Photohadronic processes and neutrinos

Summer school “High energy astrophysics”August 22-26, 2011

Weesenstein, Germany

Walter Winter

Universität Würzburg

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Contents

Lecture 1 (non-technical)

Introduction, motivation Particle production (qualitatively) Neutrino propagation and detection Comments on expected event rates

Lecture 2 Tools (more specific) Photohadronic interactions, decays of secondaries,

pp interactions A toy model:

Magnetic field and flavor effects in fluxes Glashow resonance? (pp versus p) Neutrinos and the multi-messenger connection

Lecture 1

Introduction

4

Neutrino production in astrophysical sources

Example: Active galaxy(Halzen, Venice 2009)

max. center-of-mass energy ~ 103 TeV

(for 1012 GeV protons)

5

Different messengers

Shock accelerated protons lead to p, , fluxes p: Cosmic rays:

affected by magnetic fields

(Te

resa

Mo

nta

ruli, N

OW

2008)

: Photons: easily absorbed/scattered : Neutrinos: direct path

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galactic extragalactic

Evidence for proton acceleration,

hints for neutrino production Observation of

cosmic rays: need to accelerate protons/hadrons somewhere

The same sources should produce neutrinos: in the source (pp,

p interactions) Proton (E > 6 1010

GeV) on CMB GZK cutoff + cosmogenic neutrino flux

(Source: F. Halzen, Venice 2009)

In the source:

Ep,max up to 1012 GeV?

GZKcutoff?

UHECR

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Example: Gamma-ray bursts

(Ahlers, Gonzales-Garcia, Halzen, 2011)

Direct+cosmogenic fluxes come typically together:

Neutrino flux produced within

source

Neutrons from same

interactions escape the

sources cosmogenic

neutrino flux

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Example: IceCube at South PoleDetector material: ~ 1 km3 antarctic ice

Completed 2010/11 (86 strings) Recent data releases, based on

parts of the detector: Point sources IC-40 [IC-22]

arXiv:1012.2137, arXiv:1104.0075 GRB stacking analysis IC-40

arXiv:1101.1448 Cascade detection IC-22

arXiv:1101.1692 Have not seen anything (yet)

What does that mean? Are the models wrong? Which parts of the parameter space

does IceCube actually test?

Neutrino detection: IceCube

http://icecube.wisc.edu/

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Neutrino astronomy in the Mediterranean: Examples: ANTARES, KM3NeT

http://antares.in2p3.fr/

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When do we expect a signal?[some personal comments]

Unclear if specific sources lead to neutrino production and at what level; spectral energy distribution can be often described by other radiation processes processes as well (e.g. inverse Compton scattering, proton synchrotron, …)

However: whereever cosmic rays are produced, neutrinos should be produced to some degree

There are a number of additional candidates, e.g. „Hidden“ sources (e.g. „slow jet supernovae“ without gamma-ray

counterpart)(Razzaque, Meszaros, Waxman, 2004; Ando, Beacom, 2005; Razzaque, Meszaros, 2005; Razzaque, Smirnov, 2009)

What about Fermi-LAT unidentified/unassociated sources? From the neutrino point of view: „Fishing in the dark blue

sea“? Looking at the wrong places? Need for tailor-made neutrino-specific approaches?

[unbiased by gamma-ray and cosmic ray observations] Also: huge astrophysical uncertainties; try to describe at

least the particle physics as accurate as possible!

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The parameter space? Model-independent

(necessary) condition:

Emax ~ Z e B R

(Larmor-Radius < size of source)Particles confined to

within accelerator! Sometimes: define

acceleration ratet-1

acc = Z e B/E(: acceleration efficiency)

Caveat: condition relaxed if source heavily Lorentz-boosted (e.g. GRBs)

(Hillas, 1984; version adopted from M. Boratav)

(?)

Protons to 1020 eV

„Test points“

Simulation of sources

(qualitatively)

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Photohadronics (primitive picture)

Delta resonance approximation:

+/0 determines ratio between neutrinos and gamma-rays

High energetic gamma-rays;might cascade down to lower E

If neutrons can escape:Source of cosmic rays

Neutrinos produced inratio (e::)=(1:2:0)

Cosmic messengers

Cosmogenic neutrinos

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Photohadronics (more realistic)

(Photon energy in nucleon rest frame)

(Mücke, Rachen, Engel, Protheroe, Stanev, 2008; SOPHIA)

Resonant production,

direct production

Multi-pionproduction

Differentcharacteristics(energy lossof protons;

energy dep.cross sec.)

res.

