`First ep events in the Zeus micro vertex detector in 2002`

Transcription

1 Amsterdam 18 dec 2002 `First ep events in the Zeus micro vertex detector in 2002` Erik Maddox, Zeus group 1

2 History (1): HERA I ( ) Lumi: 117 pb -1 e +, 17 pb -1 e - Upgrade (2001) HERA II ( ) Goal: 4*250pb -1 (e +/-, with longitudinal polarization) ZEUS Upgrade Improve tracking - Tagging of charm, beauty (also tau) with secondary vertexing or impact parameter methods - Build and install silicon microvertex detector (impact parameter ~100 micron) --> Large NIKHEF contribution 2

3 History (2): Start up 2002 difficult Beam related backgrounds much higher than expected Started begin Nov: `lumi-period` establish stable running (with lower beam currents) (re)comissioning of detector/trigger and new components 3

4 Introduction Scope of the talk: Focus on microvertex detector - reminder of the detector - integration in data taking and event reconstruction Cosmic runs - estimate impact parameter resolution Effect of background on MVD 2002 nice ep events 2002 data studies - de/dx of microvertex tracks - Primary and secondary vertices - K0s signal in data 4

5 Zeus microvertex detector Geometry: (barrel) Total 30 ladders fit in the barrel most tracks will pass 3 cylinders, (giving 6 hits) Ip Elliptic beam pipe (height 5 and width 12 cm) 5

6 Zeus microvertex detector Geometry: (wheels) 4 wheels in the forward (proton) direction. Wheel has two layers of silicon Total 712 silicon sensors: m thick - single sided 20 m strip pitch m read out pitch - analog read out 6

7 Zeus microvertex detector Cosmic runs (1): (Using the new ZEUS event display: zevis) Energy loss in calorimeter (CAL) Cosmic muon Measure hits and tracks in central tracker (CTD) For vertex detector (MVD) -> zoom in 7

8 Zeus microvertex detector Cosmic runs (2): MVD track fit is done using Kalman filter, developed by Nikhef group. Clear track in MVD is found Also shown are noise hits and the 'ghost' hits 8

9 Zeus microvertex detector Cosmic runs (3): Only the track hits are shown 9

10 Zeus microvertex detector Cosmic runs (4): ~100k clean cosmic events collected this year - used in alignment study (in progress) - <50 microns deviation from nominal position impact parameter study - consider cosmic as two independent tracks and study how well they match. Track 1 Z Track 2 10

11 Zeus microvertex detector Cosmic runs (5): Plot the distribution of the difference of z 0 coordinates of the 2 tracks. σ CTD = 3.27 mm For MVD cosmics impact parameter is then ~0.16 / sqrt(2) = 110 microns (without alignment corrections) σ MVD = 0.16 mm 11

12 Zeus microvertex detector Impact parameter precision: Simulation of Zeus MVD with design geometry including all detector materials Track fit is done using Kalman filter developed by Nikhef group. Single track MC study - High p: hit precision is dominant - Low p: multiple scattering dominates (1/p) 12

13 Zeus 2002 running Backgrounds: Background comes from: Proton-beamgas Off-momentum positrons Back scattered synchroton radiation --> What does this mean for the microvertex detecor? Limiting factor: Currents in the central tracker become to high Particles backgrounds give high rates and large occupancy, leads to deadtime and bad data taking efficiency Effect increase with beam currents 13

14 Zeus 2002 running Radiation monitoring: I) Instantenious radiation measurement (16 Si pin diodes positioned just outside beampipe ) - generate alarms and beam dumps - used by HERA for machine optimisation II) Integrated radiation measurement (8 radfets) Expected dose of krad for 5 years Measured dose ~ krad Beam loss Increase during normal operation Electronics: small increase in noise of modules close to the beam (~1%) Silicon: small increase in leakage currents observed Work is done to estimate the effective radiaton dose on the MVD from this 14

15 Zeus 2002 running Example: p-gas Large energy deposition in CAL Many hits and tracks in CTD Protons, 920 GeV Z-axis Enormous number of hits in MVD 15

16 Zeus 2002 running Example: off-momentum e + Electro-magnetic cluster in CAL (fakes a scattered positron) Positron, 27.5 GeV Many hits in +x side of MVD (particle showers in beampipe and MVD) 16

17 Zeus 2002 running Backgrounds and the MVD: ep events (DIS) in yellow - in center of detector - MVD occupancy is small Proton-gas background - the secondary particles hit collimator at -75 cm - large tail in number of MVD hits Z vertex [cm] High data size of background events slows down data aquistion, trigger (deadtime) and event reconstruction. --> Let's find some nice events Hits in MVD 17

18 Zeus 2002 running Physics events in the MVD (1): 18

19 Zeus 2002 running Physics events in the MVD (2): 19

20 2002 data analysis MVD de/dx: Take sample of good events and good tracks correct measured signal in silicon for pathlength plot average signal vs track momentum pion, kaon and proton band seen! From 2500 dis candidate events 20

21 2002 data analysis Primary/Secondary vertexing: Nikhef vertexing package Run vertex finder without beam constraint, plot all vertices in xy-plane. Primary vertex and interactions in the beampipe are seen. 21

22 2002 data analysis K 0 --> π + π - : Take all secondary 2 track vertices with opposite charged tracks. At least 4 MVD hits per tracks. Assume tracks have pion mass Calculate invariance mass Cut on kaon Pt: >0.25 From 4400 dis candidate events Then require kaon to point back to 1 st vertex (less background) 22

23 Summary Zeus microvertex detector fully integrated in data taking and event reconstruction. > 100K of good cosmic events recorded. Already good tracking precision obtained. Background events cause large occupancy in microvertex detecor. Clean ep events are found. No evident radiation damage observed `Luminosity runs` are used for detector comissioning and developing of new analysis methods for future running. - Primary vertex/secondary vertex - K0 peak Hunt for charm signal with microvertex detector has started 23

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25 Click Final to statement add title Many presentations at this meeting will be about the challenging and exciting future we have in 2007 or 2012 or whenever the future experiments start. But if you like to do physics and you are not afraid of challenges and you do not want to wait so long, why not join the Zeus experiment? Deze doe ik niet hoor! 25

26 Zeus microvertex detector Basic principle: 300 m thick n-type Si wafers, with p-type implanted strips every 20 m 120 m Read out strips are every 120 m. Using capacitive charge sharing the analog readout provide position resolution < 20 m 300 m 26

27 Zeus microvertex detector Signal readout: (half-module) 512 channels per sensor Z coordinate 4 * helix analog readout chip R coordinate (coupled to Z channel) Multiplexed signal (4*) to FADC (daisy chain) then to disk/trigger HV/LV/clock and control 27

28 Zeus microvertex detector Geometry: (ladder) (In reality!) 2 mirror imaged half-modules on top of each other form a module, inner and outer strips perpendicular. (charged track gives 2 hits) 5 modules are placed on stiff carbon fiber ladder (~ 6 * 60 cm 2 ), electronics, cooling & cabling mounted on the ladders 28

29 Zeus microvertex detector Geometry: (overall) Rear: place for tubes and cables Barrel: 30 ladders (in 3 cylinders), total 600 silicon sensors Forward: 4 wheels (each with 28 sensors) Wheel sensors: same technology wedge shaped Inner and outer sensor with crossing angle of 26 o 480 strips/sensor no coupling of inner and outer strips 29

30 Zeus microvertex detector Installation: (march 2001) Inside central tracker Large NIKHEF contribution (design, mechanics, electronics, testing, and event reconstruction) since 1997 Detector works fine, work is now done on fine tuning Proof it is really there! 30