On The Expected Photon Spectrum in B -> X_s + gamma and Its Uses

Measuring the photon energy spectrum in radiative B decays provides essential help for gaining theoretical control over semileptonic B transitions. The hadronic recoil mass distribution in B -> X_u \ell\nu promises the best environment for determining |V_ub|. The theoretical uncertainties are largest in the domain of low values of the lepton pair mass q^2. Universality relations allow to describe this domain reliably in terms of the photon spectrum in B -> X_s + \gamma. A method is proposed to incorporate 1/m_b corrections into this relation. The low-E_\gamma tail in radiative decays is important in the context of extracting |V_ub|. We argue that CLEO's recent fit to the spectrum underestimates the fraction of the photon spectrum below 2 GeV. Potentially significant uncertainties enter in the theoretical evaluation of the integrated end-point lepton spectrum or the B -> X_u \ell\nu width with a too high value of the lower cut on q^2 in alternative approaches to |V_ub|.Comment: 24 pages, 6 figures, LaTeX. Revised: Complete version. Numerical predictions are improved and the estimate for the decay fraction revised. The theoretical expectations for the decay fraction and the spectrum itself are given on the plot

The Two Roads to "Intrinsic Charm" in B Decays

We describe two complementary ways to show the presence of higher order effects in the 1/m_Q expansion for inclusive B decays that have been dubbed "Intrinsic Charm". Apart from the lessons they can teach us about QCD's nonperturbative dynamics their consideration is relevant for precise extractions of |V_{cb}|: for they complement the estimate of the potential impact of 1/m_Q^4 contributions. We draw semiquantitative conclusions for the expected scale of Weak Annihilation in semileptonic B decays, both for its valence and non-valence components.Comment: 17 pages, 3 figure

Precision Studies of Duality in the 't Hooft Model

We address numerical aspects of local quark-hadron duality using the example of the exactly solvable 't Hooft model, two-dimensional QCD with N_c --> infinity. The primary focus of these studies is total semileptonic decay widths relevant for extracting |V_{cb}| and |V_{ub}|. We compare the exact channel-by-channel sum of exclusive modes to the corresponding rates obtained in the standard 1/m_Q expansion arising from the Operator Product Expansion. An impressive agreement sets in unexpectedly early, immediately after the threshold for the first hadronic excitation in the final state. Yet even at higher energy release it is possible to discern the seeds of duality-violating oscillations. We find the ``Small Velocity'' sum rules to be exceptionally well saturated already by the first excited state. We also obtain a convincing degree of duality in the differential distributions and in an analogue of R_{e^+e^-}(s). Finally, we discuss possible lessons for semileptonic decays of actual heavy quarks in QCD.Comment: 45 pages, 16 eps figures include

