Introduction to High Energy PhysicsJana Schaarschmidt (Weizmann Institute of Science)

1. History and Discovery of the Standard Model Particles

2. Higgs Boson Discovery

3. New ideas

Outline 2

1. History and Discovery of the Standard Model Particles

2. Higgs Boson Discovery

3. New ideas

Outline 3

The discovery of the Atom

Size of an atom:0.1 nm = 10-10 m

20th century:

Atomic structureobservable withelectron microscope

Philisophic considerations, for example in the ancient Greece (500 bC):

→ All matter is constituted from atoms, which cannot be divided further

17th century:Existence of molecules would explain the behaviour of gas (Boyle, Avogadro, …),and they also explain observations in the early chemistry (Dalton)

19th century:Further evidence for atoms from studiesof the Brownian motion (Brown, Einstein, Perrin, ...)

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The discovery of the Electron (1897)

JJ Thomson (1856-1940)

He put a metal cathode and anode inside a vacuum tube in a high voltage field. The heated cathode starts to emit a form of glowing ray (cathode rays).

These cathode rays could be deflected by electro- magnetic fields, which means that these beams consist of charged particles electrons.→

This ended the hypothesis that atoms are fundamental! Atoms consist of other particles.

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The discovery of the Nucleus (1911)

Ernest Rutherford (1871-1937) Most alpha-particles pass through the gold foil.

But Rutherford detected that sometimes particlesare deflected backwards!

That can only be explained, if there is somethingvery heavy and very localized (small) objectinside the atom the nucleus, which contains→99.9% of the mass of the atom

Nucleus size1 fm =10-12 m

Helium

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The Units of Energy in HEP

In everyday life, we measure energy in Joule (1J=1NM), and masses in kg.In High Energy Physics, we measure energy and mass in units of electron Volts (eV).

1 eV is the energy that an electron gainswhen moved across an electric potential of 1 V.

1 eV = 1.6 * 10-19 J

1 keV = 1000 eV1 MeV = 1000 keV1 GeV = 1000 MeV = 109 eV

Since energy and mass are equivalent (A. Einstein 1905): E= mc2

we also measure masses in eV.

Examples: Proton mass 1 GeV. Electron mass = 0.5 MeV. LHC beam energy = 6.5 TeV.

We forget about the c2. We know it should be multiplied to the mass, but we are too lazyto do so :) We measure mass and energy in the exact same unit.→

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Superkamiokande Neutrino Detector:Sources forNeutrinos:

- Sun- Nuclear power plants- Accelerators- Atmosphere

Neutrinos (1956)

νe + p+ n + e→ +

Inverse b-decay

e++e- 2→ g

(Cowan-Reines-experiment 1956)

Existence of neutrinos proposed early 19th

to explain the spectrum of the electronsemitted in beta decays n p→ + + e- + ? ? = n

Neutrinos almost don't interact.They simply go through the earthand our bodies!„They have a tiny cross section“

observed

n +108Cd → 109Cd + g

Cerenkov light detectionCerenkov light detection

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The discovery of Muon and Tau

„Who ordered that?“ (Isidore Rabi)

The muon and tau are not stable, they decay. Muon decay: m- e→ - + nm + n

e (lifetime: 2.2 ms)

Electron, muon and taus are called „charged leptons“. → There are 3 of them.

Neutrinos are called „neutral leptons“. → There are also 3 of them.

Three generations of leptons...

Anderson discovered the muon in 1937 by studying cosmicrays. The muon m- is a „heavy replica“ of an electron e-.

e+ + e− → τ+ + τ− e→ ± + μ∓ + 4n

The tau (another „heavy replica“) was discovered in 1974 at SLAC (USA) in a collider experiment:

SPEAR

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The Pion discovery (1947) and Quark Model

Frank Powell used photo emulsions and light microscopes to make charged particles from cosmic rays visible. He used magnetic fields to bend the particle tracks.

In the coming years, dozens of new particles were discovered:

Kaons, Lambdas, Omegas, Deltas, Eta, Xis, Bs, J/y ...

Some of them are charged, some are neutral, some havestrange properties, some live longer than others...

→ Too much chaos! Nature prefers it simple...

In 1961 Gell-Mann and Nishijima proposed the quark modelto bring order into the chaos.

All those particles are built-up from a few elementary quarks.

Pion (p+) track visible through ionization in silver-halid:

p+

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The Strong Force, Confinement and Gluons

Quarks have a quantum number called „color“.(It is actually not a real color, we just call it like that.)

