已知单位向量$\stackrel{\rightharpoonup }{e_1}$与$\stackrel{\rightharpoonup }{e_2}$的夹角为$\alpha$，且$\cos \alpha =\frac{1}{3}$，向量$\stackrel{\rightharpoonup }{a}=3\stackrel{\rightharpoonup }{e_1}-2\stackrel{\rightharpoonup }{e_2}$与$\stackrel{\rightharpoonup }{b}=3\stackrel{\rightharpoonup }{e_1}-\stackrel{\rightharpoonup }{e_2}$的夹角为$\beta$，则$\cos \beta =$.