CP Violation - introduction

In the Big Bang model for the origin of the Universe, matter and antimatter was initially created in equal portions. Why is there now more matter in the Universe than antimatter? This question will be studied with the LHCb Experiment at CERN.



A crucial ingredient in the physics that creates this matter-antimatter asymmetry is the mechanism of CP violation. CP violation implies that some particles have different decay rates than antiparticles. B mesons contain a b-quark and another lighter (anti)quark u, d, s, c. Neutral B mesons can turn into their own antiparticles through a process called mixing.

The LHCb Experiment will study the nature of the differences between matter and antimatter. Such measurements are designed to shed light on the problem of why there is more matter in the Universe than antimatter.



The LHCb Detector will measure the decays of neutral B mesons. These particles can turn into their own antiparticles through a process called mixing.

The Microvertex Detector features multi-vertex event reconstruction and proper-time measurements, permitting measurements of B_s mixing well beyond the largest conceivable values of x_s.



Silicon detectors are used as active elements in the Microvertex Detector.

The mechanical design of the Microvertex Detector is carried out at the Vrije Universiteit in collaboration with the mechanical department at NIKHEF.

Detailed calculations are performed to estimated the interaction between the mechanical structures and the circulating protons in the Large Hadron Collider.

CP Violation - The Physics of CP Violation

The investigation of symmetries and their associated conservation laws have proven to be of fundamental interest in physics. From classical physics we know for example that the requirement that the laws of physics should be invariant under translations in space and time leads to conservation of momentum and energy, respectively. Furthermore, the invariance of the Lagrangian, that describes the dynamics of a system, with respect to spatial rotations leads to angular momentum conservation. While the conservation laws for energy, momentum, and angular momentum always hold, we now know that other symmetries are violated (or broken) in certain interactions.

At the fundamental level, charge conjugation, C, and parity, P, are symmetries of particle interactions. Although these above symmetries are perfectly realized in the strong and electromagnetic interaction, it is well known that they are maximally violated in the weak interaction. Investigation of the CP combination revealed a small CP violation, discovered by Cronin and Fitch in 1964 in experiments with neutral kaon beams. Since then no observation of CP violation was made in any other physical system. Presently, it is thought that this symmetry violation is one of the most subtle effects of nature. Although the effect is small, it implies a fundamental difference between matter and antimatter. Without the mechanism of CP violation the matter and antimatter that was created in equal portions in the Big Bang would simply have annihilated. As such it is of principal importance for cosmology, since it provides a mechanism to explain the observed matter-antimatter imbalance in the universe (there are probably no galaxies consisting of antimatter).

Furthermore, under very general assumptions one can show that the combined symmetry operation CPT is obeyed in all interactions. From CPT-invariance one derives for example that a particle and an antiparticle should have exactly the same mass and life time. Combined CPT symmetry provides the basis for our successful field theories and constitutes one of the capstones of modern physics. Consequently, a violation of CP implies a corresponding violation of time-reversal invariance.

Although violation of CP symmetry can be accommodated in the standard model, we do not understand the mechanism of this symmetry breaking. The Large Hadron Collider (LHC) at CERN will be a prolific source of B mesons, and therefore strong interest has been expressed in measuring precisely CP violation in several relevant B decay channels to determine elements of the CKM matrix (which signifies the fact that the strongly interacting quark eigenstates are not precisely the eigenstates of the electro-weak interaction). Data taking will start in 2005 at the turn-on of the LHC collider. The Letter of Intent and the subsequent research and development reports have been approved by the LHC committee of CERN. The collaboration is invited to submit a technical proposal by spring 1998. It is the responsibility of SAF - VUA group to provide contributions to this proposal in order to establish the scientific case for the experiment.

The SAF - VUA group is involved in the design of the microvertex detector. This detector facilitates multi-vertex event reconstruction and proper time measurements for the decays of neutral B mesons to final non-CP-eigenstates. Topology information on secondary vertices will be used at the trigger level. Major experimental requirements have to be met, such as low-mass, large acceptance, micrometer-precision alignment and retractability of the silicon detectors, while taking into account the effects of e.g. heat loading, beam-detector RF coupling, mechanical stresses, and other technical considerations such as high-vacuum compatibility, signal feed-through, etc. We have already started a detailed design study to address these issues.