proton-proton and deuteron-gold collision diagram (IMAGE)
Caption
A penchant for photons and neutral pions: PHENIX is particularly good at detecting photons (wavy lines, γ). To detect jets, PHENIX measures the photons that neutral pions (π0) — often the leading particles that make up jets — decay into. Distinguishing between direct photons — those that come from the interaction of quarks and gluons in the collision — and decay photons, like those from the decay of a neutral pion or other particles in a jet, is a complicated, but well-established and proven procedure and was essential to the new PHENIX result. The image shows the difference between an energetic jet emerging from a proton-proton (p-p) collision, where no quark-gluon plasma (QGP) forms, and a jet that gets "quenched" — or loses energy — in a collision of a deuteron (d) with a gold (Au) nucleus. Note that direct photons (lower γ) emerge unaffected from each collision. The combination of equal-energy direct photons with jet-energy loss is a clear sign that QGP is forming in central deuteron-gold collisions.
Credit
Valerie Lentz/Brookhaven National Laboratory
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