Experimental Tests of the Standard Model of Weak Interactions D.Zakoucky, Nuclear Physics Institute ASCR, Rez near Prague Experimental facility WITCH at ISOLDE-CERN Project: Search for new physics in  - correlations using trapped ions and a retardation spectrometer KU Leuven, ISOLDE-CERN, Uni Münster, FZK Karlsrühe, NPI-Rez Motivation - Standard Model and beyond Searches for non-SM effects Experiment WITCH Principle Realisation Current status,results Summary,Outlook

 general Lorentz invariant 4-fermion interaction Fermi, 1933 : 4-fermion interaction Lee & Yang, 1956; Wu et al., 1957 : parity violation General form of Hamiltonian with 5 possible interaction types and coupling constants C i defining their properties Scalar (parity conserving and violating) Vector Tensor Axial Vector Pseudoscalar Structure of the weak interaction in nuclear  -decay

C V =1 (CVC) C A =-1.27 (g A /g V = from n-decay) C V ΄=C V & C A ΄=C A (maximal parity violation) C S =C S ΄=C T =C T ΄=C P =C P ΄=0 (only V- and A-currents) no time reversal violation (except for the CP-violation described by the phase in the CKM matrix) BUT: experimental evidence (neutron and nuclear beta-decay)  C T ( ΄ ) /C A  < 0.08 ;  C S ( ΄ ) /C A  < 0.06 (95%CL) from: N. Severijns, M. Beck, O. Naviliat-Cuncic, Rev. Mod. Phys. 78 (2006) 991 Standard Model of Electroweak Interactions

Standard Model and beyond Standard Model : works well, but still many problems - too many ‘parameters’ (masses, fine structure constant, Fermi constant, Weinberg angle, …) -a number of not-well understood features (e.g. parity violation, baryon-antibaryon asymmetry, unification with gravitation, etc.)  General belief that SM is a “low” energy (~ 200 GeV) approximation of more fundamental theory  New physics beyond SM (e.g. neutrino oscillations !) Search for physics beyond the SM in the sector of the weak interaction : - at high energy colliders (CERN, Fermilab, DESY, …) - in neutrino physics (Antares, SuperKamiokande, AMANDA,  -beams, …) - in atomic physics (e.g. parity violation) - in nuclear beta decay (correlations, ft-values, …) High-energy and low-energy experiments are complementary – it is useful to test the SM in different energy domains.

Low energy search Correlations in  -decay – search for new time reversal invariant S- and T-interactions  -asymmetry – A parameter (Tensor interaction) Study of low-energy  -decay (the lower energy, the higher sensitivity to possible tensor contribution) Study of correlation between the spin of  -decaying nucleus and momentum of emitted  -particle  Measurement of the angular distribution of  -particles emitted during  -decay of oriented sample (nuclear orientation experiments - NICOLE)  - correlation - a parameter (Scalar, Tensor interaction) Difficulty to detect neutrinos  study recoil nuclei instead of neutrinos  Using ion or atom traps to get radioactive sources with required properties (isotopically pure, localized in small volume, negligible source scattering, decay at rest,…)  WITCH – combination of double-Penning trap + retardation spectrometer at ISOLDE-CERN – measuring energy spectra of nuclei recoiling after  -decay

High energy search search for “exotic” particles whose exchange could create possible scalar-or tensor- type interactions Limits (95%CL) on possible new bosons for S- and T- interactions, from high- energy experiments : mass limit for H  (charged Higgs): >79.3 GeV (LEP -ALEPH) mass limit for leptoquarks : >242 GeV (from pair production, combined CDF-D0) > 298GeV (from single production ZEUS) Correspondence of High x Low-energy searches (sensitivity of beta-neutrino correlation):  a  0.01  sensitive to masses of new bosons of ~ (0.01) -1/4 M W  215 GeV/c 2  a   sensitive to masses of new bosons of ~ (0.002) -1/4 M W  320GeV/c 2 (“handwaving estimate”)

e+e+ e nucleus   - correlation (search for SCALAR and TENSOR type weak interactions) (assuming maximal P-violation and T-invariance for V- and A-interactions)

