hadron separation

Nuclear Instruments and Methods in Physics Research A 389 (1997) 204-207

NUCLEAR INSTRUMENTS & METHODS IN PHYSES RESEARCH Sectron A

ELSEVIER

Search for TeV y-rays from extragalactic point sources with neural network ylhadron separation* S. Ommer, S. Westerhoff”, H. Meyer, HEGRA Collaboration Ph_vsicsDepartment, Uniuersiq of Wuppertai, D-42097 Wuppertal, German?

Abstract The neutral net (NN) based analysis of the data provided by the 17 Geiger towers of the HEGRA (High Energy Gamma Ray Astronomy) air shower detector array on the Canary Island La Palma is used to search for 2 50 TeV y-emission from the superposition of nearby northern blazars. A comparison of MC and experimental data shows good agreement if the actual detector performance is taken into account. We find weak evidence for enhanced y-emission from the detection of the blazars.

1. Introduction In spite of the enormous effort which has gone into searching for point sources of TeV cosmic rays, results are rather disappointing so far. Two extragalactic objects, Mrk 421 and Mrk 501, have been detected unambiguously at TeV energies [l-3] with Cherenkov telescopes, both belonging to the blazar subclass of galaxies showing powerful non-stellar activity characterized by rapidly variable, polarized continuum emission. But although the blazar continuum spectrum varies rather smoothly from radio frequencies to TeV y-rays suggesting that it might continue to still higher energies, no extragalactic source has been discovered above -20 TeV, the energy region of earth-bound detector arrays like HEGRA at 2200 m a.s.1. on the Canary Island La Palma. Data analysis in air shower cosmic ray physics has to overcome two basic problems: the cosmic ray primary particles are detected only indirectly by the huge amount of secondary particles they induce in Earth’s atmosphere. Measuring the arrival times of the particles in the shower front by a scintillator matrix allows the reconstruction of the primary particle’s direction, but gives no hint as to its type. This is crucial as the showers initiated by y

primaries have to be separated from the isotropic background of hadron-induced showers which are at least two orders of magnitude more abundant but have lost the information about their origin by deflections in the intergalactic magnetic fields. Apart from the 221 scintillation counters placed on a 15 m grid (including a dense inner region with 10 m spacing) on an area of 180 x 180 m’ mainly used for the determination of the shower direction and the energy of the primary particle, HEGRA therefore comprises two additional sub-arrays which allow y/hadron separation based on (nearly) orthogonal features of air showers. A matrix of 17 Geiger towers [4] placed on a 30 m grid in the central part of the HEGRA array provide a ylhadron separation based on the charged particles of the air shower. Each tower covers an active area of 270x 600cm’ and consists of 6 layers of 160 Geiger tubes with quadratic cross section (1.5 x 1.5 cm’) and 600 cm in length. After the first and the second Geiger tube layer, 5 r.1. of lead absorber separate the low-energy component of the air showers from penetrating muons and high- energy electromagnetic particles, which are indicators for hadronic showers [7]. The NN analysis of Geiger tower data is described in detail elsewhere [6,7] and is thus only shortly reviewed in the next section. In this contribution, we will concentrate on its application to data.

*This work is supported by the BMBF, FRG, under contract number 05 2 WT 164. *Corresponding author. Tel.: + 49 (0)202 4392829; fax: + 49 (0)202 4392662; e-mail: westerho@wposl. physik.uniwuppertal.de. 0168-9002/97/$17.00 Copyright PII SO168-9002(97)00109-5

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1997 Published

Finally, HEGRA also includes a 7 x 7 matrix of socalled AIROBICC detectors (Air Shower Observation by Angle Integrating Cherenkov Counters [S]), each of them containing an open photomultiplier (diameter 20 cm) with a mirrored Winston cone to increase the light collection area. These counters directly view the night

by Elsevier Science B.V. All rights reserved

S. Ommer et al. / Nucl. Ins@. and Meth. in Phw

sky to detect the Cherenkov light from air showers, thus the operation is restricted to clear, moonless nights. The y/hadron separation makes use of the different lateral distribution of Cherenkov photons.

