Search for physics beyond the Standard Model at LEP 2

Transcription

1 Search for physics beyond the Standard Model at LEP 2 Theodora D. Papadopoulou NTU Athens DESY Seminar 28/10/03 1

2 Outline Introduction about LEP Alternatives to the Higgs mechanism Technicolor Contact Interactions Leptoquarks R-parity Violation Extra Dimensions Conclusion As an Epilogue 2

3 L E P e + e - Collider at CERN 12 years of data collection with excellent performance! V *H9 V *H9 V *H9 3 ALEPH, DELPHI, L3, OPAL

4 L E P Performance The updates and combinations of the final results are at the last stage LEP Working Groups : for combination of measurements 4

5 L E P Performance A D L O 5 LEP 2 : E cm = GeV Total int. Lum ~ 2.5 fb -1 (ADLO)

6 L E P Physics High precision extensive tests of the SM have been achieved to ~ 0.1 % N = ± LEP 1 Summer

7 LEP Combined m W and W results ± ± ± ± m W (LEP) = (stat) (sys) GeV W (LEP) = (stat) (sys) GeV 7

8 Comparison with Other Results SM consistency between direct measurements and predictions from radiative corrections. Low Higgs mass preference. 8

9 Direct vs. Indirect results Electroweak fit LEP final results for SM Higgs : m H > GeV at 95 % CL Important LEP legacy! 9 Higgs limit : m H < 219 GeV at 95 % CL

10 Higgs mechanism & alternatives Elementary Higgs Spontaneous Symmetry Breaking - SM Higgs ( 1 Higgs doublet) - MSSM Higgs, 2HDM ( 2 Higgs doublets) ex: Search for Fermiophobic Higgs - other models Dynamical Symmetry Breaking ( Compositeness) - Technicolor - Chiral Color Theories (High Color scenario) ex: Search for Axigluons Physics beyond SM 10 ½ Experimental results?

11 Fermiophobic Higgs 2HDM Model : 2 Higgs doublet Type 1 Model : One doublet couples to fermions, the other to bosons 2 CP even neutral Higgs bosons : h (light), H ( Heavy) mixes with scalar field with angle coupling to fermions is cos( ) for h & sin( ) for Therefore h is fermiophobic in the limit We search for the light h : ½ h Z with h, qq, l l, ½ h A with h, bb ½ h A with h, hz, Z qq α π 11

12 Fermiophobic Higgs DELPHI limit : m h > GeV at 95% CL Assuming SM + No Couplings to ff: LEP Combined limit at 95 % CL m h > GeV/c 2 12

13 Fermiophobic Higgs = - 13

14 Technicolor New (strong) interactions : ( TC >> QCD ) Technifermions T ( coupled to W, Z ) Alternative to Higgs Mechanism EW symmetry is broken dynamically But there are problems: precision measurements of EW quantities mass of top quark flavour-changing neutral currents ( FCNC ) Extensions of TC solve the problems: topcolor- assisted technicolor (TC2) 14

15 Technicolor New particles : Technicolor scalar and vector meson light enough to be observed at LEP D techidoublets 1) Search for e + e - > * > W L, 4 jet final state semileptonic decays : l q q q q 15

16 Technicolor 2) Search for e + e - > ( ) with M < s p 0?? b b ( ~ 90 %) b tagging e + e - > ( ) with M > 2? WW W W L L additional contribution to W + W - cross section e + e - > ( ) hadrons ( q q, ) 16

17 Technicolor with DELPHI Excluded at 95% CL : 90 GeV < m < GeV/c 2 m > 79.8 GeV/c 2 17

18 Technicolor with OPAL m ( ) > 62 GeV at 95 % CL (upper limits ) ~ fb 18

19 Technicolor Comparison between LEP 2 and CDF Exclusion Plots 19

20 Contact Interactions signatures of conventional compositeness at LEP 2 Contact interactions Parametrisation of new physics due to composite fermions or New heavy mediators ( m X >> V ) is the energy scale e f of new physics J / HII = ηlm HLγ µ HL I Mγ µ I + δ Λ e L M= / 5 Limits set by looking for deviations from SM predictions f 20 M g = unknown coupling g 2 / 4 = 1 = 1 for Bhabhas, 0 otherwise Observables: 2-fermions cross-sections [ σ(e + e - f f ), dσ/dcosθ], Asymmetries, R b,r c, limits +, -, correspond to constructive, destructive interference with SM

