Cosmic ray composi,on measurements
andcosmic ray background free γ-ray observa,onswithCherenkov telescopes

Andrii Neronov
University of Geneva… and D.Semikoz, Ie.Vovk, R.Mirzoyan, arXiv:1610.01794 (Phys.Rev. D)

Overview
IntroducCon / moCvaCon: how rejecCon of cosmic ray background and measurement of cosmic raycomposiCon in space detectors and in the air shower arrays

Muon component of strongly inclined air showers

Imaging of Cherenkov light of the muon component

RejecCon of cosmic ray background in imaging Cherenkov telescopes, based on the imaging of themuon component of the air showers

Cosmic ray composiCon in the PeV-EeV range with imaging Cherenkov telescopes

Suppression of cosmic ray background for γ-ray observa,onsFermi/LAT

Cosmic ray background rejecCon ~106

HESS

Cosmic ray background rejecCon ~102

1 GeV

HAWC
300 GeV

Cosmic ray background rejecCon ~101-3~10 TeV

HAWC

CTA…..? ….. TeV-PeV?

Suppression of cosmic ray background for γ-ray observa,onsFermi/LAT

Cosmic ray background rejecCon ~106

HESS

Cosmic ray background rejecCon ~102

1 GeVHAWC
300 GeV

Cosmic ray background rejecCon ~101-3~10 TeV

HAWC

CTA…..? ….. TeV-PeV?
IceCubeSuppression of cosmic ray background for γ-ray observa,onspγ

Fermi-LAT reaches a rejecCon factor ~10-6 for the cosmic ray background through the use of(a) anC-coincidence shield(b) limited measurement of parCcle ID in the tracker and calorimeterSuppression of cosmic ray background for γ-ray observa,onspγ?

“AnC-coincidence shield” at the top of the atmosphere is hardly possible, but informaCon on theprimary parCcle ID helps to reduce the cosmic ray background in ground-based gamma-ray
observaCons.

Measurements of par,cle ID for cosmic rays

Air shower arrays derive more detailed
primary parCcle ID based on themeasurement of(a) muon content of the showers

Measurements of par,cle ID for cosmic rays

DeterminaCon of primary cosmic
ray parCcle ID is essenCal for
resoluCon of the problem of theorigin of the knee(s) in the cosmic
ray spectrum

Measurements of par,cle ID for cosmic rays

Air shower arrays derive more detailed
primary parCcle ID based on themeasurement of(a) muon content of the showers(b) measurement of the depth of
the shower maximum.

Measurements of par,cle ID for cosmic rays

DeterminaCon of primary cosmic
ray parCcle ID is also essenCal forunderstanding the nature of theAnkle and idenCﬁcaCon oftransiCon from GalacCc to
extragalacCc cosmic ray ﬂux.
DeterminaCon of primary cosmic
ray parCcle ID is essenCal for
resoluCon of the problem of theorigin of the Knee(s) in the cosmic
ray spectrum

Measurements of par,cle ID for cosmic rays

Air shower arrays derive more detailed
primary parCcle ID based on themeasurement of(a) muon content of the showers(b) measurement of the depth of
the shower maximum.

μe

Cherenkov light from muon component of the air shower

Muon component of the air showerμeMuon component of the air shower

PeV proton-induced shower incident at
87o zenith angle

Muon component is largely sub-dominant in
the shower maximum region.
It starts to dominate at large depth in theatmosphere.

μeCherenkov light from muon component of the air shower

Muons emit Cherenkov light, similarly to electrons.
Muon Cherenkov emission is widely diﬀerent for diﬀerent
primary parCcles.
Measurement of Cherenkov emission from muons could
provide primary parCcle ID, but only for strongly inclined
showers.

μe

Imaging of Cherenkov light from muon component of the air shower

Muons from strongly inclined showers reach the groundand are spread over large distance range.
Superimposed fragments of “muon rings” of individualmuons form a many-degree-long “tail” image.
To the contrary, image of electromagneCc component of
the shower is compactGamma-ray induced showers are muon-poor and,therefore, do not exhibit the “muon tail”.

μeImaging of Cherenkov light from muon component of the air shower
3 km

impact

6 km

gamma

protonironMuon “tail” or “halo” is absent in gamma showers
but is present in proton / nuclei induced showers

Cosmic ray background rejec,on for gamma-ray observa,ons

Proton and nuclei induced highly inclined shower images are strongly dominated by muon tailcomponent. Gamma-ray shower images are compact. Requirement of non-detecCon of muon tail downto certain (energy-dependent) fracCon of the electromagneCc shower size suppresses cosmic ray
backgorund by a factor up to 105.

PeV gamma-ray sky

Upper limit from KASCADE
In Northern hemisphere
Upper limit from Auger
In Southern hemisphere

The energy threshold for observaCons of nearly horizontal showers with 4 m class Cherenkov
telescopes is rather high. The 10-5 suppression factor for the cosmic ray background could be achieved
only in the PeV range.
No gamma-ray sources have previously been observed in this energy band…….

PeV gamma-ray skyFermi-LAT Southern sky spectrum
IceCube Southern sky neutrino spectrum
Possible GalacCc diﬀuse emission level?

The energy threshold for observaCons of nearly horizontal showers with 4 m class Cherenkov
telescopes is rather high. The 10-5 suppression factor for the cosmic ray background could be achieved
only in the PeV range.
No gamma-ray sources have previously been observed in this energy band…….

Observa,ons of nearly horizontal air showers

A setup opCmized for observaCons of PeV diﬀuse emission inbackground-free regime should consist of a system oftelescopes overlooking the enCer 360o strip along the horizon.
Trigger and readout system should be opCmized for relaCvely
long Cme scale of the signal: ~1 μs.

PeV gamma-ray sky

CTA SST

Dedicated telescope system

The energy threshold for observaCons of nearly horizontal showers with 4 m class Cherenkov
telescopes is rather high. The 10-5 suppression factor for the cosmic ray background could be achieved
only in the PeV range.
No gamma-ray sources have previously been observed in this energy band……. exploraCon potenCal

Cosmic ray composi,on measurements

KASCADE and KASCADE-Grande measurements of composiCon in the knee-ankle energy range show
strong dependence on the hadronic interacCon model used.
Account of LHC constraints did not improve the situaCon: ambiguity on details of light / heavy nuclei
parCal contribuCons to the ﬂux persist.

Cosmic ray composi,on measurements
Fep

Measurement of muon content of strongly inclined showers also provides informaCon on theprimary parCcle ID for the cosmic ray nuclei.

Cosmic ray composi,on measurements

Fep

Measurement of muon content of strongly inclined showers also provides informaCon on theprimary parCcle ID for the cosmic ray nuclei.
This could be used to measure the detailed composiCon of the cosmic ray spectrum in the kneeankle energy range in a way which is complementary to the KASCADE measurements: it does not rely
on measurement of parCcles surviving to the ground level

SummaryImages of strongly inclined air showers from primary cosmic
ray nuclei have strong muon “tail” component

Vetoing of the tail component
showers allows suppression of thecosmic ray background by up to 105for gamma-ray observaCons

This provides a possibility for exploraCon of the PeV
gamma-ray sky in backgorund-free regime

Measurement of the size of themuon component providesa
possibility for the study ofcomposiCon of cosmic ray
spectrum above PeV energy