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4-Apr-06 Search for the SM Higgs Boson with CMS Marco Pieri 2
Outline
LHC collider and CMS Detector
Higgs Searches at LHC
H→γγ search with CMS Trigger Ecal calibration Background simulation Photon isolation Vertex estimation Photon conversion identification and π0 rejection Selection and background measurement C.L. for discovery or exclusion Analysis optimization
Summary and Outlook
4-Apr-06 Search for the SM Higgs Boson with CMS Marco Pieri 3
LHC Collider
First beams 2007 - Pilot Run
2008 start of physics Only ~1.5 years to the
first collisions Now mainly studying
the low luminosity phase
LHC operation (pp s =14 TeV) Low luminosity phase (Pile-up ~4
events/beam crossing) ℒ ~ 2 x 1033 cm-2s-1
Int ℒ ~ 30 fb-1
High luminosity phase (Pile-up ~20 events/beam crossing) ℒ ~ 1 x 1034 cm-2s-1
Int ℒ ~ 300 fb-1
CMSCMS
AtlasAtlas
LHCbLHCb
ALICEALICE
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CMS Detector
MUON BARREL
Silicon MicrostripsPixels
ECAL Scintillating PbWO4 crystals
Cathode Strip Chambers Resistive Plate Chambers
Drift Tube Chambers
Resistive Plate Chambers
SUPERCONDUCTINGCOIL
IRON YOKE
TRACKER
MUONENDCAPS
Total weight : 12,500 tOverall diameter : 15 mOverall length : 21.6 mMagnetic field : 4 Tesla
HCAL Plastic scintillator/brasssandwich
CALORIMETERS
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CMS Magnet
Superconducting coil: length 13m, diameter 5.9 m
Magnetic field 4 Tesla Energy stored 2.5
GJoule Coil has been cooled
down to 4.5K
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DIRECT SEARCHES AT LEP GAVE NEGATIVE RESULTS SM Higgs boson
MH>114.1 GeV @95% CL Some hints of possible Higgs signal were reported the last year of
LEP running MSSM neutral Higgs bosons
Mh, MA>92.9, 93.3 GeV @95% CL
Current Status of Direct Higgs Searches
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INDIRECT CONSTRAINTS ON THE SM HIGGS BOSON
SM Electroweak fits to all high Q2 measurements give: MH=89+42
-30 GeV MH<207 GeV @ 95% CL
(taking into account direct LEP limit)
Other Constraints on Higgs Mass
MSSM HIGGS BOSON In the MSSM Mh ≲ 135 GeV In the decoupling limit, for MA≳150 GeV
h behaves like HSM
Standard model searches directly apply H→γγ channel is the most sensitive at LHC
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SM Higgs Production at LHC
NLO Cross sections M. Spira et al.
gg fusion
IVB fusion
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SM Higgs Decays
When WW channel opens up pronounced dip in the ZZ BR
…
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SM Higgs Search Channels
ProductionDECAY
Inclusive VBF WH/ZH ttH
H → γγ YES YES YES YES
H → bb YES
H → ττ YES
H → WW* YES YES YES YES
H → ZZ*, Z ℓ+ℓ-, ℓ=e,μ YES YES YES YES
Low mass MH ≲ 200 GeV
H → γγ and H → ZZ* → 4ℓ are the only channels with a very good mass resolution ~1%
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Current status of CMS analysis
Physics TDR (Technical Design Report for the LHCC)
End of last year Volume I Detector performance,
calibration, analysis tools Now Volume II
Physics channels (more or less detailed)
H→γγ search is being finalized these days
End of the year Volume III Startup physics and activities All what must be done to start
CMS data taking and analysis
Continuous effort on software, data management and computing The readiness of this (together
with hardware performance) will determine the early success of the experiment
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forward jets
Photons from Higgs decay
qqH → qqγγ MH = 120 GeV
H→ γγ Signal
SIGNAL: two isolated photons with large Et
Gluon-gluon fusion WW and ZZ fusion (Weak Boson Fusion) WH, ZH, ttH (additional leptons) Total σ x BR ~90 fb for MH = 110-130 GeV Very good mass resolution better than 1%
H → γγ MH = 115 GeV Jets from qq are at
high rapidity and large Δη
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H→ γγ Background
BACKGROUND ‘irreducible’ backgrounds, two real photons
gg→ γγ (box diagram) qq→ γγ (born diagram) pp→ γ+jets (2 prompt γ)
