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Observation of Tau Neutrino Interactions

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Observation of Tau Neutrino Interactions Observation of Tau Neutrino Interactions DONUT Collab oration 1 1 2 2 K. Ko dama ,N. Ushida , C. Andreop oulos , N. Saoulidou , 2 3 4 4 G. Tzanakos ,P. Yager , B. Baller ,D.Boehnlein , 4 4 4 4 W. Freeman , B. Lundb erg , J. Mor n , R. Rameika , 4 5 5 5 6 J.C. Yun , J.S. Song , C.S. Yoon , S.H.Chung ,P. Berghaus , 6 6 6 6 M. Kubanstev , N.W. Reay , R. Sidwell , N. Stanton , 6 7 7 8 S. Yoshida ,S. Aoki , T. Hara , J.T. Rhee , 9 9 9 9 9 D. Ciampa ,C. Erickson , M. Graham , K. Heller , R. Rusack , 9 9 9 9 R. Schwienhorst , J. Siela , J. Trammell , J. Wilcox 10 10 10 10 10 K. Hoshino , H. Jiko , M. Miyanishi , M. Komatsu ,M. Nakamura , 10 10 10 10 T. Nakano ,K. Niwa , N. Nonaka , K. Okada , 10 11 11 12 O. Sato ,T. Akdogan ,V. Paolone , C. Rosenfeld 11;12 13 13 13 13 A. Kulik , T. Kafka ,W. Oliver , T. Patzak ,J. Schneps 1 Aichi University of Education, Kariya, Japan 2 University of Athens, Greece 3 University of California/Davis, Davis, California 4 Fermilab, Batavia, Il linois 60510 5 Gyeongsang University, Chinju, Korea 6 Kansas State University, Manhattan, Kansas 7 Kobe University, Kobe, Japan 8 Kon-kuk University, Korea 9 University of Minnesota, Minnesota 10 Nagoya University, Nagoya 464-8602, Japan 11 University of Pittsburgh, Pittsburgh, Pennsylvania 15260 12 University of South Carolina, Columbia, South Carolina 13 Tufts University, Medford, Massachusetts 02155 December 14, 2000 Abstract The DONUT exp eriment has analyzed 203 neutrino interactions recorded in nuclear emulsion targets. A decay search has found evi- 1 dence of four tau neutrino interactions with an estimated background of 0.34 events. This number is consistent with the Standard Mo del exp ectation. PACS numb er 14.60.Lm 2 The was p ostulated to exist after the discovery of the lepton in 1975 [1]. Since that time, much indirect evidence has b een gathered implying that the exists as the Standard Mo del third generation neutrino. However, the charged current interactions of a third neutrino have not been observed in the same manner as the interactions of the [2] and the [3]. e The DONUT exp eriment (Fermilab E872) was designed to observe the charged currentinteractions of the by identifying the lepton as the only lepton created at the interaction vertex. At the neutrino energies in this exp eriment, the typically decays within 2 mm of its creation to a single charged daughter (86% branching fraction). Thus the signature of the is a track with a kink, signifying a decay characterized by a large transverse momentum. Nuclear emulsion was used to lo cate and resolve these decays. Acharged particle sp ectrometer with electron and muon identi cation capa- bilities provided additional information. The neutrino b eam was created using 800 GeV protons from the Fermilab Tevatron interacting in a meter long tungsten b eam dump, whichwas 36 m upstream from the emulsion target. Most of the neutrinos that interacted in the emulsion target originated in the decays of charmed mesons in the b eam dump. The primary source of is the leptonic decayofaD meson into S and , and the subsequentdecayofthe to a . All other sources of are estimated to have contributed an additional 15%. (5 1)% of all neutrino interactions detected in the emulsion were predicted to b e from with the dominant uncertainty from charm pro duction and D ! branching ratio S measurements[4]. The mean energies of the detected neutrino interactions were calculated to be 89 GeV, 69 GeV, and 111 GeV, for , , and e resp ectively. The other pro ducts from the proton interactions, mostly muons, were reduced in the emulsion region using magnets, concrete, iron and lead shield- ing, shown in Figure 1. The emulsion targets were exp osed from April to September 1997. The length of exp osure for each target was set by the track 5 2 densityfrommuons, with a limit of 10 cm that was reached within ab out one month. The detectors, downstream of the shielding, consisted of a scintillation counter veto wall, the emulsion-scintillating b er hybrid target (emulsion mo dules interleaved with scintillating b er planes), trigger ho doscop es, and the charged particle sp ectrometer. Up to four emulsion mo dules, each7cm thick, were mounted along the neutrino b eam, separated by 20 cm. Two typ es of emulsion mo dules were used as neutrino targets. The rst typ e (called ECC) had a rep eated structure of 1 mm thick 3 stainless steel sheets interleaved with emulsion plates. The