Searching for Supersymmetry using the

Higgs boson

Andrée Robichaud-VéronneauOxford University

Outline

Higgs discovery and its consequences Why Supersymmetry? ATLAS@LHC Search for SUSY decaying to Higgs Summary and Outlook

The Standard Model of elementary particles

The best description of matter and forces to date Validated by precision measurements over a large range

of energy scales

Matter made from quarks and leptons

4 elementary forces with their carriers:

- Electromagnetic (g)

- Weak Nuclear (W, Z)

- Strong Nuclear (g)

- Gravity (?)

"We found a new boson” July 4th, 2012: Announcement

of the discovery of a new boson consistent with the Higgs boson

Mass measured using ZZ->4l and gg signatures: 126.0 ± 0.4 (stat.) ± 0.4 (syst.) GeV

Combination of all channels: ZZ, WW, , , gg tt bb, using 7 and 8 TeV dataset from ATLAS

Boson properties compatible with the Standard Model Higgs

Phys. Lett. B 716 (2012) 1-29

Nobel prize winners! 2013 Nobel prize in Physics awarded to Prof.

Higgs and Englert "for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN's Large Hadron Collider”

Is that the whole story?* Not quite. We still have a few

unanswered questions: Matter/Antimatter imbalance What is Dark Matter? Hierarchy problem ...

"To infinity... and beyond!” ©

*Re

spe

cting

Hin

cliffe's ru

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SUPERSymmetry Introducing a new symmetry

of spacetime and fields Heavier superpartners with

spin-½ compared to the SM MSSM: 105 parameters to be

determined!

Introducing R-parity (aka matter parity) SM particles (+1), SUSY particles (-1) Phenomenology centered around the Lightest

Supersymmetric Particle (LSP) If conserved, protects against proton decay

How can SUSY help? In many ways:

Provides a dark matter candidate (LSP) Cancel Higgs mass corrections using

sparticle loop Unifies all forces

Now, how do we go about to look for it?

Large Hadron Collider

Proton-proton collider at 8 TeV (soon 14)

High luminosity (~1034 cm-2s-1)

4 interaction points – 7 experiments

Using the largest, coolest machine in the world! Hermetic multipurpose

particle detector Inner tracking Calorimetry Muon detection

High precision and granularity (~100 million channels)

Allow to measure passage of charged particles, leptons, photons, muons and jets

ATLAS

LHC performance

Good data-taking efficiency for the whole dataset and excellent work from the LHC team!

Multiple interactions for each proton bunch crossing → pile-up

N=σ L

ATLAS reconstruction

ATLAS performance

Excellent muon reconstruction efficiency over large range of momentum and pseudorapidity

Electron reconstruction efficiency greatly improved from 2011 (red) to 2012 (blue)

ATLAS performance

Jets can be tagged for heavy flavour, such as b or c quarks

Correction factor (data/MC) to b-tagging efficiency

Excellent agreement of data and simulation over large energy ranges

SUSY search strategy in ATLAS

Strong production Top and bottom

(charm) squarks Electroweak

production

Cross section

Various scenarios of symmetry breaking, violation of R-parity or exotic long-lived particles considered

We look in every corner!

Cross section

Higgs-aware SUSY

MSSM: Contains 5 Higgses, one of which is the SM Higgs (h0)

Knowledge of the mass of the SM Higgs provides constraints in the SUSY models

It also gives information on the couplings of the SM Higgs to sparticles

All 3 main production types can be probed using Higgs in their signatures

We'll focus here on the electroweak production

The S

US

Y H

iggses

SUSY Electroweak production

R-parity conserving models → Production of sparticle in pair

Electroweak production means sleptons, charginos and neutralinos, the SUSY partners of the weak bosons of the SM

Order by index in mass → decreasing cross section with increasing mass

Chargino-neutralino production

Chargino-neutralino production

Considering the case of lowest mass states allowing the production of a Higgs boson (Dm[χ0

2- χ01] > 130

GeV) Favoured in certains area of the MSSM

parameter space

Choosing h0 → bb, since it has the highest branching ratio.

The lepton in the W decay helps to reduce QCD background

The LSPs generate large amount of missing energy

ATLAS-CONF-2013-093

Signal simplified model Simplified models

used to generate signal points

Settings BR to 100% (for non-SM processes)

Adjusting parameters to obtain one single process (3 params for electroweak production: M

1, M

2, m

ATLAS-CONF-2013-093

0 GeV

60 GeV

∞

M1

M2

m

Signal grid

Simplified models used to generate signal points

Each red dot represent a model

Using degenerate masses between χ±

1

and χ0

2.

Scanning χ02 mass.

ATLAS-CONF-2013-093

SM Backgrounds

Many SM process have similar signatures that the one we are looking for in our signal

tt: WbWb with one W decaying to ln tt+V: Smaller cross section Single top: Mainly Wt mode W/Z+jets: Contribution from jets mistag Diboson: W(ln)W(qq) mostly W/Z+H: SM process, not missing energy

Modelled using Monte Carlo simulation

ATLAS-CONF-2013-093

Event selection Using ATLAS recommendations for physics objects reconstruction

Define baseline objects

Jets with pT > 20 GeV

Leptons (e or m) with pT > 10 GeV

Apply cleaning cut for detector defects

Reject overlapping objects (e, m, jets) in the same detector area

Extra overlap removal between e and m

DRe-m < 0.1, DRm-m < 0.05

Events are triggered by single lepton requirements

Electrons: EF_e24vhi_ medium1 || EF_e60_medium1 Muons: EF_mu24i_tight || EF_mu36_tight

