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Effect of air fluorescence properties on the reconstructed energy of UHECR

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### Effect of air fluorescence properties on the reconstructed energy of UHECR

José Ramón Vázquez, María Monasor, Fernando Arqueros

Universidad Complutense de Madrid

6th Fluorescence Workshop, L’Aquila, Italy

Summary

- Motivation
- Fluorescence yield description
- Comparison of experimental data sets
- Humidity effect
- Temperature effect
- Effect on energy reconstruction
- Results
- Conclusions and todo

Motivation

- The fluorescence yield Y is a relevant parameter in the determination of the primary energy of UHECRs and its uncertainty is one of the main sources of systematics in the energy reconstruction.
- Accurate measurements of this magnitude could improve our knowledge about the energy spectrum of UHECR and reduce the uncertainties.

Fluorescence Yield

- The fluorescence yield, at a given and conditions, over a wavelength interval can be calculated as (1):

(1) Proceedings of the 5th Fluorescence Workshop. Nucl. Instrum. Meth. A 597 (2008) 1

Comparison of experimental data sets

Three experimental data sets are widely used in UHECRs experiments based on the fluorescence technique:

- HiRes (Kakimoto-Bunner):
- Y(P0,T0) and p’(T0) from Kakimoto (1) for 337, 357 and 391 nm lines measured at P0 = 1013 hPa and T0 = 288 K.
- The remaining spectrum is distributed according to Bunner spectrum (2).

(1) Kakimoto et al. Nuclear Instruments and Methods A 372 (1996) 527-533

(2) Interpretation of Bruce Dawson in Internal Auger Note GAP 2002-067

Comparison of experimental data sets

2. Nagano:

- Y0 and p’(T0) from his own measurements at P0 = 1013 hPa and T0 = 293 K for all wavelengths (1)

3. Auger (Nagano-Airfly):

- Y0337(800hPa, 293K) from Nagano measurements (1).
- P’(T0) and relative intensities from Airfly (2) measured at 800 hPa and 293 K.

The fluorescence yield as a function of height has been evaluated for these

3 data sets using a typical atmospheric profile measured at Auger site (3).

(1) Nagano et al. Astroparticle Physics 22 (2004) 235-248

(2) Ave et al. [Airfly Collaboration] Astroparticle Physics 27 (2007) 41-57

(3) Keilhauer et al. Proc. 29th ICRC 7 (2005) 23

Comparison of experimental data sets

Average difference ~ 20% !!

Impact of detector efficiency

However, the number of observed fluorescence photons depends on detector efficiency and air transmission T (X,X0) (including both molecular and aerosol effects).

X0

Experimental data sets

Experimental data sets

Including Auger filter transmittance F

Average difference ~ 7% !!

F

Humidity effect

- The effect of water vapour on air can be accounted following the general equations (1):

(1) Proceedings of the 5th Fluorescence Workshop. Nucl. Instrum. Meth. A 597 (2008) 1

Temperature effect

- p’ measurements are obtained at a given T0, but its value depends on temperature T. In principle:
- However, an additional dependence due to collisional quenching cross section has been observed (1,2):

(1) Ave et al. Nucl. Instrum. and Meth. A 597 (2008) 50-54

- Fraga et al. Nucl. Instrum. and Meth. A 597 (2008) 75-82

Temperature effect

- The temperature dependence of p’ in dry air, considering air as 79% nitrogen and 21% oxygen, yields:

In principle the T dependence for N-N quenching is different

from that of N-O quenching.

Temperature and humidity effect

We have studied different cases:

- Temperature model 1:

Equal treatment of N-N and N-O quenching (1):

- Temperature model 2:

Different treatment of N-N and N-O quenching:

- Humidity model:

p’hum from Airfly (1)

·

(1) Ave et al. Nucl. Instrum. and Meth. A 597 (2008) 50-54

- Fraga et al. Nucl. Instrum. and Meth. A 597 (2008) 75-82
- Waldenmaier et al. Nucl. Instrum. and Meth. A 597 (2008) 67-74

Temperature and humidity effect

Again, typical atmospheric profiles for humidity,

temperature… measured at Auger site have been used.

Temperature and humidity effect

Again, typical atmospheric profiles for humidity,

temperature… measured at Auger site have been used.

Temperature and humidity effect

Again, typical atmospheric profiles for humidity,

temperature… measured at Auger site have been used.

The humidity effect is important at low height.

Temperature and humidity effect

Including Filter transmittance F

Including detector efficiency

The filter does not introduce any difference since these effects don’t modify significantly the spectrum (relative intensities).

Effect on energy reconstruction

The Gaisser-Hillas parameterization describes the longitudinal energy deposition :

Typical GH profile for a shower of 1020 eV

The total calorimetric energy E will be:

Effect on energy reconstruction

The impact on energy of using two different data sets is studied by comparing E, E’:

- The profile of deposited energy will generate a fluorescence photon profile according to a given Y(X):
- If this photon profile were generated by a different fluorescence yield Y’(X), the deposited energy profile would become:

Effect on energy reconstruction

- The effect of the fluorescence yield on energy reconstruction
- will be also dependent on:
- Detector efficiency
- Showers features:
- Zenith angle
- Primary energy and mass
- Distance shower-telescope (molecular and aerosol transmission)

Effect on energy reconstruction

- We have studied the effect of changing the fluorescence yield for three different zenith angles (0º, 30º, 60º) and three different distances shower-telescope (10, 20, 30 km) considering the Auger description as standard and comparing it with:
- Nagano
- HiRes
- Auger std + Temperature model 1
- Auger std + Temperature model 2
- Auger std + Humidity dependence
- Auger std replacing p’ from those measured by Macfly Coll.

For humidity, T and P profiles, averaged values measured at Auger site for June have been used.

Effect on energy reconstruction

Comparing Auger and HiRes Y(X) profiles including detector efficiency:

- = 30º

Ediff= -1.77%

Energy reconstruction

Comparing Auger with Auger+humidity dependence as a function of :

- = 0º

Ediff= -7.05%

- = 30º

Ediff= -3.99%

- = 60º

Ediff= -0.62%

Energy reconstruction

Comparing Auger with Auger+temp1 dependence as a function of :

- = 0º

Ediff= -2.56%

- = 30º

Ediff= -2.91%

- = 60º

Ediff= -6.04%

Preliminary Results

Relative energy difference (%) at = 0°

Preliminary Results

Relative energy difference (%) at = 30°

Preliminary Results

Relative energy difference (%) at = 60°

Conclusions

- We have developed an analytical method to study the impact of Y on the energy of UHECR showers, as a function of detector efficiency and shower geometry. No event reconstruction is needed.
- Significant differences, already pointed out, between Auger and HiRes fluorescence models have been observed. This discrepancy is strongly reduced when detector efficiency is taken into account.
- Auger and Nagano data sets don’t present significant differences (<2%).

Conclusions

- The humidity introduces a zenith angle dependent effect on energy from 7 % for 0° to 0.6% for 60°.
- Using Temperature 1 model a difference in energy of 2.7 % for 0° to 6.5 % for 60°. The separate treatment of NN and NO adds another 2% variation.
- The shower-telescope distance does not have a significant effect on the reconstructed energy.
- When p’ values from Macfly collaboration are used no significant changes in energy (<1%) are observed even when the experimental values are quite different from those obtained by Airfly collaboration.

To do:

- Study the impact of fluorescence yield properties as a function of primary energy and mass.
- The effect of distance must be evaluated for different aerosol conditions.
- Evaluate these effects under different atmopsheric conditions
- Evaluate the uncertainties.

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