The 6He nucleus is an ideal candidate to study the weak interaction. To this
end we have built a high-intensity source of 6He delivering ~10^10 atoms/s to
experiments. Taking full advantage of that available intensity we have
performed a high-precision measurement of the 6He half-life that directly
probes the axial part of the nuclear Hamiltonian. Currently, we are preparing a
measurement of the beta-neutrino angular correlation in 6He beta decay that
will allow to search for new physics beyond the Standard Model in the form of
tensor currents.Comment: 5 pages, 4 figures, proceedings for the Eleventh Conference on the
  Intersections of Particle and Nuclear Physics (CIPANP 2012

Alexander, Z. T.

Bagdasarova, Y.

Cope, T. M.

Delbridge, B. G.

Flechard, X.

Garcia, A.

Hong, R.

Knecht, A.

Lienard, E.

Mueller, P.

Naviliat-Cuncic, O.

Palmer, A. S. C.

Robertson, R. G. H.

Storm, D. W.

Swanson, H. E.

Utsuno, S.

Wauters, F.

Williams, W.

Wrede, C.

Zumwalt, D. W.

English

arXiv

A. Knecht

Z. T. Alexander

Y. Bagdasarova

T. M. Cope

B. G. Delbridge

X. Fléchard

A. García

R. Hong

E. Liénard

P. Mueller

O. Naviliat-Cuncic

A. S. C. Palmer

R. G. H. Robertson

D. W. Storm

H. E. Swanson

S. Utsuno

F. Wauters

W. Williams

C. Wrede

D. W. Zumwalt

Crossref

Weak interaction studies with [sup 6]He

The 6He nucleus is an ideal candidate to study the weak interaction. To this end we have built a high-intensity source of 6He delivering ∼1010 atoms/s to experiments. Taking full advantage of that available intensity we have performed a high-precision measurement of the 6He half-life that directly probes the axial part of the nuclear Hamiltonian. Currently, we are preparing a measurement of the beta-neutrino angular correlation in 6He beta decay that will allow to search for new physics beyond the Standard Model in the form of tensor currents. © 2013 AIP Publishing LLC

Fléchard, X.

García, A.

Liénard, E.

Palmer, A. S.C.

Robertson, R. G.H.

