Manfred Jeitler December 18, 2009 1 Particle Physics From an experiment-driven to a theory-driven...

Manfred Jeitler December 18, 2009 1

Particle Physics

From an experiment-driven to a theory-driven field

Manfred Jeitler

Institute of High Energy Physics

of the

Austrian Academy of Sciences


Particle Physicsin a nutshell

elementary particles first postulated in antiquity– Democritus called them ἄτομος (undividable = elementary), atoms

– what we call atoms today is not at all undividable!

– philosophical reasoning, no experimental evidence » maximally underdetermined

indirect evidence for atoms– elements react in ratios of small whole numbers

» John Dalton 1803: “law of multiple proportions”

– erratic movement of small objects» Brownian motion 1827 (Robert Brown)


1897

the electrone-

ThomsonThomson

direct evidence for first elementary particle: the electron


1897

the proton

e-

1900-1924

1914

RutherfordRutherford

p

another constituent of matter:the proton


1897

the photon

1900-1924

PlanckPlanck EinsteinEinstein

ComptonCompton

e-

p

the particle of light:the photon


1897

the neutron

e-

1900-1924

1914

np

1932

ChadwickChadwick

there is still something else in the nucleus:the neutron


a particle theory of the world - all aspects nicely determined

a few particles nicely described our world– very simple: for a scientist, beautiful !

– Occam’s razor

each of them had been accurately determined by experiment

almost everything explained – except some details: for instance, what keeps the particles in a nucleus

together?

but experimentalists continued their observations!


1897

the muon

e-

1900-1924

1914

µp

1932

n

1937

• Hess• Anderson, Neddermeyer

e+

Who ordered this ?

more particles appearing


1897

„I have heard it said that the finderof a new elementary particle usedto be rewarded by a Nobel Prize,but that now such a discovery ought to be punished by a $10,000 fine.“

e-

1900-1924

1914

K

p

1932

n

1937

µ

1947

e+

1947-...

In his Nobel prize speech in 1955, Willis Lamb expressed nicely the general attitude at the time:

LambLamb

too many particles?


the theorists’ turn

get some order into this “particle zoo”

after some more work (details later) everything was again classified into a model: The Standard Model (~1970)


fermions (spin ½)

charge

0

-1

+2/3

-1/3

d

uu

du

d

leptons quarks

+1 0 proton neutron

baryons

interactions

strong

weak

gravitation?

weakW, Z

electromagnetic

strongg

force carriers = bosons (spin 1)

e

e

u c t

d s b

the Standard Modeltwo extra generations


++

u

uu

u

d

d

us

c

d

D

s

u

b

b

d

uu

du

d

proton neutron

mesons

baryons

...

...

nucleus

He nucleus(-particle)

atom

matter

The Standard Model explaining all the particles


where did the “extra generations” come from?

“leptons” (like electron) and “quarks” (like in proton and neutron) have heavier, unstable partners

predicted by theory ?

found as a surprise by experiment ?


the second generation

muon (μ)– 2nd generation of “leptons” (the electron’s partner)

– observed by chance, not expected

“strange” quark (s-quark) – 2nd generation of “hadrons” (the up and down quarks’ partner)

– observed by chance, not expected

puzzle observed: no “flavor-changing neutral currents”– particle decays described by strange quark (inside an unstable particle)

decaying into up quark, but never into down quark

– why ?!


the GIM mechanismGlashow - Iliopoulos -Maiani


discovery of the “charm” quark

predicted by theory (1970) but experimentalists payed no attention !

discovered accidentally (1974) – in the “J/ψ” particle

– Sam Ting and Burt Richter


decay of a “charmed” baryon (Σc++)

Manfred Jeitler December 18, 2009 18 bubble chamber


drift chamber


another question:are particles completely symmetrical ?

e

imagine as little billiards balls

no room for any asymmetry (left - right)?

few people doubted symmetry

but this conviction was underdetermined– and turned out wrong


parity violation parity (= mirror-image) symmetry

had not been proved for all interactions

C.N.Yang and T.D.Lee conjectured that parity symmetry might be broken in Weak interactions

