Attila Abramowski, F. Acero, Felix A. Aharonian, Faical Ait Benkhali, A. G. Akhperjanian, Ekrem Oǧuzhan Angüner, Gisela Anton, Shangkari Balenderan, Arnim Balzer, Anna Barnacka, Yvonne Becherini, J. Becker Tjus, K. Bernlöhr, E. Birsin, E. Bissaldi, Jonathan Biteau, Catherine Boisson, J. Bolmont, Pol Bordas, J. Brucker, Francois Brun, Pierre Brun, Tomasz Bulik, Svenja Carrigan, Sabrina Casanova, M. Cerruti, Paula M. Chadwick, R. Chalme-Calvet, Ryan C. G. Chaves, A. Cheesebrough, M. Chretien, Sergio Colafrancesco, Gabriele Cologna, Jan Conrad, C. Couturier, M. Dalton, M. K. Daniel, I. D. Davids, B. Degrange, C. Deil, P. deWilt, H. J. Dickinson, A. Djannati-Ataï, W. Domainko, L. O'C. Drury, G. Dubus, K. Dutson, J. Dyks, M. Dyrda, T. Edwards, Kathrin Egberts, P. Eger, P. Espigat, C. Farnier, S. Fegan, F. Feinstein, M. V. Fernandes, D. Fernandez, A. Fiasson, G. Fontaine, A. Foerster, M. Fuessling, M. Gajdus, Y. A. Gallant, T. Garrigoux, H. Gast, B. Giebels, J. F. Glicenstein, D. Goering, M. -H. Grondin, M. Grudzinska, S. Haeffner, J. D. Hague, J. Hahn, J. Harris, G. Heinzelmann, G. Henri, G. Hermann, O. Hervet, A. Hillert, James Anthony Hinton, W. Hofmann, P. Hofverberg, Markus Holler, D. Horns, A. Jacholkowska, C. Jahn, M. Jamrozy, M. Janiak, F. Jankowsky, I. Jung, M. A. Kastendieck, K. Katarzynski, U. Katz, S. Kaufmann, B. Khelifi, M. Kieffer, S. Klepser, D. Klochkov, W. Kluzniak, T. Kneiske, D. Kolitzus, Nu. Komin, K. Kosack, S. Krakau, F. Krayzel, P. P. Krueger, H. Laffon, G. Lamanna, J. Lefaucheur, M. Lemoine-Goumard, J-P. Lenain, D. Lennarz, T. Lohse, A. Lopatin, C-C. Lu, V. Marandon, A. Marcowith, R. Marx, G. Maurin, N. Maxted, M. Mayer, T. J. L. McComb, M. C. Medina, J. Mehault, U. Menzler, M. Meyer, R. Moderski, M. Mohamed, E. Moulin, T. Murach, C. L. Naumann, M. de Naurois, D. Nedbal, J. Niemiec, S. J. Nolan, L. Oakes, S. Ohm, E. de Ona Wilhelmi, B. Opitz, M. Ostrowski, I. Oya, M. Panter, R. D. Parsons, M. Paz Arribas, N. W. Pekeur, G. Pelletier, J. Perez, P-O. Petrucci, B. Peyaud, S. Pita, H. Poon, G. Puehlhofer, M. Punch, A. Quirrenbach, S. Raab, M. Raue, A. Reimer, O. Reimer, M. Renaud, R. de los Reyes, F. Rieger, L. Rob, S. Rosier-Lees, G. Rowell, B. Rudak, C. B. Rulten, V. Sahakian, David M. Sanchez, A. Santangelo, R. Schlickeiser, F. Schuessler, A. Schulz, U. Schwanke, S. Schwarzburg, S. Schwemmer, H. Sol, G. Spengler, F. Spiess, L. Stawarz, R. Steenkamp, Christian Stegmann, F. Stinzing, K. Stycz, Iurii Sushch, A. Szostek, J-P. Tavernet, R. Terrier, M. Tluczykont, C. Trichard, K. Valerius, C. van Eldik, G. Vasileiadis, C. Venter, A. Viana, P. Vincent, H. J. Voelk, F. Volpe, M. Vorster, S. J. Wagner, P. Wagner, M. Ward, M. Weidinger, Q. Weitzel, R. White, A. Wierzcholska, P. Willmann, A. Woernlein, D. Wouters, M. Zacharias, A. Zajczyk, A. A. Zdziarski, Alraune Zech, H-S. Zechlin

• Axionlike particles (ALPs) are hypothetical light (sub-eV) bosons predicted in some extensions of the Standard Model of particle physics. In astrophysical environments comprising high-energy gamma rays and turbulent magnetic fields, the existence of ALPs can modify the energy spectrum of the gamma rays for a sufficiently large coupling between ALPs and photons. This modification would take the form of an irregular behavior of the energy spectrum in a limited energy range. Data from the H. E. S. S. observations of the distant BL Lac object PKS 2155 - 304 (z = 0.116) are used to derive upper limits at the 95% C. L. on the strength of the ALP coupling to photons, g(gamma a) < 2.1 x 10(-11) GeV-1 for an ALP mass between 15 and 60 neV. The results depend on assumptions on the magnetic field around the source, which are chosen conservatively. The derived constraints apply to both light pseudoscalar and scalar bosons that couple to the electromagnetic field.