Atmospheric neutrinos

1. Measurements of
atmospheric neutrinos
M. Spurio
Università e INFN Bologna
XXVIII SEMINARIO*
NAZIONALE di FISICA NUCLEARE E
SUBNUCLEARE
"Francesco Romano"
OTRANTO (Serra degli Alimini 1) 3-10 giugno 2016
*
1 Istituto per la formazione religiosa e la preparazione culturale
dei giovani aspiranti al sacerdozio
2 In ambito universitario, lezione in cui gli studenti partecipano
attivamente con relazioni e interventi; esercitazione tenuta da
1
un docente per un ristretto numero di studenti
Avvertenze
• Argomenti scelti in coordinazione con l’altro docente (FV)
• Non intendo passare in rassegna tutte le ricerche e i risultati
sperimentali connessi con la fisica del neutrini
• Ho selezionato alcuni argomenti, che tratto col taglio che ritengo
adeguato ad una scuola avanzata e per studenti con master in Fisica
• Grafica delle slides spartana (come a lezione)
• Le slides sono in inglese (possibilità di riutilizzo)
• Come in ogni corso, ci sono delle domande che possono stimolare
la discussione tra studenti, e tra studenti e docenti
• Come in ogni corso, c’e’ un libro di testo per gli
approfondimenti e ulteriori stimoli (io tratterò i
capitoli 10, 11 e 12 del libro)
• Le figure sono tratte per la maggior parte dal
libro; in tal caso i riferimenti sono omessi
• Come per ogni libro, c’e’ la possibilità di avere la
copia piratata (chiavetta USB)
• Ovviamente, questa slide scompare
2
Neutrini from the Cosmos
• Flux of neutrinos at the
surface of the Earth.
• The three arrows near
the x-axis indicate the
energy thresholds for CC
production of the
charged lepton
4
Once upon a time…
•
•
•
•



