Study on Large-Scale CR Anisotropy with ARGO-YBJ experiment Cui Shuwang1, Li Taoli1, Zhang Jianli2 On behalf of ARGO Collaboration 1Hebei Normal university 2 Institude of High Energy Physics, CAS

Outline Motivation ARGO-YBJ Experiment Analysis Method & DATA Selection Results Conclusion and Discussion

Motivation Many ground experiments have detected large-scale sidereal anisotropy of galactic Cosmic Ray. The intensity amplitude of anisotropy is an important information for understanding the cause of this phenomenon Previous investigations showed that amplitude is weak(0.1%) so that it is very difficult to identify dominant cause from many effects Some experiments have detected the variation of anisotropy amplitude as function of the energy of CR Now it is still an opening question

ARGO-YBJ Experiment A full coverage array by RPC detectors at the Yangbajing cosmic ray observatory (Tibet, P.R. China) ,4300 m a.s.l.

Layout of the ARGO-YBJ experiment

Data selection & analysis method Duty time: (2 full years ) 2008.01~2009.12 Zenth angle < 45° Nhits > 80  < 80 Analysis Method  Iteration based on Equi-zenith Angle Method Azimuth modified by different zenith angle Smooth angle 5° —— 2008 —— 2009 Azimuth distribution

Sidereal anisotropy Results B The smooth fitting line is the second-order cosine harmonics function 1 + P0cos(2π(x - P2)/360) + P1cos(2π(x - P3)/180)

Same structure for Sidereal anisotropy between two years 2008 2009

Amplitudes of anisotropy are stable in these two years 2008 2009

Anisotropy amplitude vs Energy(Number of Hits) E_m(TeV) N_hits 0.9 ——40-60 1.52 ——60-100 2.4——100-160 3.86 ——160-300 7.17 ——300-700 12.47——700-1000 23.6 ——1000~

——ARGO (ICRC2009) The amplitude of CR anisotropy intensity variation as a function of CR’s median energies in the unit of GeV compared with the results from Tibet ASγ experiment and presented by ARGO-YBJ last ICRC. The open boxes are obtained by underground muon observations(Bercovitch Agrawal 1981; Thambyahpillai 1983; Nagashima et al. 1985; Swinson Nagashima 1985; Andreyev et al. 1987; Lee Ng 1987; Ueno et al.1990; Cutler Groom 1991; Fenton et al. 1995; Mori et al. 1995; Munakata et al. 1995, 1997; Ambrosio et al. 2003) and the open circles are by air shower array experiments (Gombosi et al. 1975; Alexeenko et al. 1981; Nagashima et al. 1989;Bergamasco et al. 1990; Aglietta et al. 1995,1996; NFJ).

Two dimensions anisotropy monthly variation at solar time and sidereal time period

Monthly variation of anisotropy Amplitude Correlation of solar anisotropy and sidereal anisotropy

Conclusion & Discussion We have detected the large-scale anisotropy of CRs by ARGO-YBJ experiment with two years data from 2008 to 2009. It is stable in the two years. We report the amplitude of anisotropy energy dependent effect with seven groups events divided by the vector Nhits. It is helpful for understanding and judging the model for origin of anisotropy phenomenon. We have observed the monthly variation of anisotropy, more detailed work is ongoing……

Thank you!

2 fitting method Equal This method is developed to fit simultaneously the relative cosmic ray intensity from all directions over the sky. The principle is that at any moment, for all directions , if we scale down(or up) the observed event number by divide them by their relative CR intensity, then statistically, those scaled observed event number in an zenith belt should be equal anywhere.Here we define relative CR. Intensity as I. So, for some moment, at some zenith angle,we get an on-source window, and then we obtain some off-sources window with the same shape, the same size and the same zenith angle as on-source window, For the on source window, there are observed event number Non and relative intensity Ion, according for off-sources, Noff and Ioff.then statistically, Non/Ion and the average value of Noff/Ioff should be approximately equal, so a ki-square function can be built, this is for some certain direction and on-source window, and then for all direction and all moment, in the same way, we can get all ki-square function and then sum them,so a large ki-square function is constructed.by minimizing the function, we can get CR relative Intensity at all directions.