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  • Open Access

First Measurement of Timelike Compton Scattering

P. Chatagnon et al. (CLAS Collaboration)
Phys. Rev. Lett. 127, 262501 – Published 22 December 2021
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    Abstract

    We present the first measurement of the timelike Compton scattering process, γp→p′γ*(γ*→e+e−), obtained with the CLAS12 detector at Jefferson Lab. The photon beam polarization and the decay lepton angular asymmetries are reported in the range of timelike photon virtualities 2.25<Q′2<9  GeV2, squared momentum transferred 0.1<−t<0.8  GeV2, and average total center-of-mass energy squared s=14.5  GeV2. The photon beam polarization asymmetry, similar to the beam-spin asymmetry in deep virtual Compton scattering, is sensitive to the imaginary part of the Compton form factors and provides a way to test the universality of the generalized parton distributions. The angular asymmetry of the decay leptons accesses the real part of the Compton form factors and thus the D-term in the parametrization of the generalized parton distributions.

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    • Received 31 August 2021
    • Revised 21 October 2021
    • Accepted 11 November 2021

    DOI:https://doi.org/10.1103/PhysRevLett.127.262501

    Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.

    Published by the American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Deep inelastic scatteringPhoton & charged-lepton interactions with hadronsQuantum chromodynamics
    1. Properties
    Form factors
    Nuclear PhysicsParticles & Fields

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    Vol. 127, Iss. 26 — 24 December 2021

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    • Figure 1
      Figure 1

      Left: handbag diagram of the TCS process. Right: diagram of the BH process. t=(p−p′)2 is the squared four-momentum transfer between the initial and final protons, Q′2=(k+k′)2 is the invariant mass of the lepton pair, and Q2=−q2 is the virtuality of the real photon. ξ=Q′2/(2(s−mp2)−Q′2) is the momentum imbalance of the struck quark, s is the squared center-of-mass energy, and mp is the proton mass. x is the average momentum fraction of the struck quark.

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    • Figure 2
      Figure 2

      Relevant angles for TCS. ϕ and θ are, respectively, the angle between the leptonic plane (defined by the outgoing leptons momenta k and k′) and the hadronic plane (defined by the incoming and outgoing proton momenta p and p′, defined in Fig. 1), and the angle between the electron and the recoiling proton in the leptons center-of-mass frame.

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    • Figure 3
      Figure 3

      Invariant mass of the electron-positron pairs. The indicated peaks correspond to the ρ0/ω, ϕ, and J/ψ mesons. The TCS events are selected in the 1.5–3-GeV mass range (within the dotted vertical lines) and are compared to MC simulation of BH events. The simulation is normalized to the total number of events. The data-simulation bin-by-bin ratio agrees at the 15% level.

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    • Figure 4
      Figure 4

      A⊙U as a function of ϕ for the four t bins used in this analysis. The sine fit function is superimposed. The amplitude of the fit A⊙U is plotted as a function of −t in Fig. 5.

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    • Figure 5
      Figure 5

      A⊙U as a function of −t at the averaged kinematic point Eγ=7.29±1.55  GeV; M=1.80±0.26  GeV. The errors on the averaged kinematic point are the standard deviations of the corresponding distributions of events. The blue data points are represented with statistical error bars, horizontal bin widths, and shaded total systematic uncertainty. Red triangles show the asymmetry computed for simulated BH events. The dashed and dash-dotted lines are the predictions of, respectively, the VGG [27, 28, 29, 30] and the GK [31, 32, 33] models, evaluated at the average kinematics.

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    • Figure 6
      Figure 6

      FB asymmetry as a function of −t at the average kinematics Eγ=7.23±1.61  GeV; M=1.81±0.26  GeV. The solid line shows the model predictions of the VGG model with D-term (from Ref. [39]) evaluated at the average kinematic point. The other curves are defined in the caption of Fig. 5.

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    • Figure 7
      Figure 7

      FB asymmetry as a function of −t at the average kinematics Eγ=8.13±1.23  GeV; M=2.25±0.20  GeV. The curves are defined in the captions of Figs. 5 and 6.

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