Searches at LEP. Tom Junk Carleton University Ottawa, Canada. RADCOR 2000 September 12, 2000

Transcription

1 Searches at LEP Tom Junk Carleton University Ottawa, Canada RADCOR 2 September 12, 2 With many thanks to the ALEPH, DELPHI, L3, and OPAL collaborations, and the Accelerator Divisions at CERN Introduction and Context Higgs Searches SM, MSSM, H ±, H invisible, H γγ SUSY Searches Gauginos, squarks, sleptons, GMSB Beyond the SM Check of the photon recoil Prospects

2 Performance of LEP in 2 2/9/ Luminosity delivered per experiment (pb -1 ) Centre-of-mass energy (GeV) Follow the news at What s new in 2 More RF volts: 365 MV Bending-Field Spreading with Correctors Mini-Ramps at the ends of fills Approximately 15 pb -1 /experiment in 2 analyzed and combined for this presentation (results as of 5 Sept. LEPC) Average E CM : 25.9 GeV. 6.4 pb -1 /experiment above 27.5 GeV

3 The Standard Model Landscape at LEP2: Backgrounds Four-Fermion Backgrounds, W + W - and Z Z are important to Higgs/SUSY searches. σ~2 pb (W + W - ), 1 pb (Z Z ) Two-photon processes have large crosssections -- more important for SUSY searches with small m (to be explained later)

4 The SUSY Spectrum SM Particles +Higgs R P = 1 R P = -1 SUSY Partners γ,z,h,h,a Four Neutralinos ~ 1 ~ 2 ~ 3 ~ 4 W ±, H ± gluon Two Charginos gluino ± 1 ± 2 graviton udscbt gravitino squarks ~ u~ d ~ s ~ c ~ b ~ t e, µ, τ sleptons ~ e ~ ~ e sneutrinos ~ e ~ ~ : Search presented in this talk

5 Confidence Levels and the Likelihood Ratio First: Select a hypothesis of new physics to test. One parameter to order experimental outcomes Does the experiment look Signal-like: more candidates Background-like: fewer candidates or Somewhere in Between? Sometimes: Too many candidates even for signal! or Too few candidates even for background Constructed from binned search results: Each bin has Estimated number of signal events Estimated number of background events Number of events observed in the data L = P poiss ( data signal + background) ( data background) P poiss + data log L s tot ni log 1 + = i bins bi s

6 Confidence Levels and the Likelihood Ratio Some searches just count events -- one bin! Other searches reconstruct masses, tag B s etc -- Many bins with different s/b s. Each bin is an independent counting experiment (a separate analysis with different cuts ) Can add bins with the same s/b Use MC or analytic techniques to find the PDF of logl, given the results of all bins. -- Easy to combine many searches for the same particle -- different analyses, ECM, and experiments all can be combined for more search power. CL CL P ( L L signal background ) s + b = obs + = P ( ) L L b obs CL = CL / CL s s+ b b background

7 Electroweak Fit Constraint on m H 6 4 theory uncertainty α had = α (5).284± ±.46 χ 2 2 Excluded Preliminary m H [GeV] Logarithmic sensitivity to sin 2 θ W Current central value: m H = % CL Upper limit GeV Results as of ICHEP 2 m H <17 GeV

8 Consequences of Believing the Standard Model up to the Planck Scale Λ is the scale at which interactions beyond the Standard Model become important Upper Higgs mass bound -- Landau pole Lower Higgs mass bound -- vacuum stability

9 Fine-Tuned Radiative Corrections in the SM Kolda and Murayama, hep-ph/317 One-loop correction to µ 2 is proportional to Λ 2, which destabilizes the weak scale, unless m h is exquisitely chosen Λ ( µ µ + 2m 4 ) 2 2 W + mz + mh mt 32π v 6 Triviality 5 Higgs mass (GeV) % 1% Electroweak 1 Vacuum Stability Λ (TeV) Provocative, but experimentalists don t believe pretty theories until the particles show up.

10 Standard Model Higgs Production Higgs-Strahlung (BJ process) Has a kinematic cutoff at s m Z e + e - Z * Z (resonant) H Fusion e + W + (Z) ν e (e + ) No kinematic cutoff, but low cross-section e - W - (Z) H E C M= 26 GeV Hig g Total Fus ion sstrahlung Interference The Higgsstrahlung Wall ν e (e - ) Note: only Hνν x-sect shown. Multiply HZ by ~5 to get total.

