Cosmological neutrinos

Elisa BernardiniDeutsches Elektronen-Synchrotron DESY (Zeuthen)

The neutrino sector

Lectures overview★ Lecture 1: A brief introduction / Neutrino masses / Supernova Neutrinos

★ Lecture 2: Neutrino Oscillation experiments

★ Lecture 3: Cosmological implications of neutrinos

★ Lecture 4: High energy neutrino Astrophysics

Neutrino Masses Supernova Neutrino

Suggested reading:

C. Giunti and C.W. Kim, Fundamentals of Neutrino Physics and Astrophysics, Oxford University Press (2007; 728 pages)

This morning:

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Fundamental particles

Pictures from Understanding the Universe: From Quarks to the Cosmos By Don. Lincoln @ books.google.com

Charged Current interaction Neutral Current interaction

• Ordinary matter made of three particle:– Electron Q = -1– Up-quark Q = +2/3– Down-quark Q = -1/3

• Plus a fourth fundamental particle: the neutrino

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Cross sections• Neutrino elastic scattering:

Cross section on deuterium (below): much larger, can be used for target and relatively small detectors

Cross section on nucleon (above) as a function of energy

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Neutrino detection

1956 1970

ν

µ+

p

π+

Reines & CowanProject Poltergeist

First neutrino detection in a bubble chamber at the Zero Gradient Synchrotron

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Neutrino discovery• 1956 Nuclear reactor (strong source)• Basic reaction:• Detect annihilation γγ (511 KeV each) followed by neutron

capture reaction (few µs later) with several emitted γ-rays

• Reines 1/2 Nobel in 1995 "for the detection of the neutrino"

Reines et al., "Detection of the free antineutrino", Phys. Rev. 117, 159 (1960)

Fig. from “Neutrino Physics” Von Kai Zuber

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Fundamental Neutrino properties• Spin = 1/2• Electric Charge = 0• Strong Charge (color) = 0• Mass: VERY SMALL• Magnetic moment < 9 • 10-11 µB• The correspondence between the six

quarks in the Standard Model and the six leptons suggests that there should be exactly three types of neutrino

• How many do really exist?• Are there inactive (sterile) massive

neutrinos?

Fermion Mass

First Generation

νe < 2.2 eV

νe < 2.2 eV

Second Generationνµ < 170 KeVνµ

Third Generationντ <18.2 MeVντ

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How many?

S. Schael et al. Phys.Rept. 427 (2006) 257

• A limit can be derived from studies of the Z0 production• The invisible partial width is attributed to light neutrino

species (m << mZ)

From theoryFrom measurements

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State of the art

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State of the art

Absolute scale?

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Two ν mixing: some thoughts

a) Flavor states as a combination of mass eigenstates (the length of the box illustrates the probability to find a certain mass eigenstates in a given neutrino flavor state)

b) Flavor composition of the mass eigenstates (red=electron neutrino, green=other flavor state)

c) As a) in terms of flavor composition: the νe has a “latent” non-electron component, which manifests during propagation, when the phase differences of the νa components in ν1 and ν2 do not cancel out

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The missing ingredients

• Fundamental parameters of Standard Model:– Precision measurement of |Δm2

23| (⇒oscillations in matter)

– Precision measurement of θ23

– Measurement of unknown angle θ13

– Determination of mass hierarchy sgn(Δm223)

– Search for leptonic CP violating phase δ (⇒LBL experiments)

– Just a few experiments mentioned here …

• The absolute value of the masses• Are neutrino and anti-neutrinos the same particle?

Neutrino masses

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mν=0

mν>0

resolution

Fermi-Kurie plot: square root of the number of e- with given momentum,

divided by Fermi function, versus energy

Direct measurements• νe from β-decay spectrum:

– If electron neutrinos had a nonzero mass, the maximum electron energy would be lower compared to the case of zero neutrino mass

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Mainz/Troitsk resultsAnalyze electron energies by an electrostatic potential which sets an energy threshold just below the endpoint energy. Electrons above the threshold are counted.

– Mainz, Troitsk mβ<2.2 eV (95% C.L)

– KATRIN (Karlsruhe), expected sensitivity is 0.35 eV/c2 (0.2 eV/c2) for mass measurement (upper limit)

Probe anti-neutrinos. If CPT is violated and neutrino and anti-neutrinos have different masses, the bounds are looser

• Sensitive to the effective electron neutrino mass in β decay

KATRIN Approaching

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Pion and tau decays• Kinematic of the decay of charged pions:

• The neutrino is a superpositions of different massive states and a measurement of the mass, forces the system to collapse in one eigenstate

• For pions at rest:

• @ PSI measured:

• Similarly tau decays in the ALEPH experiment:

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Neutrinoless double-β decay 0νββ• Double β-decay (two neutrons/protons in the same

nucleus) is allowed in the SM (2νββ):

• The neutrinoless double β-decay is forbidden (0νββ):

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Majorana versus Dirac neutrinos• Differences Majorana/Dirac nature of the neutrino:

– Majorana masses violate lepton number– Majorana neutrino is equivalent to the anti-neutrino

• Only 0νββ is sensitive to the difference between the two types of neutrinos

Gedanken experiment by Strumia and Vissani: an hypothetical massive neutrino at rest, spin-down is accelerated to relativistic energies up where it undergoes a CC interaction. If accelerated down, due to helicity it can only interact if it is a Majorana particle.

