MODEL DEPENDENT SEARCHES FOR NEW PHYSICS AT HERA Giuseppe - PowerPoint PPT Presentation

MODEL DEPENDENT SEARCHES FOR NEW PHYSICS AT HERA Giuseppe Barbagli-INFN Firenze representing the collaborations Outline: Introduction. HERA Leptoquarks and LFV Excited fermions Anomalous top coupling Contact Interactions. Conclusions

INTRODUCTION. HERA Asymmetric collider of e + ( e − ) with p . E CM ≃ 320 GeV Two general purpose experiments, ZEUS and H1. Two phases 1994-2000 HERA I 2003-2007 HERA II (with longitudinally polarised e ± in most of data) End data taking on June 2007. Total integrated luminosity ≃ 1 fb − 1 for the two experiments.

LEPTOQUARKS Many extensions of the Standard Model predict Bosons with both Lepton and Baryon numbers. Phenomenological model of Buchm¨ uller-R¨ uckl-Wyler (BRW): - invariance under SU (3) C × SU (2) L × U (1) Y - L and B conservation - 7 Scalar and 7 Vector (4 decaying into both eq and νq ) - fermion number F = 3 B + L = 0 , 2 e ± ℓ - coupling to either LH or RH leptons, not both LQ - fixed branching ratio into eν 1, 1/2, νq 0, 1/2 λ ℓq j λ eq i F = 0 e − ¯ q j q or e + q ⇒ best sought in e + p q i F = 2 e − q or e + ¯ q ⇒ best sought in e − p e ± ℓ λ eq i Different regimes: M LQ << √ s : resonant production, dominated by s-channel λ ℓq j M LQ >> √ s : both s- and u-channel contribute, q i q j ¯ ¯ analysis in terms of contact interactions.

FIRST GENERATION LEPTOQUARKS-1 No excess seen in mass spectra (e-jet, ν -jet). The use of polarization of both signs within the HERA II sample enhances the sensitivity to individual LQ species. Leptoquark Search, HERA I+II Leptoquark Search, HERA I+II 4 4 10 10 Events / 20 GeV Events / 20 GeV Events / 20 GeV Events / 20 GeV H1 3 3 H1 10 10 3 3 10 10 -1 -1 e p (449 pb ) e p (428 pb ) ± ± 2 2 10 10 2 2 10 10 10 10 10 10 NC CC H1 Data (prelim.) H1 Data (prelim.) 1 1 1 1 SM SM SM Uncertainty SM Uncertainty 100 100 150 150 200 200 250 250 300 300 100 100 150 150 200 200 250 250 300 300 M M / GeV / GeV M M / GeV / GeV LQ LQ LQ LQ

FIRST GENERATION LEPTOQUARKS-2 -1 Leptoquark Search, HERA I+II (449 pb ) λ 1 1 Excluded - S (e u, d) ν -1 -1 0,L 10 10 H1 prelim. single LQ H1 (94-00) single LQ D0 pair prod. -2 -2 10 10 L3 indir. limit 200 200 250 250 300 300 350 350 400 400 M / GeV LQ

LEPTON FLAVOUR VIOLATION Mediated by LQs in family non-diagonal models. e ± replaced by a µ or a τ in the final state. H1 looked for ep → µX mediated by F = 2 LQ in e − p data. Signature: isolated muon and jet, back to back in events with high missing Pt . SM background: lepton pair production, real W, photoproduction, DIS. Search for Lepton Flavour Violation Search for Lepton Flavour Violation 1 1 eq eq λ λ Events / 50 GeV Events / 50 GeV Events / 50 GeV H1 preliminary = = 10 10 10 q q µ µ λ λ - H1 data HERA II e p d e d SM u -1 -1 10 10 l c SM uncertainty H1 preliminary x e 1 1 1 L S 0 H1 HERA I +II (prelim.) single LQ -2 -2 10 10 -1 -1 -1 10 10 10 H1 HERA I single LQ D0 pair prod. (qq → e ν ) -2 -2 -2 10 10 10 -3 -3 10 10 0 0 0 50 50 50 100 150 200 250 300 350 400 100 150 200 250 300 350 400 100 150 200 250 300 350 400 100 100 200 200 300 300 400 400 500 500 600 600 M M / GeV / GeV X X X µ µ µ M M M GeV GeV GeV LQ LQ da da da

