Gargamelle

Source: Wikipedia, the free encyclopedia.
Not to be confused with Gargamel.
View of Gargamelle bubble chamber detector in the West Hall at CERN, February 1977
The chamber of Gargamelle is currently on exhibition at CERN

Gargamelle was a

neutrinos and antineutrinos, which were produced with a beam from the Proton Synchrotron (PS) between 1970 and 1976, before the detector was moved to the Super Proton Synchrotron (SPS).[1] In 1979 an irreparable crack was discovered in the bubble chamber, and the detector was decommissioned. It is currently part of the "Microcosm" exhibition
at CERN, open to the public.

Gargamelle is famous for being the experiment where

electroweak theory
.

Gargamelle can refer to both the bubble chamber detector itself, or the

The Life of Gargantua and of Pantagruel, in which the giantess Gargamelle is the mother of Gargantua.[1]

Background

Z0 boson. Flavors
are unaffected.

In a series of separate works in the 1960s

quantum numbers unaffected—charge, flavor, baryon number, lepton number, etc. Since there is no transfer of electric charge, the exchange of a Z0 is referred to as "neutral current
". Neutral currents were a prediction of the electroweak theory.

In 1960

strange particles
. Using these new neutrino beams greatly increased the energy available for the study of the weak interaction. Gargamelle was one of the first experiments that made use of a neutrino beam, produced with a proton beam from the PS.

A bubble chamber is simply a container filled with a superheated liquid. A charged particle travelling through the chamber will leave an

CBrF3 (Freon)—increasing the probability of seeing neutrino interactions.[1]

Conception and construction

Installation of the Gargamelle chamber body. Placement of the chamber in the oblong shaped magnet coils.

The domain of

neutral kaon into two charged leptons
, had measured very small limits of around 10−7.

Due to budgetary crisis, the experiment was not approved in 1966, contrary to what was expected.

Bernard Grégory, Scientific Director, decided to commit the money themselves, the latter offering a loan to CERN to cover the instalment due for 1966.[5]
The final contract was signed on 2 December 1965, making this the first time in CERN's history that an investment of this kind was not approved by the council, but by the Director General using his executive authority.

The Gargamelle chamber was entirely constructed at

Saclay. Though the construction was delayed by about two years, it was finally assembled at CERN in December 1970, and the first important run occurred in March 1971.[5]

Experimental setup

The inside of the bubble chamber. The fish-eye lenses can be seen on the walls of the chamber.

The chamber

Gargamelle was 4.8 meters long and 2 meters in diameter, and held 12 cubic meters of heavy liquid Freon. To bend the tracks of charged particles, Gargamelle was surrounded by a magnet providing a 2 Tesla field. The coils of the magnet were made of copper cooled down with water, and followed the oblong shape of Gargamelle. In order to maintain the liquid at an adequate temperature several water tubes surrounded the chamber body, to regulate the temperature. The entire installation weighed more than 1000 tons.

When recording an event, the chamber was illuminated and photographed. The illumination system emitted light that was scattered at 90° by the bubbles, and sent to the optics. The light source consisted of 21 point flashes disposed at the ends of the chamber body and over one half of the cylinder.[8] The optics were situated in the opposite half of the cylinder, distributed in two rows parallel to the chamber axis, each rows having four optics. The objective was made by an assembly of lenses with a 90° angular field followed by a divergent lens which extends the field to 110°.

The neutrino beam

A schematic of the beam line between PS and Gargamelle bubble chamber

Gargamelle was designed for neutrino and antineutrino detection. The source of neutrinos and antineutrinos was a proton beam at an energy of 26 GeV from the PS. The protons were extracted by a magnet and then directed through an appropriate array of quadrupole and dipole magnets, providing the necessary degrees of freedom in position and orientation for adjusting the beam onto target. The target was a cylinder of

kaons, which both decay to neutrinos. The produced pions and kaons have a variety of angles and energies, and consequently their decay product will also have huge momentum spread. As neutrinos have no charge, they cannot be focused with electric or magnetic fields. Instead, one focuses the secondary particles by using a magnetic horn, invented by Nobel laurate Simon van der Meer. The shape of the horn and the strength of the magnetic field can be tuned to select a range of particles that are to be best focused, resulting in a focused neutrino beam with a chosen range of energy as the kaons and pions decay. By reversing the current through the horn, one could produce an antineutrino beam
. Gargamelle ran alternately in a neutrino and an antineutrino beam. The invention of van der Meer increased the neutrino flux by a factor of 20. The neutrino beam had an energy between 1 and 10 GeV.

The magnetic horn of Simon van der Meer used in the neutrino beam line to Gargamelle.

