PHENIX Drift Chamber operation principles

1. PHENIX Drift Chamber operation principles Modified by Victor Riabov Focus meeting 01/06/04 Original by Sergey Butsyk Focus meeting 01/14/03

2. Outline • Drift chamber (DCH) design • Construction and assembling • Operation principles • Calibration aspects • Trackfinding principles • Future goals

3. Drift Chamber for PHENIX(basic information) • Main purpose: • Precise measurement of the charged particle’s momentum • Gives initial information for the global tracking in PHENIX • Acceptance: • 2 arms 90º in f each • ±90 cm in Z • 0.7 units of h • Location: • Radial :2.02<R<2.48 m • Angular: • West: -34º < f < 56º • East : 125º < f < 215º

4. Made of titanium Conists of 20 identical keystones Weight ~ 1.5 tons Total tension of wires ~ 3 tons Keystone Wires DCH Frame Drift Chamber design

5. Lay-out of one keystone • 6 radial layers of nets (X1,U1,V1,X2,U2,V2) • Stereo nets start in one keystone (n) and end in the neighbouring keystone e.g. (n+1) for U, (n-1) for V • The tilt of UV nets along f allows measurement of Z component of the track

6. Jet-type wire structure X nets: 12 anode wires UV (stereo) nets: 4 anode wires Total of 80 anode nets of each type are evenly distributed in f ~6400 readout channels in every DC Independent signal readout from both sides (North, South) Wire net configuration

7. Mechanical design and production – PNPI (Russia) Front Electronics – SUNYSB Wire net production, assembling - PNPI,SUNYSB Construction and assembling

8. To reconstruct charged particle track DC samples a few points in space along the path of the particle. One such point is called a “HIT” Registration of one HIT is based on a few physical processes: When charged particle transverse the gas volume of the DC it creates clusters of primary ionization on its way Electrons of primary ionization drift from the point of ionization to anode wires along electric field lines Electrons of primary ionization create avalanches in the vicinity of anode wires Back drift of posistively charged ions generate measurable signal on anode wires which is amplified, shaped and discriminated To register a HIT in the DC: Carry out drift time measurements: Start - collision time measured by BBC; Stop - time when signal appears on the anode wire Drift time (t) can be tranformed into drift distance (x) if calibration curve is known x = x(t) Working gas is chosen to have an uniform drift velocity in the active region  linear xt relation can be used x = Vdr · t DCH Operation Principles

9. 50% Ar - 50% C2H6 mixture is chosen for operation based on: uniform drift velocity at E~1 kV/cm High Gas Gain Low diffusion coefficients In Year2 ~1.6% Ethanol was added to the mixture to reduce ageing and improve HV properties of the nets Gas mixture choice

10. Necessary electric field configuration is created by 5 groups of wires: Cathode Wires – Create uniform drift field between anode and cathode Field Wires – Separate adjacent anode wires and help to control gas gain Back Wires – Stop drift from one side of the anode wire Gate Wires – Localize the drift region width Termination Wires – Help to reduce boundary affects, make gas gain uniform along the plane Back Gate Field Anode Cathode Drift field configuration

11. Here is what happens when the charged particle passes through the wire cell Note that only even wires collect charge due to the back wires that block the odd anode wires ! Back wires solves left-right ambiguity problem Drift Field Configuration (II)

12. Single wire efficiency~ 90-95% Back efficiency (probability to get hit from the back closed side)< 10% Spatial Resolution~ 120 um Angular resolutionda/a ~ 1 mrad DCH Performance (Run03)

13. Idea: Substitution of real calibration dependence with a line: x(t) = Vdr·(t-T0), where T0 - effective time Vdr – effective drift velocity in the drift region Once we know these parameters <-> we know everything! Calibration aspects

14. Timing distribution for each arm have a characteristic shape By fitting the leading and trailing edge of the distribution with Fermi function we obtain time at ½ height. t0 and t1 t0 is assumed to be global T0 for the arm Vdr = <distac>/(t1-t0) is global drift velocity t1 t0 Time Global calibration

15. Slewing corrections – dependence of measured arrival time on the signal width Shape of the drift region – the wires close to the mylar windows experience distortion of the electric field Wire-by-wire t0 corrections – includes geometrical shifts of the anode wires within the net, electronics channel-by-channel variations e.t.c Plane-by-plane Vdr corrections – Vdr is electric field dependant, thus it changes significantly on the side-standing wires where edge effects distorts the field strength Global alignment to the vertex – center of the arm can be shifted from the vertex location. Translation of the arm center can be found by centering a distribution of the vertexes around zero in field-off data Other calibration effectstaken into account

16. Pick hits of the track on 3 neighboring wires (i.e. 1,3,5). Residual for wire #3 is: x3 = (x1 + x5)/2 – x3 Residual is equal to ZERO if hits are perfectly aligned Basic idea of local calibrations is to center all residuals at zero for all the parameters (i.e. align ever three neighboring wires) Residual distributions

17. Look at residuals vs. width Slewing corrections

18. Study residuals shifts within one net We can determine t0 shifts that zero the mean of residuals distributions Wire-by-wire t0 corrections

19. Shape of the drift region • The shape of the drift region was simulated in GARFIELD and parameterized in the offline software

20. Tracking principles • Main assumptions: • Track is straight in the detector region • f and a variables defined on the figure • Use hough transform – calculate f and a for all possible combinations of hits and bin those values into hough array – 2D histogram on f and a • Look for local maxima in hough array that surpass the threshold

21. Track Candidates • The results of the hough transform are track candidates • First we look for tracks with X1 and X2 hits • Remaining unassociated hits goes into X1 only and X2 only tracking • Finally we left with the following tracks • Z coordinates of tracks are dedined by PC1-UV-vertex tracking

22. Final results

23. Future goals • Calibration of the detector is a main contributor to the momentum resolution  need to improve the absolute calibration methods • Use outer detector’s matching • Online Calibration • Improve HV stability over the run • Control gas mixture properties during the run • Improve UV reconstruction