1,599 research outputs found
Determination of alpha_S using hadronic event shape distributions of data taken with the OPAL detector
The measurement of the strong coupling alpha_S using hadronic event shape
distributions measured with the OPAL detector at center-of-mass energies
between 91 and 209 GeV is summarized. For this measurement hadronic event shape
distributions are compared to theoretical predictions based on
next-to-next-to-leading-calculations (NNLO) and NNLO combined with resummed
next-to-leading-logarithm calculations (NLLA). The combined result using NNLO
calculations is
alpha_S(MZ)=0.1201+-0.0008(stat.)+-0.0013(exp.)+-0.0010(had.)+-0.0024(theo.)
and the result using NLLO and NLLA calculations is
alpha_S(MZ)=0.1189+-0.0008(stat.)+-0.0016(exp.)=-0.0010(had.)+-0.0036(theo.),
with both measurements being in agreement with the world average.Comment: 4 pages, 2 figures, contribution to the Proceedings of Rencontres de
Moriond, QCD Session, March 20th - 27th, 2011 in La Thuile, Italy; based on
arXiv:1101.147
Measurement of alpha_s in radiative hadronic events at OPAL
Hadronic final states with a hard isolated photon are studied using data
taken at centre-of-mass energies around \Mz with the OPAL detector at LEP.
The strong coupling \as is extracted by fitting event shape variables for the
reduced centre-of-mass energies ranging from 20 GeV to 80 GeV, and the energy
dependence of \as is studied. Combining all the values using different event
shape variables and energies gives:
\asmz=0.1176\pm 0.0012(stat.)^{+0.0093}_{-0.0085}(syst.).Comment: 4 pages, 1 figure, contribution to the QCD '03 conferenc
DEPFET detectors for direct detection of MeV Dark Matter particles
The existence of dark matter is undisputed, while the nature of it is still
unknown. Explaining dark matter with the existence of a new unobserved particle
is among the most promising possible solutions. Recently dark matter candidates
in the MeV mass region received more and more interest. In comparison to the
mass region between a few GeV to several TeV, this region is experimentally
largely unexplored. We discuss the application of a RNDR DEPFET semiconductor
detector for direct searches for dark matter in the MeV mass region. We present
the working principle of the RNDR DEPFET devices and review the performance
obtained by previously performed prototype measurements. The future potential
of the technology as dark matter detector is discussed and the sensitivity for
MeV dark matter detection with RNDR DEPFET sensors is presented. Under the
assumption of three background events in the region of interest and an exposure
of one kgy a sensitivity of cm
for dark matter particles with a mass of 10 MeV can be reached.Comment: submitted to EPJ
A neural network z-vertex trigger for Belle II
We present the concept of a track trigger for the Belle II experiment, based
on a neural network approach, that is able to reconstruct the z (longitudinal)
position of the event vertex within the latency of the first level trigger. The
trigger will thus be able to suppress a large fraction of the dominating
background from events outside of the interaction region. The trigger uses the
drift time information of the hits from the Central Drift Chamber (CDC) of
Belle II within narrow cones in polar and azimuthal angle as well as in
transverse momentum (sectors), and estimates the z-vertex without explicit
track reconstruction. The preprocessing for the track trigger is based on the
track information provided by the standard CDC trigger. It takes input from the
2D () track finder, adds information from the stereo wires of the
CDC, and finds the appropriate sectors in the CDC for each track in a given
event. Within each sector, the z-vertex of the associated track is estimated by
a specialized neural network, with a continuous output corresponding to the
scaled z-vertex. The input values for the neural network are calculated from
the wire hits of the CDC.Comment: Proceedings of the 16th International workshop on Advanced Computing
and Analysis Techniques in physics research (ACAT), Preprint, reviewed
version (only minor corrections
Search for direct pair production of the top squark in all-hadronic final states in proton-proton collisions at s√ = 8 TeV with the ATLAS detector
The results of a search for direct pair production of the scalar partner to the top quark using an integrated luminosity of 20.1 fb−1 of proton-proton collision data at s√ = 8 TeV recorded with the ATLAS detector at the LHC are reported. The top squark is assumed to decay via t¯ →tχ¯01 or t¯ →bχ¯±1 →bW(∗)χ¯01 , where χ¯01 (χ¯±1) denotes the lightest neutralino (chargino) in supersymmetric models. The search targets a fully-hadronic final state in events with four or more jets and large missing transverse momentum. No significant excess over the Standard Model background prediction is observed, and exclusion limits are reported in terms of the top squark and neutralino masses and as a function of the branching fraction of t¯ →tχ¯01 . For a branching fraction of 100%, top squark masses in the range 270–645 GeV are excluded for χ¯01 masses below 30 GeV. For a branching fraction of 50% to either t¯ →tχ¯01 or t¯ →bχ¯±1 , and assuming the χ¯±1 mass to be twice the χ¯01 mass, top squark masses in the range 250–550 GeV are excluded for χ¯01 masses below 60 GeV
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