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Poster excerpt 5: Muon and Neutrino Energy Reconstruction from Km3NeT

March 2, 2015

(The following text has been supplied by poster contributor Evangelia Drakopoulou)

Why should you look down to the deep sea in order to unveil the mysteries of the Universe? How can the elusive and invisible neutrinos be unique cosmic messengers for the study of galactic and extragalactic astrophysical sources? How can the neutrinos be detected? Why is it important to estimate the neutrino energy and how efficiently can you perform such an estimation?

The KM3NeT collaboration aims to answer such questions by deploying a neutrino telescope of volume of several  in the bottom  of the Mediterranean Sea. The neutral and weakly interacting neutrinos are unique cosmic messengers as they are not deflected by magnetic fields or absorbed by interstellar matter. Therefore, neutrinos can travel from the most distant places of the Universe to the earth, pointing back to the astrophysical sources and revealing the mechanisms that produced them, unveiling the nature of these sources.

Neutrinos need to travel through large volumes in order to interact with matter, so neutrino telescopes take advantage of the earth volume and search for neutrinos coming via the earth to the telescope. The telescope consists of photomultipliers that are sensitive to the light produced by charged particles, induced during the neutrino passage through sea water. These particles emit light within the characteristic Cherenkov cone which allows the neutrino direction reconstruction.

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Apart from neutrinos from astrophysical sources, there are neutrinos and muons originating from cosmic rays that interact in the atmosphere and reach the detector from above the horizon constituting the detector background. The energy estimation provides a tool to differentiate between astrophysical and atmospheric neutrinos, since astrophysical neutrinos spectrum extends to higher energies.

The study that is presented in this poster refers to neutrinos interacting to muons via Charged Current interactions. The light that is derived from the muon passage through sea water is collected by the photomultipliers allowing the muon track direction and energy estimation. A Multi-Layer Percepton Neural Network with appropriate variables is employed in order to reconstruct the muon energy and consequently, the neutrino energy. The results of the energy reconstruction are summarized in the following plot of the median of with respect to the MC Muon Energy with 68% and 90% quantiles.

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The muon energy can be reliably reconstructed and can be used as a measure of the neutrino energy. Muon and neutrino energy reconstruction can give rise to better understanding the galactic and extragalactic astrophysical sources of our Universe.     So, during this workshop I will present the method of the muon energy reconstruction and give more details about the results achieved so far.

Conference Hall

March 2, 2015

Here is the “Sala dei portici” at Palazzo Franchetti, where the conference will take place.It is the ground floor of a beautiful, historical palace on the Grand Canal. Carlo Rubbia (not shown in the pic) has just arrived and will talk first – he already took place in the front row, jokingly claiming he deserves it as he is life senator of the republic. I think we all agree he deserves it for his enormous contributions to XXth – and also XXIst – century physics, rather than for being the recipient of that award !

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BTW I just learned that Rubbia has over 50 slides for his 30′ talk – it will put my stenographic capabilities to the test!

Neutel Day 1: the program

March 2, 2015

The conference is about to start, and attendees are registering at the front desk. This afternoon session will start with big names giving opening lectures, as the program features the following:

– Muon Cooling at CERN ? by Carlo Rubbia

– The Successful Story of Neutrino Telescopes, by Francis Halzen

– Neutrinos: Projecting onto the Future, by Alexei Smirnov

– Cosmology and Particle Physics, by Licia Verde

– 3-neutrino Oscillations, Past Present and Future, by Gianluigi Fogli

I will try to report on each of these interesting topics today, so stay tuned! I also take the opportunity to encourage conference attendees to contribute to the discussion, either by commenting on posts with their impressions or notes, or by submitting material to be put online in the blog web site!

Poster Excerpt 4: Appearance of Rare Physics Phenomena in the Opera Experiment

March 2, 2015

(The excerpt below is from the poster of Marco Roda, from the Opera collaboration)

The tau neutrino is very difficult to be detected. Since it is a neutrino, it requires huge and homogeneous  detectors in order to maximize the luminosity of your experiment. Then, since its interaction produces a tau particle, you also need a tracker with a spatial resolution at the micrometer level. Once you have obtained that, you can reject your background in an extremely efficient way. So efficient that, in 2000, just 4 events were enough to claim the discovery of the tau neutrino particle.

The OPERA experiment has been designed to detect the presence of tau neutrinos in the CNGS neutrino beam, 700 km far away from the production site, the CERN.

