Energy dependence of transverse momentum fluctuations in Pb+Pb   collisions at the CERN Super Proton Synchrotron (SPS) at 20A to 158A GeV
  Results are presented on event-by-event fluctuations of transverse momenta $p_T$ in central Pb+Pb interactions at 20$A$, 30$A$, 40$A$, 80$A$, and 158$A$ GeV. The analysis was performed for charged particles at forward center-of-mass rapidity ($1.1 < y^{*}_{\pi} < 2.6$). Three fluctuation measures were studied: the distribution of average transverse momentum - $M(p_T)$ - in the event, the $\Phi_{p_{T}}$ fluctuation measure and two-particle transverse momentum correlations. Fluctuations of $p_T$ are small and show no significant energy dependence in the energy range of the CERN Super Proton Synchrotron. Results are compared with QCD-inspired predictions for the critical point, and with the UrQMD model. Transverse momentum fluctuations, similar to multiplicity fluctuations, do not show the increase expected for freeze-out near the critical point of QCD. 
The NA49 Collaboration