When atomic nuclei such as gold or lead nuclei collide at high energy in particle colliders, they can produce a quark-gluon plasma (QGP) – a hot, dense state of matter predicted to exist shortly after the Big Bang. A key feature of QGP formation in heavy ion collisions is the long-range spatial correspondence, or correlation, between the particles that arise in the collisions.
This collective phenomenon, which appears as a ridge-like shape in data plots and is known as a ridge, was first observed in 2005 in collisions of heavy ions at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory in the US, and has since been observed at the Large Hadron Collider (LHC). of the European Organization for Nuclear Research (CERN) in smaller collision systems such as collisions between protons.
At the Rencontres de Moriond conference today, the ALICE collaboration reported the observation of ridge correlation in the simplest collision system to date. The result brings physicists a step closer to finding the origin of collective QGP-like phenomena in small colliding systems.
The first observation of ridge-association in collisions other than heavy ion collisions was made in 2010 by the CMS Collaboration on ‘high-polarity’ proton collisions that produce a relatively large number of particles. Soon after, CMS, ALICE and ATLAS also observed this phenomenon in collisions between protons and lead nuclei. These observations came as a surprise—these collision systems were expected to be too small and simple to develop QGP-like collective behaviour. Other studies have shown that the observed ridge associations are in fact collective in nature, but the exact mechanisms underpinning this collective behavior in these smaller, simpler systems still need to be identified.
In their latest study, the ALICE collaboration set out to investigate whether ridge-binding also occurs in collisions of “low-multiply” protons that create a relatively small number of particles. ALICE researchers analyzed a large sample of proton collisions recorded by the collaboration during the second run of the Large Hadron Collider to explore how the effect of ridges depends on the number of particles produced in the collisions. They then plotted the number of particle pairs produced in a set of low-multiplicity collisions along two angular directions relative to the collision axis, and found a distinct ridge-like shape.
Next, the ALICE team examined how the number of edge-bound particle pairs varied with multiplicity, and compared the results with previous results from electron-positron collisions recorded by the ALEPH experiment at the Large Electron-Positron Collider, the predecessor of the Large Hadron Collider. This comparison showed that, for the same multiplicity, the correlation of ridges in proton collisions is stronger than that inferred in electron-positron collisions, as no correlation between ridges has been observed so far.
These new ALICE findings, combined with future studies based on data from the third run of the LHC, should help physicists identify the mechanisms that govern collective behavior in small colliding systems.
Alice’s cooperation: alice-collaboration.web.cern.ch/
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