the quest for the higgs - department of physics and astronomy

The Quest for the Higgs

Professor Matt Strassler

Rutgers University

©

July 4, 2012: Higgs Day

The CERN Laboratory

© Matt Strassler 2012

The “Higgs Boson”

Has (Almost Certainly) Been Discovered

BUT WHAT IS IT?

AND WHY DO SOME PEOPLE

CARE SO MUCH?




BUT WHAT IS IT?

All particles are either

• Fermions (e.g. electrons)

• Bosons (e.g. photons)

(The Higgs Particle just

happens to be a boson)




BUT WHAT IS IT?

All particles are little ripples in corresponding fields

• Electron is ripple in electron field

• Photon is ripple in electric/magnetic fields

• The Higgs Particle is a ripple in the Higgs Field




BUT WHAT IS IT?

AND WHY DO SOME PEOPLE

CARE SO MUCH?

All particles are little ripples in corresponding fields

• Electron is ripple in electron field

• Photon is ripple in electric/magnetic fields

• The Higgs Particle is a ripple in the Higgs Field

Higgs Field: essential to structure of matter & our

very existence; but barely understood it at all… yet!

These ripples give us our first chance…


The Structure of Ordinary Matter


The Structure of Ordinary Matter

Atom

Electrons

Nucleus

Neutrons

Protons

Quarks,

Antiquarks

& Gluons


Higgs: It’s All About Mass

• What IS mass?

It is that which makes an object easier or harder for you to move.

(in the absence of friction or other confusing effects!)

WET ICE (almost no friction)

Mass

Want to make it move at 3 feet per second?

Bigger mass means bigger shove required.

No mass?

always move at the universal speed limit (“c”, often called “speed of light”)

Got mass?

always move below universal speed limit;

can even be stationary.


Why the Electron Mass is So Important

Planck’s Quantum Mechanics Constant)

Atom Radius = # ------------------------------------------------------------------------------------------------

(Universal Speed Limit) x (Strength of Electromagnetism) x (Electron Mass)

If Electron Mass Zero , Atom Radius Infinity !

NO HIGGS FIELD?!? NO ELECTRON MASS! NO ATOMS!! NO US!!!

& the Higgs Field

Electrons

Nucleus


Higgs Field?! What is a field??

It Exists

Everywhere Could Be

Not Zero

Has Waves

A Quantum

Field Also

Has Particles

!!!???

May Be

Zero

May Be

Non-Zero


Wind as a “Field”

Wind Field

Exists

Everywhere

Waves:

Sound

Dead

Calm

Steady

Breeze


Electric Field

Electric Field

Exists

Everywhere

Frizzy

Hair

Waves:

Light

Particles of

Light

“Photons”

Flat

Hair

“Light” means

All

Electromagnetic

Waves

Gamma Rays, X-

rays, Ultraviolet light

Visible Light

Infrared Light,

Microwaves, Radio

Waves

Flat

Hair

Frizzy

Hair


Waves in a Quantum World

You would think you could make smaller and smaller waves;

quieter and quieter sounds; dimmer and dimmer flashes

But you’d be wrong…

… in our quantum world there’s a wave of smallest height;

a quietest sound; a dimmest flash


QUANTA



One Quantum:

Travels as a unit

Cannot be subdivided

Can only be absorbed as a unit

Can only be emitted as a unit

Carries energy and momentum

Has a definite mass (possibly zero)

Two Quanta: Can only

be subdivided in two

Three Quanta: Can only

be subdivided in three


“PARTICLES” ARE QUANTA



One Quantum:

Travels as a unit

Cannot be subdivided

Can only be absorbed as a unit

Can only be emitted as a unit

Carries energy and momentum

Has a definite mass (possibly zero)

EVERY “PARTICLE” IN NATURE IS A QUANTUM

OF A WAVE IN A CORRESPONDING FIELD

A photon is a quantum of a wave in

the electromagnetic field

An electron is a quantum of a wave in

the electron field

An top quark is a quantum of a wave in

the top-quark field

A W particle is a quantum of a wave in

the W field

A Higgs particle is a quantum of a wave in

the Higgs field


And About That Mass?

