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Higgs Boson Searches in the two photon
Taiki Yamamura (Univ. of Tokyo)
On behalf of the ATLAS collaboration
High Energy Physics
in the LHC Era
4th International workshop
06/01/2012
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Outline
Higgs boson search in the two photon
(1) Search for the Standard Model(SM) H→-1
Main topic of this presentation
with 4.9fb (= full dataset of 2011)
H→ (2 photons)
decay channel (H→ ) at ATLAS
(2) Search for the Fermiophobic H→ with 1.08fb-
(3) Summary & future prospects
3High Energy Physics in the LHC Era ー 4th International workshopTaiki Yamamura
(Univ. of Tokyo)
will be presented briefly.
(※ This is not a result with the full 4.9fb sample,-1
that has not been published yet. )
Search result with 1.08fb-1
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https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/
◆ SM H→ : ATLAS-CONF-2011-161
References
◆ Fermiophobic H→ : ATLAS-CONF-2011-149
4High Energy Physics in the LHC Era ー 4th International workshopTaiki Yamamura
(Univ. of Tokyo)
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earc or e →
5High Energy Physics in the LHC Era ー 4th International workshopTaiki Yamamura
(Univ. of Tokyo)
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Search for the SM H→
At LHC, H→ is the most sensitive
・Narrow si nal eak in M s ectrum
・Small branching ratio, but large event
Branching fraction
for SM Higgs boson
channel in the low mass range.
yield due to high selection efficiency.
(110<
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H→ analysis
◆ Backgrounds
・Irrreducible BG : Main contribution
・Reducible BG : +jets, di-jet
・Z→ee(DY) (・・・ very small contribution)
◆ M( ) reconstruction
・ M( ) = 2E1E2・(1ーcos α)2
7High Energy Physics in the LHC Era ー 4th International workshopTaiki Yamamura
(Univ. of Tokyo)
✓α ・・・ Opening angle of the two photons
・For the precise reconstruction, careful understandings
are needed for the followings :
✓Energy calibration & resolution
✓Primary vertex position (related to α)
(related to E1 and E2)
(→ For details, see the following slides.)
✓ E1 [E2] ・・・ Energy of 1st [2nd ] photon
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Energy calibration & resolution
<Z→ee peak>
Photon energy calibration
◆ MC-based calibration
(Tuned by beam-test result)
◆ After MC-based calibration,
electron energy scale corrections
8High Energy Physics in the LHC Era ー 4th International workshopTaiki Yamamura
(Univ. of Tokyo)
.(Scale factor is obtained from Z→ee.)
Energy resolution
Resolution correction is applied to MC.
(This correction factor is also determined by comparing
the Zee peak between data and MC.)
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Vertex reconstruction
Vertex position is measured by
(z-vertex)
・Unconverted photon :
“1st + 2nd layer of EM calorimeter”
“pointing method”.
・Converted photon :
“1st layer of EM calorimeter”
+ “conversion oint →ee ”
<Calorimeter
(i) Measure photon-direction.
pointing>
EM
calorimeter
Beam axis
Robust measurement against pile-up.
<H→ peak with various conditions for pile-up>
M( ) resolution also becomesstable against pile-up effect.
By using “pointing method”,
(ii) z of primary vertex
is deduced.
9
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Event selection
◆ 2-photon trigger
◆ Primary vertex selection
(for selecting collision event)
Event signature is very simple.
Event with two high-Et photons
(ET( 1, 2)>40, 25GeV )
Diphoton event candidate
(Obtained from data)
◆ Selection for di-photon event
Photon is required to satisfy :
・|η|
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(conv, η)
Event categorization
To improve sensitivity, data sample
different S/B and M( ) resolution.
is divided into 9 categories with
9 categories
(a) Conversion- η categories(5 categories)◆ Both photons unconverted
・Central ・Rest
◆ At least one photon converted
・Central ・Rest・Transition
Thrust axis
)()( 21 T T p pt rr
r
pT,t
・pT,t is the transverse component of
pT( γγ) with respect to the thrust axis.
