win 05 -delphi 10/6/2005 iacopo vivarelli-infn pisa 1 higgs physics at atlas and cms iacopo...
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10/6/2005 Iacopo Vivarelli-INFN Pisa 1WIN 05 -Delphi
Higgs Physics at ATLAS and CMS
Iacopo VivarelliINFN Pisa and University of Athens
On behalf of the ATLAS and CMS collaborations
10/6/2005 Iacopo Vivarelli-INFN Pisa 2WIN 05 -Delphi
Summary• Standard Model Higgs Boson:
- discovery channels at LHC over the Higgs mass range. Discussion on the main channels in each Higgs mass region. Overall view of the statistical significance for the discovery as a function of MH
- SM Higgs parameters measurement: determining the Higgs sector after the discovery
• MSSM Higgs: discovery perspectives for the LHC experiments
All the results assume full operative detector (full detector installed, final performances in terms of alignment, calibration, etc.)
10/6/2005 Iacopo Vivarelli-INFN Pisa 3WIN 05 -Delphi
SM Higgs boson - Introduction• The Higgs boson mass is not
theoretically foreseen. Both theoretical and experimental limits exist
From direct LEP search:
MH > 114.4 GeV
From the Electroweak fit of the standard model
MH < 260 GeV
MH > 1 TeV is theoretically forbidden
The LHC experiments have to cover from the LEP limit up to the TeV
scale
10/6/2005 Iacopo Vivarelli-INFN Pisa 4WIN 05 -Delphi
Cross sections… Higgs production cross sections
available at NLO. CMS often quote the results with K-factors included. ATLAS
usually quote the LO results (preliminary results on NLO).
KggH > 1.7; KttH~1.2;KWH~1.3;KVBF~1.1
Gluon fusion is the dominant production channel
Vector Boson Fusion (VBF) in the following is the next production
channel over the whole mass range
Though less important in terms of absolute cross section values, associated productions (ttH,WH), provide distinctive signatures. Relevant for largely
background dominated decay channels (bb,)
VBF is characterized by energetic jets in the forward region and by lack of hadronic activity in the central region
10/6/2005 Iacopo Vivarelli-INFN Pisa 5WIN 05 -Delphi
… and Branching Ratios
BR
bb
WWZZ
LEP excluded
Decays into third generation down-type fermions largely dominating at low mass
(MH < 130 GeV)
Hbb decay detection feasible only if the associated production ttH is considered.
H suppressed, but it provides a very clear signature.
Higgs decay into tau pairs important if VBF production is considered
Vector Boson decays imporant at large MH
and in the intermediate mass region
I will concentrate mainly on the MH < 200 GeV region
10/6/2005 Iacopo Vivarelli-INFN Pisa 6WIN 05 -Delphi
Detector simulationUnless differently stated, the results have been obtained with
generator (Pythia, ME) and with a fast simulation of the detector.
In fact: Full geant simulation of the
detector response
More precise detector response
Long CPU time for event simulation (O(1 min) per event)
Extract detector response from Geant simulation
Parametrize the detector response and apply the parametrization to the particle level event
Less precise detector response.
Short CPU time for event simulation
For the most impotant channels, full simulated results have been already obtained. Work in progress for the others
10/6/2005 Iacopo Vivarelli-INFN Pisa 7WIN 05 -Delphi
Higgs decay into bb
L = 30 fb-1 k = 1.5
M ~ 15 GeV The region MH<130 GeV is of particular interest (LEP EW fit of the SM)
Final state involves 2 jets, 4 b-jets, 1 lepton, ET
The lepton provides the trigger. b-tagging and jet energy scale are the crucial
detector-related issues.
Reducible background: tt+jets,W+jets production
Irreducible background: ttbb production
CMS applies KttH=1.5 S/√B = 5.3
ATLAS (no K,ttbb at ME,CTEQ5L) S/√B = 2.8
10/6/2005 Iacopo Vivarelli-INFN Pisa 8WIN 05 -Delphi
Higgs decay into
%47.0)(
%2.9
GeVEE
100 fb-1
MH=130GeV
K=1.6
S/BG ~ 1/20
M: ~1GeV
The Higgs decay into is a challenge for the EM calorimeters.
