Darmstadt Yu.Zaitsev1 The Muon System of the HERA-B Detector Yu.Zaitsev ITEP, Moscow

Darmstadt Yu.Zaitsev2 Outline Short description of detector Design requirements for the Muon System Absorbers & beam pipe Chambers: design & performance Aging studies Muon system in the first level trigger Muon identification Physics results NIM,A461(2001)104 IEEE Tr.Nucl.Sc. 48(2001)1059

Darmstadt Yu.Zaitsev3 HERA-B was a fixed target experiment on the proton beam with energy 920 GeV at the HERA storage ring at DESY The wire target consisted of two independent stations containing 4 wires each & separated by 40 mm along the beam direction Each wire could be moved independently into proton beam halo C, Ti & W wires were used Detector The spectrometer had a forward geometry, covering from 15 to 220 mrad – bending plane From 15 to 160 mrad – vertically Main systems: Vertex detector, Inner Tracker, Outer Tracker, RICH, ECAL & Muon System

Darmstadt Yu.Zaitsev4 HERA-B Detector

Darmstadt Yu.Zaitsev5 Performance Requirements (1) Muon identification played a key role in the detection of J/ψ → μ + μ - Pairs of muon with high transfer momentum (>0.7 GeV/c) with invariant mass in the region of J/ψ provided the first level trigger for the experiment The Muon System in the off-line analysis had to reject backgrounds due to hadron misidentification with factor more then 100 The Muon System also served to select additional single muons from semileptonic decays of D and B mesons

Darmstadt Yu.Zaitsev6 Performance Requirements (2) Muon system should detect muons in the momentum range GeV/c System should have a very good coverage in the whole acceptance The intrinsic efficiency of the chambers had to be high (>98%) To avoid pile-up from different bunch crossing, the response of the muon chambers had to be faster than 96 ns The transverse segmentation of the muon chambers should ensure low occupancy & fulfill the requirements of the first level trigger System should have sufficient absorber material to keep the average punch- through probability of hadrons with momenta up to 100 GeV/c at the level less than 4·10 -3 System should provide a track slope & sufficient number of hits to allow linking with tracks in the Tracker System Chambers close to beam pipe should operate at peak doses of up to 0.2 Mrad/year

Darmstadt Yu.Zaitsev7 Momentum range & occupancies Muon momentum spectra from the decays B 0 → J/ψK 0 s → μ + μ - K 0 s for different part of the Muon System at z=19 m Track density in MU1 & MU4 (5 int)

Darmstadt Yu.Zaitsev8 Side view of the Muon System

Darmstadt Yu.Zaitsev9 Hadron absorber & Beam pipe To achieve a punch-through probability of hadrons at the level of 4·10 -3, the total thickness of absorber for central region should be about 20 interaction lengths about 3 m of iron for central region This corresponds to muon momentum cut-off of about 4.5 GeV/c Momentum spectra for outer part softer - thickness of the outer part of absorber was smaller Occupancy of muon chambers around beam depends strongly on the shape of the beam pipe. Particles can produce showers when they interact with the beam pipe wall in the vicinity of chambers. Beam pipe – four sections with radii 6.0, 6.5, 7.0 & 7.5 cm plus additional shielding – 5 cm of iron.

Darmstadt Yu.Zaitsev10 Muon Superlayers & Chambers The Muon System was segmented into four superlayers which was interleaved with iron or iron-loaded concrete absorbers Additional shielding was placed between the superlaers Mu3 and Mu4 and around beam pipe Superlayers Mu1 and Mu2 (between absorber shields) consisted of three layers of muon tube chambers with 0 0 and ±20 0 stereo angles The two superlayers Mu3 and Mu4 behind the absorber were used for muon pretrigger, for the first level trigger and in the off-line analysis (0 0 layer with pad and wire readout) Mu3 & Mu4 were separated by 1.0 m in z-direction only with thin (5 cm) absorber between them to provide clean information about position and direction of muons for pretrigger to initiate the first level trigger search Four central gas pixel chambers in each superlayer were chosen to operate in the region with the highest occupancies

