higgs production at the lhc - rwth aachen universitymkraemer/higgs... · introduction: the higgs...

Theorie-Seminar Universitat Bielefeld, Mai 2005

Higgs Production at the LHC

Michael Kramer

(RWTH Aachen)

Introduction: from WW -scattering to the Higgs boson

Higgs-boson searches at the LHC

Higher-order corrections to signal and background processes

Michael Kramer Universitat Bielefeld, Mai 2005


Fermi: weak interactions described by effective Lagrangian eg. for µ decay µ− → e−νeνµ

L =GF√

2[νµγλ(1 − γ5)µ][eγλ(1 − γ5)νe]

with GF ≈ 1.17 × 10−5 GeV−2 (Fermi coupling)

Fermi theory at high energies: M[νµe− → µ−νe] ∼ GF

2√

2πs

⇒ violates unitarity

Solution: interaction mediated by heavy vector boson W±

M[νµe− → µ−νe] →

GF s

2√

2π

M2W

M2W − s

(with MW ≈ 100 GeV)

Michael Kramer Universitat Bielefeld, Mai 2005



L =GF√

2[νµγλ(1 − γ5)µ][eγλ(1 − γ5)νe]



2√

2πs




GF s

2√

2π

M2W

M2W − s


Michael Kramer Page 2 Universitat Bielefeld, Mai 2005



L =GF√

2[νµγλ(1 − γ5)µ][eγλ(1 − γ5)νe]



2√

2πs



� ��

� ��

� ��

� �

�


GF s

2√

2π

M2W

M2W − s




Consider WW → WW

��

�

� �

� ��

M[WLWL →WLWL] ∝ s ⇒ violates unitarity

Solution: − strong WW interaction at high energies or

− new scalar particle H with gWWH ∝ MW

M → GFM2H

4√

2π

Unitarity ⇒ properties of H : − coupling gXXH ∝ particle mass MX

− MH ∼< 1 TeV



Consider WW → WW

��

�

� ��

� ��

M[WLWL →WLWL] ∝ s ⇒ violates unitarity

Solution: − strong WW interaction at high energies or

− new scalar particle H with gWWH ∝ MW

��

�

� M → GFM2H

4√

2π

Unitarity ⇒ properties of H : − coupling gXXH ∝ particle mass MX

− MH ∼< 1 TeV


Introduction: The ABEGHHK’tH Mechanism(Anderson, Brout, Englert, Guralnik, Hagen, Higgs, Kibble, ’t Hooft)

Spontaneous symmetry breaking through scalar isodoublet φ

Lφ = |Dµφ|2 + gdf ψ

dLφf

dR + gu

f ψdLφf

uR − V (φ)

scalar potential V = µ2|φ|2 + λ|φ|4 � ��

��

� ��

introduce Higgs field H : φ →(

0(v+H)/

√2

)(unitary gauge)

Goldstone bosons → longitudinal components of W, Z

⇒ SM is renormalisable gauge field theory including W and Z boson masses

and a physical Higgs particle


Introduction: The Higgs boson

The Higgs mechanism is testable becauseall couplings are known:

− fermions: gffH =√

2mf/v

− gauge bosons: gV V H = 2MV /v

with vacuum expectation value

v2 = 1/√

2GF ≈ (246 GeV)2

The Higgs sector and the properties of theHiggs particle (lifetime, decay branching ratios,

cross sections) are fixed in terms of theHiggs boson mass MH .Express Higgs potential in terms of (µ, λ) → (v2, MH)


Higgs boson hunting: indirect searches

Quantum corrections to precision observables

�

!