11 Standard Model Higgs Decays Branching Ratio bb ττ gg cc WW ZZ tt m [GeV] H SM Higgs Decay, at 11 GeV < m H <115 GEV b-quarks ~8% W + W - ~8% and rising quickly tau pairs ~7% charm and gluons -- the rest B-tags are very important

12 SM Higgs Search Channels Each Higgs search channel is named after the Z decay mode. b H bb b 6 % b 18 % H bb b q Z qq q ν ν Z νν H bb b H bb( ττ) b _ l 6 % b + l Z ee, µµ 9 % τ _ Z ττ(qq) b + τ

13 Exchanging LEP Higgs Results for Combination Results are binned in an experiment s choice of discriminant variables (reconstructed mass, b-tag, kinematic variables, or a combination of these) For each bin: signal estimation background estimation data counts For each search analysis channel: signed relative error on the signal and background itemized by named source examples: B decay modeling, ZZ x-sect, MC comparisons Effect of including errors Uncorrelated: Almost no effect on limits 1% Correlated: 2-3 MeV lower expected SM limit Properly correlated: ~1 MeV lower expected SM limit We have prepared for a discovery!

14 Distributions of the Reconstructed m H Events / 3 GeV/c s = 2-21 GeV LEP S/B=.3 background hz Signal (m h =114 GeV) 5 cnd= 199 bgd= 18.5 sgl= Reconstructed Mass m H [GeV/c 2 ] Loose Cuts: Def n: s/b=.3 for m h,rec >19 GeV Signal model: m h =114 GeV Data Background Signal ALEPH DELPHI L OPAL LEP

15 " $ & Distributions of the Reconstructed m H Events / 3 GeV/c s = 2-21 GeV LEP S/B=1. background hz Signal (m h =114 GeV) 4 2! cnd= 63 # bgd= % sgl= Reconstructed Mass m H [GeV/c 2 ] Tighter Cuts: s/b=1. for m h,rec >19 GeV Signal model: m h =114 GeV Data Background Signal ALEPH DELPHI L OPAL LEP

16 * - ' ( Distributions of the Reconstructed m H Events / 3 GeV/c s = 2-21 GeV LEP S/B=2. background hz Signal (m h =114 GeV) 2 1 ) cnd= 27 + bgd= 22.19, sgl= Reconstructed Mass m H [GeV/c 2 ] Very Tight Cuts: s/b=2. for m h,rec >19 GeV Signal model: m h =114 GeV Data Background Signal ALEPH DELPHI L OPAL LEP

17 8 6 Signal, Background, and Candidates as a function of s/b 1 ADLO m H 5 =115. GeV 217 7Data Background Signal 12. Events Expected Background / Background 6 Signal+Background Expected Signal Final Variable log1(s/b) Expected Background 3 / Background 6 Signal+Background Expected Signal Three most significant candidates -- ALEPH four-jet channel taken at the highest ECM s. Two fairly significant DELPHI four-jet events too.

18 An ALEPH Four-Jet Candidate ALEPH DALI_F1 ECM=26.7 Pch=83. Efl=194. Ewi=124. Eha=35.9 mydata Run=54698 Evt=4881 Nch=28 EV1= EV2= EV3= ThT= :32 Detb= E3FFFF 16.Gev EC 7.9Gev HC Y 1cm 1cm X YX (φ 43 )*SIN(θ) x x x x o o o o o o x o o x x o x o o x o o o o o x x o o o oo x x o o o x x o x x 28. GeV θ=18 θ=

19 : Excess is mostly in the ALEPH 4-Jet Channel -2ln(Q) 1 < 7.5 ; Background Only Observed Background+Signal ALEPH M H (GeV) Systematic checks done: Cut-based vs. NN analysis Test of m rec bias towards kinematic endpoint (no effect seen at lower E CM ) B-tag modeled well, NN modeled well. Excess goes away when one cuts out events with a pairing consistent with Z Z or W + W -, but this does not separate signal from background.

