Ryan Stillwell Paper: /10/2014. Neutrino Astronomy. A hidden universe. Prepared by: Ryan Stillwell. Tutor: Patrick Bowman

Transcription

1 Neutrino Astronomy A hidden universe Prepared by: Ryan Stillwell Tutor: Patrick Bowman Paper: Date: 10 October 2014 i

2 Table of Contents 1. Introduction pg Background pg 1 2. Findings & Discussion pg What is a neutrino? pg Definition pg Discovery pg Detection pg Neutrino astronomy pg Scientific interest pg Supernovae pg The solar neutrino problem pg 4 3. Conclusion pg 4 4. Bibliography pg 5 5. Glossary pg 5 ii

3 List of Figures Figure 1: Diagram showing proton proton chain reaction pg 1 Figure 2: Remnant of SN 1987A seen in light overlays of different spectra pg 3 iii

4 1. Introduction 1.1 Background Neutrinos are one of the fundamental particles which make up the universe, yet they are one of the least understood. This report investigates their discovery and attempts to understand their importance and their role in astrophysics and astronomy. 2. Findings & Discussion 2.1 What is a neutrino? Definition Neutrinos can essentially be thought of as one of the building blocks of matter. They are electrically neutral elementary sub atomic particles which have almost no mass, and are created by radioactive decay, nuclear reactions, or when high energy cosmic rays collide with atoms in our atmosphere. They are also abundant in the universe. Currently there is only a lower limit available in regards to the total mass, but even so it is estimated to be equivalent to the total mass of all the visible stars in the universe. In particle physics neutrinos are categorised as leptons, a class of fermions. There are three variants electron neutrinos, muon neutrinos and tau neutrino each of which correspond to a charged particle (i.e. electron, muon, tau) which are also leptons and are also produced at the same time, as illustrated in the example below: Figure 1. Diagram showing proton proton chain reaction (one of several fusion reactions by which stars convert hydrogen to helium) 1

5 2.1.2 Discovery The existence of neutrinos was revealed through the study of beta decay. This is where an atomic nucleus emits an electron and the nucleus changes from one element to another. The problem was that the energy of the original nucleus did not match the combined energy of the resultant nucleus and beta particle, which is in direct contradiction to the most dearly held principal of physics that is, energy cannot be created or destroyed, only transformed. In 1931, Wolfgang Pauli (a theoretical physicist and one of the pioneers of quantum physics) postulated the existence of an undetected particle to account for missing momentum and missing energy from beta decay. This was backed up by Enrico Fermi (widely regarded as one of the very few physicists to excel both theoretically and experimentally) who, in 1934, developed a theory of beta decay which included the neutrino. However, it wasn't until 1956, 25 years after it was first hypothesised, that the first successful experiment to detect neutrinos was made by Clyde L. Cowan and Frederick Reines a result rewarded 39 years later with the 1995 Nobel prize Detection Detecting neutrinos has often been described as trying to catch a bullet with a butterfly net. Almost all we know about the universe derives from the observation of photons (light), but since neutrinos do not interact with electromagnetic forces they produce no light and we are unable to observe them directly. Also, they do no interact with strong nuclear forces, and although they do have a tiny amount of mass and still interact with gravitational forces, at a subatomic level the amount of interaction is so weak that for all practical purposes neutrinos are considered to interact only with the weak nuclear force, so we must use indirect techniques to infer our understanding. The initial experiment in 1956 was done by essentially switching a nuclear reactor off and on to produce neutrino fluxes in the order of neutrinos per cm 2 per second. A small amount of these neutrinos would then interact with protons in a tank of water, creating neutrons and positrons. When a positron interacts with an electron (it's anti particle) it is annihilated and produces gamma rays, and these gamma rays are detected by placing a scintillator material into the tank of water which gives off flashes of light in response. In 1968, Raymond Davis Jr. and John N. Bahcall conducted an experiment to test the idea that nuclear fusion reactions are the ultimate source of solar radiation, and prove the current understanding of nuclear processes in the interior of the sun was correct. Raymond Davis was a chemist by trade and he knew that neutrinos could interact with chlorine to produce argon, but he would need a huge number of atoms to make it work. Using a vat about the size of an Olympic swimming pool, they predicted for every week of operation they would be able to produce 10 atoms of argon (compared to the 9x atoms of chlorine in the tank). In fact, what they observed was only 3 atoms of argon, and many follow on experiments produced the same result, leading into the issue of solar neutrino deficit (more on this later). 2