21 2-Fermions Cross-Sections and Asymmetries 21

22 Contact Interactions > 9 TeV 22

23 Contact Interactions 23

24 Leptoquarks b e ( ) 24 LQ s ( e )

25 Leptoquark at LEP 1 m LQ > 65 GeV ( 1rst-gen) m LQ > 73 GeV ( 2nd-gen) at 95 % CL Single LQ production gives the opportunity to explore a mass region up to ~ 80 GeV/c 2 DELPHI NTU-Athens 25

26 Pair Leptoquark at LEP 2 OPAL results V = *H9 M(LQ) limits for =1: GeV Topologies l + l - q q l q q q q for l=e,, S 1/2 (-2/3) V 1/2 (-1/3) 26

27 Single Leptoquark at LEP 2 Under the same conventions for the Couplings (Chirality) both procedures are consistent. 27

28 Single LQ at LEP 2 Pertubative Approach: ERATO generator Resolved photon : PYTHIA Doncheski et al. cr-section For λ = πα : m( LQ) > GeV at 95 % CL 28

29 Event e + e - e - + Jet DELPHI ( &0 *H9 Signature : e - + jet => M (e j ) ~ 180 GeV 29

30 Single LQ at LEP 2 LQ ( Q = 1/3,5/3 ) > LQ ( Q = 2/3,4/3 ) For λ = πα : m( LQ) > GeV at 95 % CL 30

31 Indirect LQ search at LEP 2 H + H T T with LQ /squark exchanges in t/u channels t-channel exchange F = 0 u-channel exchange F = 2 For λ = πα at 95 % CL : m( LQ) > GeV (scalar) m( LQ) > GeV (vector) ½ Like four fermion contact interactions at scale >> V 31

32 LQ limits 6 / UVW JHQ From EPS-2003, A. Zarnecki 32

33 R-parity Violation What is Rp and why go beyond? ½ a discrete multiplicative symmetry in SUSY models connected to matter parity R p = 1 for SM particles R p = -1 for SUSY particles o SUSY particles produced in pairs o LSP is stable o experimental signature of SUSY E T miss fast proton decay is suppressed 33

34 L MN L M N RpV Superpotential Rp can be explicitly broken by trilinear terms in the superpotential := λ / / ( + λ / 4 ' + λ L MN L M N L MN 8 ' / / % L M 27 Couplings 9 Couplings M N 9 Couplings L M ' N o single sparticle production via a L 0 or a B 0 operator o Unstable LSP! o Signature of multilepton or multijet events in excess fast proton decay is suppressed if Lepton and Baryon number Violating Couplings not simultaneously present 34

35 Decays via trilinear RPV Couplings Decay topologies 9 ijk + 27 ijk + 9 ijk = 45 new couplings Hierarchies in RpV Couplings expected ( as for Yukawa Couplings generating fermion masses) direct decays indirect decays multileptons - multijets 35

36 LSP Decay Length Decay Length of aχ / Pa I λ *H9 Pa χ *H9 LEP analyses are sensitive only if the LSP has a negligible lifetime ( L < 1 cm ) ½ a > *H9 P [ ~ 10 5 < < Displaced Vertices < < indirect SM bounds 36

37 Search strategies ½ Suppose one Coupling i j k ( ) 0 ½ Consider that many channels have to be combined ½ Optimize Signal selection on various topologies --with sequential cuts : ALEPH, OPAL, L3, DELPHI //( --using lepton identification and lepton isolation criteria //(/4' -- with neural network methods : DELPHI -- using jet algorithm, b tagging ½ Calculate signal reconstruction efficiency -- optimization on different mass combinations depending on the decays and the kinematics ½ If no significant deviation from the SM, set 95 % CL limits on: *cross-sections * couplings and sparticle masses * exclusion plots in the MSSM regions 37 8 ' ' /4'8 ' '

38 Minimal SUSY scenario : Theoretical Framework of searches Topologies predicted in a Constrained MSSM => gaugino mass unification ( M M 2 ) at EW scale Mass universality at GUT scale => Trilinear terms are set to 0 => A b,t, = 0 ixing angles for stop and sbottom => Bounds on RpV Couplings at EW scale: : ~ : ~ (131) up to 0.56 (232) : ~ 0.5 up to ~1.23 (except 112 = 121 ~ 2(10-9 ), 131 = 113 =10-4 ) ( for a sparticle mass of 100 GeV/c 2 ), tan, m 0 φa W φa E CMSSM 38

39 resonant sneutrino production DELPHI ALEPH e + e - channel λ λ + - channel λ = λ + - channel λ = λ 39

