Alignment methods for the OPERA drift tube detector B. Büttner a , J. Ebert a , T. Ferber a,c , C. Göllnitz a,n , D. Goloubkov b , C. Hagner a , M. Hierholzer a,d , A. Hollnagel a , J. Lenkeit a , I. Rostovtseva b , W. Schmidt-Parzefall a , B. Wonsak a , Y. Zaitsev b a Institut für Experimentalphysik, Universität Hamburg, D-22761 Hamburg, Germany b ITEP Moscow, Bolshaya Cheremushkinskaya 25, 117218, Moscow c Deutsches Elektronen Synchrotron (DESY), Notkestr. 85, 22706 Hamburg, Germany d Laboratory for High Energy Physics, University of Bern, Switzerland article info Article history: Received 21 March 2013 Received in revised form 17 January 2014 Accepted 4 February 2014 Available online 15 February 2014 Keywords: Alignment OPERA Precision Tracker Drift tube Neutrino Gas detector abstract The goal of the OPERA experiment is to give the first direct evidence for neutrino oscillations in the channel ν μ -ν τ . The OPERA detector is designed to observe the appearance of tau neutrinos in the originally pure muon neutrino CNGS beam. An important part of the magnetic spectrometer is the Precision Tracker (PT), a drift tube detector consisting of 9504 drift tubes. Its main task is the determination of the muon charge and momentum. The alignment strategy for the PT consists of two parts: the hardware measurement by theodolite and the software alignment using long muon tracks. In this paper, the hardware and the software alignment are described, and the track-based alignment methods are explained in detail. Results of the software alignment are presented with a focus on the analysis of cosmic particles. & 2014 Elsevier B.V. All rights reserved. 1. Introduction The OPERA experiment is designed to study neutrino oscillations in the CNGS beam in the channel ν μ -ν τ [1]. The OPERA detector (see Fig. 1) consists of two identical super modules (SM), each composed of a target area with emulsion cloud chambers and plastic scintillators and a muon spectrometer with dipole magnets, resistive plate chambers (RPC) and the drift tube detector called the Precision Tracker (PT). The PT is composed of 9504 drift tubes of about 8 m length, vertically arranged in 12 planes [2]. The PT provides the measurement of muon charge and momentum [3]. Its main task is to suppress the background to the signal decay (τ  -μ  ν τ ν μ ) arising from topologically similar one-prong decays of charmed particles c (ν μ N-cμ  Y -μ þ Xμ  Y ) with misidentified charge of the final state μ þ [4]. According to the design specifications, the sign determination significance has to be at the 4s-level for all beam-induced tracks, corresponding to a momentum resolution Δp=p of better than 0.25 for momenta up to 25 GeV/c [2]. The resulting requirement for the precision of the final alignment of the PT is about 250 μm [2]. Furthermore, a precise alignment is important, especially for the analysis of high-energy particles such as cosmic muons that can be measured with a deep-underground detector such as OPERA. This paper describes the alignment-related considerations that affected the design and the procedures of mass-production and installation. The entire alignment strategy (both hardware and software) of the OPERA PT for achieving the design precision is summarized, and all involved methods and accomplishments are explained. 2. PT geometry and hardware alignment Each SM of the OPERA detector includes 6 planes of vertical drift tubes: 2 in front of the magnet, 2 behind the magnet and 2 in-between the magnet arms. These planes are composed of 15 or 17 modules. Due to the vertical drift tube orientation, only a two-dimensional projection of the track can be measured. Fig. 1 shows a drawing of the OPERA detector and illustrates the arrangement of the PT planes inside the detector. It is giving the global coordinate system. Fig. 2 shows the top view scheme of a super module including 6 PT planes arranged in 3 doublets around the magnet arms. Doublets are two neighbouring planes without a magnet arm in-between that are used together to reconstruct a straight track segment. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/nima Nuclear Instruments and Methods in Physics Research A http://dx.doi.org/10.1016/j.nima.2014.02.011 0168-9002 & 2014 Elsevier B.V. All rights reserved. n Corresponding author. E-mail address: christoph.goellnitz@desy.de (C. Göllnitz). Nuclear Instruments and Methods in Physics Research A 747 (2014) 56–61