Załącznik 3. Autoreferat w języku angielskim

Transcription

1 Załącznik 3. Autoreferat w języku angielskim 1. Full name: Piotr Homola 2. Diplomas and degrees: 2004 PhD, Institute of Nuclear Physics PAN, Kraków, doctoral dissertation (with distinction): Identification of photons in Ultra-High Energy Cosmic Rays MSc, Jagellonian University, Faculty of Mathematics and Physics, Kraków, specialty: astrophysics, MSc thesis: Symulacja detekcji promieni kosmicznych metodą fluorescencji powietrza w Obserwatorium Pierre Auger. 3. Information on previous employment in scientific institutions: since 2014: since 2014: specialist on research equipment, Institute of Nuclear Physics, Kraków, Department of Cosmic Ray Physics (sabbatical since to ) postdoc, Bergische Universität Wuppertal, Germany : postdoc, Siegen Universität, Germany : assistant professor, Institute of Nuclear Physics, Kraków, Department of Cosmic Ray Physics (sabbatical since January 2013) : International PhD Studies, Institute of Nuclear Physics, Kraków : researcher, Institute of Nuclear Physics, Kraków, Department of Cosmic Ray Physics. 4. Indication of achievements under Art. 16 paragraph 2 of the Act of 14 March 2003 On Academic Degrees and Titles and on Degrees and Title in Art (Dz. U. No. 65, item. 595, as amended): 4a. The title of the scientific achievement: Photons in ultra-high energy cosmic rays 1

2 4b. Publications included in the presented scientific achievement: H1. M. Risse, P. Homola [40%], R. Engel, D. Góra, D. Heck, J. Pękala, B. Wilczyńska, H. Wilczyński, Upper Limit on the Photon Fraction in Highest-Energy Cosmic Rays from AGASA Data, Phys. Rev. Lett. 95, (2005); H2. P. Homola [60%], M. Risse, Method to Calibrate the Absolute Energy Scale of Air Showers with Ultrahigh Energy Photons, Phys. Rev. Lett. 112, (2014) ; H3. M. Risse, P. Homola [10%], R. Engel, D. Góra, D. Heck, J. Pękala, B. Wilczyńska, H. Wilczyński, Photon air showers at ultra-high energy and the photonuclear cross-section, Czech. J. Phys. A327, 56 (2006); H4. P. Homola [70%], M. Risse, R. Engel, D. Góra, J. Pękala, B. Wilczyńska, H. Wilczyński, Characteristics of geomagnetic cascading of ultra-high energy photons at the southern and northern sites of the Pierre Auger Observatory, Astropart. Phys. 27 (2007) 174; H5. M. Risse, P. Homola [40%], Search for ultra-high energy photons using air showers, Mod. Phys. Lett. A 22 (2007) 749; H6. P. Homola [90%], R. Engel, H. Wilczyński, Asymmetry of the angular distribution of Cherenkov photons of extensive air showers induced by the geomagnetic field, Astropart. Phys. 60 (2015) 47, DOI: /j.astropartphys ; Corrigendum: Astropart. Phys. 65 (2015) 111, DOI: /j.astropartphys ; H7. P. Homola [90%], R. Engel, A. Pysz, H. Wilczyński, Simulation of ultra-high energy photon propagation with PRESHOWER 2.0, Comput. Phys. Commun. 184 (2013) 1468; H8. P. Homola [95%], M. Rygielski, Discrepancies in the Monte Carlo simulations of propagation of ultra-high energy cosmic-ray photons in the geomagnetic field, Astropart. Phys. 45 (2013) 28. 2

3 4c. Discussion of the aforementioned scientific publications and the results achieved, together with a discussion of their possible use. (References relate to own publications and other items listed in the Annex 4 of this application and to the other publications listed in Section 6 below.) 4c1. Introduction: scientific motivation In my research activity I have focused on ultra-high energy cosmic rays (UHECR). The research began with my Msc. thesis: "Simulation of the detection of cosmic rays with the air fluorescence method in the Pierre Auger Observatory". At that time, the Pierre Auger Observatory [A2] was still in the design phase but it was clear already then that its research potential is arge and unique. The Observatory opened up prospects for solving the most intriguing puzzles about the physics of cosmic rays of the highest energies ever observed, ie. above ev. It is not known what these rays are, how they reach us, and what is happening to them on the way to Earth. These questions have been waiting for a response from the early 60s of the twentieth century, when the first cosmic ray event was observed with an energy greater than ev [1], ie. around 10 orders of magnitude greater than could then be obtained in terrestrial accelerators. Subsequent experiments confirmed the existence of cosmic rays of such high energies, and the record energy event, ev, was registered in the Fly's Eye experiment in 1991 [2]. The explanation of these observations has become a huge challenge for science and remains so to this day. Presented work is the result of my fascination with the mystery of cosmic rays of the highest energies and a step towards an explanation of this mystery. Why energies of cosmic rays can be so high? Two classes of scenarios are considered in which particles could reach energies of the order of ev. One of them is the "bottom-up" ("astrophysical") scenarios, describing the acceleration of particles having an electric charge, mostly nuclei. One of the acceleration possibilities is a so-called Fermi diffusive process. Acceleration of this kind should occur near the fronts of shock waves related to eg. gamma ray bursts. A charged particle can be accelerated gradually during its repeated collisions with a magnetized plasma of the shock wave front, until the final escape from the acceleration region, and each transition through the shock front increases the particle energy. The maximum energy that can be achieved in this way is proportional to the shock wave speed, particle charge, magnetic field strength and the size of the area of interactions. The Fermi gradual acceleration is possible when the Larmor radius describing the particle trajectory is not greater than a size of an area where the acceleration takes place, ie. in the case of extensive regions where the magnetic field is relatively weak (eg. radiogalaxies), or within compact areas where the magnetic field is very strongs (eg. AGNs). Another example of a "bottom-up" scenario is a direct acceleration of charged particles in a very strong electromagnetic field associated with accretion disks or compact objects such as neutron stars. Rapid rotation of small and highly magnetized objects induces an electric field in which charged particles can be accelerated to giant energies in just one shot. 3

