# $\tau \rightarrow e$ misidentified as $\tau_h$

1. Aug 29, 2015

### ChrisVer

I am just looking for a verification of logic, and maybe some feedback?
I was thinking how the following decay:
$\tau \rightarrow e \nu_\tau \nu_e$
can be misidentified by the detector (let's say ATLAS) as a hadronically decaying tau (and so the electron seen as a jet).

First of all I don't think that the tracking system can help in distinguishing an electron from a single charged pion (1prong tau decay). That's because they same charge and for momenta p ~GeV they are both indistinguishable (highly relativistic). So in the tracking system it's easy to lose that information...
Then I'm looking at the calorimeters....and here is the point that confuses me....
The distinguishment can be applied by looking at jet energy deposits in the HCAL and matching it to possible energy deposits in the ECAL. The electron is not supposed to give a signal in the HCAL and so we can make sure that the charged object is a hadron.
However this logic rules out any kind of misidentification, so how does this happen in real life....? Is it because the electron's EM showers can be over-extended and so misidentified as jets? I am attaching a picture of my logic...

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2. Aug 29, 2015

### Staff: Mentor

Electrons emit more bremsstrahlung and transition radiation than pions. The calorimeters are better, however.

Calorimeters are never perfect, and you never have only the tau decay. You get energy deposits from other particles from the same primary vertex (underlying event) or from other collisions (pileup), even effects from the previous bunch crossing (out-of-time pileup). In addition, the calorimeters have some random noise. Some electromagnetic showers can penetrate more than usual just by chance, and some hadronic showers can start earlier.

3. Aug 29, 2015

### ChrisVer

yup I agree with those points.... is it bad that I don't realize how these lead to the detector ID the electron as a jet, and assign that electron event to a jet event?
Are the jets IDed in the ECAL energy deposits?

4. Aug 29, 2015

### Staff: Mentor

The amount of energy in the hadronic calorimeter is an important point in the electron identification.
This would be the opposite direction, however - the electron selection applied to pions. The pion and jet selection is usually simple - most things are pions or jets anyway.

5. Aug 29, 2015

### ChrisVer

Yup I found some references in which my idea was somehow reconfirmed about getting rid of electrons... (there are the methods of using the calorimeter alone, or calorimeter + tracking information)...
Also there is another method which uses the $Z \rightarrow ee$ tag&probe method to identify the electrons that were falsely IDed as jets...although I haven't found a good source where it explains how this is done.... are they looking for the one electron and expect to see if they will find the second (correct ID) or not (the 2nd got falsely IDed as jet) ?

6. Aug 29, 2015

### Staff: Mentor

This is just a method to estimate their number, it does not help to reduce the background.
The basic idea is to use electron+electron and jet+electron samples and to fit the size of the Z peak in both. Their relative number gives the fraction of electrons in the jet selection.