EXCITED FERMIONS Gauge mediated model for compositeness of fermions (Hagiwara et al.): excited fermions with spin and isospin 1/2 with both left-handed, F ∗ L , and right-handed components, F ∗ R ,in weak isodoublets. Transitions between known fermions, F L , and excited states F ∗ , magnetic de-excitation. Λ compositeness scale. Parameters f , f ′ and f s weight factors of the three gauge groups (U(1), SU(2), SU(3)). At least one of f , f ′ and f s must be non-zero. Relations between f , f ′ and f s , fix branching ratios of excited-fermion decays; cross section then depends only on f/ Λ.

e, ν EXCITED ELECTRONS e e * Looked for e ∗ → eγ γ , Z, W e ∗ → eZ with Z → q ¯ γ / Z γ / Z q and e ∗ → νW with W → qq ′ . p p, X No evidence. Improved limits using HERA II. -1 Search for e*, HERA I+II ( s = 320 GeV, 435 pb ) √ -1 -1 -1 -1 10 10 10 Search for e*, HERA I+II ( √ s = 320 GeV, 435 pb ) ] ] ] H1 f = + f’ -1 -1 -1 45 45 H1 [GeV [GeV [GeV Events Events H1 Data (prelim.) All SM 40 40 Λ Λ Λ 35 35 -2 -2 -2 10 10 10 Search for e* e -1 → γ CDF Run II (202 pb ) f / f / f / f / = 1/(Me*) 30 30 Λ LEP (indirect) 25 25 -1 H1 HERA I (120 pb ) 20 20 -3 -3 -3 10 10 10 15 15 -1 H1 HERA I+II (435 pb ) (preliminary) 10 10 5 5 -4 -4 -4 10 10 10 LEP (direct) 50 50 100 100 150 150 200 200 250 250 300 300 350 350 100 120 140 160 180 200 220 240 260 280 300 100 120 140 160 180 200 220 240 260 280 300 100 120 140 160 180 200 220 240 260 280 300 e* Mass [GeV] e* Mass [GeV] e* Mass [ GeV ] e* Mass [ GeV ] e* Mass [ GeV ]

ν , e EXCITED NEUTRINOS e ν * Looked for ν ∗ → νγ γ , Z, W ν ∗ → νZ with Z → q ¯ W q and ν ∗ → eW with W → qq ′ . p X No evidence. Improved limits using HERA II. e − p data have the advantage of enhancing the CC cross section. H1 paper arxiv:0802.1858 - - Search for * at HERA (e p, 184 pb -1 ) Search for * at HERA (e p, 184 pb -1 ) ν ν 1 1 1 1 ] ] ] ] H1 f = - f’ H1 f = + f’ -1 -1 -1 -1 [GeV [GeV [GeV [GeV -1 -1 -1 -1 10 10 10 10 Λ Λ Λ Λ f / f / f / f / H1 H1 -2 -2 -2 -2 10 10 10 10 f / = 1 / M f / = 1 / M Λ Λ * * ν ν -3 -3 -3 -3 10 10 10 10 DELPHI L3 100 120 140 160 180 200 220 240 260 280 300 320 100 120 140 160 180 200 220 240 260 280 300 320 100 120 140 160 180 200 220 240 260 280 300 320 100 120 140 160 180 200 220 240 260 280 300 320 * Mass [GeV] * Mass [GeV] * Mass [GeV] * Mass [GeV] ν ν ν ν