The „color“ can be: „Red“, „green“ or „blue“.

There is also „antired“, „antigreen“, and „antiblue“.

All particles must be „white“:white = red+antired, or blue+green+red

There are no free quarks!Quarks can only exist in bound states (together with other quarks).The strong force keeps them together. This is called confinement.

The gluon was discovered in 1979 atDESY (Hamburg): e+e- → U 3g→It's a „force carrier“, like the photon.

The gluon „glues“ the quarks together

The strong force acts on the „color“.Only quarks and gluons have color, leptons don't have a color.

The theory of the color-interactions is also called Quantum-Chromo-Dynamics (QCD).

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Antimatter

ASACUSA experiment at CERN:

Spectroscopy of anti-hydrogenMass measurement of anti-protons

Anderson discovered the positron (e+) in 1932 in a cloud chamber.

Antiparticle has the same mass but opposite electric charge ofthe particle, and other quantum numbers are opposite as well.

Examples: e+, p-, d, n (anti neutron is made from anti-quarks), ...

In the big bang almost all matter and anti-matter annihilatedinto energy. Gladly, there was more matter than antimatter(asymmetry). The universe is made from matter, not antimatter.

Mass of the (anti-) proton: 938,272 MeV

(mp+ – m

p-) / m

p+ < 10-10

Matter and anti-matter annihilate into energy (E=mc2)

e+ + e- → g + g g + g e→ + + e-e-

e+

g

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The Discovery of the Top Quark (1995)

Mass of the top-quark: 172 GeV

The top quark is the heaviest fundamentalparticle that we know!It is almost as heavy as a whole gold atom!

It was discovered in high energy proton-antiproton collisions with an energy of 1.96 TeV at the Tevatron collider at Fermilab (close to Chicago).

→ Now we know the full set of 6 quarks:

protons anti-protons

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The Weak Force, W and Z Bosons

The W and Z bosons were discoveredin 1983 at CERN in high energyp+- p- collisions.

The weak force is weaker than the strong force, but also only acts on very short distances.

The weak force is responsible for particle decays, and also for nuclear fusion (sun).

Example: b-decay In a b-decay a neutron converts into a proton.What really happens, is that a d-quark turns into an u-quark, and by doing so it emits a W-boson.

The weak force acts on every particle: Quarks, leptons, neutrinos.

Example: B-decay

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The Weak Force, W and Z Bosons

The discovery of the Z-boson:

Invariant mass (Special Relativity, Einstein 1905)

The Z-boson is instable, it decays immediatly.We cannot detect it directly, we cannot see it.But, we can detect its decay products.

From the kinetic energy (the momentum) of the two leptons, the invariant mass of the lepton pair can be calculated. This is the mass of the Z boson.

pp Z → → m+m-

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The Standard Model of Particle Physics

electromagnetic force (photon)

Strong force (gluon)

Weak force (W and Z)

Fundamental forces and carrier:

„But what about gravity?“

All matter and all relations between matter can be explained by this theory.All stable matter (humans, earth, trees ...) consists of only 3 particles: e, u and d.

Gravity cannot be explained by the Standard Model.For this we need the theory of general relativity (Einstein).

H0

125 GeV

0

Higgs

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1. History and Discovery of the Standard Model Particles

2. Higgs Boson Discovery

3. New ideas

Outline17

Why do we need the Higgs Boson?

In the previous part of the talk, we learned everything about the Standard Model.Almost everything.In the Standard Model, the W and Z bosons don't have a mass, just like the photon.

BUT: We have found the W and Z bosons, they are heavy! mW

=80 GeV, mZ=91 GeV

Another question:

The leptons and quarks are also massive:

Electron: 0.5 MeV … top quark: 172 GeV

We need a mechansim that explains and „creates“ mass Higgs mechanism.→

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The Higgs Mechanism

The Higgs field is a „scalar field“: It is everywhere in theuniverse exactly the same.

Particles aquire mass by interacting with the Higgs field.

Only photons don't interact with the Higgs field, they move with the speed of light.All other particles (W,Z bosons, neutrinos, top-quark etc) „feeld“ the Higgs field,they become massive.

Without Higgs field:

With the Higgs field:

v=c c = speed of lightMassless

Massivev < c

Photonv=c

„Higgs field“

When particles interactwith the Higgs field,Higgs bosons are created

→ we can observe them!

