NEUTRINO OSCILLATIONS

1
NEUTRINO OSCILLATIONS Luigi DiLella Marienburg
Castle August 2002
Content of these lectures

• Short introduction to neutrinos
• Formalism of neutrino oscillations in vacuum
• Solar neutrinos
• Production
• Results
• Formalism of neutrino oscillations in
  matter
• Future experiments
• Atmospheric neutrinos
• LSND and KARMEN experiments
• Oscillation searches at accelerators
• Long baseline experiments
• Short baseline experiments
• Long-term future
• Conclusions

2
Neutrinos in the Standard Model Measurement of
the Z width at LEP only three light neutrinos
(ne, nm, nt) Neutrino mass mn 0 by hand
two-component neutrinos helicity (spin
component parallel to momentum) 1 for
neutrinos
1
for antineutrinos
p spin
p spin
n
n
helicity 1 neutrinos helicity 1
antineutrinos
do not exist
If mn gt 0 helicity is not a good quantum
number (helicity has opposite sign in a reference
frame moving faster than the neutrino)
massive neutrinos and antineutrinos can exist
in both helicity states
Are neutrinos Dirac or Majorana particles? Dirac
neutrinos n ? n lepton number is
conserved
Examples neutron decay N ? P e ne

pion decay p ? m nm Majorana neutrinos n ?
n (only one four-component spinor field)
lepton
number is NOT conserved
3
Neutrinoless doubleb decay a way (the only
way?) to distinguish Dirac from Majorana
neutrinos
(A, Z) ? (A, Z2) e e violates lepton
number conservation can only occur for
Majorana neutrinos A second-order weak process
Process needs neutrino helicity flip between
emission and absorption (neutron decay emits
positive helicity neutrinos, neutrino capture by
neutrons requires negative helicity)
neutrinoless doubleb decay can only occur if
m(ne) gt 0 Transition Matrix Element ?m(ne) The
most sensitive search for double-b decay
76Ge32 ? 76Se34 e e E (e1) E
(e2) 2038 keV
p e p e
n n
ne

two neutrons of the same nucleus
HeidelbergMoscow experiment Five enriched 76Ge
crystals (solidstate detectors) Total mass
19.96 kg , 86 76Ge (natural Germanium contains
only 7.7 76Ge) Crystals are surrounded by
anticoincidence counters and installed in
underground Gran Sasso National Laboratory
(Italy) Search for monoenergetic line at 2038
keV No evidence for neutrinoless double-b
decay m(ne) lt 0.2 eV for Majorana neutrinos
4
Neutrino mass relevance to cosmology A
prediction of Big Bang cosmology the Universe is
filled with a Fermi gas of neutrinos at
temperature T ? 1.9 K. Density 60 n cm3 , 60
n cm3 for each neutrino type (ne, nm,
nt) Critical density of the Universe
H0 Hubble constant (Universe present
expansion rate) H0 100 h0 km s1 Mpc 1 (0.6 lt
h0 lt0.8) GN Newton constant
Neutrino energy density (normalized to rc)
Wn 1 for
Recent evidence from the study of distant
Super-Novae rc consists of 30 matter (visible
or invisible) and 70 vacuum
energy Cosmological models prefer
non-relativistic dark matter (easier galaxy
formation) with rn ? 20 of matter density
cosmological limit on neutrino masses
Direct measurements of neutrino masses ne mc2
lt 2.5 eV (from precise measurements of the
electron energy spectrum from 3H decay) nm mc2 lt
0.16 MeV (from a precise measurement of m
momentum from p decay at rest) nt mc2 lt 18.2
MeV (from measurements of t ? nt 3, 5 or 6 p
at LEP) With the exception of ne
direct measurements of neutrino masses have no
sensitivity to reach the cosmologically
interesting region
5
Neutrino interaction with matter Wboson
exchange ChargedCurrent (CC) interactions Quasi-
elastic scattering ne n ? e p ne
p ? e n nm n ? m p nm p ? m
n Energy threshold 112 MeV nt n ?
t p nt p ? t n Energy
threshold 3.46 GeV Cross-section for energies
gtgt threshold sQE ? 0.45 x 1038
cm2 Deep-inelastic scattering (DIS) (scattering
on quarks, e.g. nm d ? m u) ne N ? e
hadrons ne N ? e hadrons (N
nucleon) nm N ? m hadrons nm N ?
m hadrons nt N ? t hadrons
nt N ? t hadrons Cross-sections for
energies gtgt threshold sDIS(n) ? 0.68E x 1038
cm2 (E in GeV)
sDIS( n ) ? 0.5
sDIS(n)

Zboson exchange NeutralCurrent (NC)
interactions Flavour-independent the same for
all three neutrino types n N ? n hadrons
n N ? n hadrons Cross-sections
sNC( n) ? 0.3 sCC(n) sNC( n ) ? 0.37 sCC(
n)
Very low cross-sections mean free path of a 10
GeV nm ? 1.7 x 1013 g cm2 equivalent to 2.2 x
107 km of Iron
6
0.8 0.6 0.4 0.2 0.0
sCC(nt) sCC(nm)
Suppression of t production by nt CC
interactions from t mass effects
Neutrino electron scattering
0 20 40 60
80 100
n Z e
E (n) GeV
ne e W e ne
Cross-section s A x 1042 E cm2 (E in
GeV) ne A ? 9.5
ne A ? 3.4 nm, nt A ? 1.6
nm, nt A ? 1.3
(all three n types)
(ne only)
Note cross-section on electrons is much smaller
than cross-section on nucleons because s ? GF2
W2 (W ? total energy in the centre-of-mass
system) and W2 ? 2meEn
7
NEUTRINO OSCILLATIONS
The most promising way to verify if mn gt 0
(Pontecorvo 1958 Maki, Nakagawa, Sakata 1962)
Basic assumption neutrino mixing ?e, ??, ?? are
not mass eigenstates but linear superpositions of
mass eigenstates ?1, ?2, ?3 with masses m1, m2,
m3, respectively

• e, ?, ? (flavour index)
• i 1, 2, 3 (mass index)

