Science

Left: Scientists use the STAR detector to study gluon distributions by tracking pairs of positive (blue) and negative (magenta) pions ( π ). These π pairs come from the decay of a rho particle (purple, ρ0) — generated by interactions between photons surrounding one speeding gold ion and the gluons within another passing by very closely without colliding. The closer the angle ( Φ ) between either π and the rho's trajectory is to 90 degrees, the clearer the view scientists get of the gluon distribution. Right/inset: The measured π+ and π- particles experience a new type of quantum entanglement. Here's the evidence: When the nuclei pass one another, it's as if two rho particles (purple) are generated, one in each nucleus (gold) at a distance of 20 femtometers. As each rho decays, the wavefunctions of the negative pions from each rho decay interfere and reinforce one another, while the wavefunctions of the positive pions from each decay do the same, resulting in one π+ and one π- wavefunction (a.k.a. particle) striking the detector. These reinforcing patterns would not be possible if the π+ and π- were not entangled. Credit: Brookhaven National Laboratory

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To understand how the physicists make these 2D measurements, let's step back to the particle generated by the photon-gluon interaction. It's called a rho, and it decays very quickly—in less than four septillionths of a second—into the π+ and π-. The sum of the momenta of those two pions gives physicists the momentum of the parent rho particle—and information that includes the gluon distribution and the photon blurring effect.

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