1. 15/11/11 LHCb Liverpool Meeting
Top update
By Henry Brown
15/11/11 LHCb Liverpool Meeting

2. 15/11/11 LHCb Liverpool Meeting
Reminder
Searching for t t-bar in:
t t-bar -> W+ b, W- b-bar, W+ & W- decay leptonically to mu & e
Main background Z-> tau tau
Also background contributions from punchthrough, charge misidentification, etc
Cross section ~ 170 pb (from ATLAS, CMS, and NNLO calculations)
15/11/11 LHCb Liverpool Meeting

3. 15/11/11 LHCb Liverpool Meeting
Event Rate
Acceptance from Pythia ~= 1.7%
Branching ratio is ~= 4.5%
0.045*0.017*170*1000 = 130 top candidates in LHCb acceptance (before efficiencies – which arent 1!) (data collected)
Z->tau tau -> mu e is 10x higher event rate
15/11/11 LHCb Liverpool Meeting

4. 15/11/11 LHCb Liverpool Meeting
2011 Data
Lumi used ~= 400 pb-1
15/11/11 LHCb Liverpool Meeting

5. 15/11/11 LHCb Liverpool Meeting
2011 Data vs Angle
Mass (MeV)
Nice Z peak!
Delta Phi (electron, muon)
15/11/11 LHCb Liverpool Meeting

6. Signal Angular Distribution (from MC)
Flat in Delta Phi! Z-> tau tau cuts are > +/- 2.7 radians, so we lose ~16-20% of our signal from cutting out Z peak
Delta Phi (electron, muon)
15/11/11 LHCb Liverpool Meeting

7. 15/11/11 LHCb Liverpool Meeting
Mass vs Delta Phi
Mass (GeV)
Delta Phi (electron, muon)
15/11/11 LHCb Liverpool Meeting

8. Mu_E Invariant mass (from Pythia)
Please note: This is not normalized for expected event rate in 1 fb-1 - expect only 15% of the number of events above. This is for mu_e in our acceptance
15/11/11 LHCb Liverpool Meeting

9. All Mu_E Invariant Mass
This is for all mu_e combinations – most of these (97.3%) will be outside of acceptance
15/11/11 LHCb Liverpool Meeting

10. 15/11/11 LHCb Liverpool Meeting
Mu E + 1 b-tag
For n number of events with mu and e in acceptance, we expect ~65% of them to have a b in acceptance as well!
-> Should be much cleaner
15/11/11 LHCb Liverpool Meeting

11. 15/11/11 LHCb Liverpool Meeting
Mu E + 1 b-tag inv mass
1 fb-1 of data we expect ~120 events, with 50% b-tagging this drops to ~65 (including events with two b jets where only one is tagged)
15/11/11 LHCb Liverpool Meeting

12. 15/11/11 LHCb Liverpool Meeting
Mu E + 2 b-tag
~835 candidates
~460
~90
Probability drops off quickly – 12% of mu_e signal events have 2 b-tags
15/11/11 LHCb Liverpool Meeting

13. 15/11/11 LHCb Liverpool Meeting
Mu E + 2 b-tag inv mass
1 fb-1 of data we expect ~18 events in our acceptance. With 50% efficient b-tagging, this drops to approximately 4.5.
15/11/11 LHCb Liverpool Meeting

14. 15/11/11 LHCb Liverpool Meeting
Some backgrounds work
Some very specific (but rare) backgrounds can fake our entire signal, no matter how harshly we cut:
Have been working to generate these (in large enough event samples) and put them through LHCb software
Note: These all have associated light flavour jets too!
15/11/11 LHCb Liverpool Meeting

15. 15/11/11 LHCb Liverpool Meeting
Future work
Kurt is continuing b-tagging work
With this search for t-tbar should be much easier
With this, we expect O(75) top candidates in 1 fb-1
Working to understand potential backgrounds for this channel
Also a suggestion of just looking for mu or e + 1 b-tag. Will increase event rate a lot
15/11/11 LHCb Liverpool Meeting