Physicists in the ALICE (A Large Ion Collider Experiment) Collaboration at CERN have announced the first measurements of an elliptic-shaped flow of upsilons, bottomonium particles consisting of a bottom quark and its antiquark.
One of the first collisions of lead ions recorded by the ALICE detector. Image credit: ALICE Collaboration / CERN.
Bottomonia and their charm-quark counterparts, charmonium particles, are excellent probes of the quark-gluon plasma, a state of matter thought to have formed just after the Big Bang.
They are created in the initial stages of a heavy-ion collision and therefore experience the entire evolution of the plasma, from the moment it is produced to the moment it cools down and gives way to a state in which hadrons can form.
One indication that the quark-gluon plasma forms is the collective motion, or flow, of the produced particles.
This flow is generated by the expansion of the hot plasma after the collision, and its magnitude depends on several factors, including: the particle type and mass; how central, or ‘head on,’ the collision is; and momenta of the particles at right angles to the collision line.
One type of flow, called elliptic flow, results from the initial elliptic shape of non-central collisions.
In the new study, the ALICE physicists determined the elliptic flow of the upsilons by observing the pairs of muons (heavier cousins of the electron) into which they transform, or ‘decay.’
The researchers found that the magnitude of the upsilon elliptic flow for a range of momenta and collision centralities is small, making the upsilons the first hadrons that don’t seem to exhibit a significant elliptic flow.
“The results are consistent with the prediction that the upsilons are largely split up into their constituent quarks in the early stages of their interaction with the plasma, and they pave the way to higher-precision measurements using data from ALICE’s upgraded detector, which will be able to record ten times more upsilons,” they said.
“Such data should also cast light on the curious case of the J/psi flow. This lighter charmonium particle has a larger flow and is believed to re-form after being split up by the plasma.”
The team presented the results this month at the European Physical Society conference on High-Energy Physics (EPS-HEP) in Ghent, Belgium.
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S. Acharya et al (ALICE Collaboration). 2019. Measurement of Υ(1S) elliptic flow at forward rapidity in Pb-Pb collisions at √sNN=5.02 TeV. CERN-EP-2019-144; arXiv: 1907.03169
