DETECTORS, FIXED-TARGET

Fixed-target experiments are those that study the collisions of a highly relativistic particle beam with a target that is stationary in the laboratory. This technique is complementary to collider experiments that study the collisions of particles from two opposed beams.

Typically in a fixed-target experiment, the total momentum in the laboratory rest frame is large compared with the energy available to produce new particles in the final state, for example, the total energy of the reaction in the center of momentum frame. Therefore, high-energy particles produced in interactions at fixed-target experiments will be observed to be close in direction to the incoming relativistic particle beam, as shown in Figure 1.

Quantitatively, the γ of the Lorentz boost from the center of momentum frame into the lab frame is given bywhere m_targ is the mass of the target particle, and E_beam and m_beam are the energy and mass of the particles in the incoming beam. The approximation is valid for the case where E_beam » m_beam, m_targ. The Lorentz factor γ gives the ratio between the momentum in the laboratory along the beam direction to the center of momentum energy for the case of particles produced at right angles to the incoming beam in the center of momentum frame. 1/γ is a typical angle

FIGURE 1

with respect to the incoming particle beam for an observed particle produced in a fixed-target interaction.

The high ratio of incoming beam momentum to total center of momentum frame energy, in typical fixed-target experiments has two important consequences. First, the total energy available for creation of new particles is significantly lower then in colliding beam experiments. This limits the production of massive final states to masses typically well below E_beam. Second, the produced final states are usually highly relativistic. Because of time dilation, these particles thus have a long observed lifetime in the lab, and therefore a long flight path. The latter feature makes fixed-target experiments particularly well suited for measuring time evolution of particle states, such as in meson lifetime or mixing measurements.

Fixed-target experiments are also particularly useful when the desired reaction has a very low cross-section, such as in a neutrino scattering experiment, or when the goal of an experiment is to observe the decay of a long-lived particle, such as a weakly decaying meson. In the former case, the fixed target can be very massive in order to increase the interaction probability. In the latter case, the near collinearity of the outgoing particles in a fixed-target reaction means that particle detection apparatus need only subtend a limited solid angle from the point of particle production. This affords the opportunity to build small sophisticated detectors, for example, high resolution vertex detectors for observation of charmed particles, or allows for the decay detectors to be located a great distance from the target, as in a kaon decay experiment.

Geometry of a Fixed-Target Detector

As with collider detectors, a typical detector configuration for a fixed-target experiment is layered, with produced particles passing through a sequence of detectors. After the target, a typical detector configuration is illustrated in Figure 2. Nondestructive tracking detectors, such as segmented scintillator or proportional counters, wire chambers, ring-imaging Cerenkov

FIGURE 2

detectors, or transition radiation detectors, provide information on charged-particle direction and velocity. The addition of a magnetic field in the tracking volume allows the tracker to measure the product of particle charge and momentum via the deflection in the field. Subsequent calorimetric, or energy-measuring detectors, destructively measure the total energy carried by electrons, photons and strongly interacting metastable hadrons, such as charged pions. Finally, muon detectors identify and track particles, notably muons, that pass through the calorimeters.

A representative implementation of this geometry in a recent fixed-target experiment is the NA48 Experiment at the European Laboratory for Particle Physics (CERN) whose detector is shown in Figure 3. NA48 seeks to identify decays of neutral K mesons into π⁺π^- and π⁰π⁰ → 4γ final states. Highly relativistic K mesons with kinetic energy of approximately 100 GeV are produced in two targets, 220 m and 100 m in front of the decay. The weakly decaying kaons produce decay products nearly collinear to the beam direction. The momenta of charged pions are measured in the tracking volume, and the photons from π⁰ decays are measured in the liquid krypton calorimeter. Anticounters surrounding the tracking chambers register the escape of wide-angle particles from the detector volume.

FIGURE 3

Active Target Material

A unique feature of fixed-target detectors is the capability for target material to serve as an active detector. In the case where interactions are rare, for example, neutrinos, this is essential since interactions can occur anywhere in the target. This feature may also be desirable in identifying extremely short-lived particles, such as τ leptons or hadrons containing heavy quarks.

