The transverse momentum dependent nuclear modification factor (NMF), namely , is investigated for protons produced in Au + Au at within the framework of the isospin-dependent quantum molecular dynamics (IQMD) model. It is found that the radial collective motion during the expansion stage affects the NMF at low transverse momentum a lot. By fitting the transverse mass spectra of protons with the distribution function from the Blast-Wave model, the magnitude of radial flow can be extracted. After removing the contribution from radial flow, the can be regarded as a thermal one and is found to keep unitary at transverse momentum lower than 0.6 GeV/ c and enhance at higher transverse momentum, which can be attributed to the Cronin effect.
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