GUT theories predicted the proton decay with measurable livetime
The proton was thought to decay in (for instance) pe+π0
Detector size: 103 m3, and mass 1kt (=1031 p)
The main background for the detection of proton decay were
atmospheric neutrinos interacting inside the experiment
Water Cerenkov Experiments
(IMB, Kamiokande)
Tracking calorimeters (NUSEX,
Frejus, KGF)
Result: NO p decay ! But some
anomalies on the neutrino
measurement!
γγ
e
Neutrino Interaction
Proton decay
5
The importance of atmospheric ν’s
• “Yestelday’s signal is today’s backglound and
tomollow’s caliblation”
• It is not always true! Atmospheric neutrinos…
•
•
•
•
•
•
•
< 1998 Background to nucleon Decay
> 1998 Signal of neutrino oscillations
> 2013 Background and calibration to HE neutrino astronomy
> 20xy Signal of Earth matter effects and of ν mass hierarchy
> 20xz Background to diffuse SN neutrino signal
>20yz Signal of nonstandard neutrino states or interaction?
>20wx Background to proton decay signals?
Adapted from E. Lisi
6
General problems for ν detectors
•
•
•
•
•
•
Low cross section Large detector volume/mass
Particle identification
Energy/momentum measurement
Direction measurement
No magnetic field (ν =ν)
Backgrounds
7
The recipes for the evaluation of
the atmospheric neutrino flux
π + → µ + +ν µ
µ + → e+ + ν µ + ν e
Independently from the details
of the computation of Φνμ (E),
Φνe(E), one can obtain two very
robust properties:
1. At energies below few GeV,
the flux of νμ is approximately
twice as large as the νe, i.e.:
Φ(νμ)= 2Φ(νe)
2. The νμ, νe fluxes are up-down
symmetric in zenith θ, i.e.:
Φ ν (Eν, θ) = Φ ν (Eν, π − θ)
Question 1: why 1. does not
hold at higher (>> GeV)
energies?
8
i) The primary CR spectrum
Direct measurements
(satellites)
Indirect measurements
(Extensive Air Shower Arrays)
9
i) The primary CR spectrum
10
i) The role of primary CRs
• Primary CR attenuation as function of X (g cm-2) and E
• Boundary condition:
• From Feyman scaling:
• The dependence on X depends on an effective attenuation length ΛN:
11
ii) p-air cross section
AUGER Coll. PRL 109 (2012) 062002
12
iii) Secondary charged multiplicity
Average number of charged
hadrons produced in pp
(andpp), e+e-, ep collisions
versus center of mass
energy
13
ii+iii) The meson production
• The pion propagation in atmosphere is described by:
• Competition between interaction and decay . The decay length:
• The pion decay constant επ=115 GeV.
• Pions start to increase with increasing depth X, reach a maximum and
then decrease
14
• High-energy limit (E>> επ ): the decay term dπ can be neglected:
(the Z are the the spectrum weighted moments)
• Low-energy limit (E cosθ << επ ): we neglect the term λIπ.
• Similar equations hold for other particles with different decay constants:
15
ii+iii) The neutrino production
• Conventional muons are produced by π and K decays:
}
• Prompt muons by the decay of charmed mesons.
• The muon flux is thus described by the equation:
• The muon neutrino flux follows similarly.
16
The conventional ν flux: π and K
Solid lines: vertical,
dashed lines: zenith 60o
17
iv) Model of the atmosphere solar
effects +geomagnetic field
•high precision 3D calculations,
•refined geomagnetic cut-off treatment (also geomagnetic field in
atmosphere)
•elevation models of the Earth
•different atmospheric profiles
•geometry of detector effects
18
The conventional ν flux (Honda)
M. Honda, et al. Phys. Rev. D 92, 023004 (2015)
• One ν flux prediction
(Honda) from MC
simulations;
• Different models
exist;
Question 2: Explain
qualitatively the (νµ/νe)
ratio
Question 3: Explain
qualitatively why the
(νµ/νµ) ratio increases
with energy
19
Measurement of atmospheric ν‘s
Example: Icarus@ Gran Sasso
νe= cascade
νµ=track
Example: Soudan II@ USA
20
(Early) measurement of atmo. ν‘s
T. Kajita, New J. Phys. 6 (2004) 194.
• Tracking calorimeter: Frejus, Nusex, Soudan.
• Water Cherenkov: IMB, Kamiokande
• Measured the number of neutrino interaction in the detector,
separating tracks (=νµ) from showers (=νe)
Intergral flux of
atmo ν’s vs energy
Question 4: Evaluate the number of interaction/kton year for E> 1 GeV for
kton of fiducial mass detector, assuming detection efficiency=1
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The golden age: SK and MACRO
• SuperKamiokande (SK) is located in
Japan, 1000 m Underground
• Active since 1996
• Filled 50.000 ton water
• 11000 large PMTs +2000 PMTs
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20 years of Super-Kamiokande
1996.4 Start data taking
1998 Evidence of atmospheric n oscillation (SK)
SK-I
1999.6 K2K started
2001 Evidence of solar n oscillation (SNO+SK)
2001.7 data taking was stopped
for detector upgrade
2001.11 Accident
partial reconstruction
2002.10 data taking was resumed
SK-II
2005 Confirm ν oscillation by accelerator ν (K2K)
2005.10 data taking stopped for full reconstruction
SK-III
2006.7 data taking was resumed
SK-IV
2009 data taking
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SuperKamiokande: νe
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SuperKamiokande: νµ
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Cherenkov Radiation
 As a charged particle travels, it disrupts the
local electromagnetic field (EM) in a medium.
Electrons in the atoms of the medium will be
displaced and polarized by the passing EM field
of a charged particle.
 Photons are emitted as an insulator's
electrons restore themselves to equilibrium
after the disruption has passed.
 In a conductor, the EM disruption can be
restored without emitting a photon.
 In normal circumstances, these photons
destructively interfere with each other and no
radiation is detected.
 However, when the disruption travels faster
than light is propagating through the medium,
the photons constructively interfere and
intensify the observed Cerenkov radiation.27
llight=(c/n)∆t
wav
e fro
nt
θ
lpart=βc∆t
cos θ C =
1
nβ
with n = n(λ ) ≥ 1
• Threshold velocity βT = 1/n  θT ~ 0
• Angle of emission (β=1): θmax= arcos(1/n)
• Distribution of emitted photons:
θC
dN/dλ
1  2π ⋅ z 2α
d 2 N 2πz 2α 
2