・pT,t has more discriminative power
than pT( γγ).11
Definition of pT,t
(b) pT,t categories ( 5 → 9 categories )
◆ Low pT,t (40GeV)
(pT,t = “pT-thrust”)
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Analysis results
12High Energy Physics in the LHC Era ー 4th International workshopTaiki Yamamura
(Univ. of Tokyo)
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Background decomposition
◆ Decomposition for “ +DY”, “ +jets” and “di-jet” is performed
as a control sample.
(data-driven)
in a data-driven manner.Control sample is obtained from “anti-cut” region that is defined
with photon-ID and isolation variables for the two photons.
(※For details, see backup slide.)
◆ DY contribution is also estimated by using “e events”
※
Enriched with Z→ee where one electron
is faking as photon.
Result
The contribution from irreducible BG ( )
is dominant. (Fraction = 71% )
It could be also confirmed that the
(
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Background modeling
BG shape is defined by the fit with single-exponential
(100
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Signal modeling
◆ Signal MC
(100-150GeV with 5GeV step)
・Function : “Crystal-ball + Gaussian”
(ggF, VBF, WH/ZH, ttH)
Samples are available at 11 mass points.
◆ Peak shape modeling
Signal shape
inclusive
Peak resolution
( =120GeV)
( =120GeV)
σCB
(GeV)
・Global fit :
Simultaneous fit is performed for allHm
Hm
mH(GeV) 110 115 120 125 130 135 140 145 150
#evts 69.9 71.5 70.9 68.3 63.7 57.5 49.8 40.8 30.6
inclusive 1.7
Best category(unconv central)
1.4
Worst category(conv transition)
2.3
(for =110-125GeV)
◆ Expected # of signal events (4.9fb , inclusive)-1
mass points in each category.
The signal peak shape are parameterized
linearly as a
Nsig ~ 70evts
function of mH.
15Hm
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Main systematics
(a) Signal yield : ~20%
(b) H→
mass resolution : ~14%
(c) Migration of signal events
between categories
(Error on # of signals)
Summary of sys errors
(a)
(b)
. ,
⊿Nsig = ±8% (for high PT,t-bin)
(d) BG modeling
⊿Nsig = 0.1-5.6 events
The intrinsic difference between
・ Chosen background model (= exponential)
・ True background shape
(Depending on categories)
It is included into sys uncertainty on # of signal events.
16High Energy Physics in the LHC Era ー 4th International workshopTaiki Yamamura
(Univ. of Tokyo)
(c)
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Result for H→ search
Exclusion limit w.r.t
SM prediction
Limit setting
By using profile likelihood ratio
method, exclusion limit is
obtained with CLs.(95%C.L.)
◆ Expected limit(1.61-2.87)×SM @ = 110-150GeV
(1.61-1.78)×SM @ = 115-130GeV
◆ Observed exclusion=114-115GeV
=135-136GeV
17High Energy Physics in the LHC Era ー 4th International workshopTaiki Yamamura
(Univ. of Tokyo)
Hm
Hm
Hm
Hm
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Result for H→ search
p0-value w/o LEE
(Look-Elsewhere-Effect)
※p0 value :
Probability of seeing upward
fluctuation in the background-
only hypothesis as large as or
◆ Observed excess at mH=126GeV
・w/o LEE : 2.8σ
・w/ LEE : 1.5σ
18High Energy Physics in the LHC Era ー 4th International workshopTaiki Yamamura
(Univ. of Tokyo)
(p0=0.27%)
(p0=6.5%)
larger than the obtained excess.
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(2) Search for the Fermiophobic H→
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(Univ. of Tokyo)
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◆ Fermiophobic benchmark scenario
・Higgs does not couple to fermions
but couples only to bosons.
Branching fraction
in Fermiophobic
・Production mechanism
✓ ggF is gone.
scenario
γγ
Search for the Fermiophobic H→
20High Energy Physics in the LHC Era ー 4th International workshopTaiki Yamamura
(Univ. of Tokyo)
BF and VH become dominant.
Br (H→ ) is strongly enhanced
in the low mass range.
(
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Analysis results◆ Using 1.08fb
-1
◆Analysis entirely based on the SM analysis (SM H→ search)・Two isolated photons with pT>40 and 25GeV
・M( ) reconstruction : Using calo pointing / conv vertex
◆ Some differences w.r.t. SM analysis
・Categorization : 3 categories with pT( )
・BG model : 2nd order polynomial (Bernstein basis)
Note
Results with the full 4.9fb sample
has not been released yet.
