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Neutrino speed: a report on the  speed measurements of the BOREXINO,
ICARUS and LVD experiments with the CNGS beam
Sergio Bertoluccia
on behalf of the
BOREXINO, ICARUS and LVD Collaborations and the CERN CNGS team
a
CERN, Geneva, Switzerland
Abstract
We report the measurement of the speed of muon neutrinos performed by the Borexino, ICARUS and LVD
experiments, using narrow bunch CNGS neutrino beams with an average energy E = 17 GeV and a baseline of ~
730 km between Cern and Laboratori Nazionali del Gran Sasso (LNGS). The final result for the difference in
time-of-flight between such muon neutrinos and a particle moving at the speed of light in vacuum is δt = 0.8 ±
0.7stat ± 2.9sys ns for Borexino, δt = – 0.1 ± 0.7stat ± 2.7sys ns for ICARUS and δt = -0.3 ± 0.6stat ± 3.2sys ns for LVD,
well consistent with zero.
Keywords: neutrino, speed, CNGS
1. Introduction
The report describes a precise measurement of the
speed of CNGS muon neutrinos made with the
Borexino, ICARUS and LVD detectors at the
Laboratori Nazionali del Gran Sasso (LNGS) in Italy.
CNGS neutrinos travel about 730 km in matter,
with one of the highest relativistic γ factors ever
artificially produced. The neutrino mass is at most ≈2
eV/c2 or possibly much less, while the CNGS average
beam energy is 17 GeV, so γ is always >1011, much
bigger than that obtained in any charged particle beam.
A test of Special Relativity with these particles is
therefore meaningful. Besides, the measurement may
also put an upper limit on the effect of non-standard
propagation of neutrinos in matter.
This effort was also motivated by the claim, made
by the OPERA Collaboration in Sep. 2011 [1], that
CNGS neutrinos travel faster than the speed of light in
vacuum. This claim, however, was later withdrawn.
At the end of the 2011, CERN and the LNGS
experiments (Borexino, ICARUS, LVD, OPERA)
agreed to perform a new dedicated campaign, using a
special configuration of the CNGS beam [2] optimized
for the neutrino speed measurement.
The main parameters of this beam are: narrow
bunches (σ ≈ 2 ns), 16 bunches per batch with a bunch
separation of ~100 ns, 4 batches per extraction
separated by ~300 ns; the bunch intensity is ~1011
protons; one extraction per CNGS cycle, with a cycle
length of 13.2 s.
In this mode of operation, one can unambiguously
connect every neutrino event at LNGS with the
originating narrow proton bunch, allowing a precise
determination of the speed with a relatively small
number of neutrino events. This new measurement was
performed from May 10th to May 24th 2012.
The neutrino time of flight measurement consists
in recording any neutrino induced interaction time at
the LNGS experiments and relating it to the transit
time of a proton bunch at a BCT monitor along the
CNGS TT40 transfer tunnel at CERN, occurring ~2.44
ms earlier.
Precise geodetic estimation of the CERN-LNGS
distance for all the experiments and a precise
measurement of the time-of flight allow calculating the
actual neutrino velocity.
A sophisticated synchronization system connects
the CERN and the LNGS experiment timing systems,
providing also the online monitoring of the stability of
the timing systems.
In order to cross check possible systematic
effects, it was decided to perform a new determination
of the distance between Cern and LNGS and to realize
a new time synchronization system, certified by
different metrology Institutes.
A very thorough analysis, specific for each
experiment at LNGS and for the beam delivery system
at CERN, needs to be performed in order to determine
all the timing offsets arising from signal propagation,
signal formation, trigger and DAQ offsets, etc., as well
as to determine the main contributions to the
systematic error.
In the following, we will outline the main
common elements of the measurements, referring the
reader to the papers [3][4][5] published in the
meantime by the experiments, where an exhaustive
treatment of the specific details can be found.
2. Geodesy
The distance between the experiments reference
points at LNGS and the target at CERN has been
computed based on a geodetic survey at Gran Sasso
Laboratories and the existing target co-ordinates at
CERN.
