# Presentation: 702649

## Neutrino Physics

**Steve Elliott**
**LANL**
**Nuclear Physics Summer School 2005**

## Lecture Outline

**Neutrinos and the weak interaction**
**Neutrino oscillations**
**Experimental results on neutrinos**
**Solar-Atmospheric experiments**
**Reactor-Accelerator experiments**
**Double beta decay**
**“****Direct” neutrino mass studies**

- Steve Elliott, NPSS 2005

- June 2005

## Outlinefor this lecture

**Neutrinos in the “standard model”**
**Sources of neutrinos**
**Neutrino detection**
**Connections to other physics**

- Steve Elliott, NPSS 2005

- June 2005

## The Standard Model Particles

| u
up | c
charm | t
top | 
gamma |
| --- | --- | --- | --- |
| d
down | s
strange | b
bottom | g
gluon |
| e
 |  |  | W
W boson |
| e
electron | 
muon | 
tau | Z
Z boson |

**Force Carriers**

**Leptons**

**Quarks**

- The Neutrinos

| u
up | c
charm | t
top | 
gamma |
| --- | --- | --- | --- |
| d
down | s
strange | b
bottom | g
gluon |
| 1
 |  |  | W
W boson |
| e
electron | 
muon | 
tau | Z
Z boson |

| u
up | c
charm | t
top | 
gamma |
| --- | --- | --- | --- |
| d
down | s
strange | b
bottom | g
gluon |
| 3
 |  |  | W
W boson |
| e
electron | 
muon | 
tau | Z
Z boson |

- Steve Elliott, NPSS 2005

- June 2005

## Neutrinos mix, therefore:

**Neutrinos have mass**
**Might have non-zero magnetic moments**
**Heavier neutrinos might decay**
**Might be Majorana or Dirac**
**What are the implications for**
**unification, supersymmetry, and extra dimensions?**
**possible existence of additional species?**
**the possibility that neutrinos have something to do with the matter-antimatter asymmetry?**

- Steve Elliott, NPSS 2005

- June 2005

## Why neutrinos are unusual

**Neutrinos might be the ultimate neutral particle**
**They would not be distinct from their antiparticles.**
**If so they would be Majorana particles**
**They might also be Dirac particles**
**Like the charged quarks and leptons**

- Steve Elliott, NPSS 2005

- June 2005

## Neutrinos and the weak interaction

**The weak interaction violates parity.**
**Hence there are no right handed current interactions**
**This can be interpreted two ways.**
**There are no right handed neutrinos**
**There are RH neutrinos, they just don’t interact**

- Steve Elliott, NPSS 2005

- June 2005

## There are 3 active light neutrinos

**The width of the Z decay depends on the number of channels available for the decay.**

- Steve Elliott, NPSS 2005

- June 2005

## Dirac vs. Majorana

**(********D********,   ********D********)**

**(********D********,   ********D********)**

**(********M********,   ********M********)**

**CPT**

**CPT**

**CPT**

**Lorentz**

**Lorentz**

- ) addresses
- Dirac/Majorana
- nature of .

- Steve Elliott, NPSS 2005

- June 2005

## Typical Dirac mass term

**Quarks and leptons get their mass by a coupling to the Higgs. Here is an example (the electron): a Dirac particle.**

- Mij doesn’t have to be diagonal, although it is for the charged leptons.

- Steve Elliott, NPSS 2005

- June 2005

## For neutrinos:

- In the standard model, jR (the RH neutrino) doesn’t exist, therefore neutrinos are massless by construction.
- Now that we know that neutrinos have mass, we need to learn how to incorporate that into the model. There are many possibilities.

- Steve Elliott, NPSS 2005

- June 2005

## We could simply put in jR

- The coupling fij doesn’t have to be diagonal and in general it isn’t. To find the physical fields, those of definite mass, we need to diagonalize *M**ij*.

- Steve Elliott, NPSS 2005

- June 2005

## Such a term leads to mixing

**m******** is the ********th**** diagonal element of the mass matrix**

**The neutrinos mix.**

- Steve Elliott, NPSS 2005

- June 2005

## Shortcomings

**f****ij**** is completely arbitrary**
**Doesn’t explain why neutrinos are so much lighter than their lepton partners.**
**We have not included additional possible mass terms...**

- Steve Elliott, NPSS 2005

- June 2005

## Adding Majorana mass terms

**Ms are nxn matrices for n generations.**
******R****, ********L**** are n element column vectors from n generations.**

- From NC scattering,
- We know ML is small

- Steve Elliott, NPSS 2005

- June 2005

## Diagonalize M

**Leads to two eigenvalues m****1**** ~(M****D****)****2****/M****R**** and m****2**** ~M****R**

- Steve Elliott, NPSS 2005

- June 2005

## Leads to the seesaw mechanism

**If we take M****D**** to be order of lepton mass, and we know that M****R**** is large:**
**We have two Majorana neutrinos**
**One with a mass much less than the leptons**
**One which is very heavy.**

- Steve Elliott, NPSS 2005

- June 2005

## Phases in the mixing matrix

**For nxn unitary matrix (U):**
** ****2n****2**** parameters in a complex matrix**
** ****-n****2**** unitarity constraints**
** ****-(2n-1) unphysical phases: that can be absorbed**
**             ****into the fields, ******** and ******
** ****=(n-1)****2**** parameters (1/2)(n-1)n of these are rotation angles**

- Note however, that for Majorana fields, the phases of  and  are related. Hence there are only n unphysical phases.