After being focused, the pions and kaons were directed through a 70 m long tunnel, allowing them to decay. Pions and kaons that did not decay hit a shielding in the end of the tunnel and were absorbed. When decaying, pions and kaons normally decay in π→μ + ν and K→μ + ν, meaning that the flux of neutrinos would be proportional to the flux of muons. As the muons were not absorbed as hadrons, the flux of charged muons was stopped by an electromagnetic slowing down process in the long shielding. The neutrino flux was measured through the corresponding muon flux by means of six planes of silicium-gold detectors placed at various depths in shielding.[8]

During the years 1971-1976 large improvements factors were obtained in the intensity, first with a new injector for the PS — the Proton Synchrotron Booster — and secondly by the careful study of beam optics.

Results and discoveries

This event shows the real tracks produced in the Gargamelle bubble chamber that provided the first confirmation of a leptonic neutral current interaction. A neutrino interacts with an electron, the track of which is seen horizontally, and emerges as a neutrino without producing a muon.

The first main quest of Gargamelle was to search for evidence of hard-scattering of muon-neutrinos and

nucleons. The priorities changed in March 1972, when the first hints of the existence of hadronic neutral current became obvious.[9] It was then decided to make a two-prong attack in the search for neutral current candidates. One line would search for leptonic events — events involving the interaction with an electron in the liquid, e.g.
ν
μ +
e−
 →
ν
μ +
e−
 or
ν
μ +
e−
 →
ν
μ +
e−
. The other line would search for hadronic events — involving a neutrino scattered from a hadron, e.g.
ν
 +
p
 →
ν
 +
p
,
ν
 +
n
 →
ν
 +
p
 +
π−
 or
p
 →
ν
 +
n
 +
π+
, plus events with many hadrons. The leptonic events have small cross-sections
, but correspondingly small background. The hadronic events have larger backgrounds, most extensively due to neutrons produced when neutrinos interact in the material around the chamber. Neutrons, being of no charge, would not be detected in the bubble chamber, and the detection of their interactions would mimic neutral currents events. In order to reduce the neutron background, the energy of the hadronic events had to be greater than 1 GeV.

The first example of a leptonic event was found in December 1972 at Gargamelle by a graduate student from

Aachen. By March 1973 166 hadronic events had been found, 102 events with the neutrino beam and 64 events with the antineutrino beam.[9] However, the question of neutron background hung over the interpretation of the hadronic events. The problem was solved by studying the charged current events which also had an associated neutron interaction which satisfied the hadronic event selection.[10]
In this way one has a monitor of the neutron background flux. On the 19th of July 1973 the Gargamelle collaboration presented the discovery of neutral currents at a seminar at CERN.

The Gargamelle collaboration discovered both

W± and Z0 bosons
.

Initially the first priority of the Gargamelle had been to measure the neutrino and antineutrino cross-sections and

Stanford Linear Accelerator Center (SLAC) in the US, using an electron beam, one found that quarks had fractional charges, and experimentally proved the values of these charges: +2⁄3 e, −1⁄3 e. The results were published in 1975, providing crucial evidence for the existence of quarks.[11]

See also

References

  1. ^ a b c "Gargamelle". CERN. Retrieved 12 August 2017.
  2. ^ "The Nobel Prize in Physics 1979". Nobelprize.org. 15 October 1979. Retrieved 28 July 2017.
  3. doi:10.1103/PhysRevLett.4.306
    .
  4. ^ "Nobel Prize in Physics 1988: Press Release". Nobelprize.org. Retrieved 16 August 2017.
  5. ^ a b c Pestre, Dominique (1996). Gargamelle and BEBC. How Europe's Last Two Giant Bubble Chambers were Chosen. Amsterdam: North-Holland. pp. 39–97.
  6. ^ Haidt, Dieter (2015). "The Discovery of Weak Neutral Currents". In Schopper, Herwig; Di Lella, Luigi (eds.). 60 Years of CERN Experiments and Discoveries. Singapore: World Scientific. pp. 165–185. Retrieved 12 August 2017.
  7. ^ "Proposal for a Neutrino Experiment in Gargamelle". 16 March 1970. CERN-TCC-70-12. Retrieved 12 August 2017. {{cite journal}}: Cite journal requires |journal= (help)
  8. ^ a b c Musset, P.; Vialle, J.P. (1978). "Neutrino Physics with Gargamelle". In Jacob, M. (ed.). Gauge Theories and Neutrino Physics. Amsterdam: North-Holland Publishing. pp. 295–425.
  9. ^ a b Cundy, Donald; Christine, Sutton (25 August 2009). "Gargamelle: the tale of a giant discovery". CERN Courier. CERN. Retrieved 15 August 2017.
  10. ^ Cundy, Donald (1 July 1974). Neutrino Physics. 17th International Conference on High-energy Physics. London: CERN. pp. 131–148.
  11. doi:10.1016/0550-3213(75)90008-5
    . Retrieved 18 August 2017.

Further reading

External links

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