BIG WORLD OF small NEUTRINOS

From the very beginning, Neutrinos have remainedan enigma of particle physics. Three decades afterthe successful formulation of the Standard Modelof Particle Physics we do not understandthem fully.On the otherhand they play a fundamental rolein many phenomena of Cosmology, Astrophysics,Particle and Nuclear Physics.

This is an elementary introduction to this vastsubject and to the question why INO?

Figure 1: Cartoon depiction of various questions- a poster from Fermilab

2

• Introduction.

• Sources- Natural and Laboratory.

• Observations and Anomalies.

• Possible Solutions - Neutrino oscillations.

• New generation of precision experiments-India-based Neutrino Observatory

3

Sources: Neutrinos come from everywhere

• Starting from Big Bang- - they are still there -about 300 neutrinos per cm3.

• They come from stars - Solar and Supernovaneutrinos, for example. The sun emits around2 × 1038 neutrinos per second and the earth re-ceives more than 40 billions neutrinos per sec-ond and cm2.

• And are produced in the Earth’s atmosphereand in laboratories.

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Naturally produced Neutrino Spectrum

Laboratory Sources of Neutrinos:

• Neutrinos from Reactors (MeV).

• Neutrinos from Accelerators (GeV).

• Neutrinos from muon storage rings ( 10’s GeV)? Atleast10-15 years.

• Beta beams ( 100’s MeV) ?

5

Neutrino Time Line

• 1930 Neutrinos were proposed by Pauli-

H3 → He3 + e− + ...

the possibility that there could exist in the nu-clei electrically neutral particle ...” (Pauli, 4Dec. 1930). Fermi gave the name neutrino -

H3 → He3 + e− + νe

• 1956 Discovered experimentally through

νe + p → e+ + n

using neutrinos from fission reactor.

• 1962 Muon neutrino discovered.

• 1964 Obsevation of Atmospheric neutrinos atKGF, India.

• 1967 Ray Davis starts Solar Neutrino Experi-ments at Home stake.

• 1987 Observation of Neutrinos from SN1987Aby Kamiokande Collaboration.

• 1998 Evidence of neutrino mass– deficit of muonneutrinos from the atmosphere. Super K.

• 2001-2004 Evidence of Neutrino Oscillations–Confirmation of the Standard Solar Model bySNO.

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Neutrinos in the Standard Model

• There are three types of active neutrinos allwith it strictly zero mass in SM along withtheir anti-neutrinos.

νL−−−→CPT νR

Non-zero neutrino mass ⇒ SM is incomplete

• Interaction strength is extremely weak.The detectors have to be huge to catch them.

• Background can be enormous-Experiments located deep underground.

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Underground laboratories around the world

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The solar neutrino problem

• 1938 Bethe - Energy produced by Sun ( stars)is a result of nuclear fusion of H to He. Quasi-static equilibrium - Thermal pressure vs Grav-itational contraction.

Sun ⇒ The evolutionary sequence of a homoge-neous star after 4.6 × 109 years.

• Neutrinos are products of these fusion reactions (many)which take place in the deep interior of the Sun.The main sequence of Hydrogen fusion in toHelium may be summarised as

4p → He4 + 2e+ + 2νe + 26.7MeV (photons)

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The pp-chain

p + p → D + e+ + νe p + p + e+ → D + νe

(99.77 percent) (0.23percent)Eν ≤ 0.423MeV Eν = 1.442MeV

D + p → He3 + γ

He3 + He3 → He4 + p + p He3 + He4 → Be7 + γ He3 + p → He4 + e+νe

(84.92 percent) (15.08 percent) (10−5 percent)Eν ≤ 18.77MeV

Be7 + e− → Li7 + γ + νe Be7 + p → B8 + γ(15.07 percent) (0.01 percent)Eν = 0.861MeV

Li7 + p → He4 + he4 B8 → 2He4 + e+ + νe

Eν ≤ 15MeV

Estimate of the neutrino flux:

4p → He4 + 2e+ + 2νe + 26.7MeV

summarises the main energy production mecha-nism. Assume the entire luminosity of the sun isdue to this process:

φν = 2L¯(erg/sec)

26.7(MeV )4πD2.

L¯ = 3.8 × 1033erg/sec.

D = 1.5 × 1013cms.

φν = 6 × 1010νe /cm2/sec

The real proof of “ How the Sun shines” lies in detecting theseneutrinos.

