Abstract
In view of the large discrepancy about the associated production of a prompt $J/\psi$ and a $Z$ boson between the ATLAS data at $\sqrt{s}=8$ TeV and theoretical predictions for Single Parton Scattering (SPS) contributions, we perform an evaluation of the corresponding cross section at one loop accuracy (Next-to-Leading Order, NLO) in a quark-hadron-duality approach, also known as the Colour-Evaporation Model (CEM). This work is motivated by (i) the extremely disparate predictions based on the existing NRQCD fits conjugated with the absence of a full NLO NRQCD computation and (ii) the fact that we believe that such an evaluation provides a likely upper limit of the SPS cross section. In addition to these theory improvements, we argue that the ATLAS estimation of the Double Parton Scattering (DPS) yield may be underestimated by a factor as large as 3 which then reduces the size of the SPS yield extracted from the ATLAS data. Our NLO SPS evaluation also allows us to set an upper limit on $\sigma_{\rm eff}$ driving the size of the DPS yield. Overall, the discrepancy between theory and experiment may be smaller than expected, which calls for further analyses by ATLAS and CMS, for which we provide predictions, and for full NLO computations in other models. As an interesting side product of our analysis, we have performed the first NLO computation of $d\sigma / dP_T$ for prompt single-$J/\psi$ production in the CEM from which we have fit the CEM non-pertubative parameter at NLO using the most recent ATLAS data.
Highlights
In addition to these theory improvements, we argue that the ATLAS estimation of the Double Parton Scattering (DPS) yield may be underestimated by a factor as large as 3 which reduces the size of the Single Parton Scattering (SPS) yield extracted from the ATLAS data
Motivated by the discrepancy uncovered by ATLAS, we perform here the very first complete evaluation of the SPS yield at NLO for the production of a J/ψ associated with a Z boson in pp collisions under the assumption of quark-hadron duality which, in the case of quarkonium production, is referred to as the Colour-Evaporation Model (CEM)
This allows us to question the size of the DPS yield assumed by ATLAS based on their W + 2-jet analysis [36] and to propose a solution for the present puzzle with a DPS yield roughly three times larger
Summary
In parallel to the aforementioned advances in the study of quarkonium associated production, more precise data for single-quarkonium production are flowing from the LHC, including cross-section measurements at larger PT , improved measurements of feed-down fractions, more reliable multi-dimensional polarisation measurements (see [37] for a recent review) and, last but not least, the first measurement of the production cross-section of the spin-singlet ηc [38]. At the great surprise of some, such ηc data happened to be very well described by postdictions [39,40,41] with the sole CS contributions ( called CSM) leaving nearly no room for CO contributions in this channel Such a constraint, translated to the J/ψ case via Heavy-Quark-Spin Symmetry (HQSS), could only be met by assuming a rather small value of the LDMEs for OJ/ψ(1S0[8]) , which in turn induces, in order to reproduce the large-PT J/ψ spectra at the LHC and the Tevatron, a somewhat large value for OJ/ψ(3S1[8]) , still in the ballpark of the NRQCD Velocity-Scaling Rules (VSR) [40]. In view of all these arguments, we believe the CEM to be currently the best model to investigate the ATLAS excess for J/ψ + Z production since a complete NLO NRQCD computation is lacking and since the CEM would probably provide an upper theory limit given the predominance of the fragmentation channels for this process
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
