High-energy neutrino detection with the ANTARES underwater erenkov telescope. Manuela Vecchi Supervisor: Prof. Antonio Capone

Transcription

1 High-energy neutrino detection with the ANTARES underwater erenkov telescope Supervisor: Prof. Antonio Capone 1

2 Outline Neutrinos: a short introduction Multimessenger astronomy: the new frontier Neutrino astronomy: why, how and where? The ANTARES Neutrino Telescope Reconstruction issues in ANTARES Conclusions and perspectives 2

3 Neutrinos in the Universe expected and measured fluxes Neutrinos, being slightly massive and weakly interacting particles, can travel unscattered from the source to Earth. 3

4 Neutrinos in the Universe Neutrinos, being slightly massive and weakly interacting particles, can travel unscattered from the source to Earth. 4

5 Neutrinos in the Universe Neutrinos, being slightly massive and weakly interacting particles, can travel unscattered from the source to Earth. 5

6 Neutrinos in the Universe Neutrinos, being slightly massive and weakly interacting particles, can travel unscattered from the source to Earth. 6

7 Neutrinos in the Universe Neutrinos, being slightly massive and weakly interacting particles, can travel unscattered from the source to Earth. 7

8 Neutrinos in the Universe Neutrinos, being slightly massive and weakly interacting particles, can travel unscattered from the source to Earth. 8

9 Multimessenger astronomy Gravitational waves High-energy photons: Mpc μ-quasar Proton s: E > ev (10 Mpc) Protons and photons are absorbed or deflected since they interact with matter. Neutrinos can travel unscattered opening a new window on the far Universe. 9

10 Production of High- Energy Neutrinos The main (conventional) production channel is the pion decay p + + n/ 0 p + μ + μ e + e μ μ 0 (E ~TeV) Due to neutrino oscillations we have: o e : μ : = 1: 2: 0 at the Source o : e μ : = 1: 1: 1 on Earth The outstanding observations made by the H.E.S.S. and the MAGIC -ray telescopes can be used to search for simultaneous emission of -rays and s. The three brightest galactic TeV sources: CRAB Nebula, SNR RX J and Vela Junior seem to be within the discovery possibilities of the present neutrino telescopes. 10

11 Neutrino Astronomy why? We can understand production sites and mechanisms for Neutrinos and High Energy Cosmic Rays. 11

12 Neutrino astronomy how? They can be detected using the visible erenkov radiation produced as the high-energy charged lepton (final state of CC interactions) passes through a transparent medium with superluminal velocity. e and : electromagnetic and hadronic showers Seabed μ μ interaction 3D PMT array erenkov cone μ ~ 1,5 E [TeV ] m Due to small fluxes and interaction cross section a large detection volume is required (~ 1 km 3 ). 12

13 Neutrino Astronomy where? The easiest solution is to use an array of photomultiplier tubes located in a natural transparent medium: water/ice. We need: a target for neutrino-muon conversion a medium for production and propagation of erenkov radiation a shield against atmospheric muons (background) a large detection volume Since it s a long way to an underwater km 3 detector let s start with 13

14 Neutrino Astronomy where? The ANTARES Neutrino Telescope We need: a target for neutrino-muon conversion a medium for propagation of Cherenkov radiation a shield against atmospheric muons (background) a large detection volume 14

15 Observable sky for ANTARES Target sources for Neutrino Telescopes are the ones traditionally detected (radio, IR, visible, rays ) 15

16 Galactic -ray sources SuperNova Remnants (SNR) Detection of resolved -ray emission from shells RX J : Multiwavelenght analysis points to hadronic emission neutrinos Pulsar Wind Nebulae (PWN) -rays emission is mainly thought to be dued to electromagnetic processes, but hadronic models have also been proposed. Binary systems Strong absorbtion of radiation: neutrinos flux could be much more abundant than the -rays one. Other sources No counterpart in other wavelenghts: be open to the unexpected!! 16

17 Depth [m] How does a muon look like? A typical down-going event (atmospheric muon) and its erenkov cone as seen in the detector Atmospheric neutrino candidate (10 linesdetector) seen as an upgoing event μ Time [ns] 17

18 Research project outline The aim of my research project is the study of reconstruction algorithms for muon tracks identification and muons and showers energy estimation. Why? A fundamental issue for a Neutrino Telescope is the Angular Resolution, which depends on both the detector hardware and software issues. An efficient algorithm leads to a good pointing accuracy which is fundamental in the search for point-like sources. The event energy will help to identify an excess of neutrinos, over the atmospheric background, that can be attributed to diffuse neutrino sources in the Universe. Energy reconstruction is a hard job, but important since it is the best chance we have to be sure that a detected neutrino comes from an astrophysical source, at energies bigger than 1 TeV. How? Starting from the time-charge-position informations one can parametrise a probability function which is linked to the best values of track parameters. Track quality cuts must be developed to enhance the quality of backgroud rejection. Energy evaluation (muon and showers) starts from MonteCarlo studies, looking for reliable estimators. 18