The geodetic campaign at LNGS was carried out in
May 2012 and was performed in two steps. In the first
step two GPS networks, one local and one regional,
were established. The regional network used in this
work consists of 32 GPS permanent stations located
between CERN and Gran Sasso (including two antennas at CERN and one at LNGS) whose position in the
IGS08 reference frame has been precisely estimated by
adjusting (fitting) two weeks of GPS data.
A local 9 points GPS network has been established
in the Gran Sasso area and framed to the regional one
through 3 common stations using a four days-24 hours
campaign.
In the second step, based on the previously
estimated GPS points, a high precision traverse has
been measured also by means of gyro-theodolites
along the Gran Sasso tunnel highway (10.5 km).
This allowed the co-ordinates estimation of the
Borexino. ICARUS and LVD reference points inside
the LNGS.
The coordinates of the target point at CERN have
been supplied by the CERN geodetic team in the
ITRF97 reference system.
After datum shift to IGS08, assuming target point
coordinates precision of 0.030 m, the geometric
distance between the reference point of the experiment
(Borexino in this case) at LNGS and the target at
CERN has been estimated in 730472.082 ± 0.038 m.
Similar precisions have been achieved for the other
two experiments.
3. Synchronization between LNGS and CERN
A detailed description of the CERN and LNGS
timing systems and their synchronizations, prepared
for the 2011 campaign and used also in 2012, is given
in ref. [6]. A schematic picture of the timing system
layout, including all delays, is shown in the top part of
figure 1. The origin of the neutrino velocity
measurement is referred to the Beam Current Transformer (BCT) detector, located 743.391±0.002 m
upstream of the CNGS neutrino target.
The proton beam time structure at the BCT is
recorded by a 1 GS/s Wave Form Digitizer (WFD),
triggered by the SPS kicker magnet signal. At every
extraction, the BCT waveform is stored into the CNGS
database. Every acquisition is time-tagged with respect
to the SPS timing system, associating each neutrino
event at LNGS to a precise proton bunch.
The absolute UTC timing signal at LNGS is
provided every second (PPS) by a GPS system ESAT
2000 disciplining a Rubidium oscillator, operating on
the surface Laboratory. A copy of this signal is sent
underground every ms (PPmS) and used in the
experiments to provide the absolute time-stamp to the
recorded events.
For the 2012 CNGS bunched beam run, additional
CERN-LNGS synchronization systems have been setup. The new overall layout at CERN and LNGS is
depicted in figure 1 top and bottom, respectively.
Under the CERN responsibility, with the aim of
providing redundancy and intercalibration, two new
additional Septentrio PolarRx4 GPS receivers,
optimized for time-transfer applications, have been
installed at CERN and LNGS. As the previously
installed PolarRx2e receivers, they operate in
"common-view" mode and are connected to the same
Cs atomic clocks Symmetricom Cs4000 used to
provide the reference frequency to the PolaRx2e
receivers. The inter-calibration of the new PolarRx4
receivers is estimated to be stable within 2.0 ns.
As shown in figure 1, both the new PolarRx4 and
the old PolarRx2e synchronization paths are connected
to the DAQ systems at CERN and LNGS, through the
"Classic" 2011 protocol. In addition to it, a new
independent system for timing distribution was
deployed both at CERN and at LNGS. It is based on a
recent, still under development, open source protocol,
called "White Rabbit" (WR) [7], whose main purpose
is to constantly and accurately monitor the propagation
delay of any signal along the optical path, connecting
all nodes (PC's provided with WR hardware interface
and running the WR protocol firmware) of the WR
system. The WR system is expected to intrinsically
correct for any change of the propagation chain delay,
thus avoiding the need of periodic calibrations of the
optical fibres described in the previous section. Any
node participating in the WR system is thus phaselocked with all the others, with accuracy and stability
much better than 1 ns.
The WR protocol allows the distribution of timing
signals at various frequencies as well as the time
stamping of any pulse generated by the DAQ systems
connected to WR Nodes.
In addition to the above set-up, the Borexino
collaboration implemented at LNGS a new timing
system (High Precision Timing Facility, HPTF[8])
based on an independent additional PolarRx4 GPS
receiver. This installation, located at the LNGS outside
laboratory, provides the time stamping of signals
propagated from the experiments along the optical
fibre bundle connecting the underground and the
external Labs. For the synchronization with CERN, the
HPTF relies on the PolarRx2 GPS receiver installed at
CERN.