Uai unitary mixing matrix
8
Time evolution of a neutrino state of momentum
p created as ?? at time t0
Note
are different if mj ? mk
phases
appearance of neutrino flavour ?? ? ?? at t gt 0
Case of two-neutrino mixing
? ? mixing angle
For ???? at production (t 0)
9
Probability to detect ?? at time t if pure ?? was
produced at t 0
Natural units
?m2 ? m22 m12
(in vacuum!)
Note for m ltlt p
Use more familiar units
L ct distance between neutrino source and
detector
Units Dm2 eV2 L km E GeV (or L m E
MeV)
NOTE Pab depends on Dm2 and not on m. However,
if m1 ltlt m2 (as for charged leptons and quarks),
then Dm2 ? m 22 ? m12 ? m22
10
Define oscillation length l
Units l km E GeV Dm2 eV2 (or
l m E MeV)
Larger E, smaller Dm2
Smaller E, larger Dm2
sin2(2?)
Distance from neutrino source
11
Disappearance experiments
Use a beam of na and measure na flux at distance
L from source
Measure
Examples

• Oscillation experiments using ne from nuclear
  reactors
• (En ? few MeV under threshold for m or t
  production)
• nm detection at accelerators or from cosmic rays
• (to search for nm? nt oscillations if En is
  under threshold
• for t production)

Main uncertainty knowledge of the neutrino flux
for no oscillation the use of two
detectors (if possible) helps
n beam
Far detector measures Paa
Near detector measures n flux
n source
12
Appearance experiments Use a beam of na and
detect nb (b ? a)?at distance L from source

• Examples
• Detect ne Nucleon ? e- hadrons in a nm beam
• Detect nt Nucleon ? t - hadrons in a nm beam
• (Energy threshold ? 3.5 GeV)
• NOTES
• nb contamination in beam must be precisely known
• (ne/nm ? 1 in nm beams from high-energy
  accelerators)
• Most neutrino sources are not mono-energetic but
  have wide
• energy spectra. Oscillation probabilities must
  be averaged over
• neutrino energy spectrum.

13

• Under the assumption of two-neutrino mixing
• Observation of an oscillation signal
  allowed region Dm2 versus sin2(2?)
• Negative result upper limit to Pab
  (Pab lt P) exclusion region
  

Large Dm2 ? short l Average over source and
detector size
Small Dm2 ? long l
log(Dm2)
(the start of the first oscillation)
0
1
sin2(2?)
14

PARAMETERS OF OSCILLATION SEARCH EXPERIMENTS
Neutrino source Flavour Baseline L
Energy Minimum Dm2
Sun ne ?1.5 x 108 km
0.2 ?15 MeV ?10-11 eV2
nm ne nm ne
10 km ? 13000 km
0.2 GeV ? 100 GeV
Cosmic rays
?10-4 eV2
20 m ? 250 km
Nuclear reactors
ne
ltEgt ? 3 MeV
?10-1 ? 10-6 eV2
nm ne nm ne
15 m ? 730 km
20 MeV ? 100 GeV
Accelerators
?10-3 ? 10 eV2
EVIDENCE/HINTS FOR NEUTRINO OSCILLATIONS

• Solar Neutrino Deficit ne disappearance between
  Sun and Earth
• Atmospheric neutrino problem deficit of nm
  coming from the other side
• of the Earth
• LSND Experiment at Los Alamos excess of ne in
  a beam consisting mainly
• of nm , ne and nm

15
SOLAR NEUTRINOS
Birth of a visible star gravitational
contraction of a cloud of primordial gas (mostly
?75 H2, ?25 He) increase
of density and temperature in the core
ignition of nuclear fusion Balance between
gravity and pressure hydrostatic
equilibrium
Final result from a chain of fusion reactions
4p ? He4 2e
2ne Average energy produced in the form of
electromagnetic radiation Q (4Mp MHe4
2me)c2 ltE(2ne)gt ? 26.1 MeV
(ltE(2ne)gt ? 0.59 MeV)
(from 2e 2e ? 4g)
Sun luminosity L? 3.846x1026 W 2.401x1039
MeV/s Neutrino emission rate dN(ne)/dt 2
L?/Q ? 1.84x1038 s 1 Neutrino flux on Earth
F(ne)? 6.4x1010 cm2 s 1 (average Sun-Earth
distance 1.496x1011 m)
16
STANDARD SOLAR MODEL (SSM) (developed and
continuously updated by J.N. Bahcall since 1960)
Assumptions

• hydrostatic equilibrium
• energy production by fusion
• thermal equilibrium (energy production rate
  luminosity)
• energy transport inside the Sun by radiation

Input

• cross-sections for fusion processes
• opacity versus distance from Sun centre

Method

• choose initial parameters
• evolution to present time (t 4.6x109 years)
• compare measured and predicted properties
• modify initial parameters (if needed)

Present Sun properties Luminosity L?
3.846x1026 W
Radius R? 6.96x108 m
Mass M?
1.989x1030 kg
Core temperature Tc 15.6x106 K
Surface temperature Ts 5773 K
Hydrogen fraction in core
34.1 (initially 71)
Helium fraction in core 63.9 (initially
27.1)
as measured on surface today
17

