Why lead-lead collisions at the Large Hadron Collider?
Two physics communities coexist at the Large Hadron Collider (LHC). One studies proton-proton collisions - looking for the Higgs particle, top quark physics, SUSY particles - and other yet unknown collision products - and the existence of new symmetries in nature. The other community studies collisions of heavy ions, with the aim of creating droplets of matter as existed in the first microsecond after the Big Bang.
At much lower energies than reached at the LHC today, scientists have studied relativistic collisions of heavy nuclei with the goal to create, in as large a volume as possible, matter at extreme densities and temperatures. Such matter may exist in supernovae implosions or in the very early moments of the universe.
Researchers want to find the equation of state, the link between pressure, density and temperature, as well as to observe phase changes that such matter will undergo. As a reminder, water has three phases: ice, liquid and vapour.
One of the first lead ion collisions seen by ALICE
At the beginning of relativistic heavy ion physics, in the mid 70s, at the Bevalac at the Lawrence Berkeley National Laboratory the question was: could colliding nuclei produce a high enough density to create a fireball, or would they just pass through each other?
With beams of light ions, mass up to 40, the measured signals of the first experiments were not much different from proton-proton collisions; thus nothing was gained for the added complexity and difficulties with the acceleration of ions. Later, helium–helium (mass 4) collisions at the Intersecting Storage Rings (ISR) collider at CERN, also revealed no new physics.
However, in 1982-84, employing beams of niobium (mass 93) and later of gold ions (mass 197) at the Bevalac, at fixed-target energies from 200 MeV to 2 GeV per nucleon, two flow phenomena in the emission pattern of these collisions were discovered.
These flows became known as the side-splash and the squeeze-out. The side splash was in the reaction plane, whereas the squeeze out was perpendicular to it. Inelastic scattering of spectator particles determines the reaction plane. These discoveries gave the first evidence for collective phenomena in relativistic heavy ion collisions, which could not be explained by standard nucleon-nucleon cascades in the collision.
Nowadays these phenomena are called directed flow (v1) and elliptical flow (v2). These flow phenomena support the assumption that in the collision zone the participants interact with each other, building up pressure and thus forming matter of high density and temperature. Furthermore, the lambda, kaon and pion production changed in these heavy ion collisions compared to proton-proton collisions, pointing to an ongoing hadron chemistry inside this hot matter.
At higher energies (starting from the Super Proton Synchrotron [SPS], going to the Relativistic Heavy Ion Collider [RHIC] and now to the LHC) quarks and gluons inside the nucleons participate more in the collision; with the gluons becoming more important with increasing energy.
At the beginning of the SPS heavy ion program (with oxygen and sulphur beams) very few scientists expected to see flow features as seen at the Bevalac. The question was if the nucleon density would be sufficient for the formation of a quark-gluon plasma. It was thought by some that the transparency of the nucleons would rule out the formation of a quark-gluon fireball.
However, when lead beams became available at the CERN SPS, directed and elliptical flow phenomena were discovered at these energies. Later also these phenomena were seen at the RHIC at Brookhaven National Laboratory. At these high energies the elliptical flow is in the reaction plane.
The full beauty of elliptical flow has now been observed again with lead lead collisions at the LHC by ALICE. The lesson learned is that when heavy ions of mass around 200 collide, the observed flow supports the picture of a dense fireball of quarks and gluons: the quark gluon plasma.
All bodies radiate a photon spectrum according to their temperature: therefore the dense fireball also emits a thermal spectrum from which its temperature can be deduced. However, this spectrum is swamped by photons from hadronic decays, dominantly from two photon decays of neutral pions and etas. Experimental studies at the CERN SPS in the 90s yielded upper temperature limits of about 230 MeV.
In March 2010, in gold-gold collisions at the RHIC of more than 10 times higher energies, the thermal spectrum of fireball photons was clearly identified and a temperature of 370 MeV (approximately 4 trillion degrees Celsius) was determined. ALICE, now running at a 15 times higher energy, is able to study an even hotter photon spectrum. The Photon Spectrometer, made of lead tungstate crystals, will play a key role.
To summarize, the measurements of flow and of thermal photons (also thermal di-leptons coming from the annihilation of virtual photons) are key signals to study the equation of state of this extreme matter. Still missing is a measurement of the interaction volume and of the interaction time in order to fully define the equation of state.
Why lead ions in the LHC and not gold ions like at RHIC?
In the late 80s, after experiments with sulphur ions (mass 32), the projectiles for the CERN heavy ion upgrade were discussed. The nucleus needed to be spherical so that the collision geometry would be simple. For the acceleration of heavy nuclei it is favourable to have only one isotope in the injection chain.
Gold ions, with only one stable isotope, were selected for the program at Bevalac and at the RHIC. At CERN, however, the accelerator complex has a better mass separation at the early acceleration stage. Thus, the heavier, spherical nucleus of lead (208) was chosen for the heavy ion program at the SPS and has been kept for the LHC lead beam program.
One of the 'founding fathers' of ALICE, Hans Gutbrod has played a major role in the Collaboration all along its history. He was the first co-spokesman, and later the deputy spokesman for many years; led the design and construction of the dimuon spectrometer and was instrumental to the participation in ALICE of many of its institutions. Having left his managerial position in ALICE to be one of the leaders of the FAIR project, he has always kept a close eye on the experiment's development, and now on the data analysis.