ATLAS-CONF-2013-093

Event selection

From the baseline object, signal objects are selected

Leptons are isolated, with pT > 25 GeV

Central jets with pT > 25 GeV, |h| < 2.4

Forward jets with pT > 30 GeV, 2.4 < |h| < 4.5

Preselection

2 highest pT central jets

1 baseline && 1 signal lepton

Missing transverse energy (ET

miss) > 100 GeV

Nsignal_jets

< 4

ATLAS-CONF-2013-093

Event selection

Targetted signal cuts

0, 1 or 2 jets to be tagged as coming from a b quark (among the 2 highest p

T jets)

mjj > 50 GeV (for the 2 highest p

T jets)

Contransverse mass (mCT

) > 160 GeV

Transverse mass (mT) at varying thresholds for

background estimation and signal measurement

ATLAS-CONF-2013-093

Signal region optimisation

Optimise analysis selection cuts based on the mass splitting regions

ATLAS-CONF-2013-093

Signal region optimisation Two signal regions: SRA at low mass splittings, SRB for

high mass splittings

SRA (SRB): mT >100 (130) GeV (on top of previous m

CT)

and ET

miss cuts).

Optimised for 105 < mbb

< 135 GeV

SRA SRB

Z N=√2 erf −1(1−2pvalue )

ATLAS-CONF-2013-093

Signal predicted yields SRA has high yields in low mass splitting regions due to

cross section and high a x e in the high mass splitting region

SRB consistently has high yields and a x e in high mass splitting region

ATLAS-CONF-2013-093

Background kinematics Distributions

scaled using background fit results

ET

miss cut applied, all other three variables untouched

Main background contribution from tt before selections cuts

ATLAS-CONF-2013-093

Background estimation Strategy:

Reducible background: estimate from data Irreducible background: validate MC simulation with

data Use control regions (close kinematically to data, but

designed to target background processes) to obtain scale factors to fit MC simulation to data

Use validation regions to validate fit (obtain good agreement between data and simulation using fit results above)

Apply normalisation to signal regions to get background estimate

ATLAS-CONF-2013-093

Control and validation regions

Cut above applied to the entire plane

mbb

binning for all regions: 50-75, 75-105, 105-135, 135-165, > 165 GeV

*: signal bin not considered in background-only fit

ATLAS-CONF-2013-093

Systematic uncertainties Lepton (electron or muon) energy scale, resolution, identification and trigger

Jet energy scale and resolution, JVF

ET

miss resolution

Btagging calibration

Luminosity

Pile-up

Generator uncertainties

ISR/FSR

Parton shower

Scale uncertainties

Background s uncertainty

Signal s uncertainty

ATLAS-CONF-2013-093

Profile Likelikood Fit

Background only fit Using only control regions

without Higgs bin Obtain normalisation

factor for two main background, tt and W+jets

Used for model independent limits

Model dependent fit Using all bins of control

and signal regions Obtain normalisation

factor for two main backgrounds and the signal strength for each signal point on the grid

ATLAS-CONF-2013-093

Data/MC comparisonATLAS-CONF-2013-093

Data/MC comparisonATLAS-CONF-2013-093

Data/MC comparison Data and SM expectations in excellent agreement → No SUSY (yet)

ATLAS-CONF-2013-093

Signal region yieldsSRA (Higgs bin) SRB (Higgs bin)

Observed 4 2

Background estimate

tt 2.8 ± 2.8 1.0 ± 0.7

W+jets 0.7 ± 0.4 0.3 ± 0.2

Single top t-channel 0.26 +0.27-0.26

0

Single top Wt-mode 1.4 ± 1.3 0.6 ± 0.4

Z+jets 0.01 +0.02-0.01

0.00 +0.01-0.00

Diboson 0.01 +0.05-0.01

0.05 +0.07-0.05

WH 0.18 ± 0.10 0.12 ± 0.07

tt + V 0.01 ± 0.01 0.11 ± 0.06

Total 5.2 ± 3.0 2.0 ± 0.7

Signal prediction

(130,0) GeV 6.5 0.2

(225,0) GeV 1.9 4.1

ATLAS-CONF-2013-093

Results interpretation

No SUSY found. What do we do next? This is precious information! It should be used to

“quantify our ignorance” The same way a discovery like the Higgs boson add

additional constraints on theories, using this information, we can rule out mass range for specific models → feedback to phenomenologists

Perform likelihood fit using signal and control regions (all bins)

ATLAS-CONF-2013-093

Model independent limits

SRA SRB

Observed s95vis

(Asymptotic) 0.29 fb 0.22 fb

Expected S95exp

(Asymptotic) 6.7 +3.1-1.9

4.6 +2.5-1.5

Observed s95vis

(Pseudo-experiments) 0.31 fb 0.22 fb

Expected S95exp

(Pseudo-experiments) 6.8 +2.7-1.4

4.4 +1.8-0.8

Limits on new (non-SM) physics processes that would have been observed if existed

Estimated using asymptotic formula and pseudo-experiments (”toys”) - results consistents

ATLAS-CONF-2013-093

Exclusion contour Contour

interpolated from individual values of CLs of each model

Small grey numbers: cross sections excluded fpr each point

Compute limits using -1s line

ATLAS-CONF-2013-093

Exclusion limits in 1DATLAS-CONF-2013-093

Where do we stand?

Where do we stand?χ

±

1χ02→

W

± (l

±n) χ0

1 h0(b

b) χ0

1

Summary and Outlook The ATLAS experiment, together with the

LHC, had a very successful first run! The Higgs boson discovery has opened new

pathways to clear out, looking for SUSY Completing the spectrum of available decays

In our search for new physics at the TeV scale, no excess has been observed over the SM background so far

Looking forward to see what 14 TeV collisions will reveal!

Backup Slides

pMSSM

T. RizzoBNL13 Sep. 2012