emulsion plates had 100 m thickemulsion layers on each side of a 200 mor800m thick plastic base. The mass of the emulsion in the ECC target was only 8% of the total, so that the neutrino interactions most likely occured in the steel plates. The second typ e of emulsion target (called bulk)was constructed from emulsion plates with 350 m thickemulsion layers on b oth sides of a 90 m thick plastic base. The three designs are shown schematically in Fig. 2. Each target mo dule consisted of either: ECC typ e only, bulk only, or a combination of ECC typ e for the upstream part and bulk for the downstream part. Two pure ECC mo dules, one pure bulk mo dule and four ECC/bulk hybrid mo dules were exp osed in the neutrino b eam. The pure ECC mo dules had a mass of 104 kg, and were 3.0 radiation lengths and 0.23 interaction lengths (for +N interactions). The pure bulk mo dule had a mass of 60 kg, and was 2.0 radiation lengths and 0.13 interaction lengths. The typical ECC/bulk hybrid mo dule was 70 kg, 2.0 radiation lengths and 0.16 inter- action lengths. All of the emulsion and steel sheets were 50 cm 50 cm oriented p erp endicular to the b eamline. Distributed b etween the emulsion mo dules were 44 planes of scintillating b er tracker (SFT) of 0.5 mm diameter read out by 6 image intensi ers [8]. The SFT in the hybrid target provided the information to lo cate the inter- action vertex within the emulsion mo dule. The reconstructed tracks were used to predict a vertex p osition with a typical precision of 1 mm in the transverse co ordinates and 7 mm along the beam. The sp ectrometer pro- vided the time-stamp ed electronic information ab out the event including the trigger, and lepton identi cation. The sp ectrometer readout was triggered by three ho doscop es placed in the emulsion mo dule region consistent with more than one charged track, and no signal from the upstream veto wall. A 6 17 total of 4:0 10 triggers were recorded from 3:54 10 protons in the tung- sten b eam dump. Tracks in the SFT were pro jected downstream to those in drift chamb ers for p ossible matches. For the tracks that were connected, momenta were measured with a 1.85 m 1.45 m ap erture magnet with a strength of 0.76 T-m. In the analysis, the sp ectrometer was used mainly to measure the momenta of muons. The momenta of other tracks in selected interactions were determined by measuring displacements due to multiple scattering in the emulsion mo dules. Muons were identi ed with an arrayof prop ortional tub es b etween 3 walls of iron (from upstream) 0.4 m, 0.9 m, and 0.9 m thick. Electrons were tagged by electron-pair creation in the emulsion, shower development in the SFT, or the lead glass calorimeter, Triggers were selected for an emulsion searchby requiring that a vertex 4 reconstructed from tracks be within an emulsion target. In addition, the event trigger timing between two counters was required to be within 10 ns, and show no evidence of nearby showering from muon interactions. Remain- ing events with a visible energy more than 2 GeV and a vertex in the emulsion volume were classi ed as neutrino interaction candidates. A total of 898 such candidates were identi ed from the sp ectrometer information compared to the exp ected numb er of 1040 200. Of the 898 reconstructed neutrino interaction candidates, 698 had a pre- dicted interaction vertex within a ducial emulsion volume that included 80% of the total target mass. There were additional requirements of event top ology and predicted vertex precision to make the sample amenable to au- tomated scanning. This reduced the number of events that were scanned to 499. The rst step in lo cating an interaction vertex was scanning a small area in the relevant emulsion plates. The tracks recorded in a plate with angles 400mradwere recognized by an automated system, the Track Selector [5] [6] and stored on disk. The typical recorded volume was 5 mm 5mm 15 mm, centered on the predicted vertex p osition. This information gave 3-D charged particle tracking segments in eachemulsion layer. The 3 co ordinates and 2 angles de ning each segment were recorded for later analysis. There 4 were approximately 10 track segments found in eachemulsion plate. Tracks that were identi ed as passing through the volume were used for plate-to-plate alignment and were eliminated as candidate tracks from a neutrino interaction. The distance of closest approach between any two tracks that started in the same or neighb oring emulsion plate was calculated and those within 4 mwere retained as candidates to form a two-trackvertex.
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