11 February 2015 1Modern Physics III Lecture 6

Modern Physics for Frommies IIIA Universe of Leptons, Quarks and

Bosons; the Standard Model of Elementary Particles

Lecture 6

Fromm Institute for Lifelong Learning, University of San Francisco

11 February 2015 2

Agenda• Administrative Matters• Higgs Redux• Electroweak Unification• The Current Paradigm

Modern Physics III Lecture 6

11 February 2015 3

Administrative Matters

•Please give some thought as to what you would like me to teach next time. Give me feed back at next week’s meeting.

• A mixture of Modern Physics stuff: Atomic and molecular physics, nuclear physics, solid state physics, etc.

• Cosmology• Repeat starting with Relativity again

Modern Physics III Lecture 6

Physics and Astronomy Colloquium #2: Today, 3:30 PM – 5:00 PM, CSI 210Dr. Alexander Grutter – National Institute of Standards and Technology

“Probing Nanoscale Magnetism with Neutron Scattering at the NCNR”

Schedule for rest of spring semester will be included in next week’s handouts and will be posted in Fromm Hall..

11 February 2015 4

Higgs Redux

There is no straightforward way to put massive field quanta into gauge theories.Invariance under local transformations of the group of gauge symmetries → introduction of gauge bosons

Lorentz invariance massive gauge bosons require mc2 terms. Unfortunately, this destroys gauge invariance.

Simple cheating (ignoring mass terms until the end) doesn’t work. Renormalizability is destroyed.

Screening and “effective” mass: photon () analogy

in a conductor has short range → appears to acquire effective mass due to screening by conduction electrons.

Saying that we can treat in conductor as massive is not to say that the really does become massive, this mass is not a property of itself but derives from its environment.

Modern Physics III Lecture 6

11 February 2015 5

Fabrication or modeling tool with no physical basis, but if the whole universe were conductive (plasma) then s would appear to be massive.

Can we do something like this for W and Z?

Doesn’t affect Affects W and Z0

not coupled to electric charge must couple to weak isospin charge.

Some possibilities: Matter field associated with Matter field associated with known quarks and leptonsForce field associated with known gauge bosonsSome entirely different field

For now, let’s just call it the Higgs field.

We still have to be careful not to destroy gauge symmetry even though we are calling our masses “effective”. This is tricky!

Modern Physics III Lecture 6

11 February 2015 6

Hidden symmetry: Symmetry there but hidden by an arbitrary or capricious choice. Remember the freezing teenagers of last lecture.

We can introduce the mass terms along with additional compensating terms that restore the hidden symmetry using the all pervading Higgs field.

Introduce a new doublet, the Higgs doublet.

upper

lower

Invariant under local SU(2) transforms

Arbitrarily, pick say lower as something we can’t perceive but which fills the universe’s vacuum, exerting a “drag force” on anything that reacts with it.

This gives the W and the Z apparent masses

The symmetry of the Higgs doublet is hidden by our choice but is still there

Modern Physics III Lecture 6

11 February 2015 7

We seem to suggest that universe is not symmetric w.r.t. SU(2) transformations that swap the 2 doublet fields, The swapped universe, with upper providing the pervasive drag force is manifestly different.

The symmetry appears to be broken, but really it’s not. The W and Z still interact just as readily with upper as they did with lower. It’s just that in the swapped universe the all pervading background is provided by upper.

For some reason the universe chose to pick one of the two cases. The choice was arbitrary but had to be made.

Choice → physical state which hides the true underlying symmetry

Higgs potential makes the choice

So, we have lower giving masses to W and Z. What is the rôle of upper?

Modern Physics III Lecture 6

11 February 2015 8

Recall the 1st generation quark doubletu

d

Non zero value for u at a point in space-time 1 or more u quarks there Likewise for d

At most points u = d = 0 but they are ≠ 0 inside an atomic nucleus

lower ≠ 0 quanta of Higgs field, Higgs bosons

The Higgs doublet fields are complex so doublet represents 4 fields rather than 2. Re Imi

Just like , upper and lower have 1

1

0

upperelectric

lower

upper

lower

uq

d

Modern Physics III Lecture 6

11 February 2015 9

The representation of a massive vector particle is different than that of a massless one.

