Electroweak SUSY search with two taus & Inclusive SUSY
search with one isolated lepton
Da Xu [IHEP,CAS]
On behalf of IHEP-SUSY group
Huajie Cheng, Shan Jin, Huan Ren, Da Xu, Peng Zhang, Xuai Zhuang
Overview
This talk will be focus on the IHEP-ATLAS activities in the two topics ---
Electroweak SUSY search with 2 taus: coordinator/editor/approval talks
Run1 8TeV signature paper: JHEP10(2014)096
Run1 8TeV electroweak summary paper: arXiv:1509.07152 submitted to PRD
[Ongoing] Run2 13 TeV analysis: aiming for ICHEP2016
[Ongoing] Direct stau upgrade analysis: aiming for Moriond2016
Inclusive SUSY search with 1 lepton: editor/approval talks
Run1 8TeV signature paper: JHEP04(2015)116
Run2 13TeV sensitivity study: ATL-PHYS-PUB-2015-005
Run2 13TeV first look: ATL-PHYS-PUB-2015-029
[NEW!] Run2 13TeV: ATLAS-CONF-2015-076, paper to be submitted to
EPJC contact editor
SUSY Intro.
Strong production: gluino/squark pair
– Larger cross-section, but heavy SUSY mass scale
– Generic signatures: Multi-jets + leptons + larger Etmiss
Electroweak production: direct gaugino/slepton
– Low cross-section, but small SUSY mass scale
– Generic signatures: Low-jet multiplicity + >=2leptons + large
Supersymmetry
- A theory describing physics beyond SM - Introduce an additional sysmmetry:
fermions<->bosons
Why supersymmetry?
- Moderate the hierarchy problem
- Grand unification of gauge couplings
- Provide a suitable dark matter
Electroweak SUSY search with two harmonically decaying taus
* 𝝌𝟏±, 𝝌𝟐𝟎 decay 100%
to final state with (s)taus in simplified mode
Run1 8TeV signature paper: JHEP10(2014)096
• Grids of interest: C1N2, C1C1, direct stau
Run1 8TeV electroweak summary paper: arXiv:1509.07152 submitted to PRD
• direct stau MVA study; ditau+3l combination for C1N2
[Ongoing] Run2 13 TeV analysis: aiming for ICHEP2016
[Ongoing] Direct stau upgrade analysis: aiming for Moriond2016
𝝌𝟏±𝝌𝟐𝟎 via
stau/sneutrino 𝝌𝟏±𝝌𝟏∓ via
stau/sneutrino Direct stau pair
Di-tau analysis: cut-and-count
4 SRs designed targeting different models: one for 𝝌
𝟏±𝝌
𝟐𝟎; one for 𝝌
𝟏±𝝌
𝟏∓; two for direct stau model, targeting high/low mass region.
Main background:
W+jets(25-50%): one fake tau + one real tau
multi-jet(13%-30%): two fake taus estimated by data-driven “ABCD”
First search for 𝝌
𝟏±𝝌
𝟏∓production with hadronic tau decays from the LHC!
Illustration of “ABCD” method
Run-1
𝝌𝟏±𝝌𝟐𝟎+ 𝝌𝟏±𝝌𝟏∓ via
stau/sneutrino 𝝌𝟏±𝝌𝟏∓ via stau/sneutrino
Direct stau pair
further improvement with MVA method (next slide) pMSSM
stau-enriched
4 leptons(e/μ/τ)
No excess beyond SM expectation.
Upper limits have been set on the direct stau pair production cross-section /signal strength.
Di-tau analysis: cut-and-count Run-1
JHEP10(2014)096
Di-tau analysis: MVA
Direct stau production
Small production cross- section.
The production cross- section falls steeply with the stau mass.
Further MVA analysis performed for the
improvement of sensitivity.
Similar background estimation and combined fit methods as in the signature paper.
For Run-1 SUSY electro-weak summary paper.
Run-1
Submitted to PRD
Di-tau analysis: MVA
--MVA separation/correlation
Good S/B separation; No large/un-expected correlation.
Run-1
arXiv:1509.07152
Submitted to PRD
Di-tau analysis: MVA
--MVA signal region definition
Best sensitivity for each signal point.
