On December 27, 1956, Chinese-American physicist Chien Wu performed an experiment that shook the physics community. In her experiment, she attested that *parity is violated in the weak interaction*. This may not sound like a surprising result today, but back then, it was a solution to a mystery that physicists had been trying to solve. In this article, I will explain the whole concept of parity as simply as possible, the experiment that revealed something was wrong, and finally, the genius of the woman who solved this mystery.

**Symmetries and Conservation Laws in Physics**

Before we start learning about parity in physics, we must first understand the concept of symmetries and conservation laws in physics. What is symmetry? Mathematically, the meaning of symmetry is that one shape is exactly like the other shape when it is moved, rotated, or flipped. Rotate a square four times by 90 degrees, clockwise or anticlockwise, and it remains unchanged. In physics, symmetries have pretty much the same meaning.

Suppose you are standing on a large plain ground with a ball in your hand. Let us assume that the point where you are standing is the origin of the cartesian coordinate system. Your job is to drop the ball and note the time taken by it to hit the ground. While the time taken by it depends on the height at which you drop the ball, it surely does not depend on the three factors. First is the place where you throw the ball. The time taken by the ball to reach the ground will be the same if you throw the ball from the point you are standing now (the center of the ground), or from another point, let’s saw near the corner. This implies** translational symmetry**. The second is the time at which you throw the ball. You’ll get the same result if you throw the ball today, tomorrow, or any other time in the future. This implies** temporal symmetry** (related to time). Third, it does not depend on the direction which you are facing when you drop the ball. Whether you are facing east, west, north, or south, the ball will hit the ground in the same duration. The third point of the example signifies **rotational symmetry**.

So far, so good. One important thing related to symmetries is that “*with each symmetry, there is associated a conservation law*.” In physics, a conservation law states that an isolated physical system’s particular measurable property does not change as the system evolves. The relation between the conservation laws and symmetries comes from Noether’s theorem. Translational symmetry signifies the conservation of linear momentum, temporal symmetry implies the conservation of total energy of a system, and finally, rotational symmetry, the conservation of angular momentum.

How are they mathematically connected is not important for us in this article. For now, we just have to remember that symmetries lead to conservation laws.

**The Concept of Parity**

In our example, we considered three variables that could change while dropping the ball: location, time, and angular orientation. Another important variable to keep in mind is our forward or backward motion. If your measurement is independent of whether you are moving forward or backward, then this implies parity. This implies if you change forward or backward in your equations, the results do not change. Mathematically, if you replace x with -x, y with -y, and z with -z, your experiment should remain the same. This is analogous to taking the mirror image of your experiment.

But how do we measure the parity of particles? In quantum mechanics, every particle is described as a wavefunction, let’s say *Ψ* (x). If you apply the parity operation to this wavefunction, i.e., negate the spatial coordinates, and the wavefunction remains unchanged, it is said to have a positive parity (denoted by P = +1). On the other hand, if you apply the parity operation and get the same wavefunction with a negative sign (-*Ψ* (x)), you’ll say that its parity is negative (P = -1).

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**The** **τ-θ Puzzle**

The problem of parity came into the spotlight when physicists came across the τ-θ Puzzle. A strange thing about these particles was that they had identical mass, charge, and lifetime. One of them decayed into two pions, while the other decayed into three pions, as shown below. Pions are elementary particles made up of a quark and an antiquark.

One way to determine the parity of a particle is to observe its decay particles and multiply their parity. As shown above, θ particle decays into two pions. The parity of pions is -1, as determined experimentally. Hence, the resultant parity of θ is (-1) x (-1) = (+1). On the other hand, τ particle, decaying into three pions has a resultant parity of (-1).

It was believed that parity is conserved in every particle interaction. Back then, it was thought to be a ‘law of nature.’ Conservation of parity means that parity is the same before and after the interaction. If parity is conserved, it means that τ and θ are different particles having the same mass, lifetime, and charge. Their made of decay was different. On the other hand, if parity is violated, τ-θ are the same particles.

**Asking The Right Question**

By 1956, the τ-θ puzzle had become a great mystery in the physics community. Scientists would discuss it in conferences, collaborations, and other meetings. In April 1956, Chen Ning Yang attended the so-called Rochester Conference. The Rochester conferences were designed to bring theorists and experimenters together better to understand the pressing physics mysteries of the day. In that conference, Yang presented the τ-θ puzzle, giving an overview. Several talks followed, each presenting a way to solve the mystery. One of the proposals was that τ-θ were two ways a particular unknown particle could decay.

Since this particle could decay into combinations of particles with parity of +1 and -1, this would imply that the original particle didn’t have a unique parity or, if it did, then clearly parity was not conserved. If this explanation turned out to be true, it would be very odd. The discussion at the conference did not solve the problem, but it did feed Yang’s curiosity.

