Detection of high-energy neutrinos in the IceCube experiment

Transcription

1 University of Ljubljana Faculty of Mathematics and Physics Department of Physics Seminar 1b Detection of high-energy neutrinos in the IceCube experiment Author: Tadej Novak Advisor: prof. dr. Peter Križan Abstract Astrophysical neutrinos are perfect cosmic ray messengers. Their almost nonexistent interaction with matter helps us look for sources of cosmic rays but it also makes them very hard to observe. The IceCube neutrino observatory in the South Pole uses a cubic kilometre of ice to detect them and solve the question of their origin. April 2015

2 Contents 1 Introduction 2 2 Neutrinos Interaction with matter Sources of high-energy neutrinos Active galactic nuclei Gamma ray bursts Cherenkov radiation 6 5 IceCube The digital optical module Detection of neutrinos Results Future Conclusion 11 1 Introduction Neutrinos are neutral elementary particles with the lowest mass. A majority of them were produced around 15 billions years ago, soon after the birth of the universe, which has continuously expanded and cooled. Neutrinos constitute a cosmic background radiation whose temperature is 1.9 K [1]. Other neutrinos are constantly being produced in nuclear power stations, particle accelerators, nuclear bombs, and cosmic accelerators in stars, but also in interactions of cosmic rays with the nuclei in the atmosphere. The neutrino was first postulated in December, 1930 by Wolfgang Pauli [1] to explain the energy spectrum of beta decays, the decay of a neutron into a proton and an electron. They were needed to describe the observed difference between the energy and angular momentum of the initial and final particles. They were first observed several years later in 1956, for which Frederick Reines was rewarded with the 1995 Nobel Prize. Astrophysical neutrinos are important messenger in astro-particle physics because of their properties and specifics of their production mechanisms. They mainly originate from the decay of secondary particles, like pions, from interaction of protons with other protons in cosmic accelerators [2]. They carry information about the origin and spectrum of cosmic rays. Neutrinos are neither deflected in magnetic fields nor they interact on the way to Earth. This makes them excellent candidates for finding point sources in the sky as their trajectories point to their origin. Neutrino s unique properties, especially very low interaction probability, lead to the conclusion that an instrumental volume of at least a cubic kilometre is needed to observe neutrinos from high-energy astrophysical sources [3]. Several experiments nowadays use optical Cherenkov radiation in water or ice to detect them. In this seminar I will present the IceCube neutrino observatory in the South Pole, that has been successfully collecting data since early

3 Figure 1: The particles and interaction carriers of the Standard Model of particle physics. [4] 2 Neutrinos Neutrinos ν are elementary particles, a part of the Standard Model of particle physics. As fermions with spin 1/2 they respect the Pauli exclusion principle and have their corresponding antiparticles. Neutrinos belong to leptons which do not carry colour charge in contrary to quarks. Because they also do not carry electric charge, they can only be influenced by the weak nuclear force. There are three generations of lepton pairs - electron e, electron neutrino ν e, muon µ, muon neutrino ν µ, tau τ and tau neutrino ν τ. Each generation has greater mass than the particles of lower generations. Electrons and all three neutrinos also do not decay. The generations are graphically represented in figure 1. At first they were assumed to be massless, but now we know that neutrinos have non-zero mass, although it is very small. It is measured to be lower than m ν < 2 ev/c 2, compared to electron mass m e = 0.51 MeV/c Interaction with matter Unlike other particles, neutrinos can travel across vast distances with almost no interaction with matter. They are not deflected by electromagnetic fields and thus behave similarly as do gamma rays in vacuum. Neutrinos can interact via the neutral current or charged current weak interactions, depending if the exchanged particle is neutral Z or charged W boson. The first is a scattering of a neutrino on a charged lepton or quark where incident and scattered particles are the same, for example ν e + e ν e + e. The charged current weak interaction has a W ± boson as propagator. As a well known example we can look at the inverse beta decay. It is a scattering of an electron 3

