Historical introduction to Elementary Particles: QCD ...

PHY771, 9/4/2014 Tomasz Skwarnicki 1

Historical introduction to Elementary Particles:

QCD, Heavy Flavors, Standard Model

Tomasz SkwarnickiSyracuse University

• Griffiths, 2nd ed., 1.9, 1.11


Quark Model and SU(3)flavor symmetry

Y (=S)

Ιz

-1 0 1

+1

0

-1

Q

Y (=S)

Ιz

-1 0 1

+1

0

-1

Q

Ιz

-1 0 1

+1

0

-1

Q

I = ½

I = 0

Q= +2 / 3

Q= −1 / 3

0

-1

S=ud

s

Y (=S+1/3)

Gell-Mann Nishijimaformula

Q = Ιz+ ½ Y, Y=A+S

Fundamental

representation

- quarks (q)

Meson nonets

(qq)

A=1/3s =1/2

A=0

, q qJ S L S s s= + = +

0, 0, 0

1

L S J

n

= = =

=

0, 1, 1

1

L S J

n

= = =

=

( )ds ( )us

( )ds( )us

( )ud ( )ud

( , , )uu dd ss


Quark Model and SU(3)flavor symmetryGell-Mann Nishijima

formula

Q = Ιz+ ½ Y, Y=A+SBaryon octet

(qqq) A=1

1

2J S= =

Ιz

-1 0 1

+1

0

-1

Q

Y (=S+1)

( )udd ( )uud

( )dds ( )uus

( )dss ( )uss

( )uds

Baryon decuplet3

2J S= = Y(=S+1)

Ιz

+1

0

-1

-2

Q

-1 0 1

( )ddd ( )udd ( )uud ( )uuu

( )sss

( )dds ( )uds ( )uus

( )dss ( )uss

Mixed symmetry under exchange of quarks

Completely symmetrical under exchange of quarks

1 2 3q q qS s s s= + +

(no radial excitations or orbital angular momentum between quark in these lightest multiplets)


Color

• Dilemma with ∆−(ddd),∆++(uuu),Ω−(sss) – All spins must be aligned to produce J=S=3/2– How come this does not violate Pauli exclusion principle (two fermions

cannot be in identical quantum state)?

• Oscar .W. Greenberg proposed a solution in 1964:– Quarks have an additional property that can assume 3 different

values [notice that this has to do with baryons having 3 quarks inside, and not with existence of 3 different quark flavors: u,d,s].

– In all baryons 3 quarks have 3 different values of that property.Pauli exclusion principle remains satisfied.

– He called the new quantity “color”, and named 3 different values: red, green and blue. Mixing such colors, gives white color. Thus, the requirement that in baryons quarks always differ by a color can be expressed as a requirement for baryons to be “white” (i.e. “colorless”).

– This fits well together with mesons being qq states. In a meson, antiquark is required to have anticolor to the color of the quark (e.g. anti-red to red), to make meson “white” as well.

– Since only quarks have color, it is natural to assume that color is “strong charge”.

• In 1964 this seemed like a pretty ad hoc idea and there were lots of skeptics about existence of quarks, since isolated quarks were not observed

• Even after electron scattering on protons revealed hard scattering centers inside, people did not jump into conclusion that these were quarks (explains why in the scattering experiments they were called “partons”).


November revolution

• One of the most lively stories in experimental particle physics

• In 1973 the first e+e- collider (SPEAR) with center-of-mass energy of up to 4 GeV came into operations in a parking lot at Stanford (SLAC). The funding agency turned down the proposal to build it several times, eventually gave money for electron ring only. Agreed not to object positron ring, if built from SLAC operating funds without permanent housing.

• SPEAR was scanning cross-section (i.e. probability) for producing hadrons in a function of e+e- collision energy is search for unexpected. In Spring 1973 they hit one energy point at which cross-section was so out of range that they initially blamed it on faulty apparatus. The lead physicist, Richter, decided to recheck it, but since the pressure was to move up to higher energies, they scheduled just a quick check for a weekend in November

• Independently, physicists at Brookhaven (BNL) led by Ting took data with a new detector exposed to 30 GeV p beam from AGS (started operations in 1960) measuring e+e- mass spectrum out of pBe collisions (kind of inverse reaction to SPEAR approach). On October 31 they found a very narrow peak at M(e+e-) ~ 3.1 GeV. Since it was unexpected they wanted to double-check the results before publishing.