-1
← Func. is optimized based on the categorization strategy.
Exclusion limit w.r.t the prediction
◆ Expected exclusion
=110-116GeV
◆ Observed exclusion
= 110-111GeV,
113.5-117.5GeV
21
(95%C.L.)
Hm
Hm
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Summary
◆ Search for the Higgs boson decaying to two photons have been
made by using 2011 data with the ATLAS detector.
・ Upper limit on x-sec
✓ Expected limit
✓ Observed exclusion
◆ Search for the SM H→ (with 4.9fb )-1
H→ search at the ATLAS experiment
. - . or - e
(1.61-1.78)×SM (for 115-130GeV)
= - e
=135-136GeV
・ Observed excess at =126GeV
w/o LEE : 2.8σ
w/ LEE : 1.5σ (※LEE : look-elsewhere effect)
22High Energy Physics in the LHC Era ー 4th International workshopTaiki Yamamura
(Univ. of Tokyo)
◆ Search for the Fermiophobic H→ (with 1.08fb )
-1
・ Expected exclusion : = 110-116GeV
・ Observed exclusion : = 110-111GeV, 113.5-117.5GeV
Hm
Hm
Hm
Hm
Hm
(※Results with the full 4.9fb sample has not been released.)-1
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Future prospects for SM H→ search
◆ Improvement of analysis sensitivity
・Multivariate technique for photon-ID selection
・Exclusive analysis (H+0/1/2 jets) etc.
◆ 2012 run at LHC
・Expected to obtain ~15fb (or more?).
・
-1
The x-sec for Hi s roduction will increase
◆ Prospects for SM H→ search in 2012
・ Exclusion is possible in complete region for 110<
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24High Energy Physics in the LHC Era ー 4th International workshopTaiki Yamamura
(Univ. of Tokyo)
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ac up →
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Backgrounds
◆ Irreducible background ( )
◆ Reducible back round + et di- et
Born Brems Box
◆ Drell-Yan (Z→ee) ・・・ Very small contribution
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(Univ. of Tokyo)
di-jet+jet
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LAr electromagnetic calorimeter
◆ EM calorimeter consists of 3 layers.
・Strip (1st
layer)・Middle (2nd layer)
・Back (3rd
layer)
<ATLAS detector ><EM calorimeter >
EM calorimeter
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(Univ. of Tokyo)27
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Photon reconstruction◆ Longitudinal segmentation
・ Strip (S1)
・ Middle (S2)
・ Back (S3)
・ Pre-sampler (in |η|
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E scale
◆ Understood from Z→ee.
◆ Energy scale at m(Z) known to ~0.5%.
◆ Linearity : better than 1%
◆ Uniformity
(Constant term of resolution) :
- .
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Event categorization
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(Univ. of Tokyo)
(eta, conv)
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9 categories based on conversion status, η and pT,t
(a) Based on η and conversion (5 categories)
◆ Both photons unconverted
・Central
・Rest
◆ At least one photon converted
・Central
Event categorization
※To improve sensitivity, data sample
different S/B and M( ) resolution.
is divided into 9 categories with
・Rest
・Transition
(b) Based on pT,t ( 5 → 9 categories )
◆ “Central” and “Rest” categories are divided into “Low-pT,t” and “High-pT,t”.
Thrust axis
)()( 21 T T p pt rr
r
・pT,t is the transverse component of
pT( γγ) with respect to the thrust axis.
・pT,t has more discriminative power
than pT( γγ).
(“Transition” category is not divided.)
◆ pT,t : “pT-thrust”
pT,t 31
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# of selected events in each category
(4.9fb , 100
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M( γγ) spectrum after the event selection
(data, 4.9fb , 100
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Background decomposition
◆ Data-driven BG decomposition
・Using “ABCD” method (i.e. A=B*C/D), # of
fake photons in “tight-isolated” region can be checked.
・This method is applied to 1st and 2nd photons iteratively.