4 Figure 2: The two types of neutrino interaction with a nucleon. l represents the same generation of leptons. [5] antineutrino with a proton where a neutron and a positron are produced. A variation with an electron neutrino and a neutron is also possible. ν e + n p + e Feynman diagrams for both interaction types are presented in figure 2. At high energies as it is in our case, neutrinos scatter on individual quarks in a hadron and we talk about deep inelastic scattering. A quark can not exist as a free particle due to the properties of the strong nuclear interaction, where the force increases with distance. It is energetically more stable for quarks to hadronise and we observe so-called hadronic showers. Since these reactions are mediated by the weak interaction, their cross sections are very low. The probability for a 1 GeV neutrino to interact in 100 m of water amounts to only Sources of high-energy neutrinos We can split sources of high-energy neutrinos either by the way of observation or by the type of neutrino production. The IceCube experiment mainly searches for point sources. This helps us investigate different types of predicted cosmic ray sources. On the other hand we can also investigate a diffuse neutrino flux. It is created by the decay of pions, produced in cosmic-ray interactions within the galactic disk [2]. The neutrino production can be separated between acceleration and annihilation processes. Observations of high-energy photon sources indicate possible cosmic accelerators. High energy particles can be produced by either electron acceleration due to synchrotron radiation and inverse Compton scattering or by acceleration of protons in some astrophysical sources. The latter can interact with a target and produce pions and kaons. p + X π 0, π ±, K ± + Y. The neutral pions then decay into photons, and charged pions into neutrinos and leptons, which gives us the connection between cosmic γ-rays and neutrinos. The possible sources 4

5 are young supernova remnants, binary systems like an expanded star together with a black hole, and interactions of protons with the interstellar medium like molecular clouds. Two examples will be presented more in detail further in the seminar. Neutrinos can also be produced in annihilation or decay of heavy particles. The main three representative sources are evaporating black holes, topological defects and annihilation or decay of super-heavy particles. 3.1 Active galactic nuclei Active galactic nuclei (AGN) are the brightest sources in the universe. The beams accelerated near the central black hole are dumped on the surrounding matter. Typically two jets of high speed particles emerge in opposite directions, perpendicular to the disc of the AGN (figure 3a). Particles are accelerated by Fermi shocks. In this process they are repeatedly reflected by a magnetic mirror. A simplified illustration of the process in shown in figure 3b. The accelerated particles travel along the jets with a Lorentz factor of at least γ 10. Cosmic Accelerators Supernova Remnants (SNR) Fermi acceleration at shock front (a) (b) 1 % of the energy of all SN explosions Figure 3: (a) An illustration of an active galactic can nucleus explain (AGN). energy [6] density (b) Fermi of acceleration. [7] cosmic rays in galaxy (~ 0.5 MeV/m 3 ) However: No SNR has been 3.2 Crab Nebula Gamma (explosion ray 1054) bursts clearly pinned down as source Gamma ray bursts (GRB) are one of the best suited sources of high-energy neutrinos [2]. In a single gamma ray burst, a fraction of a solar mass of energy is released into photons with very high energy. The phenomenon can be split into three parts. First, there must be a source of energy such as a supernova, hypernova, collapsing to or merging of neutron stars. The second part is the relativistic expansion of the fireball. It has a radius of km and releases an energy of J. The observed gamma rays are the result of the third part, a relativistic shock with γ which expands the original fireball by a factor of 10 6 over 1 s. The photons are emitted in a narrow angle in space. Coll. Ljubljana, H.Kolanoski - IceCube Neutrino Observatory 19 5

6 The energetic photons will interact with nucleons if on their way through the sphere. An example of interaction that yields neutrinos is p + γ + n + π + π + µ + + ν µ µ + e + + ν e + ν µ Observation of GRB-neutrino coincidence may indicate that GRBs produce high-energy cosmic neutrinos. 4 Cherenkov radiation Cherenkov radiation is electromagnetic radiation emitted by charged particles if their velocity v = βc exceeds the velocity of light in the transparent medium with refractive index n. Classically we can describe this phenomenon as the consequence of asymmetric polarization of the medium in front and behind the charged particle. This represents an electric dipole moment varying with time which emits radiation. [8] The Cherenkov wave front can be constructed by the superposition of spherical waves produced by the particle along its trajectory. This is called the Huygens Fresnel principle. In time t the particle travels a distance tβc, and the wave a distance tc/n, which is shorter. This is presented in figure 4, which helps us derive the direction of propagation of emitted photons, cos θ C = 1 βn, (1) where θ C is the angle of the Cherenkov radiation emitted relative to the particle trajectory. This gives a lower limit to β as β > 1/n. Also the threshold velocity for Cherenkov Figure 4: Geometrical derivation of the Cherenkov radiation propagation direction. The red arrow represents the charged particle trajectory, and blue arrows represent the propagation direction of photons, perpendicular to the wave front. [9] 6

