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For the first time, scientists have observed an anti-lambda particle colliding with a proton and breaking apart into lighter particles.
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In a particle accelerator, the anti-lambda particle is a rare guest. It is born on the fly, lives for a tiny fraction of a second, and usually decays on its own. But sometimes it manages to collide with an ordinary proton, and then a whole shower of light particles is born: kaons, pions. Such collisions have been spotted for the first time in the BESIII detector at the BEPCII accelerator in Beijing.
There, electrons collided with positrons to produce J/ψ mesons, which then decayed into pairs: a lambda hyperon and its antiparticle. The anti-lambda flew through the beam pipe, where hydrogen nuclei, ordinary protons, were hiding in the cooling oil. Occasionally, an anti-lambda hit a proton dead-on, and instead of two particles, four or six emerged: kaons and pions. The authors observed two such decays for the first time and found a hint of a third.
The anti-lambda is like a mirror twin of the lambda hyperon: the same mass, but opposite charge and different quantum numbers. For it, meeting a proton is like a collision of two worlds.
They also measured the probability of such encounters. It turns out that the anti-lambda behaves with a proton almost exactly like an antineutron does with a proton at the same energy. This is important: it means we can describe all antibaryons in a unified way, rather than inventing separate rules for each.
Why does this matter? In the early Universe, there were equal amounts of particles and antiparticles. Then something went wrong, and matter won. To understand how, we need to know how antimatter interacts with matter at the most fundamental level. Each such measurement is a piece of the puzzle.
The anti-lambda lives only about 0.26 nanoseconds, but in that time it travels several centimeters in the detector — enough to meet a proton.
In 'Star Trek,' the antimatter engine works by annihilating protons and antiprotons. Here, a similar process is studied in miniature, but instead of an antiproton, it's the more exotic anti-lambda.