studies and prospects of higgs...

L. Malgeri - METU-IPM 2015 - Higgs results and prospects 1

Studies and Prospects of Higgs searches

First Joint METU-IPM Conference on LHC PhysicsICTP-ECAR - Izmir - Turkey

29/9/2015 - 3/10/2015

Luca Malgeri


A new star is bornBirth date: July 4th 2012Birth place: Geneva (LHC)

Parents (SM) and child are in perfect shape


And everybody was waiting for it

From Inspire:

• total # of papers with “Higgs” in title: 15138

• # paper from CMS: 58

• # paper from ATLAS: 74

LHC starts Discovery


…since a long time…..http://lepewwg.web.cern.ch/LEPEWWG/Last edited Dec 2011

http://lepewwg.web.cern.ch/LEPEWWG/


A more recent history: LHC Run 1

Summer 2011: nothing yet

End of 2011: some hint

Summer 2012: discovery!

End of 2012: confirmation!

From PDG

PRD 89 (2014) 092007,EPJC 74 (2014) 3076, EPJC 75 (2015) 212


Higgs is part of the SM measurements now [p

b]σ

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3−10

2−10

1−10

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210

310

410

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CMS PreliminaryJuly 2015

All results at: http://cern.ch/go/pNj7W 1j≥ 2j≥ 3j≥ 4j≥ Z 1j≥ 2j≥ 3j≥ 4j≥ γW γZ WW WZ ZZ

µll, l=e,→, Zνl→WqqWEW

qqZEW

WW→γγ

ssWW EW γWV tt 1j 2j 3j t-cht tW s-cht γtt ttW ttZ

σ∆ in exp. Hσ∆Th. ggH qqH

VBF VH ttH

CMS 95%CL limit

)-1 5.0 fb≤7 TeV CMS measurement (L )-1 19.6 fb≤8 TeV CMS measurement (L

7 TeV Theory prediction8 TeV Theory prediction

W+jet Z+jet

di-boson

tt+jet

“t”+V Higgs

https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsCombined

https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsCombined


Outline

• Why Higgs

• Recent highlights:• ATLAS+CMS mass combinations• ATLAS+CMS couplings combination

• Other results:• Spin/CP studies• Invisible Higgs• Width• Differential cross sections

• Future Higgs (Run2, Run3, HL-LHC)• Conclusions

For BSM Higgs studies: A. Nikitenko talkFor specific ATLAS Higgs studies: T. Cuhadar talk


Why were/are we so obsessed by the Higgs?

• Self consistency of SM theory• if MH would be 300 GeV, p=3x10-5

• Vacuum structure of the Universe• Its relation to Physics BSM


arXiv: 1307.3536 Higgs, Top and vacuum stability

Higgs potential at large

If there is the desert between EW scale and Planck scale, it’s “life on the edge” (of metastability). Why so?

Current WA


We know that the SM is an effective “low” energy theory that breaks down at Planck scale. In practical terms:Corrections for elementary scalar are quadratic in new-physics cutoff ΛMain loop corrections from strongest coupled particles: top, W, Z and Higgs self-couplings:

�m2H =

3

8⇡2v2(4m2

t � 2m2W � 4m2

Z �m2H)⇤2

1% fine tuning

10% fine tuning

If naturalness is taken as a criterium, a light Higgs (125 GeV) suggests that new physics should be maximum at the level of few TeV to keep fine tuning at 1%. If Λ is Mpl, tuning becomes 10-34 !!!!

Higgs and naturalness

Need a really fine tuning of the parameters to get the Higgs mass so low (quantum correction akes it diverge!)


A very short reminder/legenda

SM Higgs production SM Higgs decays

ggH

VBF

VH

ttH

mH=125.09±0.24 GeV (see later)


Recent highlights from Higgs analyses

The task of compiling a consistent list of recent results on Higgs studies has been made easy by ATLAS and CMS:two major milestones recently made public.

Mass combination (PhysRevLett.114.191803)

Couplings combination (ATLAS-CONF-2015-044, CMS-PAS-HIG-15-002)

Based on “legacy” results from the two collaborations:ATLAS:Mass: Phys. Rev. Le+. 114, 191803 Couplings: arXiv:1507.04548

CMS, mass and couplings: Eur. Phys. J. C 75 (2015) 212


Higgs mass

Most precise channels used: H→γγ and H→ZZ

• fully reconstructed mass peaks• high resolution• enough sensitivity with Run 1

statistics

PRD 90, 052004

EPJC 74, 3076


Higgs mass: inputsIn general, couplings analyses are used as starting point. ATLAS H→γγ has been designed specifically for the mass measurement.