2 more degrees of freedom for m ≠ 0 from no longer forbidden transverse polarization.

When W and Z pick up effective mass from pervasive Higgs field background, they need to incorporate more field components into their description.

W each take one of the 2 charged (upper) components. Z 0 takes 1 of the lower (neutral).

The leftover neutral gives the uniform, pervasive, ≠ 0 background that causes the W and Z to apparently have mass.

This final leftover can be excited to deviate from the uniform background presence of Higgs boson(s).

Modern Physics III Lecture 6

11 February 2015 10

The Higgs boson is an electrically neutral, spin-0 (scalar) particle.

Only fundamental scalar particle ( quarks and leptons are spin-1/2, gauge bosons are spin 1 or 2).

New kinds of weak interactions at energies high enough for H to be excited.

Cosmological implications

The nature of “mass”.

Are all masses due to Higgs or Higgs like mechanisms.

Is mass, one of the most basic and common sense attributes of a physical object, just a sham?

Modern Physics III Lecture 6

11 February 2015 11

Electroweak Unification

Steven Weinberg, 20 November 1967, Physical Review Letters

Not Steven Weinberg

Speculative but seemingly self-consistent model of interactions of e- and their corresponding partner e

Steven Weinberg

Modern Physics III Lecture 6

11 February 2015 12

Gauge theory based on local SU(2) invariancee

e

Also incorporated a U(1) gauge symmetry like that of QED,

The telling aspect of this model is that the single boson associated with the U(1) symmetry is not quite the of QED. Call it B0.

The properties of the minimal eB0 interaction vertex do not quite reproduce those of the QED vertex.

1

137

B0

≠

In addition, SU(2) generates 3 field quanta. 2 of them are identified as the W and Weinberg predicted, with no experimental basis, that the 3rd was the electrically neutral W0.

Neutral currents not observed until 1973Modern Physics III Lecture 6

11 February 2015 13

Apart from the difference in masses, U(1)’s B0 and SU(2)’s W0 are actually quite similar.

For any fundamental process one could never tell which was responsible for the interaction.

B0

W0

OR

All you observe is 2 electrons bouncing off each other

Possibly the exchanged quantum is really a mixture of the two could be a mixture of B0 and W0 Likewise, Z0 could be another mixture of B0 and W0

Modern Physics III Lecture 6

11 February 2015 14

The Glashow-Salam-Weinberg model (GSW) is that of a single interconnected electroweak force having 2 separate facets

U(1) SU(2)represents a unified electroweak interactionU(1) and SU(2) lacked Higgs

Essentially identical to Weinberg but independent

Nobel prize in physics 1979Modern Physics III Lecture 6

11 February 2015 15

The properties of the are well known, thanks to QED. GSW model allows calculation of the electroweak mixing (Weinberg) angle.

0

0 0

cos sin

sin cosW W

W W

B

Z W

or

0 0

0 0 0

cos sin

cos sin

W W

W W

B W

Z B W

The effects of the so called neutral currents were not seen until 1973.

Time for some experimental history.

Modern Physics III Lecture 6

11 February 2015 16

To cleanly study WI one needs a beam of neutrinos. J. Steinberger, M. Schwartz, L. Lederman and others designed and built such a beam ca. 1960.

p beam from AGS

Be target

Focusing and

sweeping

etc. → m

Steel wall 13.5 m thick

Detector Al plate spark chambers

10 T

Neutral currents not thought up until 1967 but the above people did show that there are two types of neutrinos, e and .

Nobel Prize in Physics 1988

Mostly

Modern Physics III Lecture 6

11 February 2015 17

In 1973, the Gargamelle1 bubble chamber (12 m3 of liquid freon) at CERN, exposed to such a beam, saw the first neutral current event

1. From the works of François Rabelais. Gargamelle, the giantess, was Gargantua’s mother

e → e

Modern Physics III Lecture 6

11 February 2015 18

Ca. 1978. C. Prescott, V. Hughes et al. Series of polarized electron scattering experiments at SLAC looking for parity violation.

