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What can we say about what we’ve found?

Alan Barr. Just find SM Higgs. Was it really SUSY?. How can we discovery SUSY at LHC?. What can we say about what we’ve found?. Your mission…. SM. SUSY. quarks (L&R) leptons (L&R) neutrinos (L&?). squarks (L&R) sleptons (L&R) sneutrinos (L&?). Spin-1/2. Spin-0. After Mixing.

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What can we say about what we’ve found?

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  1. Alan Barr Just find SM Higgs Was it reallySUSY? How can we discovery SUSY at LHC? What can we sayabout what we’vefound? 7 June 2007

  2. Your mission… SM SUSY 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) 7 June 2007

  3. Complete event Decay part Time heavyexotic lighterexotic standard Time = exotic = standard Features of RP SUSY? • RPV as a conserved QN: • Events build from blobs with 2 “exotic legs” • A pair of cascade decays results • Complicated end result Production part standard 2 exotics Time 7 June 2007

  4. General features • Complicated cascade decays • Many intermediates • Typical signal • Jets • Squarks and Gluinos • Leptons • Sleptons and weak gauginos • Missing energy • Undetected LSP • Model dependent • Various ways of transmitting SUSY breaking from a hidden sector “typical” susy spectrum(mSUGRA) 7 June 2007 LHC Pt5

  5. What do we see? Lifetimes short -> look for Standard Model decay relics + missing energy 7 June 2007

  6. Example of a search topology LSP q • No unique choice of sensitive topology • Complementary information/sensitivity • Expect SM backgrounds with similar characteristics to signal • Need to search for excesses squark q _ q _ BACKGROUND topology (QCD) q (and similar) LSP SIGNAL topology 7 June 2007

  7. Practical Problems • See only SM decay products • Expect short lifetimes • Lose information about order of decays • Jets (other than b and t) indistinguishable • Loose flavour information for other squarks • “Missing momentum” from neutralinos only determined perpendicular to beam • Individual LSP momenta not individually measurable • Z-momentum of initial state unknown (PDFs) • Can’t reconstruct from final state • Forward jets lost down beam pipe • Can’t form “invariant masses” of sparticles • No “clean” mass peaks for resonances 7 June 2007

  8. Precise measurement of SM backgrounds: the problem • SM backgrounds are not small • There are uncertainties in • Cross sections • Kinematical distributions • Detector response “Rediscover” Lower backgrounds WW ZZ “Discover” Higher backgrounds 7 June 2007

  9. Just look for jets? Big QCD background Scalar sum of transverse energy / GeV 7 June 2007

  10. Add some missing energy • Look for events with jets and missing energy • Cuts • at least two jets with: • ETJet1,2 > 150,100 GeV • |Jet1,2| < 2.5 Meff = Jets pTi + MET • But with addition of some other cuts…  Missing transverse momentum > 100 GeV  cuts based on i = |(Jet,i)-(MET)|): • R1 = (22+(-1)2) > 0.5 rad • R2 = (12+ (-2)2) > 0.5 rad • no jet with i < 0.5 rad QCD dijets Kill events with missing energy from miss-measured jets “SUSY” 7 June 2007

  11. No MT2Dijet cuts + MET +  Two-Jet Scalar sum of transverse energy / GeV Expect discovery distribution to be of something like this form: Excess of “some sort” of new physics about SM backgrounds. 7 June 2007

  12. Importance of detailed detector understanding • GEANT simulation already shows events with large missing energy • Jets falling in “crack” region • Calorimeter punch-through • Vital to remove these in missing energy tails • Large effort in physics commissioning Et(miss) Lesson from the Tevatron Rare occurrences hurt 7 June 2007

  13. Inclusive reach in mSUGRA parameter space • Map of discovery potential corresponding to a 5σ excess above background in mSUGRA m0 – m1/2 parameter space for the ATLAS experiment. L = 1033 cm-2 s-1 jets + ETmiss channel ~1 year → ~2200 GeV ~1 month → ~1800 GeV few days (< one week) → ~1300 GeV Health warning: expecting SUSY discovery in a few days will seriously damage your credibility 7 June 2007

  14. Different searches • We will be looking in many different channels • n jets + m leptons + missing energy • +- b-jets (common at large tan β) • +- tau-jets (“ “ “) • Charged stable particles • NLSP -> photon gravitino (GMSB) • R-parity violating modes • R-hadrons • … 7 June 2007

  15. Assume we have MSSM-like SUSY with m(squark)~m(gluino)~600 GeV See excesses in these distributions Can’t say “we have discovered SUSY” Can say some things: Undetected particles produced missing energy Some particles have mass ~ 600 GeV, with couplings similar to QCD Meff & cross-section Some of the particles are coloured jets Some of the particles are Majorana excess of like-sign lepton pairs Lepton flavour ~ conserved in first two generations e vs mu numbers Possibly Yukawa-like couplings excess of third generation Some particles contain lepton quantum numbers opposite sign, same family dileptons … What might we then know? 7 June 2007 Slide based on Polesello

  16. Mapping out the new world • Some measurements make high demands on: • Statistics (=> time) • Understanding of detector • Clever experimental technique 7 June 2007

  17. Constraining masses Frequently- studieddecay chain • Mass constraints • Invariant masses in pairs • Missing energy • Kinematic edges Observable: Depends on: Limits depend on angles betweensparticle decays 7 June 2007

  18. Mass determination Measure edges Try various masses in equations • Basic technique • Measure edges • Try with different SUSY points • Find likelihood of fitting data • Event-by-event likelihood • In progress Variety of edges/variables C.G. Lester • Narrow bands in ΔM • Wider in mass scale • Improve using cross- section information 7 June 2007