�

"�

∝ m2t ∝ logMH

140

160

180

200

10 102

103

mH [GeV]

mt

[GeV

]

Excluded Preliminary (a)

High Q2 except mt

68% CL

mt (TEVATRON)

⇒ Experimental data consistent with SM

⇒MH < 260 GeV (95% CL) (LEPEWWG)

Note:

− Precision data also consistent with SUSY models

− No evidence for ESWB through new strong

interactions (eg. technicolour)




#

$%

#

&#

∝ m2t ∝ logMH

140

160

180

200

10 102

103

mH [GeV]

mt

[GeV

]


High Q2 except mt

68% CL

mt (TEVATRON)



Note:







'

()

'

*'

∝ m2t ∝ logMH

140

160

180

200

10 102

103

mH [GeV]

mt

[GeV

]


High Q2 except mt

68% CL

mt (TEVATRON)



Note:







+

,-

+

.+

∝ m2t ∝ logMH

140

160

180

200

10 102

103

mH [GeV]

mt

[GeV

]


High Q2 except mt

68% CL

mt (TEVATRON)



Note:





Higgs boson hunting

To establish the Higgs mechanism we need to

– discover the Higgs boson directly at a high-energy collider

– measure the couplings gXXφ ∝ MX

– reconstruct the Higgs potential


Higgs boson hunting: collider searches

Higgs decay modes and branching ratios

[HDECAY: M. Spira et al.]

BR(H)

bb_

τ+τ−

cc_

gg

WW

ZZ

tt-

γγ Zγ

MH [GeV]50 100 200 500 1000

10-3

10-2

10-1

1

102

103

⇒ dominant decay into

bb for MH ∼< 130 GeV

WW, ZZ for MH ∼> 130 GeV


Higgs boson hunting: past and present colliders

search at the CERN LEP2 (e+e− collider with√s ∼< 200 GeV)

e+e− −→/ ZH ⇒ MH > 114.4 GeV (95% CL) (LEPHIGGSWG)

search at the Fermilab Tevatron (pp collider with√s = 2 TeV)

current∫L = 0.7 fb−1

expectation in 2008:∫L = 4 − 6 fb−1


The Large Hadron Collider LHC

− pp collider located at CERN

− circumference 27 km

−√s = 14 TeV

− ∫L = 10 − 100 fb−1/year

− in operation from April 2007


We will start in 2007We will start in 2007

CMS HB-1 CMS HB+1

Installation of readout boxes started

source calibration in 2005.

Insertion in vacuum tank: Summer 2005.


Higgs boson search at the LHC

The days of the Higgs boson are numbered!

1

10

10 2

102

103

mH (GeV)

Sig

nal s

igni

fican

ce H → γ γ ttH (H → bb) H → ZZ(*) → 4 l

H → ZZ → llνν H → WW → lνjj

H → WW(*) → lνlν

Total significance

5 σ

∫ L dt = 30 fb-1

(no K-factors)

ATLAS


Higgs boson searches at the LHC

0.1 1 1010-7

10-6

10-5

10-4

10-3

10-2

10-1

100

101

102

103

104

105

106

107

108

109

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

101

102

103

104

105

106

107

108

109

σjet(ETjet > √s/4)

LHCTevatron

σttbar

σHiggs(MH = 500 GeV)

σZ

σjet(ETjet > 100 GeV)

σHiggs(MH = 150 GeV)

σW

σjet(ETjet > √s/20)

σbbar

σtot

σ (n

b)

√s (TeV)

even

ts/s

ec f

or L

= 1

033 c

m-2

s-1

−→ QCD background: σbb ≈ 108 pb

−→ Higgs signal: σH+X ≈ 10 pb

↪→≈ 3 × 105 Higgs bosons/year

(∫L = 30 fb−1)

−→ Higgs-search through associate production or/and through rare decays


Higgs boson searches at the LHC: cross section predictions

σ(pp → H + X) [pb]√s = 14 TeV

NLO / NNLO

MRST

gg → H (NNLO)

qq → Hqqqq

_' → HW

qq_ → HZ

gg/qq_ → tt

_H (NLO)

MH [GeV]

