Measurements of Higgs Coupling Parameters at ATLAS

Measurements of Higgs Coupling Parameters at ATLAS

Jianming Qian(University of Michigan)

For the ATLAS Collaboration

Higgs Couplings 2013, Freiburg, October 14-16, 2013

Introduction, input analysesSignal strengths, coupling fits

Jianming Qian (University of Michigan)2

Why Couplings?Discovery has been made…

Nobel prize has been awarded.The question remains:

Is the new boson solely responsible for the electroweak symmetry breaking?

Two approaches to address this question:precise coupling measurements (this presentation);direct searches of additional Higgs-like bosons (Bressler’s talk).


Theoretical Uncertainties

The uncertainties in the ggF process are starting to limit the precision of the couplingmeasurements.

0.57 has a large impact on parametric uncertainties

H bbb m

Need to improve SM calculations and their inputs as we enter a new era of precision Higgs physics!

LHC cross section working group

A. Denner et al., arXiv:1107.5909


Disentangle Production Processes – Why?

Strong ProductionFermion Coupling

Electroweak Production,Vector Boson Coupling

Production processes naturally fall into two groups

Higgs candidate events are selected from their decay signatures, independent of production.

Need to disentangle the production processes using the productionsignatures (independent of decay) to study couplings.


Disentangle Production Processes – How?

ggFthe rest

From other activities in candidate events…

VHLeptons, missing ET or low-mass dijets from W or Z decays

VBFTwo high pT jets with high-mass and large Pseudorapidity separation

ttHTwo top quarks: leptons, missing ET, multijets or b-tagged jets

For example, the analysis uses BDT to isolate VBF process from the dominant ggF process.

H

These differences can be exploited using advanced techniques to enhance the separation.


H→ Analysis14 exclusive categories based on both detector performance and production processes

Category signal compositions

Production motivated categorieslepton, missing ETdijet (high and low mass)untagged (the rest)

8 TeV, in a window around m 126.8 GeV with 90% of signal.


H→ZZ*→4l Analysis3 categoriesVBF-like: Two high pT jets with large dijet mass (1 data event)*

VH-like: with additional leptons (0 data event)*

ggF-like: The rest (31 events)*

* within 1255 GeV

47+8 TeV, m 120 -130 GeV


H→WW*→lnln Analysis

3 categories of exclusive jet bins: 0, 1, and 2 jetsout of necessity due to large top background

8 TeV, 0.75,1 for 125.5 GeV T H Hm m m


Construct likelihood from Poisson probabilities with parameter of interest (signal strength m in this case):

Hypothesized value of m istested with a test statistic:

Systematic uncertainties are included as nuisance parameters constrained by chosen pdfs (Gaussian, log-normal, …)

Combination amounts to taking product of likelihoods from different channels:

Statistical Procedure

ˆ̂,2ln 2ln ˆˆ,

m

m mm

m

Lq

L

| , | |:data Poisson data signal strength; : 'nuisance' parameters (efficiencies...)

m mm

L s b p

| ,data| , datam m i i i iL L


Overall Signal Strength

. . . . .1 33 0 14 stat 0 15 systm

Single scale parameter for allproduction and decay combinations:

SM

BRBR

m

m

Systematic uncertainty:roughly equal experimental and theoretical contribution.

Consistency with the SM expectation(m 1) is about 7%.

The largest deviation is seen in H→with a significance of ~1.9.

By final states


Probing the Production…

The ratio probes production only

BRs cancel out for each final state

VBF VH

ggF ttH

mm

0.60.40.3 0.4

1.4 stat systVBF VH

ggF ttH

mm

The combination is independent of potential new physics in differentdecay final states.

Strong vs electroweak (fermion vs vector boson)


Evidence for the VBF ProductionThe signal strength of the VBF process can be extracted by profiling (factor out) the contribution from VH little effect from the profiling:

mm

0.60.40.3 0.4

1.4 stat systVBF

ggF ttH

0 0.04% a 3.3 evidence for the VBF production. m m VBF ggF ttHp

Profiling

profiledVHm

mm

VBF VH

ggF ttH


Signal Strengths by Processes

Status: ggF well established, evidence for VBF, indication for VH, not yet sensitive to ttH

Starting to isolate all four production processes…


Beyond Signal Strengths

Higgs couplings to fermions and vector bosons are at the heart of all these. Potential deviations from SM can be studied from these couplings. Using scale parameters SM: 1 to parametrized the devi ations:

2 22 22 2, ,

ff V VHff HVV Hff HVVf V

m mm mg g g g

Signal strength mixes different production processes, production and decay, tree- and loop-level Higgs couplings. Consequently it could obscure potential new physics.

t

same couplings, but a mixture of production and decay

a mixture of fermion and vector boson couplings


Rate Modifications

2 2

2g

SMH

BR gg H gg H BR H

2

No BSM decays2 2 2 2

With BSM decays 2 2

is the scale factor to the total Higgs decay width

1

H

H x H x SMx x

SMH x

BSM

BR H xx BR H xx

BR H xxBR

Example:gg H

's can then be extracted from fits to the measured rates. Theoretical cross section and branching ratio uncertainties are absorbed into the uncertainties of 's.


