Public Talk: "The Hunt for the Higgs Boson"
Speaker: Sridhara Dasu, UW-Madison department of Physics
Date: 7/16/2012
Location: UW-Madison Department of Physics
The LHC probe is studying conditions of the "Electro-weak Era" a nanosecond after the Big Bang.
Collisions:
Classical picture: Momentum and Kinetic Energy are conserved.
Increase Speeds: Kinetic energy not necessarily conserved, but total energy is.
Increase speeds near the speed of light (c): E=M (take c=1); energy can be converted into mass. In motion, |P|=E^2-(p_x)^2-(p_y)^2-(p_z)^2=m^2 (invariant mass). Units are eV (electron-Volts). Currently, LHC can accelerate particles to 4 trillion eV.
Relativistic collisions: New particles produced, often much more massive than initial colliding particles.
Fields create quanta: Matter/anti-matter manifest out of vacuum. Particles exchange quannta: photons, gluons, W, Z.
Quantum electrodynamics works well for massless quanta, but W and Z are not massless. A Higgs field is needed to provide mass and complete the Standard Model of particle physics.
Particles:
Quarks (up, down, top, bottom, strange, charm)
Leptons (electrons, muons, tau, corresponding neutrinos)
Force Creators (photon, W/Z bosons, gluons)
Omnipresent Higgs Field
Higgs Field as medium: increase interaction with higgs field, increase mass.
Requires that it is self-interactive: Higgs Boson, has mass, but we cannot determine what mass would be.
7/4/12: CMS/ATLAS experiments obverseve new particle with mass approximately 125-126 GeV.
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-LHC: 2 rings driving protons in opposing directions.
-CMS: Compact Muon Solenoid. Giga-pixel Camera.
Observes photons, electrons, muons, charged hadrons (composed of quarks: pions, K, protons), neutural hadrons (neutrons, K_L). Leave electronic signals in silicon diodes in detector used to track particle.
Muons go straight through; charged hadron deposits all energy in calorimeter; photons don't ionize gaseous medium, ineract in inner part of calorimeter.
-Measuring Heavier Particles
decay near collision
collision point found with tracks
low-momentum: "underlying event"
electron/positron momentum measured in tracker; energy measure in calorimeter.
m^2=(electron+positron)^2(E+)(E-)(1-cos(t)), 91 GeV
Gluon fusion, couple to top quarks, produce H
Associated production
Vector Boson Fusion: higgs can decay into many things depending on mass (H-->gamma gamma, H-->ZZ)
-4 Lepton Event H-->ZZ
Golden Mode: muon and elecgron well-measured. 200 events.
clearly observe2 z-bosons.
No heavy standard model Higgs, no clusters
Zoom in: rare decay of Z into 4 leptons.
Small cluster rises: 2 sigma evidence of Higgs.
Could decay to different angles.
Reduce background and keep signal like events using theta: 3.2 sigma
ATLAS evidence gives 3.4 sigma.
Di-Photon Event (ATLAS)
Large photon background, but seeking small peak
Lack of gamma tracks make theta measurement hard
4.5 sigma evidence from ATLAS
CMS:
Divide data into four categories by mass resolution
4.1 sigma; together, evidence from CMS and ATLAS give 5 sigma.
Constraint from other standard model measurements should be around where Higgs was found: 125 GeV.
Tau decays have bad resolution because neutrinos are hard to detect.
Need more data; no evidence from W-boson, b-quark, tau-lepton pairs, less sensitive modes.
Should provide confirmation of Higgs by the end of 2012.