‘reducible’ backgrounds, at least one fake photons pp→ γ+jets (1 prompt γ + 1 fake γ) pp→ jets (2 fake γ) pp→ ee (Drell Yan) when electrons are mis-identified as photons
Process Pthat (GeV) Cross section (pb) Events/1 fb-1
pp→γγ (born) >25 82 82K
pp→γγ (box) >25 82 82K
pp→ γ+jets >30 90x104 90M
pp→jets >25 1x108 1x1011
Drell Yan ee - 4x103 4M
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Cross section and K-factors
Signal cross sections and BR (NLO M. Spira)
K-factors for the background
pp→γγ (born) 1.5
pp→γγ (box) 1.2
pp→ γ+jets (2 prompt) 1.72
pp→γ+ jets (1 prompt+ 1 fake) 1
pp→jets 1
M=115 GeV M=120 GeV M=130 GeV M=140 GeV M=150 GeV
σ (gg fusion)(pb) 39.2 36.4 31.6 27.7 24.5
σ (IVB fusion) (pb) 4.7 4.5 4.1 3.8 3.6
σ (HW, HZ, Hqq) (pb) 3.8 3.3 2.6 2.1 1.7
Total (pb) 47.6 44.2 38.3 33.6 29.7
BR (H→ γγ) 2.08x10-3 2.21x10-3 2.24x10-3 1.95x10-3 1.40x10-3
Inclusive σ x BR (fb) 99.3 97.5 86.0 65.5 41.5
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Outline of the Analysis
Trigger (Level-1+High Level Trigger) Calibration of the electromagnetic calorimeter
Background Simulation
Photon isolation Vertex estimation Conversion identification π0 rejection
Selection, background estimation C.L. extraction for discovery or exclusion Effect of systematic errors Analysis optimization
Results
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CMS Trigger Design – Two stages
Level 1 hardware trigger
High Level Trigger (HLT)
“Offline” code running on PC farm
40 MHz
100 kHz
1 Tbit/s
100 Hz
Beam crossing rate 40 MHzInteraction rate 1 GHzMax Level 1 trigger rate 100 kHzEvent size 1 MByte
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DAQ ArchitectureR
ead
ou
t B
uild
ers
12.5 kHz +12.5 kHz +12.5 kHz
Dat
a to
su
rfac
e
Aggregate data flow ~1 Tbit/s In the process of ordering PCs and network elements
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Electromagnetic trigger towers are classified in three categories depending on the energy deposition in the calorimeter trigger towers: unidentified, non-isolated, isolated.
Single isolated Et>23 GeV
Double isolated Et>12 GeV
Double non-isolated Et>19 GeV
Total electron+photon Level-1 trigger rate: 4.4 kHz Level-1 trigger efficiency for H→ γγ larger than 99.5%
Level-1 Trigger
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H → γγ signal has two isolated photons Dominant background from di-jets and γ+jet has at least one candidate
from jet fragmentation that is not well isolated
We keep early conversions in the double stream
HLT trigger efficiency 88% Trigger is relatively easy for H→ γγ because of high Et photons Total rate for photons after HLT ~6 Hz
High Level Trigger for Photons
HLT photon selection
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ECAL Calibration
Use electrons from W→eν decays Match the measured ECAL cluster energy with the electron momentum
measured in the silicon tracker
Investigating alternative methods that use π0 or η
Calibration precision in barrel Effect on Higgs mass
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Events must be pre-selected events at the generator level and we only simulate and reconstruct those events that more likely pass the analysis selection
For jets also the generation is a very heavy task Generator level selection:
charged track isolation at generator level allow more than em interacting particle to contribute to the energy of the
photon candidate Results of the selection are verified after simulation and reconstruction
using data samples generated with looser pre-selection
BG Simulation
Background coming from pp→jets and pp→γ+jets contributes more than half the total background for H→γγ search
The cross section is huge for the process pp→jets
Process Pthat cut (GeV) σ (Pythia LO) Events per 1 fb-1
pp→γ+jets >25 9.0x104 pb 9.0x107
pp→jets >35 1.0x108 pb 1.0x1011
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BG Simulation II
Pthat cut (GeV)
σ gen(pb)
σ sim(pb)
Red. factor
Ineff.parton cuts (%)
Ineff.particle cuts
Ineff. total
pp→γ+jet 25 9.0x104 2.6x102 350 0.1 3.4 3.5
pp→jets 30 1.8x108 4.4x103 41000 0.0 14.1 14.1
pp→jets 40 6.0x107 4.3x103 14000 1.3 14.1 15.4
pp→jets 45 3.7x107 4.3x103 8700 3.4 14.1 17.5
