This title appears in the Scientific Report : 2022 
An exploratory study for the ω − π transition form factor with WASA-at-COSY
Khan, Farha Anjum (Corresponding author)
Goldenbaum, Frank (Thesis advisor)
Experimentelle Hadronstruktur; IKP-1
2022
168 p.
Dissertation, Bergische Universität Wuppertal, 2022
Dissertation / PhD Thesis
Cosmic Matter in the Laboratory
OpenAccess
Please use the identifier: http://hdl.handle.net/2128/33206 in citations.


The electromagnetic transition form factor allows probing the internal structure of mesonsby studying them in the rare Dalitz mode. The internal structure here reveals the in-formation on the quark composition, constituent quark mass, quark-gluon structure andtheir interactions, quark confinement mechanism, and the fact that how confinement af-fects meson internal structure. These form factors are provided as an input to the hadroniccontribution of the anomalous magnetic momentum of muon aμ = (gμ − 2)/2, which isan interesting quantity and could be a potential hint for physics beyond the StandardModel [1, 2, 3, 4, 5, 6, 7, 8, 9]. It is well known that the gyromagnetic ratio g of a lonemuon, which is its rate of precession in an external magnetic field, should be 2 accordingto the Paul Dirac formula. However, the value of the ratio g deviates from 2 due to muonsinteractions with a quantum foam of subatomic particles popping in and out of existence. Al-though, the Standard Model is able to predict this anomaly called anomalous magnetic mo-ment aμ = (gμ − 2)/2 extremely precisely, the difference between the accepted theoreticalpredictions reviewed by Aoyama et al. (2020) [7] (116591810(43)×10−11) and the experi-mental global average published by Albahri et al. (2021) [8, 9] (116592061(41)×10−11) isat a significance of 4.2 sigma. The theoretical efforts by Aoyama et al. (2020) [7] accountfor both the non-perturbative methods of computation, the dispersion relations and the lat-tice approach to QCD. This is compelling evidence of new physics and hints at the existenceof unknown interactions involving additional particles or forces that are not accounted forby the Standard Model. Subsequently, Borsanyi et al. (2021) [10] computed the value of(gμ − 2)/2, which is 7075(55)×10−11, using lattice approach to QCD. This result favoursthe experimentally measured value of (gμ − 2)/2 over the results based on the dispersionrelation.The electromagnetic transition form factors of mesons have been studied experimentallyas well as theoretically. Among various mesons, the ω is one of the mesons which showsdisagreement with the standard vector meson dominance prediction, as determined from thedecay ω → l+ l−π0 [11, 12, 13, 14, 15]. However, its form factor seems to agree with thedata except at larger four-momentum transfer (q2) when theoretical efforts attempt to gobeyond the vector meson dominance [16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26]. Earlier ex-periments provided results on the electromagnetic transition form factor for the ω-π vertex inthe ω → μ+μ−π0 mode, as reviewed by L. G. Landsberg in Ref. [11]. Thereafter, the NA60collaboration has confirmed the results in AA as well as pA collisions [13, 14, 15]. Fur-thermore, in the immediate past, the transition form factor has been recomputed using spin-improved holographic light-front wave-functions for the mesons, which shows an agreementwith the NA60 data in all invariant mass ranges [27]. Moreover, the recent measurementfrom the photon-induced reactions with A2 tagged-photon facility at MAMI shows a betteragreement with most of the theoretical calculations as compared to the previous experi-ments [28]. However, no final conclusion could be drawn from the MAMI data due to thelack of precision and NA60 measurements are limited by its inability to reconstruct the π0meson and the analysis approach, which is entirely based on the MC models. Consequently,more measurements with a different experimental approach, other than heavy-ion collision or photon-induced reactions, and with alternative analysis methods that solely not rely onthe MC models, and with much better statistics are invigorated.WASA-at-COSY detector, which consists of a forward and a central part that nearly cov-ers 4π steradian, is capable of reconstructing the recoil particle in the forward directionand the decay products, e+, e−, γ, in the central part. Thus, inclusive, as well as exclusivereconstruction of the decaying meson, is possible. This reduces the obscurities in back-ground subtraction as compared to NA60 measurements. Moreover, detecting e+e− pairsgives access to the full range of q2 due to kinematics. Furthermore, WASA-at-COSY usesa completely different experimental approach of elementary reactions (hadron-hadron colli-sions) and produces the mesons close to the meson production threshold. Considering theseadvantages, WASA-at-COSY could be proved to be a potential tool to improve our under-standing of the form factor of ω meson. This allowed WASA-at-COSY to investigate theissues noticed by other experiments.The data available for such studies are recorded with WASA-at-COSY using pd and ppcollisions with the focus on doing a feasibility study for the ω − π transition form factorin the ω → e+e−π0 decay mode. The main goal of this thesis is the feasibility study toreconstruct the ω → e+e−π0 decay with the WASA-at-COSY pd collision data recorded at1.45 GeV and 1.50 GeV beam kinetic energies. This feasibility study has been conductedby firstly studying the two major background contributions ω → π0γ and ω → π0π+π−,followed by the ω → e+e−π0 decay.As a first step, the analysis of the prominent real photon case ω → π0γ is established as areference decay for the ω → e+e−π0 mode and its branching ratio is determined. A branch-ing ratio study of the ω → π0γ mode as a reference decay will ensure the control and qualityof the analysis procedure and the data. Furthermore, the most prominent background contri-bution ω → π0π+π−, which is having the same topology as that of the signal ω → e+e−π0,has been investigated as a background study and the data quality cross-check. Finally, theanalysis of the ω → e+e−π0 exclusive final state is established.