Exploring the Higgs boson

SSDs and APDs that helped confirm the existence of the Higgs boson Unraveling the mysteries of the birth of the universe

On the outskirts of Geneva, Switzerland, the LHC project of CERN (the European Organization for Nuclear Research) stretches 27 km. For its sensors, it uses SSDs (silicon strip detectors), APDs, and PMTs. The optical technology we create at Hamamatsu Photonics is active at the forefront of high-energy physics, where human wisdom coalesces.



Keyman Interview

SSDs for the ATLAS Detector
Kazuhisa Yamamura
R&D leader for ATLAS SSD, Solid State Division of Hamamatsu Photonics

APDs for the CMS Detector
Yoshitaka Ishikawa
R&D leader for CMS avalanche photodiode, Solid State Division of Hamamatsu Photonics

What is the Higgs boson?

The Higgs boson is a particle that gives all kinds of matter mass. It is known as the "God particle," but had not been discovered until now. CERN (the European Organization for Nuclear Research) was trying to confirm the existence of the Higgs boson by continued experiments at their Large Hadron Collider (LHC), the world's largest particle accelerator, a 27 km circuit in suburban Geneva, Switzerland. In 2013, its existence was confirmed, and emeritus professors Francois Englert and Peter Higgs, who foretold its existence half a century earlier, were awarded the 2013 Nobel Prize in Physics.

Large Hadron Collider (LHC), the world's largest particle accelerator, a 27 km circuit in suburban Switzerland. (image courtesy of CERN)

The role of our sensors

When protons collide near the speed of light, the energy causes the creation of new particles. Hamamatsu Photonics' sensors are used to investigate the nature of the particles by detecting the direction (track) they fly and their energy. In the equipment for the ATLAS and CMS experiments, SSDs have been used to detect particle tracks (supplied since 1999). PMTs are used to detect minute light for calorimeters to detect particle energy in ATLAS and CMS, and APDs are used in CMS.

Simulation of Higgs boson collapse
(image courtesy of CERN)

The CMS Award received from CERN in 2003 and 2005

SSDs used in the ATLAS and CMS experimental apparatus to detect particle tracks

Approximately 14,000 SSDs are used for the ATLAS experiment, 22,000 for the CMS experiment. An SSD is a diode array in a strip shape. It detects the positions particles pass through with a resolution of several tens of μm.

Silicon strip detectors installed in the CMS experimental apparatus (image courtesy of CERN)

SSDs that can be made from one silicon wafer: 1

In high-energy physics experiments, as the energy of the particles created in collision increases, larger sensors are needed. We developed an SSD so large that only one chip can be harvested from one 6-inch silicon wafer. With our strong capacity for sensor manufacturing, we were able to supply large area, high- quality SSDs to the LHC project just about as expected.


Different shapes are used for SSDs depending on the sensor installation location

Radiation resistance of the SSDs used in the LHC accelerator: 1.5 Mrad/year = 1.7 Sv/h

In CERN's LHC project, there are 1,232 superconducting electromagnets installed, and their magnetic force bends electron beams. As a result, the radiation level at the LHC project reaches about 6 million times normal levels. Our SSDs realize high reliability lasting over five years even under such a harsh radiation level.The radiation environments of track detectors in future particle collision experiments, such as the HL (High Luminosity)-LHC project to start in 2020 or later, are planned to be 5 to 10 times as harsh as those of the LHC project, started in 2008. Therefore, we are developing next-generation SSDs that can be used even in these environments.

Superconductive magnets of accelerator(image courtesy of CERN)

Hamamatsu Photonics APDs used as calorimeters in the CMS experiment: 130,000

The APDs have a photosensitive area of 5 mm by 5 mm. They are used in combination with scintillator. To achieve optimal performance under harsh conditions in a strong magnetic field with radiation, opto-semiconductors were used, which are strong in strong magnetic fields. The specifications for the APDs demanded that they be "strong against radiation, low-noise", "high-sensitivity, low- capacitance, and uniform in characteristics." These specifications conflict with each other, yet we managed to achieve the required specifications by reviewing the structure.

APD and scintillator used for the calorimeter of the CMS experiment (image courtesy of CERN)

Quantum efficiency of the calorimeter APDs: 70%

In the CMS experimental apparatus, radiation emitted is converted into visible light by a scintillator, and our APDs detect it. To capture such minute light, the APDs achieve high sensitivity with quantum efficiency* of 70%.
* Number of electrons output per incident photon


Conceptual diagram of calorimeter APDs