Category Archives: MSci Project

Why do we need the LHCb?

This post is about the LHCb – the motivations and science behind the experiment, and what scientists hope to discover there.

The LHCb is one of four experiments at the Large Hadron Collider (LHC) at CERN in Geneva. It will study the decays of particles known as B mesons in the hope of discovering the answer to a problem known as matter-antimatter asymmetry.

The problem is this: at the big bang, matter and antimatter should have been, and in all likelihood were, created in equal amounts. However, according to what we currently know, if they had been created in equal amounts, then I wouldn’t be here to write this and you wouldn’t be here to read it either. We exist due to a tiny imbalance in the ratio of matter to antimatter at the beginning of time. This tiny imbalance meant that when most of the stuff created in the big bang was annihilated (when matter meets antimatter both are destroyed and lots of energy is released) a tiny amount of matter was left over, and this tiny amount makes up all the matter we see in the Universe today, including us.

Shortly after the Big Bang, there were roughly equal amounts of matter and antimatter...

... crucially, though, there was slightly more matter. This is why we live in a matter dominated Universe today.

To investigate the matter-antimatter asymmetry, physicists are looking at B mesons. These particles are so called because they each contain a b, or bottom, quark. After its discovery in 1977 there were some attempts to change the bottom quark’s name to “beauty”, but the original name stuck. Incidentally, the “b” in LHCb does stand for “beauty” as opposed to “bottom”.

Decays involving B mesons may hold the key to the matter-antimatter asymmetry problem because they exhibit a property known as CP violation. CP stands for “charge parity” and is used to describe the combination of two symmetries called charge conjugation symmetry and parity symmetry. If these symmetries were obeyed, the laws of physics would treat matter and antimatter exactly the same. CP violation occurs when matter and antimatter are treated differently, and as such might be able to explain why we live in a matter dominated Universe today.

It is the weak force that is responsible for the decay of B mesons, and it is the only one out of nature’s four fundamental forces that is known to violate CP. CP violation has been seen at experiments BaBar and Belle, which are located at the Stanford Linear Accelerator Centre (SLAC) in the US and the KEK laboratory in Japan respectively. However, the weak force alone is not enough to explain all the CP violation we see.

Scientists at the LHCb will search for rare decays of B mesons in order to try and find new physics to explain the asymmetry. They will be looking for new particles that have never been seen before as well as new physical phenomena. This new source of CP violation could be found in quarks, or it could be found in some other particle. If the Higgs boson is discovered, maybe it will point us in the right direction. We don’t yet know exactly where the answer lies, but there’s only one way to find out…

For more information see the LHCb website.

Images: US / LHC webpage

Advertisements

1 Comment

Filed under MSci Project, Physics