4 In another class of scenarios, the "top-down" ("exotic") models, the mechanisms of acceleration are not considered at all. Instead, disintegration of hypothetical supermassive X-particles is described. Such particles could have energies of the order of ev, ie. within the energy range of the Grand Unification Theory. X-particles could arise from the annihilation or decay of topological defects formed during phase transitions in the early Universe. Topological defects, whose existence is foreseen in the physical scenarios beyond the Standard Model, include, among others domain walls, cosmic strings and magnetic monopoles. The X-particles should decay into quarks and leptons, which in the hadronization process would create mostly pions, and these in turn would very quickly decay, mainly into photons, electrons, positrons and neutrinos. In this case, the Earth would be reached mainly by photons, neutrinos and protons, and we should not observe heavy nuclei. Another "exotic" scenario, so-called Z-burst model, describes the Z bosons decay caused by resonance annihilation of ultra-high energy neutrinos on the relic antineutrinos. The only thinkable source of these ultra-high energy neutrinos seem to be the aforementioned X-particles. According to this model the percentage of photons in the flux of cosmic rays with energies of the order of ev could be as high as approximately 75% [3]. If "top-down" scenarios are responsible for the formation of most cosmic rays with the highest energies, the flux of these cosmic rays should be dominated by photons and neutrinos. In the case of "astrophysical" scenarios, in the observed flux of UHECR should definitely prevail protons or heavier nuclei, while photons and neutrinos could provide at most a very small contribution. Identification of cosmic rays with the highest energies thus appears as a unique opportunity to distinguish between the "exotic" and "astrophysical scenarios. If the results of the observations would point out to the "top-down" models, it would mean an exciting opportunity to indirectly study particles with energies in the range of Grand Unification Theory and would open the way to test physical models beyond the Standard Model. Propagation of UHECR: sources near or far? During propagation in the interstellar and intergalactic space, cosmic ray particles can not only be accelerated, they can also lose their energies in interactions of different types. In the range of ultra-high energies the process responsible for most of the losses is the interaction of protons with photons of the cosmic microwave background (CMB). In the case of protons with energies exceeding around ev the photoproduction reaction takes place: pγ pπ 0 and pγ pπ ±. The sequence of these interactions leads to a reduction in energy of the original proton below the above threshold. Regardless, therefore, how high would be the proton energy at the source, after traveling a distance of around 100 Mpc it will be reduced to not more than ev. Thus, if protons with higher energies reach the Earth, their sources should be located nearby in the cosmological scale. Moreover, our current knowledge suggests that magnetic fields in our cosmological neighborhood are so small that the trajectories of protons emitted from sources distant from the Earth by not more than 100 Mpc should not bend on the way to us more than a few degrees. This should allow tracing back to the source by measuring the arrival direction of a cosmic ray, or, in other words, charged-particle astronomy. So far, however, even though dozens of cosmic ray events with energies exceeding the threshold of ev were detected, even a single source was not pointed back unambiguously [A4]. This may mean that the trajectory curvatures of the observed particles are larger than expected. It is also possible that the registered 4