• If a particle has a mass, it can be stationary…

• And in that case its energy E is equation to its mass M times c2

• And that’s just the energy required to turn this…

into this…

• The Higgs Field, when it’s not zero, changes the environment,

and can change that energy… and thus change the particle’s mass…


Electron

Muon

Tau Electron

Neutrino

Muon

Neutrino

Tau

Neutrino

Bottom Quark Top Quark

Strange Quark Charm Quark

Down Quark Up Quark

Weak Nuclear Force

W, Z particles

Gravitational Force

Gravitons

Electro

magnetic

Force

Photons

Strong Nuclear Force

Gluons

The Elementary Fields And Their Particles


Electron

Muon

Tau Electron

Neutrino

Muon

Neutrino

Tau

Neutrino



Down Quark Up Quark

Weak Nuclear Force

W, Z particles

Gravitational Force

Gravitons

Electro

magnetic

Force

Photons


Gluons


Electron

Muon

Tau Electron

Neutrino

Muon

Neutrino

Tau

Neutrino



Down Quark Up Quark

Weak Nuclear Force

W, Z particles

Gravitational Force

Gravitons

Electro

magnetic

Force

Photons


Gluons

Do Not

Have a

Mass

Do

Have a

Mass

If W and Z lacked mass,

weak nuclear force wouldn’t

be at all weak!

Massless

electrons can’t

form atoms!


A Problem, and a Solution

• Electron has mass, so electron must be symmetric under mirror reflection.

– Electrons must behave the same way in a mirror as electrons in nature do.

• Weak Nuclear Force is not symmetric under mirror reflection

– Weak Nuclear Force can convert electrons to neutrinos, which are asymmetric

– Therefore electron also cannot be symmetric under mirror reflection

PARADOX!!!!!

Solution: Electron may have mass, yet be asymmetric, as long as

• There is a field in nature that

– Is not zero, on average, throughout the universe

– Interacts in just the right asymmetric way with the electron

This is the ``Higgs field’’ – it compensates for the asymmetry [Weinberg 67]

Similar issues for all the other known particles with mass…


Electron

Muon

Tau Electron

Neutrino

Muon

Neutrino

Tau

Neutrino



Down Quark Up Quark

Weak Nuclear Force

W, Z particles

1 Simple Higgs

Gravitational Force

Gravitons

Electro

magnetic

Force

Photons


Gluons

The “Standard Model”

of Particle Physics

The

Simplest

Option


Electron

Muon

Tau Electron

Neutrino

Muon

Neutrino

Tau

Neutrino



Down Quark Up Quark

Weak Nuclear Force

W, Z particles

Gravitational Force

Gravitons

Electro

magnetic

Force

Photons


Gluons

Higgs Field = 0 All Known Particles Massless


Higgs Field > 0

These Known Particles Now Can Have Mass

Electron

Muon

Tau Electron

Neutrino

Muon

Neutrino

Tau

Neutrino



Down Quark Up Quark

Weak Nuclear Force

W, Z particles

Gravitational Force

Gravitons

Electro

magnetic

Force

Photons


Gluons


Higgs Field > 0

These Known Particles Now Can Have Mass

Electron

Muon

Tau Electron

Neutrino

Muon

Neutrino

Tau

Neutrino



Down Quark Up Quark

Weak Nuclear Force

W, Z particles

Q: What sets the mass for each type of particle?

A: The more strongly it interacts with Higgs Field,

the more mass it has once Higgs Field > 0.

Q: What

determines how

strongly each type

of particle interacts

with Higgs Field?

A: No idea; a big

unsolved problem.

Exception:

Strength of Weak

Nuclear Force

determines W & Z

particle masses

Q: But what determines strength of

Weak Nuclear Force?

A: No idea; a big unsolved problem.


Electron

Muon

Tau Electron

Neutrino

Muon

Neutrino

Tau

Neutrino



Down Quark Up Quark

Weak Nuclear Force

W, Z particles

1 Simple Higgs

Gravitational Force

Gravitons

Electro

magnetic

Force

Photons


Gluons

The “Standard Model”

of Particle Physics

Perhaps the Simplest

Guess is Right

But very odd if true…

Equations form consistent

set, but leave unspecified:

• 13 masses

• 4 force strengths

• 3 decay ratios

• Why these particular

fields and forces arise


The Biggest Mass Question of All

• The Higgs Field changes the masses of other particles:

• But indirectly, the reverse is also true!