19 Muon Track Reconstruction Optical Module erenkov light k C μ track ˆ q Arrival time and amplitude are stored for each hit on PMTs, together with their positions ( x ~10cm). Very good precision: angular resolution of 0.2 for E>10 TeV. t 0, r 0,E 0 C erenkov light Muon tracks reconstruction algorithms are based on maximum likelihood procedures. The arrival time of a photon on the OM is given by: Time for the μ to reach the point of light emission (v μ ~ c) Time for the to propagate in water (v = c/n) We can define time residuals: that is the difference between the observed hit time and the hit time expected for a direct photon, not scattered by the molecules in water. 19

20 Muon track reconstruction (2) The time residuals distribution contains all the instrumental and environmental informations on the photon path in water. Photomultiplier time resolution t affects the t res distribution width. Random noise modifies tails Scattering of light on water molecules + showers produce delay in the arrival time of photons For each possible set of track parameters the probability to obtain the observed events can be calculated using the likelihood function. In case of uncorrelated hits the likelihood of the event can be written as the product of the likelihood of the individual hits: P(event track) = P(hits q ˆ, r 0 ) = P(t i t th i,...) What about the background? How to take it into account? i 20

21 Physical Background rejection The performance of the reconstruction algorithm depends on the quality of the Physical background selection criteria. Atmospheric muons Nearly horizontal muons Atmospheric neutrinos Muon bundles Cascades 5-lines data: 01/02-25/05/2007 ~ 36.8 days of active time. trigger rate ~ 1 Hz Integrated rates: MC Atmospheric ~ 0.1 Hz Data ~ 0.07 Hz These are the main sources of background events in the neutrino sample 5-lines detector Data MC: atmospheric s MC: atmospheric s Downgoing events Zenith angle Upgoing events Track fit quality cut applied 21

22 Energy reconstruction Main energy loss processes for TeV muons: Ionisation (logarithmically increasing with E μ ) Radiative processes (in the HE limit E μ) We can express muon energy loss as: de dx + E μ : ionisation contribution : radiation losses contribution Radiative energy loss processes generate secondary charged particles along the μ trajectory, which also produce erenkov light. The light produced along the muon track can give an estimate of the energy of the muon. 22

23 Showers reconstruction Electromagnetic or hadronic showers can be produced during the propagation of the leptons. Their optical signature is that of an expanding spherical shell of erenkov photons with a larger intensity in the forward direction. Their detection can give fundamental informations on the electron and tau channels, provided they are energetic enough. Shower reconstruction can provide good energy resolution (~ 30% ), since the energy deposition is localised in a small region, but can give only poor information on the direction of the original lepton (angular resolution is ~ some degrees). 23

24 Conclusions & Perspectives oantares (area ~ 0.1 km 2 ) is currently the largest underwater HE neutrino telescope in operation. HE Neutrino detection in water is now known to be feasible but a larger detection volume is needed to have a reasonable detection potential. o Reconstruction is a critical issue: muons identification, together with their energy, is of crucial importance to disentangle signal from physical background (atmospheric muons and atmospheric neutrinos). Data and MC agreement start to be good but not yet completely understood. o Showers detection can give important informations on tau and electrons even if their detection is not the main target for such a detector. othe capability of the km 3 telescopes to open the new window of high-energy neutrino astronomy is good, further improvements are needed to explore Ultra High Energy domain (acoustic and radio techniques). 24

25 Conclusions & Perspectives oantares (area ~ 0.1 km 2 ) is currently the largest underwater HE neutrino telescope in operation. HE Neutrino detection in water is now known to be feasible but a larger detection volume is needed to have a reasonable detection potential. o Reconstruction is a critical issue: muons identification, together with their energy, is of crucial importance to disentangle signal from physical background (atmospheric muons and atmospheric neutrinos). Data and MC agreement start to be good but not yet completely understood. o Showers detection can give important informations on tau and electrons even if their detection is not the main target for such a detector. othe capability of the km 3 telescopes to open the new window of high-energy neutrino astronomy is good, further improvements are needed to explore Ultra High Energy domain (acoustic and radio techniques). 25

26 Conclusions & Perspectives oantares (area ~ 0.1 km 2 ) is currently the largest underwater HE neutrino telescope in operation. HE Neutrino detection in water is now known to be feasible but a larger detection volume is needed to have a reasonable detection potential. o Reconstruction is a critical issue: muons identification, together with their energy, is of crucial importance to disentangle signal from physical background (atmospheric muons and atmospheric neutrinos). Data and MC agreement start to be good but not yet completely understood. o Showers detection can give important informations on tau and electrons even if their detection is not the main target for such a detector. othe capability of the km 3 telescopes to open the new window of high-energy neutrino astronomy is good, further improvements are needed to explore Ultra High Energy domain (acoustic and radio techniques). 26

27 Conclusions & Perspectives o ANTARES (area ~ 0.1 km 2 ) is currently the largest underwater HE neutrino telescope in operation. HE Neutrino detection in water is now known to be feasible but a larger detection volume is needed to have a reasonable detection potential. o Reconstruction is a critical issue: muons identification, together with their energy, is of crucial importance to disentangle signal from physical background (atmospheric muons and atmospheric neutrinos). Data and MC agreement start to be good but not yet completely understood. o Showers detection can give important informations on tau and electrons even if their detection is not the main target for such a detector. o The capability of the km 3 telescopes to open the new window of high-energy neutrino astronomy is good, further improvements are needed to explore Ultra High Energy domain (acoustic and radio techniques). 27