Ghost hunting of the universe: unraveling the enigma of the neutrino

Abstract concept of astrophysics light dimension portal

Abstract concept of astrophysics light dimension portal

The Majorana Demonstrator, a six-year experiment conducted by Indiana University researchers and international collaborators, sought to answer significant questions about the fundamental laws of physics, particularly as they pertain to neutrinos. The study aimed to observe whether neutrinos could be their own antiparticles and the occurrence of neutrinoless double beta decay, which, although not conclusively observed, has provided valuable insights into neutrino decay times, dark matter, quantum mechanics and demonstrated that the research techniques used can be expanded for future work in understanding the composition of universes.

A team of Indiana University researchers, along with international collaborators, is actively engaged in unraveling the fundamental mysteries surrounding the fundamental laws of physics that govern our universe.

For the past six years, a team of Indiana University researchers, along with international collaborators, has been working to unravel the mysteries of the fundamental physical laws that govern our universe. They conducted an experiment known as the Majorana Demonstrator, which greatly improved our understanding of neutrinos, one of the fundamental building blocks of the universe.

The final report of the experiments was recently published in Neutrinos, tiny particles comparable to electrons but devoid of electric charge, rank as the second most plentiful particles in the universe, trailing only light. Despite this abundance, they prove challenging to study because they do not interact the way other particles do.

Neutrinos have a profound impact on the universe and physics at every imaginable scale, surprising us down at the particle interaction level and having broad impact up through the cosmic scales, said Walter Pettus, an assistant professor of physics in the IU College of Arts and Sciences. But they are also the most frustrating to study because we know so much about them, yet we have so many gaps.

Nafis Fuad

Nafis Fuad. Credit: Indiana University

The Majorana Demonstrator, a collaboration of 60 researchers from 24 institutions, was designed to fill many of those gaps at the same time, probing into the most fundamental properties of neutrinos.

One aspect they hoped to observe was whether the neutrino could be its own antiparticle a subatomic particle of the same mass but with the opposite electric charge. Since the neutrino is uncharged, it is the only particle in the universe that could be its own antiparticle. Understanding that would provide insight into why the neutrino has mass in the first place information that would have widespread impacts in understanding how the universe was formed.

To determine if the neutrino is its own antiparticle, the researchers needed to observe a rare occurrence called neutrinoless double-beta decay. However, this process takes a single atom at least 1026 years significantly longer than the age of the universe. Instead, they chose to observe nearly 1026 atoms over the course of six years.

To observe this incredibly rare decay, the researchers needed the perfect environment. In the Sanford Underground Research Facility in the Black Hills of South Dakota, located a mile underground, they built one of the cleanest and quietest environments on Earth. Extremely sensitive detectors were made of high-purity germanium and were packed in a 50-ton lead shield and surrounded by materials of unprecedented cleanliness. Even the copper used was grown underground in their lab with impurity levels so low they couldnt be measured.

Pettus and a team of IU students were responsible primarily for analyzing data from the experiment. Graduate student Nafis Fuad, undergraduate senior Isaac Baker, sophomore Abby Kickbush and Jennifer James, a student with the Research Experiences for Undergraduates Program, have been involved in the project. Their focus has been on understanding the stability of the experiment, analyzing details of the recorded waveforms, and characterizing backgrounds.

Walter Pettus

Walter Pettus. Credit: Indiana University

Its like looking for a tiny needle in a very, very, very big haystack you have to carefully get rid of all the hays (a.k.a. backgrounds) possible, and you dont even know if theres actually a needle in there in the first place or not, Fuad said. Its very exciting to be a part of that search.

While the researchers ultimately did not observe the decay they hoped for, they did discover that the neutrinos scale for decay is longer than the limit they placed on it, which they will test further during the next phase of the experiment. In addition, they recorded other scientific results ranging from dark matter to quantum mechanics that helps provide a better understanding of the universe.

Through the project, the researchers proved that the techniques they utilized could be used at a much larger scale in a potentially game-changing search that could help explain the existence of matter in the universe.

We didnt see the decay we were looking for, but we have raised the bar on where to look for the physics were going after, Pettus said. True to its name, the Demonstrator advanced critical technologies that we are already leveraging for the next phase of the experiment in Italy. We may not have broken our picture of physics yet, but weve certainly pushed the horizons, and I am very excited about what we have accomplished.

The next phase of the project, called LEGEND-200, has already begun taking data in Italy, with plans to run over the next five years. Researchers aim to observe the decay happening at a magnitude higher sensitivity than the Majorana Demonstrator. Beyond that, thanks to support from the U.S. Department of Energy, the team is already designing the successor experiment, LEGEND-1000.

Pettus is excited about the future of this work and looks forward to involving more students in the project, both in data analysis and hardware development for LEGEND-1000.

If we discover the neutrino is its own antiparticle, there will still be ground under our feet and stars in the sky, and our understanding of physics doesnt change the reality of the physical laws that always have and continue to govern our universe, Pettus said. But knowing whats down there at the most fundamental level and how the universe works gives us a richer, more beautiful world to live in or possibly just weirder and that pursuit is fundamentally human.

Reference: Final Result of the Majorana Demonstrators Search for Neutrinoless Double- Decay in 76Ge by I.J. Arnquist et al. (Majorana Collaboration), 10 February 2023, Physical Review Letters.
DOI: 10.1103/PhysRevLett.130.062501

The Majorana Demonstrator was managed by Oak Ridge National Laboratory for the U.S. Department of Energy Office of Nuclear Physics, with support from the National Science Foundation.