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Solar Neutrino Experiments

• 1964 Raymond Davis and Homestake experiment-600 tons of C2Cl4 in a tank in Homestake mine.

νe + Cl35 → e− + A35

is a direct measure of the neutrino flux. Countthe argon atoms to get neutrino flux.- Davisgathered data until 1994 and in all gathered2000 Argon atoms in three decades.

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• Water Cerenkov detectors -Kamioka and Su-perKEnormous water tank- lined with PhotomultiplierTubes placed in a Mine. (32 Kton in Super-K).

νe + e− → e− + νe

• Heavy Water detector at SNO1kTon of heavy water stored in an acrylic vesselsorrounded by water. Capable of detecting ES,CC and NC reactions.

The PMT’s detect the flashes of light createdby charged particles in real time with good di-rection.

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Summary of Solar Neutrino Observations

Experiment R = Observed/Predicted Energy Range

Davis- Homestake (1964-1994) 0.33 ± 0.03 ± 0.05 E> 0.814 MeV

Kamioka-SuperK (1986 - 2000) 0.465 ± 0.005 ± 0.015 E > 6.5 MeV

Gallex-SAGE (1988-1996) 0.60 ± 0.06 ± 0.04 E > 0.233 MeV

SNO- Sudbury (1998-2002) 0.347 ± 0.029(CC) E > 6.75 MeV

Deficit confirmed–energy dependent–Solar neutrinopuzzle.We discuss another puzzle closer home!

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The Atmospheric Neutrino Puzzle

Production of Atmospheric Neutrinos:

π+ → µ+ + νµ, µ+ → e+ + νµ + νe

π− → µ− + νµ, µ− → e− + νµ + νe

The puzzle

R =Robs

RMC= 0.65 ± 0.02 ± 0.05

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Results so far

• Neutrinos from SUN observed - confirmationof fusion as source of energy in stars. But thenumbers do not match prediction- an energydependent deficit.

• Atmospheric neutrinos also show deficit- Thereis an up-down asymmetry and is also energydependent.

• Neutrinos from Stellar Collapse observed-SN1987A.Stellar collapse scenario qualitatively confirmed.

• Physics beyond SM needed to account for puz-zles above.

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Short primer on neutrino oscillations

Simplest and by far the most appealing solution.For simplicity, consider two neutrinos νe, νµ different from

mass eigenstates: At t0.

|νe(t0) > = cos θ|ν1(t0) > + sin θ|ν2(t0) >

|νµ(t0) > = − sin θ|ν1(t0) > + cos θ|ν2(t0) >

At time t, propagated states are:

|νe(t) > = cos θ|ν1(t) > + sin θ|ν2(t) >,

= [cos2 θe−iE1t + sin2 θe−iE2t]|νe(t0) >

+ cos θ sin θ[e−iE2t − e−iE1t]|νµ(t0) >

where

Ei = (p2 + m2i )

1/2 ≈ p +m2

i

2p; mi ¿ Ei

The Probability of oscillation

Pνe→νµ = | < νe(t0)|νµ(t) > |2 = sin2 2θ sin2 ∆m2t

4p;

= sin2 2θ sin2 ∆m2L

4E∆m2 = m2

2 − m21.

Neutrinos are relativistic, v ≈ c = 1. Replace time by lengthof propagation:

Thus neutrinos oscillate IFF mass squareddifference and mixing are both non-zero

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First Second First Second

Mass states

Source

Time, t

Weak states

ν1 ν2 νe

νe

cosθ sinθ

-sinθ cosθ νµ

νe

νµ

νµ

( ) ν2( )( )=

ν1

ν1

ν2

νe

νµ

νµ

νe

νµνµ

νe

νµνµ νµ

ν2

ν1

cosθ

cosθ

sin

θ

sinθ

θ

θ

2

Pure νµ

0

1

0

Probability that νµ has become νe Probability that νµ is still νµ

Pure νµPure νµ

sin22θ

Distance, x = ct

λosc

=

P(ν

µ

ν

e)

+ P

(νµ

ν

µ)

2.5 Eν

∆m2

cosθ

-sin

θ

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Solutions to solar and atmospheric neutrinoproblems

• Solution to Solar Neutrino Puzzle: Part of theνe flux oscillated into νx which is almost unde-tected.

Relevant L for this to happen is the Sun-Earthdistance. Required ∆m2 = 7 × 10−5eV2. Furthercomplicated my matter effects on neutrinos.