Run-1
Submitted to PRD
Di-tau analysis: MVA
--SR kinematics and XS upper limit Run-1
One scenario of combined left and right stau pair production is excluded for stau~ 109 GeV and massless LSP:
Cross-sections above 0.115 pb are excluded.
arXiv:1509.07152
Submitted to PRD
Di-tau analysis:
ditau+3lepton combination Run-1
The sensitivity to
large chargino masses is improved by 20
GeV with the
combination of di-tau and 3-lepton results.
Submitted to PRD
Inclusive 1-lepton analysis
Gluino 1-step Squark 1-step
IHEP contribution@ 8TeV: mainly focus on soft lepton analysis
Run1 8TeV signature paper: JHEP04(2015)116
IHEP contribution@ 13TeV: mainly focus on hard lepton analysis
Run2 13TeV sensitivity study: ATL-PHYS-PUB-2015-005
Run2 13TeV first look: ATL-PHYS-PUB-2015-029
[NEW!] Run2 13TeV analysis: ATLAS-CONF-2015-076, paper to be submitted to EPJC
𝒈 𝒈/ 𝒒 𝒒 𝒑𝒂𝒊𝒓 𝒑𝒓𝒐𝒅𝒖𝒄𝒕𝒊𝒐𝒏 𝒅𝒆𝒄𝒂𝒚 𝒗𝒊𝒂 𝝌𝟏±
• Free parameters: m_ 𝒈 , m_ 𝝌𝟏𝟎; 𝐦_ 𝝌𝟏±fixed at (m_ 𝒈 + m_ 𝝌𝟏𝟎)/2
• Free parameters: m_ 𝒈 , m_ 𝝌𝟏±; m_ 𝝌𝟏𝟎fixed at 60GeV
Inclusive 1 soft lepton analysis Run-1
IHEP contribution in Run 1:
“Inclusive channel”: SR optimisation, Data/MC check, Background fit, Statistical interpretation etc.
“All channels”: Trigger study, Systematic, Background Fit/Interpretation
ISR
2jet
Three signal regions are defined based on cut-and-count method, targeting ISR- type and 2jet-type respectively.
Dominant background (W+jets,ttbar) estimation: normalize MC to Data in CR, later extrapolate to SR using simultaneous combined fit including all systematics as NPs.
Inclusive 1 soft lepton analysis Run-1
JHEP04(2015)116
Run 1 Interpretation:
Hard lepton (pink) is powerful in the high gluino mass region.
While soft lepton (blue) has better sensitivity in mass compressed region, which complements the whole analysis.
gluino squark
*Target semi-leptonic decay: search for 1 lepton, jet and Etmiss.
Inclusive 1 lepton analysis Run-2
paper to be submitted to EPJC
“hard lepton channel”
SUSY scenarios with large mass splitting between 𝝌𝟏± and 𝝌𝟏𝟎
Hard lepton e/μ pT > 35GeV; large meff, MET and mT
4 SRs with jet multiplicity ranging from 4 to 6
“soft lepton channel”
SUSY scenarios with compressed mass spectra: ISR and 2- jet type
Soft lepton e/μ pT: 7/6 - 35GeV; large MET
2 SRs: ≥2jets and ≥ 5jets
LHC Run 2: Cross-section for strongly produced signal grows dramatically!
Inclusive 1 lepton analysis Run-2
ATLAS-CONF-2015-076 paper to be submitted to EPJC
Main background: ttbar, W+jets
Discriminants: MET, mT, Aplanarity
Good agreement between predicted and observed event yields in all
validation regions: largest
deviations ≤2σ.
Inclusive 1 lepton analysis Run-2
paper to be submitted to EPJC
Good agreement between predictions and observed data in most signal regions.
In the 6-jet SR, a 2σ excess is observed. This mainly arises from μ channel(local significance of 2.5σ):
2.5±0.8 expected, 8 observed.
Without (hugely) significant observation, the exclusion limit has been set on the models: gluinos are excluded up to
~1.6TeV.
Outlook
Fruitful results from Run 1.
Most analyses see no excess over the SM predictions.
Some small excess observed in the inclusive one lepton region in Run 2, but significance is still very weak: need more data!