The related inter-calibration were performed by
INRiM and ROA Institutes. Thanks to a common
agreement the system was made available also to
ICARUS and LVD.
4.1. Borexino
4. Summary of the measurements
Fig. 1. Schematics of the 2012 CERN-LNGS time synchronization,
including the 2011 classical path (green), the WR system taking
advantage of both the new PolarRx4 (red) and old PolarRx2e (blue),
and the HPTF Borexino facility (violet). Top: layout at CERN.
Bottom: layout at LNGS.
In the following, we will summarize the results
obtained by the experiments.
The Borexino detector is a high-purity liquid
scintillator calorimeter (within a Stainless Steel Sphere
of ≈1300 m3, SSS) shielded by a large Water Tank
(WT, ≈3500 m3) that serves also as a muon detector. It
is installed in Hall C of the LNGS at a depth of 3800
m.w.e.
CNGS muon neutrinos are detected via charged
current interactions that mostly occur in the rock
upstream the detector. Internal events exist, but they
cannot be easily disentangled from the crossing
muons. Their number, however, is small (~ 5%).
The kinematics of the muons does not affect the
precision of the measurement. The difference in the
time-of-flight of a ≈ 10 GeV muon with respect to a
neutrino traveling a length of about 50 m (the
approximate average distance traveled by such a muon
in the rock plus the distance of the Borexino detector
from the North wall of the Hall C) is less than 0.1 ns
and can therefore be neglected.
The same argument applies to the pions generated
in the CNGS target at CERN.
Muons can be detected by Borexino, using the
Water Tank (Cherenkov light) or the scintillator detector. In this analysis only events detected by the latter
have been used, due to their higher precision.
Although only the core 270 t of PC–PPO
scintillator is normally used for solar and geo-neutrino
physics, the sensitive mass for CNGS muon neutrinos
is made by the whole ≈1300 m3 of liquid scintillator
and buffer liquid because, though quenched by small
amount of DMP (3 g/L), the buffer liquid light yield is
sufficient to detect a muon (~2 MeV/cm, equivalent in
the buffer to approximately ~50 p.e./cm).
The cross-section of the WT is 266 m2 while that of
the SSS is 147 m2. These large areas yield a large
number of events: in May 2012 a total of 291 events
have been collected, 144 crossing the SSS and 147
crossing the WT only.
This large statistics allows to apply stringent quality
cuts to select the best data sample for the
measurement.
Borexino has developed muon reconstruction software capable of determining the location of the
entrance point of a muon in the SSS with a precision
of about 50 cm.
The algorithms make use of the information
provided by both the Cherenkov detector, through the
identification of the disk-like activation profile of the
WT PMTs, and by the scintillator, through a fit of the
arrival time distribution of the photons to the SSS
PMTs as a function of the track location.
The availability of this reconstruction makes it
possible to correct for the spherical shape of the
detector, which implies a different time-of-flight for
different entrance points.
Fig. 2. Final distribution of the difference between the neutrino time-of-flight and
that expected for a particle moving at speed c (data points). The mean value is
consistent with zero and the width agrees with Monte Carlo simulation of known
time jitters (gray/yellow filled histogram).
This correction reduces the size of the data sample
because some events cannot be properly reconstructed,
but narrows the time distribution significantly,
improving the quality of the measurement.
After this final reduction, and after having taken into
account all the time offsets, the remaining data set
consists of 62 CNGS events, more than enough for a
precision measurement, as shown in figure 2.
The final result obtained in May 2012 for the
time-of-flight difference of muon neutrinos of average
energy E = 17 GeV with respect to the speed of light is
δt = 0.8 ± 0.7stat ± 2.9sys ns.
This result implies |v −c|/c < 2.1 x 10−6 , 90% C.L.
The systematic error is mainly due to the details of
the CERN beam delivery system and to the GPS clock
synchronization.
