QCD background for Higgs → γγ

Pavel Nadolsky

Argonne National Laboratory

May 16, 2006

Pavel Nadolsky (ANL) QCD background for Higgs production May 16, 2006May 16, 2006 1

Transverse momentum (qT ) distributionsin pp

(−) → γγX

We use qT resummation to compute fully differential distributionsof prompt γγ pairs (including qT dependence) at NNLL accuracy

References

1. C. Balázs, E. Berger, P. N., C.-P. Yuan, hep-ph/06030372. C. Balazs, E. Berger, S. Mrenna, C.-P. Yuan, PRD 57, 6934 (1998)3. C. Balazs, P. N., C. Schmidt, C.-P. Yuan, PLB 489, 157 (2000)4. P. N., C. Schmidt, PLB 558, 63 (2003)

The results are implemented in a MC integrator program ResBos

Pavel Nadolsky (ANL) QCD background for Higgs production May 16, 2006May 16, 2006 2

SM Higgs boson search at the LHC

1

10

10 2

102

103

Higgs mass MH (GeV)

Sig

nal s

igni

fica

nce

H → γ γ + WH, ttH (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 = 100 fb-1

(no K-factors)

ATLAS

¥ gg → H → γγ (via t-quark loop) isthe crucial and challenging searchmode for 115 < MH < 140 GeV

¥ QCD background processes forγγ production are complicated;must be understood forsuccessful Higgs discovery andmeasurement of Higgs crosssections

¥ An interesting QCD process onits own!

Pavel Nadolsky (ANL) QCD background for Higgs production May 16, 2006May 16, 2006 3

Higgs discovery significance is improved by...

Bruce Mellado, ATLAS Physics Workshop 07/06/05

Higgs -> 2gamma + 0 or 1 jetPreliminary

TDR-like analysis with NLO cross-sections

¥ ... NLO corrections¥ ... use of qT of γγ pair

as a discriminatingvariable

I Significanceincreased by20− 40%

¥ ... combination ofinclusive H → γγ andH → γγ + 1 jetanalyses

¥ ... optimization of γdetection and jetrejection

Pavel Nadolsky (ANL) QCD background for Higgs production May 16, 2006May 16, 2006 4

Higgs discovery significance is improved by...

Bruce Mellado, ATLAS Physics Workshop 07/06/05

Higgs -> 2gamma + 0 or 1 jetPreliminary

TDR-like analysis with NLO cross-sections

¥ ... NLO corrections¥ ... use of qT of γγ pair

as a discriminatingvariable

I Significanceincreased by20− 40%

¥ ... combination ofinclusive H → γγ andH → γγ + 1 jetanalyses

¥ ... optimization of γdetection and jetrejection

Pavel Nadolsky (ANL) QCD background for Higgs production May 16, 2006May 16, 2006 4

Knowledge of qT optimizes Higgs boson search(Abdullin et al., 1998; Balazs, P.N., Schmidt, Yuan; de Florian, Kunszt, 1999; Berger, Qiu, 2003)

M. Escalier, 2005; ResBos [2,3]

¥ At qT ¿ Q,(

dσdq2

T

)q2

T¿Q2≈ ∑∞

k=0 αks

[ckδ(~qT )

+q−2T ·∑2k−1

n=0 dnk lnn(Q2/q2T )

];

the large terms are summedto all orders in αs by means ofCollins-Soper-Sterman (CSS)resummation

¥ Higgs 〈qT 〉 is enhanced bya larger soft (Sudakov) factorand gluon PDF shapein gg → H scattering

Pavel Nadolsky (ANL) QCD background for Higgs production May 16, 2006May 16, 2006 5

Knowledge of qT optimizes Higgs boson search(Abdullin et al., 1998; Balazs, P.N., Schmidt, Yuan; de Florian, Kunszt, 1999; Berger, Qiu, 2003)

M. Escalier, 2005; ResBos [2,3]

¥ Higgs signal has a larger 〈qT 〉than the background(a prediction of qT resumma-tion)

¥ An efficient qT -dependentlikelihood analysis is possible

Pavel Nadolsky (ANL) QCD background for Higgs production May 16, 2006May 16, 2006 5

qT resummation for γγ production(QCD background)

¥ new PQCD corrections to production in direct qq̄ + qg andgg channels

¥ improved treatment of the fragmentation region

¥ improved model for nonperturbative resummedcontributions (A. Konychev, P. N., PLB 633, 710 (2006))

I non-pert. terms constrained by low-Q and Tevatron Z data

¥ automated matching at the ntuple level; optimizedMonte-Carlo integration

I new ResBos code is easy to use!

