On the significance of new physics in b → sℓ+ℓ−b \to s\ell^+\ell^- decays

Motivated by deviations with respect to Standard Model predictions in decays, we evaluate the global significance of the new physics hypothesis in this system by including the look-elsewhere effect for the first time. We estimate the trial-factor with pseudo-experiments and find that it can be as large as seven. We calculate the global significance for the new physics hypothesis by considering the most general description of a non-standard amplitude of short-distance origin. Theoretical uncertainties are treated in a highly conservative way by absorbing the corresponding effects into a redefinition of the Standard Model amplitude. Using the most recent measurements of LHCb, ATLAS and CMS, we obtain the global significance to be 4.3 standard deviations

Physics case for an LHCb Upgrade II - Opportunities in flavour physics, and beyond, in the HL-LHC era

The LHCb Upgrade II will fully exploit the flavour-physics opportunities of the HL-LHC, and study additional physics topics that take advantage of the forward acceptance of the LHCb spectrometer. The LHCb Upgrade I will begin operation in 2020. Consolidation will occur, and modest enhancements of the Upgrade I detector will be installed, in Long Shutdown 3 of the LHC (2025) and these are discussed here. The main Upgrade II detector will be installed in long shutdown 4 of the LHC (2030) and will build on the strengths of the current LHCb experiment and the Upgrade I. It will operate at a luminosity up to 2×1034 cm−2s−1, ten times that of the Upgrade I detector. New detector components will improve the intrinsic performance of the experiment in certain key areas. An Expression Of Interest proposing Upgrade II was submitted in February 2017. The physics case for the Upgrade II is presented here in more depth. CP-violating phases will be measured with precisions unattainable at any other envisaged facility. The experiment will probe b → sl+l−and b → dl+l− transitions in both muon and electron decays in modes not accessible at Upgrade I. Minimal flavour violation will be tested with a precision measurement of the ratio of B(B0 → μ+μ−)/B(Bs → μ+μ−). Probing charm CP violation at the 10−5 level may result in its long sought discovery. Major advances in hadron spectroscopy will be possible, which will be powerful probes of low energy QCD. Upgrade II potentially will have the highest sensitivity of all the LHC experiments on the Higgs to charm-quark couplings. Generically, the new physics mass scale probed, for fixed couplings, will almost double compared with the pre-HL-LHC era; this extended reach for flavour physics is similar to that which would be achieved by the HE-LHC proposal for the energy frontier

LHCb upgrade software and computing : technical design report

This document reports the Research and Development activities that are carried out in the software and computing domains in view of the upgrade of the LHCb experiment. The implementation of a full software trigger implies major changes in the core software framework, in the event data model, and in the reconstruction algorithms. The increase of the data volumes for both real and simulated datasets requires a corresponding scaling of the distributed computing infrastructure. An implementation plan in both domains is presented, together with a risk assessment analysis

Study of the B−→Λc+Λˉc−K−B^{-} \to \Lambda_{c}^{+} \bar{\Lambda}_{c}^{-} K^{-} decay

The decay $B^{-} \to \Lambda_{c}^{+} \bar{\Lambda}_{c}^{-} K^{-}$ is studied in proton-proton collisions at a center-of-mass energy of $\sqrt{s}=13$ TeV using data corresponding to an integrated luminosity of 5 $\mathrm{fb}^{-1}$ collected by the LHCb experiment. In the $\Lambda_{c}^+ K^{-}$ system, the $\Xi_{c}(2930)^{0}$ state observed at the BaBar and Belle experiments is resolved into two narrower states, $\Xi_{c}(2923)^{0}$ and $\Xi_{c}(2939)^{0}$, whose masses and widths are measured to be $$m(\Xi_{c}(2923)^{0}) = 2924.5 \pm 0.4 \pm 1.1 \,\mathrm{MeV}, \\ m(\Xi_{c}(2939)^{0}) = 2938.5 \pm 0.9 \pm 2.3 \,\mathrm{MeV}, \\ \Gamma(\Xi_{c}(2923)^{0}) = \phantom{000}4.8 \pm 0.9 \pm 1.5 \,\mathrm{MeV},\\ \Gamma(\Xi_{c}(2939)^{0}) = \phantom{00}11.0 \pm 1.9 \pm 7.5 \,\mathrm{MeV},$$ where the first uncertainties are statistical and the second systematic. The results are consistent with a previous LHCb measurement using a prompt $\Lambda_{c}^{+} K^{-}$ sample. Evidence of a new $\Xi_{c}(2880)^{0}$ state is found with a local significance of $3.8\,\sigma$, whose mass and width are measured to be $2881.8 \pm 3.1 \pm 8.5\,\mathrm{MeV}$ and $12.4 \pm 5.3 \pm 5.8 \,\mathrm{MeV}$, respectively. In addition, evidence of a new decay mode $\Xi_{c}(2790)^{0} \to \Lambda_{c}^{+} K^{-}$ is found with a significance of $3.7\,\sigma$. The relative branching fraction of $B^{-} \to \Lambda_{c}^{+} \bar{\Lambda}_{c}^{-} K^{-}$ with respect to the $B^{-} \to D^{+} D^{-} K^{-}$ decay is measured to be $2.36 \pm 0.11 \pm 0.22 \pm 0.25$, where the first uncertainty is statistical, the second systematic and the third originates from the branching fractions of charm hadron decays.Comment: All figures and tables, along with any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2022-028.html (LHCb public pages

