4. Algoritmo Particle Flow en la zona hacia delante

4.c. Algoritmo Particle Flow

4.a.i. Importancia del Analisis Particle Flow

As you know, The majority of the interesting physics processes at the Linear Collider involves multi-jet final states. One of the goals of the Linear Collider detector performance is to be able to separate W and Z boson in their hadronic decay modes.

In order to achieve this goal, the jet energy resolution of detectors (σE/E) is required to be as good as:

This is a factor two better than the best energy resolution achieved at LEP.

To this end, ILD and SiD^? detector will be have a finely segmented calorimeter optimized for particle flow analysis (PFA).

This high energy resolution has direct impact in physics senstitivity: If the Higgs mechanism is not responsible for the Electroweak Symmetry Breaking (EWSB) then the Quantum Gaussian Channel (QGC) process important,

and as you can see in the picture, this energy resolution will allow to separate W and Z jets

4.a.ii.Porque es interesante la zona hacia delante?

An application of the capability to distinguish between W and Z jets are the gauge boson pair production (e+e- → WW, e+e- → ZZ). The gauge boson are typically very low angle emited (see Marcel’s talk in 8th SiLC Meeting). The events for the ZZ production process are generated as:

The W pair production is generated as:

• 
  

If only leptonic decays of W and Z bosons are considered, the polar angle distribution of e- and e+ from both processes is extremely peaked in the forward direction. At Ecm=500 GeV^? two thirds of e- are emitted with < 30o or > 150o. At larger energy the tendency for e- to be emitted in the very forward direction becomes even more pronounced. The polar angle distribution of muons and -leptons is more central but still a considerable fraction is to be reconstructed in the forward detector: for the ZZ events, (~60 %).

Therefore, these processes form a very strong physics case for a low-angle reconstruction with great precision. So, we can say that where Particle Flow is really usefull for improvement the enegy resolution is in the Forward Region.

4.a.iii.Problemas en la zona hacia delante en ILD

Our area of study will be: theta 30-10 degrees, so the jets will pass throught:

• VTX: Vertex detector
• SIT: Silicon Intermediate Tracker
• FTD: Forward Tracking Disks
• TPC: Time Projection Chamber
• ETD: Endcap Tracker Disks
• ECAL Endcap
• HCAL Endcap
  

Some problem that we can find in the forward region are:

• Noise occupancy due to pair production which affect track reconstrucion: When the two bunches of the ILC collide, the electrons are disturbed by the electromagnetic field of the incoming bunch. Under this influence, the electrons radiate photons (an effect known as beamstrahlung). The produced photons may convert into pairs of an electron and a positron through one of the processes whose diagrams are depicted in the figure below:
  

The large majority is soft and/or emitted at low angle and are trapped in the “accumulation zone”.

• Less favourable orientation of the magnetic field,
• the amount of material along the particle trajectory,
• Abundant low momentum tracks: Tracks below a given pT curl up and “loop” through the detector, leaving through the endcap. For a Bfield of 4 Tesla and a radius of over 1.5 m, pT = 0.3 B R = nearly 2 GeV^?. A challenge for reconstruction due to Multiple Scattering
  

• the degradation of the electromagnetic resolution due to the TPC EndPlate (Which difficult the identification of e- and photons)…

For the aplication of the Particle Flow algorithm, we also will have the problem to extrapolate track from TPC to ECAL matching TPC hits with ECAL cluster using information from  the Endcap Tracker Disks (between TPC end-plate and the face of ECAL endcap)as well as take into account the energy loss in TPC endplate

4.a.iv.Aplicacion del algoritmo Particle Flow en la zona hacia delante de ILD

We are in a very preliminary status in the application of the Particle Flow Algorithm in the Forward Region, because I have been only 1 month using ILC software and still will be need to familiarize with the Marlin Framework and PFA code…) but ...

We want to study in the Forward Region the impact of different factors (related with the FTD) in the final energy resolution obtained applying PFA.

• The impact of the material in the FTD , evaluated in terms of:
  • Momentum resolution of the electrons
  • Efficiency in the electron detection
  • Conversions photons
• The impact of the track reconstruction efficiency , evaluated in terms of:
  • Track finding efficiency
  • fake track rate

Other analysis useful for the application of PFA in Forward Region:

• The impact of the EndCap Tracker Disck (ETD) in the matching track-cluster
• The impact of the position of the first disk of the FTD (closer of VTX) in the reconstruction of the track

And another interesting analysis can be the study of the impact of:

• The impact of the bremstralung background, because the noise occupancy due to pair production which affect track reconstrucion
• the size of the detector: the Radius, the TPC lenght...
• the magnetic Field: its value, its unfavorable orientation
• the characteristics of the calorimeters: the ECAL granularity, the HCAL depth…

-- Main.iglesias - 22 Dec 2008

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