(7) Instrumentation in high energy neutrino experiments

Transcription

1 (7) Instrumentation in high energy neutrino experiments Scientific Objectives Solar Neutrinos & Atmospheric Neutrinos High Energy Neutrino Detection in Ice (AMANDA & ICECUBE) High Energy Neutrino Detection in Water (ANTARES) 1

2 The electromagnetic and cosmic ray spectrum Solar c.r. energy: ev mev ev ν MeV Galactic c.r. GeV TeV? Galactic c.r. energy: kev solar ν TeV Extra-galactic c.r. (?) > PeV EeV ZeV 2

3 Neutrinos in the Standard Model of Particles Charged particles (leptons, hadrons): electromagnetic interaction, exchange of photons Hadrons (quarks): strong interaction, exchange of gluons All leptons, quarks: weak interaction, exchange of W and Z bosons All massive particles: gravitation In the Standard Model, there are three flavors of neutrinos, corresponding to the three flavors of the charged leptons. Neutrinos are neutral, massless particles in this model. Weak coupling is much smaller than electromagnetic coupling and strong coupling (factor ~10-5). Gravitational coupling is extremely small (factor ~10-40). Neutrinos interact only via the weak interaction. => Their interaction cross-section is very small. => Very large detector volumes are needed to observe neutrino interactions. 3

4 Scientific Interest Neutrino Astronomy could be a powerful alternative for the exploration of the Universe since: high energy photons are absorbed by pair production with IR, CMB and radio photons high energy electrons undergo bremsstrahlung, Compton effect, Synchrotron radiation and are diffused in magnetic fields. protons are diffused in magnetic fields and undergo pair production and the GZK effect nuclei undergo photo-spallation in addition neutrons have a short lifetime (~ 880 s). neutrinos are neutral and stable and interact weakly with matter. => Neutrinos are a unique probe to obtain information from dense or distant objects. 4

5 Neutrino Sources Neutrinos detected on Earth can come from several natural sources: solar neutrinos astrophysical sources (SN, AGN, GRB,...) cosmic neutrino background at < 1.9 K neutrinos from GZK interactions of cosmic rays with the CMBR (pion-decay) atmospheric neutrinos (from interactions of cosmic rays in the atmosphere) decay products of weak interaction (radioactive decays) Man-made sources are: nuclear reactors, weapons particle accelerators The sun and SN1987A are the only astrophysical neutrino sources observed so far! 5

6 Astrophysical Neutrinos GZK neutrinos pion and muon decays are important sources of neutrinos observation of neutrinos from astrophysical sources would provide proof of hadronic acceleration at the source (contrary to acceleration of only electrons). Due to their small cross-section, neutrinos probe the regions very close to the center of astrophysical sources. ν ν 6

7 Observational Methods underground chemical detectors: (~MeV) Neutrinos interact in large volumes of certain chemicals (e.g. Cl37 liquid, Ga71 solid or liquid...). Radioactive isotopes are generated and can be detected. underground water/ice Cherenkov detectors: (~MeV - ~PeV) PMTs observe a huge volume of water or ice to detect the products of neutrino interactions via Cherenkov light. air shower detectors: (>EeV) Similar to UHE cosmic ray detection. Interactions of neutrinos in the earth or atmosphere lead to secondary particles that can trigger air showers. Search for upwardgoing showers. acoustic & radio detectors: (>EeV) Detection of acoustic or radio signals from showers in water, ice, salt 7

8 Underground Detectors Chemical or water/ice neutrino detectors are usually located under ground to shield them from other particles. Downward going particles: atmospheric muons atmospheric neutrinos astrophysical neutrinos Upward going particles: atmospheric neutrinos having traversed the Earth astrophysical neutrinos having traversed the Earth muons generated in astrophys. neutrino interaction in the Earth. ( neutrinos from WIMP decay in the Earth? ) Observation of upward going particles rejects most of the background in ice/water detectors. Chemical detectors have well defined energy thresholds. 8

9 Air Shower Detectors τ decay air shower Observation of neutrino initiated air showers in the atmosphere with cosmic ray detectors like AUGER. Earth-skimming τ-neutrinos interact in the earth and generate τs. Those decay in the atmosphere (to gammas, pions...) and generate almost horizontal air showers. Only very energetic neutrinos (in UHE regime) can be observed. At these energies, the earth is opaque for electron neutrinos. Muon neutrinos can generate muons in the earth, but those will escape from the atmosphere without causing air showers. 9

