Searches for BSM Physics with the KATRIN Experiment

Snowmass 2021 | NF03 Workshop | September 17, 2020

K. Valerius for the KATRIN collaboration

• Active and sterile (sub-eV … eV … keV scale) • Right-handed currents and exotic weak interactions • Cosmic relic neutrinos • Lorentz invariance violation

KIT – The Research University in the Helmholtz Association www.kit.edu KATRIN in a nutshell

Primary science mission: measurement of effective mass through direct, kinematic method (precision β-decay spectroscopy of molecular ) Requirements: strong tritium source (~1011 β-decays/sec) at high purity & stability, high energy resolution (ΔE ~ 1 eV at E0 ~ 18.6 keV), low background rate (~100 mcps or lower)

katrin.kit.edu target m(�) sensitivity: 200 meV

Deliverable: precision β-decay spectrum measurement close to endpoint (typically E0 - 40…100 eV; extendable to ~1600 eV at reduced source strength during commissioning or to full phase space with detector upgrade)

2 Snowmass NF03, Sept. 17, 2020 First neutrino-mass result

Initial neutrino-mass data set (~ 4 weeks at reduced source strength) demonstrates excellent quality of measured spectra and model description

Improved upper limit: m(�) < 1.1 eV (90% CL) [PRL 123 (2019) 221802]

➜ Precision β-decay spectroscopy opens up sensitivity to look for a range of BSM phenomena through distortions of the spectral shape

3 Snowmass NF03, Sept. 17, 2020 SearcheV-scale sterile neutrino search for light sterile neutrinos

Mainz 95% C.L. RAA + GA 95% CL Troitsk 95% C.L. Neutrino-4 2 Prospect 95% C.L. KATRIN KSN1 95% C.L. (stat. and syst.) Initial 4-week data set: Demonstrate potential of KATRIN to DANSS 95% C.L. Projected KATRIN final sensitivity Stéréo 95% C.L. 95% C.L. (stat. and syst.) probe sterile neutrino hypothesis; complementarity with

3 High ∆= region: 10 short-baseline`L oscillation experiments üImprove exclusion with respect to DANSS, PROSPECT, Regionand SofTEREO high Δm2: 2 10 - üImproveExclude parameter space of exclusion with respectReactor Anomaly (RAA) to DANSS, PROSPECT, STÉRÉO ) 2 - Exclude large Δm2 solution preferred by reactor & gallium anomalies (eV 1 2 41 10 m 2 RegionLow ∆ =of`L lowregion: Δm : 100 - üImproveImprove limitsMAINZ byand MainzTROITSK andlimit Troitsk Preliminary - üNeutrino-4The NEUTRINO hint-4 regionhint at isthe edge of at the edgeexclusion limit of our 95% exclusion to be published soonPreliminary 10-1 -2 -1 0 10 10 10 Outlook: Large fraction of reactor/gallium anomalies and sin2(2 ) 26 Susanne Mertens ee Neutrino-4 hint will be probed with full KATRIN data set publication in preparation

4 see also Giunti++ 2020 Snowmass NF03, Sept. 17, 2020 keV-scale sterile neutrino search Search for more massive sterile neutrinos Proof of principle: Deep scan (1.6 keV below E0) with low-activity commissioning data üexcellent agreement of model and data (p-value = 0.6) 2 -3 Proof of principle: Deep scan (1.6 keV) üsensitivity to sin \ < 10 @ m4 = 0.4 keV

ROI with low-activity commissioning data - publication in U preparation Future: Novel multi-pixel Silicon Drift Detector array (TRISTAN) ! Excellent agreement of model and data 2 -3 ühigh-statistics search sensitivity illustration Sensitivity to sin � = 10 at m4 = 0.4 keV ütarget sensitivity of sin2 \ < 10-6 Future perspectives: Novel multi-pixel Silicon Drift Detector array (TRISTAN) High-statistics search, coverage of entire spectrum 2 -6 Target sensitivity of sin � < 10

Mertens et al, JCAP 1502 (2015) T. Houdy et al 29 goal: operation in Susanne MertensMertens et al, J. Phys. G46 (2019) KATRIN by 2025

5 ➜ see separate LOI (Mertens et al.) in NF02 “Sterile Neutrinos” for details! Snowmass NF03, Sept. 17, 2020 3

