The search for an electric dipole moment of the neutron at PSI Jochen Krempel ETH Zürich 1Jochen Krempel FFK Budapest On behalf of the nEDM collaboration at PSI

13 Institutions 13 Institutions 7 Countries 7 Countries 48 Members 48 Members 11 PhD students 11 PhD students The collaboration FFK Budapest Jochen Krempel 2

Outline Short Motivation Principles of nEDM measurement UCN Source Experiment & upgrades Data blinding Performance Outlook 3 Jochen Krempel FFK Budapest

4 Jochen Krempel Baryon Asymmetry of the Universe Sakharov 1967:CP-violation [JETP Lett. 5 (1967) 24] We live in a matter dominated universe Observed (WMAP) Expected (SM) [Riotto et al Ann.Rev.Nucl.Part.Sci. 49 ] [E. Komatsu et al ApJS 192]

FFK Budapest 5 Jochen Krempel EDM violates CP A permanent EDM of any fundamental particle violates CP PCT Having T-violation And assuming CPT invariance  CP must be violated Note: magnetic field is a pseudo vector (as spin)   does NOT violate CP Magnetic field or  is not necessary for CP violation. But a nice tool to control the spin. Different coupling to external fields  Different particle However, a neutron is different from anti-neutron. Use alternative way

FFK Budapest 6 Jochen Krempel History of nEDM Lamoreaux, Golub in “Lepton dipole moments“ Roberts, Marciano (eds.) 2010 World Scientific Publishing current best nEDM limit: d n < 2.9· e cm (90% C.L.) C.A.Baker et al., PRL 97, (2006) Sensitivity goals at PSI Intermediate: d n < 5 x e cm (95% C.L.) Final: d n < 5 x e cm (95% C.L.) A. Serebrov ILL – Gatchina TUM ILL RAL ORNL TRIUMF F. Piegsa ESS

Principles of nEDM measurement How to measure precession? pickup coil  not enough signal  Measure polarization of neutrons 2 sequential measurements E,B (anti-)parallel  Measure precession frequency twice 7 Jochen Krempel FFK Budapest

Principles II: Ramsey technique 8 Jochen Krempel FFK Budapest Spin up neutrons  /2 flip pulse Free precession  /2 flip pulse External clock Neutron clock  

Sensitivity Sqrt(N) and T compete (repetition rate). T is more important  Last Decades: Trap better than beam Trap walls limit E Storage time   Long T = low N Depolarization rate t 2  Long T = low  FFK Budapest Jochen Krempel 9

FFK Budapest 10 Jochen Krempel The apparatus ~1m diameter x 1.5m length 50cm diameter x 12cm height

ultracold neutrons - UCN UCN < 300neV ~ 8m/s ~ 3 mK > 50 nm ! thermal (25 meV) 2200 m/s 300 K 0.18 nm cold (5 meV) 1000 m/s 60 K 0.4 nm UCN ( 50 nm hence the name Neutrons with E kin < 300 neV are storeable e.g. air molecules at 20 ºC: ~400 m/s E. Fermi, 1946, Ya. B. Zeldovich Sov. Phys. JETP 9, 1389 (1959) vn≤vCvn≤vC vn>vCvn>vC vnvn surface description via a material optical potential Density is low  no particle-particle interaction Elastic collisions with walls  no thermalization FFK BudapestJochen Krempel11

Ultracold neutrons (UCN) can be stored - storage properties are material dependent - Ni, Ni 58, Be, DLC, steel Gravity Material Magnet for polarized UCN 60 neV T -1 magnetic V m = -  B V g = m n gh gravitation 100 neV m -1 < 250neV 300neV = 5 T one polarization 300neV = 3m FFK BudapestJochen Krempel12

Neutron production via proton spallation on lead UCN Source Proton Accelerator 590 MeV Cyclotron 2.2 mA beam current nEDM 2 experimental areas / 3 beamlines kicker to UCN FFK BudapestJochen Krempel13

Sketch of the PSI UCN source pulsed 1.3 MW p-beam 590 MeV, 2.2 mA, 1% duty cycle spallation target (Pb/Zr) (~ 8 neutrons/proton) heavy water moderator → thermal neutrons 3.6m 3 D 2 O cold UCN-converter ~30 dm 3 solid D 2 at 5 K tank 7 m DLC coated UCN storage vessel height 2.5 m, ~ 2 m 3 UCN guides towards experimental areas 8.6m(S) / 6.9m(W) SV-shutter cryo-pump FFK BudapestJochen Krempel14

Measurement in area West with detector at beam-port pilot pulse filling storage vessel emptying storage vessel up to 2x10 7 UCN /pulse closing shutter typical exp filling time ~30s

UCN-Source progress in 2015 Melting + re-freezing (4h every 4 days) Increase duty cycle (average current) FFK Budapest Jochen Krempel 16

