The Nature and Magnitude of Neutrino Mass

Kaushik Roy

Stony Brook University

September 14 2015

Outline

I What we knowOur current knowledge regarding neutrino masses.

I What we do not knowOpen questions related to neutrino masses.

I Why we need to knowImplications on the Standard Model of Particle physics.

I What is being done to know

1. Direct mass measurements - The KATRIN experiment2. Neutrinoless double beta decays - The EXO experiments.

I Summary

What we know

I Neutrino flavor eigenstates are linear superpositions ofnon-degenerate mass eigenstates.

I Neutrinos and massive and extremely light. Courtesy datafrom solar (Homestake, SNO), atmospheric(Super-Kamiokande), reactor (KamLAND, Daya Bay) andbeam (MINOS, K2K, Super-K, T2K) neutrinos oscillationexperiments.

I Oscillation data sensitive to squared mass differences, ∆m2

and mixing matrix parameters, Uij .

Oscillation parameters from global analysis after Neutrino 14 [1] conference.

What we do not know

I The absolute neutrino masses.

I The nature of neutrino masses - Dirac or Majorana.

I The mechanism for small masses.

I The neutrino mass hierarchy - NH, IH or QD.

Neutrino masses as a function of the lightest mass.

Implications of neutrino mass on the Standard Model

I Neutrino masses ⇒ Standard Model is incomplete.Augmentation of particle content required.

I SM requires RH neutrinos and unnaturally smallHiggs-Yukawa couplings to incorporate Dirac neutrino masses.

I Motivates theories beyond the Standard Model - extradimensions, supersymmetry, GUTs, see-saw mechanisms etc.

Two ambiguities to be addressed in this talk.

1. What is the absolute mass scale?

2. What is the nature of neutrino mass? Dirac or Majorana?

Attempts at measuring the absolute mass scale

Three kinds of experiments are sensitive to the absolute masses ofneutrinos.

I Cosmology: Sensitive to the sum of the three neutrino masseigenstates, for example,

∑m(νi ) < 0.66eV (from the Planck

Collaboration) [2]

I Neutrinoless double beta decay (0νββ): Forbidden in the SMand depends on Majorana nature of neutrinos.

Γ0νββ ∝ |∑

U2eim(νi )|2 = m2

ee

I Direct mass measurements: Purely based on kinematics(energy-momentum conservation) without furtherassumptions.

1. Time of flight method.2. Precision investigation of weak decays - most sensitive and

model-independent method to determine neutrino mass.

I will discuss the second method here.

Principle

I Electron spectrum in nuclear β-decay

N (A,Z )→ N (A,Z + 1) + e− + ν̄e

is investigated near the end-point, Te,max = Qβ for m(νe) = 0and Te,max = Qβ −m(νe) for m(νe) 6= 0

I Neutrinos are non-relativistic at the end-point.

I Neutrino mass is calculated from E 2 = p2 + m2 relation.

The problem in measured the end-point - very few events near theend-point. An order of magnitude calculation [6] for the relativenumber of events occurring in an energy interval ∆T below theend-point is

n(∆T )

n∝(

∆T

Qβ

)3

I Desirable to have Qβ as low as possible. Qβ=18.6 keV forTritium is one of the lowest among all β-emitters.

CharacterizationI Information about m(νe) found by the distortion of Kurie plot

near end-point, where

K (T ) =

√√√√ dΓ/dTG2Fm

5e

2π2 cos2 θC |M|2F (Z ,Ze)Eepe

=

[(Qβ − T )

√(Qβ − T )2 −m2(νe)

]1/2

The KATRIN experiment

The KATRIN experiment

I The Karlsruhe Tritium Neutrino (KATRIN) experiment is anext-gen direct neutrino mass experiment.

I Collaboration with groups from institutions all over the world.

I The experiment is housed at Tritium Laboratory, Karlsruhe(TLK) at KIT’s North Site.

I Scientific goals

1. Designed to improve the m(νe) sensitivity from present 2 eV to200 meV at 90% CL [m(νe) < 2.3eV , Mainz andm(νe) < 2.5eV , Troitsk].

2. Discovery potential of a neutrino mass of 0.35 eV at 5σsignificance and 0.30 eV at 3σ significance.

3. Analyzing interval 30 eV below end-point.

Components

The KATRIN has the following components:

1. Windowless Gaseous Tritium Source (WGTS)

2. Transport Section having differential (DPS) and cryogenic(CPS) pumping sections

3. Pre-spectrometer, main spectrometer

4. Detector.

The most important parts are the energy filters in the pre- andmain spectrometers.

MAC-E-Filter

I Magnetic Adiabatic Collimation with an electrostatic filter(MAC-E) combines high luminosity at low background andhigh energy resolution.

