September 22, 2010
The following statement describing a phenomenon observed at the CMS experiment at the Large Hadron Collider (LHC) was issued by CERN, the European laboratory for particle physics. The phenomenon is thought to be similar to one observed by physicists at Brookhaven’s Relativistic Heavy Ion Collider (RHIC) called the “ridge effect.” This effect refers to a correlation that RHIC’s experiments see in the particle debris created when two beams of gold ions (atoms stripped of electrons) collide with one another at nearly the speed of light. Some of these particles are correlated — or associated together — when they are created at the point of collision, which creates a ridge-like structure in scientists’ data plots. Because the ridge may carry information about the very early stages of the particle collision, it has become an important element in understanding the formation of the quark gluon plasma, an extremely hot and dense form of matter that existed in the first few microseconds after the Big Bang. While a number of possible theoretical explanations for the ridge effect are under consideration, Brookhaven theorists are particularly excited that their favored explanation for the observations at RHIC — involving extremely dense matter known as “Color Glass Condensate” — also predicted that the observation would be made in proton-proton collisions at LHC. That explanation is tied to tube-like regions of intense fields created in the wake of colliding dense clouds of gluons, the elusive particles responsible for the strongest of Nature’s forces.
After almost six months of operation, experiments at the LHC are starting to see signs of potentially new and interesting effects. In results announced by the CMS collaboration today, correlations have been observed between particles produced in 7 TeV proton-proton collisions.
In some of the LHC’s proton-proton collisions, a hundred or more particles can be produced. The CMS collaboration has studied such collisions by measuring angular correlations between the particles as they fly away from the point of impact, and this has revealed that some of the particles are intimately linked in a way not seen before in proton collisions.
The effect is subtle and many detailed crosschecks and studies have been performed to ensure that it is real. It bears some similarity to effects seen in the collisions of nuclei at the RHIC facility located at the US Brookhaven National Laboratory, which have been interpreted as being possibly due to the creation of hot dense matter formed in the collisions. Nevertheless, the CMS collaboration has stressed that there are several potential explanations to be considered and the collaboration’s presentation to the physics community at CERN today focused on the experimental evidence in the interest of fostering a broader discussion on the subject.
“Now we need more data to analyse fully what’s going on, and to take our first steps into the vast landscape of new physics we hope the LHC will open up,” said CMS Spokesperson Guido Tonelli.
Proton running at the LHC is scheduled to continue until the end of October, during which time CMS will accumulate much more data to analyse. For the remainder of 2010 running, the LHC will collide lead nuclei.
Another CERN experiment that will be following developments with great interest is ALICE, whose detector is optimised to study collisions of nuclei. Like the experiments at RHIC, ALICE aims to study matter in the hot dense state that would have existed just tiny fractions of a second after the Big Bang in a bid to understand how such matter evolved into the ordinary nuclear matter that makes up the Universe today. The observation of proton-proton collisions producing large numbers of particles bodes well for this new phase of LHC running.
Having presented results on known physics at the conferences earlier in the summer, LHC experiments are now probing new ground.
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