◆ Z→ee
Control sample : “e ” events Z(ee) with one electron faking as photon
(for , +jets and di-jet)
Result
・Contribution from irreducible BG ( )
is dominant. (Fraction = 71% )
・Estimated result is consistent with
the predicted one from theory and MC.
・Very small contribution from Zee.
(
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Signal MC◆ Process
◆ Generator
・ggF, VBF : POWHEG
・WH/ZH, ttH : Pythia
◆ x-sec
・ggF, VBF, WH/ZH : NNLO
・ttH : NLO
ggF, VBF, WH/ZH, ttH
◆ Branching ratio : HDECAY
Correction to MC samples
・
35High Energy Physics in the LHC Era ー 4th International workshopTaiki Yamamura
(Univ. of Tokyo)
・Reweighting for longitudinal beam spot distribution
(σZ(data)=5.6cm)・Photon ID variables (shifted to match data)
・Energy smearing (Understood from Zee)
・Isolation variable (for lateral shower leakage, UE , pile-up)
・ggF : Reweighting for pT( ) distribution to matchthe one with HqT.
・# of ggF events : Corrected by considering the interference
with gg→ (BG).
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Signal modeling
・Function : “Crystal-ball + Gaussian”
・Global fit model :
◆ Peak shape modeling
Simultaneous fit is done for all of mass points in each category
to extract the best values of all parameters.
✓ Width and position of the peak depend on mH linearly.
✓ The other parameters : constant (not depending on mH)
Result of global fit
inclusive
Peak resolution
( =120GeV)
(for =120GeV)
σCB(GeV)
inclusive 1.7
Best category
(unconv central) 1.4
Worst category
(conv transition) 2.3
Hm
Hm
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# of signal events in each category (4.9fb )-1
37High Energy Physics in the LHC Era ー 4th International workshopTaiki Yamamura
(Univ. of Tokyo)
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Systematics
(i) # of signal events
・x-sec : +15%/-11%
・Iso-cut eff. : ±5%
・photon-ID eff. : ±11%(pdf and scale variations)(from comparison between data and MC)
(material effect)
(Understood from Zee)
・Pileup effect on photon-ID : ±4%
・Luminosity : ±3.9%
・Trigger : ±1%
・Higgs pT modeling : ±1%
38High Energy Physics in the LHC Era ー 4th International workshopTaiki Yamamura
(Univ. of Tokyo)
(Diff between HqT and Resbos)
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Systematics
(ii) Resolution of signal peak
・Cluster energy resolution : ±12%(Constant term)
・Energy calibration : ±6%
・Pileup : ±3%
(Extrapolation from electron energy
measurements) (Material effect)
(Pileup noise impact on cluster energy)
・Resolution of photon-angle measurement
(pointing)
: ±1%
39High Energy Physics in the LHC Era ー 4th International workshopTaiki Yamamura
(Univ. of Tokyo)
(Studied with ⊿z between two photons)
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Systematics
(iii) Uncertainty due to event migration among the categories
Uncertainty on # of signal events in the category is also defined.
・Between converted/unconverted
・Between hi h and low T,t cate ories : ±8% for hi h T,t-bin
categories : ±4.5% (for unconv-bin)
(Comparing #evts fraction with low and high pileup condition.)
(Material effect)
40High Energy Physics in the LHC Era ー 4th International workshopTaiki Yamamura
(Univ. of Tokyo)
(Scale variation in HqT)
(photon energy scale uncertainty)
(iv) Uncertainty due to BG modeling
See the following slides.
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ncer a n y ue o mo e ng
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BG modeling
◆ In H→ analysis, the background shape is modeled by
single-exponential function.
◆ There can be intrinsic difference between the true background
shape and the chosen background model (= exponential).
(detail)
Reliability of our BG model needs to be checked.
(Check 1) Use of another function
Modeling is also cross-checked with another functions.
(2nd order Bernstein polynomial etc.)
No large difference found for sensitivity.
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(Check2) Difference between the exponential model
・It is checked by using parton-level
and the true background shape
generator(RESBOS, process).
・Integrated the residual in a sliding
window (±2GeV) over the search
range of 110-150GeV.
(= “residual”)
Largest integrated residual is
regarded as sys error.