20 > A Also a small excess in the DELPHI Four-Jet Channel -2ln(Q) 7.5? = Background Only Observed DELPHI Background+Signal MH (GeV) But not in L3 or OPAL s four-jet channels -2ln(Q) C5 Background Only Observed -2ln(Q) E5 Background Only Observed 2.5 B 2.5 D L3 Background+Signal OPAL Background+Signal M H (GeV) M H (GeV)

21 S L N Missing-Energy, Lepton, and Tau Channels -2ln(Q) 1 H7.5 G5 2.5 F Background Only Neutrinos Background+Signal Observed -1 I I J (GeV) M H -2ln(Q) N1 7.5 M5 2.5 K Background Only Leptons Background+Signal Observed -1 N N O (GeV) M H -2ln(Q) Q5 2.5 P -2.5 RBackground Only Observed 3σ Observation Probability 1 T -1 1 U4-Jet LNT VADLO Taus Background+Signal M H (GeV) M H (GeV) Combined Missing-Energy, Lepton, and Tau channels fairly powerful!

22 The Combined Likelihood Ratio All the information that s available! -2ln(Q) Background Only Observed ADLO Background+Signal M H (GeV) Higgs boson of mass 114 GeV preferred over the background hypothesis by 2.6σ -- approximately the level of significance expected for a 114 GeV signal.

23 Confidence Levels CL s and 1-CL b For the combination of all Higgs search results CLs WObserved Median Expected Exclude up to GeV Median Expectation: GeV Y ADLO X X114.5 M H (GeV) DLO only: obs exp. 1-CLb CL b Minimum at 2.6σ significance at GeV -- but -2lnL minimum at GeV ADLO Z3σ M H (GeV)

24 Standard Model Limit Summary Experiment Observed (GeV) Expected (GeV) ALEPH DELPHI L OPAL Leptons 19.9 * 18.8 Neutrinos Taus Four Jets LEP (*) Small unexcluded region below 1.7 GeV All results are preliminary, but they are computed consistently between experimets and channel.s from exchanged results. Individual experiments limits differ slightly.

25 ` ` LEP Extension Scenarios As functions of the luminosity collected at GeV, i.e., one klystron margin s = 26.6 Case 1: Assuming no signal Expected 1-CLb ^ M H =116 3σ M H =115 4σ _ M H =114 5σ M H =113 [ \ ] Additional 26.6 GeV (pb -1 ) Case 2: Assuming a signal is present Expected 1-CLb ^ M H =116 3σ M H =115 4σ _ M H =114 5σ M H =113 [ \ ] Additional 26.6 GeV (pb -1 )

26 The Neutral Higgses of the MSSM Two Higgs Doublets: 5 Higgses h light CP-even Higgs H heavy CP-even Higgs A CP-odd Higgs H +, H - Charged Higgs m h < ~135 GeV e + e - Z * sin( β α) Z h Higgs-strahlung σ = sin 2 ( β α) SM hz σ hz And fusion processes too! e + e - Z * cos( β α) A h Associated Production σ = cos 2 ( β α) λ SM ha σ hz λ : kinematic factor (mh, ma, s)

27 e c d q b i l m j k i MSSM Higgs Limits m h -max scenario -- using FeynHiggs calculations courtesy of Heinemeyer, Hollik, Weiglein including leading two-loop contributions to m h. m A (GeV/c 2 ) f h mg -max Theoretically Inaccessible tanβ 1 nmoh-max Excluded by LEP 6 tanβ 4 2 Excluded by LEP a (GeV/c 2 ) m h o -max m h 1 Theoretically Inaccessible m h h (GeV/c 2 ) Mass Limits (GeV) 1 Mq SUSY =1 TeV Mr 2 =2 GeV µ=-2 GeV m gluino =8 GeV Stop mix: X t s =2M SUSY obs. exp. m h > m A > Excluded kby LEP m A p (GeV/c 2 ) tanβ exclusion obs exp

28 u t { y z ƒ w x Š } ~ MSSM Higgs Limits No stop mixing scenario also with FeynHiggs. Soon a casualty? Or maybe not! Interesting features at low and high tanβ m A (GeV/c 2 ) No Mixing Theoretically Inaccessible tanβ 1 No Mixing Excluded by LEP tanβ 6 4 Excluded 2 by LEP t v (GeV/c 2 ) 1 m h No Mixing 1 Theoretically Inaccessible m h (GeV/c 2 ) Mass Limits (GeV) 1 Excluded by LEP SUSY M =1 TeV Mˆ 2 =2 GeV µ=-2 GeV mgluino =8 GeV No stop mixing: X = t obs. exp. m h > m A > m A (GeV/c 2 ) tanβ exclusion obs exp

