The measurement of the cosmic ray primary energy spectrum...

Elena Cantoni for the KASCADEGrande CollaborationElena Cantoni for the KASCADEGrande CollaborationCosmic Ray International Seminar 2010Cosmic Ray International Seminar 2010

University of TorinoIstituto di Fisica dello Spazio Interplanetario di Torino INAF

The measurement of The measurement of the cosmic ray primary energy spectrum the cosmic ray primary energy spectrum

at 10at 101 61 6 – 10 – 101 81 8 eV eV with the KASCADEGrande experimentwith the KASCADEGrande experiment

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OutlineOutline

Motivations for the KASCADEGrande experiment in the cosmic ray physical context

KASCADEGrande features and performances

The measurement of the cosmic ray energy spectrum

Outlook

Conclusions

KASCADE – Grande KASCADE – Grande at Karlsruhe Institute of TechnologyLarge multicomponent groundbased experiment for highquality EAS measurements in the range 101 6 – 101 8 eV

KArlsruhe Shower Core and Array DEtector (from 1996)

Grande (2003):

extension with 37 detectors fromformer EAS-TOP (1987 – 2000, LGNS)

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Motivations for the KASCADEGrande experiment Motivations for the KASCADEGrande experiment in the cosmic ray astrophysical contextin the cosmic ray astrophysical context

Study cosmic ray primary energy spectrum and composition at 101 6 – 101 8 eV → crucial range, somehow unexplored: complete “knee” studies investigate galactictoextragalactic transition

CRs spectrum: power lawJ(E) E γ with peculiarities

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“KNEE” at 3 – 5 101 5 eV:steepening of spectral indexfrom ~2.7 to ~3.1

“ANKLE” at 4 – 10 101 8 eV:Flattening by roughly the same index ~ 0.3 – 0.4

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The “knee”: → caused by steepening of

light primaries spectra

Explained with standard model for galactic CRs ? → consequence of magnetic confinement at

acceleration sites and during propagation E

m a x = Z∙E

0 with E

0 3∙10≈ 1 5 eV

→ rigidity dependence of the brake off → iron knee expected at ~ 101 7 eV ?

EASTOP & MACROEASTOP & MACRO

KASCADEKASCADE

M. Aglietta et al., N.P. B, 35 (1994) 257

T. Antoni et al., A.P., 24 (2005) 125

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The galactictoextragalactic transition:

“mixed composition model” (Hillas, Allard, et al.) → extra contribution “B” of galactic CRs up to E > 101 8 eV → raise of extragalactic component just at the highest

energies, from early active sources → sign of transition: “ankle” → sign of component B: “second knee”

“dip model” (Berezinsky et al.) → break of galactic component

at ~ 101 7 eV → raise of purely extragalactic p

(explains ankle), AGNs as sources → sign of transition: “second knee”

at 5 – 7 101 7 eV (AKENO, HiRes)

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KASCADEGrande features and performancesKASCADEGrande features and performancesKASCADE: → energy range 101 4 – 101 6 eV → 252 detector stations over 200x200 m2 → in a station: measurement of e and μ (E>230 MeV) separately with

two colocated types of scintillators

Grande: → 37 detector stations 10 m2 each spread over 700 x 700 m2 → in a station: measurement of allcharged e + μ

→ 18 hexagonal clusters. 7 outof7 coincidence triggers data acquisition

Upper view and Bottom view of a Grande stationUpper view and Bottom view of a Grande station

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Event reconstruction: → EAS observables:

. arrival direction (θ,φ) [from time differences of EAS arrival times on G st.s]

. core position (Xc,Y

c)

. total charged size Nc h

[from energy deposit in each G station]

from Grande measurements . muon size N

μ from→ KASCADE measurements

Rec.ed July 8 2005, 12:11 UTC. Core: (-155, -401) m. log10(Nch) = 7.0. log10(Nμ) = 5.7. Zenith : 24.2°. Azimuth: 28.4°

W. D. Apel et al., N.I.M. A, 620 (2010) 202 216

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datadata

datadata

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Grande reconstruction accuracies: → direct comparison with KASCADE; results in agreement with MC simulations