• On November 10, SPEAR took several energy points in the suspect region. They clearly observed a giant peak indicating a new, long-lived particle with mass of ~3.1 GeV, which they labeled ψ.

• By chance Ting arrived at SLAC for a meeting and learned about the discovery at SPEAR. He called his own peak “J” and published on Nov. 12. The SLAC team published on Nov. 13. Since nobody wanted to yield on the particle name, the community accepted it as “J/ψ”.


November revolution

Log-scale!

e+e−→ hadrons pBe → e+e− X

ψ J

Burton Richter1931 -

Samuel Ting1936 -

They co-shared Nobel Prize in 1976

Martin Perl

who was later instrumental in

discovery of heavy lepton τ

at SPEAR

(Nobel Prize in 1995)


November revolution• Further measurements at

SPEAR revealed that there was an entire family of ψstates – excitations of cc system (“charmonium”).

• Thus, the fourth quark was discovered - named “charm”.

cc

n 2S+1 L J

n=1

n=2

Hyperfine splitting

Fine splitting

J/y 1974

yí’ 1974

cc 1975

hc1980

hc í’

hc2005

2002

1 2s s i

L Si

• The 4th up-type quark was actually predicted by theorists working on weak decays (so called GIM mechanism), but nobody paid attention. Therefore, the c quark discovery was a shock to the high energy community which was quite happy with just 3 quarks of similar mass

• The c quark is so much heavier (~1.5 GeV) than u,d,s quarks (~0.1-0.4 GeV) that it does not make sense to employ (broken) SU(4)flavor symmetry. Charm is the first “heavy flavor”which was discovered (τ lepton was next).

• However, strong couplings of c quark are the same as of the lighter quarks. People understood that universal meaning of SU(3)flavor was in the flavor independence of strong interactions but the closeness in masses of u,d,s quarks was coincidental (at least it has nothing to do with strong interactions)

• 8 excitations with mass below the threshold for decay to (cq)(cq) meson pair, q=u,d. Their strong decays are suppressed by the OZI rule (see next slide) – they are long lived. Electromagnetic photon transitions compete with strong decays and were observed (like spectral lines in atomic physics)

• Unlike in light meson, quarks in charmonium are approximately non-relativistic. Simple nonrelativisticquantum mechanics (potential approach) can reproduce the observed masses with sub-percent precision. Decays and transition rates can also be well predicted. First time hadronspectroscopy became precision science.

• November Revolution left no doubt that quarks were real and not a mathematical construct.


OZI Rule (1963-1966)

• Okubo, Zweig, Iizuka invented this rule to explain why φ decays to K+K− (49%) were dominating over π+π−π0 (15%), while “phase space”(amount of energy released in the decay) strongly favors the latter

s

s

s

u

s

u

φ

K+

K−

φs

s

u

ud

d

d

d

π+

π−

π0

Strong processes with disjoint

quark diagrams are suppressed

Charmonium states with masses below 2 x D meson (cd) mass can decay

strongly only via disjoint quark diagrams – their lifetimes are relatively long

(widths small). Electromagnetic transitions can compete with strong decays

for excited states e.g.:

c

cψ(2S) c

c

γ

χc(1P)


Experimental evidence for color• R=σ(e+e−

→ hadrons)/σ(e+e−→ µ+µ− ) measured at e+e- colliders, determined c charge

to be 2/3 as predicted. • R is also a nice experimental confirmation that quark must have colors

d,u,s,c

d,u,s,c

qf

2 2 2 21 2 1 2

3.333

33 3 3

= + + + × =

color

Prediction with color, qc=2/3

Prediction with no color, qc=2/3

Actually the rate of weak π+ decays was also known to be low without quarks having color

Prediction with color, qc=1/3


Carrier of strong interactions - gluons

• Quarks are kept together in hadrons (e.g. in pions) via strong interactions. Yukawa meson (pion) cannot be mediating strong interactions between quarks!

• Mediators of strong interactions must couple to strong charge – color

• There are 3 colors, which suggests SU(3)color symmetry for these interactions (this has nothing to do with SU(3)flavor!)

• Quantum field theory based on SU(3)color symmetry has been developed –Quantum Chromo Dynamics (QCD)

• In QCD interactions are mediated by massless gluons, which are spin 1 particles, electrically neutral like photons in QCD.

