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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 This claim, however, was later withdrawn.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