Higgs discovery prospects and gg y p pstatistics at the LHC (ATLAS)

Eilam Gross Weizmann institute of Science/ATLAS

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ATLAS with a Perspective

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A Legend Comes True

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A Legend Comes True

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The team Israel and Pakistan working for the Atlas collaboration together for a family picture.

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Higgs Generates All Current MassesffvHgffgL fHfHfff )( +=Φ=

vgm Hfff = fffvmg fHff /=

Hf

Hffgf

Hffg

The heavier the particle, its coupling to Higgs

is bigger

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is bigger

Constraints from EW precision measurements

114 166Hm< <

http://arxiv.org/abs/hep-ex/0612034v2

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The Technical Challenge• ~109 proton-proton

collisions/sec

• ~1/1011 - 1/1012 collisions produce a Higgs Boson

• One can only accommodate ~200One can only accommodate 200 collisions/sec

• Rejection rate> 99.9995%

How do you make sure not to loose a Higgs• How do you make sure not to loose a Higgs Boson?

• Make sure we “push the button” (trigger) only when interesting things happen

• This is a tremendous technical challenge! Which require state of the art fast systems.

• σ ~ 100 mb σbb ~ 1 mb σj > 1 nb• σtot 100 mb, σbb 1 mb, σjet > 1 nb, σH> 1~pbThe small S/B requires control samples to understand the background

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Higgs Production @ LHC

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Some Schematic BackgroundsQCD DrellYan Z jets→ +

/Z γ /Z γ

qg Zq→

γ

gg Zqq→

/Z γ

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σ * BR – The Analysis Challenge • The Higgs decay to bb is

still dominant up to ~150 GeV the Weak BosonsGeV, the Weak Bosons then enter the game

• However, H 4 leptons has a clear signature as well asa clear signature as well as H γγ with BRs of O(10-3). This is a challenge in analysis which makes the need for measuring gbackground from data mandatory!

• The decay to ττ, thoughThe decay to ττ, though only a few % is still very appealing for the medium –light SM Higgs

• For MSSM Higgs bb and ττare dominant

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LHC – Brief Status• Engineering run originally foreseen at end 2007 now

l d d b d l i i ll i d i

As reported by Sergio Bertolucci, LP07

precluded by delays in installation and equipment commissioning.

• Beam commissioning starts May 2008

• First collisions at 14 TeV c.m. July 2008

• First month devoted to collecting millions of minimum bias events and di-jets events for first alignment, j g ,calibrations and performance studies of the detector(@ 1029)

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LHC – Brief Status• Pilot run pushed to reach 1032 cm-2 s-1 by end 2008

As reported by Sergio Bertolucci, LP07

• This is equivalent to O(100 pb-1)/experiment

• Use Z ee,μμ to calibrate, align and understand the trackers, the EM CAL, and the Muon system.

• Use W jj to calibrate jets and study b-tag performance (tt blν bjj)

• “Rediscover” SM Physics at √s=14 TeV, this is your new background

• After all, the LHC is designed to go beyond the known SM Physics, first and foremost to discover the Higgs Boson.

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Physics, first and foremost to discover the Higgs Boson.

Cosmic Ray Tests

Tracks in the Muon chambersand in theand in theTransition-Radiation Tracker TRT

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HIGGS SEARCHAnd now…. The main course

HIGGS SEARCH TECHNIQUES AND PROSPECTSPROSPECTS

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The Official References

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The Official References

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mH~120 GeV• H γγ, classical bump hunting, BR ~2·10-3

• Irreducible background (now at NLO• Irreducible background (now at NLO compared with TEVATRON)

• From the production point of view:• The reference inclusive production:p

gg H• In the last few years the VBF is stealing the

show…..• Trigger on 2 isolated Photons with pT>20 or100fb 1 • Trigger on 2 isolated Photons with pT>20 or

one very energetic photon100fb-1

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mH~120 GeV2 2~ 2 (1 cos )m E − Ψ2 (1 cos )m Eγγ γ Ψ

• To get a 1% resolution in mass d if l ti f EM

• For high luminosity vertex

needs uniform resolution of EM calorimeter in η and a good angular resolution g y

position must be taken from calorimetry ~16 mm

• Resolution in φ is obtained from EM calorimeter

• Resolution in η is achieved ηwith a fit constraining the photons to emerge from the primary vertex (zvertex known within a few 10s of mm), this will not work for high luminosity (1034)

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mH~120 GeV• Since σγj+jj ~ 106 σγγ

d j ti f t l t 1000one needs a rejection of at least 1000 against fake photons (jets) to maintain the level of the fake photons BG below the irreducible BGthe irreducible BG

• Current studies indicate σγj+jj ~ 20% σγγ maintaining an 80% photon detection efficiencyq γ photon detection efficiency.