• However, unlike electrically neutral photons, gluons carry color charge. Their charge comes from octet representation of SU(3) i.e. consist of a pair color1+anticolor2 (8 different color charge states of a gluon)


Self-coupling of gluons

• Unlike photons, which don’t directly interact with each other, gluons do.

• This opens a possibility of bound states of:– 2-gluons e.g. g(red-antiblue) + g(antired-blue) =

gg(white). So called “gluonium”.

– Quark-antiquark pair plus gluon e.g. q(red)+q(antiblue)+g(antired-blue) = qqg(white). So called “hybrid” state.

– Experimentalists have been looking for such states. There is no clear proof that they exist.

q g

q


Confinement

• Explanation why free quarks or gluons are not experimentally observed. It is a model which is not proven directly from QCD, but likely has to do with self-coupling of gluons.

• Unlike in QED where electric force between the two charges drops (to zero) when charges are separated, strong force increases its strength with separation. Color field gets stretched to a “flux tube” between the color charges (unlike in QED in which it spreads to entire space).

• For sufficiently large separation of color charges its becoming energetically preferable to pop quark-antiquark pair inside the tube and break it.

• Because of this quarks can never be separated from each other to distances larger than size of a hadron. (~1 fm). Similarly gluons are also confined.

• Experimentally, we have indications that potential of color field is growing linearly with quark separation (e.g. from charmonium mass spectrum). Linear potential at large distances is also theoretical supported by lattice QCD calculations. These support the confinement hypothesis.

• Because of confinement, strong interactions are effectively short range (even though they grow with separation).

• Since single gluon cannot penetrate beyond the extend of a hadron, interaction between nucleons must be mediated by color neutral objects like pions.


Gluon jets at high energy e+e- colliders

• Observed in late 1980is at DESY (PETRA) and KEK (TRISTAN)


Asymptotic freedom

• Coupling constant in QFT quantifies strength of intermediate boson coupling to the charge of the theory

• The coupling changes (“runs”) with energy of the boson

• This effect is rather small in QED

• Running of coupling constant in QCD is large. David Gross, Frank Wilczek, David Politzershowed in 1973 (Nobel Prize in 2004) that it decreases logarithmically.

• The experimental data agrees with this prediction.

Energy scale

Short distancesLong distances

Small

energiesLarge

energies

At low

energies (long

distances)

strong

interactions

become

stronger

(consistent with confinement)

At high energies

(short distances)

strong

interactions

become weaker

Asymptotically

interaction

disappears –

“asymptotic

freedom”.

Explains the OZI

rule!

Since coupling becomes large, perturbative

calculations not possible

at low energies – must

use numerical methods

(lattice QCD)


How was c quark predicted ? • Cabibbo angle (1963):

– Weak decays in Κ+ (us) → µ+ ν were measured to be slower than in π+ (ud) → µ+ ν.

– Nicola Cabibbo rescued universality of weak coupling by proposing:• u,d,s are eigenstates of strong interactions

• weak current (W) is coupling u and d’ = d cosθ + s sinθ . (θ − Cabibbo angle: cosθ=0.97 sinθ=0.22)

• Glashow-Iliopoulos-Maiani mechanism (1970):– Proposed existence of “up” type partner for strange quark – charm (c).

– There is also a weak current between c and s’ = - d sinθ + s cosθ

– Automatically no Flavor Changing Neutral Currents (Z0 turning s→d) at tree level (desired to stay consistent with known results)

– c quark in the FCNC box diagram (loop!) for K0L→µ+µ− decay cancels the “large”

contribution from u quark box, and explains why not observed at 10-4 level

BR ~ 10-8

Observed in 1973+

BR «10-4BR ~ 10-4


CP violation in K0 decays

• Cronin-Fitch experiment 1964 (BNL):

James Cronin (1931-),

Val Fitch (1923-)

Nobel Prize 1980

CP= +

CP= -

• Decays of K0L mesons to

ππ violate CP symmetry!

• Evidence for new type of force – “5th force”?

K0 = ( d s ) K0 = ( d s )

K0s decay quickly K0

L not expected to decay to ππ

but it does at 0.2% level

+

-CP(ππ)= +

CP(πππ)= -

0 0 0

S

0 0 0

L

1K (K K )

2

1K (K K )

2

= +

= −


CP violation in K0 decays

• Kobayashi, Maskwa 1972 (Nobel Prize 2008)

– proposed 3 quark generations to explain the Cronin-Fitch

experiment without the 5th force (before the 2nd generation c quark was discovered!)

