# Presentation: 442972

## Neutrino Conversations

- Boris Kayser
- NeutrinoFest
- April 18, 2005

## The Universe and Us

- It used to be thought that —

## Our home planet—

## — is the center of the universe.

- But we now know that the address of our home planet is —
**#3 No-Place-Special Street**.

## We, and all everyday objects, are made of 3 kinds of tiny particles:

- These are bundled together to make Atoms:

- Electrons

- Neutrons

- Protons

- Proton

- Neutron

- Electron

## It used to be thought that theWhole Universe is made of the same particles as we are:

- Electrons      Protons      Neutrons

- But we now know that in the universe as a whole, these particles are rareties.
- For every electron, proton, or neutron, the universe contains over a billion neutrinos .

## To understand the universe, we must understand the neutrinos.

- To understand the universe, we must understand the neutrinos.

## Are Neutrinos Important to Our Lives?

- If there were no  s, the sun and stars would not shine.

-  No energy from the sun to keep us warm.
-  No atoms more complicated than hydrogen.
-    No carbon. No oxygen. No water.   No earth. No moon. No us.

- No  s is very BAD news.

## Getting Acquainted With Neutrinos

- neutrï ́no: Little neutral object
- 						 Enrico Fermi

- Q:	How little are neutrinos?
- A: About the same size as electrons. Roughly 1/10,000,000,000,000,000 inch across. This is 1/1,000 the size of an atomic nucleus.

## BIG detectors are needed to stop and study neutrinos.

- A: VERY FEEBLY.

- Every day, 10,000,000,000,000,000,000,000 neutrinos from the sun pass through the Sudbury Neutrino Observatory (SNO), 1000 tons of heavy water in a sphere 40 feet across.
- Only 10 of them interact. The rest zip right through.
- BIG detectors are needed to stop and study neutrinos.

## Slide 11

## Q:	How much do neutrinos weigh?

- A: *Almost* nothing. Years of experiments yielded no evidence that neutrinos have any mass at all.

- Q:  Can a particle have *no* mass at all?
- A: A particle *can* be a bundle of pure energy, and have no mass at all.
- 	The photon—the particle of light—is like that.
- 	But in the last 7 years we have discovered that neutrinos are *not* like that. They have nonzero masses.
- 	However, the mass of each neutrino is less than one-millionth the mass of an electron.

## Q:	Are all neutrinos the same, or are there different kinds of neutrinos?

- A: Neutrinos come in three different flavors:
- 	        The electron neutrino  e
- 	        The muon neutrino      
- 	        The tau neutrino	         

- Q:	How do we know neutrinos have masses?
- A: We’ll explain that shortly.

## Q:	How do e, , and  differ from one another?

- A: There are 3 kinds, or flavors, of electron-like particles:

- e, , and  are electrically charged, and are known as the charged leptons.

- Associated_Neutrino_
- e
- 
- 

- _Particle_     _Symbol_	  _Mass_
- Electron	e	         1
- Muon			     200
- Tau			   3500

## Neutrinos are created in a variety of physical processes.

- In nature or the laboratory, a neutrino is created together with a charged lepton.
- The neutrino and charged lepton always have the same flavor.

- or

- or

- Not

- Source

## When a neutrino collides with an atom in a neutrino detector, it creates a charged lepton.

- The charged lepton always has the same flavor as the neutrino.

- 

- 

- 

- or

- or

- Not

- e

## e

- e

- Detector

- e

- e

- Source

- 

- 

- 

- 

- The flavors match.

- _Creation and Detection of a Neutrino_

- Short Journey

## The Discovery of Flavor Change

- The last 7 years have yielded compelling evidence that, given enough time, a neutrino can change from one flavor into another.

- This is surprising behavior.
- Once an electron, always an electron.
- But once a e, not always a e.

## How We Know Neutrinos Change Flavor

- — Solar Neutrinos —

## SNO detects solar neutrinos in several different ways.

- One way counts

- Number (e) .

- Another counts

- Number (e) + Number () + Number () .

