Photohadronic processes and neutrinos
Summer school
“High energy astrophysics”
August 22-26, 2011
Weesenstein, Germany
Walter Winter
Universität Würzburg
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 n fluxes
 Glashow resonance? (pp versus pg)
 Neutrinos and the multi-messenger connection
2
Lecture 1
Introduction
Neutrino production in astrophysical
sources
max. center-of-mass
energy ~ 103 TeV
(for 1012 GeV protons)
Example: Active galaxy
(Halzen, Venice 2009)
4
Different messengers
 Shock accelerated protons lead to
p, g, n fluxes
 p: Cosmic rays:
affected by magnetic fields
 g: Photons: easily absorbed/scattered
 n: Neutrinos: direct path
(Teresa Montaruli, NOW 2008)
5
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,
pg interactions)
 Proton (E > 6 1010
GeV) on CMB
 GZK cutoff +
cosmogenic
neutrino flux
galactic
extragalactic
UHECR
In the
source:
Ep,max up to
1012 GeV?
GZK
cutoff?
(Source: F. Halzen, Venice 2009)
6
Example: Gamma-ray bursts
 Direct+cosmogenic fluxes come typically together:
Neutrons from
same
interactions
escape the
sources
 cosmogenic
neutrino flux
Neutrino flux
produced within
source
(Ahlers, Gonzales-Garcia, Halzen, 2011)
7
Neutrino detection: IceCube
 Example:
IceCube at South Pole
Detector 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?
http://icecube.wisc.edu/
8
Neutrino astronomy in the Mediterranean:
Examples: ANTARES, KM3NeT
http://antares.in2p3.fr/
9
When do we expect a n 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!
10
The parameter space?
 Model-independent
(necessary) condition:
Emax ~ Z e B R
„Test points“
(?)
(Larmor-Radius < size of
source)
 Particles confined to
within accelerator!
 Sometimes: define
acceleration rate
t-1acc = h Z e B/E
(h: acceleration efficiency)
 Caveat: condition
relaxed if source
heavily Lorentzboosted (e.g. GRBs)
Protons to 1020 eV
(Hillas, 1984; version adopted from M. Boratav)11
Simulation of sources
(qualitatively)
Photohadronics
(primitive picture)
If neutrons can escape:
Source of cosmic rays
Neutrinos produced in
ratio (ne:nm:nt)=(1:2:0)
Cosmogenic neutrinos
Delta resonance approximation:
p+/p0 determines ratio between neutrinos and gamma-rays
High energetic gamma-rays;
might cascade down to lower E
Cosmic messengers
13
Photohadronics (more realistic)
D
res.
Multi-pion
production
Resonant
production,
direct production
(Photon energy in
nucleon rest frame)
(Mücke, Rachen, Engel, Protheroe, Stanev, 2008; SOPHIA)
Different
characteristics
(energy loss
of protons;
energy dep.
cross sec.)
14
Meson photoproduction
 Starting point: D(1232)resonance approximation
 Limitations:

No p- production; cannot predict p+/ p- 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 p0 production by
factor of 2.4 (independent of input spectra!)
 Solutions:
 SOPHIA: most accurate description of physics
Mücke, Rachen, Engel, Protheroe, Stanev, 2000
Limitations: Often slow, difficult to handle; helicity dep. muon decays!
from:
 Parameterizations based on SOPHIA
Hümmer, Rüger,
 Kelner, Aharonian, 2008
Spanier, Winter,
Fast, but no intermediate muons, pions (cooling cannot be included)
ApJ 721 (2010) 630
 Hümmer, Rüger, Spanier, Winter, 2010
Fast (~3000 x SOPHIA), including secondaries and accurate p+/ p- ratios;
T=10
eV
also individual contributions of different
processes
(allows for comparison
More tomorrow
with D-resonance!)
 Engine of the NeuCosmA („Neutrinos from Cosmic Accelerators“) software
15
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 coaccelerated electrons/positrons (AGN-like)
 From a more complicated combination of
radiation processes (see other lectures)
 Minimal set of assumptions for n
production?  tomorrow!
16
Secondary decays and magnetic
field effects
 Described by kinematics of weak decays
(see e.g. Lipari, Lusignoli, Meloni, 2007)
 Complication: Magnetic field effects
Pions and muons loose energy through synchroton radiation
for higher E before they decay – aka „muon damping“
Affect spectral shape and flavor composition of
neutrinos significantly
 peculiarity for neutrinos (p0 are electrically neutral!)
