Experimental Tests of the Standard Model Precision Tests of the Standard Model - History of EW theory - Discovery of the Z and W Boson by the UA1/UA2 experiments (1983) - Precision tests of the Z sector @ LEP (1990-2000) prediction of the top quark mass via radiative corrections - Precision tests of the W sector @ LEP and Tevatron prediction of the Higgs mass Public web-link: - Higgs Searches at the LHC (2012) http://cern.ch/webcast
Historic background - 1934 Fermi-Theory: point-like interaction (no q2 dependence) approach still valid for q2 < m(w) However divergence of electron-neutrino cross section at high CME ( ) - Fix: exchange of massive boson : V-A theory Still remaining problem, divergence of W pair production cross-section Standard modell of particle physics electroweak unification!
Glashow, Salam and Weinberg (1967): Electroweak Unification - in analogy to strong IA, introduce weak isospin left handed particles form isospin doublets, right handed particle form isospin singlets generators of ladder operators eigenvalue: third component of weak isospin Weak IA described by 4 mass less gauge fields and couple to left handed particles only couples to left and right handed particles
Glashow, Salam and Weinberg (1967): Electroweak Unification contains subgroup elm. IA does not couple to left handed neutrinos and Solution: is linear combination of Weinberg angle and defined by couplings of A and Z. from experiment: Problem, no mass term in Lagrangian (otherwise spoils gauge invariance, thus is not renormalizible) require dedicated mechanism to create masses HIGGS mechanism (introduced 1964)
Discovery of neutral current NC was first theoretical introduced in GSW theory in 1969 and then discovered by the Gargamell experiment in 1973 Despite this experimental proof GSW model not widely accepted before 1977 t' Hooft and Veltmann demonstrated renormalization of this theory (all divergences cancel)
1979 Nobel prize for Glashow, Salam and Weinberg 1999 Nobel prize for 't Hooft and Veltman
Measurement of mass of W and Z boson Although not yet directly measured, masses of W and Z bosons were known to be in the range 60-100 GeV, thus rather heavy (ratio of NC to CC interactions)! Easiest scenario: LEP (large electron positron) collider was under construction, but started only in 1989, no high energy electron collider available! Alternative: use protons (or antiprotons), caveat, valence quarks carry only ~12% of proton momentum
Measurement of mass of W and Z boson Highest energy accelerator at that time: SPS (Super Proton Syncroton) @ 318 GeV, working in fixed target mode Collision on fixed target : (m: mass of target particle, p or n, ignoring that quarks carry only fractional momentum) required proton (antiproton) collider SPPS with CME 540 GeV Why antiprotons? + can use same magnetic field/storage ring (+ initial state is symmetric) - how to produce and focus high enough number of antiprotons?
Production of Protons & Anti-Protons - hydrogen gas is an infinite source of protons, can be accelerated and filtered to be stored in dense bunches of same energy and direction/phase - antiprotons are produced by colliding protons on a copper target ; antiprotons cover huge range of momentum and angular distributions. Technical challenge: how to get coharent antiproton bunches stochastical cooling (Simon van der Meer) Van der Meer showed, correcting the average of a particle bunch, will on long term uniformise the particles in the bunch (spread in momentum and direction reduced) betatron oscillation due to focusing quadrople magnets (circumference ) short distance between pick-up and kicker (same statistical compositon) Long distance between kicker and pick-up (randomizing of statistical composition due to different momenta)
SPPS Accelerator Complex
Two experiments are located at : UA1 and UA2 Start of data taking: 1981 Schematic view and foto of UA1 - Detector: UA1 one was the better detector and provided results a bit faster than UA2.
(Discovery 1983)
(Discovery 1983)
Probing Z and W in e+e- collisions
Precision Test of the Z sector at LEP s-channel only!
at Z pole ( ):
at Z pole:
We like to measure: but there are many sizeable (but computable) higher order corrections:
Measurement of Z line shape Energy scan is more precise than reconstructed invariant mass, advantage of e+e-!
Resonances look the same, independent of the final state: propagator is the same!
3 light neutrino species! had
Forward-backward asymmetry and fermion couplings to Z B