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**** NEWS IN PARTICLE PHYSICS *****
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Tuesday 18 February 1997
Embargoed until: 10.00 GMT Wednesday 19 February 1997
HERA hints at new physics
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Physicists working on the HERA accelerator in Hamburg,
including many from the UK, are puzzling over a small
but unexpected effect in the collisions of positrons
(anti-electrons) and protons. While it is still too early
for the researchers to be sure, the exciting possibility
remains that they could be observing a new interaction
between the fundamental building blocks of matter.
Both the major experiments at HERA - H1 and ZEUS -
observe more "events" at high energies than the
physicists can explain through known particle
interactions. Speculation abounds on what could be
causing such an effect. One exciting possibility
is that the extra events are due to a new particle,
created in the positron-proton collisions by a previously
unseen interaction between the positron and a quark,
one of the constituents of the proton.
To be certain of what they are seeing the physicists
at HERA need more data, so they are eagerly awaiting
the next start up of the accelerator, which will be
in March. During this year's run both experiments
should at least double the amount of data they have
in the interesting high energy region.
UK Involvement
--------------
Groups from Bristol, Glasgow, Imperial College, Oxford, and
University College London are members of the ZEUS
collaboration, while teams from Birmingham, Lancaster,
Liverpool, Manchester, and Queen Mary and Westfield College
London are members of H1. Physicists from the
Rutherford Appleton Laboratory in Oxfordshire
participate in both experiments. See below for international
involvement.
For further information please contact:
H1 experiment:
Professor Erwin Gabathuler, University of Liverpool
Tel: 0151-794-3349
Dr Graham Thompson, Queen Mary & Westfield College, London
Tel: 0171-975-5045 or 0181-504-7675
Email: G.Thompson@qmw.ac.uk
ZEUS experiment:
Prof. Roger Cashmore, Oxford University, Oxford
Tel: 01865 273333
Email: R.Cashmore1@physics.oxford.ac.uk
Dr Jon Butterworth, University College, London
Tel: 0171 380 7318 or 0171 424 0712
Fax: 0171 380 7145
Email: J.Butterworth@ucl.ac.uk
Dr Kenneth Long, Imperial College, London
Tel: 0171-594-7812
Fax: 0171-823-8830
EMail: K.Long@IC.AC.UK
General information:
Dr Christine Sutton, University of Oxford
Tel: 01865-273322 or 01865-273353 (secretary) or 01235-850091
Fax: 01865-273418
Email: C.Sutton1@physics.oxford.ac.uk
For web pages see:
DESY & HERA: http://www.desy.de/pr-info/desy-forschung_e.html
H1: http://dice2.desy.de:80/
ZEUS: http://www-zeus.desy.de/
Background Information
======================
(with thanks to Dr Jon Butterworth)
HERA
----
HERA is the Hadron-Electron Ring Accelerator, a large underground particle
accelerator at the DESY laboratory in Hamburg. The accelerator tunnel
is a ring 6.3 km long passing under a park and trotting course, as well as
the home ground of the Hamburg SV football team, in the northwest of the
city.
HERA accelerates bunches of electrons or positrons (antielectrons)
as they travel in one direction round the ring, and bunches of protons
as they travel in the opposite direction, before bringing the bunches to
collide head on a hundred million times a second. By meeting head on,
the protons collide with the electrons or positrons at energies ten
times greater than ever achieved elsewhere. This allows physicists at HERA
to make a more complete study of the internal structure of the proton than
has previously been possible.
H1 and ZEUS
-----------
H1 and ZEUS are two large detectors constructed in order to study
collisions at HERA. They have been collecting data since 1992. Both
detectors were built and are operated by multinational collaborations
of around 400 particle physicists.
Collaborating countries include Canada, France, Germany, Israel,
Italy, Japan, Korea, the Netherlands, Poland, Russia, Spain, UK and
the USA. Major parts of both experiments were constructed in the UK,
and particle physicists from the UK comprise around 20% of both
collaborations. Groups from Bristol, Glasgow, Imperial
College, Oxford and University College London are members of
the ZEUS collaboration, while teams from Birmingham,
Lancaster, Liverpool, Manchester, and Queen Mary and Westfield College
London are members of H1. Physicists from the Rutherford
Appleton Laboratory in Oxfordshire participate in both experiments.