– based on experimental evidence

C.S.Wu proved this experimentally– in the same year (1956)

the world’s mirror image differs from the world itself


Chen Ning Yang and Tsung-Dao Lee(Nobel prize 1957)

Chien-Shiung Wu

parity violation


save the symmetry!

parity violation came as a shock !

physicists hoped to find the lost symmetry again on a higher level


Charge-Parity symmetry

Parity

Chargecharge conjugation:replace particles by anti-particles

CP

left-handedneutrino

right-handedneutrino

right-handedanti-neutrino

XIn “Weak Interactions”, P and C “maximally violated” but the combined CP symmetry is mostly conserved


the next shock:CP symmetry is also broken!

but (rarely) K0L also

decays into two π’s K0

L

K0S

CP = -1CP = -1

CP = +1 CP = +1

K0L

CP = -1

CP = +1


1964 experiment

the first signal:K0

L +-


CP-violation

people were unhappy and proposed other explanations for the experimental findings (1964)

but soon had to accept CP-violation as a fact

theories were developed to explain it

one theory predicted a further “generation” of quarks (1973)


Makoto Kobayashi

Toshihide Maskawa

Nobel prize 2008 (together with Yoichiro Nambu)


Vud* Vcd

* Vtd*

Vus* Vcs

* Vts*

Vub* Vcb

* Vtb*

Vud Vus Vub

Vcd Vcs Vcb

Vtd Vts Vtb

1 0 0

0 1 0

0 0 1

the Cabibbo-Kobayashi-Maskawa matrixand the”unitarity triangle“

a 3 x 3 “quark mixing matrix” can explain CP-violation

so, there should be two more quarks (“b” and “t”)– “beauty”, or “bottom”

– “truth”, or “top”

“c” (“charm”) had not yet been found in 1973 !


mechanism of CP-violaiton

one theory (“Standard Model”) predicted:– 3 generations of quarks

– ε’ not equal 0

another theory (“superweak model”) predicted:– nothing concerning a 3rd generation of quarks

– nothing concerning ε’

question to philosophers:– what would the “realist” conclude ?

– what would the “antirealist” conclude ?


three generations - why so complex?

CP-violation, three generations ... just a whim of nature?

no!

CP-violation is of fundamental importance for our universe – but nobody had thought of this before


particle physics and cosmology: the big bang


the Big Bang, antimatter, and us

according to present understanding, the Universe was created in the “Big Bang”

matter and antimatter were created in equal quantities– there is almost no antimatter in the Universe

both would have disappeared if no matter excess had developed

CP-violation is necessary condition! – Sakharov, 1965


we wouldn’t be around without CP-violation – and lots of other facts

– “fine tuning” of constants of nature

... and so we wouldn’t be able to ask these questions!

anthropic principle– don’t invoke it - it’s not politically correct!

– physicists are supposed to understand everything - not just to show it can’t be otherwise

– although we sometimes use it and don’t care» ever wondered why we live on Earth, and not on Venus or Jupiter?

– people don’t like it - but it might still be the right answer!

but why?


welcome to the Multiverse !

anthropic principle made socially acceptable

maybe our “Universe” is just a bubble – among uncountably infinitely many other bubbles in the Multiverse – in each bubble universe, one set of laws of physics and natural constants

is realized– just “one point in the parameter landscape”

unless we manage to communicate with other universes:– how?– or find some indirect proof …– is this science or theology?


completing the Standard Model:the W± and Z0 bosons (1983)


completing the Standard Model:the top quark (1995)


Standard Model:Complete and proved? No alternatives?

very good description of nature many predictions

but:

does not describe everything:– e.g., neutrino masses not predicted

one important ingredient still not found in experiment: the HIGGS BOSON– predicted by the mechanism proposed to give mass to quarks


to Higgs or not to Higgs ...

very good evidence for Standard Model in all other respects

but the “allowed mass window” (masses not yet excluded by experiments or theory) is getting smaller

also, it’s taking so long ...– merely psychological factor?

