118 research outputs found
Screening of Dirac flavor structure in the seesaw and neutrino mixing
We consider the mechanism of screening of the Dirac flavor structure in the
context of the double seesaw mechanism. As a consequence of screening, the
structure of the light neutrino mass matrix, m_\nu, is determined essentially
by the structure of the (Majorana) mass matrix, M_S, of new super-heavy (Planck
scale) neutral fermions S. We calculate effects of the renormalization group
running in order to investigate the stability of the screening mechanism with
respect to radiative corrections. We find that screening is stable in the
supersymmetric case, whereas in the standard model it is unstable for certain
structures of M_S. The screening mechanism allows us to reconcile the
(approximate) quark-lepton symmetry and the strong difference of the mixing
patterns in the quark and lepton sectors. It opens new possibilities to explain
a quasi-degenerate neutrino mass spectrum, special ``neutrino'' symmetries and
quark-lepton complementarity. Screening can emerge from certain flavor
symmetries or Grand Unification.Comment: 27 pages, 3 figures; references added, discussion of the E6 model
modifie
Thermoelectric power of MgBBe
We investigated thermoelectric power of MgBBe (,
0.2, 0.3, 0.4, and 0.6). decreases systematically with , suggesting
that the hole density increases. Our band calculation shows that the increase
occurs in the -band. With the hole-doping, decreases.
Implication of this phenomenon is discussed within the BCS framework. While the
Mott formula explains only the linear part of at low temperature,
incorporation of electron-phonon interaction enables us to explain over
wide temperature range including the anomalous behavior at high temperature.Comment: 4 pages, 4 figure
Structural and superconducting properties of MgBBe
We prepared MgBBe (, 0.2, 0.3, 0.4, and 0.6) samples where
B is substituted with Be. MgB structure is maintained up to .
In-plane and inter-plane lattice constants were found to decrease and increase,
respectively. Superconducting transition temperature decreases with
. We found that the decrease is correlated with in-plane contraction
but is insensitive to carrier doping, which is consistent with other
substitution studies such as MgAlB and MgBC.
Implication of this work is discussed in terms of the 2D nature of -band.Comment: 3 pages,4 figures, to be published in Phys. Rev.
Anisotropy of the Upper Critical Field and Critical Current in Single Crystal MgB
We report on specific heat, high magnetic field transport and
susceptibility measurements on magnesium diboride single crystals. The
upper critical field for magnetic fields perpendicular and parallel to
the Mg and B planes is presented for the first time in the entire temperature
range. A very different temperature dependence has been observed in the two
directions which yields to a temperature dependent anisotropy with 5 at low temperatures and about 2 near . A peak effect is observed
in susceptibility measurements for 2 T parallel to the axis and
the critical current density presnts a sharp maximum for parallel to the
ab-plane.Comment: 6 pages, 5 figure
Reversible magnetization of MgB2 single crystals with a two-gap nature
We present reversible magnetization measurements on MgB2 single crystals in
magnetic fields up to 2.5 T applied parallel to the crystal's c-axis. This
magnetization is analyzed in terms of the Hao-Clem model, and various
superconducting parameters, such as the critical fields [Hc(0) and Hc2(0)], the
characteristic lengths [xi(0) and lambda(0)], and the Ginzburg-Landau
parameter, kappa, are derived. The temperature dependence of the magnetic
penetration depth, lambda(T), obtained from the Hao-Clem analysis could not be
explained by theories assuming a single gap. Our data are well described by
using a two-gap model.Comment: 20 pages, 1 table, 4 figures, will be published in Phys. Rev.
Heavy quarkonium: progress, puzzles, and opportunities
A golden age for heavy quarkonium physics dawned a decade ago, initiated by
the confluence of exciting advances in quantum chromodynamics (QCD) and an
explosion of related experimental activity. The early years of this period were
chronicled in the Quarkonium Working Group (QWG) CERN Yellow Report (YR) in
2004, which presented a comprehensive review of the status of the field at that
time and provided specific recommendations for further progress. However, the
broad spectrum of subsequent breakthroughs, surprises, and continuing puzzles
could only be partially anticipated. Since the release of the YR, the BESII
program concluded only to give birth to BESIII; the -factories and CLEO-c
flourished; quarkonium production and polarization measurements at HERA and the
Tevatron matured; and heavy-ion collisions at RHIC have opened a window on the
deconfinement regime. All these experiments leave legacies of quality,
precision, and unsolved mysteries for quarkonium physics, and therefore beg for
continuing investigations. The plethora of newly-found quarkonium-like states
unleashed a flood of theoretical investigations into new forms of matter such
as quark-gluon hybrids, mesonic molecules, and tetraquarks. Measurements of the
spectroscopy, decays, production, and in-medium behavior of c\bar{c}, b\bar{b},
and b\bar{c} bound states have been shown to validate some theoretical
approaches to QCD and highlight lack of quantitative success for others. The
intriguing details of quarkonium suppression in heavy-ion collisions that have
emerged from RHIC have elevated the importance of separating hot- and
cold-nuclear-matter effects in quark-gluon plasma studies. This review
systematically addresses all these matters and concludes by prioritizing
directions for ongoing and future efforts.Comment: 182 pages, 112 figures. Editors: N. Brambilla, S. Eidelman, B. K.
