down the mass of dark matter. Meanwhile, the detector keeps listening into the darkness, waiting for another knock.
The energy of the event, 248 keV, is about a million times smaller than the energy carried by protons in the Large Hadron Collider, yet such low-energy nuclear recoils could reveal the presence of heavy dark matter particles.