10 Acoustic & Radio Detectors New Techniques use the acoustic or radio signal from high-energy neutrino induced air showers in water, ice or underground salt domes. Acoustic detection under water (e.g. SAUND project) As the shower energy is deposited in the water, the thermal expansion generates an acoustic pulse, which is then propagated (due to the elongated source shape) mostly perpendicularly to the shower axis. The signal arriving at the detector is predicted to be well within the sensitivity of good quality hydrophones, depending on the relative position of the hydrophone and the shower. Radio signals from neutrino induced showers in ice (e.g. ANITA) Earth skimming neutrinos that traverse the ice at the South Pole induce particle showers. Cherenkov emission in the radio band from those showers can be detected with radio sensors onboard a balloon. Radio emission is also observed at > 100 TeV by RICE with radio sensors buried in the ice. 10

11 A bit of history : Wolfgang Pauli proposes a neutral, very light particle to explain the continuous energy spectrum seen in the β-decay. 1987: Super Kamiokande observes for the first time neutrinos from a supernova. It confirms an anomaly in the atmospheric and solar neutrino flux. 1956: Reines and Cowan observe e--neutrinos for the first time. They detect the interaction of neutrinos from a nuclear power plant in 400 liters of water and CdCl : LSND observes neutrinos from the particle accelerator at Los Alamos. Confirmation of neutrino oscillations. 1962: Lee and Yang observe muon neutrinos from a particle accelerator beam in a large spark chamber. 1969: Ray Davis begins an experiment in the Homestake mine to observe solar neutrinos in a huge tank filled with solvent based on chlorine. He measures a flux that is only a third of that expected. 1979: Reines finds some indications of neutrino oscillations in measurements of reactor neutrinos. 1980s/90s: GALLEX, SAGE and Kamiokande observe (different) deficits in solar neutrino flux 1993: The first underwater neutrino experiment is installed in Lake Baikal, Siberia. 1993: Amanda is the first neutrino detector installed in the ice of the South Pole. It sets upper limits on astrophysical neutrino sources. 1990s: The Dumand experiment tries to detect neutrinos in sea water off Hawaii. 1999: SNO, a huge underground heavy water Cherenkov detector in Canada, begins observation of solar neutrinos. 11

12 A bit more history : observation of the tau neutrino at Fermilab. 2002: The KamLAND detector, in the Kamioka mine, starts taking data of reactor and geo-neutrinos. 2004: Construction of ICECUBE, the successor of Amanda, begins. 2008: The Antares experiment, in the Mediteranean, has been fully deployed. 12 lines with PMTs are observing the ocean to find Cherenkov signals from products of neutrino interactions. Two similar projects, Nestor and Nemo, are in a phase of Research and Development. 12

13 Detection of Neutrinos from SN1987A (Kamiokande) The Kamiokande experiment consisted of a tank filled with 3k tonnes of very pure water, at a depth of 1 km in the Kamioka metal mine in the Japanese Alps. The inner surface of the tank was covered with PMTs. (picture: SuperKamiokande, 50k tons of water, 11k PMTs) Its purpose was to search for a possible proton decay, which was postulated by the Grand Unified Theory. No proton decay was found. The lifetime of the proton was seen to be > 1032 years. Kamiokande could also detect Cherenkov emission from fast electrons from electron-neutrino scattering: e e On 23 February 1987, within about 12 seconds, 12 neutrino events (with 6.3 MeV < E < 35.4 MeV) were observed. This observation was confirmed by the IMB experiment in the USA. The neutrinos were found to come from the supernova SN1987A (in the LMC galaxy in the Local Group). 13

14 Detection of Solar Neutrinos (Homestake) Solar neutrinos were first detected in the Cl37 underground experiment in the Homestake goldmine in South Dakota (USA) in the 1980s. This detector consisted of a tank with 400k liters of C2Cl4 in a cavity 1500m below ground. Neutrinos from a side chain of the main pp chain of the sun are observed by their interaction with 37 Cl nuclei. Radioactive 37Ar nuclei are generated. The amount of Argon gas provides a measurement of the neutrino flux. Since the first observation of solar neutrinos, other experiments have observed them (SAGE, GALLEX Ga detectors; (Super)Kamiokande water Cherenkov detectors). SAGE and GALLEX also observed neutrinos from the main pp chain. Reaction used for detection: These observations have led to the Solar Neutrino Problem. The threshold for this reaction is MeV. Neutrinos from the B8 chain (right) can be detected. E < MeV E < 15 MeV 37 Cl e 37 Ar e 14