pp→jets 50 2.5x107 4.1x103 6000 6.5 14.1 20.6
With this selection it was possible to simulate 5 million jet events corresponding to 1 fb-1 integrated luminosity
Total pre-selection inefficiency ~20% Events lost are mainly with low Et photons, not very important
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Primary Vertex Determination
LHC beams have a longitudinal spread of 7.5 cm Longitudinal interaction spread ~5 cm Vertex estimated from the underlying event and recoiling jet We use a combination of the two methods:
Maximal scalar sum of tracks pt
Maximal vector sum of tracks pt
The efficiency of determining the right vertex is ~83% Higgs events after selection
Efficiency for the different types of background is similar Can also use identified converted photons – not done yet
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Primary Vertex Determination II
Process Eff (%)
H→γγ (gg fusion) 82
H→γγ (IVB fusion) 89
pp→γγ (born) 71
pp→γγ (box) 72
pp→γ+jet (2 prompt) 78
pp→γ+jet (1 prompt + 1fake) 86
pp→jets 90
Efficiency of determining the primary vertex within 5 mm from the true one
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Photon Conversions
CMS tracker is rather thick ~30% of the photons convert before reaching the ECAL Energy resolution somewhat deteriorated Conversions can be identified in the tracker Identified conversions may also help to distinguish γ’s from π0’s
Total2 tracks1 track
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R9: Sum of 9/cluster energy
Unconverted photons have large R9
Converted photons, photons in jets and electrons have small R9
π0’s also have small R9
Energy resolution is better for large R9
Shower Shape Variables
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0.88<R9< 0.95
Photon Energy Resolution
σfit=0.64%
σfit=1.35%
σfit=1.91%
σfit=0.92%
σfit=1.02%
σfit=0.78%
Barrel Endcaps
R9>0.95
R9<0.88
Photons with Et > 40 GeV
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Photon Isolation
Reducible backrounds (π0’s and mis-identified jets) have other particles near at least one photon candidate
Most of discriminating variables are built by summing up the Et or Pt of calorimeter deposits or tracks within a cone
ΔR = (Δη2+ Δφ2)
To study the performance of isolation variables we use individual photon candidates
Signal is: H→γγ gg-fusion 2nd highest Et cluster with Et>40 GeV matched with a generated photon within ΔR<0.2, background is: γ+jet 2nd highest Et super-cluster with Et>40 GeV NOT matched with a generated photon
Plot the distribution of the variables for signal and background, move the cut and compute Eff. Sig and Eff. Background
ΔR
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Combined Isolation Performance Plot
EndcapsBarrel
The sum of ECAL, HCAL and Track Et variables can be considered as a global isolation variable
Cone sizes and Et thresholds have been optimized for the different detectors
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All isolation variables are more or less correlated We used a Neural Network with 2, 3 or 5 of following inputs:
ΔR of the 1st track with Pt>1.5 GeV/c Sum ECAL Et within ΔR<0.3 The shower shape variable R9
Sum HCAL Et within ΔR<0.35 Sum tracks Et within ΔR<0.2
Did not use kinematical information, easy to combine these variables with reconstructed mass and photons Et in an optimized H→γγ analysis
Isolated Photon Identification with NN
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Selection for Inclusive Analysis
Photon selection: photon candidates are reconstructed using the hybrid clustering algorithm in the barrel and the island clustering algorithm in the endcaps ET1, ET2 > 40, 25 GeV |η|<2.5 Both photon candidates should match L1 isolated triggers
with ET > 12 GeV within ΔR < 0.5 Track isolation
No tracks with pt>1.5 GeV present within ΔR<0.3 around the direction of the photon candidate
Calorimeter isolation Sum of Et of the ECAL basic clusters within 0.06<ΔR<0.35
around the direction of the photon candidate <4 GeV Sum of Et of the HCAL towers within ΔR<0.3 around the
direction of the photon candidate<8 GeV(6 GeV) in barrel (endcaps)
