03/08/02Friday Physics, John Hobbs1 What’s the Matter? A Story of the Smallest Things in the Universe What is Particle Physics? Where are we today? –How.

03/08/02 Friday Physics, John Hobbs 1

What’s the Matter?A Story of the Smallest Things in

the Universe

• What is Particle Physics?• Where are we today?

– How did we get this far?– The Standard Model

• Speculation on what’s coming

A glimpse at aspects of a much broader field


What is Particle Physics*• A natural question: What is the world

made from, and how is it all held together?– If a break up a rock, I still get rock. If wood

is burned, is it still wood?– Is there a common basis for the apparent

variety of “stuff” we see around us and in the universe?

– Greeks: fire, earth, air, water– Now: Particle Physics

*aka High Energy Physics


The first steps toward the answer

• Mendeleev (1869): Periodic table of the Elements– Based on experiments build a catalog

of fundamental elements. Roughly 70 entries.

– Later (1914), Neils Bohr and others develop a theory which neatly explains the patterns within the periodic table.


The Beginning of Particle Physics• 1897, Discovery of the electron, e-

• 1911, Discovery of the atomic core: nucleus– What’s in an atom? Plum pudding or hard

core?

– Rutherford/Geiger/Marsden scatter a beam of particles by a gold foil

Deflection pattern indicates hard core, nucleus

– Deeper understanding to Mendeleev


deflection


More history

• Particle discoveries– 1905*(Einstein) Photon, – 1911 (Rutherford) Proton, p+

– 1932 (Chadwick) Neutron, n0

• Except for a few anomalies (Dirac) the matter world is understood…

• 1931 Discovery of antimatter, e+

What is a particle? A submicroscopic “thing” with a specific set of properties: mass, electric charge, lifetime, decay pattern(s), …

Antimatter? A particle withproperties identical toanother, except opposite charge and magenticproperties, e.g. e+ = e

*Compton, 1923


History: A glimpse of trouble

• 1937, Find a new particle: Actually, OK, ‘cause we wanted these

to explain how the nucleus stayed intact. (Aside: bar magnets)

’46 Oops, not it. “Who ordered that?”, I.I. Rabi

• 1947, A set: 0

These are the particles we wanted to explain the nuclear binding. But their properties aren’t quite right either

Also, from these studies, Neutrino,

(1931, Fermi: We need this)


History: Seeing particles

+p

p

(data from Brookhaven 7’ bubble chamber)


The Particle Zoo

• In the 1950’s and into the 60’s, experiments to measure properties of the nucleus with energetic particle beams– More new particles than we knew

what to do with

– And their antimatter partners…


The Zoo

• A wide range of properties*– Masses: 0 to 3x Mp

– Lifetimes: 10-24s to (stable) Only the e-, p+, n and ’s are

truly stable

– Decay patterns (“Strangeness”)

• A useful theory would simplify and make sense of this mishmash.

*think of it as different shapes and sizes


Towards some understanding

• 1964 (Gell-Mann/Zweig) a new theory: “periodic table” of the subatomic elements

• requires new particles (quarks)• Do quarks exist?

n p

d u

s

Explain this using only these n = udd

Diagonal, same electric chargeHorizontal, same decay featuresKnown particles

New particles


The Modern Era• Update Rutherford scattering with

higher energy beams* to probe protons to “look” for quarks. (aka Deep Inelastic Scattering)

– High energy electron beams scattering from protons/neutrons

– Deflection pattern: hard cores. Quarks are real!

• More new particles: , J/(BNL), (USB)*Particle energy related to speed E = mc2

stopped: = 1, and E = mc2

moving: speed grows, grows


What is a “standard model”?

• “a framework built from observations to correlate and predict new phenomena”

Ptolemy’s epicycles for planetsMendeleev’s periodic tableBohr model of the atomSynapse/neuron structure of the brain

• As a larger realm is explored, is it usual to find that the S.M. needs revision


What’s the Matter? I: THE (current) Standard Model

• At the smallest scale, all matter is either quarks or leptons

• 6 ‘flavors’ of each, in 3 ‘generations’ (and their antiparticle partners.)

