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Prospects for leptoquark searches with ATLAS at the LHC

Prospects for leptoquark searches with ATLAS at the LHC

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Prospects for leptoquark searches with ATLAS at the LHC

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  1. Prospects for leptoquark searches with ATLAS at the LHC N. Benekos (CERN) V. A. Mitsou (IFIC –Valencia) I. Panagoulias, Th. Papadopoulou (NTUA Athens) Physics at LHC 13-17 July 2004, Vienna, Austria

  2. Outline • Introduction • Theoretical motivation • Phenomenology • Production of leptoquarks at LHC • ATLAS sensitivity • 2 leptons + 2 jets topology • ETmiss + 2 jets topology • Conclusions Physics at LHC 2004, Vienna

  3. Leptoquarks (LQ) are hypothetical particles which appear in many SM extensions to explain symmetry between leptons and quarks SU(5) GUT model superstring-inspired models ‘colour’ SU(4) Pati-Salam model composite models technicolor LQs are coupled to both leptons and quarks andcarry SU(3) color, fractional electric charge,baryon (B) and lepton (L) numbers LQs can have: spin 0 (scalar)► couplings fixed, i.e., no free parameters spin 1 (vector)► anomalous magnetic (κG) and electric quadrupole (λG) model-dependent couplings Yang-Mills coupling: κG=λG=0 Minimal coupling: κG=1, λG=0 Experimental evidence searched: indirectly: LQ-induced 4-fermion interactions directly: production cross sections at collider experiments Theoretical motivation Physics at LHC 2004, Vienna

  4. Buchmuller, Rückl, Wyler (BRW) model (1987) Assumptions: LQs only couple to quarks, leptons and gauge bosons (with dimensionless couplings) LQ interactions invariant under SM gauge group SU(3)C SU(2)L  U(1)Y LQs are classified by: fermion number, F=3B+L ► F = 0, 2 spin ► J=0 (scalar) or J=1 (vector) charge ► Qem= ±1/3, ±2/3, -4/3, -5/3 Intergenerational mixing is severely restricted by FCNC dataLQ appear in3 quark/lepton generations LQ-mediated π and K helicity-suppressed decays not observedchiral LQ couplings to fermions Leptoquark classification 14 chiral LQ species per generation: 7 scalar LQs (3 singlets, 3 doublets, 1 triplet) 7 vector LQs (3 singlets, 3 doublets, 1 triplet) Physics at LHC 2004, Vienna

  5. Decays: LQs decay to l±q and/orνq with branching ratios βℓ, βν = 0, 0.5, 1(depending on the quantum numbers) Scalar LQs decay isotropically Vector LQs decay ~ (1+cosθ*)2 Each LQ characterized by two parameters: LQ massMLQ LQ-l-q Yukawa coupling,λ Resonance widthΓ~ λ2·mLQ couple to ℓ-q, νq - couple to ℓ+q, νq Phenomenology labeled by weak isospin and lepton helicity Physics at LHC 2004, Vienna

  6. Single production strongly depends on λ possible signatures: ℓ+ℓ- + jet ℓν+ jet νν + jet Main background: Zjet & tt Pair production Practically independent of Yukawa coupling λ (onlyg-LQ-LQvertex) Depends mainly on LQ mass LQ LQ LQ l, ν l, ν - qqLQ LQ ggLQ LQ LQ production processes at LHC qglLQ, qg ν LQ LQ • λ-dependent process • does not contributesignificantly to 2nd & 3rd generation Physics at LHC 2004, Vienna

  7. Pair production cross sections qqLQ LQ: ~30% of total production (for mLQ≈1TeV) 100 events minimal coupling 100 events 100 events Yang-Mills LQ production rates at LHC - Vector LQs Scalar LQs Blümlein et al, Z. Phys. D 76 (1997) 137 Belyaev et al, Phys. Rev. D59 (1999) 075007 Physics at LHC 2004, Vienna

  8. Scalar leptoquarks Pair production ℓℓjj channel ννjj channel independent of λ! Simulation tools: PYTHIA qq  LQ LQ gg  LQ LQ ATLAS fast simulation (ΑΤΗΕΝΑ-ATLFAST) LQ LQ  e+e-qq mLQ=1500 GeV (schematic view) Leptoquarks in ATLAS Physics at LHC 2004, Vienna

