Nobel Prize in Physics 2008: Broken Symmetry and the Origin of Quark Families

This note covers the Nobel Prize in Physics 2008: who won it, what "broken symmetry" means in particle physics, how Yoichiro Nambu's idea of spontaneous symmetry breaking works, how Makoto Kobayashi and Toshihide Maskawa explained a strange asymmetry between matter and antimatter using three families of quarks, how the discovery unfolded over decades, why it matters for the Standard Model of physics, and quick facts for exams.

What was the Nobel Prize in Physics 2008 awarded for?

The Nobel Prize in Physics 2008 was divided between three physicists for two related but separate discoveries about broken symmetry in the world of subatomic particles.

Yoichiro Nambu received one half of the prize "for the discovery of the mechanism of spontaneous broken symmetry in subatomic physics".

Makoto Kobayashi and Toshihide Maskawa jointly received the other half "for the discovery of the origin of the broken symmetry which predicts the existence of at least three families of quarks in nature".

In plain words, symmetry in physics means that something looks or behaves the same way under some change, such as being reflected in a mirror or having its matter swapped for antimatter.

Scientists long expected nature's basic laws to be perfectly symmetric. The 2008 laureates showed, in two different ways, that symmetry is sometimes spontaneously broken or hidden, and that this breaking is essential to explain why the universe looks the way it does, including why matter won out over antimatter after the Big Bang.

The official name of this award is the Nobel Prize in Physics.

Who are the laureates?

Yoichiro Nambu

Yoichiro Nambu was born on 18 January 1921 in Tokyo, Japan, and died on 5 July 2015 in Osaka, Japan.

At the time of the award he was affiliated with the Enrico Fermi Institute, University of Chicago, in Chicago, Illinois, USA, where he held the title Harry Pratt Judson Distinguished Service Professor Emeritus. He received one half of the prize.

Nambu's contribution was theoretical. As early as 1960 he worked out a mathematical description of how a symmetry present in the basic laws of a system can fail to appear in the system's actual lowest-energy state.

He first developed this idea while studying superconductivity, where electric current flows without resistance, and then carried the same mathematics into the physics of elementary particles.

Makoto Kobayashi

Makoto Kobayashi was born on 7 April 1944 in Nagoya, Japan. At the time of the award he was affiliated with the High Energy Accelerator Research Organization (KEK) in Tsukuba, Japan, where he held the title Professor Emeritus. He received one quarter of the prize.

Together with Toshihide Maskawa, Kobayashi worked out in 1972, while both were at the University of Kyoto, a mathematical explanation for an already-observed broken symmetry in the decay of particles called kaons.

Their explanation required that nature contain at least three families of quarks rather than the two known at the time.

Toshihide Maskawa

Toshihide Maskawa was born on 7 February 1940 in Nagoya, Japan, and died on 23 July 2021 in Kyoto, Japan.

At the time of the award he was affiliated with Kyoto Sangyo University and the Yukawa Institute for Theoretical Physics (YITP), Kyoto University, both in Kyoto, Japan.

He received one quarter of the prize, sharing it jointly with Kobayashi for the same citation.

What problem were the laureates trying to solve?

By the middle of the 20th century, physicists had built up a picture of matter made of atoms, with protons and neutrons in the nucleus and electrons around it.

New particle accelerators built after the Second World War then produced a flood of new, short-lived particles that did not fit this simple picture.

Eventually it became clear that protons and neutrons, and many of these new particles, were themselves built from smaller particles called quarks.

Physicists also expected nature's laws to respect certain symmetries. Three were especially important: mirror symmetry (called P, for parity, meaning events should look the same seen directly or in a mirror), charge symmetry (called C, meaning particles should behave like their antiparticles with opposite charge) and time symmetry (called T, meaning events should look the same run forwards or backwards).

These symmetries were thought to simplify calculations and to underlie important conservation laws, such as the conservation of energy and of electric charge.

In 1956, Tsung Dao Lee and Chen Ning Yang proposed that mirror symmetry could be broken in the weak force, the force responsible for radioactive decay, and this was confirmed soon afterwards in experiments on the decay of cobalt 60.

Physicists then hoped that the combined CP symmetry (mirror plus charge symmetry together) would still always hold.

In 1964, however, James Cronin and Val Fitch found that even CP symmetry was broken, very slightly, in the decay of particles called kaons.

This surprising result had no accepted explanation and threatened to unsettle the whole developing theoretical framework for particle physics, later called the Standard Model.

A further, larger mystery loomed behind these small laboratory effects: according to the Royal Swedish Academy of Sciences, in the Big Bang some 14 billion years ago, equal amounts of matter and antimatter should have been created and should have annihilated each other completely.

Instead, a tiny excess, about one extra particle of matter for every ten billion antimatter particles, survived and became everything we see today, including stars, planets and ourselves.