Exotic weak interactions

What if … … weak interactions were hiding a left-right symmetric sector? … additional, very light bosons might exist? FIG. 2. Parameters for the spectrum approximation of Eq. (A15) for various coupling structures. spectrum with vector or pseudoscalar boson X 5 Imprint of right-handed currents in tritium 10 E. Vector bosons emitted from neutrinos and 102 2 β-spectrum difficult to observe unless E0 X /g ) ) e

X 1 ¯ ⌫ 10 As mentioned above, a Z0 that is coupled to the classi- +

fixed externally + e e ¯ ⌫ cally conserved electron-number current j has a quali- + Le + e ➜ e.g. Severijns++ 2006; Bonn++ 2011 + 4 + He 10 tatively di↵erent behavior than a Z0 that only couples to + 3 He electrons or neutrinos. With 3 H

3 7 A: ⌫¯ ⌫X Picture could change in presence of ( 10 5 H B: ¯e eX ↵ ↵ ↵ 3 5

( = g j Z0 = g (¯⌫ P ⌫ +¯e e) Z0 (9) µ Le Le ↵ Le e L e ↵ C: ⌫¯µP ⌫X 10 L L sterile neutrinos and RH/LH interference 10 µ D: ¯eµeX one finds that the amplitude for Z0 emission is approx- E: jµ X ➜ see Barry, Heek & Rodejohann 1404.5955; Le µ 13 imately constant in the limit of small mZ0 , contrary to 10 ↵ Ludl & Rodejohann 1603.08690; 3 2 1 0 1 2 3 4 the two cases discussed above, because j is a classically 10 10 10 10 10 10 10 10 Le Steinbrink++ 1703.07667 boson mass mX (eV) conserved (non-anomalous) current. Fitting to the ap- proximate spectrum gives n 2.2 and K 5 10 24g2 Le ➜ see Arcadi++3 1811.035303 + ' ' ⇥ FIG. 3. Ratio of the decay width ( H He + e + for mZ0 eV, see Fig. 2. Similar to the scalar-neutrino ! 2 ⇠ ⌫¯e + X) (divided by the relevant coupling constant gX )with case there is still a logarithmic dependence on mZ0 that NB: Wider phase-space coverage of the β-spectrumrespect beyond to the SM m( width�) search as a function window of the new boson’swill is well known from Bremsstrahlung in QED. For larger mass mX .Thedi↵erent curves are for di↵erent underlying mZ0 & 10 keV, the behavior becomes identical to that of broaden the reach of BSM physics opportunities ➜ couplingextra structures,incentive see text for for details.detector upgradea Z0 coupled to neutrinos.

D. Vector bosons emitted from electrons 6 Snowmass NF03, Sept. 17,III. 2020 eV-SCALE LIGHT BOSONS IN THE CURRENT KATRIN SETUP In a similar vein, we assume the following Lagrangian for the coupling of a Z0 with electrons: In order to compare the theoretical prediction for the

µ light bosons to upcoming data taken by the Katrin ex- =¯e (g P + g P )eZ0 . (8) L eL L eR R µ periment in the current setting, we will apply several modifications to the analytical form given in Eq. (5) to In principle one can imagine di↵erent couplings for the account for experimental characteristics. We will refer chiralities, g = g , but we will take them to be equal eL eR to this as the experimental spectrum. In the following, for simplicity, 6 g = g g , corresponding to a eL eR eV we will introduce the modifications to the spectrum be- vector-like coupling. ⌘ fore stating the sensitivity of Katrin to constrain the For small m , the spectrum is identical to that of Z0 emission of a light boson in tritium b-decay. For better Eq. (7), i.e. with parameters n 4 and K 6.7 readability and overview, we summarize the possible pro- 10 22g2 (keV/m )2, see Fig. 2.' This is not' surpris-⇥ eV Z0 duction mechanisms in Tab. I which we will later refer to ing upon noting that the electron-number current j↵ = Le ↵ ↵ when stating the results. ⌫¯e PL⌫e +¯e e is classically conserved, so a light Z0 coupled to it would not lead to a 1/m2 divergence (see Z0 Sec. IIE). This merely means that the 1/m2 divergence Z0 A. Experimental characteristics of thee ¯Z/ e coupling is exactly canceled by the 1/m2 0 Z0 divergence of the⌫ ¯eZ/0PL⌫e coupling. Since Katrin employs gaseous molecular tritium (T2), There are of course di↵erences between the Z0 coupling we have to use the molecular masses to calculate the end- to electrons and neutrinos, at least for larger Z0 masses 2 point: where the 1/mZ behavior is less dramatic. From Fig. 2 0 2 2 2 we can see that the two couplings become distinguishable m (mT 3He+ + m⌫ + mX ) + m Emax0 = T2 e , (10) for boson masses above keV. e 2mT2 194 7. Physics beyond the neutrino mass

