# Presentation: 801759

## The Neutrino World:

- Present and Future

- Boris Kayser
- APS
- May 2, 2004

## Neutrinos Are Under Our Skin

- Inside each person:
- > 107 neutrinos from the Big Bang

- {

- ~1014 solar neutrinos zip through every second.

## Neutrinos are Abundant

- In the universe —
- ~ 109 neutrinos for each nucleon or electron
- Neutrinos and photons are the most abundant particles in the universe.
- If we wish to understand the universe, we must understand neutrinos.

## Neutrinos Come in at Least Three Flavors

- The known neutrino flavors:	e  ,   

## The Meaning of this Association

- e

- Detector

- e

- e

- W boson

- Short Journey

- e

- 

- 

- 

- 

- W

- 

- 

- 

- 

- W

## Do Neutrinos Have Mass?

- Neutrino mass would lead to very interesting physics.
- But neutrino masses, if nonzero, are extremely small.
- How do you look for tiny neutrino masses?

## Look for —

- Give  time to change character
- 
- The last six years have brought us compelling evidence that such flavor changes actually occur.

- 

- 

- W

- Detector

- 

- 

- 

- Long Journey

## Neutrino Masses and Mixing

- As we shall see —
- Flavor Change  Neutrino Mass and Mixing
- Neutrino mass —
- There is some spectrum of 3 or more neutrino mass eigenstates i:

- (Mass)2

- 1

- 2

- 3

- 4

- Mass ()  mi

## 			|> =  U*i |i>  .

- When W+  l+  +   ,
- 	the produced neutrino state  |> is
- 			|> =  U*i |i>  .
- 	Neutrino of	 flavor  	  	Neutrino of definite mass mi		              	Unitary Leptonic Mixing Matrix
- Flavor- fraction of i = |< |i>|2 = |Ui|2  .

- e, , or 

- i

## Neutrino Flavor Change (“Oscillation”)

- Suppose a neutrino is born with flavor  and energy E.
- The neutrino has evolved into a mixture of the flavors.

- 

- (L)

## Then—

- 	P() = sin22 sin2 1.27 m2(eV2)
- 		Probability
- 					       m2  m22 – m12

- Neutrino oscillation  Neutrino mass & mixing
- An experiment with given 		will be sensitive to m2 (eV2) > 	      .
- Tiny splittings m2 can be probed.

- Often, only 2 neutrinos need to be considered.
- Then —

- Mixing Angle

## (

- _Neutrinos_
- Solar
- Reactor(L ~ 180 km)
- Atmospheric
- Accelerator(L = 250 km)
- Stopped  Decay LSND L  30 m

- _Evidence of Flavor Change_
- Compelling
- Very Strong
- Compelling
- Interesting
- Unconfirmed

- )

## Solar Neutrinos

- Nuclear reactions in the core of the sun produce e. Only e.
- Sudbury Neutrino Observatory (SNO) measures, for the high-energy part of the solar neutrino flux:
- sol d  e p p   e
- sol d  n p   e + + 
- From the two reactions,

- Clearly, +  ≠  0 .   Neutrinos do change flavor.

- e
- e + + 

- ———————  =  0.306 ± 0.026 (stat) ±0.024 (syst)

## The now-established mechanism for solar e    is not oscillation in vacuum but the —

- 	Large Mixing Angle —
- 		Mikheyev Smirnov Wolfenstein
- 				— Effect.
- This effect occurs as the neutrinos stream outward through solar material. It requires both interactions with matter and neutrino mass and mixing.

## Reactor (Anti)Neutrinos

- The neutrino properties m2sol and sol implied by LMA-MSW 
- KamLAND, ~ 180 km from reactor e sources, should see substantial disappearance of e flux.
- KamLAND actually does see —
- 				 = 0.611 ± 0.085(stat) ± 0.041(syst)
- Reactor e do disappear.
- Flavor change, with m2sol and sol in the LMA-MSW range, fits both the solar and reactor data.

## The fit to both the solar and reactor data.

- SNO: nucl-ex/0309004 .

- sol

- sol

## Atmospheric Neutrinos

- Isotropy of the > 2 GeV cosmic rays + Gauss’ Law + No  disappearance
- 		  –––––––  = 1 .
- But Super-Kamiokande finds for E > 1.3 GeV
- 		   ––––––––––––  =  0.54 ± 0.04 .

