Speaker
Description
Precise determination of the event collision time t_{0} is essential for particle identification with time-of-flight detectors. In the ALICE experiment, the event time can be reconstructed from measured TOF signals by comparing them with expected flight times under different particle-mass hypotheses [1,2]. The corresponding optimization problem is combinatorial, and practical reconstruction procedures therefore employ restricted hypothesis searches and rejection criteria.
We present an exact polynomial-time method for global minimization of the event-time objective function. The algorithm determines the optimal particle hypotheses together with the corresponding t0 without exhaustive enumeration of all hypothesis assignments. The method was validated against brute-force enumeration at low multiplicity, where identical particle assignments, event times, and objective-function values were obtained. It was further benchmarked against the event-time reconstruction procedure implemented in the ALICE O2 framework [3,4] using simulated pp and Pb-Pb events. For typical events, the optimized O2 procedure remains highly competitive in execution time, while the exact method provides a guaranteed global optimum and can therefore serve as a reference for validating heuristic reconstruction approaches.
We also demonstrate ambiguous event configurations for which the restricted search converges to a suboptimal solution, producing a shifted reconstructed event time and incorrect particle assignments, whereas the exact algorithm recovers the global minimum. The proposed method can therefore be used both as a validation tool and as a component of future hybrid event-time reconstruction algorithms.
References
[1] ALICE Collaboration, “Determination of the event collision time with the ALICE detector at the LHC,” European Physical Journal Plus 132, 99 (2017). doi:10.1140/epjp/i2017-11279-1
[2] A. Alici for the ALICE Collaboration, “Particle identification with the ALICE Time-Of-Flight detector at the LHC,” Nuclear Instruments and Methods in Physics Research A 766, 288–291 (2014). doi:10.1016/j.nima.2014.05.059
[3] G. Eulisse and D. Rohr for the ALICE Collaboration, “The O2 software framework and GPU usage in ALICE online and offline reconstruction in Run 3,” EPJ Web of Conferences 295, 05022 (2024). doi:10.1051/epjconf/202429505022
[4] ALICE O2, TOF EventTimeMaker implementation and documentation. EventTimeMaker.h
