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Discovery of Electron Neutrino Appearance from a Muon Neutrino Beam in T2K and Future Outlook for Discovery of CP Violation in Lepton Sector

Chang Kee Jung, Stony Brook
Monday, May 11, 2015 - 4:00pm to 5:00pm
PAA A-102
Abstract:
Matter-antimatter asymmetry is one of the most outstanding mysteries of
the universe that provides a necessary condition to our own existence.
There have been various attempts to solve this mystery including
'Baryogenesis' hypothesis. However, the B-factory experiments
during the last decade showed that the observed CP-violation (CPV)
in the quark sector is not big enough for baryogenesis
to be a viable solution to the matter-antimatter asymmetry.
This leads us to the 'Leptogenesis' hypothesis,
in which CPV in the lepton section plays a crtical role to create
the matter-antimater asymmetry at the onset of the Big Bang.
Thus, experimental observation of CPV in the lepton
sector could prove to be tantamount to one of the most important
discoveries in our understanding of the universe.
In 2011, the T2K experiment published a result that provided the first
indication for a non-zero $\theta_{13}$, the last unknown mixing angle
in the lepton sector at that time, at 2.5 sigma level of significance.
In 2013, after analyzing two more years of data taken since 2011,
the experiment reported "Observation of electron neutrino
appearance from a muon neutrino beam" at 7.3 sigma level of significance.
While neutrino oscillation has been well-established since the discovery
by the Super-Kamiokande experiment in 1998, there have not been a definitive
observation of neutrino oscillation in a so-called "appearance mode", and this
new T2K observation is the first time an explicit neutrino
flavor (electron) appearance is observed from another neutrino flavor (muon).
This observation also opens the door to study CPV in neutrinos.
When incorporating recent precision measurements on $\theta_{13}$ by the
reactor experiments along with other neutrino oscillation parameter
measurements, T2K data show an intriguing initial result on the
$\delta_{CP}$.
In this talk I will present some details of this discovery
and its importance to the future CP-violation
measurements in the lepton sector. I will also present future outlook for
discovering CPV as well as measuring other neutrino oscillation parameters
using future very large underground detectors, especially the DUNE experiment
in US, that is newly established as a truly international collaboration
to pursue discovery of CPV in the lepton sector, detrmination of
mass hierarchy, discovery of proton decay and observation of neutrinos
from the Type-II supernovae.
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