What's the MATTER?

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Hello, It's been a while!

University work has taken over all my time recently, but semester one and exams are overrrr so I'm back and have some (hopefully) interesting post ideas lined up!

:)

Last semester I completed a particle physics lab project which I found super super interesting, so I thought I would attempt to explain some of the basic concepts for you!

The LHCb detector at CERN

This experiment was six weeks long and I completed it with my lab partner. We used data which was collected from the LHCb experiment at CERN. CERN is the European Organisation for Nuclear Research and aims to find out what our Universe is made up of. The Large Hadron Collider (LHC) is the world's largest particle accelerator and the LHCb experiment is one of four particle detectors conducting experiments. The 'b' stands for a type of particle called 'beauty quark' or 'b quark' as commonly known. (Will discuss quarks more later, don't panic :P ) By studying this particle, the experiment focused on the slight differences between matter and antimatter.

We all know what matter is, it makes up everything we can see and interact with in the Universe. But what is anti-matter?

Our current Big Bang Theory predicts that equal parts matter and anti-matter were produced when the Universe began, however there is an issue with this: matter and anti-matter annihilate when they come into contact with each other. Therefore according to this, there should be no matter at all! But obviously the world we live in is almost all matter. So what happened?

1967 Russian nuclear physicist Andrei Sakharov proposed three conditions which must be satisfied in order to explain this matter-antimatter asymmetry. Known as the Sakharov conditions, they are: baryon number B violation, interactions occurring out of thermal equilibrium and CP symmetry violation. 
If you are interested and want to look more into the first two conditions (don't want to overload with toooo much information :P) the references at the bottom explain (or just have a google). But I'm going to talk a bit about CP violation since that was the basis of my experiment.

The C in CP violation stands for charge conjugation. This is the transformation that changes particles into their antiparticles. The P stands for parity conjugation. Slightly more confusing, this is the transformation that changes the sign of the spatial coordinates of a particle, so a particle with (x, y, z) will become (-x, -y, -z) under the parity transformation. The combination of these, CP, was thought to be conserved in all reactions. However we now know it can be violated through certain interactions when the weak force is involved. The weak interaction is the fundamental force which acts between subatomic particles and is responsible for radioactive decay.

Now is probably a good time to introduce these subatomic particles.

The standard model 

This picture shows the standard model of particle physics. These are what we know to be the fundamental particles that make up all of matter. Many of you may have heard of the electron, but that is only one of 6 particles in the lepton family. The quarks can group into set of either 3 quarks, 3 antiquarks, or a quark and an antiquark, but can never be seen separately. for example the proton and neutron, which make up atoms with electrons are made up of uud and udd quarks respectively. The bosons are known as exchange particles. In essence they 'carry' force.

As I mentioned at the beginning, this experiment focused on particles which contain a b quark.

Searching for this CP violation in the decays of these particles can show how the matter-antimatter asymmetry arose. If CP violation occurs, it implies that matter and antimatter particles interact differently, and therefore has led to more matter being produced than antimatter.

Now the experiment involved many hours sat at a computer extracting and analysing lots of data so I won't bore you with the details, but I will say that unfortunately, we found no evidence of CP violation. However experiments such as LHCb at CERN are continuing to explore this area of physics, and hopefully one day we may understand more about how our universe came into existence.

I hope you enjoyed this lil snippet into particle physics! Below are some of the references I used in my lab report for anyone interested in the more complex stuff!

1. B. Martin and G. Shaw, Particle Physics, 4th ed. (J. Wi- ley, 2017) pp. 135–142.
   
2. 
   
3. R. Aaij, B. Adeva, M. Adinolfi, C. Adrover, A. Affolder, Z. Ajaltouni, J. Albrecht, F. Alessio, M. Alexander, S. Ali, and et al., Measurement of cp violation in the phase space of b →k±π+π− and b± →k±k+k− decays, Physical Review Letters 111, 10.1103/physrevlett.111.101801 (2013).
4. 
   
5. T. L. Collaboration, The LHCb detector at the LHC, Jour- nal of Instrumentation 3 (08), S08005. 
6. 
   
7. A. D. Sakharov, Violation of CP Invariance, C Asymme- try, and Baryon Asymmetry of the Universe, Soviet Jour- nal of Experimental and Theoretical Physics Letters 5, 24 (1967). 

Stay Spacey,

Beck

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