User:Emun44

Proton Substructure and e-p Scaling

Slac Accelerator, 1969: High energy inelastic scattering of electrons off of liquid hydrogen target resulted in many unexpected events in the deep inelastic region (a lot like what happened in the Rutherford gold foil experiment when they found a point like nucleus). Theorists proposed small point like substructures within the proton - quarks. Energy scaling was found - results are the same regardless of what energy the experiment is performed at, as predicted by the quark model. Discovery earned 1990 Nobel Prize for Physics.

J/Psi and the November Revolution

In 1974, two groups discovered a sharp spike (i.e. long lifetime) at ~3.1 GeV, which later turned out to be $c{\bar {c}}$ or "Charmonium". It was simultaneously one of the most massive and the longest lived particle found up to that time. The reason it has a long lifetime is that the main decay mechanism is OZI suppressed. Up to that time, Gell-Mann's theory of quarks was not well accepted, and field theory was only just beginning a slow revival (since t'Hooft solved many of the problems that almost killed it). Also, the GIM mechanism showed that a fourth quark would solve the problems that plagued weak force field-theoretic calculations (this was shown shortly before the J/Psi discovery, but mostly ignored). The main point is that the J/Psi discovery completely changed the field, by bringing quarks, and the whole Gauge theory formalism, to the forefront. - Adriferr

Positron

The prediction, and subsequent discovery, of the existence of the positron constitutes one of the great successes of the theory of relativistic quantum mechanics and of twentieth century physics. When Dirac (1930) developed his theory of the electron, he realized that the negative energy solutions of the relativistically invariant wave equation had a real physical meaning. He therefore postulated the Hole Theory which predicts the existence of a new particle - the positron. The positron was subsequently discovered by Anderson (1932) in a cloud chamber study of cosmic radiation, and this was soon confirmed by Blackett and Occhialini (1933), who also observed the phenomenon of pair production.

Neutrino Oscillations

In the 1960's, the standard solar model (SSM) predicted that the Sun was an important source of electron neutrinos. These electron neutrinos were first detected in the late 60's by Ray Davis using a large underground tank of Chlorine as a detector. However, he could only detect a fraction (34%) of the expected electron neutrinos flux. Other experiments done later had the same problem. This discrepancy was called the "Solar Neutrino Problem". The solution to this problem was postulated in 1969 by Gribov and Pontecorvo. They believed that neutrinos might be oscillating between the different flavours and thus the electron neutrinos leaving the Sun could reach the Earth as muon neutrinos or tauon neutrinos. The main evidence for neutrino oscillations came in 2002 from the SNO collaboration. Their detector could determine the electron and non-electron neutrino components of the solar flux. They detected 100% of the predicted flux, with a 34% electron neutrino component and a 66% muon/tauon neutrino component.

The Search for the Higgs Boson

Standard gauge theory (the theory used to describe electromagnetic and strong interactions) does not allow for massive interaction bosons. The simplest way to incorporate the massive bosons is to postulate a new spin 0 boson that breaks the gauge symmetry and in so doing provides the bosons with mass. This particle is called the Higgs boson. Theory predicts that the Higgs boson should couple strongest to the most massive particles. Thus, we look for the Higgs boson by searching for decays into massive particles such as Z bosons and bottom quarks. Searches at the LEP did not find the Higgs boson and the Tevatron experiments are still looking, however there have been bounds put on potential Higgs boson masses, as well as a prediction using standard model parameters of the mass.

The Pentaquark State

Everyone already knows about colour confinement, so I won't waste your time stating it again. Basically, nothing forbids a state that would be q-q-q-q-qbar. These states were initially proposed in 1977, but it's not until 1997 that some of their resonances were predicted to have narrow width, and thus be observable in experiments. The first theory was the chiral soliton model, which gave the expected angular properties as well as the mass and width of the resonances. Nowadays, QCD sum rules and Lattice QCD models are also used. The first observed pentaquark was the θ+ state (other have also been observed, but to a musch lesser extent and the conclusions are the same). Initially, a lot of experiments claimed the observation of this state, however, as time passed, some new facilities built for the purpose of detecting this kind of signal, found nothing but statistical error. Most of the original experiments now claim that their original discovery was inteed statistical errors. There are still inconsistencies, but it seems like any evidence for pentaquarks as they are currently defined in theory is rapidly erroding.

