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. V. A. Mitsou / ValenciaPhysics at LHC 2004, Vienna 2 Outline Introduction Theoretical motivation Phenomenology Production of leptoquarks at LHC ATLAS sensitivity 2 leptons + 2 jets topology E T miss + 2 jets topology Conclusions

3. V. A. Mitsou / ValenciaPhysics at LHC 2004, Vienna 3 Theoretical motivation 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 and carry 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 (κ G ) and electric quadrupole (λ G ) model- dependent couplings Yang-Mills coupling: κ G =λ G =0 Minimal coupling: κ G =1, λ G =0 Experimental evidence searched: indirectly: LQ-induced 4-fermion interactions directly: production cross sections at collider experiments

4. V. A. Mitsou / ValenciaPhysics at LHC 2004, Vienna 4 Leptoquark classification Buchmuller, R ü ckl, 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 ► Q em = ±1/3, ±2/3, -4/3, -5/3 Intergenerational mixing is severely restricted by FCNC data  LQ appear in 3 quark/lepton generations LQ-mediated π and K helicity- suppressed decays not observed  chiral LQ couplings to fermions 14 chiral LQ species per generation: 7 scalar LQs (3 singlets, 3 doublets, 1 triplet) 7 vector LQs (3 singlets, 3 doublets, 1 triplet)

5. V. A. Mitsou / ValenciaPhysics at LHC 2004, Vienna 5 Phenomenology Decays: LQs decay to ± q and/or νq with branching ratios β ℓ, β ν = 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 massM LQ λ LQ- -q Yukawa coupling, λ Γ~ λ 2 ·m LQ Resonance width Γ~ λ 2 ·m LQ labeled by weak isospin and lepton helicity couple to ℓ - q, νq couple to ℓ + q, νq -

6. V. A. Mitsou / ValenciaPhysics at LHC 2004, Vienna 6 LQ production processes at LHC Single production λ strongly depends on λ possible signatures: ℓ + ℓ - + jetℓ + ℓ - + jet ℓν + jetℓν + jet νν + jetνν + jet Zjettt Main background: Zjet & tt Pair production λg-LQ-LQ Practically independent of Yukawa coupling λ (only g-LQ-LQ vertex) LQ mass Depends mainly on LQ mass LQ λ λ-dependent process does not contribute significantly to 2 nd & 3 rd generation LQ, ν LQ, ν LQ qg  LQ, qg  ν LQ qq  LQ LQ gg  LQ LQ -

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

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

9. V. A. Mitsou / ValenciaPhysics at LHC 2004, Vienna 9 2 leptons + 2 jets topology Signal LQ LQ  ℓ + qℓ - q 1 st 2 nd 1 st and 2 nd generation LQs Scalar e - u & μ - c β=1  S 0 R ( -1 / 3 ) β=0.5  S 0 L ( -1 / 3 ), S 1 ( -1 / 3 ) λ ≥ 10 -6 (LQ : resonance)Background QCD: QCD: huge, but eliminated after high-p T isolated leptons and high-m ℓj cuts are applied Drell-Yan: Drell-Yan: eliminated by high-m ℓj cut First level cuts: 2 jetsp T >30 GeV |η| 30 GeV and |η|<5.0 2leptons p T >30 GeV|η| 30 GeV and |η|<2.5 M LQ (TeV) σ (fb) 1 st gener.2 nd gener. 1.05.04.8 1.21.3 0.710.68 1.50.220.21 1.70.0740.070 2.00.0150.014 Processσ×BR (pb) Zjet (ℓℓjj) ), p T > 20 GeV1 380 tt (ℓνjℓνj)11 ZZ (ℓℓjj)1.2 ZW (ℓℓjj)1.2 WW (ℓνℓν)3.3

10. V. A. Mitsou / ValenciaPhysics at LHC 2004, Vienna 10 ℓℓjj: selection variables I Signal (m LQ =1 TeV): Many high-p T jets Two high-p T leptonsBackground: Z  ℓ + ℓ - peak at m ℓℓ  90 GeV After first-level cuts signalbackgrnd

11. V. A. Mitsou / ValenciaPhysics at LHC 2004, Vienna 11 ℓℓjj: selection variables II ∑ E T ∑ E T : sum of transverse energy in the calorimeters m ℓj m ℓj : lepton-jet invariant mass for two leading jets (minimum- Δm ℓj combination) backgrndsignal