  9. Signal LQ LQ  ℓ+qℓ-q 1st and 2nd generation LQs Scalar e-u & μ-c β=1 S0R(-1/3) β=0.5  S0L(-1/3), S1(-1/3) λ ≥ 10-6 (LQ : resonance) Background QCD: huge, but eliminated after high-pT isolated leptons and high-mℓj cuts are applied Drell-Yan: eliminated by high-mℓj cut First level cuts: At least 2 jets with pT>30 GeV and |η|<5.0 2 same-flavour, opposite- sign leptons with pT>30 GeV and |η|<2.5 2 leptons + 2 jets topology Physics at LHC 2004, Vienna

  10. Signal (mLQ=1 TeV): Many high-pT jets Two high-pT leptons Background: Zℓ+ℓ- peak at mℓℓ90 GeV After first-level cuts signal backgrnd ℓℓjj: selection variables I Physics at LHC 2004, Vienna

  11. ∑ET: sum of transverse energy in the calorimeters mℓj: lepton-jet invariant mass for two leading jets (minimum- Δmℓj combination) signal backgrnd ℓℓjj: selection variables II Physics at LHC 2004, Vienna

  12. ℓℓjj: mℓj invariant mass Provides clearest signal mLQ=1 TeV (a) (b) (c) Physics at LHC 2004, Vienna

  13. Similar cuts imposed for both eejj &μμjj channels Cuts optimized to maximize significance for all leptoquark masses at least 2 jets with pT>70 GeV and |η|<5.0 2 same-flavour, opposite- sign leptons with pT>100 GeV and |η|<2.5 mℓℓ> 180 GeV(for Z peak) ETmiss< 70 GeV(for tt background) ∑ET > 570 GeV ETmiss/∑ET < 0.05 mass window: |mℓj-mLQ| < 100 GeV mLQ reconctructed from two leading jets with minimum-∆mℓj combination ℓℓjj: selection cuts (tentative) Physics at LHC 2004, Vienna

  14. After all selection criteria S+B and B is shown for 1st generation (e+e-jj) Signal can be observed for MLQ ~ 1.3 TeV Channel background-free for MLQ>1.8 TeV but signal cross section very small (<0.07 fb) ℓℓjj: signal and background Preliminary mLQ=1 TeV mLQ=1.2 TeV mLQ=1.2 TeV mLQ=1.5 TeV Physics at LHC 2004, Vienna

  15. First generation leptoquarks Integrated luminosity ∫L=30 fb-1 Signal can be clearly observed for mLQ = 1.3 TeV Similar results obtained for μμjj channel ℓℓjj: signal significance Preliminary Physics at LHC 2004, Vienna

  16. 1st and 2nd generation LQs Scalar νd & νs LQ LQ  ννqq Signal is difficult to be separated from SM background (Z jet irreducible background) 3rd generation LQs LQ LQ  νν bb β=0 S1/2(+1/3) S1(+2/3), β= 0.5  S0L(-1/3), S1(-1/3) Background Zjet background irreducible Main backgrounds: tt, ZZ, ZW(bbℓν) All other SM backgrounds areeliminated from b-tagging and lepton-veto ETmiss +2 jets topology ~ Physics at LHC 2004, Vienna

  17. ννjj: selection criteria (tentative) • Selection criteria maximize significance for high leptoquark masses • At least 2 b-jets with pT>70 GeV and |η|<5.0 • No isolated leptons • mjj > 180 GeV • ETmiss > 400 GeV • 30o<φj-j<150o for the two leading jets • φj-pTmiss>60ofor the two leading jets signal mLQ=1 TeV background Physics at LHC 2004, Vienna

  18. ννjj: selection variables • Variables shown before applied cuts (pTjet>10 GeV) • Main cut: large ETmiss due to escaping neutrinos signal mLQ=1 TeV signal mLQ=1 TeV background background Physics at LHC 2004, Vienna

  19. ννjj: significance • Third generation leptoquarks: LQ LQ  νbνb • Integrated luminosity ∫L=30 fb-1 • No mass peak is reconstructed (only excess of events) • Signal observable for masses up to mLQ=1.3 TeV • Exclusion limits (95% C.L.) up to mLQ~1.5 TeV Preliminary 95% C.L. Physics at LHC 2004, Vienna

  20. Conclusions • ATLAS at the LHC is going to explore the existence of leptoquarks with masses up to mLQ~1.5 TeV independently of the Yukawa coupling • 1st- or 2nd-generation scalar leptoquarks can be observed up to mLQ 1.3 TeV,in the LQLQℓℓqq channel (if β=1) • 3rd-generation LQs are observable up to mLQ 1.3 TeV,if they only couple to a neutrino & a b-quark, via theLQLQννbb channel • Possibility to study other species of LQs by combining/investigating other channels Physics at LHC 2004, Vienna