A new method to accurately work out the temperature of a quark–gluon plasma has been developed by researchers at the Compact Muon Solenoid (CMS) collaboration at the Large Hadron Collider (LHC) at CERN. The technique involves looking at the behaviour of certain mesons in lead–lead collisions. While a similar result was reported last year, this latest effort is claimed to be much stronger and more statistically significant.
Cosmologists and particle physicists have long been keen to understand in what state matter existed in the primordial universe. Theories suggest that in the first few microseconds after the Big Bang, the basic building blocks of matter – quarks and gluons – were not bound within composite particles such as protons and neutrons, as they are today. Instead, they existed in a “quark–gluon plasma” – a sort of hot, dense soup-like medium in which the quarks and gluons (the carriers of the strong nuclear force) exist as free entities.
As the strong force does not diminish as the distance between quarks is increased, a very large amount of energy is necessary for the bound quarks to remain free. As a result, the QGP can only exist for very short times and at very high temperatures. When heavy particles such as lead nuclei collide in the Large Hadron Collider, a QGP could form in a number of ways. But it is not easy to tell if this extreme state of matter has formed, and it is even more difficult to measure it as it is expected to be at trillions of degrees.
Excited particles
One of the ways in which the CMS collaboration looks to see if a QGP has formed is to look at the effect its formation would have on other particles. One of the signs the researchers look out for is the sequential melting of excited states of the upsilon mesons (ϒ) – a bound state of a quark and its anti-quark – that emerges from heavy-ion collisions. It exists in three states, each of which have identical properties, but different binding energies. These are referred to as 1S, 2S and 3S. The more excited a state is, the less tightly bound are its quarks, meaning that 1S is the ground state, while 2S and 3S are loosely bound excited states that would melt more easily in the presence of a QGP.
“Here at the CMS, we can distinguish the signatures of the three states very clearly and distinctly, because of the excellent mass resolution of the CMS detector,” explains Nuno Leonardo, from Purdue University, who is a member of the CMS collaboration and was one of the leaders of this experiment. The melting of these states is actually observed as a “suppression of states” – that is, fewer mesons are produced in lead–lead (Pb–Pb) collisions, compared with the number produced in proton–proton (p–p) collisions, which are known not to produce a QGP at all, making the p–p a reference system. For the three states, the fraction of ϒ(2S) and ϒ(3S) particles produced relative to ϒ(1S) in the Pb–Pb collisions should be less than the fraction for collisions between protons, where the suppression would not exist. “This is what we exploit to measure the temperature of the QGP,” says Leonardo.
Foggy effects
Ian Shipsey, another team member and the chairperson elect of the CMS Collaboration Board, refers to the suppression as a “screening effect”. He explains that the QGP screens the quark and its antiquark from their binding forces, making them fall apart even quicker than usual. “It is a bit like two people standing close to each other in a room…they form our ϒ particle. Even if there is a fog in the room, they can see each other as they are standing close,” he explains. “But for the 2S and 3S states, they are further apart and so there is more fog between them and they cannot see each other. In this case, the fog is the QGP and the two people are the quark and its anti-quark, that now act as free particles and so do not form a ϒ particle anymore,” he told physicsworld.com. Shipsey extends his fog analogy by saying that the p–p system has no fog at all, while for the Pb–Pb system the fog was expected, and now they have the evidence for it.
To determine the actual temperature of the plasma, the researchers use models that link binding energy with the temperature and the fact that the suppression of the states becomes more pronounced at higher plasma temperatures. “We know that the 3S is the least tightly bound and so if the temperature is at a certain value, the 3S will be the first to break,” says Leonardo. Similarly, at consecutively higher temperatures, the 2S and then the 1S states would be expected to break.
Suppressed states
“With the current CMS data, we found that the 3S state is completely gone, the 2S is significantly suppressed but the 1S is very subtly suppressed,” explains Shipsey. He says that the slight suppression of the 1S state may not be due to the QGP at all, but because “the amount of 1S observed depends in part on how much 2S and 3S are present as both of these states can disintegrate forming a 1S. If the 2S and 3S are suppressed the 1S is automatically suppressed”. This means that the QGP formed is at an intermediate temperature, and not at the highest temperature theoretically expected. With the new data from 2012, the statistical significance of the researcher’s findings has increased from 2.4σ to 5σ – the golden standard for a particle-physics discovery.
To ensure the effects that they have observed are really a QGP being formed, the researchers plan to look at proton–lead (p–Pb) collision taking place at the LHC early next spring, which would serve as a middle ground. These collisions would also allow the team to ensure that the fog is produced by a QGP and not a phenomenon known as “cold nuclear effects” that could produce their own fog or screening effect. So the p–Pb system would provide a final qualification.
The research is published in Physical Review Letters.
