Five miraculous catches of fish beyond the shores of the Standard Model (and General Relatity)...
Since 1998, we have discovered ﬁve pieces of empirical evidence for physics beyond the Standard Model thanks to tremendous progress in experiments.
- First, non-baryonic dark matter. Even though dark matter had been discussed since 1930’s by Fritz Zwicky, it was not clear whether dark matter would be dark astronomical objects or hidden baryons. This issue was completely settled in 2003. The search for dark astronomical objects (MACHOs = Massive Compact Halo Objects) excludes the possibility that Galactic halo consists solely of MACHOs between about ×10−7Msun and 10Msun . On the other hand, the power spectrum in the Cosmic Microwave Background (CMB) anisotropy by WMAP (Wilkinson Microwave Anisotropy Probe) excludes the baryonic dark matter completely as a discrepancy between the overall matter density... and the baryon density ... ...,
- The ﬂavor oscillation of neutrinos, and hence their ﬁnite masses, is not a part of the Minimal Standard Model either, arguably the ﬁrst established physics beyond the Standard Model in 1998 ...,
- The accelerated expansion of the Universe came as a big surprise to all of us [11, 12]. Its cause is now called dark energy, even though we are very far away from understanding what it is....
- At the same time, the observed apparently acausal density ﬂuctuations in the CMB cannot be explained by the Standard Model. The CMB photons that came from one end of the Universe have just reached us; they seem to be correlated with the CMB photons that came from the other end, when they have had no chance to meet and set up their temperatures. This is what I mean by acausal. The best explanation is that they were in fact in causal contact early on because the entire visible Universe was much maller than a nucleus; it was later stretched to a macroscopic size by an exponential expansion called inﬂation. The latest Planck data strongly supports this idea . We normally assume that it was caused by a scalar ﬁeld called the inﬂaton rolling slowly down the hill, but we don’t know what it is, nor how it couples to the Standard Model particles.
- Finally, once we accept the inﬂationary paradigm, the cosmic baryon asymmetry... cannot be assumed to be the initial condition of the Universe. This is because the enormous exponential expansion (normally assumed to be more than e60) wipes out any pre-existing asymmetry. This implies that the baryon asymmetry needs to be created after the inﬂation by a microphysical process. On the other hand, the CP violation in the Standard Model is now known to be incapable of producing enough baryon asymmetry. This is because that we now have understood the known CP violating phenomena by the Kobayashi–Maskawa theory thanks to the B-factory experiments starting in 2001 [13, 14]
The[se]... ﬁve important pieces of data ... are crying out to be explained and understood. The catch is that we don’t know the energy scale of physics relevant to these mysteries. Right now we are on ﬁshing expeditions. In particular, we are and will continue to be looking for new phenomena and new sources of CP violation in the quark sector (LHCb, SuperKEKB, rare kaon decays), lepton sector (neutrino oscillations, neutrinoless double-beta decay, and electric dipole moments), and their combination (proton decay). We try to cast a wide net, hoping to catch any interesting ﬁsh, so that we learn where the next important energy scale is. In a sense, this is what Fermi succeeded in doing; by observing rare phenomena of nuclear β-decays, which violate conservation law of neutron and proton numbers that all other known forces respect, they were caught in the net and we learned about the Fermi scale.
Approximate energy reach for expeditions. Solid arrows indicate the current reach, while the dashed arrows anticipated improvements by proposed experiments.
Hitoshi Murayama (Submitted on 6 Jan 2014)