Magnetic field stability E polarity change every ~5 hours. Keep B-field stable Passive mu-Metal shield 4 layers ~ a few * 1000 suppression Active Coils (SFC) Measure fluctuations Hg co-magnetometer Cs magnetometers FFK Budapest Jochen Krempel 17

FFK Budapest 18 Jochen Krempel The apparatus - magnetic shielding 1uT

SFC - part I 6 rectangular coils 6m * 8m, 20 windings 10 x 3 axis Fluxgate sensors earth field compensation / sultan field compensation gradient compensation not yet fully possible (lack of number of coils) static stabilisation over time Pair wise using inverse matrix  Sensor selection FFK Budapest 19 Jochen Krempel

SFC - part II >5x shielding s of real perturbations FFK Budapest 20 Afach et al., J. Appl. Phys. 116, (2014) Jochen Krempel

FFK Budapest 21 Jochen Krempel The apparatus – Hg co magnetometer

Hg co-magnetometer FFK Budapest Jochen Krempel 22 UCN Hg Centre of mass offset: ~2mm mismatch in case of dBz / dz 8Hz

Cs-magnetometer array Pump and probe at 45°  about 20ms Extra cell, special coating. High statistics High bandwidth (  L =3.5kHz) Array of 16 sensors HV-versions included  dB / dz information Vector-information (prototype) FFK Budapest Jochen Krempel 23

FFK Budapest 24 Jochen Krempel The apparatus – Hg co magnetometer 132kV / 12cm

Improve Depolarization rate FFK Budapest Jochen Krempel 25 Depolarization rate t 2  Long T = low  33 Trimcoils to tune the field. But how? Not published yet. Use Cesium-array, do some fancy things, get Trimcoil settings, Enjoy!

R-curve analysis Scan actively dB/dz by applying TrimCoils FFK Budapest Jochen Krempel 26 B0 up B0 down Prior to Cs-magnetometer array the only way to determine 0-gradient.

Gyro magnetic ratio S. Afach et al., PLB 739 (2014) FFK Budapest Jochen Krempel 27

B 0 up λ f Earth Earth rotation correction FFK Budapest Jochen Krempel 28

Spin-echo spectroscopy A spin-echo recovers energy dependent dephasing for T = 2t 1 in a magnetic field with negative vertical gradient. gzgz FFK Budapest Jochen Krempel 29

Estimation of UCN energy spectrum Access to vertical gradient (absolute value) Spin-echo results S.Afach et al., accepted by PRL( 02. October 2015), arXiv: arXiv: FFK Budapest Jochen Krempel 30

R-curve revisited Polarization and R-curve using UCN spin-echo spectrum match data taken at T=180s Gravitational depolarization of ultracold neutrons: Comparison with data S. Afach et al. Phys. Rev. D 92, – Published 22 September FFK Budapest Jochen Krempel 31

Systematic effects FFK Budapest Jochen Krempel CsM array High precision field mapping

Why blinding Avoid psychological bias during data analysis Experimenter’s bias is defined as the unintended influence on a measurement towards prior results or theoretical expectations. Which cut to apply When to stop analysis/searching for bugs "We're more than one sigma from zero; we have to look at it some more, because we must be doing something wrong..." Outside reputation  some secrecy is necessary, simply trusting everybody is not enough. Do NOT protect against: Criminal energy, e.g. somebody installing spyware on DAQ computer JR Klein A Roodman BLIND ANALYSIS IN NUCLEAR AND PARTICLE PHYSICS Annual Review of Nuclear and Particle Science 2005 Vol. 55: DOI: /annurev.nucl arXiv:physics/ v1 Blind Analysis in Particle Physics Aaron Roodman FFK Budapest Jochen Krempel 33 Blind analysis: Hide results to seek the truth R. MacCoun, S. Perlmutter Nature 526, 187–189 (08 October 2015) doi: /526187a

Blinding offset Many experiments have blinding factor nEDM measures value + uncertainty From that upper limit is derived We blind the raw data by shifting the value  Blinding offset FFK Budapest Jochen Krempel 34

Why is blinding so difficult Easy concept: Modify frequency of neutron spin flip (synchronous to HV-polarity) Must not influence measurement Fear of systematic effects is large Independently advised by review committee Write down a false frequency Immediately visible in co-magnetometers Fake the reading of a co-magnetometer Impossible to do consistently for Hg and Cs  immediately visible 35 Jochen Krempel FFK Budapest

How do we blind I 36 Jochen Krempel FFK Budapest i /

How de we blind II FFK Budapest Jochen Krempel 37 i /

How do we blind III FFK Budapest Jochen Krempel 38 Known (because chosen) phase/frequency of spin flip E-field sign of B0 direction, sign of Detector configuration sign of SpinFlipper 1 N and  are fitted per run (~1day)

