Measurement of the $e^{+}{e}^{-}\to \mathrm{\Lambda }\overline{\mathrm{\Lambda }}$ cross section from threshold to 3.00 GeV using events with initial-state radiation

Using initial-state radiation events from a total integrated luminosity of $11.957{\mathrm{fb}}^{-1}$ of $e^{+}{e}^{-}$ collision data collected at center-of-mass energies between 3.773 and 4.258 GeV with the BESIII detector at BEPCII, the cross section for the process $e^{+}{e}^{-}\to \mathrm{\Lambda }\overline{\mathrm{\Lambda }}$ is measured in 16 $\mathrm{\Lambda }\overline{\mathrm{\Lambda }}$ invariant mass intervals from the production threshold up to $3.00\mathrm{GeV}/c^2$. The results are consistent with previous results from BABAR and BESIII, but with better precision and with narrower $\mathrm{\Lambda }\overline{\mathrm{\Lambda }}$ invariant mass intervals than BABAR.

Figure 1

The leading-order Feynman diagram for the ISR process $e^{+}{e}^{-}\to \gamma \mathrm{\Lambda }\overline{\mathrm{\Lambda }}$. The ISR photon can be emitted from the electron or the positron.

Figure 2

Distribution of $M_{\overline{\mathrm{\Lambda }}}$ versus $M_{\mathrm{\Lambda }}$ of the accepted candidates in mode I from all datasets. The dashed red box encloses the signal region, while the black boxes show the sideband regions.

Figure 3

The $M_{\pi }^2$ spectrum of the accepted candidates in mode II from all datasets. The region between the red arrows is the signal region, and the regions between the blue arrows are the sideband regions.

Figure 4

The $M_{\mathrm{\Lambda }\overline{\mathrm{\Lambda }}}$-dependent selection efficiencies obtained from MC simulation. Squares (blue), triangles (black), and circles (red) with error bars represent the datasets at $\sqrt{s}=3.773$, 4.178–4.258, and 3.773–4.258 GeV, respectively. The combined efficiency is weighted according to the effective luminosity of the ISR process.

Figure 5

The $M_{\mathrm{\Lambda }\overline{\mathrm{\Lambda }}}$ spectrum for events satisfying the $\gamma \mathrm{\Lambda }\overline{\mathrm{\Lambda }}$ selection criteria from all datasets. Contributions from $J/\psi \to \mathrm{\Lambda }\overline{\mathrm{\Lambda }}$ and $\psi (2S)\to \mathrm{\Lambda }\overline{\mathrm{\Lambda }}$ decays are clearly seen.

Figure 6

The distributions of $M_{\mathrm{\Lambda }\overline{\mathrm{\Lambda }}}$ for the signal candidates and the main background events from all datasets. Black dots with error bars refer to the signal candidates, and blue, green, and magenta histograms represent the ${\pi }^0\mathrm{\Lambda }\overline{\mathrm{\Lambda }}$, $\gamma (\mathrm{\Lambda }{\overline{\mathrm{\Sigma }}}^0+\mathrm{c}.\mathrm{c}.)$, and non-$\mathrm{\Lambda }\overline{\mathrm{\Lambda }}$ background events, respectively.

Figure 7

The cross section for the $e^{+}{e}^{-}\to \mathrm{\Lambda }\overline{\mathrm{\Lambda }}$ process from this analysis (black dots with error bars) with comparison to previous works (see the legend in the figure) [6, 7, 8, 9]. Both statistical and systematic uncertainties are included. The blue dashed line is the fit result using Eq. (10), and the red solid line is the fit result using Eq. (11). The vertical dashed line is the production threshold for $e^{+}{e}^{-}\to \mathrm{\Lambda }\overline{\mathrm{\Lambda }}$. The $\chi$ distributions of the two fits are shown in the bottom panel, where the blue and red triangles represent the results of Eqs. (10) and (11), respectively.

Figure 8

Simultaneous fit (blue curve) with a double Gaussian function (red dashed curve) for the resonance and a linear function (green dashed curve) for background of the $M_{\mathrm{\Lambda }\overline{\mathrm{\Lambda }}}$ spectra at $\sqrt{s}=3.773$ and 4.178 GeV. Black dots with error bars represent data.