CP-Violation in B0 decays

CP-Violations in B0 mesons occur due to 2 superimposing effects. Those are B0/anti-B0 mixing and beauty oscillations. B/anti-B mixing occurs since the strong force eigenstate (i.e. the B0 or the anti-B0) are not eigenstates of the weak force. Therefore, the weak force "sees" a linear superposition of B0 and anti-B0. Furthermore, this linear superposition is not static (i.e. it is time dependent) as the B0 meson travels through the detector. So, B0 mesons are allowed, through weak interactions, to change from a B0 to a anti-B0 as they travel. This is called beauty oscillations. So the physical state of a B0 meson is not pure since a B0 meson mixes with the anti-B0 meson. To see how they occur, please consult figure 10.5 in Martin and Shaw p.249. Replace the s quarks in the figure with b quarks. In the actual experiment, one first identifies B0 and anti-B0 at the production point and then observes how they decay. If there is a non-vanishing difference in rate between B0 and anti-B0 going to a final states that are chosen to correspond to a CP-eigenstate, this implies that there is CP-Violation. A difference in the rates was indeed observed by the BaBar and the Belle collaboration which supports the fact that there is CP-Violation in B0 mesons. The BaBar results are described in the paper. I can't go into the actual results because that requires writing and explaining formula. For the interested, see the term paper.

Motivations. There are 2 reasons why one would study CP-Violations in B0 mesons. First, CP-Violations studies allow a better better understanding of the standard model, by imposing better constraints on the CKM matrix elements describing quark mixing. Second, a better understanding of CP-Violations allows us to understand the mechanism causing dominance of matter in our universe. Indeed, in order to have a matter-dominated universe, one needs to have a CP-Violating process.

Top Quark

The top quark was discovered in 1995 using the Tevatron proton-antiproton collider at Fermilab. Two groups were involed in this discovery, the collider detect at fermilab (CDF) and Dzero groups. They worked independently on the project, and it was not until they combined their findings that the discovery occured. Both groups used the standard model prediction of the top quark decay, where the top decays to a W boson + a bottom, from here the W boson can decay to a lepton-neutrino pair or a quark-antiquark pair. There are three decay modes for a top-antitop pair, the dilepton mode (where both bosons decay to a lepton-neutrino pair), the lepton plus jets mode (where one boson decays to a lepton-neutrino pair and the other decays to a quark-antiquark pair) and the all hadronic mode (where both bosons decay to quark-antiquark pairs). Only the first two were used in this discovery. The results gave a top mass of 174Gev/c^2 for the original paper by CDF and a top mass of 199Gev/c^2 for the first definite discovery paper by D0. Top quark experiments are still occuring today in the attempt to improve results with higher energy colliders.

Bottomonium

Bottomonium, a bound state of the b quark and its antiparticle, was discovered in 1977 by a group at Fermilab led by Leon Lederman. The search was motivated by the quark-lepton universality hypothesis and the recent discovery of the charm quark and the tauon, which suggested the existence of a bottom quark to correspond to the new tau generation.

Lederman and his group were tracking the direct production of muon pairs in proton-nucleon collisions, using the found momenta to recontruct the parent particle invariant mass. The dimuon momenta were found by bending the particles trajectory using magnets and tracking it's position using multi-wire proportional counters. It was chosen to search for muons because of their relatively high branching fraction. Open beauty states are kinematically disallowed in bottomonium decay (Zweig Threshhold), and hidden beauty states are suppressed by the OZI rule. Thus there were many more relative decays to muons and a sharp energy peak due to the increased lifetime.

Lederman found a peak in the energy spectrum at 9.5 GeV corresponding closely to previous predictions for a bb-bar bound state using the quarkonia potential discussed in class (the model works especially well for bottomonium as it is heavier than charmonium and thus a better fit to a non-relativistic calculation).

If you want a critique: one misguided group of theorists suggested that that the potential model predicted splittings that were too small. Their own interpretation, however, was clearly misguided in other ways. They suggested an octet representation of quarks with 4-quark bosons such as Qqqq and believed that there should exist a spectrum of new particles by simply adding a Q or ¯Q to any previsouly known state (where Q was the unkown quark we now know to be b).