5 cosmic rays with the highest energies have an electrical charge much larger than protons. We also cannot exclude that the current knowledge about the distribution of magnetic fields in our cosmological vicinity is incomplete. At the end it should be kept in mind that in "top-down" scenarios one does not expect correlations of UHECR arrival directions with the visible matter. The cut-off of the energy spectrum: what does it tell us? The above-described interaction of protons with the CMB should be reflected in the shape of the energy spectrum of UHECR. If protons dominate among cosmic rays of ultra-high energies and if the sources of these protons are spread uniformly in the Universe, the observed energy spectrum of UHECR should have a cutoff around ev. This effect was predicted as early as in the sixties of the twentieth century by Greisen, Zatsepin and Kuzmin [4], hence often used the name "GZK cutoff" in relation to the suppression of the energy spectrum resulting from the "GZK interactions," ie. from the interactions of ultra-high energy protons with the CMB radiation. A cutoff of the energy spectrum should also occur if UHECR are dominated by heavy nuclei. During propagation in space, nuclei should loose nucleons in the process of photodissociation so their mean free paths should be reduced, similarly as in the case of protons. As a result, the suppression of the spectrum should be observed, although not necessarily at the same energies as in the case of the dominance of protons in the UHECR flux. Observation of the spectrum cutoff consistent with the predicted GZK limit could indicate the dominance of protons or heavier nuclei among UHECR and thus it would point out to the "bottom-up" scenarios. It would also imply the expectation of the presence of photons in the UHECR flux observed on Earth. These would be secondary photons produced in the decays of neutral pions. The contribution of these photons to the observed UHECR flux should not exceed 1%. It would be very important to determine the energy at which the energy spectrum is suppressed - this would limit the class of "astrophysical" scenarios. If the energy at which the spectrum collapses, would differ from the GZK predictions for the UHECR flux dominated by protons, it would require considering alternative mechanisms leading to the cutoff, eg. those where an upper limit of the energy which can be gained by an UHECR particle at the source is postulated [5]. Such a maximum energy would then be proportional to the particle charge Z. With this assumption the UHECR flux would be dominated by stable nuclei of the highest Z, ie. those of the iron group. Hence the energy spectrum cutoff would not be a result of the processes occuring during the propagation but it would be a consequence of the properties of the UHECR sources. It turns out that the suppression of the energy spectrum of UHECR has been observed by all of the major experiments, but there is a disagreement as to the energy at which the spectrum collapses [6]. As a consequence, one cannot presently discriminate between models that describe the sources and propagation of UHECR. This situation strongly motivates efforts towards reducing the uncertainties of energy measurements in the cosmic ray experiments. Detection: a great experimental challenge 5

6 It has been explained above that solving the mysteries of UHECR requires very precise measurements of arrival directions, energies and identification of type of these particles. Above all, however, there is a need for the data: an appropriate number of evens in the ultra-high energy range. This is a considerable experimental challenge. The observations show that the differential flux of cosmic rays decreases rapidly with increasing energy: dn/de E -α, with variable exponent α which in a wide range of energies amounts to approximately 2.8. As a result, cosmic rays with energies exceeding ev come to us so rarely that one can observe them only indirectly, by detecting the so-called extensive air showers (EAS). These are the cascades of secondary particles resulting from interactions of cosmic ray particles with the components of the atmosphere. Observation and analysis of EAS properties allows conclusions about the arrival directions, energies and chemical composition of primary cosmic ray particles. The easiest task is to determine the arrival direction, a much harder to reconstruct the energy of the primary particle and the most difficult one: to determine its type. Secondary particles of an EAS form a thin, non-flat disk called the shower. The front may have a diameter of the order of kilometers and its shape can be approximated by a spherical surface. This means that large showers can be sampled using particle detectors located on ground, wherein the distance between the detectors should be appropriate for the energy range of the sampling strategy: the higher the energy of the primary particles to be investigated, the more numerous the secondary particles, and the greater diameter of the shower front. Eg. the energies between ev and ev require distances between the ground particle sensors of about 1-2 km, but the array of these sensors must be enormous to compensate the low flux: it is expected that the flux of cosmic rays with energies of ev range from 1 to 10 particles per km 2 per millennium. Therefore collecting only a few events at these enrgies would require a detector with an area of thousands of km 2. This requirement has been met by the Pierre Auger Observatory, a giant cosmic ray detector being the result of collaboration of several hundred scientists from several countries. In the last decade it has recorded many more cosmic ray events than all of the other detectors already operating within a similar energy range combined. The Pierre Auger Observatory is composed mainly of a network of ground-based particle counters, called the Surface Detector. It covers approximately 3000 km 2, while the second largest 1 detector, AGASA [8], operated with "only" 100 km 2. The ground-based particle counters that compose the Surface Detector are water Cherenkov counters. These are tanks filled with ultra-pure water observed by a set of photomultipliers. When the electrons and muons of an EAS pass through the water, the Cherenkov light is emitted and subsequently reflected from the bottom and the walls of the tank to be finally recorded by the photomultipliers. Analysis of data from many Cherenkov detectors allows to determine a geometry of an EAS and the lateral particle distribution. It is also possible to distinguish between the electron and muon shower components and to reconstruct other shower properties such as thickness and curvature of the front. The measurement of the lateral distribution of charged particles together with the recorded arrival 1 This is the "order" valid during the first years of operation of the Pierre Auger Observatory. Presently the second largest UHECR detector is Telescope Array [7] covering 730 km 2. 6

