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Transcript
What is the Higgs???
Prof Nick Evans
University of Southampton
Everything is made of…
Electrons are responsible for
most of the properties of
matter
Who ordered that???
The trouble started in 1936
when Andersson and
Nedermeyer saw tracks in
a bubble chamber due to
cosmic rays of an electron
but with 200 times more
mass…
To this day the muon
interacts exactly like an
electron only differing in
its mass….
Why’s that trouble?
Relativity
Einstein taught us that mass is
a reflection of the energy store
din a particle…
E=mc
2
Energy is all about interactions
though… for example a candle
emits heat/light as excited
electrons move to orbits closer
to the nucleus of their atom…
We need a new force!
So if the muon and electron masses are different
there must be a new interaction that distinguishes
the two…
That force needs to act everywhere in space evenly so
that the particles masses are the same throughout
space….
The Higgs Boson
In the “Standard Model” the origin of mass is
addressed using a mechanism named after the
British physicist Peter Higgs.
This predicts a spinless particle: Higgs boson
According to Higgs,
space is filled with a
new type of field
analagous to
magnetic or electric
fields…
The electron and muon couple with different strengths and so get
different masses
The Search for the Higgs
To find the higgs we must “excite” the vacuum – produce a
higgs particle… we collide electrons, protons etc so there is 100
times nuclear energy density in some region….
The Large Hadron
Collider in
Switzerland
switched on in late
2009…
CERN - Geneva
Probing the structure of matter
LHC
began
science
in 2009
The ring is 27km round and on average 100m
underground
The Large Hadron
Collider collides
the nuclei of atoms
with 10 times
higher energy than
has previously
been achieved (14
TeV)
1232, 35 ton,
superconducting dipole
magnets accelerate ions and
focus them into bunches for
collision
36,000 tons of coolant below
2K!
Proton-Proton collisions (hydrogen atom nuclei)
14 TeV centre of mass energy
100 billion protons per bunch
20 collisions per crossing
1 crossing every 25ns
600 million collisions per second
To store all collision data would involve storing 10 Petabytes of
data a year ie a 20km high stack of CDs… more than can be made
Detectors at collision
sites:
Atlas
CMS
LHC-b
Alice
Track particles
Measure energy
Measure momentum
Higgs Production
H → tt
H→ bb
H→ WW → llnn
H → ZZ(*) → 4l
……
Exclusion mass range: 110122.5, 127 – 600 GeV
5.1 s excess at 125 GeV
higgs -> gg
higgs -> ZZ
5 s excess at 126 GeV
Other decay channels are low stats… the tau
channel signal is missing…
The Higgs Discovered
As of 2013 we have a very clear experimental signal for the
standard model higgs boson… nothing else has yet shown
up at LHC…
It is a remarkable vindication of the theoretical ideas that
make up the Standard Model…
Questions remain though – is it really the standard model
higgs or is there a more complex theory of which we’ve
found just the first bit?
The theory does not explain why the electron and muon
couple differently to the higgs… there must be a reason…