• A quantum field is never really silent…

… it’s always jiggling …

… and this jiggling has a lot of energy --- which we normally don’t notice

• The amount of jiggling energy depends on how non-zero the Higgs field is

• So when a ripple in the Higgs field is present, the jiggling energy changes…

…adding extra energy to the ripple. But E of the ripple is MHiggsc2.

So the jiggling of other fields changes the mass of MHiggs!!

and makes it very odd that MHiggs isn’t enormously large

(far, far out of experimental reach.)


Electron

Muon

Tau Electron

Neutrino

Muon

Neutrino

Tau

Neutrino



Down Quark Up Quark

Weak Nuclear Force

W, Z particles

Gravitational Force

Gravitons

Electro

magnetic

Force

Photons


Gluons

Dark

Matter?

Other

Dark

Particles?

Heavier

Quark-

Like

Particles?

Heavier

Electron-Like

Particles?

Higgs

Higgs

Higgs

Multiple Higgs

Fields

Or Perhaps a More

Elaborate Structure

Explains Some of the

Standard Model’s

Puzzles?

No Evidence In LHC

Data…Yet…

New

Forces? Heavier

Neutrino-

Like

Particles?


Saga of a Century!! And Not Over Yet

• 1897 – Electron discovered, mass measured, source of mass unknown

• 1905-20 – Massless photon suggested; discovered 1924

• 1957 – Discovery that weak nuclear force is mirror-asymmetric!

• 1964 – Higgs Field papers (Higgs, Brout & Englert, and Guralnik, Kibble & Hagen)

• 1967 – Weinberg (and Salam) theory of weak nuclear force, based on crucial work by

Glashow, using Higgs Field to give masses for the then-known particles

• Mid-1970s – Serious consideration of how to make/discover Higgs Particle

• 1980s–90s – proposal of the U.S. SSC, European Large Hadron Collider (LHC)

• 1990s–2000s– searches elsewhere for simplest Higgs: 0 – 115, 140 – 170 GeV/c2

• 2012 LHC data reveals new particle consistent with Higgs at about 125 GeV/c2

Proton mass = 0.938 GeV/c2


The Many People Behind the Higgs Idea

Englert Guralnik

Anderson Nambu Goldstone

Salam

Glashow

Brout Kibble Hagen

Weinberg


So Much We Still Don’t Know

• Is there one Higgs field, or several, each with its own type of Higgs particle?

• Is it an elementary field

– (so its particle is an elementary particle, like an electron?)

• Or is it made from other elementary fields

– (so its particle is a more complicated composite object, like a proton?)

• Is it possible the Higgs field has no particle at all? (It was; but data says no!)

How can we answer these questions?

• Often we understand a mysterious material by studying the ripples in it!

– Learn about air, other gasses? Make sound waves!

– Learn about earth rock? Study earthquake waves!

– Learn about guitar? Pluck its strings and listen!

– Learn about piece of metal? Hit it with a hammer and listen!

– Learn about Higgs field? Make ripples in it and study their properties!


An Excellent Analogy

A. Strike the metal

to make waves in it!

C. Detect the air waves

with your ear and

listen to the tone…

B. The waves in the metal

make waves in the air (sound)

1. What is its frequency (pitch)?

2. How loud is it?

3. How quickly does it die away?

The answers tell you about

the properties of the metal!



Strike the metal to

make waves in it!

The waves in the metal make

waves in the air (sound)

A. Slam protons together to

try to make the Higgs field

vibrate --- i.e., try to make a

quantum of the Higgs field [a

Higgs particle!]

B. Any Higgs particle

immediately disintegrates

into other particles that

rush outward

Detect the air waves

with your ear and

listen to the tone…

C. Detect the outgoing

particles with a giant

particle detector

1. What is the Higgs

particle’s mass?

2. How often are

Higgs particles

made?

3. How does the Higgs

particle decay?

The answers tell you about the

properties of the Higgs Field(s)!

What is its frequency (pitch)?

How loud is it?

How quickly does it die away?

Bring on the

Large Hadron Collider



Strike the metal to

make waves in it!


try to make the Higgs field

vibrate --- i.e., try to make a

quantum of the Higgs field [a

Higgs particle!]


LHC: proton-proton collider

• Most of the year: proton proton collisions

– Why? quark antiquark, gluon gluon mini-collisions

– Goal: create new types of particles!

But how do you make such incredibly tiny objects collide?!

OOPS!!