Darmstadt Yu.Zaitsev11 Detector Modules (1) Three kinds of detectors made up the Muon System: tube, pad & pixel chambers “Fast” gas mixture: Ar/CF 4 /CH 4 (74:20:6) Tube chambers – closed-cell proportional wire chambers Cell dimensions – 14.2x12 mm 2 to have response of muon chambers faster than 96 ns with “fast” gas mixture. Gold-plated tungsten wire of 45 μm diameter Module of tube chambers consists of two monolayers of 16 drift cells each which were shifted by half of the cell size Results of calculation with GARFIELD

Darmstadt Yu.Zaitsev12 Schematic view of the Tube chamber

Darmstadt Yu.Zaitsev13 Pad Chambers The pad chambers were assembled using two aluminum profiles with one open side Wire readout of pad chambers was performed identically to tube chambers The pad structure were made using double Cu covered G10 boards G10 readout board were placed 1 mm above the inner walls of profile using plastic spacers There was two rows of pads with size 10x12.8 cm 2 for Mu3 and 10.8x12.8 cm 2 for Mu4 to provide projective geometry in y-direction Two opposite pads in one module were connected together – “or” Preamplifiers were mounted directly on the pads and connected with twisted pair cables to connector positioned at the second end of the chamber There was 60 pads on each module

Darmstadt Yu.Zaitsev14 Schematic view of the Pad Chamber

Darmstadt Yu.Zaitsev15 Gas Pixel Chamber Four single-layer gas pixel chambers with overlapping in every superlayer Square chamber cells were formed by one sense wire and for potential wires with a length of 30 mm, oriented along the beam direction The drift cell sizes 9x9 mm 2 for Mu1-Mu3 and 9.4x9.4 mm 2 for Mu4 to provide a signal between two bunch crossing (96 ns) One to six pixel cells were grouped as one readout channel NIM,A368(1995)252 Results of calculations with GARFIELD

Darmstadt Yu.Zaitsev16 Schematic view of the Pixel Chamber Positioning of Pixel Chambers around beam pipe Mounting plate for wiring of Pixel Chamber module

Darmstadt Yu.Zaitsev17 Readout Electronics The HERA-B detector contained several large systems requiring high gain, front-end amplifiers with discriminated (yes/no) signal Total number of channels was about Such large number dictated for cost effective solution HERA-B used ASD-08 integrated circuit developed at the University of Pennsylvania All details of this electronic can be found in paper: IEEE Tr.Nucl.Sc.46(1999)126

Darmstadt Yu.Zaitsev18 Muon Pretrigger Muon pretrigger provided a seed for the first level trigger for the first level trigger Each pad of the superlayer Mu3 was put into coincidence with six pads in superlayer Mu4 These coincidences gave position and direction information and start the algorithm of track finding in Mu1 using wire information from 0 0 planes in Mu3 and Mu4 The coincidence were vertically in a projective geometry. In this direction deflection of muon can only occur due to multiple scattering In horizontal direction, muons are deflected by the magnetic field

Darmstadt Yu.Zaitsev19 Aging studies Three different methods had been performed for aging studies 1.Laboratory tests with strong Fe 55 & Ru 106 sources 2.Test with using intensive α -particle beam in Karlsruhe 3.Aging test in HERA-B environment Taking into account these results, needed drift velocity and actually used interaction rate about 7 MHz was chosen a gas mixture Ar/CF 4 /CH 4 (74:20:6) hep-ex/ hep-ex/ (78) NIM,A494(2002)236 NIM,A515(2003)202 hep-physics/