4.2. ICARUS
The ICARUS T600 detector consists of a large
cryostat split into two identical, adjacent modules with
internal dimensions 3.6x3.9x19.6 m3 filled with about
760 tons of ultra-pure liquid Argon. Each module
houses two TPCs separated by a common cathode. A
uniform electric field (Edrift = 500 V/cm) is applied.
There are three parallel planes of wires, 3 mm apart
with lengths up to 9 m, facing the drift volume 1.5 m
long. By appropriate voltage biasing, the first two
planes provide signals in a non-destructive way
allowing to collect the ionization charge on the third
plane. Wires are oriented on each plane at different
angles (0, ±600) with respect to the horizontal
direction. Combining the wire coordinate on each
plane at a given drift time, a three-dimensional image
of the ionizing event is reconstructed. A remarkable
resolution of ~ 1 mm3 is uniformly achieved over the
whole detector active volume (~340 m3 corresponding
to 476 t).
Scintillation light in LAr is abundantly produced by
ionizing events (~2.5x104 photons/MeV at 128 nm
wave length); it exhibits two distinct decay
components: the fast component has a decay time of 6
ns and accounts for ~25% of the total light emission.
In the ICARUS LAr-TPC the scintillation is recorded
with 74 photomultipliers of 8 inch diameter, organized
in horizontal arrays of 9 PMTs each, located behind
the wire chambers.
The PMT spacing in each array is 2 m. All PMTs
are deposited with wavelength shifter (tetra-phenylbutadiene, TPB) able to convert with high efficiency
(close to 100 %) the 128 nm VUV scintillation light to
420 nm, matching the PMT photo-cathode spectral
response. The overall estimated quantum efficiency is
about 4 %.
The sums of the signals from the PMT arrays are
used for the ICARUS global trigger and to locate the
event within the drift volume ("T=0"). The trigger
threshold, set at about 100 photoelectrons, allows full
detection efficiency for events with energy deposition
as low as few hundreds MeV. CNGS neutrinos are
recorded requiring a coincidence between a 60 s gate,
opened according to the “Early Warning Signal" for
proton extraction from CERN-SPS, and the PMT-Sum
signals. Given the geometry of the ICARUS LAr-TPC
and the PMT spacing, in the case of multi-GeV CNGS
neutrino induced events, several hundred photoelectrons are produced in the PMT's closer to the ionizing
event, within 1 ns from the ionization process. This
feature makes the ICARUS LAr-TPC very well suited
for timing measurement. In addition, the possibility to
visually scan the associated 3D event image permits to
measure the path of the photons from the interaction
vertex to the PMT location with mm accuracy (i.e. sub
ns accuracy on photon propagation time).
In order to measure the neutrino arrival time in the
ICARUS detector, the propagation time of the scintillation light signals from the PMTs to the DAQ boards,
including the transit time within the PMTs, the overall
cabling (~ 44 m) and the delay through the signal
adders, have to be calibrated.
The propagation along the cabling has been measured
with an accuracy of ~ 0.5 ns by means of standard
reflection techniques on sharp signals (few ns risetime)
with a 1 GHz bandwidth oscilloscope. All 74 PMT
cables resulted of the same length, equivalent to 233
ns, within the precision of the measurement.
The measurement of the PMT transit time was
carried out in laboratory tests at room temperature on a
spare PMT, covered with TPB wavelength shifter.
Being the PMT transit time a purely geometrical
and electrostatic effect, the laboratory measurement
allowed determining the transit time of all the PMT
installed in the T600, given the applied biasing
voltage. The residual associated error of 1 ns is related
to the oscilloscope sampling frequency of 1 GHz.
During the 2012 two weeks of data taking with the
CNGS in bunched mode, the ICARUS T600 detector
collected 25 beam-associated events, consistent with
the CNGS delivered neutrino flux.
The events consist of 8 neutrino interactions (six
CC and two NC) with vertex contained within the
ICARUS fiducial volume and 17 additional throughgoing muons (one of which stops within the LAr
active volume) generated by CNGS neutrino interactions in the upstream rock. Events in the standard
ICARUS DAQ and the PMT-DAQ have been associated through their common absolute time-stamp.
