Abstract
Bottomonia are important probes of the quark-gluon plasma since they are produced at early times and propagate through the medium, mapping its evolution. The three ϒ states (1S, 2S, 3S) were measured separately using the Compact Muon Solenoid (CMS) experimental apparatus and observed to disappear sequentially in PbPb collisions at 2.76 TeV. However, recent measurements in pp and pPb collisions, at 2.76 and 5.02 TeV respectively, show a surprising dependence of the excited state (2S or 3S) over the ground (1S) state ratio, as a function of event activity. The three ϒ states are also observed to be individually more produced in events with more activity. We review the latest results from pp, pPb and PbPb collisions and highlight their possible interpretations.
Highlights
Quarkonium suppression is known to be one of the experimental signatures of the formation of a Quark-Gluon Plasma [1]
In the context of pPb collisions, the total transverse energy deposit in the pseudorapidity range 4.0 < |η| < 5.2 is used to separate the collision data in classes of forward event activity. This sampling at forward pseudo-rapidities provides an event activity measurement that should be independent of the measurement of the hard probe
We have presented the measurement of event-activity dependent production of Υ states by the Compact Muon Solenoid (CMS) experiment in pp, pPb and PbPb collisions at the LHC
Summary
Quarkonium suppression is known to be one of the experimental signatures of the formation of a Quark-Gluon Plasma [1]. From these values, one can observe that the excited states are more suppressed than the ground state over all centrality ranges. Comparing the RAA of excited states to that of the ground state can be done by measuring the double ratio χPbPb(2S/1S) = RAA(2S)/RAA(1S) = N (2S)/N (1S)|P bP b/N (2S)/N (1S)|pp In this ratio most of the detector effects cancel, as well as parton energy loss and gluon shadowing, which are expected to affect all bb states the same way. The double ratio values are χpPb(2S/1S) = 0.83 ± 0.05 ± 0.05, χpPb(3S/1S) = 0.71 ± 0.08 ± 0.09 where the quoted uncertainties are statistical first and systematic second This result indicates the excited states are more affected than the ground state. This sampling at forward pseudo-rapidities provides an event activity measurement that should be independent of the measurement of the hard probe
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