BEGIN:VCALENDAR VERSION:2.0 PRODID:-//CERN//INDICO//EN BEGIN:VEVENT SUMMARY:Particle multiplicity fluctuations of quantum origin and high-mu ltiplicity pp collision events DTSTART;VALUE=DATE-TIME:20260923T075000Z DTEND;VALUE=DATE-TIME:20260923T081000Z DTSTAMP;VALUE=DATE-TIME:20260914T053145Z UID:indico-contribution-523@indico.bitp.kiev.ua DESCRIPTION:Speakers: Sergiy Akkelin (Bogolyubov Institute for Theoretical Physics of the National Academy of Sciences of Ukraine)\nThe multiplicity dependence of the two-particle Bose-Einstein momentum correlation radii i n pp collisions at the LHC has been measured at a fixed energy of collisio ns. One notable feature of these measurements is that the effective system ’s volume\, when extracted from the correlation radius parameters\, appe ars to scale nearly linearly with charged-particle multiplicity [1-3]\, ex cept for low multiplicities where initial state effects dominate and\, qui te unexpectedly\, except for very high multiplicities where\, at variance with the expected behavior for emission from hydrodynamically expanding sy stems\, one sees systematic deviations from such a scaling behavior. That is\, the striking feature of the data is that the correlation radius param eters become approximately independent of the particle multiplicity in the limit of very high multiplicities [1\,2]. The question naturally arises: How can this controversy with hydrodynamics be resolved?\nIn Ref. [4]\, we argue that quantum-origin multiplicity fluctuations dominate the fluctuat ions in particle multiplicity in high-multiplicity pp collision events at a fixed LHC energy. The quantum-to-classical transition of multiplicity fl uctuations happens when the interference term between the corresponding st ates becomes negligible. Until this happens\, superposition cannot be inte rpreted as a classical ensemble of its component states. To connect quantu m fluctuations of particle multiplicity to event-by-event multiplicity flu ctuations\, we introduce the statistical ensemble of the multiplicity fluc tuations of particles after the system’s freeze-out. We utilize the maxi mum entropy prescription to determine a quantum state at kinetic freeze-ou t and derive expressions for multiplicity-selected particle momentum spect ra and correlation functions by applying a fixed particle-number constrain t to this state. Then\, we study how multiplicity fluctuations at high mul tiplicities impact the multiplicity-selected particle momentum spectra and correlation functions. In particular\, we find that the effective system ’s volume should become approximately independent of the particle multip licity in the limit of very high multiplicities in multiplicity-selected c ollision events.\n\n\nReferences\n\n[1] ATLAS Collaboration\, Eur. Phys. J . C 75\, 466 (2015)\, https://doi.org/10.1140/epjc/s10052-015-3644-x\; 82\ , 608 (2022)\, https://doi.org/10.1140/epjc/s10052-022-10472-0 .\n[2] A. M. Sirunyan et al. (CMS Collaboration)\, Phys. Rev. C 97\, 064912 (2018)\, https://doi.org/10.1103/PhysRevC.97.064912 \; J. High Energy Phys. 03 (2 020) 014\, https://doi.org/10.1007/JHEP03(2020)014 .\n[3] S. Acharya et al . (ALICE Collaboration)\, Phys. Rev. C 109\, 024915 (2024)\, https://doi.o rg/10.1103/PhysRevC.109.024915 .\n[4] S.V. Akkelin\, Phys. Rev. D 112\, 11 6007 (2025)\, https://doi.org/10.1103/k7mv-57bt .\n\nhttps://indico.bitp. kiev.ua/event/18/contributions/523/ LOCATION:Bogolyubov Institute for Theoretical Physics Conference Hall URL:https://indico.bitp.kiev.ua/event/18/contributions/523/ END:VEVENT END:VCALENDAR