Sum of the 2 less than 10 GeV(8 GeV) in barrel (endcaps)
L1 + HLT inefficiency negligible after selection
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Mass Spectrum of Selected Events
All plots are normalized to an integrated luminosity of 1 fb-1 and the signal is scaled by a factor 10
Fraction of signal is very small (signal/background ~0.1) Use of background MC can be avoided when we will have data Data + signal MC can be used for optimizing cuts, training NN and
precise BG estimation
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Results
Higgs efficiency for MH=120 GeV
Background expectation at MH=120 GeV
Before isolation BG expectation ~ 300 times larger
Box(fb/GeV)
Born(fb/GeV)
γ+ jets 2 prompt (fb/GeV)
γ+ jets 1 prompt+ 1 fake(fb/GeV)
Jets(fb/GeV)
Total(fb/GeV)
H→γγ (MH=120 GeV) eff. in window of 2.5 GeV (%)
32 46 62 51 64 255 24.4
MH After photon selection Final
120 GeV 52.7% 34.4%
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Discovery Significance/Exclusion C.L.
Use Log Likelihood Ratio frequentistic approach Log likelihood ratio between the signal+background hypothesis
and the BG only hypothesis
Mean/Median BG only Mean/Median Sig + BG
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Effect of Systematic Errors
Input for CL calculation is: Background expectation from fit to the data (sidebands) Signal expectation from MCOrigin of systematic errors Error on the BG estimation (statistical from fit of sidebands +
uncertainty of the form of the fitted function) Error on the signal (theoretical σxBR, integrated luminosity,
detector + selection efficiency)Effect of systematic errors Systematic errors on the signal does not change the expected
discovery CL Systematic error on the signal makes exclusion more difficult Systematic error on the BG makes exclusion and discovery
more difficult
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Main Systematic Errors
SIGNAL Theoretical error on cross
section times BR (~15%) Integrated luminosity (5%) Higgs Qt distribution – effect
to be evaluated Selection efficiency (≲5%)
For now assume a total of 20% (anyway not important in case of discovery)
BACKGROUND Statistical error on the fit of
the sidebands (~0.3% for ~20 fb-1)
Systematic error on the shape of the fitted function (~0.3%)
No other errors when data available
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Large effect on the 5σ discovery of the systematic error on the background (due to the small s/b in this channel)
A signal/background cut is applied at 0.02
Results for Discovery/Exclusion
MH=120 GeV 3σ evidence
Int L (fb-1)
5σ discovery
Int L (fb-1)
95% exclusionInt L (fb-1)
Counting 11.3 31.5 3.6
Mass distribution 10.7 30 3.4
Counting WITH 0.5% error on BG, 20% Error on Signal
13.2 53.5 4.2
Mass distribution WITH 0.5% error on BG , 20% Error on Signal
12.7 47 4.0
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How to Improve the Sensitivity
We consider the following: ‘Reducible’ background tends to have smaller R9
‘Reducible’ background contribution is larger in the endcaps Split the sample into 4 by using the 4 combinations of the
requirements: Both photons in the barrel or not min(R91, R92) larger or smaller than 0.93
this is equivalent to having different channels
Can also split more the sample by dividing the ECAL into 4 pseudo-rapidity regions and for each region have progressively tightening cuts
When increasing the number of categories the uncertainty on the BG fit should approximately increase by (number of categories)
The uncertainty is basically uncorrelated between the different categories
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Four Categories
Barrel, min(R91, R92)>0.93
Endcap, min(R91, R92)>0.93 Endcap, min(R91, R92)<0.93
Barrel, min(R91, R92)<0.93
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Discovery/Exclusion with More Categories
MH=120 GeV 3σ discovery
Int L (fb-1)
5σ discovery
Int L (fb-1)
95% exclusionInt L (fb-1)
4 Categories 8 22 2.616 Categories 6.6 18.5 2.2
4 Categories WITH 1% error on BG, 20% Error on Signal
8.8 30 3.1
16 Categories WITH 1.5% error on BG, 20% Error on Signal
7 21.5 2.6
