Current Status on ZHH Analysis

’08 5/31 GLD ZHH-group

Analysis menu

MH v.s. (ZHH)

500GeV

750GeV

1TeV

ZHH study

ZHH analysis was started to cover wide Higgs-mass region.

• ECM=500GeV is the best for MH=120GeV.

• ECM>750GeV is preferable to study for MH>160GeV.

Our analysis menu• Light Higgs : MH=120GeV, ECM=500GeV

• Heavy Higgs : MH=170GeV, ECM=500GeV~1TeV

Study of Light Higgs Case

Introduction

Status at TILC08

• Analysis was started for HH-mode.

• Signal significance of 1.4 was obtained against ZZbbbb. Signal : 12 events ZZbbbb : 64 events

Today’s topic

• Re-optimization of the selection cut.

• Analysis with B.G. of ZZbbbb and tt. (ZZ) = 395.8 fb (tt) = 583.6 fb

Selection cutsThe new selection cuts are applied to improve B.G. rejection.

Primary cut for reconstructed particles

• Energy cut (E1,2 < 250GeV)

• Momentum cut (P1,2 < 200GeV)

• cut (1,2 < 0.9)

Fine selection cut

• 2 cut (2 < 13)

• Higgs mass cut (95GeV < MH1,2 < 125GeV)

• Missing mass cut (90GeV < Mmiss < 200GeV)

• Angle cut (|cos1,2| < 0.9)

b-tag cut

New selection cuts

Example of new cuts (Momentum and cut) is shown.

Momentum cutMomentum of the reconstructed particles was checked.

• Higgs-momentum of the signal is below 200 GeV/c.

• The momentum above 200 GeV/c was rejected.

P1(GeV/c)0 100 200 300 400

HH B.G.

P1(GeV/c)0 100 200 300 400

ZZtt

cut(=p/E) of the reconstructed particles was checked.

• of the signal is below 0.9.

<0.9 was selected.

The fine selection cuts were applied after these primary cuts.

10.3 0.5 0.7 0.9 10.3 0.5 0.7 0.9

HH B.G.

ZZtt

B-tag cutAfter the fine selection cuts, b-tagging was applied.

• Nb-tag ≧3 was selected. b-tag: 3 tracks with 3 separation from IP.

• 23.5 events of ZZbbbb still contaminates for 2 ab-1. All tt events are rejected.

Nb-tag0 1 2 3 4 5

HH

Nb-tag0 1 2 3 4 5

B.G.

ZZtt

Signal significance

The signal significance was estimated for 2ab-1

• Signal : 12.1 events

• B.G. : 23.5 events

Significance : 2.0

Rec. (MH1+MH2) for 2ab-1

TILC08

• Signal : 12 events

• B.G. : 64

1.4

ZZttSignal + BG

GeV

Reduction summary

• No cut

• E1,2 < 250GeV

• P1,2 < 200GeV

• 1,2 < 0.9

• 2 < 13

• 95GeV < MH1,2 <125GeV

• 90GeV < Mmiss < 200GeV

• |cos1,2| < 0.9

• Nb-tag ≧3

: 77.6

: 70.8 (x 0.91)

: 70.8 (x 1.00)

: 60.9 (x 0.86)

: 33.8 (x 0.56)

: 26.7 (x 0.79)

: 25.5 (x 0.96)

: 22.2 (x 0.87)

: 12.1 (x 0.55)

18,100

14,022 (x 0.77)

4,876 (x 0.35)

2,033 (x 0.42)

686 (x 0.34)

223 (x 0.33)

130 (x 0.58)

41.6 (x 0.32)

23.5 (x 0.56)

HH ZZbbbb

• The signal efficiency is 16%.

• 55% of signal is lost at b-tag cut. However, b-tag is necessary to reject tt-B.G..

1,167,200

35,950 (x 0.031)

29,063 (x 0.81)

25,678 (x 0.88)

6886 (x 0.27)

1517 (x 0.22)

817 (x 0.54)

467 (x 0.57)

0 (x 0.0)

tt

Next step is to consider tbtb-B.G. (0.7fb).

Summary for HH• The selection cut was re-optimized.

