searches for new physics at the large hadron...
TRANSCRIPT
SearchesforNewPhysicsattheLargeHadronCollider
Jeffrey D. Richman Department of Physics
University of California, Santa Barbara
Scottish Universities Summer School in Physics, St. Andrews, 19 August – 1 September 2012
Lecture 3: odd things
Outline
• SUSYsignatureswithleptons;direct(EW)producBonofneutralinos&charginos– Charginoshidinginplainsight?
• HidingSUSY(“exoBcmodels”)– LonglivedparBcles(e.g.,long‐livedgluinosinsplitSUSY)
– R‐parityviolaBngSUSYsearches• Largeextradimensions(monojets...)
• Blackholes• Conclusions
ExoBca‐fromareviewtalkatICHEPSteve Worm – Searches for Physics Beyond the Standard Model, ICHEP
Several key topics covered in other talks at this school (e.g., SM physics): dijet mass & angular distrib, Z’ l+l-, ttbar
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ThinkingaboutEWproducBon(√s=8TeV)
Courtesy T. Plehn (http://www.thphys.uni-heidelberg.de/~plehn/)
σ pb( )
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ThinkingaboutEWproducBon(√s=8TeV)
Courtesy T. Plehn (http://www.thphys.uni-heidelberg.de/~plehn/)
σ pb( ) As we push up the allowed mass range for the strongly interacting SUSY particles (gluinos & squarks), searches for potentially lower mass EW SUSY particles become competitive.
Thefamousneutralinodileptoncascade
p
p
χ20
χ1±
χ10
χ10
±
ν
ν
Opposite-sign, same flavor leptons
The can be produced in any process, not just direct EW production.
χ20
ThefamousSUSYtrileptonsignature
p
p
χ20
χ1±
χ10
χ10
±
ν
ν
The can be produced in any process, not just direct EW production. Extensive searches for trilepton signatures, including tau leptons.
χ20
Foramusement...http://arxiv.org/abs/1206.6888
ATLAS:σ(pb) CMS:σ(pb)
Measuredcrosssec.
53.4±2.1±4.5±2.1 52.4±2.0±4.5±1.2
Theorycrosssec.NLO
45.1±2.8 47.0±2.0
ppW+W‐kinemaBcdistribuBonshttp://cdsweb.cern.ch/record/1430734/files/ATLAS-CONF-2012-025.pdf
Main selection requirements • Opposite-sign dileptons (ee, emu, mumu), leading lepton pT>25 GeV • No additional leptons • Exclude Z mass window (±15 GeV) for same flavor leptons • No jets with pT > 25 GeV (suppresses ttbar); no b-jets pT>20 GeV • ETmiss_Rel > 25 -55 GeV
EWKSUSYcancontributea“background”toppW+W‐
m( χ1
± ) ≈ 112 GeV
m( χ1
0 ) ≈ 15 GeV
tanβ = 10
σ (pp→ χ1
+ χ1− ) = 2.8 pb
parameters used for plots
ExcessintheW+W‐crosssecBon?http://arxiv.org/abs/1206.6888
Whatdoesitmean?
Ihavenoidea.Firstofall,itisamodesteffectrelaBvetotheuncertainBes.LotsofreasonsthiscouldhavenothingwhatsoevertodowithanaddiBonalphysicsprocessinthedata.ButitdoesshowthatwehavetobeverycarefulaboutSUSY...itmightappearinplacesthatwearenotexpecBng.Wealsohavetobecarefulaboutourcontrolsamples.
Directgauginosearches(ATLAS,7TeV)
• CombinaBonof2‐leptonand3‐leptonsearchesforleptonsproducedincascadesstarBngfrom,,producBon.
+
− χ1+ χ2
0 χ1+ χ1
−
Dilepton search χ1+ χ1
−
https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/SUSY-2011-23/
χ1+ χ2
0Trilepton search
https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/SUSY-2012-13/
Opposite‐signdileptons+jets+MET
EventselecBon• 2opp‐signleptons• ee,μμ,eμ(control)• eτ,μτ(sepcuts)• ≥2jets,pT>30GeV• pT(lep1)>20GeV• pT(lep2)>10GeV• HT>100GeV,MET>50GeV
• Zvetoregion•
CMS SUS-11-011 http://arxiv.org/abs/arXiv:1206.3949
Thefamousneutralinodileptoncascade
p
p
χ20
χ1±
χ10
χ10
±
ν
ν
Opposite-sign, same flavor leptons
The dominant background (ttbar) produces different flavor leptons as well use eµ control sample!
