1 Bottle, or “disappearance” measurements of the neutron lifetime - PowerPoint Presentation

1. 1Bottle, or “disappearance” measurements of the neutron lifetimeAlexander SaundersLos Alamos National LabLA-UR-14-26812

2. 2OutlineWhy study weak interactions via the neutron beta decay lifetime?Because of the scientific reachBecause of the impact across nuclear and particle physicsHow to measure the neutron lifetimePresent storage lifetime experimentsThe UCNt experimentStorage lifetime at the SNS STS?

3. 3Neutron Decay ParametersSemi-leptonic decayLifetime 880 sEndpoint energy 782 keVJust two free parameters in SMCKM mixing matrix elementRatio of weak coupling constantsUncertainty comes from radiative corrections

4. 4Neutron beta decay can inform many areas of physicsMany reactions share the same Feynman diagram as neutron beta decayDubbers 2011

5. Neutron lifetime affects Big Bang NucleosynthesisLifetime is on same time scale as the formation of helium and other light elements after BBAnd much of the rest of nuclear and particle physics, when taken with beta decay correlationsTest for BSM physics via CKM unitarity5~150 sThanks to N. Fomin, S. Baessler

6. Rev. Mod. Phys. 83, 1173 (2011)How to measure neutron lifetime6Using the “beam method”:Cold Neutron BeampeCounterCounterAbsolute monitor efficiency needed!PRC 71, 055502 (2005)A. T. HolleyUsing the Ultra-cold neutron “bottle” method:

7. Tension between appearance and disappearance techniques7

8. Tension between appearance and disappearance techniques8Included in 2019 PDG average of 879.4 s

9. 9GravitrapArzumanov

10. 10Material bottle experiments involved 100 s extrapolations due to wall lossesTypical bottle experiment

11. 11Solution: eliminate wall losses using magnetic bottleA new crop of experiments using magnetic traps is now under development or taking dataStern-Gerlach effect repels polarized neutrons from wallsILL Ezhov Bottle filled with vacuumTauspect uses permanent magnets for radial trapping and aspect SC magnet for longitudinalEzhov et al., JETP 107 671, 2018.

12. slit for filling1.2 msuperconducting coilsB  2 T (at wall) focusing coilsproton detectorsvolume ~ 700 lUCNUCN detectorneutron absorberUCN = 103 – 104 cm-3 (PSI /FRM II): Nstored = 107 – 108Statistical accuracy:n ~ 0.1 s in 2-4 daysSystematics:Spin flips negligible (simulation)use different values Bmax to check expected EUCN independence ofProposed large volume magnetic storage experimentR. Picker et al., J. Res. NIST 110 (2005) 357S. Paul et al.PENeLOPE Magnetic storage of UCN & proton extraction Source not yet ready. Cryogenic experiment adds challenges. Symmetric trap.12

13. New GraviTrap13A. Serebrov et al.,(2017) arXiv:1712.05663 [nucl-ex]Preliminary result: t=881.5 +/- 0.7 +/- 0.6 s(between beam and previous bottle)

14. Other storage UCN lifetime effortsHope: vertical Halbach arrayTauspect: uses Aspect magnet and Mainz TRIGA sourceEzhov: Conical Halbach arrayBuilding larger trapProbeUCNt

15. Different approaches to loading trap loading methods15Gravitrap “Ice Cream Scoop”UCNt “Trap Door”Penelope “Ramping coils”TauSpect “Spin flipper” Ezhov “Elevator”Flipper onFlipper off

16. Measure using UCNs if =(from Bottle), then unaccounted systematic error in beam method > then possible new physics Requires absolute measurements of two quantitiesNumber of neutrons in the trapNumber of neutrons that decayed (measurement of charged particles)Goal for the LDRD ER (FY19-FY21):Demonstrate electron and neutron detector efficiencies can be measured to 0.1% levelPreliminary measurement of  to 10 s level to demonstrate experimental feasibilityUpdates:dPS scintillator Fermi potential measured to 169 +/- 2 neVCharacterized UCN losses on scintillator => f = (3.8 +/- 0.7) x 10-4 (similar to stainless steel )Procuring deuterated scintillators for the final experimentFinished design of prototype SiPM readout system UCNProbe experiment aims to measure  to ~1 s precision via the “beam” methodElectron measurementNeutronmeasurement

17. The UCN apparatus17D. Salvat, PRC 89, 052501 (2014)

18. Pairs of short-long storage times18 N: UCN countsM: Monitor counts

19. Flux Monitoring19All monitors are 10B/Zns scintillators *No 3HeUse ratio of monitors at different heights to correct for spectral effects

20. 20A typical lifetime run:

21. First science run published 2018Pattie et al., Science 360, p. 627 (2018).877.7 +/-0.7 +0.4-0.2 s

22. UCNt path forward22Only correction, for residual gas interactions, is smaller than statistical and systematic uncertainties: no extrapolation!All major systematics appear to scale with statisticsData on tape for 0.4 s total uncertainty, acquisition continuesGoal for UCNt is 0.2 s

