Speaker
Description
In ultraperipheral collisions (UPCs) of relativistic heavy ions, the strong interaction is suppressed, providing a clean environment to study photon-nucleus and photon-photon interactions. Based on the 2025 Pb-Pb UPC data collected by the CMS experiment and developing a dedicated Monte Carlo event generator, this thesis focuses on the photoproduction mechanism of the vector meson excitation state ρ′in the four-pion (4π) final state.
The analysis starts with data reconstruction and event selection, classifying events via neutron multiplicity using the Zero Degree Calorimeters (ZDC) and separating coherent and incoherent processes through transverse momentum spectra. Subsequently, the invariant mass spectrum and transverse momentum spectrum of the 4π final state are analyzed in detail. Addressing the complex intermediate states potentially involved in ρ′ decays, this study places particular emphasis on the polarization angular distribution analysis.
The results reveal a significant cos(2φ) angular modulation in the 4π final state, confirming the effective transfer of photon polarization to the final-state particles. It is found that the magnitudes and signs of the Fourier coefficients A2 and A4 vary significantly with transverse momentum and impact parameter (selected via neutron multiplicity), providing experimental evidence to distinguish contributions from different orbital angular momentum states such as ρ(1450) and ρ(1700). Furthermore, this study successfully develops and validates a Monte Carlo event generator incorporating quantum interference effects, whose preliminary simulation results are consistent with experimental trends.
This work validates the performance of the CMS detector in forward physics and polarization measurements, offering new experimental insights into the photoproduction mechanism of vector mesons and hadron spectroscopy.
| Academic Status | I am a master student |
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