The Curious Handedness of Matter and Antimatter

 

The lectures at the New Scientist Live were incredible. Due to parallel tracks I could not attend all the lectures and I hope to catch them up with my online access. I attended most of the Physics lectures in the Universe Stage. I thoroughly enjoyed the lecture by eminent Frank Close (I grew up reading his books), astrophysicist Chris Lintott, theoretical physicist Claudia de Rham (I try to attend her talks whenever possible), Jo Dunkley (this is the first time I attended her lecture — she was so lucid in her explanation of the Dark Matters.), and Or Graur.

I have never thought much about the left-handedness of our material universe. As I am flying 33000 feet above and the outside temperature is minus 62 degree Fahrenheit, I like to ponder about the weird handedness of Matter and Anitmatter.

As I mentioned above, one of the most intriguing mysteries is the apparent preference for “handedness” in matter and antimatter. This phenomenon, known as chirality, has profound implications for our understanding of the universe and the fundamental laws that govern it. The asymmetry in the behavior of particles and antiparticles challenges our notions of symmetry and raises fundamental questions about the origin and evolution of the cosmos.

What is Handedness?

In particle physics, handedness or chirality refers to a property of particles that describes how their intrinsic spin aligns with their motion. It’s analogous to left and right hands, which are mirror images but cannot be perfectly superimposed. In the context of particles:

• Left-handed particles: Spin is opposite to the direction of motion.
• Right-handed particles: Spin is in the same direction as the motion.

Mathematically, handedness is connected to the concept of helicity, which is the projection of a particle’s spin vector onto its momentum vector. For massless particles like photons, helicity and chirality are equivalent and invariant under Lorentz transformations. However, for massive particles, helicity depends on the observer’s frame of reference, while chirality remains a Lorentz-invariant property defined through the Dirac equation and gamma matrices.

The Dirac spinor ψ\psiψ can be decomposed into left-handed and right-handed components using the projection operators:

The Left-Handed Nature of Matter

Observations have shown that in our universe, matter particles involved in weak interactions are predominantly left-handed. This means that their spin is oriented opposite to their direction of motion. This phenomenon was first experimentally confirmed in the famous Wu experiment in 1957, conducted by physicist Chien-Shiung Wu. She observed that electrons emitted in the beta decay of cobalt-60 nuclei were preferentially left-handed, demonstrating that parity symmetry is violated in weak interactions.

Neutrinos, elusive fundamental particles with very small masses, are always observed to be left-handed in weak interactions. In the Standard Model of particle physics, only left-handed neutrinos and right-handed antineutrinos participate in weak interactions. The absence of right-handed neutrinos and left-handed antineutrinos in observed processes is a peculiar feature that distinguishes the weak force from other fundamental forces.

This left-handed preference is encoded in the weak interaction through the V-A (vector minus axial vector) structure of the weak currents. The weak force couples to particles via the weak isospin SU(2) gauge symmetry, where left-handed fermions are organized into doublets, and right-handed fermions are singlets (non-interacting under SU(2)).

For example, the left-handed electron and its associated neutrino form a weak isospin doublet:

The Right-Handed Nature of Antimatter

Conversely, antimatter particles involved in weak interactions tend to exhibit right-handed properties. When observing antiparticles like positrons (the antiparticles of electrons), their spin aligns with their direction of motion, making them right-handed.

In beta-plus decay, a proton in a nucleus transforms into a neutron, emitting a positron and a neutrino:

The emitted positron is right-handed, and the neutrino is left-handed. This right-handedness of antiparticles in weak interactions mirrors the left-handedness of matter particles and highlights the asymmetric nature of the weak force.

The preference for right-handed antiparticles is also a consequence of the V — A structure of the weak interaction. The coupling of the weak force to fermions involves the projection operators that select left-handed components for particles and right-handed components for antiparticles.

Why Does This Matter?

