Research at VATLY : main themes and recent results

1. Research at VATLY: main themes and recent results PN Diep, PN Dong, PTT Nhung, P Darriulat, NT Thao and VV Thuan VATLY, INST, Hanoi, Vietnam Hoi An, 2007

2. VATLY, Vietnam Auger Training LaboratorY, is a cosmic ray laboratory located in Hanoi aimed at creating a group contributing to the research activity of the Pierre Auger Observatory with which we are associated.

3.  (degrees) Azimuthal distribution of cosmic muon flux in Hanoi Our first measurements were of the atmospheric muon spectrum in Hanoi. They contributed useful data to the understanding of atmospheric neutrino oscillation experiments. They show a strong east-west asymmetry due to the effect of the Earth magnetic field which is particularly important in Hanoi (rigidity cutoff of 17GeV).

4. Since then our studies have been made in closer connection with the Pierre Auger Observatory (PAO), both at the Institute (using equipment installed on the roof of the laboratory) and from data collected there. Summary 1. The Pierre Auger Observatory 2. Studies using laboratory equipment Muon rates/ Response to muons of a replica of an Auger tank and refurbishment/ Multitank detection of air showers/ PMT studies 3. Studies using Auger data Simulation of FD reconstruction/Study of FADC traces, muon to photon ratio/ The PMT asymmetry at early times 4. Teaching and Training 5. Summary and perspectives 6. Publications

5. PIERRE AUGER OBSERVATORY High energy cosmic rays are observed from the extensive air showers that they produce when entering the atmosphere. One method consists in sampling the particle density on ground, another method consists in detecting the fluorescence light produced on nitrogen molecules along the shower axis. In both cases timing gives the direction and intensity gives the energy but both methods suffer of very different systematic sources of errors. The Pierre Auger Observatory is the first large hybrid detector ever built: it combines the strengths of Surface Detector Array & Air Fluorescence Detectors

6. THE PHYSICS OF THE PIERRE AUGER OBSERVATORY • ULTRA HIGH ENERGY COSMIC RAYS (UHECR’s) • Accurate measurement of the high • end of the energy spectrum • Settlement of the GZK controversy • Identification of possible sources • Nature of the primaries

7. Auger Surface Detector The giant detector array of the PAO is made of water Cherenkov counters and covers 3000 km2 with a triangular grid having a 1.5 km mesh size (1600 detectors of 10m2 area each). Around 1020eV showers involve 15 to 20 detectors. Typical angular and energy resolutions are 1.5° and 20%.

8. Surface array view ENERGY MEASUREMENT IN THE SURFACE DETECTOR (SD) The energy measurement obtained from the surface detector relies on the dependence of the measured signals on the distance to the shower axis (so-called lateral distribution function, LDF). Lateral distribution function fit

9. THE AUGER FLUORESCENCE DETECTOR

10. Four stations of six eyes each, each eye covering a field of view of 30°×28° with a mirror focusing on an array of 22×20 pixels (photomultiplier tubes), each having 1.5° aperture. They measure the induced fluorescence of nitrogen molecules (near UV). 11 m2 mirror UV-Filter 300-400nm camera440 PMTs

11. the first quadruple

12. Studies using laboratory equipment They aim at getting familiar with the behavior of the Auger detectors 2.1Response to muons of a replica of an Auger tank 2.2Multitank detection of air showers 2.3Refurbishing of the Hanoi Auger tank 2.4 Photocathode efficiency measurements

13. 120 360 162 1 2 3 4 5 6 203 20x20 7 8 9 120 10 11 12 40 RESPONSE TO MUONS OF A REPLICA OF AN AUGER TANK We have constructed a replica of one of the 1600 Auger surface detectors on the roof of our laboratory in order to get familiar with its behaviour and performance. A study of its response to muons was performed using a movable scintillator hodoscope located below it in the laboratory. cm

14. RESPONSE TO MUONS OF A REPLICA OF AN AUGER TANK The pulse heights measured in each phototube are observed to be essentially proportional to the length of the muon track in water. The pulse height distribution obtained for vertical traversing muons was wider and its average value lower than achieved in Auger, implying the need for a factor 4 increase of the collected light. We have now improved the water purity and the quality of the wall surface in order to reach this goal.

15. Refurbishing the Hanoi Auger tank The Hanoi Auger tank has now been refurbished: new Photonis phototubes have been installed, the walls have been coated with aluminized mylar, a newwater filtering system has been installed, the counter has now been run in.

