ESTIMATE OF THE FRACTION OF PRIMARY PHOTONS IN THE COSMIC-RAY FLUX AT ENERGIES - 1017 eV FROM THE EAS-MSU EXPERIMENT DATA
Yu. A. Fomina, N. N. Kalmykova* G. V. Kulikova, V. P. Sulakova, S. V. Troitskyh**
a Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University 119991, Moscow, Russia
b Institute for Nuclear Research, Russian Academy of Sciences 117312, Moscow, Russia
Received May 27, 2013
We reanalyze archival EAS-MSU data in order to search for events with an anomalously low content of muons with energies Efl, > 10 GeV in extensive air showers with the number of particles Ne. > 2 • 10'. We confirm the first evidence for a nonzero flux of primary cosmic gamma rays at energies E ~ 101' eV. The estimated fraction of primary gamma rays in the flux of cosmic particles with energies E > 5.4 • 101(> eV is e-, = (0.431",'^) %, which corresponds to the intensity L, = (Oiy^) • 10-1(> on"2 • s-1 • sr-1. The study of arrival directions does not favor any particular mechanism of the origin of the photon-like events.
DOI: 10.7868/S0044451013120043
1. INTRODUCTION
The study of the primary mass composition of ultra-high-onorgy (UHE) cosmic rays (CR) is one of the topical problems of astroparticle physics because these experimental results are of crucial importance for understanding the theory of both cosmic-ray generation in their sources and their subsequent propagation to Earth. The low UHECR intensity makes their study by-direct methods impossible, and hence the only available method is the study of extensive air showers (EASs).
The dominant part of EASs is caused by primary nuclei (from protons to iron), but there is a considerable interest in the possible presence of very different particles, e.g., UHE gamma rays, among them. First works on the subject already appeared half a century ago (see, e. g., Ref. [1]), but definitive quantitative results are still lacking (cf. review [2] and the references therein). Indeed, the highest-energy cosmic photons firmly detected had the energy of ~ 50 TeV [3]. The searches for gamma rays in the energy ranges 3 • 1014 eV< E < 5 • 1016 eV (the EAS-TOP [4], CASA-MIA [5], and KASCADE [6] experiments) as well as at E > 1018 eV (the Haverah Park [7], AGASA
E-mail: kalm'ffleas.sinp.msu.ru
** E-mail: sergey.troitskyfflgmail.com
[8 10], Yakutsk [11, 12], Pierre Auger [13, 14], and Telescope Array [15] experiments) did not find any signal and resulted in upper limits on the photon flux only. A few claims of the experimental detection of 101'4 eV< E < 1017 eV photons (Mt. Chacaltaya [16], Tien Shan [17], Yakutsk [18], and Lodz [19]) had low statistical significance. At the same time, a certain flux of UHE photons is predicted in many models of both the conventional and "new" physics. In particular, the flux of secondary photons from interactions of extreme-energy particles with cosmic background radiation, the so-called Greizen Zatsepin Kuzmin (GZK) photons, may serve as a tool to distinguish various models of cosmic rays at energies > 5 • 101B eV because the photon flux is very sensitive to the primary composition at these energies: a predominantly light composition at GZK energies results in a much higher flux of secondary photons. Given the present contradictory situation with the mass composition at UHE (see, e.g., Ref. [20] for a detailed review and Ref. [21] for a brief update), searches for GZK photons are now considered very important. Also, a significant contribution to the UHE gamma-ray flux is predicted in particular top-down mechanisms of the CR origin ([22] and the references therein), in particle physics models with Lorentz invariance violation [23], and in models with axion photon mixing [24].
One of the most promising approaches to the search of primary gamma-rays is the study of the EAS muon component. The number of muons in a gamma-ray-induced EAS is an order of magnitude smaller than in a usual hadronic shower. Therefore, one may hope to find photon showers by selecting those that have an unusually low muon content.
In this paper, we study the muon content of showers with the estimated number of particles Ne > 2 • 107 and zenith angles 6 < 30° detected by the EAS-MSU array [25] in 1982-1990. We demonstrate that the number of muonless events significantly exceeds the background expected from random fluctuations in the development of showers caused by primary hadrons. This result can be interpreted as an indication of the presence of gamma rays in the primary cosmic radiation with energies of the order of 1017 eV, which confirms and strengthens the first evidence for UHE cosmic photons [26].
