A proton of mass $m$ collides elastically with a particle of unknown mass at rest. After the collision, the proton and the unknown particle are seen moving at an angle of $90 ^ { \circ }$ with respect to each other. The mass of unknown particle is:\\
(1) $\frac { \mathrm { m } } { \sqrt { 3 } }$\\
(2) $\frac { \mathrm { m } } { 2 }$\\
(3) 2 m\\
(4) m