Recently a new interesting event has been discovered in OPERA with a very peculiar topology: instead of one quickly decaying particle, it has two!

The figure shows the structure of the recorded events. There is a longer decaying track (4) which decays after 1 mm in only one charged track (6) and in photons which create an electromagnetic shower after 2 mm. The second decaying particle has a very short path before it goes into 2 tracks (1 and 3).

This kind of event was supposed to be so rare that it was not taken into account in the experiment proposal. That’s why a new dedicated analysis has to be prepared. The possible interpretations are all intriguing and on top of them there is a never observed process: tau neutrino charged current  interaction with charm production.

At present the analysis is still blind, since we want to apply the analysis only when a complete strategy will be defined. What is already clear is that the interesting signals can be efficiently separated from the possible backgrounds.

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Poster Excerpt 3: Searches for Short-Baseline Neutrino Oscillations with the T2K off-axis Near Detector

March 2, 2015

(The article below is from Stefania Bordoni, on behalf of the T2K collaboration)

Neutrinos are light and neutral particles interacting via the weak force. The standard model predicts three types (flavors) of neutrinos, each one corresponding to one of the 3 lepton families: electron neutrino (νe), muon neutrino (νµ) and tau neutrino (ντ). A peculiarity of neutrinos  is that they may change their flavor during propagation: given a neutrino of a certain flavor (e.g. νµ), there is a non-vanishing probability to detect that neutrino as another flavor (e.g νe or ντ ). This is the so-called “neutrino oscillation phenomenon”, a nice name hiding a quantum interference process.

To investigate such phenomenon, the parameters of interest are the mixing angles (sin22ϑ), and the mass squared differences (Δm2) giving information respectively about the amplitude and the frequency of the oscillation.

Figure 1 (below): Neutrino oscillation probability in the approximation of two neutrino flavors

bordoni1Neutrino oscillations seem to be very well described by the Pontecorvo-Maki-Nakagawa-Sakata matrix, however in the last 15 years the completeness of the three-neutrino paradigm has been challenged. Deficits or excesses in the expected number of events have been reported by a number of experiments studying the oscillations at a short distance (short-baseline) from the neutrino production point [1][2]. Those anomalies may be explained by introducing one or more neutrinos with some different properties with respect to the already well known standard (νe, νµ and ντ) neutrinos. These additional particles are expected to not interact via the weak force and for that reason they are usually called “sterile neutrinos”. This label clearly distinguishes them from the other three, usually labeled as “active neutrinos”.  Another characteristic of sterile neutrinos is their expected large mass (on the order of 1eV/c2). Such large mass would suggest an oscillation at short-baseline (SBL), as can be deduced from the expression for the  appearance or disappearance probability.

What makes the neutrino oscillations field even more challenging, is the fact that the anomalies  observed so far only concern electron (anti-)neutrinos. No hints of short-baseline oscillations have been observed for muon neutrinos. Several experiments [3-5] searching for muon neutrino short-baseline oscillations have started to set exclusion limits on these processes.

T2K (Tokai-to-Kamioka) is a long-baseline oscillation experiment located in Japan, studying neutrino oscillations starting from a beam of muon neutrinos [6].

Neutrinos are produced from the decay of pions and kaons generated by primary collisions of high energy protons with a graphite target. For that reason, the amount of data collected by an experiment is usually expressed in units of  “proton on target” or POT.

Apart from the well known Super-Kamiokande “far” detector located at about 300 km away from the point where neutrinos are produced, the experiment has also “near” detectors. ND280 is one of those:  it is located at 280m from the neutrino production point and has an angle of 2.5 degrees with respect to the direction of the neutrino beam (off-axis detector).  Thanks to its position this detector offers good conditions to search for short-baseline neutrino oscillations. The T2K collaboration has then the possibility to add new measurements and/or new exclusions limits.

bordoni2Towards this goal, the collaboration has indeed recently published the measurement of νe disappearance at ND280 [7].  A new analysis,  searching for muon neutrino disappearance is now also on-going. The poster presented at the conference summarizes these two analyses.

 Right: The T2K experiment

The searches for short-baseline neutrino oscillations for both νe and νµ analyses are performed considering the minimal extension of the standard three neutrino paradigm, the so-called 3+1 model.  In such model a sterile neutrino (νs) with a mass of order 1eV/c2 is added to the three active neutrinos (νe, νµ and νt).

