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Z bb measurement in ATLAS (with ATLFAST)

Z bb measurement in ATLAS (with ATLFAST). Iacopo Vivarelli, Alberto Annovi. Scuola Normale Superiore,University and INFN Pisa. Introduction. The possibility of measuring the Z bb peak would be extremely helpful in the search for bb final state (ttHttbb, WHl ν bb, A/Hbb)

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Z bb measurement in ATLAS (with ATLFAST)

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  1. Zbb measurement in ATLAS (with ATLFAST) Iacopo Vivarelli, Alberto Annovi Scuola Normale Superiore,University and INFN Pisa

  2. Introduction The possibility of measuring the Zbb peak would be extremely helpful in the search for bb final state (ttHttbb, WHlνbb, A/Hbb) The “golden” channel for the b calibration in ATLAS is PT balance in bZbμμ events at present (easy to trigger, high purity). The Zbb sample can be a very important complementary tool to cross check b-jet calibration. Due to the huge QCD background (tens of mb of cross section), the reconstruction of such a signal is difficult. Work in this sense has been done looking for a trigger with a μ6 (but this introduces a bias in the invariant mass reconstruction) The aim of the present work is to check the feasibility of the Zbb measurement with a jet trigger, i.e., no requests on leptons in the final state Very hard to trigger.

  3. First trigger considerations Since we want to trigger on low-medium Pt range jets, the present LVL1 menus are less than satisfactory. In fact, although the ATLAS LVL1 trigger is designed to work at a maximum output rate of 75(100) KHz (the input is 40 MHz), the staging of the LVL2 allows only a 25 KHz LVL1 output. Since the LVL2 rejection on jets is limited, the thresholds on the jets are very high. • How can FTK contribute? • high quality tracks @ LVL2 means high rejections against multijet background for final state with b-jets • track fitting not done @ LVL2  less computing power required Lower thresholds can be used - more LVL1 bandwith available

  4. Signal and background generation • Both signal and background generated with PYTHIA 6.203 • Signal (Zbb): MSEL=11 ; σ = 9.1 nb • Background (Generic QCD): MSEL=1 with in 6 QT bins (beginning with QT > 10 GeV) ; σ = 9.45 mb. Cross check on the accepted cross section made with Alpgen generator. Results are consistent. • Underlying event in PYTHIA tuned on the CDF data (A. Moraes et. al.) • Simulation/reconstruction made using ATLFAST-OO in Athena version 7.0.2 with FastShower included.

  5. Few words on ATLFAST…. • The ATLAS fast simulation program (ATLFAST) is a particle level simulation. Calorimeters are simulated by a grid of cells for geometric acceptance. The EM and HAD resolution is parametrized using the TDR results with full simulation. • No detailed simulation of the shower profile. • All the predicted rates in the following are evaluated using ATLFAST. The extimated rates could be underestimated by a factor 2-8 • The b-tagging efficiencies are parametrized as well. Those used are • εb = 50% εc=9.2% εj=0.5% • ASSUMPTIONS IN THE FOLLOWING: • Offline b-tag quality available at LVL2 • Few KHz available at LVL1 output

  6. Quest for high PT thresholds… First attempt: try a 2 jet selection. Rate limitations call for high jet thresholds. Since the PT of the signal jets is expected harder than background, a hard cut on the PT of the jets would also improve the S/B ratio.

