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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