QCD background for Higgs γγ · ¥ Higgs signal has a larger hqTi than the background (a...
Transcript of QCD background for Higgs γγ · ¥ Higgs signal has a larger hqTi than the background (a...
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Backup slides
Pavel Nadolsky (ANL) QCD background for Higgs production May 16, 2006May 16, 2006 15
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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
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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
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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
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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
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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
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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
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