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The magnet assembly for the ALICE experiment at CERN's Large Hadron Collider. Researchers using this detector have confirmed Brookhaven National Lab's early experiments finding that the primordial particle soup that appeared directly after the Big Bang behaves almost like an ideal fluid with minimal viscosity. Image: CERN |
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The
three LHC experiments that study lead ion collisions all presented
their latest results today at the annual Quark Matter conference, held
this year in Annecy, France. The results are based on analysis of data
collected during the last two weeks of the 2010 LHC run, when the LHC
switched from protons to lead-ions. All experiments report highly subtle
measurements, bringing heavy-ion physics into a new era of high
precision studies.
“These
results from the LHC lead ion program are already starting to bring new
understanding of the primordial universe,” said CERN Director General
Rolf Heuer. “The subtleties they are already seeing are very
impressive.”
In
its infancy, just microseconds after the Big Bang, the universe
consisted of a plasma of quarks and gluons (QGP), the fundamental
building blocks of matter. By colliding heavy ions, physicists can turn
back time and recreate the conditions that existed back then, allowing
us to understand the evolution of the early universe.
The
LHC heavy-ion program builds on experiments conducted over a decade ago
at CERN’s Super Proton Synchrotron (SPS) accelerator, which saw hints
that the plasma could be created and studied in the laboratory. Then, in
1999, the baton passed to the Relativistic Heavy-Ion Collider (RHIC) at
the U.S. Brookhaven National laboratory, which firmly established that
QGP could be created on a miniscule scale. This year’s Quark Matter
conference is the first in the series to feature results from the LHC.
Results
from the ALICE experiment have provided evidence that the matter
created in lead ion collisions is the densest ever observed, over
100,000 times hotter than the interior of the sun and denser than
neutron stars. These conditions allow the properties of the plasma to be
studied with unprecedented detail. ALICE has confirmed the RHIC
experiments’ finding that QGP behaves almost like an ideal fluid with
minimal viscosity. ALICE’s presentation also discussed the behavior of
energetic particles in the QGP medium.
“We
are very excited about the plethora of observables challenging many of
the theoretical interpretations,” said ALICE spokesperson Paolo
Giubellino. "The extraordinary capability of our detector to provide
detailed information about the thousands of particles created in each
collision proves to be essential for the understanding of the QGP.”
The
ATLAS collaboration has performed a comprehensive study of heavy-ion
collisions. The experiment’s analysis includes global properties, such
as the number and distributions of charged particles emerging from the
plasma, which elucidate the collision dynamics and transport properties
of the medium, as well as so called hard-probes of the medium, which
include measurements on the production of W and Z bosons, charmonium and
particle jets.
“The
first LHC heavy-ion run was a great success for ATLAS,” said
co-convener of the collaboration’s heavy-ion group, Peter Steinberg of
Brookhaven. “Combining global measurements and hard probes in LHC
heavy-ion collisions is leading to greater insight into both the nature
of the hot, dense medium and the QCD processes that lead to jet
quenching.”
Jet
quenching is the phenomenon, first reported by ATLAS last year, whereby
so-called jets of particles formed in the collision are broken up as
they cross the turbulent region of plasma.
CMS
has seen a number of new phenomena including the production of W and Z
bosons. Novel studies have been produced on jet quenching and to
characterize the behavior of matter that reproduces the extreme
conditions just after the universe’s birth. The most striking
observation from CMS is that weakly bound states of the b-quark are
heavily suppressed in lead-lead collisions. This phenomenon is important
for understanding the properties of the QGP.
“We
are entering a new era of high precision studies of strongly
interacting matter at the highest energies ever,” said CMS spokesperson
Guido Tonelli. “By deploying the full potential of the CMS detector we
are producing unambiguous signatures of this new state of matter and
unraveling many of its properties.”
CERN presents new results
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