Two fusion reaction cycles pp cycle (98.5 of
L?)
p p ? e ne d p p ? e ne
d or (0.4) p e p ? ne d p d ? g
He3 p d ? g He3
He3 He3 ? He4 p p or
(?2x10-5) He3 p ? He4 e ne
85
p p ? e ne d p d ? g He3 He3 He4 ? g
Be7
p Be7 ? g B8 e Be7 ? ne Li7
B8 ?
Be8 e ne p Li7 ? He4 He4
Be8 ? He4 He4
15
or (0.13)
CNO cycle (two branches)
p N15 ? C12 He4 p N15
? g O16 p C12 ? g N13
p O16 ? g F17 N13 ? C13 e ne
F17 ? O17 e ne p C13 ?
g N14 p O17 ? N14
He4 p N14 ? g
O15 O15 ? N15 e ne
NOTE 1 in all cycles 4p ? He4 2e 2ne NOTE
2 present solar luminosity originates from
fusion reactions which occurred
106 years ago. However, the Sun is practically
stable over 108 years.
18
Expected neutrino fluxes on Earth (pp cycle)
Notations pp p p ? e ne d 7Be e
Be7 ? ne Li7 pep p e p ? ne d 8B
B8 ? Be8 e ne hep He3 p ? He4 e
ne
Line spectra cm-2 s-1 Continuous spectra cm-2
s-1 MeV -1
Radial distributions of neutrino
production inside the Sun, as predicted by the
SSM
19
The Homestake experiment (19701998) first
detection of solar neutrinos A radiochemical
experiment (R. Davis, University of Pennsylvania)
ne Cl 37 ? e Ar 37
Energy threshold E(ne) gt 0.814 MeV Detector 390
m3 C2Cl4 (perchloroethylene) in a tank installed
in the Homestake gold mine (South Dakota, U.S.A.)
under 4100 m water equivalent (m w.e.) (fraction
of Cl 37 in natural Chlorine 24) Expected
production rate of Ar 37 atoms ? 1.5 per day
Experimental method every few months extract Ar
37 by N2 flow through tank, purify, mix with
natural Argon, fill a small proportional counter,
detect radioactive decay of Ar 37 e Ar 37 ?
ne Cl 37 (half-life t1/2 34 d) (Final state
excited Cl 37 atom emits Augier electrons and/or
X-rays) Check efficiencies by injecting known
quantities of Ar 37 into tank Results over more
than 20 years of data taking
SNU (Solar Neutrino Units) the unit to measure
event rates in radiochemical experiments 1 SNU
1 event s1 per 1036 target atoms Average of all
measurements R(Cl 37) 2.56 ? 0.16 ? 0.16 SNU
(stat) (syst) SSM
prediction 7.6 SNU
Solar Neutrino Deficit
1.3 1.1
20
Real-time experiments using water Cerenkov
counters to detect solar neutrinos Neutrino
electron elastic scattering n e ? n e
Detect Cerenkov light emitted by recoil electron
in water (detection threshold 5 MeV)
Cross-sections s(ne) ? 6 s(nm) ? 6 s(nt)
(5MeV electron path in water ? 2 cm)
W and Z exchange
Only Z exchange
Two experiments Kamiokande (1987 94). Useful
volume 680 m3
Super-Kamiokande (1996 2001). Useful volume
22500 m3 installed in the Kamioka mine (Japan) at
a depth of 2670 m w.e.
Verify solar origin of neutrino signal from
angular correlation between recoil electron and
incident neutrino directions
cosqsun
21
Super-Kamiokande detector
Cylinder, height41.4 m, diam.39.3 m 50 000 tons
of pure water Outer volume (veto) 2.7 m
thick Inner volume 32000 tons (fiducial mass
22500 tons) 11200 photomultipliers, diam. 50
cm Light collection efficiency 40
Inner volume while filling
22
Recoil electron kinetic energy distribution
from ne e elastic scattering of mono-energetic
neutrinos is almost flat between 0 and 2En/(2
me/En) convolute with predicted
spectrum to obtain SSM prediction for electron
energy distribution
En
SSM prediction
Events/day
Data
6 8 10 12 14
Electron kinetic energy (MeV)
Results from 22400 events (1496 days of data
taking) Measured neutrino flux (assuming all
ne) F(ne) (2.35 ? 0.02 ? 0.08) x 106 cm-2 s
1


(stat) (syst) SSM prediction F(ne) (5.05 )
x 106 cm-2 s 1 Data/SSM 0.465 ?
0.005 (stat)

1.01 0.81
0.093 0.074
ne DEFICIT
(including theoretical error)
23
Comparison of Homestake and Kamioka results with
SSM predictions
0.465 ? 0.016
2.56 ? 0.23
Homestake and Kamioka results were known since
the late 1980s. However, the solar neutrino
deficit was not taken seriously at that time. Why?
24
The two main solar ne sources in the Homestake
and water experiments He3 He4 ? g Be7
e Be7 ? ne Li7 (Homestake)
p Be7 ? g B8 B8 ? Be8 e
ne (Homestake, Kamiokande, Super-K)
Fusion reactions strongly suppressed by Coulomb
repulsion
Ec
Z1Z2e2/d
R2
d
R1
Potential energy
Z1e
Z2e
d
R1R2
(R1 R2 in fm)
Ec ? 1.4 MeV for Z1Z2 4, R1R2 4 fm Average
thermal energy in the Sun core ltEgt 1.5 kBTc ?
0.002 MeV (Tc15.6 MK)
kB (Boltzmann constant) 8.6 x 10-5 eV/deg.K
Nuclear fusion in the Sun core occurs by tunnel
effect and depends strongly on Tc
25
Nuclear fusion cross-section at very low energies
Nuclear physics term difficult to
calculate measured at energies 0.1 0.5 MeV and
assumed to be energy independent
Tunnel effect v relative velocity

• Predicted dependence of the ne fluxes on Tc
• From e Be7 ? ne Li7 F(ne) ? Tc8
• From B8 ? Be8 e ne F(ne) ? Tc18
• F ? Tc N DF/F N DTc/Tc

How precisely do we know the temperature T of
the Sun core?
Search for ne from p p ? e ne d (the
main component of the solar neutrino spectrum,
constrained by the Sun luminosity)
very little theoretical uncertainties
26

• Gallium experiments radiochemical experiments to
  search for
• ne Ga71 ? e
  Ge71
• Energy threshold E(ne) gt 0.233 MeV
  reaction sensitive to solar neutrinos
• from p p ? e ne d (the dominant component)
• Three experiments
• GALLEX (Gallium Experiment, 1991 1997)
• GNO (Gallium Neutrino Observatory, 1998 )
• SAGE (Soviet-American Gallium Experiment)

In the Gran Sasso National Lab 150 km east of
Rome Depth 3740 m w.e.
In the Baksan Lab (Russia) under the Caucasus.
Depth 4640 m w.e.

• Target 30.3 tons of Gallium in HCl solution
  (GALLEX, GNO)
• 50 tons of metallic Gallium
  (liquid at 40C) (SAGE)
• Experimental method every few weeks extract Ge71
  in the form of GeCl4 (a highly volatile
• substance), convert chemically to gas GeH4,
  inject gas into a proportional counter, detect
• radioactive decay of Ge71 e Ge71 ? ne
  Ga71 (half-life t1/2 11.43 d)
• (Final state excited Ga71 atom emits X-rays
  detect K and L atomic transitions)
• Check of detection efficiency
• Introduce a known quantity of As71 in the tank
  (decaying to Ge71 e Ge71 ? ne Ga71)
• Install an intense radioactive source producing
  mono-energetic ne near the tank
• e Cr51 ? ne V51 (prepared in a nuclear
  reactor, initial activity 1.5 MCurie equivalent
• to 5 times the solar neutrino flux), E(ne)
  0.750 MeV, half-life t1/2 28 d

