Unexpected ‘ridge’ seen in CMS collision data again

An event display for a high-multiplicity p–Pb collision at 5.02 TeV, recorded by CMS on 13 September 2012. (Courtesy: CERN/CMS collaboration)

Jon Cartwright
The first data from proton–lead collisions at the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC) at CERN include a “ridge” structure in correlations between newly generated particles. According to theorists in the US, the ridge may represent a new form of matter known as a “colour glass condensate”.

This is not the first time such correlations have been seen in collision remnants – in 2005, physicists working on the Relativistic Heavy-Ion Collider (RHIC) at Brookhaven National Laboratory in New York found that the particles generated in collisions of gold nuclei had a tendency to spread transversely from the beam at very small relative angles, close to zero. A similar correlation was seen in 2010 at CMS in proton–proton collisions and then later that year in lead–lead collisions. (See image below, parts a and b.)

Observing ridges

When a graph is plotted of the fraction of particles versus the relative transverse emission angle and the relative angle to the beam axis, the correlation appears as a distinct ridge. Now, this ridge has been seen in proton–lead collisions for the first time – within a week of data collection at CMS (see image below, part c) (arXiv:1210.5482).

These three plots show the correlation between pairs of particles seen in the CMS detector. (a) shows proton–proton collisions, and the arrow points to the ridge; (b) shows the lead–lead collisions where a similar ridge emerged once more; and (c) denotes the most recent proton–lead collisions where the ridge is seen once more. Δη is the angle in that plane measured between the two particles in the longitudinal plane. ΔΦ represents the difference between the angles of the two particles in question in the transverse plane. R is a function of both Δη and ΔΦ. (Courtesy: CERN/CMS collaboration)

Although observations of ridges in different experiments would suggest a single cause, theorists believe there may be more than one explanation. When pairs of nuclei (such as gold or lead) collide, they can produce a hot, dense medium similar to quark–gluon plasma, a type of matter thought to have existed very soon after the Big Bang. The motion of this plasma probably correlates the underlying particles into the ridge structure.
Proton–proton collisions, on the other hand, are not expected to form a quark–gluon plasma, so theorists have come up with other explanations. One idea, presented by Raju Venugopalan at Brookhaven National Laboratory in the US and Kevin Dusling at North Carolina State University in Raleigh, US, is that the ridge correlation is an unusual type of quantum entanglement in which generated particles carry information about the state of protons before those protons collided……………
Read more: physicsworld.com

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