Explaining the origin of broken symmetry was therefore tied to explaining why the universe contains matter at all.

How did Nambu explain spontaneous broken symmetry?

Nambu's key insight, worked out from 1960 onward, was that the equations describing a system can be perfectly symmetric even though the system's actual, observed state is not.

The Royal Swedish Academy of Sciences used a simple picture for this: a pencil balanced upright on its point is in a completely symmetric situation, since every direction it could fall is equally likely.

But once it falls, it picks one particular direction, and the original symmetry is hidden, not destroyed, in the fallen state.

Nambu first noticed this pattern while studying superconductivity, the phenomenon in which electric current flows through certain materials without any resistance.

He then translated the same mathematics into elementary particle physics, proposing that the vacuum itself, the lowest-energy state of a quantum field, need not share the full symmetry of the underlying physical laws.

1. Start from a theory whose equations are symmetric under some transformation, such as swapping left and right, or rotating a field in an abstract internal space.
2. Find the lowest-energy, or "ground", state of the system, which in quantum field theory is called the vacuum.
3. Check whether this ground state itself still obeys the full symmetry of the equations, or whether it "chooses" one particular direction or configuration, as the fallen pencil does.
4. If the ground state breaks the symmetry while the equations do not, the symmetry is said to be spontaneously broken, and new physical consequences follow, such as new light or massless particles appearing in the theory.

Draw a pencil balanced upright on its sharp point, with arrows pointing outward in every direction to show that all directions are equally likely while it stands.

Then draw the same pencil fallen flat, lying along one particular direction, with the other possible directions shown as faint dashed arrows to show that the original symmetry is now hidden rather than gone.

This idea of spontaneous symmetry breaking turned out to be far more than a mathematical curiosity.

It became one of the central tools used to build the Standard Model of particle physics, the theory that unifies the smallest building blocks of matter with three of nature's four fundamental forces, the electromagnetic, weak and strong forces, into one framework.

How did Kobayashi and Maskawa explain the broken symmetry in quarks?

Kobayashi and Maskawa tackled a different, more specific puzzle: the broken CP symmetry that Cronin and Fitch had observed in kaon decays in 1964.

By 1972, working within the developing Standard Model, Kobayashi and Maskawa showed mathematically that the existing theory, built on only two families of quarks, could not generate the kind of complex coupling needed to break CP symmetry at all.

Their solution was to extend the model to at least three families of quarks. A kaon particle, they explained, is made of a quark and an antiquark, and the weak force can make the quark and antiquark repeatedly swap identities, turning the kaon into its own antiparticle and back again.

Kobayashi and Maskawa's calculation, expressed through a mathematical matrix describing how different quark types mix and transform into each other, showed that if there were three quark families rather than two, a tiny asymmetry between matter and antimatter would naturally appear in this process.

At the time, only two families of quarks were known to exist, so proposing a third family was a bold step.

The particles in these extra families were discovered one by one afterwards: the charm quark in 1974, the bottom quark in 1977, and the top quark, the last and heaviest, as late as 1994.

The prediction was tested still more precisely when two enormous particle-detector experiments, BaBar at the Stanford accelerator in the United States and Belle at the KEK accelerator in Tsukuba, Japan, together produced more than a million B-meson particles a day.

As late as 2001, both detectors independently confirmed the symmetry violation in B-mesons, matching almost exactly what Kobayashi and Maskawa had predicted nearly three decades earlier.

How did the discovery unfold?

Why does this discovery matter?

Nambu's idea of spontaneous symmetry breaking became one of the pillars of the Standard Model of particle physics, the theory that unifies the electromagnetic, weak and strong forces with all the known elementary particles.

According to the Royal Swedish Academy of Sciences, Nambu's mathematical tools "permeate" this model, and the same approach is used today to calculate the effects of the strong force.

Kobayashi and Maskawa's work completed an important missing piece of the same model by showing why matter and antimatter do not behave in perfectly mirrored ways, and it correctly predicted the existence of particles that were only discovered decades later, the last one in 1994.

Their confirmation in B-meson experiments in 2001 is cited by the Academy as settling a long-standing question about the structure of matter.

A bigger mystery remains open. The small CP violation seen in kaons and B-mesons is, according to the Academy, far too small to explain why the universe is made almost entirely of matter rather than being annihilated along with an equal quantity of antimatter.

The sources state that the question of exactly how this larger imbalance came about "still remains unanswered", and physicists have hoped that experiments at large accelerators such as the LHC at CERN might shed further light on it.

How does this connect to what you study?

Symmetry is also a familiar idea in school mathematics and physics, where students study mirror symmetry, rotational symmetry and conservation laws such as the conservation of energy and conservation of electric charge.