Exciting time to study SUSY: looking forward
to ICHEP2016 and beyond!
Extra
slides
1. CONF note ATLAS-CONF-2013-028: Search for electroweak production of supersymmetric particles in final states with at least two hadronically decaying taus and missing transverse momentum with the ATLAS detector in proton-proton collisions at √s=8 TeV
2. CONF note ATLAS-CONF-2013-062: Search for squarks and gluinos in events with isolated leptons, jets and missing transverse momentum at √s = 8 TeV with the ATLAS detector
3. Paper JHEP10(2014)096: Search for direct production of charginos, neutralinos and staus in final states with at least two hadronically decaying taus and missing transverse momentum in pp collisions at 8TeV with the ATLAS detector
4. Paper JHEP04(2015)116: Search for squarks and gluinos in events with isolated leptons, jets and missing transverse momentum at 8TeV with the ATLAS detector
5. Paper JHEP10(2015)134: Summary of the ATLAS experiment’s sensitivity to supersymmetry after LHC Run 1 - interpreted in the phenomenological
6. Paper CERN-PH-EP-2015-218(submitted to PRD): Searches for electroweak production of supersymmetric particles with the ATLAS detector at 8 TeV
7. PUB note ATL-PHYS-PUB-2015-005: Expected sensitivity studies for gluino and squark searches using the early LHC 13 TeV Run-2 dataset with the ATLAS experiment
8. PUB note ATL-PHYS-PUB-2015-029: Distribution of the transverse mass in events with an isolated electron or muon in proton-proton collisions at √s = 13 TeV
9. [New! ATLAS-CONF-2015-076, paper to be submitted to EPJC]: Search for gluinos in events with an isolated lepton, jets and missing transverse momentum at √s = 13 TeV with the ATLAS
Publications
SM Background Modelling
SUSY searches rely on accurate modelling of the Standard Model backgrounds.
(CR)
(VR)
(SR)
Discriminating variables
Inclusive one lepton
--Run-2 PUB note (new!)
ATL-PHYS-PUB-2015-005
Run-2
Towards the 2015 Moriond PUB note
provide 13 TeV sensitivity projections based on DC14 samples.
Targeting most sensitive model: gluino-gluino 1-step decay.
Targeting points around/in the extension line of the old exclusion contours.
Fixed gluino mass Fixed chargino mass
8 TeV exclusion contours
Inclusive one lepton
--Signal region optimisation Run-2
Cut combination after pre-selection (1 signal lepton +
>=3jets)
Scanned cut combinations per luminosity & signal point
Luminosity: 1,2,5,10fb-1 @13TeV
Significance/p-value calculation
RooStats::NumberCountingUtils::BinominalExpZ
RooStats::NumberCountingUtils::BinominalExpP
Uncertainties
Systematical uncertainty: 25 (similar to Run-1), 50%
Require background statistical uncertainty <30%
Not consider signal statistical uncertainty
Optimisation work:
fully done by IHEP
Inclusive one lepton
--Best discovery sensitivities Run-2
Neutralino mass sensitivity for fixed gluino mass Gluino mass sensitivity for fixed neutralino mass
Discovery sensitivity above 3sigma exceeds m~gl
= 1300 GeV at 2 fb-1 in case of 25%
background systematics.
-> main results in PUB note, fully done by IHEP
ATL-PHYS-PUB-2015-005
Inclusive tau+x analysis
Joined this analysis for Run-1 inclusive SUSY summary paper(published):
JHEP10(2015)054
Summary of the searches for squarks and gluinos using 8TeV pp collisions with the ATLAS experiment at the LHC
IHEP Response for:
Combined CR & SR fit
Workspace combination
Interpretation
Run-1
Inclusive tau+x analysis
𝟐 − 𝒔𝒕𝒆𝒑 𝒈𝒍𝒖𝒊𝒏𝒐, 𝒔𝒒𝒖𝒂𝒓𝒌 𝒑𝒂𝒊𝒓 𝒑𝒓𝒐𝒅𝒖𝒄𝒕𝒊𝒐𝒏
Strategy in Run-1 signature paper
SR only fit in HistFitter
Issue:
large signal contamination in CR
New strategy fo Run-1 summary paper:
