Lead-ion collisions: the LHC achieves a new energy record
After the Bevatron (Berkeley, 1954) – which broke the energy barrier of billions of electronvolts – and the Tevatron (Fermilab, 1987) – which reached a trillion electronvolts – the LHC is now reaching the peta- (quadrillion) electronvolt level with its heavy-ion collisions (see here). However, one should remember that the average energy per colliding nucleon pair, within the 1 PeV “fireball”, is 5 TeV (compared to 13 TeV in the recent proton-proton collisions).
Heavy-ion collision events from the ALICE, ATLAS, CMS and LHCb experiments.
Two of the great particle accelerators of the past were named after the symbolic energy barrier that they broke. The Bevatron (for "billions of electronvolts synchrotron"), at Berkeley in 1954, was the first to break the barrier of a billion electronvolts or BeV (now known as a gigaelectronvolt or GeV) in the centre-of-mass, by a large enough margin to create the laboratory’s first anti-protons. Three decades later, in 1987, the Tevatron at Fermilab breached the barrier of 1 teraelectron volt or TeV, a trillion electron volts or 1000 GeV, at the centre-of-mass. The Tevatron beam energy itself was almost 1 TeV, yielding almost 2 TeV in the collisions of opposing beams.
Just under three decades since the Tevatron reached 1 TeV, the LHC has resumed its programme of colliding lead nuclei at a new energy, enabled by the work done on the LHC during Long Shutdown 1. The total centre-of-mass energy in the collisions will be 1045 TeV, breaking the symbolic barrier of a quadrillion electronvolts, or 1 PeV (petaelectron volt). However, the lead isotope accelerated in the LHC contains, besides its 82 protons, 126 neutrons that have no electric charge for the accelerating fields to work on. So, the total energy of the nucleus is shared among 208 nucleons, each of which has 82/208 or 39.4% of the energy that the LHC imparts to single protons. In nuclear physics literature, it is customary to quote the average centre-of-mass energy of pairs of colliding nucleons, which will be 5.02 TeV.
On the other hand, with all due respect to our colleagues in the experiments, this convention is a perennial nuisance in accelerator physics, where we consider the dynamics of particles to be based on a certain mass, charge and energy and the “energy per nucleon” does not appear naturally in the equations. Observant watchers of the “LHC Page 1” display will have noticed a “Z” inserted into the beam energy value to take care of this (the preceding number being the energy per charge which is the same as for protons). The same display worked neatly for both beams when we collided protons with lead in 2012 and 2013.
The SPS, for its part, has been sending lead ions at 36.9 TeV (or 177 GeV per nucleon) to the LHC and to fixed target experiments for many years.
From the perspective of the early 1950s, the energies attained by the Tevatron and the LHC would have seemed like science fiction. But thanks to breakthroughs in accelerator physics and technology in subsequent decades, they are now real. In the case of the LHC’s heavy-ion collisions, the concentration of so much energy into the tiny nuclear volume is enough to create huge particle densities and temperatures about a quarter of a million times greater than those at the core of the Sun. In this way, heavy-ion collisions recreate the quark-gluon plasma, the extreme state of matter that is thought to have filled the universe when it was only microseconds old. The LHC experiments study the collective behaviour of quarks and gluons when they form this state.
Therefore, although we are far from having the capability to collide single protons at 1 PeV (the “Pevatron” perhaps?), we can still celebrate the breaking of a new symbolic energy barrier.
by John Jowett