Pavel Nadolsky (ANL) QCD background for Higgs production May 16, 2006May 16, 2006 6

Direct diphotons

The dominant production mode; evaluated here up to fullNLO/NNLL accuracy

Balazs, Berger, Nadolsky, Yuan, 2006

Collinearfragmentationmodel

Direct γγproduction(resummed)

¥ qg and ggchannels areenhanced atx ∼ Q2/s ¿ 1 bylarge gluon PDF

¥ qq̄ and qgchannels at NLO:Aurenche et al.; Bailey, Owens,Ohnemus

¥ gg channel at NLO:Balazs, PN, Schmidt, Yuan; deFlorian, Kunzst; Bern, De Freitas,Dixon; Bern, Dixon, Schmidt

Pavel Nadolsky (ANL) QCD background for Higgs production May 16, 2006May 16, 2006 7

gg → γγX in the qT → 0 limit

+ ...

¥ O(α3s ) 1-loop 5-leg (pentagon) diagrams

I are computed in the helicity amplitudeformalism

I numerically checked against the“sector decomposition” calculation(Binoth, Guillet, Mahmoudi)

I small-qT limit is derived at thematrix-element level using the splittingamplitude method

¥ 2-loop box diagrams are added atqT = 0; the full cross section is resummedin the qT → 0 limit

Pavel Nadolsky (ANL) QCD background for Higgs production May 16, 2006May 16, 2006 8

Fragmentation model

The qg fragmentation collinear singularity is removed bycombination of quasi-experimental and smooth-cone isolation

Balazs, Berger, Nadolsky, Yuan, 2006

Collinearfragmentationmodel

Direct γγproduction(resummed)

¥ sufficient fordescription ofsmallfragmentationcontributionsrelevant for theTevatron and LHC

¥ good agreementwith the Tevatrondata

Pavel Nadolsky (ANL) QCD background for Higgs production May 16, 2006May 16, 2006 9

qT resummation for γγ production(QCD background)

¥ new PQCD corrections to production in direct qq̄ + qg andgg channels

¥ improved treatment of the fragmentation region

¥ improved model for nonperturbative resummedcontributions (A. Konychev, P. N., PLB 633, 710 (2006))

I non-pert. terms constrained by low-Q and Tevatron Z data

¥ automated matching at the ntuple level; optimizedMonte-Carlo integration

I new ResBos code is easy to use!

Pavel Nadolsky (ANL) QCD background for Higgs production May 16, 2006May 16, 2006 10

Comparison with the CDF Run-2 data

pp_ → γγX, CDF Run-2, 207 pb-1

QT (GeV)

dσ/d

QT (

pb/G

eV)

∆Rcone = 0.4, ∆Rγγ > 0.3ResummedFinite-order

10-1

1

0 5 10 15 20 25 30 35 40

¥ The NLO prediction (blue)diverges at low qT

¥ The resummed prediction(red) agrees with the dataat all qT ; matches the NLOprediction at large qT

Pavel Nadolsky (ANL) QCD background for Higgs production May 16, 2006May 16, 2006 11

γγ production at the LHC

pp → γγX, √S = 14 TeV

qT (GeV)

dσ/d

q T (

pb/G

eV)

115 < Q < 130 GeV

ResummedResummed, qq

_+qg only

ET iso = 15 GeV

10-2

10-1

0 20 40 60 80 100 120

qq̄ : qg : gg = 30:50:20

(compare withqq̄ : qg : gg =70:20:10at the Tevatron)

O(α2s ) corrections to qg are

likely important(σNNLO/σNLO ∼ 20%?)

