Review on the LHC Run2 results
KMI2017, 5th-7th January 2017
Yu Nakahama (KMI, Nagoya University)

Outline of this talk• Introduction to LHC and the ATLAS Run2 experiment• Review of physics results– Standard Model measurements– Higgs physics– Searches for New Physics phenomena; Exotics, SUSY

• Summary

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Review of the LHC Run2 results2Large Hadron Collider• LHC is an energy-frontier
collider at CERN, delivering
proton-proton collision events

LHC ring at CERN:
27 km circumference70-140m in depth– to two multi-purpose detectors;
ATLAS and CMS– to dedicated detectors;
LHCb for B-physics studies andALICE for heavy-ion studies• Recent history and future– LHC Run1 (2009-2012) with p-p collisions at √s = 7-8 TeV– Currently in a high-energy phase at √s = 13-14 TeV, Run2 (2015-2018),
followed by a bit higher luminosity phase Run3 (2021-2023)• I review recent physics highlights in this talk
– For future, high-luminosity LHC (2026-) and FCC (far beyond) is planned• B. Petersen will present the future physics program• M. Tomoto will present muon trigger upgrade (KMI’s contribution)
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The ATLAS experiment• The ATLAS detector is a multi-purpose detector that candirectly explore energy scale from O(10) GeV to O(10) TeV
Muon Detector
Calorimeter+ Trigger andData acquisition
systemInner Detector• The ATLAS experiment has around 2900 collaborators from180 institutions across 38 countries• KMI has been contributing to ATLAS– with primary focuses on “Muon Detector and Trigger system”, as wellas on “Top physics and New Physics searches”

KMI2017, 05/01/2017Review of the LHC Run2 results

4Physics highlights with the Run1 dataset• Higgs discovery– Higgs-like particle discovery was announced
on July 4th 2012 - Phys. Lett. B 716 (2012) 1-29
– March 2013: key papers on particle properties.
New particle declared as “a Higgs boson”– Citation for the 2013 Nobel Prize in Physics• Deep and broad range of experimental
results, all beautifully fitting in the Standard
Model framework of elementary particle
physics

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5√s = 13 TeV: parton luminosity increase• aKMI2017, 05/01/2017

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ATLAS in Run2
• Primary physics focuses– on New Physics searches using early datasets and– on measurements of Higgs properties using higher-statistics datasets• New detectors installed– Innermost pixel layer (IBL); 3.4cm from the interaction point– Forward proton detectors (AFP);
210m from IP• In addition, various consolidations provide
improved running at high luminosities and rates
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ATLAS data taking in 2016
• The 2016 pp data taking lasted until 26th October– Set a record in peak luminosity: 1.4e34– Total dataset recorded ~36 fb-1 with excellent data quality; >90% of
data collected usable for analyses
Operations in the ATLAScontrol room

– 2016 was an extremely productive year, comparing with previous years
Delivered luminosities
to ATLAS (per year)

8Trigger challenge in high luminosities• Critical contribution from KMI, including menu-coordination role
• Designed and operated a complex list of trigger selections tomeet varied physics, monitoring and performance requirements- Typically ~2000 active
trigger selections, driven
by the priorities in theATLAS physics program- Stable primary triggers

Main primary trigger
single leptons

Trigger thresholds at HLT [GeV]Isolated µ, pT > 26
Isolated e, ET > 26

ETmiss> 110 (in MHT, jet-based)

single jet

pT > 380

single photon

ET > 140- Peak output rate at
di-muon for B-phys
Level-1; 90kHz
di-tau- Ave. output rate at Highdi-photon
Level to storage; 1.0kHzà meeting detector operation
and DAQ requirementspT(µ1, µ2)> 6, 6 + topological/mass cutspT(τ1, τ2)> 35, 25+jet requirement at L1
pT(γ1, γ2)> 35, 25- Trigger menu for the rest of Run2 is being prepared for 2e34
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9Physics Results• ATLAS has submitted 605 papers in total– including 51 papers using Run2 dataset
From varied physics areas– the full list here: https://twiki.cern.ch/twiki/bin/view/AtlasPublic• Following slides review a few selected highlights, mostly using
13 fb-1 dataset, collected up to 2016 summer– Disclaimer; Topics here are driven by the priorities in the Run2 ATLAS
physics program, by my tastes, as well as by the KMI’s contributions– Apologies if your favorite topics are not covered
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10• Measurements

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11Inclusive cross-section measurements: broad range• あKMI2017, 05/01/2017

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12Inclusive cross-section measurements: broad range• あ
These many measurements from Run1 and Run2 are important
steps forward in physics modeling in the last years:• NLO event generators – now standard• (N)NNLO calculations increasingly available - These help toface the challenge of the precision of the LHC data
Measuring complex topologies bench-test Monte Carlo models
of backgrounds to New Physics searches

