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Observation of sequential Υ suppression in PbPb collisions
S Chatrchyan1, V Khachatryan, A M Sirunyan
1Yerevan Physics Institute, Yerevan, Armenia.
Physical Review Letters
|February 2, 2013
Summary
The suppression of bottomonium (Υ) states in lead-lead (PbPb) collisions was measured. Results show sequential suppression of Υ(1S), Υ(2S), and Υ(3S) states, indicating heavy quarkonium dissociation in the quark-gluon plasma.
Area of Science:
- High-energy nuclear physics
- Particle physics
- Quantum chromodynamics
Background:
- Heavy quarkonium states like bottomonium (Υ) are sensitive probes of the quark-gluon plasma (QGP).
- Previous studies at lower energies indicated suppression of Υ states in nucleus-nucleus collisions.
Purpose of the Study:
- To measure the suppression of individual Υ(nS) states (n=1, 2, 3) in lead-lead (PbPb) collisions.
- To compare the suppression patterns of different Υ states to understand the mechanisms of QGP formation and heavy quarkonium interaction.
Main Methods:
- Analysis of dimuon invariant mass spectra from PbPb and proton-proton (pp) collision data collected by the CMS experiment at the LHC.
- Measurement of Υ(nS) yields and calculation of the nuclear modification factor, R(AA), as a function of collision centrality.
Main Results:
- Significant suppression of Υ(1S), Υ(2S), and Υ(3S) states observed in PbPb collisions relative to pp collisions.
- The nuclear modification factor R(AA) values are approximately 0.56 for Υ(1S), 0.12 for Υ(2S), and <0.10 for Υ(3S).
- A clear sequential suppression pattern is observed, with higher-mass states (Υ(2S), Υ(3S)) being more suppressed than the ground state (Υ(1S)).
Conclusions:
- The observed sequential suppression provides strong evidence for the dissociation of excited Υ states in the hot and dense medium created in PbPb collisions.
- These results support the color screening model and provide insights into the properties of the QGP at LHC energies.
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