15 The Solar Neutrino Problem The solar neutrino flux observed by the Cl37 experiment was only about 1/3 of that expected from the Standard Solar Model. Later experiments confirmed that the observed flux was significantly below the expected flux. The Standard Solar Model is quite well understood and this discrepancy could not be explained by modifications of the model. Another possibility was that something happened to the neutrinos on their way from the source to the detector. Cl37, Sage and Gallex observe only νe. Super(Kamiokande) observes all three flavors, but the cross-section of electron scattering of νμ and ντ is ~6 times smaller than for νe. If the neutrinos could change their flavor on the way to the detector, the problem could be solved. 15

16 Neutrino Oscillations (1) In the Standard Model of Particle Physics, neutrinos are massless particles. In this case, they cannot change their flavor. "Neutrino Oscillations", i.e. transitions between the different neutrino flavors, are only possible if one modifies the Standard Model. If neutrinos are massive particles, their flavors are no longer preserved. Eigenstates with different masses propagate at different speeds. The heavier ones lag behind while the lighter ones pull ahead. Since the mass eigenstates are combinations of flavor eigenstates, this difference in speed causes interference between the corresponding flavor components of each mass eigenstate. Constructive interference makes it possible to observe a neutrino created with a given flavor to change its flavor during its propagation. 16

17 Neutrino Oscillations (2) In the extended Standard Model, the transition probability between two flavor eigenstates is described by: P = 1 L [GeV ] sin2 2 1 cos 2.54 m2 2 2 E [ev ][ km] The maximum probability for flavor change is given by sin22θ. θ is a parameter (the "mixing angle") that has to be determined from experiments. 2 2 The probability for flavor change oscillates with a frequency given by Δm L/E. Δm is the squared mass difference between the two mass eigenstates. These mass eigenstates do not match the flavor eigenstates, which causes the oscillations. If one wants to observe oscillations, L/E needs to be large if the mass difference is small. L/E is different for atmospheric and solar neutrinos. If neutrinos travel through matter, the oscillation probability is enhanced (MSW effect). 17

18 Neutrino Oscillations in Experiments Solar Neutrino & Long Baseline Reactor Experiments : νe <-> ντ, νμ ( νsterile?) E ~ MeV => Δm2 ~ ev2 ( Δm122 ) Atmospheric Neutrino Experiments: νμ <-> ντ E > GeV => ev2 < Δm2 < ev2 ( Δm232, Δm132) LSND Experiment (Los Alamos): νμ <-> νe Large Scintillator Neutrino Detector located ~30 m from νμ source. νe were observed at 30<E<60 MeV. => 0.2 < Δm2 < 2 ev2 => evidence for a 4th, sterile neutrino? (could not be confirmed by MiniBoone) 18

19 Modern "low" energy neutrino detectors KamLAND: located in the old Kamioka site, oil shield against external radiation, water Cherenkov detectors as muon veto counters and radiation shields. KamLAND observed neutrinos from Japanese power plants and from geological radioactivity. Sudbury Neutrino Observatory (SNO): located in a mine in Canada, uses heavy water D2O for detection of solar neutrinos. 19

20 High Energy Neutrino Observations in Ice (Amanda) Amanda is a neutrino detector located ~ 1.5 km below the surface of the antartic ice. Neutrinos interact in the ice or in the earth and produce muons, electrons or tauons that can be observed in the detector volume. The particles that traverse the detector volume generate secondary particle cascades. Cherenkov light from these cascades is observed with photomultipliers attached to strings deployed in the ice. Amanda has been taking data since 98/99. It is now incorporated into its (much larger) successor experiment, ICECUBE. Construction on ICECUBE began in 2004 and is predicted to finish in

21 Instrumentation (Amanda-II) In its final configuration, called Amanda-II, the detector consists of 19 strings, carrying 667 optical modules in total. The optical modules are photomultiplier tubes in a special housing. Data transmission via optical fibers. The detector volume is 1.6 x 107 m3. the SPASE air shower array on top of Amanda records air showers with scintillators and air Cherenkov detectors. When an air shower is detected by SPASE, the muonic component can be observed with Amanda. This helps to calibrate Amanda and to determine its angular resolution. LED beacons and lasers are deployed for calibration and measurements of the characteristics of the ice. Sources of noise/background: electronic noise, 40K decay in the glass encasing of the PMTs downgoing muons, muons from downgoing atmospheric neutrinos muons from upgoing atmospheric neutrinos 21

22 Optical Modules Holes are drilled into the ice with the help of a hot water drill. Once the optical module has been deployed, it has to remain in the ice. 22