• Fake photons are produced by leading pions from fragmenting jets

qg

γ

γπ0q

γ

• Need high performance of EM calorimeter to tell photons from Pions(e.g. use shower shapes based on d t t l it )detector granularity)

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Photon Conversions• Photon conversions

before the calorimeter~30% conversions in t ktracker

• Single and Double conversions due to a lotconversions due to a lot of material in the Atlas Silicon trackers

• Performance of channel depends a lot on the percentage of restored conversions (construction f t d h t )of converted photons)

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mH~120 GeV

• S/B~1:20. requires a full control of the background100fb-1

• So how do we do it?

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Side Bands Background Analysis

• Toy model with ySIGNAL

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Side Bands Background Analysis• Toy model with SIGNAL • Only one way around.• FITS!• Fit the data Signal and BG• Fit the data Signal and BG

altogetherThis is the only way to reduce the

t tisystematics• Hard to believe but if we assume a

120 GeV Higgs, this toy MC gg , ydelivers a median significance of 3.9σ (with no systematics but BG fitted with data and taking shapesfitted with data, and taking shapes into account with the Profile likelihood method)L k l h ff t i i l• Look elsewhere effect is crucial here

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mH~120 GeV• Sensitivity can be

increased by inclusiveincreased by inclusive searches separating to – H+0jj– H+1j– H+2j (dominated by

VBF qq qqH)VBF, qq qqH)• Resolution on mass

~1.6 GeV(@ mH=120)

• Excellent prospects• Excellent prospects, especially for L>10fb-1

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The Golden Channel H ZZ* 4 leptons

• Very low BR of H ZZ* for low-medium mHlow-medium mH

• Yet, very clean and therefore appealing

i ll H ZZ* 4especially H ZZ* 4μbut also the 4e

• Trigger by requiring two• Trigger by requiring two high pT electrons or muons (~10,15) or one high pT electron or muong p(~20,25)

• Success of channel d d d /depends on good e/μidentification, efficiency for isolated leptons and energy resolution

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energy resolution• For muons, eff close to

100%

Reducible Background• Some collisions produce

similar topologies to the i l

Irreducible

ZZ i i d ibl

signal

• ZZ is an irreducible background dominant after selection. XSC known to NLONLO.

• Zbb or tt are reducibleZbb or tt are reducible backgrounds(mainly by requiring isolated leptons [trackerisolated leptons [tracker and calorimeter] with a pair emerging from an on shell Z boson and anti-b IP

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Z boson and anti b IP based cuts)

Irreducible Background

• ZZ is an irreducible background dominant after selection XSCdominant after selection. XSC known to NLO.

• Background shapes and normalization from DATA sidebands (reduce PDF andluminosity uncertainties)luminosity uncertainties)

• Clean channel (but low statistics)

• With 30 fb-1 a 150 GeV Higgs Boson can be observed with a significance of 7σ and a resolution of

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7σ and a resolution of 1.8 GeV(Very Preliminary, just to give an idea)

The Golden Channel• H 4 leptonsp

• Sensitivity bestSensitivity best for ~160 GeV and heavierand heavier Higgs Boson

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A Lesson in Systematic• In absence of F 5 dsystematics

significance can be

• For 5σ one needs5s

b> Δg

approximated to be• For 10% systematics

sb

• However if there is systematics say Δb

this implies 0.5s

b>systematics, say, Δb

the significance is reduced to

b

reduced to

( ) ( )2 22 (1 )

s s sbb bb b

= →Δ ⋅+ Δ ⋅+ Δ ⋅

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( ) ( )

A Lesson in Systematics, ttHb

b

• ttH, H bb• A multi-jets final state• Looked promising until fast

simulation was replaced by full simulation andfull simulation and systematics killed it(FOR THE TIME BEING) due to uncertainties in thedue to uncertainties in the shapes of the combinatorial 4j BG

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An Essential Tool: Transverse Mass

• In hadron colliders the interaction is between 2 quarks somewhere along the beam…..