• The signal significance of 2.0 was obtained against the ZZbbbb and tt.

This result is comparable to Djamel’s report at LCWS07, after scaling (ZZ) to the correct value (395.8fb).

• The other B.G. modes like tbtb should be taken into account.

Study of Heavy Higgs Case

ZHH signal has small xsec ~0.1fb @ 750 GeV, sensitive to HHH coupling

many possible final states with different characteristics:from 10 hadronic jets, to 4 charged leptons + 6 neutrinos,

and almost everything in between!

Many backgrounds with orders of magnitude higher xsec

so far, considered only subset with true Z bosonZWW (this has largest xsec), ZZZ, Ztt, ZZWW

events generated by MadGraph, studied @ QuickSim level

Introduction

Fit events to ZHH->ZWWWW hypothesismany possible signal final states need to be considered

look for Z->e+e-, mu+mu-if not found, try Z->tau tau, Z->nu nu and Z->jj possibilities

look for additional e, mu, assume from W decayidentify tau-like jets

# charged tracks = 1 or 3invariant mass < m_taujet charge = +-1

try two possibilities: true tau jet, or hadronic jetfor other Ws, try both tau decay and hadronic decay possibilities

for each possible decay topologyforce non-leptonic part of event into required # jetsask if invariant masses of Z, W, H->jets consistent with expectation

Event reconstruction

require that event fits at least one signal final state hypothesis

use kinematic variables to decide if event is real ZHH event or backgroundvisible energy, momentum vs. # assumed neutrinosYcut of jet clustering required to force # jets

no single variable gives good separation->combine into Neural Network

train network to separate signal from dominant ZWW background

try at 750 GeV & 1 Tev – slightly different event topologies

Signal/background discrimination

Train neural network to separate signal from ZWW backgrounduse TMultiLayerPerceptron package (in root)

inputs:#jets, #el+#mu, #taus, Z decay type, Ycut of jet forcing, |total momentum|, Z miss. mass

1 hidden layer, with 5 nodes

5k training events (50% ZHH, 50% ZWW)

5k testing events

2k training cycles

reached minimum

no significant overtraining

Some NN inputs @ 1TeV centre-of-mass energy ZHH, ZWW

Z decay (invis, lept, jet) # taus # hadronic jets

Jet forcing ycut

NN output

NN output @ 750 GeV

NN output @ 1000 GeV

reminder:total # signal events~100 / ab-1

Both look ~impossible...

ZHH production sensitive to gHHHsmall xsec, large backgrounds

for mH = 170 GeV, many possible final states

trained NN to separate ZHH & ZWW- 170 GeV higgs- 750 GeV & 1 TeV centre-of-mass

some separation, but signal still swamped by huge

backgrounds

maybe this measurement is not possible...

Summary for heavy Higgs analysis

Supplement

Energy cutEnergy of the reconstructed particles was checked.

• Higgs-energy of the signal is below 250 GeV.

• The energy above 250 GeV was selected.

E1(GeV)0 100 200 300 400

HH

E1(GeV)0 100 200 300 400

B.G.

ZZtt

2 cut2 distribution was checked after momentum and selection.

• The signal has the peak at 2 =0.

• B.G. has the broader shape than the signal.

2<13 was selected.

20 20 40 60 80

HH

20 20 40 60 80

B.G.

ZZtt

Mass cutThe Higgs mass cut is applied to select well-reconstructed events.

• Selection: 95GeV < MH < 125GeV

MH1(GeV)0 40 80 160 200120

HH

MH1(GeV)0 40 80 160 200120

B.G.

ZZtt

Missing mass cutThe missing mass was reconstructed.

• The peak is observed at Z-mass in HH events.

• Selection: 90GeV < Mmiss < 200GeV

Mmiss(GeV)0 100 200 400 500300

HH

Mmiss(GeV)0 100 200 400 500300

B.G.

ZZtt

Angle cutThe angular distributions of the reconstructed particles are checked.

• Selection: |cos|<0.9

cos1

-1 -0.6 -0.2 0.2 0.6 1

HH

cos1

-1 -0.6 -0.2 0.2 0.6 1

B.G.

ZZtt