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Improving trigger for B  J/ (e + e - )K s

Improving trigger for B  J/ (e + e - )K s. So far, only highest P T L0-electron has been of interest. Is there a point in looking at the 2 nd and 3 rd highest P T electrons?

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Improving trigger for B  J/ (e + e - )K s

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  1. Improving trigger for BJ/(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. Aras Papadelis

  2. 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? Aras Papadelis

  3. 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: Aras Papadelis

  4. 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 Aras Papadelis

  5. 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. Aras Papadelis

  6. 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? Aras Papadelis

  7. Applying charge tag • Not very impressive results, we cut the high mass by selecting low E-object? Aras Papadelis

  8. NEXT STEPPT-cut on electrons. • We expect the J/-electrons to have a high PT • A study of the MC data confirms this. Aras Papadelis

  9. 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. Aras Papadelis

  10. 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 Aras Papadelis

  11. 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 Aras Papadelis

  12. 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 Aras Papadelis

  13. 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… Aras Papadelis

  14. Summary • In ~ 30 % of BJ/(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… Aras Papadelis

  15. Outlook • Use VELO tracks to improve E measurement and charge tagging. • Add lost energy from clones to improve energy measurement. Aras Papadelis

  16. … 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. Aras Papadelis

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