Exploring the hidden magic of quark-gluon plasma

Alice explores the hidden magic of quark-gluon plasma

An illustration of the effect of quark-gluon plasma on the formation of charmonia in lead nuclei collision. When the temperature of the plasma increases, the weakly bound (2S) state is likely to be more likely to be “checked”, and thus not formed, due to the greater number of quarks and gluons in the plasma (colored circles). An increase in the number of charm quarks and antiquarks (c and c̄) can lead to the formation of additional charmonia by recombination of quarks. Credit: Alice Collaboration

Quark-gluon plasma is an extremely hot, dense state of matter in which the elementary components – quarks and gluons – are not confined within complex particles called hadrons, as they are in the protons and neutrons that make up the nuclei of atoms. Thought to exist in the early universe, this special phase of matter can be recreated at the Large Hadron Collider (LHC) in collisions between lead nuclei.

A new analysis from the international ALICE collaboration at the LHC looks at how the different bonding states of the charm quark and its antimatter counterpart, also generated in these collisions, are affected by the γ-quark.gluon plasma. The results open up new ways to study the strong interaction – one of the four fundamental forces of nature – under conditions of extreme temperatures and density of quark-gluon plasmas.

The bound states of the charm quark and the anti charm quark, known as charmonia or cryptic particles, are bound together by strong interaction and are excellent sensors for quark-gluon plasma. In plasma, its production is suppressed due to “sorting” by the large number of quarks and gluons present in this form of matter.

Assay, and thus suppression, increases with increasing plasma temperature and is expected to affect different types of charmonia to varying degrees. For example, the production of the ψ(2S) state, which is 10 times weaker and 20% larger than the J/ state, is expected to be more suppressive than that of the J/ state.

This hierarchical suppression is not the only fate of chromium in quark-gluon plasma. The large number of charm quarks and antiquarks in plasma – up to about a hundred head-to-head collisions – also gives rise to a mechanism called recombination, which forms new charms and resists suppression to a certain extent.

This process is expected to depend on the type and momentum of the charmonia, with the weakest charmonia likely to be produced by recombination later in plasma evolution, and the charmonia with the least (transverse) momentum having the highest recombination rate.

Exploring the hidden magic of quark-gluon plasma

A lead collision occurred that was recorded by ALICE in 2015. Credit: ALICE Collaboration

Previous studies, using data from CERN’s Super Proton Synchrotron and then from the LHC, have shown that the production of the ψ(2S) state is indeed suppressed more than that of J/ψ. ALICE has also previously provided evidence for the recombination mechanism in J/. But, to date, no studies on ψ(2S) production at low particle momentum have been accurate enough to provide conclusive results in this momentum regime, preventing a complete picture of ψ(2S) production from being obtained.

The ALICE collaboration has now reported the first measurements of ψ(2S) production down to zero transverse momentum, based on lead–lead collision data from the LHC collected in 2015 and 2018.

Measurements show that, regardless of the particle’s momentum, the ψ(2S) state is suppressed about two times more than that of J/ψ. This is the first time that a clear hierarchy of repression of the overall production of chromium in the LHC has been observed. A similar observation was previously reported by the LHC Collaborations for the bound states of the down quark and its antiquark.

Upon further study as a function of particle momentum, the ψ(2S) suppression is seen to be reduced towards the decrease in momentum. This feature, previously noted by ALICE in the case of J/, is a signature of the recombination process.

Future high-resolution studies of this and other charmonia using data from the LHC Run 3, which began in July, may lead to a definitive understanding of the modulation of occult magic particles and, as a consequence, of the strong interaction that holds them together, in the harsh environment. quark-glon plasma.


First direct observation of the dead cone effect in particle physics


more information:
ALICE cooperation, suppression of ψ(2S) in Pb–Pb collisions in the LHC, arXiv (2022). arXiv: 2210.08893 [nucl-ex] doi.org/10.48550/arXiv.2210.08893

Journal information:
arXiv


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