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separation

The Geiger tower y/hadron separation is based on the number of reconstructed tracks and the different energy distribution of electromagnetic particles (e * , y) in y- and hadron-induced air showers. The complex calorimetric information contributed by individual Geiger towers in different distances to the shower core is analyzed using feed-forward neural networks [5]. For the simulation of the cosmic ray showers, we use the CORSIKA 4.06 air shower generation code in combination with a full Geiger tower simulation program based on the GEANT 3.21 package. As input variables we use 4 values for each of the 17 towers: the number of hits in the first and in the second layer, the average number of hits in layers 3-6, and the number of tracks, thus covering both the calorimetric information and the track information in e~lch 10~~. A three-layered NN with topology 68-17-t is trained using the backpropagation algorithm to give the desired output value 0 for y- and 1 for proton-induced showers. After 500 epochs of training with 1500 y- and 1500 proton-induced showers, the NN performance on test showers shows no further improvement. The NN output for 3000 y- and 3000 proton test showers is shown in Fig. 1. Before applying the NN analysis to experimental data, we have to take into account the differences between data and MC in terms of detector performance. Although the NN tolerates missing information to a certain degree, the quality decreases with the inevitable number of non-working towers, layers or tubes. As a method for monitoring the y-efficiency, c,, and the hadron rejection for individual runs, we use the experimental data to determine the actual efficiency of every single Geiger tube. These Geiger tube efficiencies are then included in the full Geiger tower detector simulation, so the MC data represent the actual status of the detector matrix as closely as possible. Thus, the expected NN performance can be determined for every individual run. Applying this folding procedure allows to discern runs with deteriorated performance and to verify that the y-efficiency is stable for all data runs used in this analysis. In Fig. 1 the NN output distributions of MC showers and experimental data are compared. As an important conclusion, we note that in order to explain the experimental data it is not necessary to include nn~ y-showers in the cosmic radiation at all. There is no significant excess in the region where y-showers are supposed to pile up. Nevertheless, Fig. 1 also shows that in spite of the complexity and the huge number of simulated processes

I 1

1 1

net output Fig. I. NN output for MC y-showers (dashed line). MC proton showers (dots with error bars), and experimental data (solid line) normalized

Table 1 The sources

to the number

of MC proton

and their redshifts

events.

I

Source

2

Name

Source

:

Name

0055 +300 2201 +044 1101 +384 0430+05’ 1652 +398 2344 + 513

0.017 0.028 0.031 0.033 0.034 0.044

NGC315

1514 1727 0402 0116 0802 1214

0.052 0.055 0.055 0.059 0.060 0.062

1zw 187 4C +37.1 I MS01166+31 3C 192.0 MS 12143 i-38

Mrk 421 3C120.0 Mrk501

+004 +502 +379 +319 +243 +381

the MC simulation chain with CORSIKA air shower generation and GEANT detector simulation describes the experimental data to a satisfying degree. As only a marginal signal is expected even after y/hadron separation, we apply the source stacking method, a common tool in astronomy, and search for a cumu~uti~ y-excess from the superposition of n physically equivalent sources, thus imitating an n-fold observation of a single source which may be regarded as “generic”. On the basis of the paper of Mannheim et al. [9] which comprises nearby blazars as likely candidates for TeV emission. we create a sample of sources (see Table 1) which - show blazar properties, i.e. are OVV (optically violently variable) quasars or BL Lac objects, ~ have a redshift 2 <0.062, i.e. are relatively nearby so as to account for the absorption of TeV y’s by pair production with photons of the diffuse cosmic infrared background.

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_ are in the field of view of the HEGRA

detector array, i.e. 0” < 6 I 55” where 6 is the source declination. The following search is based on 48 180 324 events taken with the HEGRA array in October-December 1994 and in March-April 1995. The shower core position and the incident zenith and azimut angle 0 and 4 of the incoming primary are determined from scintillator data. Events with zenith angle 6 2 40” are discarded as well as showers with N, I 10 000 and N, 2 30 000, where N, is the reconstructed number of electromagnetic particles reaching the observation level. As N, is roughly proportional to the energy of the primary particle, these cuts restrict analysis to energies between 50 and 100 TeV. Whereas the lower limit guarantees a stable separation quality, the upper limit is introduced to account for the fact that due to the inevitable y-cutoff caused by the interaction with photons of the universal 3 K background, no y-rays are expected above approx. 100 TeV. We choose a radial source bin with the exact source position in the center and a radius crT2%= 1.0” (containing 72% of the source events) corresponding to an angular resolution [lo] for y-induced showers of oe3% = 0.87”. As even after y/hadron separation the source bin events are dominated by background, we have to estimate and subtract the residual hadronic background. For each event, n additional events are created by connecting the actual event time with incident angles taken at random from the (Q, 4)-distribution of the data. These generic events are treated like real events, including all cuts. This procedure allows to automatically remove systematics resulting from the dependence of the detector efficiency on local coordinates [ll]. In order to have a small statistical error on the background estimate, n = 20 is chosen in the following analysis. Fig. 2 (top) shows the NN output for all events within one of the 12 source bins together with the NN output for the background estimate. Both distributions significantly differ in the region where y-showers are expected, i.e. at NN outputs below N 0.4 (Fig. 2 (bottom)). Although there is no single source with a significance 230 in the sample, the superposition of the sources yields a yshower excess in the source bins of the order 3.5-4.00 depending on the NN cut. In order to check whether the expected efficiencies for y-detection and hadron rejection are actually achieved, we treat the excess as a signal and calculate the efficiencies as a function of the cut in the NN output. As shown in Fig. 3, results are compatible with MC expectations, but again only if the actual detector performance is taken into account. Fig. 4 shows the number of entries as a function of the angular distance to the exact source position (i.e. the first bin is the “source bin”) after y/hadron separation together with the background estimate for a network cut value <,,, = 0.4 which yields a y-efficiency sy = 0.89 f 0.01 f 0.09 and a hadron efficiency chad = 0.79 fO.O1 f 0.02. The y/hadron separation power is often

