NuMI Neutrino Beams
On-axis neutrino beam
The NuMI neutrino beam is being built as a flexible beam. By re-arranging
the relative positions of the target and two focusing horns it will be
possible to produce beams with different neutrino spectra. These beams
are usually referred to as: low/medium/high energy beams.
Charged current event rates expected at
the far detector position for different beam configurations under the no
oscillations hypothesis are given for 1 kton/year exposure assuming 4x10**20
protons per year on the NuMI target.
Event rates in the near detector hall are approximately 1,000,000 times
higher. Neutrino energy spectra at the near detector are very similar to
those in the far detector.
Off-axis neutrino beams
Neutrinos are produced by the decays of pions and kaons focused into a
nearly parallel beam by magnetic horns. The neutrino spectrum is therefore
a reflection of the energy distribution of the decaying parent mesons,
modulo a factor of 0.43.
What is special about off-axis beams? For low energy beams,
the energy of the neutrinos produced at 'large'angles is only weakly
dependent on the parent pion momentum, as illustrated in fig
. Lines labeled '10 km' and '20 km' represent decay angles pointing
to the detectors located at these transverse distances from the beam axis
at the distance of 735 km from the source. (This phenomenon
was pointed out in the Long Baseline Neutrino Oscillation Proposal for
Experiment, E889, at Brookhaven.)
Beam spectra at off-axis detectors
The spectra of numu CC events expected (in the absence of oscillations)
for 10 kton*year exposures at distances of 5, 10 and 20 km (transversely)
from MINOS for a nominal medium energy beam are shown in Event
spectra . The event energy spectra do not depend on the beam
setting (low/medium/high energy), but the event rates do. This can
be seen by looking at the corresponding event spectra for the
low
and high energy beams. The significant
reduction of the off-axis neutrino flux in the case of the nominal high
energy beam is primarily due to the kinematic
reduction of the flux as a function of the decay angle for higher energy
pions.
Disappearance experiment with off-axis detectors
Detectors located at the same distance from Fermilab (taken to be 735 km)
but at different distances from the beam
axis will probe neutrino oscillations at different L/E ranges. Comparison
of the expected (open histogram) and the observed (hatched histogram) CC
event spectra for 10 kton*years exposures for maximal mixing angles and
different dm**2's
Electron neutrino background rates
Electron neutrino background
is at the level of 0.5% of the numu event rates (at the peak of the numu
spectrum).
This background originates from Ke3 decays and from muon decays. At
the peak of the expected numu neutrino event energy distribution this background
is dominated (at 90% level) by the neutrinos
produced in muon decays (open histogram shows all nue background event,
shaded histograms show a contribution of Ke3 decays).These neutrinos are
therefore directly related to the muon neutrinos observed in the near detector.
Numu event rates at different detector positions
New detectors can be located at different distances from Fermilab, hence
optimizing the oscillation probability and/or the matter effects, and at
different distances from the nominal beam axis, hence selecting the beam
energy and spectrum. Event rates are given in 200 MeV neutrino energy bins
for 4x10**20 protons on target and 1 kton detector .
vec files are in ascii format, eps files show the corresponding distribution.
Nue background event rates at different detector positions
Electron neutrino (and antineutrino) events spectra for different detector
locations. Event rates are given in 200 MeV neutrino energy bins for 4x10**20
protons on target and 1 kton detector.
vec files are in ascii format, eps files show the corresponding distribution.
Nue signal event rates at different detector positions
Assume dm**2=3x10-3 eV**2 and full oscillation of muon neutrinos into electron
neutrinos. Electron neutrino events spectra for different detector locations
are convolutions of the numu neutrino flux shape and the oscillation probability.
Event rates are given in 200 MeV neutrino energy bins for 4x10**20 protons
on target and 1 kton detector .
vec files are in ascii format, eps files show the corresponding distribution.
Baseline/position optimization
This is just an example of a possible optimization scheme. Assume
that we want to maximize the sensitivity of the experiment to nue
appearance. Assume that the neutral current background is reduced below
the nue contamination of the beam and that the latter is known. The sensitivity
of the appearance experiment is limited by the statistical fluctuations
of the (otherwise known) background. The figure of merit can be defined
as the ratio of the number of expected nue signal events to the square
root of number of the predicted background events. Furthermore, for
every detector position one may define different energy ranges to maximize
the experiment sensitivity at this location.
The table shows the result of such an optimization procedure. The entries
are:
-
figure of merit (assuming full mixing angle),
-
number of nue signal events,
-
number of nue background events
-
energy window, lower limit, GeV
-
energy window, upper limit, GeV
All numbers refer to a 1 kton*year exposure.
| L/DR [km] |
5 |
8 |
9 |
10 |
11 |
12 |
15 |
20 |
| 900 |
136.6
140.9
1.1
1.8
5.2 |
146.9
90.8
0.4
2.0
3.6 |
144.6
81.9
0.3
2.0
3.4 |
139.0
68.6
0.2
1.8
3.0 |
129.9
56.4
0.2
1.8
2.8 |
118.6
43.8
0.1
1.8
2.6 |
71.2
18.9
0.1
1.4
2.0 |
15.9
4.8
0.1
0.6
2.0 |
| 850 |
135.3
135.4
1.0
1.8
4.8 |
145.6
88.3
0.4
2.0
3.4 |
146.1
83.4
0.3
1.8
3.2 |
139.3
64.4
0.2
1.8
2.8 |
129.8
55.3
0.2
1.6
2.6 |
118.6
43.5
0.1
1.6
2.4 |
67.5
18.9
0.1
1.4
2.0 |
15.5
5.3
0.1
0.6
2.4 |
| 800 |
134.6
144.8
1.2
1.6
4.8 |
146.5
98.0
0.4
1.8
3.4 |
145.0
80.1
0.3
1.8
3.0 |
137.5
58.9
0.2
1.8
2.6 |
130.6
50.8
0.2
1.6
2.4 |
116.5
38.0
0.1
1.6
2.2 |
63.6
16.9
0.1
1.2
1.8 |
15.1
5.0
0.1
0.2
2.2 |
| 735 |
133.2
148.0
1.2
1.6
4.6 |
146.9
88.8
0.4
1.8
3.0 |
142.6
80.1
0.3
1.6
2.8 |
138.6
58.9
0.2
1.6
2.4 |
126.1
45.0
0.1
1.6
2.2 |
110.9
36.1
0.1
1.4
2.0 |
57.7
16.3
0.1
1.2
1.8 |
14.4
4.8
0.1
0.2
2.2 |
| 650 |
130.8
139.5
1.1
1.6
4.0 |
143.4
81.1
0.3
1.6
2.6 |
143.0
68.5
0.2
1.6
2.4 |
133.4
58.6
0.2
1.4
2.2 |
115.9
46.9
0.2
1.2
2.0 |
101.7
32.9
0.1
1.2
1.8 |
49.8
13.9
0.1
1.0
1.6 |
13.7
4.4
0.1
0.4
2.0 |
Last updated on March 30, 2002.
Questions or comments?