Pavel Nadolsky (ANL) QCD background for Higgs production May 16, 2006May 16, 2006 12

γγ production at the LHC

pp → γγX, √S = 14 TeV

qT (GeV)

dσ/d

q T (

pb/G

eV)

115 < Q < 130 GeV

ResummedResummed, qq

_+qg only

DIPHOX

ET iso = 15 GeV

10-2

10-1

0 20 40 60 80 100 120

¥ DIPHOX agrees with theresummation at large qT ;exhibits integrable logarithmicsingularities at qT < E iso

T (E isoT is

the isolation energy)

¥ RESBOS shows a milddiscontinuity at qT = E iso

T

Pavel Nadolsky (ANL) QCD background for Higgs production May 16, 2006May 16, 2006 12

γγ production at the LHC

pp → γγX, √S = 14 TeV

qT (GeV)

dσ/d

q T (

pb/G

eV)

115 < Q < 130 GeV

ResummedResummed, qq

_+qg only

DIPHOX

ET iso = 15 GeV

10-2

10-1

0 20 40 60 80 100 120

pp → γγX, √S = 14 TeV

Q (GeV)

dσ/d

Q (

pb/G

eV)

qT < Q

ResummedResummed, qq

_ + qg only

DIPHOX, qq_ + qg only

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

40 60 80 100 120 140 160 180 200 220 240

Pavel Nadolsky (ANL) QCD background for Higgs production May 16, 2006May 16, 2006 12

Conclusions

¥ Knowledge of qT distributions is important for Higgs bosonsearches

I Significance of H → γγ discovery is improved by includinginformation about qT distributions (ongoing ATLAS studies)

I The shape of dσ/dqT affects dσ/dQ and other observables

I Theory predictions for QCD γγ production are most reliable atE iso

T < qT < Q

Pavel Nadolsky (ANL) QCD background for Higgs production May 16, 2006May 16, 2006 13

Conclusions II

¥ Resummation predicts normalization and shape ofdσ/d3pγ1d3pγ2 for direct γγ production at NNLL/NLO.

¥ Large logs are resummed at qT ¿ Q

¥ At qT & Q, our results agree with NLO, as well as with DIPHOXcalculation within the fragmentation model uncertainty ofDIPHOX

¥ At qT < E isoT , fragmentation contributions are removed by

smooth-cone isolation

I no logarithmic singularities at qT 6= 0!

¥ Streamlined matching of resummed and NLO cross sections;optimized phase space integration

Pavel Nadolsky (ANL) QCD background for Higgs production May 16, 2006May 16, 2006 14

Conclusions II

¥ Resummation predicts normalization and shape ofdσ/d3pγ1d3pγ2 for direct γγ production at NNLL/NLO.

¥ Large logs are resummed at qT ¿ Q

¥ At qT & Q, our results agree with NLO, as well as with DIPHOXcalculation within the fragmentation model uncertainty ofDIPHOX

¥ At qT < E isoT , fragmentation contributions are removed by

smooth-cone isolation

I no logarithmic singularities at qT 6= 0!

¥ Streamlined matching of resummed and NLO cross sections;optimized phase space integration

Accurate predictions for future measurements at the Tevatronand LHC!

Pavel Nadolsky (ANL) QCD background for Higgs production May 16, 2006May 16, 2006 14

Backup slides

Pavel Nadolsky (ANL) QCD background for Higgs production May 16, 2006May 16, 2006 15

Universality of nonperturbative contributions

A. Konychev, P. N., PLB 633, 710 (2006)Nonperturbative Gaussian smearing a(Q)

5 10 20 50 100 200Q @GeVD

0

0.2

0.4

0.6

0.8

1

1.2

a@G

eV

2D

E288E605

CDF ZD0 ZR209

b max = 1.5 GeV-1

a2 = 0.19 GeV2

¥ qT factorization: initial-statenonperturbative contributions(∼“intrinsic” 〈k2

T 〉 ≡ a) followuniversal quasi-linear dependenceon ln Q; this expectation isconfirmed by the global analysis ofDrell-Yan and Z boson data

¥ the observed ln Q dependence agrees with therenormalon/lattice estimate

¥ at Q ∼ MZ , soft NP corrections dominate over collinear NPcorrections

¥ the model is employed to predict γγ cross sections

Pavel Nadolsky (ANL) QCD background for Higgs production May 16, 2006May 16, 2006 16

Resummation vs. CDF Run-2 data (PRL 95, 022003 (2005))

pp_ → γγX, CDF Run-2, 207 pb-1

QT (GeV)

dσ/d

QT (

pb/G

eV)

