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Longlived Particle Searches at Colliders
Philippe Mermod (Oxford University)
Seminar DPNCGeneva, 14 October 2009
• Fundamental physics puzzles• SplitSUSY and longlived gluinos• Magnetic Monopoles• Past searches – LEP, Tevatron• ATLAS search plans
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History of new particles• 19501974 :
➢ hadron jungle, neutrino➢ quark model confirmed
• 19741994 : ➢ heavy quarks, tau, W and Z... ➢ 3 generations, EW unification confirmed
• 2010 and onward : ➢ We generally expect new particles to be
heavy and shortlived (e.g., Higgs boson)➢ Longlived ? (e.g., WIMP)
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Today's particle physics puzzles• Origin of mass, mass hierarchy• Unification of forces• Matterantimatter asymmetry• Dark matter
Proposed searches• Higgs boson• Precision measurements• Exotic phenomena, e.g.,
Supersymmetry
New data needed !
Chemistry
Nuclear force
Energy frontier
QCD
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SplitSUSYarXiv:hepth/0405159
• Accept the unnaturalness of the Higgs mass
• Supersymmetry breaking occurs at Ms >> 1 TeV
• Scalars have mass at this scale
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Nice features of splitSUSY • Unification of couplings• Long proton lifetime• Longlived gluino
→ RHadrons !
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Examples of wellmotivated exotic longlived particles
• Neutral LSP dark matter (Rparity cons.)• Metastable squark or gluino (kinematics)• Metastable stau NLSP (LSP gravitino)• Magnetic Monopole (charge conservation)• Leptoquark (weak coupling)
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Magnetic Monopoles• Dirac's argument (1931)
– Angular momentum of field of electronmonopole system :
• “explain” charge quantization• Symmetrize Maxwell Equations• Ingredient in Grand Unification Theories
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Monopoles – kinematics• Direct pair production processes at colliders :
• Higherorder corrections ? – Strong g coupling → nonperturbative dynamics !!! – Must rely on models for the cross section and kinematics
calculations !
photon fusion DrellYanM
M
M
M_
_
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Monopoles ionization
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Monopole bending in a magnetic field
• Acceleration along beam axis
• Parabolic trajectory in rz plane
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SQUID technique• At HERA and the Tevatron• Search for trapped Monopoles• Old beam pipe and detector material in a
superconducting coil to sense flux jump
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Plastic tracketch detectors• Pits due to highly
ionizing particles• Tevatron• LEP
Phys. Rev. D 46, R881 (1992)
• MoEDAL (LHC)● At Point 8● Run in 2010
LHCb
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Current cross section limits for Dirac Monopoles
• Limits only valid for given mass ranges
• Unwise to quote mass limits (must assume production model)
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Longlived particles carrying colour charge
• Colourtriplets : – Spin0 : leptoquark, squark– Spin1/2 : KKquark, 4th gen quark
• Colouroctets : – Spin1/2 : gluino– Spin1: KKgluon
• Large production cross sections• RHadrons with integer charges
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Heavy Coloured objects at the LHC• Strong process, e.g. gluino pair production
– mass 300 GeV more than → 100000/fb1
(14 TeV pp collisions)
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RHadrons
• Longlived > 50 ns (size of ATLAS)
• Heavy > 250 GeV (current limit)
• Coloured
Generic signature : slow and high momentum
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RHadrons : mass spectra• Quarklike : similar to charmed and bottomed
hadrons (e.g. Λc+ )
• Gluonlike : assume a model– Lightest state neutral or charged ?→
– Gluino balls ?
arXiv:0908.1868
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CDF search using dE/dx in inner detector
Phys. Rev. Lett. 90, 131801 (2003)
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CDF search using TOF in muon detector
• 250 GeV mass limit for a longlived scalar top quark
Phys. Rev. Lett. 103, 021802 (2009)
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Published mass limits
● Assume standard couplings
• What are the limits for a coloroctet ?
