The Quest for the Higgs - PDF Free Download

The Quest for the Higgs July 4, 2012: Higgs Day The CERN Laboratory Professor Matt Strassler Rutgers University

The Higgs Boson Has (Almost Certainly) Been Discovered BUT WHAT IS IT? AND WHY DO SOME PEOPLE CARE SO MUCH?

The Higgs Boson Has (Almost Certainly) Been Discovered 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)

The Higgs Boson Has (Almost Certainly) Been Discovered All particles are little ripples in corresponding fields Electron is ripple in electron field Photon is ripple in electric/magnetic fields BUT WHAT IS IT? The Higgs Particle is a ripple in the Higgs Field AND WHY DO SOME PEOPLE CARE SO MUCH? 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 Electrons Atom Nucleus Protons Quarks, Antiquarks & Gluons Neutrons

What IS mass? Higgs: It s All About Mass It is that which makes an object easier or harder for you to move. (in the absence of friction or other confusing effects!) Want to make it move at 3 feet per second? Bigger mass means bigger shove required. Mass WET ICE (almost no friction) 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.

& the Higgs Field Why the Electron Mass is So Important Electrons Nucleus 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!!!

Higgs Field?! What is a field?? It Exists Everywhere Could Be May Be Zero May Be Non-Zero Not Zero Has Waves A Quantum Field Also Has Particles!!!???

Wind as a Field Wind Field Exists Everywhere Dead Calm Steady Breeze Waves: Sound

Electric Field Electric Field Exists Everywhere Light means All Electromagnetic Waves Waves: Frizzy Flat Flat Hair Hair Frizzy Hair Particles of Gamma Rays, X- rays, Ultraviolet light Visible Light Infrared Light, Microwaves, Radio Waves Light Light Photons

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 Three Quanta: Can only be subdivided in three 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 in our quantum world there s a wave of smallest height; a quietest sound; a dimmest flash

PARTICLES ARE QUANTA 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 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) in our quantum world there s a wave of smallest height; a quietest sound; a dimmest flash

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 c 2 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

Gravitational Force Gravitons Strong Nuclear Force Gluons Top Quark Charm Quark Up Quark Bottom Quark Strange Quark Down Quark The Elementary Fields And Their Particles Electro magnetic Force Photons Tau Muon Electron Muon Neutrino Tau Neutrino Electron Neutrino Weak Nuclear Force W, Z particles

Gravitational Force Gravitons Strong Nuclear Force Gluons Top Quark Charm Quark Up Quark Bottom Quark Strange Quark Down Quark Electro magnetic Force Photons Tau Muon Electron Muon Neutrino Tau Neutrino Electron Neutrino Weak Nuclear Force W, Z particles

Gravitational Force Gravitons Strong Nuclear Force Gluons Top Quark Bottom Quark Do Not Have a Mass Electro magnetic Force Photons Tau Muon Electron Massless electrons can t form atoms! Charm Quark Up Quark Strange Quark Down Quark Muon Neutrino Tau Neutrino If W and Z lacked mass, weak nuclear force wouldn t be at all weak! Electron Neutrino Do Have a Mass Weak Nuclear Force W, Z particles

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

Gravitational Force Gravitons Strong Nuclear Force Gluons Top Quark Bottom Quark The Simplest Option Electro magnetic Force Photons Tau Muon Electron Charm Quark Up Quark 1 Simple Higgs Strange Quark Down Quark Muon Neutrino Tau Neutrino The Standard Model of Particle Physics Electron Neutrino Weak Nuclear Force W, Z particles

Gravitational Force Gravitons Strong Nuclear Force Gluons Top Quark Charm Quark Up Quark Bottom Quark Strange Quark Down Quark Higgs Field = 0 All Known Particles Massless Electro magnetic Force Photons Tau Muon Electron Muon Neutrino Tau Neutrino Electron Neutrino Weak Nuclear Force W, Z particles

Gravitational Force Gravitons Strong Nuclear Force Gluons Top Quark Charm Quark Up Quark Bottom Quark Strange Quark Down Quark Higgs Field > 0 These Known Particles Now Can Have Mass Electro magnetic Force Photons Tau Muon Electron Muon Neutrino Tau Neutrino Electron Neutrino 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 Top Quark Muon Electron Charm Quark Higgs Field > 0 These Known Particles Now Can Have Mass Tau Up Quark Bottom Quark Strange Quark Down Quark Muon Neutrino Tau Neutrino Electron Neutrino Q: But what determines strength of Weak Nuclear Force? A: No idea; a big unsolved problem. Weak Nuclear Force W, Z particles

Gravitational Force Gravitons Strong Nuclear Force Gluons 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 Electro magnetic Force Photons Top Quark Tau Muon Electron Charm Quark Up Quark 1 Simple Higgs Bottom Quark Strange Quark Down Quark Muon Neutrino Tau Neutrino The Standard Model of Particle Physics Electron Neutrino Weak Nuclear Force W, Z particles

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 M Higgs c 2. So the jiggling of other fields changes the mass of M Higgs!! and makes it very odd that M Higgs isn t enormously large (far, far out of experimental reach.)

