Measurement of the $e^{\mathbf{+}}{e}^{\mathbf{-}}\to p\overline{p}$ cross section in the energy range from 3.0 to 6.5 GeV

The $e^{+}{e}^{-}\to p\overline{p}$ cross section and the proton magnetic form factor have been measured in the center-of-mass energy range from 3.0 to 6.5 GeV using the initial-state radiation technique with an undetected photon. This is the first measurement of the form factor at energies higher than 4.5 GeV. The analysis is based on $469{\mathrm{fb}}^{-1}$ of integrated luminosity collected with the BABAR detector at the PEP-II collider at $e^{+}{e}^{-}$ center-of-mass energies near 10.6 GeV. The branching fractions for the decays $J/\psi \to p\overline{p}$ and $\psi (2S)\to p\overline{p}$ have also been measured.

Figure 1

The Feynman diagram for the ISR process $e^{+}{e}^{-}\to p\overline{p}\gamma$.

Figure 2

The distribution of the $p\overline{p}$ transverse momentum for simulated $e^{+}{e}^{-}\to p\overline{p}\gamma$ events. The arrow indicates $p_{\mathrm{T}}=0.150\mathrm{GeV}/c$.

Figure 3

The $M_{\mathrm{miss}}^2$ distribution for simulated $e^{+}{e}^{-}\to p\overline{p}\gamma$ events, where $M_{\mathrm{miss}}^2$ is the missing mass squared recoiling against the $p\overline{p}$ system. The arrows indicate $|M_{\mathrm{miss}}^2|=1{\mathrm{GeV}}^2/c^4$.

Figure 4

The $p\overline{p}$ invariant-mass spectrum for selected data $p\overline{p}\gamma$ candidates.

Figure 5

The $M_{\mathrm{miss}}^2$ distribution for data events (points with error bars) selected using all the criteria described in Sec. 3 except $|M_{\mathrm{miss}}^2|<1{\mathrm{GeV}}^2/c^4$. The solid histogram represents the distribution for simulated $e^{+}{e}^{-}\to e^{+}{e}^{-}p\overline{p}$ events. The simulated distribution is normalized to the number of data events with $M_{\mathrm{miss}}^2>30{\mathrm{GeV}}^2/c^4$.

Figure 6

The distributions of $M_{\mathrm{miss}}^2$ versus $p_T$ (a) for data events with $M_{p\overline{p}}>3.2\mathrm{GeV}/c^2$, (b) for simulated signal events, and (c) for simulated ISR background events.

Figure 7

The $p\overline{p}$ invariant-mass dependence of the detection efficiency obtained from MC simulation in the model with $|G_E|=|G_M|$.

Figure 8

The ratio of the detection efficiencies obtained from MC simulation using $G_E=0$ and using $|G_E|=|G_M|$. The solid curve is drawn to guide the eye.

Figure 9

The $p\overline{p}$ invariant-mass spectrum in the invariant-mass region near (a) the $J/\psi$, and (b) the $\psi (2S)$. The curves show the results of the fits described in the text.

Figure 10

The $e^{+}{e}^{-}\to p\overline{p}$ cross section measured in this analysis [BABAR (SA ISR)] and in other experiments: BES [9], CLEO [10], NU [11], and BABAR (LA ISR) [2].

Figure 11

The proton magnetic form factor measured in this analysis [BABAR (SA ISR)] and in other experiments: BES [9], CLEO [10], NU [11], E835 [13], E760 [12], BABAR (LA ISR) [2].

Figure 12

The proton magnetic form factor measured in this analysis [BABAR (SA ISR)] and in other experiments: BES [9], CLEO [10], NU [11], E835 [13], E760 [12], BABAR (LA ISR) [2]. Points denoted by “SLAC 1993” represent data on the spacelike magnetic form factor obtained in $ep$ scattering [26] as a function of $\sqrt{-q^2}$, where $q^2$ is the momentum transfer squared. The curve is the result of the QCD-motivated fit described in the text.