ANOMALOUS TOP COUPLING At HERA top quarks can only be singly produced. SM, single-top production proceeds due to (CC) reaction ep → νt ¯ b X. SM cross section at HERA is less than 1 fb ⇒ any observed single-top event must come from physics beyond the SM. FCNC reaction: incoming lepton exchanges a γ or Z with an up-type quark in the proton, yielding a top quark in the final state most sensitive to a coupling of the type tqγ . u -quark dominant at large x ⇒ production of single top quarks is most sensitive to a coupling of the type tuγ . Deviations in terms of couplings of the type tuV (with V = γ, Z 0 ). Effective Lagrangian contains magnetic coupling k g and vector coupling v z assumed to be real and positive. k g and v z zero at tree level and extremely small at the one-loop level in SM.

RESULTS e e H1 searched for single top: t → bW → eP tmiss jet or → µP tmiss jet in a multivariate analysis γ/ Z 0 Background from SM : mainly real W production. q / ¯ ¯ l u t q ′ /ν κ tuγ /v tuZ W + | | H1 Preliminary (HERA I+II) tuZ tuZ b v v | | Excluded 1 1 σ ep → etX < 0.16 pb Excl. by ZEUS Excl. by CDF Excl. by L3 -1 -1 10 10 κ = v = 0 Most stringent tc γ tcZ m = 175 GeV t limit | | | | -1 -1 κ κ 10 10 1 1 γ γ tu tu to date: k tuγ < 0.14

VARIABLES ELECTRON CHANNEL MUON CHANNEL Events Events Events Events Events Events H1 Data (prelim.) 5 5 5 10 10 10 All SM 4 4 4 8 8 8 W MC Top MC 3 3 3 6 6 6 2 2 2 4 4 4 2 2 2 1 1 1 0 0 0 0 0 0 0 0 0 20 20 20 40 40 40 60 60 60 80 80 80 0 0 0 20 20 20 40 40 40 60 60 60 80 80 80 b b b b b b P P P / GeV / GeV / GeV P P P / GeV / GeV / GeV T T T T T T 8 8 8 Events Events Events Events Events Events 2.5 2.5 2.5 7 7 7 6 6 6 2 2 2 5 5 5 1.5 1.5 1.5 4 4 4 3 3 3 1 1 1 2 2 2 0.5 0.5 0.5 1 1 1 0 0 0 0 0 0 100 100 100 150 150 150 200 200 200 250 250 250 100 100 100 150 150 150 200 200 200 250 250 250 M M M / GeV / GeV / GeV M M M / GeV / GeV / GeV b b b e e e ν ν ν b b b µ µ µ ν ν ν 4 4 4 Events Events Events Events Events Events 6 6 6 3.5 3.5 3.5 5 5 5 3 3 3 2.5 2.5 2.5 4 4 4 2 2 2 3 3 3 1.5 1.5 1.5 2 2 2 1 1 1 1 1 1 0.5 0.5 0.5 0 0 0 0 0 0 -1 -1 -1 -0.5 -0.5 -0.5 0 0 0 0.5 0.5 0.5 -1 -1 -1 -0.5 -0.5 -0.5 0 0 0 0.5 0.5 0.5 e e e µ µ µ cos cos cos θ θ θ cos cos cos θ θ θ W W W W W W

DISCRIMINANT ELECTRON CHANNEL MUON CHANNEL Events Events Events Events Events Events H1 Data (prelim.) All SM 10 10 10 W MC Top MC 1 1 1 1 1 1 10 10 10 -1 -1 -1 -1 -1 -1 10 10 10 0 0 0 0.2 0.2 0.2 0.4 0.4 0.4 0.6 0.6 0.6 0.8 0.8 0.8 1 1 1 0 0 0 0.2 0.2 0.2 0.4 0.4 0.4 0.6 0.6 0.6 0.8 0.8 0.8 1 1 1 D D D D D D