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Hunting down elusive axion particles: Experiments suggest better ways to explore the dark sector

Hunting down elusive axion particles: Experiments suggest better ways to explore the dark sector

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The 90% predicted and effective CL from CCM120 for ALP-photon coupling gA. Also included is the projection region for the three-year run of the CCM200 using background taken from the CCM120 spectrum reduced by two orders of magnitude for various conservative improvements (dashed green line) and a no-background hypothesis (extension of the shaded green region). The parameter space of the QCD axion model for the reference scenario KSVZ extends in the region indicated by the arrows. Credit: Physical review D (2023). DOI: 10.1103/PhysRevD.107.095036

Since axions were first predicted by theory nearly half a century ago, researchers have searched for evidence of the elusive particle, which may exist outside the visible universe, in the dark sector. But how to find particles that cannot be seen?

The first physics results of the Coherent CAPTAIN-Mills experiment at Los Alamos just described in a publication in the journal Physical review Dsuggest that liquid argon accelerator-based experimentation, initially designed to search for similar hypothetical particles such as sterile neutrinos, could also be an ideal setup for stealthy axion searches.

“Confirmation of dark sector particles would have a profound impact on understanding the Standard Model of particle physics, as well as the origin and evolution of the universe,” said physicist Richard Van de Water. “A major focus of the physics community is exploring ways to detect and confirm these particles. The Coherent CAPTAIN-Mills experiment pairs existing predictions of dark matter particles such as axions with high-intensity particle accelerators capable of producing this obscure hard to find question.”

Demystifying the dark sector

Physics theory suggests that only 5% of the universe is made up of visible matter atoms that form things we can see, touch and feel, and that the remaining 95% is the combination of matter and energy known as the dark sector. Axions, sterile neutrinos, and others can account for and explain all or part of that missing energy density.

The existence of axions could also solve a long-standing problem in the Standard Model, which delineates the known behavior of the subatomic world. Sometimes referred to as “fossils” of the universe, hypothesized to have originated just a second after the Big Bang, axions could also tell us a lot about the universe’s founding moments.

The Coherent CAPTAIN-Mills experiment was one of several projects to receive funding from the Department of Energy for dark sector research in 2019, along with substantial funding from the laboratory-led research and development program at Los Alamos. A prototype detector dubbed CCM120 was built and operated during the 2019 Los Alamos Neutron Science Center (LANSCE) beam cycle. Physical review D publication describes the results of the initial engineering run of the CCM120.

“Based on the first round of CAPTAIN-Mills searches, the experiment demonstrated the ability to perform axion search,” said Bill Louis, also a project physicist at Los Alamos. ‘We are realizing that the energy regime provided by the LANSCE proton beam and liquid argon detector design offers an unexplored paradigm for axion-like particle research.’

Experiment design

Stationed in the Lujan Center adjacent to LANSCE, the Coherent CAPTAIN-Mills experiment is a 10-ton, supercooled, liquid argon detector. (CAPTAIN stands for Cryogenic Apparatus for Precision Tests of Argon Reactions with Neutrinos.)

High-intensity 800 megaelectron-volt protons generated by the LANSCE accelerator strike a tungsten target in the Lujan Center, then travel 23 meters through a large steel-and-concrete shield to the detector to interact with liquid argon.

The prototype detector’s inner walls are lined with 120 sensitive eight-inch photomultiplier tubes (hence the CCM120 nickname) that detect single-photon light flashes that occur when a normal or dark sector particle pushes an atom into the argon tank. liquid.

A special material coating on the inner walls converts the light output of the argon into visible light which can be detected by the photomultiplier tubes. The rapid timing of the detector and beam helps remove the effects of background particles such as beam neutrons, cosmic rays and gamma rays from radioactive decays.

Pieces of the puzzle

Axions are of great interest because they are “highly motivated”; that is, their existence is strongly implied by theories beyond the Standard Model. Developed over more than 70 years, the Standard Model explains three of the four known fundamental forces: electromagnetism, the weak nuclear force and the strong nuclear force that govern the behavior of atoms, the building blocks of matter. (The fourth force, gravity, is explained by Einsteinian relativity.) But the model is not necessarily complete.

An unsolved problem in Standard Model physics is known as the “strong CP problem”, where “CP” stands for charge-parity symmetry. Essentially, particles and their antiparticle counterparts are similarly affected by the laws of physics. However, nothing in Standard Model physics mandates that behavior, so physicists should see at least occasional violations of that symmetry.

In weak force interactions, charge parity symmetry violations occur. But no similar violations were observed in strong-force interactions. That puzzling absence of theoretically possible behavior poses a problem for Standard Model theory. What prevents charge parity symmetry violations from occurring in strong-force interactions?

Abundant, nearly weightless, and electrically neutral, axions can be an important part of the puzzle. Axion earned its nickname in 1978, coined by physicist Frank Wilczek after a brand of laundry detergent because such a particle could “clean up” the strong CP problem. Physicists speculate that they are components of a dark matter force that preserves charge parity symmetry and that they can pair or interact with photons and electrons.

Next steps

If axions exist, finding them may be a matter of devising the right experimental setup.

“As a result of this initial run with our CCM120 detector, we have a much better understanding of the signatures associated with axion-like particles coupled to photons and electrons as they move through liquid argon,” said Louis. “This data gives us the insight to upgrade the detector to be an order of magnitude more sensitive.”

More information:
AA Aguilar-Arevalo et al, Prospects for the detection of axion-like particles in the Coherent CAPTAIN-Mills experiment, Physical review D (2023). DOI: 10.1103/PhysRevD.107.095036

About the magazine:
Physical review D

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