Williams, William

Smith College: Smith ScholarWorks

Smith ScholarWorks Physics: Faculty Publications Physics 12-16-2013 Weak Interaction Studies ith 6He A. Knecht University of Washington Z. T. Alexander University of Washington Y. Bagdasarova University of Washington T. M. Cope University of Washington B. G. Delbridge University of Washington See next page for additional authors Follow this and additional works at: https://scholarworks.smith.edu/phy_facpubs  Part of the Physics Commons Recommended Citation Knecht, A.; Alexander, Z. T.; Bagdasarova, Y.; Cope, T. M.; Delbridge, B. G.; Fléchard, X.; García, A.; Hong, R.; Liénard, E.; Mueller, P.; Naviliat-Cuncic, O.; Palmer, A. S.C.; Robertson, R. G.H.; Storm, D. W.; Swanson, H. E.; Utsuno, S.; Wauters, F.; Williams, William; Wrede, C.; and Zumwalt, D. W., "Weak Interaction Studies ith 6He" (2013). Physics: Faculty Publications, Smith College, Northampton, MA. https://scholarworks.smith.edu/phy_facpubs/92 This Conference Proceeding has been accepted for inclusion in Physics: Faculty Publications by an authorized administrator of Smith ScholarWorks. For more information, please contact scholarworks@smith.edu Authors A. Knecht, Z. T. Alexander, Y. Bagdasarova, T. M. Cope, B. G. Delbridge, X. Fléchard, A. García, R. Hong, E. Liénard, P. Mueller, O. Naviliat-Cuncic, A. S.C. Palmer, R. G.H. Robertson, D. W. Storm, H. E. Swanson, S. Utsuno, F. Wauters, William Williams, C. Wrede, and D. W. Zumwalt This conference proceeding is available at Smith ScholarWorks: https://scholarworks.smith.edu/phy_facpubs/92 Weak Interaction Studies with 6HeA. Knecht∗, Z. T. Alexander∗, Y. Bagdasarova∗, T. M. Cope∗,B. G. Delbridge∗, X. Fléchard†, A. García∗, R. Hong∗, E. Liénard†,P. Mueller∗∗, O. Naviliat-Cuncic‡, A. S. C. Palmer∗, R. G. H. Robertson∗,D. W. Storm∗, H. E. Swanson∗, S. Utsuno∗, F. Wauters∗, W. Williams∗∗,C. Wrede‡ and D. W. Zumwalt∗∗Department of Physics and Center for Experimental Nuclear Physics and Astrophysics,University of Washington, Seattle, Washington 98195, USA†LPC-Caen, ENSICAEN, Université de Caen, CNRS/IN2P3-ENSI, Caen, France∗∗Physics Division, Argonne National Laboratory, Argonne, Illinois 60439, USA‡Department of Physics and Astronomy and National Superconducting Cyclotron Laboratory,Michigan State University, East Lansing, MI 48824, USAAbstract. The 6He nucleus is an ideal candidate to study the weak interaction. To this end we havebuilt a high-intensity source of 6He delivering ∼1010 atoms/s to experiments. Taking full advantageof that available intensity we have performed a high-precision measurement of the 6He half-life thatdirectly probes the axial part of the nuclear Hamiltonian. Currently, we are preparing a measurementof the beta-neutrino angular correlation in 6He beta decay that will allow to search for new physicsbeyond the Standard Model in the form of tensor currents.Keywords: helium-6, nuclear beta decay, half-life, weak axial coupling constantPACS: 23.40.-s, 27.20.+nINTRODUCTIONElectroweak currents are at the basis of many interesting, fundamental processes suchas, e.g., solar fusion, neutrino interactions, or muon and pion interactions [1]. As not allthose processes are easily accessible directly, studies of nuclear beta decays can be usedinstead. Especially, measurements of half-lives and correlation coefficients have a longstanding history in pinning down the underlying mechanism of the weak interaction.The 6He nucleus in particular is a very attractive system for such studies:• The 6He nucleus decays through a pure Gamow-Teller transition to the ground-state of 6Li with a 100% branching ratio (except for a small beta-delayed deuteronemission branch of ∼10−6 [2]).• The half-life of ∼807 ms allows enough time to manipulate the atoms whileensuring high decay rates.• The 6He nucleus is a simple few-body system for which ab initio calculations canbe performed with precision (see, e.g., Refs. [3, 4, 5, 6]).• The 6He mass has recently been measured in a Penning trap [7] providing a reliableand high-precision Q-value of 3505.208(53) keV.• With helium being a noble gas, effects due to chemical interactions are minimized.arXiv:1208.6433v2  [nucl-ex]  19 Oct 2012In this paper, we will briefly present our 6He production source, our measurement ofthe 6He half-life and the current status of the measurement of the beta-neutrino angularcorrelation in 6He beta decays.PRODUCTION OF 6HEA detailed description of our high-intensity source for 6He can be found in Ref. [8].We produce 6He using the tandem Van de Graaff accelerator available at the Centerfor Experimental Nuclear Physics and Astrophysics of the University of Washington. Adeuteron beam impinges on molten lithium held in a stainless steel cup producing 6Heprimarily via the reaction 7Li(2H, 3He)6He. The 6He atoms subsequently diffuse outinto vacuum and are transported to the experimental area by means of a turbo-molecularpump.While we reported in Ref. [8] extracted 6He rates in the experimental area of109 atoms/s, we have in the meantime reached values in excess of 1010 atoms/s byincreasing the deuteron beam currents up to 14 µA and improving our beam andproduction diagnostics. This is currently the highest intensity