Physics principle  - correlation ++ e Vector Scalar V S ++ e Fermi  + -decay

++ e Vector Scalar V S ++ e WITCH – Weak Interaction Trap for CHarged particles cooler & decay Penning trap + retardation spectrometer Ideal radioactive sample : - sample is isotopically pure - localized in small volume - source scattering negligible - atoms/ions decay at rest - potential for polarized sample Main goal : search for scalar weak interaction by measuring shape of recoil ion energy spectrum after  -decay Fermi  + -decay

REXTRAP Horizontal beam line 90  bender Pulsed drift tube Cooler trap Decay trap 9T magnet 0.1T magnet Radial into axial motion conversion  -detector Post acceleration Energy analysis by retardation Einzellens Detector (MCP) Experimental set-up Vertical beam line

~7m Overview of the set-up WITCH: Weak Interaction Trap for Charged particles

Rextrap decay trap (20 cm) cooler Trap (20 cm)  m/m  5 x pulsed drift tube (78 cm)

Retardation Spectrometer

WITCH proof-of-principle experiment (recoil spectrum of 124 In) 124g,m In First observation of recoil ions with WITCH

integral recoil spectrum WITCH proof-of-principle experiment (recoil spectrum of 124 In)

Choice of isotope Interesting from a physics point of view Production ISOLDE ~ 10 6 / 10 7 particles per second Half-life: order of 1 s Low ionisation potential Stable daughter isotope Decay mode: β - (± 10 times more recoil ions than β + ) Minimal isobaric/isomeric contamination Simple decay scheme => 35 Ar

First run with 35 Ar failed due to: - isobaric contamination with stable 35 Cl - 25 times more Cl than Ar - target group dealing with this - losses of 35 Ar due to charge exchange in REXTRAP - improvements planned - losses of 35 Ar due to charge exchange in WITCH - we couldn’t cool the ion cloud, because the ions were neutralized before being cooled - vacuum upgrade necessary - ‘secondary ions’, not created by beta decays (noise/discharges) Necessary improvements - Improvement of the vacuum - reduce charge exchange in the traps - reduce the secondary ionisation problem - Improve buffer gas system - redesign, (electro)polish electrodes - remove sparking & filling of unwanted traps - Install magnetic shielding and RFQ → be independent from the REX planning

General improvements Vacuum –General purity of WITCH was improved –More pumps –installation of NEG (Non Evaporable Getters) coated chambers –NEG foils + resistive heater around the traps –All teflon has been replaced with ceramics (electrical insulation of wires, teflon electrode connections gone, kapton wires replaced with ceramics) –buffer gas system is ‘all-metal’ now ↓ - overall pressures went down ~ few x mbar → few x mbar Electrodes, traps redesign of some electrodes, electrodes polished and electropolished, Ti instead of Al structures for electrodes and traps

Trap structure Kapton and teflon wires replaced by ceramics Whole structure redesigned, made of titanium (instead of Al) Buffer gas line in metal NEG coated foil around traps Heater to bake the system and activate the NEG

View of an electrode Teflon gone, polished and electropolished, …

35 Ar experiment (2007) ‘Charge exchange half-life’ in REXTRAP = 75 ms in WITCH = 8 ms (= not enough to cool) REXTRAP WITCH

Ar experiment (2009) REXTRAP WITCH Charge exchange (with 36 Ar) : REXTRAP situation is basically the same WITCH: charge exchange is not a problem anymore

Summary The overall efficiency of the setup has been improved by a factor of ~60 Mass dependent purification of the ion beam was achieved for the first time First recoil ions were seen!!! First spectrum was measured!!! Full data analysis is ongoing, systematic effects under investigation New problems were identified and are being looked into Everything we aimed for 2 years ago was achieved

Accomplish the main goal of the WITCH experiment: real precision measurement with 35 Ar, getting statistics for recoil spectrum, good accuracy for a parameter => improve the limits for scalar interaction Many additional prospects Tensor interactions F/GT mixing ratio EC/β + ratio Search for heavy neutrinos - kinks in the recoil spectrum Tape station on top of WITCH - trap assisted spectroscopy - using the traps to purify the beam, study pure samples of exotic nuclei Outlook

Backup

precision : for recoil ion endpoint energy of 300 eV e+e+ e nucleus 

Isotope Separator ISOLDE

N. Severijns, GSI-Colloquium, 21 Nov 2006, Darmstadt