Constraints from "Higgs mass and Cross-Section Measurements at a 500GeV CLIC Machine, Operating at sqrt(s)= 350GeV and 500GeV"

The 350 GeV HZ Recoil Analysis:

For both muon and electron channels, the background has a total cross section of roughly 1000 times greater than the signal. Given the small signal cross-sections, excess events in the Higgs note were generated, simulated and reconstructed. A safety factor of 50 was applied, and events were then weighted to the correct normalisation throughout the analysis. This helped remove the effects of statistical fluctuations in the signal sample.

For the background sample, cuts at generator level are required:

• Transverse momentum of the leptons (l+ l-) must be greater than 10 GeV- Higgsstrahlung process will to produce a high Z transverse momentum, and hence a high lepton transverse momentum.
• Ιcos (θ)Ι of l+ l- < 0.95. Cross-section for Higgsstrahlung process are expected to decrease the forward/backward regions, whilst their directions are favoured for processes involving the production of W or Z pairs.

Following the selection of the lepton pair: the Transverse momentum and polar angle of the lepton pair were calculated. Also Acollinearity and Acopolanarity of the leptons were calculated.

Cuts from ZFinder

The ZFinder works by using the fact that the electric calorimeter does not detect any muons.

Vector _pfovec is the detected particle vector.

If the energy of _pfovec is less than the momentum cut for fermion from Z decay then it is not a muon. If the electric calorimetry is greater than the cut on the muon ECAL energy then not a muon. If the hadronic calorimetry is greater than the cut on the muon HCAL energy then not a muon.

If the energy of _pfovec is less than the momentum cut for fermion from Z decay then it is not an electron. If the electric calorimeter is less than the cut on the electron ECAL energy, then not an electron. If the hadronic calorimeter is greater than the cut on the electron HCAL energy, then not an electron.

• Momentum cut for fermion from Z decay, ' _momentumCut ' = 10
• Cut on muon ECAL energy, ' _muonEcalEnergyCut '= 2.5
• Cut on muon HCAL energy, ' _muonHcalEnergyCut '= 10
• Cut on electron ECAL energy, ' _electronEcalEnergyCut ' = 10
• Cut on electron HCAL energy, ' _electronHcalEnergyCut ' = 10

According the Readme, Bremstrahlung and FSR recovery is on.

NB: ZFinder tautau decay mode and qq decay mode not implemented.

Background Rejection

In mumuX channel, the mass of the selected lepton pair, Transverse momentum of lepton pair, and the recoil mass were investigated. The made distributions made from the properties motivated the cuts:

Selection Cuts:

• 40 Gev < M (l+ l-) < 120 GeV
• 95 GeV < M (recoil) < 290 GeV
• Transverse momentum of l+ l- > 60 GeV

500 GeV HZ Recoil Analysis

Hard selection cuts:

• 30 GeV < M (l+ l-) < 120 GeV
• 0 Gev < M (recoil) < 380 GeV
• Transverse momentum of l+ l- > 65 GeV

500 GeV HZqq Analysis

Jet reconstruction: 3 different values for jet cone size:

R=0.7, 1.0 and 1.3

Kinematic Fit:

• Energy conservation ∑ Ei = 500 GeV
• Momentum conservation ∑ (pxi, pyi, pzi)= (5,0,0) GeV
• Mass constraint: mass of one pair of jets is contrained to be equal to Z mass.

The variation of the jet parameters, and calculation of the fit probability used the following jet energy and angular resolutions:

• σ(e) = 4.5% . Ejet
• σ(θ) = 0.27 rad GeV / (sqrt(Ejet))
• σ(Φ) = 0.25 rad GeV / (sqrt(Ejet))

Fit Procedure

To get the mass of the Higgs, Mh, Simplified Kernal Estimation used the shape of the fitted Higgs mass distribution for the signal sample.

Mass range 90 GeV < Mh < 200 GeV

500 GeV Hmumu Analysis

Monte Carlo fit: if invariant mass was within 5 GeV of Z mass the event was declared to be Higgsstrahlung. The remaining were flagged as WW fusion.

Project Work

Lepon Identification

The ZFinder selects suitable Z boson candidates.

Reconstruction of H mass from inferred vector

Given the initial state four vector and the four vector of the Z boson it is possible to infer the four vector of the recoil mass. The only two variables of the Higgs recoil mass are the momenta of the pair of leptons from the Z decay.

Note that (besides the background) precision of the Higgs recoil mass measurement can be affected by both the smearing and radiative effects.

The cross section can also precisely be measured to examine the coupling between the Higgs and the Z boson, given by:

g^2(HZZ) : sigma = Ns/ eL

where Ns is the number of events of Higgs-strahlung signal, e is the total efficiency of signal reconstruction, and L is the luminosity.

Signal & Background Analysis

Number of events run over: 5000

mumuX

• Signal underflow : 0
• Signal overflow: 435

90.5% of entries used.

• Background underflow : 174
• Background overflow : 313

35% of entries used.

eeX

• Signal underflow : 47
• Signal overflow : 591

86.21% of entries used.

• Background underflow : 299
• Background overflow : 303

40.39% of entries used.

Number of events run over: 50,000

mumuX

• Signal underflow : 7
• Signal overflow : 4565

90.01% of entries used.

• Background underflow : 1720
• Background overflow : 3077

34.56% of entries used.

eeX

• Signal underflow : 500
• Signal overflow : 5615

86.88% of entries used.

• Background underflow : 3119
• Background overflow : 2985

41.61% of entries used.

Eta-Phi plots

The momentum of each particle produced in a collision can be decomposed in:

• Component parallel to the beams (longitudinal, parallel to z)
• Component perpendicular to the beams (transverse, in x-y plane)
• The transverse component is: PT = P sin(θCM)

Pseudorapidity (eta) = -ln(tan(θCM)/2)

Report

Final report (40 credits) on Moodle

-- DominicSmith - 2012-11-20