Higgs Results from LEP2. University of Wisconsin January 24, 2002 WIN 02

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1 Higgs Results from LEP2 Peter McNamara University of Wisconsin January 24, 22 WIN 2

2 No Higgs? In December, 2, New Scientist wrote: The legendary particle that physicists thought explained why matter has mass probably does not exist This prompted a Times article claiming:... the whole investigation has been a wild goose chase. Has LEP demonstrated that the Higgs does not exist??

3 Electroweak Predictions New Scientist s claim was based on the LEP Electroweak Working Group fit 6 4 theory uncertainty α had = α (5).276± ±.2 χ 2 2 Excluded Preliminary 2 4 = [GeV] GeV From EW fit Direct Search Limit: > 4. GeV BUT EW fit upper limit: < 96 GeV Still some room for a Higgs boson!

4 Direct Searches Additionally, in November 2, LEP reported a 2.9 sigma excess in the Higgs search -2 ln(q) LEP TOTAL Nov Observed Expected background Expected signal + background (GeV/c 2 ) The data was consistent with the presence of a Higgs boson of mass 5 GeV How does this lead to claim that there is no Higgs? Take a closer look at the Higgs search

5 LEP For the last several years, the Large Electron-Positron Accelerator (LEP) outside Geneva, Switzerland has been used to search for the Higgs boson. LEP has been run with steadily increasing collision energy, and thus Higgs mass sensitivity, since 996. Year Energy (GeV) Lower Limit (GeV/c 2 ) < ??? After very successful years of operation, LEP was shut down in November 2.

6 Higgs Production At LEP, Higgs is predominantly produced via the Higgs-strahlung process e - Z * Z e + H Because of need to produce Z Bosons, Higgs production is limited to s m Z cross section (pb) =3 =4 = center-of-mass energy

7 Topologies branching fraction... _ bb τ + τ - gg _ cc - + W W ZZ In the LEP range, the Higgs decays predominantly to b b and τ + τ γγ 2 m H (GeV/c2 ) When decays of the Z are considered, several different topologies are possible Four Jet (65%) b q Missing Energy (2%) b ν b q b ν Leptons (7%) - - b e, µ Taus (9%) τ - b e +, + µ τ +

8 Events / 3 GeV/c Events Selected Applying the analyses to LEP data collected in 2, s = 2-2 GeV all cnd= 87 bgd= 88. sgl=.4 LEP loose background hz Signal (m h =5 GeV) > 9 GeV Events / 3 GeV/c 2 5 s = 2-2 GeV all cnd= 6 bgd= 6.32 sgl= 5.75 LEP medium background hz Signal (m h =5 GeV) > 9 GeV Events / 3 GeV/c s = 2-2 GeV LEP tight background hz Signal (m h =5 GeV) 2 all cnd= 22 bgd= 2.6 sgl= 2.69 > 9 GeV Reconstructed Mass [GeV/c 2 ]

9 Likelihood Ratio Interpretation of data is not simple event counting Instead, a Likelihood Ratio is constructed: ln Q = ln L(s+b) L(b) = s + ( + sf s(m i ) bf b (m i ) ) ln(+s/b) ln(+s/b) This is a sum of event weights: ALEPH (GeV) 2 7 L (GeV) ln(+s/b) ln(+s/b) DELPHI (GeV) OPAL (GeV) Excess primarily from a few high weight candidates

10 ALEPH ALEPH bbqq Candidate = 4.3 GeV DALI_F ECM=26.7 Pch=83. Efl=94. Ewi=24. Eha=35.9 BEHOLD Run=54698 Evt=488 Nch=28 EV= EV2= EV3= ThT= 6 4 2:32 Detb= E3FFFF Gev EC (φ 38)*SIN(θ) 5 Gev HC o o o o x x ox o x o x oo ox o x x o oo xo o x 5 GeV x cm cm X" P>.5 Z< D<2 F.C. imp. θ=8 θ= RO TPC.3cm.6cm Y" µ o o o o o x x o x o x x o o o o o x End of detector tracks

11 Summer 2 Update In July 2, LEP updated the Higgs combination All data now included ALEPH and L3 included improved treatment of very high weight candidates L3 final result, including reanalysis of Four Jet and Missing Energy channels -2 ln(q) LEP Observed Expected background Expected signal + background Test signal + background (GeV/c 2 ) Minimum shifted from 5 GeV 5.6 GeV

12 Result by Experiment Excess comes predominantly from ALEPH data -2 ln(q) ALEPH -2 ln(q) DELPHI ln(q) Observed Expected background Expected signal + background (GeV/c 2 ) L3-2 ln(q) Observed Expected background Expected signal + background (GeV/c 2 ) OPAL Observed Expected background Expected signal + background (GeV/c 2 ) Observed Expected background Expected signal + background (GeV/c 2 ) At 5.6 GeV: Experiment 2 ln Q ALEPH 5.59 DELPHI +3.4 L3.26 OPAL.48

13 Probability density Significance Large numbers of simulated experiments are generated for each Higgs signal mass Observed LEP Expected for background Expected for signal ( =5.6 GeV/c 2 ) + background ln(q) At 5.6 GeV: signal+background and background-only Integrating over the log-likelihood ratio distribution for background-only -CL b Observed Expected for signal+background Expected for background LEP (GeV/c 2 ) 2σ 3σ 4σ At 5.6 GeV, observed log-likelihood corresponds to 2.σ Excess

