Letter of Intent for the LHCb Upgrade

The primary goal of LHCb is to measure the effects of new particles or forces beyond the Standard Model. Results obtained from data collected in 2010 show that the detector is robust and functioning well. While LHCb will be able to measure a host of interesting channels in heavy flavour decays in the upcoming few years, a limit of about 1 fb^ 121 of data per year cannot be overcome without upgrading the detector. The LHC machine does not face such a limitation. With the upgraded detector, read out at 40MHz, a much more flexible software-based triggering strategy will allow a large increase not only in data rate, as the detector would collect 5 fb^ 121 per year, but also the ability to increase trigger efficiencies especially in decays to hadronic final states. In addition, it will be possible to change triggers to explore different physics as LHC discoveries point us to the most interesting channels. Our physics scope extends beyond that of flavour. Possibilities for interesting discoveries exist over a whole variety of phenomena including searches for Majorana neutrinos, exotic Higgs decays and precision electroweak measurements. Here we describe the physics motivations and proposed detector changes for exploring new phenomena in proton-proton collisions near 14 TeV centre-of-mass energy

Precision measurement of the Xi(++)(cc) mass

A measurement of the Ξ++cc mass is performed using data collected by the LHCb experiment between 2016 and 2018 in pp collisions at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 5.6 fb−1. The Ξ++cc candidates are reconstructed via the decay modes Ξ++cc→Λ+cK−π+π+ and Ξ++cc→Ξ+cπ+. The result, 3621.55 ± 0.23 (stat) ± 0.30 (syst) MeV/c2, is the most precise measurement of the Ξ++cc mass to date

Search for A^{'}→μ^{+}μ^{-} Decays.

Searches are performed for both promptlike and long-lived dark photons, A^{'}, produced in proton-proton collisions at a center-of-mass energy of 13 TeV. These searches look for A^{'}→μ^{+}μ^{-} decays using a data sample corresponding to an integrated luminosity of 5.5  fb^{-1} collected with the LHCb detector. Neither search finds evidence for a signal, and 90% confidence-level exclusion limits are placed on the γ-A^{'} kinetic mixing strength. The promptlike A^{'} search explores the mass region from near the dimuon threshold up to 70 GeV and places the most stringent constraints to date on dark photons with 214<m(A^{'})≲740  MeV and 10.6<m(A^{'})≲30  GeV. The search for long-lived A^{'}→μ^{+}μ^{-} decays places world-leading constraints on low-mass dark photons with lifetimes O(1)  ps