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Measurement of the Cosmic Ray energy spectra by the ARGO-YBJ experiment A. Surdo

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Measurement of the Cosmic Ray energy spectra by the

ARGO-YBJ experiment

- A. Surdo

IstitutoNazionale di Fisica Nucleare

Sezione di Lecce, Italy

(on behalf of the ARGO-YBJ Collaboration)

ARGO-YBJ

Astrophysical Radiation

with Ground-based

Observatory at YangBaJing

ARGO-YBJ

High Altitude Cosmic Ray Observatory @ YangBaJing,Tibet, China

Site Altitude: 4,300 m a.s.l. , ~606 g/cm2

1/20

ARGO-YBJ

- VHE g-Ray Astronomy:

study of point-like (and diffuse) galactic and

extra-galactic sources with few hundreds GeV energy threshold

- Cosmic ray physics:
- energy spectrum and composition
- study of the shower space-time structure
- flux anisotropies at different angular scales
- p-Air cross section measurement
- hadronicinteraction studies
- anti-p / p ratio at TeV energies,
- geomagnetic effects on EAS

…..

- Search for GRB’s (full GeV / TeV energy range)
- …

throughthe…

Observation of Extensive Air Showers produced in the atmosphere by primary g’s and nuclei

NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ

2/20

BP Amplitude :

mV to many Volts

ARGO-YBJ

Detector layout

(5,800 m2)

74 m

99 m

8 Strips

(6.5 x 62 cm2)

for each Pad

10 Pads

(56 x 62 cm2)

for each RPC

1 CLUSTER = 12 RPC

(43 m2)

78 m

111 m

RPC

BigPad

Strip = spatial pixel

Pad = time pixel

Time resolution ~1.8 ns

BigPad =CHARGE readout PIXEL, 123 x 139 cm2, 3120 (central carpet)

+ Analog RPC charge read-out

NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ

EAS reconstructionbydigitalreadout

ARGO-YBJ

Data takingwith full configuration:

November 2007- February 2013

Event Rate ~ 3.5 kHz for Nhit≥20 - Duty cycle ~ 86% - 1011evts/yr

duty-cycle 86 %

Space/time granularity

+ full coverage

+ high altitude

event imaging and EAS space/time structure study with unprecedented details

NumberoffiredStrips

NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ

ARGO-YBJ

- Analogreadout pixel = BigPad (half RPC chamber)
- Extended energyrange (above 100 TeV)
- Access to the LDF down to the showercore
- (particledensities up to ~ 2x104/m2)
- Sensitivity toCR primarymass
- Info/checks on Hadronic Interactions

Saturated digital signal

Same event in analogview

NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ

5/20

The RPC analog readout (II)

ARGO-YBJ

- Eightgain scales (G0, G1, … G7) ensuregoodlinearity up toabout
- 2 x 104 particles/m2
- G7 data overlap the digital-mode linearity range, and have been
- used for intercalibration and cross checks
- G0 allowsto cover the energyrangeuptto 10 – 15 PeV
- importantregionfor the estimationofatmosfericnflux
- Here we use G4and G1scales
- to cover the 50 TeV – 5 PeV
- range with high efficiency
- and without saturation
- Eventselection:
- Corereconstructed in a central
- detector fiducial area
- - Reconstructedzenith angle <15o

trigger

effect

Log (particle maximum density/(part/m2))

NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ

(mass dependent) energyestimator

ARGO-YBJ

- Np8(number of particles within 8m from the core):
- well correlated with primary energy
- not biased by finite detector size effects
- weakly affected by shower fluctuations

Onlyeventswithzenith angle lessthan15 degrees in thisanalysis

Look for information on the showerage in ordertohave a mass independentenergyestimator

P

He

CNO

Fe

NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ

7/20

Lateral Distribution Function (LDF)and shower age

ARGO-YBJ

With the analog data

the LDFcan be studied

nearthe core

withoutsaturation

- LDF is well fitted by
- a modified NKG function

The LDF slope (s’) is

related to the shower

age and does not depend on the primarymass (‘universalityproperty’)