For all the collected events, the visual scanning
and 3D reconstruction are exploited to determine the
distance of the interaction vertex from the ICARUS
"reference entry point" and to quantify the shortest
path of the scintillation light to the nearest PMT. In
LAr the 128 nm scintillation photons propagate at 4.0
ns/m (refraction index = 1.20) with a Rayleigh
scattering length as large as 80 cm hence comparable
with the typical distance from ionization track to the
PMTs..
Then the actual neutrino time of flight is measured
on an event-by-event basis: (1) the transit time of the
proton beam bunch at the BCT in the CERN time-base
is used as starting time; (2) the neutrino arrival time is
taken at the ICARUS "reference entry point" in the
LNGS time base; (3) the alignment of the LNGS and
CERN time bases is calculated following the different
synchronization paths available for the 2012 run. The
first term is determined picking up in the BCT waveforms the proton bunch giving the neutrino time of
flight value closest to expectation for v = c.
The systematic errors of each branch of the
measurement have been used to perform the averages
along the different timing paths. Cross-correlations,
essentially due to the common use of the same GPS
signals by the various receivers and the use of the
same PolarRx2 receiver at CERN by HPTF and the
Classical/WR path at LNGS, have also been
considered. The final timing is obtained as weighted
average of the HPTF timing with that obtained with
the alternative paths. The overall systematic error
derived within the averaging procedure is ~ 2.39 ns.
The expected event distribution width is ~ 3.0 ns,
dominated by the detector response (mainly related to
PMTs), the width of the proton beam bunches and
residual synchronization jitter.
Fig. 3. Event distribution in ICARUS T600 for t = tofc - tof,
according to the averaging procedure of all synchronization paths
described in the text
The difference between neutrino time of flight
from the BCT to the ICARUS "reference entry point"
and the expected time of flight based on the speed of
light (tofc = 2439096.1 ns) is shown in figure 3.
The resulting value t = 0.10 ± 0.67stat ± 2.39sys ns,
is fully compatible with the neutrino propagation at the
speed of light.
This measurement excludes neutrino velocities exceeding the speed of light by more than 1.35 x10-6 c at
90% C.L.
4.3. LVD
The Large Volume Detector (LVD), in the INFN
Gran Sasso National Laboratory (LNGS), is a 1 kt
liquid scintillator detector whose major purpose is
monitoring the Galaxy to study neutrino bursts from
gravitational stellar collapses. It has started operation
in 1992, and since 2006 it has been acting as a remote
monitor of the CNGS beam.
LVD is sensitive to neutrino interactions with
protons and carbon nuclei in the liquid scintillator and
with the iron of the detector structure. Muons,
produced by charged current interactions of muon
neutrinos in the rock, can also be detected and are
responsible for the bulk of CNGS events in LVD.
LVD consists of 840 scintillator counters, 1.5 m3
each. The array is divided in three identical ‘‘towers’’
with independent high voltage power supply, trigger,
data acquisition and absolute clock (ESAT Slave
RAD100) connected, through a 8 km optical link, to
the Master clock (ESAT RAD100) located in the
external buildings of the LNGS. Each tower consists
of 35 ‘‘modules’’ hosting a cluster of 8 counters. Each
counter is viewed from the top by three 15 cm
photomultiplier tubes (PMTs).
In view of the new bunched beam measurement
campaign, a subset of the LVD counters has been
modified, in order to improve their timing performances.
The CNGS neutrino data collected since 2006
allowed to choose 58 counters (the Super-Set),
constituting the back face of the first tower the CNGS
beam, which are involved in ~ 40% of the CNGS
events detected by LVD, while representing only less
than 7% of the whole array.
To avoid time fluctuations in the trigger formation
at the single-counter level, we have delayed the signal
of one PMT for each counter, therefore granting that
the threefold coincidence among the PMTs in every
counter is always driven by the same tube.
The transit time, LVD, (i.e. the time between the
light generation inside a counter and the formation of
the trigger signal), was measured by means of a LED
based system.
Trigger signals are sent to the HPTF timing system, which time-stamps them with the high precision
(50ps) Time Interval Counters (TIC) Pendulum CNT91.
This provides a high precision time difference,
TIC, between the LVD trigger and the absolute GPS
time. Thanks to this improvement, the absolute time
accuracy of LVD is of the order of few nanoseconds.
