1. Weak Interaction Trap for Charged Particles
WITCH: Testing the Standard Model using a beta-recoil spectrometer with a trapped ion cloud as source
A. Lindroth1*, F. Ames2, D. Beck1, M. Beck1, G. Bollen3, B. Delauré1, J. Deutsch4, V.V. Golovko1, V. Yu. Kozlov1, I. Kraev1, T. Phalet1, K. Reisinger2, P. Schuurmans1, N. Severijns1, S. Versyck1.
1Instituut for Kern- en Stralingsfysica, K. U. Leuven, Celesijnenlaan 200D, B-3001 Leuven, Belgium
2LMU, München, Germany
3MSU, East Lansing, MI, USA
4Louvain-la-Neuve, Belgium
*  :
Hi! I’m Axel Lindroth I’m gonna talk about WITCH: Testing the Standard Model using a beta-recoil spectrometer with a trapped ion cloud as source
Our group at Katholieke Universiteit in Leuven, Belgium, is presently constructing this experiment in CERN. The active team members are two postdocs; myself, Axel Lindroth, and Marcus Beck, our professor Nathal Severijns, and two PhD students Bavo Delaure and Valentin Kozlov.
Witch stands for Weak Interaction Trap for Charged Particles.
Our logotype is naturally a witch on a broom consisting of the three particles flying apart in the beta-decay process. We will measure the energies of recoiling nuclei, which is directly related via kinematics to the beta-neutrino angular correlation.
This short talk will be very straight-forward., but let me point out that the physics motivation is only for the first and possibly most important experiment planned. After describing the set-up I therefore end the talk by listing other experiments that should be feasible with WITCH.
Weak Interaction Trap for Charged Particles
CONTENTS OF THIS TALK
Weak Interaction Motivation
The Set-Up
Using the Set-Up
Other Possible Experiments with WITCH

2. Scalar contributions to pure Fermi decays
The experimental limits for S- and T-interaction leave room for considerable contributions, of the order of 10% for the V and A coupling constants. The first WITCH aim is improving these limits by measuring the e-antineutrino angular correlation, i.e. the a coefficient, in pure F decays. Later also in GT decays.
In the Standard Model V-A description a=1 as we se from the formula. This general expression shows that scalar contributions (the only allowed ones with the same selection rules as V) would change the value of a to be <1. We also have to take into account the possibility of a Fiertz interference term when we fit the recoil spectrum. It is left out for clarity.
Scalar contributions can be due to e.g. leptoquark or Higgs boson exchange.
Since the neutrino can not be measured directly other observables like recoil spectrum shape must be used for the correlation studies. Such experiments have long been hampered by the fact that the beta-emitter is usually embedded in a solid matrix, which leads to changes in the spectra due to energy losses. Recoiling daughter nuclei will most times be stopped already in the source due to their low energy, E, of usually E<1keV.
A number of experiments have been performed- mostly long before I was borne – on noble gases with pure F or GT transitions. Those experiments contributed important information. The latest experiment, by Adelberger et al, is the best present limit on S in a Fermi transition. But pure F and GT transitions in noble gases are rare , and (% to excited states in 6He (0%), 32Ar(about 50% to proton group and 50% to bound states), 19Ne(0%), 23Ne(to two states ~equally strongly)) (check which are PURE F or GT!) not necessarily optimal.
The WITCH set-up will of course also be able to study pure GT transitions. And possibly even to study F/GT ratios, but that is only far away on our planning horizon.
Johnson et al 6He GT A/T
Adelberger et al 32Ar F V/S
Allen et al 6He, 19Ne, 23Ne, 35Ar F & GT

3. Nuclear recoil measurements of a
The beta and neutrino emitted in beta-decay have opposite chirality for V interaction, and the same chirality for S interaction. In superallowed 0+0+ Fermi decays where the lepton spins must couple to zero they will be emitted preferentially into the same (V) direction/opposite(S) directions. This leads to an on average larger recoil energy for V than for S interactions. The figure shows the extreme cases of a recoil spectrum. We saw in the expression for aF, aF<1 for S contributions.
By constructing a novel type of set-up that can measure precisely the beta-recoil spectrum of any element, we are free to choose among the best transitions available. The example on the transparency happens to be a noble gas, but also Al and Vanadium are of prime interest to us. 35Ar has a well-known a from logft measurements. And in addition opens for general studies/experiments where beta-recoil spectra of nuclei are of interest.
The problem of resolution, and even stopping, in solid elements is in WITCH circumvented by the use of a cylindrical Penning trap to store the radioactive ions. There they constitute a low-density ion cloud of basically zero thickness. This is the source of the WITCH experiment.

