Preliminary work

Our group’s main emphasis lies on angular correlations involving the neutron spin and electron, neutrino, and proton momenta. We measure the coefficients a (neutrino electron correlation coefficient) [Bae08,Kon09], A (beta asymmetry parameter) [Abe97,Abe02,Abe09], B (neutrino asymmetry parameter) [Kre05a,Sch07], and C (proton asymmetry parameter) [Sch08] with increasing accuracy. Independent measurements of one of these observables, sensitive to λ (ratio of the axial to the vector coupling constant), and of the neutron lifetime τn allow the determination of Vud. Each observable brings a different sensitivity to non-Standard Model (non-SM) physics, such that comparing the various values of λ provides an important test of the validity of the SM (unitarity of the Cabibbo-Kobayashi-Maskawa matrix). Precise measurements of correlations in neutron decay can as well be used to search for evidence of possible extensions to the SM, like right-handed currents, scalar and tensor interactions [Abe09,Kon11] and many more, as described in the Introduction.

The spectrometer aSPECT, opens an external URL in a new window [Glü05] has been built at Mainz University to perform a precise measurement of the proton spectrum shape in free neutron decay. aSPECT is a retardation spectrometer which measures the proton recoil spectrum by counting all decay protons that overcome an electrostatic barrier. Such a measurement allows the determination of the correlation coefficients a and C in decays of unpolarized and polarized neutrons, respectively. The present best experiments have an uncertainty of Δa/ a = 5% and since the seventies there is no substantial improvement [Str78,Byr02]. The aSPECT collaboration identified and fixed a problem in the readout electronics which caused a significant systematic error in our 2008 beam time. With a measurement in 2011 the aSPECT collaboration aims to improve the uncertainty on the correlation coefficient a to 1%.

The instrument PERKEO II , opens an external URL in a new window[Abe97] has been designed at Heidelberg University to measure the correlation coefficients A, B, and C with reduced corrections while maintaining the PERKEO I principle of a 4π electron detection. In PERKEO II, in a superconducting split pair configuration with a coil diameter of about 1 m, the magnetic field is oriented perpendicular to the neutron beam, guiding the charged decay products, electrons and protons, away from the neutron beam to regions of low background. In a collaboration with the Institut Laue Langevin (ILL), opens an external URL in a new window in Grenoble, France, major improvements both in neutron flux and degree of neutron polarization have been made: The new ballistic supermirror guide H113 at ILL gives an increase of about a factor of four in the cold neutron flux [Abe06] and an X-arrangement of two supermirror polarizers makes it possible to achieve an unprecedented degree of neutron polarization P of 99.7(1)% over the full cross-section of the beam [Kre05b]. Table 1 presents the correlation coefficients measured in decays of polarized neutrons with PERKEO II.

Table 1: Angular correlation coefficients measured in decays of polarized neutrons with PERKEO II.





A - 0.1189(12) 1997



- 0.1189  (7)



A - 0.1198  (5) 2006



0.967  (12)



B 0.9802(50) 2007


C - 0.2377(26) 2008


The main corrections in the recent beta asymmetry experiment [Mun06] are due to neutron beam polarization (0.3%) and background (0.1%). As a consequence, the uncertainties in reducing the total correction on A0 are less than 0.4% with an error of 0.33%. Averaging over PERKEO II measurements [Abe97,Abe02,Mun06] gives a preliminary A0 = -0.11933(34) (Publication of the result of the last PERKEO II run [Mun06] is underway). From this value the ratio λ of the axial to the vector coupling constant can be derived λ = -1.2750(9) with a precision of Δλ/ λ = 7 x 10-4.  The PERKEO collaboration favors these results over earlier experiments [Bop86,Yer97,Lia97], where large corrections had to be made for neutron polarization, electron-magnetic mirror effects, or background, which were all in the 15 - 30% range. From the new neutron decay data on a, A, B, C, and the neutron lifetime τn, e.g., a limit on right-handed tensor currents has been derived [Abe09] which is comparable with a recent survey of nuclear and neutron beta decays [Sev06].

The successor PERKEO III , opens an external URL in a new window[Mae06], built by B. Märkisch at Heidelberg University, serves to measure neutron decay correlation coefficients using a pulsed neutron beam [Mae09]. This further reduces the sources of error typical for correlation coefficient experiments using magnetic fields, such as a magnetic mirror effect, edge effects, beam related background, and degree of polarization, while vastly increasing statistical accuracy. With a measurement in 2008/2009 the PERKEO III collaboration improved the uncertainty on A by about a factor of 5 (preliminary) [Mes11], compared to the Particle Data Group’s 2010 average [Nak10]. The results of the 2008/2009 measurement will be published soon.



Within the Priority Programme SPP 1491 "Precision experiments in particle- and astrophysics with cold and ultracold neutrons, opens an external URL in a new window" of the German Research Foundation (DFG), opens an external URL in a new window we focus on novel experiments in neutron beta decay to study the structure and nature of the weak interaction. PERKEO II and PERKEO III have motivated the new instrument PERC (Proton Electron Radiation Channel) [Dub08] improving the sensitivity of neutron decay studies by up to two orders of magnitude compared to the best experiments. PERC will be built in collaboration, opens an external URL in a new window with the Universities of Heidelberg and Mainz, the Technical University Munich, and the ILL in Grenoble. The instrument will be set up at the beam facility MEPHISTO of the Forschungs-Neutronenquelle Heinz Maier-Leibniz, opens an external URL in a new window (FRM II) in Munich, Germany.

In PERC, the charged decay products are collected by a strong longitudinal magnetic field directly from inside a neutron guide, see Fig. 2. This combination provides the highest phase space density of decay products. A magnetic mirror serves to limit the phase space precisely, reducing related systematic errors. Systematic errors related to electron spectroscopy have been shown to be on the level of 10-4, more than 10 times better than that achieved today [Dub08].

