INSTRUCTIONS: This page explains how to carry out the activities. Whenever you need to perform a task, it will be indicated in blue. Whenever you need to answer a question, it will be indicated in red. It is recommended that students work in groups of two if possible.
Whenever a charged particle goes through water or ice, it produces light known as Cherenkov radiation. As was shown during the introductory presentation, IceCube is able to detect high energetic neutrinos since it observes the light emitted by the lepton created when a neutrino hits the ice. This light is captured by a sort of camera called a photomultiplier and, subsequently, a digital image that physicists call an “event” is recorded. The real challenge is to distinguish the light that is produced due to a neutrino hitting the ice (what we will call a “signal”), against the light emitted by any other electromagnetically charged particle that was not produced from a neutrino (what we will call “background”).
IceCube detects around 75 neutrinos per day, which are mainly produced when cosmic rays hit the gases that make up the atmosphere. After this collision, new particles are created which ultimately can decay into neutrinos. The Sun is another important source of neutrinos, however, of all the neutrinos detected by IceCube, only one per month arrives from beyond our solar system (a.k.a. an extraterrestrial neutrino) and has a higher energy than those produced in the Sun.
The amount of light captured by the photomultipliers and, consequently, the image recorded at the detector depend on the energy of the particle that goes through the detector. In this way, the larger the energy of the neutrino, the larger the energy of the particle produced and the more light is released in the detector.
TASK 1: Click here and try to identify which columns within the pair of images correspond to extraterrestrial neutrinos (“signals”) or to any other particles (“background”). When you choose your answer, click on “Reveal signal” to find out whether your guess was correct or not!
Are the images very different?
How can we identify extraterrestrial neutrinos?
The first step is to distinguish the images produced by neutrinos from images due to other charged particles detected at IceCube. We, therefore, need to make sure that the light is created inside the detector, rather than outside. This means that the neutrino has entered the detector, has hit the ice and thus created the charged lepton that produces the light.
Furthermore, as we already know, extraterrestrial neutrinos have much larger energy than the neutrinos produced in the atmosphere. Consequently, in order to make sure we have detected an extraterrestrial neutrino, we need to satisfy the following two conditions:
1) The light originates within the detector (“passes veto”).
2) The neutrino deposits enough energy in the detector (“passes the charge cut”).
TASK 2: Classify the 24 different events you can find here. In order to “pass veto”, light has to be created inside the detector. In order to “pass the charge cut”, the total charge recorded at the detector must be larger than 6000 pe (pe is just a type of unit that measures the amount of charge deposited in the detector). When you finish this task, write down the number of events guessed correctly in the second question of the questionnaire. Note that the colour of the light represents time (red is earlier times and blue is later times), while the size of the image is related to the amount of energy left in the detector (the bigger the circle, the larger the energy).
This very same exercise was done by physicists working at the IceCube experiment. After classifying the different events, they selected 37 high energetic neutrinos from the few millions of events detected at IceCube between May 2010 and May 2013. These events were the only events that were created inside the detector (passed veto) and also had an energy above 30 TeV (passed the charge cut). Note that 1 TeV is a unit of energy relevant to particle physics and it is equivalent to 1012 electron-volts or 1.6 x 10-7 joules. These are the events you will proceed to analyse!
TASK 3: Go to this website and click on “IC 86 – Year 2”, and then on “Start drawing”. Look at the different images, select the 5 events with the largest energy and write down their names in the third question of the questionnaire. Then, click “Check selection” . How many events did you guess correctly? Write down your answer in the fourth question of the questionnaire.
With this data, we will try to answer the two following questions:
1) Were these neutrinos produced beyond our solar system?
2) Can we identify the exact point in the sky where these neutrinos were produced?
TASK 4: Look at the different events in more detail by clicking here and write down in the fifth question of the questionnaire the name, energy, and the declination of the neutrino with the largest energy.
Now we know the most important features of the observed neutrinos. With this information, we can compare the data with the theoretical predictions. These theoretical predictions are based on simulations that reproduce the signals we expect from neutrinos produced in different ways and places. In this way, we compare what we observe with what we expect to observe. When doing so, we can choose to focus only on the observed neutrinos which have an energy where the “background” is small. Physicists call this an “energy cut” and it helps us to better understand our signal we are interested in.
TASK 5: Click here and compare the observed and simulated neutrino events. In the figure on the left, you can find the number of observed and simulated neutrinos as a function of energy. In the figure on the right, you can find the number of observed and simulated neutrinos as a function of the declination with respect to the equatorial plane. As a quick reminder, a negative declination means that the neutrinos arrive from the southern hemisphere, while a positive declination implies that the neutrinos arrive from the northern hemisphere. Look at how the declination changes when you modify the energy cut. What is the ideal energy cut that makes the observed data match the simulations best? Write down this ideal energy cut in the sixth question of the questionnaire. Based on the observed data and looking at the figure on the right, do a large number of neutrinos arrive from a specific declination? Answer this in the seventh question of the questionnaire. What do you think this means?
Congratulations! You have now reproduced the analysis carried out by IceCube scientists which was published in the famous scientific journal Science in November 2013. Do not forget to click on “Done” to send in your answers.
Now we will discuss the main conclusions from this analysis.
If you are brave enough, you can take this quick quiz in order to asses how much you have learnt about neutrinos and the IceCube detector.