group at KEK/IPNS.
Our group, together with Tokyo Metropolitan Univ. and Meiji Gakuin Univ.,
joins the international collaboration ZEUS experiment at the e-p collider
HERA in DESY, Hamburg.
This page is still preliminary, but you can access following information.
Collection of some event pictures from ZEUS experiment (postscript).
Please read the next section to learn about the details of the events.
Articles on HERA physics (in Japanese) appeared on High Energy News.
Brief description of the experiment.
HERA (Hadron Elektron Ring Anlage) is the only electron(positron)-proton
collider accelerator in the world. It collides 27GeV electrons(positrons)
and 820GeV protons, producing the center-of-mass energy of 300GeV.
ZEUS is an omnipurpose particle detector sitting at one of the
HERA interaction points, characteristic
with superconducting solenoid and uranium calorimeter.
The collaboration consists of about 450 physicists from 12 countries.
The experiment can be analogized to a gigantic electron microscope, proving
the inside of the proton with electrons. The wavelength by which one
proves the proton is inversely proportional to the square root of the value
Q-square, the momentum transfer squared. For example, Q-square of 10000GeV^2
corresponds to a wavelength of 0.01fm, where 1fm=10^-15m.
(The size of the proton is about 1fm.)
With a high Q-square value which
HERA can achieve, the electron strikes the "partons" in the proton,
the quarks and gluons. Here one observes interactions(events) in which
the electron is scattered in a large angle and one of the quarks in the proton
is scattered to balance the momentum, detected as a "jet" of particles in the
detector. The rest of the proton is also broken up, as two quarks cannot
remain as a particle, and escapes as a jet in the forward direction.
Such an interaction is called "Deep Inelastic Scattering (DIS)" and gives us
information on the structure of the proton and dynamics between the partons
known as Quantum Chromodynamics(QCD). Typically electron and quark interacts
via an exchange of the photon (neutral current), but at a high Q-square also an
exchange of W boson can happen, in which the electron becomes a neutrino and
escapes the detector undetected.