The Higgs Boson (Finally) Explained in Depth（原声文稿）

In the main auditorium at CERN near Geneva, two teams of physicists show their data, one after the other.

In the audience sits an 83 -year -old theorist from Edinburgh.

He had proposed the particle they're closing in on 48 years earlier and never expected to live long enough to see it.

When the last speaker finishes and the room rises, Peter Higgs wipes a tear from his eye.

Finding it took a ring 27 kilometers around and, by one estimate, about 13 billion dollars at the time.

And you've probably heard why it mattered.

The Higgs is the particle that gives everything mass. Usually a picture comes with it.

Space is full of an invisible syrup and particles wading through it get heavy. It's a lovely picture. It's also wrong. long.

Syrup drags on anything that moves and wears its energy away.

As physicist Matt Strassler argues, if space really were syrup, then over billions of years the drag would have sent the earth spiraling into the sun. It hasn't.

Mass does neither of those things.

It only fights changes in motion. So today, no syrup.

Just the real mechanism, in four steps until you could explain it to a friend.

And one more thing, think of the number on your bathroom scale.

The Higgs is responsible for only about 1 % of it, yet without the Higgs, you wouldn't have a shape.

So if the Higgs field isn't syrup, what is it actually doing?

In the fall of 1934, a young physicist in Osaka, Hideki Yukawa, asked an odd question.

If a force reaches only across an atomic nucleus, what must its carrier weigh?

The answer rests on a rule you can picture.

Two astronauts float in space, passing a ball. With a tennis ball, they can be far apart.

With a bowling ball, they'd have to be almost touching. In physics, the rule is real.

The heavier the messenger, the shorter its reach.

Yukawa worked out that his messenger had to weigh about 200 electrons.

13 years later the particle turned up.

In today's terms the world is made of fields, something with a value at every point in space.

A particle is the smallest possible ripple in its field.

Kick the electron field and the ripple is an electron.

Forces have fields too and their ripples are the messengers. Light carries electromagnetism.

The weak force, behind some kinds of radioactivity, is carried by three particles, the W plus, the W minus, and the Z.

The W is about 86 times heavier than a proton. The Z about 97 times. Bowling balls.

The weak force reaches only about a thousandth of a proton's width. Heavy messengers also make it feeble.

That's part of why the sun burns slow and steady.

Its first fusion step needs the weak force, and a typical proton in the core waits about 9 billion years to take part. Here's the problem.

The best theory physicists had said the W and Z must weigh exactly nothing. The math demanded it. So why are they bowling balls?

February 1954, Princeton.

Chen Ning Yang is presenting a new idea about forces when Wolfgang Pauli interrupts.

What's the mass of your new field? Yang says they don't know.

As Yang later recalled, Pauli's comeback was, that is not sufficient excuse.

To see why that question mattered, picture a tiny clock hand at every point in space, a hidden feature of the electrons quantum wave.

No one can read a hand directly, so the laws ought to work however you set each one.

Demand that rule, let the hands twist differently everywhere, and the equations break, unless you add one new field that keeps track of the twisting.

That field is electromagnetism.

Its ripples are light. Light falls out of a symmetry rule.

In 1961, Sheldon Glashow used a rule like this to sketch electromagnetism and the weak force as two faces of one structure with four messengers.

The rule says all four must be massless, which was exactly what Pauli had been poking at. In reality, only the photon is.

Glashout typed the other masses in by hand, and that breaks the rule.

Do it anyway, smash W particles together at about a trillion electron volts, and the equations spit out probabilities above 100%.

Something new has to show up by then. It gets worse.

The same rule forbids mass for electrons and quarks, the very particles you're made of. Step one, the rule.

So how do you give something mass without breaking the rule that forbids it?

July 1964, Edinburgh.

As Higgs recalled it, he opens the mail and He finds a paper that seems to shut the door on the whole approach.

His first reaction was a curse.

Over one weekend, he saw it didn't.

Here's the approach. Balance a pencil on its tip.

The laws of physics prefer no direction, but the pencil has to fall somewhere.

The laws stay symmetric. The outcome doesn't.

So a field could obey the rule perfectly while the state it settles into doesn't.

physicists call this spontaneous symmetry breaking. The catch?

A theorem from 1962 said that whenever a symmetry breaks like this, a massless particle must appear. None had ever been seen.

In 1964, three teams found a loophole, separately.

Let the field talk to a force field and the massless particle gets absorbed.

Robert Brout and François Englert in Brussels published first, then Higgs, then Gerald Guralnick, Carl Hagen, and Tom Kibble in London. None of them was aiming at the weak force. That would take three more years.

Higgs' second paper, with the actual model, came back rejected from the journal Physics Letters.

He later said he was rather shocked.

By his account, the journal's editor sat at CERN near Geneva.

And a colleague later told him the people there who read it saw no relevance to particle physics. Hold on to that.

Higgs added a paragraph spelling out what the theory predicts, a leftover massive particle.

He sent it across the Atlantic to physical review letters, which published it.

Higgs himself said that paragraph is perhaps why he got the credit. it.

Historians add a 1972 talk by physicist Ben Lee. But what does it actually mean for a field to fall over?

In 1960, as Higgs told it, a young physicist named Jeffrey Goldstone drew the picture, an energy landscape shaped like the bottom of a wine bottle, or a sombrero.

The Higgs field comes to rest in a valley of its landscape.

On the sombrero, the peak in the middle is unstable. The lowest ground is the ring around it. Think of the field as a dial.

For the other elementary fields, the dial rests at zero. In empty space, they're off.

The Higgs dial is turned up, everywhere, and it stays there.

Physicists say it has a non -zero vacuum value.

Think of it less as stuff in space, space, and more as a setting of space itself. It wasn't always turned up.

In the hot early universe, the landscape was a simple bowl, and the field rested in the middle at zero.

As the universe cooled, a bump rose in the middle, and the field's resting place slid out into the trough.

That happened about 10 trillionths of a second after the Big Bang, at nearly two quadrillion degrees.

And in the standard model, physicist's best theory of particles, it happens smoothly, like a thermostat turning down, not a pot boiling over.

Before it, electromagnetism and the weak force were two faces of one structure. After it, they split.

Step two, the fall. Now shake the field.

Push it up the wall of the hat, and that costs energy.

You get a heavy ripple, the Higgs boson.

push it around the trough and it costs nothing. You get a ripple with no mass at all.

The real hat has three of these round directions, too many to draw.

So we came looking for mass and the math just handed us three massless ripples. Nobody has ever caught one. Where did they go? Fall 1967.

Stephen Weinberg later recalled that probably on his drive to MIT, it occurred to him that he had been applying the right ideas to the wrong problem.

The right problem was the weak force joined with electromagnetism.

Of their four messengers, three would get heavy and one would stay massless.

Abdus Salam reached the same model on his own.

But how does a messenger get heavy without breaking the rule. Picture a long spring.

Shake the end sideways in either of two directions and a ripple runs down it. That's all a massless messenger can do.

Push and pull the end along the spring and you get a third kind of ripple, the one only a heavy messenger has. Light has exactly two. Now the trick.

Those three free ripples from the hat never get to exist on their own.

The w plus, the w minus, and the z each take one, and it becomes their third wiggle. They get heavy in the same stroke.

Physicists like to say the ripples get eaten.

Four messengers, three ripples to swallow.

One blend of the messengers is never fed, and that blend is the photon.

The field's Steady value carries no electric charge, so it leaves the photon alone.

Light stays massless, and electromagnetism keeps its endless reach, while the weak force can't even cross a proton. Step 3.

On January 25, 1983, CERN announced it had found the W, at about 80 billion electron volts, right where the theory said. A few months later came the Z.

that explains the messengers but you're not made of W particles so where does the electron get its mass?

December 1956, Washington, Qian Xiong Wu's team cools radioactive cobalt to a few thousandths of a degree above absolute zero, lines up its nuclei and counts the electrons flying out. They come out mostly one way. Nature can tell left from right.

Every electron and every quark comes in two versions, left -handed and right -handed.

One way to picture it, which way the particle spins compared with the way it's heading.

The W only grabs left -handed particles, and even the Z treats the two hands differently.

To the weak force, a left -handed and a right -handed electron are different particles. That's why the rule forbids the electron's mass.

A mass would glue its left and right halves together.

But the Higgs field carries weak charge, and it's switched on everywhere.

It supplies exactly the missing charge. That makes the join legal. The join is the mass.

One way to picture it, from physicist Roger Penrose.