7 times at the ground detectors enable determination the arrival direction of a shower, while with the comparison of the measured lateral distribution with theoretical predictions it is possible to determine the primary particle energy. The theoretical predictions for the lateral particle distributions in EAS come from Monte Carlo modeling, and in this modeling, as mentioned earlier, we are dealing with large uncertainties associated with unknown cross sections for hadron interactions. Additionally, to make a comparison of the measured lateral distribution with the distribution obtained through simulations, one should assume the primary particle type in the latter. It means that on the basis of one measured lateral distribution a variety of primary energies can be reconstructed, depending on the assumptions taken in the simulations. Detailed analysis shows that for the lateral distribution, assuming that the primary particles are nuclei, the reconstructed energy remains at the same level regardless of the weight of the primary nucleus. However, assuming that the primary particle was a photon, one reconstructs mostly higher energies. This effect is related to the differences in the development of showers initiated by hadrons and photons. The lateral distributions of particles carry information about the type of particle initiating an air shower. An example of a shower feature that can be observed on ground and which allows to distinguish between different types of primary particles is the total number of muons, N μ. The muon signal can be recorded by muon counters built specifically for this purpose or, as in the case of Cherenkov counters in the Pierre Auger Observatory, it can be determined on the basis of the collective distribution of charged particles arriving at the ground. In the latter case, the muon component can be exctracted from the collective signal on the basis of the time structure of the record. The signal from electrons is much more extended in time than that coming from muons. This is due to the higher mass of muons as compared to electrons. Larger mass means less scattering in the atmosphere and, consequently, shorter arrival time at the detector and a smaller spread of arrival times. Separation of the muon signal in the manner described is very complicated and a subject to considerable uncertainty. Accordingly, the unambiguous identification of the primary type for a single shower is currently very difficult. In the near future, thanks to the installation of muon counters, the accuracy of the muon data from the Pierre Auger Observatory will be significantly improved. We hope that it will help to identify the primary particles of individual air showers. In addition to ground-based array of particle counters, the Pierre Auger Observatory uses another completely independent detection method. The so-called air fluorescence method is to record the fluorescence light induced by the propagation of a shower through the atmosphere. The air shower particles excite molecules of nitrogen which is the principal component of the atmosphere, and the deexciting nitrogen molecules emit part of the excitation energy in the form of an isotropic fluorescence light in the range of nm. The fluorescence light is emitted at every stage of a shower development and it is to a good approximation proportional to the energy left in the atmosphere by shower particles. This light can be recorded by optical telescopes, often called fluorescence telescopes. Fluorescence telescopes used in the Pierre Auger Observatory are grouped in four stations, called eyes, forming the so-called Fluorescence Detector. Fluorescence telescopes can capture fluorescence light emitted even at a distance of tens of kilometers, depending on the size of a shower and atmospheric conditions. The measurements can be carried out only during clear and moonless nights, which reduces the 7

8 operating time of the detector to 10-15% per annum. Based on measurements of fluorescence light at different stages of an air shower development, it is possible to reconstruct the properties of the particle initiating this shower. The geometry of a shower, ie. its arrival direction and location of the axis, is determined based on the time structure and shape of the light pulse recorded. This reconstruction is more accurate when a shower is observed by more than one detector eye. On the basis of the measured intensity and geometry of an EAS one can determine the total energy deposited in the atmosphere by secondary particles and on this basis reconstruct the energy of the primary particle. The hardest task is to determine the type of particle initiating an air shower. The analysis of fluorescence light profile enables determination of the direction of observation for which this light has the largest intensity. On this basis, knowing the geometry of a shower and the structure of the atmosphere, it is possible to determine the atmospheric depth, where a shower reaches the maximum of its development. This depth, briefly called Xmax, is measured in the Fluorescence Detector with the accuracy of approximately 20 g/cm 2. From the modeling of extensive air showers we know that X max is a parameter sensitive to the type of primary particle. For example, it is expected that a shower initiated by a photon reaches the maximum of its development up to 200 g/cm 2 deeper in the atmosphere than an EAS induced by a proton of the same energy. From the simulations it is also known that the distributions of Xmax look different for nuclei with different masses: the heavier primary nucleus (lower energy per nucleon), the earlier a shower developes. For example, it should be expected that a shower initiated by an UHE iron nucleus will reach its maximum development stage on the atmospheric depth on average 60 g/cm 2 shallower than an EAS induced by a proton of the same energy. Large fluctuations of X max, approximately 30 g/cm 2 in the case of iron nuclei, and approximately 60 g/cm 2 in the case of protons, makes it very difficult to reconstruct the primary mass of a single shower. However, one can successfully conclude on the composition of cosmic rays on the basis of the measured distribution of Xmax, comparing it to the expected distribution for certain types of primary particles. Thus, to reconstruct the primary type one should first reconstruct its energy, then determine X max and then compare the Xmax value with the results of Monte Carlo simulations for a variety of primary particles with energies such as the reconstructed energy. As can be seen, an accuracy of EAS modeling is an issue of a huge importance. Inevitably, however, this modeling contains necessary extrapolations of the cross sections for the interactions of hadrons with energies exceeding the scope of accelerator experiments. These extrapolations are the source of presently unavoidable uncertainty in determining the types of UHECR. If a shower is initiated by a photon, the uncertainty associated with the modeling is smaller than in the case of EAS indued by nuclei. If the first interaction that initiates an air shower is the conversion of a primary photon into an electron-positron pair, a relatively small hadronic component is formed with a delay and the maximum energies of secondary hadrons are lower than in the case where a shower is initiated by a hadronic or photonuclear interaction. Smaller maximum energies of hadrons mean a narrower energy range of extrapolation of the hadronic interaction models used and, consequently, smaller uncertainty of X max. In addition to the fluorescence light, the Fluorescence Detector receives also the light of the stars and other bright objects in the sky. Another important component of the signal, indistinguishable 8