• Solution to Atms. Neutrino Puzzle: Part of theνµ flux oscillated into νy- which is not detected.

Relevant L for this problem is 400-13000 kms.Required ∆m2 = 3 × 10−3eV2.

• Combined solution to Solar and AtmosphericNeutrino puzzles requires the mixing of threeflavours of Neutrinos and atleast two non-zero masses.

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What we know so far

• Definite evidence for neutrino oscillations ⇒ neu-trinos have non-zero, however small, mass

• There are two mass squared differences:

∆m221 = m2

2 − m21, ∆m2

32 = m23 − m2

2

and three mixing angles:

θ12, θ13, θ23

.

• Solar neutrino and KAMLAND reactor neutrino experiments:

∆m221 = 7 × 10−5 eV2; sin2 2θ12 = 0.82

• Atmospheric Neutrino experiments:

|∆m232| = 2 × 10−3 eV2; sin2 2θ23 = 1

• Reactor Neutrino experiments:

sin2 2θ13 < 0.16

First clear evidence of physics beyond theStandard Model of Particle Physics is established

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Open Problems OR Goals of INO

There is an Indian initiative to set up a neutrino detector at asuitable location- INO (India-based Neutrino Observatory)

• Establishing neutrino oscillation to a greater pre-cision beyond SK results–observation of osilla-tion minima and maxima

• The question of hierarchy? –Magnetized iron

ν

ν

ν

ν

ν

ν

1

2

3

2

1

3

7x10 −5

2x10

2x10

−3

−3Direct Hierarchy

Inverted Hierarchy

calorimeter is the answer.

• Improve limits on θ13

sin2 2θ13 < 0.16

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Magnetised iron calorimeter at INO: StatusReport

• Site Survey

• Detector R & D

• Simulations

• Physics Goals

INO is now a major collaborative projectinvolving as many as 15 institutions. The

collaboration has about 60 members and manymore will be needed. A Memorandum of

Understanding (MOU) was signed by variousparticipating Institutions in September 2002 witha mandate to prepare a detailed project report

within two years.

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The Collaboration

National Neutrino Collaboration1

B.S. Acharya1, C.V.K. Baba2, Sudeshna Banerjee1, A. Bhadra15, P.N. Bhat1,Pratap Bhattacharya3, Sudeb Bhattacharya3, Dhiman Chakraborty1,

Subhasish Chattopadhyay4, Sukalyan Chattopadhyay3, V.M. Datar15, S.R. Dugad1,Anindya Datta6, Raj Gandhi6, B. Ghosh15, P. Ghosh1, P. Ghosh4, Ambar Ghoshal3,

Srubabati Goswami6, Asimananda Goswami3, K.S. Gothe1, S.K. Gupta1,M.M. Gupta12, D. Indumathi7, A.S. Joshipura14 S.D. Kalmani1 Kamales Kar3,

N. Krishnan1, D. Majumdar3, M.R. Dutta Mazumdar4, Poonam Mehta6,D.P. Mohapatra11, Naba K. Mondal1 (Spokesperson), A. Mukherjee15,

G.S.N. Murthy4, M.V.N. Murthy7, B.K. Nagesh1, P. Nagaraj1, Tapan Nayak4,Palash B. Pal3, Biswajit Paul1, S.C. Phatak11, S. Rakshit6, A.K. Ray1,

Amitava Raychaudhury8, S.D. Rindani14, Amit Roy2, B. Satyanarayana1,Satyajit Saha3, Sandip Sarkar3, S.K. Sarkar15, Swapan Sen3, Manoj Sharan3,S.D. Sharma13, J.B. Singh12, S. Umasankar16, S. Upadhya1, Piyush Verma1,

Y.P. Viyogi4

Scientific Advisors:

Ramanath Cowsik9, H.S. Mani10, V.S. Narasimham1, G. Rajasekaran7, Amit Roy2,Bikash Sinha3

1Tata Institute of Fundamental Research, Mumbai, India2Nuclear Science Centre, New Delhi, India

3Saha Institute of Nuclear Physics, Kolkata, India4Variable Energy Cyclotron Centre, Kolkata, India5Bhabha Atomic Research Centre, Mumbai, India

6Harishchandra Research Institute, Allahabad, India7The Institute of Mathematical Sciences, Chennai, India8Dept. of Physics, University of Calcutta, Kolkata, India

9Indian Institute of Astrophysics, Bangalore, India10S.N. Bose Centre for Basic Sciences, Kolkata, India

11Institute of Physics, Bhubaneswar, India12Panjab University, Chandigarh, India

13H.P. University, Shimla, India14Physical Research Laboratory, Ahmedabad, India

15North Bengal University, Siliguri, India16Indian Institute of Technology, Mumbai, India

1This is an open Collaboration and experimentalists are especially encouraged to join in.