Heltsley, R. Vogt. Section Coordinators: G. T. Bodwin, E. Eichten, A. D.
Frawley, A. B. Meyer, R. E. Mitchell, V. Papadimitriou, P. Petreczky, A. A.
Petrov, P. Robbe, A. Vair
Search for the standard model Higgs boson decaying into two photons in pp collisions at sqrt(s)=7 TeV
A search for a Higgs boson decaying into two photons is described. The
analysis is performed using a dataset recorded by the CMS experiment at the LHC
from pp collisions at a centre-of-mass energy of 7 TeV, which corresponds to an
integrated luminosity of 4.8 inverse femtobarns. Limits are set on the cross
section of the standard model Higgs boson decaying to two photons. The expected
exclusion limit at 95% confidence level is between 1.4 and 2.4 times the
standard model cross section in the mass range between 110 and 150 GeV. The
analysis of the data excludes, at 95% confidence level, the standard model
Higgs boson decaying into two photons in the mass range 128 to 132 GeV. The
largest excess of events above the expected standard model background is
observed for a Higgs boson mass hypothesis of 124 GeV with a local significance
of 3.1 sigma. The global significance of observing an excess with a local
significance greater than 3.1 sigma anywhere in the search range 110-150 GeV is
estimated to be 1.8 sigma. More data are required to ascertain the origin of
this excess.Comment: Submitted to Physics Letters
Measurement of the Lambda(b) cross section and the anti-Lambda(b) to Lambda(b) ratio with Lambda(b) to J/Psi Lambda decays in pp collisions at sqrt(s) = 7 TeV
The Lambda(b) differential production cross section and the cross section
ratio anti-Lambda(b)/Lambda(b) are measured as functions of transverse momentum
pt(Lambda(b)) and rapidity abs(y(Lambda(b))) in pp collisions at sqrt(s) = 7
TeV using data collected by the CMS experiment at the LHC. The measurements are
based on Lambda(b) decays reconstructed in the exclusive final state J/Psi
Lambda, with the subsequent decays J/Psi to an opposite-sign muon pair and
Lambda to proton pion, using a data sample corresponding to an integrated
luminosity of 1.9 inverse femtobarns. The product of the cross section times
the branching ratio for Lambda(b) to J/Psi Lambda versus pt(Lambda(b)) falls
faster than that of b mesons. The measured value of the cross section times the
branching ratio for pt(Lambda(b)) > 10 GeV and abs(y(Lambda(b))) < 2.0 is 1.06
+/- 0.06 +/- 0.12 nb, and the integrated cross section ratio for
anti-Lambda(b)/Lambda(b) is 1.02 +/- 0.07 +/- 0.09, where the uncertainties are
statistical and systematic, respectively.Comment: Submitted to Physics Letters
Search for new physics in events with opposite-sign leptons, jets, and missing transverse energy in pp collisions at sqrt(s) = 7 TeV
A search is presented for physics beyond the standard model (BSM) in final
states with a pair of opposite-sign isolated leptons accompanied by jets and
missing transverse energy. The search uses LHC data recorded at a
center-of-mass energy sqrt(s) = 7 TeV with the CMS detector, corresponding to
an integrated luminosity of approximately 5 inverse femtobarns. Two
complementary search strategies are employed. The first probes models with a
specific dilepton production mechanism that leads to a characteristic kinematic
edge in the dilepton mass distribution. The second strategy probes models of
dilepton production with heavy, colored objects that decay to final states
including invisible particles, leading to very large hadronic activity and
missing transverse energy. No evidence for an event yield in excess of the
standard model expectations is found. Upper limits on the BSM contributions to
the signal regions are deduced from the results, which are used to exclude a
region of the parameter space of the constrained minimal supersymmetric
extension of the standard model. Additional information related to detector
efficiencies and response is provided to allow testing specific models of BSM
physics not considered in this paper.Comment: Replaced with published version. Added journal reference and DO
Measurement of isolated photon production in pp and PbPb collisions at sqrt(sNN) = 2.76 TeV
Isolated photon production is measured in proton-proton and lead-lead
collisions at nucleon-nucleon centre-of-mass energies of 2.76 TeV in the
pseudorapidity range |eta|<1.44 and transverse energies ET between 20 and 80
GeV with the CMS detector at the LHC. The measured ET spectra are found to be
in good agreement with next-to-leading-order perturbative QCD predictions. The
ratio of PbPb to pp isolated photon ET-differential yields, scaled by the
number of incoherent nucleon-nucleon collisions, is consistent with unity for
all PbPb reaction centralities.Comment: Submitted to Physics Letters
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