measurement of the branching ratios for standard model higgs decays into muon pairs and into z boson...

Measurement of the branching ratios for Standard Model Higgs decays into muon pairs and into Z boson pairs at 1.4 TeV CLIC

Gordana Milutinovic-Dumbelovic, I. Bozovic-Jelisavcic, C.Grefe, G.Kacarevic, S.Lukic,

M.Pandurovic, P.Roloff, I. Smiljanic

On behalf of the CLICdp collaboration

9th International Physics Conference of the Balkan Physical Union – BPU9, 24-27 August 2015, Istanbul University, Istanbul, Turkey

2 Overview

Introduction to the CLIC project

Higgs production at 1.4 TeV CLIC

Motivation for the measurements

Simulation and reconstruction

H→μ+μ- analysis

H→ZZ* analysis

Conclusions

Gordana Milutinovic-Dumbelovic BPU9 , 24-27 August 2015 , Istanbul, Turkey

3 Introduction to the CLIC project

The Compact Linear Collider (CLIC) is an option for a future multi-TeV linear electron-positron collider.

Gradient of 100 MV/m

High luminosity (~1034 cm-2s-1)

Energy staging:

Stage 1: 380 GeV (500 fb-1) SM Higgs physics, top threshold scan

Stage 2: 1.4 TeV (1.5 ab-1) start of BSM physics, ttH, Higgs self coupling, rare Higgs decays

Stage 3: 3 TeV (2 ab-1) BSM physics, Higgs self coupling, rare Higgs decays

(each stage corresponds to 4-5 years)

CLIC vs. LHC- known initial state- no QCD background

CLIC will be a Higgs factory

4 Higgs production at 1.4 TeV CLIC

At 1.4 TeV WW-fusion is dominant

Higgs production channel.

Polarisation can enhance statistics.

σ(e+e-→Hν)

Possibility to study rare Higgs decays (BR10-4) like Zγ, γγ, μμ

Relative couplings gHWW(gHZZ)can be determined at percent level

5 Motivation for the measurements

σPRODBR Higgs couplings

Higgs BRs measurements are potential probe for the New Physics

Measurement of gHµµ is challenging (BR10−4)

gHµµ can not be accessed at LHC

H→ZZ* at 1.4 TeV provides input to the overall fit of data obtained at all CLIC energy stages.

The fit can be performed in a model independent (dependent) manner, providing precision of the Higgs to EW boson couplings at the percent (sub-percent) level.

6 Simulation and reconstruction

Full CLIC_ILD detector simulation of signal

and background events

Particle reconstruction and identification

using PFA approach jet energy

resolution 3.5 %

Muon identification and reconstruction (provided by

TPC and instrumented yoke) allow 99% reconstruction

efficiency (barrel) and )~10-5 GeV-1

7 H→μ+μ- analysis

BR(H→) ≈ 2 × 10-4 σ⇒ HWW × BR ≈ 0.05 fbNs(H→) ≈ 78/1.5 ab−1

Uncertainty is dominated by limited statistics.

𝜎×𝐵𝑅gHWW

2 ∙ gHμμ2

Statistical uncertainty of σWWH×BR() is obtained as RMS of the number of signal measurements.

8 Signal and background processes The most distinguishing kinematical property

of the signal is missing energy.

Process with the same

signature as signal gives irreducible background.

Processes

forward electron signature - can be reduced by

electron tagging.

Process similar

final state as signal, can be reduced in MVA approach.

- The cross-sections for all processes with photons in the initial state include both Beamstrahlung and processes with EPA photons.

- Signal muon pair is generated in the Higgs mass window (100-150) GeV.

Preselection and MVA analysis

PRESELECTION:

Two reconstructed muons

>5 GeV

Di-muon invariant mass window (105-145) GeV

Forward electron tagging (absence of tagged electrons with E>200 GeV and θ>1.7º)

Preselection efficiency 82% Overall signal efficiency 24%

Preselection only Preselection + MVA

MVA (6 sensitive observables):

Evis, (μμ), μ1)+μ2), β(μμ), θ(μμ),

MVA signal selection efficiency is 32% due to the limited discriminating power of sensitive observables

Two major steps: - Preselection (serves to remove beamstrahlung and background with forward electrons) - MVA (background suppression due to different kinematics)

10 Signal and background PDFs

Fully simulated samples of signal and background are generated to extract PDFs.

PDFs are used to describe pseudo-data (both signal and background).

Signal Background

11 Toy MC experiments Pseudo-data are made of: randomly sampled fully simulated signal events and background generated

with PDFs

Function f is built from signal and background PDFs to describe pseudo-data

Expected shape of data (signal + background) for each Toy MC is fitted with f to extract the number of signal Ns

12 Statistical uncertainty 5000 Toy MC experiments are performed to extract statistical uncertainty.

RMS of the number of signal distribution gives statistical uncertainty of the measurement δ(σBR)=38%.

13 Result

signal efficiency (εs) 24%

δ(σWWH×BR()) 38%

Uncertainty is dominated by the limited signal statistics and the irreducible background.

Systematic uncertainties (uncertainties of the integrated luminosity and muon identification efficiency, uncertainty on the transverse momentum resolution, uncertainty of the signal count caused by the fit) are negligible compared to the statistical uncertainty.

Result is obtained with unpolarised beams. Employment of beam polarisation (-80%) can enhance the Higgs production cross-section by a factor 1.8.