Rayleigh fitRayleigh fit

σ(Nc h

): syst. < 8%; stat. < 15%

at full efficiency i.e. Nc h

> 106σ(θ,ϕ): ≈ 0.8°

σ(Xc,Y

c): ≈ 6 m

W. D. Apel et al., N.I.M. A, 620 (2010) 202 216

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KASCADE Grande muon reconstruction accuracy: → from Monte Carlo simulations (FLUKA/QGSjetII) it is found:

Mean ∆Nµ/N

µt r u e ≤ 20% satisfactory, but features of inaccuracies well understood, so

→ application of correction function: N

µc o r r = N

µ∙f(θ,r,N

µ)

calibrated with simulations H, He, C, Si, Fe mixed → total syst. with correction within 10% at all N

µ

W. D. Apel et al., N.I.M. A, 620 (2010) 202 216

simsimsimsim

simsim

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The events selection: → Runs with detector in stable performance and fully operational → Events passing successfully the full KG reconstruction procedure → 0° ≤ θ ≤ 40° [to avoid reconstruction uncertainties of higher angles] → fiducial area (see figure)

[to discard border effects and under and overestimation of Nµ]

→ events at full reconstruction efficiency (E0 ? 101 6 eV from simulation studies)

Finally, for this analysis: → 1173 days of data → 2∙109 cm2∙sr total acceptance → 2∙101 7 cm2∙s∙sr total exposure

simsim

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Reconstruction of the cosmic ray energy spectrumReconstruction of the cosmic ray energy spectrum

Three approaches are used: → use of the observable N

c h

→ use of the observable Nµ

→ use of the combination of Nc h

and Nµ

This makes possible a number of important cross checks: → reconstruction procedures → systematic uncertainties → effectiveness of hadronic interaction models in describing the data

Moreover: → Test sensitivity to composition

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The first two methods can be described in parallelThe first two methods can be described in parallel

charged size spectra muon size spectra

in different angular ranges of equal acceptance

→ the size spectra must be corrected for attenuation effects in the atmosphere → the Constant Intensity Cut method is used

datadatadatadata

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The Constant Intensity Cut Method: → advantage: independent from MC simulations → assumption: cosmic rays fall isotropically and flux behaves monotonically with primary energy. Then particles with the same frequency have got the same energy at different zenith angles

→ fixed integral intensities are chosen → attenuation curves for each intensity are built → global fit to all curves applied (2nd deg. P(secθ)) → for each shower: correction to reference angle is obtained (θref,ch = 20°; θref,µ = 22°)

datadatadatadata

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Size – Energy calibration (QGSjetII/FLUKA used): → disadvantage: dependent from primary composition assumed in MC simulations → assumption: dependence E

0 ∝ N

c h , µ

αc h

,αµ

Fluctuations bigger than bin size

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simsim

simsim

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Unfolding procedure (QGSjetII/FLUKA used. H, He, C, Si, Fe in 20% mix): → to reduce effect of fluctuations on the determined energy spectrum → response matrix R

i j constructed with simulations

Ri j → probability that event with energy in bin Log[E

r e c(i)]

has true energy in bin Log[Et r(j)]

n

jt r = Σ

i = 1N R

i j∙n

i e x with:

→ N total number of bins → n

i e x exp number of events

corresponding to Log[Er e c

(i)]

→ njt r true number of events

corresponding to Log[Et r(j)]

→ Σi = 1

N Ri j = 1

Matrix is smoothed to avoid random fluctuations caused by MC limited statistics

simsim

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2dimentional reconstruction of the energy spectrum with the2dimentional reconstruction of the energy spectrum with the NNc hc h

NNµµ technique technique

→ with MC simulations (QGSjetII/FLUKA) a formula is obtained to calculate the primary energy per individual shower on basis of N

c h & N

µ

→ advantage: primary composition dependency minimized taking into account the mass sensitivity → model dependency of correction for atmospheric attenuation 5 different zenith angle intervals analyzed independently and combined after differences in results systematic uncertainty→ (1)

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simsimsimsim

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Comparison of the reconstructed intensity in different angular bins: → semidifference between 1s t and 5t h angular bin data points estimates (1): uncertainty on relative energy calibration among angular bins systematic due to differences in shower attenuation between real and sim. data ~ 10% at all energies, uncertainty on selfconsistency of the technique and the model

