ETD

Archivio digitale delle tesi discusse presso l'Università di Pisa

Tesi etd-11192018-125425


Tipo di tesi
Tesi di laurea magistrale
Autore
DELLA GATTA, MARTA
Indirizzo email
andreaagudelovalencia@ymail.com, mdg1990@hotmail.it
URN
etd-11192018-125425
Titolo
Timelike and Spacelike evaluation of the leading order hadronic contribution to the muon g-2
Dipartimento
FISICA
Corso di studi
FISICA
Relatori
relatore Dott. Venanzoni, Graziano
relatore Dott. Passera, Massimo
Parole chiave
  • muon
  • magnetic moment
  • spacelike
Data inizio appello
10/12/2018
Consultabilità
Completa
Riassunto
The subject of the thesis ”Timelike and spacelike evaluation of the leading order hadronic contribution to the muon g-2” is the calculation of the leading hadronic contribution to the anomalous magnetic moment (g-2) of the muon. The measurement of the g-2 of the muon represents a very stringent test of the Standard Model (SM). The most accurate measurements obtained at Brookhaven National Laboratory, Upton, USA show a discrepancy of more than 3 standard deviations with the theoretical prediction of the SM and experiments at Fermilab and in Japan are under way to reduce the experimental error by a factor of 4. The theoretical prediction is limited by the hadronic leading order contribution (Hadronic Vacuum Polarization, HVP) and by the light-light contribution (Hadronic Light-by-Light). The dominant part aμHLO is calculated with help of a dispersion integral over experimental measurements of the total e+e− → hadrons cross-sections, where the contribution to the central value and its error come mostly from the low energy region. Specifically the e+e− → π+π− channel below 2 GeV contributes with 73% to aμHLO. To date the most precise measurements of this cross section, which have achieved a systematic uncertainty up to 0.5%, come from the CMD2, KLOE and BaBar experiments. The calculation of aμHLO via dispersion integral suffers from the complications due to the combination of different data sets with their systematics and correlations and the presence of resonances and threshold effects which make the cross sections highly fluctuating, especially at low energies ( <10 GeV). An alternative way to calculate aμHLO with experimental input from direct measurement of the hadronic part of the photon vacuum polarization in the spacelike region has been recently proposed [1]. In particular a proposal exists, the MUonE experiment [2], which plans to measure the hadronic part of the running of the electromagnetic coupling constant ∆αhadQED(t) as a function of the squared momentum transfer t = q2 < 0, by extracting it from the differential cross section of the elastic scattering μe → μe process. In the Section 1 of this Thesis we will review the status of the theoretical calculation of the muon g-2. In Section 2 we will focus on the hadronic contribution, and will present the main formula for computing aμHLO in timelike and spacelike region. The calculation of aμHLO timelike in low energy region employs the ratio of the experimental cross section for the process e+e− → π+π− and the theoretical approximated cross section for e+e− → μ+μ− which is denoted as R(s). It is integrated from the threshold up to a certain value of s (the center of mass energy) while at high energies it is calculated using the perturbative QCD. The spacelike formula is obtained from the timelike one exchanging the integrals. In Section 3 we will compare the two methodologies for the calculation of the HVP leading order contribution to the muon g-2. For this purpose, the timelike hadronic contribution is calculated, with particular reference to the 2π channel, for 3 different datasets (BaBar, KLOE and CMD2). Through the Gounaris-Sakurai parametrization of the form factor and using the dispersion relation that makes it possible to obtain the hadronic spacelike contribution from the timelike data, the corresponding spacelike predictions of aμ2π are obtained. These predictions differ at percent level in magnitude and shape and represent a limitation for an improvement of the theoretical prediction of the muong-2, which is necessary in view of the new g-2 experiments. On the contrary the evaluations of aμ2π in the spacelike region are in perfect agreement with the timelike ones which is a confirmation of the correctness of the procedure. Future experiments, like MuonE , should be able to distinguish between the different determination of the π+π− hadronic contribution and therefore clarify the current experimental situation.

References
[1] C.M. Carloni Calame, M. Passera, L. Trentadue, G. Venanzoni, A new approach to evaluate the leading hadronic corrections to the muon g-2, Phys.Lett. B746 (2015) 325-329.
[2] G. Abbiendi et al., Measuring the leading hadronic contribution to the muon g-2 via μe scattering, Eur.Phys.J.C 77 (2017) no.3, 139
File