(Brout, Englert, Guralnik,Hagen, Higgs, Kibble)

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The Higgs Mechanism

Anology:

Pencil standing on the tip, it falls down.When it falls down, the symmtry is broken.

Symmetricunstable

Not symmetricstable

In the early universe there was no Higgs field. All particles were massless.

Then something happened, we call it „spontaneous electroweak symmetry breaking“.Since then, the vacuum has changed, we have the Higgs field.

All interactions can be connected to a potential V.The potential for the Higgs field looks like that:

„Mexican hat“ (1961)

The early universe was symmetric but instable.Now we have taken a stable but non-symmetric ground state, and the Higgs field is present.

(Brout, Englert, Guralnik,Hagen, Higgs, Kibble)

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The Large Hadron Collider, ATLAS and CMS

8.5 km diameter

Proton-proton collider 7-13.5 TeV energy. Largest machine ever built.

CMS DetectorCMS DetectorATLAS DetectorATLAS Detector

>Video<

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Discovery of the Higgs Boson

The invarant mass of the photon pairgives the Higgs boson mass

(Remember the Z-boson discovery?)

The Higgs boson mass is 125 GeV

We need to collect a lot of data to see this.

This decay is very rare: Only 0.2% of all Higgs bosons decay to photons.

g1

g2

Higgs decay to photons: Histogram: We count events with a certain photon-pair mass

Background eg. q+q → gg

Signal

>VIDEO<

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Discovery of the Higgs Boson

This picture visualizes a Higgs boson decay seen in theATLAS detector. It is a real „Higgs to 4 muons“ data event.

The invariant mass ofthe 4 leptons gives the Higgs boson mass.

Higgs boson decay to twoZ Bosons, each Z deacysinto 2 leptons:

Muon 1

Muon 2 Muon 3

Muon4

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The Nobel Prize of Physics 2013

The Higgs boson discovery is the most important discovery since 50 years.

For the prediction of this particle („theoretical discovery“), Peter Higgs and Francois Englert have been awarded the nobel prize of physics in 2013.

ATLAS and CMS collaborations were mentioned in theNobel prize announcement.

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1. History and Discovery of the Standard Model Particles

2. Higgs Boson Discovery

3. New ideas

Outline25

Dark Matter

Dark matter could be detected at the LHC.

It could be created in the proton-antiprotoncollisons, and then indirectly detected.

The rotationial velocity of stars around galactic centers can be measured.

The velocity can be predicted from the mass of the (luminous) stars.

The result is in huge disagreement withthe expectation.

→ There must be massive matter in the universe that we we cannot see, dark matter.

observablesingle jet

escapedetection

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Supersymmetry (SUSY)

Imagine, there could be a whole new world out there!

To each particle of the Standard Model, there could be a SUSY partner particle.This theory is motivated by the wish to unify all forces into only one. One force that existed at high temperatures.All attempts to unify gravity with the Standard Model require SUSY.

We have not found any SUSY particles, maybe they are extremely heavy? „Null result“Maybe we find them with the Large Hadron Collider RUN-2 ?

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Quark-Substructure, Lepton-Substructure

What if quarks and leptons are not really fundamental, theyconsist of even smaller particles?

A proton is fundamentally different from an electron, sowhy is the electric charge of proton and positron identical?

Why do quarks have „fractional“ electrical charge,either 1/3 or 2/3 ?

Idea is that „preons“ are the elementary bulding blocksof leptons and quarks.

This can be experimentally tested at the LHC:

If leptons and quarks havesubstructure, they can be„excited“, just like atoms.

This would then show up in themass spectrum as „bumps“.No „bump“ has been found.Anology

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Extra Dimensions

Space has 3 dimensions, time is the 4th dimension What if there were extra dimensions?→

Lisa Randall (*1962)

The discovery of a gravitonwould be evidence for such a theory.

Theory by Randall and Sundrum:

There is a 5th dimension, bounded by 2 „branes“We live on the „weak brane“. We cannot leave it.Gravity lives on the „gravity brane“, it exponentiallygets weaker when it reaches into our brane .

Raman Sundrum (*1964)

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5-dim space

SUMMARY

High energy physics is a science which aims to understand the fundamentalbuilding blocks of our world, and the interactions between them.

Our best understanding is a theory called „Standard Model“.This theory has been tested and confirmed in countless experiments.

Recently we found the Higgs boson, which was predicted by that theory.

There are many things we don't understand yet:eg. dark matter, parameters of the Standard Model, matter-antimatter asymmetry, gravity, ...

→ The journey continues!

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