• A typical event will have a large imbalance in theA typical event will have a large imbalance in the Z direction but in principle should be balanced and therefore could be reconstructed (up toand therefore could be reconstructed (up to traceless particles like neutrinos) in the transverse plane e g qq H+1jettransverse plane, e.g. qq H+1jet

HH

jpZmiss

jTransverse Plane

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An Essential Tool: Transverse Mass• This is why in many cases people use quantities which

are well defined in the Transverse plane first andare well defined in the Transverse plane, first and foremost the transverse missing energy (missing ET ) or MTT

• W lν• What is the source of the Transverse mass?

2 2 (1 cos )TT Tm p E ϕ= − Δ

• In this example the source of the missing TRANSVERSE energy is (only) a neutrino with a zero mass

• There are two unknown variables, with one constraint 2 0miss =p

,zmiss missp E

• The mass cannot be determined, but using Lagrange multipliers we can find an extremum for th f ti f th d k titi

2 2( )lep missM = +p p

the mass as a function of the measured known quantities 2 2 (1 cos )

TT Tm p E ϕ= − Δ2 2 2( ) ( ) ( )T TT miss lep lep missm P P= + − +T Tp p

An Essential Tool: Transverse Mass• Example (parton level) tt bqqb ν→Example (parton level) tt bqqb ν→

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mH~160, Gauge Bosons Playground• H WW lνlν

with l-l spin correlationH WW νν→ →

with l l spin correlation

• Dominant for mH~2mW

• Signature of 2 isolated leptons + missing ET

• No mass peak use• No mass peak, use transverse massneed to understand missing ET

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mH~160, Gauge Bosons Playground• H WW lνlν

with l-l spin correlationH WW νν→ →

with l l spin correlation

• Dominant for mH~2mW

• Signature of 2 isolated leptons + missing ET

• No mass peak use• No mass peak, use transverse massneed to understand missing ET

W+ e+ν

Higgs

36W- e-ν

ggSpin 0

mH~160, Gauge Bosons Playground• Main background from

Wt(b) and tt, rejected viaH WW νν→ →

Wt(b) and tt, rejected via jet veto, yet systematics is still >10%

Si f 2 i l d• Signature of 2 isolated leptons with small acoplanarity+ missing ET

O• VETO on hard jets

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mH~160, Gauge Bosons Playground• H WW lνlν

ATLASM=160GeV

30fb-1

•Main interest is near MH~160 GeV (BR H WW 95%)•Sensitivity in the lower mass

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region can be extended looking at VBF production

VBF – Central Jets Veto( )

VBF qq qqH→ Forward j t

Han, Valencia, Willenbrock (1992);Figy, Oleari, Zeppenfeld (2003,2004)

H/Z W

φ

jets

/Z W

φ

ηHiggs DecayDecay

t bW

tW

W

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tb

The Breakthrough: VBF( )

• qqH, H WW,ττ

Han, Valencia, Willenbrock (1992);Figy, Oleari, Zeppenfeld (2003,2004)

Jet• Signature:

– central jet veto ( l i l d

Jet

(only signal decay products in the central region)two high p jets with

Jet

– two high pT jets with large Δη separation

• Success of channel

Full line Parton level,“data” is the reconstructed

Success of channel relies on understanding of the Missing Energy Forward

jets• Note also the massive

use of central jet veto necessiates its

φ

jets

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necessiates its understanding! ηHiggs

Decay

VBF H WW Backgrounds2qq qqWW hard jetsνν→ → +qq qq j

• tt– Central jet veto

• QCD WW– Use reconstructed Central jet veto

– Anti b-tag missing ET

– Use di-lpton mass as a

• Try to estimatediscriminator

Try to estimate background from data

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An Essential Tool: Collinear Approximation

• The Taus are boostedA th l t d• Assume the leptons and neutrinos are collinear with the Taus l