Darmstadt Yu.Zaitsev20 Aging test in HERA-B environment Two chambers 0.5 long were irradiated in front of the muon absorber: one filled with Ar/CF4/CH4 (67:30:3) & with Ar/CF4/CO2 (65:30:5) Each chamber was divided in 5 HV zone with different HV: 2.25 kV - closest to the beam pipe; 2.5 kV – nominal HV: 2.55 kV, 2/6 kV and 2.65 kV; two zones were interleaved with a reference wire Maximum current density ~ 500 nA/cm: Gas flow two volumes per hour Results Ar/CF4/CH4 (67:30:3) without additional water Rapid aging effects for all wires irradiated at HV more than 2.5 kV starting from the collected charges about 20 mC/cm Ar/CF4/CH4 (67:30:3) ppm of H2O Collected charge ~ 400 mC/cm Wires with HV=2.6 kV and 2.65 kV were dead Gas gain loss for wire with HV=2.55 kV : δG/G ~ 50% Ar/CF4/CH4 (67:30:3) ppm of H2O Collected charge ~ 400 mC/cm HV 2.25 kV and 2.55 kV Partial recovery of gas gain for HV=2.6 kV No efficiency drop was seen for wires with HV=2.5 kV

Darmstadt Yu.Zaitsev21 Aging tests with α beam Aging test using intensive beam of α particles in Karlsruhe show a relatively fast aging on the equivalent level of the dose about 500 mC/cm It clearly seen that deposits on the wire grow in direction of the avalanche

Darmstadt Yu.Zaitsev22 Chamber performance Chamber efficiency were measured using 3 GeV electron beam at DESY Efficiency of the tube double module within gate of 96 ns was more then 99% The final efficiency measurements had been perform using muons from the decay J/ψ->μ + μ - For all working modules wire efficiency was more the 98%

Darmstadt Yu.Zaitsev23 Efficiency of the Pad chambers Efficiency of the pad chambers had been measured also using muons from J/ψ decays Average efficiency was ε=92%

Darmstadt Yu.Zaitsev24 Backgrounds for Muon Identification There are several sources of background for muon identification: Muons from hadronic showers in the absorber and hadron leakage through the muon shielding (hadronic punch-through) Muons from π/K decays-in-flight in the region between target & beginning of the muon system Accidental coincidence of track in the tracker system and hits in the muon system (including background in the hall due to neutrons and soft photons) Overlapping of hits in the muon system from real muon and extrapolation real hadron track from the tracker system (see later) Background from the proton beam pipe shielding in vicinity of chambers

Darmstadt Yu.Zaitsev25 Software for muon identification Use the main tracker tracks as seeds for muon reconstruction Tracks extrapolated to the muon detector and the hits were linked to these seeds using Kalman filter Knowledge of the momentum & track impact point allowed to calculate the multiple scattering errors precisely Muon likelihood probability was calculated using assigned hits together with errors due to wire position alignment and extrapolation errors

Darmstadt Yu.Zaitsev26 Study of muon misidentification Muon misidentification was investigated with the clean pion, kaon and proton samples obtained with the RICH detector and using K 0 ->π + π - and Λ->pπ - decays It was found that isidentification probability (η) dependent on cutoff on muon likelihood and momentum of particles:. π + π - K + K - p p^ Lhmu= Lhmu= Lhmu>0.1 π + p=15 GeV/c η=0.026 p=50 GeV/c η=0.004 π K p=35 GeV/c K p(p^) p=25 GeV/c p=75 GeV/c < 0.001

Darmstadt Yu.Zaitsev27 Background from the “muon clones” Distance between two muon candidates in MU1 (cm) Opposite sign tracks could produce a sharp “false” signal The same sign track could give a double entries in the real resonances

Darmstadt Yu.Zaitsev28 Muon “clones” (2) Two possibility to reduce this background: 1.To improve algorithm of muon identification & assign muon flag to the track with the best muon likelihood – but even in this case some wrong assignation is possible 2. To cut such combinations and introduce such losses in efficiency

Darmstadt Yu.Zaitsev29 Published HERA-B results with muons During run , 164 million dilepton-triggered events (μμ/ee) were recorded Containing about J/ψ mesons: about μ+μ- Physics results: B → J/ψ+x → μ+μ-x EPJ,C26(2003)345 PR,D73(2005) χc → J/ψγ → μ+μ-x PL,B561(2003)61 Search for D0 → μ+μ- PL,B596(2004)173 Υ → μ+μ-x PL,B638(2006)13 J/ψ → μ+μ- PL,B638(2006)407 ψ(2S) → μ+μ- hep-ex/