The beauty and charm of LHCb

The LHCb experiment is designed to study particle decays involving ‘beauty‘ (b) and ‘charm‘ (c) quarks that are produced in quark-antiquark pairs in the collisions at CERN’s Large Hadron Collider (LHC). However, these fundamental particles cannot be observed directly, but rather in composed states called hadrons consisting out of two (mesons), three (baryons) or more quarks. These hadrons are mostly unstable and decay till they produced protons, neutrons or electrons.
Instead of focusing on the Standard Model of particle physics, the following sections give more details on the beauty and charm quark.

b stands for beauty

The beauty quark is the second heaviest quark and the heaviest that does build a hadron before it decays enabling to study a wide range of physics. It is a so-called down-type quark with an electric charge of -1/3e.

The LHC beams are accelerated close to the speed of light and smashed together, recreating the conditions that existed when the Universe was a hundredth of a billionth of a second old and producing many particles that can otherwise not be observed. An important example are particles known as ‘beauty quarks’ that were common in the aftermath of the Big Bang, but absent in today universe. They are generated in their billions by the LHC, along with their antimatter counterparts, the anti-beauty quarks.

b and anti-b quarks are combined with other quarks in unstable and short-lived bound states called hadrons, which decaying rapidly into a range of other particles. However, b hadrons have a longer lifetime compared to some other combined quark states. Therefore, they fly a certain distance away from the beam collision point before they decay. This unique feature helps to select them from other decays. Physicists believe that by comparing these b hadron decays, they may be able to gain useful clues as to why nature prefers matter over antimatter.

Some other experiments in this area, called flavour physics, have already met with success. But these experiments are mostly based at lepton collider producing much fewer b and anti-b quarks compared to hadron colliders like the LHC. Therefore, by harnessing the power of the LHC, the LHCb experiment is able to study many more b and anti-b quark decays than ever before.

c’s to charm them

The prediction and discovery of the charm quark in the 1970, was critical to deepen the understanding of the fundamental forces which navigate the interactions of particles. Despite its discovery many decades ago, there are still many unanswered questions explaining the charm of studying charm physics.

The charm quark is the third heaviest quark and an up-type quark carrying an electric charge of +2/3e. Similar to the beauty quarks, the charm quarks exist as mesons or baryons, containing one or several (for baryons) charm quarks, or as charmonium states which are bound states of c and anti-c quark pairs. Additionally, several states containing charm quarks have been observed that cannot be classified as conventional mesons, baryons, or their bound states. The precise nature of these states remains one of today’s unresolved questions.

In charm physics, the unique decays of composite particles containing charm quarks enable to probe the strong and weak interactions in the standard model and beyond. This is a result of the fact that the charm quark can only decay via weak decays, mediated by a W-boson, into a strange or down quark. The important exception are the decays of charmonium mesons, which decay via annihilation of the c and anti-c quarks. Thus, particles consisting of charm quarks are the only ones allowing the study of weak decays of an up-type quark in a bound state.