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Starting point: (1232)-resonance approximation

Limitations:- No - production; cannot predict / - ratio (affects neutrino/antineutrino)- High energy processes affect spectral shape (X-sec. dependence!)- Low energy processes (t-channel) enhance charged pion production Charged pion production underestimated compared to production by

factor of 2.4 (independent of input spectra!) Solutions:

SOPHIA: most accurate description of physicsMücke, Rachen, Engel, Protheroe, Stanev, 2000Limitations: Often slow, difficult to handle; helicity dep. muon decays!

Parameterizations based on SOPHIA Kelner, Aharonian, 2008

Fast, but no intermediate muons, pions (cooling cannot be included) Hümmer, Rüger, Spanier, Winter, 2010

Fast (~3000 x SOPHIA), including secondaries and accurate / - ratios; also individual contributions of different processes (allows for comparison with -resonance!)

Engine of the NeuCosmA („Neutrinos from Cosmic Accelerators“) software

Meson photoproduction

T=10 eV

from:Hümmer, Rüger, Spanier, Winter,

ApJ 721 (2010) 630

More tomorrow

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Typical source models

Protons typically injected with power law (Fermi shock acceleration!)

Target photon field typically: Put in by hand (e.g. obs. spectrum: GRBs) Thermal target photon field From synchrotron radiation of co-

accelerated electrons/positrons (AGN-like) From a more complicated combination of

radiation processes (see other lectures)

Minimal set of assumptions for production? tomorrow!

?

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Secondary decays and magnetic field effects

Described by kinematics of weak decays(see e.g. Lipari, Lusignoli, Meloni, 2007)

Complication: Magnetic field effectsPions and muons loose energy through synchroton radiation for higher E before they decay – aka „muon damping“

(example from Reynoso,

Romero, 2008)

Dashed:no lossesSolid:with losses

Affect spectral shape and flavor composition ofneutrinos significantly

peculiarity for neutrinos (0 are electrically neutral!)… more tomorrow …

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Astrophysical neutrino sources producecertain flavor ratios of neutrinos (e::):

Pion beam source (1:2:0)Standard in generic models

Muon damped source (0:1:0)at high E: Muons lose energy before they decay

Muon beam source (1:1:0)Cooled muons pile up at lower energies (also: heavy flavor decays)

Neutron beam source (1:0:0)Neutron decays from p (also possible: photo-dissociationof heavy nuclei)

At the source: Use ratio e/ (nus+antinus added)

Flavor composition at the source(Idealized – energy independent)

Neutrino propagation and detection

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Neutrino propagation (vacuum)

Key assumption: Incoherent propagation of neutrinos

Flavor mixing: Example: For 13 =0, 23=/4:

NB: No CPV in flavor mixing only!But: In principle, sensitive to Re exp(-i ) ~ cos

(see Pakvasa review, arXiv:0803.1701,

and references therein)

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Earth attenuation

High energy neutrinos interact in the Earth:

However: Tau neutrino regeneration through (17%) + +

(C. Quigg)

Earth

Detector

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Neutrino detection (theory)

Muon tracks from Effective area dominated!(interactions do not have do be within detector)

Electromagnetic showers(cascades) from eEffective volume dominated!

Effective volume dominated Low energies (< few PeV) typically

hadronic shower ( track not separable)

Higher Energies: track separable

Double-bang events Lollipop events

Glashow resonace for electron antineutrinos at 6.3 PeV

NC showers

(Learned, Pakvasa, 1995; Beacom et al, hep-ph/0307025; many others)

e

e

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Neutrino detection: Muon tracks

Number of events depends on neutrino effective area and observ. time texp:

Neutrino effective area ~ detector area x muon range (E); but: cuts, uncontained events, …

Time-integrated point source search, IC-40 (arXiv:1012.2137)

Earth opaque to

: via

[cm-2 s-1 GeV-1]

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Computation of limits (1) Number of events N can be translated into

limit by Feldman-Cousins approach

(Feldman, Cousins, 1998)

This integral limit is a single number given for particular flux, e.g. E-2, integrated over a certain energy range

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Computation of limits (2)

Alternative: Quantify contribution of integrand in

when integrating over log E:

Differential limit: 2.3 E/(Aeff texp)

Is a function of energy, applies to arbitrary fluxes if limit and fluxes sufficiently smooth over ~ one decade in E

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Comparison of limits (example)

(arXiv:1103.4266)

IC-40

Differential limitFluxes typically

below that

Integral limit

Applies to E-2 flux only

Energy range somewhat arbitrary (e.g. 90% of all events)

NB: Spectralshape important

because ofinstrumentresponse!

