Birth and end of the universe is a challenging problem for physics. The quest started till the known time. Lots of ideas about the creation of the universe came in the course of time till modern cosmology. Most of the earlier Cosmological models describe the universe as static. After the discovery of Cosmic Background Radiation which is thought to be the relic of Big Bang Cosmology, the Big Bang theory of creation of the universe is accepted although lots of questions are yet to be answered like what causes Big Bang or who is the First Mover. Discovery of Hubble law and redshift indicated the universe is expanding. The ultimate fate of the universe is one of the most important searches of present day cosmology. Modern Cosmology provides more and more evidence for the universe with finite life. The scenario of Proton Decay has something to add to this proposition.
Proton is a fundamental constituent of matter. Since the discovery of radioactivity in 19th century scientists are compelled to recognize the fact that not only the radioactive element but much of the matter decays eventually. High energy physics reported the decay of many so called elementary particles. Huge numbers of particles which are composed of more fundamental entities like quarks of different flavours disintegrate into lighter particles. Leptons like muon, tau disintegrates to other particles. Free neutron which is a component of the nucleus also decays with half life ~ 10.3min but it is stable when in the nucleus. The neutron is 0.2% more massive than the proton. The lightest composite stable particle is Proton which consists of two up(uu) quarks and one down(d) quark.
Big Bang theory of the universe is successful in explaining two facts, one is 3-K background radiation as remnant of primordial radiation emitted by Big Bang and the second one is current mass ratio of Helium to Hydrogen in terms of nucleosynthesis in first 100second of Big Bang. The major problem is the explanation of ratio of matter (baryon density) to photon density nB/nγ and preponderance of matter over antimatter. Proton Decay was first proposed by Andrei Sakharov in 1967. He put forward three points which are called Sakharov conditions to generate baryon asymmetry of the Universe. He argued that if the universe starts from Hot Big Bang, three fundamental considerations must be taken into account. First is the CP asymmetry which must account for (one of the outstanding problems in modern cosmology) the particle antiparticle asymmetry of the universe via CP violating processes. Second is the non-equilibrium dynamics which accounts for the galaxy formation and third is the baryon number violating decay to account for the baryonic matter and anti matter asymmetry of the universe. The universe is globally anti symmetric in matter. He observed that baryonic matter must decay to explain the origin of baryonic matter of the universe.
All decay processes in nature are abide by some basic interactions and corresponding conservation laws. There are four known basic interactions in nature such as Strong, Electromagnetic, Weak and Gravitational interactions. Conservation laws are critical to an understanding of Particle physics. Standard Model (SM) which unifies electromagnetic and weak interactions is supposed to be the gauge theory of Strong Interaction. SM observes baryon number (B) and lepton number (L) symmetry so that B – L are exactly conserved and a proton is stable in the framework of SM. Grand Unification Theory (GUT) unites Strong, Electromagnetic and weak interactions with quarks and leptons and their anti particles. When strong and electro-weak interactions are unified, quarks and leptons appear as members of the same irreducible representation of gauge group. The gauge boson which mediates the interactions transform quark into leptons or anti-quarks and violate baryon number (B). The unification procedure of GUT resulted in the coincidence of the coupling constants of strong, electromagnetic and weak interactions at a certain high energy known as grand unification scale. In GUT quarks could transform into lepton by the exchange of extremely heavy intermediate particles in the energy scale 1016 GeV. One of the implications of Grand Unified Theory (GUT) is the prediction that proton decay with a half life ~ 1032 years. Protons being composed of three quarks can decay via exchange of extremely massive particles called X and Y bosons with life time that is 20 orders of magnitude longer than the age of the universe. X and Y bosons can mediate the interactions which violate the baryon number B causing proton to decay.
The prominent decay mode of proton (p) is p→ e+ + π0 i.e. proton ( a baryon) decays into positron (a lepton) and a neutral pion (meson; Fig.1). There are other decay modes of protons but SU(5) predicts two body decay modes of proton to positron and meson will dominate. The new interaction in the framework of GUT predicts Baryon number and Lepton number violating four Fermions interaction via exchange of super heavy particles. The effective coupling constant of such a type of four fermions interaction is very large and makes the proton decay detectable by present generation experiments.
