1. Stefania Ricciardi RAL, March 2009
A short journey to the infinitely small Fundamentals of Particle Physics
Building blocks: particles and forces
Current areas of research
Stefania Ricciardi
RAL, March 2009

2. Warning Stay awake and keep an open mind!
This journey may change your vision of the Universe.
What you will hear may alter your perception of reality.
Stay awake and keep an open mind!
We are entering a Quantum World..

3. We and all things around us are made of atoms
Human Hair
~ 50 mm = m
= m
Atom ~ m
= m
Magritte

4. Atoms Atoms are all similarly made of:
- protons and neutrons in the nucleus
- electrons orbiting around
proton
The electron was the
first elementary
particle to be
discovered
(JJ Thomson 1897)
Protons, neutrons are made up of quarks
electron
neutron

5. From the atom to the quark
How small are the smallest constituents of matter?
<10-18 m
< m
~ m
~ m
~ m
Atoms and sub-atomic particles are much smaller than visible light wave-length
Therefore, we cannot really “see” them (all graphics are artist’s impressions)
To learn about the sub-atomic structure we need particle accelerators

6. Quarks detected within protons
Freeway 280
End Station A
experimental area
2 miles long accelerator
Stanford (SLAC), California, late 1960s
Fire electrons at proton: big deflections seen!

7. Is the whole Universe made only of quarks and electrons?
No! There are also neutrinos!
Electron, proton and neutrons are rarities!
For each of them in the Universe there is 1 billion neutrinos
Neutrinos are the most abundant matter-particles in the Universe!
n n n n n nnnnnnnn
nn n n nn n nnnnnnn
n nn nn nn n n n n n
nn n n nn n nnnnnn
n nn nn nn nnnnnnn
nn nn n nnn nnnnnnn
Within each cm3 of
space: ~300 neutrinos
from Big Bang
1 cm
1 cm
Neutrinos are everywhere!
in the outer space, on Earth, in our bodies..

8. Neutrinos get under your skin!
per second from Sun
are zipping through you n
Every cm2 of Earth surface is crossed every second by more than 10 billion (1010) neutrinos produced in the Sun
Within your body at any instant: roughly 30 million neutrinos from the Big Bang
No worries!
Neutrinos do not harm us.
Our bodies are transparent to neutrinos

9. Protons and neutrons in the quark model
Quarks have fractional electric charge!
u electric charge + 2/3
d electric charge -1/3
proton (charge +1)
neutron (charge 0) u u u d
So we know now that protons and neutrons are made of quarks. d d

10. The particles of ordinary matterne e- u d
-1/3
+2/3
charge
Leptons:
n = neutrino
e = electron
Quarks:
u = up
d = down -1
All stable matter around us
can be described using
electrons, neutrinos, u and d “quarks”

11. 3 Families (or Generations)
1st generation
2nd generation
3rd generation ne e- u d
-1/3
+2/3
+2/3
+2/3 nm m- c s nt t- t b -1 -1
-1/3 -1
-1/3
Ordinary matter
Cosmic rays
Accelerators
3 generations in everything similar but the mass
We believe these to be the fundamental
building blocks of matter

12. Quark masses 175 GeV Top (discovered 1995)
E= mc2
1 proton mass ~ 1GeV (10-27 Kg)
0.003
0.006
4.5
0.095
1.2
The mass grows larger in each successive family

13. Anti-matter
For every fundamental particle of matter there is an anti-particle with same mass and properties but opposite charge
Matter
Anti-Matter ne e- u d
-1/3
+2/3 e+
+1/3
-2/3 +1 -1
Bar on top
to indicate
anti-particle
positron
Correspondent anti-particles exist for all three families
Anti-matter can be produced using accelerators

14. Matter-antimatter pair creation
Electron-positron pair created out of photons hitting the bubble-chamber liquid
Example of conversion of photon energy into matter and anti-matter
Matter and anti-matter spiral in opposite directions in the magnetic field due to the opposite charge
Energy and momentum is conserved

15. Building more particles
B mesons (bq) c c
J/y b b Y
Many more mesons and baryons…

16. Quark confinement
There are no free quarks, quarks and antiquarks are “confined”
in colourless doublet (mesons) or triplets (baryons) by the exchange of gluons
Decay Z0
Gluon hold quarks
together as they move
further apart
until the gluon connection
snaps, and other quark-antiquark
pairs are created out of the
energy released
The new quarks bound
to the old quarks
and form new mesons
® S.Ward

17. Quarks and colour
All quark flavours come in 3 versions, called “colours” u d
+2/3
-1/3 up
down
Quarks combine together to form colourless particles
Baryons (three quarks: red+ green + blue = white)
Mesons (quark-antiquark pair)
such as red u+anti-red ubar state
Strong forces “glue” quarks together in bound states
proton p u
pion u p

18. Confinement
Color confinement, often simply called confinement, is the phenomenon that color charged particles (such as quarks) cannot be isolated singularly, and therefore cannot be directly observed.[1] Quarks, by default, clump together to form groups, or hadrons. The two types of hadrons are the mesons (one quark, one antiquark) and the baryons (three quarks).