40 Single gaugino production a ν Resonant sneutrino production µ ν τ probes masses up to E cm DELPHI a Single gaugino production Analysis of final states : 3 topologies σ λ L M N ½ 2 leptons and E miss ½ 4 or 6 leptons (with or without E miss ) ½ leptons + jets Couplings 121, 131 limits on for tan =1.5 or

41 Single sneutrino production a ν a ν a µ ν τ single sneutrino production : ALEPH H H γ a ν O M N Couplings 1jk and 231 multi lepton final states ( direct / indirect ) 6 analyses Upper limits on ~

42 2, 4 and 6 leptons (+ Emiss) Final states Gaugino pair production 2, 4 jets +lepton(s) (+ Emiss) multijets + leptons (+ Emiss) a χ ± indirect decay is the dominant decay channel in almost all the MSSM parameter space 42

43 Limits from Gaugino searches Limits in MSSM parameter space DELPHI Scans in, 2 for different values of m 0 and tan 43

44 Gaugino mass limits A D L O ass limits at 95 % CL in GeV/c 2 aχ aχ aχ L D L L ± aχ A D L

45 ah a µ a τ slepton pair production + a a H H O O Direct decay : Indirect decay : H + H O O a χ a χ IHUPLRQV IHUPLRQV 45 =200 GeV/c 2, tan =1.5, BR=1

46 slepton pair production MSSM exclusion contours, at 95 % C.L. Cross-section upper limits, at 95 % C.L. 46

47 a H ν a a ν ν µ τ Sneutrino pair production Direct decay : Indirect decay : H H + + H H a ν a ν a a ν ν χ χ IHUPLRQV IHUPLRQV ν a ν a ν Excluded cross-section ( 4 jets ) 47 Excluded contour, at 95% CL for for indirect decays ν a H

48 Squark pair production Direct decays : T a T a => => 4 fermions Indirect decays : T T a χ a χ => => 8 fermions SSM Exclusion contours, at 95% CL 48

49 a W a E Squark pair production Mixing : a W = a W a / FRVφ + W Ta 5 VLQφ Ta a E a W φa E = Exclusion contours at 95% CL a a W E φa W = Mixing angle to vanishing coupling to Z for => min 49

50 Conclusions for RpV searches RpV has inspired new interesting scenarios of SUSY searches RpC and RpV are two complementary ways of SUSY searches Searches for SUSY with RpV performed by all LEP collaborations ( ADLO) in many channels No evidence for SUSY with RpV so far at LEP Limits on SUSY particles and RpV Couplings are set at 95% CL Limits from RpV are in the majority of cases more conservative than the RpC ones 50

51 Extra Dimensions Why extra dimensions? Why not? String people believe that d = 10 Extra dimensions compact on some size R ~1 / M p Why is gravity so weak? M P = 1/ G ~ N 1018 GeV Scenario ½ SM particles are confined to space time dimension ½ Gravity propagates in 4 + dimensions 51

52 Extra Dimensions : Motivation ½ Gauss law must work in 4 and 4 + dimensions M P r vs m D r M D is the fundamental Planck scale when r = R Hence ' 5 If M D ~ 1 TeV, we have = 1 Å R = m Å excluded by macroscopic gravity d = 2 Å R = 1.0 mm Å limit of small-scale gravity experiments d = 7 Å R = 1.0 Fm => Extra dimensions are compactified over R < 1 mm 52

53 Extra - Dimensions Arkani-Hamed, Dimopoulos, Dvali ( ADD ) G G Bulk Our world is ( 4 + d ) dimensional SM fields live on a 4 brane while gravity can propagate in the d extra compactified dimensions( bulk ) ½ The gravity is strong in the bulk but weak at the 4-brane ½ R can be large ( ~1 mm ) Infinite tower of KK excitations of Graviton G k investigation in the production cross-section of pairs at the LHC W W 53

54 Extra - Dimensions signature is + miss Direct search at DELPHI 54

55 Indirect Search in LEP Virtual G Exchange : Extra - Dimension H H * + I I = -1 =+1 Best fit 55

56 Extra Dimensions Combined Limits at LEP for indirect search OPAL preliminary M D ( =+1) > 1.03 TeV M D ( =-1) > 1.17 TeV 56

57 Conclusions Results from 4 LEP experiments are consistent and in good agreement with each other. Preliminary LEP combined results are in good agreement with SM expectations No evidence for New Physics at LEP Interesting new physics is expected to appear at the TeV range at LHC & NLC 57

58 As an Epilogue HERA Tevatron LEP 2 LHC & NLC??? desert? New Physics? ½All hopes are shifted to future colliders! Ithaki gave us the rewarding travel! C. Kavafys 58