9 from the fluorescence one, are Cherenkov photons, the ones reaching the detector directly and those scattered by air molecules. Cherenkov light is emitted when electrons and muons of an air shower travel with velocities greater than the speed of light in air. This causes a temporary polarization of atmosphere molecules and the acceleration of molecular charges connected to this polarization produces electromagnetic waves at frequencies in the UV range. Slower muons and electrons also generate electromagnetic waves, however in this case the polarized molecules are arranged in a spherical symmetry with respect to the particle which induces the polarity and the resultant interference is destructive. The flux of Cherenkov light is very high compared to the fluorescent signal, nevertheless the Fluorescence Detector sees mainly the latter. The most of Cherenkov photons are emitted along the direction of an air shower propagation, unlike the isotropic fluorescence. Thus, if the viewing direction is sufficiently 2 distant from the propagation direction of an EAS, the fluorescence photons can dominate the light flux that reaches the detector. Knowledge of the processes responsible for the emission of Cherenkov light and accurate modeling of the atmosphere allows a satisfactorily accurate determination of the Cherenkov background and separating it from the fluorescence signal. Hybrid Detection: improved data quality Hybrid detection of air showers, ie. the simultaneous observation with the two detectors, the Surface and the Fluorescence one, allows for a more accurate interpretation of measurements. Both detectors operate independently, both allow for reconstruction of the arrival direction, energy and particle type of the primary particle, but each of them measures different shower observables. This improves the precision of determining the reconstructed EAS properties. In particular, the uncertainties of the primary energy reconstruction are smaller thanks to hybrid detection technique. As mentioned above, determination of the energy of an EAS based on the signal registered only by the Surface Detector is quite uncertain. However, the use of hybrid detection technique can significantly increase the accuracy of the energy reconstruction For this purpose, the signal from the Surface Detector recorded at a specific distance from the shower axis is calibrated using the value of energy determined based on the data from the Fluorescence detector. This calibration is used in reconstructing the primary energies registered only by the Surface Detector. In this way the energy of all the registered air showers can be reconstructed with satisfactory precision. Data interpretation: do we record photons of ultra-high energies? So far no extensive air shower that could be unambiguously classified as initiated by photons have been registered [9]. On the other hand, as predicted by the scenarios of the origin of cosmic rays, photons of ultra-high energies should reach the Earth and initiate air showers. Detection of cosmic-ray photons with extremely high energies would mean the start of a new channel for observation of the Universe. It would be a very important channel, because it would give a chance to point back to the astrophysical objects being the sources of the observed particles. Photons, like neutrinos, are not deflected in a magnetic field during their propagation between the 2 In the Pierre Auger Observatory the light recorded by the Fluorescence Detector is dominated by fluorescence photons if the viewing direction is inclined with respect to the shower propagation direction at an angle greater than about 20. 9

