SEARCH FOR AGN N m ' '! I N O S WITH THE SOUDAN 2 DETECTOR

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1 PDK-671 M Goodman 5/22/97 HE 4125 SEARCH FOR AGN N m ' '! I N O S WITH THE SOUDAN 2 DETECTOR ANL-HEP-CP DM DeMuth' for the Soudan 2 Collaboration CONF- 5 V 0 7 / 0 3 'School of Physics & Astronomy, University of Minnesota, Minneapolis, MN, USA ABSTRACT Several authors have presented models for neutrino production from Active Galactic Nuclei (AGN) that allow for the possibility of,agn neutrinos outnumbering the atmospheric neutrino flux for energies in excess of 30 TeV We present preliminary results from a search for high energy neutrinos from AGN using the underground Soudan 2 Detector NEUTRINOS FROM ACTIVE GALACTIC NUCLEI Active Galactic Nuclei have been established as intense acceleration sites of extremely highenergy photons[l] and therefore should be production sites of high-energy neutrinos [2] in excess of 10 TeV Albeit that the identification of individual point source detections of neutrinos from AGN are unlikely because of the limited effective detection area of present day experiments, a diffuse flux of extra-galactic neutrinos within an assumed isotropic distribution of active galaxies may be a tractable measurement Several authors have offered predictions: in particular, Szabo & Protheroe [3] and Stecker, Done, Solomon & Sommers [4] thus providing experimentalists with an impetus to search for AGN v's With these ultra-high-energy (TeV-PeV) neutrino flux predictions along with contemporary neutrino-muon cross sections [5] and a muon energyrange relationship [6] an underground muon flux from AGN neutrino interactions has been calculated [5, 61 For muon energies in excess of several TeV, the predicticted flux of muons produced from AGN neutrinos dominates the background of atmospheric v-induced muons THE SOUDAN 2 DETECTOR The Soudan 2 experiment [7] is located near Soudan, Minnesota at a depth of 2100 meters of water equivalent (mwe) medium and has been in operation since 1988 The Soudan 2 detector is an iron tracking calorimeter comprised of an array of individual modules each consisting of ' I steel sheets interleaved with proportional drifting tubesthe apparatus consists of the central ' detector measuring 5 x 8 x 14 m3 and an enclosing proportional tube detector[8] which is used to improve the discrimination of v-induced muons and for background rejection for contained-event analyses A mixture of gaseous argon and CO;! is I used as the active component in both detectors The sampling rate of the central detector is W 200 ns and ionization pulse-shape information is recorded The detectors angular resolution is -1" and the event rate of muons is roughly 02 Hz 7 The detector geometry imposes no additional limitations on the measurement of horizont a1 Fig 1: The Soudan 2 Detector muons with the exception of the tiny fraction of solid angle associated with trigger irregularities The submitted manuscript has been authored m m Submitted to 25th International Cosmic Ray Conference, Durban, South Africa, July 28-August 8, 1997 ul& QUA%&AA'& &ww&j&jj 4 by a contractor of the U S Government under contract No W ENG-38 Accordingly, the U S Government retains a nonexclusive, royalty-free licenre to publish or reproduce the published form of this contribution, or allow others to do so, for

2 DISCLAIMER This report was prepared as an account of work sponsored by aa agency of the United States Government Neither the United States Government aor any agency thereof, nor any of their employees, makes any warranty, cxprcss or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, pmduct, or procey disclosed, or represents that its usc would not infringe privately owned rights Reference h m i n to any spccsc commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its U I ~ O K C I Uream~~~, mend;rtion or favoring by the United States Government or any agency thereof The vim and opinions of authors exprrssed hercia do not aeccssarily state or reflect those of the United States Government orany agency thereof

3 HORIZONTAL MUONS AT SOUDAN In order to resolve a neutrino-induced signal we limit our detection aperture to the horizontal direction where the atmospheric muon contribution is significantly diminished It is the detector's inability to resolve up from down-ward going muons that leads to this exclusive horizontal measurement Underground muon vertical intensities have been compiled by Crouch[S, 101 From Figure 2, two distinct muon spectra are evident; 1) atmospheric muons which with increasing slant depth fall off quickly as the expected power law and 2) for large slant depths, a relatively flat population of muons considered to be induced from atmospheric neutrino interactions in the rock near the detector It is apparent the atmospheric neutrino induced muon component of the underground muon spectra dominates the atmospheric muon component for slant depths in excess of 14 kmwe of standard rock loa 10' los DEPTH (mwr) HMU Data Stream At run-time, data is stored on disk and subsequently processed using an efficient track- Fig 2: Vertical Intensity Slant Depth, finding algorithm which utilizes the detector Soudun zenith angles are superimpased to space Point and Pulse shape information to indicate their relationship to slant depth produce a Horizontal Muon (HMU) data stream composed of events having as zenith After initial processing is (relative to the vertical) an angle in excess of 60" complete the data is stored on 8mm Exabyte format for retrieval on-demand Filter software which incorporates standard reliability cuts has been developed to extract candidate v-induced muon events which are then verified by visual scanning techniques First Results As proto-type, we made a first pass at the data in order to develop our search techniques We considered data from selected! 360 HMU data tapes which spanned the dates! 15-Apr-1992 to 26-Nov-1996 corresponding t,o a detector exposure of seconds The detectors response to v-induced muons ' 2w ~,,,,,,,,, : ~ has been calculated with a slant depth algorithm that uses USGS digitized surface 100 elevation data Muon events which have a slant depth in excess of 14 kmwe are so considered to be from the population of 818 SZ 82 muons induced from atmospheric neutrinos Acceptance to this population of muons varies slightly with zenith owing to topographical Fig 3: v-induced muon acceptance effects on the surface but beyond 827", all muon events detected are classified as vinduced We display this variation in Figure 3 The solid angle for horizontal v-induced muons is 177 sr The effective detection area for horizontal muon events was calculated using

