  The preliminary NA49 results on the energy dependence of transverse momentum
fluctuations over the whole SPS energy range exhibit an unexpected effect. The
$\Phi_{p_{T}}$ fluctuation measure - used by the NA49 experiment - manifests a
different behavior for different charge combinations. Whereas the
$\Phi_{p_{T}}$ is consistent with zero and independent of energy for negatively
charged particles, it significantly increases for both all charged and
positively charged particles at lower SPS energies. The string-hadronic model
UrQMD is applied here to explain this effect. The UrQMD simulations show that
the number of protons is strongly correlated with impact parameter and that the
event-by-event impact parameter fluctuations are responsible for the
event-by-event transverse momentum fluctuations of positively charged and all
charged particles where protons are included. The observations presented in
this article are important for all experiments measuring event-by-event
fluctuations, especially for those using all charged particles in the analysis.
The results can be also crucial for detectors with acceptances extending to the
beam/target spectator domains.
