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The unprecedented size of the LHCb dataset gives an opportunity to study very rare decays of heavy quarks and leptons. Searching and measuring these suppressed decays allow us to precisely test the mechanisms that suppress them in the Standard Model and have the potential to uncover dynamics beyond the Standard Model. The study of rare decays represents an important part of the LHCb physics program (see the publications of the LHCb Rare Decays Working Group). The Bicocca group contributes to various analyses, as detailed in the following.
Search for the Lepton Flavor Violating decay τ→μμμ
Search for the Lepton Flavor Violating decay τ→μμμ
Lepton flavour is foreseen to be strictly conserved in the SM. In the SM extension, needed for the introduction of massive neutrinos, charged lepton decays with a violation of lepton flavour conservation remain highly suppressed. Several models of New Physics that explain possible violations of lepton universality also foresee a LFV. Any observation of a LFV decay would be a signal of New Physics. Particularly interesting for a search at LHC is the decay of a tau lepton into three muons. Previous searches by several experiments have set upper limits on the branching fraction at the 10⁻⁸ level. Improvements in the current LHCb result obtained on Run1 data are foreseen with the analysis of available Run2 data and future Run3 data.
Ongoing Analysis
Ongoing Analysis
Search for the lepton flavour violating decay τ− -> μ−μ+μ− with Run2 data.
Bibliography
Bibliography
Search for the lepton flavour violating decay τ− → μ−μ+μ− LHCb Collaboration, R. Aaij et al JHEP02(2015)121
Test of lepton universality with rare b→sℓℓ transitions
Test of lepton universality with rare b→sℓℓ transitions
Measurements of the ratio R_K⁽*⁾ between B→K⁽*⁾µµ and B→K⁽*⁾ee decays.
Decays b→s of a beauty quark to a strange quark are allowed only at the loop level in the SM and therefore are very rare. They are sensitive to small contributions of physics beyond the SM.
In particular, if the SM symmetry of lepton universality is broken at a very high energy scale, the effect could be detected by testing the symmetry precisely in b→s transitions accompanied by charged leptons of different flavours such as electrons and muons. LHCb made these tests with world-leading precision and did not observe any deviation from lepton universality.
The Bicocca group is involved in further tests of this kind using angular observables as well as the difficult phase space region of high q².
Ongoing Analysis
Ongoing Analysis
Lepton Universality test through an angular analysis of B→K*ℓℓ at central q².
First Lepton Universality test R_K* at high q².
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