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13ttbar cross-section measurements• Nagoya’s contributions since early Run1
• Precise measurements ofinclusive ttbar cross-sections– Precisions ±(3.9-4.4)% (7-13 TeV)
is better than theory NNLO+NNLL
predictions (~5%)• High statistics allow detailed studies of production properties– e.g. differential cross-section measurements (Check poster by K.
Kawade, the Nagoya ex-member)

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W-boson mass measurement

ATLAS-CONF-2016-113• Precise measurement is motivated for a consistency test ofthe SM and a probe of BSM physics – yet with slow progress• After long/huge efforts, the first ATLAS result using the 7 TeV
dataset was released in Dec 2016
– m(W)= 80.370 ± 0.019 GeV, a competitive measurement, dominated byphysics modeling uncertainties as expected– Consistency tests of the SM

SM prediction for m(W) vs m(t),assuming m(H) = 125.09 ± 0.24 GeV
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SM prediction for m(W),assuming m(H) = 125.09 ± 0.24 GeVm(t) = 172.84 ± 0.70 GeV

Review of the LHC Run2 results15• Understanding the 125 GeV Higgs

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Physics of the 125 GeV Higgs• Higgs discovery opened the door on the scalar sector• In Run1 we measured:– Its spin-parity, and its mass precisely (±0.2%)– Production via gluon-fusion, vector-boson fusion, and with a W or Z
– The decays to γγ, WW, ZZ, and the fermionic decay to ττà These proved true the BEH mechanism to break gauge symmetryà However, apart from the mass, none of these measurements areyet very precise and leave room for significant deviations. Also,
important modes are still unseen (bb, ttH, µµ, Zγ, HH)• Run2 priorities:– Establish and measure at 13 TeV– Search for ttH production to probe ttH coupling directly– Search for Hàbb decays– Search for rare decays– Refine measurements of couplings, mass, etc– Expand use of H as a tool to find New PhysicsKMI2017, 05/01/2017

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Clear Higgs observation at 13 TeV in bosonic channels
Hàγγ

Hà4ℓCombining γγ+4ℓ channels
σ(ppàH+X, 13 TeV)=SM prediction
Overall significance at 13 TeV ~10σ
18First search for ttH production at 13 TeV by CMS• Direct measurement of Higgs-top coupling• CMS showed preliminary results for ttH in all
major Higgs decay channels: γγ, multi-lep, bb• Highly complex analyses, huge effort to get these done soquickly after data taking (already by Moriond 2016)
H → γγ,tt → 0 & 1 leptons

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ttH → multi-leptons
2 same charge (+ 𝜏) / 3 leptons

Review of the LHC Run2 results

H → bbtt → 1 & 2 leptons

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First search for ttH production at 13 TeV by ATLAS• Direct measurement of Higgs-top coupling• ATLAS showed preliminary results for ttH in
all major channels and their combination atICHEP 2016
Individual channels and combination

à Observed significance 2.8σ
(expect 1.8σ, improved sensitivityover Run1)

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20Searches for rare Higgs decays e.g. Hàµµ	• Beyond SM reach at present, but could have New Physics
contributions?
– Strongly resolution dependent, improve sensitivity by categorising
events (low/high p , central/forward, VBF)
– Obs limit: 4.4 times SM. Needs ~300 fb-1 data to reach SM (Run3)– Can already exclude universal Higgs coupling to fermions• Would have observed Hàµµ, if same BR as HàττKMI2017, 05/01/2017

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Beyond the Standard Model Higgs physics• Higgs sector may be non-minimal and/or Higgs boson maycouple to New Physics• Diverse search program is in place;

No sensitivity yet for SM di-Higgs production(Primary target for High-luminisity LHC)No deviations from SM found in any of these searches
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22• New Physics searches

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23New Physics searches in Run2
• Broad search program– from inclusive surveys of basic event topologies– to dedicated searches ruling out corners of phase space• Major extension of reach compared to Run1
• All results shown here include 2016 data
• They probe well into the TeV, even multi-TeV, mass scale
range

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24Exploring Exotics• Guiding physics models• Signatures– Technicolor– Di-lepton / di-jet / di-b-jet
resonances (Z’, W’, ...)– Warped Extra Dimensions– Di-boson resonances– Grand Unified Theories– WZ,ZZ,Wγ,Zγ– Heavy Vector Triplet– Multi-jet resonances– Quantum Black Holes• One model with– tb,Ht,Zt,Zb (Z’,W’,T,B,... )– Hidden valley
different signatures– Excesses in high tails (non– Contact Interactions• /One signature probingresonances)
Compositenessvarious models– Long lived particles– Exited Fermions– Large missing energy– Leptoquarks– …
– …

