Pursuing forbidden beauty:Search for the lepton-flavour violating decays B0 → e± μ∓ and Bs0 → e± μ∓ and study of electron-reconstruction performance at LHCb

Physicists have developed a successful theory, called the Standard Model, describing all known fundamental particles and interactions, except gravity, up to high energies and small length scales. Despite its success, it is when extrapolating to astronomical length and time scales that the theory fails. To alleviate these issues, physicists search for more fundamental laws of physics by exploring higher energies. One such approach is the study of heavy and therefore energetic particles called beauty mesons. Since interactions of higher energies mediating decays of particles are relatively suppressed, decays of particles that are rare or forbidden according to the Standard Model are of particular interest, as such interactions can have a relatively large effect. At the particle accelerator LHC at CERN, beauty mesons are produced by the trillions a year. Hence, rare and forbidden beauty decays can be searched for and studied to great precision.This dissertation covers the search for the in the Standard Model forbidden decays of neutral beauty mesons to an electron and muon (a heavier sibling of the electron) with the data collected in 2011 and 2012 by the LHCb detector operating at the LHC. No such decays were found, but its increased precision of the upper limit of its probability is used to constrain new models. In addition, a study of the performance of the reconstruction of electrons at the LHCb experiment is presented, which will allow to determine and reduce systematic uncertainties in future analyses of beauty decays with electrons in the finalstate

B(s)→eμ: search for lepton flavour violation at LHCb

The existence of three flavors of fundamental fermions is one of the great mysteries of the standard model of particle physics. At the same time it is at the basis of a variety of exciting phenomena, such as CP violation and neutrino flavor oscillations. Hints are mounting that the assumption of lepton universality and charged lepton flavour conservation, as currently incorporated in the standard model, may not be valid. We report on our test of charged lepton flavour conservation in the decay of B mesons, as measured at LHCb

Search for K S (L)0 →μ+μ-μ+μ- decays at LHCb

A search for KS(L)0→μ+μ-μ+μ- decays is performed using proton-proton collision data collected by the LHCb experiment at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 5.1 fb-1. No evidence for signal is found. The 90% confidence level upper limits are the first set for both decays and are B(KS0→μ+μ-μ+μ-)<5.1×10-12 and B(KL0→μ+μ-μ+μ-)<2.3×10-9

Measurement of antiproton production from antihyperon decays in pHe collisions at √sNN=110GeV

The interpretation of cosmic antiproton flux measurements from space-borne experiments is currently limited by the knowledge of the antiproton production cross-section in collisions between primary cosmic rays and the interstellar medium. Using collisions of protons with an energy of 6.5 TeV incident on helium nuclei at rest in the proximity of the interaction region of the LHCb experiment, the ratio of antiprotons originating from antihyperon decays to prompt production is measured for antiproton momenta between 12 and 110GeV\!/c . The dominant antihyperon contribution, namely Λ¯ → p¯ π+ decays from promptly produced Λ¯ particles, is also exclusively measured. The results complement the measurement of prompt antiproton production obtained from the same data sample. At the energy scale of this measurement, the antihyperon contributions to antiproton production are observed to be significantly larger than predictions of commonly used hadronic production models

Search for the baryon- and lepton-number violating decays B0 →pμ- and Bs0 →pμ

A search for the baryon- and lepton-number violating decays B0→pμ- and Bs0→pμ- is performed at the LHCb experiment using data collected in proton-proton collisions at s=7, 8 and 13 TeV, corresponding to integrated luminosities of 1, 2, and 6 fb-1, respectively. No significant signal for B0→pμ- and Bs0→pμ- decays is found and the upper limits on the branching fractions are determined to be B(B0→pμ-)<2.6(3.1)×10-9 and B(Bs0→pμ-)<12.1(14.0)×10-9, respectively, at 90% (95%) confidence level. These are the first limits on these decays to date

First observation of the B+ → Ds+ Ds- K+ decay

The B+→Ds+Ds-K+ decay is observed for the first time using proton-proton collision data collected by the LHCb detector at center-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of 9 fb-1. Its branching fraction relative to that of the B+→D+D-K+ decay is measured to be B(B+→Ds+Ds-K+)B(B+→D+D-K+)=0.525±0.033±0.027±0.034, where the first uncertainty is statistical, the second systematic, and the third is due to the uncertainties on the branching fractions of the Ds±→K∓K±π± and D±→K∓π±π± decays. This measurement fills an experimental gap in the knowledge of the family of Cabibbo-favored b¯→c¯cs¯ transitions and opens the path for unique studies of spectroscopy in future. © 2023 CERN.<br/

First observation of the B+ → Ds+ Ds- K+ decay

The B+→Ds+Ds-K+ decay is observed for the first time using proton-proton collision data collected by the LHCb detector at center-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of 9 fb-1. Its branching fraction relative to that of the B+→D+D-K+ decay is measured to be B(B+→Ds+Ds-K+)B(B+→D+D-K+)=0.525±0.033±0.027±0.034, where the first uncertainty is statistical, the second systematic, and the third is due to the uncertainties on the branching fractions of the Ds±→K∓K±π± and D±→K∓π±π± decays. This measurement fills an experimental gap in the knowledge of the family of Cabibbo-favored b¯→c¯cs¯ transitions and opens the path for unique studies of spectroscopy in future. © 2023 CERN.<br/