The Standard Model of fundamental interactions (SM) provides a unified description of electromagnetic, weak and strong interactions within the framework of gauge theories. In the last decades, the SM has proven incredibly successful in predicting a large variety of phenomena observed in experimental particle physics. Thanks to the great performances of the Large Hadron Collider (LHC) at CERN, the SM picture has been completed by exploring particle physics up to energies at the TeV scale. The last missing particle of the SM, the Higgs boson - which is responsible for the generation of particle masses via the mechanism of spontaneous symmetry breaking - has been discovered at the LHC. Its properties, still under accurate study, seem to correspond closely to the ones formulated in the sixties.
In spite of its success, many are the theoretical limitations of the SM and the questions it leaves open. Amongst them, the unexplained big number of input parameters, the values of masses and couplings that span order of magnitudes, the presence of massive neutrinos, the explanation of the origin of dark matter and baryon asymmetry. For such reasons, the SM is not considered to be the "fundamental" theory of elementary particle physics. It is generally believed to be an effective theory, substantially valid up to energies larger than those tested so far, which is the low-energy limit of a more fundamental theory, that could include gravity and describe physics up to the Planck scale.
Our research project wants to address some of these issues, getting insights in a variety of phenomena by using different strategies which correspond to the different expertises of the members of our group. In particular, our research will focus on: i) Flavour Physics within the SM and beyond; ii) Phenomenology and precision physics at colliders; iii) Exotic hadron spectroscopy; iv) Dark matter; v) Non-perturbative aspects of strong interactions.
In view of the structure of our project - naturally divided in 5 research lines - we proceed by presenting the subjects of the last two sections of the proposal line by line, redistributing the 5 parts amongst the last two available slots.
i) Flavour Physics within the SM and beyond;
On the phenomenology side, the main point are:
- continue improving our Unitarity Triangle analysis in the Standard Model and beyond, by adding new experimental and theoretical results and obtaining more stringent bounds on NP or, hopefully, indirect signals of it;
- investigate the impact of forthcoming experimental data on B anomalies on new physics models;
- derive constraints on the SM Effective Field Theory by combining data on electroweak precision observables, Higgs production and decay, weak gauge boson scattering, top quark production and decay, and flavour observables.
Most of the above objectives will be reached by improving the HEPfit code, which has been developed by our group and which features state-of-the-art calculations in both flavour and electroweak phenomenology.
The topics described above are at the forefront of indirect searches for NP, and any advance in this direction might bring us to the discovery of physics beyond the SM (L. Silvestrini and E. Franco with G. Martinelli).
On the lattice QCD side, we plan to compute the matrix elements of Delta S=2 four-fermion operators. Access to the CLS ensembles of gauge configurations with Nf=2+1 dynamical flavours of light quarks guarantees us the forefront of lattice QCD simulations. In order to compute the renormalisation and RG-running of the operators in a non-perturbative way, we plan to use the Chirally rotated Schroedinger Functional (X-SF) scheme, which is a recently proposed variant of the SF scheme - a finite volume renormalisation scheme which allows for the non-perturbative computation of RG-running over a range of scales which span 2 order of magnitudes (between Lambda_QCD and the electroweak scale). The X-SF has several advantages over the SF, namely that of being automatically O(a) improved and of having a lower numerical statistical variance. For the renormalisation of the Delta S=2 operators we have access to the SF ensembles generated by the Alpha Coll. for the renormalisation of the quark mass.
In order to obtain the full RG-running of the Delta S=2 operators, we also needs the perturbative NLO anomalous dimension matrix in the X-SF scheme which we will obtain from a 1-loop perturbative computation of the matrix of renormalisation constants (M. Papinutto).
ii) Phenomenology and precision physics at colliders;
For the computation of higher-order perturbative corrections, the main points are:
- the investigation of the role of heavy quarks in the NLO production of a Higgs boson in correspondence with a jet. A precise QCD prediction of the pT distribution of the Higgs boson will improve for instance the understanding on the possibility for the Higgs boson to be a composite state.
- we recently proposed an analytical method for the calculation of massive NLO QCD corrections performing a kinematic pT expansion. We applied the method to the calculation of the cross section of production of two Higgs bosons in hadronic collisions. In the near future, we plan to use this technique for the determination of the production cross section of other important processes for the LHC physics programme, as H+j, H+Z, Z+Z and W+W production.
- the Lepton-pair hadro-production at large pt (Drell-Yan) mediated by Z and W bosons is a very important process at hadron colliders, since it enables a precise determination of the sine of the Weinberg angle and the weak-boson masses. We plan to complete the calculation of the mixed NNLO QCD-EW corrections to the
production cross section of an on-shell Z boson, and to extend the calculation to the complete 2->2 process, with leptons in the final state.
The precision studies will be performed using the most recent techniques for the computation of higher-order quantum corrections, namely the reduction to the Master Integrals and the evaluation of the latter using systems of differential equations. The analytic evaluation of massive corrections will involve new functions for the mathematical expression of the results, as the elliptic polylogarithms, that were recently introduced and are under study now
(R. Bonciani).
Concerning the resummation of small-x logarithms to the next-to-leading logarithmic (NLL) accuracy, this is crucial both for improving the accuracy of theoretical predictions and to understand the reliability of the expansion. Reaching NLL accuracy requires extending the factorization at the basis of the LL resummation to the next order and, due to the complications arising from loop contributions, it is far from trivial. We will start from deep inelastic scattering and then move to the more interesting processes in proton-proton collisions (M. Bonvini).
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