High Energy Physics (OG 3202)

The High Energy Physics Group addresses fundamental questions concerning the structure of matter at high energies. This includes searches for yet undiscovered particles as well as precision measurements which might give hints to "new" physics which is not described by the standard model of particle physics.

The group made major contributions to several large scale multi purpose experiments, like L3 at CERN-LEP, H1 at DESY-HERA and CMS at CERN-LHC. The latest large project was the development and construction of the CMS pixel detector. The hostile environment close to the collision point posed unprecedented requirements for the inner tracking device in terms of radiation hardness and data rates. Those required technical developments in several areas like readout electronics, interconnection techniques and particle detection. The pixel detector was installed in CMS in summer 2008. Its performance exceeds expectations and it became a key device for the CMS track and vertex reconstruction which is essential for many physics analysis.

This success is now followed by a project to replace this detector by an upgraded one which is about two times larger. This effort is led by the PSI-LTP-HEP group and aims for the installation of the new pixel detector in 2017.

A second major contribution to the CMS detector was the proposal to use Avalanche Photo-Diodes (APDs) to read out the signals from the lead tungstate crystal calorimeter. The APDs were developed by Hamamatsu Photonics in collaboration with CMS, with PSI being the lead partner in CMS, responsible for guiding the development. The use of APDs enabled CMS to build a high performance calorimeter within a compact CMS, whose performance is key to the search for the Higgs particle in the mass region where it is now expected.

In addition to the detector development and construction the group is involved in the analysis of the CMS data making special use of the pixel detector. The first measurement of the b-quark production cross section in CMS was performed with the 2010-data set using the muon transverse momentum relative to a close-by jet. More recently the group searched for the very rare decay Bs0→μ+μ-, which is suppressed in the standard model of particle physics. Using the CMS data collected in 2011-2012 the branching fraction for the decay Bs0→μ+μ- was measured for the first time with a statistical significance exceeding four standard deviations. Furthermore, we obtained an upper limit for the B0→μ+μ- decay. Both measurements are statistically compatible with standard model predictions and allow stringent constraints to be placed on theories beyond the standard model. The LHC experiments have resumed taking data in 2015, recording proton–proton collisions at a centre-of-mass energy of 13 teraelectronvolts, which will approximately double the production rates of Bs0 and B0 mesons and lead to further improvements in the precision of these crucial tests of the standard model.

CMS pixel test beam measurement at the PiE1 area at PSI

CMS_pixel_PiE1_testbeam.gif