Ricerca di nuova fisica a HERA

Transcription

1 Ricerca di nuova fisica a HERA Incontro sulla Fisica delle Alte Energie, Lecce aprile 2003 S.Dusini, INFN Padova 1

2 Overview Introduction to Hera Results from HERA I Contact Interaction: Compositness, Large extra dimensions, Quark radius eq resonances: Leptoquarks, Squark in Rp violating SUSY models, Excited fermions Single top production (FCNC) Summary for HERA I running Outlook to the future running 2

3 HERA kinematics and luminosity e ± 27 GeV p 820, 920 GeV s=318 GeV s=300 GeV Q 2 = -q 2 four momentum transfer x = Q 2 /2pq quark momentum fraction of the proton y energy transferred from lepton to γ s = Q 2 /xy ep (CM energy) 2 Data sample: e + s=300 GeV : 40 pb -1 e + s=318 GeV : 66 pb -1 e - s=318 GeV : 16 pb -1 Two experiments: ZEUS, H1 3

4 Search for new physics at HERA Direct search for new states with definite lepton and/or baryon number: eq resonances: Lepto-quark, R p violating SUSY Excited fermion FCNC rare processes Search for deviations, at high Q 2, of the measured ep cross section from the SM predictions: Contact interaction approach New physics would produce deviation from SM at high Q 2 LargeQ 2 range covered by HERA up to GeV 2 Standard Model shows good agreement over 6 orders of magnitude Spatial resolution: 1/Q max ~ cm eq ex eq νx 4

5 Contact Interaction CI formalism allows to parameterises low energy effects of new physics at TeV scale: Compositness Massive boson exchange Gravitational effect of large extra dimensions Quark radius Effective Lagrangian: (vector term only) 8coupling: 2 q g η =± 2 Λ ij q ij µ ( )( ) i i j j q L L η eγ e q γ µ q = SM + q= u, d i, j= L, R g 2 =4π ij Constrain from APV measurements q q q q η + η η η = 0 LL LR RL RR Several CI scenarios depending fermion chirality combinations: e.g. VV : η =+ η =+ η = η ; AA : η = η = η = η LL LR RL LL LL LR RL LL 5

6 CI: Q 2 fits and Λ limits Measure dσ/dq 2 and use statistical analysis method to evaluate modification w.r.t. SM expectation due to CI Curves are 95% C.L. exclusion limits for positive (+Λ) or negative ( Λ) interference term Limit range of effective mass scale Λ eeqq : ZEUS 2 7 TeV H TeV LEP & Tevatron similar results 6

7 CI: Large extra dimensions Arkani-Hamed et al. [hep-ph/ ]: SM particles propagate in 4-D, gravitons in (4+n)-D M2 p = R n M 2+n s M s scale of the 4+n theory (~1 TeV) R= size of n compact extra dim Virtual Kaluza-Klein graviton exchange interferes with γ,ζ Effective coupling η G = λ/m 4 s ZEUS limits: M S = 0.82 TeV, λ=-1 M S = 0.81 TeV, λ=+1 7

8 Quark radius Assuming electron point-like SM d σ d = σ 1 RQ 2 2 dq dq 6 2 Quark charge radius probed down to <10-16 cm, 1/1000 of the proton radius 8

9 Leptoquark Coloured bosons with B and L 0 predicted by many models (GUT, Tecnicolor, Compositeness) which extend the SM Characteristics: (BRW model) Fermion number F = L + 3B = 0,2 LQ interaction preserve SM gauge symmetry Left or Right coupling with leptons 7 scalar + 7vectorstate with fixed branching ratios to eq and νq LQ Final similar to DIS (e,ν + jet) but with different angular distribution: Scalar: flat in y Vector: (1-y) 2 DIS: 1/y 2 9

10 ν-jet and e-jet invariant mass Good agreement with SM expectations in both eq and νq channel Excess observed in NC data at high x and Q 2 not confirmed by the full HERA I statistics 10