16. After refurbishing Data have been collected with an independent trigger on extensive air showers (see later). - First results show major improvement of the optical quality of the counter. In particular strong correlations between the signals recorded in each of the 3 PMTs are now observed.- A precise measurement of the light yield remains to be done.

17. 185 140 325cm 40 305 217 115 100 112 392 MULTITANK DETECTION OF AIR SHOWERS We have installed and are currently operating three additional smaller water tanks (3000 liters each), each seen by two photomultiplier tubes, around the Auger Cherenkov counter. The aim is to detect extensive air showers giving coincident signals in several of these.

18. MULTITANK DETECTION OF AIR SHOWERS • The system is now operational and provides a trigger with a triple coincidence rate of 0.1 Hz with a core particle density of some 2-3 m-2 such that each trigger is associated with a signal in the Auger Cherenkov tank • The trigger selects showers within 0.4 sr from the vertical having typical energy of 200 GeV. They represent only 1‰ of the primary cosmic flux.

19. MULTITANK DETECTION OF AIR SHOWERS • Requiring high charges in the small trigger tanks increases the charge deposited in the main tank and improves the accuracy of the charge and timing measurements. - A factor of two reduction of the data sample gives a 15% improvement on the timing measurement and increases the main tank charge by 38%.

20. Dynode e- e-  K  Photocathode efficiency measurements - We measured the response of the photocathode to single photons at IPN Orsay using a scintillator exposed to Americium as a light source. - The fine structure of the time distribution and its dependence on incidence angle have been studied. - Evidence for a dependence of the PMT gain on geometry is calling for additional measurement as a function of impact point which are currently undertaken in Hanoi.

21. Studies using Auger data 3.1Simulation of FD reconstruction 3.2Study of FADC traces, muon to photon ratio Theses under joint supervision with foreign universities.

22. FD SIMULATION AND SHOWER RECONSTRUCTION The pixel pattern defines the shower detector plane (accurately), the time distribution along the track locates the shower within this plane (not accurately)

23. Our simulation program has been used to understand in simple terms specific features of the detection of UHE showers in a fluorescence detector. The Pierre Auger Observatory was used as an illustration. In addition to the difficulty of positioning accurately the shower in the shower-detector plane in the case of monocular observation (pointing to the importance of making binocular, or better hybrid observations) other effects were considered: -The compatibility between fluorescence and surface detections was studied and found to be very good in the PAO design; -Considerations on the necessity to extrapolate the observed shower profile outside the field of view shed some light on the different requirements implied by a good energy measurement; -Parameters such as the spans in time and in atmosphere thickness covered by the field of view were identified as good indicators of the quality of the measurements performed.

24. STUDY OF FADC TRACES, MUON TO PHOTON RATIO: • REDUCING FADC TRACES TO A SUM OF PEAKS • The surface detector signals are stored in flash analog to digital converters (FADC, 25ns bin size). • The time distribution of the charge (averaged over the three PMTs) is the sum of muon and electron-photon signals; • the former cluster around a mean charge corresponding to muons traversing the whole water volume; • the latter have a rapidly decreasing exponential distribution starting at very low values defined by the detection threshold (typically 1/30 of the muon signal). • Disentangling the two and reducing the FADC traces to a sum of peaks is therefore an essential preliminary to a detailed understanding of the shower properties.

25. FADC spectrum of a big photon event (data & fit) Time in bins of 25 ns

26. FADC spectrum of a four muon event (data & fit) Time in bins of 25 ns

27. FADC spectrum of a complex event (data & fit) Time in bins of 25 ns

28. FADC spectrum of another complex event (data & fit) Time in bins of 25 ns

29. Properties of the Cherenkov signals • Each particle producing Cherenkov light in a PAO Cherenkov tank generates a signal in each of the PMTs that starts abruptly and decays exponentially with a decay time of some 75ns (typically 14 diffusions). • We have written a Monte Carlo simulation of the optical properties of the tank that reproduces this general behavior.

30. Early time asymmetry Moreover, the light reaching the PMT’s in the first 25ns or so is not equally shared between the three PMTs. It displays a strong asymmetry that is correlated with the azimuth of the incoming particles and with their impact points. Data Monte Carlo

31. Early time asymmetry • We are currently elaborating an algorithm that uses this early time asymmetry to tag the start of individual signals in the FADC traces to help disentangling their pattern. • The ultimate aim is to identify and separate muon signals and photon signals in order to obtain information on the nature of the primary (proton or heavier nucleus).

