Roots of Riemann zeta: only minus evens and half + imaginary included – that’s where roots come from! (7)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

H[alf] I[maginary] in R[iemann] Z[eta] O[nly] M[inus] E[vens]. The surface is referring to the Riemann hypothesis , a famous unsolved problem in mathematics