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New two-particle correlations observed in CMS detector at the LHC

The CMS Collaboration at CERN released a paper entitled “Observation of Long-Range Near-Side Angular Correlations in Proton-Proton Collisions” that details signs of a new phenomenon in proton interactions.

A study of “high multiplicity” collisions, where a hundred or more charged particles are produced, has revealed indications that some particles are somehow “correlated” – associated together when they were created at the point of collision.

It was considered natural to search for these correlations in the highest multiplicity proton-proton collisions at LHC as the particle densities begin to approach those in high-energy collisions of nuclei such as copper, where similar effects have already been seen...

By Christophe Delaere on September 23, 2010
Additional information:

  In the analysis presented in the paper, all pairs of charged particles in a collision were selected and the differences in the directions of the two particles measured.  These differences are denoted by Δη and Δφ. For each pair a “correlation function” was also computed.

The most notable effect is the curious addition of an elongated ridge at Δφ=0 for all Δη. This means that particles in some pairs at large Δη are receding from each other at close to the speed of light, but are oriented along the same φ angle – as if the particles were somehow associated together when they were created at the point of collision.

CMS Spokesperson Guido Tonelli commented: “We were actively looking for such a phenomenon, which has never been seen before in proton-­proton collisions. Additional statistics will shed more light on the origin of this effect. This observation demonstrates the power and versatility of the CMS detector, as well as of the physicists exploiting it. We are now on our way to exploring, inch by inch, the new territory made accessible by the LHC.”

This is the first observation of such a phenomenon in proton-­proton collisions and many different interpretations on the origin of the effect are possible. Although there is not yet a definitive explanation for the cause of this effect, the novel structure is reminiscent of similar features seen by experiments at RHIC (Relativistic Heavy Ion Collider in the USA) that were interpreted as being due to the presence of hot and dense matter formed in relativistic heavy ion collisions.

More detailed analyses with increased statistics have been initiated for this class of high multiplicity events. The increase in intensity of the LHC beams in the coming months will provide at least a hundred times more data with which to study this effect in greater detail and elucidate the mechanism behind it. Similar studies in CMS are planned for the upcoming heavy‐ion run of the LHC.

Belgian groups often played a key role in the results published so far. Indeed, teams from Universiteit Antwerpen, Université Catholique de Louvain, Université Libre de Bruxelles, Universiteit Gent, Université de Mons and Vrije Universiteit Brussel are actively involved both at the level of data taking and quality assessment and at the level of data analysis and interpretation.

Belgian groups also contributed to the building of the CMS detector, some for more than 15 years. UCLouvain in particular played a major role in design and commissioning of the silicon strip tracker, made of nearly 15 000 silicon detectors, that is especially relevant for the observation of the correlations presented today.

Reference url: http://cms.web.cern.ch/cms/News/2010/QCD-10-002/index.html

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