'

'V

d d

s s

=

u

c

W

weak

force

Cabibbo + GIM

† † 1VV V V= =To conserve probability the quark mixing matrix V must be unitary:

V has 1 free parameter:

rotation angle between flavors -

Cabibbo angle!

All Vji elements can be made real.

– the complex phases of Vji not observable unless two

amplitudes (i.e. processes) interfere

'

'

'

d d

s

b

sV

b

=

u

c

t

W

weak

force

+ Kobayashi,Maskawa

NP!

V has 4 free parameter:

e.g. 3 rotation angles (Euler angles)

+ 4th must be in a complex phase.


Loops: CP violation in K0 decays

• The interference term produces CP violation and depends on the complex

phase of the mixing diagram (φM) minus phase of decay diagram (φA≈0)

• Observation of CP violation in K0 decays to ππππππππ was effectively the first indirect observation of t quark

• t quark observed directly via tree diagram in 1995 (CDF&D0); b quark in

1977 (Lederman ϒϒϒϒ)

Vtd

st

wd

t

d

s

wK0K0

Vtd*Vts

*

Vts

s

d

K0w

u

u

d

d

π−

π+

Vus*

Vud

s

dw

u

d

dπ−

π+

Vus

Vud*

u

Box diagram responsible

for K0-K0 mixing

Impossible

to tell which

happened

- the two

processes

interfere

Mixing + decay: “indirect” CPV

CP eigenstate


Discovery of b-quark 1977

• Upsilon discovery at Fermilab in 1977. Similar to J discovery.

pA → µ+µ− X

M(µ+µ− )

ϒϒϒϒ,ϒϒϒϒ’,ϒϒϒϒ’’

Leon Lederman1922-

Nobel Prize 1988

400 GeV p beam

(In 1976 Lederman published ϒϒϒϒdiscovery at mass of 6 GeV – this was a statistical fluctuation which went away with more data)


Upsilons

• Upsilon system (bottomonium) is even more non-relativistic than charmonium. More long lived states.

cc

n 2S+1 L J

S= 0 1 0 1 0 1 0 1

L= 0 1 2 3

n=1

n=2

n=3

n=4

Hyperfine splitting:1 2S S⋅

Fine splitting:

1 2 1 2

,L S

S r S r S S

⋅

⋅ ⋅ − ⋅

n=1

n=2

Hyperfine splitting

Fine splitting

J/y 1974

yí’ 1974

cc 1975

hc 1980

hc í’2002

hc2005

° 1977

°í’ 1977

°’’ 1979

cb 1983

χb’1982

°2 2002

bbχb’’2012

hb 2008

hb’2012 hb

2012

hb’

2012


Top quark discovery 1995

• Top discovered in 1995 at Tevatron (pp at 1800 GeV) at Fermilab

eν

jet jet

jet

jet

Top decays weakly before it can form a hadron! (also no topomonium). Weak decays

are not “weak” at energies larger than mass of intermediate weak bosons (W,Z)

(after 3 lower energy accelerators were built in hope of its discovery but failed to reach its mass)

t

b

W−

W+

t

b


3 generations of quark and leptons

• Weak interactions group 6 known quarks and 6 known leptons into 3 generations of partners in charged weak currents (mediated by W):

• We don’t know: – Why there is more than one generation of either quarks or

leptons

– Why number of quark and lepton generations is equal

– Why there are 3 generations

– Are there more generations yet to be discovered? (see next)

(Only left-handed fermions couple to W)


Neutrino counting via Z0 width• LEP (Larger Electron-Positron) collider was the highest energy e+e- collider ever built

(CERN)

• In 1989-1995 it operated at collision energies to directly produce Z0 boson.

• Z0 decays to charged leptons and quarks directly visible

• Z0 decays to neutrinos undetectable, but rate of such decays affects the observed width and height of the Z0 peak when changing center-of-mass energy: can experimentally determine to how many neutrino generations Z0 decays to:

• From this measurement we know for sure that there are exactly 3 generation of leptons with light neutrinos (i.e. below half of the Z0

mass; known neutrinos have masses which are undetectably small)

• There are also no hints for 4th

generation of quarks via rare processes involving b-quark decays(could contribute via generation mixing in a virtual form)

• Whenever new higher energy colliders are built, we do look for direct evidence for more quark generations

LEP operated above Z0 (up to 209 GeV) until 2000. LHC

replaced it in the same tunnel.