as combinations of both, range from 1 to 106 [RW04, LVV08, F¨as+13]. In the scope of this thesis, also CnB-adopted MTDs are tested for their e↵ect on the sensitivity on ÷. However, the increase in sensitivity is found to be only minor. No order of magnitude increase can be achieved, even with a “CnB-focused” MTD. The reason is the dominant e↵ect of the background rate, which generally limits the Katrin sensitivity for the CnB(also see ref. [Hei15]). For larger neutrino masses, Search for capture of relic neutrinosthe statistical sensitivity is found to improve marginally by about a factor 1.4. While Katrin most likely will not be able to detect the CnB, it can rule out spe- culative models about the origin of the GZK cut-o↵ [HM05] and improve existing b-decay limits on ÷ by at least one order of magnitude. 7.3.Possibility Relic neutrinos of relic neutrino capture in KATRIN’s189 gaseous T2 source discussed in several Note that only the statistical sensitivity is investigated in this thesis. In order to works (e.g., Kaboth et al. 2010, Fässler et al. 2013,derive Heizmann an upper limit from 2018) the neutrino mass data, also systematic e↵ects need to be studied very carefully. E E (eV) 0 20 15 10 5 0 5 1016 17 10 Robertson et al. [Rob+91] std di↵ spec 1014 C⌫B di↵ spec Hwang et al. [HM05] η 12 1 18 10 10 relic ν capture peak Lobashev et al. [Lob+99b] 10 exaggerated for 10 this work, PhD Heizmann [2018] (eVs)

E Kaboth et al. [KFM10] illustration! 8 d 19 10 / 10 F¨assler et al. [F¨as+13] tritium β-spectrum 106 rate d 4 ↵ 20 10 10

di Lazauskas et al. [LVV08] localoverdensity 102 Ringwald and extrapolation from [RW04] Wong [RW04] 21 0 10 10 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 separationstd int spec 2mν m⌫ (eV) 101 C⌫B int spec combined int spec Figure 7.18.: Statistical sensitivity for relic neutrinos. Shown is the statisti- cal sensitivity of Katrin for the detection of the CnB (solid blue). “Target”0 mass not likely to support detection, but could constrain local relic overdensities. 10 The values obtained by Robertson et al. [Rob+91] and Lobashev et (cps)

˙ al. [Lob+99b] present upper limits obtained from b-decay spectroscopy. N The other values for the CnB overdensity are predictions from various 1 7 10 models. Snowmass NF03, Sept. 17, 2020 int rate

2 10

3 10 20 15 10 5 0 5 qU E (V) 0

Figure 7.15.: Basic spectrum - Upper panel: the relic neutrino capture signal (red) 194

is modelled as a Gaussian peak at E0 + m⌫ (see eq. (7.35)). Lower panel: the peak manifests as a shoulder in the spectrum as measured 12 by Katrin. Spectra for m⌫ =1eVand ÷ =10 , no background rate included.

Doppler broadening, the width of the CnB-Gaussian only influences the di↵erential spectrum and its integrability. Due to the signal being a normalised Gaussian, the integration will yield the same result for the count rate even if ‡CnB di↵ers. Therefore, ‡CnB = 10 meV was chosen because it increases the numerical stability of the integration. The capture (and therefore decay) rate for the CnB induced b-decay

189 How to think about Lorentz violation?

Rotations: - anisotropies ⇔ preferred directions → parametrization by Probing Lorentz invariance with KATRIN (more generally: 3-tensors) “Countershaded” LIV in neutrino sector: Oscillations and direct kinematics can probe complementary quantities (oscillation-free parameters accessible in endpoint experiments) Lorentz transformation (rotations+boosts):Immediate consequences of this idea: Standard Modelexperiments Extension with intrinsic (SME), direction based desirable on effective field theory + background fields: - generalized rotations in 4-dim MinkowskiAnisotropic effects space could be observable at KATRIN (“intrinsic direction” via acceptance cone) → 4-dim anisotropies ⇔ preferred 4-directionsKATRIN example: direction of acceptance cone → parametrization by Possible impact on tritium β-spectrum: - Global shift of endpoint E0 more generally: - Sidereal oscillation of E0: can be looked 4-tensors for in repeated spectrum scans (typ. scan sequence ~2 hrs) illustrationsLehnertbyR.

