Transcript T2K 2km WC detector

Ultra-high energy upward going muons in Super-Kamiokande II
Ariana S. Minot (Duke University)
Ultra-high energy upward-going muons (E>1 TeV) are created by the interactions of very high energy neutrinos inside the Earth. They could be produced
by astrophysical sources (AGN, etc.). Improvements to the track fitters were made to reduce backgrounds.
Reconstructing muons tracks: OD fit
Superk-Kamiokande II
Super-Kamiokande is a large water Cherenkov detector located in Kamioka
In the Japanese Alps. It is famous for its discovery of neutrino oscillations in
1998.
It is a 50 kton cylinder filled with ultra-pure water. There are 2 optically
separated regions :
the Inner Detector (ID) contains over 11000 photo-multiplier tubes (PMTs)
the Outer-Detectpr (OD), a 2m-thick region surrounding the ID,
contains 1885 smaller PMTs. Its’ purpose is to tag entering and exiting
particles.
After an accident destroyed most of the ID in 2001, Super-Kamiokande was
partially rebuilt with half the tubes. This phase is known as SK-II.
OD fit performs a linear fit using only hit tubes in the OD. A linear fit to the
charge-times distribution is performed:
x(t) = x0 + vx t
y(t) = y0 + vy t
z(t) = z0 + vz t
The slope of the best line is an estimate of -cos() where  is the zenith
angle of the fitted muon.
Results for SK-II
Ultra-high energy upward going muon selection cuts :
Total charge > 866000 photo electrons
Difference between Entry and Exit times > -40 ns
More than 10 hits around entry and exit points
Fitted track length > 7 m
Fitted -cos() <- .1
Efficiency of cuts on mono-energetic isotropic Monte-Carlo :
Super-Kamiokande is exposed to a large flux of downward going muons
---> We must eliminate this background to find the upward going muons.
Improvements to OD fit
Super Kamiokande
Tokyo
OD fit can be confused by large isolated charges and can mis-identify proper
upward-going events.
Why search for ultra-high energy upmu’s ?
Ultra high energy upward going muons are produced by very high energy
neutrino interactions in the rock below the detector. Such neutrinos can be
atmospheric neutrinos, but at these high energies, there will be very few.
More importantly, high energy neutrinos are thought to come from
astrophysical sources such as active galactic nuclei and gamma ray bursts.
We searched for such neutrinos in our data, using SK as a telescope for
exotic astrophysical objects.
Water Cherenkov Imaging method
Actual track
OD fit mistake
Selection cuts are applied on SK-II data and SK-II atmospheric neutrino
simulation. Upward going muons from the high-energy tail of the atmospheric
neutrinos are the main background for this search for astrophysical
neutrinos.
After selection the events are eye-scanned:
Horiz- Stopp Down Up
Multi Junk
ontal -ing
Going Going Muon
Muon
Improvement: check that there are at least two neighboring hits within 3m
and 50 ns of every hit, to remove outliers.
The direction fit is improved by a factor of 2.
Data
3
2
17
1
10
2
MC:
0
0
0
9
0
0
2
10
1
19
0
0
Upmu
Rck
MC:
All
When charged particles travel in the water at velocity higher than the speed
of light in water ( c/n ), an electromagnetic shockwave is produced. This
phenomenon (similar to a sonic boom for sound waves) is known as the
Cherenkov effect.
Wtr
The photons are emitted around the direction of the track,
making a cone with opening angle cos() = 1/ n .
We find one candidate in the data:
These photons are detected by the PMTs on the wall : the cones intersect
with the walls of Super-K, making ring patterns
However, when particles of ultra-high energies enter the detector, all the
PMTs of the ID are saturated, because the amount of light is enormous.
In order to search for muons that enter and leave the detector, it is better to
use the information provided by the OD.
Angle between true and fitted direction