Measurement of Promptψ(2S)toJ/ψYield Ratios in Pb-Pb and

Abstract

The ratio between the prompt ψð2SÞ and J=ψ yields, reconstructed via their decays into μ þ μ − , is measured in Pb-Pb and p-p collisions at ffiffiffiffiffiffiffi ffi s NN p ¼ 2.76 TeV. The analysis is based on Pb-Pb and p-p data samples collected by CMS at the Large Hadron Collider, corresponding to integrated luminosities of 150 μb −1 and 5.4 pb −1 , respectively. The double ratio of measured yields ðN ψð2SÞ =N J=ψ Þ Pb-Pb = ðN ψð2SÞ =N J=ψ Þ p-p is computed in three Pb-Pb collision centrality bins and two kinematic ranges: one at midrapidity, jyj < 1.6, covering the transverse momentum range 6.5 < p T < 30 GeV=c, and the other at forward rapidity, 1.6 < jyj < 2.4, extending to lower p T values, 3 < p T < 30 GeV=c. The centralityintegrated double ratio changes from 0.45 AE 0.13ðstatÞ AE 0.07ðsystÞ in the first range to 1.67 AE 0.34ðstatÞ AE 0.27ðsystÞ in the second. This difference is most pronounced in the most central collisions.

Prospects for Quarkonium Measurements in p–A and A–A Collisions at the LHC

“…The experimentally more challenging ψ(2S) plays a crucial role to establish the physics picture of the different models and providing also discriminative power between them, see for instance in [24]. First measurements of the excited ψ(2S) have been published currently strongly limited by statistics [25,10,11] awaiting a coherent phenomenological interpretation. The degree of thermalisation of the quarkonium states in A-A collisions can be also probed by measuring their azimuthal correlation in momentum space with respect to the bulk of soft particles emitted in the collision.…”

“…The comover model which incorporates a late-stage interaction of the cc pair leading to the suppression of the excited state could as a first model account for the experimental observation [45]. The measured behaviour of ψ(2S) production in A-A collisions [25,10] was also interpreted as a sign of latestage interactions like in p-A collisions within an in-spirit similar approach than the comovers and a later freeze-out of the ψ(2S) [46]. This interpretation will need to be reassessed with the Run 2 data, which starts to become available [25].…”

“…The measured behaviour of ψ(2S) production in A-A collisions [25,10] was also interpreted as a sign of latestage interactions like in p-A collisions within an in-spirit similar approach than the comovers and a later freeze-out of the ψ(2S) [46]. This interpretation will need to be reassessed with the Run 2 data, which starts to become available [25]. In addition, a decrease of the yield ratio between the excited ϒ states normalised to the ground state as a function of midrapidity multiplicity has been observed in p-A and in pp [47] data.…”

“…For the charmonium, the acceptance limit implies a p T cut-off of about 7-10 GeV/c for the dimuon p T in the measurements at midrapidity. At the forward rapidity edge of the muon systems, the acceptance of the dimuon trigger reaches lower in p T as demonstrated, e.g., in [25] also in Pb-Pb collisions. The performance of the muon offline reconstruction even in most central Pb-Pb collisions allows for not strongly mitigated background conditions and similar resolutions as in pp collisions for tight track selections in case of CMS.…”

“…Since LHCb only joined recently the A-A programme, it is not possible to be quantitative about the certainly strong potential of LHCb in heavy-ion collisions for quarkonium studies and the delivered luminosities. [12,25,11]. Minimum bias is abbreviated by MB.…”