-  (Up)
-  (Down)

- ~

- Detector

- Cosmic ray

- 

- 

-  (Up)
-  (Down)

## (

- Voluminous atmospheric neutrino data are well described by —
- 
- with —
- 1.9  10-3  <  m2atm  <  3.0  10-3 eV2
- and —
- sin2 2atm >  0.90
- Super-K
- 90%CL

- )

## z

- (Super-K)

## L(iquid) S(cintillator) N(eutrino) D(etector)

- Unconfirmed e signal

- Confirmation of the LSND oscillation would imply the existence of a fourth neutrino species:			     the sterile neutrino.
- Unlike e, , or , a sterile neutrino does not interact even weakly.

- 30m

- 

- e+

- e

- e

- 

- Detector

## What Have We Learned?

- If LSND is confirmed, there are at least 4 neutrino species.
- If LSND is not confirmed, nature may contain only 3 neutrinos.
- Then, from the existing data, the neutrino spectrum looks like —

## }

- m2atm

- e [|Uei|2]

- [|Ui|2]

- [|Ui|2]

- 

- 

- 

- (Mass)2

- m2sol

## — The Future —What We Would Like to Find Out

- How many neutrino species are there?Are there sterile neutrinos?
- MiniBooNE will confirm or refute LSND.
- What are the masses of the mass eigenstates i?

- Is the spectral pattern            or           ?
-  behavior in earth matter can distinguish.
- How far above zero is the whole pattern??

**( ****—**** )**

- 3

- 0

## (

- Cosmological Data + Cosmological Assumptions 
-  mi < 0.71 eV .
- 						95% CL	       Mass(i)		Spergel et al.
- If there are only 3 neutrinos,
- 0.04 eV < Mass[Heaviest i] < 0.23 eV
- 		           m2atm	  Cosmology

- )

- ~

##  Does —

-  i = i 	(Majorana neutrinos)
- or
-  i ≠ i	(Dirac neutrinos) ?
- e+ ≠ e–   since Charge(e+) = – Charge(e–).
- But neutrinos may not carry any conserved charge-like quantum number.
- A conserved Lepton Number L defined by—L() = L(l–) = –L() = –L(l+) = 1  	may not exist.
- If it does not, then we can have —

## It is more practical to seek —

- Neutrinoless Double Beta Decay

- Observation would imply  i = i , making the neutrinos very different from the charged leptons and quarks.

- i

- W–

- W+

- e+

- e–

- i

- W–

- W–

- e–

- e–

- Nuclear Process

- Nucl

- Nucl’

## This preponderance of Matter over antimatter could not have developed unless the two behave differently.

- 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.
- The observed difference between Quark and antiquark behavior, as described by the Standard Model, is inadequate.
- Could the interactions of Matter and antimatter with neutrinos provide the crucial difference?

- Matter

- Antimatter

## }

- The most popular theory of why neutrinos are so light is the —
- See-Saw Mechanism

- 

- N

- Very heavy neutrino

- Familiar light neutrino

- {

- The heavy neutrinos N would have been made in the hot Big Bang.

## If Matter and antimatter interact differently with these heavy neutrinos N, then we can have —

- Probability [ N   e- + ... ]  ≠  Probability [ N   e+ + ... ]
- 		Matter			antimatter
- in the early universe.
- This phenomenon (leptogenesis) would have led to a universe containing unequal amounts of Matter and antimatter.

## We cannot repeat the early universe.

- But we can lend credibility to the hypothesis of leptogenesis by showing that Matter and antimatter interact differently with the light neutrinos .

- Source

- Detector

- e-

- -

- 

- Source

- Detector

- e+

- +

- 

- A neutrino flavor change involving Matter:

- A neutrino flavor change involving antimatter:

- If these two flavor changes have different probabilities, then quite likely so do —
-  	       N   e- + ...       and	N   e+ + ...

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

## Conclusion

- Beautiful experiments have led to the discovery of neutrino mass.
- This discovery has raised very interesting questions that we must now try to answer.

## are sponsoring a year-long study, aimed at laying the groundwork for a sensible future program to answer the open questions.

-  Particles and Fields
-  Nuclear Physics
-  Astrophysics
-  Physics of Beams

- Session L4 (Today at 14:30):	Highlights of the study				working group explorations
- Session N1 (Today at 20:00):	Town Meeting on 				“Our Neutrino Future”
- Please Come!