Benchmark Models

2

2

Parameter definitions: coupling scale parameter

and

i iij ii

j H

Current statistics not sufficient to fit the most general model, reducing number of parameters through benchmark models.

A few selected models following the prescriptions of arXiv:1209.0040 (Thanks to the LHC cross section working group!)

All models assume no BSM productions and decays


Decomposing Loops…

t/b

In SM, the cross section can be brokeninto three pieces: SM tt bb tb

gg H

2 2With coupling modifications, the cross section becomes tt bbt b t b tb

The effective coupling scale parameter is

Hgg2 2

2

2 2 * 1.058 0.007 0.065

tt bb tb

SM tt b

t b t bg

t b t b

b tb

2 22

2 2 * 0.07 1.59 0.66

tt WW tW

SM ttt W t W

t

W

W

W

W

W

t

t

* 125.5 GeVHm


Inputs to Coupling Fits

At the LHC, only the products of BR's are measured. There is no model-independent way to separate and BR.

Measured rates of different production and decay combinationsalong with their estimated compositions

and a lot of interesting discussions…


Fermion and Boson Couplings

: for all fermions ...: for all vector bosons and are decomposed to their tree-level couplings

F F t b

V V W Z

g

, F V

Interference between W- and top-loop in H decay

two minima in the contour

68% CL intervals: 0.76, 1.181.05, 1.22

F

V

is largely determined by the production since no fermion decays are included.

F

2 2 20.75 0.25H F V

(Contours include theoretical uncertainties)


Fermion-Boson Coupling Ratio2

2Reparameterized model: , F VFV VV

V H

0.70, 1.01FV consistency with the SM: 12%

, FV VV

The ratio is independent of the Higgs total width assumption.FV


Probing Vertex Loops…

, g and for H and H couplings; All other tree-level scale parameters 1.

g

i

gg

1.04 0.141.20 0.15

g

Consistency with the SM expectations 1, 1 at 14%.

g

Note that the fit attributes theobserved high rate in the channel mostly to the decay.

2 2 20.91 0.085 0.0023H g

Probe production and decay loops, sensitive to potential new physics in these loops.

gg H H


Custodial Symmetryis required by the SM and imposes 1, can be tested from the measured rates (mostly from decays).

W Z WZ

, , , WZ FZ Z ZZ All 's are normalized to ;Universal fermion scale parameter.

Z

Potential new physics in the H decay loops may result in apparent deviation from unity. WZ

Without the decomposition: 0.82 0.15WZ

Consistency with the SM: 20%

Two models:with and withoutthe loop decomposition.


Summary of Coupling Fits

Coupling parameters are determined with precisions ~10%.

Fits to different models are not independent, they often represent different parameterizations of the same information with varying assumptions.

The bottom line is that the data isconsistent with the SM expectationat ~10% level.

SM


Expected Coupling DeviationsTypical effect on coupling from heavy state (or new physics scale) M:

(Han et al., hep-ph/0302188, Gupta et al. arXiv:1206.3560, …)

2

5% @ M 1 TeVM

Typical sizes of couplingmodification from someselected BSM models

To be sensitive to a deviation , the measurement precision needs to be much better than , at least /3! Challenging to measure absolute couplings, better precisions can be achieved for some ratios of couplings.

Snowmass Higgs report


Non-SM DecaysHiggs could have decays that are not accounted for in SM. The decays do not have to be invisible. They could be decays not detectable at LHC. modified total Higgs decay width and therefore BRs of other decays, effectively leave the total decay width free.

2 2

2inv,undet.inv,undet.

, 11

SMH H SM

H x

H

BR H xx BR H x Rx BBR

A model allows for potentialnew physics in vertex loops and additional decays

inv,undet., , g BR +0.32-0.14+0.16-0.14

inv,undet.

1.081.24

0.6 @ 95% CL

g

BR

95% CL upper bound


ConclusionThe couplings have been measured at ~10% precisions and are Consistent with the expectation of the Standard Model.

Within a short year, we have gone from the discovery of a Higgs-like boson to a SM-like Higgs boson.

to tell whether Dr. Sheldon Cooper is right.

Is the particle the SM Higgs boson? will need more data as well as improved theory calculations…

It was yesterday’s discovery,

It will be tomorrow’s background,

It is today’s playground!


The results summarized in this presentation are described in Combined coupling measurements of the Higgs-like boson withthe ATLAS detector using up to 25 fb-1 of proton-proton collision data ATLAS-CONF-2013-034

Measurement of the Higgs boson production and couplingsin diboson final states with the ATLAS detector at the LHCPhys. Lett. B 726 (2013), pp. 88-119

Please see also ATLAS and CMS presentations at this workshop

References

https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2013-034/

http://www.sciencedirect.com/science/article/pii/S0370269313006369

Documents

Measurements of Higgs Coupling Parameters at ATLAS