wop particles of the standard model

VU lecture – World of Particles Thomas Gajdosik, FI & VU

World of Particleso d o a t c es

Th P ti l Z Thomas Gajdosik

Vilnius Universitetas, Teorinės Fizikos Katedra

The Particle Zoo Symmetries The Standard Model


Particles of the Standard Model:Particles of the Standard Model: Fermions

1. reminder about the particles• from the historical introduction

2. the ordering principle• example: electron and neutrinoexample: electron and neutrino

3. the systematics• extending the ordering to all fermions• extending the ordering to all fermions• counting the degrees of freedom

4 i4. overview


th l te- the electrone-

ThomsonThomsonThomsonThomson

1897


the positron (anti-matter)e+ p ( )

e-

γ

discovery

γp

nnµ

π

AndersonAndersonprediction π

DiracDirac

1897

1900-1924

1914 19321937

1947


the neutrinoν

e-

γγp

nFermiFermiPauliPauli

nµ

ππ

e+

1897

1900-1924

1914 19321937

1947


Reminder:Are symmetries perfect?

P violation - but maybe a CP symmetry?y y y

νν νright-handed left-handed left-handed

• there is no left-handed anti-neutrino, but there is a left-handedneutrino (and only a such-handed!)

right handedanti-neutrino

left-handedanti-neutrino

left-handedneutrino

neutrino (and only a such handed!)• obviously, this violates C-symmetry (symmetrie between matter and anti-matter)

• BUT: the combined symmetry transformation CP (exchangey y ( gmatter/anti-matter plus mirroring) works:

ν νCPCPνright-handedanti-neutrino

νleft-handed

neutrino

CPCP↔↔


Ordering principle g p pdiscreet symmetries

• Parity P• Parity P– left handed or right handed

eL eR

• Charge Conjugation C– particle or antiparticle

νν_

particle or antiparticle

• Charge Q or Flavour 0 -1 ⅔ -⅓– possible values:

• Generationν e u d

– first – second – third e µ τ


Particles of the Standard Model: Fermions

RightLeft

cle

eR-νe eL

-

Par

tie e+e+ _

tipar

ticle eReL νe

Ant


the protonp p

e-

γγ

RutherfordRutherford

1897

1900-1924

1914


the neutronn the neutron

e-

γChadwickChadwick

γp

1897

1900-1924

1914 1932


partons / parton modelp p

e-

γRichard Feynman1969 γ

pn

1969

nµ

ππ

e+

νν

1900-1924

1897 1914 19471932

1937 1955

1947

1969



RightLeft

cle

eR-νe eL

- uL dL uR dR

Par

tie e+e+ _

tipar

ticle eReL νe

Ant


the pionπ the pion

e-

γ

π

prediction discoveryγ

pnn

µPowellPowellYukawaYukawa PowellPowellYukawaYukawa

1897

1900-1924

1914

1932

1937 1947



RightLeft

cle

eR-νe eL

- uL dL uR dR

Par

tie e+e+ u

_ _d u

_ _d

tipar

ticle eReL uL dL uR dR

Ant


the muonµ the muon

e-

γ

µWho ordered that one? γ

pnn

k h b

Hess

spark chamber

• Hess• Anderson,

Neddermeyer

1897

1900-1924

1914

1932

1937 • Street, Stevenson



RightLeft

--cle

eR-νe eL

- uL dL uR dR

µR-µL

-

Par

tie e+e+ u

_ _d u

_ _d

µR+µL

+

tipar

ticle eReL uL dL uR dR

Ant


RochesterΛΛ

strange particlese-

γ

KKRochester,Butler,...

ΣΣ γp

n

ΣΣ

nµ

ππ

e+

νν

1897

1900-1924

1914 1932

1937

1947 1947-...