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Generic signaturebased longlived particle searches in ATLAS
• Displaced vertices / kinks• Nonpointing photons• Low EM calorimeter fraction• Outofbunchcrossing• Slow tracks• High invariant mass dimuons• Highly ionizing tracks
decaying
nondecaying
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Early data : Generic & Simple
• Systematics– Efficiency– Robustness– Backgrounds
• Nonoptimum conditions• Look for extreme signatures• Use several independent
variables
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Expected signature : low speed
• Measure timeofflight of highmomentum objects– Calorimeter – muon RPC
• If correlation– Slow massive
particle !
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Expected signature :reconstructed mass
RHadrons
muons
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Expected signature : invariant mass of two muons
Z resonance
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RHadrons : possible selection criteria
1. Low β in muon system and/or calorimeter● Fakes : muon β distributions obtained from Z→μμ
2. HighpT muon track without associated ID track
● Fakes : ID efficiency vs. pT obtained from Z→μμ
3. Additional muon and high invariant mass ● Fakes : highp
T tails obtained from Z→μμ
4. A combination of the above
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What could happen• Look for anomalies in spectra of muon objects
– All variables behave as expected for ordinary muons → cross section and mass limits
– Excess Detector effect ? Unexpected backgrounds ? →New physics ?
• Combine all variables : unmistakable events ?
high pT
delayed
anomalous ionization
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HighQ/Monopoles in ATLAS
100% TRT HT hits
E/p >> 1E/p ~ 1 E/p → 0
high E in small area
XXXX
X
XX
XX
X β < 1β ~ 1 β < 1
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Summary• Fundamental physics puzzles
– We expect new physics at the TeV scale– LHC will probe these regions– Signatures : electrons, jets, missing energy,
longlived particles !
• Search techniques with generalpurpose experiments (experience from LEP and Tevatron)
– Late arrival / late decays– Energy loss– Special event topologies
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Outlook• As soon as the LHC machine runs at high
energies– Look at all possible signatures– Can we do something even with non
optimized data ? – e.g. RHadrons, Monopoles in ATLAS
striking events→
– Possibly large cross sections high masses accessible early→
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Extra slides
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Free quarks / lowcharge objectsPhys. Lett. B303, 198 (1993) / Phys. Lett. B572, 8 (2003)
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SUSY models giving rise to SMPs
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Gluino lifetime sin splitSUSY
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Interactions of RHadrons with detector material
• Heavy parton unlikely to interact (cross section suppressed by 1/m2)
• Effectively lowenergy (~GeV) interactions involving light quarks
– Regge Theory
• Light quark flavor can change several times during the passage through the detector
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Nonpointing photons
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Displaced vertices
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+
Event selection(SUSY chapter in CERNOPEN2008020, arXiv:0901.0512)
Search for stop and gluino RHadrons, optimized for 1 fb1 of integrated luminosity for pp collisions at 14 TeV• Trigger : muon trigger• Selection : jet veto + one of the following
criteria– Hard (pT>250 GeV) muon track lacking inner track
– Two hard backtoback inner tracks with few highthreshold (HT) hits
– Two hard backtoback likesign muon tracks– One hard muon track with inner track of opposite
charge
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Simulated data• Signal samples
– Gluinos 300, 600, 1000, 1300, 1600, 2000 GeV– Stops 300, 600, 1000 GeV
• Background samples– QCD dijets (PYTHIA) with pT > 140 GeV
– Top pairs (semileptonic)– W and Z with muons in final state
• Full ATLAS simulation• Standard ATLAS reconstruction
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Final state observables• pT of muon tracks
(normalized to 1 fb1) • Highthreshold (HT) hits in inner tracker
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Chargeflipping signatureqIDpT,ID/qμpT,μ
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Results
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Mu20 trigger
Use for Z mass tagging
Is the muon track consistent ?
Require another muon track with pT > 20 GeV
• Use Z μμ → tagandprobe – HighpT tail fractions
– Charge misidentification probability– Standalone track reconstruction efficiency
• In situ determination of both ID and muon system performances at high pT
Instrumental backgrounds+