Yang was friends with Tsung-Dao (T.D.) Lee, a professor at Columbia University. Yang would often meet Lee to discuss theoretical problems in particle physics. The τ-θ puzzle had intrigued them to a great extent. The τ and θ decayed via the weak force, one of the four fundamental forces in nature, besides gravity, strong, and electromagnetic force. Assuming a particle could decay into two modes that violated parity conservation, is there a possibility that parity is not conserved in weak decay? Has anyone experimentally verified the parity conservation in weak interaction? Lee and Yang were now asking the right question.

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**Wu Saves The Day**

Lee and Yang were theoretical physicists. They turned to another professor at Columbia University, Chien Wu, an experimental expert in weak decay. Wu was an exceptionally great experimentalist. During that time, there was a saying among the physicists that if Wu performs an experiment, it must be correct. Wu gave the two theorists a book that contained the results of all beta decay experiments in the past 40 years. Lee and Yang went through the book and found that no one had verified parity conservation in weak decay.

A curious experimentalist as she was, Wu decided to test the hypothesis. The most likely result would be that parity is conserved. If it isn’t, it would change everything.

**Wu’s Experiment To Test Parity Conservation**

Before I jump to the experiment performed by Chien Wu, I will briefly introduce the concept of quantum mechanical spin, which will help us understand what Wu was trying to achieve. Temporarily, think of a particle as a tiny rotating sphere. This isn’t accurate but it will serve our purpose. The spin axis can have only two directions…along or against the direction in which the particle is moving. If we use our thumb to point in the direction of motion, we see that we must use either our right or left hand to simultaneously wrap our fingers in the way that the particle is rotating. Depending on which hand is needed, we say that the particle is right- or left-handed.

Suppose your thumb is pointing upwards, in the direction of motion. If the particle is spinning clockwise, you’ll need your left hand to simultaneously wrap the fingers in how the particle is rotating. If it is spinning anti-clockwise, you will need your right hand. Try it! Once you have understood this concept, we are ready to learn the experiment.

To test the hypothesis, Wu needed three things. The first was a nucleus that decayed due to weak force (beta decay). The second was that the nucleus must have an intrinsic quantum mechanical spin. The third and the tricky thing was that all the nuclei spins must be made to point in the same direction. So why is this? This is because you test parity violation by defining a direction and looking at the decay particles that come out of the nucleus. For instance, do the decay products come out aligned with the nuclei’s spin, in the opposite direction, or at 90. If parity symmetry isn’t violated, you shouldn’t be able to tell if you flip all directions to their opposite.

Wu decided to take the nuclei of Cobalt-60 (Co^{60}). It undergoes a beta decay into Nickel-60 (Ni^{60}), a positron e^{+} (positive electron), and an electron neutrino ν_{e} as shown in the image below. Co^{60} has spin 5, and Ni^{60} has spin 4. Since the spin must be strictly conserved, the positron and the neutrino must have a combined spin of 1. This is simple physics. The cool part is yet to come. Another important thing to note is that particles tend to come out along or against the direction of nickel’s spin due to quantum mechanics laws.

Now, look at the left-hand diagram image above. The yellow arrow shows the direction of spin, and the red arrow shows the direction of motion. Consider neutrinos. In this diagram, on the left, the neutrino is traveling in the opposite direction of its spin. It’s a ‘left-handed’ neutrinos. On the right-hand side, the direction of neutrino’s motion coincides with its direction of motion: a right-handed neutrino.

The idea of the experiment is to ‘count’ the number of left-handed and right-handed neutrinos. If their frequency is almost the same, then nature does not differentiate between the system’s ‘handedness’ in weak decay. So what was the result?

To Wu’s surprise, the left-handed neutrinos had more flux than the right-handed neutrinos. This was the first compelling evidence that parity is violated in weak decay. The orientation of the system does matter. Not everyone believed the initial results. In a letter to Victor Weisskopf, Wolfgang Pauli wrote, “I can’t believe that the Lord is a weak left-hander.” But quickly, the experimental results were confirmed by groups around the world.

**The Nobel Prize**

Lee and Yang were later awarded the Nobel Prize in Physics in 1954 to disprove the Conservation of parity. But sadly, Wu’s efforts in proving their theory right went unacknowledged.

She was excluded from the well deserved Nobel, as were many other female scientists during that time. Wu was well aware of gender-based injustice, and at an MIT symposium in October of 1964, she stated, “I wonder whether the tiny atoms and nuclei, or the mathematical symbols, or the DNA molecules have any preference for either masculine or feminine treatment.” Although her experiments did not win her a Nobel in Physics, they definitely gave her the title “The First Lady of Physics.” Wu was decorated with honors in every other way, including the National Medal of Science and the Wolf Prize in Physics. She even had an asteroid named after her in 1990.

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