Measuring the Higgs trilinear self-coupling at a high energy Muon - - PowerPoint PPT Presentation

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Measuring the Higgs trilinear self-coupling at a high energy Muon Collider

[Preliminary] Alexander Conway aconway@fnal.gov

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Introduction

• Measuring the tri-linear Higgs self-coupling

parameter, “λ,” is a crucial test of SM Higgs electroweak symmetry breaking [2].

• In the Standard Model:
• Best-case LHC limits on λ uncertainty are ~+30% and

~-20% [2].

• Best-case CLIC limits are ~11% [1].
• How (well) could a muon collider measure this?

V (ηH)=1 2 mH

2 ηH 2 +λ HHH v ηH 3 + 1

4 λHHHH ηH

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[1] λHHH=λ HHHH=λSM=mH

2 /2v 2

[1]

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Introduction

• High energy (1.5 – 10 TeV):

– WW double-Higgs fusion:

• mu+ mu- → h h nu nu~

– Cross section increases with

(sqrt?)ecm

• Higher cross section than e+e-

because of tighter beam spread

• Muon collider can go to higher

energies with less beam spread

– Process is more forward-

boosted at higher energies

• Muon collider needs bigger cone;

how does this affect signal?

• Low energy (~2*m_h):

– S-channel:

• mu+ mu- → h* → h h

– Inaccessible at e+e-

collider.

– More or less isotropic. – Analytic expression for

cross section:

• ~1.4ab: too small to use

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What to Measure?

WW Fusion

• These three diagrams interfere with each other.

– Cross section of (1) is directly dependent on λ. – Therefore, total cross section of these

diagrams is sensitive to the value of λ.

• Decay angle θ* distribution is different for (1)

– Distribution of |cosθ*| is sensitive to λ.

• Combined template fit to cross section and

|cosθ*| distributions.

(1) (2) (3)

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h* h h

θ θ

*

̂ pz

Decay Angle

Decay angle is the angle the decay products make with respect to the boost direction in the h* reference frame.

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Decay Angle

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Cone Angle Effects @ 3TeV

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How to Measure?

WW Fusion

• Do template fit with λ as independent parameter.
• Cross section:

– Not significantly larger than e+e- – Requires good ID of 2-Higgs events. – Strube et al. [1] used full jet/flavor/PFA recon. as inputs for ANN to tag Higgs

events.

• Hard to replicate with bigger cone in short time.
• How else to estimate effect of cone on this measurement?
• Decay angle:

– Requires good angular resolution. – How does cone affect resolution in decay angle?

• Generator level study of reconstruction using MC data?
• Bottom line:

– Muon collider not significantly different from e+e- for this channel. – CLIC and ILC studies approximate the muon collider's potential.

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Comparison of Cone Effects

• Look at h h → b + b~ + b + b~ at 3 and 6 TeV

– Require all four b's can theoretically be tagged using tracker.

• Each b decay must have at least one lepton track or two other charged

particle tracks.

• These tracks must have at least 5 (25) GeV of energy.
• Initial particle momentum must have |cos(theta)| < cos(theta_cone)

– Compare acceptance for different cone half-angles

• 10 deg for high-energy MuC
• 2 deg for CLIC (From [4], p. 52: “95% electron tagging efficiency down

to ≈ 40 mrad polar angle”)

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Comparison of Cone Effects

Self- Coupling Process 3 TeV 6 TeV 2 Deg 93.6% 94.4% 10 Deg 90.2% 87.4% Other hh Processes 3 TeV 6 TeV 2 Deg 93.8% 94.6% 10 Deg 87.4% 78.0%

• Acceptances for four 'taggable' b's

– 5 GeV energy cut – 25 GeV energy cut

Self- Coupling Process 3 TeV 6 TeV 2 Deg 22.0% 34.6% 10 Deg 18.6% 24.6% Other hh Processes 3 TeV 6 TeV 2 Deg 27.1% 36.8% 10 Deg 20.0% 20.9%

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3TeV vs. 6TeV Visible Energy in Self-Coupling Process

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S-Channel

σ(μ

+μ

• →h

*→h 0h 0)=

9mμ

2 mh 4(

λ λSM ) 64 πv

4(s−mh 2) 2√1−4mh 2/s

σ(μ

+μ

• →h

*→h 0h 0)=σ(τ + τ

• →h

*→h 0h 0)×(

mμ mτ)

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σ(τ

+ τ

• →h

*→h 0h 0)=0.4fb

σ(μ

+μ

• →h

*→h 0h 0)=0.4fb×3.6×10 −3=1.4ab

• Easier: calculate cross section for tau+tau- in

MadGraph5* and use mass ratio to get muon cross section approximation:

– Above expression has been confirmed to

give same results [3]

*MadGraph5 does not have mu+mu- → h vertex by default

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What We Learned

• S-channel is indeed too small to use.

– ~1 event per 500fb^-1!

• 10 degree cone has non-negligible effect on signal.
• Muon Collider can still do this physics at 6 TeV
• All about decay angles and template fitting
• Physics backgrounds at 3TeV should be very similar at MuC

and CLIC

– Lepton universality means only difference in cross sections comes

from beam spread.

– Machine background is the bigger issue.

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My Focus Moving On:

• Continuing development and documentation of MCD analysis

toolchain/environment.

– Event generation, hadronization, simulation, background overlay,

reconstruction, analysis.

– Have to be using it to know what will be needed. – Continue providing support for people to work on analyses.

• Physics studies:

– Develop understanding of/motivation for segmentation, fast timing and DR

• Demonstrate background reduction, show effect on signal, etc.

– Full simulation, look at jet resolution

• Demonstrate W and Z separation

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References

(1)

• T. Laštovička, J. Strube. Measurement of the trilinear Higgs

selfcoupling at CLIC. Slides accessed online on 08/13/13 at link. (2)

• F. Goertz et al. Higgs Boson self-coupling measurements using

ratios of cross sections. MITP/13-002 (2013) (3)

• S. Dawson et al. Higgs Working Group Report. Draft accessed
• nline 08/13/13 at link

(4) "Physics and Detectors at CLIC - CLIC Conceptual Design Report" is now published, see: https://edms.cern.ch/document/1180032/ (290 pages, 20 Mb)