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Short-distance constraints on the hadronic light-by-light contribution to the muon g-2

Applicant Colangelo Gilberto
Number 178915
Funding scheme Project funding
Research institution Institut für Theoretische Physik Universität Bern
Institution of higher education University of Berne - BE
Main discipline Theoretical Physics
Start/End 01.04.2018 - 31.03.2022
Approved amount 208'945.00
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Keywords (3)

anomalous magnetic moment of the muon; operator product expansion; Quantum chromodynamics

Lay Summary (Italian)

Lead
L'obiettivo di questo progetto e` l'applicazione di metodi di calcolo perturbativi per derivare il limite di alta energia del contributo adronico al processo di diffusione fotone su fotone, e divalutarne l'impatto sul calcolo del contributo adronico al momento magnetico anomalo del muone.
Lay summary
Questo progetto ne affianca uno gia` approvato, il cui obiettivo e` di migliorare 
la precisione delle predizioni teoriche del modello standard per il momento di dipolo 
magnetico del muone che sta per essere rimisurato al "Fermilab" negli USA. 
L'obiettivo della nuova misura e` una riduzione di un fattore 4 dell'errore 
sperimentale. La precisione della misura attuale e` arrivata a 0.6 parti 
per milione e i calcoli teorici hanno raggiunto una precisione simile. 
Un miglioramento sul fronte sperimentale va quindi accompagnato da un 
analogo miglioramento teorico.

Diversi gruppi di teorici in tutto il mondo stanno investendo tempo e 
risorse nel cercare di migliorare questi calcoli. Anche il nostro 
dell'universita` di Berna si impegnato su questo fronte ed e` riuscito a 
sviluppare un nuovo metodo sistematico per calcolare un contributo che 
finora era stato stimato solo sulla base di modelli. Questo contributo, che e` 
noto con il nome di "hadronic light-by-light", e` uno dei due maggiormente 
responsabili dell'incertezza teorica. L'approccio sviluppato ha il vantaggio 
di essere sistematico, ma e` anche notevolmente complicato. 

Il presente progetto ha come specifico obiettivo quello di derivare, per poi tenerne
conto nel calcolo, il limite di alta energia del contributo adronico all'ampiezza 
di diffusione di fotone su fotone. In questo limite il calcolo si puo` fare con 
metodi perturbativi in cromodinamica quantistica, un approccio che non puo` essere
utilizzato a basse energie, da dove proviene la maggior parte di questo contributo
al momento magnetico anomalo del muone. Il calcolo del contributo di basse energie e`
fatto con altri metodi ed e` obiettivo dell'altro progetto, ma e` importante avere
una descrizione coerente a tutte le energie.
Direct link to Lay Summary Last update: 11.04.2018

Responsible applicant and co-applicants

Employees

Publications

Publication
2-loop short-distance constraints for the $g-2$ HLbL
Bijnens Johan, Hermansson-Truedsson Nils, Laub Laetitia, Rodríguez-Sánchez Antonio (2021), 2-loop short-distance constraints for the $g-2$ HLbL, in 24th High-Energy Physics International Conference in Quantum Chromodynamics, 1.
Short-distance constraints for the longitudinal component of the hadronic light-by-light amplitude: an update
Colangelo Gilberto, Hagelstein Franziska, Hoferichter Martin, Laub Laetitia, Stoffer Peter (2021), Short-distance constraints for the longitudinal component of the hadronic light-by-light amplitude: an update, in Eur. Phys. J. C, 81(8), 702-702.
Short-distance HLbL contributions to the muon g-2
Bijnens Johan, Hermansson-Truedsson Nils, Laub Laetitia, Rodriguez-Sanchez Antonio (2021), Short-distance HLbL contributions to the muon g-2, 312-317, 180-184, Elsevier, Nuclear and Particle Physics Proceedings 312-317, 180-184.
The two-loop perturbative correction to the $(g − 2)_μ$ HLbL at short distances
Bijnens Johan, Hermansson-Truedsson Nils, Laub Laetitia, Rodríguez-Sánchez Antonio (2021), The two-loop perturbative correction to the $(g − 2)_μ$ HLbL at short distances, in JHEP, 04, 240-240.
Longitudinal short-distance constraints for the hadronic light-by-light contribution to $(g-2)_μ$ with large-$N_c$ Regge models
Colangelo Gilberto, Hagelstein Franziska, Hoferichter Martin, Laub Laetitia, Stoffer Peter (2020), Longitudinal short-distance constraints for the hadronic light-by-light contribution to $(g-2)_μ$ with large-$N_c$ Regge models, in JHEP, 03, 101-101.
Short-distance constraints on hadronic light-by-light scattering in the anomalous magnetic moment of the muon
Colangelo Gilberto, Hagelstein Franziska, Hoferichter Martin, Laub Laetitia, Stoffer Peter (2020), Short-distance constraints on hadronic light-by-light scattering in the anomalous magnetic moment of the muon, in Phys. Rev. D, 101(5), 051501-051501.
Short-distance HLbL contributions to the muon anomalous magnetic moment beyond perturbation theory
Bijnens Johan, Hermansson-Truedsson Nils, Laub Laetitia, Rodríguez-Sánchez Antonio (2020), Short-distance HLbL contributions to the muon anomalous magnetic moment beyond perturbation theory, in JHEP, 10, 203-203.
The anomalous magnetic moment of the muon in the Standard Model
Aoyama T., others (2020), The anomalous magnetic moment of the muon in the Standard Model, in Phys. Rept., 887, 1-166.