23 Neutrino Transmission through the Earth (1) Neutrino detectors of dimension ~km3 have a lower energy threshold for up-going neutrinos of ~ 100 GeV. The detection efficiency depends on: the cross-section of the neutrinos, which increases with energy (higher E -> detection more probable) the range of muons (for muon neutrinos), which increases with energy (higher E -> longer muon tracks) the lifetime of the tau (for tau neutrinos), which increases with energy (higher E -> longer tau tracks) BUT, the earth is essentially opaque for e-- and muonneutrinos with energies > ~ 10 PeV. Tau neutrinos are not absorbed by the earth, but they loose energy. X Y ; Z "re-generation": 23

24 Neutrino Transmission through the Earth (2) interaction cross-section for muon neutrinos as a function of energy. (taken from: astro-ph/ ) muon range as a function of energy (taken from: astro-ph/ ) 24

25 Detection of a νμ Muon neutrinos yield the clearest signal. They generate muons by the "Charged Current" (CC) interaction with nucleons: N X The muons generate secondary particle cascades along their track through bremsstrahlung, muonic pair production and nuclear interactions. These cascade particles emit Cherenkov light along the muon track in the ice, which is recorded by PMTs on several strings. The angle between the neutrino and the muon is < 1o at TeV energies. The direction of the original muon neutrino can thus be determined with a high angular precision. (~1o in ICECUBE). Unlike electron neutrinos, muon neutrinos need not interact within the detector volume. Only the muon they generate needs to cross the detector volume. In "Neutral Current" interactions, muon neutrinos can also initiate cascades of secondary particles (see section on electron neutrinos). 25

26 Detection of a νμ Simulations of muon Cherenkov tracks in ICECUBE. 26

27 Detection of a νe Electron neutrinos produce electrons in CC interactions with nucleons: e N e X Unlike muons, electrons will interact quickly in the ice and generate cascades of secondary particles. The Cherenkov light of those secondary particles can be observed. Since the length of these cascades is of the order of 10 m in ice (~km for muons), one does not see a nice track, but a nearly spherical distribution of light. => The energy estimate of electron neutrinos is very good, but their angular resolution is much worse than for muon neutrinos. Electromagnetic and hadronic cascades can be generated by neutrinos of all flavors. Simulation of an electron neutrino signal in ICECUBE. 27

28 Detection of a ντ Apart from the detection via secondary particle cascades, the tau neutrino can also lead to a "double-bang" signature. The signature comes from two separate cascades. The first shower is generated in the interaction of the tau neutrino with a nucleon: N X Since the tauon has a relatively high mass, the created X particle will be accelerated and initiate a hadronic cascade. The second shower comes from the tau decay, in which the decay product initiates a cascade. For these events, angular and energy reconstruction are expected to be both very good. However, tauons need high energies for "double bangs" and the events are thought to be rare. In general, the tau neutrino does not need to interact in the detector volume. Only the tau has to reach the detector volume. Simulation of a "double-bang" event in ICECUBE. 28

29 Scientific Results (Amanda) "The (...) neutrino sky as seen by AMANDA-II using data from just the first year of operation (Feb -Oct of 2000). (...)Nearly all events in the northern sky are compatible with atmospheric neutrinos (plus a small admixture of poorly reconstructed atmospheric muons). While the angular distribution of this data reveals NO evidence for extraterrestrial neutrino sources, it provides important constraints on theoretical models." (from the Amanda website) The same events in a contour plot: No statistically significant excess is seen. The units of the color legend are 10-7 cm-2s-1. The horizontal units are hours of right ascension and vertical units are degrees of declination. 29

30 Sensitivity WB: upper limit estimate of the neutrino flux derived from cosmic ray flux measurements. estimates of neutrino fluxes from different sources: 1) (StSa) AGN 2) (MPR) AGN 3) (RB) GZK 4) (GRB) GRB (from astro-ph/ ) 30

31 Current and Expected Neutrino Flux Limits Limits at 90% CL for each flavour, white lines: (top to bottom) SHDM, AGN, GZK (horizontal) GRB (diagonal) TD yellow lines: preliminary AUGER limit ( from 2007 ) 31

32 Neutrino Flux Limits expected from IceCube from the IceCube website 32

33 The Future of Neutrino Experiments in Ice: ICECUBE 59 strings and 120 tanks deployed, > 2/3 of ICECUBE (status 01/09) 33

34 High Energy Neutrino Observation in Water (ANTARES) ANTARES is a large area water Cherenkov detector in the deep Mediterranean Sea, off the coast of Marseille, optimised for the detection of muons from high-energy astrophysical neutrinos. ANTARES has a surface area of 0.1 km2, a first step toward a kilometric scale detector. The detector consists of an array of approximately 1000 photomultiplier tubes in 12 vertical strings, spread over an area of about 0.1 km2 and with an active height of about 350 metres. On May 30th, 2008, the last line was connected. Antares consists of 12 detection line + 1 instrumentation line. 34