Average Xmax (g/cm2)

Assume anexponentialabsorptionafter the showermaximum. Get the correctsignal at maximum (Np8max) byusing Np8 and s’ (fitparameter) measurementforeachevent

Checks in progress with Gaisser-Hillas profile

NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ

Mass independent Energy reconstruction

ARGO-YBJ

MC sample followingHorandelmodelspectra and composition

SimilarresultswithGaisser-Stanev-Tilav (GST) model

In excellent agreement withtotal-size-vs-Etheoretical plot.

The shiftissimply due to the factthatwe are using the truncatedsize

Measurement energy range

The measurementof Np8 and the (agecorrelated) LDF slope

allowsestimating the truncatedsize at the showermaximum.

Thisensures a mass independent Energy determination.

NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ

9/20

Trigger and eventselectionefficienciesfor the allparticlespectrum

ARGO-YBJ

G4 U G1

in full efficiencyforallspeciesfrom

300 TeVto 5 PeV

Energy range for the spectrum measurement

G4

P

He

Fe

CNO

Average (Hoerandel)

Average (GST)

Low MC statistics.

Fit with Fermi functions.

Enlarged MC data sample on the way.

Energy range for the spectrum measurement

G1

NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ

10/20

The allparticlespectrum

ARGO-YBJ

- Picture consistentwithmodels and previousmeasurements
- Niceoverlapbetweenthe twogainscales (differentdata samples, …)
- Resultssuggestspectralindex-2.6 below 1 PeVand -2.8 from 1 to 5 PeV
- G0 wouldextend the energyrange up to 10-15 PeV
- About a factor 5 shouldbegainedbyconsideringinclinedevents
- The extensionstohigherenergywouldbe the subjectof a future work

Preliminary

Extent of the possible shift due to the energy scale uncertainty

NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ

11/20

p and Heselection(MC Hoerandelspectra and normalizations)

ARGO-YBJ

s’ vs Log(Np8)

He

protons

CNO

Fe

~ 15% contaminationfrom CNO and heavier elements

NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ

The p+Hespectrum (1st)

ARGO-YBJ

- Overlap of the points with different gain scales
- Overlap with direct measurements at low energy
- Gradual change of the slope starting around 700 TeV: possible (p+He) knee !?!
- Consistent with previous hints (MACRO, CASA-MIA, Chacaltaya, EAS-TOP, …)
- and YAC-Tibet spectrum
- Flux systematics + CNO contamination Overall uncertainty < 20 %

Preliminary

Extent of the possible shift due to the energy scale uncertainty

NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ

13/20

p+Hespectrum (2nd):Bayesian unfolding of analog data

ARGO-YBJ

Phys. Rev. D 85, 092005 (2012)

Bayesian analysis

for ARGO-YBJ digital data

NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ

14/20

p+Hespectrum (2nd):Bayesian analysis of analog data

ARGO-YBJ

- The results are consistent with previous analysis
- The approach is fully Bayesian
- Different fiducial cuts, also more inclined events (qzenith<35°)

Preliminary

NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ

15/20

p+He hybrid measurement (3rd):Cerenkov telescope + ARGO-YBJ array

ARGO-YBJ

Wide Field of view Cerenkov Telescope Array (WFCTA)

5 m2spherical mirror

16×16 PMT array

Field of View: 14° × 16°

Elevation angle: 60°

Chinese Phys. C 38 (2014) 045001

Hybrid analysis for lower energy

showers

Cerenkov signal: energy measurement

Hillas parameters

ARGO-YBJ analog data: core position

particle number at maximum

shower direction

Light elements are selected according to particle density

near the core and shape of the Cerenkov image (L, W)