7 emission is defined as v s = c/n with corresponding relativistic γ factor γ s = 1 1 1/n 2. (2) A large volume of clear material, e.g. water or ice, is commonly used by big detector experiments for the detection of neutrinos. Water-filled detectors of this type recorded the neutrino burst from supernova 1987a. The largest such detector is the Super-Kamiokande. The Sudbury Neutrino Observatory (SNO) uses heavy water. Besides the interactions in a regular water detector, neutrinos can also beak up the deuterium in the heavy water. The resulting free neutron releases a burst of gamma rays when captured [1]. The IceCube project takes advantage of the Cherenkov radiation on a much larger scale by using ice instead of water. 5 IceCube IceCube is a neutrino observatory located at the South Pole built upon its considerably smaller predecessor AMANDA. It is able to detect neutrinos over a wide energy range from about 100 GeV to more than 10 9 GeV [10]. The main goal is the search for highenergy astrophysical neutrino point sources, which are believed to be connected with cosmic ray production sites. The detector has to be as large as possible due to extremely small cross-sections for neutrino interactions, as well as the expected low fluxes. The base of the experiment is positioned from 1450 m to 2450 m below the surface of ice, and is shown on figure 5a. The entire instrumented volume is 1 km 3. IceCube consists of 86 strings, horizontally positioned in a triangular grid over a square kilometre. The distance between 80 of them is 125 m (figure 5b). Each detector string contains 5 60 digital optical modules placed 17 m apart. A subset of the detector called DeepCore 510 J. Ahrens et al. / Astroparticle Physics 20 (2004) Old Pole Station line AMANDA-II SPASE-2 South Pole Grid north 125 m Runway Dome FIG. 2: IceCube Neutrino Observatory and its component arrays. B. Muon Neutrino Detection Muon neutrinos undergoing CC interactions in the ice produce muons. The muons on average carry about 75% of the initial neutrino energy [22]. Simulation studies indicate that muon angular resolution is typically between 0.5 and 1, depending on the angle of incidence and the muon energy. The energy loss per meter, for a muon propagating through the ice, is related to its energy [6]: de = α(e)+ β(e)e, (1) dx where E is the muon energy, α 0.24 GeV/m is the ionization energy loss per unit propagation length, and β m 1 is the radiative energy loss through bremsstrahlung, pair production, and photonuclear scattering, (α and β are both weak functions of energy). For muon energies less than about a TeV, energy loss is dominated by ionization, and the light produced is nearly independent of energy. However, for higher energy muons, there are many stochastic interactions along the muon s (a) Fig. 1. Schematic view of the planned arrangement of strings of the IceCube detector at the South Pole station. The existing AMANDA-II detector will be embedded in the new telescope, and the SPASE-2 air-shower array will lie within its horizontal boundaries. Figure 5: (a) IceCube Neutrino Observatory strings in the Antarctic ice. [10] (b) The triangular grid of detector strings at the South Pole station. [11] ergy loss per meter and the muon UVenergy. to bluemost varies ofbetween the 50 and 150 m, dependingmuon s depth. path Light comes scattering, on the other hand, Cherenkov light emitted along the from the secondary particles produced will result in radiative in strong losses. dispersion of the Cherenkov An estimation of de/dx, basedsignal ontheover amount large ofdistances, detected light, the event geometry, information and the ice properties, carried by the photons. This scatter- diluting the timing was used in the energy spectrum ing unfolding effect increases discussed with in the average distance at Sect. V. The energy of individual events was not estimated. Rather, the distribution of neutrino energies was which photons are collected, but is somewhat compensated directly inferred from the distribution of reconstructed for by the information contained in the muon de/dx values. time structure of the recorded PMT pulse, since, e.g., its length 7 is a measure for the distance to the point of light emission. As a significant improvement over the The detection rate for high energy AMANDA νµ ( νµ) istechnology, aided by each IceCube OM will the fact that the CC interaction cross housesection, electronics as well toas digitize the PMT pulses, so the range of the resultant muon, are thatproportional the full waveform to theinformation is retained [17]. neutrino energy. High energy muons have a significant The waveforms will be recorded at a frequency of path-length and can reach the detector even if produced about 300 mega-samples per second, leading to an outside of the detector, hence increasing the effective volume. Muons in earth or ice can have intrinsic a track timing length accuracy from for a single pulse measurement kilometers, of 7depend- ns. The digitized signals will be several tens of meters, up to several ing on the muon energy and the detection transmitted threshold. to the data The acquisition system, located (b) with its nearest neighbors by means of a dedicated copper-wire pair. This enables the implementation of a local hardware trigger in the ice, such that digitization occurs only when some coincidence requirement has been met [16]. This is particularly important in order to suppress the transmission of pure noise pulses, which, unlike photon pulses from high-energy particles, are primarily isolated, i.e., occur without correlation to pulses recorded in neighboring and nearby OMs. (The dark noise rate of an OM will be as low as Hz, due to the sterile and low-temperature environment.) Local triggers will be combined by surface processors to form a global trigger. Triggered events will be filtered and reconstructed on-line, and the relevant information will be transmitted via satellite to research institutions in the northern hemisphere. The complete detector will be operational perhaps as soon as five years after the start of construction, but during the construction phase all