The rest of the paper is organized as follows. In Sec. 2, we briefly review the experimental setup (Sec. 2.1), then discuss the data set we study, and muonless events in particular (Sec. 2.2). Section 3 is devoted to the estimate of the number of background muonless events for hadronic showers (Sec. 3.1) and to the derivation of the estimated photon flux under the assumption that all muonless events not accounted for by the hadronic background are caused by primary gamma rays (Sec. 3.2). Possible systematic errors in the determination of the flux are discussed in Sec. 3.3. In Sec. 4, we present a detailed study of the distribution of the arrival directions of muonless events on the celestial sphere and test various models of the origin of primary photons. We put our results in the context of the present-day state of the art and briefly conclude in Sec. 5.
2. EXPERIMENT AND DATA
2.1. The EAS-MSU array
The description of the EAS-MSU array is given in [25]. The array had the area of 0.5 km2 and contained 77 charged-particle density detectors (consisting of Geiger-Miiller counters) for determination of the EAS size Ne using an empirical lateral distribution function [27] and 30 scintillaion detectors that measured particle arrival times necessary for determination of the EAS arrival direction. In addition to the surface detectors that mostly recorded the electron-photon component of an EAS, the array also included four underground muon detectors, also consisting of
y, m_
300 - D □ D □
-200 - □ □ □ □ □ □ □ □
-300 -
_I_I_I_I_L
-400 -200 0 200 400
X, m
Fig. 1. The EAS-MSU array setup. Muon detectors (circles) are denoted by ¡ii, i = 1,...,4; surface detector stations are represented by squares
Geiger-Miiller counters, located at the depth of 40 meters of water equivalent. These detectors recorded muons with energies above 10 GeV. A muon detector with the area of 36.4 m2 was located at the center of the array while the other three stations had the area of 18.2 m2 and were located at the distances between 150 m and 300 m from the center (see Fig. 1). To select the sample of showers with the number of particles Ne > 2 • 107 that we use in this work, 22 scintillaion detectors, each of the area of 0.5 m2, were used. The scintillaion detector threshold was set at the level of 1/3 of a relativistic particle. The temporal resolution was ~ 5 ns. The 22 stations formed 13 systems of 4-fold coincidences between counters located at the vertices of tetragons with sides between 150 m and 300 m, which allowed efficiently selecting the showers on the full array area. The scintillaion detectors were located at the same points as the Geiger-Miiller counters. The master criterion was determined by the firing, in the time gate of ~ 6 /is, of at least one of the 4-fold coincidence systems.
With these selection criteria implemented, the probability of detection of a shower with Ne > 2 • 107 falling to any place of the array was not less than 95 %. The position of the shower axis was determined with the precision of ~ 10 m. The precision of determining the arrival direction was ~ 3°. The number of particles in the shower was determined with the accuracy - (15-20)%.
Fig. 2. The distribution of muonless events over the Fig. 3. Cumulative distribution of muonless events in
distance R between the shower axis and the muon de- the LDF-based Poisson probability P(m = 0)
tector. Line: data; shadow: expectation for hadronic primaries
2.2. The data set and muonless events
The presence of muon detectors in the EAS-MSU array allows searching for primary gamma rays. The method is based on the fact that for Ne > 107 and for an hadronic primary, it is highly improbable to have zero muons in the central, 36.4 m2, detector if the shower axis is within ~ 240 m from it. At the same time, these muonless events are fully consistent with the conjecture of primary gamma rays. The total number of events with Ne > 2 • 107 in the data set is 1679; 48 of them are muonless.
Figure 2 presents the distribution of muonless events over the distance R between the shower axis and the muon detector. Most of the muonless events correspond naturally to large R; however, there are a certain number of events close to the axis, which are very difficult to explain by random fluctuations of the hadronic background. We note that the real number of muonless events is larger than the observed one because of the non-EAS background that results in firing of each counter in the central muon detector with the average frequency of 4.6 Hz. In three other muon detectors, the frequency of random firing was 2 to 3 times higher, and in this work, we use only the data of the central detector. It consisted of 1104 counters. For the time of EAS detection ~ 15 ¿¿s, we expect 0.076 random firings. Therefore, we assume that the probability of the absence of random firing was 0.93.
To obtain a very rough estimate of the probability to have a muonless hadronic event, we can start with the (experimentally known) mean muon lateral
distribution function [27] and estimate the expected muon density p^{NeiR) for a given core distance R. Then, using the Poisson distribution, we can calculate the probability P(m = 0) to have no muons in the detector at this distance. In Fig. 3, the distribution of m = 0 events in P(m = 0) is shown. T
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