Opposite‐signdileptons:m(l+l‐)
Fitsignalandcontrolregionsjointlytoshapesdescribingnbar+DY+signal(smearedtriangle).
eµ control region: ttbar (+WW)
signal region
Signal contrib. from fit (2.1 σ) local signif.
Signal shape reflects kinematics of sequential two-body decay (mmax=280 GeV)
Opposite‐signdileptons:METpredicBon
HT > 300 GeV
In SM events, can use lepton spectrum to predict the MET spectrum! In general need suitable corrections for W polarization in W+jets and ttbar, as well as resolution and threshold effects.
UsingtheleptonspectrumtopredictMETinsingle‐leptonevents
• InnbarandW+jetsevents,thelepton&neutrinoareproducedtogetherinWdecay.
• InmanySUSYmodelstheleptonandMETaredecoupled.decoupled.
CMS-PAS-SUS-12-010 http://cdsweb.cern.ch/record/1445275
UsingtheleptonspectrumtopredictMETinsingle‐leptonevents
• The MET distribution for SM events is dominated by ttbar and W+jets. • The MET is dominated by the neutrino. • The neutrino spectrum can be predicted from the lepton spectrum, taking into account W polarization in both cases! MET resolution also included.
CMS-PAS-SUS-12-010 http://cdsweb.cern.ch/record/1445275
Searchforlong‐lived,stoppingparBcles
• ImagineaparBclethatliveslongenoughthatitdoesnotdecayduringthebeamcrossingintervalwhenitwasproduced,butstopsinthedetector!
• Itdecays(asynchronouslytobeamX‐ing.)
• SuchparBclesarepredictedinseveralmodels.
• Doweeventriggeroneventslikethis?• “Ifitdidn’ttrigger,itdidn’thappen.”
– oritmightaswellnothavehappened...
Searchforlong‐lived,stoppingparBcles
Somereferences• M.J.StrasslerandK.M.Zurek,“Echoesofahiddenvalleyat
hadroncolliders”,Phys.Len.B651(2007)374,arXiv:hep‐ph/0604261.
• N.Arkani‐HamedandS.Dimopoulos,“SupersymmetricunificaBonwithoutlowenergysupersymmetryandsignaturesforfine‐tuningattheLHC”,JHEP06(2005)073,arXiv:hep‐th/0405159.
• P.Gambino,G.F.Giudice,andP.Slavich,“Gluinodecaysinsplitsupersymmetry”,Nucl.Phys.B726(2005)35,arXiv:hep‐ph/0506214.
• R.MackeprangandA.Rizzi,“InteracBonsofcolouredheavystableparBclesinmaner”,Eur.Phys.J.C50(2007)353,arXiv:hep‐ph/0612161.
Examplescenario:splitSUSY
SUSY scalar particles (including squarks) are at extremely high mass scale
gluino neutralino LSP
LHC energy scale g→ g χ1
0 (via loops)
g→ qq χ1
0
huge gap in Split SUSY
Compare with lifetime of free neutron!
Whathappenstoalong‐livedgluino?
• HadronizaBonturnsgluino/stopinto“R‐hadron”
• TheR‐hadroninteractswiththematerialofthedetector.SomefracBonwillstop,typicallyinthedensestregonsinthedetector.Probtostop∼0.07.
• Eventuallythegluinodecays.
gg gqq gqqq ...
M ( g), M (t )
tq tqq ...
Gluinodecayinhadroniccalorimeter(MC)Trigger = CALO cluster + no incoming p bunches + no muon segments
Trigger: Calo jet ET>50 GeV + veto on signals from Beam Position and Timing Monitors (BPTX) 175 m on either side of CMS. Don’t want either proton bunch present (beam gas events can be produced with just one p bunch). Also veto on beam halo forward muon trigger.
CMS simulation
EventselecBonforstoppingparBcles
• During2011run,numberbunches/beamvariedfrom228to1380.