I Two superconducting solenoids produce a magnetic guidingfield.

I β-electrons from source magnetically guided in a cyclotronmotion into the forward hemisphere → accepted solid angle ofnearly 2π.

I B decreases smoothly by several orders of magnitude keepingthe magnetic moment

µ =E⊥B

= constant

I Broad beam of electrons flying almost parallel to the magneticfield lines.

I Energetically analyzed by applying an electrostatic barrier withcylindrical electrodes. Relative sharpness given by

∆E

E=

Bmin

Bmax

Attempts at determining the nature of neutrino massNeutrinoless double beta decay

I Most promising way to find Majorana nature of neutrinos.

I Majorana imposition breaks matter-antimatter distinction i.eν = νC = CνT [Majorana condition]

I Possible for neutrino because of electrical charge neutrality.

N (A,Z )→ N (A,Z + 2) + 2e−

Forbidden in the SM due to lepton number violation.

I ββ decay observed only if the corresponding β-decay isenergetically forbidden or strongly suppressed due to spin.

I The ννββ and 0νββ decays are distinguished by the β-energyspectra- the former being a continuous distribution and thelatter a line at Q-value.

Why neutrinos should be massive and Majorana for 0νββ

I In the SM, 0νββ is not allowed due to particle-antiparticlemismatch and helicity mismatch.

I For Majorana neutrinos, νe = ν̄e . Upper leptonic vertex canemit a neutrino with –ve helicity with relative amplitudem(νe)/Ee which is absorbed by the lower leptonic vetex withrelative amplitude unity (+ve helicity).

The EXO experiment

The EXO experiment

I Looking for 0νββ in Xe-136. It has two parts

1. EXO-200: A 200-kg prototype currently operating at WasteIsolation Pilot Plant (WIPP).

2. nEXO (next EXO): A tonne-scale experiment using Xe-136.Extensive R and D going on to design the detector and developBa-tagging techniques.

Mode of operation

I Detector has a back to back Time Projection Chamber (TPC)with high efficiency UV light detection.

I When a particle deposits energy in the liquid xenon, it ionizesthe xenon atoms, knocking electrons off.

I Electrons liberated during ionization drift towards wire grids(anode).

I Recombination of Xe-atoms with e− and consequentde-excitation to ground states releases UV light.

I Photons are recorded using a multidiode array of large areaavalanche photodiodes.

I The time between the light signal (nearly instantaneously) andthe ionization signal (takes microsecond due to drift) allows toreconstruct the full 3D location of the event when combinedwith the 2D position from the wire grids. [5]

Experimental results

I First ννββ decay observed in Xe-136 using EXO-2011.

I No signature of 0νββ observed yet.

I Date analysis gives T1/2 > 1.6× 1025 yrs at 90 % CL for0νββ decay of Xe-136.

I The m(νe) mass limit obtained using nuclear shell model,interacting boson model, RQRPA and QRPA-2 is 140-380meV. [5]

Summary

Present mass regimes of the direct mass measurements and 0νββ-decay experiments [3] [6]

I KATRIN will investigate the entire parameter space of QDneutrino masses.

I The ultimate challenge in β-spectroscopy is to push theν-mass sensitivity to the lowest values possible (≈ 4× 10−3

eV).

I The EXO collaboration is planning a detector using 5 tonnesof Xe, that would allow a very substantial advance in thesensitivity to a Majorana neutrino mass.Only by comparing high precision results from direct massmeasurements and 0νββ searches, we can obtain thecomplete picture of ν-masses to assess the unique role ofneutrinos in particle physics and cosmology. [4]

Bibliography

1. D. V. Forero, M. Tortola, J. W. F. Valle, “Neutrino oscillationsrefitted”, Phys. Rev. D 90, 093006, 2014.2. P.A.R. Ade et al., [Planck Collab.], arXiv:1303.5076, Astronomyand Astrophysics, Volume 571, November 2014.3. K. Zuber, “Long term prospects for double beta decay”,arXiv:1002.4313 [nucl-ex]4. G. Drexlin, V. Hannen, S. Mertens, C. Weinheimer, “Currentdirect neutrino mass experiments, arXiv:1307.0101[physics.ins-det].5. Giorgio Gratta and David Sinclair, Present Status and FuturePerspectives for the EXO-200 Experiment, Advances in HighEnergy Physics, vol. 2013, 2013.6. C. Giunti and C. W. Kim, “Fundamentals of neutrino physicsand astrophysics”, OUP, 2007.

“Without experimentalists, theorists tend to drift”....“Without theorists, experimentalists tend to falter”

T.D Lee

Thank you!!!