HCP: Particle Physics Module, Lecture 3 Plan for today: Review where we are on the Standard Model Chart 1. Discuss the strong force between quarks: gluons and QCD 2. Introduce the weak force 3. Discuss the unification of electromagnetic and weak forces predictions and observations unobserved (so-far) predictions

Fermions (half-integer spin particles) satisfy the Pauli Exclusion Principle : no more than one identical particle can occupy the same quantum state

For the -, all the quarks are identical (sss) and in the same quantum state: isn’t this a problem (ie. conflict with Pauli Exclusion Principle)?

Quarks and “color” States like ++ = (u u u) or - = (s s s) with three identical fermions in the same quantum state violate the Pauli principle UNLESS the three quarks inside the baryon are somehow different This problem was dealt with in 1964 by introducing the concept of Quark color: each type of quark comes in three colors Red Blue Green Observed hadrons are “color neutral” Baryons: (q R ) (q B ) (q G ) R + B + G = “white” (color neutral) Mesons: (q R ) (q R ) or B B G G When it was introduced (1964), color seemed “ad hoc”; doesn’t it have some kind of real physical meaning?

Quantum Chromodynamics (QCD) Theory of how the strong interaction between quarks works (analogous to QED – quantum electrodynamics – of the electromagnetic interaction) The strong force between quarks is carried by exchange particles called gluons (analogous to the photons in QED) The gluons are attracted to the color charge (just like the photons in QED are attracted to the electric charge) There are 8 types of gluons (they come in different color – anticolor combinations): R B , R G , B G , etc. Example of two colored quarks interacting via gluon exchange: u R u B RB R B u u Gluons (like photons) are massless BUT gluons carry color charge (compare to photons which have zero electric charge)

Strength of the Interaction Between Quarks Confinement: At larger (say ~ 1 F) distances the force between quarks becomes VERY LARGE. The quarks are permanently confined inside the hadrons. confinement strength of the force strength of the force between two quarks asymptotic freedom 1. Asymptotic freedom: At short (< 0.3 F) distances, the force between quarks is very weak. They essentially move about freely. distance between particles distance between quarks Electromagnetism: (interaction between proton and electron in atom, say) Quarks and strong interaction (QCD): (interaction between two quarks) Two special properties of Quantum Chromodynamics (QCD)

Discovery of Asymptotic Freedom in QCD – awarded Nobel Prize in Physics 2004

Why Don’t We Observe Free Quarks? Confinement Electromagnetism (QED): F  1/R2 weaker at larger separations Strong (QCD): F  R stronger at larger separations Confinement: We can never observe a free quark. attempt to pull two quarks apart so much energy is stored that a new q q pair is created

Trying to pull two quarks apart results in more quarks being formed  JETS of hadrons What evidence do we have that gluons exist? Processes like below where a gluon is “radiated” lead to 3 JET events 3 Jet Event 2 Jet Event

Think about the proton’s mass  something doesn’t add up

“Picture” of the Proton in QCD Recall that the proton is made up of three quarks: proton: u u d electric charge: +2/3e +2/3e -1/3e = +e d u “bag” radius ~ 1 F Picture: The quarks are trapped inside a spherical “bag” with a radius of ~ 1 F (CONFINEMENT) inside of which they move freely at nearly the speed of light (ASYMPTOTIC FREEDOM). Most of the mass of the proton is in the kinetic energy of the quarks (not in the rest mass of the quarks). Recall the Einstein equation E=mc2.

Return to Discussing Forces Note: any number of identical bosons can exist in the same quantum state

LIGO: Laser Interferometic Gravitational Wave Observatory: The search for gravity waves

LISA: Laser Interferometer Space Antenna

Return to Discussing Forces Note: any number of identical bosons can exist in the same quantum state

How Do We Classify the “Strength” of Interactions? Consider decays of particles: Decay Force Lifetime of particle ++  + + p strong 7 x 10-24 seconds o   +  electromagnetic 8 x 10-17 seconds -  e- + e +  weak 2 x 10-6 seconds Force Typical particle decay lifetime strong 10-23 - 10-24 seconds electromagnetic 10-17 - 10-20 seconds weak minutes - 10-10 seconds Basic point: particle lifetime  (1 / strength of interaction) So stronger forces  shorter lifetimes and vice versa

Weak Force Examples: Neutron beta decay: n  p + e- + e lifetime = 900 seconds d u d (-1/3) neutron u (+2/3) proton ? -1 time e- (-1) e (0)  decay:   p + - lifetime = 2.6 x 10-10 seconds d u s (-1/3)  u (+2/3) proton ? -1 time d (-1/3) u (-2/3) - We need a new force carrier to describe the weak interaction. Note that it has to have electrical charge in order for the above processes to work.