10-4

10-3

10-2

10-1

1

10

10 2

100 200 300 400 500 600 700 800 900 1000

/01

1

2 34

55

55

2 34 /

2 3 4

565

7 89/

11

060

/


Higgs boson searches at the LHC: signal significance

1

10

10 2

100 120 140 160 180 200 mH (GeV/c2)

Sig

nal s

igni

fican

ce H → γ γ ttH (H → bb) H → ZZ(*) → 4 l H → WW(*) → lνlν qqH → qq WW(*)

qqH → qq ττ

Total significance

5 σ

∫ L dt = 30 fb-1

(no K-factors)ATLAS

MH ∼< 2MZ →

gg → H (H → γγ, ZZ∗,WW (∗))

gg/qq → ttH (H → bb, ττ)

qq → qqH (H → γγ,WW ∗, ττ)

qq′ →WH (H → γγ)

MH ∼> 2MZ →{

gg → H (H → ZZ,WW )

qq → qqH (H → ZZ,WW )

[ + diffractive Higgs production]


Higgs boson searches at the LHC: why precision calculations?

Higgs discovery through resonance peak, eg. H → γγ

10000

12500

15000

17500

20000

105 120 135mγγ (GeV)

Eve

nts

/ 2 G

eV

0

500

1000

1500

105 120 135mγγ (GeV)

Sign

al-b

ackg

roun

d, e

vent

s / 2

GeV

However, need precision calculations for signal and background processes

– for Higgs discovery in WW decay channels (no reconstruction of mass peak possible)

– for a reliable determination of the discovery/exclusion significance

– for a precise measurement of the Higgs couplings

↪→ test of the Higgs mechanism

↪→ discrimination between SM and BSM (eg. SUSY)


Higgs boson searches at the LHC: why precision calculations?

Example: MSSM Higgs sector

two Higgs doublets to give mass to up- and down-quarks → 5 physical states: h, H , A, H±

MSSM Higgs sector determined by tanβ = v2/v1 and MA

Consider

σMSSM(gg→h→γγ)

σSM(gg→h→γγ)

→ differences ∼< 10%

Needs precision measure-

ments & calculations for

production cross sections

and branching ratios

[Dedes, Heinemeyer, Su, Weiglein]

0 200 400 600 800 1000 1200 1400 1600 1800 20000

10

20

30

40

50

60

MA (GeV)

tanβ

mSUGRA gg−>h−>γγ

< 0.50.5−0.80.8−1.01.0−1.21.2−1.5>1.5


Particle production at hadron colliders

Example: Drell-Yan process

:;=< ;?>

@@

ABDCB

EEC F=GHI JK FH JLM N N

Cross section: σpp→l+l− =∑

q

∫

dx1dx2 fq(x1) fq(x2) σqq→l+l−

− fq,q(x) dx: probability to find (anti)quark with momentum fraction x

→ process independent, measured in DIS

− σqq→l+l− : hard scattering cross section

→ calculable in perturbation theory




OP=Q P?R

SS

TUDVU

WWV X=YZ[ \] XZ \^_ ` `


q

∫









ab=c b?d

ee

fgDhg

iih j=klm no jl npq r r


q

∫









st=u t?v

ww

xyDzy

{{z |=}~� �� |~ ��


q

∫








Factorization is non-trivial beyond leading order

− virtual corrections

→ UV divergences

→ IR divergences

− real corrections

→ IR divergences

→ collinear divergences

UV divergences → renormalization (αs(µren) etc.)

IR divergences → cancel between virtual and real (KLN)

collinear initial state divergences → can be absorbed in pdfs




− virtual corrections�

��

��

→ UV divergences

→ IR divergences


→ IR divergences








− virtual corrections�

��

��

→ UV divergences

→ IR divergences


��

��

→ IR divergences







Initial state collinear singularities,eg.

