Quark-gluon plasma in nuclear collisions

5 problems

• Does the chiral magnetic effect occur in heavy-ion collisions?

  Open

  In the strong magnetic field of a heavy-ion collision, regions with unequal numbers of left- and right-handed quarks should push positive and negative charges in opposite directions along the field, an effect called the chiral magnetic effect. Measured charge-separation signals can so far be explained by ordinary background effects.

• How the quark-gluon plasma becomes a fluid so quickly

  Partially resolved

  Fluid models describe heavy-ion collisions starting about $3 \times 10^{-24}$ seconds after impact, which seems too early for the matter to have settled down. How the system starts behaving as a fluid so soon is not fully understood.

• Do jets lose energy in proton-nucleus and high-multiplicity proton collisions?

  Partially resolved

  In large collisions, fast quarks lose energy crossing the plasma, which suppresses high-momentum particles. This suppression has now been seen in collisions of oxygen nuclei, but whether it also occurs when a proton hits a nucleus, or when two protons collide, is not established.

• Origin of the large global spin alignment of $\phi$ mesons

  Contested

  In off-center heavy-ion collisions the produced matter rotates, and spinning particles should line up slightly with the rotation. STAR found that $\phi$ mesons (strange quark plus strange antiquark) line up far more than expected, while $K*0$ mesons do not.

• Why small collision systems show collective flow

  Contested

  When two lead nuclei collide, the produced particles flow together as if from a drop of liquid. The same flow pattern appears in collisions of single protons, where few expected a liquid to form.