HZ Recoil Analysis: First Look at Samples

John Marshall HZ Recoil Analysis 1

HZ Recoil Analysis:First Look at Samples

LCD WG6 Meeting, 13/03/2012

J.S. Marshall, University of Cambridge


HZ Recoil Analysis - Reminder

sEMsM ZZrecoil 222

s = 500GeVLint = 500fb-1

mH =120GeVNo polarization

• Relevant processes for this study are the recoil reaction e+e-HZHff, 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 Zee: “X” and “eeX” channels.

• Signal is selected by identifying two well-measured leptons in final state, yielding Z mass. Can then compute recoil mass:


Requests and Production Status

Process description

Whizard process ID

Cross-section

Cross-section

after pre-cuts

Events requested

X (signal) hzmumu 2.18 fb 2.18 fb 54,500

e2e2nn 160.05 fb 160.05 fb 8.0 104

ff e2e2ff 4592.55 fb 776.14 fb 3.9 105

eeX (signal) hzee 2.18 fb 2.18 fb 54,500

ee eenunu 419.78 fb 419.78 fb 2.1 105

eeff e1e1ff 6047.78 fb 1965.53 fb 9.8 105

Generator-level cuts for background samples ff and eeff

pT l+l- > 10GeV, cut on transverse momentum, calculated from vector sum of two leptons

|cos l+/l-| < 0.95, cut on angle of either of leptons

Status on 09/03/2012:

Events available

Percentage of total

59,381 100%

81,337 100%

6,000 1.5%

60,379 100%

221,058 100%

581,000 59%

Test samples of ee, and also in production, to

justify neglecting 2-fermion bkg.


Lepton Identification

• Can now look into properties that could be exploited to perform background rejection...

• Procedure for selecting signal events and leptons produced by Z-decay:

• Efficiency of the lepton id procedure (without quality cuts) for different samples:

X ff eeX ee eeff

Efficiency: 98.2% 69.2% 70.7% 97.9% 73.1% 75.7%

1. Loop over SelectedPandoraPFANewPFOs.2. Populate two lists of particles: negatively/positively

charged leptons of specified flavour.*3. If both lists are populated, event will be selected as a

signal candidate.4. If either list contains more than one entry, investigate all

possible di-lepton combinations.5. Select lepton pair producing invariant mass closest to the Z

mass.

*Could apply quality cuts at this point, to ensure leptons are well-reconstructed.


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.


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.

sEMsM ZZrecoil 222


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 PT for these samples.

zTdl

Tdlzz

Tdl

z

TdlZTdl PP

PPPP

PP

sMPf

;)/( )(

222

22 1

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.

2

2

1dl

Tdldl

PP

cos sMZHee

Z /sin~cosd

)(d 222 8


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.

)/.( cosacol -12121 PPPP


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.

)/.( cosacop -12121 TTTT PPPP


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.

TTdlTbal PPP


Next Steps...

1. 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.

http://www-flc.desy.de/lcnotes/notes/LC-PHSM-2009-006.pdf

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HZ Recoil Analysis: First Look at Samples