Cosmic-Ray Physics with Air-Shower Arrays - Methods & Discoveries

1. Cosmic-Ray Physics with air-shower arrays P. Camarri University of Roma “Tor Vergata” INFN Roma Tor Vergata

2. Outlook • Introduction • Methods of measurement • Extensive Air Showers (EAS) • EAS arrays WAPP 2011 - Darjeeling 20/12/2011

3. 1 particle /(m2 s) 1 particle / (m2 year) 1 particle / (km2 year) Energy spectrum of primary CRs WAPP 2011 - Darjeeling 20/12/2011

4. Energy spectrum of primary CRs About 2 orders of magnitude are lost each decade !! E > 1014 eV: only indirect, ground-based measurements are possible WAPP 2011 - Darjeeling 20/12/2011

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6. Indirect observations At present, the indirect observation is the unique solution to overcome the poor primary flux above 100 TeV. The different approaches to investigate the chemical composition are commonly based on the fact that the inelastic cross section of a nucleus of mass A is proportional to A2/3, which leads to a long interaction mean free path (m.f.p.) of protons and a short m.f.p. of nuclei. • Characteristics of early development: • Large lateral spread • Muon rich events • Soft secondary energy spectrum Short m.f.p. of nuclei From these observations EAS experiments can extract information on the primary mass WAPP 2011 - Darjeeling 20/12/2011

7. Different mean free path WAPP 2011 - Darjeeling 20/12/2011

8. How to detect Extensive Air Showers The classic method to detect air showers is to build an array of detectors over an area of the order of 104-6 m2. EAS arrays are characterized by the number of detectors, the distance step in the array and the total covered area. EAS-TOP: LNGS KASCADE: Karlsruhe Tibet ASγ: still operating The detectors are chosen according to the observable to be detected: e, γ, μ, h, Cherenkov light, fluorescence. WAPP 2011 - Darjeeling 20/12/2011

9. Arrays of particle detectors WAPP 2011 - Darjeeling 20/12/2011

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11. Observables in EAS Charged particles: e.m., m, hadrons Cerenkov light Fluorescence light + Monte Carlo simulations E0, A hadronic interaction models + detector response Energy spectrum Composition Test of hadronic interaction models WAPP 2011 - Darjeeling 20/12/2011

12. EAS – an example EAS measured by the ARGO-YBJ experiment with high space-time granularity and unprecedented details. time (ns) Shower core position meters Arrival time vs position Lateral distribution WAPP 2011 - Darjeeling 20/12/2011

13. EAS – an example EAS measured by the ARGO-YBJ experiment Real event !! WAPP 2011 - Darjeeling 20/12/2011

14. Extensive Air Showers - EAS An EAS is the result of nuclear interactions with air nuclei WAPP 2011 - Darjeeling 20/12/2011

15. EAS – Energy Flow • Hadrons provide energy to muonic and e.m. components • “One Way” for energy into e.m. particles • Details of energy transfer processes are important (Particle Physics) WAPP 2011 - Darjeeling 20/12/2011

16. Discovery of Extensive Air Showers It was Bruno Rossi in 1933 that noticed coincidences between several counters placed in a horizontal plane, far in excess of chance coincidences. From observation in Eritrea he noted: “It would seem... that from time to time there arrive upon the equipment very extensive groups of particles (‘sciami molto estesi di corpuscoli’) which produce coincidences between counters even rather distant from each other”. Supplemento a la Ricerca Scientifica, 1 (1934) 579. To investigate these observations B. Rossi sent a young student, Giuseppe (Beppo) Occhialini, to the Cavendish Laboratory (U.K.) to work with Blackett. Occhialini discovered that CRs occasionally produce complex events with many particles. WAPP 2011 - Darjeeling 20/12/2011

17. Occhialini and Blackett Blackett named “showers” the groups of particles observed in their detectors as an english translation of the word “sciami” used by Occhialini and Rossi in their discussions. Rossi invented the coincidence circuit. It employed triode vacuum tubes and was capable of registering coincident pulses from any number of counters with a ten-fold improvement in time resolution over the mechanical method of Bothe. The Rossi coincidence circuit was the first effective electronic device of particle physics. Rossi also invented the method of anti-coincidence. WAPP 2011 - Darjeeling 20/12/2011

