In the paper of Green and Tao "Restriction Theory of the Selberg Sieve, with applications," their theorem 6.1 states: Let $N$ be a large integer. Then the number of Chen primes in the interval $(N/2,N)$ is at least $c_1N/\ln^2N$, for some absolute constant $c_1>0$.

My question is, what the heck is $c_1$? Is it Brun's constant, or is that just wishful thinking?