à Search as broad and general as possible (signatures,masses, rates) covering physics without guiding models
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Review of the LHC Run2 results25Di-photon searches

2015 ATLAS data: localised excess seen
• 2.1σ global (3.9σ local) significance at750 GeV (spin-0 search), width ~50
GeV
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2016 data: no clustering around 730-750
GeV, and 3.8x more data
• 2016 data consistent with 2015 at 2.7σ
• Appears that the 2015 excess was astatistical fluctuation

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Finally with 2015+2016 data

• Small excess at 710 GeV (Г/m~10%)• Local significance 1.4σ, global <1σ
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Massive di-boson searches• Search for resonant di-boson production– Boosted (“fat jet”) selections vital at high-pT– Explored many channels with 2016 data

– Overlaying limits from all WZ searches – no persistent excesses, incontrary to the Run1 excesses
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Review of the LHC Run2 results28Di-jet searches• Search for New Physics in di-jet mass spectrum; bump hunter– Sensitivity based on signal shapes– Di-jet: qq, qg, gg (high mass
> ~1 TeV). High priority in early Run2 including large low mass tails

For low-mass search, a dedicated trigger technique “datascouting at trigger level” allowed lower threshold by keeping
partial info. and online calibration

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Review of the LHC Run2 results29Di-jet searches: Dark Matter Interpretation• Complementary to mono-(jet, γ,W, Z, b, t, H, ...) searches– Tag DM pair (ETmiss) via Initial state radiation– Mediator (Z’) mixing with Z, H

• Mediator (Z’)– Couples to quarks(gq) and DM(gDM)– Dijet searches cover a broad mediator
mass range– DM interpretation of Z’ searches

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Review of the LHC Run2 results30Di-jet searches: Dark Matter Interpretation• a• Complementary to mono-X searches; cover a broad mediator
mass range – Results highly depend on choice of coupling
parameters (L = gqγµq Z’µ)

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SUSY searches• The case for SUSY is well known- Natural Dark Matter candidate in neutralino1- Can solve the fine-tuning problem – if t~ and g~ are light
Higgsinos Gauginos• In Run1 we identified:the Super Symmetry

Higgs
Higgs
Gauge bosons
SfermionsFermions
Fermions

the Standard Model– m(g~)>1.4TeV for low-mass neutralino1– m(t~)>700GeV if m(neutralino1)<100GeV• Run2 priorities:– Extend mass reach according to √s increase– Fill “kinematic holes” where particles are soft– “Leave no stone unturned” e.g. displaced decays from long-livedparticles
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SUSY search results
Run1 data Run2 data

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SUSY search results
Run1 data Run2 data

• In brief, huge number of signature-based searches for various
production and decay processes within SUSY, in many finalstates, providing stringent limits under various assumptions;- Many of these limits reach ~1 TeV in “easy” inclusive signaturesusing the Run2 data
- More searches will come for smaller-cross section processes(EW) or for difficult signatures (Long-lived particles)

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Example: g~/q~ search with jets+ETmiss• Event selection: require 2-6 jets and veto isolated leptons– Sensitive to g~ and q~ production– High priority in early Run2

• Total of 30 signal regions to cover various
mass spectra– No significant excesses in any signal region overall– For small m(neutralino1)
exclusion reach extendedm(g~) < 1.4 TeV (Run1)
à < 1.86 TeV (2015+16)– Check details in posterby Y. Sano (a NagoyaPhD student)

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t~ search summary• Search for top squark pair productions– Explored many channels with 2016 data

For m(neutralino1) < 200 GeV,m(t~) < 800 GeV excluded
except in rather small regions– Developing dedicated searches for such compressed spectraà Check details in poster by K. Onogi (a Nagoya PhD student)

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36Conclusion• The LHC is performing extremely well
• A broad range of LHC Run2 results using early 2016 dataset– Starting precise measurements of H(125) at 13 TeV• Concluding on the Standard Model ttH sensitivity– 13 TeV in Run2 brings significant increase in sensitivity, shaping thesearch program in 2015 and early 2016
• The limits set significantly extend the Run1 results, thus furtherconstraining various models. No significant excesses, though some~3σ effects as expected. More data will tell which, if any, will remain• The search program is moving toward more challenging signaturesand scenarios• Expecting exciting future– New results with ~40 fb-1 in early spring of 2017
– Data analysed so far: a small fraction of total Run2 (100 fb-1)
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37• Backup slides

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pMSSM scans

JHEP 10 (2015) 134• LHC: simplified model → BR=1 (even for cascade decays)• setting limits on various pMSSM using all ATLAS search
results in Run1:- Good coverageof pMSSM,
dependence onLSP nature(via signatures)- Similar scan will be repeatedwith results using the Run2
full dataset

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