B-Physics in Lattice QCD and at Belle II

Theory and experiment both have key roles to play in our understanding of the Universe. In flavour physics, semileptonic and leptonic decays of B mesons let us access CKM matrix elements, and anomalies in these decay processes offer tantalising hints of new physics. New measurements of these processes with improved statistical and systematic errors are expected over the coming years using data from the Belle II experiment. New lattice calculations (with a greater understanding of QCD effects in the Standard Model) will be needed to support this increase in experimental precision. Precise measurement of rare processes with unreconstructed energy from neutrinos such as B→D(*)τν and B→τν requires the entire BB‾ event to be constrained. This is achieved via reconstruction of the companion B meson in the decay, the so-called tag B. In this work, we describe Belle II's reconstruction process for the tag B, and prepare the skimming to collect data for analysts ahead of Belle II data production. Measurements of B→τν can be used to resolve the anomaly between inclusive and exclusive measurements of CKM matrix elements, if a suffciently precise value of fB is available from the lattice QCD community. As fB is often calculated on the lattice via the ratio fBs/fB, it is important to understand and control SU(3) breaking effects in the light and strange quarks, and study how these affect extrapolations of fBs/fB. In this work, we compute fB and fBs using a set of gauge field configurations that break SU(3) flavour in a controlled way, keeping the average of the lighter quark masses held fixed at the physical value.Thesis (Ph.D.) -- University of Adelaide, School of Physical Sciences, 202

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

Investigation of cryogenic Er:YAG lasers for gravitational wave interferometry

High power, stable single frequency laser sources are required for gravitational wave interferometry. The next generation of interferometers may require laser sources in the 1.3-1.65 µm band for use with Si test masses and InGaAs photodetectors. We propose a high power cryogenic Er:YAG laser operating at 1.618 µm for this purpose, adapting existing knowledge about cryogenic Yb:YAG lasers developed at the University of Adelaide. To produce such a laser, further information is required about the viability of Er:YAG in high power, single frequency operation. In this thesis, I report this investigation of the spectroscopy of Er:YAG at room temperature and at cryogenic temperatures (≈ 77 K) and investigate a variety of wavelengths for diode pumping of an Er:YAG slab laser. Spectroscopy indicates that diode pumping for the 77 K laser slab will be most effective in the 1450-1480 nm absorption band, most specifically at the 1453 nm absorption peak. I describe methods for cooling a 1470 nm diode below 0 °C to pump this 1453 nm Er:YAG absorption. The cooled diode exhibits up to 9 % increase in slope effciency and improved beam divergence compared to room temperature operation. I then describe the construction and characterisation of CW Er:YAG lasers at both 300 K and 77 K, tuning the pump wavelength in the 1450-1480 nm band. At 300 K, I demonstrate an Er:YAG laser with 4.5 W output power when pumped with 30 W of diode power at 1468 nm, and just under 4 W of output power when pumped with 34Wof diode power at 1456 nm. Both lasers have a threshold of approximately 12 W incident pump power. The laser pumped at 1468 nm also demonstrates a greater slope effciency relative to incident pump power: 28 % compared to 20 % when pumped at 1456 nm. The development of a preliminary cryogenic Er:YAG laser is also reported. Despite sub-optimal mounting materials and geometries, we demonstrate a cryogenic laser with 5.5 W output power and approximately 6 W threshold under comparable pumping conditions to the 4.5 W 300 K laser pumped at 1468 nm. Unfortunately subsequent studies of the cryogenic slab laser are not comparable to the 300 K Er:YAG laser due to electrical damage to the diode that significantly reduced diode power and changed pumping conditions. Nevertheless, these results provide valuable information on the sensitivity of end-pumped cryogenic lasers to mounting conditions and pump focusing that are useful for a future high power design.Thesis (M.Phil.) -- University of Adelaide, School of Physical Sciences, 2015