Both the electron and muon neutrino event selections are based on the signals from the ND280 tracker:  three Time Projections Chambers (TPCs) allowing the reconstruction of the track of charged particles and two Fine Grain Detectors offering active targets for the neutrino interactions. For the electron neutrino selection also the information coming from the Electromagnetic Calorimeter (ECAL) surrounding the tracker is used.  The TPCs and the ECAL offer very good performance on particles identification. Thanks to these detectors it is then possible to disentangle electrons from muons, which are key elements of the signature of electron and muon neutrino events.

SBL νe disappearance :

The search for νe disappearance has been performed using the data collected between January 2010 and May 2013. This amount of data correspond to 5.9×1020 protons on target (POT). This is the very first results of T2K on these searches for short-baseline neutrino oscillations.

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Figure 2:  ND280 exclusion region at 95% CL for SBL electron neutrino disappearance compared to other experiments results [7].

Figure 2 shows the limits (95%CL) set by this analysis (green-shaded area).  One can note that the T2K results is covering part of the regions delimited by the other experiment: this means that with this analysis some potential values of the interesting parameters sin22ϑ and Δm2 are now excluded. More precisely the T2K results exclude parts of the allowed regions coming from the gallium anomaly [1] and the reactor anomaly [2].

SBL νµ disappearance :

A new analysis focusing on the search for νµ  disappearance is under development in the T2K collaboration.  Such measurement aim to verify and extend the current limits[3][4][5].

For this analysis only Monte Carlo-based studies are developed so far. The Monte Carlo simulation used in the study is normalized to the current T2K collected statistics, 6×1020 POT.

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Figure 3: Expected ND280 Monte Carlo sensitivity at 90%CL to SBL muon neutrino oscillations compared to other experiment limits [3-5].

As shown in Figure 3, the expected sensitivity (90%CL) to the muon neutrino disappearance obtained by this study looks promising if compared to the results from the previous experiments. As previously mentioned, no  hints of short-baseline muon neutrino disappearance has been reported so far. The regions delimited by the black line and gray area show the regions for which the values of the parameters sin22ϑ and Δm2 are already excluded. One can note than that in the high Δm2 region in fact, T2K may be able to extend the existing exclusion limits.

To conclude, the collection of further data will reduce the statistical uncertainty, which is an important limitation especially for the neutrino disappearance analysis.

It has to be noticed that deep correlations exist in the 3+1 model between the oscillation parameters of the different channels (νe disappearance, νµ disappearance and νe appearance). A joint analysis, aiming to fit simultaneously electron and muon neutrino signals, is foreseen for the near future and it promises to be interesting.

References :

[1] Kaether et al., “Reanalysis of the Gallex solar neutrino flux and source experiments”, Phys. Lett. B 685 Issue 1 (2010)

[2] Muller et al., “Improved predictions of Reactor Antineutrino Spectra“, Phy.Rev.C 83 054615 (2011)

[3] Mahn et al.,  “Dual baseline search for muon neutrino disappearance at 0.5 eV2 < Δm2 < 40 eV2”, Phys. Rev. D 85 032007 (2012)

[4] Dydak et al.,  “A search for νµ oscillations in the Δm2 range 0.3-90 eV2 ”, Phys Lett B 134, 281 (1984)

[5] Stockdale et al. , “Limits on Muon -Neutrino Oscillations in the large Mass Range 30< Δm2 < 1000 eV2/c4”,  Phys Rev Lett 52 1384 (1984)

[6] Abe et al. , “The T2K experiment”, Nucl. Instrum Meth A 659, 106 (2011)

[7] Abe et al.,  “Search for short baseline νe disappearance with the T2K near detector”, arXiv:1410.8811 (2014),  accepted by PRD.

Poster Excerpts 2: A Novel Spectrometer for Neutrino Experiments

March 1, 2015

(Author: Laura Pasqualini, for the NESSiE Collaboration)

The WA104 – NESSiE Collaboration aims to develop a high precision muon spectrometer  in the 0.5-5 GeV/c range for applications in future neutrino experiments. Preliminary studies have shown that a tracking system with a spatial resolution of about 1 mm, operating inside a 0.12 T magnetized air volume would allow a charge mis-identification of ~ 1 % at 1 GeV/c, required for an accurate determination of the neutrino beam components.

A R&D program was carried out to prove that an innovative use of planes of triangular scintillator bars (of cm size) equipped with Silicon Photomultipliers (SiPM) in analog mode read out could achieve a spatial resolution of the order of  1 mm.

