1. Art and Diseño

Physics Letters B 545 (2002) 57–61
www.elsevier.com/locate/npe
A new measurement of the ν e e− elastic cross
section at very low energy
C. Amsler d , M. Avenier a , C. Broggini c,∗ , J. Busto b , C. Cerna c , Z. Daraktchieva b ,
G. Gervasio b , P. Jeanneret b , G. Jonkmans b , D.H. Koang a , J. Lamblin a , D. Lebrun a ,
O. Link d , F. Ould-Saada d , G. Puglierin c , A. Stutz a , A. Tadsen c , J.L. Vuilleumier b
a Institut des Sciences Nucléaires, IN2P3/CNRS-UJF, 53 Avenue des Martyrs, F-38026, Grenoble, France
b Institut des Physique, rue A.L. Breguet 1, CH-2000, Neuchâtel, Switzerland
c Istituto Nazionale di Fisica Nucleare, Sezione di Padova, via Marzolo 8, I-35131 Padova, Italy
d Physik Institut, Winterhurerstrasse 190, CH-8057 Zürich, Switzerland
Received 3 July 2002; received in revised form 25 July 2002; accepted 31 July 2002
Editor: L. Rolandi
Abstract
We have built a low background detector, a time projection chamber surrounded by an active anti-Compton, to measure the
ν e e− elastic cross section down to the antineutrino energy of 900 keV. With our detector, running at 18 m from the core of a
nuclear reactor in Bugey, we could detect reactor antineutrinos by measuring both the energy and the direction of the recoiling
electrons. We report here on a first analysis of the data using an automatic scanning procedure. The results we obtain are 1.5σ
higher than the ones predicted by the standard model.
 2002 Elsevier Science B.V. All rights reserved.
PACS: 13.15; 14.60.5; 26.65; 96.60.J; 29.40.G
Keywords: Neutrino–electron scattering; Neutrino properties; Solar neutrinos
1. The experiment
MUNU was designed to study ν e e− → ν e e− scattering with the antineutrinos from a nuclear reactor.
Technical details of MUNU have already been reported [1,2].
* Corresponding author.
E-mail address: [email protected] (C. Broggini).
Briefly, the detector consists of a 1 m3 cylindrical
time projection chamber (90 cm diameter and 162 cm
long) filled with CF 4 at the pressure of 3 bar. CF 4
was chosen because it has at the same time a low
atomic number and a high density (3.7 gr/l at 1 bar
pressure and room temperature). The z coordinate
along the TPC axis is determined by the drift time.
The transverse x and y coordinates are obtained with
two planes of 256 orthogonal strips.
The acrylic vessel TPC is mounted inside a steel
vessel (2 m diameter and 3.8 m long) filled with 10 m3
0370-2693/02/$ – see front matter  2002 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 0 - 2 6 9 3 ( 0 2 ) 0 2 5 6 0 - 1
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C. Amsler et al. / Physics Letters B 545 (2002) 57–61
Thanks to the tracking capability of the detector
we have a simultaneous measurement of signal plus
background events in the forward solid angle (along
the antineutrino direction) and background events
only in the backward one. Background can thus
be measured on-line, while the reactor is on, and
subtracted.
The detector is running at 18 m from the core
of a 2800 MW power reactor in Bugey, at a depth
equivalent to 20 meter of water. The antineutrino flux,
1.2 × 1013 cm−2 s−1 at the detector location, increases
rapidly below 1 MeV and is negligible above 10 MeV.
To the flux we assign a total error of 5%, which takes
into account both the error on the measured spectrum
above 1.8 MeV [3] and the estimated error in the
energy region from 900 keV to 1.8 MeV [4].
Fig. 1. 860 keV electron (37 cm range): xz (top) and yz (bottom)
projections. The binning is 3.5 mm for x and y and 1.8 mm for z,
corresponding to 80 ns drift time bins.
2. Event rate
of liquid scintillator. The liquid scintillator, read out
by 48 photomultipliers (20 cm diameter), works as a
muon veto and as an anti-Compton shielding. It gives
134 ± 8 photoelectrons for a 1 MeV energy deposition.
The steel vessel is surrounded by 8 cm thick boron
loaded polyethylene shielding and a 15 cm thick
lead shielding to absorb external neutrons and γ rays
entering the detector from outside.
Since the expected event rate is low it has been
necessary to minimize all possible background sources
and to construct each part of the detector from selected
low radioactivity material.
The electrons of the TPC filling gas (1.06 ×
1027 ) are the target for ν e e− scattering. In each
interaction the direction of the recoiling electron can
be reconstructed, together with its energy (Fig. 1).