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Site Surveys

• Singara(PUSHEP)- On the northern foothills ofNilgiri mountains; 90km south of Mysore and60km north of Coimbatore. Good infrastruc-ture and access. Single depth (1400m)

• Rammam (Darjeeling)–River valley in the Dar-jeeling Himalayas. Multiple depths (1500m –1800m), excellent support,

• Rohtang Tunnel– Near Manali–futuristic–rightnext to a 9km road tunnel (ala Gran Sasso)–Multiple depths (1500m–2100m).

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ICAL detector at INO

• Detector ICAL— 16 m X 32 m X 11.9 m (height)–6 cm thick iron plates–2.5 cm air gap– contain-ing active detector elements– about 32 kTons.

• Active detector are Glass RPC’s (Resistive PlateChambers) –nanosecond timing to provide up-down discrimination.

• The detector will be sensitive to charged parti-cles –muons in particular.

• Magnetic field of about 1–1.3 Tesla for efficientenergy-momentum resolution and charge iden-tification-crucial to solve hierarchy problem andfor long-baseline expts.

• Design is modular. Additional modules can beadded– infact necessary.The lab design will haveadequate space for this.

• Emulsion sandwich and scintillator blanket toimprove detection efficiency for leptons.

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12m

16m 16m6cm2.5cm

16m

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Physics possibilities with ICAL at INO

The results from Solar, Atmospheric, Accelerator and Reactorexperiments till date clearly indicate that the neutrinos have

non-zero mass and they mix.

• While there is strong case for flavour oscilla-tions, the oscillation pattern needs to be seenclearly, with atleast one clear minimum and max-imum.

• Set limits on |δ32| and θ23 with better sensitivitythan superK.

• What is the sign of δ32 = m23 − m2

2 ?

• What is the mixing angle θ13? There exists onlyan upper limit on the 1–3 mixing angle, θ13 < 12◦.

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Future goals

• A non-zero θ13 allows the possibility of leptonicCP violation, new physics.

• Future long baseline experiments with ICALtype detector can measure both θ13 and CP phaseδ to a good precision .

• Is there a fourth neutrino - even if sterile ?

• Physics beyond oscillations ?

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Phase I: Atmospheric Neutrinos

The atmospheric neutrino physics program possible with amagnetised iron calorimeter is substantial and germane.

• Establish the oscillation pattern–by identifyingthe minima and maxima in the event-rate vs.L/E with better sensitivity than Water Cerenkovdetectors.

• Study of earth induced matter effects with up-ward going neutrinos.

• Better handle on θ23 and the magnitude and signof δm2

32 = m23 − m2

2. Settle the question of Hier-archy Possible only if θ13 > 6◦.

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Detector and Physics Simulations

• NUANCE Event generator: Generates atmo-spheric neutrino events inside ICAL

• GEANT Monte Carlo package: Simulates thedetector response for the neutrino event

• Event Reconstruction: Fits the “raw data” toextract neutrino energy and direction

• Physics Performance: Analysis of reconstructedevents to extract physics

• One-line status summary: programs in placeand being tested; “data” being analysed.

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Physics results with neutrino event generator

• Nuance event generator → Analysis: in the fol-lowing events are directly analysed with realis-tic resolution functions for direction and E in-cluded.

• Analysis done for different input values of oscil-lation parameters, δ23 and sin θ23 interms of Up-Down events.

down

up

L

−

θπ θ

Figure 2: A schematic of the relation between up-coming and down-going neutrinos with θ ↔ (π − θ).

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Results for up/down event ratio

δ = 2 × 10−3 eV2

1 2 3 40

100

200

1 2 3 40

0.5

1

1.5

0.8 0.9 1

δ = 3 × 10−3 eV2

1 2 3 40

100

200

1 2 3 40

0.5

1

1.5

0.8 0.9 1

Note the up-going neutrinos are depleted – effectof oscillations

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Sign of δ32 ?

