Figure. Directed flow (v1) versus pseudorapidity for Au + Au collisions
at
intermediate centrality. Also shown are data from the NA49 experiment at
lower energy.
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Directed Flow: the First Harmonic of Azimuthal Anisotropy
If we focus on a subset of particles emerging from a collision with
about the same beam-axis velocity (or rapidity), then directed flow
represents a preferential emission towards a single direction in the
transverse plane (aligned with the plane of the colliding nuclei),
whereas the much larger elliptic flow represents a preferential
emission in two directions 180 degrees apart in the transverse
plane. Models predict that directed flow at RHIC should be almost
zero over a range of rapidities centered on midrapidity. Nonetheless,
whatever small departures from flat behavior might be observed over
this range can reveal important clues regarding stopping of the
incoming matter and space-momentum correlations, and also have
implications in the search for Quark Gluon Plasma.
This is the first measurement of directed flow (v1) at
RHIC. We observe the predicted flat region centered at zero on the
horizontal axis (midrapidity), whereas at rapidities further from
the center, the signal rises relatively steeply. We note that
v1 at rapidities within a couple of units from beam
rapidity appears unchanged as a function of y - ybeam
if the beam energy is lowered by a factor of more than 10
hence the motivation for mapping NA49 (CERN) data from the green
points (observed rapidities) to the blue points (shifted left and
right by the difference in beam rapidity between RHIC and CERN).
The 2004 RHIC run has accumulated much improved statistics, and
forthcoming analysis of these data is expected to address the
remaining interesting physics issues associated with the nearly
flat v1 region centered on midrapidity.
Related STAR papers
Azimuthal anisotropy at the Relativistic Heavy Ion Collider: the first and fourth harmonics
Phys. Rev. Lett. 92 (2004) 062301
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