proton-nucleus collision with pi zero pair (IMAGE)
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STAR scientists searched for signs of saturation in collisions of a proton (black) with a nucleus (multicolored). By tracking events where a pair of neutral pion particles (π0) strikes a forward detector at back-to-back positions, they select for interactions between a high-momentum-fraction quark from the proton and a low-momentum-fraction gluon from the nucleus. In large nuclei they saw suppression of this back-to-back signal. This suppression—a key prediction of models describing a saturated state of gluons—likely results from multiple gluon scatterings and recombination of abundant overlapping gluons.
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Brookhaven National Laboratory
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