23. Key strengths of UCNt experimentMagnetic+gravity trap: no material interactions during holding periodAsymmetric rippled trap: near- or superbarrier neutrons cleaned rapidlyVery long storage time: “other losses” have greater than three weeks characteristic time~1e-7 Torr vacuum~zero depolarizationNo neutron heating observed (yet!)In situ survivor detection: detector efficiency almost independent of phase space distributionActive time-resolved detection: neutrons can be detected as function of time and height, including heated or uncleaned neutrons23UCNt

24. And one major limitationUCNt experiment is, as far as we know, statistically limited: ultimate reach, 0.2 s total uncertainty24EffectUpper bound (s)DirectionMethod of evaluationDepolarization0.07+Varied external holding fieldMicrophonic heating0.24+Detector for heated neutronsInsufficient cleaning0.07+Detector for uncleaned neutronsDead time/pileup0.04±Known hardware dead timePhase space evolution0.10±Measured neutron arrival timeResidual gas interactions0.03±Measured gas cross sections and pressureBackground variations<0.01±Measured background as function of detector positionTotal0.28 (uncorrelated sum)Set by statistics of systematic measurements taken during production: these uncertainties will automatically reduce as statistics improveStatistical uncertainty on this data set (2016-2017) was 0.7 s, much larger (worse) than systematic uncertainties, and limits total uncertainty(Science 2018, arxiv https://arxiv.org/abs/1707.01817)

25. The UCNt experiment uses only a small fraction of the UCNs produced by the LANSCE source25UCN spectrum produced by LANL sourceUCN spectrum counted by UCNt (38 cm)UCN spectrum available to be counted by Tau2# of UCNSpectrum cut off by 180 neV potential of stainless steel UCN guides

26. Optimizing the trap depthUCNt has trap depth of 38 cm (~38 neV UCN energy)Arriving neutrons must be split between three destinations:Stored in trap for countingCounted in superbarrier normalization detectorLost over rim of trapCan vary trap depth to minimize overall statistical uncertainty as function of relative normalization detector efficiency and guide cutoff energyAnswer: ~120 neV (cm) trap optimizes use of UCNs26SSNiPDLC180 neV50% efficiency

27. Optimizing the trap depthUCNt has trap depth of 38 cm (~38 neV UCN energy)Arriving neutrons must be split between three destinations:Stored in trap for countingCounted in superbarrier normalization detectorLost over rim of trapCan vary trap depth to minimize overall statistical uncertainty as function of relative normalization detector efficiency and guide cutoff energyAnswer: ~120 neV (cm) trap optimizes use of UCNsBut requires superconducting magnets to achieve required >2 T field strength27SSNiPDLC180 neV50% efficiency

28. Monte Carlo simulation of trap loading in expanded geometriesAs a first look, we tried expanding a simplified trap in MCSimulation includes UCN source and transport all the way from production“Small” = UCNt“Wide” = 1.5x wider and longer“Tall” = 1.5x deeper“Big” = bothNote the conceptual Tau2 geometry would be another factor of 2 larger in all directions28Thanx to S. Clayton, E. M. Fries and V. SuPreliminary

29. Schedule, budget, reachApproximately 10x improved neutron utilization versus UCNtSo approximately 3x better sensitivity in same running period (nominally 4 years), or ~0.06 sLeading unresolved systematic uncertainties:residual gas upscattering will be improved by cold bore superconductorsDepolarization will be improved by stronger holding fieldDead time/pileup can be managed by detector design and insertion rateCost dominated by magnet: of order 1e7 $2920192028Pre-conceptual design Conceptual Design Design and construction Commissioning and DAQ20222025

30. Tau2 at SNS STS?Concept: attach a superconducting version of UCNt to a superfluid He UCN source coupled to STSAssume UCNt technique remains statistically limitedSet volume by matching filling time to system lifetime; ~200 sThis gives a factor of ~2 advantage in reach over the same experiment installed at LANSCE at same production density (LANSCE system lifetime ~= 60 s)So a precision of about 0.02 s in three years runningCombines knowledge base of Supersun, SNS-EDM, and UCNtAllows next order of magnitude search for BSM physics compatible with reach of PERC in lRequirements:About 20%-50% duty factor use of beam during run10 m x 10 m x 5 m experiment hall spaceFunctioning LHe external UCN source ala SupersunSubstantial cryogenic infrastructure for source and trap30Superfluid He UCN SourceSTS CN BeamSuperconducting trap~3 m x 4 m x 1.5 m deepReactors? ESS?

31. ConclusionsNeed to settle lifetime technique tension and develop the next generation experiment to go beyond 0.1 sI suggest that both appearance and disappearance experiments will be needed at next levelMultiple experiments underway using variety of techniques, with different systematicsTau2 (superconducting UCNt) would benefit greatly from closely coupled long lifetime UCN source at STS31