H→ZZ* analysis

BR(H→ZZ*) ≈ 2.89% σ⇒ HWW × BR ≈ 7.05 fbBR(Z→qq) ≈ 70 % Ns(ZZ* → qqqq ) ≈ 5175/1.5 ab−1

BR(Z→e+e−, Z→µ+µ−, Z →ττ) ≈ 10% Ns(ZZ*→qqe+e−, ZZ*→qqµ+µ−, ZZ*→ qq ττ ) ≈ 1500/1.5 ab−1

Reconstruction of multi jet final states.

𝜎×𝐵𝑅gHWW

2 ∙ gHZZ2

Statistical uncertainty of σWWH×BR() is extracted as *100%.

𝑔𝐻𝑊𝑊𝑔𝐻𝑍𝑍

15 Signal and background processes

Processes with a large cross-section like

can be reduced by requiring high-pT jets.

Processes with two jets in the final state:

can be removed

with MVA analysis.

Event selection is performed in two major steps:

preselection and MVA separation

Beamstrahlung and EPA are considered.

16 Preselection Main aim of the preselection is to reduce large cross-section background (and ).

Remaining background:

For qqqq final state: ,

For qqll final state: , , Gordana Milutinovic-Dumbelovic BPU9 , 24-27 August 2015 , Istanbul, Turkey

qqqq final state qqll final state

45GeV<<110GeV

<65GeV

90 GeV <<165 GeV

-log10y34<3.5

-log10y23<3.0

100GeV<<600GeV

>80 GeV

P(b)jet1 <0.95

P(b)jet2 <0.95

17 MVA analysis (4-jet final state) TMVA input variables (, -log10y34, -log10y23, , P(b)jet1, P(b)jet2, P(c)jet1, P(c)jet2).

Due to high pT jet selection, to reduce large cross-section background (

most of the signal is also cut-off by preselection .

Irreducible background comes from and

Preselection efficiency 30.2% Overall signal efficiency 18%

18 MVA analysis (2-jet 2l final state)

TMVA input variables (, P(b)jet1, P(b)jet2, P(c)jet1, P(c)jet2, , θHiggs, Evis- EHiggs, NPFOs).

gives irreducible background due to the similar signature final state (+ limited lepton finding efficiency).

Preselection efficiency 62% Overall signal efficiency 30.4%

19 Results

δ(σWWH×BR()) 18.3%

δ(σWWH×BR()) 5.6%

For the qqqq final state, uncertainty of the measurement is dominated by background with large x-section and by irreducible background with the same topology as the signal.

For the qqll final state, uncertainty of the measurement is also dominated by background with similar signature as the signal.

Unpolarised beams are assumed.

Uncertainties are statistical only.

20 Conclusions- CLIC energy staged program is capable to provide precision measurements of Higgs couplings.

- Two of the analysis at 1.4 TeV CLIC have being presented here: σWWH×BR() and σWWH×BR() .

- Assuming unpolarised beams, corresponding statistical uncertainties are:

a. 38% (σWWH×BR(

b. 18.3% (σWWH×BR( qqqq) )

c. 5.6 % (σWWH×BR( qqll))

- Statistical uncertainties are dominantly coming from irreducible background and/or limited statistic of the signal.

Systematic uncertainties are smaller than the statistical ones.

Higgs couplings to the second generation fermions () that can not be accessed at the present machines can be precisely measured at CLIC (δ()= 5.6%, 1.5 ab-1 at 1.4 TeV + 2ab-1 at 3 TeV + 80% electron beam polarisation) .

Higgs couplings to EW bosons can be determined in an overall fit with the precision of 0.9% and 0.8% for gHZZ and gHWW respectively, in a model independent way.

Teşekkürler

THANK YOU

22 List of all background processes for analysis

Process σ(fb) 17 358 84.5 1942.2

Most relevant processes

Other considered processes

Tau decays become relevant if both taus decayinto two muons which happens in ~3% of cases. The invariant mass of the di-muon system does not match the Higgs mass window considered in this analysis.

23 Electron tagging

Forward region calorimetry plays an important role to veto electron spectators from 4-f and processes.

Energy dependent tagging is introduced in LumiCal and BeamCal:

- Take 5 mrad cone particles (e, gamma) to construct electron,

- Require 4σ deviation from the background (converted pairs) energy in the layer with the maximal deposition. Energy resolution is taken into account, as well as fluctuations of background deposition over the θ range.

24 Lepton identification

We have to identify e- and μ form the qqll final state

Track energy > 7 GeV

Energy contained in a cone around the track: cos θ<0.995

Impact parameters: d0<0.2 mm, z0<0.2 mm, R0<0.2 mm

ECAL/HCAL depositions: 0.025< μ ECAL to HCAL fraction<0.3,

e- ECAL to HCAL fraction>0.9

leptons Other PFOs

Steps to reconstruct a tau:

Look for tau ‘seed’ (a high energy, charged track)

Add all particles within search cone to seed

Check number of charged tracks, isolation, tau mass

Initial pT cut for all tracks > 4 GeVpT cut for seed > 10 GeVImpact parameter R0: 0.01 - 0.5Search cone angle < 0.15 radIsolation energy < 3 GeVRing particles < 5Invariant mass < 2. GeV/c2

37% efficiency in reconstruction of tau pair87% efficiency in reconstruction of the lepton pair

25 Model independent fit

From individual measurements of σ and σ x BR, couplings are determined via a global fit using:

Model independent fit is only possible at lepton colliders.

All results are limited by σ(HZ) measurement.

The Higgs width is extracted with 5.0% - 3.5 % precision.

width as a free parameter

26 Model dependent fit

LHC-like constraints: no invisible decays, fixed total width

Sub-percent precision, but strongly depend on fit assumptions

Higgs width extraction with 1.6-0.22% precision

width constrained

measurement of the branching ratios for standard model higgs decays into muon pairs and into z boson...

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