UNDERSTANDING THE ELECTRONIC PROPERTIES OF QUANTUM MATERIALS BY MEANS OF PHOTOEMISSION WITH ANGULAR AND SPIN RESOLUTION

This thesis contains a selection of the results on the shallow electron states of quantum materials that I obtained as doctoral student of the Scuola di Dottorato in Fisica, Astrofisica e Fisica Applicata at the Universit\ue0 degli Studi di Milano. I carried out my doctoral research activity mostly at the TASC-IOM CNR laboratory, in the framework of the NFFA and APE-beamline facilities (Elettra Sincrotrone Trieste), as well in dedicated sessions at the I2; beamline of the Diamond light source, Harwell Campus, UK. To access the electronic properties of materials I specialised myself in photoemission spectroscopy techniques. High quality samples are a prerequisite for any attempt to study quantum materials so that a major effort in my PhD project has been to master the growth of novel quantum materials by means of Pulsed Laser Deposition (PLD). Given that the PLD is integrated in the suite of UHV facilities attached in-situ to the APE beamline, I directly characterised the electronic properties of the PLD grown samples exploiting both the spectroscopic techniques available at the beamline (ARPES, X-ray photoemission and absorption spectroscopies: XPS and XAS), either ex-situ structural characterisation tools (X-ray diffraction \u2013XRD\u2013 and X-ray reflectivity, XRR). Photoemission spectroscopy is a powerful and versatile experimental tool for understanding the electronic properties of materials, providing deep insight into various physical and chemical phenomena, ranging from electronic correlations to surface reactions. Angle-resolved photoemission spectroscopy (ARPES) provides direct insight in the dispersion of extended valence electronic states. Its combination with spin selective detection (spin-ARPES) gives access to the spin polarisation of the photoelectrons at specific points in the Brillouin zone, i.e. to the spin texture of the corresponding band structure. The photoemission intensity is modulated by the "matrix element effects", which express the probability of photoelectron transition from the initial to the final state and link the specific experimental geometry to the symmetry properties of the electron states. This implies that a change in the light polarisation or incident/emission angle induce variations in the spectral intensity and can be misleading in the interpretation of ARPES and spin-ARPES results. In this scenario, a fruitful approach is the simultaneous detection of all quantum numbers of the final state photoelectrons together with the exploitation of light polarisation and photoionisation cross-section tuning. I directly addressed the matrix-element effects performing measurements on the spin polarised states of NbSe2 single crystals while varying the photon energy and polarisation at the highly efficient apparatus for vectorial spin-polarization analysis available at APE-LE beamline. The second part of my thesis work was dedicated to the investigation of the electronic properties of two oxide systems SrNbO3 and anatase TiO2, whose peculiar properties are exploited in catalysis. While TiO2 is a very well-known catalytic material, SrNbO3 has only recently been proposed for applications due to visible- light photo-catalytic properties driven by plasmonic resonances. Up to now, systematic experimental investigation of such a system with X-ray/UV electron spectroscopies was missing. Xray absorption and photoemission spectroscopies were used to probe the chemical states of the samples and the changes induced by different oxygen pressure during the growth. I further performed detailed ARPES investigation of the electronic band structure of SrNbO3. I found that the Fermi surface is made up by three bands mainly originating from t2g orbitals of Nb 4d, as reported for 3d based perovskite systems. The experimental results are consistent with the band dispersion predicted by bulk Density Functional Theory (DFT) calculations that I performed by means of the open source QuantumEspresso software. The narrower bandwidth observed in the ARPES spectra with respect to calculations suggests mass renormalisation arising from electronic correlations. Anatase TiO2 is utilised in a number of applications ranging from photocatalytic devices to sensors as well as solar cells. To efficiently tailor high performing devices, the understanding and control of carrier concentration in the material is a key aspect. I therefore investigate the role of oxygen vacancy defects at the (001) anatase surface that are known to induce extended metallic states on the surface of this nominally insulating material, by means of ARPES and Resonant-ARPES. I observed both localised and metallic delocalised electronic states and investigated the evolution of the spectral intensity as a function of varying oxygen vacancies. I found that the excess oxygen, provided by O2 dosing at the surface, significantly quenches the localised states, whereas O2 reaction has weak impact on the delocalised electronic states: the number of free carriers is reduced but could not be suppressed. These results are very promising for future applications as they may be exploited to tune the excess carriers\u2019 concentration in novel anatase-based devices

A Vademecum on Quark-Hadron Duality

We present an elementary introduction to the problem of quark-hadron duality and its practical limitations, in particular as it concerns local duality violation in inclusive B meson decays. We show that the accurate definition of duality violation elaborated over the recent years allows one to derive informative constraints on violations of local duality. The magnitude of duality violation is particularly restricted in the total semileptonic widths. This explains its strong suppression in concrete dynamical estimates. We analyze the origin of the suppression factors in a model-independent setting, including a fresh perspective on the Small Velocity expansion. A new potentially significant mechanism for violation of local duality in \Gamma_sl(B) is analyzed. Yet we conclude that the amount of duality violation in \Gamma_sl(B) must be safely below the half percent level, with realistic estimates being actually much smaller. Violation of local duality in \Gamma_sl(B) is thus far below the level relevant to phenomenology. We also present a cautionary note on the B->D^* decay amplitude at zero recoil and show that it is much more vulnerable to violations of quark-hadron duality than \Gamma_sl(B). A critical review of some recent literature is given. We point out that the presently limiting factor in genuinely model-independent extraction of V_cb is the precise value of the short-distance charm quark mass. We suggest a direct and precise experimental check of local quark-hadron duality in semileptonic B->X_c l\nu decays.Comment: 48 pages, 4 figures; LaTe