H→γγ• event categorizations based on quality of

photons• event vertex chosen according to MVA

techniques• sgn+bkg shapes: Gauss/CB + data driven H→ZZ• event categories based on final states (4e, 4µ,

2e2µ, 2µ2e)• dominant bkg: diboson production• lepton calibration systematics negligible w.r.t.

statisticsSystematics:dominant →energy scale for e,µ and γsubdominant→Jet energy scalenegligible→theory

JINST 10 P08010

Phys. Rev. D. 90, 052004


Higgs mass: systematicsLe

pton

mom

entu

m

scal

e/re

solu

tions

Oth

er e

xp

syst

emat

ics

The

ory

syst

emat

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Higgs mass: resultsThe ATLAS+CMS combination technique is based on profile likelihood ratio:

Cross-section signal modifiers are left to vary independently

Nuisance parameters (~300 in total) take into account all systematics and their correlations.Note: correlation among experiments as well as internal!


Higgs mass: results

Full consistency:• between experiments• inside experiments• between decay channels

mH=125.09 ± 0.24(±0.21 ± 0.11)GeV


The experienced gained in the mass combination paved the way for a larger step: couplings!• Use as input the published results from ATLAS and CMS (in some cases improved with

latest greatest analyses)• The mass is fixed to the combined mass value (improves SM couplings measurements)

Higgs signal strength and couplings


The inputs from the different ATLAS and CMS analyses are used in a global fit (profile-likelihood ratio)

Signal strength modifiers used both for production and decays:

Note: other production modes are included in the fit but not explicitly tagged (i.e. are part of a category above)

Higgs strength and couplings



Inputs to the fit: • most sensitive final discriminants distributions,

usually:- mass distribution for fully reconstructed channels- BDT output for more complex topologies

Total of 4200 nuisance parameters used in the global fit

SystematicsFour main categories:• finite MC statistics (uncorrelated between

experiments)• Signal theory normalization uncertainties

(largely correlated)• Bkg theory uncertainty (largely correlated)• Experimental uncertainty (largely

uncorrelated)



Signal strength

A single parameter fit assuming either BR or cross section SM-like, or two signal strength modifiers for “bosons” and “fermions” productions mechanism.

µi=1µf=1



Testing compatibility with µi and µf = 0

• Combining the two experiments we reach “5σ” in VBF production and H→ττ decay mode.

• We also see a ttH excess of 2.3σ w.r.t. SM (and a deficit in H→bb)


Higgs couplings and new physicsThe LHC Higgs cross section working group proposed a framework to test deviations from SM by “modifying” the couplings by a multiplicative modifier k:

In this context BRBSM is either non-SM decay or undetected decays

• Loops can be resolved in effective k’s:

• k’s that are not accessible are just assumed to be : kc=kt, kµ=kτ,ks=kb

• To first order we are not sensitive to sign(k), but we can access the sign through interference. For example:

* all interference terms contain top couplings. Its sign its arbitrary in the fit (set >0 from now on)


Higgs couplings to fermions and bosonsAssuming universal coupling modifiers for fermions and bosons

Negative kF disfavored

GlobalPer decay channel


Higgs couplings to fermions and bosonsOr simply test scaling of coupling with mass (Yukawa coupling behaviour)

Assuming only SM particles in loop

Just a different (and more appealing) way to plot the same


Does the Higgs couples to BSM?How much room we have for beyond SM Higgs decay?

• use effective couplings for γ and g, i.e. allow new particles in the loop

• allow BRBSM to increase the Higgs width

Given that we really measure only σxBR, some assumption should be made on ΓH (or BRBSM)

kV≤1 (e.g. 2HDM)BRBSM=0

BRBSM < 0.34 at 95% C.L.

�H(~) =2H(~) · �SM

H

1�BRBSM

�i ·BRf =�i(~) · �f (~)

�H(~)


Many more studies than just mass and couplings!


Higgs spin/CP fits

Bosonic channels are the most sensitive:

• ZZ is fully reconstructed, polarisation derived from kinematics of decay products

• γγ fully reconstructed but polarisation is not accessible (pT and cosϑ* give some sensitivity)

• WW not fully reconstructed but kinematic variables (Mll, Δφ and MT) sensitive to Higgs spin/CP

Global fits are are usually based on comparisons of kinematic variables vs matrix elements calculations


Higgs spin/CP fits

• The SM hypothesis (0+) is tested against many alternative ones.• JP=0-, 1±,2± excluded at more than 3σ level

• CP mixture and alternative tensor structure also tested w/o any sign of non-SM behaviour

ATLAS: arXiv:1506.05669 CMS: PRD 92 (2015) 012004


Higgs width - 1Direct limits from H→γγ and H→ZZ are limited by detector resolutions and not sensitive to the O(MeV) region:

• ATLAS: 2.6 GeV (6.2 GeV exp) in ZZ 5.0 GeV (6.2 Gev exp) in γγ)

• CMS: 1.7 GeV (2.3 GeV exp) (γγ and ZZ combined)

But ingenuity came to rescue: off-shell vs on-shell production is sensitive to Higgs total width (under certain assumptions/SM dependence)

JHEP 08 - 116 (2012), PRD 88 - 054024 (2013)JHEP 04 - 060 (2014)

Their ratio depends directly on ΓH, provided couplings ratio are unchanged on-shell and off-shell.