32. TEACHING AND TRAINING • VATLY has been actively contributing to the teaching of particle physics and astrophysics in the Ha Noi universities (national and pedagogic) and Institute of Physics. Modern astrophysics lectures were delivered for the first time to Vietnamese university students in 2005. • VATLY is providing supervision for graduation, master and PhD theses in collaboration with the Hanoi Universities and with the Institute of Physics. The PhD program is under joint supervision with western universities yielding to a PhD degree in both Vietnam and the foreign country.

33. Summary Cosmic ray studies have been performed by VATLY, including muon flux measurements, studies of the response of an Auger Cherenkov counter to muons and to extensive air showers using three additional trigger counters in coincidence. The future of the VATLY Laboratory is with the Pierre Auger Observatory, this is the aim of most of our present activities. Simulation studies of the surface and fluorescence detectors have been performed with particular emphasis on the understanding of systematic uncertainties. Studies are underway to learn about the nature of the primaries by reducing the FADC traces to a sum of muon and electron-photon peaks. Contribution is also being given to the reduction of the fluorescence detector data. Our association with the PAO is an endless source of research subjects in collaboration with prestigious universities from all over the world. VATLY is actively contributing to University training and teaching. We are indebted to CERN, RIKEN, the World Laboratory, the EU-Asia link program, the French CNRS, the Rencontres du Vietnam and the Vietnam Academy of Science and Technology for invaluable support. We are particularly grateful to the Pierre Auger Collaboration for their constant interest and support in the development of our group.

34. VATLY PUBLICATIONS • Measurement of the Vertical Cosmic Muon Flux in a Region of Large Rigidity Cutoff, P.N. Dinh et al., Nucl. Phys., 627B (2002) 29 • Measurement of the Zenith Angle Distribution of the Cosmic Muon Flux in Hanoi, P.N. Dinh et al., Nucl. Phys. 661B (2003) 3 • Dependence of the cosmic muon flux on atmospheric pressure and temperature, P.N. Diep et al., Com. Phys. Vietnam 14 (2003) 57 • The cosmic ray research in Hanoi: The Auger experiment and measurements made at home, P.N. Dinh, Nucl. Phys.722A (2003) 439 • Measurement of the east-west asymmetry of the cosmic muon flux in Hanoi, P.N. Diep et al., Nucl. Phys. 678B (2004) 3 • Properties and performance of the prototype instrument for the Pierre Auger Observatory, Auger Collaboration, J. Abraham et al., Nucl. Inst. Meth A523 (2004) 50 • Atmospheric muons in Hanoi, P.N. Diep et al., Com. Phys. Vietnam 15 (2005) 55 • On the detection of ultra-high energy cosmic rays: Fluorescence detection, P.N. Diep et al., Com. Phys. Vietnam, 16 (2006) 129 • Anisotropy studies around the galactic centre at EeV energies with the Auger Observatory, J. Abraham et al. [Pierre Auger Collaboration], Astropart.Phys.27(2007) 244-253 • An upper limit to the photon fraction in cosmic rays above 1019 eV from the Pierre Auger Observatory, J. Abraham et al. [Pierre Auger Collaboration], Astropart.Phys.27 (2007) 155-168 • RECENT PRESENTATIONS TO CONFERENCES • Atmospheric muons in Hanoi, D.Q.Thieu at the Vth Rencontres du Vietnam, Hanoi, August 2004 and by P.N.Diep at the IXth APPC conference, Hanoi, October 2004. • Cosmic ray studies at VATLY, P.T.T.Nhung at the Hanoi-Osaka Forum, Hanoi, September 2005. • VATLY, a cosmic ray laboratory in Hanoi: a progress report, P.N. Diep at the VIth Rencontres du Vietnam, Hanoi, August 2006. • In addition, on various occasions, the Auger Collaboration has been giving numerous presentations and progress reports with which some VATLY members are associated. • THESES • PhD theses: Dang Quang Thieu, 2005 • Master theses: Nguyen Hai Duong, 2004; Pham Thi Tuyet Nhung, Pham Ngoc Diep, Pham Ngoc Dong 2006; • +two master theses currently underway • Diplom works: Pham Ngoc Diep and Pham Thi Tuyet Nhung, 2003; Dinh Lam Anh Huyen, 2004; Nguyen Viet Phuong and Kim Thi Phuong, 2006; Doan Thi The 2007;

35. Thank you for your attention! Information and documentation concerning VATLY is available on our site http://www.inst.gov.vn/inst/English/About/VATLY/Vatly.htm