1. University of Florence, Department of Physics, Via Sansone 1, I-50019 Sesto Fiorentino, Florence, Italy
2. INFN, Sezione di Florence, Via Sansone 1, I-50019 Sesto Fiorentino, Florence, Italy
3. University of Naples "Federico II", Department of Physics, Via Cintia, I-80126 Naples, Italy
4. INFN, Sezione di Naples, Via Cintia, I-80126 Naples, Italy
5. Lebedev Physical Institute, Leninsky Prospekt 53, RU-119991 Moscow, Russia
6. University of Bari, Department of Physics, Via Amendola 173, I-70126 Bari, Italy
7. INFN, Sezione di Bari, Via Amendola 173, I-70126 Bari, Italy
8. INFN, Sezione di Trieste, Padriciano 99, I-34012 Trieste, Italy
9. Ioffe Physical Technical Institute, Polytekhnicheskaya 26, RU-194021 St Petersburg, Russia
10. KTH, Department of Physics, AlbaNova University Centre, SE-10691 Stockholm, Sweden
11. INFN, Sezione di Roma "Tor Vergata", Via della Ricerca Scientifica 1, I-00133 Rome, Italy
12. IFAC, Via Madonna del Piano 10, I-50019 Sesto Fiorentino, Florence, Italy
13. University of Rome "Tor Vergata", Department of Physics, Via della Ricerca Scientifica 1, I-00133 Rome, Italy
14. Moscow Engineering and Physics Institute, Kashirskoe Shosse 31, RU-11540 Moscow, Russia
15. Universität Siegen, D-57068 Siegen, Germany
16. KTH, Department of Physics and The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova University Centre, SE-10691 Stockholm, Sweden
17. INFN, Laboratori Nazionali di Frascati, Via Enrico Fermi 40, I-00044 Frascati, Italy

Correspondence to: P. Picozza^11,13 Correspondence and requests for materials should be addressed to P.P. (Email: Piergiorgio.Picozza@roma2.infn.it).

Antiparticles account for a small fraction of cosmic rays and are known to be produced in interactions between cosmic-ray nuclei and atoms in the interstellar medium¹, which is referred to as a 'secondary source'. Positrons might also originate in objects such as pulsars² and microquasars³ or through dark matter annihilation⁴, which would be 'primary sources'. Previous statistically limited measurements^{5, 6, 7} of the ratio of positron and electron fluxes have been interpreted as evidence for a primary source for the positrons, as has an increase in the total electron+positron flux at energies between 300 and 600 GeV (ref. 8). Here we report a measurement of the positron fraction in the energy range 1.5–100 GeV. We find that the positron fraction increases sharply over much of that range, in a way that appears to be completely inconsistent with secondary sources. We therefore conclude that a primary source, be it an astrophysical object or dark matter annihilation, is necessary.

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