Transcript TESLA - The TEV-Energy Superconducting Linear Accelerator Light Higgs Production at the Tesla Photon Collider Aura Rosca DESY Zeuthen Amsterdam, Netherlands, 1-4 April 2003

TESLA - The TEV-Energy Superconducting Linear Accelerator
Light Higgs Production at the Tesla
Photon Collider
Aura Rosca
DESY Zeuthen
Amsterdam, Netherlands, 1-4 April 2003
TESLA - The TEV-Energy Superconducting Linear Accelerator
Motivation
• Measure the two-photon partial
width:
– Contribution to the two photon
decay width from any kind of
massive charged particles. Any
deviation of the partial width from
SM prediction:
• Evidence for new physics;
• Can be directly compared to
predictions of alternative models
(MSSM, NMSSM, general 2HDM).
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TESLA - The TEV-Energy Superconducting Linear Accelerator
How to Get Widths?
• The Higgs mass peak gives
Γ(h  γγ)  BR(h  bb) (mh  140 GeV)
• Taking BR(h  bb) and
from LHC or LC,
Γ γγ 
BR(h  γγ)
Γ(h  γγ)  BR(h  b b )
BR(h  b b )
Γ γγ
Γ tot 
BR(h  γγ)
• This is proposed as the way to get the
total width. This would be a modelindependent result.
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TESLA - The TEV-Energy Superconducting Linear Accelerator
•
Realistic Luminosity Spectra and
Polarization for the Photon Beams
Circe 2.0
dL
 1.57 fb-1 /GeV
d s γγ
• Luminosity spectra for J=0,2 with
s ee  210 GeV
2λ e  85%, Pc  100%
x  1.8
• Total luminosity for
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z  s γγ  80 GeV,
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L  80 fb-1 .
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TESLA - The TEV-Energy Superconducting Linear Accelerator
The Signal
• Adjust beam polarization to
increase the signal cross
section;
• If m h know, adjust photon
energies to have s   mh .
mh  120 GeV
• Higgs boson has spin-0
Γ tot  4 MeV
– It is produced from J=0
NS ( γγ  h  b b ) 
(1  λ1 λ 2 ),
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dL γγ
d s γγ

mh
BR(h  b b )  68%
4 π Γ(h  γγ)BR(h  b b ) BR(h  γγ)  0.22%

2
m
dL
-1
2
λ i photon helicities
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d s γγ
 1.57 fb /GeV
NS  20000 events/year
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TESLA - The TEV-Energy Superconducting Linear Accelerator
Higgs Decay
• Light Higgs (mh  140 GeV)
BR(h  bb) dominant
• Signature
– 2-jet events
• Background
b
b
γγ  b b (g)
γγ  cc(g)
– large cross sections, can be suppressed exploiting
the polarization dependence of the cross section.
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TESLA - The TEV-Energy Superconducting Linear Accelerator
The Background
2
mq
12πα Q β
dσ (J  0)

(1  β ) 
dcosθ
s (1  β cos θ)
s γγ
2
LO
z
4
q
2
4
2
γγ
12πα2 Q q4β 3
dσLO (J z  2)
2
2
2

(1

cos
θ
)(2

β
(1

cos
θ))
2
2
2
dcosθ
s γγ (1  β cos θ)
β  1  4m2q /s γγ
σ  Qq4 ,
•
σ(γγ  cc)  σ(γγ  bb)
Need b-tag to reduce
σ(J z  0) 
mq2
s γγ
cc
bkg.
γγ  bb(g)
Need to take into account
the NLO corrections!
in respect t o σ(J z  2)
Photons of the same helicity
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•
Suppression removed for
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
suppress continuum
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bb.
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TESLA - The TEV-Energy Superconducting Linear Accelerator
Background Cross Sections
• NLO cross sections
include:
– Exact one-loop QCD
corrections (Jikia,
Tkabladze)
– Non-Sudakov form factors
(Melles, Stirling, Khoze)
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TESLA - The TEV-Energy Superconducting Linear Accelerator
Simulation
• Results include realistic photon spectrum for ++ and
+- helicities simulated with Circe 2.0;
• Signal MC generated with Pythia and passed through
the TESLA fast simulation, Simdet 4.02;
• Background MC generated with Pythia and passed
through the TESLA fast simulation:
– s γγ  80 GeV
– Convolution with the realistic photon spectrum for ++ and +helicities
– Events weighted by the NLO Xsec for ++ and +- helicities
• B-tagging
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TESLA - The TEV-Energy Superconducting Linear Accelerator
Cross Sections
s γγ  80 GeV
Cross section
(pb)
Number of
expected ev.
Number of
generated ev.
Signal process
 eff , L  80fb 1
0.25
~20 000
50 000
Background
(from Pythia)
0.75
44 175.0
(L J0  58.9fb 1 )
600 000
  h  bb
  b b (g)
J=0
  b b (g)
4.79
102 314.4
(L J2  21.1fb 1 )
600 000
  cc (g)
13.4
789 260.0
600 000
  cc (g)
85.1
1 817 734.0
600 000
J=2
J=0
J=2
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TESLA - The TEV-Energy Superconducting Linear Accelerator
Selection Requirements
• Isotropic angular distributions for signal and forward
peaked for the background:
• cosθT  0.7
• Jet clustering using Durham with y=0.02; N jet  2,3
• Other cuts on E vis  95 GeV, E long/E vis  0.1
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TESLA - The TEV-Energy Superconducting Linear Accelerator
B-tagging Performance
e  e   Z  qq, s ee  91.2 GeV
bb
cc
Events containing at least one jet with two
reconstructed vertices:
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NNout  0.95  ε bb  70 %
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purity  98%
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Invariant Mass Spectrum
• It is possible to isolate the signal from the background.
N sig  6018 events
N bkg  7111 events
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TESLA - The TEV-Energy Superconducting Linear Accelerator
Partial Width Uncertainty



Δ Γ(h  γγ )BR(h  bb

Γ(h  γγ )BR(h  bb)


N obs
N obs  N b
 1.9%
• This is one of the main justifications for a
Photon Collider.
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TESLA - The TEV-Energy Superconducting Linear Accelerator
Summary
• Measure Γ(h  γγ)  BR(h  bb) with a
precision of 1.9% by:
– Taking into account the QCD radiative corrections
to the background process γγ  qq (Pythia + NLO
Xsec.) through a reweighting procedure;
– Adopting a b-quark tagging algorithm based on a
neural network.
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DESY-Zeuthen
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TESLA - The TEV-Energy Superconducting Linear Accelerator
Summary
• Measure Γ(h  γγ)  BR(h  bb)
with a precision of 1.9% by:
– Taking into account the QCD radiative
corrections to the background
γγ  +
qqNLO
process
(Pythia
Xsec.) through a reweighting
procedure;
– Adopting a b-quark tagging algorithm
based on a neural network.
N sig  6018 events
N bkg  7111 events
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