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SUSY Spin Measurements with ATLAS

hep-ph/0405052. SUSY Spin Measurements with ATLAS. So … is it really supersymmetry?. Alan Barr. “If it quacks like SUSY …”. “What else could it possibly be?”. “Don’t be so sure … ”. (S)particle reminder. SM. SUSY. “Left squark” = scalar SUSY partner of left quark.

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SUSY Spin Measurements with ATLAS

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  1. hep-ph/0405052 SUSY Spin Measurementswith ATLAS So … is it really supersymmetry? Alan Barr “If it quacks like SUSY …” “What else could it possibly be?” “Don’t be so sure … ” UK ATLAS Physics

  2. (S)particle reminder SM SUSY “Left squark” = scalar SUSY partner of left quark quarks (L&R)leptons (L&R) neutrinos (L&?) squarks (L&R)sleptons (L&R)sneutrinos (L&?) Spin-1/2 Spin-0 AfterMixing Z0W± gluon BinoWino0Wino± gluino BW0 Spin-1 4 x neutralino Spin-1/2 gluino ~ h0 H0 A0 H± H0H± ~ 2 x chargino Spin-0 Extended higgs sector (2 doublets) UK ATLAS Physics

  3. [RPC SUSY] 1-year low-lumi mSUGRA reach • Sparticle decays generate: • Missing ET (LSP) • Jets (squarks/gluino) • Leptons (sleptons) • “If we see these at LHC we have discovered SUSY” • Not necessarily… Gaugino mass term (GeV) Plot by D. Tovey Scalar mass term (GeV) UK ATLAS Physics

  4. Universal Extra Dimensions Cheng, Matchev “How to get fooled at the LHC” hep-ph/0205314 • TeV-scale universal extra dimension model • Kaluza-Klein states of SM particles • same QN’s as SM • mn2 ≈ m02 + n2/R2 [+ boundary terms] • same spin as SM • KK parity • Lightest KK state stable, and weakly interacting • 1st KK mode pair-produced • First KK level looks a lot like SUSY Radius of extra dimension ~ TeV-1 KK tower of masses n=0,1,… Dubbed “Bosonic Supersymmetry” UK ATLAS Physics

  5. UED KK spectrum Cheng, Matchev hep-ph/0205314 Example decay spectrum 1st excited KK level (example masses) Stable lightestKK particle(weakly interacting) After adding boundary terms -> SUSY-like spectrum Need to measure spins! UK ATLAS Physics

  6. SUSY decay chain Spin-0 • Use mSUGRA LHC point 5 as example • Well studied in context of sparticle masses • Nice leptonic signal at LHC Spin-½ Spin-½, mostly wino Spin-0 Spin-½, mostly bino Final state = jet + l+ + l- + ET ( + decay of other sparticle) UK ATLAS Physics

  7. Spin projection factors P In rest frame of with +ve z-axis aligned with qL S Approximate SM particles as massless -> okay since m « p UK ATLAS Physics

  8. Spin projection factors P In rest frame of with +ve z-axis aligned with qL S Σ=0 S Spin-0 Approximate SM particles as massless -> okay since m « p UK ATLAS Physics

  9. Spin projection factors In rest frame of with +ve z-axis aligned with qL θ* p S Approximate SM particles as massless -> okay since m « p UK ATLAS Physics

  10. Spin projection factors P In rest frame of with +ve z-axis aligned with qL mql - measure S Σ=0 θ* p S S Spin projection factor Spin-0 Approximate SM particles as massless -> okay since m « p UK ATLAS Physics

  11. lnearq invariant mass (1) Back to backin 20 frame nearlepton quark Probability l+ θ* Phase space χ20 l- Phase space -> factor of sin ½θ* Spin projection factor in |M|2: l+q -> sin2½θ* l-q ->cos2½θ* m/mmax = sin ½θ* UK ATLAS Physics

  12. lnearq invariant mass (2) (Lepton from 2 decay only – not directly measurable) 0 1 0 1 squarkdecay anti-squarkdecay l+ l- l+ l- Obvious asymmetry! Opposite asymmetry! -> if same # squarks as anti-squarks effect cancels UK ATLAS Physics

  13. How to measure? • lnearq shows nice charge asymmetry • Excellent probe of spin of • Experimental problems in measurement: • Can’t tell near lepton from far lepton • Plot l+q and l–q instead -> contributions from both near and far • Can’t tell quark jet from anti-quark • Sum of q and q-bar is measurable • pp collider -> get squark asymmetry from PDFs UK ATLAS Physics

  14. Production asymmetry qg -> squark + gluino x1 and x2 sampled by MC Valence quarks Peak near 0.1, 0.1 Parton distribution functionhas more q than q-bar for Bjorken x ≈ 0.1 -> pp collider produces more squarks than anti-squarks UK ATLAS Physics (Twice as much for our point)

  15. Parton Level l- l+ spin-0=flat Chargeasymmetry Experimentally measurable-> q and q-bar -> near and far leptons difference/sum Shape indicates that20 is spin-½ UK ATLAS Physics

  16. After detector simulation (ATLFAST) Change in shape due to charge-blind cuts l- parton-level * 0.6 Events spin-½ Charge asymmetry, spin-0 l+ detector-level Invariant mass -> Charge asymmetry survives detector sim-> Same shape as parton level (but with BG and smearing) UK ATLAS Physics For cuts see C. Lester thesis

  17. Cross-check • Monte Carlo is HERWIG (6.5) • Can turn off spin correlations • distribution for “scalar 20” • consistent with flat • not consistent with spin-½ 20 of previous page (linearly increasing) No asymmetry if spincorrelations turned off spin-0=flat spin-½ -> spin-0 -> production asymmetry UK ATLAS Physics

  18. Further evidence - slepton spin Straightline distn • Dilepton invariant mass. • “Right-handed” slepton • l+ and l- are right handed • might expect pronounced spin effects (as for lq distributions) • none because slepton is scalar • Scalar particle carrying lepton number • sounds like a slepton to me! (phase-space) Back-to-backin slepton frame Shape after different-family subtraction UK ATLAS Physics

  19. Conclusions • Method for measuring spins of SUSY particles at the LHC • Measure l±q invariant mass distributions • equivalent to angular distributions in • Look for lepton charge asymmetry • Requires initial asymmetry in squark vs anti-squark production • valence quarks in PDFs at x ~ 0.1 • Measure spin-½ nature of 20 • Also spin-0 slepton from mll • Should be enough to kill “straw man” e.g. universal extra dimensions UK ATLAS Physics

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