Reconstruction and Identification of Hadronic Decays of Taus using the CMS Detector

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


Outline

• The CMS detector

• Physics with hadronic at CMS

• Hadronic reconstruction

• Hadronic identification

• trigger

• Conclusions


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


Physics with SM Higgs

• qqqqH() VBF

Forward jet tagging

Central Higgs decay products to trigger


Physics with MSSM Higgs

Neutral Higgs (A.H,h) Charged Higgs (H±)

5 discovery potential

Production mechanism


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


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


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.


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


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


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.


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


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


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%


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


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


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


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

Documents

Reconstruction and Identification of Hadronic Decays of Taus using the CMS Detector