27
Ge71 production rate 1 atom/day
6.5 6.1
SAGE (1990 2001) 70.8
SNU SSM PREDICTION 128 SNU
Data/SSM 0.56 ?? 0.05
9 7
28
0.465?0.016

• Data are consistent with
• Full ne flux from p p ? e ne d
• 50 of the ne flux from B8 ? Be8 e ne
• Very strong (almost complete) suppression
• of the ne flux from e Be7 ? ne Li7

The real solar neutrino puzzle There is evidence
for B8 in the Sun (with deficit 50), but no
evidence for Be7 yet Be7 is needed to make B8 by
the fusion reaction p Be7 ? g B8 Possible
solutions

• At least one experiment is wrong
• The SSM is totally wrong
• The ne from e Be7 ? ne Li7 are no longer
  ne when they reach the Earth and become
• invisible ne OSCILLATIONS

29
Unambiguous demonstration of solar neutrino
oscillations SNO (the Sudbury Neutrino
Observatory in Sudbury, Ontario, Canada)
SNO a real-time experiment detecting Cerenkov
light emitted in 1000 tons of high purity heavy
water D2O contained in a 12 m diam. acrylic
sphere, surrounded by 7800 tons of high
purity water H2O Light collection 9456
photomultiplier tubes, diam. 20 cm, on a
spherical surface with a radius of 9.5 m Depth
2070 m (6010 m w.e.) in a nickel mine Electron
energy detection threshold 5 MeV Fiducial
volume reconstructed event vertex within 550 cm
from the centre
30

• Solar neutrino detection at SNO
• (ES) Neutrino electron elastic scattering
  n e ? n e
• Directional, s(ne) ? 6 s(nm) ? 6 s(nt)
  (as in Super-K)
• (CC) ne d ? e p p
• Weakly directional recoil electron
  angular distribution ? 1 (1/3) cos(qsun)
• Good measurement of the ne energy
  spectrum (because the electron takes
• most of the ne energy)
• (NC) n d ? n p n
• Equal cross-sections for all three
  neutrino types
• Measure the total solar flux from
  B8 ? Be8 e n in the presence of
• oscillations by comparing the rates
  of CC and NC events
• Detection of n d ? n p n
• Detect photons (? ee) from neutron capture at
  thermal energies
• First phase (November 1999 May 2001)
• n d ? H3 g (Eg 6.25 MeV)

31

• SNO expectations
• Use three variables
• Signal amplitude (MeV)
• cos(qsun)
• Event distance from centre (R)
• (measured from the PM relative times)

cos(qsun)
(R/Rav)3 (proportional to volume)
(Rav 6 m radius of the acrylic sphere)
Use b and g radioactive sources to calibrate the
energy scale Use Cf252 neutron source to measure
neutron detection efficiency (14) Neutron signal
does not depend on cos(qsun)
32
From 306.4 days of data taking Number of events
with kinetic energy Teff gt 5 MeV and R lt 550 cm
2928 Neutron background 78 ? 12 events.
Background electrons 45 events
18 12
Use likelihood method and the expected
distributions to extract the three signals
33
Solar neutrino fluxes, as measured separately
from the three signals FCC(ne) 1.76
x 106 cm-2s-1 FES(n) 2.39
x 106 cm-2s-1 FNC(n) 5.09
x 106 cm-2s-1
0.06 0.09 0.05 0.09
Note FCC(ne) ? F(ne)
Calculated under the assumption that all incident
neutrinos are ne
0.24 0.12 0.23 0.12
FSSM(n) 5.05 x 106 cm-2s-1
0.44 0.46 0.43 0.43
1.01 0.81
stat. syst.
stat. and syst. errors combined
FNC(n) FCC(ne) F(nmt) 3.33 ?? 0.64 x 106
cm2 s 1 5.2 standard deviations from zero
evidence that solar neutrino flux on Earth
contains sizeable nm or nt component (in any
combination)
Write FES(n) as a function of F(ne) and F(nmt)
(because
)
F(n) F(ne) F(nmt)
34
Interpretation of the solar neutrino data using
the two-neutrino mixing hypothesis Vacuum
oscillations ne spectrum on Earth F(ne)
Pee F0(ne) (F0(ne) ? spectrum at
production) ne disappearance
probability L 1.496 x 1011
m (average Sun Earth distance with 3.3 yearly
variation from eccentricity of Earth
orbit) Fit predicted ne spectrum to
data using q, Dm2 as adjustable parameters
L m E MeV Dm2 eV2
4x1010 eV2 1010 4x1011
Regions of oscillation parameters consistent with
solar neutrino data available before the end of
the year 2000
35
NEUTRINO OSCILLATIONS IN MATTER
(L. Wolfenstein, 1978)
Neutrinos propagating through matter undergo
refraction.
p neutrino momentum N density of scattering
centres f(0) forward scattering amplitude
(at ? 0)
Refraction index
In vacuum
Plane wave in matter ? ei(npr Et)
(for e ltlt 1)
But energy must be conserved!
Add a term V ? neutrino potential energy in
matter
V lt 0 attractive potential (n gt 1) V gt 0
repulsive potential (n lt 1)
36
Neutrino potential energy in matter
1. Contribution from Z exchange (the same for all
three flavours)

n n
Z
GF Fermi coupling constant Np (Nn) proton
(neutron) density ?w weak mixing angle
e,p,n e,p,n
2. Contribution from W exchange (only for ne!)
ne
e-
W
matter density g/cm3
electron density
ne
e-
NOTE V(n) V( n )
37

Example two-neutrino mixing between ne and nm in
a constant density medium (same results for
mixing between ne and nt)
Use flavour basis
Evolution equation
2x2 matrix
(Remember
for M ?? p)
NOTE m1, m2, ? are defined in vacuum
38

Rewrite
diagonal term no mixing
term responsible for nenm mixing
r constant time-independent
H Diagonalize non-diagonal term in H to obtain
mass eigenvalues and eigenstates
Eigenvalues in matter
eV2
(r in g/cm3, E in MeV)
Mixing angle in matter

