1. First Performance studies of jet rejection using a pixel based detector
Michele Pioppi*
on behalf of PRIN _005 group
*Università degli studi di Perugia &
INFN Perugia
7th International Conference on Large Scale Applications
and Radiation Hardness of Semiconductor Detectors
Firenze, 05 October 2005
RD05-Firenze, 5 Oct 2005
Michele Pioppi-Univ.&INFN Perugia

2. Michele Pioppi-Univ.&INFN Perugia
Outline
Motivations
Pixel detector overview
Pixel performances
Trigger strategies
Results
Plans & Conclusions
RD05-Firenze, 5 Oct 2005
Michele Pioppi-Univ.&INFN Perugia

3. Michele Pioppi-Univ.&INFN Perugia
Motivations I
LHC
p-p collider
Low luminosity
L = 2x1033 cm-2s-1 -> ~108 events/s
High Luminosity
L = 1034 cm-2s-1
Mostly due to QCD background processes
RD05-Firenze, 5 Oct 2005
Michele Pioppi-Univ.&INFN Perugia

4. Michele Pioppi-Univ.&INFN Perugia
Motivations II
Main goal of LHC and CMS:
Discovery of Higgs boson
DAQ rate ~ 100 Hz
4*105 rejection power
2 trigger levels:
L1 (40 MHz  100 kHz) HW
Use of fast subdetectors
HLT (100 kHz  100 Hz) SW
Calorimetry Hgg
CMS
Muon chambers H4 m
No L1trigger for multi hadronic jets final states ttH8 jets
PIXEL COULD HELP!!!
RD05-Firenze, 5 Oct 2005
Michele Pioppi-Univ.&INFN Perugia

5. Michele Pioppi-Univ.&INFN Perugia
Pixel Overview
3 barrel layers : r ~ 4.4cm
r ~ 7.3 cm
r ~ 10.2 cm
|h|<1.6
endcap
barrel
53cm
ROC
8x8 mm2
Each ROC is a matrix of 52x80 pixel units(150x100 mm2) arranged in 26 doublecolumns.
ROC has a fast output that provides:
ROC address
# hit doublecolumns
# clusters of doublecolumns
16 x
RD05-Firenze, 5 Oct 2005
Michele Pioppi-Univ.&INFN Perugia

6. Pixel performanceses at full granularity
How to build Pixel Jets :
Pt measurement from pixel tracks
1)Pixel tracks from fit to triplets of pixel hits
Good Pt meas. for Pt<10GeV
2) Vertex reconstruction
3) Pixel jets with at least 5 tracks
Leading track (seed for jet) r
Pixel tracks
Rejected track
()jet= 
Σi i·Pti
Σi Pti z
()jet= 
Σi ηi·Pti
Σi Pti ZV
(Pt)jet = Σi Pti
RD05-Firenze, 5 Oct 2005
Michele Pioppi-Univ.&INFN Perugia

7. Full granularity L1 calo/pixel jets comparison
Studies performed on bb 100 GeV sample
Test sample used to exploit algorithm performances pt f
<Njet>=2,2 h
Pixel Jet  
PixelJet
L1 calo jet
<Njet>=1,7 Et f h
L1 reconstruction:
+ cone algorithm 0.5
+ Et>10 GeV
+ eta, phi bin = 0.087
RD05-Firenze, 5 Oct 2005
Michele Pioppi-Univ.&INFN Perugia

8. Full granularity resolutions
Geometrical associations between b partons and pixeljets
Dz vtxrec-vtxsim
Dh pixeljet-parton
Df pixeljet-parton
sh ~ 50 mrad
sf ~ 50 mrad
sz ~ 47 mm
To be compared to 120 mrad spatial resolution of L1calo jet
Improvement of jet finding & fake jet rejection
RD05-Firenze, 5 Oct 2005
Michele Pioppi-Univ.&INFN Perugia

9. Full granularity selection performances
+ Try to combine calorimetric and pixel information
+ Evaluate selection power on the ttH VS direct QCD production
Fully hadronic final state (t->b, W->qq’ H->bb) 8 jets in the final state
Soft jet (20-30 GeV)
The pixel jet could help
All events that can be considered not-signal from pp interaction
We expect best performance on 3 and 4 jet-stream L1 selection
RD05-Firenze, 5 Oct 2005
Michele Pioppi-Univ.&INFN Perugia

10. Full granularity combining calo and pixel trigger
L1 QCD rate (Low luminosity)
3 jets stream
Ncaljet(E>Etr) >1,2,3,4
(Ncaljet(E>Etr) >1,2,3,4) && (Npixjet(E>Etr) >1,2,3,4)
Considering the same QCD rate, the ttH efficiency gain is 20-40%
Each pixel jet with at least 5 tracks
RD05-Firenze, 5 Oct 2005
Michele Pioppi-Univ.&INFN Perugia