19. Confinement
The constituent quarks in a group cannot be separated from their parent hadron, and this is why quarks currently cannot be studied or observed in any more direct way than at a hadron level

20. Confinement
The force responsible for holding quarks together is the strong nuclear force, which is notable as the only fundamental force that does Not decrease with distance (a property known technically as color confinement).

21. Confinement
For these reasons, as more and more energy is pumped into a system in an attempt to separate the quarks, we find that eventually it takes so much energy that we end up producing a new quark pair (quark and antiquark) before getting the quarks separated beyond the strong force range.

22. The Particle Physicist’s Bible: Particle Data Book https://pdg.lbl.gov
"Young man, if I could remember
the names of these particles,
I would have been a botanist!“
E.Fermi to his student
L. Lederman (both Nobel laureates)
The Particle Physicist’s Bible: Particle Data Book
Most particles are not stable and can decay to lighter particles..

23. The 4 forces of Nature Weak Electromagnetic weak charge Electric
Beta-decay
pp fusion
Electromagnetic
TV, PCs
Magnets
e- e+ creation
weak
charge
Electric
charge
Strong
Quark binding
Gravity
Responsible of
Keeping us
well-planted on earth
strong
charge
mass

24. THE WEAK FORCE Beta Decayn p
Antineutrino
Electron

25. A (free) neutron decays after 15 min
Neutron b-decay
At quark level:
d→ u e- ne u d p u d n
15 min e- ne
A (free) neutron decays after 15 min
Long life time (15min is an eternity in particle physics!)  “weak”
without such weak interactions the Sun would shut down!

26. Electromagnetic force
The repulsive force that two approaching electrons “feel” e- e-
Photon is the particle
associated to the electromagnetic force
“smallest bundle” of force
Photon

27. Photon exchange Feynman Diagram

28. Weak force: W-,W+,Z0 b-decay n→pene W- Electric charge
conserved at each
vertex

29. Strong force: gluons Gluons interact with quarks
Gluons interact with other gluons

30. Feynman diagrams
Reference the Feynman videos!

31. Force Particles (summary)
Particles interact and/or decay thanks to forces
Forces are also responsible of binding particles together
Strong: gluons
Only quarks
(because of their
colour charge)
Weak: W+, W-, Z0
Leptons and quarks (only force for neutrinos)
Electromagnetic:g
Quarks and charged leptons (no neutrinos)
Gravity: graviton?
Still to be discovered
Negligible effects on particles

32. The Standard Model Framework which includes: Not gravity! No quantum
Matter
6 quarks
6 leptons
Grouped in three generations
Forces
Electroweak:
g (photon)
Z0, W±
Strong
g (gluon)
Not gravity! No quantum
field theory of gravity yet..
H= the missing ingredient: the Higgs Boson
Very successful to describe all observed phenomena in the
subatomic world so far. But there ought to be more..

33. Beyond the Standard Model:Unification of forces
ELECTRO-
MAGNETIC
UNIFIED
FORCE?
GRAVITY
STRONG
WEAK
Looking for a simple elegant unified theory

34. Recap Leptons: electrons, (tau) 𝜏 or (mu) μ and associated neutrinos
Hadrons: either mesons or baryons
Mesons are made from an antimatter-matter pair
Baryons are made from 3 quarks with colour confinement rules

35. Conservation Laws
Lepton number, strangeness, baryon number and charge are all conserved in particle physics interactions (strong force).
All quarks have a strangeness of 0, except the strange particle (naturally) with a strangeness of -1.
All leptons have a lepton number of 1 and anti-leptons have a lepton number of -1.
All quarks have a baryon number of 1/3.So all baryons have a baryon # = 1.
Mesons have a baryon # = 0.

36. Conservation Laws
Example: Determine the quark content of a meson with charge +1 and strangeness 0 and that of a baryon with charge –1 and strangeness –3.
Meson:
Baryon:

37. Conservation Laws
Example: Determine the quark content of a meson with charge +1 and strangeness 0 and that of a baryon with charge –1 and strangeness –3.
Meson: made of matter-antimatter. 𝑢 𝑑
Baryon: made of 3 quarks but strange particle only one with strangeness of -1: so it must be sss

38. Kaons A Kaon is a meson made with strange quarks. It’s a K-meson.
K+ is 𝑢 𝑠
K0 is d 𝑠 or s 𝑑
K- is 𝑠 𝑢

39. Conservation Laws
Example 2: The following particle interaction is proposed. p + π– → K– + π+ In this interaction, charge is conserved. State, in terms of baryon and strangeness conservation, whether the interaction is possible.