4/8/03 Aras Papadelis

Improving trigger for BJ/(e+e-)Ks

So far, only highest PT L0-electron has been of interest. Is there a point in looking at the 2nd and 3rd highest PT electrons?

Yes! As in J/ there might be a possibility to use the electrons to reconstruct the invariant mass of J/. If this can be done successfully, it can increase the L1 trigger efficiency for B J/(e+e-) Ks.

4/8/03 Aras Papadelis

Topics of Interest

Properties of the three highest PT electrons Clones – their origin and how to get rid of

them Charge tagging – essential for mass

calculation ET-cuts Bremsstrahlung recovery J/-mass reconstruction Ways of improvement?

4/8/03 Aras Papadelis

Properties of L0 electrons (1)

1st, 2nd and 3rd highest PT electrons are retrieved from the full list of L0 candidates.

Tagged as J/-electrons if their mother or grandmother is from a MC-J/

To remind you of L0 and offline selection rates:

4/8/03 Aras Papadelis

Properties of L0 electrons (2)

As we see, by using the 3rd candidate in the offline selected events we go from a ~ 19 % to ~ 29 % fraction with two J/-tagged electrons.

The following plots are based on 1st and 2nd .

Fraction of the different combinations of PT electrons where both electrons are J/ -tagged. Large errors on offline selection is due to poor statistics

4/8/03 Aras Papadelis

Clones – origin and exclusion

When an electron hits the ECAL close to a border between two cards the energy is shared over both cards and this sometimes manifests itself as two L0-particles.

About 10 % of all L0-candidates are clones. This is quite a common phenomenon. An analysis of the distance distribution between the clones

suggests a distance cut of 20 cm to be applied when selecting 1st, 2nd and 3rd. In practice this means: If two L0-electrons differ less than 20 cm in distance from each other, reject the

one with lowest PT. Problem remains though, the electrons with clone partners are measured to have less PT than they

actually have lower energy resolution, lower J/-mass resolution.

4/8/03 Aras Papadelis

Charge tagging

To calculate J/-mass we need to know the charge of the two electrons.

VELO tracks is a natural way to do this. Due to lack of time I instead assume that all particles with x>0 are e+ and vice versa.

Although the asymmetry isn’t big, can it still be used to enhance the resolution of the mass peak?

4/8/03 Aras Papadelis

Applying charge tag

Not very impressive results, we cut the high mass by selecting low E-object?

4/8/03 Aras Papadelis

NEXT STEPPT-cut on electrons.

• We expect the J/-electrons to have a high PT

• A study of the MC data confirms this.

4/8/03 Aras Papadelis

PT-cut on electrons (2)

As we see, this cut is efficient on getting rid of bg in m <2 GeV region.

I have chosen the second cut out of the three listed.

4/8/03 Aras Papadelis

Bremsstrahlung recovery (1)

If we study the energy resolution for J/Psi-tagged electrons we see a tail energy is measured to be too low!

Three probable reasons: - Clones - Bremsstrahlung - Saturation at 5.1 GeV

4/8/03 Aras Papadelis

Bremsstrahlung recovery (2)

How is the bremsstrahlung found? With measured electron energy, predict where

on the ECAL the photons (emitted BEFORE magnet) should be found. This gives two points, one for each charge of the electron.

Define a window in the ECAL around predicted points and scan it for photon candidates.

Window size is defined with respect to the error of the energy and position measurements of the ECAL

4/8/03 Aras Papadelis

Bremsstrahlung recovery (3)

Definitions: -Pos/Neg. Window = window around predicted point for a

positive/negative electron In ~25 % of the cases there is something in the window.

About 70 % of these are true bremsstrahlung photons. Too few too give a significant effect on the J/-mass

4/8/03 Aras Papadelis

A first look at minimum bias

Plot shows the mass for a neutral charge assumption with a PT-cut.

Too much mbias left, room for improvement…

4/8/03 Aras Papadelis

Summary

In ~ 30 % of BJ/(ee) both the J/ electrons are among the three with the highest PT

A PT-cut improves the mass peak Charge must come from tracks Few bremsstrahlung photons among L0-

candidates. Mbias bg is too big but…

4/8/03 Aras Papadelis

Outlook

Use VELO tracks to improve E measurement and charge tagging.

Add lost energy from clones to improve energy measurement.

4/8/03 Aras Papadelis

…

Thanks to Niels for all his help and support during these 7 intense weeks. Now I am not afraid anymore to try a career a particle physicist.