14 Higgs Limit We can also set a lower limit on the Higgs mass CL s - LEP Observed Expected for background (GeV/c 2 ) Higgs masses below 4. GeV/c 2 are excluded at the 95% confidence level (with a limit of 5.4 GeV/c 2 expected)

15 Recent Developments In December, ALEPH released their final results Result incorporated: Final detector alignment Improved LEP energy calibration Increased simulation statistics Beam background rejection Preliminary ALEPH results confirmed 2.82σ excess seen near = 5 GeV (was 2.96σ from ALEPH in November 2) Final Result of L3 was included in July combination Results of DELPHI and OPAL are expected to be finalized soon A final LEP combination should follow shortly thereafter

16 MSSM Higgs In addition to the Standard Model Higgs, there are a large number of other Higgs models which have been examined at LEP For example, in the MSSM, Higgs is produced either through Higgs-strahlung as in the SM or through associated pair production e - Z * Z e - Z * A e + h e + h These processes are complementary: σ hz = σ HZ sin 2 (β α) σ ha σ HZ cos 2 (β α)

17 -CL b ha Results Looking for the production of ha only LEP GeV Preliminary Observed Expected, Signal Expected, SM 5σ m h +m A (GeV/c 2 ) An excess near m h + m A = 87 GeV causes limit to be 7 GeV below expected CL s CL s =.5 Observed Expected m h +m A (GeV/c 2 )

18 MSSM Exclusion tanβ In the conservative max-m h benchmark LEP GeV Preliminary m h -max Excluded by LEP Theoretically Inaccessible m h (GeV/c 2 ) m h > 9. GeV (m h > 94.6 GeV expected) m A > 9.9 GeV (m A > 95. GeV expected).5 < tan β < 2.4 Excluded

19 MSSM Exclusion tanβ In the no mixing benchmark scenario LEP GeV Preliminary No Mixing Excluded by LEP Theoretically Inaccessible m h (GeV/c 2 ) m h > 9.5 GeV (m h > 95. GeV expected) m A > 92.2 GeV (m A > 95.3 GeV expected).7 < tan β <.5 Excluded

20 Charged Higgs In the MSSM, the Charged Higgs is usually too heavy to be produced at LEP However, light Charged Higgs can be produced in general 2 Higgs Doublet models e - Z * H + e + H It is assumed that the Charged Higgs decays only to c s or τ + ν The branching ratio is left free, and the search is performed over a range of + and BR(H + τ + ν)

21 -CL b -CL b L3 Excess L3 sees a large ( 4σ) excess in H + H c s cs at + = 68 GeV Observed Expected background L3 preliminary Br(H τν)= (GeV/c 2 ) Other experiments see no excess Observed Expected background Expected signal ADO Br(H τν)= (GeV/c 2 ) When combined, excess reduced below 2σ and region is easily excluded.

22 Charged Higgs Exclusion Br(H τν) LEP GeV charged Higgs mass (GeV/c 2 ) BR(H + τ + ν) Observed Expected Limit Limit Any

23 XSEC/SM BR(h hadrons) Flavor Independent Decays In 2 Higgs Doublet Models, the Higgs coupling to b b can be suppressed Higgs with then decay to light quarks or gluons - LEP PRELIMINARY HZ FLAVOUR BLIND HYPOTHESIS s 29 GeV observed expected for background m h (GeV) For σ/σ SM BR(h hadrons) =, m h > 2.9 GeV (m h > 3. GeV expected)

24 Yukawa production Another production process possible in 2HDM s is Yukawa production e - Z * f h/a e + f Cross section for this process can be large For f = b or τ: σ Y ukawa,h sin 2 α/ cos 2 β (ξ h d )2 σ Y ukawa,a tan 2 β (ξ A d )2

25 Yukawa production Loop diagrams involving h or A in such a scenario could explain the recent (g-2) measurement ξ d 95 ξ d 95 a µ (exp) a µ (SM) = 43 ± a) OPAL Observed Limit Expected Limit δa µ * - Two-Loop Contribution b) Observed Limit Expected Limit δa µ * - One-Loop Contribution m A [GeV] m h [GeV] δa µ * δa µ *

26 Invisible Higgs In some models (e.g H neutralinos), the decay products of the Higgs will be invisible Ratio to SM rate Observed Median Background M H, GeV/c 2 LEP sets a 95% CL limit of > 4.4 GeV ( > 3.6 GeV expected)

27 Fermiophobic Higgs Although H γγ has a very low branching ratio in the Standard Model In some models the process is significantly enhanced Upper Limit on B(h γγ) - -2 Photonic Higgs Search ADLO Combined Preliminary Excluded Region +2 sigma expected median expected -2 sigma expected Fermiophobic BR Limit = 8.2 GeV Expected Limit = 9. GeV M h (GeV)

28 Conclusion Rumors of the death of the Higgs have been greatly exaggerated In the Standard Model: EW fit: < 96 GeV Direct Search: > 4. GeV 2.σ excess at 5.6 GeV Max m h m h > 9. GeV m A > 9.9 GeV tan β excluded: In the MSSM: Other Higgs Models: No Mixing m h > 9.5 GeV m A > 92.2 GeV tan β excluded:.7-.5 Charged Higgs Flavor Independent Invisible Higgs Fermiophobic + > 78.6 GeV m h > 2.9 GeV > 4.4 GeV m h > 8.2 GeV