Physicists have found the strongest sign yet of a fabled four of a kind. For six decades, researchers have hunted for clusters of four neutrons called tetraneutrons. But evidence for their existence has been shaky. Now, scientists say they have observed what appear to be tetraneutrons. The result strengthens the case that the fab four is more than a figment of physicists' imaginations. But some scientists doubt that the claimed tetraneutrons are really what they seem.
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