[Translate to English:] Scheme of the facility PERC

© Atominstitut

Scheme of the facility PERC: Cold neutrons (green) pass through the decay volume where only a few neutrons decays. The decay products (red) are guided by the strong magnetic field towards the detector (blue). The superconducting coils are drawn in grey. The equipment for neutron beam preparation, like velocity selector, polarizer, spin flipper, or chopper, is located in front of the instrument (to the left of the scheme) and not shown here. For details see [Dub08]

The main goal within the years 2010-2013 is the design and the construction of the joint project PERC. The first milestone (the design together with magnetic field calculations for strong magnetic fields) has been achieved:  At its exit, PERC delivers neutron decay products, electrons and protons, under well-defined and precisely variable conditions. The next step is systematic studies for the analysis of the extracted electrons and protons. Depending on the decay parameters studied, this analysis must be performed with different and specialized detectors. We focus on electron energy spectroscopy, simultaneous electron and proton momentum spectroscopy, and proton spectroscopy for PERC.

For most correlation coefficient measurements, the detectors are energy sensitive detectors for decay electrons. The decay electrons are guided by the strong magnetic field to, e.g., a scintillation detector with photomultiplier readout. The advantage of scintillation detectors is a short readout time with time resolution of  1 ns, which is needed for high count rate spectroscopy.

For a measurement of the Fierz interference term b, a new dedicated magnetic spectrometer must be built. A great advantage of PERC is that it allows for the first time a precise momentum spectroscopy of electrons and protons. The problem so far was that systematic errors are considerably larger for absolute count rate spectra N(E) than they are for measured asymmetry spectra Aexp(E). Figure 3 shows the configuration of a magnetic spectrometer coupled to PERC in the analyzing area, which simultaneously, but not in coincidence, measures electron and proton momentum spectra via position sensitive detectors.

[Translate to English:] Sketch of a magnetic spectrometer

Sketch of a magnetic spectrometer for neutron decay products. Electrons and protons delivered by PERC are detected in position sensitive e- and p+ detectors after momentum analysis in the magnetic field [Dub08]

High-precision measurements with PERC demand a perfect knowledge of the key neutron beam parameters, i.e., wavelength distribution, degree of polarization, and time structure. Therefore, we plan a novel design of a spatial magnetic spin resonator, allowing precise velocity selection as well as the accurate definition of the neutron beam’s time structure by only tuning electronic parameters [Goe11], see Fig. 4. The concept is based on the Drabkin [Dra63] neutron resonator combined with travelling magnetic fields.

[Translate to English:] Scheme of wavelength selection by spatial magnetic spin resonance

© Atominstitut, Photo: Hartmut Abele

Scheme of wavelength selection by spatial magnetic spin resonance [Goe11]


[Abe09]  H. Abele, Nucl. Instr. Meth. A 611, 193-197 (2009).

[Abe06]  H. Abele et al., Nucl. Instr. Meth. A 562, 407 (2006).

[Abe02]  H. Abele et al., Phys. Rev. Lett 88, 211801 (2002).

[Abe97]  H. Abele et al., Phys. Lett. B 407,212 (1997).

[Bae08]  S. Baeßler et al., Eur. Phys. J. A 38, 17 (2008).

[Bop86]  P. Bopp et al., Phys. Rev. Lett. 56, 919 (1986).

[Byr02]  J. Byrne et al., J. Phys. G: Nucl. Part. Phys. 28 (2002).

[Dra63]  G.M. Drabkin, Sov. Phys. JETP 16, 282 (1963).

[Dub08] D. Dubbers at al., Nucl. Instr. Meth. A 596, 238 (2008),
for an extended version, see arXiv:0709.4440.

[Glü05]  F. Glück et al., Eur. Phys. J. A 23, 135 (2005),
see also O. Zimmer et al., Nucl. Instr. Meth. A 440, 548 (2000).

[Goe11]  C. Gösselsberger et al., Physics Procedia, accepted,
see also G. Badurek et al., Physica B 406, 2458 (2011).

[Kon11]  G. Konrad et al., in World Scientific ISBN 978-981-4340-85-4 (2011)
and arXiv: 1007.3027v2 (2010).

[Kon09]  G. Konrad et al., Nucl. Phys. A 827, 529 (2009),
see also M. Simson et al., Nucl. Instr. Meth. A 611, 203 (2009).

[Kre05a]  M. Kreuz et al., Phys. Lett. B 619, 263 (2005).

[Kre05b]  M. Kreuz et al., Nucl. Instr. Meth. A 547, 583 (2005).

[Lia97]  P. Liaud et al., Nucl. Phys. A 612, 53 (1997).

[Mae09]  B. Märkisch et al., Nucl. Instr. Meth. A 611, 216-218 (2009).

[Mae06]  B. Märkisch, Dissertation, University of Heidelberg 2006.

[Mes11]  H. Mest, Dissertation, University of Heidelberg, 2011.

[Mun06]  D. Mund, Dissertation, University of Heidelberg 2006.

[Nak10]  K. Nakamura et al J. Phys. G: Nucl. Part. Phys. 37, 075021 (2010).

[Sev06]  N. Severijns et al., Phys. Rep. 78, 991 (2006).

[Sch08]  M. Schumann et al., Phys. Rev. Lett. 100,151801 (2008).

[Sch07]  M. Schumann et al., Phys. Rev. Lett., Phys. Rev. Lett. 99, 191803 (2007).

[Str78]  C. Stratowa et al., Phys. Rev. D 18, 3970 (1978).

[Yer97]  B.G. Yerozolimsky et al., Phys. Lett. B 412, 240 (1997).