RightLeft

--cle

eR-νe eL

- uL dL uR dR

µR-µL

-

Par

ti sL sR

e e+e+ u_ _

d u_ _

d

µR+µL

+

tipar

ticle

_sL

_sR

eReL uL dL uR dR

Ant


antineutrino reactors:_ν

e-

γ

Clyde Cowan, Frederick Reines

γp

nnµ

ππ

e+

νν

1897

1900-1924

1914 1932

1937

1956

1955

1947



RightLeft

--cle

eR-νe eL

- uL dL uR dR

µR-νµ µL

-

Par

ti sL sR

e e+e+ _u_ _

d u_ _

d

µR+µL

+

tipar

ticle

νµ__

sL

_sR

eReL νeuL dL uR dR

Ant


charm quark: J/Ψc q

e-

γγp

nnµ

πBurt Richter (SLAC), Samuel Ting (BNL) π

e+

Samuel Ting (BNL)1974

νν

1900-1924

1897 1914 19471932

1937 1955

1947

1969

1974



RightLeft

--cle

eR-νe eL

- uL dL uR dR

µR-νµ µL

-

Par

ti cL sL cR sR

e e+e+ _u_ _

d u_ _

d

µR+µL

+

tipar

ticle

νµ_

cL_ _

sL cR_ _

sR

eReL νeuL dL uR dR

Ant


tau lepton: ττ p

e-

γMartin Perl (SLAC LBL) γ

pn

(SLAC-LBL)1975

nµ

ππ

e+

νν

19751900-1924

1897 1914 19471932

1937 1955

1947

1969

1974



RightLeft

--cle

eR-νe eL

- uL dL uR dR

µR-νµ µL

-

Par

ti cL sL cR sR

τR-τL

-

e e+e+ _u_ _

d u_ _

d

µR+µL

+

tipar

ticle

νµ_

cL_ _

sL cR_ _

sR

eReL νeuL dL uR dR

Ant

τR+τL

+


bottom quarkb q

e-

γγp

nnµ

ππ

e+

νE288 (Fermilab)1977

ν

19751900-1924

1897 1914 19471932

1937 1955

1947

1969

1974 1977



RightLeft

--cle

eR-νe eL

- uL dL uR dR

µR-νµ µL

-

Par

ti cL sL cR sR

τR-τL

- bL bR

e e+e+ _u_ _

d u_ _

d

µR+µL

+

tipar

ticle

νµ_

cL_ _

sL cR_ _

sR

eReL νeuL dL uR dR

Ant

τR+τL

+_bL

_bR


top quarktCDF D0p q

e-

γ

CDF, D0 (Fermilab)1995 γ

pnn

µππ

e+

νν

19751900-1924

1897 1914 19471932

1937 1955

19951947

19831969

1974 1977

1979



RightLeft

--cle

eR-νe eL

- uL dL uR dR

µR-νµ µL

-

Par

ti cL sL cR sR

τR-τL

- tL bL tR bR

e e+e+ _u_ _

d u_ _

d

µR+µL

+

tipar

ticle

νµ_

cL_ _

sL cR_ _

sR

eReL νeuL dL uR dR

Ant

τR+τL

+ tL

_ _bL tR

_ _bR


tau neutrino: ντντ

e-

γ

DONUT (Fermilab)2000 γ

pn

2000

nµ

ππ

e+

νν

19751900-1924

1897 1914 19471932

1937 1955

19951947

19831969

1974 1977

20001979



RightLeft

--cle

eR-νe eL

- uL dL uR dR

µR-νµ µL

-

Par

ti cL sL cR sR

τR-ντ τL

- tL bL tR bR

e e+e+ _u_ _

d u_ _

d

µR+µL

+

tipar

ticle

νµ_

cL_ _

sL cR_ _

sR

eReL νeuL dL uR dR

Ant

τR+τL

+ ντ_

tL

_ _bL tR

_ _bR


neutrino oscillations: νe ↔ νµ↔ ντνx

• solve the “solar neutrino puzzle”

µ

solve the solar neutrino puzzleneutrinos have a tiny mass

there exist also right-handed neutrinos

• but they have:

no charge, no hypercharge, and no color

no interaction except the mass termno interaction, except the mass-term

their existence does not change the Standard Model!



RightLeft

--cle

eR-νeReL

- uL dL uR dRνe

µR-νµRµL

-

Par

ti cL sL cR sR

τR-ντRτL

- tL bL tR bR

νµ

ντ

e e+e+_u_ _

d u_ _

d_

µR+µL

+

tipar

ticle

νµR_

cL_ _

sL cR_ _

sR

eReLνeR uL dL uR dR

νµ_νe

Ant

τR+τL

+ντR_

tL

_ _bL tR

_ _bRντ

_


Particles of the Standard Model:Particles of the Standard Model: Gauge Bosons

1. Gauge Theory (wop-sm2.pdf)

2. screening in QED2. screening in QED• Vacuum polarization• running coupling constantrunning coupling constant

3. anti-screening in QCD• asymptotic freedom• asymptotic freedom• confinement

4 i t b4. massive vector bosons


screeningscreening• the effective charge of an electrong

in dielectric media is reduced by dielectric molecules surrounding the charge

• the same happens in the vacuum: ppif one looks at the charge with sufficient energy to see virtual electron-positron pairs:

Vacuum polarization!Vacuum polarization!


screeningscreening• the energy dependence of the gy p

effective charge in the vacuumdue to Vacuum polarizationis described by the

running coupling:running coupling:


anti screeninganti-screening• the self couplings in QCDp g

have the opposite effect for the color charges:

• the closer one looks, the weaker the charges seem to gbecome:

asymptotic freedom!asymptotic freedom!


anti screeninganti-screening• but that also means:

th l th• the lower the energy becomes, the strongerthe charges seem to be!the charges seem to be!

