  According to \emph{Wind} observations between June 2004 and May 2019, this
Letter investigates the proton and alpha particle temperatures in the space of
($\theta_d$, $V_d/V_A$) for the first time, where $\theta_d$ and $V_d$ are the
radial angle and magnitude of alpha$-$proton differential flow vector
respectively, $V_A$ is the local Alfv\'en speed. Results show that the
temperatures significantly depend on $\theta_d$ as well as $V_d/V_A$. In case
of low proton parallel beta ($\beta_{p{\parallel}} < 1$), it is found that the
proton perpendicular temperature is clearly enhanced when $\theta_d$ is small
($\lesssim 45^\circ$) and $V_d/V_A \gtrsim 0.5$. On the contrary, the
perpendicular temperature of alpha particles is considerably enhanced when
$\theta_d$ is large ($\gtrsim 90^\circ$) or $V_d/V_A$ is sufficiently small.
The maximum of proton parallel temperature takes place at $\theta_d \sim
90^\circ$ accompanied by higher $\beta_{p{\parallel}}$ and by larger turbulence
amplitude of magnetic fluctuations in inertial range. This study should present
strong evidence for cyclotron resonance heating of protons and alpha particles
in the solar wind. Other mechanisms including Landau resonance and stochastic
heating are also proposed, which tend to have different ($\theta_d$, $V_d/V_A$)
spaces than cyclotron resonance heating.
