Difference between revisions of "First measurement of near-threshold J/ψ exclusive photoproduction off the proton"

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{{DISPLAYTITLE: First measurement of near-threshold J/ψ exclusive photoproduction off the proton}}
 
{{DISPLAYTITLE: First measurement of near-threshold J/ψ exclusive photoproduction off the proton}}
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== Abstract ==
 
== Abstract ==
  
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== Figures and captions ==
 
== Figures and captions ==
  
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{| border="1" cellpadding="2" valign="top" style="text-align: center; width: 85%"
! style="width:200px" | Figures
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| Electron-positron invariant mass spectrum from the data. The insert shows the J/&psi; region fitted with a linear polynomial plus a Gaussian (fit parameters shown). || <span class="plainlinks">[https://halldweb.jlab.org/wiki/images/5/5a/Fig1.pdf pdf]</span>
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| style="padding: 10px" | Electron-positron invariant mass spectrum from the data. The insert shows the J/&psi; region fitted with a linear polynomial plus a Gaussian (fit parameters shown).  
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| style="padding: 10px" | <span class="plainlinks">[https://halldweb.jlab.org/wiki/images/5/5a/Fig1.pdf pdf]</span>
 
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|  Differential cross section for J/ψ photoproduction as a function of −(t − tmin) for 10.00 &lt; E<sup>&gamma;</sup> &lt; 11.80 GeV. || <span class="plainlinks">[https://halldweb.jlab.org/wiki/images/f/f5/Fig2.pdf pdf]</span>
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| style="padding: 10px" |  Differential cross section for J/ψ photoproduction as a function of −(t − tmin) for 10.00 &lt; E<sub>&gamma;</sub> &lt; 11.80 GeV.  
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| style="padding: 10px" | <span class="plainlinks">[https://halldweb.jlab.org/wiki/images/f/f5/Fig2.pdf pdf]</span>
 
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|  GlueX results for the J/ψ total cross section vs beam energy, compared to the Cornell [15] and SLAC [16] data, the theoretical predictions [11, 13], and the JPAC model [6] corresponding to B(P<sup>+</sup><sub>c</sub>(4440) &rarr; J/&psi;p) = 1.6% for the J<sup>P</sup> = 3/2<sup>−</sup> case as discussed in the text. All curves are fitted/scaled to the GlueX data only. For our data the quadratic sums of statistical and systematic errors are shown; the overall normalization uncertainty is 27%. || <span class="plainlinks">[https://halldweb.jlab.org/wiki/images/2/2a/Jpsi_Fig3.pdf pdf]</span>
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|  style="padding: 10px" |  GlueX results for the J/ψ total cross section vs beam energy, compared to the Cornell [15] and SLAC [16] data, the theoretical predictions [11, 13], and the JPAC model [6] corresponding to B(P<sup>+</sup><sub>c</sub>(4440) &rarr; J/&psi;p) = 1.6% for the J<sup>P</sup> = 3/2<sup>−</sup> case as discussed in the text. All curves are fitted/scaled to the GlueX data only. For our data the quadratic sums of statistical and systematic errors are shown; the overall normalization uncertainty is 27%.  
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| style="padding: 10px" | <span class="plainlinks">[https://halldweb.jlab.org/wiki/images/2/2a/Jpsi_Fig3.pdf pdf]</span>
 
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Latest revision as of 13:48, 12 June 2019


Abstract

We report on the measurement of the γp → J/ψp cross section from Eγ=11.8 GeV down to the threshold at 8.2 GeV using a tagged photon beam with the GlueX experiment. We find the total cross section falls toward the threshold less steeply than expected from two-gluon exchange models. The differential cross section dσ/dt has an exponential slope of 1.67 ± 0.39 GeV−2 at 10.7 GeV average energy. The LHCb pentaquark candidates P+c can be produced in the s-channel of this reaction. We see no evidence for them and set model-dependent upper limits on their branching fractions B(P+c → J/ψp.

Publication Information

Summary

In summary, we have made the first measurement of the J/ψ exclusive photoproduction cross section from Eγ = 11.8 GeV down to the threshold, which provides important inputs to models of the gluonic structure of the proton at high x. The data were collected by the GlueX experiment located in Hall D at Jefferson Lab during 2016 and 2017, representing about 25% of the total data accumulated by the experiment to date. We measure the exclusive reaction γp → J/ψp, and identify the J/ψ by its decay into an electron-positron pair. We normalize the J/ψ total cross section to that of the non-resonant Bethe-Heitler process in the invariant mass range 1.20−2.50 GeV, thus canceling uncertainties from factors like luminosity and common detector efficiencies. The measured cross section is also used to set model-dependent upper limits on the branching fraction of the LHCb P+c states 90% confidence level of <4.6%, <2.3%, and <3.8% for P+c (4312), P+c(4440), and P+c(4457), respectively, which allow to discriminate between different pentaquark models.

Figures and captions

Caption Figures
Electron-positron invariant mass spectrum from the data. The insert shows the J/ψ region fitted with a linear polynomial plus a Gaussian (fit parameters shown). pdf
Differential cross section for J/ψ photoproduction as a function of −(t − tmin) for 10.00 < Eγ < 11.80 GeV. pdf
GlueX results for the J/ψ total cross section vs beam energy, compared to the Cornell [15] and SLAC [16] data, the theoretical predictions [11, 13], and the JPAC model [6] corresponding to B(P+c(4440) → J/ψp) = 1.6% for the JP = 3/2 case as discussed in the text. All curves are fitted/scaled to the GlueX data only. For our data the quadratic sums of statistical and systematic errors are shown; the overall normalization uncertainty is 27%. pdf