Gravitational Force Gravitons Dark Matter? New Forces? Other Dark Particles? Electro magnetic Force Photons Top Quark Tau Muon Electron Charm Quark Up Quark Heavier Electron-Like Particles? Higgs Higgs Heavier Quark- Like Particles? Multiple Higgs Bottom Quark Fields Higgs Strange Quark Down Quark Muon Neutrino Tau Neutrino Strong Nuclear Force Gluons Electron Neutrino Or Perhaps a More Elaborate Structure Explains Some of the Standard Model s Puzzles? No Evidence In LHC Data Yet Heavier Neutrino- Like Particles? Weak Nuclear Force W, Z 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/c 2 2012 LHC data reveals new particle consistent with Higgs at about 125 GeV/c 2 Proton mass = 0.938 GeV/c 2

The Many People Behind the Higgs Idea Glashow Anderson Nambu Goldstone Englert Brout Guralnik Kibble Hagen Weinberg Salam

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! 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? C. Detect the air waves with your ear and listen to the tone The answers tell you about the properties of the metal!

An Excellent Analogy Strike the metal to make waves in it! 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!] The waves in the metal make waves in the air (sound) 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? What is its frequency (pitch)? How loud is it? The answers tell you about the properties of the Higgs Field(s)! How quickly does it die away?

Bring on the Large Hadron Collider

An Excellent Analogy Strike the metal to make waves in it! 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!]

LHC: proton-proton collider Most of the year: proton proton collisions Why? quark antiquark, gluon gluon mini-collisions Goal: create new types of particles! Make New Particle(s) Here But how do you make such incredibly tiny objects collide?! OOPS!!

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

An Excellent Analogy Strike the metal to make waves in it! The waves in the metal make waves in the air (sound) Detect the air waves with your ear A. Slam protons together to try to make a quantum of the Higgs field [a Higgs particle!] B. Any Higgs particle immediately disintegrates into other particles that rush outward 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! B. Lots of sound and noise emerges! C. Try to listen for a faint tone

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 Signal 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? Background In any given collision, we can t tell. But

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 = M Higgs c 2 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/c 2

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

EXTRA SLIDES

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 Z mu DECAY! mu Z particle muon + antimuon Z particle up quark + up antiquark and many more Matthew Strassler Rutgers University 53

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 And It Runs In Reverse Too! up Z DECAY! Z particle muon + antimuon CREATE! Example: up Z particle up quark + up antiquark and many more Z particle up quark + up antiquark Matthew Strassler Rutgers University 54

Put these together! Example: The universe rings in the key of Z mu up quark + up antiquark Z particle, then Z particle muon + antimuon motion-energy mass-energy motion-energy up DECAY! Z particle muon + antimuon Z CREATE! Example: up Z particle up quark + up antiquark and many more mu Z particle up quark + up antiquark

ATLAS May 10, 2010 Proton-Proton Collision Produces a Z Particle

But how do we know it is a Z particle?! There are other ways to make a muon and an antimuon mu up up mu

But how do we know it is a Z particle?! There are other ways to make a muon and an antimuon Albert Einstein: E = m c 2 for a particle (at rest) Emmy Noether: Energy is always conserved [unchanged over time] Total Energy of the Debris = Energy of the Debris Source mass-energy of heavy particle motion-energy of lightweight particles Z E = M Z c 2 Energy = (Mass of Z) x (speed of light) 2

But how do we know it is a Z particle?! There are other ways to make a muon and an antimuon Albert Einstein: E = m c 2 for a particle (at rest) Emmy Noether: Energy is always conserved [unchanged over time] Total Energy of the Debris = Energy of the Debris Source mass-energy of heavy particle motion-energy of lightweight particles mu E = ½ M Z c 2 E = M Z c 2 mu E = ½ M Z c 2 Energy = (Mass of Z) x (speed of light) 2

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 up + up Z mu + mu These particles are measured in the detectors From these particles, we infer what happened in the mini-collision deep inside the protons

mu What s true for the Z is true for the H mu up Z up

What s true for the Z is true for the H photon Make Higgs! gluon + gluon Higgs gluon higgs gluon Break Higgs! Higgs photon + photon (if lightweight) Higgs Z + Z (if heavyweight) Higgs many other options (must consider all of them!) photon

mu What s true for the Z is true for the H Make Higgs! gluon + gluon Higgs mu Z gluon higgs gluon Z Break Higgs! Higgs photon + photon (if lightweight) Higgs Z + Z (if heavyweight) Higgs many other options (must consider all of them!) mu mu

IS THAT A HIGGS BOSON??!??!? mu Maybe but there are other ways to make two Z particles mu up Z up Z mu mu

Natural

What Is The Underlying MECHANISM?!? Not Natural

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

What Is The Underlying MECHANISM?!? Not Natural Little Higgs or Extra Lattice Dimensions Supersymmetry or Weird Extra Dimensions of Space The small Higgs field value is protected by a new symmetry The Higgs field IS a bit protected Technicolor or Warped Extra Dimensions of Space The Higgs field value is naturally pinned to a small value Flat Extra Dimensions of Space The Higgs field IS at its natural large value!

Other Stuff the LHC Could Find! 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?!??!

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

Doesn t give mass to other particles Find The Higgs Particle Understand The Higgs Field Does give mass to other particles