Particle Physics II

Chris Parkes Room 455, chris.parkes@cern.ch

Experimental electroweak physics: W &Z

•Resonances

•Measuring the number of neutrinos from the width of the Z0

•W mass & branching ratio

1st Handout

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Course Content• Experimental Electroweak: W & Z

– Number of generations from Z resonance– Triple boson vertices

• Top Quark & Higgs– Direct & indirect searches

• Heavy Flavour Physics– Flavour changing interactions

• CP Violation– matter anti-matter asymmetry

• Beyond the Standard Model– Neutrinos– Supersymmetry– Astro-particle physics

Text books:1.Alessandro Bettini, Introduction to Elementary Particle Physics, Cambridge

University Press (2008)2. B.R. Martin and G. Shaw, Particle Physics (2nd edition), Wiley (2001)3. Donald H. Perkins, Introduction to High Energy Physics (4th edition),

Cambridge University Press (2000).

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Resonances and particles• Many particles are too

short-lived to be observed directly– Observe from decay

particles– e.g. ppX+other stuff,

Xµ+µ-

Three resonances

=2ΔEFWHM

Energy uncertainty ΔE is known as the particles width .

Uncertainty principle relates width and mean lifetime τ of particle

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tE

Exponential decay process for particles

/toeNN

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Particle Width/lifetimeDepends on:• Strength of interaction•Strong G~10-100MeV t~10-22-10-24s •EM G~10-100keV t~10-16-10-20s•Weak G<0.01eV t~10-8-10-13s •Number of different states the particle can decay into

•Can see long-lived particle through their decay distance•Can see short-lived particles through their width

Flightdistance

STRONG

EMAG

WEAK

Shorter lifetime, larger width

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Breit-Wigner

2221 2

1 2

i

(2 1)

(2 1)(2 1)( )

4intial momentum

resonance spin

, spins of incoming particles

, widths of initial and final states

i fif

i

i

f

j

q s sE M

q

j

s s

•The total width is related to the strength of the interaction•The particle can usually decay toA number of different final states•Each individual decay mode has a width, i, such that sum of “partial widths”=total width.•Branching Ratio

•Fraction of decays to that state• BR=i/

•Total width from width of invariant mass distribution•Mass from centre of distribution•Partial width of initial and final states from height of distribution

•Shape is same for all decays

spins Energy

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Z0 production

• e+e- has two diagrams and Z0 exchange

• Photon exchange dominates at low c.m. energies

• Z0 exchange results in a resonance at Mz

• At High cm energy, both photon and Z exchange– EW unification

• Calculation of relative strength of xsection

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Much Ado about Nothing:

Getting N from Z

Final states X in Z-decays Counting states

e== l

e==

u=c u

d=s=b d

Z=had+3l+N

How to get N

had

0 02

2 2 2 2 2

12 Z

cm cm Z Z Z

Z e e Z XMe e X

E E M M

0 0

max 2 2

12

Z Z

Z e e Z Xe e X

M

Total width from shape in any decay channelPartial widths from all seen decay channels

What isn’t seen is the neutrinos

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Number of neutrinos• Mass – peak of distribution

– MZ=91.1876±0.0021GeV (0.02% error!)

• Total width - width

Z=2.4952±0.0023GeV (0.1%)

• Partial Widths – number events

had=1.741±0.006

l=0.0838±0.0003

• Neutrino width from theory– determine number

• N=2.97±0.07

LEP

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What does it mean?

• Proves there are only three generations of particles

• 3 neutrino families 3 generations

– Satisfies anomaly condition

• What is the catch?– Assumed neutrinos massless in

theory width– Neutrinos now know to have

mass (more later) but very small

– Mass hierarchy

• We have summed Z X+X-

If there is an extra neutrino XWith mass > Z mass/2Z could not decay to it

quarksquarks

leptonsleptons

e

QQ

b

t

s

c

d

u

e

3

Z0

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How is Z mass / lineshape reconstructed ?

• Energy of electron&positron beams in accelerator adjusted

• Cross-section measured at different energies

• Breit-wigner fitted to cross-sections

Peak value gives mass

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Aside: Measuring the LEP beam energy

The accuracy with which the LEP beam energy is known controls the accuracy of MZ

Return current for train –TGV- changed magnet currents

Tidal effects change the shape of the ring

TIDE TIDE

TRAINS

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W Discovery • UA1, UA2 1983• Proton anti-proton collider

• We-+ν• Lots of soft tracks +

electron

                             ‘transverse’ massE2-p2,

taking only componentsIn transverse plane

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LEP e+e-W+W-

• Can be used to make precision tests of electroweak theory

Recall: 3 diagrams contribute

'' qqqqWWee

First Event: Me et al. !

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•All three diagrams contribute and interference•Total cross-section agrees with theory, requiresTriple boson vertex

Demonstration of triple boson vertex

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W boson: mass measurementHow:• Measure momentum of all

charged particles– Highly relativistic E≈p

• Energy of neutrals in calorimeters– Less well measured than p

• Cluster particles into jets– Obtain quark momenta

• ‘Missing’ momenta– Gives neutrino

• Pair jets/leptons– Form W

• Reconstruct mass from– E2-p2=m2

• Use known constraints to improve– Total energy from beams– Total momentum=0

Z

WW M

Mcos

Why:• Test of SM, recall

• Allows measure top mass, higgs mass in SM– See later

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W boson: branching fractions• Count possible decays of W-• Leptonic:• Hadronic:

[Hadronic is approximation – discuss CKM matrix later] – but 3 colours

• 3 leptonic+2 hadronic *3 colours =9 states– Approx 1/9 B.R per state– 6/9= 67% hadronic

• Width per state =225 MeV– Total width W=9*225MeV~2 GeV

, ,e

Q)What is lifetime ? Q)What fraction e+e-W+W- are fully hadronic/leptonic/mixed decays?Which can be used for mass measurement ?

sc d,u :qq Not as too heavybt