1. Cosmic muon signal and its seasonal modulation at Gran Sasso with the Borexino detector Davide D’Angelo for the Borexino Collaboration Università degli Studi di Milano Istituto Nazionale di Fisica Nucleare sez. di Milano ICRC 2011 – Beijing, 11-18 th Aug 2011

2. Why a seasonal modulation? (in a nutshell) Muons originate from π and K meson decay high in the atmosphere. The muon flux observed underground depends on the fraction of mesons that decay before first interaction Hotter air is less dense leading to reduced interaction chance ICRC11 - Beijing, 11-18 Aug 2011 D. D'Angelo higher temperature  more visible muons

3. a little bibliography... Known since ‘50s: ◦ Barrett et al. Rev. Mod. Phys., 24:133, 1952. First observed at LNGS by MACRO in ‘97: ◦ M. Ambrosio et al. Seasonal variations in the underground muon intensity as seen by macro. Astropart. Phys., 7:109–124, 1997. ◦ Revised in: M. Ambrosio et al., Phys. Rev. D67, 042002 (2003). Best work now by MINOS: ◦ P. Adamson et al. (MINOS coll.). Observation of muon intensity variations by season with the MINOS far detector. Phys. Rev. D81, 012001, 2010. arXiv:0909.4012 [hep-ex]. Also observed at LNGS by LVD: M.Selvi (for the LVD coll.) Analysis of the seasonal modulation of the cosmic muon flux in the LVD detector during 2001-2008. In Proc. 31 st ICRC, 2009. ICRC11 - Beijing, 11-18 Aug 2011 D. D'Angelo Thanks to E.W. Grashorn of CCAPP, Ohio State University π-only modelling

4. A Low energy neutrino and anti-neutrino detector based on elastic scattering on electrons in highly purified organic liquid scintillator Solar neutrinos: 7 Be, 8 B, pep, CNO and possibly pp. Geo-neutrinos, reactor, supernova explosion, … D. D'Angelo ICRC11 - Beijing, 11-18 Aug 2011 Why Borexino?

5. Water Tank: γ and n shield μ water Č detector 208 PMTs in water 2100 m 3 20 legs Carbon steel plates Scintillator: 270 t PC+PPO (1.4 g/l) Stainless Steel Sphere: ● 2212 PMTs ● ~ 1000 m 3 buffer of pc+dmp (light queched) Nylon vessels: (125 μm thick) Inner: 4.25 m Outer: 5.50 m (radon barrier) Detector layout ICRC11 - Beijing, 11-18 Aug 2011 D. D'Angelo

6. A Low energy neutrino and anti-neutrino detector based on elastic scattering on electrons in highly purified organic liquid scintillator Solar neutrinos: 7 Be, 8 B, pep, CNO and possibly pp. Geo-neutrinos, reactor, supernova explosion, … Also a powerful detector for muons, neutrons and cosmogenic backgrounds. Muon detection occurs with both Inner and Outer detector D. D'Angelo ICRC11 - Beijing, 11-18 Aug 2011 Why Borexino? Spherical detector: no systematics due to angular dependence of the acceptance

7. D. D'Angelo ICRC11 - Beijing, 11-18 Aug 2011 Abruzzo, Italy 120 Km from Rome Laboratori Nazionali del Gran Sasso Assergi (AQ) Italy 1400m of rock shielding ~3800 m.w.e. Borexino Detector and Plants External Labs The experimental site

8. The Borexino 4y muon signal Continuous data taking: May 16 th 2007 - May 15 th 2011 Rate: (4310±10 syst ) cpd Flux: (3.41±0.01 syst )· 10 -4 m -2 s -1 Period = (366±3)d D. D'Angelo ICRC11 - Beijing, 11-18 Aug 2011 Amplitude ~1.3% Phase ~ Jun 28 th (±6d) Systematics: Efficiency Threshold Detector diameter

9. ...or folded to 1y D. D'Angelo ICRC11 - Beijing, 11-18 Aug 2011 Phase ~ Jun 28 th Amplitude ~1.3% [period fixed to 365d] same fit results

10. Air temperature data Macro and LVD use data from Aeronautica MiIitare Italiana: ◦ @ Pratica di Mare, ~130km away from LNGS ◦ very incomplete data set, both in number of measures and atmospheric depth coverage. European Centre for Medium-range Weather Forecast (ECMWF): ◦ variety of measures used  global model of atmosphere. ◦ 4 daily measures: 0h, 6h, 12h, 18h. ◦ 37 discrete pressure levels: [1-1000]hPa. ◦ interpolated to exact LNGS coordinates: 13.578E, 42.454N. ICRC11 - Beijing, 11-18 Aug 2011 D. D'Angelo thanks to Scott Osprey, NCAS, University of Oxford (UK)

11. Correlating with air temperature ICRC11 - Beijing, 11-18 Aug 2011 D. D'Angelo only site-related parameter T(X) W(X)

12. Air temperature data Data come without errors! ◦ We average the 4 daily measures and use their spread: underestimation Sinusoidal fit is poor (small scale fluctuations), however: ◦ amplitude and phase consistent with muon data, yearly period. ICRC11 - Beijing, 11-18 Aug 2011 D. D'Angelo

13. Air temperature data ICRC11 - Beijing, 11-18 Aug 2011 D. D'Angelo So-called Sudden Stratospheric Warming (SSW): ◦ ~1-2w winter maxima due to artic cyclone deformations. ◦ Geophys. Res. Lett. 36 L05809 (2009) ‏

14. Lomb-Scargle periodograms D. D'Angelo ICRC11 - Beijing, 11-18 Aug 2011

15. Superimposing data sets ICRC11 - Beijing, 11-18 Aug 2011 A positive correlation is indeed evident D. D'Angelo Temperature Muon signal

16. … or side-by-side D. D'Angelo ICRC11 - Beijing, 11-18 Aug 2011

17. Measuring the correlation ICRC11 - Beijing, 11-18 Aug 2011 Linear regression accounting for errors on both axes Achieved correlation : Pearson coefficient (R-value) = 0.62 D. D'Angelo previous measurement at LNGS: α T = 0.94 ± 0.07 (MACRO `03) predicted value at LNGS depth: α T = 0.92 ± 0.02 (MINOS `10)

18. Summary of existing measurements ICRC11 - Beijing, 11-18 Aug 2011 D. D'Angelo no systematics due to acceptance, thanks to spherical symmetry

19. Conclusions The spherical Borexino detector observes the cosmic muon signal without systematics due to angular dependence of the acceptance. We measure the muon flux at LNGS – Hall C to be: (3.41±0.01 syst )· 10 -4 m -2 s -1 We observe indeed a modulation with yearly period, amplitude ~1.3% and a phase ~ Jun 28 th (±6d) We relate the fluctuations to atmospheric temperature variations, obtaining correlation 0.62 We measure the “effective temperature coefficient”: consistent with theoretical expectations and with smaller error if compared with MACRO-`03 measurement. ICRC11 - Beijing, 11-18 Aug 2011 D. D'Angelo