11. Full granularity Pileup rejection
Primary vertex finding is an essential tool to reduce secondary p-p interaction
Approach:
L1 calo/ pixel jet association
Consider for L1 calo trigger, only jets that can be associated to a PV built with pixel
RD05-Firenze, 5 Oct 2005
Michele Pioppi-Univ.&INFN Perugia

12. Full granularity pixel vertex
Jets vtx in 22mm PV window
Jets vtx out of 22mm PV window
Vertex width~22 mm
10*ptqual
(PVpixel-PVtracker) before cuts
(PVpixel-PVtracker) after cuts
Selection:
Ptqual>0.9
Dr(cal,pix)<0.5
RD05-Firenze, 5 Oct 2005
Michele Pioppi-Univ.&INFN Perugia

13. Reduced granularity Idea:
Use fast output of PixelROC to determine a L1 decision
Worst scenario:
At L1 only 52x80 pixel(8x8 mm2)
geometry is available .
RD05-Firenze, 5 Oct 2005
Michele Pioppi-Univ.&INFN Perugia

14. Reduced granularity hit selection and track reconstruction
ROCi= ROC of ith layer
ROCHit selection
#DoubleColumnROC1>1
#DoubleColumnROC2-3>0
ROCTrack preselection
|fROC1 –fROC2 | <0.3
|Z0| from ZROC1 and ZROC2<15 cm
|Z0| from ZROC1 and ZROC3<15 cm
ROCTrack construction
Linear and circular fit (Pt,h,f,Zv,)
Tracks c2-ordered
Only ROC1 could belong to more than 1 track
<ROCHit>qcd80120 ~ 800
<ROCTracks>qcd80120 ~ 120
RD05-Firenze, 5 Oct 2005
Michele Pioppi-Univ.&INFN Perugia

15. Reduced granularity Validation and angular resolution
Study performed with a tighter preselection
x-y plane
Pixel jets
ROC Hits
ROC Tracks
L1 Calo Jet
sf =142 mrad
bb jet 100 Gev events
RD05-Firenze, 5 Oct 2005
Michele Pioppi-Univ.&INFN Perugia

16. Reduced granularity Vertex reconstruction
Algorithm:
Histogram of Zv of all ROCtracks (Width of the bin = 1cm)
Ntr(Pt>0.3GeV)>4
PVz =<Zroctk> of the
most populated bin
# vertices
Primary Vertex resolution
tth
qcd5080
qcd80120
sz ~ 1.6 cm cm
RD05-Firenze, 5 Oct 2005
Michele Pioppi-Univ.&INFN Perugia

17. Reduced granularity ROCtracks in L1 trigger
L1 trigger efficiency
@ Etresh ~ 70 GeV
Effqcd <0.1%
Tth
qcd5080
qcd80120
New L1 request:
Ncalo (Et>Etresh) > 3 && Npixtr>90
Next step:
Build ROCjets pointing at PV and use them at L1
RD05-Firenze, 5 Oct 2005
Michele Pioppi-Univ.&INFN Perugia

18. Reduced granularity what is needed?
Module trigger chip to combine fast output from all the ROC
HW to build ROC tracks
Pattern recognition (Associative memory)
Histogramming vertex finder
RD05-Firenze, 5 Oct 2005
Michele Pioppi-Univ.&INFN Perugia

19. Reduced granularity alternative approach - strategy
From ROC -> seed in f -> region finder
Regional full granularity (DR=0.2) reconstruction
Tracks are used for direction, PVz and IPz
sf ~ 100 mrad
qcd80-120
75% of events:
2 seeds found in DR<0.5 wrt MC
RD05-Firenze, 5 Oct 2005
Michele Pioppi-Univ.&INFN Perugia

20. Reduced granularity alternative approach - resolution
s ~ 140 mm
80% Pv finding efficiency
Good resolution on IPz
For b-trigger application
RD05-Firenze, 5 Oct 2005
Michele Pioppi-Univ.&INFN Perugia

21. Michele Pioppi-Univ.&INFN Perugia
Plans and conclusions
Plans:
Timing evaluation
HW feasibility
Conclusions:
Pixel could contribute to L1 trigger especially for multihadronic jets final states
At full granularity pixel can increase significantly ttH8jets efficiency maintaining the same QCD rate
Reduced granularity:
In the worst scenario (8x8 mm2) <ROChits>qcd ~800
Each ROChit is characterized by position(layer,phi,z), #doublecolumns and #clusters
Tracks ~120 and vertices can be reconstructed at L1
L1 trigger efficiency for QCD reduced
ROC could provide a seed in f for a regional full reconstruction
RD05-Firenze, 5 Oct 2005
Michele Pioppi-Univ.&INFN Perugia