• All Forces are carried by bosons

• Quarks feel strong force,– Leptons do not


What are the forces?Apparent in “nature”: In the S.M. 1. Gravity ? 2. Magnetism 3. Electricity Electroweak (, W, Z) 3. Weak force 4. Strong force (nucleii) Strong (gluon, g)

Einstein’s dream: UNIFICATION: all apparent forces are described by a theory with one real force…

Relative strengths: Strong 10 EM 1/100 Weak 1/10000000000000 Gravity 1/1042 (but in the end, gravity wins…)

E&M

What’s a force? a push or a pull…


How Particles Interact(anti)matter boson matter

Coupling constant How strong is the force? Depends on both matter particle type and boson type

In general

Specific example decay, ala carbon dating

ud u

ud d

W-

e-

Force carriers: bosons , m = 0 W, m = 83x Mp

Z, m = 91x Mp

g, m = 0


• Everyday life, hold atomic nucleii together– We and our earth require the strong force

• It’s so strong, that we never “see” directly the quarks or the gluons– Forced to infer their existance from

Rutherford scattering experiments (q) and jets (g)

Regarding the Strong Force

What’s a jet?

Z0

e+

e-

u

u

a jet

a jet


How do we know this? I: Counting

• The S.M. has 3 generations. Are there more?– Conclusively determined there are

only 3 (light) generations*Compared Z lifetime with

observed decaysCompared ee+ events with events

– Work at CERN ’89-’91

*Worth a talk in itself


How do we know this? II: New Particles

• In ’70’s when the S.M was being formulated, the t, W, Z and H were required to make an internally consistent theory. But, never “seen”.

• In ’83, the W and Z were first “seen” at CERN, a European physics lab.

• What about t?


Search for the Top Quark

• Going on seriously since early/mid 80’s• Since it’s clearly heavy (M is large),

need high energy beams

• By ’94, Mt > 130 Mp

• In ’95, simultaneous announcement by D and CDF experiments at Fermilab – Mt = 175x Mp (like lead)

– Stony Brook major contributor to D


How do we “see” a top quark?


The (current) Standard Model

– Explains most phenomena observed to date with extraordinarily high precision

The whole “Zoo” can be described by the quarks, with the “strange” decay patterns arising from inter-generational reactions!

– Some things are hard to calculate from theory, so less well tested

– Some theoretical aspects are put in “by hand” because of preexisting results.


The State of the Standard Model

• We’ve accomplished a lot– Found W, Z, top quark– Confirmed 3 generations (of light ’s)– Internal consistency checks passed

with nearly unmatched precision. Many other beautiful results

100’s of measurements18 (25?) parameters

• Higgs boson?

Internal consistency check: cake baking: The recipe says bake for 2 hours at 350o. But, just to make sure, stick a knife in to see if it comes out dry…


Searching for the Higgs Boson• Why should we care?

– The initial form of the theories has ,W,Z massless (but not so!). If there is a Higgs, then we get massive W, Z for free…

– In S.M. all masses are put in “by hand”, but the ability to have any masses comes via Higgs particle

– No Higgs = No Standard Model

SYMMETRY BREAKING Symmetry breaking? (ball on a wire)


Current Situation

• Current Knowledge– Direct searches, especially at CERN / LEP

MH > 113x Mp

A hint of a signal(!) at MH = 115 GeV

– Internal consistency checks (assume SM)MH < 200x Mp

• Very soon– Tevatron at Fermilab (D0/CDF)– LCH at CERN (2007)


What’s the Matter? II: Shortcomings• There are some problems

– No gravity!– No unification between EW and strong– Neutrino masses? (SuperK and K2K/SB)– No intrinsic explanation for mass values– …

• What’s the answer? Don’t know…– Supersymmetry (SuSy), Extra dimensions of

space (>3D), strings, technicolor, …

• Stay tuned: Fermilab is gearing up; CERN, too


Conclusions• Particle physics has a Standard

Model which describes most of the known matter and forces with

• Missing some key elements? There is more to come…

• Stay tuned: new experiments starting now

6 quarks (u, d, s, c, b, t)6 leptons (e, , , , , )4 force carriers (,W,Z,g)1 Higgs

28Friday Physics, John Hobbs03/08/02

More information?General Resources http://particleadventure.org/particleadventure/index.html http://www.fnal.gov/pub/inquiring/matter/index.html http://www2.slac.stanford.edu/vvc/ http://public.web.cern.ch/Public/

Stony Brook Involvement: http://www-d0.fnal.gov http://www-d0.fnal.gov/public/highlights/highlights1.html http://neutrino.kek.jp/ http://pdg.lbl.gov/atlas/atlas.html

http://particleadventure.org/particleadventure/index.html

http://www.fnal.gov/pub/inquiring/matter/index.html

http://www2.slac.stanford.edu/vvc/

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

http://www-d0.fnal.gov/

http://www-d0.fnal.gov/public/highlights/highlights1.html

http://neutrino.kek.jp/

http://pdg.lbl.gov/atlas/atlas.html


03/08/02Friday Physics, John Hobbs1 What’s the Matter? A Story of the Smallest Things in the Universe What is Particle Physics? Where are we today? –How.

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