Can you break a lease if you reasonably determine that other tenants in your building are dealing drugs
Recently my family and I have relocated from out of state and rented an apartment in Massachusetts. The apartment building has 4 apartments with 3 occupied and one vacant.
Since our arrival we have seen a constant parade of cars pulling up and visiting our upstairs neighbors for 3-4 minutes then leaving - these "visits" are usually at night 7-9pm and the number of visits substantially increase on Friday and Saturday nights (and extend to all hours) In addition, these same neighbors are frequently found in the back of the building at night, with grow lights doing "gardening" work. The plants are obviously marijuana.
We have informed the landlord who told us that he knows all about it and that the police were also aware of the problem and were "investigating". Other residents on the street where the building is located have told us that the building we live in is "a known hangout" for illegal activity and at one point was condemned as a "crack house"
We have reported this activity to the police who would only INFER that they were investigating but would not say if and when anything would be done about it.
Is there any provision under Massachusetts law which allows a tenant to terminate a lease under these conditions or do we have to act under the "quiet enjoyment" provisions; and, can we just move out. Maybe this is a bit paranoid, but we think that eventually we will be drawn into this either by someone breaking into the wrong apartment (ours) looking for drugs or that there will be some kind of shooting - or other violent incident.
Thank you in advance for any advice you can offer.

Drugs in apartment unit
If the situation is bad enough and the landlord fails to correct the problem then, in theory, you could ask a court to terminate your tenancy based on a breach of your right to quiet enjoyment. However, your question raises other issues. I find it hard to believe that the landlord and the police are aware that the upstairs tenants are growing marijuana in the back yard and selling it but are taking no action, unless the house is the subject of an ongoing investigation and the police are working with the landlord.
Drugs are a thorny problem for landlords, not only because bad tenants can chase away good tenants, like you, but because Massachusetts law requires landlords to evict a tenant if the landlord has evidence of illegal drug activity. If the landlord does not take "all reasonable measures" to evict the problem tenant, in a legal manner, he can be subject to fines and even a prison sentence. The problem is: What type of evidence does the landlord need? Must there be an arrest? A conviction? I believe most housing courts will act on arrest and allow the landlord to use the expedited eviction process for drug activity provided in Massachusetts General Laws Chapter 139, Section 19.
In short, I think you need to press the landlord and the police for more information.