  19. SUSY mass measurements Tryvariousdecaychains Look forsensitive variables (many of them) • Extracting parameters of interest • Difficult problem • Lots of competing channels • Can be difficult to disentangle • Ambiguities in interpretation • Lots of effort has been made to find good techniques Extractmasses 7 June 2007

  20. SUSY mass measurements: • LHC clearly cannot fully constrain all parameters of mSUGRA • However it makes good constraints • Particularly good at mass differences [O(1%)] • Not so good at mass scales • [O(10%) from direct measurements] • Mass scale possibly best “measured” from cross-sections • Often have >1 interpretation • What solution to end-point formula is relevant? • Which neutralino was in this decay chain? • What was the “chirality” of the slepton “ “ “ ? • Was it a 2-body or 3-body decay? 7 June 2007

  21. The defining property of supersymmetry Distinguish from e.g. similar-looking Universal Extra Dimensions Difficult to measure @ LHC No polarised beams Missing energy Indeterminate initial state from pp collision Nevertheless, we have some very good chances… SUSY spin measurements 7 June 2007

  22. Measuring spins of particles • Basic recipe: • Produce polarised particle • Look at angular distributions in its decay spin θ 7 June 2007

  23. Revisit “Typical” sparticle spectrum Left Squarks-> strongly interacting -> large production -> chiral couplings LHC point 5 20 = neutralino2–> (mostly) partnerof SM W0 Right slepton(selectron or smuon) -> Production/decay produce lepton -> chiral couplings Right slepton(selectron or smuon) -> Production/decay produce lepton -> chiral couplings mass/GeV 10–> Stable -> weakly interacting 10 = neutralino1–> Stable -> weakly interacting 7 June 2007 Some sparticles omitted

  24. Spin projection factors P S Chiral coupling Approximate SM particles as massless -> okay since m « p 7 June 2007

  25. Spin projection factors P S Σ=0 S Spin-0 Produces polarised neutralino Approximate SM particles as massless -> okay since m « p 7 June 2007

  26. Spin projection factors Fermion θ* p S Scalar Polarisedfermion Approximate SM particles as massless -> okay since m « p 7 June 2007

  27. Spin projection factors P mql – measure invariant mass S θ* p S Approximate SM particles as massless -> okay since m « p 7 June 2007

  28. lnearq invariant mass (1) Back to backin 20 frame quark Probability θ* l+ lepton Phase space Invariant mass l- m/mmax = sin ½θ* Phase space -> factor of sin ½θ* Spin projection factor in |M|2: l+q -> sin2½θ* l-q ->cos2½θ* 7 June 2007

  29. After detector simulation (ATLFAST) Change in shape due to charge-blind cuts l- parton-level * 0.6 Events Charge asymmetry, spin-0 SUSY l+ detector-level Invariant mass -> Charge asymmetry survives detector simulation-> Same shape as parton level (but with BG and smearing)  detector effects  cuts to greatly reduce SM 7 June 2007

  30. Interesting questions • Can we test gaugino universality? • Can we constrain the neutralino mass mixing matrix? • Can we measure sparticle splittings? • JMR: Htt coupling interesting • Can we “predict”/confirm dark matter density? • Can we measure mass scale to better than ~10% • Precision measurement/prediction for cross-sections? • Can we confirm spin(s)? 7 June 2007

  31. Extras 7 June 2007

  32. Example: SUSY BG Missing energy + jets from Z0 to neutrinos Measure in Z -> μμ Use for Z ->  Good match Useful technique Statistics limited Go on to use W => μ to improve m m n n Standard Model backgrounds: measure from LHC DATA • Measure in Z -> μμ • Use in Z -> νν R: Z -> nn B: Estimated 7 June 2007

  33. W contribution to no-lepton BG • Use visible leptons from W’s to estimate background to no-lepton SUSY search Oe, Okawa, Asai 7 June 2007

  34. Normalising not necessarily good enough Distributions are biased by lepton selection  7 June 2007

  35. Need to isolate individual components… 7 June 2007

  36. Then possible to get it right… Similar story for other backgrounds – control needs careful selection 7 June 2007

  37. l+ e+ q ~ Z/γ e+ _ _ q q θ* ~ e- q l- e- Direct slepton spin determination • Spin important in sleptonproduction • Occurs through s-channel spin-1 process only • Characteristic angular distribution in production ~ ~ 7 June 2007

  38. Spin-0 SUSY Sleptons “perpendicular” to beam Spin-½ UED KK leptons “parallel” to beam ~ l+ _ q q θ ~ l- Distributions @ parton level Parallel Perpendicularto beam Parallel red=UED blue=PS σtotal not to scale black=SUSY 7 June 2007

  39. θ1lab θ2lab l1 l2 η1lab l1 η2lab l2 Δη θl* θl* l2 l1 Δη Sensitive variables? • cos θlab • Good for linear collider • Not boost invariant • Missing energy means Z boost not known @ LHC • Not sensitive @ LHC • Δη • Boost invariant • Sensitive • Not easy to compare with theory • cos θll* • 1-D function of Δη: • All benefits of Δη • Interpretation as angle in boosted frame • Easier to compare with theory (A) (B) boost (C) N.B. ignores azimuthal angle 7 June 2007

  40. Some results • “SPS5” point • Below: spectrum • Right: results • Good stat. discrimination “Data” = inclusive SUSY after cuts 7 June 2007

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