Transcript Lecture 3
From Quarks to the Cosmos Lecture III – Where do we come from? The Higgs Field Where does mass come from? Acknowledgement: Special thanks to Dr. David Heddle, CNU, for many of the slides! July 4, 2012 Newspapers around the world herald the discovery of the Higgs Boson—the so-called “God Particle” that explains the origin of mass. Goals for this Lecture • To give you a hint as to how the Higgs particle is responsible for mass. • Short answer: It isn’t—it is the Higgs field that creates mass, not the recently discovered Higgs particle—but they are closely related. Outline 1. 2. 3. 4. 5. What are fields? What is mass? How can a field produce mass? The discovery What next? What is a field? • Not a field: has a value at one location • Field: has a value at all locations in space A Field Not a Field Fields are Everywhere Electric field from opposite charges Magnetic field from a bar magnet Gravitational field near the Earth’s surface Fields: more fundamental than forces Charges: Recall that likes repel, opposites attract. If charge 1 is there and we drop charge 2 in, it feels the force instantly. This is bad! I’m Here! 1 2 Go Away! Nothing, not even information, can travel faster than the speed of light, so there should be a delay before 2 “feels” the force from 1! “Action at a distance”—a very serious problem. Fields solve the “action at a distance” problem No action at a distance---the field from charge Q1 is there even when Q2 is not. Information does not have to make a round trip. 2 1 I feel nothing! No Charge Don’t know where it’s coming from, but I feel a field pushing me away! The field is invisible to objects with no charge. What is Mass? A) How much an object weighs? B) An object’s resistance to acceleration? B) It is a resistance to acceleration. From Newton’s 2nd Law: The bigger the mass, the smaller the acceleration for the same force. The Force of Gravity A) A great way to measure mass. B) A lousy way to measure mass. B) A lousy way. Under gravity alone, an object’s motion is independent of its mass. How can a field “make mass”? Q + + + + + + + + + + + Q Electric Field Two identical blocks, but one has a charge hidden inside. This block is easier to move—it seems to have has less mass! You could say the electric field has reduced its mass! The Higgs Field • It is a field that is everywhere in the universe. • Objects that do not interact with the field are massless and travel at the speed of light (such as the photon) They can never slow down. • The stronger the interaction, the more massive the particle. • It’s really the God “field,” not the God particle! Something like… The Higgs field means the universe is kind of like velcro. Everywhere. These kind of particles see the Higgs field. They have mass. They can come to rest. These kind of particles don’t see the Higgs field. They have no mass. They must travel at the speed of light. Particles and Fields Excitations in fields are what we call particles—like pulses in stretched ropes electron photon When an electron accelerates, its electric field “pulses”, and it shakes off a photon. This is how radio works. When the Higgs field pulses, it shakes off a Higgs particle. The Higgs particle is an artifact of the Higgs field. The actual math, in case you want to work it out before the next class Discovery of the Higgs Particle • The Higgs particle has a mass of about 130 × the proton mass. • It takes a huge amount of energy to create one—which is why we need a massive particle accelerator. • You will also make lots and lots of other particles—it’s worse than a needle in a haystack Large Hadron Collider • Collides proton beams at 12 trillion electron volts • The most complicated machine ever built Size of the LHC (17 mi diameter) Compare to the 1st Accelerator The Cyclotron, 1932 (about 4 inches in diameter) Civic Pride! Jefferson Lab in Newport News LHC Jefferson Lab 4,000,000,000, 000 eV 12, 000,000,000 ev But… The LHC accelerates protons, J-lab accelerates electrons, 2000 times lighter, so the LHC needs higher energy electron proton About a 1 mile circumference LHC proton speed J-Lab electron speed 0.99999997 × speed of light 0.999999999 × speed of light J- Lab accelerates particles closer to the speed of light! We win! So – how do you see the Higgs? p Higgs Outgoing particles p Particle detectors p Not Higgs p Same outgoing particles Particle detectors Protons collide, produce Higgs (with very low probability) but mostly produce other stuff—and detectors measure the outgoing particles. How are particles detected and identified? “Calorimeter” stops particles and records their energy Magnetic field bends the particles path. Curvature give the particle’s speed × Outgoing particle leaves “hits” in the tracking detector p p So: “tracking” gives speed, “calorimetry” gives energy Einstein told us how to use this to determine mass, and hence what particle is produced What it really looks like What we really see These bumps in the data are the signatures of the Higgs particle! The Announcement On July 4th 2012, scientists announced that two independent experiments have seen the signature of the Higgs particle, to a confidence of “5 sigma.” That means: less than 1 in a million chance that we are not really seeing the Higgs. Confidence Probability of Error 1 Sigma 0.32 2 Sigma 0.046 3 Sigma 0.0027 4 Sigma 0.000063 5 Sigma 0.00000057 What’s Next? • The discovery of the Higgs was 50 years in the making! • There are still great mysteries in physics, including – Dark Matter (what is it?) – Dark Energy (the universe is expanding faster and faster! Is it because of dark energy?) • We know virtually nothing about 94% of the universe (dark matter and energy). • Next class, we will talk about all this!!!