The electron is two light speed partners. A left -handed zig and a right -handed zag.

The Higgs field keeps turning one into the other, so on average, the electron never quite reaches the speed of light.

Nobody has ever seen an electron do this. But notice what's missing.

Nothing holds the electron back. No syrup, no crowd.

How strongly an electron or a quark connects to the field is another dial. Turn it up and the particle is heavy. Turn it down and it's light.

A single top quark weighs about as much as a whole atom of tungsten.

In the standard model, the electrons connection is more than 300 ,000 times weaker. Why each dial sits where it does is still a mystery.

None of this means bumping into Higgs particles.

In this picture, mass isn't a label stuck on a particle.

It's what the endless back and forth with the field's steady value adds up to.

But if the field is everywhere, can we make it ring?

The Higgs boson is the field's smallest ripple. Hit the field hard enough, and it rings.

So the Large Hadron Collider smashed protons together at nearly the speed of light, about half a billion times a second in 2012.

Only about one collision in 3 billion made to Higgs, and theory says each one lives less than a six -tillionth of a second.

Detectors never see it directly.

They catch what it falls apart into, and physicists look for a bump at one mass. July 4th, 2012, back in that auditorium.

Every time a new plot with the bump went up, the room cheered.

The CMS bump sat near 125 billion electron volts. Atlas saw a matching bump.

When the last speaker finished and the room rose, Higgs wiped that tear.

Later, he said, it felt like being knocked over by a wave.

Fair enough for a man who had predicted a ripple.

Cern's director general, Rolf Hoyer, told the room, As a layman, I would now say, I think we have it. And he asked, do you agree?

Officially, Cern announced a new particle consistent with the Higgs boson. And remember that rejected paper?

The particle was found at CERN, the lab where, as Higgs later heard it, his idea hadn't seemed worth a letter.

In 2013, Englert and Higgs shared the Nobel Prize.

Brout had died two years earlier, and the prize isn't given after death.

Higgs died in April 2024 at 94, and Englert in June 2026 at 93.

We found the ripple, but have we seen the hat? Not yet.

We've measured where the trough sits and how steeply the walls rise right there.

To map the rest, you have to watch the Higgs interact with itself, which means making two Higgs bosons at once. No Higgs pair has been confirmed yet.

Catching them is one of the headline goals of the rebuilt collider due back around 2030. Then there are neutrinos. Remember the zigzag?

In the standard model as written, a neutrino has no right -handed partner to flip into, so the theory says its mass is exactly zero.

But in 2015, a Nobel Prize went to the discovery that neutrinos change type, which means they have mass. Where it comes from, nobody knows yet.

And our valley may not be the last one.

Run the measured Higgs and Topcork masses through the standard model up to enormous energies, and there may be a deeper one.

Then our vacuum would be metastable.

Stable for any practical purpose, maybe not forever.

It's a calculation, and anything new up there could change it.

Whether our valley is the deepest one, or just a very long lived one is a question for another day.

That's the mechanism, loose ends and all. So, could you explain it to a friend now? Let's check. The rule.

Symmetry forbids the messengers, electrons and quarks, any mass. The fall.

As the universe cooled, the Higgs dial turned up. Everywhere. And stayed there. The meal.

The W and Z swallow the field's free ripples and get heavy, while the photon stays massless. The zigzag.

The field links each particle's left and right halves, and that link is its mass. No syrup anywhere. Now, that number on your scale. It's your mass, felt as weight.

About 1 % of it is the Higgs is doing, the built -in masses of your quarks and electrons. electrons. About 99 % is strong force energy.

Corks and gluons buzzing inside your protons and neutrons, wade through E equals MC squared.

That 99 % is the strong force's own doing. But look at the electron.

According to the standard model, all of its mass comes from the field, although nobody has measured that link for the electron itself yet.

And the The width of every atom is set by the electron's mass.

A lighter electron means a bigger, more loosely held atom.

Physicist Juan Malacena has a warning for dieters.

Turn down the Higgs field and your weight would barely change, but you'd become much bigger.

Now imagine the electron had no mass at all. Atoms would grow without limit.

No compact atoms, no chemical bonds, no solids, no liquids, no you. It's a thought experiment. It can't actually happen.

A man wiping a tear, a ring 27 kilometers around, and a number on your bathroom scale.

The Higgs doesn't give you your weight. It gives you your shape.