Discovering the Mysterious Muon: the Anomaly of the Particle That Passes Through Our Bodies Every Day

This series of research tales told by young protagonists stems from the @UniboPER/PhD Storytelling initiative, which has brought together PhD students, science popularisers, professionals from the Italian Union of Science Journalists (UGIS) and UniboMagazine. The author of this article is Eugenia Spedicato, PhD student at the Department of Physics and Astronomy of the University of Bologna

Good morning, Mr. Muon! Who are you and where are you from?
I have just been born! I popped out when my mother Lucilla’s cosmic ray touched the atmosphere! Yes, I am indeed the child of an interaction between a cosmic ray and the Earth’s atmosphere. This is where my journey begins!

Welcome! Tell us a little more about yourself.
I’m tiny — a particle, one of the smallest pieces that make up the universe. And I am a muon. We muons have a very short life span. But when we are flying, at speeds close to that of light, something strange happens to us. If we were on the ground and we behaved like a vehicle, we could only travel 660 metres at such speed. But this is not the case: due to the very high speed at which we fly, it is as if the space we cross becomes shorter. I believe you humans call this strange phenomenon the theory of relativity, discovered by the great scientist Albert Einstein. Thanks to this principle of physics, we can travel as far as 15,000 metres from where we were born to the Earth.

Fascinating! I bet you have more great stories to share!
Of course! Do you want to know the most bizarre things I’ve seen on my journey? Like many of us, when I arrived on Earth, I went right through your head, Eugenia, and discovered that you study at the University of Bologna. I say “like many of us” because, as you are reading this, one of my newborn siblings is likely passing through your ear, hand, or some other part of your body. We particles have no obstacles.

You went through my head? A bit intrusive, aren’t you? What about privacy?
I’m sorry, but it’s in our nature. We’re actually worse than cats, we get into everything! For example, this morning I discovered that Eugenia had many “cosmic thoughts”. That’s when I realised she is a researcher in particle physics in the Department of Physics and Astronomy at the University of Bologna. As chance would have it, she studies us muons! Her experiment, called MUonE, seems to investigate the mystery surrounding our nature. Specifically, what scientists from the Earth call the anomalous magnetic moment of the muon... Quite a complicated name! Yet it’s easy to explain: you know those magnets that stick on your fridge? Well, whenever you bring a magnet close to the surface of the appliance, it creates a magnetic field. Imagine it as an invisible force that pulls two bodies together. When a muon like me enters a magnetic field, it starts to spin like a top. The magnetic moment is how fast we spin! Let’s be clear, for me it’s just one of the many traits I was born with. But for researchers, it seems to hide one of the secrets behind understanding our universe.

I see you’re an expert! What else have you discovered about what we know about you?
It seems that in America this trait of ours was measured with unparalleled precision just a few years ago! But, mysteriously, the number found does not align with what the theory predicted. Simply put, theories represent what you humans understand about how reality works and is structured. The fact that there is a difference between what is measured and what was predicted by the theory is a mystery. It is as if your doctor, while measuring your height, realised that you are 5 centimetres taller than expected. Their theory — what they initially thought — is different from the actual measurement. Something’s missing! And that’s what happened with my magnetic moment too. The theory, therefore, could be wrong or incomplete. Either way, this difference could point to the existence of new particles or theories still unknown to you humans!

Impressive, you are up to date! What else have you discovered, about me this time?
I know that Eugenia is curious. Like a detective, she thinks that discovering our world piece by piece is exciting! That’s why she studies physics and believes that the MUonE project can help everyone take a step closer to understanding the universe. And I know your experiment will be conducted at Europe’s most important laboratory for studying us particles: the CERN in Geneva. I had heard rumours that humans here make my siblings and cousins race through long underground tubes... but I didn’t want to believe it. Yet, during my journey, I saw with my own eyes the huge blue tunnels I had always heard about. And, right there, I met my distant relative, the “proton”! He’s a particle, just like me! You can even find him in the cells of your body, since he’s in all the elements of nature. Before saying goodbye, proton predicted that the MUonE experiment would take more time. So, it’s still too early to know whether these humans’ studies will solve the mystery of us muons. What’s sure is that they need young researchers, curious and brave enough to join them on this adventure!

You got me all figured out! But everything you said is true. We humans are drawn to the unknown — we’re always thirsty for knowledge. Maybe you don’t know this but, many muons ago, one of our poets wrote: “There are more things in heaven and earth, than are dreamt of in your philosophy.”
I couldn’t agree more! Now I must say goodbye. My journey is over. After travelling 15,000 metres, I have given birth to new particles, just as my mother Lucilla did with me. I can finally rest, look within myself and wonder who I am. I’ll dream about my essence and everything that surrounds us. I can’t wait for the day that some young human helps Eugenia uncover the mystery behind my magnetic moment. I’m rooting for them. And you?