2L

2W

NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ

16/20

Hybrid measurement (3rd):Cerenkov telescope + ARGO-YBJ array

ARGO-YBJ

- The results are consistent with previous analyses
- Possible shape difference
- Different data sample and introduction of another detector
- Different analysis cuts (also inclined events)

Preliminary

NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ

17/20

ARGO-YBJ (+ WFCTA): p+He spectrum

ARGO-YBJ

Published ARGO-YBJ

(strip) spectrum

Preliminary

Preliminary

- The resultsare consistent withARGO-YBJ measureat lowerenergies (strip data)
- The 3 new analyses are consistent within systematics
- (further cross-checks in progress)

NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ

18/20

Comparisonwith other p+He measurements

ARGO-YBJ

Preliminary

Preliminary

These results are consistent with direct (i.e. below 200 TeV)

and YAC-Tibet measurements

NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ

19/20

Summary and Outlook

ARGO-YBJ

- ARGO-YBJ measured the CR spectrum in the TeV – 5 PeV range.

All-particle spectrum consistent with other experiments

- Evidence for a bending in the p+He spectrumbelow 1 PeV

(two different analyses of ARGO-YBJ data in agreement

within quoted uncertainties)

- Consistent results from a third independenthybridanalysis

(RPC + Cerenkov signal)

- Further cross checks, larger data and MC statisticsforfinalresults
- Clueforanimprovedestimationofatmosphericnflux

(spectrumofnucleonsdeterminesfluxesofatmosphericn and m)

NOW-2014 A. Surdo:Measurement of CR energy spectra with ARGO-YBJ

20/20

Bruno Rossi conceptual EAS detector

4000

3500

3000

2500

2000

Analog view of a shower

1500

1000

500

3-D view of a shower detected in ARGO-YBJ

The analog readout system

ARGO-YBJ

Eight gain scales (G0, G1, … G7) ensure good linearity up to

about 2 x 104 particles/m2

G7 data overlap the digital-mode linearity range, and have

been used for intercalibration and cross checks

Here we use G4 and G1 scales to cover the 50 TeV – 5 PeV

range with high efficiency and without saturation

Np8 = how many

particles within

8 m from the core

Lateral Distribution Function (LDF)

G1 scale

G4 scale

Linearity

range

5x104 < Np8 < 105

5x103 < Np8 < 104

Linearity

range

Sensitivity threshold

Sensitivity threshold

ARGO-YBJ

Np8:particle size truncated at 8m of core distance

Not affected by possible saturation of Analog System

Log(Np8) distributions for DATA from G4 and G1 scales

ARGO-YBJ

- Simulated air shower samples:
- (a) p showers (1- 10,000)TeV, Theta<45°
- (b) He showers “ “
- (c) CNO showers “ “
- (d) Fe showers “ “
- produced using CORSIKA code (QGSJET-II.03 + Fluka)
- Also p and He showers (1- 3000)TeV, Theta<45°
- produced using a different hadronic model: SIBYLL-2.1 (+ Fluka)
- Simulated showers (sampled on large areas) given in input to the ARGO MC (based on Geant-3) fully simulating the detector response (analog charge trigger and readout system included)
- MC data reconstructed by using the same program as for real data.
- Event selection: core inside a fiducial area Afid = (64 x 64) m2
- (qzen < 15° used in this analysis)

ARGO-YBJ

DLog(Np8) = (4.7 - 5.0)

Fit with r’NKG

±10% band

Comparison of residuals from different function fits:

The all-particle spectrum (II)

ARGO-YBJ

- Picture consistent with models and previous measurements
- Nice overlap with the two gain scales (and different data)
- The plot suggests spectral index -2.6 below 1 PeV and -2.8 from 1 to 5 PeV
- G0 would extend the energy range up to ~15 PeV
- About a factor 5 should be gained by considering inclined events
- The higher energies would be the subject of a future work

