Figure 4

One-dimensional Gaussian HBT radius in $pp$ collisions at $\sqrt{s}$ = 900 GeV, determined using pion pairs with $kt = 0.1-0.55$ GeV/$c$, $\langle$\kt$\rangle$ = 0.32 GeV/$c$, and shown as a function of the charged-particle multiplicity at midrapidity(full dots) The shaded band represents the systematic errors (see text) For comparison, open symbols, red stars, and green filled boxes represent the data taken at the ISR [24],RHIC [14],and Tevatron [25], respectively