2,519 research outputs found
Role of bound pairs in the optical properties of highly excited semiconductors: a self consistent ladder approximation approach
Presence of bound pairs (excitons) in a low-temperature electron-hole plasma
is accounted for by including correlation between fermions at the ladder level.
Using a simplified one-dimensional model with on-site Coulomb interaction, we
calculate the one-particle self-energies, chemical potential, and optical
response. The results are compared to those obtained in the Born approximation,
which does not account for bound pairs. In the self-consistent ladder
approximation the self-energy and spectral function show a characteristic
correlation peak at the exciton energy for low temperature and density. In this
regime the Born approximation overestimates the chemical potential. Provided
the appropriate vertex correction in the interaction with the photon is
included, both ladder and Born approximations reproduce the excitonic and free
pair optical absorption at low density, and the disappearance of the exciton
absorption peak at larger density. However, lineshapes and energy shifts with
density of the absorption and photoluminescence peaks are drastically
different. In particular, the photoluminescence emission peak is much more
stable in the ladder approximation. At low temperature and density a sizeable
optical gain is produced in both approximations just below the excitonic peak,
however this gain shows unphysical features in the Born approximation. We
conclude that at low density and temperature it is fundamental to take into
account the existence of bound pairs in the electron-hole plasma for the
calculation of its optical and thermodynamic properties. Other approximations
that fail to do so are intrinsically unphysical in this regime, and for example
are not suitable to address the problem of excitonic lasing.Comment: 14 pages, 12 figure
Thermodynamic properties and thermal correlation lengths of a Hubbard model with bond-charge interaction
We investigate the thermodynamics of a one-dimensional Hubbard model with
bond-charge interaction X using the transfer matrix renormalization group
method (TMRG). Numerical results for various quantities like spin and charge
susceptibilities, particle densities, specific heat and thermal correlation
lengths are presented and discussed. We compare our data also to results for
the exactly solvable case X/t=1 as well as to bosonisation results for weak
coupling X/t << 1, which shows excellent agreement. We confirm the existence of
a Tomonaga-Luttinger and a Luther-Emery liquid phase, in agreement with
previous studies at zero temperature. Thermal singlet-pair correlation lengths
are shown to dominate density and spin correlations for finite temperatures in
certain parameter regimes.Comment: 13 pages, revte
New data on OZI rule violation in bar{p}p annihilation at rest
The results of a measurement of the ratio R = Y(phi pi+ pi-) / Y(omega pi+
pi-) for antiproton annihilation at rest in a gaseous and in a liquid hydrogen
target are presented. It was found that the value of this ratio increases with
the decreasing of the dipion mass, which demonstrates the difference in the phi
and omega production mechanisms. An indication on the momentum transfer
dependence of the apparent OZI rule violation for phi production from the 3S1
initial state was found.Comment: 11 pages, 3 PostScript figures, submitted to Physics Letter
Measurements of the reaction of antiproton annihilation at rest at three hydrogen target densities
The proton-antiproton annihilation at rest into the final state
was measured for three different target densities: liquid hydrogen, gaseous
hydrogen at NTP and at a low pressure of 5 mbar. The yield of this reaction in
the liquid hydrogen target is smaller than in the low-pressure gas target. The
branching ratios of the channel were calculated on the basis of
simultaneous analysis of the three data samples. The branching ratio for
annihilation into from the protonium state turns out to be
about ten times smaller as compared to the one from the state.Comment: 10 pages, 3 Postscript figures. Accepted by Physics Letters
Evidence for t\bar{t}\gamma Production and Measurement of \sigma_t\bar{t}\gamma / \sigma_t\bar{t}
Using data corresponding to 6.0/fb of ppbar collisions at sqrt(s) = 1.96 TeV
collected by the CDF II detector, we present a cross section measurement of
top-quark pair production with an additional radiated photon. The events are
selected by looking for a lepton, a photon, significant transverse momentum
imbalance, large total transverse energy, and three or more jets, with at least
one identified as containing a b quark. The ttbar+photon sample requires the
photon to have 10 GeV or more of transverse energy, and to be in the central
region. Using an event selection optimized for the ttbar+photon candidate
sample we measure the production cross section of, and the ratio of cross
sections of the two samples. Control samples in the dilepton+photon and
lepton+photon+\met, channels are constructed to aid in decay product
identification and background measurements. We observe 30 ttbar+photon
candidate events compared to the standard model expectation of 26.9 +/- 3.4
events. We measure the ttbar+photon cross section to be 0.18+0.08 pb, and the
ratio of the cross section of ttbar+photon to ttbar to be 0.024 +/- 0.009.
Assuming no ttbar+photon production, we observe a probability of 0.0015 of the
background events alone producing 30 events or more, corresponding to 3.0
standard deviations.Comment: 9 pages, 3 figure
A search for resonant production of pairs in $4.8\ \rm{fb}^{-1}p\bar{p}\sqrt{s}=1.96\ \rm{TeV}$
We search for resonant production of tt pairs in 4.8 fb^{-1} integrated
luminosity of ppbar collision data at sqrt{s}=1.96 TeV in the lepton+jets decay
channel, where one top quark decays leptonically and the other hadronically. A
matrix element reconstruction technique is used; for each event a probability
density function (pdf) of the ttbar candidate invariant mass is sampled. These
pdfs are used to construct a likelihood function, whereby the cross section for
resonant ttbar production is estimated, given a hypothetical resonance mass and
width. The data indicate no evidence of resonant production of ttbar pairs. A
benchmark model of leptophobic Z \rightarrow ttbar is excluded with m_{Z'} <
900 GeV at 95% confidence level.Comment: accepted for publication in Physical Review D Sep 21, 201
Precision Top-Quark Mass Measurements at CDF
We present a precision measurement of the top-quark mass using the full
sample of Tevatron TeV proton-antiproton collisions collected
by the CDF II detector, corresponding to an integrated luminosity of 8.7
. Using a sample of candidate events decaying into the
lepton+jets channel, we obtain distributions of the top-quark masses and the
invariant mass of two jets from the boson decays from data. We then compare
these distributions to templates derived from signal and background samples to
extract the top-quark mass and the energy scale of the calorimeter jets with
{\it in situ} calibration. The likelihood fit of the templates from signal and
background events to the data yields the single most-precise measurement of the
top-quark mass, \mtop = 172.85 \pm\pmComment: submitted to Phys. Rev. Let
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