# Presentation: 123939

## Physics Implications of the AFE II Upgrade

- Mike Hildreth
- with
- Juan Estrada, Charly Garcia, Bruce Hoeneisen
- in consultation with the hardware experts

## Questions

- To narrow the focus we are trying to answer two questions:
- What is the physics cost of *NOT* doing the AFE upgrade?
- Model all effects of the current system plus all those that degrade the tracker performance at high luminosity
- Use *b*-tagging performance in *Z**bb* events as a benchmark
- Studies of luminosity cost for top mass, Higgs searches
- What are the benefits of adding timing information to CFT hits?
- Realistic model of timing with known electronics behavior
- provide performance basis for previous tracking studies

## Overview

- Summary of CFT simulation details
- “basis of estimate” for all results
- signal generation
- degradation at high luminosity
- min-bias model
- Results for Question 1: Cost of not doing the upgrade
- Simulation of timing information from the Trip-T
- Results for Question 2: Benefits of the upgrade

## Default CFT Monte Carlo

**Default CFT Monte Carlo**

- Energy Deposition of Geant particles, mapping to fibers, photon generation:
- Photons ordered by arrival time at VLPC, converted to photo-electrons
- Total charge calculated, including sampling distribution

- single Geant volume

- All of this is incorporated in the “Digitization” portion of the CFT MC

## Modifications to the CFT Simulation

- The results presented here are based on a new version of the CFT digitization code that includes:
- realistic light yields
- realistic gain spectra
- realistic thresholds
- small amount of pedestal noise to simulate threshold spread
- (readout thresholds have one setting for 64 channels)
- This simulation provides the “Current CFT” detector model
- (One other deleterious effect missing: tick-by-tick pedestal shifts)
- Not simulated: any degradation of SMT, improvement from Layer 0

## Modifications for High-Lumi CFT

- For purposes of comparison, we have created a “CFT 2007” simulation to model the “un-upgraded” AFE system at high luminosities.  This includes:
- degradation of CFT light yield by 18% from radiation damage
- VLPC gain is lowered by 18% due to high singles rates
- (total of ~30% signal loss)
- SVX pedestal shifts as a function of discriminator occupancy
- at 32% occupancy (linear with occupancy)
- SIFT1: -1pe, SIFT2: -0.5pe, SIFT3: +1pe, SIFT4:+3pe
- higher readout threshold (1.5  2.5 p.e.)
- this would be implemented to fight the occupancy caused by the pedestal shifts
- Saturation of SVX response according to bench tests

## Minbias Simulation

- Total Pythia cross section for latest tune is 59 mb
- includes single- and double-diffractive events

- comparison of minbias data taken at low luminosity with Pythia events
- “proper” extraction of occupancy, subtracting zero-bias and correcting for multiple interactions
- occupancy for only those (single) MC events that pass minbias trigger
- Here, a mean of 12 min-bias interactions/crossing corresponds to approximately =3x1032/cm2/s

## Simulation Results: Saturation Effect

- ttbar events interspersed with “single neutrino” events to simulate saturation effects vs. beam crossing number
- Poisson <N> minbias events overlaid on this pattern
- same 1000 ttbar events used in each sample
- Drastic reduction of track multiplicity, number of CFT hits/track; lumi-dependent
- effect much worse at higher luminosities

- Number of Tracks per beam crossing

- Number of CFT Hits per beam crossing

- ttb+jjb

- Mean Number of Tracks per Beam Crossing

- Mean Number of CFT Hits/Track per Beam Crossing

## Simulation Results: Single Hit Efficiency

- Degraded events have more clusters (~10% at highest luminosities), produce up to 40% more tracks per event
- CFT Clusters are split as effective light-yield drops, resulting in many narrow or single-fiber clusters.
- Due to lower light yield, higher thresholds to mitigate the effects of SVX pedestal shifts, loss of hits due to saturation

- ttb+jjb

## Simulation Results: b-tagging

- Efficiency of saturated samples relative to “current CFT” identical events
- this is relative efficiency at fixed luminosity
- B-Tag: one “loose” and one “tight” tag, Sec. Vtx. Algo.
- ttbar: b-tagging useful to reduce combinatorics in jet pairing for mass measurement
- ZH: b-tagging necessary for background suppression
- Drastic reduction in efficiency with saturation effects