18. Coincidence Rate The most systematic investigation on these showers was undertaken by Pierre Auger in 1938. He recorded coincidences between counters with a horizontal separation of 75 meters. The Observed Rate was found to be much higher than the Calculated Chance Rate, even when the counters were placed 300 m apart. WAPP 2011 - Darjeeling 20/12/2011

19. Particles discovered in CRs WAPP 2011 - Darjeeling 20/12/2011

20. The shower core and the shower size • For the first time the existence of a single shower core was established. • For the first time the total number of charged particles (“shower size”) was calculated by integrating the measured densities. • For the first time the absolute number of showers with a given size (“size spectrum”) was evaluated. WAPP 2011 - Darjeeling 20/12/2011

21. The EAS thickness: a new dimension A new dimension was added to air-shower research when it was established by Bassi, Clark and Rossi (1953) that the thickness of the EAS disc was quite small and all the particles crossed the detector in less than a few nanoseconds. This led them to suggest that, by recording the arrival times of the shower particles at different scintillators in the array, it would be possible to determine the primary direction with an accuracy of ~ 5º. WAPP 2011 - Darjeeling 20/12/2011

22. The EAS analysis The direction determination together with a particle-density measurement with the different array detectors provided an elegant method of determining the shower parameters (core position, shower size, direction of arrival). Density Sampling + Fast Timing In 1956 for the first time in the “Agassiz” experiment B. Rossi’s group applied simoultaneously both methods in the EAS analysis. With this approach (the so-called “MIT Standard” ) showers with more than 109 particles (E ≥ 1018 eV) were observed. Therefore, to detect such very high energy events Rossi’s group built larger arrays. The most important was the one operated at Volcano Ranch (New Mexico). This experiment could detect an EAS induced by a 1020 eV primary CR thus representing a milestone in the history of CRs. WAPP 2011 - Darjeeling 20/12/2011

23. 1800 m (3600 m) Volcano Ranch - MIT “Desert Queen” 1957-1963:exagonal array, 19 plastic scintillators 1959:John Linsley and LivioScarsi detected an EAS induced by a CR with energy E0= 6·1019eV containing 3·1010 particles. 1962:The array was enlarged up to 3.6 km and Linsleydetected the first CR with energy E0= 1020eV. The EAS contained 5·1010 particles ! WAPP 2011 - Darjeeling 20/12/2011

24. Volcano Ranch results This paper described the first event believed to be due to a cosmic ray of energy > 1019 eV. WAPP 2011 - Darjeeling 20/12/2011

25. Volcano Ranch results The first deep study of the time structure of muons and electrons in cosmic ray showers. “...is a seminal paper – and a required read for all who work on surface detectors in air-shower arrays...” Alan Watson, 2007 WAPP 2011 - Darjeeling 20/12/2011

26. EAS longitudinal development • The number of particles in the EAS increases with the atmospheric depth, due to the different interactions, up to a maximum value. • Multiplication stops when the individual particle energies drop below the Critical Energy, where collisional energy losses exceed radiative losses. • The number of secondaries gradually decreases and the EAS can be completely absorbed before reaching the ground. 1. 2. 3. Shower Age s: s < 1 s = 1 s > 1 WAPP 2011 - Darjeeling 20/12/2011

27. EAS – toy model for e.m. component After traveling X0 (radiation length) the particle number doubles and the energy is equally divided X0for bremsstrahlung 0.78 X0for pair production The shower behaviour is reproduced quite well Reality is more complicated,…needs a MC simulation WAPP 2011 - Darjeeling 20/12/2011

28. Basic features of e.m. showers Despite its limitations, the Heitler model reproduces 2 basic features of the e.m. shower development which are confirmed by accurate MC simulations and observations: The maximum size of the shower Is proportional to the primary energy E0. The depth of the maximum increases logarithmically with the energy, at a rate of about 85 g/cm2 per decade of the primary energy. Elongation Rate WAPP 2011 - Darjeeling 20/12/2011