29 Ž MSSM Higgs Limits Large-µ Scenario -- Uses Subhpole2 by Carena and Wagner --RG-improved 1-loop calculation. More reliable for high tanβ and large µ. Designed to be challenging! Br( h bb ) for some points, and the tau b.r. is not enhanced. W + W -, charm, and gluons make up the difference. Need to combine flavor-independent searches. -- LEP has these, but we need to combine them. tanβ 1 Unexcluded Large Mu Scan M SUSY=4 GeV M 2 =4 GeV µ =1 TeV mgluino =2 GeV =-3 GeV X t 1 Excluded by LEP Œ β below.7 not scanned tan m A 4 5 (GeV/c 2 )

30 Limits on Charged Higgs Production from Combining LEP Searches Large background from W + W - production -- easiest to overcome in the τντν channel. Assume Br H for the limits. ( + + ) + τ ν + Br( H cs) = 1 τ Br(H τν) ŸLEP GeV September š œ ž ž mh (GeV) Observed (median expected) Limits (GeV) vs. Br ( ) H + τ + ν τ Br = Br = 1 Any Br 8.5 (79.8) 89.2 (9.9) 78.7 (78.5)

31 ª Combination of H γγ Searches Upper Limit on B(h γγ) ADLO Combined 5 September Update Excluded Region +2 sigma expected median expected -2 sigma expected Fermiophobic BR Limit = 17.7 GeV Photonic Higgs Search M (GeV) h Really the cross-section limits assuming Br( H γγ ) = 1, normalized to the SM x-sect Can derive a mass limit assuming Br( H fermions) = Observed limit: 17.7 GeV Median expected: 15.8 GeV

32 Searches for H Invisible Particles e.g., h χ 1 χ 1 with χ 1 = LSP but no restrictions on the model. Search: similar to the SM missing-energy channel, but the visible system has mass M Z. Br H to Invisible 1 «.8 «.6 «.4 «.2 «95% CL 99% CL 9% CL Expected 95% CL LEP M H, GeV/c 2 Assuming SM production x-section and 1% invisible decays, obtain the limits Observed Limit: Median Expected: GeV GeV

33 Chargino Search Assume: ~χ 1 + has a short lifetime, R-parity conserving decays, is the LSP ~χ 1 Production: (similar to W + W - CC3 diagrams) e + + ~χ 1 e - (γ,z) * e + e - Decays: ~ + + ~χ ~ 1 ν e ~χ 1 ~χ 1 χ + χ W + or 1 1 ~ χ ~ ~ ~ Destructive interference! Diagram important for small m ~ + + ν, + χ B.R. s depend on slepton masses, m

34 Chargino Search Modes q Same as W + W - channels, but with missing energy q q ~χ 1 q ~χ 1 q q ~χ 1 ~χ 1 ~χ 1 ~χ 1 Kinematics depend strongly on M M χ M = + 1 χ 1 Low M: Background dominated by untagged two-photon processes High M: Background dominated by W + W - production. f f e + unmeas. e - unmeas.

35 ± An Acoplanar Dilepton Y Z X µ + µ with large missing p t SM interpretation: W + W µ + ν µ µ ν µ

36 Chargino Production Limits m(χ 1 ) [GeV] 1 75 (a) OPAL Preliminary ² σ<.2pb ² σ<.4pb 5 ² σ<.6pb 25 ² σ<2.pb ³ 11 m(χ ± ) [GeV] 1 Plot does not assume CMSSM Best sensitivity away from very low, very high M Extra data at 28 GeV makes a lot of difference! OPAL has an excess of 5 obs/.74 bg for the M~1 GeV search in the highest energy data, but it is not confirmed by the other experiments. Would like to keep looking!

37 Neutralino Production and Decay e + Still assume ~χ 1 is the LSP Seach for associated production of in s-channel Z exchange e - Z * ~χ 1 ~χ 2 e + + e ~ e - ~ χ ~ χ 1 2 ~χ 1 ~χ 1 q Decays: ~ χ χ ~ Z 2 1 ν ~χ 1 ~χ 1 ~χ 1 ν ~χ 1 q ~χ 1 ~χ 1 Challenging!