… more tomorrow …
Dashed:
no losses
Solid:
with losses
(example from
Reynoso,
Romero, 2008)
17
Flavor composition at the source
(Idealized – energy independent)
 Astrophysical neutrino sources produce
certain flavor ratios of neutrinos (ne:nm:nt):
 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 pg
(also possible: photo-dissociation
of heavy nuclei)
 At the source: Use ratio ne/nm (nus+antinus added)
18
Neutrino propagation and
detection
Neutrino propagation (vacuum)
 Key assumption: Incoherent propagation of
neutrinos
(see Pakvasa review,
arXiv:0803.1701,
and references therein)
 Flavor mixing:
 Example: For q13 =0, q23=p/4:
 NB: No CPV in flavor mixing only!
But: In principle, sensitive to Re exp(-i d) ~ cosd
20
Earth attenuation
 High energy neutrinos
interact in the Earth:
(C. Quigg)
Earth
Detector
 However: Tau neutrino regeneration
through nt  t  (17%) m + nm + nt
21
Neutrino detection (theory)
 Muon tracks from nm
Effective area dominated!
(interactions do not have do be
within detector)
 Electromagnetic showers
(cascades) from ne
Effective volume dominated!
 nt: Effective volume dominated
 Low energies (< few PeV) typically
hadronic shower (nt track not
separable)
 Higher Energies:
nt track separable
 Double-bang events
 Lollipop events
 Glashow resonace for electron
antineutrinos at 6.3 PeV
 NC showers
t
nt
nt
e
ne
m
nm
(Learned, Pakvasa, 1995; Beacom et
al, hep-ph/0307025; many others)
22
Neutrino detection: Muon tracks
 Number of events depends on neutrino
effective area and observ. time texp:
[cm-2 s-1 GeV-1]
 Neutrino effective area
~ detector area x muon
range (E); but: cuts,
uncontained events, …
 Time-integrated point
source search, IC-40
nt: via
tm
(arXiv:1012.2137)
Earth opaque
to nm
23
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
24
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
25
Comparison of limits (example)
(arXiv:1103.4266)
Differential limit
Fluxes typically
below that
IC-40 nm
Integral
limit
Applies to E-2
flux only
NB: Spectral
shape important
because of
instrument
response!
Energy range somewhat
arbitrary (e.g. 90% of all events)
26
Neutrino detection: backgrounds
 Backgrounds
domination:
Earth
Atmospheric
neutrino
dominated
Detector
Cosmic
muon
dominated
 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)
27
Measuring flavor? (experimental)
 In principle, flavor information can be
obtained from different event topologies:




Muon tracks - nm
Cascades (showers) – CC: ne, nt, NC: all flavors
Glashow resonance (6.3 PeV): ne
Double bang/lollipop: nt (sep. tau track)
t
(Learned, Pakvasa, 1995; Beacom et al, 2003)
nt
 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
28
Flavor ratios at detector
 At the detector: define observables which
 take into account the unknown flux normalization
 take into account the detector properties
 Example: Muon tracks to showers
Do not need to differentiate between
electromagnetic and hadronic showers!
 Flavor ratios have recently been discussed for many
particle physics applications
(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…)
29
New physics in R?
Energy dependence
flavor comp. source
Energy dep.
new physics
(Example: [invisible] neutrino decay)
1
Stable state
1
Unstable state
Mehta, Winter,
JCAP 03 (2011) 041; see
also Bhattacharya,
Choubey, Gandhi,
Watanabe, 2009/2010
30
How many neutrinos do we
expect to see?
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 fp<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 energy
 Muon neutrinos take 0.05 Ep
fp = 1
fp ~ 0.2
 Warning: bound depends on flavors considered, and
whether flavor mixing is taken into account!
32
Comments on statistics
 At the Waxman-Bahcall bound:
O(10) events in full-scale IceCube per year
 Since (realistically) fp << 1,
probably Nature closer to O(1) event
Do 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!
33
Diffuse flux (e.g. AGNs)
(Becker, arXiv:0710.1557)
Comoving
volume
Single source
spectrum
Source
distribution
in redshift,
luminosity
Decrease
with
luminosity
distance
 Advantage: optimal
statistics (signal)
 Disadvantage:
Backgrounds
(e.g. atmospheric)
34
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 g-rays: Neutrino results will likely be based
on accumulated statistics. Therefore: use input from g-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
35
Summary (lecture 1)
 Neutrino observations important for
 Nature of cosmic ray sources
 Hadronic versus leptonic models
 Neutrino observations are qualitatively
different from CR and g-ray observations:
 Low statistics, conclusions often based on
aggregated fluxes
 Charged secondaries lead to neutrino
production: flavor and magnetic field effects
36