Scientific events

Active participation

Title Type of contribution Title of article or contribution Date Place Persons involved
INT-19-74W Workshop Hadronic contributions to (g-2)μ Talk given at a conference Pseudoscalar poles and longitudinal short-distance constraints 09.09.2019 Seattle, United States of America Laub Laetitia;


Associated projects

Number Title Start Funding scheme
175791 Strong interaction at low energy, flavour physics and the search for new physics 01.11.2017 Project funding
200553 Strong interaction at low energy, Muon g-2 and the search for new physics 01.10.2021 Project funding

Abstract

Anomalous magnetic moment of leptons have played a central role inestablishing quantum field theory as the successful framework fordescribing the fundamental laws of Nature. Nowadays, they still representbenchmark observables being among the most accurately measured andpredicted quantities in particle physics.The anomalous magnetic moment of the muon $a_\mu=(g-2)_\mu/2$, in particular, enjoys aspecial status for two reasons. First, as compared to its electron sibling,it shows an enhanced sensitivity to weak and strong interaction effects andmore importantly to New Physics scales. Secondly, $a_\mu$ is one of thefew observables to exhibit a significant discrepancy (of about$3\,{\rm{\sigma}}$) with respect to its Standard Model (SM) determination~\cite{Hagiwara:2011af,Jegerlehner:2009ry}.The origin of this discrepancy is unknown and its explanation is a toppriority for particle physicists worldwide. According to our presentunderstanding, it could be due to poorly understood effects on theexperimental and/or SM theory side, or could represent a genuine hint ofNew Physics. In order to clarify this issue, two new experiments have beendesigned (FNAL E989 and J-PARC E34) with the goal to improve in the nextfew years the already astonishing accuracy of 0.54 parts per millionreached by previous measurements. This calls for improved theorypredictions. SM uncertainties are dominated by virtual low-energy stronginteraction effects that cannot be computed using perturbative quantumchromodynamics (QCD). These are dominated by the hadronic vacuumpolarization (HVP) as well as the hadronic light-by-light (HLbL) scatteringcorrections. The former is straightforwardly related to measurable hadronproduction cross sections while to compute the latter people had to rely onhadronic models, which introduce systematic errors that are impossible toquantify~\cite{Jegerlehner:2009ry,Nyffeler:2010rd}. In a recent theory breakthrough, we have set up a noveldispersive formalism for the first data-driven determination of \amu~andproduced first numerical estimates within this approach~\cite{Colangelo:2014dfa,Colangelo:2014pva,Colangelo:2014qya,Colangelo:2015ama,Colangelo:2017qdm,Colangelo:2017fiz}. In the proposedresearch, we will carefully analyze and take into account all possible short distance constraints on the hadronic light-by-light tensor. This represents a crucial step for the completion of our evaluation of \amu~with better controlled uncertainties, with the aim ofbeing sufficiently accurate to make experimental measurements of the muonanomalous magnetic moment a decisively stringent test of the SM.
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