Bose-Einstein condensation in liquid 4He near the liquid-solid transition line

We present precision neutron scattering measurements of the Bose-Einstein condensate (BEC) fraction n ₀(T) and the atomic momentum distribution n ^→(k) of liquid 4He as a function of temperature at pressure p=24 bars. Both the temperature dependence of n ₀(T) and of the width of n ^→(k) are determined. The n ₀(T) can be represented by n ₀(T)=n ₀(0)[1- ⁽T ^/T _λ ⁾γ] with a small n ₀(0)=2. 80±0.20% and large γ=13±2 for T<T _λ, indicating a strong interaction. The onset of BEC is accompanied by a significant narrowing of the n ^→(k). The narrowing accounts for 65% of the drop in kinetic energy below T _λ and reveals an important coupling between BEC and k>0 states. The experimental results are well reproduced by path integral Monte Carlo calculations.