Make New Particle(s) Here


Hundreds of proton bunches each with

~100 billion protons, organized, accelerated,

steered, and aimed using Giant Magnets (and

many other large devices)

Protons Collide!!!


Protons Collide!!!

Hundreds of proton bunches each with

~100 billion protons, organized, accelerated,

steered, and aimed using Giant Magnets (and

many other large devices)


Detecting the Debris

CMS

ATLAS



Strike the metal to

make waves in it!

The waves in the metal make

waves in the air (sound)


try to make a quantum of

the Higgs field

[a Higgs particle!]

B. Any Higgs particle

immediately disintegrates

into other particles that

rush outward

Detect the air waves

with your ear…

C. Detect the outgoing

particles with a giant

particle detector…


Making A Higgs Particle Just Once is

Not Enough to Discover It

Maybe this is a Higgs Particle –

but probably it isn’t

A. Strike it!

C. Try to listen for a faint tone…

B. Lots of sound and noise emerges!


The Strategy for Discovery

Smash two protons together again and again

• Sometimes, two gluons, one from each proton, will hit each other hard

– Sometimes, the two gluons will annihilate and make a Higgs particle.

• Sometimes, the Higgs will fall apart into something striking

1. Two photons

2. Two muon-antimuon or electron-antielectron pairs

Extremely Rare! 1 in 100,000,000,000,000 proton-proton collisions at LHC!

• Unfortunately there are other ways LHC collisions can produce

1. Two photons

2. Two muon-antimuon or electron-antielectron pairs

• So how can we tell Higgs particles are being produced?

– In any given collision, we can’t tell. But…

“Signal”

“Background”


• The Background is Random, but the Signal is Regular

• Plot number of events versus energy [really “invariant mass”; too long a story for now]

• Expect excess of events, if signal is present, at a single energy E = MHiggs c2

Collisions with

two photons Collisions with two

muon-antimuon or

electron-antielectron

pairs

Non-Random Bumps Non-Random Bumps

ATLAS and CMS

each observe 2

small bumps

consistent with

Higgs particle of

mass 125 GeV/c2


Higgs Particle Found! Now What?!

• Remember what we want to understand is the Higgs Field!!

1. What is the Higgs

particle’s mass? Done!

2. How often are Higgs

particles made? Started…

3. How does the Higgs

particle decay? Started…

4. Are there other types

of Higgs particles?

5. Is the Higgs ever

produced, or does it ever

decay, in an unexpected

fashion?

The answers tell you about the

properties of the Higgs Field(s)!

The LHC’s enormous data set over

the coming decade will help answer

these questions.

But that will be just the

end of the beginning…


Quests and Questions

• The Higgs Field is a crucial part of nature

– nonzero value allows mass for many types of particles

– thereby ensuring that atoms exist, weak nuclear force is weak, etc.

• The Higgs Particle (a “boson”) is a ripple in the Higgs Field

– its properties can give insight into the still mysterious Higgs Field(s)

– new particle closely resembling simplest possible Higgs has been found

• if it’s what it looks like, it confirms Higgs Field exists

• assures much will be learned about Higgs Field(s) over coming years

– ends a quest of five decades, opens a new era

– century-long saga to understand particles & their masses is not over

• Standard Model now (apparently) complete; is it everything? (at least at LHC?)

– No sign yet of anything amiss with its predictions; studies continue

– But if so, profoundly puzzling; so many unspecified quantities and features


The Quest to Find the Higgs Particle

May Have Finally Come to an End

(unless there are more types of Higgs particles yet to be found!)

But the Quest to Understand the Higgs Field

And the Quest to Understand

the Masses of the Known Particles

Are Just Beginning!

Website/Blog: Of Particular Significance

profmattstrassler.com

www.facebook.com/ProfMattStrassler

Twitter: @MattStrassler

http://www.physics.rutgers.edu/~strassler

http://www.facebook.com/ProfMattStrassler


EXTRA SLIDES


• A wave of one type can dissipate into waves of other types

• A particle of one type can decay into 2 (or more) particles of other types

Most Particles, Including Higgs, Can Decay

Matthew Strassler Rutgers University 53

DECAY!

Z particle muon + antimuon

Z particle up quark +

up antiquark

and many more …

Z

mu

mu


Most Particles, Including Higgs, Can Decay

• A wave of one type can dissipate into waves of other types

• A particle of one type can decay into 2 (or more) particles of other types

Matthew Strassler Rutgers University 54

DECAY!



up antiquark

and many more …

CREATE!

