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Tau Neutrino Physics Introduction

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Tau Neutrino PhysicsIntroduction

Barry Barish

18 September 2000

D, 3He, 4He and 7Li primordial abundances

- abundances range over nine orders of magnitude
- Y < 0.25 from number of neutrons when nucleosynthesis began (Y is the 4He fraction)
- Yobserved = 0.2380.0020.005
- presence of additional neutrinos would at the time of nucleosynthesis increases the energy density of the Universe and hence the expansion rate, leading to larger Y.
- YBBN= 0.012-0.014 N

1.7 N 4.3

- most precise measurements come from Z e + e
- invisible partial width, inv, determined by subtracting measured visible partial widths (Z decays to quarks and charged leptons) from the Z width
- invisible width assumed to be due to N
- Standard Model value ( l)SM = 1.991 0.001 (using ratio reduces model dependence)

N = 2.984 0.008

- Existence was indirectly established from decay data combined with reaction data (Feldman 81).
- DIRECT EVIDENCE WAS PRESENTED THIS SUMMER FROM FNAL DONUT EXPERIMENT

- Observe thet and its decays from nt charged current interactions

- calculated number of interactions = 1100 ( nm, ne, nt)
- total protons on target = 3.6 1017
- data taken from April to September 1997

Spectrometer

Emulsion-Vertex Detectors

- 6.6 106 triggers yield 203 candidate events

4 events observed

4.1 1.4 expected

0.41± 0.15 background

J = ½

- J = 3/2 ruled out by establishing that the is not in a pure H -1 helicity state in

magnetic moment

- expect for Majorana or chiral massless Dirac neutrinos
- extending SU(2)xU(1) for massive neutrinos,

- where m is in eV and B eh/2me Bohr magnetons.
- using upper bound mt < 18 MeV < 0.6 10-11mB
- Experimental Bound < 5.4 10-7mB from e e (BEBC)

electric dipole moment

< 5.2 10-17 e cm from (Z ee) at LEP

nt charge

< 2 10-14 from Luminosity of Red Giants (Raffelt)

lifetime

> 2.8 1015 sec/eV Astrophysics (Bludman) for mn < 50 eV

- direct bounds come from reconstruction of multi-hadronic decays
- LEP (Aleph)
- from 2939 events 2 + +< 22.3 MeV/c2
- and 52 events 3 + 2 + () + < 21.5 MeV/c2
- combined limit< 18.2 MeV/c2

- method
- two body decay
- t(Et,pt) h (Eh,ph) + nt (En,pn)
- tau rest frame – hadronic energy
- Eh* = (mt2 mh2 +mn2) / 2mt
- laboratory frame
- Eh = (Eh* + ph* cos)
- interval bounded for different mn
- Ehmax,min = g (Eh* b ph*)

two sample events

3 + 2 + () +

events & contours

0 MeV/c2 and 23 MeV/c2

Log-likelihood fit vs mn

Unstable nt

- bounds on mnt from cosmology
- combined with non observation of lepton number violating decay and direct mass limits

Path length from ~20km to 12700 km

- ratio-of-ratios (reduces systematics):
- R = (nm/ne)obs / (nm/ne)pred

hint #1

ratio lower than expected

Hint #2

anisotropy up/down and distortion of the angular distribution of the up-going events

Superkamiokande

Hint #3

anomalies have been found in a consistent way for all energies

Detectors can detect internal of external events produced in the rock below the detector – 100 MeV to 1 TeV

SuperKamiokande

MACRO Detector

MACRO at Gran Sasso

- Detector mass ~ 5.3 kton
- Event Rate:
- up throughgoing m
- (ToF) ~160 /y
- (2) internal upgoing m
- (ToF) ~ 50/y
- (3) internal downgoing m
- (no ToF) ~ 35/y
- (4) upgoing stopping m
- (no ToF) ~ 35/y

MACRO results

Probabilities of nm nt oscillations

(for maximal mixing)

- the peak probability from the angular distribution agrees with the peak probability from the total number of events
- probability for no-oscillation: ~ 0.4 %

SuperKamiokande

MACRO

- ratio (Lipari- Lusignoli, Phys Rev D57 1998) can be statistically more powerful than a c2 test:
- 1) the ratio is sensitive to the sign of the deviation
- 2) there is gain in statistical significance
- disadvantage: the structure in the angular distribution of data can be lost.
- nm nt oscillation favoured with large mixing angle:m2 ~ 2.5x10-3 eV2
- sterile n disfavoured at ~ 2 slevel

test of oscillations

the ratio vertical / horizontal

SuperKamiokande

- excluded regions using combined analysis of low energy and high energy data
- Sobel n2000 stated ….

SN1987a

What can be learned about the nt from the next supernovae ….??

- direct eV scale measurements of m(nm) and m(nt) from Supernovae neutrinos
- early black hole formation in collapse will truncate neutrino production giving a sharp cutoff
- allows sensitivity to m(ne) ~1.8 eV for SN at 10 kpc in Superkamiokande detector
- (Beacom et al hep-ph/0006015)

Events in SK

Low: 0 < E < 11.3 MeV

mid: 11.3 < E < 30 MeV

High: 30 < E <

- rate in OMNIS, a proposed supernovae detector
- tail: 6.1 eV 2.3 events

OMNIS

delayed counts vs mass nt

OWL - Airwatch

GZK cutoff – neutrinos ??

OSCILLATIONS

FLUXES OF nt AND nm

ARE EQUAL

- neutrinos from interactions of ultrahigh energy cosmic rays with 3 K cosmic backgrond radiation
- neutrinos from AGNs, GRBs, etc
- Zbursts – relic neutrinos from big bang cosmology

- high energy n’s E > 106 GeV
- neutrinos from proton acceleration in the cores of active galactic nuclei
- vacuum flavor neutrino oscillationsenhance nt / nm ratio
- detectable in under water / under ice detectors
- (Athar et al hep-ph/0006123)

- ntidentified by characteristic double shower events
- charged currect interaction + tau decay into hadrons and nt
- second shower has typically twice as much energy as first
- “double bang”

- shower size vs shower separation
- identified events will clearly result from vacuum neutrino oscillations, since without enhancement expect nt / nm < 10-5
- nt events can be identified in under water/ice detectors

MINOS

K2K

CERN GS

nt appearance

muon collider

Example

7400 km baseline

Fermilab Gran Sasso

“world project”

neutrino beams

select nm’s or anti nm’s

- accurately determine n mixing matrix
- perhaps even measure CP violation in n sector

- direct observation of the tau neutrino by DONUT is an important milestone
- properties of tau neutrino like other neutrinos ne, nm, nt
- neutrino oscillations open up a variety of new future possibilities for nt in cosmology, astrophysics and future accelerators