  7. …quest for low PT thresholds The final S/B ratio is expected to be 0.1-1%. A MC prediction at this level of precision is impossible. A background subctration a la UA2 is needed. This means we need to have a lower and a higher side band to evaluate the background contribution. In order not to destroy the low-mass side band, taking into account that the Z0 peak has long radiative tails at low masses, we need to avoid too high thresholds on the Pt of the jets. Signal subtraction requirements call for low PT thresholds σ(M)/M~13% μ = 90.47 GeV dN/dMbb Mbb

  8. Minv distributions 1j20-1j15 1j25-1j15 2j25 1j25-1j15 nr A scan on the thresholds of the 2 leading jets has been made. Even with a very loose selection, the background mass distribution peaks around 50 GeV. 50 GeV Even in the (very) optimistic scenario of a full efficient LVL1 on very low Pt jets, the background subctraction is made difficult by poor low-mass side band. The 2σ window for the signal is also drawn for completeness Mbb

  9. 2 jet selection results All the channels need a strong prescale factor already for the ATLFAST estimate. High significances can be reached with the two hardest selections, but the peak of the background invariant mass is too high. The signal to background ratio is around 0.2%

  10. 3 jet selection 2j selection 3j selection We look for different strategies. We require a leading non-b jet. This decreases the LVL1 rate, and moves at low masses the trigger  turn-on in the background invariant mass distribution The reason is the following: requiring the leading jet to be non-b, one strongly reduces the contribution from direct bb production and selects mainly gluon splitting events (mainly ggggbbg). They are characterized by low invariant mass of the bb couple, because of the small angle between the b-jets Rbb

  11. 3 jet selection (2) μ=90.23 GeV σ=10.74 GeV The first selection tried is 1j40-2j25. The LVL1 ATLFAST rate estimate goes down to 25 KHz. The signal to background ratio is 0.4%, while the significance is 24 for a LVL1 prescale of 2.5. 50 Gev Mbb Mbb

  12. 3 jet selection (3) Signal QCD background To reduce further the LVL1 rate & the trigger turn-on mass, a hard selection on the leading jet has been tried. A leading jet of PT > 80 GeV is required. This reduces the value of the peak in the background invariant mass. The LVL1 rate is 2.6 KHz. 50 Gev Mbb Rbb

  13. 3 jet selection (4) The selection can be refined further by raising the thresholds on the b-jets to 30 GeV. While the background mass does not change much, the Z mass distribution becomes narrower.

  14. 3 jet selection (5) 1j80-2j30 1j80-2j25 1j40-2j25 In general, the application of a hard cut on the leading jet of the events leads to more reasonable trigger configurations, to better S/B, even if the statistical significance decreases over a certain threshold. The Z mass shape get better as the selection gets harder, with less signal in the low tail, which should lead to a better background subtraction Significance S/B LVL1 prescale for 2.6 KHz included Mbb

  15. LVL1 trigger considerations The main caveats are about the LVL1 efficiencies and rates - Can we trigger efficiently on the hard PT 3jet selections? - Which is the rate for them? I am currenly analysing the LVL1 performances with the full G3 detector simulations. Unfortunatly the software is not the final one Going back to the TDR results With a 5 GeV tower threshold, a 30 GeV jet can be identified (i.e., the LVL1 will provide a Region Of Interest –ROI) with ~95% efficiency

  16. LVL1 trigger considerations (2) At present, the LVL1 multijet trigger rates are given in terms of symmetric thresholds (i.e. 3 jets of the same energy) The trigger menu for this channel should include a high PT LVL1 jet plus 2 very low PT jets.

  17. Background subtraction • We tried to address the following question: is it possible to extract a signal with high significance when the signal to background ratio is below 1%? • The background distributions is regular down to (at least) 40 GeV in the 1j80-2j30 case. It is well fit by an exponential, but the statistics is limited. • Even using the fast simulation, a big amount of CPU time is needed to generate the statistics of the background a for some fb-1 of integrated luminosity • We made the following: • Physicist A was generating histograms with parametrized (and unknown to the physicist B) distributions and “real” statistics and statistic fluctuations for the background for 10 fb-1. Then the signal is added using a sample a factor 10 lower than the one that will be collected in 10 fb-1. • Physicist B fitted the background using the sidebands, a la UA2 • The function that fits the sidebands was subctracted from the histogram and the residual is fitted with a gaussian

