Nuclear Instruments and Methods in Physics Research A 461 (2001) 311–313

Shower reconstruction in the CLUE experiment

B. Bartolia, D. Bastierib, C. Bigongiarib,*, M.A. Cioccic, D. Cosulichb, M. Crestib,V. Dokoutchaevad, D. Kartashovd, F. Lielloe, N. Malakhovd, M. Mariottib,G. Marsellaf, A. Menzioned, R. Paolettic, G. Parlavecchiog, L. Peruzzob,

A. Picciolic, R. Pegnag, F. Rossog, R. Saccob, A. Saggionb, G. Sartorib, P. Sartorib,C. Sbarrab, A. Scribanoc, E. Smogailovd, A. Stamerrah, N. Turinic

aDipartimento di Fisica, Universit "a di Napoli and INFN, sezione di Napoli, ItalybDipartimento di Fisica, Universit "a di Padova and INFN, sezione di Padova, Via F. Marzolo 8, 35131 Padova, Italy

cDipartimento di Fisica, Universit "a di Siena and INFN, sezione di Pisa, Italyd INFN, sezione di Pisa, Italy

eDipartimento di Fisica, Universit "a di Trieste and INFN, sezione di Trieste, ItalyfDipartimento di Fisica, Universit "a di Lecce and INFN, sezione di Lecce, ItalygDipartimento di Fisica, Universit "a di Pisa and INFN, sezione di Pisa, ItalyhDipartimento di Fisica, Universit "a di Torino e INFN, sezione di Torino, Italy

Abstract

The CLUE experiment studies primary cosmic rays ðE � 2 TeVÞ by detecting UV (190–230 nm) Cherenkov lightproduced by atmospheric showers. Since atmospheric absorption in the UV range is higher than in the visible range,CLUE cannot apply algorithms normally used in IACT1 experiments to determine primary cosmic-ray direction. In this

paper, we present a new method developed by CLUE. The algorithm performances were evaluated using simulatedshowers. Preliminary results of the source analysis using this new method are shown.# 2001 Elsevier Science B.V. Allrights reserved.

PACS: 95.75.Pq

Keywords: Cosmic ray; Reconstruction algorithm

1. Introduction

In the past, CLUE [1,2] used a maximumlikelihood method approach [3] to determine

primary cosmic-ray direction comparing observeddistributions of Cherenkov photons in CLUEchambers with simulated ones. The best angularresolution on shower direction is 0:88 for energyE > 2 TeV and the reconstructed angle is affectedby a large systematic error for off-axis showers(18 systematic error for showers with 28 off-axisangle). Recently, CLUE developed a new recon-struction algorithm, called Thrust, adapting a

*Corresponding author. Tel.: +39-049-827-7211; fax: +39-

049-827-7102.

E-mail address: bigongiari@pd.infn.it (C. Bigongiari).1The Imaging Atmospheric Cherenkov Telescope.

0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 1 2 5 3 - 5 SECTION V.

method [4] widely used in accelerator physicsexperiments to determine particle jet directionand collimation.

2. Thrust method

The Thrust method relies on shower symmetryproperties, assuming that the momentum ofCherenkov photons is distributed with axialsymmetry around the primary direction. Thismethod determines the shower direction estimatingthe versor which maximizes the overall long-itudinal momentum of photons. We introduce aversor #nT , called Thrust axis, and a scalar T , calledThrust, defined by

T ¼1

PNck¼1Qk

XNc

k¼1

Qkj#rk � #nT j ð1Þ

where Nc is the number of charge clusters inCLUE chambers, #rk is the direction versor of thekth cluster and Qk its charge. The #nT whichmaximizes T is the Thrust estimate of the showeraxis (primary cosmic-ray direction).

3. Test and performances

This method has been applied and tested onsimulated proton and g shower samples (11 000showers). The MC showers were generated with anenergy between 1 and 10 TeV, sampled accordingto primary cosmic-ray spectrum (spectral index�2:7) and with a zenith angle between 08 and 48(mirror axes are assumed to be vertical). Showerimpact point was fixed in the CLUE array centeror randomized on a 300 m� 300 m squarecentered on it. The full detector simulation wasapplied to MC showers.The angular resolution (RMS) on the recon-

structed direction for triggered2 showers withimpact point in CLUE array center is 0:78. Thisimproved result was found for showers initiatedboth by protons and g’s. A very similar result isobtained for showers with randomized impactpoint when a proper correction is applied.The angular resolution improves with the

number of clusters as expected. Furthermore, theThrust angle is affected by a systematic error for

Fig. 1. Zenith angle distribution for events collected by tracking sources (Mrk421 on the left and Crab on the right), dots, and for

events collected tracking a region of the sky where no gamma ray source is known, filled area. Distributions are normalized in the

region y > 1:38.

2We required at least one cluster in three chambers.

B. Bartoli et al. / Nuclear Instruments and Methods in Physics Research A 461 (2001) 311–313312

off-axis showers with a much weaker dependenceon off-axis angle than the old likelihood method:only 0:48 for showers with 28 off-axis angle.

4. Conclusion

A new algorithm to estimate atmosphericshower direction was described and tested onsimulated data. It does not depend on MCsimulation and its performances are an improve-ment on the old likelihood method.The Thrust method was applied on the data

collected tracking the AGN Mrk421 and Crab. Aclear signal is visible from both sources (Fig. 1),even when no correction for non-zero shower

impact parameter was applied and only a smallfraction of available data was used.

References

[1] D. Alexandreas et al., Nucl. Instr. and Meth. A 409 (1998)

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[2] D. Alexandreas et al., Nucl. Instr. and Meth. A 409 (1998)

679.

[3] D. Bastieri et al., The CLUE experiment running with

8 telescopes; observation of gamma sources and runs

on Moon, in: B.L. Dingus et al. (Eds.), Proceedings of

GeV–TeV Gamma Ray Astrophysics Workshop, 2000,

pp. 436–440.

[4] S. Brandt et al., Phys. Lett. 12 (1959) 57.

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