Force Carriers of the Weak Interaction Steven Weinberg (1967) proposed three force carriers for the weak interaction: particle electric charge mass W+ +e 80.4 GeV/c2 W- -e 80.4 GeV/c2 Z0 0 91.2 GeV/c2 The W+ and W- were necessary to explain already known charged current processes like: d u d (-1/3) neutron u (+2/3) proton W- -1 time e- (-1) e (0) neutron beta decay: n  p + e- + e The prediction of the Z0 implied the existence of new, previously unobserved neutral current processes like:    + e-   + e- Z0 e- e-

neutral current processes like: The prediction of the Z0 implied the existence of previously unobserved neutral current processes like: Z0  e-  + e-   + e- These processes were first discovered in 1973:

Experimental Confirmation of Electroweak Theory Many predictions of the electroweak theory (like neutral currents) were experimentally confirmed in the 1970’s BUT The masses of the W+- and Z0 bosons were predicted, but no accelerator was energetic enough to create them at that time. Discovery of W and Z (1983) by Carlo Rubbia and many others p + p  W+- + “stuff”  “stuff” p + p  Z0 + “stuff” Done at CERN in Geneva, Switzerland W and Z observed with predicted masses Triumph of electroweak theory!

Observation of the Z0 particle at CERN The energies of the observed e- and e+ decay pair added up to the rest mass of the Z0

Desire for Simplicity in Particle Physics The ultimate goal of particle physics is to describe Nature in terms of the smallest number of particles interacting through the smallest number of forces. Particles (2005): 6 quarks 6 leptons Forces (2005): Gravity Weak, Electromagnetic (unified as Electroweak) Strong  Goal is to eventually unify all four forces into one force.

Electroweak theory (unification of electromagnetism and the weak force) Weinberg and Salam (1967) proposed that the electromagnetic and weak forces were really two aspects of the same underlying force. How can this be? Consider the relation between the range of a force carrier and its mass: At very high energies (>> 100 GeV) we probe very short distances (<< 10-18 m). At such short distances even the weak force appears “long-range” like the electromagnetic interaction.

How Does the Unification Work? At high energies (>> 100 GeV) and short distances (<< 10-18 m)  4 massless exchange particles W+ W0 B0 W- “triplet” “singlet” Symmetric  all massless, all interacting with the same strength Spontaneous symmetry breaking Low energy world we live in  3 massive particles 1 massless particle (weak force) (electromagnetism) W+ Z0  W-  Symmetry broken. The fact that the electromagnetic and weak forces are the same is hidden from us due to the low energy world we live in.

Example of spontaneous symmetry breaking – the bar magnet Iron bar magnet at high temperature Spins randomly oriented Symmetric  no special direction Reflects the underlying symmetries of the laws of electromagnetism Iron bar magnet at low (room) temperature Spins all oriented along the same direction Special direction picked out  not symmetric Broken symmetry  the symmetries of the underlying laws of electromagnetism are hidden

How does the spontaneous symmetry breaking occur in electroweak theory? Higgs mechanism There are postulated to exist 4 Higgs particles 3 of them are absorbed (“eaten”) by the W+ , W-, and Z0 to give them their mass Since the photon has no mass, there should be one remaining, “uneaten” particle the Higgs boson  Not yet observed, but expected to be more massive than the W+- , Z0 The primary goal of the world’s newest, biggest accelerator will be to detect the Higgs boson: LHC: Large Hadron Collider 7 x 1012 eV = 7 TeV proton – proton collider  will start in 2010 to take data at CERN in Geneva, Switzerland