Active target material may either be fully active or part of a "sampling" detector, one that observes particle interactions in only a fraction of its total material. The FOCUS experiment at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, is an example of the latter type of active target applied to the detection of short-lived particles. FOCUS produces charmed hadrons, with a typical lifetime of 0.5 × 10^-12 s, from selected interactions of a wide-band photon beam with an average energy of 180 GeV. Produced charm hadrons have a mean flight path before decay of ∼2 cm. As shown in Figure 4, the segmentation of the target, along with the active silicon planes inside the target region itself, allows for identification of multiparticle vertexes from decays of charm in the empty regions of the target.

A classic fully active detector is a bubble chamber or emulsion detector. In such detectors, the target and detection media are the same material that fills the volume of the detector. Modern realizations of this technology include large emulsion experiments (CHORUS at CERN), cryogenic time projection chambers (ICARUS at Gran Sasso), solid-state detectors (CDMS at Soudan, CRESST at Gran Sasso), and water Cerenkov detectors (Super-Kamiokande at Kamioka, SNO at Sudbury). The experiments noted all search for very weakly interacting particles, such as neutrinos or dark matter candidates, which can interact anywhere in the volume of the detector. Fully active detectors typically utilize sensor technology that is not local in space-time to the observed interaction. For example, when produced particles travel through water Cerenkov detectors, such as the SNO detector shown in Figure 5, they create cones of visible light (Cerenkov radiation) that are observed with photomultiplier tubes located at the outer perimeter of the detector. These Cerenkov cones appear as rings when projected onto the perimeter and allow for measurement of the velocity and the identification of the produced particles.

Physics Goals of Planned Fixed-Target Experiments

Planned fixed-target experiments exist that take advantage of all the unique aspects available to fixed-target experiments. High statistics K meson decay experiments, which study rare decays of long-lived kaons and take advantage of high-rate kaon production on fixed targets, are planned to study the rare semileptonic decays K → πv̄ in charged kaons

FIGURE 4

(CKM at Fermilab) and neutral kaons (KOPIO at Brookhaven National Laboratory and E391A at the Japanese High-Energy Accelerator Research Organization [KEK]). Experiments studying high statistics lepton interactions on polarized electron (E158 at Stanford Linear Accelerator Center) and polarized nuclear targets (COMPASS at CERN and a number of experiments at Jefferson National Accelerator Laboratory) will investigate electroweak interference and nuclear structure.

A large number of planned neutrino fixed-target experiments with very large active targets are currently under construction. Large detectors devoted to the study of neutrino oscillation phenomena in long-baseline accelerator neutrino beams include K2K at KEK/Kamoika (50 kilotons of water), Mini-BooNE at Fermilab (100 tons of mineral oil), MINOS at Fermilab/Soudan (10 kilotons of steel-scintillator sampling calorimeter), and OPERA and ICARUS at CERN/Gran Sasso (2 kilotons of lead-emulsion and up to 5 kilotons of liquid argon, respectively). In addition, a variety of planned experiments searching for very rare ultrahigh energy neutrinos in cosmic rays employ targets of unprecedented mass to search for these rare events. As an example, the IceCube experiment will search for these high-energy interactions in a cubic kilometer of Antarctic ice.

FIGURE 5

Bibliography

Barker, A. R., and Kettell, S. H. "Developments in Rare Kaon Decay Physics." Annual Review of Nuclear & Particle Science50 , 249–297 (2000).

Boger, J., et al. "The Sudbury Neutrino Observatory: The SNO Collaboration." Nuclear Instruments and Methods in Physics Research A449 , 172–207 (2000).

Hughes, E. W., and Voss, R. "Spin Structure Functions." Annual Review of Nuclear & Particle Science49 , 303–339(1999).

Primack, J. R.; Seckel, D.; and Sadoulet, B. "Detection of Cosmic Dark Matter." Annual Review of Nuclear & Particle Science38 , 751–807 (1988).

Kevin McFarland

Detectors, Fixed-Target

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