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0νββ and Majorana neutrinos• The 0νββ is possible only if the neutrino is a Majorana

particle• The amplitude of the process (lifetime of this decay mode)

depends on the mass of the neutrinos, since it involves flipping the spin of the “internal” neutrino

To connect the two antineutrino lines is needed:

1. Particle-antiparticle matching:

a Majorana particle2. Helicity matching: a massive particle

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0νββ direct measurement • Must use nuclei for which the 2νββ is suppressed

• Search for sharp lines in the β-decay spectrum:

Collect ionization charge produced by electrons HM, IGEX, GERDA, Majorana, GEM

Single electron tracking NEMO3

Bolometric detector

CUORICINO/ CUORE

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Results• No signal reported yet except a controversial claim

(Heidelberg-Moscow)• Independent experiments are running and new proposals

are discussed to test the claim

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Supernova neutrinos• Extremely powerful explosions terminating some stars • Few solar masses ejected with kinetic energies ~1053 erg• Several types of SN:

– Type Ia have regular light curves, Type Ib and II not – Type Ib and II produce a huge flux of neutrinos (all types)

• Collapse of the core of massive stars for > 8 solar masses

Hubble Space Telescope image of the Crab Nebula (SN 1054)

Chandra /Hubble Space Telescope images of the SN1987A more than 16 years after

Stellar evolution: Nuclear fusion reactions

• For fusion reactions to be ignited, the Coulomb repulsion must be overcame (Tunnel effect)

• The first series of nuclear reactions in stars can be activated at a temperature of about 107 K, in which hydrogen is converted in helium

• Neutrinos are also emitted, with an average energy of 0.26 MeV

The CNO-cycle

The pp-chain

Giant phase• When the hydrogen fuel runs out and the star moves to the next

phase• The electrostatic potential barrier for helium burning is higher and the

nuclear fusion reactions cannot start immediately• The Star contracts and the heats up: the surrounding shell will also be

heated and nuclear reactions (hydrogen burning) can be ignited in the shell of the star

• The luminosity increases and the pressure pushes away the shell from the core: red giant phase

• The collapse of the core to a sufficiently high density will trigger the next set of nuclear reactions, which produce carbon from helium

Following stages

• Eventually also the helium in the core gets exhausted and a similar course of actions takes place, with a carbon inert core, a helium burning shell and a hydrogen to helium in a even outer one

• If the mass is high enough (> 8 solar masses) the star can continue to burn:

– Hydrogen for about 7 106 yr– Helium 5 105 yr– Carbon 600 yr– Neon 1 yr– Oxygen 0.5 yr– Silicon 1 day

• Leading to a iron core

Nuclear fusion reaction

Binding energy per nucleon, as a function of the mass number A: it shows a maximum at iron, which is therefore the most stable nuclei (A=56). For nuclei lighter than iron, the nuclear fusion process is exothermic, for nuclei heavier than iron endothermic.

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(1.4 solar masses)

large mass

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SN1987 A• 99% of the gravitational binding energy liberated during

the collapse is carried away by the neutrinos • SN1987 A: unusual number of events in underground

detectors (E~10 MeV, Dt ~10 sec)

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SN 1987 A

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Supernovae neutrinos• An extremely relativistic neutrino (m<<E) propagates with

velocity:

• Time-of-flight delay from distance D compared to massless particle:

• For simultaneous neutrinos with mean energy E and sigma ΔE

• For a burst of intrinsic duration ΔT0

• SN 1987 A:

Exercise

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Some constraints from SN 1987A• Neutrinos must have propagated for a distance of 50 kpc

• The total emitted energy, as estimated from the observed neutrinos, is compatible with the binding energy of a neutrino star only if:

• A magnetic moment could flip the helicity (making neutrinos sterile) and let neutrinos escape freely. From the observed cooling time:

• The galactic magnetic field would lengthen the path of neutrinos if they have a charge. From the measured spread of the arrival time with energy:

Large Detectors for Supernova Neutrinos

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Super-Kamiokande (104)KamLAND (400)

MiniBooNE(200)

LVD (400)Borexino (100)

IceCube (106)

Baksan (100)

In brackets eventsfor a “fiducial SN”at distance 10 kpc

G. Raffelt

CoincidenceServer @ BNL

Super-K

Alert

Others ?

LVD

IceCube

SuperNova Early Warning System (SNEWS)

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Summary neutrino masses

Phenomenon Measure Sensitivity Dirac vs Majorana

Flavor oscillations No

β-decay 0.2 eV No

Cosmology 0.1 eV No0νββ 0.01eV Yes

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Cosmogenic neutrinos• 238U and 232Th decay of within the Earth produce anti-νe

• Can Earth composition models be improved with neutrino detection?

• First hint from Kamland