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 July 2004, Vienna, Austria

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

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

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)

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 -

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 -

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)

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)

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 ≥ (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 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

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

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

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

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

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

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 Preliminary

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) Wjet (ℓνjj) tt (ℓνbℓνb)51.6 ZZ (ννbb)0.6 ZW (bblν)1.3 ZW (ννjj)3.6 WW (ℓνjj)30.5

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

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

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 % C.L. Preliminary

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