1. CERN Summer Student Session
PH - CMM
Università di Torino (Italy)
Serena Oggero
August 14th, 2007
CERN Summer Student Session

2. CERN Summer Student Session 2007
Outline
The CMS High Level Trigger
My physics channel
My work on CMS software with MC data
First results: efficiency measurements
Summary
Serena Oggero
CERN Summer Student Session 2007

3. CERN Summer Student Session 2007
The CMS HLT
CMS detector
(40 MHz)
What?
CMS online event selection code
L1 Trigger
(100 kHz)
Why?
most of events produced at LHC are not interesting
the interaction rate is larger than what can be recorded
Custom-made Hardware
HL Trigger
(100 Hz)
Pc farm
CMS Software
How?
takes events already accepted by the first level trigger (L1)
decides whether the event should be kept, based on sequences of more elaborated algorithms
...so any offline analysis depends on the HLT efficiency…
Serena Oggero
CERN Summer Student Session 2007

4. CERN Summer Student Session 2007
My physics channel
140 < MH < 700 GeV
One of the most promising channel
H  Z Z*  l+l- l+l-
and I focused on
H  Z Z*  e+e- e+e-
How it is seen by the CMS detector:
Serena Oggero
CERN Summer Student Session 2007

5. CERN Summer Student Session 2007
My work (I)
Event sources
(Montecarlo)
Set of complete events of:
H  Z Z*  l+l- l+l- Higgs mass 170 GeV
H  Z Z*  e+e- e+e- Higgs mass 190 GeV
Filter Module
selection of at least 2 e- and 2 e+ per event
acceptance cuts: pt > 5 GeV and |η| < 2.7
to preserve electron reconstruction efficiency
Serena Oggero
CERN Summer Student Session 2007

6. CERN Summer Student Session 2007
My work (II)
Analyzer
module
- Offline electron reconstruction in several steps:
starting from the SuperCluster
using information from pixel detector, tracker, HCAL
- Run HLT Egamma paths: each path iterates over a collection of reconstructed physics objects and applies cuts
Root Ntuple
Serena Oggero
CERN Summer Student Session 2007

7. CERN Summer Student Session 2007
HLT table for L = 1032cm-2s-1
HLT paths
used
Single Electron
Double Electron
Relaxed Single Electron
Relaxed Double Electron
L1 seed
A_SingleIsoEG12
A_DoubleIsoEG8
A_SingleEG15
A_DoubleEG10
|η|
< 2.5 Et
< 15 GeV
< 10 GeV
< 17 GeV
< 12 GeV
Tk isol
< 0.06
< 0.4
HCAL isol barrel
< 3 GeV
< 9 GeV
HCAL isol endcaps
E/P barrel
< 1.5 -
E/P endcaps
< 2.45
..relaxed paths only differ for Et thresholds that are matched with L1..
Serena Oggero
CERN Summer Student Session 2007

8. Results: efficiency tests
Efficiency measured on 2 MC samples (~730 events for mH = 190 GeV and ~370 events for mH = 170 GeV):
mH = 170 GeV
Single Electron
Relaxed Single Electron
Double Electron
Relaxed Double Electron
Logical OR
Absolute
96,0%
96,2%
92,2%
95,0%
(98,4±0,7)%
On L1 seed
93,2%
HLT_events
L1_events
ε =
HLT_events
All_events
ε =
Single Electron
Relaxed Single Electron
Double Electron
Relaxed Double Electron
Logical OR
Absolute
97,6%
97,3%
95,8%
97,4%
(98,9±0,4)%
On L1 seed
95,9%
mH = 190 GeV
Serena Oggero
CERN Summer Student Session 2007

9. Main sources of inefficiency
L1 double seed (double electrons)
Pixel match (single and double electrons)
Electron E/p cut (single and double electrons)
Checking HLT report:
Serena Oggero
CERN Summer Student Session 2007

10. CERN Summer Student Session 2007
Some H  4e plots
Pt and eta spectrum for events chosen by my first Filter:
Serena Oggero
CERN Summer Student Session 2007

11. Many thanks to Roberta Arcidiacono and Nadia Pastrone!
Summary & Outlook
Written modules for HLT studies on H4e channel
efficiency measured on two different MC samples
results look promising…
…still to do:
improve Ntuple structure for a more detailed study
study signal and background rates
work with higher statistics and other Higgs mass points
Many thanks to Roberta Arcidiacono and Nadia Pastrone!
Serena Oggero
CERN Summer Student Session 2007