Discovery of the Higgs Boson

Gavin Lawes Department of Physics and Astronomy

Includes slides and images from:

Robert Harr, Paul Karchin, and Sean Gavin (Wayne State University)

LHC (CMS Collaboration)

Note: There is a lot more information about the LHC and the Higgs Boson at home.web.cern.ch

Acknowledgements

What do we know about matter?

Mass.

Spin. (Zero spin particles are bosons, spin ½ particles are fermions)

Electric charge.

Color charge.

(Other)

What properties define matter particles?

Particles in the Standard model

Photon γ (massless, no charge)

Electron e (massive, charged)

Muon µ (massive, charged)

Proton p (massive, charged)

Gluon g (massless, “color charge”)

Higgs H (massive, ?????)

Particle crib sheet

Particles exert forces on one another. These forces are transmitted by gauge bosons.

Electromagnetic force (carried by photons [1])

Strong nuclear force (carried by gluons [8])

Weak nuclear force (carried by W and Z particles [3])

What are gauge bosons?

e- e- γ

Gauge bosons reflect underlying symmetries of the universe.

The number of gauge bosons for each force is the number of generators for each symmetry group (e.g. the symmetry group for the strong nuclear force has 8 generators, hence 8 gluons).

These gauge bosons should be massless.

However, the W+, W-, and Z bosons (which mediate the weak nuclear force) are massive.

Where do gauge bosons come from?

F = ma

F = GMm

r2

Classical mass

Rab �12Rgab =

8⇡G

c4Tab

E = mc2

Mass in Relativity (special and general)

Binding Energy

Proton mass, mP=1.6726x10-27 kg Neutron mass, mN=1.6749x10-27 kg

4He mass=6.6446x10-27 kg

Less than 2mP+2mN

Dynamics 98%

Quarks

Proton mass

The mass of a proton is much larger than the mass of the quarks that make up the proton.

The Heisenberg Uncertainty Principle states:

ΔEΔt≥h/2π with the Planck constant h=6.636x10-34 m2 kg/s

This means that particles are popping in and out of existence continuously (so the vacuum is actually fluctuating).

The probability distribution of particles everywhere in space is called a field.

Quantum fluctuations

In 1964, 6 authors in 3 different papers (Brout and Englert, Higgs, and Guralnik, Hagen, and Kibble) proposed a mechanism for making the weak force carriers massive.

Depended on having these gauge bosons couple to something called the Higgs field, which has a fourfold symmetry.

When the vacuum fluctuations of the Higgs field become non-zero, the symmetry is broken, which makes the W+, W-, and Z bosons massive and leaves a single massive particle called the Higgs boson.

Other particles (electrons, quarks, etc) can also acquire mass by coupling to the Higgs field.

Higgs Mechanism

Build a giant particle collider. Fortunately, there was already a giant tunnel at CERN.

27 km ring filled with superconducting magnets cooled to just above absolute zero.

Large Hadron Collider (LHC) costs about $10B over 20 years. Note: Hadrons are particles containing quarks.

ATLAS and CMS projects involve over 3,000 physicists

How can we find the Higgs boson?

H γγ

Same event, different angle

H ZZ µµµµ

H ZZ eeee

H γγ

ATLAS

CMS

Even

ts /

2 G

eV

2000

4000

6000

8000

10000

ATLAS PreliminaryaaAH

-1Ldt = 4.8 fb0 = 7 TeV, s-1Ldt = 20.7 fb0 = 8 TeV, s

Selected diphoton sampleData 2011+2012

=126.8 GeV)H

Sig+Bkg Fit (mBkg (4th order polynomial)

[GeV]aam100 110 120 130 140 150 160Ev

ents

- Fi

tted

bkg

-200-100

0100200300400500

Two different experiments (ATLAS and CMS) find a new particle with a mass of 125.6 GeV/c2.

This is a spin 0 boson, with properties consistent with the Standard Model Higgs boson.

The existence of this particle confirms the point of view that mass is an acquired property (through coupling to the Higgs field) and not an intrinsic property of particles.

Higgs boson summary

Want to investigate the properties of the Higgs boson in more detail (e.g. decay paths, coupling to other particles, etc).

Are there other Higgs-like particles? The Standard Model assumes a Higgs field with fourfold symmetry, but there are other models that include more Higgs terms.

Also, other interesting physics problems to study at the LHC.

What next?

Higgs search concentrated on p-p collisions (total of 6 quarks).

When colliding nuclei (each with hundreds of quarks) together, one can produce a “quark-gluon” plasma, consisting of quarks and gluons mixed together but not bound into hadrons.

Understanding this special state of matter can provide insight into the evolution of the universe, which is believed to have passed through a similar high density/high temperature state nanoseconds after the Big Bang.

Quark-gluon plasma

The phase diagram of QCD Te

mpe

ratu

re!

baryon density! Neutron stars!

Early universe!

nuclei!nucleon gas!

hadron gas!color !

superconductor!

quark-gluon plasma!Tc!

ρ0!

critical point ?!

vacuum!

CFL!

Note: 1 TeV=1,000 GeV

Nuclear Collisions

Note: Spherical nuclei look like pancakes because of relativistic length contraction.

Hydrodynamics of quark-gluon plasma

Quark-gluon plasma acts like a perfect liquid

Viscosity of quark-gluon plasma

The LHC provides a tool for measuring the properties of fundamental particles at high energies and high densities.

Because of (possible) phase transitions (e.g. weak symmetry breaking, quark-gluon plasma, etc) the properties of particles under these extreme conditions (high density and high energy) may be very different than their properties under ambient conditions (low density and low energy).

This can potentially change our understanding of the fundamental symmetries that govern physical law.

Other new physics

The end