Heavy Quark Expansion and Preasymptotic Corrections to Decay Widths in the 't Hooft Model

We address nonperturbative power corrections to inclusive decay widths of heavy flavor hadrons in the context of the 't Hooft model (two-dimensional QCD at N_c->oo), with the emphasis on the spectator-dependent effects sensitive to the flavor of the spectator. The summation of exclusive widths is performed analytically using the `t Hooft equation. We show that the 1/m_Q expansion of both the Weak Annihilation and Pauli Interference widths coincides with the OPE predictions, to the computed orders. Violation of local duality in the inclusive widths is quantified, and the new example is identified where the OPE prediction and the actual effect are completely saturated by a single final state. The qualitative aspects of quark hadronization emerging from the analysis in the 't Hooft model are discussed. Certain aspects of summation of spectator-independent hadronic weak decay widths are given in more detail, which were not spelled out previously. We also give some useful details of the 1/m_Q expansion in the 't~Hooft model.Comment: 54 pages, 8 figures in the text. Version to be published in Phys. Rev. D. A number of typos are corrected and relevant references added. Clarifications in Conclusions, Appendices 2.1 and 3 are adde

Theoretical Uncertainties in Gamma_sl(b->u)

I review the existing theoretical uncertainties in relating the semileptonic decay width in b->u transitions to the underlying Kobayashi-Maskawa mixing element |V_ub|. The theoretical error bars are only a few percent in |V_ub|, with uncertainties from the impact of the nonperturbative effects nearly negligible.Comment: 12 pages, LaTeX; discussion of 1/m_b^3 effects is expanded. Contribution to Workshop on the Derivation of |V_cb| and |V_ub|: Experimental Status and Theory Uncertainties, CERN, May 28 - June 2, 199

Non-Abelian Dipole Radiation and the Heavy Quark Expansion

Dipole radiation in QCD is derived to the second order in $\alpha_s$. A power-like evolution of the spin-singlet heavy quark operators is obtained to the same accuracy. In particular, ${\cal O}(\alpha_s^2)$ relation between a short-distance low-scale running heavy quark mass and the \barMS mass is given. We discuss the properties of the effective QCD coupling \aw(E) which governs the dipole radiation. This coupling is advantageous for heavy quark physics.Comment: 12 pages, Late

Semileptonic B Decays and Determination of |Vub|

Semileptonic decays of the B mesons provide an excellent probe for the weak and strong interactions of the bottom quark. The large data samples collected at the B Factories have pushed the experimental studies of the semileptonic B decays to a new height and stimulated significant theoretical developments. I review recent progresses in this fast-evolving field, with an emphasis on the determination of the magnitude of the Cabibbo-Kobayashi-Maskawa matrix element |Vub|.Comment: 16 pages, 4 figures, accepted by Mod. Phys. Lett.

Analytical Heavy Quark Expansion in the 't Hooft Model

We present a number of exact relations for the heavy quark limit and develop an analytical 1/m_Q expansion for heavy mesons in the 't Hooft model. Among the new results are relation 3\mu_\pi^2=\bar\Lambda^2 - m_{sp}^2+\beta^2, 1/m_Q corrections to the decay constants, to the kinetic expectation values and 1/m_Q^2 nonperturbative corrections to the B->D amplitude at zero recoil. The properties of the IW functions are addressed and the small velocity sum rules are verified.Comment: 22 pages, plain LaTeX (no figures