of 6He atoms available forexperiments world-wide and even surpasses the 6He intensities reached in the 1960s athigh power nuclear reactors [9].MEASUREMENT OF THE 6HE HALF-LIFEThe pure Gamow-Teller decay of the 6He decay allows to probe axial currents in a simplenuclear system. Contrary to the vector currents that are protected by the ConservedVector Current hypothesis, axial currents could be changed in the nuclear mediumleading to a renormalization of the axial coupling constant found in the decay of freeneutrons. Several works have calculated the 6He half-life using ab initio methods [3, 4,5, 6] and found agreement between the experimental and theoretical matrix elements atthe percent level allowing for only minimal “quenching” of the axial coupling constant[10, 11].With theoretical calculations pushing towards precisions of one percent and below andwith previous half-life measurements being discrepant we decided to perform a high-precision measurement of the 6He half-life that was recently published in Ref. [12]. The6He atoms from our source were transported to the experimental area and confined in astainless steel cylinder closed off on one side by a valve and on the other by a thin copperwindow. In front of the copper window we had mounted two scintillators detecting thedecay electrons in coincidence. We took data in cycles in which we filled the countingvolume with 6He for 8 s, closed it off and then counted the decay electrons for 16 s.In order to study systematic effects we performed the measurements at various initialdecay rates, checked for helium diffusion effects by inserting a stainless steel rod intothe counting volume, and performed background runs with the inlet valve closed. Inaddition, several offline measurements were performed to determine dead times, afterpulsing, and other effects .Time [ms]10000 12000 14000 16000 18000 20000 22000Events/10 ms110210310410510610Time [ms]10000 12000 14000 16000 18000 20000 22000Norm. Residuals-3-2-10123Time [ms]14 0 16 0 18000 2 00 22000Norm. Residuals--Year of Publication1954 1962 1974 1981 1982 [ms]1/2T796798800802804806808FIGURE 1. Left: 6He decay curve data including a fit and the corresponding residuals. Right: Compi-lation of the previous five precision measurements of the 6He half-life [14, 15, 16, 17, 18] with the dashedred band depicting the value for the 6He half-life obtained in our measurement. Both figures are takenfrom Ref. [12].Figure 1 (left) shows the summed decay curve for part of the data together with a fitand its corresponding residuals. Due to the coincidence requirement between the twoscintillators the background rate is very low allowing to follow the decay curve overfour orders of magnitude. We performed a careful analysis of possible systematic effectspresented in Refs. [12, 13]. The final result for the 6He half-life is:T1/2 = 806.89±0.11stat +0.23−0.19 syst ms (1)The previous five precision results of the 6He half-life [14, 15, 16, 17, 18] are showntogether with our result in Fig. 1 (right). There is a clear discrepancy between our resultand three of the previous results. Because the possibility of diffusion out of the targetwas not explicitly addressed in these experiments, we suspect that this may be the causeof the discrepancy.From our measurement we calculated the f t-value to be 803.04+0.26−0.23 s and extractedthe experimental Gamow-Teller matrix element |MGT|= 2.1645(43) using the value forthe axial coupling constant obtained from free neutron decay gA = −1.2701(25) [19]providing a precise and reliable value to be used in comparison with theoretical calcula-tions.TOWARDS A MEASUREMENT OF THE 6HE BETA-NEUTRINOANGULAR CORRELATIONA measurement of the beta-neutrino angular correlation probes the symmetry of theunderlying interaction. In the case of 6He – a pure Gamow-Teller decay – the interactionis purely axial in nature according to the Standard Model. Testing for deviations fromthe Standard Model prediction can uncover contributions from new physics in the formof tensor currents. A review of the status of the field can be found in Refs. [20, 21].The current best measurement for 6He – reaching a precision of 0.9% – was performedDetuning(MHz)-40 -20 0 20 40Photon counts50001000015000200002500030000FIGURE 2. Left: Fluorescence photon counts as a function of the wavelength of a laser probing thetrapped 6He atoms. Right: Cross-sectional view of the detection MOT with a MCP on the bottom, a2 kV/cm electrode structure in the middle, and a proportional counter and thick scintillator on the top.The 6He atom cloud is located in the center of the electrode structure.in 1963 by measuring the energy distribution of the recoil ions [22]. A 3.1%-precisionvalue for the correlation coefficient from a coincidence measurement of the emitted betaparticle and recoil ion using LPCTrap at GANIL has been reported recently [23] withdata to reach a statistical accuracy of 0.7% currently under analysis.The 6He atoms coming from our source are extracted into the experimental area andpass through an RF discharge source to get excited into the metastable 2s-state. Atthis point we can access the 2s-2p cycling transition using a laser at 1083 nm. Themetastable 6He atoms are