• Oscillation probability in vacuum depends onlyon |δ32|.

• Therefore matter effects are essential: The sur-vival probability of νµ and νµ may be approxi-mated as (away from any resonance)

Pmµµ(A, δ) ≈ P (2)

µµ − sin2 θ13[A

δ − AT1

+

(

δ

δ − A

)2(

T2 sin2[(δ − A)x] + T3

)

] ,

Pmµµ(A, δ) ≈ P (2)

µµ − sin2 θ13[−A

δ + AT1

+

(

δ

δ + A

)2(

T2 sin2[(δ + A)x] + T3

)

] ,

x ≡ 1.27L/E; P (2)µµ = 1 − sin2 2θ23 sin2(xδ)

A = 7 × 10−5ρE eV2

T1 = xδ sin2 2θ23 sin 2δx ,

T2 = 4 sin4 θ23 ,

T3 = sin2 2θ23

(

sin2 Ax − sin2 δx)

.

• Note the difference

Pmµµ − P

mµµ

is zero for vacuum and sign dependent in mat-ter.

• Define the asymmetry in terms observables!

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Asymmetry to determine the hierarchy

• Define the asymmetry:

A =U

D(νµ) −

U

D(νµ)

U : Up-going events (through the earth); DDown-going events (no or little matter).

• The ratio U/D is an experimentally determinedquantity, separately for neutrinos and antineu-trinos.

• Theoretical asymmetry:

Figure 3: The difference asymmetry as a function of L/E for E > 4 GeV. θ13 = 5◦−−11◦ with 7◦ and 9◦ in between.

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Events from Monte-Carlo generator

The event data is generated for about500kTon-year exposure.

Figure 4: |δ32| = 2 × 10−3 eV2.

Best case scenario is shown here. Crucial point ishow to bin the data to maximise the asymmetry

and cuts (E > 4GeV ?).

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Effect of detector resolution for E and L

Figure 5: A comparison of the difference asymmetry a with (right) and without (left) inclusion of resolution functionsfor δ ≡ |δ32| = 2 × 10−3 eV2 and θ13 = 5, 11◦.

35

Phase II: Far–end Detector for NeutrinoFactories

• Good location for a far–end detector: Baseline of about6600 km from JHF to PUSHEP and 4900 kms from JHF toRammam.

• Magic baseline of 7200 km from CERN to either PUSHEPor Rammam. No CP contamination, extract pure mattereffect.

3.4−4.0

2.6−2.9

12.71−13.1

9.9 − 12.15

4.4−5.6

(11300)FERMI LAB

(7145)CERN

PUSHEP

The Ultimate Neutrino Collaboration

Rammam

JHF(4828)

CERN(6871)

FERMI LAB(10480)

(6556)JHF

• Reach and measure of θ13.

• Sign of ∆m223 if not already measured.

• Glimpse of leptonic CP phase δ.

• Improved statistics for L/E oscillations at low values.

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Other Physics Issues at ICAL

• Cosmic ray muon energy spectrum: Muons in the multi-GeVto TeV range, astrophysical point sources, upward muonshowers, multi-muon events, etc.

• UHE neutrino events.

• Kolar events: Unusual phenomenon- not well understood-discovery possibility.

• Low energy events: A one KTon liquid scintillator detectorwill be able to observe around 200 events due to all speciesof neutrinos and antineutrinos for a supernova explosion ata distance of 10 Kpc at 10 MeV threshold.

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Current status

• A prototype detector of size 1m × 1m × 1m isbeing built. There will be 14 detector layerswith 6 cm iron plates, with 28 readout planes,alternately placed in the x and y directions.Electronics components such as discriminators,latches, TDCs, trigger, monitoring etc., will betested at this time.

• Site survey of PUSHEP, RAMMAM and pre-liminary survey of Rohtang sites are complete.A site selection committee (SSC) is working onsite selection. A comparative study of the twomain sites will be made, with respect to time-scale involved, depth available, other facilities,and corresponding physics issues.

• Simulations and physics calculations are in progress.A GEANT-based fortran simulations package isin place for ICAL. Event generation is done withNUANCE neutrino generator. Track recogni-tion and fitting programmes have been devel-oped and testing is going on–will be used tooptimise the detector geometry and field.

• Human resources generation: This is an impor-tant issue. We are actively campaigning for col-laboration with various Institutions both in In-dia and abroad. More people needed.

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