Multidifferential study of identified charged hadron distributions in ZZ-tagged jets in proton-proton collisions at s=\sqrt{s}=13 TeV

Jet fragmentation functions are measured for the first time in proton-proton collisions for charged pions, kaons, and protons within jets recoiling against a $Z$ boson. The charged-hadron distributions are studied longitudinally and transversely to the jet direction for jets with transverse momentum 20 $< p_{\textrm{T}} < 100$ GeV and in the pseudorapidity range $2.5 < \eta < 4$. The data sample was collected with the LHCb experiment at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 1.64 fb$^{-1}$. Triple differential distributions as a function of the hadron longitudinal momentum fraction, hadron transverse momentum, and jet transverse momentum are also measured for the first time. This helps constrain transverse-momentum-dependent fragmentation functions. Differences in the shapes and magnitudes of the measured distributions for the different hadron species provide insights into the hadronization process for jets predominantly initiated by light quarks.Comment: All figures and tables, along with machine-readable versions and any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2022-013.html (LHCb public pages

Measurement of the ratios of branching fractions R(D∗)\mathcal{R}(D^{*}) and R(D0)\mathcal{R}(D^{0})

The ratios of branching fractions $\mathcal{R}(D^{*})\equiv\mathcal{B}(\bar{B}\to D^{*}\tau^{-}\bar{\nu}_{\tau})/\mathcal{B}(\bar{B}\to D^{*}\mu^{-}\bar{\nu}_{\mu})$ and $\mathcal{R}(D^{0})\equiv\mathcal{B}(B^{-}\to D^{0}\tau^{-}\bar{\nu}_{\tau})/\mathcal{B}(B^{-}\to D^{0}\mu^{-}\bar{\nu}_{\mu})$ are measured, assuming isospin symmetry, using a sample of proton-proton collision data corresponding to 3.0 fb${ }^{-1}$ of integrated luminosity recorded by the LHCb experiment during 2011 and 2012. The tau lepton is identified in the decay mode $\tau^{-}\to\mu^{-}\nu_{\tau}\bar{\nu}_{\mu}$. The measured values are $\mathcal{R}(D^{*})=0.281\pm0.018\pm0.024$ and $\mathcal{R}(D^{0})=0.441\pm0.060\pm0.066$, where the first uncertainty is statistical and the second is systematic. The correlation between these measurements is $\rho=-0.43$. Results are consistent with the current average of these quantities and are at a combined 1.9 standard deviations from the predictions based on lepton flavor universality in the Standard Model.Comment: All figures and tables, along with any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2022-039.html (LHCb public pages

An experimental and phenomenological dissection of beauty-quark decays into light leptons