- ttb+jjb

- ZHbb

## Luminosity Cost of Status Quo

- The efficiency drop vs. luminosity can be convoluted with the luminosity profile of an average run to calculate the effective loss in luminosity due to AFE effects
- (~20% drop in tracking efficiency for single muons in top events not included)
- Huge reduction in effective luminosity from degradation in CFT performance due to AFE I
- up to 40% of post FY05 luminosity is compromised

- ZH

- top

- design

- top: 33% loss

- ZH: 40% loss

## Improvements: Trip-T Timing Studies

- “Real” CFT MC used as a basis for detailed studies of photo-electron charge deposition and timing:
- “standard” CFT simulation has always included the generation and propagation of individual scintillation photons
- directions, capture in fiber, reflection off of polished end, etc.
- waveguide lengths/losses, quantum efficiency, etc., also simulated
- statistical sampling of charge generation
- Time over discriminator threshold is encoded with ADC (or p.e.) information so it is available to Reco
- individual photons sorted by their arrival time
- all photons from all MC particles hitting a fiber are generated before any charge deposition simulation takes place
- proper simulation of charge deposition with “real” thresholds
- “Time” is set when total charge crosses the channel threshold

## Trip-T Timing Studies II

- Motivations for this study:
- Allows “confirmation” of expected time resolution
- previous studies based on toy MC without real detector geometry, photon propagation, etc.
- Study timing performance, time “purity” of hits in high-occupancy events
- evaluate prospects for time-based track reconstruction without rewriting the entire tracking code first
- validate earlier studies of tracking timing improvement
- Event samples: 15 high-pT muons per event
- zero minbias baseline
- 3, 6, 9, 12 minbias overlay
- see effects of increasing occupancy on timing info

## Timing Data in CFT Monte Carlo

- sigma of time resolution for single muon tracks is 1.8- 2.2 ns
- close to expected value
- expected evolution of resolution with  of track
-  (z)=2 ns * 18cm/ns = 36 cm

## Timing Data in CFT Monte Carlo

- Expected correlation (“time-walk”) of low-ADC signals clearly seen in simulation
- can be corrected on average for better timing resolution for all hits
- not done yet for these studies

## Z position Reconstruction

- Track Z position compared with time of hits (no minbias overlayed here)
- Only hits with large charge deposition (>10 p.e.) are used to avoid “time-walk” effects
- works as expected
- “wiggle” due to timing effects from reflected photons (poorer resolution beyond z=0)

- axial fibers

## Timing Results at Higher Occupancy

- Clearly, the width of clusters increases as occupancy goes up
- same events, same muon tracks, higher occupancy
- must be from background hits
- What happens to time resolution?

## Time Resolution vs. Occupancy

- For events with 9mb overlaid, timing (z) resolution vs. CFT layer.
*not* ruined by occupancy
- outer layers fare better than inner layers, as expected

- rms = 28.2cm

- rms = 28.0cm

- rms = 24.9cm

- rms = 26.2cm

## Good vs. “Spoiled” Hits

- time residuals for good hits (top), spoiled hits (from other MC particles, bottom)
- many “spoiled” hits are close in apparent z position to correct hits
- some tails
- (hatched area is 98% efficiency for good hits)
- (muon events, 12mb)

## Good vs. “Spoiled” Hits

- calculated z position vs. time for “good” hits (matched to MC track) and “spoiled” hits (contaminated by other MC particles)
- Spoiled hits are often close in true z position, or at least correlated
- less true on inner layers
- (muon events, 12mb)

- “spoiled” hits

- “good” hits

## Timing in Zbb events

- Zbb, 12 minbias overlaid
- time resolution of hits on found tracks
- note tails on inner layers
- outer layers still relatively clean
- core resolutions similar in all cases, slightly worse than single muons

- Layer 1

- Layer 3

- Layer 8

- Layer 6

## Comments on Tracking Improvements

- Guennadi Borissov’s Tracking Study (DØ Note 4497):
- Done with 25cm resolution in *z* position of hits (cut at 3)
- c.f. ~40cm
- Done with 15 minbias events
- Done by assigning the *z* position of the first photon in the fiber to that hit (no time-over-threshold simulation)
- Occupancy effects included
- Used for forming 2-D (axial+stereo) hits only
- no attempt to “break” clusters that have fibers with different times associated with them
- resulted in a 40% reduction in cpu time, with no eff. loss
- Conclusions would not be very different with current MC