10 source and the observer, and thus the observed arrival direction should indicate the source. Moreover, to determine the fraction of photons in the observed UHECR flux together with a precise measurement of the energy spectrum would allow to identify the processes responsible for the formation of cosmic rays at ultra-high energies. If these processes turn out to be the exotic ones it would be a great scientific discovery that would open the door to testing physical models beyond the Standard Model of Particles. Confirmation of the absence of photons among cosmic rays of ultra-high energies would imply consequences of equally fundamental importance. To understand such an observation one would need physical models beyond the Standard Model of Particles and beyond the General Relativity. Consequently, one should re-interpret all astronomical observations based on the new models. One of the examples of how one could use the information on the lack of photons among UHECR is testing the hypothesis of the Lorenz invariance violation [10]. A significant violation of the Lorenz invariance would lead to an abnormal dispersion relation resulting in the suppression of the conversion of photons into electron pairs. Consequently, the number of highest-energy photons observed on Earth should be larger than if the Lorenz invariance is not violated. The upper limit for the presence of photons among the highest-energy cosmic rays thus allows determination of upper limits on terms that modify the dispersion realtion in the models where the violation of Lorentz invariance is postulated. Such restrictions have been determined on the basis of the results obtained in the Pierre Auger Observatory [10]. Identification of photons: importance of modeling the preshower effect As mentioned earlier, modeling of EAS is of great importance in the studies of cosmic rays at extremely high energies. The standard and also the most popular simulation tool is CORSIKA [11]. It allows performing a full Monte Carlo modeling of extensive air showers initiated by particles of different types and energies. A popular program that allows less accurate but much faster simulations is CONEX [12]. This program utilizes cascade equations which significantly reduces the computation time. The main competition for CORSIKA is AIRES [13]. It is particularly convenient that the mentioned key simulation codes are open and freely available allowing the users participation in the development. The air shower simulations start by default at the border of the Earth's atmosphere, but when it comes to the analysis of the showers induced by UHE photons the modeling have to begin well before the primaries enter the Earth's atmosphere. A cosmic ray photon of energy exceeding ev propagating in the geomagnetic field is likely to convert into an electron-positron pair. These electrons in turn should emit bremsstrahlung (synchrotron) photons, which also could convert into electron-positron pairs if only their energy is high enough. As a result of these interactions, instead of one high energy photon at the border of the atmosphere one can expect a cascade of low energy particles. This cacade is often called a preshower, since it origiantes and develops well before the air shower is initiated. The preshower effect is fairly well described in the literature [14, 15]. Properties of air showers initiated by preshowers differ significantly form the properties of showers induced by a photon that has not undergone the conversion. The former develop much earlier in the atmosphere than the latter and the difference in Xmax exceed even 10

11 200 g/cm 2, an order of magnitude more than the uncertainty of Xmax measurement in the Pierre Auger Observatory. Another important prediction related solely to ultra-high energy photons and preshowers is a dependence of <X max > on the arrival direction and on the geographical location of the observatory. It is implied by the dependence of the probability of the preshower occurence on the primary photon energy and on the intensity of the geomagnetic field component B perpendicular to the direction of propagation of the photon. Consequently, if the photons constitute a significant component of the observed UHECR flux, <X max > measured for the arrival directions along which B is small should be larger than in the case of directions with strong B. Similarly, in observatories, where the geomagnetic field intensity is low, the overall <X max > of EAS at ultra-high energies should be larger than in the observatories, where the intensity of the geomagnetic field is strong. Taking the above into account it is clear that including the simulation of the preshower effect in the widely accessible programs used for modeling EAS is a matter of great importance. However, all the analyses concerning preshowers published before I started my research in this field were based on calculations made using private programs, inaccessible to the public. The lack of an open access to the existing preshower tools did not allow the inclusion of these programs to the popular and public programs as CORSIKA or AIRES. The lack of access to the codes of the existing programs also prevented their verification and comparison of results. There was a need to improve this inconvenient situation and I was given an opportunity to work in this direction within my doctoral studies. The aim was to create a program to simulate the preshower effect and make the tool public and freely available with the "open source" licence so that it could be included as a module in the popular programs like CORSIKA or AIRES. The task proved to be highly non-trivial, mainly because of the pitfalls in publications I used as basic references. Upon completion of the program called by me PRESHOWER, I started to test it intensively trying to use the available references as benchmarks. Interestingly, my analysis did not allow a confirmation of a full convergence between the PRESHOWER results and any of the results obtained with private programs [16-18]. Difficulties in comparisons were mainly due to the lack of complete information about private programs, eg. about physical processes taken into account, models applied and their simplifications, numerical solutions used, and even about all the basic input parameters of the simulations presented in the publications. In addition, in cases where the relevant information could be found, it turned out that the applied physical models were unnecessarily imperfect. This issue concerned mainly too simplistic models of the geomagnetic field. However, contacting the authors of the "disputed" results did not bring much to my work I failed to explain the reasons for the differences. Continuing the tests of PRESHOWER, I discovered a significant inaccuracy in the basic reference describing the preshower effect, by T. Erber [14]. In this publication one can find incorrectly calculated values of the so-called magnetic pair production function T used to calculate the probability of the preshower effect occurence. Function T contains a modified Bessel function of the order of 1/3, K 1/3. I found out that Erber incorrectly calculated the values of K 1/3. As a result, the values of T presented by Erber in Table VI of his work [14] were understated by a factor of 2-3. Most of the aforementioned private programs were based on the Erber's publication, but none of the authors referred to this mistake, which made it very difficult to trust 11