The 2008 Physics prize shows how the same basic idea, that a law can be symmetric even when its outcome is not, scales up from simple shapes to the deepest questions about why matter exists at all. A student who has drawn a shape with a line of symmetry, or balanced a ruler on a point, has already met the same principle that Nambu applied to the whole universe of particles.

Learning that quarks come in families, and that particles like the kaon and the B-meson can act as windows into very small, rare violations of a symmetry, gives a sense of how experiments in giant accelerators test ideas first worked out on paper decades earlier.

This also links to basic chemistry and physics ideas about the structure of matter: protons and neutrons, which make up the nucleus of every atom studied in school science, are themselves built from quarks. The forces covered in physics classes, electromagnetic, and later the stronger nuclear forces, are the same forces that the Standard Model, strengthened by Nambu's and by Kobayashi and Maskawa's work, tries to unify into one single description.

Quick facts for exams

The Nobel Prize in Physics 2008 was announced on 7 October 2008 by the Royal Swedish Academy of Sciences. It was shared between three Japanese-born physicists for work on broken symmetry in subatomic physics.

Yoichiro Nambu, based at the University of Chicago in the USA, received one half of the prize for discovering the mechanism of spontaneous broken symmetry.

Makoto Kobayashi, based at KEK, and Toshihide Maskawa, based at Kyoto Sangyo University and the Yukawa Institute, shared the other half for explaining the origin of broken symmetry that predicts at least three families of quarks. The total prize amount was 10,000,000 Swedish kronor.

Note: Source. The prize facts in this note are from the Nobel Prize's official site, nobelprize.org.

Glossary

• Symmetry — a situation in which something stays the same under a particular change, such as reflection in a mirror or rotation.
• Spontaneous symmetry breaking — when a system's lowest-energy state fails to show a symmetry that is present in the underlying physical laws.
• Parity (P) — mirror symmetry, the idea that a process and its mirror image should be equally likely.
• Charge symmetry (C) — the idea that particles should behave the same as their antiparticles, which carry opposite charge.
• Time symmetry (T) — the idea that physical events should look the same whether time runs forwards or backwards.
• CP symmetry — the combination of mirror symmetry and charge symmetry acting together.
• Quark — one of the fundamental particles that combine to form protons, neutrons and many other particles.
• Quark family — a pair of related quark types; the Standard Model needs three families for Kobayashi and Maskawa's explanation to work.
• Kaon — a particle made of a quark and an antiquark, whose decay first revealed broken CP symmetry in 1964.
• B-meson — a particle about ten times heavier than the kaon, used to test predictions about broken symmetry.
• Standard Model — the theory unifying the elementary particles with the electromagnetic, weak and strong forces.
• Vacuum — in quantum physics, the lowest-energy state of a system, not simply empty space.
• Superconductivity — a phenomenon in which electric current flows through certain materials with no resistance; it inspired Nambu's ideas.
• Antimatter — matter made of antiparticles, which have the same mass as ordinary particles but opposite charge.

Common errors and misconceptions

• Misconception: Nambu, Kobayashi and Maskawa won the prize for discovering quarks themselves. Correct: quarks had already been proposed and discovered by other scientists; Kobayashi and Maskawa's contribution was predicting that a third family of quarks must exist, to explain an already observed broken symmetry.
• Misconception: "broken symmetry" means a symmetry has been destroyed or disproved. Correct: in spontaneous symmetry breaking, the underlying laws remain symmetric; it is only the observed, lowest-energy state that hides the symmetry.
• Misconception: the 2008 prize fully explains why the universe contains far more matter than antimatter. Correct: the Academy stated that the small CP violation observed is not enough to explain this, and the question remains open.
• Misconception: Nambu, Kobayashi and Maskawa worked together on one shared project. Correct: Nambu's work on spontaneous symmetry breaking (from 1960) and Kobayashi and Maskawa's work on quark families (from 1972) were separate discoveries, recognised together under the broader theme of broken symmetry.
• Misconception: the broken symmetry Cronin and Fitch found in 1964 was predicted in advance. Correct: their discovery came as a complete surprise, and Kobayashi and Maskawa's explanation for it came eight years later, in 1972.
• Misconception: all three laureates shared the prize money equally. Correct: Nambu received one half of the 10,000,000 Swedish kronor prize, while Kobayashi and Maskawa each received one quarter.

Exam-style questions with model answers

Q1. In which year was the Nobel Prize in Physics 2008 announced? [1 mark]

1. It was announced on 7 October 2008, by the Royal Swedish Academy of Sciences.

Q2. State the exact citation for Yoichiro Nambu's share of the 2008 Nobel Prize in Physics. [2 marks]

1. Nambu was awarded his half of the prize "for the discovery of the mechanism of spontaneous broken symmetry in subatomic physics".