- Steve Elliott, NPSS 2005

- June 2005

## Sources of neutrinos

- Big Bang
- Radioactive decays
- Stars
- Supernovas
- Cosmic rays
- Reactors
- Accelerators

- Steve Elliott, NPSS 2005

- June 2005

## Big Bang

**Relic neutrinos contribute at least as much mass to the Universe as all the stars.**
**There are as many leftover neutrinos as photons.**
**N******** ~420/cc**
**Photon energy: 2.728 K**
**Neutrino energy: 2 K**
**There are no viable ideas for detecting such low energy neutrinos.**
**Note that neutrinos are studied via their particle nature**
**The microwave background was discovered by the wave nature of photons.**

- Steve Elliott, NPSS 2005

- June 2005

## Radioactive Decays

**MCi sources have been made**
**Mostly for use by solar neutrino radiochemical experiments for efficiency measurements.**
**Electron capture isotopes provide a monoenergetic neutrino.**
**51****Cr**
**37****Ar**

- Steve Elliott, NPSS 2005

- June 2005

## Stars (our Sun)

_**	**__**Features**_
**Very long baseline, ********e**** disappearance, ********x**** appearance**
**Low energy, spectral shape well known**
**L/E is large so sensitive to small ********m****2**
**Large Flux**
**Matter enhancement**
_**	**__**Data**_
**Rates from several experiments**
**Energy dependence**
**Day vs. Night**
**Seasonal**

- Steve Elliott, NPSS 2005

- June 2005

## Supernovas

_**	**__**Features**_
**~ Very long baseline**
**~ ************'s**
**~ Complicated and poorly understood source**
**~ Target cross sections not all well understood**
_**	**__**Data**_
**~ Not a common phenomenon **
**	****once ~30 years in our galaxy**
**~ SN1987A provided little ******** physics data**
**~ SN1987A did give hope for the future **

**My personal prediction is that neutrinos will teach us a lot about supernovae, but the inverse will be much harder.**

- Steve Elliott, NPSS 2005

- June 2005

## Supernovas

**By using various targets with different energy- and **
**flavor-dependent cross sections, one may be able to**
**de-convolute the various fluxes.**

- Steve Elliott, NPSS 2005

- June 2005

## Cosmic Rays

- Steve Elliott, NPSS 2005

- June 2005

## Reactors

_**Features**_
**Complicated but well-understood source.**
**Low energy**
**Short, medium, long baselines**
** ****disappearance experiments**
_**Data**_
**Several at short baselines; 10-250 m**
**CHOOZ/Palo Verde at ~1 km**
**KamLAND at ~250 km**

- Steve Elliott, NPSS 2005

- June 2005

## Accelerators

_**Features**_
**Usually appearance**
**Various baselines and wide energy range**
**Controlled experimental conditions**
_**Data**_
**Oscillation limits for many species**
**Lots of experimental results**

- Steve Elliott, NPSS 2005

- June 2005

## Neutrino detection

**Targets**
**H****2****O**
**D****2****O**
**Scintillator**
**Ga**
**Cl**
**Emulsion**
**Ice**
**Iron**
**Rock**

**ES on e****-****:**
**  ********x**** + e****-****-> ********x**** + ****e****-**
**CC on Nucleus:**
**  ********l**** + A-> ****A’****+ ****l**
**NC on Nucleus:**
**  ********x**** + A-> ****A’****+ ********x**

- Steve Elliott, NPSS 2005

- June 2005

## Cross sections

**10,000 light years of Pb to stop half of solar neutrinos**
**Beta decay provides estimate of strength**

- Neutron beta decay

- Anti-neutrino absorption

- Steve Elliott, NPSS 2005

- June 2005

## Cross Sections

- The small size of these cross sections is what led early researchers to believe they had postulated an undetectable particle.

- Steve Elliott, NPSS 2005

- June 2005

## Hard experiments

**Rates are very low**
**Big detectors**
**Background difficulties**
**Signal may not be very distinct**
**Other more common processes can mimic signal**
**Rare variations of common phenomena...**

- Steve Elliott, NPSS 2005

- June 2005

## Connections to other physics

** ****Cosmology**
** ****Large scale structure**
** ****Baryon asymmetry**
** ****Nuclear and Particle physics**
** ****Incorporating mass into the standard model **
** ****Astrophysics**
** ****Nucleosynthesis**
** ****Supernova dynamics**

**Neutrinos are very practical**

- Steve Elliott, NPSS 2005

- June 2005

## A summary of the questions

**Are neutrinos Majorana or Dirac?**
**What is the absolute mass scale?**
**How small is ********13****?**
**How maximal is ********23****?**
**Is there CP violation in the neutrino sector?**
**Is the mass hierarchy inverted or normal?**
**Is the LSND evidence for oscillation true? Are there sterile neutrinos?**

- Steve Elliott, NPSS 2005

- June 2005

## References

**Mohapatra/Pal book**
**Kayser book**
**Bahcall book**
**Boehm/Vogel book**

- Steve Elliott, NPSS 2005

- June 2005