4. Overview of the WITCH set-up
Our set-up has to be vertical due to limited space in the ISOLDE hall at CERN. The entire set-up is ~7m high.
The ions are produced and mass separated by ISOLDE, CERN – where an extraordinary range of nuclides is available
REXTRAP traps and cools the beam, after which it is delivered in bunches to WITCH.
These ion bunches of 60keV pass through a HBL section, are decelerated in 3 steps in the VBL, one of which is a PDT used to avoid a HV cage. Some fine retardations lead finally to injection with a suitable energy into the first Penning trap, called the cooler trap. Here the ions are cooled to room temperature. The cooler trap is emptied into the second trap, which we call the decay trap. Here the ions undergo beta decay and half of them end up in the spectrometer.
The spectrometer is based on the combination of two principles:
inverse magnetic mirror to convert radial energy of recoils into axial energy, gives a <~2PI acceptance
a retardation voltage that allows only recoils with sufficient energy to pass
There are 2 super-conducting magnets to create a homogenous magnetic field for the traps and a magnetic field gradient for the inverse magnetic mirror. They are housed in a common cryostat. The whole magnet system was delivered by Oxford Instruments last week and is now being tested at CERN.
The retardation voltage allows only recoiling ions with enough energy to continue to the top of the spectrometer, where these ions are reaccelerated by ~10kV and focused by an Einzel lens onto an MCP. By scanning the retardation voltage and simply counting events in the MCP we will get an integral recoil energy spectrum.

5. The System of 2 Traps
I shall now describe how we will use our two traps:
~50eV ion energy when entering the cooler trap
The ions are cooled and centered in the first trap. The cooling is mass selective. It is assisted by buffer gas – He – present in the first trap. This first trap then helps us get rid of isobaric impurities, and to rid the ion cloud of some energy so that it can be more localized and still.
ejected through a differential pumping barrier into the decay trap. The ions are kept 1 to a few half-lives in the decay trap (few seconds), most ions will decay during this time. The ion cloud in the decay trap constitutes the source of the experiment.
The two traps have been selected to be of Penning type since that means no restrictions on elements that can be trapped, and the cylindrical versions have an open end.

6. The Trap Structure Magnetron Motion Cyclotron Motion Axial Motion
Penning traps should be hyperbolical in shape in order to allow the separation into three independent motions for the ions in the trap: cyclotron, magnetron and axial. (The leftmost figure shows these motions.) Cylindrical Penning traps are equipped with correction electrodes to make the potential well hyperbolic enough. Enough in this case means that it allows the motion of the ions in the trap to be separated into those independent motions.
The cooling is mass selective since it depends on an RF quadrupole field applied to the ring electrode –which is segmented into 8 pieces. The RF field couples magnetron and cyclotron motions so that energy can be transferred between them.
The central figure show how cooling by only a buffer gas transfers energy from cyclotron into the magnetron motion – the ion spirals out into the trap electrodes and is lost. The rightmost figure shows how the RF coupling of the motions helps centering and cooling the ions at the same time.
Our particular pair of Penning traps are modified versions of the cooler trap for the ISOLTRAP high-precision mass measurement experiment in CERN.
They are separated by a diff pumping barrier which keeps the He-buffer gas away from the decay trap. The Pressure difference will be ~few*100.
The axial confinement is achieved by the endcap electrodes, whereas CE – denotes correction electrode – and is for shaping the potential well.
both traps: cylindrical shape  open! Correction electrodes needed especially in the cooler trap
A boxlike potential shape should be enough for the decay trap.
The Trap Structure
Magnetron Motion
Cyclotron Motion
Axial Motion