• SelectBmeintervalsforanalysisbetweenbunchcrossings.
• VetoanyeventwithintwoLHCclockcycles(BX=25ns)ofeitherpbunchpassingthroughCMS.
• Get85%oforbitBmefor228bunchfills;16%oforbitfor1380bunchfillsforthesearch249hoursliveBme.LHCorbitperiodis89μs.
• Cutstorejectbeamhalomuons,cosmics,HCALnoise.Finalrate:(1.5±2.5)×10‐6Hz.
Stoppedgluinosearch:Background&observedyields
Estimate of background contributions over total live time.
Estimate of background contributions for live-time intervals chosen for each lifetime hypothesis.
For lifetimes shorter than one LHC revolution time, search in an time window of 1.3τ after beam xing.
CrosssecBonexclusionfromstoppedgluinosearch
Hypothetical lifetime
Masslimitsonstoppingand g t
Hypothetical lifetime
Massexclusionfromstoppedgluinosearch
Monophotonsearch:interpretaBoninLargeExtraDimensionsmodels
MPl2 ≈ MD
n+2Rn
• Try to explain difference between Planck and EW scales. • n extra compact spatial dimensions, characteristic scale R • Gravity propagates in the (4+n) dimensional bulk of space-time; SM fields are confined to four dimensions. Graviton production seen as missing momentum. qq → γG qq → gG gg→ gG
R‐parityviolaBngSUSY
• WhatifSUSYviolatesR‐parity?
• Mainissue:canhaveverylinleMET.SomeexisBngSUSYsearcheswith“strong”signaturescanworkwithlooseMETrequirements(e.g.,same‐signdileptons).
CMS multilepton analysis: http://arxiv.org/abs/1204.5341 CMS three-jet search: https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsEXO11060
(GeV)jjjM0 200 400 600 800 1000
!(D
ata
- F
it)/
-4
-2
0
2
4
6
8
10
12
14
16
18
Data300 GeV gluino450 GeV gluino
= 7 TeVs
(b)
-1CMS, 5.0 fb
3 jet search
Excludes gluino masses below ~460 GeV.
Searchfor“microscopic”blackholes
• Signatureoflow‐scalequantumgravity.
• Butmanydifferentscenarios–smallindustryofsimulaBons/models.
• PhysicsofblackholeformaBonandevaporaBonhasseveralsubtleBes.(E.g.,whatfracBonoftheiniBalpartonenergyistrappedintheeventhorizon,rotaBngvs.non‐rotaBng,etc.)
CMS black hole search: http://arxiv.org/abs/1202.6396
• ObjectselecBonissimple– Leptons(e,mu):pT>50GeV
– Photons(e,mu):pT>50GeV– Jets:pT>50GeV– NonoverlappinginconeΔR=0.3.
• Computetotalscalarsumoftransversemomentaintheevent.
• StudySTasafuncBonofobjectmulBplicity,whichdoesnotincludeMET.
Searchformicroscopicblackholes
ST = pTj
j= jets, leptons, photons, MET∑
Blackholes:backgroundesBmaBonCMS black hole search: http://arxiv.org/abs/1202.6396
• Background shape is obtained from fit to low-multiplicity (N) events and restricting ST to range 1200 < ST < 2800 GeV. • Shapes in N=2 and N=3 samples are very similar. • Dedicated search for new physics in N=2 sample shows no signal.
Searchformicroscopicblackholes
750 MC samples for the signal scenarios considered... Excluding black hole masses below 4-6 TeV.
Cross sections vs. black hole mass Example of high-multiplicity sample
Blackholesearch:highSTevent
Conclusions
• ThisisauniqueperiodinthehistoryofparBclephysics.
• Wedon’tknowwhatwewilldiscover–thatisthefundamentalnatureofscience.
• Therearenoguarantees,butthepotenBalforbreakthroughshasneverbeengreater.
• YourworkandleadershiparecriBcaltothefutureofhighenergyphysics.
• Manythankstoalltheorganizers,staff,postdocs,andstudents!
SearchforZ’e+e‐,μ+μ‐
SearchforZ’e+e‐,μ+μ‐
DatawithsimulatedADDsignal