The east-west anisotropy, caused by the deflection of primary cosmic rays in
the Earth's magnetic field, is observed for the first time in the flux of
atmospheric neutrinos. Using a 45 kt-year exposure of the
  Super-Kamiokande detector, 552 e-like and 633 mu-like horizontally-going
events are selected in the momentum range between 400 and 3000 MeV/c.
  The azimuthal distribution of e-like and mu-like events agrees with the
expectation from atmospheric neutrino flux calculations that account for the
geomagnetic field, verifying that the geomagnetic field effects in the
production of atmospheric neutrinos in the GeV energy range are well
understood.Comment: 8 pages,3 figures revtex, submitted to PR

A. Habig

A. Hasegawa

A. Kibayashi

A. L. Stachyra

A. Okada

A. Sakai

A. Suzuki

A. T. Suzuki

B. K. Kim

B. Rossi

B. Viren

C. K. Jung

C. Mauger

C. McGrew

C. Saji

C. W. Walter

C. Yanagisawa

D. Casper

D. Kielczewska

D. Takemori

E. Blaufuss

E. Kearns

E. Sharkey

F. Tsushima

G. W. Sullivan

H. Fujiyasu

H. Ishino

H. Ogawa

H. Okazawa

H. Takeuchi

H. W. Sobel

J. A. Goodman

J. G. Learned

J. Hill

J. Kameda

J. Kanzaki

J. L. Stone

J. S. George

J. Shirai

K. Higuchi

K. Inoue

K. Ishihara

K. K. Young

K. Kaneyuki

K. Kobayashi

K. Martens

K. Miyano

K. Munakata

K. Nagashima

K. Nakamura

K. Nishijima

K. Nishikawa

K. Okumura

K. S. Ganezer

K. Scholberg

L. L. Wai

L. R. Price

L. R. Sulak

M. A. Stephens

M. D. Messier

M. Earl

M. Etoh

M. Goldhaber

M. Honda

M. Kirisawa

M. Koga

M. Kohama

M. Koike

M. Koshiba

M. L. Chen

M. Miura

M. Nakahata

M. Nemoto

M. R. Vagins

M. Sakuda

M. Shiozawa

M. Smy

M. Takahashi

M. Takahata

M. Takita

M. Yoshida

N. Sakurai

O. Sasaki

P. Lipari

R. J. Wilkes

R. Sanford

R. Svoboda

R. W. Ellsworth

S. B. Kim

S. Echigo

S. Hatakeyama

S. Inaba

S. Kasuga

S. Matsuno

S. Mine

S. Nakayama

S. Tasaka

S. Yamada

T. Barszczak

English

arXiv

Etoh, M.

Hasegawa, A.

Hasegawa, T.

Hatakeyama, S.

Iwamoto, T.

Koga, M.

Maruyama, T.

Ogawa, H.

Shirai, J.

Suzuki, A.

Tsushima, F.

et, al.

Super-Kamiokande, Collaboration

Institutional Repositories DataBase (IRDB)

