A photon of initial wavelength \(\lambda_0\) travels in the positive \(x\)-direction and undergoes a head-on collision with an electron that is initially at rest. As a result of the Compton scattering process, the photon is backscattered directly along the negative \(x\)-direction with a longer wavelength \(\lambda\). Which of the following expressions correctly represents the magnitude of the recoil momentum \(p_e\) of the electron immediately after the collision?