Atmospheric neutrinos are the result of cosmic rays (typically protons) striking a nucleus in the upper atmosphere. The shower of particles starts with pions, which decay via the weak interaction to muons and muon neutrinos. The muons subsequently decay to electrons, an electron neutrino and another muon neutrino. The Super-Kamiokande experiment did not measure the expected two-to-one ratio of muon to electron neutrinos, and this provided an important element of their evidence for neutrino oscillation in 1998.

 

Primary cosmic rays arrive at the near vicinity of the earth isotropically, having been randomized by interstellar magnetic fields. The Super-K group relied on the up-down symmetry of high energy neutrinos (with energy greater than 1 GeV) from high energy cosmic rays as another element of their evidence for neutrino oscillation. However, low energy cosmic rays are deflected by the earths magentic field and are therefore more complicated to study.

In particular, low energy cosmic rays from the east are suppressed compared to those from the west, because the presence of the earth effectively shadows certain trajectories, which are therefore forbidden. In the 1930's this east-west effect was detected in charged secondary cosmic rays and was used to infer that the sign of the primary cosmic rays charge must be positive. Super-K has for the first time detected this asymmetry in the flux of atmospheric neutrinos.

Scientists at Lawrence Berkeley National Lab have discovered elements 118 and 116 by colliding beams of krypton atoms with a target made of lead atoms.

(a) The neutral K meson and its antimatter counterpart can both be thought of as a combination of a short-lived particle K1 (green squiggle) which mostly decays into two pions (each indicated by the letter p) and a long-lived particle K2 (red squiggle) which decays mostly into three pions. (b) In some rare cases, however, the K2 (CP= -1) turns into a K1 (CP= 1), which then decays into two pions. This is evidence for indirect CP violation. (c) To illustrate how K mixing comes about, consider the analogy with polarized light. Ordinary light from the sun contains light of all different polarizations (the direction of the light wave's electric field). But if the light is passed through a Polaroid filter oriented vertically, some of the light will be blocked and only that portion with a vertical polarization will emerge. In this beam there can be no light with a horizontal orientation. Next pass the light through a filter oriented at 45 degrees to the vertical. The light that emerges (at even lesser intensity) will now be oriented at the same 45 degrees; this light can be said to have a component which has vertical polarization and a component with horizontal polarization. The proof that some of the beam is now horizontally polarized (whereas a moment before the light was exclusively vertical) is that some light does emerge from a third polarizer oriented horizontally. Something like this is at work in converting K1's and K2's into each other just as vertically polarized light is turned into horizontal. Instead of polarizers, however, the K's are made to pass through thin slabs of matter, in which beams of short-lived K's are "regenerated" from beams of pure long-lived K's. (d) The recently observed case in which K2's are seen to be decaying directly into two pions. This is evidence of direct CP violation.

This illustration depicts how hypothetical particles known as "monopoles" would participate in interactions between protons (p) and antiprotons (p with a horizontal bar on top) in experiments at the Fermi National Accelerator Laboratory in Illinois. This illustration is an example of a Feynman diagram, a pictorial representation of interactions between subatomic particles. At the high energies used at Fermilab, it is often the case that a single quark (q) inside the proton scatters from a single antiquark (q-bar) inside the antiproton. Since these quarks are the chief players in the interaction, the diagram shows the quark q and antiquark q-bar indicated alongside the respective proton p and antiproton p-bar to which they belong. These interactions can, according to theoretical expectation, occasionally happen through the emission of "virtual photons" (photons which exist artificially for only a brief instant and which cannot be directly measured; indicated by the Greek letter gamma) which in turn connect with a virtual monopole loop (M). The net result of this rare interaction scheme would be the release of two "real" photons which could be detected in the laboratory. Physicists at Fermilab are currently searching for such signs of monopoles, whose existence is a subject of great debate. (Illustration by Malcolm Tarlton, AIP.)

An aerial view of the Relativistic Heavy Ion Collider (RHIC) being built at the Brookhaven National Laboratory in Upton, NY. Using the existing Alternating Gradient Synchrotron as an injector, heavy atoms will be accelerated to high energies and then smashed head-on in the hope of making quark-gluon plasma, a state of matter in which the quarks inside protons and neutrons will blend together in a high-energy soup.

The Rarest Observed Decay of the K⁺ Meson

An elementary particle such as a meson (a quark-antiquark pair) can decay in many different ways. For each possible decay path, physicists can measure the "branching ratio," a number that describes the relative likelihood for that particular decay path to occur against all other possible modes of decay.

Newly Identified Top-Quark Decay Modes

Produced in a high-energy collision between protons and antiprotons, a top quark-antitop pair (t, t bar) decays into a total of four particles--two W bosons, a bottom quark (b) and an antibottom quark (b bar). In the "all-hadronic" decay mode, pictured above, each W boson decays into a pair of quarks, for a total of four quarks (q1, q2, q3, q4). These quarks, plus the bottom and antibottom quarks, each produce an energetic spray of particles, or "jet," that is detected experimentally. Therefore, in this type of decay the nominally expected signal is six jets. (Illustration by Malcolm Tarlton and Elliot Plotkin, AIP.)

"Dilepton" mode in which each W particle decays into a lepton [such as an electron (e) or muon (ľ)] plus a neutrino (). The bottom quark (b) and antibottom quark (b bar) each produce an energetic spray of particles, or "jet" that is detected experimentally.(Illustration by Malcolm Tarlton and Elliot Plotkin, AIP.)

Emerging from the fireball of a proton-antiproton collision, a top quark and its antitop twin quickly decay into a total of four particles--a pair of W bosons, a bottom quark (b), and an antibottom quark (b bar). Discovered in 1995 at the Fermi National Accelerator Laboratory, the top quark continues to be studied by two Fermilab groups, known as D0 and CDF. These groups classify top quark events into three categories according to how the Ws decay:

(1) The most common decay mode, but somewhat difficult to distinguish from non-top particle decays, is the "all-hadronic" mode in which each W decays into a pair of quarks. A new mass measurement (F. Abe et al., 15 Sept. Physical Review Letters) by the CDF group is based on the first identification of all-hadronic top decays.

(2) A rare decay mode is the "dilepton mode" in which each W particle decays into a lepton (such as an electron or muon) plus a neutrino. D0 has reported calculations of the top mass based on the dilepton events.

(3) In the "lepton-plus-jets" mode, one W decays into a lepton (such as an electron or a muon) plus a neutrino while the other W decays into two quarks, which subsequently produce a "jet," or spray of particles. The first identified decay mode for the top quark when it was discovered back in 1995, the lepton-plus-jets mode is the basis of a new, record-high-accuracy top mass determination from the D0 experimental group at Fermi National Accelerator Laboratory (S. Abachi et al., Physical Review Letters, 18 August).

A stylized diagram representing the production of a top-antitop pair of quarks in the high-energy collision between a proton and an antiproton. The proton is a composite particle, consisting of three quarks (q). One of these annihiliates with one antiquark from the antiproton. The energy of this annihilation rematerializes as a top and an antitop quark, each of which rapidly decays into a W boson and the next lightest quark, a bottom (b) quark. These particles in turn quckly decay into lighter particles such as electrons (e), muons (ľ), neutrinos (), and other quarks. In their detectors, physicists search for top quarks by looking for characteristic patterns of electrons or muons and clumps of quarks.