  18. Background subtraction First example: simple exponential (known physicist B). On the left the 1j40-2j25 case (low sideband is 56-68 GeV, well inside the signal window) leads to a symmetric peak for the signal. The effect of the low radiative tails is clearly visible also in the 1j80-2j25 selection (low sideband is 40-50) on the right. On the right bottom plot the found peak is compared to the MC generated Z0 1j40-2j25: μ=88.7 GeV σ=10.7 GeV S/√B=20.5 1j80-2j25 selection

  19. Background subtraction In the case of the best considered selection (1j80-2j30) and a low sideband window 38-52 GeV (achievable in the analysis context), we obtain good results on the peak reconstruction. The radiative tails are well reproduced after the background subtraction. Good reconstruction of the peak details. The points reproduce the MC Z0 peak shown in the right low plot. The fit is done in the 80-110 GeV window

  20. Background subctraction First unknown (to the physicist B) distribution: Fit in the sidebands made with 1j40-2j25 selection The fit of the sidebands (54-68 GeV and 116-160 GeV) gives a good Χ2 (1.3) The fitted gaussian (black Line) is very similar to the fit found with the exponential background (red line)

  21. Background subctraction Same background as before but with a different fitting function: Because of the presence of a non negligible amount of signal in the low sideband, part of the signal is lost. systematic uncertainties on the Z significance and parameters

  22. Background subtraction Using the same background distribution as before. Using the fit function F1. Check the results with the 1j80-2j30 distribution. Better low sideband and narrower signal  Less signal in the low mass sideband  the Z parameters and the significance are less dependent on the fit function chosen F2 used F1 used

  23. Background subctraction New unknown distribution: a/M4 NO SIGNAL INCLUDED Fit made with both F1 and F2 F1 does not fit the low sideband. As a consequence, a fake bump is produced. F2 does fit the low sideband. No signal is found.

  24. Conclusions • Different trigger/selection strategies are available • The signal can be triggered (more investigation with full simulation needed) • The signal can be reconstructed • The amount of found signal and the parameter accuracy depends on the sample selection and reconstruction algorithm • A very simple procedure gives good results if signal-free low mass side band can be used. To evaluate the parameter stability we have to fit the reconstructed signal with a Landau, not a gaussian shape, however the comparison with the MC generated Z0 is significant.

  25. Some work done also in the Vector Boson Fusion H production with Hbb. It is one of the most promising channels for the measurement of the WWH coupling. A measurement with a precision of 20% is expected after 600 fb-1 of integrated luminosity. Impossible to trigger in the ATLAS enviroment without good b-tagging at LVL2. See http://agenda.cern.ch/askArchive.php?base=agenda&categ=a036321&id=a036321s1t29/transparencies http://agenda.cern.ch/askArchive.php?base=agenda&categ=a04587&id=a04587s1t10/transparencies There are preliminary results also in the bbA/H4b MSSM channel. This can be used as a complementary channel for the MSSM heavy neutral Higgs discovery. See http://agenda.cern.ch/askArchive.php?base=agenda&categ=a041955&id=a041955s1t1/transparencies

  26. BACKUP

  27. Tile Calorimeter EM barrel and EndCap Hadronic EndCap Forward Calorimeter Calorimeters in ATLAS EM LAr || < 3 : Pb/LAr 24-26 X0 3 longitudinal sections1.2   = 0.025  0.025 Central Hadronic || < 1.7 : Fe(82%)/scintillator(18%) 3 longitudinal sections 7.2   = 0.1  0.1 End Cap Hadronic1.7 <  < 3.2 : Cu/LAr – 4 longitudinal sections  < 0.2  0.2 Forward calorimeter3 <  < 4.9 : EM Cu/LAr – HAD W/Lar 3 longitudinal sections EM LAr + TileCal resolution (obtained at 1998 Combined TestBeam) Linearity within ±2% (10-300 GeV)

  28. Background subctraction

  29. FIT RESULTS

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