Observation of the East-West Anisotropy of the Atmospheric Neutrino Flux

application/pdf学術論文 (Article)60820 bytesjournal articl

Tohoku University Repository (TOUR) / 東北大学機関リポジトリ

Observation of the East-West Anisotropy of theAtmospheric Neutrino Flux著者 白井  淳平journal orpublication titlePhysical review lettersvolume 82number 26page range 5194-5197year 1999URL http://hdl.handle.net/10097/35024doi: 10.1103/PhysRevLett.82.5194VOLUME 82, NUMBER 26 P H Y S I C A L R E V I E W L E T T E R S 28 JUNE 1999Observation of the East-West Anisotropy of the Atmospheric Neutrino FluxT. Futagami,21 Y. Fukuda,1 T. Hayakawa,1 K. Inoue,1 K. Ishihara,1 H. Ishino,1 Y. Itow,1 T. Kajita,1 J. Kameda,1S. Kasuga,1 K. Kobayashi,1 Y. Kobayashi,1 Y. Koshio,1 M. Miura,1 M. Nakahata,1 S. Nakayama,1 Y. Obayashi,1A. Okada,1 K. Okumura,1 N. Sakurai,1 M. Shiozawa,1 Y. Suzuki,1 H. Takeuchi,1 Y. Takeuchi,1 Y. Totsuka,1S. Yamada,1 M. Earl,2 A. Habig,2 E. Kearns,2 M. D. Messier,2 K. Scholberg,2 J. L. Stone,2 L. R. Sulak,2 C. W. Walter,2M. Goldhaber,3 T. Barszczak,4 D. Casper,4 W. Gajewski,4 W. R. Kropp,4 S. Mine,4 L. R. Price,4 M. Smy,4H. W. Sobel,4 M. R. Vagins,4 K. S. Ganezer,5 W. E. Keig,5 R. W. Ellsworth,6 S. Tasaka,7 A. Kibayashi,8 J. G. Learned,8S. Matsuno,8 V. J. Stenger,8 D. Takemori,8 T. Ishii,9 J. Kanzaki,9 T. Kobayashi,9 K. Nakamura,9 K. Nishikawa,9Y. Oyama,9 A. Sakai,9 M. Sakuda,9 O. Sasaki,9 S. Echigo,10 M. Kohama,10 A. T. Suzuki,10 T. J. Haines,11,4E. Blaufuss,12 B. K. Kim,12 R. Sanford,12 R. Svoboda,12 M. L. Chen,13 J. A. Goodman,13 G. W. Sullivan,13 J. Hill,14C. K. Jung,14 K. Martens,14 C. Mauger,14 C. McGrew,14 E. Sharkey,14 B. Viren,14 C. Yanagisawa,14 W. Doki,15M. Kirisawa,15 S. Inaba,15 K. Miyano,15 H. Okazawa,15 C. Saji,15 M. Takahashi,15 M. Takahata,15 K. Higuchi,16Y. Nagashima,16 M. Takita,16 T. Yamaguchi,16 M. Yoshida,16 S. B. Kim,17 M. Etoh,18 A. Hasegawa,18 T. Hasegawa,18S. Hatakeyama,18 T. Iwamoto,18 M. Koga,18 T. Maruyama,18 H. Ogawa,18 J. Shirai,18 A. Suzuki,18 F. Tsushima,18M. Koshiba,19 Y. Hatakeyama,20 M. Koike,20 M. Nemoto,20 K. Nishijima,20 H. Fujiyasu,21 Y. Hayato,21 Y. Kanaya,21K. Kaneyuki,21 Y. Watanabe,21 D. Kielczewska,22,4 J. S. George,23,* A. L. Stachyra,23 L. L. Wai,23,†R. J. Wilkes,23 and K. K. Young23,‡(Super-Kamiokande Collaboration)1Institute for Cosmic Ray Research, University of Tokyo, Tanashi, Tokyo 188-8502, Japan2Department of Physics, Boston University, Boston, Massachusetts 022153Physics Department, Brookhaven National Laboratory, Upton, New York 119734Department of Physics and Astronomy, University of California, Irvine, Irvine, California 92697-45755Department of Physics, California State University, Dominguez Hills, Carson, California 907476Department of Physics, George Mason University, Fairfax, Virginia 220307Department of Physics, Gifu University, Gifu, Gifu 501-1193, Japan8Department of Physics and Astronomy, University of Hawaii, Honolulu, Hawaii 968229Institute of Particle and Nuclear Studies, High Energy Accelerator Research Organization (KEK),Tsukuba, Ibaraki 305-0801, Japan10Department of Physics, Kobe University, Kobe, Hyogo 657-8501, Japan11Physics Division, P-23, Los Alamos National Laboratory, Los Alamos, New Mexico 8754412Department of Physics and Astronomy, Louisiana State University, Baton Rouge, Louisiana 7080313Department of Physics, University of Maryland, College Park, Maryland 2074214Department of Physics and Astronomy, State University of New York, Stony Brook, New York 11794-380015Department of Physics, Niigata University, Niigata, Niigata 950-2181, Japan16Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan17Department of Physics, Seoul National University, Seoul 151-742, Korea18Department of Physics, Tohoku