Table of contents List of figures ix List of tables xvii 1 Introduction 1 1.1 History of the Standard Model..................................................................... 2 1.1.1 A new player inthe game .............................................................. 2 1.1.2 Objects in the mirror don’t look as you’d expect......................... 3 1.1.3 A heavier sibling of the electron..................................................... 5 1.1.4 The "strange” path to new fundamental particles......................... 7 1.1.5 Symmetry as a guiding principle..................................................... 10 1.1.6 A “massive” problem........................................................................ 12 1.1.7 The colour and flavour of particle physics..................................... 13 1.1.8 Yet another symmetry is questioned............................................... 15 1.1.9 A third heavier family.................................................................... 17 1.2 The Standard Model.................................................................................... 20 1.2.1 The electroweak sector..................................................................... 20 1.2.1.1 Assigning particles to representations of SU(2)l x U(l)y 20 1.2.1.2 Electroweak unification .................................................. 22 1.2.2 Spontaneous symmetry breaking..................................................... 26 1.2.3 Fermion mass generation and the flavour sector......................... 30 1.2.3.1 The CKM matrix............................................................... 31 1.2.3.2 Lepton masses................................................................. 32 1.3 Beyond the Standard Model........................................................................ 35 1.3.1 Unanswered questions in the Standard Model............................ 35 1.3.2 The Standard Model as a low energy effective theory................ 38 1.3.2.1 A systematic approach to identifying the New Physics structure ........................................................................... 38 1.3.2.2 b —> sí+ír transitions......................................................... 39 1.3.2.3 Sandwiching the currents ................................................ 41 1.4 The Large Hadron Collider.......................................................................... 43 1.4.1 The LHCb experiment at the LHC............................................... 44 1.4.1.1 Tracking system ................................................................. 46 1.4.1.2 Particle identification ........................................................ 49 1.4.1.3 Calorimeters ........................................................................ 51 1.4.1.4 The muon system................................................................. 52 1.4.1.5 The trigger system.............................................................. 53 1.4.2 Overview of latest b —» sO.+t~ results............................................... 54 1.4.2.1 Differential semi-leptonic branching fractions ................ 54 1.4.2.2 Angular observables........................................................... 56 1.4.2.3 Purely leptonic branching fractions.................................. 58 1.4.2.4 Ratios of branching fractions ............................................ 60 1.5 Personal contributions................................................................................. 64 References 67 Appendix l.A Appendix to Chapter 1............................................................... 79 l.A.l The optical theorem ....................................................................... 79 l.A.2 Applications of the optical theorem............................................... 81 l.A.2.1 Decay widths and unstable particles ............................... 81 l.A.2.2 Unitarity bound ............................................................... 82 l.A.3 Generalities about SU(N)............................................................... 85 l.A.4 Symmetries and interactions............................................................ 87 l.A.4.1 global vs. local gauge invariance...................................... 87 l.A.4.2 Non-abelian (Yang-Mills) gauge theories...................... 89 2 QED in />’ - LFU ratios:Theory versus Experiment, a Monte Carlo Study 93 2.1 Introduction....................................................................................................... 93 2.2 Monte Carlo Framework................................................................................. 96 2.2.1 Generalities.......................................................................................... 96 2.2.2 Basic strategy of the Monte Carlo approach................................ 97 2.2.3 Numerical procedure........................................................................... 98 2.2.3.0.1 2.3 Direct Comparison with PHOTOS at the Short Distance Level................... 100 2.3.1 Parameterisation of the short distance amplitude ...................... 100 2.3.2 Comparison of our Monte Carlo with PHOTOS..................................100 2.4 Adding Long Distance (Charmonium Resonances)..................................... 104 2.4.1 Parameterisation of the charm amplitude........................................ 105 2.4.2 Study of the J/T-resonance interference term in our Monte Carlo 106 2.4.3 J/T and T(2S), including the resonant mode via a semi-analytic approach................................................................................................ 109 2.5 Outlook and Conclusions ..............................................................................113 References 115 Appendix 2.A Appendix to Chapter 2................................................................. 118 2.A.1 Kinematics .......................................................................................... 118 2.A.2 More Detail on the CharmParameterisation . . .............................. 119 2.A.3 Supplementary plots.......................................................................... 121 2. A.4 Values of /th used in theMonte Carlo Simulations......................... 121 3 Test of lepton flavour universality in beautyquark-decays 125 3.1 Methods............................................................................................................ 135 3.1.1 Experimental setup............................................................................. 135 3.1.2 Analysis description............................................................................. 136 3.1.2.1 Event selection.................................................................... 136 3.1.2.2 Multivariate selection ........................................................138 3.1.2.3 Calibration of simulation......................................................139 3.1.2.4 Likelihood fit........................................................................ 140 3.1.2.5 Additional cross-checks........................................................ 145 3.1.2.6 Systematic uncertainties..................................................... 145 References 149 Appendix 3.A Appendix to Chapter 3................................................................ 163 Appendix 3.B Branching fraction measurements.............................................. 163 Appendix 3.C Fits to the B+ —> ^(2S)K+ resonant mode ........................ 164 Appendix 3.D Effect of q2 migration................................................................... 166 Appendix 3.E Overview of RK measurements.....................................................168 4 On the significance of new physics in b —> si+l~ decays 171 4.1 Introduction...................................................................................................... 171 4.2 Effective Lagrangian and selection of the observables...............................173 4.3 Statistical Method ..........................................................................................176 4.4 Results............................................................................................................... 178 viii Table of contents 4.5 Conclusion and discussion................................................................................179 References 183 5 A general effective field theory description of b —» si+i~ lepton universality ratios 189 5.1 Introduction..........................................................................................................189 5.2 General expression of Rx in terms of Wilson coefficients ......................... 190 5.2.1 Numerical estimate of the .............................................................194 5.2.2 Impact of four-fermion operators.......................................................... 195 5.3 Global combination of current measurements................................................197 5.4 Impact of future measurements...................................................................... 199 5.5 Conclusions......................................................................................................... 201 References 205 Conclusions 20