ATLAS Questions and Answers John Rominger-Watts PHYS 129, Fall 2010
Questions: - What is mass? - Why is gravity weak compared to the weak force? - Does dark matter exist? - Why is the universe mostly matter, not antimatter? - Is supersymmetry real? - What does the top quark look like? - How did the universe behave at the Big Bang?
Answers: We have some... - (Supersymmetric) String theory - Higgs mechanism - CP violation - Other modifications to Standard Model but how do we know if they're right?
Large Hadron Collider - Replaced the outdated Large Electron-Positron collider (LEP) - Can accelerate protons and lead ions - Currently the largest and most powerful particle accelerator
ATLAS - One of six main detectors - ~2000 scientists/engineers - Involves 35 countries - 44 m long, 25 m diameter - Weighs 7000 metric tons - Detected first events in 2008
ATLAS Detector Systems Inner Detector:
ATLAS Detector Systems Calorimeter:
ATLAS Detector Systems Muon Spectrometer:
Question: What is mass? - Inertial mass: resistance to acceleration Image: nasa.gov Image: sparknotes.com - Gravitational mass: attraction to other massive objects Image: mit.edu Image: toothpastefordinner.com
Answer: The Higgs Mechanism (maybe) - Predicted by the Standard Model - Explains why photons are massless, but W and Z bosons aren't - Uses spontaneous symmetry breaking to give W and Z bosons mass - Can detectably manifest as a Higgs boson
The Higgs Mechanism As explained by Prof. David Miller, University College London: - Evenly-distributed politicians - Politicians gather around an important figure or or - Higgs field permeates all space - The field is distorted by a moving particle Images: David J. Miller
The Higgs Boson As explained by Prof. David Miller, University College London: - Evenly-distributed politicians - People crowd around the source of a rumor or or - Higgs field permeates all space - A wave of Higgs field clustering propagates Images: David J. Miller
Cool! So, where is the Higgs? We've been searching through every energy range accessible... ATLAS can look in the unexplored regions! Directly: when combined with other particles, can form an almost unique decay signature Indirectly: can measure corrections to W and Z bosons caused by Higgs loops Images: wikipedia.com
And if we're wrong? Image: xkcd.com
And if we're wrong? ATLAS should confirm or deny some other theories... - Technicolor: - New gauge interaction coupled to technifermions - Extra dimensions - Kaluza-Klein Theory (1923) - Top quark condensate: - Replaces Higgs field with bound top and antitop quarks
So, what about the top quark? Image: FNAL - Recently discovered experimentally (Fermilab, 1995) - The heaviest elementary particle observed so far - Bare mass around 171 GeV/c², comparable to a tungsten nucleus - Difficult to produce and measure because of its mass
Question: Why is the universe matter? (Rather, why is it not antimatter, too?) We have two options: 1.) The universe was imbalanced from the beginning 2.) The universe can produce matter and antimatter at different rates - To do this, Sakharov (1967) says we must: - Not conserve baryon number (not observed) - Break CP symmetry (observed) - Not have thermal equilibrium (OK, but slow)
Answer: CP violation (maybe) Here's the problem: the Standard Model allows two types of CP violation - Strong interactions: - Never been seen experimentally - Would make neutron EDM ~10^-18 e*m! (exp. < 10^-28 e*m) - Would be much too large - Weak interactions: - Seen experimentally - Not enough violation!
Seeing CP violation in the LHC LHCb (CERN) Tevatron (FNAL) - Directly: detection of new particles - Indirectly: B-meson oscillation (LHCb) - Bº mesons transform into their antiparticles and back - Decay into matter 1% more than antimatter (FNAL, 2010)
Review and Conclusion: ATLAS is poised to unveil many mysteries of the universe: - Higgs boson and the origin of mass - CP violation and the origin of the universe - The top quark as a stepping stone