- SNO finds

- Number (e)

- Number (e) + Number () + Number ()

** ****=  **1/3 .

## Neutrinos change flavor.

**Neutrinos change flavor.**

## — Atmospheric Neutrinos —

- — Atmospheric Neutrinos —

- Cosmic rays come from all directions at the same rate.
- So atmospheric neutrinos are produced all around the earth at the same rate.
- But Number ( Up) / Number ( Down) = 1/2.

- Detector

- Cosmic ray

- 

- 

## 		              

- The detailed data show that the disappearance is due to —
- 		              

## Evidence For Flavor Change

- _Neutrinos_
- Solar
- Atmospheric
- Reactor
- Accelerator

- _Evidence of Flavor Change_
- Compelling
- Compelling
- Compelling
- Strong

## Neutrino Flavor Change Implies Neutrino Mass 

- The neutrinos we study pass through matter between their source and our detector.
- Can’t their *interactions* with the matter change their flavor?
- In  practice, **no**.
- We have confirmed that the interactions between neutrinos and matter are very well described by the Standard Model of elementary particle physics.
- Standard Model neutrino interactions do not change neutrino flavor.

## Therefore, a neutrino must have a mass.

- We observe that they do this *over time*.
*A **** has a sense of time.*

- Only particles with masshave a sense of time.

**Therefore, a neutrino must have a mass.**

## The Physics of Neutrino Flavor Change

- If particles like the electron never morph into something else, how can a  morph into a ?
- Answer: A  is not a particle to begin with.
- There *are* neutrino particles:

- (And maybe more.)

## e, , and  are different MIXTURES of 1, 2, and 3.

- In each of—

- 

- 

- 

- 

- e

- e

- the emitted neutrino is actually a 1, 2, or 3.

-  is:

- maybe 1

- maybe 2

- maybe 

## The world of the subatomic particles is governed byQUANTUM MECHANICS.

- An object can be maybe here and maybe there. It can be maybe this and maybe that.It can be maybe a 1, maybe a 2, and maybe a 3.

- Quantum mechanics involves *uncertainty* at its core.

## Voyage of a Neutrino

- 

- 

- 

- Long Journey

- Original 1, 2, 3 mixture

- New, different 1, 2, 3 mixture

- 1, 2, 3 travel differently because they have different masses.

- The  mixture of 1, 2, 3 has turned into the  mixture.

- Neutrino flavor change is a quintessentially quantum mechanical phenomenon.
- It occurs over VERY LARGE distances.

## NEUTRINO OSCILLATION

- Thus, flavor change is called —
**NEUTRINO OSCILLATION**

- An example, starting with a :

## Neutrino Oscillation

## What We Have Learned

**What We**** Have Learned**

## The Neutrino Mass Spectrum

- There are at least 3 neutrino particles: 1, 2, 3.
- Neutrino oscillation results have revealed the *differences* between the squares of their masses.
- The spectrum of squared masses looks like —

## Not above (Electron mass / 1,000,000)2. {From Cosmology}

- (Mass)2

- or

- m2atm

-         m2sol = m2atm **/** 30

-  m2atm  = (Electron mass  **/** 10,000,000)2

- Not above (Electron mass **/** 1,000,000)2. {From Cosmology}

## When one of the neutrino particles (1, 2, or 3) interacts in a detector and makes a charged lepton, this charged lepton could be an e, a , or a . 

- It’s that quantum-mechanical uncertainty again!
- But, for each neutrino particle, we know the probability that the charged lepton it produces will be of any particular flavor.

## 1

- 1

- 2

- 3

**The Probabilities of Making ****e****, ********, and ******

## The Open Questions

**The Open Questions**

## How many different  neutrino particles are there?

- CERN: If there are more than 3, then at least one mixture of them does not participate in any of the known forces of nature except gravity.
- All known particles participate in some force besides gravity. e, , and participate in the weak nuclear force.An object that doesn’t experience any of the known forces except gravity would be ***very*** different.
- LSND (Liquid Scintillator Neutrino Detector): There are more than 3 neutrino particles.
- MiniBooNE (in progress): Is the LSND experiment right or wrong?

