Could scientist initiate a proton decay and create a mini black hole?

Even if a scientist could initiate proton decay, the process is extremely slow.

Black holes have to do with mass and gravity, not proton decay.

Side fact:  If all the mass in the earth was pushed into the size of a marble, it would be a black hole.

Protons decay all the time.  Over a very long period, they decay into neutrons after losing their positive charges.   This happens when the two up and one down quarks become one up and two down quarks.  

They do not create black holes as I suspect you realize at this time.  If they did, there would be a heck of a lot more of black holes than there are.

Black holes are produced when mass is squeezed down to a point size in space under its own gravitational pull.  When massive stars die out and collapse in on themselves, they can form small black holes.  

But when that collapse is in or near the center of a galaxy, where stars are tightly packed, that fledgling black hole can begin to swallow up the nearby stars, adding to the captured and squeezed mass.  And that adds to the size of the hole and its gravitational pull, which then pulls in more stars from farther out.

The so-called accretion of mass is self limiting.  The black hole can only swallow so much.  Then it sort of burps and stops eating.  Even so they can reach the equivalent of 100 to 1000 Suns in equivalent mass, which puts out a lot of gravity field as you might imagine.

In the end, most if not all galaxies have at least one of what is known as a massive black hole at its center.  This includes our own Milky Way galaxy.  More than one black hole is possible, but eventually, one or the other black hole will merge with its companion, making an even more massive single black hole in the center of the galaxy.