35 Instrumentation - Layout 35

36 Instrumentation PMTs & Trigger The optical modules are grouped together in `storeys' of three modules and interconnected via an electro-mechanical cable. Each of the 12 strings has a total height of about 350 metres and consist of 25 storeys spaced vertically by 14.5 metres. The strings are distant by about 70 metres. The optical modules in a storey are arranged with the axis of the photomultiplier tubes 45 degrees below the horizontal. 10-inch Hamamatsu photomultiplier tubes are used. The angular acceptance of the optical modules is broad, falling to half maximum at around 70 from the axis. This means that the proposed arrangement of OMs detects light in the lower hemisphere with high efficiency, and has some acceptance for muon directions above the horizontal. The first-level trigger requires a coincidence between any two OMs in a single storey. The second-level trigger is based on combinations of first-level triggers. Following a second-level trigger the full detector will be read out. A more refined third-level trigger, imposing tighter time coincidences over larger numbers of optical modules, will be made in a farm of processors on shore. The readout rate is expected to be several khz, and the corresponding data recording rate less than 100 events per second. 36

37 Instrumentation - Positioning " Two independent systems have been incorporated to provide a precise knowledge of the relative position of each OM at any time: The first system is based on a set of tiltmeters and compasses which measure local tilt angles and orientations on the string. The reconstruction of the line shape, as distorted by the water current flow, is obtained from a fit of measurements taken at different points along the line. A maximum error of 1 m on the reconstructed shape is estimated. The second system, based on acoustic triangulation, is more precise but requires more complex and expensive electronics. In this system, rangemeters (hydrophones) placed on the string send an acoustic signal to a minimum of three transponders fixed to the sea bed. Each transponder replies with its characteristic frequency. A global fit of the measured acoustic paths gives the precise three-dimensional position of the rangemeters, provided that the positions of the transponders and the sound velocity in water are known. => Reproducibility of ~ 1 cm in the acoustic path length. In order to exploit such a system fully, a precise knowledge of the sound velocity in water along the acoustic path is required. This depends strongly on water temperature and also on salinity and depth. The prototype string is thus equipped with sound velocimeters, which measure the local sound velocity with a precision of 5 cm s-1, and with Conductivity Temperature Depth devices (CTDs) to observe the variations of temperature and salinity. " (from the Antares website) 37

38 Energy reconstruction The muon energy reconstruction is based on the fact that the energy loss along its track increases with energy (radiative energy losses). The muon looses energy along its track, which leads to lower energy cascades. E loss ~ E This leads to a decrease in the Cherenkov signal along the track. p - pair production, b - bremsstrahlung, pn - photo nuclear The method is only valid above the critical energy ( 600 GeV), where energy losses caused by radiative processes dominate over ionization processes. 38

39 Backgrounds same backgrounds as in Amanda: noise: - electronic noise - 40K radioactive emission from the PMT glass encasing down-going events: - atmospheric muons (from air showers) - muons from atmospheric neutrinos up-going events: muons from atmospheric neutrinos additional backgrounds: bio-luminiscence from organisms in the sea. optical fouling (bacteria and sediments covering the optical modules) influence light transmission to PMTs. 39

40 The Future of Neutrino Observations in Water: KM3NET " KM3NeT, an European deep-sea research infrastructure, will host a neutrino telescope with a volume of at least one cubic kilometre at the bottom of the Mediterranean Sea (...). The kilometer-sized KM3NeT will search for neutrinos from distant astrophysical sources like gamma ray bursters, supernovae or colliding stars and will be a powerful tool in the search for dark matter in the Universe. (...) The design, construction and operation of the KM3NeT neutrino telescope will be pursued by a consortium formed around the institutes currently involved in the ANTARES, NESTOR and NEMO pilot projects. Based on the leading expertise of these research groups, the development of the KM3NeT telescope is envisaged to be achieved within a period of three years for preparatory R&D work plus five years for construction and deployment. With an angular resolution for muon events of better than 0.1 degree for neutrino energies exceeding 10 TeV, an energy threshold of a few 100 GeV and a sensitivity to neutrinos of all flavours and to neutral-current reactions, the KM3NeT neutrino telescope will be unique in the world in its physics sensitivity and will provide access to scientific data that will propel research in different fields, including astronomy, dark matter searches, cosmic ray and high energy physics. " (from the KM3NET website) 40

41 More Information Amanda: ICECUBE: Antares: KM3NET: 41