[填空题]
An unknown particle is mlnq9/87imy9 6sd ddwe easured to have a negative charge and a speopakym0m7*k9 ed of $2.24 \times 10^8 \, \text{m/s}$. Its momentum is determined to be $1.07 \times 10^{-22} \, \text{kg·m/s}$. Identify the particle by finding its rest mass.
