Number density in Toward (top), Transverse (middle) and Away (bottom) regions at $\sqrt{s}=0.9$ TeV (left) and $\sqrt{s}=7$ TeV (right). Right and left vertical scales differ by a factor 2. Shaded area in upper plots: systematic uncertainties. Shaded areas in bottom plots: sum in quadrature of statistical and systematic uncertainties. Horizontal error bars: bin width.