Finally, the CNGS-LVD baseline, namely, the
distance between the center of the Beam Current
Transformer (BCT) detector at CERN and the LVD
Super-Set upstream entry wall, is found to be
731291.87 ± 0.04 m .
The corresponding time-of-flight at speed of light,
when including the 2.2 ns contribution due to Earth
rotation, is tofc = 2439329.32 ± 0.13 ns.
During the May 2012 bunched beam period, 190
events have been recorded in all LVD, consistent with
the 1.89 x 1017 protons on target (p.o.t.) delivered.
For the measurement of the neutrino velocity,
only events involving at least one of the Super-Set
counters have been selected. This reduces the sample
to 79 events, consistent with the expectations.
Fig. 4. Distribution of t, the difference between the time-of-flight
of neutrino and the time-of-flight at speed of light, for the 48
selected events (black histogram), compared with the superposition
of the peaks of the waveforms correlated to detected events (gray
histogram).
In order to improve time resolution, further quality cuts are applied on the muon deposited energy,
selecting events with a deposited energy E > 50 MeV.
The distribution of t for the survived 48 events is
shown in figure 4 (solid histogram) compared with the
superposition of all the peaks of the waveforms
correlated to detected events (gray curve).
The positive tail in the gray histogram is an
artifact of the transfer function of the BCT system.
This effect does not influence our measurement since
time calibrations are performed with respect to the
position of the maximum.
The mean value of the measured distribution is
t = 0.3 ± 0.6stat ± 3.2sys ns , compatible with zero.
The corresponding limit on the speed of neutrino, at
99% C.L., is: – 3.3 x10 –6 < (v– vc) < 3.5 x 10 – 6
5. Conclusions
The measurements of the speed of muon neutrinos
performed by the Borexino, ICARUS and LVD
experiments at LNGS, using a bunched CNGS
neutrino beam with an average energy E = 17 GeV
over a baseline of ~730, have confirmed that the
difference in time-of-flight between such muon
neutrinos and a particle moving at the speed of light in
vacuum is δt = 0.8 ± 0.7stat ± 2.9sys ns for Borexino,
δt = – 0.1 ± 0.7stat ± 2.7sys ns for ICARUS and
δt = – 0.3 ± 0.6stat ± 3.2sys ns for LVD, well consistent
with zero.
These measurements exclude neutrino velocities
exceeding the speed of light by more than 1.0 x10-6 c
at 90% C.L.
Acknowledgments
I would like to acknowledge the fundamental support
of W. Fulgione, M.Pallavicini, F. Pietropaolo, G.C.
Trinchero in the preparation of the talk given at the
Neutrino 2012 Conference, which was based on a data
taken only a few days before.
The CNGS team at Cern and in particular E.
Gschwendtner should be commended for their brilliant
preparation and operation of the bunched neutrino
beam.
P. Alvarez Sanchez and J. Serrano have played a
decisive role in the establishment of the timing system,
which has been key for the success of the
measurement.
References
[1] OPERA collaboration, T. Adam et al., Measurement of the
neutrino velocity with the OPERA detector in the CNGS beam,
JHEP 10 (2012) 093 [arXiv:1109.4897].
[2] General Description of the CNGS Project, http://projcngs.web.cern.ch/projcngs/;
[3] P. Alvarez Sanchez et al., Borexino Collaboration, Physics
Letters B 716(2012) 401–405
[4] M. Antonello, et al., ICARUS Collaboration, JHEP11 (2012)
49.
[5] N.Yu. Agafonova, et al., LVD Collaboration, PRL 109(2012)
[6] P. Alvarez and J. Serrano, Time transfer techniques between
CERN and LNGS , CERN/BE-CO-HT Internal Note (2011),
http://www.ohwr.org/projects/cngs-time-transfer/documents .
[7] M. Lipinski et al., Performance results of the first White Rabbit
installation for CNGS time transfer , private communication, to
be submitted to IEEE Transactions (2012).
[8] B. Caccianiga et al., GPS-based CERN-LNGS time link for
Borexino , 2012 JINST 7 P08028