Experimental Searches of Proton Decay:
Baryon numbers violating decay of protons have studied widely both theoretically and experimentally. GUT predicts protons will decay at a very small rate but quite possibly measurable rates determined by the mass MX of the postulated gauge boson of grand unifying symmetry with MX ~1015GeV. A number of experimental efforts were going on till 1980 to detect the proton decay. The average lifetime of protons is very large but according to quantum physics the time of decay is random and if a tiny fraction of protons decay long before the average lifetime, the decay can be detected. The only requirement is a huge number of protons sources which enhances the probability of detection. Water is used as a source of protons. A proton either from Hydrogen or Oxygen decay into positron e+ and π0 which eventually decays into two photons π0 → 2γ (Fig.2). These showers can be observed in detectors and act as a tool for proton decay observation.
The search for proton decay has been done in Kolar Gold Field (KGF) Mine in South India 2300 meters below the surface of the earth.140 tonnes of iron which can provide 60,000 billion, billion protons, at least six of which are expected to decay annually. The interference due to cosmic ray background radiation has been minimized. The analysis of data puts the limit on the lifetime of protons as ~ 7 x 1030 years whereas the Mont Blanc NUSEX detector experiment puts the limit on proton lifetime τp ~1030 years. A large water Cherenkov detector was constructed in a deep mine at Park City, Utah. The detector is sensitive to different decay modes of protons. The detector is protected from background cosmic ray radiation of muon and neutrino by natural walls of mines and using active shields. The Water Cherenkov Detector predicts that the bound proton decay to a lifetime ~ 7 x 1033years. The IMB experiment was done at Morton Salt Mine at Cleveland, Ohio. Observation was made of 1570 meter of water equivalent to 8000 metric ton water Cherenkov detector. They put the limit on τp ~ 1.9 x1031.
Most recently the experiment on proton decay has been done by Super Kamiokande, Japan which started observation from 1996. It is a large water Cherenkov detector which is the most sensitive detector in the world used to examine proton decay with huge source with 7.5×1033 protons or 50,000 tons water Cherenkov detector. The proton decays via most favoured channel in p→ e+ + π0 in several GUTs model. The neutral pion subsequently decays into two photons as π0 → 2γ. So three Cherenkov rings can be observed in the detector (Fig 2.). All the particles can be detected by the Cherenkov detector. The detector has made observations for more than 12 years and put the lower limit on the lifetime of protons as >1.6 1034 years. Till now there is no concrete evidence of proton decay from any experimental group.
Proton Decay and age of the universe:
Proton decay has immense cosmological implications. Protons which are the basic building blocks of all matter decay into lighter particles and supposed to be unstable. It may have a very long lifetime but still it is finite and cannot exist for ever. So the matter around us which is composed of protons must have been produced no more than a finite time and it cannot be infinite in time. It has been suggested that true eternal life depends on whether or not protons can decay and there may be inexorable decay of matter all over the universe. In billions of years from now the 10 billion galaxies that constitute the known universe may no longer exist. Proton decay has not been detected experimentally till now probably because of the fact that the event is extremely rare. Super Kamiokande observed no event in the limit of 1033 years. At present time there is neither convincing evidence for believing protons and bound nucleons in nucleus undergo decay nor any good reason for believing that they live forever. No fundamental laws of nature prevent protons from decay. It is based on compelling theory. Science is a way of thinking. Failure of any experiment leads the new way. Thomas Alva Edison once said, ’I have not failed. I’ve just found 10,000 ways that won’t work.’ New experiments are being designed. Hyper Kamiokande experiment for detecting proton decay is on line. This experiment aimed to explain GUT and to reveal the evolution of the universe through proton decay and CP violation. Hyper-Kamiokande has sensitivity up to more than one order longer than the current lower lifetime of protons. We are hopeful that Hyper-Kamiokande will discover proton decays and will elucidate the root of materials and mysteries in the genesis of the universe beyond the Standard Model. A Baryon non-conserving process with a small asymmetry in the early universe may make the universe evolve from a symmetric state of matter to an asymmetric state of matter. Symmetry and asymmetry together make our beautiful universe. Universe is full of magical thing beyond our imagination. There are lots of questions to answer and the quest will continue. In the end let us quote from Heisenberg “Not only is the universe stranger than we think, it is stranger than we ‘can’ think”!!
Courtesy : Pictures from Google.
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