Physics 116 Session 40 Particle physics Dec 6, 2011 Email: - - PowerPoint PPT Presentation

Email: ph116@u.washington.edu

Physics 116

Session 40

Particle physics

Dec 6, 2011

Announcements

• Final exam: Monday 12/12, 2:30-4:20 pm
• Same length/format as previous exams (but you can have 2 hrs)
• Kyle Armour is away this week; see TAs in study center
• JW will have extra office hours Thu-Fri this week:
• 12:45-1:15pm before class,
• 2:30-3pm after class (my office B303 PAB, or B305 conf room next door)
• Practice questions will be posted tomorrow (Weds) evening

TODAY: YOUR CHANCE FOR REVENGE – COURSE EVALUATION Pick up now: bubble sheet, yellow sheet and pencil but wait to fill out until I leave the room (or, if you must leave early, leave forms with a neighbor to turn in) Leave completed forms in box at front of room, at end of class. News items worth reading: Today’s NY Times, see

• Astronomers Find Biggest Black Holes Yet, By DENNIS OVERBYE (more
• n this today)
• Quantum Computing Promises New Insights, Not Just Supermachines, By

SCOTT AARONSON (contemporary atomic physics)

3

Lecture Schedule

(to end of term)

Today

4

What’s a black hole?

• Stars die in 3 ways:

– Mass smaller that 1.4 Sun’s mass: as nuclear fuel burns out, becomes a “brown dwarf” (blob of frozen gas) – Mass between 1.4 and about 3 Msun : supernova explosion

• As fusion fuel runs out, star collapses under its own gravity
• Eventually density/temperature so great it reignites in a

massive sudden explosion: supernova

• Most of its mass (heavy elements!) is expelled into its galaxy
• Remnant is a neutron star (solid blob of neutrons*!!) or other

“compact object”, typically radiates lots of energy (“pulsar”) – Mass greater than about 3 Msun: collapses too quickly for the supernova stage, into a “black hole”

• General relativity says light is bent by strong gravity
• When density is so big, g field gets so intense that light cannot

escape: black hole

• Black hole’s intense g field sucks up all nearby matter…
• Most galaxies have a BH at their center

* Density >1017 kg/m3 = Earth crushed to size of Manhattan

“Not on exam”

Today’s NY Times

Artists’ conception of Black Hole with M=1010 Msun What is required to make gravitational escape velocity = speed of light? Recall from PHYS 114: vESC =

2GM EARTH REARTH c = 2GM BH R

BH

R

BH = 2GM BH

c2 ? We used classical physics, but GR

calculation gives the same result Schwarzschild radius: “horizon” of BH of mass M “event horizon” = radius such that nothing can escape (all light cones bend into the BH; “future” of any object there lies entirely inside the BH Most BHs have spin – stars spiral into them

• ur solar system on

the same scale

“Not on exam”

6

Some terminology...

• "Elementary particles" = objects that make up atoms (n,p,e) or

are produced when atoms are smashed (over 200 identified)

– "elementary" because thought to be fundamental in 1950s

• "Fundamental" particles or constituents of matter

– Truly no known substructure (as of today!)

• Hadrons = elementary particles subject to strong nuclear force

(Greek: hadros = strong)

– protons, neutrons; plus pions, kaons, lambda particles...etc – now known to be made of fundamental particles: quarks

• Leptons = elementary particles subject to weak nuclear force

(Greek: leptos = weak)

– responsible for radioactive decays – electrons, plus muons, taus and associated neutrinos

• All leptons are considered fundamental (as of today!)

yesterday

7

But that’s not all...

• Antimatter : Each elementary particle has an “antiparticle”

counterpart

Electron Antielectron (Positron) Proton Antiproton Neutron Antineutron

• etc. anti-etc.

Antiparticles have opposite electric charge (and other properties) but are otherwise identical

• E=mc 2 says matter and energy are interchangeable

– It’s just as easy to make antimatter as matter

• Happens all the time in nature - and we can do it in labs

– But: if particle and antiparticle meet – annihilation! – How come we live in a universe where there is almost no antimatter? (luckily - or we might not survive long…) What caused the Big Bang to create much more matter than antimatter?

8

The “Standard Model” of Particle Physics

Basic ingredients of matter are the fundamental particles: quarks and leptons

6 quarks 6 leptons

+ their antiparticles

(Symmetry!)

These types of particles are called 'fermions'

(from http://www.fnal.gov)

Fundamental forces are mediated by photons, gluons, Z’s and W’s These types of particles are called 'bosons'

(after Enrico Fermi) (after Satrendyanath Bose)

!"#$%&'((

Fundamental particles: graphic where size is proportional to rest energy (mass)

9

(Neutrinos are invisibly tiny on this scale)

10

All Forces are Mediated by Exchanged Particles

• Electrical and Magnetic forces – photon (massless)
• Strong nuclear force – “gluons” (massless – but have 3 “colors”)
• Weak nuclear force – W, Z (massive)
• Gravity – graviton (massless, although no one has yet seen one)

– LIGO experiment in Hanford, WA will try!

• search for gravity waves
• Laser beams in 4km-long tunnels
• Look for changes in length of 10-15 m!

LIGO Hanford Observatory http://www.ligo-wa.caltech.edu/

• The range (reach) of the force depends on the mass of the

exchanged particle

– Gravity and electromagnetic forces extend infinitely far (though weaken with increased distance) – Strong and weak nuclear forces are remote from everyday experience: only come into play at distances like nuclear size

11

Hot Quark Soup

• 6 quarks, in 3 'generations' with increasing mass

– up/down, charm/strange, top/bottom

• Combinations of quarks make up (explain properties of) the

entire 'zoo' of particles cataloged since the 1950s

– 6 quarks, 6 anti-quarks, grouped in twos and threes

• baryons = 3 quarks
• mesons = quark + antiquark

– Many dozens of combinations, but only 1 or 2 stable – Charges always come out as multiples of e charge! – Model explained newly discovered particles too!

• For example:

– Up, charm, top quarks have +2/3 charge – Down, strange, bottom quarks have –1/3 charge – 2u + 1d (uud) proton, with +1 charge

• 2(+2/3) + 1(-1/3) = 3/3 = +1

– 2d + 1u (ddu) neutron, with 0 charge

• 2(-1/3) + 1(+2/3) = 0/3 = 0

12

Need heavy-duty equipment to make high energy particle beams!

CERN, Switzerland

Geneva airport CERN proton accelerator and p/anti-p collider rings (LHC = Large hadron collider)

Photo of CERN (EU particle physics lab) near Geneva, Switzerland

See http://public.web.cern.ch/

ATLAS detector @ LHC

13

Size of humans! UW members: Profs. Henry Lubatti, Anna Goussiou, and Gordon Watts and their students

Data from ATLAS: p+anti-p go to Z boson + 2 electrons

14

What are they looking for?

15

• Ideally, all quarks should have zero mass: perfect symmetry!!
• How come we live in a Universe where things have mass?
• Symmetry must be broken somehow…
• “Higgs boson” = as-yet undetected particle that is the “messenger”
• f the symmetry-breaking that gives everything else mass
• Higgs is expected to be very massive itself: beyond energy reach of previous

generations of particle accelerators

Hot results expected from LHC next week…

What to look for in the news…

16

Actual data (as of early November If p+anti-p make a Higgs, we expect to see a bump in the probability vs total energy

• graph. The rumor mill says the

bump below has become much bigger (>3) with more data.

= std. deviations