• when we try to separate color charges, we havecolor charges, we have no problems at high energies colliders

• but at low energies, the force is strong enough, g , g g ,that the potential (= force * distance) can createa quark-antiquark pair, that restores color neutrality

color confinement!


color confinementcolor confinement• low energy states have to be color neutralgy

we can only observe color neutral particles

• the strong force hides inside the nucleons

the nuclear force is more like a• the nuclear force is more like a van der Waals force:

– mediated by mesons (quark – antiquark pairs)

• Baryons and Mesons are color singletsBaryons and Mesons are color singlets


Particles of the Standard Model:

first hint for

massive vector bosons• first hint for

neutral currents: G llGargamelle Bubble chamber,1973


Particles of the Standard Model:

W and Z bosons

massive vector bosons• W- and Z-bosons

detected in 1983b UA1 d UA2by UA1 and UA2

W+W- event in Aleph (LEP)


Particles of the Standard Model: W- and Z-bosons

precision studies by LEP:by LEP:


Particles of the Standard Model: W- and Z-bosons

precision studies by LEP:


Particles of the Standard Model:Particles of the Standard Model: Higgs Boson

1. Why a Higgs Boson?2. The Higgs mechanism2. The Higgs mechanism

• … again formulas … …

3 Systematics:3. Systematics:• counting the degrees of freedom

4 E i t l id4. Experimental evidence• how it is seen• exclusion / hints


Why a Higgs Boson ?• The Standard Model is a chiral theory

it is described with massless fermion fieldsit is described with massless fermion fields• Gauge Symmetry enforces

massless vector bosonsmassless vector bosons• But we have

i f i l t d k• massive fermions: leptons and quarks• massive vector bosons: W± and Z0

• Solution: the Higgs mechanism


The Higgs mechanism

• Ingredients:• a scalar field • continuous symmetries = gauge symmetries• vacuum

• Result:• the symmetry is spontaneously brokeny y y• the scalar field develops a vacuum expectation

value (vev)• other fields can acquire masses due to the vev


symmetry breakingy y g

example: chess

• the rules of chess are in principle• the rules of chess are in principleabsolutely symmetric for both players

• i.e. the rules how the pieces move arethe same for black and whitethe same for black and white

but:

• symmetry is broken at the beginning• symmetry is broken at the beginning,due to the initial setup of the pieces

• therefore, e.g. a bishop never canchange the color of the field it ischange the color of the field it isstanding on


symmetry breaking

In the standard model, the particle‘s masses are an effect of symmetry breaking:

y y gthe origin of mass

• originally, all particles are massless • but there is an additional interaction with the so-called Higgs-field• if there were no Higgs-field, the interaction would have no effect• however due to a spontaneous symmetry breaking the whole universe• however, due to a spontaneous symmetry breaking, the whole universe is filled with a non-zero Higgs-field

• the interaction with this omni-present field produces what we know as mass of particles

energy hot universe(shortly after big bang)

particles are massless

Higgs-field0

cold universe(condensed into anasymmetric state)

particles get a mass

spontaneous symmetry breaking

gg


degrees of freedom:degrees of freedom: (only SU(2) x U(1) bosons)

massive theorymassless theory

1 complex scalar 4 1 real scalar field 11 pdoublet 4

4 massless gauge 8

1 (Higgs) 1

1 massless gauge 24 g gboson (B, Wi) 8

0 massiv gauge 0

1 g gboson (photon) 2

3 massiv gauge 90 g gbosons 0

12

3 g gbosons (W±,Z0) 9

1212 12


Production at LEP:Production at LEP: Higgs production cross section :Higgs-strahlung:

Higgs fusion:Higgs-fusion:


Production at LEP:Production at LEP: Higgs branching ratio :Higgs branching ratio :

seen in the decay to:

• bottom quarks

b-tagged jets

• tau leptons

reconstructed τ

IF the m > 161 GeVIF the mH > 161 GeV• W-pair

• Z-boson pair


exclusion by LEP I & II:exclusion by LEP I & II:

comparison between an

expected (calculated)

distribution and the

measured distribution of

events

← measured mass distribution


exclusion by LEP I & II:exclusion by LEP I & II: statistically better

evaluated by a likelihood ratio Q:

comparison between the

distribution, depending

on a “test mass” and theon a test mass and the

measured distribution of

events


exclusion by LEP I & II:exclusion by LEP I & II: then one compares the likelihood oflikelihood of signal+background, measured in confidence levels CLs+b with thelevels CLs+b, with the likelihood of only backgroundand defines the signal as

CL CL / CLCLs = CLs+b / CLb

exclusion of the signal is given if g

CLs < 0.05

LEP excludes a 114.4 GeV Higgs boson at 95% CL


exclusion by Tevatron:exclusion by Tevatron:


hints:hints:electroweak

precision measurementsprecision measurements

precise measurements ll th i ithallow the comparison with

precise calculations:

but all loop calculations depend on the masses of the particles in the loop!the particles in the loop!

sensitivity to particles,th t ld t t bthat could not yet beproduced!


hints:hints:blueband-plot:

• giving the most probable value for the mass of thethe mass of the Higgs boson from electro-weak precision measurements:

• the most probable value is already excluded by direct non-observation.

wop particles of the standard model

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