8 consists of 6 strings specialised for lower energies, which are placed closer together around the centre of the experiment. The location and type of experiment was chosen due to low light absorption of the deep Antarctic ice. The absorption length from UV to blue varies between 50 and 150 m, depending on the depth. Atmospheric background coming from the ice surface above the detector array is suppressed by a deep layer of ice to have representative signals from cosmic neutrinos coming through the earth i.e. from below. The main IceCube detector is complemented with the IceTop tanks. Two tanks with two digital optical modules each are placed on top of each IceCube string. They are used for detection of particle showers in Earth s atmosphere, a consequence of high-energy cosmic rays. It is used as a standalone detector, but also helps IceCube with signalbackground separation. 5.1 The digital optical module IceCube takes advantage of the Cherenkov light produced by charged particles, the products of neutrinos interaction with ice. The photons coming from below need to be detected and processed into an useful digital signal. This is the purpose of the digital optical module. The digital optical module (DOM) is the main detector element of IceCube. It consists of a photomultiplier tube (PMT) with 25 cm diameter and a suite of electronic boards. All is contained in a glass pressure housing with 35.6 cm diameter shown on figure 6 [3]. DOMs have been designed to operate reliably for 15 years in a cold, high-pressure environment. Photomultiplier tubes are one of the most common instruments used in particle detectors. Visible light liberates electrons by the photoelectric effect from a thin photocathode layer in the internal surface of an evacuated glass quartz tube. The photocathodes are usually semiconducting alloys containing one or more metals from the alkali group e.g. Na, K and Cs. Various geometrical arrangements of electrodes are used to collect, focus and accelerate the photoelectrons onto the first dynode. It is an electrode made from 158 A. Achterberg et al. / Astroparticle Physics 26 (2006) Fig. 1. A schematic view of an IceCube digital optical module. Figure 6: The IceCube digital optical module schematic view. [3] includes power distribution, bidirectional data transmission, and timing calibration signals. Most of the electronics reside on the main board (MB), which holds the analog front-end and two digitizer systems. The fast digitizer system uses a 128-sample switched-capacitor array, implemented in a custom analog transient waveform digitizer (ATWD) chip, which can run between 200 and 700 mega-samples per second (MSPS). The ATWD sampling frequency is controlled with a digital-to-analog 8 on a separate, dedicated circuit board. The delayed signal is split among three (of 4) input channels of each one of the ATWDs with gains differing by successive factors of 8. In this manner the digitizers cover the entire dynamic range of the PMT, which is linear up to currents of 400 PE/15 ns. The fourth ATWD channel is used for calibration and monitoring. The ATWD sampling speed is variable and is currently set at 3.3 ns/sample, allowing acquisition of 422 ns long

