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Manuscript Title: PHOTOS Interface in C++
Authors: N. Davidson, T. Przedzinski, Z. Was
Program title: Photos++
Catalogue identifier: AEYF_v1_0
Distribution format: tar.gz
Journal reference: Comput. Phys. Commun. 199(2016)86
Programming language: C++.
Computer: PC.
Operating system: Linux, MacOS.
RAM: Bytes. Libraries take less than 2MB. Memory complexity is O(n) with around 2-4MB for events with 10k particles.
Keywords: PHOTOS, QED, Bremsstrahlung, FSR, Photon emission in decays, Final state radiation.
Classification: 11.1, 11.2.

Nature of problem:
Algorithm described in this paper can be used to add final state radiation to the event generated by external software using selected event record format. It can also be used on a sample of events loaded from data file. User can define parts of the decay tree on which algorithm can be invoked. The influence of the next-to-leading-order corrections, along with other options regarding electron-positron pair, muon pair and photon emission, can be studied.

Solution method:
The event record is traversed and a list of all decaying particles is created. Decays where program is not supposed to act and decays excluded by the user are removed from the list. The photon and pair adding algorithm is invoked separately on each remaining decay. If one or more particle is added to the decay, the kinematic of the whole decay tree is updated.

Application of the algorithm strongly depends on the content on the event record. Insufficient precision or missing information may deteriorate quality of the results of the algorithm or prevent algorithm from working on the event or its parts. See Section 2 for more details.

Running time:
10-30 seconds per 100k events for small events (less than 1k particles). The complexity strongly depends on the event content and user selection of excluded decays. The theoretical pessimistic complexity of the algorithm is O(n2). However, such cases are highly unrealistic. In our tests, the average complexity is around O(n1.2).