Because of the scintillation properties of CF 4 [2]
the electron energy is obtained not only from the
charge collected at the TPC anode plane but also from
the amplified light emitted at the anode plane and detected by the photomultipliers which read out the liquid scintillator (the electron primary scintillation light
is too faint to be seen in our detector because of the
acrylic wavelength cut below ∼400 nm). The energy
resolution provided by the two different methods is
very similar.
The counting rate of the anti-Compton above
100 keV is 700 s−1 : 400 due to natural radioactivity
and 300 from cosmic muons. Both rates are in agreement with the measured radioactive contaminants [1]
and the muon flux (32 m−2 s−1 ).
The rate of the events we are interested in, i.e.,
electrons in the TPC not due to Compton scattering
from gamma rays, needs a more detailed discussion.
First the neutrino trigger acquires the events which
have an energy deposition larger than 300 keV in the
TPC and have not produced any light signal above
90 keV in the anti-Compton during the 80 µs which
precede the start of the TPC event (the ‘length’ of
the TPC amounts to 75 µs). The Compton suppression
efficiency of the neutrino trigger is more than 99.99%
for external γ rays. The actual neutrino trigger rate
is 0.9 Hz with a 40% dead time, mostly due to event
writing to disk.
In this Letter we give the results obtained with
the data collected in the year 2001, after the detector
improvement made the year before [2]. They refer to
68 live days with reactor on and 23 days with reactor
off.
We apply a 700 keV cut on the electron kinetic
energy, which corresponds to neutrino energies larger
than 900 keV. This way we are above the end-point
energy from the β decay of 85 Kr, which, with an
C. Amsler et al. / Physics Letters B 545 (2002) 57–61
estimated activity of 0.15 Bq, is probably the most
severe background source at low energy.
3. Data analysis
A track recognition program finds the events with
one contained electron and fits the vertex and the
direction of the track with 80% efficiency.
The energy and angular r.m.s. resolution of the detector, measured with radioactive sources, amounts to
10% and 16 degrees, respectively, at 700 keV (7%
and 12 degrees at 1.2 MeV). The detector containment efficiency varies from 57% at 700 keV to 35%
at 1.2 MeV.
The event rate of single reconstructed electrons
fully contained inside the TPC volume is 303.6 ± 2.1
counts per day (cpd), i.e., 27 cpd/kg, very close to
the rate of very well shielded ultra-low-background
germanium detectors operating in an underground
laboratory [5].
From the energy spectrum and angular distribution
of the electrons we conclude that more than 90% of the
background is due to the β decay of the 210 Bi nuclei
implanted on the anode plane; 20 cpd are from the
radon in the gas (2 mBq) whereas about 2 cpd can be
ascribed to external γ rays.
The events, stored as a function of the electron
energy and scattering angle, undergo two different
types of analysis: forward–backward and kinematic
one. The key point exploited by the two analysis is
the forward–backward symmetry of both the detector
and the background (as compared to the antineutrino
direction).
3.1. Forward–backward analysis
From the Monte Carlo simulation we expect that
33% of the ν e e− scattering events are in the cos(θ )
region between 0.8 and 1 (where θ is the angle
between the antineutrino and the electron directions).
Such a spread comes from the shape of the differential
cross section, the angular resolution of the detector,
the track reconstruction algorithm, the size of the
reactor core and the unknown absolute z position of
the interaction vertex.
In the forward solid angle region, 0.8 cos(θ ) 1,
the rate is 19.7 ± 0.5 cpd, as compared to 17.9 ±
59
0.5 cpd in the backward one, −1 cos(θ ) −0.8.
The difference between the two gives a rate of 1.8 ±
0.7(stat) cpd.
To prove that these events are due to antineutrino
interactions we have first to show that the background
is forward–backward symmetric when the reactor is
off.
The detector is absolutely symmetric by construction with regards to the reactor–detector axis. For instance, the intrinsic radioactivity of each of the pieces
used for the TPC vessel construction has been measured by neutron activation and all the resistors connecting the field shaping rings of the TPC are mounted
in the plane perpendicular to the reactor–detector axis.
The electrons from a beta contamination on the
inner surface of the TPC walls would be suppressed
by the TPC fiducial volume cut (no hits within 3 cm
from the TPC walls), whereas a gamma ‘hot spot’ on
the anti-Compton vessel is excluded because of the
uniform count rate of the anti-Compton along the z
direction.
As final check we make the same analysis with the
data taken during the reactor off period. The rate we
obtain, 0.1 ± 1.4 cpd, is fully consistent with zero.
We have now to prove that the event excess with
reactor on is due to ν e e− → ν e e− . To achieve this
result we have considered and excluded the following
background:
• ν e + p → e+ + n in the TPC gas. It is non-existent
because there are no free protons in pure CF 4
and the event would be in any case vetoed due
to the gamma rays from the positron annihilation.