subsequently transversely cooled, focused, slowed down andfinally trapped in a magneto-optical trap (MOT). We detect the number of atoms inthe trap by tuning a second laser onto the 2p-3s resonance at 706 nm and collecting thefluorescence signal with a photomultiplier tube (PMT). Figure 2 (left) shows the numberof photons detected by the PMT as the wavelength of the probe laser is varied. We haveachieved a total number of 500 atoms in our trap and still have ways to increase thenumber of atoms to our design goal of 1000 trapped atoms.Currently we are working on instrumenting a second MOT with the necessary de-tectors in order to register the emitted beta particle and recoil ion in coincidence. The6He atoms will be transferred from the trapping MOT through a small opening to thedetection MOT in order to reduce the amount of background present from untrappedatoms. On one side of the detector chamber we will mount a micro-channel plate (MCP)detector that records the position and time-of-flight of the recoil ions (with respect tothe detection of the beta particle). An electrode structure providing an electric field of2 kV/cm will collect and accelerate all the ions onto the MCP. On the other side betaparticles passing through a thin beryllium window will be detected by a ∆E-E detectorconfiguration consisting of a proportional counter and a thick scintillator read out bya PMT. The setup can be seen in Fig. 2 (right). In this configuration we are aiming atperforming a 1% measurement within the next year. At a later stage we will change thedetection MOT to a dipole trap allowing for better control of the atom cloud and betteraccess for particle detectors. This should then lead to the final anticipated precision of0.1%.SUMMARYThe 6He nucleus is an ideal system to probe the weak interaction in detail. To this end wehave developed a high-intensity source delivering ∼1010 6He atoms/s to experiments.We have performed a high-precision measurement of its half-life that allows forimproved comparisons between experiment and ab initio theory calculation, deepeningour understanding of the nuclear Hamiltonian.With 500 6He atoms trapped in a MOT we are ready to pursue the next steps in ourproject of measuring the beta-neutrino angular correlation. This will allow for searchesfor new physics beyond the Standard Model in the form of tensor currents.ACKNOWLEDGMENTSWe thank the excellent staff at the Center for Experimental Nuclear Physics and As-trophysics. This work has been supported by the US Department of Energy under DE-FG02-97ER41020 and under Contract No. DEAC02-06CH11357.REFERENCES1. K. Kubodera, and M. Rho, “Effective Field Theory and High-Precision Calculations of NuclearElectroweak Processes,” in From Nuclei to Stars, Festschrift in Honor of Gerald E. Brown, editedby S. Lee, World Scientific, Singapore, 2011.2. R. Raabe, et al., Phys. Rev. C 80, 054307 (2009).3. R. Schiavilla, and R. B. Wiringa, Phys. Rev. C 65, 054302 (2002).4. P. Navrátil, and W. E. Ormand, Phys. Rev. C 68, 034305 (2003).5. M. Pervin, S. C. Pieper, and R. B. Wiringa, Phys. Rev. C 76, 064319 (2007).6. S. Vaintraub, N. Barnea, and D. Gazit, Phys. Rev. C 79, 065501 (2009).7. M. Brodeur, et al., Phys. Rev. Lett. 108, 052504 (2012).8. A. Knecht, et al., Nucl. Instr. Method in Phys. Res. A 660, 43 (2011).9. F. Pleasonton, and C. H. Johnson, Rev. Sci. Instrum. 35, 97 (1964).10. W.-T. Chou, E. K. Warburton, and B. A. Brown, Phys. Rev. C 47, 163 (1993).11. E. Caurier, A. Poves, and A. P. Zuker, Phys. Rev. Lett. 74, 1517 (1995).12. A. Knecht, et al., Phys. Rev. Lett. 108, 122502 (2012).13. A. Knecht, et al., Phys. Rev. C (2012), accepted for publication.14. R. M. Kline, and D. J. Zaffarano, Phys. Rev. 96, 1620 (1954).15. J. K. Bienlein, and F. Pleasonton, Nuclear Physics 37, 529 (1962).16. D. H. Wilkinson, and D. E. Alburger, Phys. Rev. C 10, 1993 (1974).17. P. H. Barker, T. B. Ko, and M. J. Scandle, Nuclear Physics A 372, 45 (1981).18. D. E. Alburger, Phys. Rev. C 26, 252 (1982).19. K. Nakamura, et al., J. Phys. G 37, 075021 (2010).20. N. Severijns, M. Beck, and O. Naviliat-Cuncic, Rev. Mod. Phys. 78, 991 (2006).21. N. Severijns, and O. Naviliat-Cuncic, Annual Review of Nuclear and Particle Science 61, 23 (2011).22. C. H. Johnson, F. Pleasonton, and T. A. Carlson, Phys. Rev. 132, 1149 (1963).23. X. Fléchard, et al., Journal of Physics G: Nuclear and Particle Physics 38, 055101 (2011).

Weak Interaction Studies ith \u3csup\u3e6\u3c/sup\u3eHe

5 pages, 4 figures, proceedings for the Eleventh Conference on the Intersections of Particle and Nuclear Physics (CIPANP 2012)International audienceThe 6He nucleus is an ideal candidate to study the weak interaction. To this end we have built a high-intensity source of 6He delivering ~10^10 atoms/s to experiments. Taking full advantage of that available intensity we have performed a high-precision measurement of the 6He half-life that directly probes the axial part of the nuclear Hamiltonian. Currently, we are preparing a measurement of the beta-neutrino angular correlation in 6He beta decay that will allow to search for new physics beyond the Standard Model in the form of tensor currents

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