4 l u J the Soudan 2 monte-carlo code A large number of muons, uniformly distributed in zenith in the interval (60",90") were generated For each muon track, the effective area was determined by calculating the 2-D projected areas orthogonal to the track direction The detection area for muons with zenith in excess of 82"is 8486 m2 Trigger and filter efficiencies have been estimated by monte-carlo to be 81% for the proto-experiment Our initial search has produced 14 events which pass the v-induced muon criteria which includes an 175 cm track-length cut corresponding to a muon energy threshold of 600 MeV Expressing the v-induced HMU flux as cp- dnp dtdadfldc where dt is the experimental exposure, d A is the detection area, dr is the solid angle subtended cm-2 sr-l s-' and dc as the detection efficiency we calculate Q P = (412 f 11f 058) x Measurement of the Neutrino Induced Muon Flux Several underground experiments have reported upward-going muon fluxes which intrinsically include detection thresholds These thresholds are related to the minimum energy-deposition observed in the detector Because detection medium varies, comparisons between experiment can often be misleading In order to avoid this confusion, it is the custom to scale the detector energy threshold to that of Kamiokande, or 3 GeV In addition, up-going muon fluxes are averaged over the full range of zenith angles corresponding to the upwards direction To make a comparison to horizontal fluxes special care must be taken to realize the horizontal component We have considered up-going muon fluxes from Baksan[l4] and Kamiokande[l5] by averaging only the horizontal points from the up-going data to give 372 & 330 x cm-2 sr-l s-l respectively Several other experiments have reported exclusive horizontal muon fluxes In particular, Frejus[ll], LVD[12], and Crouch[l3] report horizontal intensities as 367f066, ~ sr-l s-l respectively 83f26 & 459f042 ~ 1 O - lcm-2 AGN NEUTRINO INDUCED MUON FLUX AT SOUDAN As UHE energy neutrinos (E>10 TeV) traverse the earth, they can undergo charged-current interactions and produce muons These muons are likely to undergo a catastrophic loss of energy by radiative processes which dominate for energies in excess of 1 TeV Muon-lepton pair production and bremsstrahlung processes are evidenced as multiple electro-magnetic bursts along the trajectory of the muon By determining the shower energy we expect to estimate of the muon energy and thus discriminate an extra-galactic origin by inferring a neutrino energy Table 1: Preliminary AGN-Muon Flux TeV % Efficiency x10-13 cm-2 sr-l s-l We have employed a Geant simulation of the Soudan 2 detector [16, 171 to determine the detection efficiency for measuring in excess of 10 GeV loss of energy on 5, 10, and 100 TeV muons After interrogating our horizontal muon database for events that exhibit a catastrophic loss of energy within the fiducial volume of the Soudan 2 detector we observe zero events with energy loss in excess of 10 GeV and set a preliminary 90% confidence limit on AGN v induced muons at Soudan 2 as 151, 083, and ~ lcm-2 ~ sr-l s-l respectively

5 Remarks and Summary We have made a preliminary study resulting in an estimation of atmospheric and AGN muon fluxes using horizontal muons at Soudan from an exposure of 422 x IO9 m-2 sr-l s-l We are in the process of analyzing a larger data set totaling 196 x 1O1O m-2 sr-l s-l We hope to have conclusive results regarding the neutrino fluxes measured at Soudan in the near future ACKNOWLEDGEMENTS The collaborating institutions for the Soudan 2 experiment include Argonne National Laboratory, the University of Minnesota, Oxford University, the Rutherford-Appleton Laboratory, Tufts University and Western Washington University This work was undertaken with the support of the the US Department of Energy and the State of Minnesota We wish to thank the Minnesota Department of Natural Resources for allowing us to use the facilities of the Tower-Soudan State Park and the dedicated laboratory workers, W Miller, J Beaty, J Meier, G Benson, D Carlson, B Anderson, J Proton, and B Dahlin * REFERENCES [l] D JThompson et al,the Second EGRET Catalog of High-Energy Gamma-Ray Sources AstroPhys J, Suppl Ser 101,259 (1995) [2] Gaisser, Stanev Phys Rev Lett 54,20 (1985) [3] AP Szabo & RJ Protheroe Astro Phys 2, 2222 (1995) [4]FWStecker et al, Phys Rev Lett 66,2697 (1991) & Phys Rev Lett 69,2738(E) (1992) [5] Ghandi et al, Fermilab Pub 95/221-T,(1995) [6] TK Gaisser, F Halzen, T Stanev Phys Rept 258, (1995) [7]WWM Allison et al, The Soudan 2 Detector Nucl Instrum Methods A, 376 (36)1996 [8] WP Oliver et al, Nucl Instrum Methods A, 276 (371)1989 [9] MF Crouch Proc 20th ICRC 6,165 (1987) [lo] TStanev, private communication [ll] W Rhode et al, Astropart Phys 4, 217 (1996) [12] M Aglietta et al, Astropart Phys 3,311 (1995) [13] MF Crouch Phys Rev D 18, 2239 (1978) [14] MM Boliev et al, Proc 3rd Int Workshop on Neutrino Telescopes, 235 (1991) I151 M Mori et al, Phys Rev Lett B210, 89 (1991) [16] JHTrost Internal PDK Note 473,1-25 (1994) [17] MC Goodman et al, Proc of the Workshop on High-Energy Neutrino Astrophysics, Hawaii, 243 (1991)

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