Addressing the MiniBooNE anomaly

1
The MicroBooNE Experiment A Liquid Argon TPC
Joshua Spitz Yale University MicroBooNE
Collaboration
Cross sections and the beams
Addressing the MiniBooNE anomaly
The detector
MicroBooNEs excellent efficiency, low energy
threshold, and unique ability to separate photons
from electrons will be utilized to study cross
sections important to NOvA in the 0-2 GeV range.
These measurements are not only vital to model
signal processes, they are important in
background rejection as well. MicroBooNE will
record 8000 neutral current pi0 interactions
from the BNB (peaked at 0.8 GeV) and 5400
events from the NuMI off-axis beam. Furthermore,
the detector will study a large number of
nue charged current events.
In pure liquid argon, the charge created due to
ionizing tracks is drifted along electric field
lines to XY collection and induction wire planes.
In MicroBooNEs baseline design, 30 PMTs will
provide triggering and the time when the event
begins. A bubble-chamber-like 3D photograph of
a neutrino event is obtained. The 73 ton fiducial
volume MicroBooNE detector will give uniform
imaging with 3 mm spatial resolution will also
measure dE/dx over the track length.
MiniBooNE has ruled out a two-neutrino
interpretation of the LSND result by seeing
events consistent with background above E 475
MeV 1. However, a 96 19stat 21syst event
excess from 300-475 MeV and a 91 19stat
25syst event excess from 200-300 MeV suggest an
unpredicted background or new physics
2,3,4,5,6,7,8,9. This excess must be
understood.
The MicroBooNE detector. The cryostat,
feed-throughs, on-detector electronics, wire
chambers, field cage, PMTs, and cold electronics
can be seen.
A simulated neutral current pi0 interaction in
MicroBooNE
A particle track in a LArTPC
LArTPC milestones
Electron and gamma separation
MicroBooNE is the vital next step in a phased
program of RD for Liquid Argon Time Projection
Chambers (LArTPCs) for use in future
(CP-violation, , mass-hierarchy) long
baseline experiments. The DOEs Neutrino
Scientific Assessment Group (NuSAG) states
12 A phased RD program with milestones and
using a technology suitable for a 50-100 kton
detector is recommended for the liquid argon
detector option. Upon completing of the existing
RD project to achieve purity sufficient for long
drift time, to design low noise electronics, and
to qualify materials, construction of a test
module that could be exposed to a neutrino beam
is recommended
.
MicroBooNE 10 will be ideal to explain
MiniBooNEs anomalous energy region. Utilizing a
Liquid Argon Time Projection Chamber (LArTPC)
design, MicroBooNE will combine low-background
bubble-chamber-like resolution and efficiency
with the ability to differentiate electrons from
photons. With pre-cuts and the track based
analysis, MiniBooNEs nue
efficiency is lt 40. The efficiency for nue
CCQE events within the fiducial volume in LArTPCs
is 80 at worst 11. As discussed to the right,
backgrounds from electron and gamma separation
will be reduced by gt94 with MicroBooNEs dE/dx
measurement capabilities. Furthermore, MicroBooNE
will not contend with uncertainties in the light
propagation associated with Cherenkov detectors.
The ability to differentiate an electron track
from a gamma track is one of the most important
advantages a LArTPC has over other detectors. The
energy deposition in the first few centimeters of
track (dE/dx) will be about one minimum ionizing
particle (MIP) for electrons. Since photons
convert to electron-positron pairs, dE/dx will be
about twice as large for gamma tracks. This
particle identification tag will be useful for
Argon purity is essential to the success of
MicroBooNE. Drift electrons need to be able to
freely propagate across the volume in order to be
detected. Utilizing an ICARUS-style purity
monitor 14, a 5.7 ms electron lifetime has been
demonstrated at FNAL. The monitor works as
follows photons from a Xenon flash lamp impinge
upon a photocathode. Photoelectrons are drifted
along electric field lines from a cathode to an
anode. The drift time is found from the amplitude
of the cathode and anode signals and the time
between the signals. The efficacy of this type of
purity monitor has also been confirmed in a test
at Yale University.

• A neutral current pi0 can be misidentified as an
  electron event if only one of the decay photons
  converts.
• About 75 of dirt neutrino events are neutral
  current pi0 with only one gamma entering the
  detector.
• Radiative delta decays produce only one photon
  which may appear electron-like.

Anode
Cathode
For a 6E20 POT beam request, the same signal
MiniBooNE observes in the 200-475 MeV range
corresponds to 53.4 5.6stat 1.9syst events in
MicroBooNE, a 9.1s effect. This value has been
obtained after simple fiducial volume scaling
(73tons500tons) from MiniBooNE, a conservative
90 electron/gamma separation efficiency estimate
and assuming x2 detection efficiency to signal
and intrinsic events.
1 MIPgreen 2 MIPred
Yales purity monitor above and FNALs electron
lifetime measurement below.
Events in the ICARUS LArTPC
A pioneer in LArTPC techniques, the ICARUS13
experiment has demonstrated purity repeatedly.
A strong demonstration of purity, cosmic-ray
tracks have been seen for the first time in the
US with a LArTPC developed at Yale University
14.
Simulated electron and gamma separation for
neutrino energies close to the MiniBooNE
anomalous region. 94 separation efficiency is
obtained.
Simulated gamma and electron tracks. The color is
representative of the energy deposition in the
wires.
A hadronic shower in Yales LArTPC
References
ArgoNeut15 is a mini-LArTPC that will begin
taking data in February 2008 upstream of the
MINOS near detector. The 176 L active volume will
see 175K neutrino interactions per year with the
current NuMI LE and 2E20 POT setup. This small
detector will do a nice job of containing protons
and charged pion tracks. Long-range muon tracks
can be tagged with the help of the downstream
MINOS detector. Although neutral pions will be
hard to tag as the radiation length in liquid
argon is 14 cm, the event sample will be quite
large. ArgoNeut will include a purification and
recirculation system along with a purity monitor
and will act as a test stand for MicroBooNE.
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hep-th 10 The MicroBooNE Collaboration, A
Proposal for a new experiment using the Booster
and NuMI beamlines MicroBooNE, submitted to the
PAC. 11 E. Kearns et al. T2K Collaboration A
Proposal for a detector 2 km away from the T2K
neutrino source 12 Recommendations to the
Department of Energy and the National Science
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physics.ins-det 15 http//t962.fnal.gov
ArgoNeut sitting upstream of the MINOS
detector. Picture courtesy of Bartoszek
engineering.