12 their results. Information about the mistake I discovered was published [A65] and presented at international conferences. Another major doubt about the private codes was the use of simplifified formula for bremsstrahlung emission given by Erber in his publication [14]. This formula was used at least in one of the private programs and it could potentially be used in the other codes where no description or information was available. The point is that the formula can be applied only for photon energies much smaller than the energy of the emitting electrons. The use of such an approximation seems to be unjustified. With a more complete description of magnetic bremsstrahlung ([19]) it can be seen that the energies of photons emitted by extremely energetic electrons are often comparable to the energy of the emitters. In general, the probability of such a catastrophic emission increases with the energy of the emitting electron. All the encountered difficulties and inconsistencies found in the literature confirmed my conviction that an open source program to simulate the preshower effect is very necessary. In addition it was clear to me that an accurate description of the program containing its applications and tests performed should be published in the peer-reviewed journal. And so it happened - the publication describing PRESHOWER was released in 2005 [A65] and the code has been placed in the public program library of the Computer Physics Communications journal. PRESHOWER was also presented at international conferences (eg. [A61]). Thanks to its open code and reliable tests described in the refereed publication, PRESHOWER was implemented in CORSIKA and CONEX. Both integrations have been made by the authors of those two programs. The PRESHOWER module became thus a standard and indispensable tool used to simulate EAS initiated by ultra-high energy photons. What is very important, it is a tool available to the whole community. Examples of applications of PRESHOWER can be found, among others, in publications [20-23]. Search for Answers The enormous size of the Pierre Auger Observatory and its hybrid detection of extensive air showers promised a chance to get closer to the explanation of the mystery of the most energetic particles in the Universe. That is why at the beginning of my scientific activity I decided to do a research concerning the UHECR physics. Over the years that followed I realized that the field I had chosen is even more interesting than it looked at the beginning. Do photons contribute to the flux of UHECR observed on Earth? As explained above, the answer to this question is of great importance. The work presented in this proposal is a step towards obtaining this answer. 4c2. Description of the publications. H1. M. Risse, P. Homola [40%], R. Engel, D. Góra, D. Heck, J. Pękala, B. Wilczyńska, H. Wilczyński, Upper Limit on the Photon Fraction in Highest-Energy Cosmic Rays from AGASA 12

13 Data, Phys. Rev. Lett. 95, (2005) My first important post-doctoral scientific achievement was to make a full analysis of the most energetic showers recorded by the AGASA experiment. The study I performed, which began even before the doctorate, enabled determination of the first upper limit on the fraction of photons in cosmic rays with energies above ev. In the AGASA experiment, extensive air showers were recorded with an array of scintillators covering an area of approximately 100 km 2. These scintillators recorded the densities of muons. As noted in the Introduction, the number or density of muons in an air shower are good signatures of primary particles, similarly to Xmax. Modeling of air showers and numerous analyzes show that air showers initiated by photons should contain on average approximately 6 times less muons than those initiated by protons or heavier nuclei. I uesd this interesting property of air showers in my analysis of the highest energy AGASA events. I was interested in air showers with energies above ev with a precisely determined density of muons at the distance of 1000 m from the EAS axis. There were six such events [24]. For these six cases I performed Monte Carlo simulations of showers initiated by photons. I used the CORSIKA program with the PRESHOWER module, using the energies and reconstructed arrival directions of the primary cosmic rays as an input. It should be emphasized here that Monte Carlo simulations at so high energies would not be sufficiently accurate without taking into account the preshower effect the probability of occurrence of this effect is close to one for all the AGASA events considered. The simulations gave me the distributions of muons expected for photon primaries. Then I compared these expected values with the experimental ones assuming that the uncertainty of each measurement can be described by a normal distribution with a width of 40% for each of the six events. In this way, for each of the six events I determined the probability that the measured density of muons is consistent with the expectations in the case of photon primaries. Then I calculated the probability of occurrence of a certain fraction of photons in the available sample, which allowed the determination of the upper limit on the occurrence of photons in cosmic rays with energies exceeding ev. Then, with M. Risse and other co-authors of [H1], we applied an advanced analytical method to account for a possible reinterpretation of the energy scale determined in the AGASA experiment. As mentioned in the Introduction, to reconstruct the energy of the primary particle of a shower recorded by a ground-based array of particle counters, one has to compare the measured lateral distribution of particles to the theoretical distribution resulting from modeling of extensive air showers. In modeling, however, the type of primary particle has to be assumed. The authors of the AGASA experiment reconstructed the primary energies assuming that the primary particles are protons. If it would have been photons then the reconstructed energies should be scaled up by approximately 25%. Such a rescaling affects both the determination method of the upper limit on the fraction of photons with specific energies and its value. As mentioned above, the determination of this fraction requires performing accurate Monte Carlo simulations of extensive air showers with specific arrival directions, energies and types of primary particles. Hence, changing assumptions about the energy scale affects the results of the simulations. Taking into account this change, it allowed determination of the first upper limit on the fraction of photons in 13

