searches for heavy resonances at the lhc tetiana berger-hryn’ova (lapp, cnrs-in2p3) for the atlas...
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Searches for Heavy Resonances
at the LHC
Tetiana Berger-Hryn’ova(LAPP, CNRS-IN2P3)
For the ATLAS and CMS collaborations
Moriond QCD 2014
Higgs Discovery & Searches• Following the discovery of a scalar boson Standard
Model (SM) is complete (but for an axion) and self-consistent
• A lot of questions still need answers:– Why is Higgs light?– What is dark matter? – How to accommodate gravity?– Why are there three generations? – …– Is there a theory which answers all our questions?
• We are looking for something – we do not know what – neither where to look – nor if there is any chance of finding it…
Does it remind you of something?2T. Berger-Hryn'ova Moriond QCD 2014
How do we search
Approach 1• Pick a Theory• Pick a model• Look at various production/decay modesof new states! (model-independently)
Approach 2Pick a final state & look(model-independently)
Get lucky!It sounds like a quest
Go there- I don’t know where,
Bring that – I don’t know what
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Look for Signatures!Interesting Models:• Extended Gauge Groups• Compositeness• Wraped Extra Dimentions• Technicoloretc …
Interesting Signatures:• Dibosons• /W/Z+jet(s)• Di/Tri-jet• Dileptons (ll, )etc…
Look for bumps on smooth SMbackgrounds!
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• Focus on selection of results• Apologies if your favourite is left out• All limits are quoted at either 95% Confidence Level or Credibility Level (CL)
Today’s menu• Searches in +jet
final state• Dilepton searches• Diboson searches• Tri-jet resonant
searches• XHHbbbbT. Berger-Hryn'ova Moriond QCD 2014 5
Searches in +jet final stateSelection CMS(ATLAS): pT
j/>175(125) GeV + iso;
||<1.44(1.37), |j|<3.0(2.8) |,j|<2.0(1.6)No significant excess seen.Exclude m(q*)<3.5 TeV @ 95% CL
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Background fit function:
ATLAS: PLB 728, 562 (2013)
+jet limits• Limit behaviour quite similar
for various models: • (QBH/q*)~83-72/85-80%• What happens at high mass?
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QuantumBlackHole
(QBH)
q*
Effect on limits from Signal Shapes
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• Increased fraction of the off-shell production for the signal events (“parton-luminosity tail”) as the kinematic production limit is approached • size increases with the resonance width. • Noticeable impact of this tail on cross-section limit values when >5-10% of events in the tail
This affects most of the limits (q*, Z’ etc)!
Usual signalBreit-Wigner
shape
Parton distribution functions
Parton-luminosity tail
Effect on limits from Signal Shapes
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• For thresholds or resonances with suppressed low-mass contributions (Z*) “parton-luminosity tail” effect is not present
• It is also not present it the resonance shape is input “by hand”, e.g. assuming a Gaussian or Breit-Wigner of certain width, instead of using shape from simulation
QBH Productionis a threshold!
MD=MTH Mass
MD=fundamental scaleof quantum gravity
MTH=threshold for black hole production
+jet limits:width effects
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Width assumptions matter at low masses: smaller width = better limits
Also parton luminosity tail is larger for wider resonances, which leads to worse limits
Dilepton Resonant Searches
Data consistent with SM predictions
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CMS signal shape assumption: Breit-Wigner with Z’ width
(/M=0.5%) convolutedwith Gaussian of signal resolution
ATLAS signal shape assumption:Monte-Carlo based
Z’SSM shape (with width /M~3%)
Dilepton Resonant Search Results
Limits @ 95% CL are: M(Z’SSM) > 2.96 TeV (CMS) & 2.8 TeV (ATLAS)
M(Z’)>2.60 TeV (CMS) & 2.38 TeV (ATLAS)
Those assumptions affect the final results, making limits not comparable directly: effects of parton luminosity tail visible
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VV & Vq (V=W/Z) Resonant Search
Jets Cambridge-Aachen R=0.8, W/Z-tag jet, isolated leptons, kinematic cutsNo excess over SM backgrounds is seenxBR(XWW)>70 (3) fb @ 0.8(2.5) TeV; m(G*ZZ)>710 GeV for k/MPl=0.5
Different channels to enhance sensitivityT. Berger-Hryn'ova Moriond QCD 2014 13
See also ATLAS-CONF-2012-150 (ZZlljj) & ATLAS-CONF-2013-015 (WWlll)
k/MPl=0.1
k/MPl=0.1
Search for XHH(bb)(bb)
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2 back-to-back boosted doubly btagged di-jet systems
4 b-jets (MV1, eff 70%) anti-kt 0.4 pT>40 GeV, R(2b)<1.5, pT
jj>200 GeVHadronic ttbar veto
Background: multijet (90%); ttbar (<10%); Z+jets
No evidence of signal is foundExclude @95% CL for k/MPl=1.0
590<M(G*RS)<710 GeV
<1.6
XX(3j)(3j) Resonant Search
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No excess over SM backgrounds is seen.Limits on m(gluino) of 650GeV at 95% CL is set for light flavour jets 835GeV at 95% CL for 2 light and 1 b-jets
See also ATLAS-CONF-2013-091, dijet: CMS PAS EXO 059, ATLAS-CONF-2012-148
We should notgive up!
Perspectives/Conclusions• Resonant searches are very powerful
– Many final states have been explored
• But no new physics seen in 8 TeV data yet…
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LHC Run 2 will provide us with more luminosity to explore lower couplings & LHC energy increase to higher masses.
We should notgive up!
Perspectives/Conclusions• Resonant searches are very powerful
– Many final states have been explored
• But no new physics seen in 8 TeV data yet…
17T. Berger-Hryn'ova Moriond QCD 2014
LHC Run 2 will provide us with more luminosity to explore lower couplings & LHC energy increase to higher masses.
Stay tuned to new exciting results in 2015!
Documentation
• https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsEXO
• https://twiki.cern.ch/twiki/bin/view/AtlasPublic/ExoticsPublicResults
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Dijet Search
T. Berger-Hryn'ova Moriond QCD 2014 22CMS PAS EXO 059, ATLAS-CONF-2012-148
Z’ hadronic search
Z’hadhad(~42% of the BF)
No excess over SM backgrounds seen.Excluded M(Z’SSM)<1.89TeV @ 95%CL
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