  Although it is widely accepted that the electromagnetic spectrum from radio
to very-high-energy $\gamma$-rays of pulsar wind nebulae (PWNe) originates from
leptons, there is still an open question that protons (or more generally, ions)
may exist in pulsar wind and are further accelerated in PWN. The broadband
spectrum of the prototype PWN Crab, extended recently by the detection of the
Tibet AS$\gamma$ and HAWC experiments above 100 TeV, may be helpful in
constraining the acceleration efficiency of ions. Here, we model the broadest
energy spectrum of Crab and find that the broadband spectrum can be explained
by the one-zone leptonic model in which the electrons/positrons produce the
emission from radio to soft $\gamma$-rays via the synchrotron process, and
simultaneously generate the GeV-TeV $\gamma$-rays through inverse Compton
scattering including the synchrotron self-Compton process. In the framework of
this leptonic model, the fraction of energy converted into the energetic
protons is constrained to be below $0.5\ (n_{\rm t}/10\ {\rm cm}^{-3})^{-1}$
per cent, where $n_{\rm t}$ is the target gas density in the Crab. However,
this fraction can be up to $7\ (n_{\rm t}/10\ {\rm cm}^{-3})^{-1}$ per cent if
only the $\gamma$-rays are used.
