... constant.

Global charge conservation would normally imply that $N_+ - N_-$ is contsant and equal to the number protons in the colliding nuclei. However, the particles observed in the central region of phase space reflect the participant parts of the nuclei rather than the whole nuclei. It is thus reasonable to expect that $N_+ - N_-$ is proportional to $N_{ch}$, which is also supported by the experimental data.

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