Example:


up antiquark

And It Runs In Reverse Too!

Z

up

up


CREATE!

Example:


up antiquark

Put these together!


Example: The universe rings in the key of Z

up quark + up antiquark Z particle , then

up up

DECAY!



up antiquark

and many more …

motion-energy mass-energy motion-energy

Z

mu

mu


ATLAS May 10, 2010

Proton-Proton Collision

Produces a Z Particle


• There are other ways to make a muon and an antimuon…

But how do we know it is a Z particle?!

up up

mu

mu




• Albert Einstein:

– E = m c2 for a particle (at rest)

• Emmy Noether: Energy is always conserved [unchanged over time]

– Total Energy of the Debris = Energy of the Debris Source

Z E = MZ c2

Energy = (Mass of Z) x (speed of light)2

mass-energy of heavy particle motion-energy of lightweight particles




• Albert Einstein:

– E = m c2 for a particle (at rest)

• Emmy Noether: Energy is always conserved [unchanged over time]

– Total Energy of the Debris = Energy of the Debris Source

mu

E = MZ c2

Energy = (Mass of Z) x (speed of light)2

E = ½ MZ c2 E = ½ MZ c2

mass-energy of heavy particle motion-energy of lightweight particles

mu


That’s what is done at a hadron collider

• Protons collide with protons

• Inside, quarks, antiquarks and gluons collide

• Occasionally a collision creates something new – ringing in the key of Z

• The new particle falls apart right away into known long-lived particles

• These particles are measured in the detectors

• From these particles, we infer what happened

in the mini-collision deep inside the protons

up + up Z mu mu +


up up Z

mu

mu

What’s true for the Z is true for the H


– Make Higgs!

• gluon + gluon Higgs

– Break Higgs!

• Higgs photon + photon (if lightweight)

• Higgs Z + Z (if heavyweight)

• Higgs many other options (must consider all of them!)


gluon gluon higgs

photon

photon


– Make Higgs!

• gluon + gluon Higgs

– Break Higgs!

• Higgs photon + photon (if lightweight)

• Higgs Z + Z (if heavyweight)

• Higgs many other options (must consider all of them!)


mu

gluon gluon higgs Z

Z

mu

mu

mu


IS THAT A HIGGS BOSON??!??!?

• Maybe… but… there are other ways to make two Z particles…

up up Z

Z

mu

mu mu

mu


Natural


Not Natural What Is The

Underlying

MECHANISM

?!?


• Any reasonable person would expect Higgs field to be either

ZERO (There would be no atoms)

Or

10,000,000,000,000,000 times larger than it is.

(Protons would almost be mini-black holes)

WHY IS IT SO SMALL AND YET NOT ZERO???

• Is there a mechanism keeping it there??

– Many suggestions – we have no idea at this point… but…!

– Every mechanism so far conceived of would be detectable at the LHC…


Not Natural What Is The

Underlying

MECHANISM

?!?

Technicolor or

Warped Extra Dimensions of

Space

The Higgs field value is naturally

pinned to a small value

Supersymmetry or

Weird Extra

Dimensions of Space

The small Higgs field

value is protected by a

new symmetry

Flat Extra Dimensions of

Space

The Higgs field IS at its

natural large value!

Little Higgs or

Extra Lattice Dimensions

The Higgs field IS a bit

protected


• Particles of Dark Matter? – Most of the matter in the universe is unfamiliar stuff

• New differences between matter and antimatter? – Need to explain why the universe is full of matter and not antimatter

• New forces of nature? – Why not? Might have something to do with dark matter.

• Strings? (as in String Theory?) – Maybe…

• Something that no one has ever previously suggested?!??!

Other Stuff the LHC Could Find!


What It’s Not: The “God Particle”

• Experimental particle physicist Leon Lederman

– Nobel Prize 1988

created this unfor-gettable/-givable moniker in 1993

• Impre$$ive mockery of both science and religion

– Bad because

• it implies science can do things that it cannot

• it misleads public about how science works

• it gives license to creation “science”, etc.

• it reduces religion to something material


Find The

Higgs Particle

Understand The

Higgs Field

Doesn’t give mass

to other particles

Does give mass to

other particles

the quest for the higgs - department of physics and astronomy

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