For x Dm2cos2q ? x res mixing becomes maximal
(qm 45) even if the mixing angle in vacuum is
very small MSW resonance (discovered by
Mikheyev and Smirnov in 1985)
Notes MSW resonance can exist only if q lt 45
(otherwise cos2q lt 0) For ne x lt
0 no MSW resonance if q lt 45
39
M2 M22 M12
Mass eigenvalues versus x
Oscillation length in matter
(l ? oscillation length in vacuum)
At x x res
40
Matter-enhanced solar neutrino oscillations
Solar neutrinos are produced in a high-density
medium (the Sun core) and travel through
variable density r r(t) Use formalism of
neutrino oscillations in matter
Evolution equation Hn i ?n / ? t H (2 x
2 matrix) depends on time t through r(t)
H has no eigenstates Solve the
evolution equation numerically
solar density vs. radius
100 10 1 0.1
r g/cm3
R/RO
0. 0.2 0.4 0.6 0.8
(pure ne at production)
(d very small time interval)
(until neutrino escapes from the Sun)
41
It is always possible to write
(a12 a22 1)
where n1, n2 are the local eigenstates of the
time-independent Hamiltonian for fixed r
At production (t0, in the Sun core)
n1(0), n2(0)
eigenstates of H for rr(0)
Assume q (mixing angle in vacuum) lt 45 cosq gt
sinq in vacuum qm gt 45 at production if x gt
xres x gt xres

( Dm2 in eV2, r in g/cm3)
A simple class of solutions ( adiabatic
solutions) a1 ? a1(0), a2 ? a2 (0) at all
t (if r varies slowly over an oscillation
length) At exit from the Sun (ttE)
M2
n1(tE), n2(tE) mass eigenstates in vacuum In
vacuum (because q lt 45 in vacuum)

qm lt 45
qm gt 45
ne DEFICIT
42
Regions of the (Dm2 , sin22q) plane allowed by
the solar neutrino flux measurements in the
Homestake, Super-K and Gallium experiments
Different energy thresholds
different regions of the (Dm2 , sin22q) plane
Super-K
The regions common to the three
measurements contain the allowed oscillation
parameters
43
Matter-enhanced solar neutrino oscillations
(MSW solutions) (using only data available
before the end of the year 2000)
Survival probability versus neutrino energy
LMA
SMA
105 eV 2
LOW
sin22q
103 102 101
LMA Large Mixing Angle SMA Small Mixing Angle
44

• Additional experimental information
• Energy spectrum distortions

Super-K 2002
Data/SSM
Electron kinetic energy (MeV)
SNO recoil electron spectrum from ne d ? e p
p
SNO data/SSM prediction
ne deficit is energy independent within errors
(no distortions)
45
Seasonal variation of measured neutrino flux in
Super-K
Yearly variation of the Sun-Earth distance 3.3
? seasonal variation of the solar neutrino flux
for some vacuum oscillation solutions
Note expected seasonal variation from change of
solid angle ? 6.6
Days since start of data taking
The observed effect is consistent with the
variation of solid angle alone
46
Day-night effects (expected for some MSW
solutions from matter-enhanced oscillations when
neutrinos traverse the Earth at night
increase of ne flux at night)
Subdivide night spectrum into bins of Sun zenith
angle to study dependence on path length
inside Earth and density
cos(Sun zenith angle)
SNO Day and Night Energy Spectra (CC ES NC
events) Difference Night Day
47
SK data comparison with oscillations
Sun zenith angle distributions for different
electron energy bins
Electron energy distribution
Vacuum oscillation SMA LMA LOW

• Vacuum oscillation and SMA solution disagree
  with electron energy distribution
• LMA and LOW solutions describe reasonably well
  the zenith angle distributions
• No dependence on zenith angle within errors

48
Global fits to all existing solar neutrino
data 48 data points, two free parameters (mixing
angle q, Dm2) ? 46 degrees of freedom LMA
solution ?2 43.5 Dm2 6.9x10 5 eV2 q
31.7 (BEST FIT) LOW solution ?2 52.5 Dm2
7.2x10 8 eV2 q 39.1
D?2 9 Prob(D?2 ? 9) 1.1 (marginally
acceptable)
LMA
Dm2 eV2
The present interpretation of all solar neutrino
data using two-neutrino mixing
Note variable tan2q is preferred to sin22q
because sin22q is symmetric around q 45 and
MSW solutions are possible only if q lt 45
tan2q
49
Verification of the LMA solution using
antineutrinos from nuclear reactors Nuclear
reactors intense, isotropic sources of ne from
b decay of neutron-rich fission fragments ne
production rate 1.9x1020 Pth s1
(Pth GW reactor thermal power) Broad energy
spectrum extending to 10 MeV, ltEgt ? 3
MeV Uncertainty on the expected ne flux 2.7
Detection ne p ? e n (on the free
protons of hydrogen rich liquid scintillator)

thermalization by multiple
collisions
(lttgt ?180 ms),
followed by capture
e e ? 2g n p ? d
g (Eg 2.2 MeV)
prompt signal
delayed signal E En
0.77 MeV
KAMioka Liquid scintillator Anti-Neutrino
Detector (KAMLAND) ne source several nuclear
reactors surrounding the Kamioka site Total power
70 GW average distance 175 ? 35 km (long
baseline) Expected ne flux (no oscillations) ?
1.3 x 106 cm2 s1 550
events/year Average oscillation length ltloscgt ?
110 km for Dm2 6.9 x 105 eV 2 (LMA)
expect large ne deficit with measurable
energy modulation
50
KAMLAND detector
1000 tons liquid scintillator Transparent
balloon Mineral oil Acrylic
sphere Photomultipliers (1879) (coverage 35 of
4p) Outer detector (pure H2O) 225
photomultipliers
13 m 18 m
51
KAMLAND sensitivity to ne oscillations
Fiducial mass 600 tons
Exclusion regions if no ne deficit is observed
?1 s regions after 3 years
Data taking in progress since January 2002
results expected soon
52
Borexino experiment (at Gran Sasso National
Lab) Study of the elastic scattering reaction
n e ? n e Recoil electron
detection threshold 0.25 MeV
sensitivity to from e Be7 ? ne Li7
(En 0.861 MeV) 300 tons of
ultra-pure liquid scintillator
isotropic light emission no
directionality Expected event rate ( electron
energy 0.25 0.8 MeV) No oscillations 55
events/day LMA 35 events/day
(? 3s ) Expected background 15
events/day Start data taking mid 2003
5 3
53
Primary cosmic ray interacts in upper atmosphere
ATMOSPHERIC NEUTRINOS
e
The main sources of atmospheric neutrinos p?, K
? ? m ? nm( nm) ? e ?
ne( ne) nm(nm)
At energies E lt 2 GeV most parent particles decay
before reaching the Earth
At higher energies, most muons reach the Earth
before decaying
(increasing with E)
Energy range of atmospheric neutrinos 0.1 100
GeV Very low event rate 100 /year for a
detector mass of 1000 tons Uncertainties on
calculations of atmospheric neutrino fluxes
typically 30 (from composition of primary
spectrum, secondary hadron distributions,
etc.) Uncertainty on the nm/ne ratio only 5
(because of partial cancellations)
54
Detection of atmospheric neutrinos nm Nucleon ?
m hadrons presence of a long, minimum
ionizing track (the m) ne n ? e p, ne p ?
e n presence of an electromagnetic
shower (ne interactions with multiple hadron
production is difficult to separate from neutral
current events for atmospheric ne only
quasi-elastic interactions can be studied)