11 Leptoquark: exclusion limits BRW model: β=br(lq eq,νq) fixed If we release the BRW conditions allowing for free β F=2 LQ NC+CC Tevatron: pair production independent of λ LEP: virtual effect in e + e - hadrons strongly depend on λ For β(eq)+b(νq)=1 limits almost independent from β Unique sensitivity for β(eq)<<1 e + p data more sensitive F=0 LQ, e - ptof=2 11

12 μu i μ D j μ D k + μ i L i H 2 R-parity violating SUSY General Superpotential contains: Relevant for HERA: 00 0 i L j μ E k + ijk L iq j μ D k + ijkl = p R= W + ed % c L Which violates Lepton and Baryon number In MSSM R p = (-1) 3B+L+2S is +1 for SM and -1 for SUSY particle If R-parity is not conserve Resonant production of SUSY particle at HERA Lightest Supersymmetric Particle (LSP) decays (not necessary P miss t topology) + + Main signatures: e + + jet (LQ-like) e + + multijets - e - + multijets wrong sign lepton signature (background free) 12

13 Limits for Rp violating SUSY λ 121 λ 111 Unconstrained MSSM Squark mass vs coupling varying the SUSY parameters λ 121 < M=200 GeV Similar results for λ 131 Minimal SuperGravity Model (msugra) - Constrained by: m 0 univ. sferimon mass at GUT scale EW symmetry breaking driven by radiative corrections M% < 260GeV nearly fully excluded u L 13

14 Excited fermions Compositeness of fermion would manifest in fermion excited states f* which decay to ground state emitting a gauge boson (magnetic coupling) Phenomenological model Hagiwara et al. σ s and BR s depends on coupling f, f, f s of the corresponding gauge group SU(2) L, U(1) Y, SU(3) c and Λ compositeness scale. Assuming relations between f, f, f s production cross section (f/ Λ) 2 f =f no νν γ vertex, f =-f no ee*γ vertex 14

15 Excited fermions: limits for f/λ Delphi, OPAL Hera is sensitive above LEP2 c.m.e. For ν* the e - p data set gives higher sensitivity: u-quark is involved instead of a d helicity suppression in e + p due to W exchange 15

16 Search of single top production in FCNC In the Standard Model Flavour Changing Neutral Current transitions are forbidden at tree level and suppressed at one-loop (GIM mechanism). Top sector is a good place to look for FCNC FCNC top decay extremely rate [BR(t cγ,z)<10-12 ] top mass close to EW scale SM single top HERA σ<1fb Extensions of SM predict anomalous effective couplings which enhance FCNC process [BR up to 10-2 ] k tuγ,, v tuz anomalous magnetic and vector FCNC top coupling to the γ and Z Leptonic & hadronic W decay considered 16

17 Summary for HERA I running HERA I : ~ 110 pb -1 e + p and ~ 15 pb -1 e - p data So far no clear evidence for physics beyond SM New constrains have been set on: Compositness: Λ eeqq > 2 7 TeV Large Extra Dimensions: M s > 0.8 TeV quark form factors: R < cm Leptoquark: M LQ > 280 GeV (λ=0.3) Squark in Rp violating SUSY: M > 260 GeV (λ=0.3) Excited fermion states: f/λ < 0.01 for M ν or M e* = 200 GeV FCNC transitions: k tuγ < 0.2 Limits are comparable/complementary to LEP and Tevatron What next. 17

18 HERA II prospective In summer HERA will restart operation after the Spring shutdown Increase in luminosity: Possibly increase e - pdatasetwhich is more sensitive to new physics Keep testing the SM and eventually set new limits Longitudinal polarisation: Enhance/disentangle possible signal since in many cases chiral leptons are involved Control background since NC and CC DIS depend on lepton chirality Upgrade of the detectors Silicon microstrips vertex detector for flavour tagging Improvement in the forward tracking 18

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