29. EAS – toy model for hadronic showers Interaction length λπ-air ~ 120 g/cm2 λp-air ~ 85 g/cm2 Critical Energy: energy at which the decay length < distance to the next interaction Eπcrit ~ 20 GeV In each interaction: Nchπ±and ½ Nchπº Ntot = 3/2 Nch Nch ~ 10 WAPP 2011 - Darjeeling 20/12/2011

30. Basic features of hadronic showers The primary energy is finally divided between Nπ pions and Nee.m. particles in subshowers. The number of muons is Nμ = Nπ. energy conservation The relative magnitude of the contributions from Nμ and Ne is determined by their respective critical energies: the energy scale at which hadronic and e.m. multiplication cease. The importance of this relation is that E0 is simply calculable if both Neand Nμare measured. In addition, this linear relation is insensitive both to fluctuations and to the primary mass A. CASA-MIA WAPP 2011 - Darjeeling 20/12/2011

31. μ and electron sizes Muon size grows with primary energy more slowly than proportionally. The exponent depends on the division of energy between charged and neutral daughter particles in each interaction. α~ 1.03 The e.m. fraction is 72% at E0 = 1014 eV, rising to 90% at E0 = 1017 eV. WAPP 2011 - Darjeeling 20/12/2011

32. Depth of shower maximum Xmaxis the atmospheric depth at which the e.m. component of the shower reaches its maximum. Interaction length Depth of γ-induced shower maximum The elongation rate: Λp is reduced from Λγ for e.m. showers by two effect: larger multiplicity Nchand larger cross-section (smaller λInt) Linsley’s elongation rate theorem (1977): Λγ for e.m. showers represents an upper limit to the elongation rate for hadron showers. WAPP 2011 - Darjeeling 20/12/2011

33. Superposition model A nucleus with atomic number A and total energy E0 is taken to be A individual single nucleons, each with energy En = E0 / A, and each acting independently. The resulting EAS is the sum of A separate p-induced EAS all starting at the same point. For any additive measurable quantity Q the model predicts A times the average value for the quantity computed in a proton shower of energy Ep = EA / A <QA(E)> = A · <Qp(E/A)> α and βdepend on the nature of hadronic interactions. WAPP 2011 - Darjeeling 20/12/2011

34. Nucleus-air interaction Increasing the mass A • More secondary particles with less energy → less electrons (after max), more μ • Surviving hadrons have less energy • Larger deflection angles → flatter lateral distributions of secondary particles The lower energy nucleons generate fewer interactions and so lose less energy to e.m. components. Showers by nuclei dissipate their energy faster than protons, thus having shallower (smaller) Xmax . J. Matthews, Astrop. Phys. 22 (2005) 387 J. Linsley, 15th ICRC, 12 (1977) 89. WAPP 2011 - Darjeeling 20/12/2011

35. Energy flow in EAS WAPP 2011 - Darjeeling 20/12/2011

36. Shower fluctuations The main source of the shower-to-shower fluctuations is due to the distribution of the depth of the first interaction. In the superposition model the resulting EAS is the sum of A separate p-induced EAS all starting at the same point. In reality, showers exhibit significantly larger fluctuations than the ones expected in the superposition model. J. Engel et al., PRD 46 (1992) 5013. WAPP 2011 - Darjeeling 20/12/2011

37. Fluctuations and lateral distribution Different first-interaction atmospheric depth → different lateral distribution Therefore, the lateral distribution is sensitive to Xmax (i.e. to particle type). In fact, nuclei develop higher in the atmosphere (smaller Xmax) than protons, producing flatter lateral distributions. Difficult to disentangle fluctuation effects from different primary chemical composition. WAPP 2011 - Darjeeling 20/12/2011

38. Lateral distribution: the key for EAS reconstruction LDF = Lateral Distribution Function • LDF important since first evidence of EAS (Auger + Kolhoster 1938) • Measurement of particles at ground is a calorimetric method • Chudakhov principle: number of particles primary energy • LDF is the basis for integration → energy and composition • Which function is the best ? • It depends on: • Which secondary EAS component is measured • Energy threshold of measured particles • What kind of detectors are used • At which distance from the core it is measured • Energy range of the primaries EAS-TOP WAPP 2011 - Darjeeling 20/12/2011

