Search for eV (pseudo)scalar penetrating particles in the SPS neutrino beam
Introduction
Neutral spin-zero scalar or pseudoscalar particles a's (this notation will be used for both cases) of nearly zero mass are predicted in many theories. The most motivated pseudo-Nambu–Goldstone bosons, for example, arise in models constraining spontaneously broken symmetry, see e.g. [1]. They couple to the divergence of the current whose charge generates the symmetry which is spontaneously broken. The most popular light pseudoscalar, the axion, postulated [2]to provide a solution of the “strong CP” problem, emerges as a consequence of the breaking of the Peccei–Quinn symmetry [3]. It is now believed that the axion has a mass much smaller than that was originally expected 4, 5, 6. The axion two-photon interaction is given by the Lagrangian
where gaγγ is the coupling constant, are the electric and magnetic fields, a is the axion field. If is an external magnetic field, a's interact with the photon electric field component parallel to .
An example of a scalar particle weakly coupled to two photons is the dilaton, which arises in super-string theories and interacts with matter through the trace of the energy-momentum tensor [7]. In particular its interaction with photons is given by the LagrangianHere again, gaγγ is the coupling constant and a is the dilaton field. If is an external magnetic field a's interact with the photon electric field component orthogonal to . Usually it is assumed that gaγγ=O(M−1Pl) and that the dilaton mass ma=O(MPl), where MPl is the Planck mass. However, in recent models with large compactification radii (see e.g. [8]), the dilaton could be rather light and since there are no firm predictions for the coupling gaγγ the searches for such particles become very interesting and actual.
Several experiments have placed limits on possible (pseudo)scalar bosons [1]. Experimental bounds on gaγγ for light a's can be obtained from laser experiments 9, 10, from experiments on J/ψ and ϒ particles [11], and from orthopositronium decays [12]. The best direct experimental limit, for the eV mass range, has been extracted from the recent limit of the CLEO Collaboration Br(ϒ(1S)→aγ)<1.3×10−5 [13]by assuming this decay to occur through a virtual photon [14].
The best limit on the axion-photon coupling comes from astrophysical limits on anomalous energy loss by stars [15]. However, such astrophysical constraints, although more stringent, are model-dependent and have various uncertainties. For example, as has been demonstrated in [16], the inclusion of additional fermions which strongly interact with (pseudo)scalars of mass may evade the astrophysics constraints. Hence, it is important to perform independent laboratory tests on the existence of such particles in the mass range discussed above.
Experimental techniques used for searching for light (pseudo)scalars are based either on the measurement of vacuum birefringence 17, 18, 19, or on the Helioscope method 20, 21, or on what is called “photon-regeneration” [22]. Here we describe a direct experimental search for a particles which are weakly coupled to two photons and which might be present in the SPS neutrino beam. The experiment is performed by using elements of the CERN West Area Neutrino Facility (WANF) beam line and the NOMAD neutrino detector and is based on the photon-regeneration method, used for the first time at high energy. In the analysis we do not assume any relationship between particle mass and coupling to photons but we assume that a's are rather long-lived particles. The present analysis as well as the experimental signature of the signal events are similar to those of our previous light gauge boson search [23].
Section snippets
WANF and NOMAD detector
The present WANF beam line [24](schematically shown in Fig. 1) provides an essentially pure νμ beam for the CERN neutrino experiments. It consists of a beryllium target irradiated by 450 GeV protons from the CERN SPS. The secondary hadrons are focused with two magnetic elements, the horn and the reflector, located in front of a 290 m long vacuum decay tunnel. Protons that have not interacted in the target, secondary hadrons and muons that do not decay are absorbed by a 400 m shielding made of
Detection principle
This experiment was possible due to the unique combination of several factors, namely, the presence of: i) the high regions both in the WANF horn and in the NOMAD detector, where z is the coordinate along the magnet axis; ii) the relatively transparent DC target, used to reject efficiently interactions in the magnet coils; iii) the PRS and ECAL to detect photons. The experimental set-up consisting of WANF elements and the NOMAD detector and the detection principle are schematically
Axion and regenerated photon spectra
The energy spectra of photons produced in the neutrino target mainly through π0 decays have been obtained with the same detailed GEANT [30]simulation used to predict the neutrino flux distributions at the NOMAD detector. The effects of pion re-interactions in the target, photon conversion and bremsstrahlung of electrons or positrons have been explicitly taken into account, as well as the correct material composition of the horn for cascade development. Fig. 2 shows the energy spectrum of π0's
Event samples and selections
We study events taken with the ECAL trigger during the 1996 run period. The selection criteria for a→γ events are based on a full Monte Carlo simulation of a→γ conversions in the NOMAD detector. These criteria are similar to those used in our previous search for a light gauge boson [23]. The difference between the two analyses is due to the lower ECAL energy cut and the additional background simulations described in Section 6.
Candidate events were identified by the following criteria:
- •
: no
Background events
The relative neutrino beam composition in the NOMAD detector is predicted to be νμ : : νe : = 1.00 : 0.061 : 0.0094 : 0.0024, with average energies of 23.5, 19.2, 37.1, and 31.3 GeV, respectively [32]. The main background to a→γ events is expected from neutrino processes with a significant electromagnetic component in the final state and with no significant energy deposition in the HCAL.
The following neutrino processes occurring either in the PRS region or in the upstream region, which includes the
Results
Fig. 4 shows the overall background and candidate event energy spectra in the ECAL. The agreement between data and Monte Carlo is reasonable. The overall efficiency for single high energy photon detection in the PRS/ECAL was found to be ∼25% for axion masses . The inefficiency is mostly due to the requirement of photon conversion in the PRS, the ECAL energy cut and PRS/ECAL matching.
By subtracting the number of expected background events from the number of candidate events we obtain Na→γ
Acknowledgements
We gratefully acknowledge the CERN SPS accelerator and beam-line staff for the magnificent performance of the neutrino beam. The experiment was supported by the following funding agencies: Australian Research Council (ARC) and Department of Industry, Science, and Resources (DISR), Australia; Institut National de Physique Nucléaire et Physique des Particules (IN2P3), Commissariat à l'Energie Atomique (CEA), France; Bundesministerium für Bildung und Forschung (BMBF, contract 05 6DO52), Germany;
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