NOMAD
Neutrino Oscillation MAgnetic Detector
Back to NOMAD home page... Updated : september 1997

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A complete description of the NOMAD detector can be found in "The NOMAD experiment at the CERN SPS", submitted to Nuclear Instruments and Methods Phys. Res. A (CERN-PPE/97-059, May 26th, 1997) ( 23146 Ko).

The NOMAD collaboration is composed of about 150 physicists...
Every 14s, 1013 muon neutrinos (muon nu) coming from the CERN SPS go through the 3 ton target of the NOMAD detector. At each burst only one or two neutrino interact. Despite this low rate NOMAD can register up to 500,000 neutrino interactions per year. The hope of physicists is that among these interactions some are not due to muon nu but to tau-neutrino (tau mu).
This discovery would confirm the neutrino oscillation hypothesis : this fundamental phenomena implies that neutrino are massive particles. They could then contribute to the hidden mass of the Universe. Moreover, the standard model of particle physics does not predict any masses for neutrino and therefore neutrino oscillation would allow an extension of this model.

Finally, in NOMAD if a muon nudouble arrowtau nu oscillation is seen, this will lead to the first direct detection of the tau nu (up to now it has only been indirectly detected via the presence of the tau lepton). If no tau nu is seen , NOMAD will have contributed to the exploration of the oscillation parameters as already done by many other experiments these last years. Moreover, thanks to its intrinsic characteristics, NOMAD will have improved previous studies made by various experiments (CDHS, CHARM and bubble chamber experiments) on neutrino interactions.

muon nu beam, tau nu appearance

The neutrino beam is produced from 450 GeV protons extracted from the CERN SPS synchroton. The protons interact on a beryllium target and produce pion and kaon hadrons that essentially decay into muons and muon nu. At the NOMAD position (940 m further) the beam contaminations with respect to the main muon nu component, with a mean energy of 24 GeV, are the following:

  • anti-anti muon nu = 0.07
  • electron nu = 0.01
  • anti-electron nu = 0.003
  • tau nu = negligible (a few times 10-6 for the ratio of interaction rates)

    • Therefore the observation of tau nu in the detector undoubtly signs an oscillation.

    tau nu detection

    When an electron-neutrino (electron nu) or a muon nu interacts by exchanging a W boson (charged current reaction) a lepton is produced. This lepton has the same flavor as the incident neutrino, and it signs the neutrino interaction. The electron is stable, and the probability for a muon to decay within the detector is very small (life time of 2.2 microseconds). In the case of the tau nu, the tau lepton which is produced immediately decays : its life time is of the order of 10-13 seconds, and in NOMAD its track is less than 1 mm long. The tau lepton cannot therefore be directly detected. This lepton decays essentially as described in the following table :

    Main decay modes of the tau lepton
    Mode Probability (%)
    muon, anti-muon nu, t ALIGN= 17.4
    electron,anti-electron nu,tau nu 17.8
    pion,tau nu 12
    rho,tau nu 26

    It is clear that in the two first modes the electron or the muon which is emitted can be simulated by a electron nu or a muon nu interaction. The crucial difference comes from the fact that in the decay of the tau lepton produced in a tau nu interaction there are two neutrinos escaping thus creating an important missing energy. This missing energy is the way the tau nu interaction is signed in NOMAD.

    Detector characteristics

    schematic view of the detector

    Schematic view of the NOMAD detector.
    The magnet creates a magnetic field perpendicular to that picture. This field bends the particle trajectories. A numu interaction by charged current is shown with identification of the muon inside the muon chambers.

    1. Veto
    2. Drift chambers
    3. T1 (First trigger plane)
    4. Transition Radiation Detector (TRD)
    5. T2 (Second trigger plane)
    6. Preshower
    		 7. Electromagnetic calorimeter
    8. Hadronic calorimeter
    9. Muon chambers
    10. Forward calorimeter
    11. Field return yoke
    12. Magnet coil

    In order to identify the tau nu candidate, NOMAD has therefore to identify electrons and muons, and has to measure the energy and the momentum of all the tracks produced during the neutrino interaction in order to be able to compute with accuracy the missing momentum. The detector needs therefore to have the following characteristics :

  • It has to be massive to count a large number of neutrino interactions
  • It has to measure with an excellent accuracy the momentum of all particles
  • It has to be able to identify electrons and muons
  • It has to be as hermetic as possible for the missing momentum not to be due to particles that would have escaped.

