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This book reports on a new result from the KL 0 search at the
J-PARC KOTO experiment, which sets an upper limit of 3x10-9 for the
branching fraction of the decay at the 90% confidence level,
improving the previous best limit by an order of magnitude. To
explain the matter-antimatter asymmetry in the universe, still
unknown new physics beyond the standard model (SM) that breaks CP
symmetry is necessary. The rare decay of a long-lived neutral K
meson, KL 0 , is a CP-violating decay. It is an excellent probe to
search for new physics because new physics can contribute to the
decay and change its branching fraction, while the SM is as small
as 3x10-11. However, it is extremely difficult to search for
because all of the decay products are neutral and two neutrinos are
undetectable. The KL 0 signal is identified by measuring two
photons from a 0 with a calorimeter and confirming the absence of
any other detectable particles with hermetic veto counters. The
book contributes to the analysis of neutron-induced backgrounds
which were the dominant background sources in the search. For the
background caused by two consecutive hadronic showers in the
calorimeter due to a neutron, the author evaluated the background
yield using a data-driven approach. For another background caused
by an meson production- decays two photons-by a neutron that hits a
veto counter near the calorimeter, the author developed an original
analysis technique to reduce it. The book also contributes to the
analysis of the normalization modes (KL 3 0, KL 2 0, KL 2 ) to
measure KL yield, the estimation of the signal acceptance based on
a simulation, and the evaluation of the trigger efficiency. As a
result, significant improvements in the measurement were achieved,
and this is an important step in the continuing higher sensitivity
search, which can reach new physics with the energy scales up to
O(100-1000 TeV).
This book reports on a new result from the KL 0 search at the
J-PARC KOTO experiment, which sets an upper limit of 3x10-9 for the
branching fraction of the decay at the 90% confidence level,
improving the previous best limit by an order of magnitude. To
explain the matter-antimatter asymmetry in the universe, still
unknown new physics beyond the standard model (SM) that breaks CP
symmetry is necessary. The rare decay of a long-lived neutral K
meson, KL 0 , is a CP-violating decay. It is an excellent probe to
search for new physics because new physics can contribute to the
decay and change its branching fraction, while the SM is as small
as 3x10-11. However, it is extremely difficult to search for
because all of the decay products are neutral and two neutrinos are
undetectable. The KL 0 signal is identified by measuring two
photons from a 0 with a calorimeter and confirming the absence of
any other detectable particles with hermetic veto counters. The
book contributes to the analysis of neutron-induced backgrounds
which were the dominant background sources in the search. For the
background caused by two consecutive hadronic showers in the
calorimeter due to a neutron, the author evaluated the background
yield using a data-driven approach. For another background caused
by an meson production- decays two photons-by a neutron that hits a
veto counter near the calorimeter, the author developed an original
analysis technique to reduce it. The book also contributes to the
analysis of the normalization modes (KL 3 0, KL 2 0, KL 2 ) to
measure KL yield, the estimation of the signal acceptance based on
a simulation, and the evaluation of the trigger efficiency. As a
result, significant improvements in the measurement were achieved,
and this is an important step in the continuing higher sensitivity
search, which can reach new physics with the energy scales up to
O(100-1000 TeV).
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