373 research outputs found
On the nature of Bose-Einstein condensation in disordered systems
We study the perfect Bose gas in random external potentials and show that
there is generalized Bose-Einstein condensation in the random eigenstates if
and only if the same occurs in the one-particle kinetic-energy eigenstates,
which corresponds to the generalized condensation of the free Bose gas.
Moreover, we prove that the amounts of both condensate densities are equal. Our
method is based on the derivation of an explicit formula for the occupation
measure in the one-body kinetic-energy eigenstates which describes the
repartition of particles among these non-random states. This technique can be
adapted to re-examine the properties of the perfect Bose gas in the presence of
weak (scaled) non-random potentials, for which we establish similar results
Flavoured Large N Gauge Theory in an External Magnetic Field
We consider a D7-brane probe of AdS in the presence of pure
gauge -field. In the dual gauge theory, the -field couples to the
fundamental matter introduced by the D7-brane and acts as an external magnetic
field. The -field supports a 6-form Ramond-Ramond potential on the D7-branes
world volume that breaks the supersymmetry and enables the dual gauge theory to
develop a non-zero fermionic condensate. We explore the dependence of the
fermionic condensate on the bare quark mass and show that at zero bare
quark mass a chiral symmetry is spontaneously broken. A study of the meson
spectrum reveals a coupling between the vector and scalar modes, and in the
limit of weak magnetic field we observe Zeeman splitting of the states. We also
observe the characteristic dependence of the ground state
corresponding to the Goldstone boson of spontaneously broken chiral symmetry.Comment: 22 pages, 7 figures, references added, few typos correcte
Search for single top quarks in the tau+jets channel using 4.8 fb of collision data
We present the first direct search for single top quark production using tau
leptons. The search is based on 4.8 fb of integrated luminosity
collected in collisions at =1.96 TeV with the D0 detector
at the Fermilab Tevatron Collider. We select events with a final state
including an isolated tau lepton, missing transverse energy, two or three jets,
one or two of them tagged. We use a multivariate technique to discriminate
signal from background. The number of events observed in data in this final
state is consistent with the signal plus background expectation. We set in the
tau+jets channel an upper limit on the single top quark cross section of
\TauLimObs pb at the 95% C.L. This measurement allows a gain of 4% in expected
sensitivity for the observation of single top production when combining it with
electron+jets and muon+jets channels already published by the D0 collaboration
with 2.3 fb of data. We measure a combined cross section of
\SuperCombineXSall pb, which is the most precise measurement to date.Comment: 12 pages, 5 figure
b-Jet Identification in the D0 Experiment
Algorithms distinguishing jets originating from b quarks from other jet
flavors are important tools in the physics program of the D0 experiment at the
Fermilab Tevatron p-pbar collider. This article describes the methods that have
been used to identify b-quark jets, exploiting in particular the long lifetimes
of b-flavored hadrons, and the calibration of the performance of these
algorithms based on collider data.Comment: submitted to Nuclear Instruments and Methods in Physics Research
Measurement of the dijet invariant mass cross section in proton anti-proton collisions at sqrt{s} = 1.96 TeV
The inclusive dijet production double differential cross section as a
function of the dijet invariant mass and of the largest absolute rapidity of
the two jets with the largest transverse momentum in an event is measured in
proton anti-proton collisions at sqrt{s} = 1.96 TeV using 0.7 fb^{-1}
integrated luminosity collected with the D0 detector at the Fermilab Tevatron
Collider. The measurement is performed in six rapidity regions up to a maximum
rapidity of 2.4. Next-to-leading order perturbative QCD predictions are found
to be in agreement with the data.Comment: Published in Phys. Lett. B, 693, (2010), 531-538, 8 pages, 2 figures,
6 table
Measurement of Z/gamma*+jet+X angular distributions in ppbar collisions at sqrt{s}=1.96 TeV
We present the first measurements at a hadron collider of differential cross
sections for Z+jet+X production in delta phi(Z, jet), |delta y(Z, jet)| and
|y_boost(Z, jet)|. Vector boson production in association with jets is an
excellent probe of QCD and constitutes the main background to many small cross
section processes, such as associated Higgs production. These measurements are
crucial tests of the predictions of perturbative QCD and current event
generators, which have varied success in describing the data. Using these
measurements as inputs in tuning event generators will increase the
experimental sensitivity to rare signals.Comment: Published in Physics Letters B 682 (2010), pp. 370-380. 15 pages, 6
figure
Search for the standard model Higgs boson in tau final states
We present a search for the standard model Higgs boson using hadronically
decaying tau leptons, in 1 inverse femtobarn of data collected with the D0
detector at the Fermilab Tevatron ppbar collider. We select two final states:
tau plus missing transverse energy and b jets, and tau+ tau- plus jets. These
final states are sensitive to a combination of associated W/Z boson plus Higgs
boson, vector boson fusion and gluon-gluon fusion production processes. The
observed ratio of the combined limit on the Higgs production cross section at
the 95% C.L. to the standard model expectation is 29 for a Higgs boson mass of
115 GeV.Comment: publication versio
Search for scalar bottom quarks and third-generation leptoquarks in ppbar collisions at sqrt(s) = 1.96 TeV
We report the results of a search for pair production of scalar bottom quarks
(sbottom) and scalar third-generation leptoquarks in 5.2 fb-1 of ppbar
collisions at the D0 experiment of the Fermilab Tevatron Collider. Scalar
bottom quarks are assumed to decay to a neutralino and a quark, and we set
95% C.L. lower limits on their production in the (m_sbottom, m_neutralino) mass
plane such as m_sbottom>247 GeV for m_neutralino=0 and m_neutralino>110 GeV for
160<m_sbottom<200 GeV. The leptoquarks are assumed to decay to a tau neutrino
and a quark, and we set a 95% C.L. lower limit of 247 GeV on the mass of a
charge-1/3 third-generation scalar leptoquark.Comment: Published by Phys. Lett.
Search for a scalar or vector particle decaying into Zgamma in ppbar collisions at sqrt(s) = 1.96 TeV
We present a search for a narrow scalar or vector resonance decaying into
Zgamma with a subsequent Z decay into a pair of electrons or muons. The data
for this search were collected with the D0 detector at the Fermilab Tevatron
ppbar collider at a center of mass energy sqrt(s) = 1.96 TeV. Using 1.1 (1.0)
fb-1 of data, we observe 49 (50) candidate events in the electron (muon)
channel, in good agreement with the standard model prediction. From the
combination of both channels, we derive 95% C.L. upper limits on the cross
section times branching fraction (sigma x B) into Zgamma. These limits range
from 0.19 (0.20) pb for a scalar (vector) resonance mass of 600 GeV/c^2 to 2.5
(3.1) pb for a mass of 140 GeV/c^2.Comment: Published by Phys. Lett.
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