Drawing of the Belle II detector

The Belle II experiment at the SuperKEKB collider is a major upgrade of the Belle experiment at the KEKB asymmetric e+e- collider at KEK in Tsukuba, Japan. The experiment will focus on the search for new physics beyond the standard model via high precision measurement of heavy flavor decays and search for rare signals.

[Photo of Dr. Lucien Cremaldi]
Lucien Cremaldi
[Photo of Dr. Robert Kroeger]
Rob Kroeger
[Photo of Dr. David Sanders]
David Sanders
[Photo of Dr. Donald Summers]
Don Summers

Motivation and Physics

    At the time of the Big Bang, 13.7 billion years ago, heavy matter and antimatter particles, whose nature still remains unexplored to date, were created in equal numbers from a tremendous energy density. All these heavy particles decayed within a very short period of time into well-known particles of matter, namely quarks, leptons, and their antiparticles. Particles and antiparticles annihilated each other converting their masses into light (photons). According to this theory, today's matter-dominated Universe, with its atoms, molecules, stars and galaxies, should not exist at all. Scientists explain this “victory” of matter over antimatter, to which we owe our existence, with a tiny imbalance (asymmetry) in the decay between the heavy particles and their antiparticles: In their decays, the heavy particles ultimately produced more protons than the heavy antiparticles produced anti-protons. The causes of this imbalance will be investigated with the new Belle II experiment at the SuperKEKB accelerator in Japan.

    The new SuperKEKB accelerator and the large detector, Belle II, constitute a milestone in the investigation of the matter excess (CP violation) in the Universe. In SuperKEKB, bunches of particles of matter (electrons) and their antiparticles (positrons), with energies up to 8 GeV, are brought to collision at rates which are 40 times larger than in the previous KEKB accelerator. The particles being created in the collisions and the decay products formed are measured and analyzed in the Belle II experiment. With the high statistics provided by SuperKEKB, scientists are hoping to finally find deviations from the predictions of the Standard Model. B mesons, composed of a “heavy” b quark and a “light” anti-quark play a special role here. In SuperKEKB the energies of the electrons and positrons are chosen such that exactly one B meson and one anti-B meson are produced per collision. Due to the high density of the colliding bunches, made possible by the extremely small beam cross sections of SuperKEKB, the B meson pairs are produced in unprecedented large numbers. For this reason, the SuperKEKB accel- erator is also called a “Super B factory”.

    CP violation is the subject of various experimental investigations. With the forerunner experiments Belle at the KEKB accelerator and BaBar at SLAC (California), scientists were able to describe the fundamental mechanisms for the decay of B mesons and their anti- particles within the framework of the Standard Model. For the development of the basic theoretical principles of CP violation in the Standard Model M. Kobayashi and T. Maskawa were awarded the Nobel Prize for Physics in 2008. Never-the-less, the Standard Model cannot deliver a satisfactory explanation for the excess of matter observed in the Universe. In the future, the Belle II detector will supply scientists with plenty of much more precise data in order to search for new sources for CP violation beyond the Standard Model.

Time-of Propogation Particle Identification System

The University of Mississippi's High Energy Physics group's at the Belle II exeriment includes work on the imaging Time-of-propagation (iTOP) detector. This subdetector of Belle II will perform an integral role in Particle identification (PID). It will comprise 16 modules between the tracking detectors and calorimeter; each module consisting of a quartz radiator, approximately 2.5m in length, instrumented with an array of 32 micro-channel plate photodetectors (MCP-PMTs). The passage of charged particles through the quartz will produce a cone of Cherenkov photons that will propagate along the length of the quartz, and be detected by the MCP-PMTs. The excellent spatial, and timing resolution (of 50 picoseconds) of the iTOP system will provide superior particle identification capabilities, particularly allowing for enhanced discrimination between pions and kaons that will be essential for many of the key measurements to performed.

iTOP quartz bar module
The quartz bar in a module of the iTOP system

iTOP Gas System

    The iTOP quartz bar optics must be maintained in stable and pristine environmental conditions for its many year life at the Belle II detector. The bars must be maintained in a low moisture environment free of dust and particulates. Dr. Lucien Cremaldi is heading Mississippi's work on the iTOP Gas System.

    A good choice is to use dry nitrogen gas from liquid nitrogen (LN2) boil-off. This will flow through each box at the rate of 8-10 L/h. The gas flush is monitored for humidity. To remove particulates the gas is filtered through a molecular sieve and several mechanical filters to remove particulates. We expect dew points on the gas exhausted from the iTOP bar boxes to be of order of −60 °C at the exhaust.

      Some goals are to:
    1. measure the flow to know if there are leaks.
    2. prevent overpressure in the iTOP quartz bar boxes (QBB).
    3. sample the gas and send for RGA or Mass Spectrometry to see if there are contaminants.
    4. consider strong changes in barometric pressure.
    5. prevent back flow.
    6. assess failure modes.

iTOP Optics Studies

    There are two separate studies that need to be carried out. The quality of the joint between the quartz and the photodetector (PMT) and the optical properties of the quartz itself. Dr. Robert Kroeger has been conductiong these studies of the iTOP system at the University of Mississippi.

    During the lifetime of the Bell II experiment some of the PMT's may fail. At some point these PMTs will have to be replaced, in situ. Thus it is necessary to monitor the quality of the joint between the cookies that interface the PMTs to the quartz prism and the prism itself. Small cameras are to be placed within the iTOP housing for purposes of monitoring the joint betweenthe PMT and the quartz prism as the new PMT is in the process of being mounted via cameras inside the iTOP housing. The issue was to study the choice and placement of the cameras under constraints of very small available spaces and the required focal lengths, field of view and depth of field. Under the space constraints, the cameras must view the cookies and their internal reflection off the top of the prism at a very oblique angle to the bottom surface of the prism.

    The optical properties of the quartz under the influence of a strong magnetic field must be studied because the fact that strong magnetic effects within the quartz have a pronounced effect on the polarization of the Cherenkov photons and the fact that the quantum efficiency for the photocathode of the iTOP PMT has a very strong dependence on the polarization state of these incident photons. The quantum efficiency asymmetry between the TM and TE states is especially strong for angles near the critical angle for fused quartz, 43 degrees from the longitudinal. This is also the angle for which these gyroelectric effects most strongly affect the phase changes on total internal reflection.