7. The Retardation Spectrometer
This transparency depicts the retardation spectrometer. Note: PEKA!
The two magnet coils
the magnetic fieldlines in green
the two traps
the retardation electrodes
the einzel lens
and the MCP detector
The magnet system has been designed for maximum homogeneity in the two trap centers, and for a suitable gradient from the strong to the weak field region. This gradient serves as an inverse magnetic mirror, converting radial energy of recoils into axial. This leads to much increased acceptance. We will collect <~2PI, which spiral from 9T to 0.1T. Due to the isotropic emission from beta-decay: backward  lost. In the B-field 98.9% of the radial energy of ions is converted into axial energy.
This is of course paramount as it is then enough to probe the axial energy of ions in order to extract a recoil spectrum. This is done by a retardation voltage of up to a few hundred volts, corresponding to the recoil energies. The retardation voltage is applied in steps of increasing radius in order that the electric field lines are parallel with the B-field lines. This ensures that electric retardation is focused on the axial energy only.
After the retardation, the ions are accelerated to ~10keV for two reasons:
to get off the B-field lines non-adiabatically
to reach energies where the MCPs are efficient /flat efficiency vs E curve
There is an einzel lens for focusing onto the MCP
Then, simply varying Vret and counting in the MCP gives the spectrum. It will, due to the retardation principle, naturally be an integral spectrum.
The Mainz and Troitsk neutrino mass experiments use this same spectrometer principle.

8. Set-up Behavior & Measurement
Simulated response function for different pressures in the decay trap.
This shape is due to the larger solid angle for betas emitted in the radial direction, for which the conversion of radial into axial energy is incomplete.
One sees clearly how the response function deteriorates for too high pressures. Due to the factor of several 100 in Pressure difference over the differential pumping barrier we can achieve ~10-6 mbar or better while still having enough buffer gas of ~10-5 to 10-3 in the cooler trap.
The bottom panel shows two realistic recoil spectra. One for a pure F transition (46Vanadium) – thus with a=1, and the other is from the mixed F/GT transition of 35Ar. S contributions will have the same effect as for 35Ar in this figure, which has a<1.
46V (a=1.0)
35Ar (a=0.908)
Retardation Voltage

9. Precision of S measurement with WITCH
We need 107 events in the differential recoil spectrum.
4 days measurement time needed!
From this we should get Da/a=0.5% at 1s.
The important factors for our total statistics are things like
the number of ions in the bunches from rextrap
The number of ions that can fit in our traps: this is limited by space-charge effect: a hot topic in trap technology
transfer efficiencies
we loose half the ions because they have a momentum component in the downward/backward direction
a fraction of the ions will not decay within the time they are kept in the decay trap, and some will decay before reaching it
We measure the integral spectrum (due to the retardation principle). We will measure ~20 channels in the upper part  here the effect is stronger, and some problems in the lower part (see next slide) can be avoided counts in the differential recoil spectrum is achievable under reasonable assumptions.
From the plot we see that then the sensitivity of WITCH will be delta-a/a=0.5% (1sigma). This is at the forefront of what is possible at present. Naturally, the aim is to improve on this after the principle has been seen to work.
Test experiments will be run next year, and real experiments in the end of 2003 or in 2004.
107
105
106
Events in continuous spectrum for different fitting intervals.

10. A General b-recoil Spectrometer
T interaction
F/GT ratios
Measure excited states
Q values
Charge state distributions
EC/b+ ratios
Polarized Nuclei
In-trap spectroscopy
In the physics motivation I discussed only briefly the possibility to find Standard Model violating S components in F decay. But WITCH opens for the first time a general way for recoil spectroscopy in beta decay.
The previous recoil spectrum was a simplification, it should look something like the bottom panel here. It is a calculated differential recoil energy spectrum including: continuous spectrum, EC-peak, charge states
The maximum is actually at charge states higher than one. We will be able to measure this charge state distribution.
b+ is always accompanied by EC decay, albeit usually with small branching ratios. From the position of these EC peaks the Q value can be inferred. A precision of few keV should be possible. This is nearly as good as ISOLTRAP.
In which of these measurements the WITCH experiment will be competitive remains to be seen.
To summarize: by the novel idea of combining a trap structure for the radioactive source with a retardation spectrometer with an inverse magnetic mirror principle for converting radial energy into axial energy we are taking the step from measuring the very rare cases amenable to study previously – a step to a general b recoil spectrometer!
Finally I’d like to invite you to have a look at out poster – which describes an experiment at PSI with which Krakow, ETH Zurich and ourselves from KU Leuven are going to study the R-correlation in b-decay of the polarized free neutron (related to T invariance).
1 e+5 EC
80000
60000 1+
Intensity [events]
40000 2+ 3+
20000
100
200
300
400
Retardation Voltage