Scatter polarized electrons of opposite helicities from LH2 and LD2 targets

Z0

AEM ANC

Interference: R L

PVR L

A

Modern Physics III Lecture 6

11 February 2015 19Modern Physics III Lecture 6

11 February 2015 20

Finally the W and the Z 0 were directly observed

1976: Carlo Rubbia suggests running the CERN SPS as a proton – antiproton collider in hopes of attaining sufficient energy.

Simon van der Meer figures out how to make the accelerator do this, stochastic coolingCollider starts running in 19811983: The 100 member WA1 collaboration sees the W and the Z.

1984 Nobel Prize in Physics

Carlo Rubbia Simon van der Meer

Modern Physics III Lecture 6

11 February 2015 21

Parity and Other ViolationsConsider the reflection of a movie in a mirror

Physical laws in the reflected scene appear to be identical to those in the direct scene.Conclusions: (1) Laws of physics invariant under reflection.

(2) There is no experiment that can distinguish our universe from the one in the mirror.

Mirror reflection a.k.a. parity inversion is a discrete symmetry operation.

Symmetry group which leaves physical quantity invariant or symmetrical Reflection or parity group has only 2 elements:

invert

do nothing

“discrete” all or nothing

2 note that =1 1

PP P

Modern Physics III Lecture 6

11 February 2015 22

Noether’s theorem: Symmetry conserved quantity, parity, with 2 and only 2 possible values, “even” and “odd”.

Any system, with say odd parity, obeying laws that are parity invariant, as familiar laws are, remains odd, unless there’s something fishy going on.

Mid 1950’s: puzzle ( these are mesons, is not the lepton) Same mass and lifetime but,

0 even parity

odd pari y t

Physicists uncomfortable

Allowing a parity violating process would be an easy fix.

Flies in the face of physical intuition. Rotational invariance, why not reflection.

Never the less, physicists began thinking about it.

Modern Physics III Lecture 6

11 February 2015 23

lifetime (10-8 sec), long by particle physics standards WI

1956: T, D. Lee and C. N. Yang looked carefully at WI, No evidence one way or the other re parity invariance

Spin, Helicity and Parity: Consider e- (s=1/2) moving with momentum p

ˆHelicity , space quantization 1 2s p H

Apply right and left hand rules, s = 1/2 is RH helicity, s = -1/2 is LH. Now, reflect in a mirror

spin spin

Direct Reflected

Observer

Parity inversion changes the handedness of spinning particlesObserver

RH LH

Modern Physics III Lecture 6

11 February 2015 24

A set of particles is invariant under a parity invariant interaction process must not change if the handedness of every particle is changed

OR

Characteristics of a process change when all spins are flipped parity is not conserved.

C. S. (“Madame”) Wu at Columbia and E. Ambler at NBS60 6027 28Co Ni ee Nucleons align so net nuclear spin = 5 ħ

Using a strong magnetic field and extremely low temperature it was possible to polarize the 60Co sample, i.e. most of the nuclear magnetic moments aligned with the field.

Modern Physics III Lecture 6

11 February 2015 25

C. S. Wu 1912 - 1997

Modern Physics III Lecture 6

11 February 2015 26

T. D. Lee and C. N. Yang Nobel Prize 1957

The Wu experiment showed WI have a strong penchant for LH particles How strong is this preference?

1957: R. Garwin et al. looked at decay at the Nevis cyclotron

Modern Physics III Lecture 6

11 February 2015 27

stopped in carbon absorber.

Measure angle of e- emission w.r.t flight direction

2 for RH only# forward

1 for no preference# backward

1 2 for LH only

Measured ratio: 1/2 10 %

WI has only LH currents. Note, antiparticles are RH

Note that the universe is matter rather than antimatter.

Parity violation is maximal for WI

is now known as 0

via WIK us

Modern Physics III Lecture 6

11 February 2015 28

------ time

Modern Physics III Lecture 6