810 research outputs found
Electronic Correlations and Unconventional Spectral Weight Transfer in the High-Temperature Pnictide BaFe2-xCoxAs2 Superconductor Using Infrared Spectroscopy
We report an infrared optical study of the pnictide high-temperature superconductor BaFe1.84Co0.16As2 and its parent compound BaFe2As2. We demonstrate that electronic correlations are moderately strong and do not change across the spin-density wave transition or with doping. By examining the energy scale and direction of spectral weight transfer, we argue that Hund\u27s coupling J is the primary mechanism that gives rise to correlations
An infrared probe of the insulator-to-metal transition in GaMnAs and GaBeAs
We report infrared studies of the insulator-to-metal transition (IMT) in GaAs
doped with either magnetic (Mn) or non-magnetic acceptors (Be). We observe a
resonance with a natural assignment to impurity states in the insulating regime
of GaMnAs, which persists across the IMT to the highest doping
(16%). Beyond the IMT boundary, behavior combining insulating and metallic
trends also persists to the highest Mn doping. Be doped samples however,
display conventional metallicity just above the critical IMT concentration,
with features indicative of transport within the host valence band
High energy pseudogap and its evolution with doping in Fe-based superconductors as revealed by optical spectroscopy
We report optical spectroscopic measurements on electron- and hole-doped
BaFe2As2. We show that the compounds in the normal state are not simple metals.
The optical conductivity spectra contain, in addition to the free carrier
response at low frequency, a temperature-dependent gap-like suppression at
rather high energy scale near 0.6 eV. This suppression evolves with the
As-Fe-As bond angle induced by electron- or hole-doping. Furthermore, the
feature becomes much weaker in the Fe-chalcogenide compounds. We elaborate that
the feature is caused by the strong Hund's rule coupling effect between the
itinerant electrons and localized electron moment arising from the multiple Fe
3d orbitals. Our experiments demonstrate the coexistence of itinerant and
localized electrons in iron-based compounds, which would then lead to a more
comprehensive picture about the metallic magnetism in the materials.Comment: 6 pages, 7 figure
Heavy Quark Spectroscopy -- Theory Overview
Some recent discoveries in the spectroscopy of hadrons containing heavy
quarks, and some of their theoretical interpretations, are reviewed.Comment: 13 pages, 6 figures. Presented at Second Meeting of APS Topical Group
on Hadron Physics, Nashville, TN, 22-24. Proceedings to be published by
Journal of Physics (UK), Conference Series. Uses jpconf.cls, jpconf11.clo.
Some corrections; references update
Operational experience, improvements, and performance of the CDF Run II silicon vertex detector
The Collider Detector at Fermilab (CDF) pursues a broad physics program at
Fermilab's Tevatron collider. Between Run II commissioning in early 2001 and
the end of operations in September 2011, the Tevatron delivered 12 fb-1 of
integrated luminosity of p-pbar collisions at sqrt(s)=1.96 TeV. Many physics
analyses undertaken by CDF require heavy flavor tagging with large charged
particle tracking acceptance. To realize these goals, in 2001 CDF installed
eight layers of silicon microstrip detectors around its interaction region.
These detectors were designed for 2--5 years of operation, radiation doses up
to 2 Mrad (0.02 Gy), and were expected to be replaced in 2004. The sensors were
not replaced, and the Tevatron run was extended for several years beyond its
design, exposing the sensors and electronics to much higher radiation doses
than anticipated. In this paper we describe the operational challenges
encountered over the past 10 years of running the CDF silicon detectors, the
preventive measures undertaken, and the improvements made along the way to
ensure their optimal performance for collecting high quality physics data. In
addition, we describe the quantities and methods used to monitor radiation
damage in the sensors for optimal performance and summarize the detector
performance quantities important to CDF's physics program, including vertex
resolution, heavy flavor tagging, and silicon vertex trigger performance.Comment: Preprint accepted for publication in Nuclear Instruments and Methods
A (07/31/2013
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
Search for the Higgs boson in events with missing transverse energy and b quark jets produced in proton-antiproton collisions at s**(1/2)=1.96 TeV
We search for the standard model Higgs boson produced in association with an
electroweak vector boson in events with no identified charged leptons, large
imbalance in transverse momentum, and two jets where at least one contains a
secondary vertex consistent with the decay of b hadrons. We use ~1 fb-1
integrated luminosity of proton-antiproton collisions at s**(1/2)=1.96 TeV
recorded by the CDF II experiment at the Tevatron. We find 268 (16) single
(double) b-tagged candidate events, where 248 +/- 43 (14.4 +/- 2.7) are
expected from standard model background processes. We place 95% confidence
level upper limits on the Higgs boson production cross section for several
Higgs boson masses ranging from 110 GeV/c2 to 140 GeV/c2. For a mass of 115
GeV/c2 the observed (expected) limit is 20.4 (14.2) times the standard model
prediction.Comment: 8 pages, 2 figures, submitted to Phys. Rev. Let
Measurements of branching fraction ratios and CP-asymmetries in suppressed B^- -> D(-> K^+ pi^-)K^- and B^- -> D(-> K^+ pi^-)pi^- decays
We report the first reconstruction in hadron collisions of the suppressed
decays B^- -> D(-> K^+ pi^-)K^- and B^- -> D(-> K^+ pi^-)pi^-, sensitive to the
CKM phase gamma, using data from 7 fb^-1 of integrated luminosity collected by
the CDF II detector at the Tevatron collider. We reconstruct a signal for the
B^- -> D(-> K^+ pi^-)K^- suppressed mode with a significance of 3.2 standard
deviations, and measure the ratios of the suppressed to favored branching
fractions R(K) = [22.0 \pm 8.6(stat)\pm 2.6(syst)]\times 10^-3, R^+(K) =
[42.6\pm 13.7(stat)\pm 2.8(syst)]\times 10^-3, R^-(K)= [3.8\pm 10.3(stat)\pm
2.7(syst]\times 10^-3, as well as the direct CP-violating asymmetry A(K) =
-0.82\pm 0.44(stat)\pm 0.09(syst) of this mode. Corresponding quantities for
B^- -> D(-> K^+ pi^-)pi^- decay are also reported.Comment: 8 pages, 1 figure, accepted by Phys.Rev.D Rapid Communications for
Publicatio
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
- …