Title: Title
Author: Philippe CHOMAZ
Last saved by: VOLPE
Company: GANIL

Fundamental tests with low energy beta-beams

Abstract  

Neutrino beams produced with low energy beta-beams are used to perform fundamental interaction studies, in particular a measurement of the Weinberg angle at low momentum transfer and a test of the Conserved-Vector-Current (CVC) hypothesis on the weak magnetism form factor. 

Keywords 

Beta-beams, weak interaction, Weinberg angle, CVC hypothesis

Physics case
 
The measurement of the Weinberg angle represents a crucial test of the Standard Model of the electroweak theory. Several measurements exist at different momentum transfer and are consistent with the expected running of the weak mixing angle. However, a recent measurement of the neutral- to charged-current ratio in muon anti-neutrino nucleon scattering at the NuTEV experiment disagrees with these results by about 3σ [1]. A complete understanding of the NuTeV anomaly is still lacking. Probing the Weinberg angle through additional experiments with different systematic errors would be very useful.
	Low energy beta-beams are neutrino beams in the 100 MeV energy range [2], produced by the decay of boosted radioactive ions circulating in a storage ring [3]. Such beams could be used to carry our such a test, through scattering of neutrinos on electrons at Q^2 = 10^-4 GeV^2  in a water Cherenkov detector. 

 The Conserved-Vector-Current (CVC) hypothesis relates the weak to the electromagnetic currents and tells us, in particular, the values of the vector, pseudo-scalar and weak magnetism form factors. Several tests of CVC have been performed in the past. In particular, the CVC prediction on the vector form factor has been extensively studied in super-allowed nuclear beta-decays [4]. 
Verifying that the CVC hypothesis correctly predicts the weak magnetism term is of fundamental importance. So far, such a prediction has been tested in an experiment involving the beta-decay of Gamow-Teller transitions in mirror nuclei in the A=12 triad [5]. A test based on neutrino-nucleon scattering at low momentum transfer would be particularly interesting. In addition to providing a test of the CVC hypothesis, such a measurement is of direct interest for astrophysics, as these terms play an important role in the dynamic of core-collapse supernova.   

Observables

For the measurement of the Weinberg angle, the scattering of neutrinos on electrons should be measured.  Keeping systematic errors as low as 10%, a measurement of the Weinberg angle with a precision of 10% could be within reach [6].

For the CVC test, the scattering of neutrinos on protons should be measured. The angular distribution is needed since it is more sensitive to the weak magnetism form factor than the number of events. If systematic errors are kept as low as 5%, a one year measurement of the weak magnetism term is possible at a 1σ level of 9% (with the ions boosted at γ = 12) [7].
  	
Proposed experiment

An (anti)neutrino beam should be produced by the decay of ^6He ions, boosted at γ = 7-14, and circulating into a storage ring. 	Preliminary calculations show that 2.7x10^12 ions/s can be accumulated in the storage ring which has 1885 m total length and 678 m straight sections. A 1 kton water Cherenkov detector is located at 10 meters from the storage ring.  The (anti)neutrinos impinging in the water detector will interact mainly with the protons, with the oxygen nuclei and the electrons. Since neutrino-electron scattering is forward peaked, such events can be selected by choosing an angular cut of cosθ>0.9. 
	By selecting (anti)neutrino-proton events, the same apparatus can be used to perform the CVC test. 

Requirements

Beam properties

	 A 6He ion beam of 2x10^13 ions/s is needed. This ion beam should be stored in a storage ring to get a pure (anti)neutrino beam of well-known flux. Some characteristics of the storage ring need still to be studied (in particular the space-charge effects and the duty cycle).

Detection

	A 1 kton water Cherenkov detector is used. Detailed background simulations are required to assess the finally achievable sensitivities.  

References
[1] G.P. Zeller et al (NuTeV Collaboration), Phys. Rev. Lett. 88 (2002) 091802, hep-ex/0110059 [Erratum-ibid. 90 (2003) 239902].
[2] C. Volpe, J. Phys. G 30 (2004) L1, hep-ph/0303222.
[3] P. Zucchelli, Phys. Lett. B 532 (2002) 166.
[4] J.C. Hardy and I.S. Towner, Phys. Rev. C 71 (2005), nucl-th/0412056.
[5] Y.K. Lee, L.W. Mo and C.S. Wu, Phys. Rev. Lett. 10 (1963) 253.
[6] A.B. Balantekin, J.H. de Jesus, C. Volpe, Phys. Lett. B 634 (2006), hep-ph/0512310.
[7] A.B. Balantekin, J.H. de Jesus, R. Lazauskas, C. Volpe, Phys. Rev. D 73 (2006) 073011.