�

¡

¢£ ¤

− process independent divergence in∫

dk2T as k2

T → 0

→ absorb singularity in parton densities:

fq(x, µfac) = fq(x) +{

divergent part of∫ µ2

fac0 dk2

T

}

Hadron collider cross section

σ =∫

dx1fPi (x1, µF )

∫

dx2fPj (x2, µF )

×∑

n

αns (µR) Cn(µR, µF ) + O(ΛQCD/Q)

(Collins, Soper, Sterman ’82-’84 and many others)




¥

¦¦¦§

¨© ª


dk2T as k2

T → 0




fac0 dk2

T

}


σ =∫

dx1fPi (x1, µF )

∫

dx2fPj (x2, µF )

×∑

n






«

¬¬¬

®¯ °


dk2T as k2

T → 0




fac0 dk2

T

}


σ =∫

dx1fPi (x1, µF )

∫

dx2fPj (x2, µF )

×∑

n





Scale dependence

σ =

∫

dx1fPi (x1, µF )

∫

dx2fPj (x2, µF )

×∑

n

αns (µR)Cn(µR, µF )

finite order in perturbation theory

→ artificial µ-dependence:

dσd ln µ2

R

=

N∑

n=0

αns (µR) Cn(µR, µF )

= O(αs(µR)N+1)

⇒ scale dependence ∼ theoretical

uncertainty due to HO corrections

Example: rapidity distribution in pp→W +X

[Anastasiou, Dixon, Melnikov, Petriello ’03]

⇒ significant reduction of µ dependence at

(N)NLO



pdf uncertainties

[Martin, Roberts, Stirling, Thorne ’03]

2.50

2.55

2.60

2.65

2.70

2.75

2.80

W @ Tevatron

NLO

Q2cut = 7 GeV2

Q2cut = 10 GeV2

NNLO

xcut = 0 0.0002 0.001 0.0025 0.005 0.01

σ W .

Blν

(nb)

MRST NLO and NNLO partons

→ ∆ pdf ∼< 5% (CTEQ, MRST, Alekhin,...)



Precision physics at hadron colliders requires

– the calculation of QCD corrections at (N)NLO;

– the precision determination of input pdfs;

– the resummation of large logarithmic corrections;

– matching of fixed order calculations with parton showers & hadronization;

– the inclusion of electroweak corrections.


Higgs production cross sections: theoretical status

Higgs production in gluon-gluon fusion

±²

³³

− NLO corrections: KNLO ≈ 1.7[Spira, Djouadi, Graudenz, Zerwas; Dawson, Kauffman]

− NNLO corrections:

KNNLO(mtop �MH) ≈ 2[Harlander, Kilgore; Anastasiou, Melnikov ’02; Ravindran et al. ’03]

consistent with calculations based on soft/collinear gluon

approximations [MK, Laenen, Spira; Catani et al.]

→ NNLO scale dependence ∼< 15%

[Harlander, Kilgore ’02]

1

10

102

100 120 140 160 180 200 220 240 260 280 300

σ(pp → H+X) [pb]

MH [GeV]

LONLONNLO

√s = 14 TeV




´µ

¶¶








1

10

102

100 120 140 160 180 200 220 240 260 280 300

σ(pp → H+X) [pb]

MH [GeV]

LONLONNLO

√s = 14 TeV




·¸

¹¹








1

10

102

100 120 140 160 180 200 220 240 260 280 300

σ(pp → H+X) [pb]

MH [GeV]

LONLONNLO

√s = 14 TeV




º»

¼¼








1

10

102

100 120 140 160 180 200 220 240 260 280 300

σ(pp → H+X) [pb]

MH [GeV]

LONLONNLO

√s = 14 TeV




½¾

¿¿








1

10

102

100 120 140 160 180 200 220 240 260 280 300

σ(pp → H+X) [pb]

MH [GeV]

LONLONNLO

√s = 14 TeV



Vector boson fusion

À Á?Â

ÃÃ

ÃÃ

À Á?Â Ä

− discovery channel & measurement ofHiggs couplings[Kauer, Plehn, Rainwater, Zeppenfeld]