Key points: calorimeter resolution (and linearity), id of photons vs rejection against π0
CMS(barrel, measured with 5x5 crystal array)
ATLAS (barrel, module 0 at TestBeam, η = 0)
Main backgrounds: irreducible dominant. j and jj together are half the irreducible background
Dedicated algorithm for the recovery of photon conversion (~1/3) in the material in front of the
EM calorimeter
Calorimeter segmentation is critical to reject jets. For ~80% efficiency, the jet rejection factor is 1000
to 4000, depending on the ET
%55.0)(
155.0
)(
%7.2
GeVEGeVEE
10/6/2005 Iacopo Vivarelli-INFN Pisa 9WIN 05 -Delphi
Higgs decay into ZZFinal states investigated: 2e2μ, 4μ, 4e
Irreducible background coming from direct ZZ production
Reducible background coming mainly from tt,Zbb
The reducible background are strongly reduced by isolation criteria on the leptons
and by b-veto
The channel is useful for the Higgs discovery in the ranges
130 GeV < MH < 150 GeV
and
180 GeV < MH < 600 GeV (HWW is opened at ~160 GeV)
10/6/2005 Iacopo Vivarelli-INFN Pisa 10WIN 05 -Delphi
Higgs decay into ZZ(2)The H4μ decay is largely studied in full
simulation (ATLAS).
Efficiency of the muon spectrometer slightly affected by the layout change after
the TDR
The studies confirm the results published in the TDR
ATLAS
pT(GeV)
ATLAS
ATLAS
10/6/2005 Iacopo Vivarelli-INFN Pisa 11WIN 05 -Delphi
Vector Boson FusionDuring the last years a lot of work has been put on the VBF
production channels.
Jet
Jet
One expects two hard jets in the forward and backward regions of the detector forward jet tagging
Lack of color exchange between partons in the initial state reduced hadronic activity in the central region
central jet veto
Forward tagging jets
Higgs Decay
Extremely useful for Higgs discovery in the low and
intermediate mass region
It can be used for several Higgs decay channel useful to measure Higgs coupling
10/6/2005 Iacopo Vivarelli-INFN Pisa 12WIN 05 -Delphi
Vector Boson Fusion (2)Implementation of fwd jet tagging and jet veto algorithms:
•Associated jets are “forward”. They are well separated in pseudorapidity
•Veto any jet in the central region (except decay products of the Higgs)
Forward jets are the “signature” of VBF
Central jet veto effective for QCD background rejections, in particular against the inclusive tt production, which is a common
background for all the VBF channels
10/6/2005 Iacopo Vivarelli-INFN Pisa 13WIN 05 -Delphi
Vector Boson Fusion (3)Detailed tuning of the fast
simulation on the full simualtion results has
been done.
Study undergoing for the new software/new
detector layout/new simulation
Pileup is a critical issue for the central jet veto. Fake veto rate strongly
depends on the amount of pileup and on the veto threshold applied. Results
for the new simulation are being produced
ATLASATLAS
ATLAS
10/6/2005 Iacopo Vivarelli-INFN Pisa 14WIN 05 -Delphi
VBF HWW
MH=160 GeVWWe
ATLAS
10 fb-1
CMS
60 fb-1
The best results are obtained if the leptonic decay is considered for both the Ws.
Dominant backgrounds come from tt production (reduced by the central jet veto and b-jet veto), WW production
Selection includes fwd tagging, central and b jet veto, spin correlations between leptons
Careful study on the bakground systematics due to generator uncertainties most of the multijet final states generated with ME generators. There are preliminary
results for NLO tt calculation.
The channel is one of the most promising for
135 GeV < MH <190 GeV
The normalization of the background value can be
estimated at 10% level from data. Background shape taken from MC
10/6/2005 Iacopo Vivarelli-INFN Pisa 15WIN 05 -Delphi
VBF H
missl
li
ii
i
PP
Px
Investigated both in the llETmiss and in the ljETmiss channel
ATLAS: it provides the cleanest signal for the discovery in the low mass range (MH< 140-150 GeV)
CMS: useful complementary tool to the H channel
The difference relies on the difference calorimeter and muon spectrometer design
Main backgrounds: Z+nj, tt, W+nj (hadronic decay of one of the two )
The Higgs mass can be completely reconstructed using the collinear approximation for the decay.
21xx
mm ll
Defining:
It can be shown that
10/6/2005 Iacopo Vivarelli-INFN Pisa 16WIN 05 -Delphi
VBF H(2)Etmiss (for the ll and lj channel) and hadronic tau reconstruction and resolution (for lj) are
key points for the mass measurement
Etmiss resolution related to jet measurement and to non-associated clusters in the
calorimeters
Hadronic tau decay: does the tracker improve the energy resolution?
QCD eventsG4 simulationATLAS
Hμ μG4 simulationATLAS
PRELIMIN
ARY
PRELIMIN
ARY
ETeflow/ET
truth
1 prong
Z
ATLAS
PRELIMIN
ARY
10/6/2005 Iacopo Vivarelli-INFN Pisa 17WIN 05 -Delphi
VBF H(3)
CMS
ATLAS30 fb-1
30 fb-1
Statistical significance (Poisson statistics) above 5 for 30 fb-1 if 110 GeV<MH<140 GeV
Background normalization control at 10% level from the expected Z peak and from the high sideband
10/6/2005 Iacopo Vivarelli-INFN Pisa 18WIN 05 -Delphi
Overall view for the SM HiggsLHC is forseen to run for a period (1y?) at reduced luminosity. The discussed
results have been obtained assuming L = 2x1033 cm-2s-1 (and the corresponding pile-up)
The present estimations show that 30 fb-1 of “good” data would be enough for the SM Higgs discovery
Above MH = 200 GeV (not shown in the plot) the HZZ decay provides a clear, almost background free signature.