Secrecy of data files Raw data (secret) + blinded data (public) Backup scheme: Encrypt secret data with RSA (asymmetric) Use public key for encryption Private key is kept in sealed envelope Do not trust encryption -> keep additional copy (NOT encrypted) at hidden place 39 Jochen Krempel FFK Budapest Slow control Main DAQ Neutron Intern PSI PSI -AFS France PSI tape GRIS Poland

Performance FFK Budapest Jochen Krempel 40

Performance FFK Budapest Jochen Krempel 41

Pseudo magnetic field from a spin-dependent exotic force Axion Window where UCN are a sensitive probe “Constraining interactions mediated by axion- like particles with ultracold neutrons” Physics Letters B Volume 745, 18 May 2015, Pages 58– /j.physletb Poster by Beatrice Franke

Light axions – axion wind FFK Budapest Jochen Krempel 43 Y. V. Stadnik and V. V. Flambaum PHYSICAL REVIEW D 89, (2014)

The future - n2EDM Double Chamber E-B parallel antiparallel at same time Horizontal guide Higher N due to better use of spectrum Higher E More Cs magnetometers (vector information) FFK Budapest Jochen Krempel 44

Summary The collaboration produced many papers during the last year. Since August 2015 very stable running with best daily sensitivity world wide. Systematic effects studied in detail FFK Budapest Jochen Krempel 45

Acknowledgements Special thanks for slides to Philipp Schmidt-Wellenburg Bernhard Lauss Martin Fertl Georg Bison Michał Rawlik Dieter Ries The whole collaboration FFK Budapest Jochen Krempel 46 Thank you for your attention

Backup Slides FFK Budapest Jochen Krempel 47

Simultaneous spin detection ~20% increase in sensitivity (for 2014) sequential A device for simultaneous spin analysis of ultracold neutrons EPJ A Accepted 12 October 2015

Cs OPM Servo FFK Budapest Jochen Krempel 49

UCN Production – “Conventional” 300 K 30 K  0 [cm -2 s -1 ] cm -3  (UCN)=70x  0 [cm -2 s -1 ] cm -3 Using an adequate moderator and extracting the low-energy tail of the Maxwell-Boltzmann distribution: (first UCN observed in the Russia) 00 Moderator Experiment liquid deuterium FFK BudapestJochen Krempel50

4 He F. Atchison et al., PRL99(2007) D2D2 C.A. Baker et al., PLA308(2003)67 dispersion relation How can we obtain more UCN ? Superthermal UCN production: Golub & Pendlebury FFK BudapestJochen Krempel51

Hg-Level Scheme FFK Budapest Jochen Krempel 52

Allan Deviation with VCsM FFK Budapest Jochen Krempel 53

Secrecy of blinding offset Blinding program needs to know it Randomly generate offset, store it with (2 nd ) public key private key is injected into blinding program during compile process Blinding offset is never written in clear text (nobody can accidentally read it) Blinding code can be modified at any time and recompiled - one trustful person necessary to keep private key 54 Jochen Krempel FFK Budapest

Secrecy implementation Step 0) Get their public key openssl rsa -in id_rsa -pubout -outform pem > id_rsa.pub.pem Step 1) Generate a 256 bit (32 byte) random key openssl rand -base64 32 > key.bin Step 2) Encrypt the key (asymmetrically) openssl rsautl -encrypt -inkey id_rsa.pub.pem -pubin -in key.bin -out key.bin.enc Step 3) Actually Encrypt our large file (symmetrically) openssl enc -aes-256-cbc -salt -in SECRET_FILE -out SECRET_FILE.enc -pass file:./key.bin Step 4) Put key.bin.enc and SECRET_FILE.enc to Archive Decryption (in 2 years) openssl rsautl -decrypt -inkey id_rsa.pem -in key.bin.enc -out key.bin openssl enc -d -aes-256-cbc -in SECRET_FILE.enc -out SECRET_FILE -pass file:./key.bin 55 Jochen Krempel FFK Budapest

BACKUP RSA algorithm Calculate private key Chose 2 large prime numbers p = 11 and q = 13 RSA-Modulus is N = p * q = 143 Euler's totient function phi(N) = phi(143) = (p-1)(q-1) = 120 Chose e coprime to 120. We chose e = 23. e = 23 and N = 143 are the public key. Calculate public key (Inverse of e): e * d + k * phi(N) = 1 = ggT(e,phi(N)) (Greates Common Divisor) 23 * d + k * 120 = 1 = ggT(23,120). Extended Euclidean algorithm  d=47 and k=-9 23 * 47 + (-9 )* 120 = 1 d=47 is the secret key, k can be disposed encryption m=7 with public key (e,N) (m < N) c = m ^ e mod N 7 ^ 23 mod 143 = 2 decrypt m = c ^ d mod N 2 ^ 47 mod 143 = 7 56 Jochen Krempel FFK Budapest