University, Sendai, Miyagi 980-8578, Japan19The University of Tokyo, Tokyo 113-0033, Japan20Department of Physics, Tokai University, Hiratsuka, Kanagawa 259-1292, Japan21Department of Physics, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan22Institute of Experimental Physics, Warsaw University, 00-681 Warsaw, Poland23Department of Physics, University of Washington, Seattle, Washington 98195-1560(Received 13 January 1999)The east-west anisotropy, caused by the deflection of primary cosmic rays in the Earth’s magneticfield, is observed for the first time in the flux of atmospheric neutrinos. Using a 45 kt yr exposure ofthe Super-Kamiokande detector, 552 e-like and 633 m-like horizontally going events are selected in themomentum range between 400 and 3000 MeVc. The azimuthal distributions of e-like and m-likeevents agree with the expectation from atmospheric neutrino flux calculations, verifying that the flux ofatmospheric neutrinos in the GeV energy range is reasonably well modeled by calculations that accountfor the geomagnetic field. [S0031-9007(99)09471-5]PACS numbers: 96.40.Tv, 14.60.Pq, 95.85.RyThe primary cosmic ray flux approaching the Earth isknown to be almost isotropic [1]. However, since theEarth has a magnetic field and the primary cosmic rays5194 0031-90079982(26)5194(4)$15.00are positively charged, an angular anisotropy is producedfor those primaries which reach and interact in the at-mosphere. This azimuthal anisotropy, called the east-west© 1999 The American Physical SocietyVOLUME 82, NUMBER 26 P H Y S I C A L R E V I E W L E T T E R S 28 JUNE 1999effect, was discovered in the 1930s as a deficit of sec-ondary cosmic rays (muons) arriving from the easterly di-rection compared to a westerly direction [2]. In fact, thiseffect was used to infer that the primary cosmic rays arepositively charged. The azimuthal anisotropy varies ac-cording to the position on the Earth and the momentumand charge of the primary cosmic ray. The anisotropy islargest for the lowest momenta particles. The anisotropycan be characterized by a cutoff momentum, the maxi-mum momentum at which the incoming particle can-not reach the atmosphere. Since Super-Kamiokande islocated close to the geomagnetic equator (25.8±N geo-magnetic latitude), the cutoff momentum for horizontallyarriving protons from the east is 50 GeVc, consider-ably higher than the average over directions [3,4]. Fig-ure 1 illustrates typical allowed and forbidden tracks ofcosmic rays. This results in the depletion of primary cos-mic ray interactions directed at Kamioka from the east.The flux of neutrinos produced in the atmosphere by thesecosmic rays should likewise be depleted.In this paper we report the first observation of the east-west anisotropy in the atmospheric neutrino flux. Weobserve a neutrino east-west effect which is in agreementwith expectations based on detailed flux calculations [3,4]and a Monte Carlo simulation of neutrino interactions[5,6]. This confirms that the angular dependence of theatmospheric neutrino flux calculations in the GeV energyrange is reasonable.The Super-Kamiokande detector is a 50 kt waterCherenkov detector located at the Kamioka Observatory,ICRR, University of Tokyo. The center of the detectoris at 36±2503300 N, 137±1803700 E, 371.8 m above the sealevel. To reduce the rate of cosmic ray muons, it isplaced at 2700 meters-water-equivalent below the peak ofMt. Ikenoyama. It is cylindrically shaped with a heightof 41.4 m and a diameter of 39.3 m, and divided intothe inner and outer detectors by a stainless steel supportnorth polethe EarthB→SKνforbidden track (low energy)allowed track (high energy)FIG. 1. Typical allowed and forbidden tracks of the incidentcosmic rays. The curved line shows