The ATLAS B-trigger - exploring a new strategy for J/  (ee)

1. The ATLAS B-trigger - exploring a new strategy for J/(ee) • A brief introduction to B-physics on ATLAS • The ATLAS B-trigger – an overview • A new trigger for Bd J/(e+e-)Ks

2. B-physics on ATLAS • LHC is expected to run at a 'low' luminosity during its first years, ~2 x 1033 cm-2 s-1good opportunity to measure CP-violation in the B-system with ATLAS before the higher design luminosity is reached. A high lumi degrades the B-layers that are necessary for B-physics. • The asymmetry in Bd J/ K0s can give us a clean measurement of sin(2) in the unitarity triangle. It is expected that the reconstructed background will be relatively low. • J/ (mm) decay is easy to reconstruct.J/(ee) is harder due to bigger background and bremsstrahlung losses. Since rate of CPV is the same , it is important to measure both decay modes, as a cross check (to identify systematic errors in measurements, etc) and to increase the statistics.

3. The ATLAS trigger • At 40 Mhz and low lumi, ~ 5 events / bunch crossing. O(108 Hz) • Event size ~ 1 MB • Maximum output to tape ~ 100 MB/s, O(102 Hz) • Trigger reduction factor of 106 needed.

4. Level 1 • Level 1 (LVL1) – hardware triggerMaximum output rate of 75 kHz.Uses reduced granularity measurements from calorimeters and muon stations to form decision. • Output consists of Regions of Interest (RoI) to LVL2.

5. High Level Trigger • LVL2 – software trigger with output rate of ~ 1 kHz • Uses software algorithms and full granularity in calorimeter to validate LVL1 objects like muons and EM clusters. • Full-scan of Inner Detector (ID) for tracks. OR • Use RoIs from LVL1. Scan in limited volume of ID. • Event Filter – output rate of ~ 100 Hz. Uses offline algorithms to refine LVL2 selection.

6. The trigger costs money... • Previous trigger studies have already covered Bd →Je+e-)KS with promising results. • But... due to recent developments (increased start-up lumi, less money to DAQ, possibilites of reduced detector at start-up), a new trigger strategy is needed. • Previous trigger strategy for Bd →Je+e-)KS : • LVL1: with pT > 6 GeV (MU6), |h|<2.4 • LVL2: perform a full-scan of the Inner Detector to find interesting tracks. • Gives good efficency but requires a lot of computing power, which costs money.

7. A new ATLAS B-trigger strategy • A RoI-guided trigger is suggested instead. • Different strategies are being considered, one of them for Bd →Je+e-)KS is: • LVL1: require MU6 (or MU8) + at least 1 EM cluster (RoI) • LVL2: Only scan for tracks in regions around the RoIs. only ~10 % of the ID has to be searched = less computing power needed • If only one RoI is found in LVL1, an extended scan around the RoI is needed to locate both electrons.

8. Can this be efficient? Mean: 3.0656 • In the new strategy, at least one of the electrons in a signal event has to be detected by calorimeter at LVL1. • On the other hand, if the multiplicity in background events is too high, a lot of of resources are needed anyway. • A look at the signal and bg RoI-multiplicities give promising results. Bd→J/(ee)KS

9. Know your enemy - background events in LVL2 • At LVL2 we will (ideally) only deal with events of typeb bbar→ mX. Events that pass LVL1 will satisfy a pT-cut and an h-cut. sbg=s( b bbar→ mX | pT(m) > 6 GeV, |h| < 2.4 )≃ 2.3 mb • BR(b →J/ ) ≃ 0.01 • BR(J/ →ee) ≃ 0.06Þs ( b →J/ (ee) X) ≃ 2.3 b· 0.01 · 0.06 = sbg · 6·10 -4 • Of the events that pass LVL1, only a very tiny fraction (6·10 -4) are events we're looking for, so the signal trigger efficiency has to be maximised while the background retention has to be minimised.

10. How much can background is tolerated? • The output of LVL2 is ~1 kHz, this is to be shared between all decay channels that ATLAS is looking for. • If Bd →Je+e-)KS is allowed to occupy ~1% of this bandwith, i.e. 10 Hz, we can estimate the upper limit of the background retention.With L = 2 x 1033 cm-2 s-1 we find that the LVL1 rate for b-events will be 4.6 kHz. To get a LVL2 output of ~10 Hz we need to suppress the background retention to the order of eLVL2,bg≃ 0.1 % • Is this possible without 'killing' the signal efficiency?

11. What can we do at LVL2? • The main theme of the LVL2 trigger: • Scan for tracks in a volume around the EM RoIs (e.g. ΔΔ=0.20.2) • Select tracks that match certain criteria (cuts) • Calculate invariant mass for combinations of tracks from different RoIs. • If the invariant mass is sufficiently close to the nominal mass of particle we want to trigger on, the event is chosen to pass to the next trigger level, the Event Filter. • In this trigger, the electrons from Jare used for the invariant mass plot. (Ks is reconstructed at a later stage)

12. Trigger tools • To reduce the number of tracks, one studies different properties of the real J-electrons. This results in a list of cuts applied to each event. • PT-cut on RoIs and tracks • Cut on angle between different tracks, i.e. , ΔΔ or ΔR(Δ2+Δ2) • Cut on ratio Ecal/Ptrack (me << peEp) • Demand different charge for the tracks

13. An example of E/p-cut • E/p  1 does not only indicate that we are dealing with a light particle, it also indicates that the a correct match has been made between the track and the EM-cluster. • By for example applying some cuts on E/p we see that a great deal of the background tracks would be sorted out, while keeping most of the signal.

14. Correlations between J/ -electrons • J/ electrons are emitted in a such a way that their mutual 'angular' distance stays between 0 < ΔR < 1.5 • Combinations of background tracks span the entire spectrum. • By applying an appropriate cut, there is a lot to gain: • Combinations between background tracks are rejected • Choosing ΔR > 0 we ensure ourselves that no combinations of the same track occur.

15. Electron identification • The transition radiation tracker (TRT) is able to provide e/π-separation. TRT-tracking is time and resource consuming. Unclear if this can be used in LVL2. • Electron identification can be done instead by looking at different shower shape variables in the ECAL. • Harder to separate e/π

16. Results so far • Problems with ATLAS software have not yet permitted a proper trigger study. Only offline algorithms (for tracking, calorimeter etc) have been available so far. • Preliminary studies based on offline algorithms have been done, indicating that this particular strategy gives quite a low efficiency but also a low background retention:eLVL2, signal ~ 25 %eLVL2, background ~ 0.1 % • The low efficiency primarily comes from scanning for tracks in a too small part of the ID. This is a problem, but it can hopefully be solved anyway.