Limits:CMS: 22 (33) MeVATLAS: 23 (33) MeV

Theory

PRD 92 (2015) 012004PLB 736 (2014) 64 Exp.


Higgs width - 2 (an interesting curiosity) …..ingenuity calls more ingenuity…..

Recently few theorists have also realised that we had already a very large sample of off-shell Higgs, even though not at high mass but a low mass: LEP!

arXiv:1504.02458

From a simplified fit:

Rh = �H

�SM

3 MeV

7.7 MeV


Guiding principle: Stirling plot, cross sections increase by factor 2-3 @13 /14 TeV(up to a factor of 4 for ttH)

1fb-1 equivalent reach to Run1

2 fb-1 equivalent reach to Run110 fb-1 equivalent reach to Run1

Higgs prospects


Higgs prospects

LHC timeline in terms of integrated luminosity


Higgs prospects @ 300 fb-1

34

Higgs Physics expectations

• ~5 M Higgs events produced

• ~50 K events useful for precision measurements (x 40 w.r.t. now)

Physics subjects

• Higgs precision measurement - Mass (100 MeV reachable?)- Cross-sections- Couplings➡H→μμ might be measured

at 30% level

• Possible (but very difficult) WLWL scattering ?

Assumptions on systematic uncertainties:

Scenario 1: no changeScenario 2: theory unc. / 2, rest goes like 1/√L

CMS and ATLAS white papers: arXiv:1307.7135 and 1307.7292


Higgs prospects @ 3000 fb-1

35

Higgs Physics expectations • 50 M Higgs events produced• 500 K events useful for

precision measurements (x 400 w.r.t. now)

Physics subjects

• Higgs precision measurement - Mass (100 MeV syst. limited?)- Cross-sections- Couplings➡H→μμ might be measured

at 10% level ?- di-Higgs (bbγγ and bbττ) up

to 2σ

• Possible (but very difficult) WLWL scattering ?

Assumptions on systematic uncertainties:

Scenario 1: no changeScenario 2: theory unc. / 2, rest goes like 1/√L

CMS and ATLAS white papers: arXiv:1307.7135 and 1307.7292


Conclusions - 1

The last three years have been a fantastic ride for Higgs physics at LHC:

• mass measured @0.2% accuracy• signal strength measured @10% accuracy• Yukawa couplings behaviour verified at percent accuracy

It was also an exemplary exercise where both experiments managed to work together and delivered combined results in record time!

Many other studies performed and on-going, all of them showing that “this” particle is indeed the SM Higgs boson:• spin/CP, width, differential xsec, etc.

The Run2 and Run3 Higgs studies are expected to bring another order of magnitude in accuracy


From S. Gori talk at LHCP15

The prospects for (future) Higgs studies are more than optimistic despite early “warnings”:

Conclusions - 2


Additional material


Higgs couplings: generic parametrizationsGeneric parametrisations are based on ratios of parameters that are less affected by theoretical uncertainties:• ratio of cross sections and BR• ratio of coupling modifiersin both cases the gg→H→ZZ production/decay is used as reference (cleanest channel)

reference


Higgs couplings: generic parametrizations

• 2.4σ discrepancy in BRbb/BRZZ driven by large ZH and ttH

• Double minima seen in λtg and λWZ


Invisible Higgs searchesSeveral channels and production mechanisms used by both ATLAS and CMS:

VBF production gives largest sensitivity (clean signature):• Large Δη(jj), m(jj) and large missing energy

ATLAS arXiv: 1508.07869 CMS-‐PAS-‐HIG-‐14-‐038

ATLAS: BR(H -‐> inv.) < 25% (27% exp.)

CMS: BR(H -‐> inv.) < 47% (35% expected)


Differential distributions/XsecStatistics is not yet really discriminating but we are already starting to go differentials:• Fiducial region defined to minimise acceptance

corrections (unfolding applied for final results)• Kinematics sensitive to PDF, production mode (i.e.

VBF) , QCD effects• Affected by spin/CP and possible BSM• Extrapolation to full phase space (first real Xsec

measurements)

ATLAS:arXiv:1504.05833 CMS: PAS-HIG-14-028

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