日期:2022/09/18   IAE

來自宇宙的高能中微子 / High energy neutrinos from the cosmos

by Per Olof Hulth*

* Per Olof Hulth 出生於 1943 年。他是斯德哥爾摩大學的實驗天體粒子物理學教授。 他獲得了博士學位。 在斯德哥爾摩大學獲得基本粒子物理學博士學位，論文關於質子-質子相互作用中的奇怪粒子產生。 1976 年，他以研究員身份加入歐洲核子研究中心，開始研究中微子物理學，與 BEBC 氣泡室一起工作。

他參與了歐洲核子研究中心 LEP 對撞機的 DELPHI 實驗，以尋找希格斯粒子。 在歐洲核子研究中心之後，他轉向實驗天體粒子物理學，從宇宙中尋找中微子，並成為南極 AMANDA 中微子實驗的瑞典團隊負責人。 他是南極新的大型中微子望遠鏡項目 IceCube（2001-2005）的第一位發言人。 自 2004 年 4 月起，他成為瑞典皇家科學院院士。

數千年來，人類通過仰望迷人的夜空，在無數恆星和其他現象發出的可見光的引導下，研究了宇宙。在上個世紀，科學家們使用肉眼無法看到的不同波長的光，如無線電波、紅外光、X 射線和伽馬射線，發現了新的夜空圖像。每次打開天空中的新窗戶，都會發現新的意想不到的現象，如大爆炸的微波背景、中子星、活動星系核 (AGN)、黑洞、伽馬射線暴 (GRB) 和其他令人興奮的物體。今天，科學家們開始打開一個全新的窗口，使用另一種基本粒子中微子，而不是光子，光子是用於研究宇宙的光的基本粒子。這個稱為中微子天文學的新領域有望揭示新的未知現象並幫助我們回答我們今天遇到的幾個問題。

中微子
中微子是一種基本粒子，由 1945 年諾貝爾物理學獎獲得者沃爾夫岡泡利在 1930 年提出，以解決核物理學中的能源危機。科學家們很難在放射性衰變中找到能量，泡利認為存在一種粒子，他認為這種粒子正在帶走丟失的能量。但在中微子被發現之前花了幾年時間。 1965 年，Clyde Cowan 和 Frederick Reines 首次發現並確定了這種粒子。由於他的貢獻，Reines 獲得了 1995 年的諾貝爾物理學獎。

中微子是一種不起眼的粒子，不帶電荷，僅通過弱核力與物質相互作用。近年來，人們發現中微子的質量很小，推翻了早先的假設，即它是無質量的。在太陽中，當四個氫原子轉變為一個氦原子時，會在聚變過程中產生大量的中微子。儘管中微子數量眾多，但平均只有其中一個會在一生中與人的身體相互作用。地球表面來自太陽的中微子通量為每平方厘米每秒 6×1010 個中微子。太陽中聚變過程產生的中微子在被物質吸收之前可以穿過數光年的固體鉛。然而，中微子與物質相互作用的可能性隨著中微子的能量而增加。

已觀察到三種不同類型的中微子：
電子中微子電子中微子
介子中微子 介子中微子
和 tau 中微子中微子

這些中微子與三種帶電粒子有關，即電子、μ子和τ。所有六個粒子都稱為輕子。當中微子與物質相互作用時，它既可以在相互作用後繼續作為中微子（“中性電流相互作用”），也可以產生相應的帶電粒子（“充電電流相互作用”）。電子中微子產生一個電子，μ子中微子產生μ子，τ中微子產生τ輕子。

從宇宙中尋找高能中微子的工作剛剛開始，在不久的將來，我們有望了解關於我們迷人宇宙的新有趣事實。

Mankind has studied the universe for thousands of years by looking at the fascinating night sky, guided by the visible light emitted from myriads of stars and other phenomena. During the last century new pictures of the night sky have been discovered by scientists using different wavelengths of light which the naked eye cannot see, such as radio waves, infrared light, x-rays and gamma rays. Each time new windows in the sky are opened, new unexpected phenomena have been discovered, like the microwave background from Big Bang, neutron stars, active galactic nuclei (AGN), black holes, gamma ray bursts (GRB) and other exciting objects. Today, scientists are starting to open up a completely new window by using another elementary particle, the neutrino, instead of the photon, which is the elementary particle of light used to investigate the universe. This new field called neutrino astronomy, will hopefully reveal new unknown phenomena and help us answer several of the questions we have today.

Neutrinos
The neutrino is an elementary particle, which was postulated in 1930 by Wolfgang Pauli, 1945 Nobel Laureate in Physics, in order to solve an energy crisis in nuclear physics. Scientists had difficulty finding energy in radioactive decays and Pauli suggested the existence of a particle which he believed was carrying away the missing energy. But it took some years before the neutrino was discovered. It was Clyde Cowan and Frederick Reines who first detected and identified this particle in 1965. For his contribution, Reines was awarded the 1995 Nobel Prize in Physics.

The neutrino is an obscure particle with no electric charge and which only interacts with matter via the weak nuclear force. In recent years it has been discovered that neutrinos have a small mass, debunking the earlier assumption that it was massless. In the sun, an enormous number of neutrinos are produced in the fusion process when four hydrogen atoms transform into one helium atom. Despite the large number of neutrinos, an average of only about one of these will interact with a person’s body during a lifetime. The flux of neutrinos from the sun at the surface of the earth is 6×1010 neutrinos per square centimeter and second. The neutrinos from the fusion process in the sun can pass through several light years of solid lead before being absorbed by matter. The probability for a neutrino to interact with matter increases, however, with the energy of the neutrino.

Three different kinds of neutrinos have been observed:
the electron neutrinoelectron neutrino
the muon neutrinomuon neutrino
and the tau neutrinoelectron neutrino

These neutrinos are related with three electrically charged particles, the electron, the muon and the tau. All six particles are called leptons. When a neutrino interacts with matter, it can either continue as a neutrino after the interaction (“neutral current interaction”) or create the corresponding charged particle (“charge current interaction”). The electron neutrino creates an electron, the muon neutrino a muon, and the tau neutrino a tau lepton.

Summary
The hunt for high energy neutrinos from the cosmos has just started and, in the near future, we will hopefully learn new interesting facts about our fascinating universe.