9 material with a high coefficient of secondary electron emission. Electrons get multiplied over a series of dynodes to amplify the original signal. For a series of 14 dynodes with potential differences from 150 to 200 V between stages, the multiplication of the number of electrons reaches 10 8 [8]. Besides the PMT, the digital optical module also contains high voltage generator for the PMT, the main electronics board, which acquires and digitises the signal, and also the flasher-board, an electronic chip controlling light-emitting diodes. Its 12 LEDs pointing radially outward from the DOM are used for calibration of local coincidence, timing, geometry and surrounding DOMs, but also for verification of optical properties of ice. Data acquisition is initiated when the PMT signal exceeds a programmable threshold. Typically this is 0.3 photoelectrons for DOMs in the ice. A local timestamp is also given at the trigger time by a 40 MHz clock. The analogue signal from the PMT is then digitised with a sampling speed of 3.3 ns/sample. The electronics allow the acquisition of 422 ns long waveforms. The DOMs operate in one of the coincidence modes to reduce false trigger related data traffic to the surface. One possibility is to check the validity of the trigger with the nearest neighbours, above and below. Data is transmitted only if a DOM receives a local coincidence pulse within 800 ns of the trigger. 5.2 Detection of neutrinos IceCube detects neutrinos indirectly by the products of their interaction with ice. The produced charged particles emit Cherenkov radiation (figure 7a) which is then detected by the digital optical modules, which is oriented Neutrino with induced the photomultiplier muon tracks. downwards. The reason for it is the use of the Earth as a filter, and to reduce the atmospheric background from above. Three different neutrino signals can be observed in the detectors, depending on the cos θ c = (βn) -1 Detection of a Neutrino θ c (β (a) H.Kolanoski - 'Origin of Cosmic Rays' - II: Neutrinos 20 θ c Figure 7: (a) Detection of neutrinos by Cherenkov radiation made by a muon produced in neutrino interaction. [7] (b) An exampleljubljana, muon March track 2015 reconstruction, H.Kolanoski where - 'Origin of colours Cosmic Rays' - II: Neutrinos indicate amount of deposited energy. [7] (b) Only C Angular Energy m only de mi sign ener the d Effective instrume 9

10 neutrino flavour and type of interaction. The simplest signal is a cascade, either electromagnetic or hadronic. The latter is a result of neutral current interaction of all neutrino flavours. An electromagnetic cascade can only be a result of the inverse beta decay of electron neutrino or antineutrino. The produced electron or positron has enough energy to cause an EM cascade and then stops after a short distance. The other two signal types also consist of a single track. Muon or tau neutrino interact via the inverse beta decay and produce a muon or a tau lepton. Muon interaction with matter is much weaker than electron one, so muon continues on its path through the detector and leaves behind a track. Tau leptons have very short lifetime so they leave only a short track. If they decay into an electron or a hadron, they produce cascades, or they decay into a muon which then leaves behind a track. An example of a muon track is displayed in figure 7b. To improve the background identification, only muon tracks that start in the detector are considered. 5.3 Results The main search of the IceCube experiment are point sources of cosmic rays. During the analysis astrophysical neutrino signals need to be separated from background of atmospheric neutrinos. In the map of the sky we search for an excess of events within an angular interval of 2-3 [7]. No significant signals were found in 4 years of data. The main problem is low statistics. We can describe it on an example of average background of 3 and a signal of 7 events. If we look into each search window as an independent result, a probability that 7 events is background is only 3.3 %. When we increase the number of search windows, the probability rises. For about 30 windows, it rises to 60 %. One of the recently published analysis [12] focused on neutrino energies above 60 TeV. In a 988-day sample there were 37 events observed consistent with expected background of 8.4 ± 4.2 cosmic ray muon events and atmospheric neutrinos. Collected events were then compared with predictions that astrophysical neutrino flux is proportional to Figure 8: Deposited energies of observed events with predictions in three years of data.[12] FIG. 2. Deposited energies of observed events with predictions. The hashed region shows uncertainties on the sum of all backgrounds. Muons (red) are computed from simulation to overcome statistical limitations in our background measurement and scaled to match the total measured background rate. Atmospheric neutrinos and 10 uncertainties thereon are derived from previous measurements of both the /K and charm components of the atmospheric µ spectrum [9]. A gap larger than the one between 400 and 1000 TeV appears in 43% of realizations of the best-fit continuous spectrum. FIG. 3. Arrival angles of events w in our fit and above the majority of ground. The increasing opacity o neutrinos is visible at the right o spheric neutrinos by muons from t presses the atmospheric neutrino b data are described well by the ex hard astrophysical isotropic neutr ors as in Fig. 2. Variations of th thresholds are in the online supple for maximal robustness. The