The overall effect of splitting into more categories is a decrease of the effect of the systematic error on the CL (mainly because of the increase of s/b for a fraction of events)
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Discovery/Exclusion with 16 Categories
With ~25 fb-1 we can discover the Higgs boson with mass between 115 and 140 GeV
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Optimized Analysis
Can do better using the optimized signal/background ratio ordering method technique
I introduced this method in L3 for Higgs searches and has been later used for all Higgs searches at LEP
First of all split sample into 6 categories by using the 6 combinations of the requirements: Both photons in the barrel or not min(R91, R92) larger than 0.95, between 0.90 and 0.95 and
smaller than 0.90 Then, for barrel and endcap events use a NN with the following
mass independent input variables: Isolation NN γ1
Isolation NN γ2
Et1/M Et2/M PLHiggs
|Δη| Train the Neural Network using data as background outside the
mass region under study
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NN Output and Mass
mass distribution
category 1category 1
Background
Signal
category 1category 1
NN output without mass information
Compute log(s/b) for each event Add the NN output and Mass log(s/b) Than combine all 6 categories into one single plot Use LLR frequentistic method to evaluate the sensitivity
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Results of Optimized Analysis
Now get 5σ significance at MH= 120 GeV with an integrated luminosity of 7 fb-1
Important source of improvement is the exploitation of high s/b Weak Boson Fusion events
Expect some degradation from error on the background but less than in the conventional analysis
4% s/b cut
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Summary and Outlook
SM Higgs boson (or MSSM h) can be discovered with CMS in the H→γγ channel with 25 fb-1 at low luminosity, with a conventional cut based analysis in the mass range 115-140 GeV
Optimized likelihood analysis needs ~10 fb-1 luminosity for a 5σ signal in the same mass range
Further optimization Include additional tools for:
Photon conversion identification π0 rejection
Largest improvement expected from: Separation of other γγ channels, WBF, WH, ZH, ttH, exploiting their additional
signatures Then obviously from combination with H→ZZ*→4 lepton channels
Only one and a half years from the beginning of LHC operation, must continue to prepare the real analysis: based on data with minimal use of MC information using all possible control samples to verify the performances of the detector
We will hopefully discover soon a low mass Higgs boson at LHC
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Shower Shape Variable R9
rr99=S9/E=S9/ESCSC
non-converted
Note that s/b also improves as we select photons that
didn’t convert.
Note that s/b also improves as we select photons that
didn’t convert.
Jet backgroundJet background
Signal
categories
For highest EtFor highest EtFor highest EtFor highest Et
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Analysis flow for each category
Isolation NN photon 1
Isolation NN photon 2(same)
ET1/M
ET2/M
PLhiggs
Neural Net
Mass
SmoothBkgd
SmoothBkgd
Bin Signal andBkgd In LLR(same way)
Define a function of the mass and the NN output and plot events
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NN training
Use the background outside the mass region + signal MC When data available use data for the BG
Train
Validate Validate
Train
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Combined Signal/Background Variable
ln(s/b) from gg mass
From binned histogram.
ln(s/b) from gg mass
From binned histogram.
ln(s/b) from Neural Net
From fits to s and b.
ln(s/b) from Neural Net
From fits to s and b.
XXXX
====Log Likelihood per
eventLog Likelihood per
event
Rapidly falling Rapidly falling background background
distribution in region distribution in region with significant with significant
signal. signal.
Rapidly falling Rapidly falling background background
distribution in region distribution in region with significant with significant
signal. signal.