• Particle identification in a water Cerenkov
  counter
• muon track
• dE/dx consistent with minimum ionization
• sharp edges of Cerenkov light ring
• electron shower
• high dE/dx
• fuzzy edges of Cerenkov light ring
• (from shower angular spread)
• Measure electron/muon separation by exposing a
  1000 ton water Cerenkov counter
• (a small Super-K detector) to electron and muon
  beams from accelerators.
• Probability of wrong identification 2
• Measurements of the nm/ne ratio first hints for
  a new phenomenon
• Water Cerenkov counters Kamiokande (1988), IMB
  (1991), Super-K (1998)
• Conventional calorimeter (iron plates
  proportional tubes) Soudan2 (1997)
• (nm/ne)measured
• (nm/ne)predicted

42
R 0.65 0.08
55
Atmospheric neutrino data from Super-K Distance
between event vertex and inner detector wall ?1
metre
(April 96 July 01)
PC events are all assumed to be m-like
Lepton (e/m) energy GeV
56
Classification of Super-K events
(m/e)Data (m/e)MC
0.638 0.016 0.05
(m/e)Data (m/e)MC
0.030 0.028
0.658 0.078
57
An additional event sample Upward-going muons
produced by nm interactions in the rock
Note downward going muons are dominated by
high-energy cosmic ray muons traversing
the mountain and reaching the detector
58
Measurement of zenith angle distribution
Definition of zenith angle q Polar axis along
the local vertical axis, directed downwards
Earth atmosphere
Down-going q 0º
detector
Up-going q 180
Horizontal q 90
Baseline L (distance between neutrino production
point and detector) depends on zenith angle
Earth
local vertical axis
104 103 102 10
L varies between 10 and 12800 km as q
varies between 0º and 180º search for
oscillations with variable baseline Strong
angular correlation between incident neutrino and
outgoing electron/muon for E gt 1 GeV n
L Km

• a ? 25 for E 1 GeV
• a ? 0 as E increases

a
e/m
5 km uncertainty on n production point
1. 0.5 0. 0.5
1.
cosq
59
Super-K zenith angle distributions
No oscillation (c2 456.5 / 172 degrees of
freedom) nm nt oscillation best fit Dm2
2.5x103 eV2, sin22q 1.0
c2 163.2 / 170 degrees
of freedom
60

• Super-K zenith angle distributions
• evidence for nm disappearance over distances of
  1000 10000 km
• Oscillation cannot be nm ne
• Excluded by reactor experiment CHOOZ (see
  later)
• Zenith angle distribution for e-like events
  would show opposite sign up-down asymmetry
• (more upward-going e-like events) because
  nm/ne ? 2 at production
• a nm nt oscillation is the most
  plausible solution

(nt N ? t X requires E(nt) gt 3.5 GeV and t ?
m decay fraction ? 18 only)
Super-K
Combined region (90 CL) Dm2(1.3 3.9) x 103
eV2 sin22q gt 0.92
61
CHOOZ a long baseline ne disappearance
experiment sensitive to Dm2 gt 7 x 104 eV2
Two reactors at the Chooz EDF power plant (total
thermal power 8.5 GW) L 998, 1114
m Detector 5 tons of Gadolinium-loaded liquid
scintillator (neutron capture in Gd ? gs with
total energy 8.1 MeV) 17 tons unloaded
scintillator (to contain the grays) 90 ton
liquid scintillator (for cosmic ray
rejection) Detector installed in an underground
site under 300 m w.e. Data taking
1997-98 (Experiment completed in 1998)
62
Event rate with reactors at full power 25 /
day Background rate (reactors off) 1.2 /
day Positron energy spectrum (prompt signal
from ne p ? n e) and comparison with
expected spectrum without oscillation
Measured spectrum Expected spectrum
(no oscillation) Ratio (integrated over energy
spectrum) 1.010 0.028 0.027
no evidence for ne disappearance
Positron energy
63
CHOOZ experiment
Excluded region for ne nx oscillations
Dm2 eV2
Super-K nm nt oscillation
64

• Distinguishing nm nt from nm ns oscillations
• (ns sterile neutrino, a hypothetical neutrino
  with no coupling to W and Z
• no interaction with matter)
• Two methods
• Select a sample of multi-ring events with no
  mlike ring (event sample enriched
• in neutral-current events n N ? n hadrons)
• nm nt oscillation no up down asymmetry in
  the zenith angle distribution
• (nm and nt
  have the same neutral-current interaction)
• nm ns oscillation up down asymmetry
  similar to that of mlike events
• Matter effects when neutrinos traverse the
  Earth
• Potential energy in matter V(nm) V(nt)
  VZ, V(ns) 0
• nm nt oscillation no matter effects
• nm ns oscillation

neutron density
density g/cm3
(VZ lt 0 for
neutrinos, VZ gt 0 for anti-neutrinos) Matter-effe
cts are important when VZEn ? Dm2 (En ? 20 GeV
for r ? 5 g/cm3) Study
high-energy m-like events
65

Fit Super-K data with nm ns oscillations
No oscillation nmns oscillation
(nmnt oscillations c2min163.2/170 dof)
66
Try nm n oscillation with n cosx nt sinx ns
pure nt
sin2x lt 0.19 (90 confidence)
67
LSND and KARMEN experiments search for nm ne
oscillations Conceptual design
Anti-coincidence counter
800 Mev protons
p
q
n
target beam dump
Detector
shielding
Neutrino sources
Decay At Rest (DAR) 75
DAR 100
p
nm e ne
7090
nm m
Decay In Flight (DIF) 5
800 MeV (kin. energy) proton-nucleus
collision
20
m p ? nm n
nuclear absorption
capture?90
DIF few
p
nm m
DAR ?10
The only source of ne
3010
nm e ne
ne ne
? 103
68
Parameters of the LSND and KARMEN experiments