➜ See, e.g.: Colladay & Kostelecký 1998; Díaz, Kostelecký & Lehnert 1305.4636 analysis in progress ➜ Presentation by J. Díaz in this workshop

8 Snowmass NF03, Sept. 17, 2020 Summary

Direct kinematics of weak decays offer intriguing opportunities for BSM physics searches!

KATRIN experiment (tritium β-decay): First data release (2019) allowed for new neutrino-mass upper limit and demonstration of potential for sterile neutrino search. Studies of more BSM cases ongoing (e.g. relic neutrinos, right-handed currents & light extra bosons, Lorentz invariance violation).

… further ideas & proposals welcome! :) https://physics.anu.edu.au

Data-taking in progress (goal: 1000 measurement days or ~5 calendar years in total), plans for subsequent detector upgrade will further boost BSM search at KATRIN.

9 Snowmass NF03, Sept. 17, 2020 References*

- KATRIN collaboration, Prospects for keV sterile neutrino searches with KATRIN, LOI submitted to Snowmass 2021 (NF02) - KATRIN collaboration, Searches for BSM physics with the KATRIN experiment, LOI submitted to Snowmass 2021 (NF03) - J. A. Formaggio & J. Barrett, Resolving the reactor neutrino anomaly with the KATRIN neutrino experiment, Phys. Lett. B 706 (2011) 68 - A. Sejersen Riis & S. Hannestad, Detecting sterile neutrinos with KATRIN like experiments, JCAP 02(2011)011 - A. Esmaili & O. L. G. Peres, KATRIN sensitivity to sterile neutrino mass in the shadow of lightest neutrino mass, PRD 85 (2012) 117301 - C. Giunti, Y. F. Li & Y. Y. Zhang, KATRIN bound on 3+1 active-sterile neutrino mixing and the reactor antineutrino anomaly, JHEP 05(2020)061 - KATRIN collaboration, Search for eV sterilen neutrinos with KATRIN, in preparation - J. Stephenson et al., Tritium , neutrino mass matrices, and interactions beyond the standard model, Phys. Rev. D 62 (2000) 093013 - N. Severijns et al., Tests of the standard electroweak model in nuclear beta decay, Rev. Mod. Phys. 78 (2006) 991 - J. Bonn et al., The KATRIN sensitivity to the neutrino mass and to right-handed currents in beta decay, Phys. Lett. B 703 (2011) 310

10 *) no attempt at completeness; rather a starting point for further reading Snowmass NF03, Sept. 17, 2020 References*

- J. Barry, J. Heeck & W. Rodejohann, Sterile neutrinos and right-handed currents in KATRIN, JHEP 07(2014)081 - P. O. Ludl & W. Rodejohann, Direct neutrino mass experiments and exotic charged current interactions, JHEP 06(2016)040 - N. M. N. Steinbrink et al., Statistical sensitivity on right-handed currents in presence of eV scale sterile neutrinos with KATRIN, JCAP 06(2017)015 - G. Arcadi et al., Tritium beta decay with additional emission of new light bosons, JHEP 01(2019)206 - F. Heizmann, PhD thesis, Karlsruhe Institute of Technology, 2018 - A. Kaboth, J. A. Formaggio & B. Monreal, Sensitivity of neutrino mass experiments to the cosmic neutrino background, PRD 82 (2010) 062001 - A. Fässler, R. Hodak, S. Kovalenko, F. Simkovic, Tritium and rhenium as a probe of cosmic neutrino background, J. Phys. G 38 (2011) 075202 - A. Fässler, R. Hodak, S. Kovalenko, F. Simkovic, Search for the Cosmic Neutrino Background with KATRIN, Rom. J. Phys. 58 (2013) 1221, arXiv:1304.5632 - D. Colladay & V. A. Kostelecký, Lorentz-violating extension of the standard model, PRD 58 (1998) 116002 - J. S. Díaz, V. A. Kostelecký & R. Lehnert, Relativity violations and beta decay, PRD 88 (2013) 071902

11 *) no attempt at completeness; rather a starting point for further reading Snowmass NF03, Sept. 17, 2020