3,078 research outputs found
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 . A
power-like evolution of the spin-singlet heavy quark operators is obtained to
the same accuracy. In particular, 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
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