Preliminary

possible shift

due to energy uncertainty

Efficiency of trigger and event selectionfor the p+He spectrum

ARGO-YBJ

range for p+He spectrum measurent

Efficiency plateau

above 200 TeV

range for p+He spectrum measurement

- H
- He
- Fe
- CNO
- Average (Hörandel)
- Average (GST)

Finding the best labsparameter

ARGO-YBJ

Furtherimprovements in progress

LogE resolution at 270 TeV vs labs

- p
- He
- CNO
- Fe
- Total

Correction with l=120 g/cm2

1 s of Log(Erec/Etrue)

l=120 g/cm2

Small residual shift with LogA as foreseen by theory

l→∞

- p
- He
- CNO
- Fe

No correction

( l→∞ )

Energy reconstruction:bias and resolution

ARGO-YBJ

The response function is Gaussian in LogE.

The spectra are then given in LogE bins, much larger

than the estimated bias and well above the LogE

resolution, in the considered energy range.

Bin size chosen for the energy spectrum

Bias < +/- 0.05

Measurement energy range

Measurement energy range

MC Energy distributions

ARGO-YBJ

MC sample following Horandel model spectra and composition

Similar results with Gaisser-Stanev-Tilav (GST) model

Measurement energy ranges

MC true energy

MC reconstructed energy

Systematic uncertainty evaluations

ARGO-YBJ

- Flux
- Geometrical aperture : (5 % in/out contamination) (2.5% angular contamination) = 5.6 %
- Efficiency: (5% from MC samples) (<10% efficiency estimation of the mixture) = 5.0-11.2 %
- Unfolding: 3 %
- Hadronic interaction model < 5 %
- TOTAL: 8.1 % - 13.8 % TOTAL (conservative) = 14%

- Energy scale
- Gain of the analog system: 3.7 %
- Energy calibration: 0.03 in LogE = 6.9 %
- Hadronic interaction model: 5 %
- TOTAL: 9.3 % TOTAL (conservative) = 10%

In the flux plots an over-conservative ± 14% shaded area has been

temporarily drawn on the flux measurements

Error bars show the statistical uncertainties

Systematics from the hadronic interaction models

ARGO-YBJ

The dependence on the adopted hadronic interaction model is small.

The differences among the QGSJET-II.03 and Sibyll-2.1 are within few percent

in the explored energy range (no bias due to muon number).

All further results shown here were obtained with QGSJET-II.03.

Mass selection for the hybrid measurement

ARGO-YBJ

2L

Log(RPCmax) – 1.44 Log(E)

2W

L/W – 0.0091 Rp – 0.14 Log(E)

Rp – shower impact parameter

E – reconstructed energy

RPCmax – maximum RPC signal

The ARGO-YBJ measurements of the p+He spectrum

Results also consistent with measurement at lower energies, done with the strip data.

Consistent picture within systematics. Further cross-checking still ongoing.

Preliminary

3 TeV – 3 PeV

Update of the analysis published in

PRD 85 (2012) 092005

Gaisser- Stanev- Tilav p+He

Hoerandel p+He

with Ec= Z PeV

Hoerandel p+He

I. De Mitri: Measurement of CR energy spectra with ARGO-YBJ

The p+Hespectrumindex

Preliminary ARGO-YBJ analogdata (G4and G1)

CREAM p+He

Preliminary

p+Hespectralindex

Direct measurements combination

Log(E/TeV)

Comparison with other p+He measurements

Consistent results with direct measurements (i.e. below 200 TeV) and YAC-Tibet

Preliminary

Gaisser- Stanev- Tilav p+He

Hoerandel p+He

with Ec= Z PeV

Hoerandel p+He

I. De Mitri: Measurement of CR energy spectra with ARGO-YBJ

CREAM1.09 x 1.95 x 10-11 (E/400 TeV)-2.62

- ARGO-YBJ 1.95 x 10-11 (E/400 TeV)-2.61
- Hybrid 0.92 x 1.95 x 10-11 (E/400 TeV)-2.63

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