25 sep 2008 [email protected] 1/18
Reconstruction and Identification
of Hadronic Decays of Taus
using the CMS Detector
Michele Pioppi – CERN
On behalf of the CMS collaboration
TAU 08
Novosibirsk – Russian Federation
25 September 2008
25 sep 2008 [email protected] 2/18
Outline
• The CMS detector
• Physics with hadronic at CMS
• Hadronic reconstruction
• Hadronic identification
• trigger
• Conclusions
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The CMS detector
||<2.4Muon
||<3.0 barrel
||< 5.0 forward
HCAL
||<3.0ECAL
||<2.4Tracker
CoverageResolution
%5.0%3
E
E
%4%100
E
E
TT
P
PPT
%51
TT
P
PPT
%10
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Physics with SM Higgs
• qqqqH() VBF
Forward jet tagging
Central Higgs decay products to trigger
25 sep 2008 [email protected] 5/18
Physics with MSSM Higgs
Neutral Higgs (A.H,h) Charged Higgs (H±)
5 discovery potential
Production mechanism
25 sep 2008 [email protected] 6/18
Physics with SUSY
5 discovery potential
01
02
~~~~ qqqq In mSUGRA models, light mass SUSY can be discovered soon in di- final states through the decay chain
25 sep 2008 [email protected] 7/18
reconstruction strategy
1. Particle Flow reconstruction High efficiency, low fake rate and optimal
resolution for each kind of particle
2. Common selection used as a basis for all the final states
Robustness wrt unexpected detector effects, high reconstruction efficiency and sufficient QCD background rejection
3. Sophisticated identification Suitable and tunable reco and id algorithms
for each individual analysis
25 sep 2008 [email protected] 8/18
The particle flow algorithm
Particle Subdetectors
Muons TK-Ecal-Hcal-Mu system
Electrons TK-Ecal-Hcal
Photons TK-Ecal
Charged hadrons TK-Ecal-Hcal
Neutral hadrons TK-Ecal-Hcal
Particle Flow consists in identifying and reconstructing each particle in an event followed by the best possible determination of the energy and direction, by including the information of all CMS subdetectors.
Jet, tau and missing transverse energy reconstruction is then made from these reconstructed and calibrated particles directly.
25 sep 2008 [email protected] 9/18
Pre-selection Sample Entries
Signal Z 250K
Background QCD(22) 750K
Jet PT >15 GeV/c
Lead track PT >5 GeV/c
Lead track cone R<0.1
An iterative tracking approach (allows to have good tracks with only 3hits) is significantly improving the leading track finding
25 sep 2008 [email protected] 10/18
Isolation algorithm
All the decay products are expected to be in a narrow signal cone around the leading track.
If the is isolated an isolation annulus, expected to contain little activity, is defined.
candidates with charged particles (Pt>1GeV/c) and neutrals (Pt>1.5GeV/c) in the isolation cone are rejected
25 sep 2008 [email protected] 11/18
Shrinking vs fixed cone
In both the cases the Isolation cone = 0.5
CDF implements a 3D signal cone that shrinks as a function of jet E-1,while historically CMS uses a fixed signal cone (R=0.07)
The shrinking cone is defined in -plane and scales as E-1 to be extended in the forward region.
25 sep 2008 [email protected] 12/18
Common selection performance
The shrinking cone algorithm improves signal efficiency at the cost of increasing QCD fake rate.
The range affected by the cone algorithm is between 20 and 60 GeV/c
25 sep 2008 [email protected] 13/18
Secondary background sources
• The common selection is aimed to fight the main source of background (QCD jets)
• Secondary sources are in order of importance:– Electrons
Due to the high material budget in the tracker, several electrons often emit a significant fraction of energy by radiation. A special treatment is needed to reduce electron contamination
– PhotonsPhotons convert frequently, and the isolation is much more difficult for such photons (under study).
– Muons Very high identification efficiency
25 sep 2008 [email protected] 14/18
Electron rejection
• A veto is applied to all the tracks pre-identified as electrons in the particle-flow
• The electron pre-identification is aimed to identify electrons (isolated and within jets) in a wide range of transverse momentum, pseudo-rapidity and physics case
• The algorithm uses a multi-variate analysis of information from calorimeters and tracker(more efficient for electrons emitting high-energy Bremmstrahlung photons)
• Eff(e)>95%• Eff()=5%
25 sep 2008 [email protected] 15/18
Electron rejection
The main electron source of background for isolated are isolated electrons.For such electrons the rejection can be improved by using inclusive calorimetric information.
EEcal= sum of the cluster energy in a window (around the extrapolated impact point of the leading track) ||<0.04 and <0.5 in the direction of the expected brem photon deposition
EHcal= sum of the cluster energy in a window (around the extrapolated impact point of the leading track) in a window R <0.184
25 sep 2008 [email protected] 16/18
trigger
The hadronic trigger is crucial for final states with a single (e.g H±)
Level1 trigger relies on pure calorimetric information
Three different paths dedicated to have been designed
25 sep 2008 [email protected] 17/18
trigger
• HLT is composed by 3 steps(Lvl2, Lvl2.5,Lvl3) of increasing complexity– Lvl2 is based on jet reconstruction and isolation– Lvl 2.5/3 is based on tracking seeded by the jet direction
Efficiency for SingleTau path
25 sep 2008 [email protected] 18/18
Summary
• physics program in CMS is ambitious• A common and robust selection for hadronic
has been developed– (Pt>40 GeV/c) eff > 50% – QCD (Pt>40 GeV/c) eff<3%
• Sophisticated identification to reduce secondary source of background– Z eff 92% – Zee eff 1%
• Dedicated trigger for hadronic in place to achieve the physics goal