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highmultiplicity

High multiplicity refers to events in high-energy collisions that produce a large number of final-state particles in a single interaction. In collider experiments, multiplicity is often quantified by the number of charged particles produced within a defined region of phase space, such as a pseudorapidity interval. High-multiplicity events are of particular interest because they probe the regime of high parton density and large energy deposition, and they can reveal collective or emergent phenomena not evident in more typical events. These events occur in proton-proton, proton-nucleus, and nucleus-nucleus collisions, with their prevalence increasing with collision energy and system size.

Experimental measurements focus on observables such as the charged-particle multiplicity N_ch, the differential multiplicity distribution P(N_ch),

Theoretically, high multiplicity tests involve quantum chromodynamics in dense parton systems and the modeling of multiple

and
the
density
of
particles
as
a
function
of
pseudorapidity,
dN_ch/deta.
In
heavy-ion
collisions,
centrality
classes
correlate
with
the
degree
of
overlap
of
the
colliding
nuclei
and
with
higher
multiplicities.
Correcting
for
detector
acceptance
and
efficiency,
unfolding,
and
pile-up
effects
are
essential
for
accurate
interpretation.
High-multiplicity
data
are
also
used
to
study
correlations
and
collective
behavior,
including
flow-like
signatures
and
long-range
correlations,
which
challenge
traditional
expectations
from
simple
superposition
of
independent
nucleon-nucleon
collisions.
parton
interactions,
saturation
effects,
and
hadronization.
In
heavy-ion
physics,
such
events
are
central
to
investigations
of
the
quark-gluon
plasma
and
its
possible
collective
dynamics,
while
in
small
systems
they
contribute
to
debates
about
the
origin
of
apparent
collectivity.
Event
generators
and
phenomenological
models
are
tuned
to
reproduce
multiplicity
distributions
and
related
observables,
making
high-multiplicity
measurements
a
key
input
for
understanding
strong
interaction
dynamics.