P x Pτ± ±±=

• Solution is valid if the scale 0<x<1

l τ

1 PEx− • One finds• Define

• Given the missing momentum

1

l l

PExx E P

ννννα ±

± ±

±

±

−= = = • One finds

and the resolution( )( )2 2 1 1H l l

M P Pα α + −+ −= + + ⋅Given the missing momentum in the transverse planeone can solve for

,x yP/α±

P P/ /

and the resolution1 1

sin2

HT T

x yH l l l l

MP PM P PP P

φ+ + + −

Δ∝ =

x y

x yl l

P PP PP P

α±

/ /±

=∓ ∓

• i.e. No collinear mass for back to back leptonsA th i Th lli i th

x yl lP P− −

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x yl l

x yl l

P PP P

+ +

− −

• Another view: The collinear mass is the mass of the Higgs Boson needed in order to boost the system to the Higgs rest frame

An Essential Tool: Collinear Approximation

• One finds

( )( )2 2 1 1H l lM P Pα α + −+ −= + + ⋅

• The resolution2 2 2 21

2det 1 1 1 1H H H H H

x y x y y xl l l l

M M M M MP P P P P PM α α α α

− + + −

⎡ ⎤⎛ ⎞ ⎛ ⎞Δ≈ Δ / + + Δ / +⎢ ⎥⎜ ⎟ ⎜ ⎟+ + + +⎝ ⎠ ⎝ ⎠⎣ ⎦2det 1 1 1 1

det sin

H

x y T Tl ll l

x yl l

M

P PP P

P P

α α α α

φ+ +

+ −

− −

+ − + −+ + + +⎝ ⎠ ⎝ ⎠⎣ ⎦

≡ =

2ν+l

depends on the acoplanarity angle and

x yl l

τ

2ν+l

the missing ET resolutionτφ

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2ν+l

τ

2ν+l

τφ

2ν+l

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1H jττ→ +

0H jττ→ +

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H ττ

• qqh, H ττqqh, H ττ• Channels

lepton-leptonlepton hadronlepton-hadronhadron-hadron

• The ττ decay mode extends the sensitivity toextends the sensitivity to lower Higgs masses where there is a BR(H ττ)M b t t d• Mass can be reconstructed for ττ using the collinear approximation (for lepton-lepton and lepton hadronlepton and lepton-hadron

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H ττ Dominant Backgrounds

R d ibl I d iblReducibleDrell Yan Z ll+jetstt+jets where the b jets

• IrreducibleDrell Yan Z ττ ll+jets

tt+jets where the b-jets mimic forward jets (tthas a huge crosshas a huge cross section)QCD & EW WW

e Eττ μ→ +

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130Te E

fb

ττ μ−

→ +

Use of Control Data Samplesp

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The Breakthrough: VBF( )

• qqH, H WW,ττ

Han, Valencia, Willenbrock (1992);Figy, Oleari, Zeppenfeld (2003,2004)

qq , ,• The ττ decay mode

extends the sensitivity to lower Higgs masses wherelower Higgs masses where there is a BR(H ττ)

• Mass can be reconstructed for ττ using the collinearfor ττ using the collinear approximation

130Te E

fb

ττ μ−

→ +

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Very Preliminary Combination• Based on ATLAS TDR and ATLAS VBFBased on ATLAS TDR and ATLAS VBF

Scientific NoteD ith th P fil Lik lih d th d• Done with the Profile Likelihood method assuming the asymptotic chi squared behavior

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The Profile Likelihood for Significance Calculationfor Significance Calculation

2ˆ2 log ( )N Nλ μ σ± =ˆ2 log ( )

2 log ( )

N N

Nμλ μ σ

λ μ

− ± =

= −

• In particular if we generate background onlyexperiments, λ(μ=0) is distributed as χ2 with 1 d.o.fp , (μ ) χ

• Discovery has to do with a low probability of the background only experiment to fluctuate and give us abackground only experiment to fluctuate and give us a signal like result….

