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C4 Lecture 1 - Jim Libby 1

Particle physics option overview

• Introduction, some tools and basics [JL]• Particle detectors [Roman Walczek]• Relativistic Quantum Mechanics [RW]• Dynamic quarks and introduction to Quantum Chromodynamics

(QCD) [Robin Devenish] • Particle accelerators [RW]• Weak interactions [Giles Barr]• Oscillation phenomena – neutrinos and mesons [John Cobb]• Electroweak physics and the Standard Model [GB]• Top quark [Todd Huffman]• Why we know that this is not the whole story and what it might it

be [TH and Andre Lukas]

http://www.physics.ox.ac.uk/users/Devenish/Teaching/C4-home.htm


Things we hope you will take away1. The elegance of what has been discovered in the past

40 years– The standard model of electroweak interactions and Quantum

Chromodynamics2. The importance of sophisticated accelerators and

detectors in allowing this progress-you are being lectured by experimentalists!

3. What we don’t know and why this is importantHealth warning: this is not a Part A or B course

– Everything you learn is on the syllabus but not necessarily examinable

– For example, it is important that you understand how a calorimeter works but there will not be a whole finals question on this topic

– Aim to give you an overview beyond ticking the boxes for an exam• prepare some of you for projects and PhDs


The Standard Model of particle physics

• Describes the basic building blocks of matter and three of the forces that act between them:– Electromagnetism– Weak– Strong

• Unbroken for nearly thirty-five years


The outstanding issues with the SM

• Despite its success there are still many outstanding issues:– What is the origin of mass?

• Higgs boson of SM the only missing piece– Why three generations and the hierarchy of masses?

• 22 parameters in the Standard Model– Hierarchy problem?

• Fine tuning to the 26th decimal place– Why do we live in a matter dominated universe?

• Where did all the antimatter go? – What is dark matter? – Can we incorporate a quantum theory of gravity?


Large Hadron Collider location

LHCbATLAS

ALICECMS

Lake Geneva

Alps

27 km circumference

~100 m


The accelerator • 14 TeV pp collisions• Superconducting dipoles:

– 15 m long– 1232 cooled to 1.9 K– Max. field 8.33 T

• Ultra-high vacuum– 10−13 atm

• Vital statisics– 11,000 revolutions/s – 2808 bunches/beam– 1.2×1011 protons/bunch– Collisions rate of 40 MHz at

each interaction point

• 10th Sept. beam circulated successfully in both directions– No more running this year due

to a cryogenic power connection failure


ATLAS and CMSATLAS: 46 m × 25 m × 25 m CMS: 21 m × 15 m × 15 m

Direct searches for Higgs and non-Standard Model physics (i.e. Dark Matter)

Large Oxford involvement


The LHCb Experiment Dedicated experiment for precision measurement of CP violation

(matter-antimatter difference) and rare decays of b-hadrons at the LHC– Indirect searches for non-Standard Model physics

p

p

Interaction point

10 – 300 mrad forward acceptance

Number of b quark pairs

Dipole magnet

Large Oxford involvement


Real data Events generated by cosmic rays, collisions with residual gas in LHC beam pipe or beam stoppers close to detector


Overview of this course1. Introduction to the particle physics option

and the quark model of hadrons2. Isospin (including revision of the addition of

angular momentum and SU(2))3. Rotations and symmetries4. Heavy quark spectroscopy (including

revision of the Breit Wigner resonance)5. Lorentz invariant phase space6. Some of this in action: introduction to

colliders; total cross sections in e+e- and hadron collisions; selecting events in real time (triggering)


Quark model of hadrons


The elementary fermions of nature

2/3~1.3c [charm]0<0.00019νµ

-1/3~0.005d [down]-10.000511e-

2/3~0.003u [up]0<2×10-9νe

-1/3~4.3b [bottom]-11.7770τ

2/3174t [top]0<0.0182ντ

-1/3~0.1s [strange]-10.106µ

Charge (e)

Mass (GeV/c2)

FlavourCharge(e)

Mass (GeV/c2)

