853 research outputs found
Freezing and melting of 3D complex plasma structures under microgravity conditions driven by neutral gas pressure manipulation
Freezing and melting of large three-dimensional complex plasmas under
microgravity conditions is investigated. The neutral gas pressure is used as a
control parameter to trigger the phase changes: Complex plasma freezes (melts)
by decreasing (increasing) the pressure. Evolution of complex plasma structural
properties upon pressure variation is studied. Theoretical estimates allow us
to identify main factors responsible for the observed behavior.Comment: Phys. Rev. Lett. (in press); 4 pages, 4 figure
Fluid-solid phase transitions in 3D complex plasmas under microgravity conditions
Phase behavior of large three-dimensional complex plasma systems under
microgravity conditions onboard the International Space Station is
investigated. The neutral gas pressure is used as a control parameter to
trigger phase changes. Detailed analysis of structural properties and
evaluation of three different melting/freezing indicators reveal that complex
plasmas can exhibit melting by increasing the gas pressure. Theoretical
estimates of complex plasma parameters allow us to identify main factors
responsible for the observed behavior. The location of phase states of the
investigated systems on a relevant equilibrium phase diagram is estimated.
Important differences between the melting process of 3D complex plasmas under
microgravity conditions and that of flat 2D complex plasma crystals in ground
based experiments are discussed.Comment: 13 pages, 10 figures; submitted to Phys. Rev.
Measurement of the semileptonic charge asymmetry in B0 meson mixing with the D0 detector
We present a measurement of the semileptonic mixing asymmetry for B0 mesons,
a^d_{sl}, using two independent decay channels: B0 -> mu+D-X, with D- ->
K+pi-pi-; and B0 -> mu+D*-X, with D*- -> antiD0 pi-, antiD0 -> K+pi- (and
charge conjugate processes). We use a data sample corresponding to 10.4 fb^{-1}
of ppbar collisions at sqrt(s) = 1.96 TeV, collected with the D0 experiment at
the Fermilab Tevatron collider. We extract the charge asymmetries in these two
channels as a function of the visible proper decay length (VPDL) of the B0
meson, correct for detector-related asymmetries using data-driven methods, and
account for dilution from charge-symmetric processes using Monte Carlo
simulation. The final measurement combines four signal VPDL regions for each
channel, yielding a^d_{sl} = [0.68 \pm 0.45 \text{(stat.)} \pm 0.14
\text{(syst.)}]%. This is the single most precise measurement of this
parameter, with uncertainties smaller than the current world average of B
factory measurements.Comment: Version includes minor textual changes following peer review by
journal, most notably the updating of Ref. [21] to reflect the most recent
publicatio
Measurement of Leptonic Asymmetries and Top Quark Polarization in ttbar Production
We present measurements of lepton (l) angular distributions in ttbar -> W+ b
W- b -> l+ nu b l- nubar bbar decays produced in ppbar collisions at a
center-of-mass energy of sqrt(s)=1.96TeV, where l is an electron or muon. Using
data corresponding to an integrated luminosity of 5.4fb^-1, collected with the
D0 detector at the Fermilab Collider, we find that the angular distributions of
l- relative to anti-protons and l+ relative to protons are in agreement with
each other. Combining the two distributions and correcting for detector
acceptance we obtain the forward-backward asymmetry A^l_FB = (5.8 +- 5.1(stat)
+- 1.3(syst))%, compared to the standard model prediction of A^l_FB (predicted)
= (4.7 +- 0.1)%. This result is further combined with the measurement based on
the analysis of the l+jets final state to obtain A^l_FB = (11.8 +- 3.2)%.
Furthermore, we present a first study of the top-quark polarization.Comment: submitted versio
Journal Staff
We present the first measurements of the differential cross section d sigma/dp(T)(gamma) for the production of an isolated photon in association with at least two b-quark jets. The measurements consider photons with rapidities vertical bar y(gamma)vertical bar < 1.0 and transverse momenta 30 < p(T)(gamma) < 200 GeV. The b-quark jets are required to have p(T)(jet) > 15 GeVand vertical bar y(jet)vertical bar < 1.5. The ratio of differential production cross sections for gamma + 2 b-jets to gamma + b-jet as a function of p(T)(gamma) is also presented. The results are based on the proton-antiproton collision data at root s = 1.96 TeV collected with the D0 detector at the Fermilab Tevatron Collider. The measured cross sections and their ratios are compared to the next- to- leading order perturbative QCD calculations as well as predictions based on the k(T)- factorization approach and those from the sherpa and pythia Monte Carlo event generators
A search for charged massive long-lived particles
We report on a search for charged massive long-lived particles (CMLLPs),
based on 5.2 fb of integrated luminosity collected with the D0 detector
at the Fermilab Tevatron collider. We search for events in which one
or more particles are reconstructed as muons but have speed and ionization
energy loss inconsistent with muons produced in beam collisions.
CMLLPs are predicted in several theories of physics beyond the standard model.
We exclude pair-produced long-lived gaugino-like charginos below 267 GeV and
higgsino-like charginos below 217 GeV at 95% C.L., as well as long-lived scalar
top quarks with mass below 285 GeV.Comment: submitted to Phys. Rev. Letter
Precise measurement of the top quark mass in the dilepton channel at D0
We measure the top quark mass (mt) in ppbar collisions at a center of mass
energy of 1.96 TeV using dilepton ttbar->W+bW-bbar->l+nubl-nubarbbar events,
where l denotes an electron, a muon, or a tau that decays leptonically. The
data correspond to an integrated luminosity of 5.4 fb-1 collected with the D0
detector at the Fermilab Tevatron Collider. We obtain mt = 174.0 +- 1.8(stat)
+- 2.4(syst) GeV, which is in agreement with the current world average mt =
173.3 +- 1.1 GeV. This is currently the most precise measurement of mt in the
dilepton channel.Comment: 7 pages, 4 figure
Search for right-handed W bosons in top quark decay
We present a measurement of the fraction f+ of right-handed W bosons produced
in top quark decays, based on a candidate sample of events in the
lepton+jets decay mode. These data correspond to an integrated luminosity of
230pb^-1, collected by the DO detector at the Fermilab Tevatron
Collider at sqrt(s)=1.96 TeV. We use a constrained fit to reconstruct the
kinematics of the and decay products, which allows for the
measurement of the leptonic decay angle for each event. By comparing
the distribution from the data with those for the expected
background and signal for various values of f+, we find
f+=0.00+-0.13(stat)+-0.07(syst). This measurement is consistent with the
standard model prediction of f+=3.6x10^-4.Comment: Submitted to Physical Review D Rapid Communications 7 pages, 3
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