Upload
nora-logan
View
221
Download
0
Embed Size (px)
Citation preview
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)
Gordana Milutinovic-Dumbelovic BPU9 , 24-27 August 2015 , Istanbul, Turkey
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
Gordana Milutinovic-Dumbelovic BPU9 , 24-27 August 2015 , Istanbul, Turkey
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.
Gordana Milutinovic-Dumbelovic BPU9 , 24-27 August 2015 , Istanbul, Turkey
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
Gordana Milutinovic-Dumbelovic BPU9 , 24-27 August 2015 , Istanbul, Turkey
7 H→μ+μ- analysis
BR(H→) ≈ 2 × 10-4 σ⇒ HWW × BR ≈ 0.05 fbNs(H→) ≈ 78/1.5 ab−1
Gordana Milutinovic-Dumbelovic BPU9 , 24-27 August 2015 , Istanbul, Turkey
Uncertainty is dominated by limited statistics.
𝜎×𝐵𝑅gHWW
2 ∙ gHμμ2
Г H
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.
Gordana Milutinovic-Dumbelovic BPU9 , 24-27 August 2015 , Istanbul, Turkey
9
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
Gordana Milutinovic-Dumbelovic BPU9 , 24-27 August 2015 , Istanbul, Turkey
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
Gordana Milutinovic-Dumbelovic BPU9 , 24-27 August 2015 , Istanbul, Turkey
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%.
Gordana Milutinovic-Dumbelovic BPU9 , 24-27 August 2015 , Istanbul, Turkey
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.
Gordana Milutinovic-Dumbelovic BPU9 , 24-27 August 2015 , Istanbul, Turkey
14
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
Gordana Milutinovic-Dumbelovic BPU9 , 24-27 August 2015 , Istanbul, Turkey
Reconstruction of multi jet final states.
𝜎×𝐵𝑅gHWW
2 ∙ gHZZ2
Г H
Statistical uncertainty of σWWH×BR() is extracted as *100%.
𝑔𝐻𝑊𝑊𝑔𝐻𝑍𝑍
15 Signal and background processes
Processes with a large cross-section like
and
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
Gordana Milutinovic-Dumbelovic BPU9 , 24-27 August 2015 , Istanbul, Turkey
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
find
two
isol
ated
lept
ons
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%
Preselection only Preselection + MVA
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%
Preselection only Preselection + MVA
Gordana Milutinovic-Dumbelovic BPU9 , 24-27 August 2015 , Istanbul, Turkey
19 Results
signal efficiency (εs) 18%
δ(σWWH×BR()) 18.3%
signal efficiency (εs) 30%
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.
Gordana Milutinovic-Dumbelovic BPU9 , 24-27 August 2015 , Istanbul, Turkey
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.
Gordana Milutinovic-Dumbelovic BPU9 , 24-27 August 2015 , Istanbul, Turkey
21
Teşekkürler
THANK YOU
Gordana Milutinovic-Dumbelovic BPU9 , 24-27 August 2015 , Istanbul, Turkey
22 List of all background processes for analysis
Gordana Milutinovic-Dumbelovic BPU9 , 24-27 August 2015 , Istanbul, Turkey
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.
Gordana Milutinovic-Dumbelovic BPU9 , 24-27 August 2015 , Istanbul, Turkey
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.
Gordana Milutinovic-Dumbelovic BPU9 , 24-27 August 2015 , Istanbul, Turkey
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
Gordana Milutinovic-Dumbelovic BPU9 , 24-27 August 2015 , Istanbul, Turkey
width constrained