The prevalence of left-handed matter and right-handed antimatter in our universe has significant implications:

1. Violation of Parity Symmetry (P-Symmetry): Parity symmetry suggests that physical laws should be invariant under spatial inversion (mirror reflection). The discovery that weak interactions violate parity, as shown in Wu’s experiment, was a groundbreaking revelation that nature inherently distinguishes between left and right.
2. Violation of Charge-Parity Symmetry (CP-Symmetry): While the violation of parity was accepted, it was initially thought that the combination of charge conjugation ©, which swaps particles with antiparticles, and parity (P) would restore symmetry. However, in 1964, James Cronin and Val Fitch discovered CP violation in the decay of neutral K-mesons (kaons). This violation implies that the laws of physics are not entirely symmetric when both charge and spatial coordinates are inverted.
3. Matter-Antimatter Asymmetry: The observed CP violation is a key ingredient in explaining the dominance of matter over antimatter in the universe. According to the Sakharov conditions proposed by Andrei Sakharov in 1967, CP violation is necessary for a matter-antimatter imbalance to develop in the early universe. The preference for left-handed matter and right-handed antimatter could be linked to mechanisms that led to baryogenesis, the process that produced the excess of baryons (matter particles) over antibaryons.
4. Weak Interaction Uniqueness: The weak force is the only fundamental interaction that violates parity and CP symmetry significantly. This uniqueness makes it a critical area of study for understanding the fundamental asymmetries in nature.
5. Implications for New Physics: The violations of CP symmetry observed are not sufficient to account for the observed matter-antimatter asymmetry. This suggests that there may be additional sources of CP violation beyond the Standard Model, possibly involving right-handed neutrinos or other exotic particles.

Ongoing Research

While we have observed this handedness preference, the underlying reason for it remains one of the great unsolved mysteries in physics. Researchers continue to explore various theories and conduct experiments to understand this phenomenon better. Some areas of ongoing research include:

• Studying Neutrino Oscillations and Masses: Neutrino oscillation experiments like T2K in Japan, NOvA in the USA, and the upcoming DUNE experiment aim to measure neutrino mixing parameters and CP violation in the lepton sector. The discovery that neutrinos have mass (requiring physics beyond the original Standard Model) opens up possibilities for right-handed neutrinos and new interactions.
• Investigating CP Violation in B-Meson Systems: Experiments like LHCb at CERN and Belle II in Japan study CP violation in the decays of B-mesons. Precise measurements of CP-violating parameters help test the predictions of the Standard Model and search for signs of new physics.
• Searching for Right-Handed Currents: Some theories beyond the Standard Model, such as left-right symmetric models, propose the existence of right-handed weak interactions mediated by additional W bosons. Experiments aim to detect these currents through rare decay processes or deviations in expected particle interactions.
• Exploring Supersymmetry and Grand Unified Theories (GUTs): These theories attempt to unify the fundamental forces and often predict additional particles and interactions that could explain the origin of handedness. Supersymmetry, for instance, introduces superpartners for all Standard Model particles, potentially affecting CP violation and matter-antimatter asymmetry.
• Investigating Leptogenesis: This is a theoretical framework where an asymmetry in the lepton sector (due to CP violation involving heavy right-handed neutrinos) leads to the observed baryon asymmetry through sphaleron processes in the early universe. Testing leptogenesis requires understanding neutrino masses and mixing in great detail.
• Double Beta Decay Experiments: Searches for neutrinoless double beta decay aim to determine whether neutrinos are Majorana particles (identical to their antiparticles). The observation of this rare decay would have profound implications for the nature of neutrinos and could provide insights into the handedness problem.
• Precision Measurements of Fundamental Constants: Experiments that measure electric dipole moments (EDMs) of particles like the electron or neutron test CP violation at very high precision. Non-zero EDMs would indicate new sources of CP violation beyond the Standard Model.
• Collider Searches for New Particles: High-energy colliders like the Large Hadron Collider (LHC) continue to search for signs of new particles that could explain the handedness of matter and antimatter. This includes searches for heavy neutrinos, additional gauge bosons, and supersymmetric particles.

Understanding the handedness of matter and antimatter could provide crucial insights into the nature of our universe, potentially shedding light on its origins and evolution. The interplay between theoretical predictions and experimental discoveries continues to drive the field forward, with the hope of unraveling one of the most fundamental questions in physics: Why does the universe exhibit a preference for left-handed matter over right-handed antimatter in weak interactions?

The resolution of this mystery may require a new framework that goes beyond the Standard Model, incorporating novel particles and symmetries. As technology advances and experimental techniques improve, physicists are optimistic about making breakthroughs that could redefine our understanding of the fundamental forces and particles that constitute reality.