The detectors have tracking chambers to detect the directions of
charged particles as well as large devices called calorimeters to
measure the energies of the particles produced in the collisions.
Huge superconducting magnets provide a magnetic field to bend the
charged particles and allow a measurement of their momentum to be made.
The high rate of crossings between particle bunches in HERA requires that
both the experiments have extremely sophisticated arrays of electronics
to read out and make sense of "events" (collisions) sufficiently quickly.
Large farms of computers sit at the end of the data cables, processing events
and saving the interesting ones on disks and tapes for later analysis by
physicists.
QUARKS, GLUONS AND THE PROTON
-----------------------------
The proton is one of the ubiquitous building blocks of atomic nuclei;
indeed, the nucleus of the lightest element, hydrogen, consists of
a single proton. The proton is not, however, a fundamental particle.
It is made up of three smaller particles called quarks, which are bound
together by the aptly named strong force - the strongest of nature's
fundamental forces. The quarks stick together by continually swapping
gluons (the carriers of the strong force) and other quarks and antiquarks
among themselves.
The high energies available at HERA mean that the electron (or positron)
beam probes the proton a distances small enough for the individual quarks
and gluons to be seen. In fact, the electron can hit a single quark or gluon
and knock it out of the proton. The quark and electron recoil from
each other, and the remaining quarks continue on their way almost
undisturbed. However, as the quarks separate the force between them
grows, rather like the force between the two ends of a stretched piece of
elastic. The energy of the recoil is so large that the elastic quickly
snaps, leading to more pairs of quarks at the ends of smaller pieces
of elastic. The process continues, and the end result is a spray or
jet of particles made from quarks ("hadrons") in one side of the detector,
balanced by the electron in the other side.
Much has been learnt, about the quarks and gluons inside the proton,
and the force which binds them, by studying such collisions.
HIGH X, HIGH Q2
---------------
The parameters physicists use to describe electron-proton collisions
are x and Q^2 (Q squared). In simple terms, x is the fraction of the
proton's energy that was carried by the struck quark or gluon, and Q^2
is related to the distance between the electron and the quark or gluon
at the point at which the interaction occurs. High x means very
energetic quarks, and high Q^2 means very short distances.
The experiments at HERA have made a number of important discoveries and
measurements over a very wide range x (from 0.00001 to 0.5) and Q^2
(from close to zero up to 10,000). However, the rarest and in many
ways the most interesting events are those at high Q^2 and high x,
where the most energetic quarks interact with the electron at the
smallest distances.
THE NEW RESULTS
---------------
Recently in this region, at high x and high Q^2, both experiments
have seen more events than expected. If the result is confirmed, this
will imply that there is something new happening in the interaction
between electrons and quarks at very small distances and high
energies. Such a new interaction is impossible to explain in the
current `Standard Model' of particle physics.
In the Standard Model, there are six quarks (down, up, strange, charm,
bottom and top, in order of increasing mass) and three charged leptons, of
which the electron is the lightest. Each charged lepton comes with an
uncharged partner - a neutrino - and all these particles come with
antiparticles of equal but opposite charge. (The positron is the antiparticle
of the electron.) There is an appealing symmetry between the six quarks
and the six leptons (including neutrinos). Yet although physicists see
many bound states (particles) composed of quarks, they never see bound
states of quarks and leptons.
In the Standard Model, quarks and leptons interact by exchanging of one of
the force carriers of the electroweak force (photons, or the W and Z bosons).
If a new interaction occurs between them, this implies either a new force
(and therefore a new force-carrying boson) or a direct interaction between
the quark and the lepton. Either could lead to a new particle which could
be created when a quark and electron come together. These new particles are
generically referred to as `leptoquarks'. They could be either a bound state
of quarks and leptons, or a new elementary particle created by the
annihilation of a quark and lepton. In either case the new particle lives
only very briefly, decaying rapidly back to the lepton and quark from which
it was created.
The fact that a new particle of this kind will have a fixed mass means
that `unexpected' events could show up at HERA around common value of x.
The data from both H1 and ZEUS suggest that this could be the case, but more
data from the forthcoming run are needed to ensure that the observed effect
is not simply a statistical fluctuation.