Tag Archives: high-energy particle physics

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

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First results from the LHC

The LHC has been subject to much media coverage and criticism since it was started up last September only to break down little more than a week later, but the first results from the record-breaking particle accelerator are finally in. Admittedly, nothing earth-shattering has been discovered yet, but the turnaround speed of the paper alone must be a record.

The first collisions took place on Monday 23rd November and since then the twin proton beams have reached an energy of 1.18TeV, smashing the previous record of 0.98TeV. Researchers working on ALICE, one of the six experiments within the LHC, however, took data from some of the very first collisions when the protons were circulating at only 450GeV per beam. They had their paper accepted by the European Journal of Physics just 8 days after the collisions took place. Presumably, most of the paper was already written before the LHC was even fired up, with gaps left to fill in the results when they came in.

Before being accepted by the European Journal of Physics, the paper was published online at arXiv.org, an open access repository for scientific papers not yet published elsewhere. Anyone interested in the paper can read it online, but be warned, you’ll have to skip to page 6 to even get to the abstract due to the list of authors (over two pages worth) and involved institutes (more than 100 in total).

In the paper, Aamodt and colleagues describe how some 284 events recorded in the first collisions were used to measure something called the pseudorapidity density of the charged particles. Pseudorapidity is used in particle physics to describe the angle of the particle beam relative to the axis. This may not sound very exciting, but the ALICE collaboration are pleased as the results agree with theory and previous experiments, meaning that the LHC is working well and should provide high quality data to work with when they get to the really interesting stuff.

The first super high energy collisions at the LHC are on track to start in early 2010, and will reach energies of 3.5TeV per beam.

Reference: arXiv:0911.5430v2

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