38 Neutralino Production Limits m(χ 1 ) [GeV] 1 75 OPAL Preliminary (b) σ<.24pb σ<.3pb σ<.5pb m(χ 2) [GeV]

39 » º À Ã Ä Á Slepton Searches e e ~ ~ + + ~ + ~ χ 1 Experimental signature : + + E/ T Neutralino Mass (GeV/c 2 ) 1 ¼ ¹ Staus  s = GeV ADLO Preliminary Observed Expected ½» ¾ ¼ Excluded at 95% CL ÀStau Mass (GeV/c 2 ) ADLO Y2K data (Osaka) Limits for m > 15 GeV m~ e m m ~ µ ~ τ > 98 GeV > 94 GeV > 79 GeV assuming tanβ=1.5, µ=-2 GeV, m~ >> m~ L R Stau excess in 1999 data not confirmed in 2 data.

40 Í Ö Ø Ì Ô stau Õ Stau CL b and also 2 M χ (GeV/c 2 ) 1 8 È6 Ç4 Staus CL(NoExcess) s = GeV Exp:ADLO and 1999 data.1% CL no signal Æ2 1-3 Å É È Ê Ë 1 M stau (GeV/c 2 ) data (Osaka result) M χ (GeV/c 2 ) 1 8 Ð6 Staus CL(NoExcess) s= GeV Exp:ADLO CL now fine. Ï2 1-3 Î Ñ Ð Ò Ó 1 M (GeV/c2 ) 1-4

41 Check of Light Sbottom Searches July 2 LEPC: Very very very preliminary request from ALEPH to DLO to check a fresh analysis. b-jets with leptons at LEP2: 56 obs/33.6 expected in 58 pb -1 or 39 obs/23. expected in 411 pb -1 check analysis. Since then, OPAL and DELPHI have sought the same signal. OPAL sees a deficit: 15/2.5, somewhat lower efficiency: 15% (vs. ALEPH ~25%) DELPHI also does not see an excess. ALEPH updated with new study with more appropriate lepton ID for leptons in jets, finds no excess for September 5 LEPC 24 obs/2 expected in 411 pb -1 Rumors travel too quickly! and leptons in jets are hard to identify and model

42 Ù Þ Model-Independent Check of the Photon Recoil Distribution Sensitive to a variety of new signals possible, and can be dominant 2 χ1γ in the MSSM, depending on params ~ χ ~ ~ χ G ~ 1 γ ν * νγ With GMSB, the Gravitino can be LSP Radiatively decaying excited neutrinos Anything invisible + ISR. Events / (4 GeV/c 2 ) Ýpreliminary ÚÚννγ(γ) Û 13 Ü Û s 28 GeV ALEPH DELPHI L3 OPAL Events / (4 GeV/c 2 ) ãpreliminary ßßννγγ(γ) á â á s 28 GeV à13 ALEPH DELPHI L3 OPAL Recoil mass (GeV/c 2 ) Recoil mass (GeV/c 2 )

43 Interesting Signatures of GMSB Gravitino LSP -- Search for the NLSP NLSP lifetime is arbitrary! Prompt decays -- reinterpret standard SUSY searches Delayed decays -- interesting class of new analyses: Tracks with large impact parameter Kinked tracks non-pointing photons Anomalous ionization of stable particles ~ τ R mass limit GeV/c 2 è1 ç9 æ8 7 å6 ä5 ALEPH - Preliminary Combination of searches NLSP mass limits m m ~ 1 >> χ ~ τ GeV GeV é lifetime [s]

44 LEP Extension Plans Uncertain! September 16 to Oct 1 is already an extension over the original LEP plan. Granted shortly after the July 2 LEPC meeting. Dismantling scheduled to start Oct 1. Four experiments and the LEP Higgs Working Group jointly request an extension to double the integrated luminosity at 26.6 GeV, and review the situation then. Accelerator division says the request can be fulfilled by the beginning of November (we already have some of this luminosity) LHC Schedule would be affected by a LEP extension as requested. Delayed contracts increased costs Meeting with the CERN research council on 14 September to decide.