The X position of a crossing particle is reconstructed from the pulse height in each channel, namely:

laurapasqualini_1

where xi is the fiber nominal position and wi is the pulse height in the i-th channel.

Tests triggering on cosmic rays muons were performed requiring at least a signal over threshold in each plane of triangular bars.

This experimental approach will be implemented in a multiplane (meter scale) prototype to be tested with charged beams to set the ultimate achievable spatial accuracy.

laurapasqualini_2

Figure: (a) Sketch of the R&D layout. The tracking system is composed by 2 modules of triangular scintillator bars. Each module has 2 detector planes, 4 channels each. (b) Picture (adapted from Ref. [1]) of scintillator bars with embedded Wavelength Shifter. (c) Residuals between the reconstructed X positions and  fitted track projections in each plane of triangular bars. The achieved spatial resolution is better than 2 mm.

References:

[1] arXiv:1305.5199 [physics.ins-det].

Posters Excerpts 1: CALDER – Cryogenic Light Detectors for Rare Event Searches

February 28, 2015

While we wait for the start of Neutrino Telescopes XVI, we start today with an asynchronous series of posts describing the posters competing for the “best poster” award. The text below is from the author, Lorenzo Pagnanini.

The purpose of the CALDER project (Cryogenic wide-Area Light Detector with Excellent Resolution) is to develop new cryogenic light detectors to be used in CUORE and LUCIFER to improve the sensitivity in the search of neutrinoless double beta decay (0νββ) and dark matter.

The CUORE experiment is designed to search for the 0νββ decay of the 130Te (Q = 2528 keV). For this process we expect a monochromatic line at the Q-value of the decay in the sum spectrum of the two electrons, as neutrinos don’t subtract energy. A detector with high resolution and efficiency is required to separate the 0νββ signal from the 2νββ background, for this reason in CUORE we use crystals grown by Tellurium as bolometers. The detector of CUORE consists of 988 crystals of Tellurium dioxide (TeO2) in which 34% of Tellurium is 130Te; each crystal is a 0.75 kg cube with 5 cm of side, the total mass is 741 kg (208 kg of 130Te). The expected sensitivity on the half-life of the 0νββ decay of 130Te is about 1026 years in 5 years of data taking.  As shown in the figure 1 the sensitivity of CUORE can be increased by a factor of 3, thanks to the reduction of the α background, obtained by detecting the Cherenkov light (∼100 eV) emitted by βs events and not by the α-background.

The LUCIFER project explores the possibility to build a scintillating bolometer experiment to search for 0νββ decay of isotopes with Q-values higher than 2615 keV (end of natural radioactivity) . There are three main possibilities for this kind of detectors: ZnSe crystals enriched in 82Se (Q = 2997 keV), ZnMoO4 crystals enriched in 100Mo (Q = 3034 keV) and CdWO4 crystals enriched in 116Cd (Q = 2814 keV). The β/γ background will decrease with respect to CUORE because the Q-values are above the bulk of the environmental γ radioactivity. On the other hand the use of scintillating crystals will help to reject the α-background, because these particles have a different light yield than the one of β/γ. This technique, in principle, allows to eliminate all the α background in LUCIFER and reach the so-called zero background condition. As a result in 5 years of data taking, LUCIFER is expected to get a sensitivity comparable to the CUORE one, but with a much smaller mass detector (36 bolometers with a total isotope mass of about 9.8 kg).

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LUCIFER could have a broader physics potential if equipped with sensitive light detectors. Scintillating bolometers are widely used for dark matter searches, as the simultaneous read-out of heat and light allows to disentangle the energy region where WIMPS interactions are expected from the background due to electrons and gamma. The light detectors with an energy resolution of 20 eV RMS, if used in LUCIFER, allow to achieve enough sensitivity to discriminate nuclear recoils (due to WIMP interactions) from β/γ background in the low energy region (see figure 2).

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New light detectors with an active area of 25 cm2, a baseline energy resolution of ∼ 20 eV RMS and a working temperature of 10 mK are mandatory. The technology chosen is based on the phonon-mediated kinetic inductance detectors (KIDs), in which the interacting photons are converted in phonons that break the Cooper Pairs condensed in a LC superconductor circuit as shown in detail in figure 3.

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The best prototypes tested so far have a resolution of 230 eV on the baseline (see figure 4) and 365 eV (580 eV) to the 6.4 keV peak (14 keV) of 55Co source.

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