Project

CP violation; effective field theories; dispersion relations; chiral perturbation theory; flavour mixing; isospin symmetry breaking

Lead
In fisica delle particelle si cerca da molti anni di scoprire dei segnali di nuova fisica: effetti incompatibili con il "modello standard" delle interazioni fondamentali. Questa ricerca procede lungo due direttrici principali: alta energia e alta precisione. Aumentando l'energia di collisioni ad acceleratori si cercadi produrre nuove particelle, mentre aumentando la precisione nello studio di fenomeni noti, si cercano piccole deviazioni rispetto allepredizioni del modello standard.
Lay summary
Obiettivo di questo progetto è un miglioramento della precisione di calcoli teorici relativi a osservabili che sono già stati misurati o che verranno misurati con precisione crescente. Solo con questo miglioramento sul fronte teorico sarà possibile ottenere la prova di effetti di nuova fisica da nuove misure sperimentali più precise. La difficoltà dei calcoli teorici è tutta legata alla presenza delle cosiddette interazioni forti. Nonostante esista una teoria che le descrive con successo (la cromodinamica quantistica o QCD), ci è spesso impossibile calcolare delle grandezze all'interno di questa teoria con il desiderato livello di precisione. Per questa teoria non è possibile applicare uno degli strumenti principali e più efficaci nell'effettuare calcoli in teoria dei campi: la teoria delle perturbazioni. Metodi alternativi a questo e che useremo all'interno di questo progetto sono le teorie di campo effettive, le relazioni di dispersione e i metodi numerici collegati alla formulazione della QCD su reticolo. Uno degli obiettivi principali di questo progetto è il miglioramento dei calcoli del momento magnetico anomalo (detto il g-2) del muone , ed in particolare dei contributi adronici a questa grandezza. Recenti sviluppi sperimentali hanno portato a raggiungere la precisione di 0.6 parti per milione nella misura del g-2 del muone, e si è evidenziata una differenza rispetto alle predizioni teoriche all'interno del modello standard di più di tre deviazioni standard. Sul fronte teorico le difficoltà maggiori e il maggior contributo all'incertezza vengono dal contributo adronico, ed è su questo che concentreremo i nostri sforzi.

Direct link to Lay Summary

Last update: 06.11.2013

Lead
Since long the particle physics community is trying to findevidence of effects not explained by the "standard model" of particle interactions. This effort is carried out along two main alleys: the high-energy and the high-precision ones. By increasingthe energy of collisions at accelerators one tries to produce new,yet unseen particles. By increasing the precision of measurements of known phenomena one tries to see tiny deviations from what the standard model predicts.
Lay summary
The efforts carried out at the LHC have so far lead to the discovery of the only missing particle in the standard model: the Higgs boson. Signs of completely new particles, however, are still missing. The goal of this project is to improve the precision of the theoretical calculations of quantities which have been and will be measured with increasing accuracy. Only in this manner one can obtain convincing evidence of new effects from precise experimental measurements. The difficulty of the theoretical calculations lies in the presence of strong interactions -- although we have a theory which we believe describes them exactly (quantum chromodynamics, QCD), often we are not yet able to calculate observables, within this theory, to the desired level of precision. We will use methods like effective field theories or dispersion relations and collaborate with colleagues which use the numerical approach to deal with the strong interactions (lattice QCD). One of the goals of the project is to improve the evaluation of hadronic contributions to the gyromagnetic ratio of the muon. This quantity has been measured with very high accuracy (to about $0.6$ parts per million for $(g-2)_\mu$) and is different by more than three standard deviations from what the standard model predicts. The most delicate part of the standard-model calculations is precisely the hadronic contributions, whose evaluation we aim to improve.

Direct link to Lay Summary

Last update: 06.11.2013

Active participation

Title	Year

Number	Title	Start	Funding scheme

The LHC has been recently shut down and therewith its first period ofrunning brought to an end. The highlight of the past three years is thediscovery of the Higgs boson, thus completing the experimental confirmationof the Standard Model (SM) in all its building blocks and structure. Adetailed experimental study of the couplings of the Higgs boson to itselfand to the other SM particles is a future goal of the second phase of theLHC and of future accelerators, but from measurements performed so far, itseems that the SM predictions are confirmed at the current level ofprecision. This great success of the LHC and of its experiments has itscounterpart in the lack of any sign of new physics -- not that theexperiments are to blame for this, since they have worked even better thanexpected. Nature has not fulfilled our (presumably too naive) expectationsand presents us with the problem of thinking with a fresh mind aboutpossible extensions of the SM to higher energy scales.There are however a few puzzles which are left unexplained. One of the mostresistant to theoretical and experimental scrutiny remains that of the$(g-2)$ of the muon. While ten years ago the discrepancy betweenmeasurement and SM prediction has been called a ``harbinger of newphysics'', the increased bounds on all beyond-the-Standard-Model (BSM)particles makes the same discrepancy a true puzzle. For example, the``natural'' explanation of the discrepancy within a supersymmetric (SUSY)extension of the SM has now become very problematic because the bounds onthe SUSY particles -- especially those of the first and second generation,which are those needed to explain the $(g-2)_\mu$ puzzle -- are now toohigh. Some of the other puzzles have been generated at the LHC itself, like themeasurement of CP violation in $D$-meson decays, which seems to be higherthan what the SM predicts -- here a cautious formulation of the puzzle(``seems'') is mandatory, because of the difficulty in making a reliable SMprediction. A difficulty which at present we do not even know how to solvein the near or medium-term future.In summary, after the first running phase of the LHC the confirmations ofthe SM have increased both in precision and in number. The fundamentalscalar assumed to be responsible for electroweak symmetry breaking has beendiscovered and the flavour and CP violations observed are all compatiblewith the SM. The possibility that the SM is valid up to a scale much higherthan the TeV, despite naturalness arguments, has now to be considered moreseriously. In any case, we will not have new insights on this fundamentalquestion for the coming three years.The current situation makes it even more urgent to continue to work onreducing hadronic uncertainties in observables where there is an apparentdiscrepancy between measurement and SM prediction which is not yet fullyunderstood. This will be the main goal of this project.