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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
le
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