“theoreticians are getting cold feet” – John Ellis (one of the chief theoreticians at CERN)

lots of new theories (no Higgs after all, invisible Higgs, little Higgs, ... you name it) – much time and little data - theoreticians leave no stone unturned (?)


bosons

SUSY

SUSY particles. green: known particles of the Standard Model red: hypothetical new particles

for each known elementary particle there should exist a supersymmetric partner

fermions

Supersymmetry


Atome (Bekannte Materie)

3% Dunkle Materie23%

Dunkle Energie74%

massive astrophysical cosmic halo objects?weakly interacting massive particles?

questions of cosmology to particle physics:Why is there more matter than anti-matter in the universe?What is the universe made of? What is dark matter?What is dark energy?

answers to these questions concerning the largest scales might come from the physics of the smallest scales - elementary particle physics

dark matter: MACHOS vs WIMPS


... and much more

superstrings

extra dimensions


data dearth

so many theories

so many explored alternatives

probably some unexplored alternatives (?)

give us data !!


CMS

just

sta

rtin

g !


how to observe particles

Tracks of particles in a typical collider experiment (CMS, CERN)

just

sta

rtin

g !


just

sta

rtin

g !


LIGO (Laser Interferometer Gravitational Wave Observatory, USA)

measurement of gravitational waves

still

wai

ting

for

sign

al!


important questions of today’s particle physics(ongoing experiments)

• Where do particles get their mass from? (by interaction with the Higgs particle?)

• Why are these masses so different?

• Is there an overall (hidden) symmetry such as supersymmetry (SUSY) “mirror world” of all known particles?.

• What is the nature of “dark matter” and “dark energy” in the universe?

• Why is there more matter than anti-matter?

• Why have neutrinos such small mass?

• Is there a Grand Unification which combines all interactions, including gravitation?

• Are there extra dimensions, D > 4 ? ( string theory, …)


where are we now ?

many theories in particle physics are strongly underdetermined at present

numerous conceived alternatives– in violation of Latin grammar, and often also common sense

maybe nature has some unconceived alternatives in store for us?

finally, the Large Hadron Collider is online, and we may hope for some answers


THANK YOU


BACKUP


parity violation parity (= mirror-image) symmetry

had not been proved, however, for all interactions

C.N.Yang and T.D.Lee conjectured that parity might not be conserved in Weak interactions (1956)

– θ 2π (positive parity) and – τ 3π (negative parity) – “τ-θ puzzle”– K+ 2π (positive parity) and – K+ 3π (negative parity)

they convinced C.S.Wu to test parity conservation in experiment

β-decay of 60Cobalt the world’s mirror image differs

from the world itself


CP-eigenvalue particles can be attributed a

“CP-eigenvalue”– like charge, mass, parity

this eigenvalue is multiplicative:– CP () = -1

– CP () = +1

there are 2 kinds of “neutral K-mesons”

– the (long-lived) K0L decays into

3 -mesons

– the (short-lived) K0S decays into

2 -mesons

K0L and K0

S differ by their CP-eigenvalue !

CP(K0L) = -1 CP(K0

S) = +1

K0L

K0S

CP = -1CP = -1

CP = +1 CP = +1


CP-violation

K0L

K0S

K0L

CP = -1CP = -1

CP = +1 CP = +1

CP = -1

CP = +1

1964: sometimes (0.3 percent) also


proton decay?

if baryon number conservation is violated (Sakharov condition #1), the proton could decay– for example, p e+ π0

– no other conservation law forbids this

another argument for possible proton decay: there is no field that corresponds to baryon number conservation– in gauge theories, long-range fields give rise to absolutely conserved

quantities

proton decay is definitely very slow– good for us: else, we would have problems with radiation damage!

experimental limits very low– half-life > 1036 years (“Superkamiokande” detector, Japan)

spin-off: neutrino detectors


• the Standard Model works only with particles which are originally massless!• mass is created through interaction with a (hypothetical) Higgs field• due to spontaneous symmetry breaking this field is present everywhere in the universe• “oscillations” in the Higgs field manifest themselves as Higgs particles, which should be observed at LHC / CERN over the next few years

spontaneous symmetry breaking

energy

Higgs field

hot universe(soon after big bang)

cold universe(condensates in an asymmetric state with Higgs field)0

v

particles are massless

particles acquire mass

the Higgs boson

Manfred Jeitler December 18, 2009 1 Particle Physics From an experiment-driven to a theory-driven...

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