the allowed trajectoryof the cosmic ray which has momentum larger than cutoffmomentum, and the dotted curved line shows a forbiddentrajectory. “SK” represents the Super-Kamiokande detector.structure with a pair of opaque sheets. The inner detectoris viewed by 11 146 photomultiplier tubes (PMTs) of50 cm diameter, facing inward to detect the neutrinoevents. The outer detector, a layer of water of 2.6 to2.75 m thick, has 1885 PMTs of 20 cm diameter used toveto entering events and to tag exiting events.The energy and direction of a neutrino are measuredfrom the momentum of the final state charged leptonwhich is produced in a charged current interaction ofthe neutrino on nucleus, n 1 N ! l 1 X. The flavorof the final state lepton is used for tagging the flavor ofthe neutrino. With the 45 kt yr exposure of the Super-Kamiokande, a total of 4077 fully contained single-ringatmospheric neutrino events were observed with the fol-lowing requirements: (1) total number of hits in outerdetector less than 25, and no spatial cluster with morethan 10 hits, (2) total charge collected in the inner de-tector more than 200 photoelectrons (pes), (3) the ratio(maximum pe in any single PMT)(total pes) less than0.5, (4) the time interval from a preceding event largerthan 100 ms, (5) the vertex position should be insidethe fiducial volume, 2 m inside from the inner detec-tor wall, and (6) single-ring events with the momentumhigher than 100 and 200 MeVc for e-like and m-likeevents, respectively. To determine the direction of neutri-nos from the observed leptons, only single Cherenkov ringevents were used. Then they were separated into e-likeand m-like events according to their showering or non-showering signature. The particle identification efficiencywas estimated to be better than 99% [5].For analysis of the east-west anisotropy, the followingadditional cuts were applied: (7) momentum was requiredto be between 400 and 3000 MeVc for both e-like andm-like events and (8) the cosine of the zenith angle ofthe leptons was required to be between 20.5 and 0.5.Criterion (7) selects the momentum region where the east-west anisotropy is expected to be most significant. Theeast-west anisotropy of the flux is expected to be larger forlower energy neutrinos because of the cutoff momentum.On the other hand, the lepton direction is more poorlycorrelated with the neutrino direction at low energy, whichtends to wash out the flux anisotropy. The estimated meanscattering angle in the momentum region of criterion (7)is 36± for both ne and nm charged current events. Thecontamination of neutral current events in these e-likeand m-like events was estimated to be 9.0% and 1.3%,respectively. Criterion (8) is to select neutrino eventsfrom near the horizon where the effect of the geomagneticcutoff is maximum.After these criteria, 552 e-like and 633 m-like eventsremained. These data were compared with Monte Carlosimulations using two independent flux calculations [3,4].These calculations represented the geomagnetic field mapby a multipole expansion of the spherical harmonic func-tion. Cutoffs were calculated for all zenith and azi-muthal angles at the Super-Kamiokande detector site by5195VOLUME 82, NUMBER 26 P H Y S I C A L R E V I E W L E T T E R S 28 JUNE 1999backtracing antiprotons through the three dimensional mapof the geomagnetic field. Figure 2 shows the azimuthaldistributions for e-like and m-like events. The number ofMonte Carlo events (494.3 e-like and 820.7 m-like basedon the flux of Ref. [3], and 487.0 e-like and 790.5 m-likebased on the flux of Ref. [4]) was normalized to the num-ber of data events to compare the azimuthal shape with-out regard to the 20% uncertainty in absolute flux and thedeficit of m-like