12. V. A. Mitsou / ValenciaPhysics at LHC 2004, Vienna 12 ℓℓjj: m ℓj invariant mass m ℓj combination |m ℓj -m LQ | < 100 GeV # events% (a) all combinations13634% (b) two leading jets12645% (c) two leading jets; minimum- ∆m ℓj combination 9870% (a)(b)(c) m LQ =1 TeV Provides clearest signal

13. V. A. Mitsou / ValenciaPhysics at LHC 2004, Vienna 13 ℓℓjj: selection cuts (tentative) eejjμμjj Similar cuts imposed for both eejj & μμjj channels Cuts optimized to maximize significance for all leptoquark masses 2 jetsp T >70 GeV|η| 70 GeV and |η|<5.0 2leptons p T >100 GeV|η| 100 GeV and |η|<2.5 m ℓℓ > 180 GeV m ℓℓ > 180 GeV (for Z peak) E T miss < 70 GeV E T miss < 70 GeV (for tt background) ∑ E T > 570 GeV E T miss / ∑ E T < 0.05 |m ℓj -m LQ | < 100 GeV mass window: |m ℓj -m LQ | < 100 GeV m LQ reconctructed from two leading jets with minimum-∆m ℓj combination

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

15. V. A. Mitsou / ValenciaPhysics at LHC 2004, Vienna 15 ℓℓjj: signal significance First generation leptoquarks ∫ L =30 fb -1 Integrated luminosity ∫ L =30 fb -1 m LQ = 1.3 TeV Signal can be clearly observed for m LQ = 1.3 TeV μμjj Similar results obtained for μμjj channel M LQ (TeV)SignalBackgroundS/√B 1.01264.6558 1.227.64.1414 1.316.13.4610.7 1.54.491.865.9 Preliminary

16. V. A. Mitsou / ValenciaPhysics at LHC 2004, Vienna 16 E T miss +2 jets topology 1 st and 2 nd generation LQs Scalar νd & νs LQ LQ  νν qq Signal is difficult to be separated from SM background (Z jet irreducible background) 3 rd generation LQs LQ LQ  νν bb β = 0  S 1/2 ( +1 / 3 ) S 1 ( +2 / 3 ), β = 0.5  S 0 L ( -1 / 3 ), S 1 ( -1 / 3 ) Background Zjet background irreducible Main backgrounds: tt, ZZ, ZW(bbℓν) All other SM backgrounds are eliminated from b-tagging and lepton-veto ~ Processσ×BR (pb) Zjet (ννjj)22 000 Wjet (ℓνjj)38 400 tt (ℓνbℓνb)51.6 ZZ (ννbb)0.6 ZW (bblν)1.3 ZW (ννjj)3.6 WW (ℓνjj)30.5

17. V. A. Mitsou / ValenciaPhysics at LHC 2004, Vienna 17 ννjj: selection criteria (tentative) Selection criteria maximize significance for high leptoquark masses 2 b-jets At least 2 b-jets with p T >70 GeV and |η|<5.0 No isolated leptons m jj > 180 GeV E T miss > 400 GeV 30 o <φ j-j <150 o 30 o <φ j-j <150 o for the two leading jets φ j-pTmiss >60 o φ j-pTmiss >60 o for the two leading jets signal m LQ =1 TeV background

18. V. A. Mitsou / ValenciaPhysics at LHC 2004, Vienna 18 ννjj: selection variables Variables shown before applied cuts (p T jet >10 GeV) large E T miss Main cut: large E T miss due to escaping neutrinos signal m LQ =1 TeV background signal m LQ =1 TeV

19. V. A. Mitsou / ValenciaPhysics at LHC 2004, Vienna 19 ννjj: significance LQ LQ  νbνb Third generation leptoquarks: LQ LQ  νbνb Integrated luminosity ∫L=30 fb -1 No mass peak is reconstructed (only excess of events) m LQ =1.3 TeV Signal observable for masses up to m LQ =1.3 TeV m LQ ~1.5 TeV Exclusion limits (95% C.L.) up to m LQ ~1.5 TeV m LQ (TeV) σ x BR (fb) eff. (%) signal SM bgd S/√B 1.0 4.8448.770.73.438.3 1.21.2855.521.33.411.5 1.30.6859.112.13.46.5 1.50.2161.63.93.42.1 1.70.0764.31.43.40.7 95% C.L. Preliminary

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