− NLO QCD corrections (cf DIS) ≈ +10%[Han, Valencia, Willenbrock]

Associated VH production

− crucial for Tevatron search (MH ∼< 130 GeV)

− NLO QCD corrections ≈ +(30 − 40)%[Han, Willenbrock]

− NNLO QCD corrections ≈ +10%[Brein, Djouadi, Harlander ’03]

− NLO electroweak corrections ≈ −10%[Ciccolini, Dittmaier, MK ’03]



Vector boson fusion

Å Æ?Ç

ÈÈ

ÈÈ

Å Æ?Ç É

− discovery channel & measurement ofHiggs couplings[Kauer, Plehn, Rainwater, Zeppenfeld]

− NLO QCD corrections (cf DIS) ≈ +10%[Han, Valencia, Willenbrock]

Associated VH production

Ê ËÍÌ

ÎÏÎ

Ð ÑÓÒÔ

− crucial for Tevatron search (MH ∼< 130 GeV)

− NLO QCD corrections ≈ +(30 − 40)%[Han, Willenbrock]

− NNLO QCD corrections ≈ +10%[Brein, Djouadi, Harlander ’03]

− NLO electroweak corrections ≈ −10%[Ciccolini, Dittmaier, MK ’03]


Associated WH and ZH production: QCD + EW corrections

1

1.05

1.1

1.15

1.2

1.25

1.3

1.35

80 100 120 140 160 180 200MH[GeV]

KW

H(L

HC

)

QCD+EW

QCD

NNLO

1

1.1

1.2

1.3

1.4

1.5

1.6

80 100 120 140 160 180 200MH[GeV]

KW

H(T

evat

ron)

QCD+EW

QCD

1

1.1

1.2

1.3

1.4

1.5

1.6

80 100 120 140 160 180 200MH[GeV]

KZ

H(L

HC

)

QCD+EW

QCD

1

1.1

1.2

1.3

1.4

1.5

1.6

80 100 120 140 160 180 200MH[GeV]

KZ

H(T

evat

ron)

QCD+EW

QCD



Higgs production in association with top quarks

ÕÕ

Ö×Ö

Ø

− σ(ttH) ∼< 1 pb (LHC, MH > 115 GeV)

but: distinctive final state:

pp→ t(→W+b) t(→W−b) H(→ bb)

→ important contribution to 5σ discovery

for MH ∼< 130 GeV

− cross section ∝ g2ttH

⇒ direct measurement of top Yukawa coupling

δgttH/gttH ≈ 15% (exp.)[Drollinger, Muller, Denegri]

− NLO scale dependence ∼< 15%[Beenakker, Dittmaier, MK, Plumper, Spira, Zerwas ’01;

Dawson, Jackson, Orr, Reina, Wackeroth ’01-’03]

[Beenakker, Dittmaier, MK, Plumper, Spira, Zerwas]

σ(pp → tt_ H + X) [fb]

√s = 14 TeV

MH = 120 GeV

µ0 = mt + MH/2

NLO

LO

µ/µ0

0.2 0.5 1 2 5200

400

600

800

1000

1200

1400

1




ÙÙ

ÚÛÚ

Ü

− σ(ttH) ∼< 1 pb (LHC, MH > 115 GeV)




for MH ∼< 130 GeV








√s = 14 TeV

MH = 120 GeV

µ0 = mt + MH/2

NLO

LO

µ/µ0

0.2 0.5 1 2 5200

400

600

800

1000

1200

1400

1




ÝÝ

ÞßÞ

à

− σ(ttH) ∼< 1 pb (LHC, MH > 115 GeV)




for MH ∼< 130 GeV








√s = 14 TeV

MH = 120 GeV

µ0 = mt + MH/2

NLO

LO

µ/µ0

0.2 0.5 1 2 5200

400

600

800

1000

1200

1400

1




áá

âãâ

ä

− σ(ttH) ∼< 1 pb (LHC, MH > 115 GeV)