In addition, the reaction is isotropic, apart from a
small positron backward asymmetry;
• ν e + p → e+ + n in the TPC acrylic vessel. They
do take place but they are removed by the TPC
fiducial volume cut;
• events due to neutron capture. About 52000 neutrons per day are produced by ν e + p → e+ + n
in the liquid scintillator and in the acrylic vessel.
Neutrons can then be captured either by hydrogen, giving a 2.2 MeV gamma ray followed by a
possible Compton inside the TPC, or by a fluorine
nucleus of CF 4 , giving an electron from the beta
decay of 20 F. Such events give rise to less than
0.3 cpd with an isotropic angular distribution inside the TPC [1].
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C. Amsler et al. / Physics Letters B 545 (2002) 57–61
Fig. 2. Spectrum of the forward minus backward electrons above
700 keV as compared to the one predicted by the weak interaction.
Moreover, the reactor associated neutron flux in
the Bugey laboratory has been evaluated in a previous
experiment and found to be negligible [6].
We can now ascribe the event excess with reactor
on to ν e e− elastic scattering and compare the measured signal, 1.8 ± 0.7 cpd, to the one expected from
weak interaction: 0.71 ± 0.05 cpd. The Monte Carlo
error comes from uncertainties in the antineutrino
flux and spectrum (5%), track reconstruction (3%),
electron containment (3%) and target electron density (1%).
In Fig. 2 we compare also the measured spectrum
to the predicted one. We conclude that our result is
∼1.5 standard deviations higher than predicted but
compatible with weak interactions. There is so far no
evidence for a finite neutrino magnetic moment.
We observe that our result is in agreement with
the one of Reines and collaborators [7], which, with
a 1.5 MeV threshold on the electron kinetic energy, is
still the best documented one. Actually the analysis of
Vogel and Engel [8] shows that they also got a result
1–2σ above the weak prediction.
3.2. Kinematic analysis
From the electron energy and scattering angle
we reconstruct the antineutrino energy. The electron
energy threshold corresponds to a minimum cos(θ )
cut of 0.64. For positive values the event enters the
kinematic forward spectrum, else we swap the sign
Fig. 3. Spectrum of the kinematic forward minus backward electrons
as compared to the ones predicted by the weak interaction (squares)
and by the weak plus electromagnetic interactions (triangles)
assuming a magnetic moment of 2 × 10−10 Bohr magnetons.
of cos(θ ) and for positive antineutrino energy the
event enters the kinematic backward spectrum. The
difference between the forward and the backward
distributions is given in Fig. 3.
The integrals of the forward and backward spectrum are 44.7 ± 0.8 cpd and 41.9 ± 0.8 cpd, respectively. Their difference, 2.8 ± 1.1 cpd, gives the number of reactor events. Also in this analysis we find that
this difference with reactor off is consistent with zero:
−0.9 ± 2.1 cpd.
We now compare the measured signal, 2.8±1.1 cpd,
with the one expected from the weak interaction:
1.19 ± 0.08 cpd. Our result is ∼1.5σ higher than the
expected weak one. Fig. 3 shows the measured and
predicted spectra.
A χ 2 analysis is also performed which is sensitive
to the measured spectrum, as compared to predicted
one. This way we are sensitive both to the integral and
2 of 9.6
to the shape of the spectrum. We find a χmin
over 10 degrees of freedom corresponding to an upper
limit to the antineutrino magnetic moment of 2.3 ×
10−10 Bohr magnetons (90% C.L., Fig. 3). Such a
value is the consequence of the somewhat larger cross
section we measure for elastic ν e e− scattering.
Our limit is larger than the one obtained with a
reactor on-off analysis in the Rovno experiment, 1.9 ×
10−10 Bohr magnetons [9]. However, our signal to
noise ratio is one order of magnitude better.
C. Amsler et al. / Physics Letters B 545 (2002) 57–61
The best existing limit is the astrophysical one of
0.02 × 10−10 Bohr magnetons, which is coming from
the red giant luminosity [10] and which is not model
independent. Equally model dependent, this time on
the oscillation scenario used, is the limit of 1.5 ×
10−10 Bohr magnetons obtained from the shape of the
solar neutrino spectrum in Superkamiokande [11].
61
track reconstruction program, which exploits the experience gained from further detailed hand scanning
of the events, is now under development. Better acceptance, higher background rejection efficiency and
thus smaller errors are expected.
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4. Summary and conclusions
Thanks to the tracking capability of our detector we
were able to measure both the angle and the energy of
the recoil electron and to extract the reactor signal by
subtracting the background measured at the same time
as the signal.
For antineutrino energies higher than 900 keV (corresponding to electron energies above 700 keV) our
result is compatible with the predicted weak rate, even
if 1.5σ higher. An improved background rejection and
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