On the path to new physics

The Standard Model of Particle Physics

Experiments in particle physics can be effectively characterized through the so-called Standard Model of the three fundamental interactions – strong, weak, and electromagnetic. This extends from low-energy phenomena as in beta decay (a few megaelectronvolts, MeV) up to high-energy processes (several hundred gigaelectronvolts, GeV) as in the production of the weak gauge bosons W and Z and the top quark. The last missing piece of the Standard Model, the Higgs boson, was also recently detected at the LHC. Thus there is little doubt that the Standard Model correctly describes particle phenomena, at least up to the energies attainable today.

Despite this success, further questions arise that the Standard Model of particle physics in its current form does not answer. It does not explain, for example, why there appear to be exactly three quark-lepton families. Similarly, it is unclear how the mass hierarchies of individual quarks and leptons, as well as the different mixtures within the quark and lepton families, come about. Finally, the Standard Model does not include the fourth fundamental interaction: gravitation, with which presumably the dark matter and possibly also the dark energy are connected.

Matter-antimatter asymmetry in the universe

An important consequence of the mixing of the three quark families is the violation of symmetry between matter and antimatter: Experiments with K and B mesons have shown that particles of matter, such as B mesons for example, exhibit decay patterns slightly different from those of their antiparticles, the anti-B mesons. A difference in decay patterns between particles and antiparticles ("CP violation") is the prerequisite to explain the observed matter-antimatter asymmetry in the universe.

B mesons are made of one anti-B quark and one light quark – for example a u, d, or s (up, down, or strange) quark. Anti-B mesons consist of one b quark and one anti-u, anti-d, or anti-s quark. The phenomenological model developed by Kobayashi and Maskawa was able to explain the observed quark mixture within the framework of the Standard Model through the introduction of a few parameters (three quark mixing angles and one CP phase).

Since that time, the experiments BaBar and Belle at the two "B-factories" at SLAC (USA) and KEK (Japan) have provided impressive proof of CP violation in B meson decays – thereby confirming the theory of Kobayashi and Maskawa. For this, the two researchers received the Nobel Prize in Physics in 2008.

Nevertheless, the CP violation anchored in the Standard Model does not come near accounting for the matter-antimatter asymmetry in the universe. The inescapable conclusion is that "new physics" beyond the Standard Model – that is, at higher energy scales – must exist. The other shortcomings of the Standard Model mentioned earlier also argue in favor of this view. At the Large Hadron Collider (LHC), following the discovery of the Higgs boson, an intensive search is now under way for such new heavy particles.

Search for new physics in quantum loops

The LHC works with energies up to 13 teraelectronvolts (TeV) and could detect particles with a mass of up to 5 TeV. The high-energy region is not, however, the only option in the search for new physics. New particles could also be accessible at lower energy through virtual (quantum loop) effects in reactions with known particles. In the history of particle physics, this has often been the case: The heavy charm quark was predicted through suppressed decays of the light, neutral K mesons; and the existence of a third generation of quarks and leptons was suggested, through the Kobayashi-Maskawa theory of CP violation in the K system, long before it was observed.

The masses of the top quark and more recently the Higgs particle were also correctly predicted through virtual effects from low-energy data. Since quantum loop effects typically become smaller as the mass of the virtually exchanged particles increases, high precision is the crucial point for such physics. In fact, high-precision experiments can search for new physics on energy scales that are inaccessible to present-day and even next-generation colliders.

Precision experiments with B mesons are the best choice in the search for new physics in quantum loops: In particular, b quark transformations with equal charge (for example, from bottom to strange) – thus, neutral, flavor-changing currents – are only possible in the Standard Model on a higher order (that is, in quantum loops, on the "loop level") and therefore are strongly suppressed.

As a consequence, potentially large effects could be expected through new physics. If some generic flavor-violating new physics with interconnections of the order of magnitude 1 are incorporated into the Standard Model, the present-day flavor data allow such additional contributions only for scales beyond 10 to 100 TeV. This shows how sensitive precision experiments can be on very high mass scales.

Super flavor factories

To take a significant step forward in measurement precision and put the Standard Model to the test, the luminosity of the B-factories must be drastically increased: In the Japanese particle research center KEK, the former KEKB facility, which was in service until 2010, is currently being upgraded to the new SuperKEKB collider. Like KEKB, SuperKEKB consists of two separate rings for electrons and positrons with different energies (7 GeV for electrons, 4 GeV for positrons). Indeed, a new focusing system to be installed at SuperKEKB will increase the luminosity by a factor of 40 or more in comparison to the old world record set by KEKB. At the same time, the old Belle detector will be updated to Belle II. One essential new component of Belle II is the pixel vertex detector.

With integrated luminosity beyond 50 ab-1 (per attobarn), the experimental uncertainties of the Belle II experiment will be reduced by nearly an order of magnitude in comparison to the currently published data. With that, completely novel measurements specifically oriented toward new physics are possible. Depending on how strongly the new physics is linked to familiar particles, mass scales in the domain beyond 100 TeV could also be accessible.