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Strong interaction at low energy, Muon g-2 and the search for new physics

English title Strong interaction at low energy, Muon g-2 and the search for new physics
Applicant Colangelo Gilberto
Number 200553
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 01.10.2021 - 30.09.2025
Approved amount 648'621.00
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Keywords (10)

standard model; dispersion relations; chiral perturbation theory; new physics searches; isospin symmetry breaking; effective field theories; muon; anomalous magnetic moment; hadronic physics; quantum chromodynamics

Lay Summary (Italian)

Obiettivo di questo progetto è di migliorare la precisione delle predizioni teoriche per il dipolo magnetico del muone che è stato appena misurato al "Fermilab" negli USA, confermando la precendente misura dei laboratori di Brookhaven (USA) vent'anni fa. Il confronto tra teoria ed esperimento mostrauna differenza di 4,2 deviazioni standard, vicina quindi alla soglia di 5, al disopra della quale si puo` parlare di "scoperta" di nuovi effetti.
Lay summary
Obiettivo finale della nuova misura è una riduzione di un fattore 4 
dell'errore sperimentale rispetto alla misura precedente, che era di 0.6 parti 
per milione. Perché la precisione della misura sperimentale venga sfruttata 
appieno è necessario che l'incertezza teorica sia inferiore o simile a quella 
sperimentale, come è attualmente. In vista dell'obiettivo finale della 
nuova misura, è quindi necessario affinare i calcoli teorici.

Il lavoro in questa direzione procede già da tempo e vede coinvolto
il nostro gruppo di ricerca dell'università di Berna, che è riuscito a 
sviluppare un nuovo metodo sistematico per calcolare un contributo che 
finora era stato stimato solo sulla base di modelli. Questo contributo, che è 
noto con il nome di "hadronic light-by-light", è uno dei due principali responsabili 
dell'incertezza teorica. L'approccio sviluppato ha il vantaggio di essere 
sistematico, ma è anche tecnicamente complicato. Il completamento di questo 
calcolo entro i prossimi quattro anni è uno dei principali obiettivi di 
questo progetto. Il secondo è di migliorare il calcolo dell'altro contributo 
principale responsabile dell'incertezza teorica ("hadronic 
vacuum polarization") sulla base di metodi analoghi.

Se i valori centrali, sia della misura sperimentale che del calcolo teorico,
verranno confermati, e gli errori ridotti di un fattore 4 per entrambi, 
questo rappresenterà una chiara evidenza sperimentale dell'esistenza
di nuove particelle, non incluse nel modello standard (la teoria che descrive
in maniera matematicamente consistente le interazioni fondamentali delle 
particelle elementari), ed ancora mai osservate. Questa sarebbe una scoperta
di fondamentale importanza e darebbe nuovo stimolo alla ricerca di queste 
particelle agli acceleratori come l'LHC.
Direct link to Lay Summary Last update: 30.08.2021

Responsible applicant and co-applicants


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
178915 Short-distance constraints on the hadronic light-by-light contribution to the muon g-2 01.04.2018 Project funding


The present high-energy frontier (LHC) hasn't brought the discoveries whichwere hoped for, besides the Higgs. The search continues on that fronttoo, but until a further significant increase in energy will come, the LHCwill slowly move to become part of the high-precision frontier. Indeedseveral of the puzzling and interesting results coming from the LHC are dueto high-precision measurements, and will continue to be improved in thenear future. The high-precision frontier is explored and pushed furtherelsewhere too. $B$-physics, $K$-physics as well as lepton (both charged aswell as neutral) physics experiments are carried out at various labs aroundthe world. They are all addressing interesting questions or puzzles, whichwill help bring our knowledge further.One of the most significant and long-standing discrepancies with the SMremains that of the $(g-2)$ of the muon. As it first showed up, thediscrepancy between measurement and SM prediction was called a ``harbingerof new physics'', but now that the bounds on the masses of allbeyond-the-Standard-Model (BSM) particles have moved up as a consequence ofthe unsuccessful searches at the LHC, the same discrepancy has become atrue puzzle. To clarify it, a new generation of $(g-2)_\mu$ measurementshas been planned: the Fermilab Muon $(g-2)$ experiment is running andshould soon deliver its first result and a new experiment (Muon $g-2$/EDMexperiment), based on a completely different experimental set-up, iscurrently being developed at J-Parc in Japan.On the theory side, similar efforts are being made, in particularfor the hadronic contributions which are responsible for almost 100\% ofthe uncertainty in the SM prediction. In my previous SNF project I set outto carry further the dispersive approach to the so-called hadroniclight-by-light scattering (HLbL) contribution to the $(g-2)_\mu$. The finalgoal is a model-independent evaluation and a significantly reduceduncertainty with respect to earlier model calculations. In addition, wehave shown how the dispersive approach can improve the evaluation of thetwo-pion contribution to the hadronic vacuum polarization (HVP)contribution, beyond what is usually done in a pure data-driven approach.We have also started an effort to apply these techniques to the evaluationof radiative corrections to the HVP contribution---at the level ofprecision reached, these have become critical, and have so far beencalculated only on the basis of models.Much progress has been made by our group in the past three years, most ofwhich has played a significant role in building the new theoretical SMprediction for the $(g-2)_\mu$, which has recently appeared as a ``WhitePaper'', whose writing I have coordinated togetherwith Aida El-Khadra and Christoph Lehner (chairs) and the other colleaguesof the Steering Committee of the Muon $g-2$ Theory Initiative. Even though much progress has been made, we are not finished yet. A betterunderstanding of the role of short distance in HLbL is urgently needed,much as an improved evaluation of a number of other contributions which arelikely small, but which are particularly difficult to calculate in areliable, systematic way. Our work on the two-pion contribution to HVPneeds to be integrated with the full evaluation of this contribution, and arigorous estimate of radiative corrections needs to be completed. Moreover,a recent lattice calculation of the HVP, with much reduced uncertainties,would bring the SM in agreement with experiment, but is in flatdisagreement with dispersive evaluations. While this needs validation fromother lattice calculations, the disagreement with the dispersive approachalso needs to be understood and constrained: we are already working onthat. These are the main goals of this project. The coming four years willbe crucial for these tasks, which we aim to complete before the Fermilabexperiments releases its final result.