10/6/2005 Iacopo Vivarelli-INFN Pisa 19WIN 05 -Delphi
Determination of Higgs parametersOnce the Higgs boson would be discovered, we want to measure its
parameters (mass, spin, partial widths, coupling constants)
Most of the coupling measurements will be possible because in the whole
Higss mass range more than one decay is accessible at one time
Spin determination examples:
•spin 1 rouled out if H or ggH is observed
• HZZ: spin correlations can be observed in the φ distribution
-
TL
TLR
LTG
22 sin)cos1()(
10/6/2005 Iacopo Vivarelli-INFN Pisa 20WIN 05 -Delphi
Determination of Higgs parameter(2)
Precision on the mass: it assumes 0.1% precision on lepton measurement, 1% precision on jet measurement (probably optimistic….)
Width determination: detector dominated is MH < 200 GeV. If MH> 200 GeV the H4l is used for direct determination.
10/6/2005 Iacopo Vivarelli-INFN Pisa 21WIN 05 -Delphi
Ratios of Higgs decay widths and couplingsPerformed a maximum likelihood fit to all channels, taking into account cross talk between signal channels, systematics
(luminosity, reconstruction efficiencies, backgrounds) Assuming that only SM
particles couples to the Higgs, the ratios between coupling constants can be
extracted. With additional hypothesis (couplings to W and Z as in the SM, no new particles enter in the loops for the decay)
the absolute coupling can be determined
10/6/2005 Iacopo Vivarelli-INFN Pisa 22WIN 05 -Delphi
MSSMMinimal Supersymmetric extension: two
Higgs doublets 8 degrees of freedom (5 particles):
CP-even : h,H CP-odd: A Charged: H+,H-
gu gd gV
h cos/sin -sin/cos sin(-)
H sin/sin cos/cos cos(-)
A 1/tg tg 0
Couplings to SM particles modified w.r.t. SM. Decay into third generation
fermions enhanced at high tgβ
At high MA the heavy bosons degenerate in
mass while the h saturate at a limit value (around
130 GeV)
10/6/2005 Iacopo Vivarelli-INFN Pisa 23WIN 05 -Delphi
h search at LHCMost of the studies done in the LEP maximal mixing scenario (μ=-200 GeV, MSUSY = 1 TeV, M2 =
200 GeV, Mg = 800 GeV)
In the decoupling limit (high MA), the h behaves like a SM Higgs boson. Decay into bb enhanced at high tgβ
h,hbb only
Most of the parameter plane covered by hbb
hμμ important at high tgβ at low MA
VBF channels not included in the plot
10/6/2005 Iacopo Vivarelli-INFN Pisa 24WIN 05 -Delphi
Heavy Neutral Higgs Bosons bbH/A bbμμ: cross section higher than in the SM at high tgβ.
Studied performed in full simulation by CMS.
Main backgrounds coming from Drell-Yan production and tt.
At 130 GeV, the resolution (1%) is not enough to resolve the mass difference between H and A
Most of the background rejected with the requirement of b-tag.
Since the associated b are soft (ET < 50 GeV), a special tagging algorithm (use of secondary vertex and impact parameter without the jet requirement in the calorimeter) has been used to
increase the b-jet ID
10/6/2005 Iacopo Vivarelli-INFN Pisa 25WIN 05 -Delphi
H/A decay into ’sThe tau decays provides the cleanest signature for the havy Higgs discovery at
high mass (and relatively high tgβ)
Investigated all the final states (ll, lj, jj). All of them contribute (at different MA).
The associated production (bbA/H) provides additional rejection against the main backgrouds (Z+jet, WW, QCD (double hadronic tau decay only)
Trigger issues solved w.r.t. the full hadronic final state
10/6/2005 Iacopo Vivarelli-INFN Pisa 26WIN 05 -Delphi
Overall coverage of the MA-tgβ plane for A/HThe A/H Higgs boson can be detected thanks to the A/Hμμ
decay in the low mass- high tgβ region
The decay into allows (with different final states) to cover a large part of the plane
There is no coverage (with 30 fb-1) for the large mass-intermediate tgβ region
The very high mass region can be reached only by the full
hadronic final state of the
Similar plot for ATLAS
10/6/2005 Iacopo Vivarelli-INFN Pisa 27WIN 05 -Delphi
• H+; hadr
Reconstruction of the top decay and of the
hadronic tau.
tt and Wt backgrounds suppressed
exploiting spin correlations One single π
has to carry the largest part of the tau
energy.
large background (tt+jets)
reconstruct tops and Higgs MH
• H+tb t bW blnqq
CMS L= 30 fb-1
Charged Higgs decayTwo channel are of particular interest
if MH > Mt.