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Neutrino detection: backgrounds

Backgrounds domination:

Background suppression techniques: Angular resolution (point sources) Timing information from gamma-ray counterpart

(transients, variable sources) Cuts of low energy part of spectrum (high energy

diffuse fluxes)

Earth

Detector

Atmosphericneutrinodominated

Cosmicmuondominated

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Measuring flavor? (experimental)

In principle, flavor information can be obtained from different event topologies: Muon tracks - Cascades (showers) – CC: e, , NC: all flavors Glashow resonance (6.3 PeV): e

Double bang/lollipop: (sep. tau track)(Learned, Pakvasa, 1995; Beacom et al, 2003)

In practice, the first (?) IceCube „flavor“ analysis appeared recently – IC-22 cascades (arXiv:1101.1692)

Flavor contributions to cascades for E-2 extragalatic test flux (after cuts):

Electron neutrinos 40% Tau neutrinos 45% Muon neutrinos 15%

Electron and tau neutrinos detected with comparable efficiencies Neutral current showers are a moderate background

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At the detector: define observables which take into account the unknown flux normalization take into account the detector properties

Example: Muon tracks to showersDo not need to differentiate between electromagnetic and hadronic showers!

Flavor ratios have recently been discussed for many particle physics applications

Flavor ratios at detector

(for flavor mixing and decay: Beacom et al 2002+2003; Farzan and Smirnov, 2002; Kachelriess, Serpico, 2005; Bhattacharjee, Gupta, 2005; Serpico, 2006; Winter, 2006; Majumar and Ghosal, 2006; Rodejohann, 2006; Xing, 2006; Meloni, Ohlsson, 2006; Blum, Nir, Waxman, 2007; Majumar, 2007; Awasthi, Choubey, 2007; Hwang, Siyeon,2007; Lipari, Lusignoli, Meloni, 2007; Pakvasa, Rodejohann, Weiler, 2007; Quigg, 2008; Maltoni, Winter, 2008; Donini, Yasuda, 2008; Choubey, Niro, Rodejohann, 2008; Xing, Zhou, 2008; Choubey, Rodejohann, 2009; Esmaili, Farzan, 2009; Bustamante, Gago, Pena-Garay, 2010; Mehta, Winter, 2011…)

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New physics in R? Energy dependenceflavor comp. source

Energy dep.new physics

(Example: [invisible] neutrino decay)

1

1

Stable state

Unstable state

Mehta, Winter, JCAP 03 (2011) 041; see also Bhattacharya, Choubey, Gandhi, Watanabe, 2009/2010

How many neutrinos do we expect to see?

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Upper bound from cosmic rays Injection of CR protons inferred from observations:

(caveats: energy losses, distribution of sources, …) Can be used to derive upper bound for neutrinos (Waxman,

Bahcall, 1998 + later; Mannheim, Protheroe, Rachen, 1998)

Typical assumptions: Protons lose fraction f<1 into pion

production within source About 50% charged and 50%

neutral pions produced Pions take 20% of proton energy Leptons take about ¼ of pion energyMuon neutrinos take 0.05 Ep

Warning: bound depends on flavors considered, and whether flavor mixing is taken into account!

f = 1

f ~ 0.2

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Comments on statistics

At the Waxman-Bahcall bound: O(10) events in full-scale IceCube per year

Since (realistically) f << 1, probably Nature closer to O(1) eventDo not expect significant statistics from single

(cosmic ray) source!Need dedicated aggregation methods:

Diffuse flux measurement Stacking analysis, uses gamma-ray counterpart

(tomorrow)

However: WB bound applies only to accelerators of UHECR! Only protons!

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Diffuse flux (e.g. AGNs)

Advantage: optimal statistics (signal)

Disadvantage: Backgrounds(e.g. atmospheric)

(Becker, arXiv:0710.1557)

Single sourcespectrum

Sourcedistributionin redshift,luminosity

Comovingvolume

Decreasewith

luminositydistance

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Consequences of low statistics[biased]

Neutrinos may tell the nature (class) of the cosmic ray sources, but not where exactly they come from

Consequence: It‘s a pity, since UHECR experiments will probably also not tell us from which sources they come from …

Comparison to -rays: Neutrino results will likely be based on accumulated statistics. Therefore: use input from -ray observations (tomorrow …)Clues for hadronic versus leptonic models?Again: probably on a statistical basis …

Consequences for source simulation? Time-dependent effects will not be observable in

neutrinos Spectral effects are, however, important because of

detector response

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Summary (lecture 1)

Neutrino observations important for Nature of cosmic ray sources Hadronic versus leptonic models

Neutrino observations are qualitatively different from CR and -ray observations: Low statistics, conclusions often based on

aggregated fluxes Charged secondaries lead to neutrino

production: flavor and magnetic field effects