events most likely due to neutrino oscil-lations [5,7,8]. The Monte Carlo events we used here as-sumed no neutrino oscillations. The lengths of the flightpaths of the neutrinos vary with zenith angle but do notvary with the azimuthal angle. Therefore, neutrino flavoroscillations have little effect on east-west anisotropy. Theestimated effect from neutrino oscillations is far below thestatistical accuracy of the data.The azimuthal angles are divided into eight bins, 45±each. Agreement between the data and the Monte Carlois good: the x2 values were 5.17 DOF and 2.67 DOFfor e-like and m-like events, respectively.A quantitative comparison between the data and MonteCarlo using the flux of Ref. [3] was performed using aKuiper test [9,10] (similar results were obtained usingthe flux of Ref. [4]). This test calculates a binning andstarting-point free probability that observed data are theresult of an assumed distribution. The Kuiper statistic Vis defined asV  max0,f,2pSN f 2 Pf1 max0,f,2pPf 2 SN f ,where f is the azimuthal angle, SN f is a cumulativeprobability function from data, and Pf is the one from0204060801000 π/2 π 3π/2 2πe-likeφnumber of events0204060801000 π/2 π 3π/2 2πµ-likeφnumber of eventsFIG. 2. Azimuthal angle distributions of e-like and m-likeevents. The crosses represent the data points, and the histogramdrawn by solid line (dashed line) shows the prediction of theMonte Carlo based on the flux of Ref. [3] ([4]). Data are shownwith statistical errors. The Monte Carlo has 10 times morestatistics than data. The Monte Carlo histogram is normalizedto the total number of the real data. f represents the azimuthalangle. f  0, p2, p, and 3p2 show particles going tonorth, west, south, and east, respectively.5196Monte Carlo. The significance is obtained from the statis-tic V   V pn 1 0.155 1 0.24pn , and defined asProb  2X`j14j2V 2 2 1 exp22j2V 2 ,where n is the number of events.From this test, the probability that the azimuthal distri-bution of the data originated from a flat parent distributionwas 0.0008% (20%) for e-like (m-like) events. The dif-ference in probability can be traced to the run of 4 lowbins in e-like data compared to only 2 low bins in m-likedata. The probability that the data matches the MonteCarlo in shape with the flux of Ref. [3] was 42% fore-like events and 92% for m-like events. For a MonteCarlo with neutrino oscillations with Dm2, sin22u 2.2 3 1023 eV2, 1.0 [8], the probabilities were the samewithin 1% for both e-like and m-like events. As expected,the probabilities did not change much with the addition ofneutrino oscillations.With current data, the deficit of the westward-goingneutrinos is more significant in the e-like data sample thanin the m-like data sample. We define an east-west asym-metry by NE 2 NW NE 1 NW , where NENW  rep-resents the number of eastward(westward)-going events.Here the azimuthal distribution was divided into two bins,and the same zenith angle cut (8) was used. The observedasymmetry in m-like data, 0.08 6 0.04, is smaller than ine-like data, 0.21 6 0.04. However, both are consistentwith expectation: 0.11 for m-like and 0.13 for e-like basedon the flux of Ref. [3]; 0.15 for m-like and 0.17 for e-likebased on the flux of Ref. [4].The study of the east-west anisotropy as a function oflepton momentum is important for our understanding of theneutrino-lepton angular correlation and the geomagneticfield effect on the production of atmospheric neutrinos.Figure 3 shows the east-west asymmetry as a function oflepton momentum. The data and the Monte Carlo agreedwell; the x2 value was 5.96 DOF (5.85 DOF) for e-like(m-like) events. The x2 value of a comparison of thedata and a straight line at NE 2 NW NE 1 NW   0was 26.56 DOF (10.55 DOF) for e-like (m-like) events.As expected from the large neutrino-lepton scatteringangle, the measured east-west asymmetry was small below400 MeVc. Above 3 GeVc, the neutrinos originatefrom primary cosmic rays that are minimally affected bythe geomagnetic field, so the flux is symmetric from theeast and west.In summary, we have observed for the first time clearevidence for the east-west anisotropy in the atmosphericneutrinos, based on the data from 45 kt yr of exposure ofthe Super-Kamiokande detector. A deficit of westward-going e-like events was seen at more than 99% confidencelevel, while for m-like events, it was statistically lesssignificant. However, the azimuthal distributions of theseevents and the momentum dependence of the east-westanisotropy for both e-like and m-like agreed well withVOLUME 82, NUMBER 26 P H Y S I C A L R E V I E W L E T T E R S 28 JUNE 1999-0.4-0.3-0.2-0.100.10.20.30.410 2 10 3 10 4(NE-N W)/(NE+N W) e-like-0.4-0.3-0.2-0.100.10.20.30.410 2 10 3 10 4Lepton Momentum (MeV/c)(NE-N W)/(NE+N W) µ-likeFIG. 3. East-west asymmetry NE 2 NW NE 1 NW  as afunction of lepton momentum for e-like and m-like events,where NE and NW represent the number of eastward andwestward-going events, respectively (see the text for thedefinition). The crosses with error bars represent the data andthe hatched region represents the prediction based on the fluxof Ref. [3]; error bars are statistical.the prediction based on detailed neutrino flux calculations.This observation suggests that the geomagnetic fieldeffects in the production of atmospheric neutrinos in theGeV energy range are reasonably well modeled.We gratefully acknowledge the cooperation of theKamioka Mining and Smelting Company. The Super-Kamiokande experiment was built from, and has beenoperated with, funding by the Japanese Ministry ofEducation, Science, Sports and Culture, and the UnitedStates Department of Energy.*Present address: Space Radiation Laboratory, CaliforniaInstitute of Technology, Pasadena, CA 91125.†Present address: Department of Physics, StanfordUniversity, Stanford, CA 94305.‡Deceased.[1] See, for example, K. Nagashima, K. Fujimoto, andR. M. Jacklyn, J. Geophys. Res. 103, 17 429 (1998);K. Munakata et al., Phys. Rev. D 56, 23 (1997).[2] T. H. Johnson, Phys. Rev. 43, 307 (1933); 43, 381 (1933);48, 287 (1935); see also B. Rossi, Cosmic Rays (McGraw-Hill, New York, 1964), and references therein.[3] M. Honda et al., Phys. Rev. D 52, 4985 (1995).[4] P. Lipari, T. K. Gaisser, and T. Stanev, Phys. Rev. D 58,073003 (1998).[5] Y. Fukuda et al., Phys. Lett. B 433, 9 (1998).[6] M. Nakahata et al., J. Phys. Soc. Jpn. 55, 3786 (1986).[7] Y. Fukuda et al., Phys. Lett. B 436, 33 (1998).[8] Y. Fukuda et al., Phys. Rev. Lett. 81, 1562 (1998).[9] M. A. Stephens, J. R. Statist. Soc. B 32, 115 (1970).[10] W. H. Press, S. A. Teukolsky, W. T. Vetterling, and B. P.Flannery, Numerical Recipes in FORTRAN (CambridgeUniversity Press, Cambridge, 1992), 2nd ed.5197

Physical Review Letters

T.  Futagami

Y. Fukuda

T. Hayakawa

Y. Itow

T. Kajita

Y. Kobayashi

Y. Koshio

Y. Obayashi

Y. Suzuki

Y. Takeuchi

Y. Totsuka

W. Gajewski

W. R. Kropp

W. E. Keig

V. J. Stenger

T. Ishii

T. Kobayashi

Y. Oyama

T. J. Haines

W. Doki

Y. Nagashima

T. Yamaguchi

T. Hasegawa

T. Iwamoto

Crossref

https://tohoku.repo.nii.ac.jp/?action=repository_action_common_download&item_id=4714&item_no=1&attribute_id=18&file_no=1

Observation of the east-west anisotropy of the atmospheric neutrino flux

Observation of the east-west anisotropy of the atmospheric neutrino flux

Abstract

Similar works

Full text

Available Versions

Institutional Repositories DataBase (IRDB)

Tohoku University Repository (TOUR) / 東北大学機関リポジトリ

Crossref