14 cosmic rays with energies above ev at 67%. This first complete analysis for photons with extremely high energies was very important for the verification of the "top-down" scenarios. As discussed in the Introduction, according to these scenarios photons should be an important component of cosmic rays with the highest energies, and the fraction of photons in the observed flux should increase with energy. For example, in the SHDM (Super Heavy Dark Matter) model [25] the expected fraction of photons in cosmic rays with energies above ev is about 25%, for energies in excess of ev the photon fraction is approximately 50%, while at energies greater than ev it is already approximately 62%. In the already mentioned version of the Z-burst model it is expected that the photons should constitute 75% of the flux of cosmic rays with energies exceeding ev. The upper limit of 67% therefore restricts this particular version of the Z-burst scenario and other models with similar expectations about the fraction of photons. Our analysis showed for the first time that the "top-down" scenarios can be verified experimentally. Today one can already tell that this was the first step on the road to obtaining reliable experimental support for the elimination of "exotic" scenarios. Due to the increasingly stringent limits on the presence of photons in cosmic rays obtained in next years, the "top-down" models are presently practically not considered as possible explanations of the UHECR origin, while a few years ago both classes of models, the "top-down" and "bottom-up" scenarios were considered on an equal rights. The method of data analysis developed in [H1] has been successfully used for data collected by the Pierre Auger Observatory. This largest ever instrument to study the highest-energy cosmic rays has been recording air showers already for 10 years and now the data statistics by far exceeds what was collected by all the other experiments together, including both those working presently and the ones from the past. A natural step was thus an attempt to identify the photons among the primary particles initiating the showers observed in the Pierre Auger Observatory or to place the appropriate upper limits. The first in a series of publications on the subject was published in 2007 and concerned the energies in excess of ev [A57]. At these energies it is necessary to take into account the preshower effect so my experience from the previous studies was useful. The analysis of the data collected by the Pierre Auger Obseratory and other experiments have not led so far to the identification of ultra-high energy photons among primary cosmic rays, increasingly stringent upper limits on photon fractions are determined instead. It is ipmortant to keep in mind that the presence of photons in the cosmic ray flux is predicted by all models of cosmic ray origin, also by standard scenarios. In these models it is assumed that protons or heavier nuclei of high energies propagating through the Universe interact with photons of the cosmic microwave background. One of the products of these interactions should be neutral pions which quickly decay into photons. Those photons should be seen on Earth if our assumptions about the interactions of extremely-high energetic particles are correct. This means that by verifying the presence of photons among cosmic rays we actually test the models of particle interactions at highest energies. A very important example of such tests are the analyses in which on the basis of experimental constraints on the existence of photons the upper limits on violation of Lorentz invariance are determined [10]. As predicted by a number of the "new physics" 14

15 theories, including the theories assuming extra space dimensions, this invariance could be broken in the case of particles of very high energies. In such situations, for an example, the dispersion relation of photons would have to be modified in a way which would inhibit the process of converting photons to electron-positron pairs. Consequently, more ultra-high energy photons should be observed on Earth than it is predicted by the models in which Lorentz invariance is preserved. However, because so far high-energy photons are not observed at all, it is possible to determine on the basis of this fact the experimental upper limits on factors modifying dispersion relations, ie. determining the energy up to which Lorentz invariance is preserved. This shows the importance of the search for photons among cosmic rays and how wide may be the use of the results. The work [A57] and the subsequent analyses of data from the Pierre Auger Observatory allowed determination of very stringent upper limits on the occurrence of photons among cosmic rays of extremely high energies. These limits now begin to probe the photon predictions of the standard models, and thus it is likely that the identification of photons is a matter of the next few years. H2. P. Homola [60%], M. Risse, Method to Calibrate the Absolute Energy Scale of Air Showers with Ultrahigh Energy Photons, Phys. Rev. Lett. 112, (2014) ; The shapes of the energy spectrum of UHECR determined in a variety of experiments are very similar, which means that the relative energies measured in each of the experiments are quite well determined. There is no agreement, however, about the absolute energy values: the lack of the absolute energy scale calibration makes the spectra of different detectors shifted relative to each other [6]. Uncertainties regarding the energy scale claimed by the main experiments are quite substantial, in the range 14-32% [26-29]. Such a small precision does not allow for a sufficiently precise interpretation of the spectrum features, and in particular for an identification of the GZK effect. Identifying the GZK cutoff in the UHECR energy spectrum is fundamental for understanding the physical mechanisms leading to the emission of particles of ultra-high energies and the processes during the propagation of these particles in space. Therefore, the possibility of the absolute calibration of the energy scale is very much needed. Given the perspective of identifying the photons among the cosmic rays of extremely high energies, in [H2] with M. Risse we presented a completely new method of absolute calibration of the energy scale for EAS based on the analysis of the properties of showers initiated by ultra-high energy photons. The method involves the use of the threshold-like dependence of the preshower effect on the energy of a primary photon. The preshower effect is a manifestation of well-known electrodynamic processes, thus the primary photon energy at which such the effect should occur can be predicted with a great precision. Therefore, the evidence for occurrence or the lack of occurence of the preshower effect would give an information on the energy of the primary photon. Observation of a single shower identified as being initiated by a preshower allows to determine the lower limit of the primary photon energy and this limit would be independent of the energy values determined with standard methods on the basis of the signal recorded by the 15