tracking with the

Th.Naumann, DESY Zeuthen, HERMES Tracking meeting, 27.03.20031

Tracking with the

Silicon Detectors

Th.Naumann H1 DESY Zeuthen

• A short collection of experiences

• What would I/we do better now ?


FST CST BST

The H1 Si detectors :


• 5 APC’s with 128 channels = 640 channels per disk

• CST: double sided double metal 2 planes• BFST: single sided

• 12/16 phi sectors with overlap• u/v planes back-to-back with crossing angle

• FST: 2 r planes for redundancy• BST: trigger pads

u/r u/v

CST

FST

BST

H1 Si detectors :


BST CST FST

Planes u/v r phi z min.

pitch/um 75 96 50 88 hits

FST 5 2 3BST 6 0 2 incl. bwd calorimeterCST 2 2 2 picked up by jet chamber

The Silicon Trackers


FST CST BST

A high Q2 NC DIS event in the Forward Silicon Tracker

• E jet > 100 GeV @ x > 0.1• E e > 100 GeV @ Q2 > 10.000 GeV2

=> high track densities => real redundancy => u/v + r planes !


• x > 0.1 => E jet > 100 GeV

• high track densitiesand multiplicities

• need real redundancy

• add r to u/v planes

u/v ghost hits ...

... rejected by r detectors


A low Q2 scattered electron in the BST :• mostly only 1 scattered lepton or• some hadrons @ low x < 10-3• not much redundancy needed => u/v planes only• incl. bwd calorimeter + vertex: also use 2 hit tracks (acceptance gain!)


Track finding before

pattern recognition to reject : • noise and• background• create track candidates

Track fitting :

• optimum track model• constraints• outlier rejection


From charge to space information :

Online readout Power PCs calculate :

• strip wise :• pedestal + noise

• APC wise for 128 channels fit coherent readout problems :• common mode + slope + ... parabola ?

Decisive and tricky procedure to get bestSignal / Noise

• dead strips e.g. don’t follow common mode => pseudo-noisy + destroy common mode

Coherent readout losses => external inefficiency !

Monitor occupancy, noise, S/N, sagitta, efficiencies as early as possible !

Clustering of strip charges by sliding window in 2 steps:

• in Power PCs with loose S/N cuts• in reconstruction with tight cuts


BFST r detectors:

double metallization (as in CST)

• S/N ~ 13• pat rec has to fight noise• cut into efficiency = > 92 %

BFST u detectors:

simple single sided

• S/N ~ 30• nearly noise free• efficiency ~ 99 %


Si+1Si

• s = (x1 + x3) / 2 - x2

• zero if no curvature• constant on a circle : si = si+1

• easy to calculate:can work combinatoricallyif nr of measurements small(HERMES recoil)

• biasfree: no cuts + assumptions

• monitors detectorwise :• mapping• alignment• noisy channels

Pat Rec: Triplet Sagitta

x1

x2

x3s


s1

s2

HERMES recoil measurement:

E < 1 GeV:

model dE/dx by decreasing sagitta

si+1 < si < s i-1

heavy background:

ask for samecircle origin


Track model: 2D or not 2D ... ?

• solenoid field => circle in x-y , linear in s-z (s = arc length)

• H1 Central Tracker :• Jet Chamber: r-phi + z chambers: r-z + CST• precisions in r-phi and r-z differ by 100 =>• 3D=2D+2D: circle fit in x-y , linear fit in s-z

• H1 B/FST:• precision alignment dominated => similar in x-y-z• now: combine u-v to x-y:

• dead u disk kills v disk• no detectorwise residuals for alignment + efficiency =>

• try: directly fit measured quantities in a u-v-z track model !

• HERMES recoil: ....


Use all constraints :• vertex:

• L2 : large gain in precision

• H1 x-y run vertex: y < 30 m (better than HERA prediction)

• H1 z event vertex: used to reject FST x-y ghost tracks

• momentum conservation:• HERMES: pt or phi balance in elastics ?


Improve momentum by dE / dx ?


• Si: double sided/metal vs single sided ?

• guarantee real redundancy

• get best S/N: fight for pedestal, noise, common

mode

• clearly separate pattern recognition + track

fitting

• design robust pattern recognition

• track model: think twice

• use all constraints

• Good luck, HERMES !

Summary

tracking with the

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