Measuring the Higgs Boson at ATLAS ?

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Measuring the Higgs Boson at ATLAS ?

Measuring the Higgs Boson at ATLAS

Christine Kourkoumelis

University of Athens

On behalf of the ATLAS collaboration

10/25/2007 C.Kourkoumelis,13th

LomonosovConference

1


Limits on SM Higgs(1)

‣Direct from LEP : m H >114.4 GeV @95% CL

‣ Indirect from EW precision data :

m H < 144 GeV LP ’07 (w/o the new LP07 M top )

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Limits on SM Higgs (2)

NEW

From the Tevatron

Now:

LP07 Korea, Aug ’07

( Previous from ICHEP’06 with L=0.3-1 fb -1 )

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Limits on SM Higgs (3)

‣Tevatron searches : future

‣Accumulated 2.5 fb -1 on tape (now shown~2fb -1 )

‣Expect 6-7 fb -1 by 2009 (no 5σ sensitivity)

L(fb -1 )/exp

Competition with

LHC in 2009

2007

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• LHC pp collisions @ √ s=14 TeV

ATLAS (+CMS) detector optimised for l’s,γ’s,

jets,b-jets,measurements with the SM and beyond

• Rapidity : |η|< 2.7 (l’s, γ’s) |η|< 5 (calorimeter coverage)

• Momenta : soft to 5 TeV

The underground

cavern at point-1

for the ATLAS

Detector

Length = 55 m

Width = 32 m

Height = 35 m

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The surface

The tunnel

~9000 magnets!!

6


The ATLAS Detector

Excellent Standalone Muon

measurement

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Most spectacular magnet

Oct 2005

8


The calorimeter inserted

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The inner detector (pixel) inserted

28 June 2007

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The 1 st End-Cap toroid

240 ton

12m diameter

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The 2 nd End-Cap (and last!!) Toroid 12 th July

240 ton

12m diameter

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The official LHC schedule

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LHC machine scenario:

•The Low energy run of this year cancelled

•July 2008 : two beam-collisions (low luminosity)@14TeV

•End 2008 : accumulate < one fb -1

•2009 : L~ few*10 33 cm -2 sec -1

•End 2009 :accumulate ~ few fb -1

Initial run

Assume: 10 pb -1 /month @10 30 cm -2 sec -1

100pb -1 /few days @ 10 32 (eff~50%)

Assume: 6*10 6 sec/year (duty factor 20%)

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pp cross-sections

10 8 events/sec

W,Z

1 event/sec

Higgs 150 GeV

10 -5 events/sec

15


Process

W→ eν, μν

15

Z→ ee, μμ

1

Η m=130 GeV

Indicative Rates

10 -2

10 -4

~

Η m=180 GeV

g g ~

m=1 TeV

tt →WbWb → μν+X

bb → μν +X

Minimum bias

QCD jets p T >150 GeV

Events/s

10 -3

1

10 3

>10 8

10 2

Events for

1fb -1 (end 2008 )

7*10 6

~10 6

6*10 3

60

10 2 -10 3

8*10 4

10 6

>10 6 *10 -2

~10 6

16


2008 running : pp collisions @ √ s= 14TeV

L~10 30 32

cm -2 s -1 Statistics of

For detector

calibration

‣ QCD jets

‣ Z’s

‣ W’s

‣ top

One month @10 30

and < 2weeks @10 31 Few days @10 32

End of 2008

Tevatron now

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SM Higgs at the LHC

(several production/decay channels)

gg fusion

associated

t t H

VBF WW/ZZ fusion

associated WH, ZH

Uncertainties on x-sections

• gg 10-20 (NNLO) k~2.0

• VBF ~ 5%(NLO) k~1.1

• WH,ZH ~


Cannot cover all channels

will give some examples with the following channels:

‣H → γγ “benchmark”

•ttH, H →bb (exclusive) Low mass Higgs

•qqH, H →ττ VBF (exclusive)

•H →ZZ (*), Z →4l Golden discovery channel

•qqH, H →WW(*) VBF (exclusive)

•H → WW(*) →llvv

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LOW MASS

For the mass range m H ~ 115-130 GeV

Need FEW channel combination

Best the following three:

1. H→ γγ S/B ~10 -2 (excellent calorimetry)

σ/m


H γγ for m H ≤150 GeV

Recent developments:

•Irreducible diphoton background computed to NLO improvement of significance

by a factor~1.5 : TDR(LO)~3.9 (NLO)6.3 @30fb -1

•Analysis using discriminant variables, shape of kinematical variables

•Direction given from Primary Vertex : σ=40μm z-position from ID tracks @ low L

σ=1.6cm z-position from calo @ high L

ATLAS preliminary

ATLAS

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Carminati L., Physics at LHC 2006


BR

Uncertainties

on branching ratios

few % (NLO)

SM Higgs

Decay

channels

(A.Djouadi et al)

INTERMEDIATE MASS

For the mass range 130 < m H

< 180GeV, the Higgs decays to:

‣H→ ZZ*→ 4l

(clean signature and rather low background)

‣H→ W W*→ ll ν ν ( E Tmiss

)

‣qqH→qqττ

(ll+ l-had)

m=130GeV @10fb -1 each 3σ

total S/√B ~ 6 σ

22


H → ZZ (*) → 4 leptons

130 ≤ m H < 700 GeV

g

g

t

H

Z (*)

Z

e, μ

e, μ

e, μ

e, μ

“golden channel ” for the Higgs discovery at LHC

ZZ* → 4l irred.bkg

qq known to NLO

+20%for gg

Zbb → 4l, tt →4l

m Z

Simulation of H → μμμμ

Discovery with less

than 10 fb -1 for

130< m H


H→WW→l + νl - ν

Important channel for

m H ~160 GeV (H→WW BR ~ 95% )

•No mass peak

•Need exact knowledge of bgk shape

Backgrounds:

tt, tWb : rejected by jet-veto

WW,WZ,ZZ: rejected by kinematical cuts

Recently:

gg → WW contribution

tt, single top@NLO

l + l - +MET

Extra sensitivity by adding exclusive VBF →WW

24


VBF channels qqH →qqττ

or qqH →qqWW

•qq jets go in the forward region

•No central jet

•At least one τ/W → leptons

•Irreducible bkg qqW/Z

‣Important for low masses

‣x-section is about 20% of gg

ATLAS

30fb -1

sn-atlas-2003-024

25


ATLAS discovery potential in 3 yrs low L

LEP limit

Beware:

2003 result

LO calculation

New on-going

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ONE YEAR

Covers almost

all mass region

(if detector known…)

m H > 114.4 GeV

here discovery “easy”

with H → 4

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MSSM Higgs ( h,H,A, H ± ) 2 doublets

A large variety of channels (M SUSY >1TeV) ,two parameters m A ,tanβ

m h < 135 GeV

m A ~m H at large m A

Minimal mixing mh


Charged Higgs

Detection beyond SM

UNAMBIGUOUS

+ H ± → χ0, χ1

5σ discovery contour

Eur.Phys.J.C44(205)s11

30


if the Higgs is discovered:

(if not study the WW scattering at high mass → difficult. needs lots of L).

‣Measure mass and width to determine if SM or not

‣Measure coupling strength

ATLAS 300fb -1

‣Measure Spin, CP etc

Mass ~0.1%; Calibrate the detector →

find the peak of H → γγ and/or

H → ZZ(*) “complete reconstruction”

→ fit the peak

(if not SM measure the mass

from other decay channels → harder

“event counting”)

Width → experimental resolution is ~1 GeV

cannot measure width directly for m H


Couplings~10-20% → measure from rates σ *BR

(But for need width m H


Conclusions (1)

If the SM Higgs exists and

If the detector is well understood

The Higgs will be detected with < 5fb -1

Properties

Mass, couplings, spin..

Is it SM

Do more exist

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Conclusions (2)

First data (@ 14 TeV) starting summer 2008

With < 1 fb -1 by the end of 2008…

Standard Model physics:

Underlying event, min. bias, QCD x-section ~30%

W, Z production ~ 10%

→ use for detector, MC understanding

→ improve knowledge of PDFs

Measure top events, x-section ~20%

→ use for calibration, top mass σ ~3-7 GeV

First searches:

Probe SUSY in the ~ 1 TeV scale

Look for Z’,W’ ~ 1 TeV resonances

Higgs (with a bit of luck. If it has an intermediate

mass)

34


Back-up slides

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Only h

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Not all mixings studied!!


tanβ = 3 MSSM neutral Higgs production tanβ=30

BR’s

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More details on Tevatron searches

Summer 2006

SM

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