The electron(positron)-proton collider HERA was shut down at the
end of June 2007. HERA was a unique instrument which made a major
contribution to high energy particle physics and, in particular, to
confirming aspects of quantum chromodynamics (QCD). The knowledge
obtained with HERA will be essential for discovering the meaning
of data obtained from the large hadron Collider (LHC) at CERN in
Geneva.
HERA was the first and only ring to provide longitudinal
electron(positron) spin polarisation at high energy (27.5 GeV). This
necessitated the installation of pairs of spin rotators near the
interaction points and a special choice of the optical configuration
of the ring. Not only was high polarisation attained, but longitudinal
polarisation was provided simultaneously at 3 interaction points.
Longitudinal polarisation with one pair of spin rotators was provided for the HERMES experiment in 1994.
Longitudinal polarisation at 3 interaction points, with 3 three pairs of spin rotators,
was provided in the second phase of HERA, HERA-II, in 2003:
The longitudinal polarisation provided something for the text books by way
of this plot showing the agreement between measurements and the predictions
for the spin dependence of the electro-weak interaction in the Standard Model.
In the last days of HERA, a recalibration of the electron(positron)
polarimeters was carried out. This curve, obtained with the H1 and
ZEUS solenoids running and with 3 pairs of rotators, shows the
usual and expected time dependence for the rise of the polarisation. But
this measurement displays the very small statistical errors available
with the recently commissioned Compton polarimeter which utilised the
very high flux of photons accumulated in a Fabry-Perot cavity.
Sadly, this polarimeter will now never be in routine use.
In principle HERA could have been upgraded to store
high energy deuterons for studies of the structure of the
neutron. This was called the HERA-III project and there was plenty of support in the
community of people wishing to make further tests of QCD.
However, PETRA, one of the pre-accelerators needed by HERA,
was given to the community of users of synchrotron radiation, thereby
ending over 40 years of experimental high energy particle physics on
the DESY site.