for MH ∼< 130 GeV








√s = 14 TeV

MH = 120 GeV

µ0 = mt + MH/2

NLO

LO

µ/µ0

0.2 0.5 1 2 5200

400

600

800

1000

1200

1400

1




åå

æçæ

è

− σ(ttH) ∼< 1 pb (LHC, MH > 115 GeV)




for MH ∼< 130 GeV








√s = 14 TeV

MH = 120 GeV

µ0 = mt + MH/2

NLO

LO

µ/µ0

0.2 0.5 1 2 5200

400

600

800

1000

1200

1400

1



Recent progress for SM Higgs production includes

– NNLO QCD calculations for pp → H[Harlander, Kilgore; Anastasiou, Melnikov; Ravindran, Smith, van Neerven; Anastasiou, Melnikov, Petriello;Blumlein, Ravindran; . . . (02-05)]

– NNLO QCD calculations for pp → HZ, HW[Brein, Djouadi, Harlander (04)]

– (N)NLO QCD calculations for pp → QQH[Beenakker, Dittmaier, MK, Plumper, Spira, Zerwas; Dawson, Jackson, Orr, Reina, Wackeroth; Harlander, Kilgore;Campbell, Ellis, Maltoni, Willenbrock; . . . (01-05)]

– NLO QCD calculations for pp → qqH[Figy, Oleari, Zeppenfeld; Berger, Campbell (03-04)]

– NLO EWK calculations for pp → HZ, HW[Ciccolini, Dittmaier, MK (03)]

– (N)NLL resummation for pp → H[Kulesza, Sterman, Vogelsang; Berger, Qiu; Catani, de Florian, Grazzini; . . . (03-04)]

– matching of NLO calculation with parton shower MC Herwig for pp → H[Frixione, Webber (04)];

– NNLO splitting functions and PDF fits, error estimates for PDF fits[Moch, Vermaseren, Vogt; MRST; CTEQ (02-05)].


Higgs production in the MSSM: associated bbh production

Associated bbh/H production is crucial for MSSM Higgs searches at large tan β

Bottom-Higgs Yukawa coupling:

gMSSMbbh = − sin α

cos βgSM

bbH

gMSSMbbH = cos α

cos βgSM

bbH

}

tan β � 1-

{

tan β gSMbbH (Mh'MA'MZ)

tan β gSMbbH (MH'MA�MZ)

At tanβ � 1 associated bbh/H pro-

duction becomes the dominant MSSM

Higgs production mechanism

[Spira]

10-3

10-2

10-1

1

10

10 2

10 3

10 4

102

gg→H (SM)

gg→H

Hbb–

Htt–

Hqq

HZ HW

tan β = 30Maximal mixing

Ù H

Ù

h

mh/H (GeV)

Cro

ss-s

ectio

n (p

b)


Higgs production in the MSSM: associated bbh production

Associated bbh/H production is crucial for MSSM Higgs searches at large tan β

Bottom-Higgs Yukawa coupling:

gMSSMbbh = − sin α

cos βgSM

bbH

gMSSMbbH = cos α

cos βgSM

bbH

}

tan β � 1-

{

tan β gSMbbH (Mh'MA'MZ)

tan β gSMbbH (MH'MA�MZ)

At tanβ � 1 associated bbh/H pro-

duction becomes the dominant MSSM

Higgs production mechanism

[Spira]

10-3

10-2

10-1

1

10

10 2

10 3

10 4

102

gg→H (SM)

gg→H

Hbb–

Htt–

Hqq

HZ HW

tan β = 30Maximal mixing

Ù HÙ

h

mh/H (GeV)

Cro

ss-s

ectio

n (p

b)


Associated bbh production mechanism

At leading order

︸︷︷︸︸︷︷︸︸︷︷︸

MQ � MH : ∼ α2s ln2(MH/MQ) ∼ α2

s ln(MH/MQ) ∼ α2s

Summation of ln(MH/MQ) terms by using heavy quark PDFs[Collins, Olness, Tung; Barnett, Haber, Soper; Dicus, Willenbrock,. . . ]