∆Rcone = 0.4, ∆Rγγ > 0.3ResummedFinite-order

10-1

1

0 5 10 15 20 25 30 35 40

¥ The NLO prediction (blue)diverges at small qT

¥ The resummed prediction(red) agrees with the data at allqT ; matches the NLO predictionat large qT

Pavel Nadolsky (ANL) QCD background for Higgs production May 16, 2006May 16, 2006 17

Resummation vs. CDF Run-2 data (PRL 95, 022003 (2005))

pp_ → γγX, CDF Run-2, 207 pb-1

qT (GeV)

dσ/d

q T (

pb/G

eV)

∆Rcone = 0.4, ∆Rγγ > 0.3

ResummedDIPHOX, ET

iso = 1 GeV

DIPHOX, ET iso = 4 GeV, µF = Q/2

10-1

1

0 5 10 15 20 25 30 35 40

The large-qT “shoulder” in thedata occurs atQ . 25 GeV, Q . qT , ∆ϕ < π/2The DIPHOX cross section¥ agrees with NLO RESBOS for

the nominal E isoT = 1 GeV, same

µF

¥ reproduces the “shoulder”for E iso

T = 4 GeV, smaller µF

N for E isoT = 4 GeV, the 1-fragmentation cross section increases by 400%

Pavel Nadolsky (ANL) QCD background for Higgs production May 16, 2006May 16, 2006 17

Resummation vs. CDF Run-2 data (PRL 95, 022003 (2005))

pp_ → γγX, CDF Run-2, 207 pb-1

qT (GeV)

dσ/d

q T (

pb/G

eV)

∆Rcone = 0.4, ∆Rγγ > 0.3

ResummedDIPHOX, ET

iso = 1 GeV

DIPHOX, ET iso = 4 GeV, µF = Q/2

10-1

1

0 5 10 15 20 25 30 35 40

The large-qT “shoulder” in thedata occurs atQ . 25 GeV, Q . qT , ∆ϕ < π/2The DIPHOX cross section¥ agrees with NLO RESBOS for

the nominal E isoT = 1 GeV, same

µF

¥ reproduces the “shoulder”for E iso

T = 4 GeV, smaller µF

N for E isoT = 4 GeV, the 1-fragmentation cross section increases by 400%

Pavel Nadolsky (ANL) QCD background for Higgs production May 16, 2006May 16, 2006 17

Resummation vs. CDF Run-2 data (PRL 95, 022003 (2005))

pp_ → γγX, CDF Run-2, 207 pb-1

qT (GeV)

dσ/d

q T (

pb/G

eV)

∆Rcone = 0.4, ∆Rγγ > 0.3

ResummedDIPHOX, ET

iso = 1 GeV

DIPHOX, ET iso = 4 GeV, µF = Q/2

10-1

1

0 5 10 15 20 25 30 35 40

¥ Other large corrections areknown to contribute at Q . qT(not in the existing theorycalculations)

¥ The CDF “shoulder” is a low-Qeffect, not relevant for the LHCHiggs searches

N can be removed by aqT ≤ Q cut

Pavel Nadolsky (ANL) QCD background for Higgs production May 16, 2006May 16, 2006 17

Azimuthal angle separation ∆ϕ

pp_ → γγX, CDF Run-2, 207 pb-1

∆ϕ (rad)

dσ/d

(∆ϕ)

(pb

/rad

)

∆Rcone = 0.4, ∆Rγγ > 0.3

RESBOS, ET iso = 1 GeV

µF = Q (lower), Q/2 (upper)

DIPHOX, ET iso = 1 GeV

DIPHOX, ET iso = 4 GeV, µF = Q/2

10-1

1

10

0 0.5 1 1.5 2 2.5 3

pp_ → γγX, √S = 1.96 TeV

∆ϕ (rad)

dσ/d

(∆ϕ)

(pb

/rad

)

∆Rcone = 0.4, ∆Rγγ > 0.3

RESBOS, ET iso = 1 GeV

µF = Q (lower), Q/2 (upper)DIPHOX, ET

iso = 1 GeV

DIPHOX, ET iso = 4 GeV, µF = Q/2

QT < Q

10-1

1

10

0 0.5 1 1.5 2 2.5 3

Pavel Nadolsky (ANL) QCD background for Higgs production May 16, 2006May 16, 2006 18