Understanding High Energy Neutrinos

Paolo Lipari: INFN Roma “Sapienza”

NOW-2014Conca Specchiulla 12th september 2014

An old “dream” is becoming a reality :

Observing the Universe with Neutrinos

(“A new way to look at the Sky”)

The Sun

SuperNova 1987A

The Earth (geo-neutrinos)

Neutrinos from the “High Energy Universe”(recent result from IceCube) [!?]

“A glimpse of the promised land”

Christian Spiering:

“A glimpse of the promised land”

1. Could the “glimpse” be a Mirage ? (playing the “devil's advocate”)

2. What can we say about the “high energy neutrino land” from the shape of the very first outline ?

3. When can we expect the first “landing” ? [the first detection of an astrophysical source]

Neutrinos from the “High Energy Universe”

Neutrino associated to the production and propagationof very high energy relativistic particles (hadrons).

1. Astrophysical object (or event) that accelerates protons and nuclei to relativistic energies.

2. Target material (gas, or radiation field)

3. Interactions of the relativistic particles with the production of unstable [weakly decaying] particles.

4. The decays produce neutrinos.

Jakob van Santen

Atmospheric Neutrinos

Foreground to the astrophysicalneutrino signal

Neutrino Flux:decomposition of the flux into an astrophysical signal of extraterrestrial neutrinos and a “foreground” of atmospheric neutrinos

Flavor composition

Angular distribution

Energy distribution

Each component of the neutrino fluxhas characteristics:

The atmospheric neutrino flux

depends only on the zenith angle.

The astrophysical neutrino fluxes

depend on celestial coordinates.

The establishment of a celestial coordinate dependence of the neutrino fluxes would beunambiguous evidence for an astrophysicalsignal.

But: largest predicted astrophysical neutrino fluxis expected as a featureless – isotropic extragalacticflux.

125 m string separation17 m between PMT's

IceCube

Analysis of “contained events”in ICECUBE

SCIENCE (16 december 2013)

“Evidence for High Energy Extraterrestrial Neutrinos in the IceCube Detector”

New publication Phys.Rev.Lett astro-ph/1405.5303

2 years → 3 years of data

Contained events

“TRACK”

“Shower”

Total Number of Events : 36(+1)

Background from Down-going Muons

Atmospheric Neutrinos

Excess = 5.7 sigmas

IceCube contained events 3-years

Total Number of Events : 37Background from Down-going Muons

Atmospheric Neutrinos

Excess = 5.7 sigmas

“conventional”

“charm”

IceCube contained events 3-years

Estimate of the neutrino flux by IceCube

in the energy range: 40 TeV - 2 PeV(isotropic, equal flux for all flavors)

“conventional”

“charm”

Playing the “Devil's Advocate”

Atmospheric Neutrinos

Could the “extraterrestrial neutrino” excessbe attributed to atmospheric neutrinos ?

Are these theoreticalpredictions

(and their estimateduncertainties)

correct ?

Significance of the excess of events:naive (incorrect!) way.

Poisson statistics [37 events, background of 15=8.4 + 6.6]

Significance =

Flavor composition

Angular distribution

Energy distribution

Each component of the neutrino fluxhas characteristics:

Analysis of Energy Spectrum. angular distribution

flavor composition of the data

A “caveat”

from theold experts

Predictions of “Magister Eligius”:

Neutrino induced muons

ConfirmingEvidence fromneutrino-induced muons !

IceCube 3 years data [Contained events (from PRL)]

Excess at HIGH ENERGY

Excess at down-going “vertical” direction

Estimating the Fluxes of Atmospheric Neutrinos

1. Cosmic Ray fluxes

2. Modeling of Hadronic Interactions.

3. “Geometry” of the Earth atmosphere (density profile)

Primary Cosmic Ray Flux [only some data points are shows]

p

He Air Shower Measurements “all particles”

Direct Measurements

Primary Cosmic Ray Flux [Spectral features]

p

He

“Knee”

“Ankle”

Primary Cosmic Ray Flux

p

He

“Knee”

“Ankle” Is this the flux (and energy density) of extragalactic Cosmic Rays ?

1. Readjust the results of different experiments

2. Composition model

[3 source components] [3 “Peters cycles”

p

All nucleons(bound + free)

Nuclei very ineffectivefor neutrino production

p

He

Proton Flux

“All nucleon Flux”

Gaisser, Stanev, Tilav(fit + composition model)

Angle integrated Neutrino fluxes

Angle integrated Neutrino fluxes

Charm decay component

IceCube fit of the “extraterrestrial component”(per each flavor)

“GZK neutrinos”

Flavor Content[flux ratios]

Flavor Content

1. More than

2. Absence of

3. More than

2-body decay of pion/kaon

Decay forbidden (tau mass)

Kinematics of pion decay

[2-body decay V-A structure of the interaction]

Charged Pions decay into muon-neutrinos

Electron neutrinos generated by

3-body decays of kaons.

Flavor Content

1. Equal and

2. Small (10%)

3. slightly less than

Universality

2 body decay of Ds

Kinematics of charm decay

Flavor Content

1. Equal and ,

Oscillation Probability

Average to zero(for size of source region)

Relative fluxes at the observersstarting from 2 nu_mu and 1 nu_e(standard mixture for a pion chain-decay origin)

= best fit

Significant presence oftau-neutrinos

More in general:For an arbitrary flavor composition emission

In principle key role for

Flavor Content

Tau neutrinos can be directly detected[“Double Bang events”]

But also the ratio “Shower” / “Tracks”is sensitive to the presence of tau neutrinos.

Is there an excess of “showers” versus “tracks”

in the IceCube data ?

Result is “compatible” with Equal fluxes for all neutrino flavors.