LSND
KARMEN
Accelerator Los Alamos
Neutron Neutron Spallation
Facility
Science Centre
ISIS ar R.A.L. (U.K.) Proton kin. energy
800 MeV
800 MeV Proton current
1000 mA
200 mA Detector
Single cylindrical tank

filled with liquid scintillator 512
independent cells
Collect both scintillating
filled with liquid scintillator
and
Cerenkov light Detector mass
167 tons
56 tons Event localisation
PMT timing
cell size Distance from n
source 29 m
17 m Angle q
between proton 11
90
and n direction Data taking period
1993 98
1997 2001 Protons on target
4.6 x 1023
1.5 x
1023

Neutrino energy spectra from p ? m nm decay at
rest
e nm ne
MeV
69
ne detection the classical way
ne p ? e n

delayed signal from np?? gd (Eg 2.2
MeV) KARMEN has Gd-loaded paper between adjacent
cells ? enhanced neutron capture, SEg 8.1 MeV
prompt signal
KARMEN beam time structure Repetition rate 50 Hz
Expect nm ? ne oscillation signal within 10 ms
after beam pulse
LSND beam time structure Repetition rate 120 Hz
0 600
ms
no correlation between event time and beam pulse
time ms
70

LSND final results evidence for nm ne
oscillations Positrons with 20 lt E lt 200 MeV
correlated in space and time with 2.2 MeV
g-ray from neutron capture N(beam-on events)
N(beam-off events) 117. 9 22.4 events
Background from DAR n 29.5
3.9 Background from
DIF ne 10.5 4.6
ne signal 87. 9
22.4 6.0 events

(stat.) (syst.)
Posc( nm ne)
(0.264 0.067 0.045) x 10-2 Tighter event
selection (less background) Positrons with 20 lt E
lt 60 MeV N(beam-on) N(beam-off) 49.1 9.4
events n-induced background 16.9 2.3
ne signal 32.2
9.4 events
71
KARMEN final results Events selection criteria
space and time correlation between prompt and
delayed signal
time correlation between prompt signal
and beam pulse
16 lt E(e) lt 50 MeV Number of selected
events 15 Expected backgrounds
Cosmic rays 3.9 0.2
Random coincidences between two
ne ? e events 5.1 0.2 Random coincidences
between ne ? e and uncorrelated g 4.8
0. 3
Intrinsic ne contamination 2.0
0. 2 Total background 15.8 0. 5 events
no evidence for nm ne oscillations Posc(
nm ne) lt 0.085 x 10-2 (90 confidence) LSND
value (0.264 0.067 0.045) x 10-2
Consistency between KARMEN and LSND is only
possible for a restricted region of oscillation
parameters because the baseline L is different
for the two experiments L 29 m (LSND) L 17
m (KARMEN)
LSND allowed region and KARMEN exclusion region
72

• LSND evidence for nm ne oscillations a very
  serious problem
• Define Dmik2 mk2 mi2 (i,k 1, 2, 3)
• Dm122 Dm232 Dm312
  0
• Evidence for neutrino oscillations
• Solar neutrinos Dm122 ? 6.9 x 105
  eV2
• Atmospheric neutrinos Dm232 ? 2.5 x 103 eV2
• LSND Dm312 0.2
  2 eV2
• Dm122 Dm232 Dm312
  0.2 2 eV2
• If all three results are correct, at least
  one additional neutrino
• is needed.
• To be consistent with LEP results (only
  three neutrinos),
• any additional neutrino, if it exists,
  must be sterile
• (no coupling to W and Z bosons ? no
  interaction with matter)
• LSND result needs confirmation

73

• MiniBooNE (Booster Neutrino Experiment at
  Fermilab)
• Goal to definitively confirm (or disprove) the
  LSND signal
• start with nm ne appearance search
• then search for nm ne search
• if a positive signal is found, build a second
  detector at different L

50 m decay region
450 m earth
Fermilab 8 GeV proton synchrotron
Beryllium target
focuses p in an almost parallel beam
n flux (arbitrary units)
Neutrino beam flux calculations
En GeV
74
MiniBooNE detector

• 12 m diameter spherical tank
• 807 tons mineral oil used as
• Cerenkov radiator
• fiducial mass 445 tons
• optically isolated inner region
• with 1280 20 cm diam. PM tubes
• external anticoincidence region
• with 240 PM tubes

Particle identification based on different
behaviour of electrons, muons, pions and pattern
of Cerenkov light rings
75
MiniBooNE expectations for two years of data
taking (1021 protons on target) 500K nmC ? mX
events, 70K nC ? nX events Background to the nm
ne oscillation signal 1500 neC ? e X
events (from beam contamination) 500
mis-identified m 500 mis-identified p
1000 neC ? e X events if the LSND result is
correct Note the electron energy
distributions from nm ne oscillations
and from the ne contamination in the
beam are different because the nm
and contamination ne have different
energy spectra
MiniBooNE exclusion region after two years of
data taking if no oscillation signal is observed
LSND allowed region 90 C.L.
99 C.L.
Start data taking June 2002
sin22q
76

• Long baseline experiments at accelerators
• Purpose to provide definitive demonstration that
  the atmospheric nm deficit
• is due to neutrino oscillations using
  accelerator-made nm .

Super-K L/E distribution does not
show oscillatory behaviour expected from
oscillations because of poor resolution on the
L/E variable at large L/E values
Data Prediction
Ideally
Maximum L ? 12800 km to study the
region L/E gt 104 km need events with E lt 1 GeV
for which the angular correlation between the
incident neutrino and the outgoing muon is weak
poor L/E resolution
L / E km/GeV
Planned measurements at long baseline
accelerator experiments

• Distortions of the nm energy spectrum at large
  distance (measurement of Dm2 and sin22q)
• Ratio of neutral current to charged current
  events (to distinguish nm nt oscillations
• from oscillations to a sterile neutrino ns)
• nt appearance at large distance in a beam
  containing no nt at production.

