the muon system of the hera-b detector

Darmstadt 16.10.06 Yu.Zaitsev 1

The Muon System of the HERA-B Detector

Yu.ZaitsevITEP, Moscow


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


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 directionEach wire could be moved independently into proton beam

halo

C, Ti & W wires were used

DetectorThe spectrometer had a forward geometry,

covering from 15 to 220 mrad – bending planeFrom 15 to 160 mrad – vertically

Main systems:Vertex detector, Inner Tracker, Outer Tracker,

RICH, ECAL & Muon System


HERA-B Detector


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


Performance Requirements (2)

• Muon system should detect muons in the momentum range 5-200 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


Momentum range & occupancies

Muon momentum spectra from the decays B0→J/ψK0

s→μ+μ-K0s for

different part of the Muon System at z=19 m

Track density in MU1 & MU4 (5 int)


Side view of the Muon System


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.


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 00 and ±200 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

(00 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


Detector Modules (1)

• Three kinds of detectors made up the Muon System: tube, pad & pixel chambers

• “Fast” gas mixture: Ar/CF4/CH4(74:20:6)

• Tube chambers – closed-cell proportional wire chambers

Cell dimensions – 14.2x12 mm2 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


Schematic view of the Tube

chamber


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 cm2 for Mu3 and

10.8x12.8 cm2 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


Schematic view of the Pad Chamber


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 mm2 for Mu1-Mu3 and 9.4x9.4 mm2 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)252Results of calculations with GARFIELD


Schematic view of the Pixel Chamber

Positioning of Pixel Chambers around beam

pipe

Mounting plate for wiring of Pixel Chamber module


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 188000 • 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


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 00 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


Aging studies

Three different methods had been performed for aging studies

1. Laboratory tests with strong Fe55 & Ru106 sources2. Test with using intensive α-particle beam in Karlsruhe3. 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/CF4/CH4(74:20:6)hep-ex/0107080 hep-ex/0111077 (78) NIM,A494(2002)236 NIM,A515(2003)202 hep-physics/0403055


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

ResultsAr/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) + 500 ppm of H2OCollected charge ~ 400 mC/cm

Wires with HV=2.6 kV and 2.65 kV were deadGas gain loss for wire with HV=2.55 kV : δG/G ~ 50%

Ar/CF4/CH4 (67:30:3) + 1000 ppm of H2OCollected charge ~ 400 mC/cm

HV 2.25 kV and 2.55 kV Partial recovery of gas gain for HV=2.6 kVNo efficiency drop was seen for wires with HV=2.5 kV


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


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%


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%


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


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


Study of muon misidentification

Muon misidentification was investigated with the clean pion, kaon and proton samples obtained with the RICH detector

and using K0->π+π- and Λ->pπ- decaysIt was found that isidentification probability (η) dependent on cutoff

on muon likelihood and momentum of particles: . π+ π- K+ K- p p^

<mom> Lhmu=0.1 0.030 0.030 0.010 0.010 0.003 0.002 . Lhmu=0.9 0.012 0.014 0.004 0.004 0.001 0.001

Lhmu>0.1 π+ p=15 GeV/c η=0.026 p=50 GeV/c η=0.004 π- 0.028 0.003 K+ 0.012 p=35 GeV/c 0.005 K- 0.012 p(p^) p=25 GeV/c 0.003 p=75 GeV/c < 0.001


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


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


Published HERA-B results with muons

During run 2002-03, 164 million dilepton-triggered events (μμ/ee)

were recorded

Containing about 300000 J/ψ mesons: about 160000 μ+μ-

Physics results:B→J/ψ+x→μ+μ-x EPJ,C26(2003)345

PR,D73(2005)052005

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/0607046

the muon system of the hera-b detector

Documents

muon systemyu

pairs of muon

ecal muon system yu

gevc system

muon momentum cutoff

tracker system chambers

beam pipe wall

proton beam haloc