11 E 2 (figure 8). The result has a significance of 5.7σ which confirms first observation of astrophysical flux of high energy neutrinos. The three neutrinos with highest energy detected until now have energies of 2.00 PeV, 1.14 PeV and 1.04 PeV. 5.4 Future Current search for point sources of cosmic rays has only resulted in upper limits on the flux of individual galactic and extragalactic source candidates. This may suggest that the observed cosmic neutrinos originate from relatively weak sources [13]. The increase of sensitivity of a neutrino observatory would improve the measurements of their fluxes. To improve existing searches and introduce new which had not been anticipated in the initial design, physicists are already planning the IceCube-Gen2, a large upgrade of the IceCube experiment. Work is now actively under way with the goal to instrument a volume of 10 km 3 and deliver a substantial increase in sensitivity to astrophysical neutrinos of all flavours. 6 Conclusion Astrophysical neutrinos are important cosmic ray messengers. With the help of their unique properties we try to confirm our predictions about cosmic ray sources. The main purpose of a large volume neutrino observatory IceCube is their detection. A flux of high-energy neutrinos is now confirmed but there are still many questions to answer about its sources. Up until now there were no point sources confirmed as statistics are too low. Statistical significance can be improved by using pre-defined source positions from gamma-ray, optical, and X-ray telescopes. A reverse option is also promising, to point the telescopes in the direction of individual neutrinos with highest energies. A lot of further observations are needed. With the updated IceCube detector we will hopefully detect much more neutrinos and look for their sources outside of our galaxy. References [1] All about neutrinos. [ ]. [2] Kai Zuber. Neutrino physics. CRC Press, [3] Abraham Achterberg, M Ackermann, J Adams, J Ahrens, K Andeen, DW Atlee, J Baccus, JN Bahcall, X Bai, B Baret, et al. First year performance of the icecube neutrino telescope. Astroparticle Physics, 26(3): , [4] Standard model. [ ]. [5] Neutrinos. [ ]. [6] Galaxies. Chapter24.html [ ]. 11

12 [7] Herman Kolanoski. Origin of cosmic rays. Lecture, [8] Konrad Kleinknecht. Detectors for particle radiation. Cambridge University Press, [9] Cherenkov radiation. [ ]. [10] R Abbasi, Yasser Abdou, T Abu-Zayyad, J Adams, JA Aguilar, M Ahlers, K Andeen, J Auffenberg, X Bai, M Baker, et al. Measurement of the atmospheric neutrino energy spectrum from 100 gev to 400 tev with icecube. Physical Review D, 83(1):012001, [11] J Ahrens, John N Bahcall, X Bai, RC Bay, T Becka, K-H Becker, D Berley, E Bernardini, D Bertrand, DZ Besson, et al. Sensitivity of the icecube detector to astrophysical sources of high energy muon neutrinos. Astroparticle physics, 20(5): , [12] MG Aartsen, M Ackermann, J Adams, JA Aguilar, M Ahlers, M Ahrens, D Altmann, T Anderson, C Arguelles, TC Arlen, et al. Observation of high-energy astrophysical neutrinos in three years of icecube data. Physical review letters, 113(10):101101, [13] MG Aartsen, M Ackermann, J Adams, JA Aguilar, M Ahlers, M Ahrens, D Altmann, T Anderson, G Anton, C Arguelles, et al. Icecube-gen2: A vision for the future of neutrino astronomy in antarctica. arxiv preprint arxiv: ,