CERN Accelerating science

LHCb collaboration meets to discuss recent results and future prospects

by Monica Pepe-Altarelli (LHCb collaboration)

A record number of over three hundred physicists from the LHCb collaboration and the theory community got together on 8-10 November at CERN for the seventh edition of the workshop on the “Implications of LHCb measurements and future prospects”. The very accurate results obtained by LHCb in a broad range of topics have made a large impact on the flavour physics landscape and have implications on classes of extensions of the Standard Model (SM), bearing in mind the interplay with the results of searches for on-shell production of new particles at ATLAS and CMS.  

This series of joint LHCb–theory workshops is aiming at facilitating informal discussions between LHCb experimentalists and theorists, leading to a fruitful, mutual exchange of information. The format has proved to be very successful. This year, the attendance was record-breaking, so that the meeting venue had to be moved from the usual “Filtration Plant” to the more spacious Main Auditorium.

The workshop was organized in four streams covering “Mixing and CP violation in beauty and charm”, “Semileptonic decays, rare decays, and tests of lepton flavour universality”, “Electroweak physics, heavy flavour production, implications for PDFs, and exotic searches” and “QCD spectroscopy and exotic hadrons”.

Each stream was introduced by an experimental overview presenting the current status and prospects. This was followed by a series of theoretical presentations typically covering the latest, state-of-the-art calculations, or suggesting interesting observables or analysis methods to test new theoretical ideas.

Examples of recent results that have attracted a lot of interest and were extensively discussed include spectroscopy of conventional and exotic hadrons, as the emergence of exotic states, such as four-quark and five-quark hadrons, has provided new challenges for QCD. Measurements of CP-violating observables in B(s) meson decays are used to determine the angles of the Unitarity Triangle and hence probe for manifestations of New Physics (NP)  beyond the Cabibbo-Kobayashi-Maskawa SM paradigm. Unfortunately, the data present an overwhelming agreement with the SM, thus placing severe constraints on NP scenarios. The good news is that the majority of these measurements are statistically limited, with theoretical uncertainties on the interpretation of the physical observables much smaller than the attainable experimental precision, even at the end of the LHCb Phase I upgrade. This was extensively discussed by Greig Cowan, from the University of Edinburgh, in the keynote session, in which he presented challenges and ideas for opportunities in flavour physics and beyond, in the HL-LHC era.

A significant part of the workshop was devoted to the discussion of a few exciting and intriguing anomalies in the b-quark sector, when performing tests of Lepton Flavour Universality (LFU). These anomalies can naturally be grouped into two categories, according to the underlying quark-level transition:

  • those arising in b → sl+l- flavour-changing neutral-currents at one-loop level, when measuring B→ K*l+l-, or B→ K+l+l-, with l = e or μ;
  • those arising in b → c l ν charged-currents at tree level, when measuring B→ D*l ν, or B+c → J/Ψ l ν, with l=τ or a light lepton (μ or e).

As discussed by Gino Isidori, from the University of Zurich, in his keynote speech, taken together these anomalies represent the largest coherent set of possible NP effects in the present data. He described a well-motivated model with NP coupled mainly to the third generation of quarks and leptons that describes both charged- and neutral- current anomalies while being consistent with the absence of deviations from the SM so far observed in other low- and high-pT observables.

It is certainly too early to draw any definite conclusions. In fact, it should be pointed out that so far not a single LFU measurement exhibits a deviation with respect to the SM above the 3σ level. However, what is particularly interesting, is that these anomalies challenge the LFU assumption, which we have taken for granted for many years. Furthermore, these measurements have been performed so far with Run-1 data only. Updates with Run-2 data are under way and should allow LHCb to rule out the possibility of weird statistical fluctuations. LHCb’s results are interesting and make searches for these and other similar processes well worth pursuing.