Average from all LEP experiments


Standard Model

• Periodic table?

• Some of outstanding questions:– Why 3 generations of fermions? (“Generation or flavor problem”)

– Why such a large and bizarre variation in masses (also in CKM and neutrino mixing matrix)?

– Can all interactions be unified (described by one theory)?

Unified already

Spin-2

massless

graviton ?


Fermion masses visualized

• In Standard Model quarks and leptons acquire mass by interacting with Higgs field. However via ad hoc (i.e. not

explained) couplings.

• The origin of the observed mass pattern is a mystery!

tb

c

ττττ

Why (almost) massless neutrions?

Why is top super heavy?


Quark mixing matrix visualized

• CKM elements in Standard Model also have to do with Higgs interactions, though they are ad hoc parameters.

• The origin of the observed mixing pattern is a mystery!

Not far from the diagonal

matrix!

Neighboring generations mix

more.

3rd generation almost

decoupled.


Neutrino masses and mixing

• Only upper limits on neutrino masses exist from direct measurements

• In the initial formulation of Standard Model (late 1970ies) neutrinos were assumed to be strictly massless

• In 1957 – 1967 a theoretical description of a possible neutrino flavor oscillations, between neutrions of nearly equal masses, was developed by Pontecorvo, Maki, Nakagawa and Sakata

• In 1968 first indications that there was a deficit of solar electron neutrinos reaching the Earth was reported. Neutrino oscillations was a possible explanation.

• Using atmospheric neutrinos Super Kamiokande experiment in Japan announced in 1998 the first significant observation of explicit neutrino flavor oscillations – this implies some non-zero neutrino masses

• Reactor neutrino and accelerator (neutrino beams) experiments followed. Very active experimental field in Japan, US and Europedevoted to precise measurement of neutrino mixing matrix, including its complex phase(s) (CP violation in neutrino sector?).


Neutrino mixing matrix visualized

• The origin of neutrino masses and mixing matrix could be the same or different than for quarks. In any case, the mixing pattern is very different than for quarks. None of this is understood.

• Neutrino mixing violates lepton flavor conservation. However, unlike quark mixing, neutrino mixing does not lead to observable particle-level violation of lepton flavor:

Mass eigenstates

Flavor (i.e. weak)

eigenstates(Pontecorvo-Maki-Nakagawa-Sakata matrix)

SM diagram for

b→sγ

Rare but observable!(CLEO B →K*γ 1993)

SM diagram for

µ→eγ

Too small to be ever measureable(in W-)

“mass insertions” ~ mf

Off diagonal mixing matrix elements are large and lead to large neutrino flavor oscillations.

(happens thanks to neutral Higgs vacuum)


Confirmation of two-neutrino hypothesis 1962

• Hypothesis that there are two distinct neutrinos:

electron neutrino and muon neutrino (each with

its own antiparticle) was confirmed experimentally

by Lederman–Schwartz–Steinberger in 1962:

π− → µ− νµ

An accelerator based experiment at Brookhaven (NY), in which muon

anti-neutrino beam was produced via:

and directed into proton-reach target:

ννννµµµµ pppp→ n µµµµ++++

ννννµµµµ pppp→ n eeee++++

Observed

Not observed

Outcome of this observation could have been different if done over a different distance to allow for muonneutrino to oscillate to electron neutrino. However, the fact that outcome depends on the length of neutrino beam does mean that there are different neutrino flavors (which oscillate). Net conclusion is the same!

Previously shown


Road ahead

• We have finished historical introduction (Ch.1):– Important to memorize type of interactions, related conservation

laws, fundamental fermions and bosons, lightest versions of mesons and baryons.

– You should know major milestones in development of particle physics. No need to memorize exact dates, but knowing rough periods is desirable.

– We will be coming back to many topics through the semester to learn more details.

• We will cover Ch.2: (Introduction to) “Elementary Particle Dynamics” on Thursday:– We have discussed different types of forces and their carriers

already. This will be mostly reiteration of what we have alreadysaid with only a few new topics.

– Please do Problem 2.7 before Thursday! (exercise in use of conservation laws). This is an important practice before the exam next Tuesday.

Historical introduction to Elementary Particles: QCD ...

Documents

Transcript of Historical introduction to Elementary Particles: QCD ...