The groundbreaking discoveries made by Takaaki Kajita and Arthur B. McDonald earned them the prestigious Nobel Prize in Physics in 2015. Their contributions led to a paradigm shift in our understanding of neutrino oscillation, a phenomenon that transformed the field of particle physics. This blog post aims to explore the historical context, delve into the physics underlying neutrino oscillation, and examine the experimental principles employed in the Sudbury Neutrino Observatory (SNO) and Super-Kamiokande.
The Solar Neutrino Problem:
In the mid-1960s, physicist Raymond Davis Jr. embarked on a series of experiments aimed at detecting solar neutrinos—neutrinos emitted from the Sun. Employing an underground detector filled with an abundance of chlorine, Davis Jr. sought to capture the elusive interactions between neutrinos and chlorine atoms. However, the outcome of his experiments consistently revealed a strikingly lower count of observed neutrinos compared to the theoretical predictions of the Sun's neutrino production.
This puzzling discrepancy became widely recognized as the solar neutrino problem, as the theoretical models of the Sun's nuclear reactions anticipated a significantly higher flux of neutrinos, particularly electron neutrinos, than what Davis Jr.'s experiments recorded.
Neutrino Oscillation:
Unraveling the Mystery: Neutrino oscillation emerged as a groundbreaking solution to the solar neutrino problem, shedding light on its perplexing nature. Prior to the discovery of this problem, theoretical physicist Pontecorvo introduced the concept of neutrino oscillation. While standard models assumed that neutrinos were devoid of mass, Pontecorvo postulated that these elusive particles could undergo flavor transitions as they traversed space, provided they possessed mass.
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| Fig.1. Schematic picture of neutrino oscillation. |
The three known flavors of neutrinos—electron, muon, and tau—fall under the purview of quantum mechanics due to their status as fundamental particles. Pontecorvo's proposal posited that these three flavors of neutrinos could mix in the presence of minuscule masses. Consequently, as a result of this mixing phenomenon, neutrinos would oscillate or fluctuate during propagation, leading to a potential alteration in their flavor composition. When solar neutrinos, originating from the Sun's core, reached Earth, their flavors underwent transformation, thereby accounting for the discrepancy observed between predicted and observed solar neutrino quantities.
The problem is how to verify the hypothesis experimentally.
Sudbury Neutrino Observatory (SNO)
To verify the phenomenon of neutrino oscillation, the Sudbury Neutrino Observatory (SNO) was established in Ontario, Canada, in the 1990s. This extraordinary neutrino observatory, situated 2,100 meters underground within Vale's Creighton Mine in Sudbury, featured a target comprising an astounding 1,102 short tons of heavy water contained in a 20-ft acrylic vessel. The activation of this monumental neutrino detector took place in May 1999. The decision to locate the neutrino observatory deep underground holds great significance due to the minuscule nature of neutrino interactions. It is crucial to minimize background signals, which necessitates shielding from cosmic rays. Thus, the choice of an underground location effectively reduces unwanted interference, ensuring a more precise and accurate measurement of neutrino properties.
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| Fig 2. SNO Experiments to observe the solar neutrinos |
The experiment at SNO involved studying three distinct types of neutrino interactions. The first type is the Charged-Current (CC) interaction, which exclusively involves electron neutrinos ($\nu_e$). As electron neutrinos pass through the heavy water tank, they interact through the CC process, where they collide with deuterium (D) nuclei, resulting in the production of two protons (p) and an electron ($e^-$). The high-speed electron emits Cherenkov radiation, an electromagnetic wave, which is observed in the SNO detector as a signal of the electron neutrino's propagation.
The second interaction is the neutral current (NC) interaction, in which all flavors of neutrinos can participate. When electron, muon, or tauon neutrinos ($\nu_e$, $\nu_\mu$, and $\nu_\tau$) traverse the heavy water tank, they interact with deuterium, generating a proton, a neutron, and a neutrino. The produced neutrons from this NC interaction are captured by deuterium nuclei, emitting gamma rays. These signals are observed by the SNO detector, allowing for the detection of both CC and NC interactions from solar neutrinos. The key reactions can be specifically expressed as:
CC interaction: $\nu_e + D \rightarrow p + p + e^-$
NC interaction: $\nu_{x = e, \mu, \tau} + D \rightarrow p + n + \nu_x$
By comparing the flux of electron neutrinos from CC interactions with the total flux of all neutrino flavors from NC and elastic scattering (ES) interactions, SNO researchers discovered a deficit in the number of electron neutrinos reaching the detector. This deficit indicated that neutrinos were indeed oscillating and undergoing flavor transformations during their journey from the Sun to Earth.
Super-Kamiokande:
During a similar period, the Super-Kamiokande experiment commenced in Japan. The Kamikande detector had been established in the 1980s, successfully detecting energetic neutrinos from supernova 1987A. Subsequently, the larger Super-Kamiokande detector was built to study solar neutrinos. The name "Super-Kamiokande" refers to the Super-Kamioka Neutrino Detection Experiment.
Like its counterparts, Super-Kamiokande is also located underground, specifically 1,000 meters beneath the surface in the Mozumi Mine, in Hida's Kamioka area. It consists of a colossal cylindrical stainless steel tank measuring 41.4 meters in height and 39.3 meters in diameter, containing 50,220 metric tons of ultrapure water. The tank is equipped with 11,146 photomultiplier tubes (PMTs) measuring 50 cm in diameter. The immense size of the detector is necessary to observe the minute interactions of neutrinos.
Super-Kamiokande follows a similar principle to SNO but employs a different target material. The detector utilizes ultra-pure water within its vast cylindrical tank, which is lined with photomultiplier tubes capable of detecting the faint flashes of light, known as Cherenkov radiation, emitted when neutrinos interact with the water.
Super-Kamiokande is designed to detect two types of neutrinos: muon neutrinos and electron neutrinos. While it can detect solar neutrinos, it also made a significant discovery regarding atmospheric neutrinos.
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| Fig.3. Atmospheric neutrino detection in Super-Kamiokande. |
Atmospheric neutrinos result from the decay of secondary cosmic rays generated by interactions between primary cosmic rays, mostly protons, and the Earth's atmosphere. The predicted number of neutrinos observed should remain consistent, irrespective of the zenith angle. However, in 1998, Super-Kamiokande observed that the number of upward-going muon neutrinos (generated on the opposite side of the Earth) was half the number of downward-going muon neutrinos. This phenomenon can be explained by neutrinos oscillating or changing into undetectable states. The Super-Kamiokande collaboration successfully detected this phenomenon, providing compelling evidence of neutrino oscillation.
Conclusion
The groundbreaking contributions of Takaaki Kajita, who led the Super-Kamiokande experiment, and Arthur B. McDonald, who directed the SNO experiments, were recognized with the Nobel Prize in Physics in 2015. These monumental neutrino detectors unveiled the mysteries of neutrino oscillation, revealing that neutrinos must possess mass. However, there is still much work to be done. Given that the standard model suggests neutrinos are massless, physicists continue to explore the origin of neutrino masses and methods for measuring their absolute values.
Moreover, the development of neutrino observatories has opened up new opportunities in multi-messenger astrophysics. Neutrinos generated by the Sun reach Earth in a mere 8 minutes due to the minuscule nature of neutrino interactions, whereas photons take approximately 200,000 years. This implies that if we can detect neutrino signals more rapidly than photons, we can gain insights into astrophysical events ahead of traditional observations. We now find ourselves in an era of multi-messenger astrophysics, where the detection of various observational signals, including neutrinos, will further expand our knowledge and understanding of the universe.
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