## How much do the neutrino particles 1, 2, and 3 weigh?

- Can we use cosmology?
- Can observations of the structure of the universe tell us, not just an upper limit on the mass of any neutrino particle, but the actual masses of these particles?
- Can we use laboratory experiments?

## Does the neutrino mass spectrum look like        or like        ?

- Grand Unified Theories: The neutrinos and the charged leptons are cousins of the quarks.
- The quark spectra look like       .
- So, if these theories are right, the neutrino spectrum should look like        too.
- To find out if it does, pass a beam of neutrinos through more than 500 miles of earth matter.
-  The behavior of the neutrinos in matter will depend on which kind of a spectrum we have.

## Are neutrinos identical to their antiparticles?

- For every particle, there is a corresponding antiparticle.

- _Particle_

- _Antiparticle_

|  Electron |    Positron | Electric Charge |
| --- | --- | --- |
|   Proton |  Antiproton | Electric Charge |
|  Neutron |  Antineutron | Baryonic Charge |
| Neutrino  | Antineutrino  |             ?? |

- _Difference_

*Matter**                 **Antimatter*

## e

- 	 L() = L(e–) = –L() = –L(e+) = 1 ?
- That would explain why —

- e

- e+

- but

- But if there is no such leptonic charge, then there is nothing to distinguish a  from a .

## Then, unlike all the other constituents of matter — the charged leptons, and the quarks that make up protons and neutrons — the neutrinos are identical to their antiparticles:

- 			  = .
**This would make neutrinos very distintive.**

- How can  we confirm that  =  ?
- “Charges,” such as the hypothetical leptonic charge L, are conserved quantities:

- Process

- L(in)

- L(out) = L(in)

## e–

- New Nucleus

- Nucleus

- e–

- e–

-       Neutrinoless Double Beta Decay (0)

## What is the origin of neutrino mass?

- Observation of neutrinoless double beta decay would

- The origin of neutrino mass is different from the origin of the masses of electrons, quarks, protons, neutrons, humans, the earth, and galaxies.

## This preponderance of Matter over antimatter could not have developed unless the two behave differently (“CP violation”).

- The universe contains Matter, but essentially no antimatter.
- Good thing for us:

- This preponderance of Matter over antimatter could not have developed unless the two behave differently (“CP violation”).
- A difference not involving neutrinos has been seen, but it is way too small to explain the universe.

- Matter

- Antimatter

## Does Matter interact with neutrinos differently than antimatter does? 

- Could this difference explain the universe?

## Slide 49

## N   e- + ...   and    N   e+ + ...

- Then they would have disintegrated into lighter particles:

- N   e- + ...   and    N   e+ + ...
-        Matter                              antimatter

- If Matter and antimatter interact differently with neutrinos, both heavy and light, then one of these disintegrations can be more likely than the other.
- Then we would get a universe with unequal amounts of Matter and antimatter.

## Do these processes have different rates?

## If N decays led to the present preponderance of Matter over antimatter, then we are all descendants of heavy neutrinos.

## Recommendations of the APS Multi-Divisional Study

## High priority: Searches for neutrinoless double beta decay, to see if  = . 

## High priority: A program to — 

- find out how big the small e-flavored wedge in 3 is
- determine whether the mass spectrum looks like       or like
- search for CP violation in neutrino flavor change

- CP violation: Neutrinos interact differently with matter than with antimatter.
- There can be no CP violation unless the pie chart for every neutrino particle involves all three colors.

## Important: Develop an experiment that can make detailed studies of the neutrinos from the primary fusion process that we think powers the sun.

- These neutrinos have lower energy than those studied in detail so far.
- Now that we understand *neutrinos* much better, we can use them to test whether we truly understand how the *sun* works.

## Conclusion

- There has been an explosion in our knowledge of the neutrinos in the last seven years.
- The recent discoveries have raised very interesting questions that we must now try to answer.
- Exciting, challenging, experiments to answer them will be launched in the coming years.