The default Latex font is known as Computer Modern:
from matplotlib import rc
import matplotlib.pylab as plt
rc('font', **{'family': 'serif', 'serif': ['Computer Modern']})
rc('text', usetex=True)
x = plt.linspace(0,5)
plt.plot(x,plt.sin(x))
plt.ylabel(r"This is $\sin(x)$", size=20)
plt.show()