6 Today, there are several observatories in operation (such as the Antarctic IceCube Neutrino Observatory and Kamiokande located under Mount Kamioka in Japan) that have been set up using arrays of optical modules to detect Cherenkov radiation. This is produced when light is passed through a medium, such as a large tank of pure water or ice, slowing it down. Neutrinos are then detected when they collide with electrons in the water, giving the electrons more energy and causing them to move faster than the light does. When this happens the electron gives off a weak glow, known as Cherenkov radiation, which allows us to detect the neutrino. 2.2 Neutrino astronomy Scientific interest Although they are tiny, Neutrinos are plentiful in the universe. They also travel close to the speed of light and are unaffected by magnetic fields, which makes them prime candidates for astronomical observation. There are many outstanding mysteries in astrophysics may be hidden from our sight at all wavelengths of the electromagnetic spectrum due to absorption by matter and radiation between us and the source. The unique advantage that being able to measure and understand neutrinos provides is that they are essentially unabsorbed as they travel cosmological distances between their origin and us Supernovae In February 1987, two detectors located in the US (IMB) and Japan (Kamiokande) recorded a total of 19 neutrino interactions over a span of 13 seconds. This burst of neutrinos alerted astronomers to a supernova event occurring in the large Magellanic clouds 168,000 light years away. Neutrino emissions occur simultaneously with core collapse, but precede emission of visible light as they are unabsorbed by matter. This acted as an early warning system and, 2 hours later, allowed astronomers to fully observe the first supernova visible to the unaided eye (pictured below) since the days of Kepler, in Figure 2. Remnant of SN 1987A seen in light overlays of different spectra. ALMA data (radio, in red) shows newly formed dust in the center of the remnant. Hubble (visible, in green) and Chandra (X ray, in blue) data show the expanding shock wave. 3

7 This event was the first time neutrinos known to be emitted from a supernova had been observed directly and is considered to be the birth of neutrino astronomy. It lead onto many deductions about the nature of neutrinos, such as limits on mass, charge, gravitational attraction, magnetic moment, and many publications were produced as a result. Neutrino detection of astronomical events still continues today The solar neutrino problem The experiment performed in 1968 had a surprising result. Based on calculations from the standard solar model, it appeared as though only about a third of the expected solar neutrinos were reaching the Earth, leaving a huge deficit of energy. As neutrino detectors became sensitive enough to measure the flow of neutrinos from the Sun, it became clear that the number detected was lower than that predicted by models of the solar interior. In various experiments, the number of detected neutrinos was between one third and one half of the predicted number. This came to be known as the solar neutrino problem. The accepted Standard Model of particle physics at the time showed neutrinos were massless and that they were able to travel at the speed of light. This would mean the type of neutrino was fixed when it was produced (i.e. the Sun should emit only electron neutrinos which are produced by H H fusion). This was the source of the solar neutrino problem and it wasn't until 30 years later that this was found to be incorrect. Neutrino oscillation was theorised by Bruno Pontecorvo (a nuclear physicist and one time assistant to Enrico Fermi) in 1957, but the first compelling evidence for it didn't come until 1998 after the discovery (at Kamiokande) of a non zero neutrino mass. This allowed for a neutrino created with a specific lepton flavour (electron, muon or tau) to be later measured to have a different flavour. Conclusive evidence for this came in 2001 from the Sudbury Neutrino Observatory in Canada, who were able to detect and distinguish between all types of neutrinos coming from the sun and found the total number of neutrinos to be consistent with earlier predictions based on the standard Solar model. 3. Conclusion Neutrinos are as elusive and mysterious as they are abundant. In the last century, our model of particle physics has changed dramatically through experimentation and understanding, largely due to the discovery of neutrinos. By detecting neutrinos we have been able to monitor astrological events with greater depth and gain insight into parts of the universe which were unknown to us before. These ghost like particles have revealed a hidden universe for us to study and although we are in its infancy, the era of neutrino astronomy has begun. (1552 words) 4

8 4. Bibliography Glossary beta particle cosmic rays fermions leptons positrons high energy, high speed electrons or positrons emitted by certain types of radioactive nuclei immensely high energy radiation, mainly originating outside the Solar System a subatomic particle, such as a nucleon, which has half integral spin and follows the statistical description given by Fermi and Dirac a subatomic particle, such as an electron, muon, or neutrino, which does not take part in the strong interaction a subatomic particle with the same mass as an electron and a numerically equal but positive charge 5