excess

0

0.2

0.4

0.6

0.8

network

output

Fig. 2. Top: NN output distribution for events in the source bins (ON) of the 12 blazars and for the estimated background (OFF). Bottom: Significance of the difference between ON and OFF NN output as a function of the NN output.

_.

1.8 ,

E

1.6

m

A! .E

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MC

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data

(y-like)

D data (hodronic

0.2

0.3

0.4

0.5

background)

0.6

0.7

0.8

cut in network

0.9

output

Fig. 3. y- and hadron efficiency as a function of the cut in the NN output for MC showers (solid lines) and data under the assumption that the observed excess is a signal (dots).

S. Ommer et al. / Nucl. Instr.

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in Phys. Res. .4 389 (1997) .?04&207

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should also be a valid tool in this pattern recognition task. 6000 y- and 6000 proton-induced showers with energies between 30 and 1000 TeV are trained for 500 epochs, and the net performance is checked with 2500 showers of both types. As input information, the net receives the anode current of the 49 photo multipliers after calibration. These values are proportional to the number of Cherenkov photons detected per shower per hut. Again, the performance of the NN is promising. and quality factors up to Q = 2.5 seem feasible. At low energies, the AIROBICC separation clearly outperforms the Geiger separation, but in contrast to the Geiger tower analysis, separation quality rlecru~t.s~.swith energy, as the electromagnetic part of hadron showers becomes increasingly more dominant with higher energy. thus covering the hadronic part which is responsible for the separation. First tests using both the Geiger and the Cherenkov data as input show that both separation techniques are almost independent. thus the combined analysis is likely to considerably improve Q over a wide range of energies.

6300~....b~~.-‘...‘~““““““““‘~~~’~~~~ I 1.5

1

2

2.5

radial distance

3

3.5

from source

4

*I,.

4.5

bin [“I

Fig. 4. Number of entries as a function of the radial distance to the source position after yjhadron separation (<,., = 0.4) for the blazar superposition and the background estimate. The variable bin size accounts for the increasing area of the ring segments.

References [I] C.W. Punch et al.. Nature

of the so-called quality factor as the significance of an excess is a linear function of Q. With the HEGRA Geiger tower analysis, we achieve a quality factor of Q e 2 at 50 TeV in agreement to MC estimates. quoted

Q = L,i,,J

in

i

1 -

3. AIROBICC

terms

Chad.

yihadron

separation

Additional information on the type of the primary particle comes from AIROBICC. As the y/hadron separation is mainly based on the Cherenkov light yield at different distances to the core, the network approach

[2] 133 [4] [5] [6] [7] [X] [9] [lo] [ll]

35X (1993) 477. J. Quinn et al.. Astrophys. J. 456 (1996) L83. D. Petry et al.. Astron. Astrophys. 31 I (1996) Ll3. W. Rhode et al.. Nucl. Instr. and Meth. A 37X (1996) 399. L. Liinnblad, C. Peterson and T. Ragnvaldsson. Comput. Phys. Comm. 81 (1994) 185. S. Westerhoff and H. Meyer. Proc. 4th tnt. Workshop AIHENP, ed. B. Denby, Pisa. 1995. S. Westerhoff et al.. Astroparticle Phys. 4 (1995) 119. A. Karle et al.. Astroparticle Phys. 3 (1995) 321. K. Mannheim. S. Westerhoff, H. Meyer and H.-H. Fink. Astron. Astrophys. 3 I5 (1996) 77. H. Krawczynski et al.. Nucl. Instr. and Meth. A 3X3 (1996) 431. D.E. Alexandreas et al., Nucl. Instr. and Meth. A 32X (199’) 570.

IIId. NN:

DATA ANALYSIS