    The event kinematics is reconstructed thanks to a B=0.4 tesla magnetic field, horizontal and perpendicular to the beam. The magnetic field bends the trajectories of charged particles : the curvature radius R is related to the momentum (p goes like B.R)

    Inside the magnet are placed some proportional drift chambers, a transition radiation detector, a preshower detector and an electromagnetic calorimeter.

  • Drift chambers are used to reconstruct the charged tracks and are the target. They have to be massive enough to obtain an important number of neutrino interactions but light enough to minimize the multiple scattering. The active target (3 tons) is composed of 44 chambers. 5 additional chambers are installed individually inside the TRD region (see below) and are used to improve the lever arm for tracking and for a better extrapolation of the tracks to the rest of subdetectors. Each chamber is composed of 3 planes of sensitive wires. The precision on the position of a hit in the chambers is 200 micrometers vertically and 2 millimeters horizontally (perpendicular to the beam). The drift chambers were build in CEA-Saclay by the DAPNIA.
  • The transition radiation detector allows to make the difference between pions and electrons with a rejection factor greater than 1000. It is composed of 9 modules at the end of the target. Each module is composed of a radiator and of a sensitive plane. When a highly relativistic particle, mainly electrons in NOMAD, goes through the radiator it emits a transition radiation (few keV photons) due to the sequence of different dielectric constant materials (315 polypropylene foils in air). This radiation is absorbed in the sensitive planes and produces an additional amount of energy that characterises the electrons. The TRD was built by CERN and LAPP d'Annecy.
  • The preshower is composed by a Pb-Sb foil (96%-4%) that initiate the electromagnetic showers (electrons and photons) and two planes of sensitive tubes, respectively vertical and horizontal. The shower generally extends into the electromagnetic calorimeter. The energy measurement inside the preshower allows, like in the case of the TRD, to distinguish electrons from pions (this identification is improved when done together with the electromagnetic calorimeter). The preshower was built by Université de Lausanne.
  • The electromagnetic calorimeter measures photon and electron energies. It is composed of lead glass blocks ; the electromagnetic particles going faster than the velocity of light in that medium generate a Cerenkov effect (light emitted near the visible spectrum) which is detected by tetrodes (special photomultipliers designed for the magnetic field). The electromagnetic calorimeter was built by the Italian groups of the collaboration

    • Outside the magnet can be found the forward calorimeter, the hadronic calorimeter and the muon chambers

  • The forward calorimeter (to be written...)
  • The hadronic calorimeter detects the hadrons and measures their energy, in particular neutral hadrons (neutrons, K0) that were not detected upstream. It permits a more detailed study of the two last modes described above. The hadronic calorimeter is composed of plastic scintillator planes installed in the iron block that closes the magnetic field. Hadrons interact in the iron and produce showers that are detected by scintillators.
  • The muon detector is composed of 5 modules containing each four sensitive wire planes. This system allows to determine the muon trajectories with an intrinsic resolution of 400 micrometers. The muons interact only weakly with matter and go through the whole detector. The iron block upstream (i.e. the hadronic calorimeter) serves as a filter for all other particles.

    • A neutrino interaction is signed using 3 scintillator planes : one veto plane V upstream, a T1 plane just before the TRD region and a T2 plane just before the preshower detector. A neutrino interaction in the target does not produce any signal in V but a signal in T1 and T2.

    Most of the texts of this page are a translation of an article published in 1994 in the Bulletin de la Société Française de Physique : "Nomad : de la masse des neutrinos à la masse de l'Univers", Th. Stolarczyk, Bulletin de la Société française de Physique, 104 (1996) 6.

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