MQ �MH-


Associated bbh production: two calculational schemes

4-flavour scheme

+ exact g → bb splitting & mass effects

− no summation of ln(MH/Mb) terms

5-flavour scheme

+ summation of ln(MH/Mb) terms

− LL approximation to g → bb splitting

The 4- and 5-flavour schemes

– are both theoretically consistent & well-defined

– represent different ways of ordering perturbation theory

– should agree at sufficiently high order

– do not match exactly at finite order


Associated bbh production: two calculational schemes

Comparison at leading order

→ strong scale dependence

→ σ(bb→ H) � σ(gg → bbH) at µ = MH

→ discrepancy reduced at µF = MH/4 (→ Harlander, Kilgore)

[see also Spira; Maltoni, Sullivan, Willenbrock; Boos, Plehn]


Associated bbh production: (N)NLO corrections

Comparison of 4- and 5-flavour schemes at (N)NLO (SM Higgs, LHC)

[Harlander, Kilgore; Dittmaier, MK, Spira]

σ(pp → bb_

h + X) [fb]

√s = 14 TeV

µ = (2mb + Mh)/4

Mh [GeV]

bb_

→ h (NNLO)

gg → bb_

h (NLO)10

10 2

10 3

100 150 200 250 300 350 400


Higgs background calculations

Example: pp → H → γγ/WW backgrounds

γγ background

– NLO parton level MC program pp→ γγ (DIPHOX) [Binoth, Guillet, Heinrich, Pilon, Werlen]

– gg → γγ at NLO (2-loop) [Bern, De Freitas, Dixon, Schmidt ’02]

WW background

– jet veto effects [Catani, De Florian, Grazzini]

– NLO parton level MC program pp→W+W−[Frixione, Nason, Ridolfi; Baur, Han, Ohnemus]

– NLO spin correlations in leptonic W decays [Dixon, Kunszt, Signer]

– tt backgrounds including finite width effects [Kauer, Zeppenfeld]

– NLO parton level MC program pp→W/Z + bb [Campbell, Ellis, Veseli]

– gluon-gluon induced contributions to pp→WW → lνlν [Binoth, Ciccolini, Kauer, MK ’05]


WW production: a background to Higgs searches

pp → H → WW → lν l′ν ′ is an important Higgs search channel

– WW pair production is the dominant background: σ× BR ≈ 5 times larger than signal

– a Higgs mass peak cannot be reconstructed from leptonic W decays

⇒ theoretical control of the background is important (≈ 5%!)

experimental selection cuts to enhance the signal/background ratio may amplifythe importance of higher-order corrections for background processes

example: gg →WW → lν l′ν′ [Binoth, Ciccolini, Kauer, MK (05)]

W−

νµ

µ+

W+

e−

νe γ, Z

νe

e−

νµ

µ+

W+γ, Z, H

g

gq

q

q

g

gq

q

q

g

g W+

νµ

µ+

e−

νe

d

d u

W−d

→ formally NNLO

→ but enhanced by Higgs search cuts

gg qq (NLO) corr.

σtot 54 fb 1373 fb 4%

σbkg 1.39 fb 4.8 fb 30%


Summary and Conclusions

The LHC will find the (or a or various) Higgs boson(s) (or some new strong inter-

actions in the gauge boson sector)

Higgs physics is exciting:

– reveals the mechanism of electroweak symmetry breaking

– points towards physics beyond the SM (hierarchy problem)

Exploring Higgs physics at the LHC needs precision measurements and precision

calculations

– signal cross sections are now known within 15% or better

– more work is needed to improve prediction of background processes

Exciting times ahead!


higgs production at the lhc - rwth aachen universitymkraemer/higgs... · introduction: the higgs...

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