WIN 05 -Delphi 10/6/2005 Iacopo Vivarelli-INFN Pisa 1 Higgs Physics at ATLAS and CMS Iacopo...

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


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.)


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


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


… 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


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


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


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


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)


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


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


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


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


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


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


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


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


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.


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()(


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.


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


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)


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


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


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


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


• 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.


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….


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….


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


BACKUP


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


MT>175 GeVMT<175 GeV

Signal Region

Outside Signal Region


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





What’s new for Muon System since PhysicsTDR (1999)?

Provide access to EndCap Calorimeter and ID

Central crack

~45 m

~25

m



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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