| Preprint | |
| Report number | DIRAC-NOTE-2023-01 |
| Title | The investigation of the Coulomb interaction in $\pi^+ \pi^-$ pair production and application of the effect to the event simulation and setup description |
| Author(s) | Benelli, A. (CTU, Prague) ; Nemenov, L. (JINR, Dubna) ; Pentia, M. (IFIN-HH, Bucharest) ; Smolik, J. (CTU, Prague) |
| Publication | 2023 |
| Imprint | 23 July 2023 |
| Number of pages | 16 |
| Subject category | Particle Physics - Experiment |
| Accelerator/Facility, Experiment | CERN PS ; DIRAC PS212 |
| Abstract | In this work we present the Coulomb effects in the π +π − pair analysis and show the application of these effects in high energy physics investigations. The π +π − pairs were generated in p − Ni interaction at the proton momentum of 24 GeV/c and detected by the DIRAC setup installed on the PS CERN beam. The Coulomb effects (Coulomb correlations) were studied using experimental pair distributions in Q, the relative momentum in the pair center of mass system (c.m.s), and its projections QL (longitudinal component) and Qt (transverse component). The major part of pion pairs π +π − was produced by decay of ρ,ω and ∆ and other short-lived sources (Coulomb pairs). In these pairs the significant Coulomb interaction at small Q occurs. The minor part of the pairs are produced if one or both pions arose from long-lived sources like η,η 0 . In this case the distance between particles will be larger, and the Coulomb interaction in the final state is practically absent. These pairs are defined as non-Coulomb pairs. The Q, QL and Qt distributions of Coulomb pairs in the c.m.s. were simulated assuming that they are described by the phase space modified by the known Coulomb correlation function AC(Q). The same spectra of the non-Coulomb pairs were simulated without AC(Q). The experimental π +π − pairs were analyzed in the intervals 0 < Qt < 5 MeV/c and -20 MeV/c < QL <20 MeV/c. All the events were divided into nine Qt intervals, and for each interval the QL spectrum was obtained which shows peak around QL = 0 caused by the Coulomb final state interaction. The full width at half maximum height increases with Qt from 3 MeV/c for (0 < Qt < 0.25 MeV/c) to 11 MeV/c for (4.0< Qt <5.0 MeV/c). The employed matrix element described the experimental distribution shape as the sum of the Coulomb and non-Coulomb pairs with two free parameters: the ratio of the short-lived and long-lived sources and the normalization constant. The matrix element contribution to the error in the number of Coulomb pairs is less than 2%. It was shown that with this matrix element, the number of pairs in all Qt intervals, including the small Qt , is calculated with the theoretical precision better than 2%. The Qt values in the center of mass system and in the laboratory system (l.s.) are the same. Therefore the pairs with the minimal Qt and the maximum total pair momentum in the l.s. have the minimum opening angles θ and the distance D in the same system. The comparison of the simulated number of Coulomb pairs and the experimental number of pairs at small Qt allows checking and to correcting the detection efficiency for the pairs with small θ (0.05 mrad and smaller) in the laboratory system. |
Zapis kreiran 2024-02-16, zadnja izmjena 2024-02-16