To estimate a discovery sensitivity we simulate a data• To estimate a discovery sensitivity we simulate a data compatible with a signal (s+b) and evaluate for this data λ(μ=0). For this data, the MLE of μ is 1

Comb Stat Forum Sep07 - Eilam Gross and Ofer Vitells52

ILLUSTRATIVEILLUSTRATIVEE.G. & o. Vitells

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Exclusion with Profile Likelihood• Exclusion is related to the probability of the p y

“would be” signal to fluctuate down to the background only region (i.e. the p-value of the g y g ( ps+b “observation” )

• Here we suppose the data is the background only and the exclusion sensitivity is given byonly and the exclusion sensitivity is given by

2 ( 1)N λ μ= − =

• Exclusion at the 95% C.L. means N=2

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ATLAS Exclusion Sensitivity

ILLUSTRATIVEILLUSTRATIVEE.G. & o. Vitells

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Cases for ILC• OK, so LHC discovered a Higgs Boson, now

h t?what?

• What happens if we observe one Higgs and• What happens if we observe one Higgs and nothing more’?– Is it a SM Higgs Boson?Is it a SM Higgs Boson?– What is its width?– What is its spin?– What are its couplings?

• Even though some properties can be probed• Even though some properties can be probed with the LHC, if the LHC is a Higgs hunter, the ILC would be a Higgs Probe…..

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From LHC to ILC

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From LHC to ILCK. Desch, LCWS 07

Also workshop for the LHC early phase for the ILC, Fermilab, 07

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The Case of Observing One Scalar

• Even if the Higgs is supersymmetric there is a large region in the

t hparameters space where the LHC can observe only one of the Higgs Bosonsone of the Higgs Bosons

• An accurate measurements of the couplings & BRs of the observed Higgs can reveal its nature

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Some Things ILC Does Better • Here mh=120 • LHC: for 300 fb-1 and

110<mH<190 GeVWWHBRbbHBRR )(/)( →→=

Moriond QCD 2006 - Helenka Przysiezniak

110<mH<190 GeV– Δg2/g2 ~ 10%-45% (except for b)– ΔΓH/ΓH ~ 10%-50%

[ ]SMWWHBRbbHBRR

)(/)( →→=

LCLHC hep-ph/0406323

Dührssen2.5-3.5%

20%

ΔmA = 30% for mA = 800 GeValso in parameter regions whereLHC is blind

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LHC/LC interplay in the MSSM Higgs sectorKlaus Desch ,E.G. ,Lidia Zivkovic ,Sven Heinemeyer and Georg Weiglein

J. High Energy Phys. 2004 JHEP09(2004) (September 2004) 062

The Ultimate Goal: Probing the SSB Sector

• Derive 95%CL bounds from χ2 fit to mvis shape

Moriond QCD 2006 - Helenka Przysiezniak

mvis shape

• SM assumed to be valid except for self coupling.

• Assume mH precisely known, and BR(H→WW) known to 10% or better.

• gg→HH→(W+W-)(W+W-)→(jjℓ±ν) (jjℓ’±ν) (ℓ = e,μ)for mH>150 GeV/c2.

• The self coupling λ is determined to-60-300% within 1σ for 150<mH<200 GeV

• This will have to wait for a future collider

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The Ultimate Goal: Probing the SSB SectorK. Desch, LCWS 07

Also workshop for the LHC early

phase for the ILC, Fermilab, 07

• Self Higgs coupling is not a piece if cake for a LC as well!

• Needs >1 TeV with a polarized electron beam and 1 ab-1 to probe it to the level of ~10%of ~10%

• This by itself might justify aThis by itself might justify a LC (If we observe a scalar)

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Some Things LHC Can Also Do• Combining CMS and ATLAS

with high luminosity (yet

Moriond QCD 2006 - Helenka Przysiezniak

with high luminosity (yet accessible within a few years) with channels like H→γγ,H→ZZ(*)→4ℓ, WBF H ℓ h d fH→ττ→ℓ+hadr an accuracy of 0.1% might be achieved for the mass with mH~120-400 GeV, that is not so different from LCthat is not so different from LC

• This is under the condition that the systematics on the energy y gyscale is below the 0.1% for photons/electrons and below 1% for jets….

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Conclusions• LHC is a discovery machineLHC is a discovery machine• LHC is scheduled to deliver data starting

d f 2008end of 2008• The optimist expects to see a hint of a p p

Higgs Boson by the end of 2009• The pessimist will wait another year• The pessimist will wait another year• SUSY might come earlier

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