Download Modern Physics

Modern Physics
LECTURE III
Standard Model
 6 quarks (and 6 anti-quarks)
 6 leptons (and 6 anti-leptons)
 4 forces
 Force carriers (g, W+, W-, Zo, 8 gluons, graviton)
What is the Standard
Model?
 A single and very elegant theoretical
framework.
 Can describe “everything except
gravity” in terms of about 20
parameters.
 Has been tested to astonishing
precision.

1
 137.0359895  0.0000061
Standard Model (SM)
Standard Model has been introduced
by Glashow, Salam and
Weinberg, based on Relativity &
Quantum Theory.
Nobel prize 1979
Symmetry
 Symmetry is abundant in Nature.
 Some symmetries relate to shapes in
space whilst others are more abstract.
e.g. Triangle
e.g. Average A Level score
is same for females as
for males. Not an exact symmetry.
Standard Model (SM)
 But, the wekness of the SM is the lack of
symmetry.
 The mediating particles i.e. photon, gluon,
W and Z differ in their masses! The
symmetry is broken!
 Where does mass come from?
 Higgs: From the non-trivial action of
the vacuum.
Peter Higgs
Gerardus ‘t Hooft
Higgs’ mechanism
 Higgs proposed that empty space
(vacuum) is not really empty.
 Some particles move around
unhindered (massless) whilst others are
dragged back by the vacuum (massive).
 In this way the symmetry is more
“hidden” rather than “broken”.
Broken versus Hidden
symmetry
 A block of ferromagnetic material is
unmagnetised at high temperature:
A lump of ferromagnetic material
is made of a myriad of tiny magnets
(one for each atom).
At high temperature the magnets
point randomly so the net
magnetisation is zero.
Broken versus Hidden
symmetry
 A block of ferromagnetic material is
magnetised at low temperature:
At low temperature the magnets
all line up so the net
magnetisation is not zero.
Broken versus Hidden
symmetry
 A block of ferromagnetic material is
magnetised at low temperature:
After heating the magnet and
then cooling it again the
magnetisation points in a
different direction.
 The basic laws which govern the ferromagnet
have a rotational symmetry.
Since there is no preferred direction in space.
 But at low temperatures the “ground state” of the
ferromagnet is NOT rotationally symmetric.
 The symmetry is said to be hidden.
 The Higgs mechanism is analogous: an
“invisible” field (analogous to the magnetic field of the ferromagnet)
permeates all space, selectively hindering
certain particles.
 As a result the symmetry is not
really broken at all….
 And particles can therefore be massive.
 There is a consequence: There ought to
be a new particle: the Higgs Boson.
The Higgs boson is the “footprint” of the pervasive field which permeates the vacuum.
Hunting the Higgs
 Modern day particle physics
experiments are busy searching for
the higgs particle.
 CERN (Geneva)
 Fermilab (Chicago)
Particle Accelerators
 The Standard Model of particle physics has been
tested by many experiments performed in particle
accelerators
 Accelerators come in two types – hadron and lepton
 Heavier particles can be made by colliding lighter
particles that have added kinetic energy (because
E=mc2)
 Detectors are used to record the shower of new
particles that results from the collision of the
particle/anti-particle beams
The Cyclotron
F  q( vxB)
Particle Accelerators-SLAC
 2 mile long accelerator which
can make up to 50 GeV
electrons and positrons
FermiLab
 5 accelerators which
collide protons and
anti-protons at 2 TeV
Colliding Detector
at Fermilab (CDF)
D0
Picturing Particles Activity
 Analyze the events that are seen in different





chambers of a detector
Determine the particles that could have made these
tracks
Remember that positively charged particles curve
opposite to negatively charged particles due to the
magnet in the detector
Muons are not stopped by any of the layers – they
travel through the entire detector
Electrons (positrons) and photons are stopped in the
electromagnetic calorimeter layer
Hadrons are stopped in the hadron calorimeter
CERN
 European Center for Particle
Physics
 Near Geneva, on France-Swiss
border
 CERN had both electronpositron collider (LEP) and
hadron collider (SPS)
 LHC will be the world’s highest
energy accelerator – now
under construction
In Search of the Higgs Boson
 CERN LEP Turned off on 11/2/00 to build LHC –
confirmed precise details of standard model
 LEP’s last run produced hints for Higgs Boson
at 115 GeV
 Higgs boson is “cosmic molasses” – the Holy
Grail of particle physics
 Interactions with the Higgs Field are theorized to
give all the particles their masses
 LHC detectors should be able to confirm or
disprove initial hints for Higgs at E=115 GeV
Plasma=
electrons
+ions
RHIC
The Relativistic Heavy Ion Collider
at Brookhaven National Laboratory
was built expressly to produce the
quark-gluon plasma. In the RHIC 2.4mile-long ring, fully-ionized gold ions
go in both directions at once and can
meet at six places around the ring for
collisions.
 When gold nuclei collide head-on, their kinetic energy
dissociates many nucleons and forms the hot, dense
plasma of quarks and gluons, which must
immediately begin to expand and cool. The hot
plasma lasts only 10-23 seconds, and only when the
plasma cools sufficiently do the quarks and gluons
“freeze out,” leaving a spray of thousands of
elementary particles carrying the signature of the hot,
dense plasma that led to their production.
The spray of thousands of particles produced from collisions of colliding
gold nuclei. (image courtesy of Brookhaven National Laboratory)
Web Resources
 The Particle Adventure http://particleadventure.org/
 SLAC http://www.slac.stanford.edu
 FermiLab http://www.fnal.gov/pub/tour.html

Virtual Space time travel machine
http://www.lactamme.polytechnique.fr/Mosaic/descripteurs/
demo_14.html
 CERN http://public.web.cern.ch/Public/
Particle Physics Education Sites
http://particleadventure.org/particleadventure/other/othersites.html