Q3. Explain, in your own words, what "spontaneous symmetry breaking" means, using an everyday example. [4 marks]

1. Spontaneous symmetry breaking occurs when the underlying equations of a system are perfectly symmetric, but the system's actual lowest-energy state is not. A standard everyday example is a pencil balanced upright on its point: every direction it could fall is equally likely, so the standing situation is symmetric. Once the pencil falls, it picks one particular direction, and the symmetry seems lost, even though it was present in the original situation. Nambu applied this same mathematical idea, first studied in superconductivity, to elementary particle physics from 1960 onward, and it later became a key tool for building the Standard Model.

Q4. Why did Kobayashi and Maskawa propose that nature needed at least three families of quarks? [4 marks]

1. By 1972, physicists knew that the weak force broke CP symmetry very slightly in the decay of kaons, as discovered by Cronin and Fitch in 1964. Kobayashi and Maskawa showed mathematically that the existing theory, built using only two known quark families, could not produce the kind of complex coupling needed to generate this broken symmetry. By extending the model to three quark families, they found a natural way for the required symmetry breaking to appear in the mathematics describing how quarks mix and transform into each other during weak decays.

Q5. Trace the sequence of experimental discoveries that confirmed Kobayashi and Maskawa's 1972 prediction. [5 marks]

1. Kobayashi and Maskawa's 1972 theory required particles from a third quark family that had not yet been observed. The charm quark was discovered in 1974, followed by the bottom quark in 1977, and finally the top quark, the heaviest and last to be found, in 1994. These discoveries confirmed that the extra family of quarks genuinely existed. The theory's prediction about broken symmetry itself was tested still more directly in 2001, when the BaBar detector at the Stanford accelerator in the USA and the Belle detector at the KEK accelerator in Tsukuba, Japan, each independently observed broken symmetry in heavy particles called B-mesons, matching Kobayashi and Maskawa's calculations from almost thirty years earlier.

Q6. Discuss why the Royal Swedish Academy of Sciences considered broken symmetry important for understanding the origin of the universe. [6 marks]

1. According to the Royal Swedish Academy of Sciences, the Big Bang some 14 billion years ago should have created equal amounts of matter and antimatter, which should then have completely annihilated each other, leaving only radiation. Instead, a tiny excess, roughly one extra particle of matter for every ten billion antimatter particles, survived, and this small surplus became the seed of all the galaxies, stars, planets and living things that exist today. The broken symmetries studied by Nambu, Kobayashi and Maskawa are examples of the kind of asymmetry between matter and its mirror or antimatter counterpart that could, in principle, help explain such an imbalance. However, the broken symmetry actually observed in kaons and B-mesons is far too small by itself to account for the full imbalance seen in the universe. The Academy stated that exactly how this larger asymmetry arose "still remains unanswered", making it an active area of ongoing research in particle physics and cosmology.

Key takeaways

• The Nobel Prize in Physics 2008 recognised two separate discoveries about broken symmetry in subatomic physics.
• Yoichiro Nambu received one half of the prize for discovering spontaneous symmetry breaking, first studied through superconductivity.
• Makoto Kobayashi and Toshihide Maskawa shared the other half for explaining broken symmetry using at least three families of quarks.
• Their 1972 theory predicted new quarks before they were found: charm in 1974, bottom in 1977 and top in 1994.
• The BaBar and Belle experiments confirmed the predicted symmetry breaking in B-mesons in 2001.
• Spontaneous symmetry breaking became a key tool for building the Standard Model of particle physics.
• The broken symmetry observed so far is too small to fully explain why matter outnumbers antimatter in the universe.
• All three laureates were born in Japan; Nambu was affiliated with the University of Chicago, while Kobayashi and Maskawa worked in Japan.

Test yourself

Who shared the Nobel Prize in Physics 2008?

Yoichiro Nambu, Makoto Kobayashi and Toshihide Maskawa shared the prize, with Nambu receiving one half and the other two one quarter each.

Where was Yoichiro Nambu affiliated at the time of the award?

Yoichiro Nambu was affiliated with the Enrico Fermi Institute, University of Chicago, in Chicago, Illinois, USA.

What does "spontaneous symmetry breaking" mean?

It means the underlying laws of a system are symmetric, but the system's actual lowest-energy state is not, hiding the symmetry rather than removing it.

Which particle decay first revealed broken CP symmetry, and in which year?

The decay of kaons revealed broken CP symmetry, discovered by James Cronin and Val Fitch in 1964.

How many families of quarks did Kobayashi and Maskawa say nature needed?

They said nature needed at least three families of quarks to explain the observed broken symmetry.

Which two experiments confirmed Kobayashi and Maskawa's prediction in 2001?

The BaBar detector at Stanford, USA, and the Belle detector at Tsukuba, Japan, independently confirmed the predicted broken symmetry in B-mesons in 2001.

What field of physics did Nambu study before applying his ideas to particle physics?

Nambu first studied superconductivity, where electric current flows without resistance, before translating his mathematics into elementary particle physics.