10/6/2005 Iacopo Vivarelli-INFN Pisa 28WIN 05 -Delphi
Overall view (10 fb-1)
No VBF with VBF
Few statistics (of good data) is enough to cover most of the MA-tgβ plane
But a lot of questions to address before: machine operations, detector understanding, background measurement….
10/6/2005 Iacopo Vivarelli-INFN Pisa 29WIN 05 -Delphi
Overall view (300 fb-1)Can we disentangle between SM and MSSM just from the Higgs
sector?
In most of the parameters space more than 1 Higgs boson is
detectable.
In the yellow region just the h is detectable
Still in a region we can disentangle looking at the ratio
between the BR into and WW. Anyway, it is difficult….
10/6/2005 Iacopo Vivarelli-INFN Pisa 30WIN 05 -Delphi
Conclusions
-Standard Model Higgs detectable with few tens of fb-1 over all the mass range. In the most likely region, VBF production plays a
relevant role
-Spin measurement feasible in its ZZ decay.
-Coupling measurement will require full luminosity in most of the mass range. Achievable precisions: 15-50% depending on the
channel
-MSSM plane coverage requires few tens of fb-1 in a maximal mixing scenario. Higgs decays into third generation fermions play
an extremely relevant role. VBF important for the h in the decupling limit
10/6/2005 Iacopo Vivarelli-INFN Pisa 31WIN 05 -Delphi
BACKUP
10/6/2005 Iacopo Vivarelli-INFN Pisa 32WIN 05 -Delphi
Signal(mH=130GeV)ZZ 4 ZZ 22 ttbarZbb
3) Results for L=30 fbResults for L=30 fb-1-1
Signal (mH=130GeV)(eff=81.2%) 4.16
ZZ 4 1.36
ZZ 22 0.15
Zbbar 4 0.31
ttbar 4 0.01
BKG 1.83
Signal and background rates after overall analysisin mass window ±5 GeV around mH
Higgs Mass (GeV) Significance (Poisson)
130 2.322.32
150 5.125.12
180 2.242.24
By combining the channels H ZZ* 4, H ZZ* 4e, H ZZ* 2e2, the Higgs signal can be observed with a better than 5σ significance over most of the range 130<mH<180 GeV for an integrated luminosity of 30 fb-1
Before
after
10/6/2005 Iacopo Vivarelli-INFN Pisa 33WIN 05 -Delphi
MT>175 GeVMT<175 GeV
Signal Region
Outside Signal Region
10/6/2005 Iacopo Vivarelli-INFN Pisa 34WIN 05 -Delphi
2) Analyse: Background rejection
Signal and backgrounds after kinematic cuts ( integrated over a mass window of ± 5 GeV around mH=130 GeV)
2) Rejection of irreducible backgroundRejection of irreducible background Use Likelihood function & Neural Network with discriminating variables based on Higgs properties
Aim : Bring the reducible bkg down to ~ 10% of irreducible bkg (protection vs theoretical uncertainties) Rejection ~ 100 is needed
process σXBR(fb)
Signal : H 4 µ (mH=130 GeV) 0.100.10
Irreducible : (Z/*)(Z*/ *) 4 µ 0.040.04
Irreducible : (Z/*)(Z*/ *) 2µ2 0.010.01
Reducible : (Z/*)bb 0.400.40
Reducible : tt 0.270.27
1) Rejection of reducible backgroundRejection of reducible background
10/6/2005 Iacopo Vivarelli-INFN Pisa 35WIN 05 -Delphi
10/6/2005 Iacopo Vivarelli-INFN Pisa 36WIN 05 -Delphi
10/6/2005 Iacopo Vivarelli-INFN Pisa 37WIN 05 -Delphi
10/6/2005 Iacopo Vivarelli-INFN Pisa 38WIN 05 -Delphi
What’s new for Muon System since PhysicsTDR (1999)?
Provide access to EndCap Calorimeter and ID
Central crack
~45 m
~25
m
10/6/2005 Iacopo Vivarelli-INFN Pisa 39WIN 05 -Delphi
10/6/2005 Iacopo Vivarelli-INFN Pisa 40WIN 05 -Delphi
Masses and tan from neutral Higgs bosons
Error on masses
m/m = 0.1 to few %
Error on tan
tan/tan
= 15 to 5 %
from rates of H/A
VBF h/Hnot studied yet ATLAS TDR
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