A more quantitative statement would be desirable

Angular Distribution :

Atmospheric standard:

Atmospheric charm: quasi-isotropic

characteristic enhancement for horizontal directions

Decay probability:[Competition between decay and interaction]

Geometry of Particle DecayZenith angle dependence

Characteristic zenith angle distribution ofStandard atmospheric neutrinos

Absorption of neutrinos in the Earth

Up/Down Ratio

Tom Gaisser

From Jacob Van Santen

Effect of VETO on rejecting atmnospheric neutrinos

Effect allowsto separate

Atmospheric-charm

from isotropicastrophysical

[2 years data] Down-going 24

Up-going 4

Very large asymmetry [why ?]

[2 years data] [3rd year]

Down-going 24 4

Up-going 4 5

Time dependence ?!(at 3 sigma level)

…. very unlikely ….

Energy distribution of Atmospheric Neutrinos.

(and absolute normalization of the fluxes)

Reflects:

1. The shape (and normalization) of the primary “all nucleon flux”

[Problem of CR at “Knee”]

2. The properties of particle production in Hadronic interactions.

[Large uncertainties for charmed particle production]

All particle spectrum

All nucleon spectrum

proton spectrum

“Devil's advocate” “ad hoc” additional proton component

Dynamics of charm productionin hadronic interactions

Perturbative QCD calculation (gluon flusion dominant)

Recent measurements of charm cross section at LHC(small phase space coverage).

Possibility of “Intrinsic charm”

Qualitative idea: Large component of charm in the Proton Parton Distribution Function.

(Very) speculative possibility to increase significantlythe charm neutrino production,without direct conflict with the data.[example P.L. astro-ph/1308.2086].

The study of neutrinos around 1 PeVsees the intersection of 3 problems:

1. Astrophysical Neutrino Sources

2. The determination of the ALL NUCLEON flux [proton component and mass composition above the knee]

3. The dynamics of non-perturbative charm production in hadronic interactions

The available information is sufficiently redundant to “solve” the three problems simultaneously.

Super-Kamiokande

Determination of Oscillation Parameters:

“

Set of parametersthat describe the MC prediction

Analogy with the discovery of Neutrino oscillations from the studyof atmospheric neutrinos,

and the determination of the oscillation parameters.

Simultaneous Fit of

Astrophysical Component(with a number of parameters)

Atmospheric Component(set of parameters)

Estimating the All-nucleon flux(from the VHE atmospheric flux) is very interesting for an understanding of VHE cosmic rays

[5.7 sigma evidence of Astrophysical neutrinos. (what are the chi2 of the fits ?)

Interpreting an Astrophysical Neutrino Signal

Compare the (high energy) Neutrino Sky

with the Gamma Ray sky

Prediction on the “Neutrino Sky”based on observations of Gamma Rays

[in the 0.1-100 GeV energy range]

FERMItelescope

Superposition of a “diffuse flux” (disk of the Galaxy)and an ensemble of point-like or quasi point-like sources

1. Ensemble of (quasi)-point sources

2. Diffuse Galactic Flux

(generated by cosmic rays magnetically confined in the Milky Way)

3. Isotropic flux.

(attributed to an ensemble of unresolved extragalactic sources)

2FGL

2nd FERMICatalog

24 months of observations

1873 sources

E > 100 MeV

Diffuse Emission

Galactic coordinates

Inverse Compton

BremsstrahlungDescription reasonably successful.[but several ambiguities and open problems remain.]

Angle integrated (4pi) Gamma Ray fluxes

Remarkable matching between the isotropic gamma-ry flux and the neutrino-flux.

Same origin ?

Extragalactic Flux of Neutrinos

Isotropic flux of particles(reflecting the isotropy of the Universe)

Neutrino injection

[Particles injected per unit volume, unit timeand unit energy. ]

Time (redshift) dependenceCosmological evolution.

Relation between the injection and the neutrino flux

Injection is a power law

Flux is a power law:

Power Density of the neutrino sources

Cosmological evolution

Constant injection

Gamma Rays sources in the FERMI-LAT 2nd catalog. (galactic declination distribution)

FLUX of all sources

Flux (E = [1-100 GeV]) distribution of the 1015 sources in the Galactic Pole region

Flux (E = [1-100 GeV]) distribution of the 1015 sources in the Galactic Poles region

2 brightest sources PSR J1836+5295 (galactic) Blazar 3C 454.3

Brightest extragalactic sourceIn the FERMI-LAT catalog

blazar 3C454.3

z=0.859

Cumulative Flux (1015 sources)

Gamma Rays in [1-100 GeV] energy interval.

Resolved flux

Unresolved flux

Gamma Rays in [1-100 GeV] energy interval.

Resolved flux

Unresolved flux

Indicationfor IceCube

Brightest extragalactic source 2% of total flux ?

EXTRA-GALACTIC or include

GALACTIC contribution ?

Galactic versus extra-Galactic

Distribution in Galactic Latitude(concentration on the galactic plane?)

Distribution in Longitude(concentration near galactic center ??)

Fluxes from resolved sources in the FERMI-LAT 2nd catalog

Extrapolation of flux pf the FERMI-LATGalactic sources of IceCube energies

IceCube Astrophysical Neutrinos

Diffuseisotropic flux

ExtragalacticResolved sources

Milky Wayresolved sources

The evidence for an astrophysical componentin the IceCube data is becoming compelling.

The disentangling of the different componentsis delicate Astrophysical/galactic, Astrophysical/extragalactic Atmospheric/conventional Atmospheric/charm

need to take into account carefully all systematic uncertainties.

The astrophysical neutrino land is in sight.[Obviously] more data is needed.Perhaps source identification soon.