θC
1
sin
=
=
−
2
2 2 
2

λ  β n 
λ
dxdλ
d 2 N z 2α
sin 2 ϑc .
=
dxdE c
λ
dN/dE
Question 5: Evaluate the number of Cherenkov photons in
water in the λ=300-600 nm interval for a relativistic single
charged particle
Ε
28
ν
ν energy: event topology
Fully
Contained
ν
Partially
Contained
Stopping µ
µ
Through going µ
µ
ν
µ
ν
Energy spectrum (Monte
Carlo) of atmospheric ν
seen with different event
topologies (SuperKamiokande, MACRO)
29
cosΘ>0
SuperKamiokande I-IV: results
cosΘ<0
Θ
30
MACRO @ Gran Sasso
• Liquid scintillator counters, (3 planes)
for the measurement of time and dE/dx.
• Streamer tubes (14 planes), for the
measurement of the track position (<1o);
• Detector mass: 5.3 kton
• Downward going muons ~ 106 upward
going muons
• Different neutrino topologies
31
Neutrino induced events are upward throughgoing
muons, Identified by the time-of-flight method
1
T2
Streamer tube track
β
=
(T1 − T2 ) ⋅ c
T1
µ from ν: upgoing
1
β
=
(T1 − T2 ) ⋅ c
L
L
=
+1 µ
-1 µ
Atmospheric µ:
downgoing
=
32
The MACRO neutrino deficits
•
•
•
•
•
•
Completely different topology w.r.t. SK
Different experimental technique
Deformation of the angular distribution w.r.t. expectation
Missing events from the vertical direction
Interpretation: oscillations
The same oscillation parameters!
33
The Soudan II neutrino deficit
e-like
µ-like
• Iron tracking calorimeter
• 770 t fiducial mass
• Active from 1989 to 2001
in the Soudan Mine (USA)
• (P)contained events
• µ-like deficit from below
34
Neutrino oscillations…
 Idea of neutrinos being massive was first suggested
by B. Pontecorvo
 Prediction came from proposal of neutrino
oscillations
|νe> , |νµ> , |ντ> =Weak Interactions (WI) eigenstats
|ν1> , |ν2> , |ν3> =Mass (Hamiltonian) eigenstats
35
..with atmospheric
neutrinos
• ∆m2, sin22θ  from Nature;
• Eν = experimental parameter
(energy distribution of neutrino
giving a particular configuration of
events)
• L = experimental parameter
(neutrino path length from
production to interaction)
Pν µν µ
2

∆
m
⋅L
2
2
= 1 − sin 2θ ⋅ sin 1.27

Eν 

36
Discovery of neutrino oscillations
37
Why not νμ→νe ?
Apollonio et al., CHOOZ Coll.,
Phys.Lett.B466 (1999) 415
PDG value:
0.095±0.010
38
Measurement of energy spectra
• See: neutrino
telescopes
(part 3)
39
The 2015 Nobel Prize
40
41
42
From E. Lisi
Now and next..
43
From E. Lisi
44
45
End of part1
46
Solution of question 3)
• At low energy, neutrinos exceed antineutrinos due to the fact that
CR protons are more aboundant than neutrons
• Above few hundreds GeV, neutrinos from K decay are more
abundant than from pions. Thus, conservation of the
strangeness (S) and baryon (B) quantum numbers are
responsible for the difference
• Consider the production of charged kaons from pp interactions:
– Κ+ (B=0, S=1) is produced in association with Λ (B=1, S=-1);
– Κ- (B=0, S=-1) requires at least one associated baryon (B=1) and an
additional strange meson (S=1).
• Κ+ are generated much more frequently than Κ- because of the
associated production with the Λ.
47
Solution of question 4)
1. Flux: Φν ~ 1 cm-2 s-1
2. Cross section (@ 1GeV):
σν~0.5 10-38 cm2
3. Targets M= 6 1032 (nucleons/kton)
4. Time t= 3.1 107 s/y
Nint = Φν (cm-2 s-1) x σν (cm2)x M (nuc/kton) x t (s/y) ~
~ 100 interactions/ (kton y)
48
Solution of question 5)
• In water (n=1.33) for z=1 and β=1 the number of photons for
energy interval and unith path length is:
d 2 N z 2α
sin 2 ϑc .
=
dxdE
c
• With: cos ϑc = (1 / n) = 0.75 → ϑc = 42o
• The ∆λ=(600-300) nm corresponds to ∆E=2 eV, thus
d 2N
(1 / 137)
2eV × (0.67) 2
330
2
=
⋅ ∆E ⋅ sin ϑc =
=
−13
3
6
dx
(197 MeV fm)
27 ⋅10 ×10 eV ×10 cm cm
49