## Comments on Tracking Improvements

- AFEII has several additional benefits for tracking algorithms:
- improvements in single-hit resolution by forming CFT clusters only from fibers that are contiguous in time
- Better ADC information:
- removes saturation effects on ADC
- removes pedestal shifts
- removes split pedestals
- removes tick-to-tick variation
-  recoups much of lost tracking, b-tagging efficiency
- May allow cluster-splitting, or better single hit resolution using charge sharing

## Conclusions

- Current SVX-SIFT AFE will seriously degrade the tracking performance at high occupancies
- Saturation is the dominant effect
- 40% loss of tracks at highest luminosities
- effects worsened by pedestal shifts and required high thresholds
- Bottom line: up to 40% effective loss of luminosity due to lower efficiencies
- Trip-t timing *will* be useful for pattern recognition
- will eventually be limited by multi-hit fibers
- outer layers can still benefit even to the highest luminosities
- one more handle to combat high occupancies
- 40% improvement in Reco time = $$$$ in computing

## Backup Slides

## Degradation in Tracking

- Loss of high-pT muons in top events:
- degradation of single-hit efficiency due to saturation effect, higher thresholds, begins to effect high-pT isolated tracks

## Necessary Software Modifications

- Unpacking:
- unpack_ft_fe
- AFE-board classes
- cft_unpdata
- raw  fiber space
- sftdigi_evt
- SFTDigiChunk
- Monte Carlo
- sftdigi
- CFTReadoutSim
- packer for MCRaw
- sftdigi_evt
- SFTDigiMCChunk

- Reco:
- cft_evt
- CftClusterChunk
- d0om packing
- CftClusterMCChunk
- cft_reco
- Clustering algorithms
- (Tracking Algorithms)
- Databases:
- t0 per channel or sector
- additional constants

## More Timing Results

- Timing cut: efficiency vs. rejection for various layers, multi-muon events, 9mb events overlaid (12mb looks similar, slightly worse)
- here, “fake” hits are those on found tracks which are caused by another MC particle besides one of the muons

- Layer 1

- Layer 4

- Layer 8

- (.9,.32)

- (.9,.42)

- (.9,.74)

## Z  bb Event Simulation

- Several samples generated so that the progressive addition of defects could establish a clear trend
- Same 1000 Zbb events used for each sample
- Here, 12mb corresponds roughly to 3x1032 instantaneous Lumi
- Samples:
- “default” MC, 0 minbias overlay	     = “perfect”
- “default” MC, 12 minbias overlay	     = “messy”
- “realistic” 2004 CFT, 12 minbias	     = “current + messy”
- “bad sift” 2007 CFT, 12 minbias	     = “degraded + messy”
- “saturated” 2007 CFT, 12 minbias	     = “worst performance”

## b-tagging results

- SVT Tagger per-jet forward-tag efficiencies for all B jets:
- no discernible difference in backwards tags
- up to 40% loss in relative efficiency compared with 2004 MC
- AFEII recoups about 75% of this efficiency loss

| Sample: | Loose Tag | Tight Tag |
| --- | --- | --- |
| Default MC, 0mb | 0.416 | 0.322 |
| Default MC, 12mb | 0.418 | 0.321 |
| 2004 CFT MC, 12mb | 0.404 | 0.318 |
| Bad SIFT MC, 12mb | 0.377 | 0.287 |
| Saturated SVX, 12mb | 0.335 | 0.217 |

## b-tagging results

- JLIP Tagger per-jet forward-tag efficiencies for all B jets:
- no discernible difference in backwards tags
- up to 15% loss in relative efficiency compared with 2004 MC
- AFEII recoups about 75% of this efficiency loss

| Sample: | Loose Tag | Tight Tag |
| --- | --- | --- |
| Default MC, 0mb | 0.460 | 0.364 |
| Default MC, 12mb | 0.460 | 0.371 |
| 2004 CFT MC, 12mb | 0.442 | 0.360 |
| Bad SIFT MC, 12mb | 0.438 | 0.343 |
| Saturated SVX, 12mb | 0.409 | 0.308 |