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WBF analysis
May tag forward jets and apply the additional following cuts pT
jet > 20 GeV
|jet| < 4.5
Rjet > 0.5
|jet1 –jet2| > 4.0
jet1 *jet2 < 0
pTHiggs> 50 GeV and
Mj1j2> 500 GeV
CompHEP background samples + 3 jets and + 2 jets were produced
PYTHIA underestimates QCD and prompt photon backgrounds when two forward jets are detected
We need more statistics to improve our results with CompHEP
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WBF Results
Integrated luminosity needed to reach 5 significance for three different scenarios
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H → ZZ* → 4ℓ
H → ZZ* → ℓ+ℓ-ℓ+ℓ- ℓ=e,μ Irreducible background:
ZZ production Reducible backgrounds
tt and Zbb Very good mass
resolution ~1%
In this channel (and in the H→ γγ) background can be easily estimated from data by fitting the sidebands
Above MH ~ 2MZ the two Z bosons are real and σxBR is larger
Golden channel for Higgs discovery at LHC
Branching ratio dip due to opening of WW channel
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Low mass discovery
With 30 fb-1 more than 5 sigma significance for MH>100 GeV Higgs boson can be discovered in more than one channel, possible to
measure its couplings
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Whole mass range discovery
All mass range accessible at 5σ significance with 10 fb-1
With a few fb-1 possible to discover the Higgs boson with mass between ~150 and ~500 GeV in the WW and ZZ channels
For mass larger than ~200 GeV use ZZ and WW leptonic decays For mass larger than ~700 GeV use qqH, H → ZZ → ℓℓνν and H → WW → ℓνqq
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After discovering the Higgs boson we should measure its parameters Studies for high luminosity (Int L = 300 fb-1)
SM Higgs boson mass direct reconstruction: 4ℓ, γγ, bb likelyhood fit WW
Measurement of Higgs bosons parameters
INT L = 300 fb-1
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MSSM Higgs Searches
Two Higgs doublets model 5 Higgs bosons: 2 Neutral scalars h,H 1 Neutral pseudo-scalar A 2 Charged scalars H±
In the Higgs sector all masses and couplings are determined by two independent parameters
Most common choice: tanβ – ratio of vacuum
expectation values of the two doublets
MA – mass of pseudo-scalar Higgs boson
In the MSSM: Mh ≲ 135 GeV
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Neutral MSSM Higgs bosons
Decoupling limit (MA≳150 GeV) h behaves like HSM
Standard model searches directly apply MH~MA~MH
±
MA=O(MZ) and large tanβ H behaves similarly to SM Higgs (SM searches apply)
In other cases for large tanβ h(H) → WW,ZZ highly suppressed (A → WW,ZZ never allowed at tree
level) h(H),A almost exclusively decay into bb and ττ and are produced in
association with bb pair Large MA small tanβ
H,A decays almost 100% into tt for lower masses (200-300 GeV) also H → hh and A → Zh
If SUSY particles are light the Higgs bosons may decay into s-particles
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h,H production and decay
Decoupling region
Large tanβ mainly
bb, ττ decays Large tanβ hbb, Hbb (and
Abb) production dominates
h,H decays h,H productiontanβ = 30
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Results from SM Higgs Searches
In a large part of the MSSM parameter space SM Higgs searches are effective to find the MSSM h boson
In the decoupling region if h observed hard to distinguish SM from MSSM
CMS 5σ discovery contours
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MSSM h,H decays
Decoupling region
Large tanβ
bb, ττ decays
Small tanβ H decays into tt when allowed
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MSSM Production processes
Large tanβ hbb, Hbb and
Abb production dominates
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Higgs Bosons visibility in the MSSM
All the plane is covered but there is a large area where only h can be seen
4 Higgs observable
3 Higgs observable
2 Higgs observable
1 Higgs observable
5σ discovery regions in the MSSM tanβ – MA plane for MH
MAX scenario
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Beam Spread
LHC beams have a longitudinal spread of 7.5 cm Longitudinal interaction spread ~5 cm Vertex estimated from the underlying event and recoiling jet
Can also use identified converted photons – not done yet
Higgs signal (MH=120GeV)
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Vertex Determination
We use a combination of the two methods: Maximal scalar sum of tracks pt
Maximal vector sum of tracks pt
The efficiency of determining the right vertex is ~83% Higgs events after selection
Efficiency for the different types of background is similar
Process Eff (%)
H→γγ (gg fusion) 82
H→γγ (IVB fusion) 89
pp→γγ (born) 71
pp→γγ (box) 72
pp→γ+jet (2 prompt) 78
pp→γ+jet (1 prompt + 1fake) 86
pp→jets 90
Efficiency of determining the primary vertex within 5 mm from the true one