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Response of the entire procedure checked (using QGSjetII/FLUKA simulations): (also here, reconstructed flux treated with unfolding)

→ < 10% systematic uncertainty (2)

→ relative uncertainty

on energy assignment (3): 26% energy resolution (RMS) at threshold < 20% at highest energies

simsim

simsim

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Capability to reconstruct extreme cases (QGSjetII/FLUKA): → using simulations, a primary composition with an abrupt change is constructed and the formula is used to reconstruct the correspondent total flux → also in cases like this, the systematic uncertainty of flux_rec / flux_true < 10% (4)

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(1) →(2) →

(4) →(5) →

(3) →

Table of systematics on the flux cases: (numbered the discussed uncertainties)

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Hadronic interaction models: → calibration depends on simulations so other models will change data interpretation → to study effects on spectrum method is performed with EPOS 1.99 based calibration function (left plot) and also using EPOS data as “experimental” and analyzing them using QGSjetII (right plot)

EPOS leads to ≈ 10 – 15 % lower flux motivation: EPOS richer in muons and poorer in charged particles (at KG experimental conditions)

simsim

simsim

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Comparing the different approaches: → using one observable: large dependence on primary composition. True spectrum has to be a solution inside the range spanned by two methods → larger uncertainty from using both observables compensated taking into account their correlation at individual events and composition dependence strongly decreased → relatively heavy composition in the frame of QGSjetII

syst. band

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Discussion of the result using the residual plot: → smaller structures under overall power law behavior, spectrum not describable with single power law over 101 6 eV: concavity expected if rigidity dependence is assumed around 101 7 eV: small break composition studies on the way!!→ structures reliable independently from interaction model used for analysis

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Comparison with KASCADE and EASTOP at lower energies: → initial concave part in agreement

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About the break at 101 7 eV: → Fisher test applied to check incompatibility between break and single power law

→ the two power laws are significantly incompatible with each other

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Comparison with the other experiments: → lower energies: agreement with EASTOP, KASCADE & c. → middle energies: bit lower flux than GAMMA, AKENO and YAKUTSK → highest energies: agreement with HiRes, Auger

(blue band is total systematic uncertainty)

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OutlookOutlook

Composition analysis will be from now crucial to understand origin of spectral structures

Again different approaches are under study based on N

c h N

µ and QGSjetII model:

separation in light and heavy spectra using k parameter KNN technique study of ratio N

µ/N

c h and relative abundances

of different primaries in bins of Nc h

unfolding techinque

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Study of ratio Nµ/N

c h and relative abundances of different primaries in bins of N

c h

→ data and simulations compared at different arrival directions and energies. With Chi Square Test, the number of mass groups and their relative abundances

describing fairly the data distributions are found.

3 mass groups, light, medium, heavy needed, QGSjetII consisitent with data Analysis repeated on KASCADE as reference is consistent Heavy dominant, but light still needed around 101 7 eV

0º-24º 29º-40º

KASCADE

E. Cantoni et al., Proc ICRC 2009

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ConclusionsConclusions

KASCADEGrande main observables shower size and muon size could be reconstructed with high precision and low systematic uncertainties

Different methods are applied for the allparticle energy spectrum reconstruction and compared for crosschecks of: reconstruction systematic uncertainties validity of the hadronic interaction model

The cosmic ray energy spectrum could be reconstructed in the range 101 6 – 101 8 eV within a 10 – 15 % uncertainty, being based on the hadronic interaction model QGSjetII which shows its intrinsic consistency

Tests with EPOS show the possibility of a shift in the absolute energy scale, but the use of different models would not change the overall spectral structure

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Regarding the allparticle energy spectrum: agreement between different reconstruction approaches No single power law, structures at threshold and around 101 7 eV agreement with KASCADE and EASTOP at threshold (and with HiRes and Auger at the highest energies) medium/heavy primary composition preferred assuming QGSjetII

Bases are put to look for the iron knee and the galactictoextragalactic transition features of cosmic rays work is in progress for → composition studies using different approaches

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The measurement of the cosmic ray primary energy spectrum...

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