77
Long baseline accelerator experiments (in
progress or in preparation)
Project
Baseline L ltEngt
Status K2K (KEK to KAMIOKA)
250 km 1.3 GeV Data
taking since June 99 MINOS (Fermilab to Soudan)
735 km few GeV Start
data taking 2005 CERN to Gran Sasso
732 km 17 GeV
Start data taking 2006

• Threshold energy for nt N ? t X En gt 3.5
  GeV
• Typical event rate 1 nm ? m event / year
  per ton of detector mass
• need detectors with masses of
  several kilotons
• nm beam angular divergence

beam line
q
p
nm from p ? m nm decay
Beam transverse size 100 m 1 km at L gt 100
km no problems to hit the far detector
but neutrino flux decreases as L2 at large L
78
K2K
12 GeV proton synchrotron
L250 km
Neutrino beam composition 95 nm 4 nm 1 ne
K2K Front Detector neutrino flux monitor and
measurement of nm interactions without
oscillations 1 Kton Water Cerenkov
detector Similar to Super-K fiducial mass 25
tons Scintillating Fibre Water Detector (SciFi) D
etect multi-track events fiducial mass 6
tons Muon chambers Measure m range from p
decay mass 700 tons nm beam monitor
79
beam spill duration
1Rm 1ring m-like events
80
Expected Posc(nmnm) versus En at L 250 km for
Dm2 3x103 eV2, sin22q 1
Posc 0
Expected shape of the nm spectrum in Super-K with
and without nm disappearance
En GeV
Beamassociated events in Super-K June 1999
July 2001 (4.8x1019 protons on target) FCFV
events, Evis gt 30 MeV Expected (Posc 0)
80.1 events
Observed 56 events

(probability of a statistical fluctuation 3
if Posc 0) Nov 1999 July 2001 (stable beam
conditions) 1Rm events
Observed 29 events

6.2 5.4
81
Measurement of the nm energy distribution in
Super-K using 1Rm events (assumed to be
quasi-elastic events nm n ? m p)
m
Incident nm direction (precisely known)
q
Recoil proton (not detected because under
Cerenkov threshold)
Expected shape (no oscillation) Expected
shape for nm disappearance Dm2 3x103
eV2 sin22q 1 (Best fit)
Assume target neutron at rest and apply
two-body quasielastic kinematics to extract
incident nm energy
(M ? nucleon mass)
Measured En distribution shows distortion
consistent with oscillation with Dm2 3x103
eV2, sin22q 1, as suggested by atmospheric
neutrino data Probability for no oscillation
0.7 (combining event deficit and distortion of
spectral shape)
En GeV
82
MINOS experiment Neutrino beam from Fermilab to
Soudan (an inactive iron mine in Minnesota) L
730 km
Accelerator Fermilab Main Injector (MI) 120 GeV
proton sinchrotron High intensity (0.4
MW) 4x1013 protons per cycle Repetition rate
1.9 s 4x1020 protons on target / y Hadron decay
pipe 700 m
83
MINOS Far Detector

• 8 m octagonal steel tracking calorimeter
• Magnetized steel plates 2.54 cm thick
• 4 cm wide scintillator strips between plates
• 2 modules, each 15 m long
• 5400 ton total mass (fiducial mass 3300 tons)
• 484 planes of scintillator strips (26000 m2)
• Steel plates are magnetized toroidal field,
• B 1.5 T
• Far Detector is half-built, to be completed by
• June 2003
• Now recording cosmic ray events

• MINOS Near Detector
• 3.8x4.8 m octagonal steel tracking calorimeter
• Same basic construction as Far Detector
• 282 magnetized steel plates
• 980 ton total mass (fiducial mass 100 tons)
• installed 250 m downstream of the end of the
  decay pipe
• First protons on target scheduled for December
  2004

84
MINOS Expected energy distributions for nm ? m
events Low energy beam, exposure
of 10 kton x year
Histogram no nm disappearance Data points
oscillation with sin22q 0.9
Dm2 is measured from position of minimum in the
ratio versus E plot sin22q is measured from
its depth.
85
MINOS distinguishing between nm nt and nm
ns oscillations Compare ratio NC/CC defined as
Rate of muonless events Rate of m
events in Far and Near Detector. nm nt
oscillations nt is under threshold for t
production no charged current
events same neutral current
events as nm nm ns oscillations ns does
not interact with matter no
charged current events no
neutral current events

MINOS excluded region for nm nt
oscillations
if (NC/CC)
is found to be the same within errors

in the Near and
Far Detector
10 kton x year
Beam energy Low Medium High
86
CNGS (CERN Neutrinos to Gran Sasso) Search for nt
appearance at L 732 km Expected number of nt
N ? t X events (Nt)
Normalization constant contains detector
mass, running time, efficiencies, etc.
cross-section for t production
nm flux
nm nt oscillation probability Pmt
Good approximation for L 732 km, E gt 3.5 GeV,
Dm2 lt 4x103 eV2

• Disadvantages
• L 732 km is too short to reach the first nm
  nt oscillation maximum
• Nt depends on (Dm2) 2 very low
  event rates at low values of Dm2
• Advantages
• Beam optimization does not depend on Dm2 value

87
400 GeV proton beam from the CERN SPS
Neutrino beam layout at CERN
88
Neutrino beam energy spectra and
interaction rates at Gran Sasso Primary
protons 400 GeV 4x2.3x1013 / SPS cycle SPS
cycle 26.4 s Running efficiency 75 Running
time 200 days/year Protons on target
4.5 x 1019 / year (sharing SPS with other users)

With SPS in dedicated mode (no other user)
expect 7.6 x 1019 protons on target / year
89
Search for nt appearance at Gran Sasso Two
detectors (OPERA, ICARUS) No near detector
Gran Sasso National Laboratory and the two
neutrino detectors
90
OPERA experiment t detection through the
observation of one-prong decays Typical t mean
decay length 1 mm need very good space
resolution Use photographic emulsion (space
resolution 1 mm)
Plastic base
50 mm thick emulsion films
Brick 56 emulsion films separated by 1 mm
thick Pb plates packed under vacuum
Internal brick structure
Bricks are arranged into walls of 52 x 64
bricks Walls are arranged into supermodules 31
walls / supermodule Two supermodules, each
followed by a magnetic spectrometer 206 336
bricks, total mass 1800 tons Track detectors
(orthogonal planes of scintillator strips) are
inserted among brick walls to provide trigger and
to identify the brick where the neutrino
interaction occurred. The brick is immediately
removed for emulsion development and automatic
scanning and measurement using computer-controlled
microscopes
91
OPERA supermodule
Magnetic spectrometer magnetized iron dipole
12 Fe plates 5 cm thick equipped with trackers
(RPC)
92
OPERA backgrounds