39. LDF: electron/charged particles Nishimura – Kamata – Greisen formula: s = age parameter: describes the shape of the particle distribution rM= Moliere radius (79 m at s.l.) Nch = total number of charged particles C(s) = normalization factor This formula (with modifications) is used in many EAS cosmic ray experiments WAPP 2011 - Darjeeling 20/12/2011

40. LDF: muons KASCADE KASCADE experiment: rM = 89 m for electrons rM = 420 m for muons Greisen function (few GeV muons): Hillas function (Haverah Park): α= Slope parameter r0 = 600 m (= 74 m in Darjeeling 1990) Valid for Eμ = 2.5 - 54 GeV, 104 < Nch < 106 WAPP 2011 - Darjeeling 20/12/2011

41. EAS - Time profile WAPP 2011 - Darjeeling 20/12/2011

42. Basic features of time profile Due to geometrical reasons, the arrival of the first particles at lateral distance r from the core is expected to be delayed with respect to an (imaginary) planar shower front. Shower core on the detector (r = 0) s delay for a particle produced at H moving along the shower axis Detector at a distance r from the core (if axis vertical) (r « H) The delay increases with r. The delay decreases with increasing height H. EAS with the first ground particles coming from large heights will have smaller delays at fixed distance r compared to EAS where the measured particles originated from smaller heights. WAPP 2011 - Darjeeling 20/12/2011

43. Time spread Spread in time = thickness of the shower disk at r Difference of the arrival times of particles generated in the height interval [H1, H1 - ΔH1]: The spread of the arrival times of these particles at fixed distance from the core increases for smaller production heights. EAS from nuclei develop higher in the atmosphere (smaller Xmax) compared to γ–induced EAS: smaller delays are expected. WAPP 2011 - Darjeeling 20/12/2011

44. EAS morphology (r « H) Very “young” shower measured by ARGO-YBJ WAPP 2011 - Darjeeling 20/12/2011

45. EAS direction The study of the EAS time profile is important because the EAS direction is reconstructed by means of the “fast timing” technique developed by Bassi, Clark and Rossi in 1953 (PR 92 (1953) 441). For an EAS falling with zenith angle θ two particles separated by d arrive on a horizontal plane with a time difference of d · sinθ/c (provided that all particles in an EAS are contained in a thin disk). v ~ c WAPP 2011 - Darjeeling 20/12/2011

46. Cosmic Rays as tool for Particle Physics • The main characteristics of hadronic interactions that are relevant for EAS physics are: • Cross sections (p-air, π-air, N-air) • Inelasticity of the collisions • Multiplicity/composition of secondaries • Transverse momentum distribution • Fraction of diffractive dissociation Elab ~ 3 · 1015 eV Elab ~ 1020 eV Still ~30 times lower than the GZK energy. WAPP 2011 - Darjeeling 20/12/2011

47. Hadronic interactions The situation is much worse than it may appear from energy considerations. Measurements at colliders are limited to an angular region that excludes the beam pipe, and therefore a very large majority of the high energy particles that are emitted at small angles are unobservable. These particles carry more than 90% of the energy in a collision and are clearly crucial in determining the EAS properties. In EAS physics the fragmentation region is more important than the central region. WAPP 2011 - Darjeeling 20/12/2011

48. Fragmentation regions Central region Fragmentation region The particles here get most of the momentum of the primary CR WAPP 2011 - Darjeeling 20/12/2011

49. Inelasticity The spectrum of nucleons produced in hadronic interactions pays a fundamental role in the development of EAS. In ND interactions the nucleons carry ~40% of the initial state energy. The energy fraction carried by nucleons is the “elasticity” of the interaction. Inelasticity k = 1 - Elead / Ep Leading energy fraction Elead / Ep ~ 1 These high-energy nucleons feed energy deeper into the EAS clearly playing a very important role in the EAS development. At the colliders most of these nucleons are unobserved ! WAPP 2011 - Darjeeling 20/12/2011

50. proton-air cross section λint = 40 g/cm2 λint = 96 g/cm2 WAPP 2011 - Darjeeling 20/12/2011