ALICE, ATLAS, CMS: Large Hadron Collider Experiments Presented New Results
The three LHC (Large Hadron Collider) experiments that study lead ion collisions all presented their latest results 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.
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 US 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 100000 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.
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.
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.