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HZ Recoil Analysis: First Look at Samples

HZ Recoil Analysis: First Look at Samples. LCD WG6 Meeting, 13/03/2012 J.S. Marshall, University of Cambridge. HZ Recoil Analysis - Reminder. Relevant processes for this study are the recoil reaction e + e -  HZ  Hff , commonly called Higgsstrahlung.

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HZ Recoil Analysis: First Look at Samples

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  1. HZ Recoil Analysis:First Look at Samples LCD WG6 Meeting, 13/03/2012 J.S. Marshall, University of Cambridge

  2. HZ Recoil Analysis - Reminder • Relevant processes for this study are the recoil reaction e+e-HZHff, commonly called Higgsstrahlung. • By detecting decay products of the Z, can search for Higgs signals without further assumption about Higgs decay modes: “model independent analysis”. • For CDR V3, will search for decays Z and Zee: “X” and “eeX” channels. • Signal is selected by identifying two well-measured leptons in final state, yielding Z mass. Can then compute recoil mass: s = 500GeV Lint = 500fb-1 mH= 120GeV No polarization

  3. Requests and Production Status Status on 09/03/2012:

  4. Lepton Identification • Procedure for selecting signal events and leptons produced by Z-decay: Loop over SelectedPandoraPFANewPFOs. Populate two lists of particles: negatively/positively charged leptons of specified flavour.* If both lists are populated, event will be selected as a signal candidate. If either list contains more than one entry, investigate all possible di-lepton combinations. Select lepton pair producing invariant mass closest to the Z mass. *Could apply quality cuts at this point, to ensure leptons are well-reconstructed. • Efficiency of the lepton id procedure (without quality cuts) for different samples: • Can now look into properties that could be exploited to perform background rejection...

  5. Di-lepton Mass • Normalised signal and background distributions of the invariant mass of the di-lepton system for the X (left) and eeX (right) channels. Pre-cuts have been applied to ff and eeff samples. • Note impact of radiative effects, introducing tails to lower energies in the distributions. In the eeX channel, accumulation of events at low mass is likely due to selection of photon conversions.

  6. Recoil Mass • Normalised signal and background distributions of the recoil mass system for the X (left) and eeX (right) channels. This distribution is a convolution of smearing and radiative effects. • Beamstrahlung and ISR cannot contribute to the distribution above the mass threshold MH+MZ, calculated to be ~410GeV, at which point see sudden decrease in X channel. • Decrease in eeX channel hidden by effects of Bremsstrahlung.

  7. Di-lepton PT • Normalised signal and background distributions of the PT of the di-lepton system. • Higgsstrahlung can be interpreted as a two-body process; both bosons gain equal PT, which is conserved by decay products. Distribution increases until reach maximum at PTdl = PZ  226GeV. • Two-fermion backgrounds have little PT although ISR can lead to a non-zero PTfor these samples.

  8. Di-lepton Polar Angle • Normalised signal and background distributions of the cosine of the polar angle of the di-lepton system for the X (left) and eeX (right) channels. This variable is strongly correlated with PTdl. • Cross-section for Higgsstrahlung process decreases towards forward/backward directions, whilst these directions are favoured for background processes proceeding via production of W or Z pairs.

  9. Di-Lepton Acollinearity • Normalised signal and background distributions of the acollinearity of the di-lepton system for the X (left) and eeX (right) channels. • Opening angle between the lepton trajectories is sensitive to the boost of the di-lepton system. Decay products from Z pairs should be boosted more strongly than those from Higgsstrahlung.

  10. Di-Lepton Acoplanarity • Normalised signal and background distributions of the acoplanarity of the di-lepton system. This is simply a projection of the acollinearity to the X-Y (or R-) plane: acop = |1 - 2| • If leptons are produced via an intermediate particle, with given PT, expect to see structure in the azimuthal angle measured between the leptons.

  11. ISR Photon Detection • Final state can gain sizeable PT due to ISR. If can detect ISR photon, can compare PT of di-lepton system with that of the photon: • Expect to see a correlation for background samples, particularly for 2-fermion samples e.g. only a lepton pair produced at Z* vertex. Expect no correlation for signal samples.

  12. Next Steps... • Investigate recovery of Bremsstrahlung photons in the eeX channel: • Four momenta of selected electrons is combined with those of photons with small angular distances to the electrons. • If combined objects form the Z mass, they are included in the Z reconstruction. • Can increases signal size, but can also reduce momentum resolution, due to inclusion of low energy photons. http://www-flc.desy.de/lcnotes/notes/LC-PHSM-2009-006.pdf 2. Investigate background rejection procedures: cut-based, multivariate technique, etc. 3. Extraction of Higgs mass and production cross-section from recoil spectrum of selected events.

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