Flavour

Quarks (spin =1/2) Leptons (spin =1/2)

• Elementary particles: occupy exact point in space-time• This course: composite structures (hadrons) built from quarks


Hadrons• Bound states of quarks:

– Held together by the strong force– Mesons: quark and anti-quark pairs

• Bosons (J=total angular momentum=0,1,2,….)• Unstable: longest lived is the pion

– Baryons: three quarks or antiquarks• fermions (J=total angular momentum=1/2, 3/2, 5/2,….)• proton stable • neutron almost stable when bound in a nucleus• Baryon number conservation ≡ quark number conservation

– Exotics: pentaquarks, D molecules, glueballs,……• This classification arose from similarities amongst the

hadrons observed empirically during the 1950s and 1960s– Example: three Σ baryons of near identical mass

qq

qqqqqq or

( ) ( ) ( ) 2202 /MeV1197;/MeV1192;/MeV1189 cmcmcm =Σ=Σ=Σ +−


Baryon octet

0-1/2-1 11/2

S

I30

-1

-2

n

udd

p

uud

−Σ

dds 0Σ

uds

0Λ

uds +Σ

uus

−Ξ

dss0Ξ

uss

Spin-parity JP=½+

•Two quark spins aligned the third antiparallel•Parity from +ve intrinsic parity of quarks

S and I 3 are additive quantum numbers•S=strangeness (S=−1 for a strange quark and +1 for an anti-strange quark) •I3= third component of isospin(I3=+½ for up quarks and −½ fordown quarks)


Baryon decupletSpin-parity JP=3/2+

All quark spins aligned

Quark compositions |qq′q′′> are not the same as quark wavefunctions

The symmetry of these wavefunctions are the reason for •both the Σ0 and the Λ0 in the octet and •the offset in masses between the Σs and the Ξs between octet anddecuplet

0-1/2-1 11/2

S

I30

-1

-2

0∆

udd+∆

uud

)1385(−Σ

dds

)1385(0Σ

uds

)1385(+Σ

uus

)1535(−Ξ

dss

)1535(0Ξ

uss

−∆

ddd++∆

uuu

-3/2 3/2

-3−Ω

sss


Meson nonet: JP=0-

1

-1

S

ηηπ ′,,

,,

0

ssdduu

0-1/2-1 11/2

I30

0K

sd+K

su

−π

ud+π

du

−K

us

0K

ds

Spin-parity JP=0-

Quark spins antiparallel

Pseudoscalars: zero spin (scalar quantity) but antisymmetric under parity operation

The three S=I3=0 statesquark wavefunctions are different admixtures of

ssdduu and ,


Meson nonet: JP=1-

Spin-parity JP=1-

Quark spins parallel

Vector particles

The three S=I3=0 statesquark wavefunctions are different admixtures of

-1

S

φωρ ,,

,,

0

ssdduu

0-1/2-1 11/2

I30

0*K

sd+*K

su

−ρ

ud+ρ

du

−*K

us

0*K

ds

1

ssdduu and ,


Quantum numbers

−1/3+1/31

−1/3−2/3-1

−1/31/21/2+1/3-1

−1/3−1/21/2−2/3-1

1/3−1/3-1s

1/3+2/31c

1/3−1/21/2−1/31d

1/31/21/2+2/31u

BI 3IQCSDU

u

d

sc

•I and I3 are the isospin and its third (‘z’) component, respectively•B is the Baryon number•Not worrying about the heaviest quarkst and b - similar assignments to c and s


Quantum numbers

• Some relations amongst the quantum numbers– B=(U+D+C-S)/3

• By design to get the correct assignment:

– I3=(U-D)/2 • Important: intimately related to upness and downness

• All a little ad hoc but works in that all these quantum numbers are conserved in strong interactions

• Very useful because exact solution of strong dynamics very difficult because its strength means perturbation theory breaks down

• Before continuing with isospin and providing some insight into the octets, nonets and decuplets we will revise the addition of angular momentum and SU(2)

03/)11(

13/))1(11(

=−=∴=

=−−+=∴=Λ+ Bdu

Buds

π

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