<Uncertainty on # of signal events due to BG modeling>
(in each of 9 categories)
43
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Limit calculation on H→
◆ Profile likelihood ratio method is used.
(Limit setting is done with CLS.)◆ Unbinned maximum likelihood fit is performed simultaneously
in 9 categories.
・ Signal parameters : fixed
・ BG parameters : free
・ → .
◆ PDF for (S+B)-fit
bb s spr spr s signal f n f f n pdf )(
Signal model(Defined by MC)
BG term
Signal term which is produced by
the bias due to BG modeling.
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List of nuissance parameters
45High Energy Physics in the LHC Era ー 4th International workshopTaiki Yamamura
(Univ. of Tokyo)
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0
46High Energy Physics in the LHC Era ー 4th International workshopTaiki Yamamura
(Univ. of Tokyo)
BG model
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p0 calculation with alternative BG model
Two alternatives have been also tried for BG modeling.
(i) Hybrid model
・High pT,t category : Bernstein
・Others : Exponential
(ii) Bernstein-only model
47High Energy Physics in the LHC Era ー 4th International workshopTaiki Yamamura
(Univ. of Tokyo)
.
◆ Uncertainties due to BG modeling
The values when using single exponential are
set to be zero or doubled.
◆ Photon energy scale uncertainty of 0.5%
is introduced in the likelihood fits.
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Result for p0-value (with alternative BG models)
The obtained results are quite
to the nominal one.
The largest change in the
observed significance at
=126GeV isH
m
48High Energy Physics in the LHC Era ー 4th International workshopTaiki Yamamura
(Univ. of Tokyo)
p0-value for the 126GeV excess
nominal Hybrid Bernstein-only
Local p0 0.27%(2.8σ) 0.38%(2.7σ) 0.25%(2.8σ)
Global p0 6.5%(1.5σ) 8.9%(1.3σ) 6.0%(1.5σ)
0.16 standard deviations.
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Combined result for SM Higgs boson in ATLAS
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Combined result for SM Higgs boson in ATLAS
50High Energy Physics in the LHC Era ー 4th International workshopTaiki Yamamura
(Univ. of Tokyo)
Excluded region (at 95% C.L.)
112.7<
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ac up erm op o c →
51High Energy Physics in the LHC Era ー 4th International workshopTaiki Yamamura
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◆ Fermiophobic benchmark scenario
・Higgs does not couple to fermions
but couples only to bosons.
Branching fraction
in Fermiophobic
・Production mechanism
✓ ggF is gone.
scenario
γγ
Search for the Fermiophobic Higgs boson
52High Energy Physics in the LHC Era ー 4th International workshopTaiki Yamamura
(Univ. of Tokyo)
BF and VH become dominant.
Br (H→ ) is strongly enhanced
in the low mass range.
(
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Search for the Fermiophobic Higgs boson at ATLAS
◆ Using 1.08fb-1
◆ Analysis entirely based on the SM analysis (SM H→ search)・Two isolated photons with pT>40 and 25GeV
・M( ) reconstruction : Using calo pointing / conv vertex
◆ Some differences w.r.t. SM analysis
・3 categories with pT( ) :
(H→ )
BG is modeled by 2nd order polynomial (Bernstein basis)
pT( )
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M( ) spectrum
◆ low pT( )
(100
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Systematics
Most of the uncertainties are the same as for SM analysis
・Theory : ±9%
±6.5evts (low pT( ))
・BG modeling :
except for :
±2.2evts (middle pT( ))
55
±0.65evts (high pT( ))
・Migration : ±2% between low-pT and high-pT categories
High Energy Physics in the LHC Era ー 4th International workshopTaiki Yamamura
(Univ. of Tokyo)
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◆ Expected exclusion :
◆ Observed exclusion :
Exclusion limit
Exclusion limit w.r.t the prediction (95%C.L.)
=110-116GeVHm
56
・CMS (H→ ) :
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https://twiki.cern.ch/twiki/bin/view/AtlasPublic/HiggsPublicResults
H→ public results
(1) Search for the Standard Model(SM) H→
57High Energy Physics in the LHC Era ー 4th International workshopTaiki Yamamura
(Univ of Tokyo)
(2) Search for the Fermiophobic H→