On September 21st 2010 the CMS Collaboration gave a seminar at CERN detailing the observation of long-range near-side angular correlations in p+p collisions at the LHC. A preprint was submitted to arXiv the same day:
The significance of these two particle correlations in azimuthal angle and pseudo-rapidity space is that they are thought to be the result of a collective, hydrodynamic, response of a medium to fluctuations of a multi-particle initial state. The medium in question of course most likely being a Quark-Gluon-Plasma.
The basis for this tantalizing speculation of QGP formation lies in the previous observation of such specific features of this correlation function in Cu+Cu and Au+Au collisions at the Relativistic Heavy-Ion Collider by the PHOBOS and STAR collaborations:
- System size dependence of cluster properties from two-particle angular correlations in Cu+Cu and Au+Au collisions at s(NN)**(1/2) = 200-GeV.
- High transverse momentum triggered correlations over a large pseudorapidity acceptance in Au+Au collisions at s(NN)**1/2 = 200 GeV.
- Three-particle coincidence of the long range pseudorapidity correlation in high energy nucleus-nucleus collisions.
One should note that CMS was able to use a high multiplicity trigger, i.e. perform their analysis for several bins in measured particle multiplicity with the highest bin containing events in excess of 110 particles. The observed modification of the two particle correlation function only developed for the high multiplicity bins, with multiplicities similar to those found in semi-central Cu+Cu or peripheral Au+Au collisions at RHIC. One should also note that standard Monte-Carlo event generators such as PYTHIA and HERWIG failed to describe this effect.
The main item of interest here is that the long-range ridge-like structure observed in the correlation function can be explained by the presence of fluctuating initial conditions followed by a collective, hydrodynamic evolution – in particular triangular flow, which is caused by initial density fluctuations in the transverse plane:
- Collision geometry fluctuations and triangular flow in heavy-ion collisions.
- Triangular flow in hydrodynamics and transport theory.
The hydrodynamic evolution of course implying the formation of a thermalized strongly interacting QCD medium: a Quark-Gluon-Plasma. While the space-time volume of QGP matter would not be large in a p+p collision, its existence in that system would certainly constitute a significant discovery.
The idea of using hydrodynamics for the description of p+p events is actually not as exotic as it may seem at first glance and even dates back to Landau [Izv. Akad. Nauk SSSR, Physics Series, Vol 17, 51 (1953)]. In more modern times this was studied in
Recently, possibly spurred by the realization that the number of particles produced in p+p collisions at the LHC can equal that in nucleus-nucleus collisions at RHIC, there have been multiple applications of hydrodynamics to p+p reactions at LHC:
- Viscous Hydrodynamic Predictions for Nuclear Collisions at the LHC.
- Elliptic flow (v(2)) in pp collisions at LHC energy : A Hydrodynamical approach.
- Observation of the collective flow in proton-proton collisions.
- Eccentricity fluctuations make flow measurable in high multiplicity p-p collisions.
- Elliptic flow in high multiplicity proton-proton collisions at sqrt(1/2) = 14-TeV as a signature of deconfinement and quantum energy density fluctuations.
While the two particle correlation functions measured by CMS are indicative of a hydrodynamic expansion, there are a variety of other predictions by hydrodynamic models for p+p reactions at the LHC which have yet to be verified or falsified. In the absence of other corroborating evidence the conclusion of QGP formation in p+p collisions at the LHC remains speculative.
As a historical side-note, it should be added that there have been repeated reports on a search for QGP formation in proton + anti-proton reactions at the TEVATRON:
- Recent Results From E735 At The Fermilab Tevatron Proton Anti-Proton Collider With S**(1/2) = 1.8-Tev.
- A Quark – gluon plasma search in anti-p p at s**(1/2) = 1.8-TeV.
However, the experiment never found conclusive evidence, possibly as a consequence of the lack of a high multiplicity trigger.