SEARCH FOR 4 TH GENERATION QUARKS AT CMS

Transcription

1 SEARCH FOR 4 TH GENERATION QUARKS AT CMS Kai-Feng Chen National Taiwan University LHC symposium in PSROC annual meeting January 6 th 0, National Normal University, Taipei

2 4TH GENERATIONS: WHY? WHY NOT? u c t tʹ d s b bʹ The possibility of 4 th generation is not really excluded by the current experimental data. Small mass splitting between the 4 th generation quarks is preferred: M t -M b <M W. Flavor physics data and the tests for unitarity triangle provide some information regarding the CKM4 matrix, but it is only weakly constrained due to the uncertainties.

3 4TH GENERATIONS: WHY? WHY NOT? The direct measurement of invisible Z width from LEP: Nν =.9 ± 0.05, but it does not guarantee that N(gen) = 3 exactly, e.g. heavy neutrino with mass > 0.5 M Z. arxiv: th gen. The electroweak fits constrain the available phase space allowed for the 4 th generations. Large impact on the Higgs sector: Heavy Higgs (up to 500 GeV) is allowed. 3

4 4TH GENERATIONS: WHY? WHY NOT? SM: sinφ Bs ~0 add ~500 GeV t : sinφ Bs ~-0.33 References: Hou et. al. arxiv: Adding 4th generation quarks will pull down the sinφ Bs value from the 3 generation SM. Agreement with data is improved, but the tension is reduced since recent Tevatron updates. Wait for the results from LHCb to verify it. 4

5 A BIG MOTIVATION: BAU Ingredients of CPV in the Standard Model: #: At least THREE generations; #: Non-trivial CP phase; Non-trivial unitarity triangle. #3: Non-degenerate like-charge quarks. Jarlskog Invariant proportional to quarks masses and triangle area A: V ud V* ub J =(m t m u)(m t m c)(m c m u)(m b m d)(m b m s)(m s m d)a 3 ( ) ( ) V cd V* cb V td V tb * ( ) The SM contributes only J/T ~ 0 n(b) n(γ) =( ) (WMAP) Something is definitely necessary to enlarge the asymmetry by O( )! 5

6 A BIG MOTIVATION: BAU If we simply shift the invariant by one generation: [ ] u c t t d s b b [ ] u c t t d s b b J =(m t m c)(m t m t )(m t m c)(m b m s)(m b m b)(m b m s)a J J m t m c m t m t By inserting M(b,t ) ~ 300~600 GeV/c, it already gives us a huge boost on J, of O( 3 ~ 5 ) m 4 b m b m s A A References: Hou arxiv: Replacing the unitary triangle contributes a factor of 30. A low cost solution: only needs heavier quarks! 6

7 FOUR STATEMENTS ABOUT THE FOURTH GENERATION Ref. Holdem et al. arxiv: ) The 4 th generation is not excluded by EW precision data; ) SM4 addresses some of the currently open questions; 3) SM4 can accommodate emerging possible hints of new physics; 4) LHC has the potential to discover or fully exclude SM4! 7

8 THE DECAY PATTERN tʹ bʹ Mostly t bw (simply a heaver top quark) or t b W Depends on the mass hypothesis of b : Rich Signatures tw mass threshold ) Larger X-sec; ) For sizable V cb b cw >> t ( * ) W ( * ) 3) Suppressed V cb b cw << t ( * ) W ( * ) 4) FCNC: b bz, bh Light Scenario Heavy Scenario b tw dominance ) Lower X-sec; ) Larger mass coverage. Today we only focus on the heavy scenario, b tw decays. 8

9 SIGNATURE OF HEAVY b b twtw The full decay chain: b b twtw bbw + W W + W (4 W-bosons + b-jets) b t b W + l + ν q q Possible final states: L: Lepton, J: Jet, MET: Missing Energy b t W W + W b l ν q q or l + ν q q l ν q q 0L+J L+8J+MET L+6J+MET 3L+4J+MET 4L+J+MET 5 possibilities in total Look for same-sign dileptons and trileptons for the first probing of the signal 9

10 CMS EXPERIMENT From Taiwan: NTU and NCU 370 scientists and engineers from 69 institutes in 39 countries.

11 ANALYSIS OF HEAVY b b twtw Ref. CMS PAS EXO--08 Data set analyzed: 34 pb at 7 TeV recorded by the CMS detector. Trigger: double electron trigger or single muon trigger. Lepton selections: Muons: cut-based ID, isolated from other activities, p T > 0 GeV/c. Electrons: cut-based ID, isolated from other activities, p T > 0 GeV/c. Requiring exact L with the same charge, or 3L in the final state. Jet selections: Anti-K T algorithm R = 0.5 with particle flow candidates. Same-sign L: at least 4 or more jets p T > 5 GeV/c. 3L: at least or more jets p T > 5 GeV/c. Other requirements: A Z-boson veto: M(ll)-M Z > GeV/c. Objects isolation: ΔR(e,μ) > 0. and ΔR(jet,l) > 0.4. S T [= MET + Σp T (jets) + Σp T (leptons)] > 350 GeV

12 RESULTING FIGURES (MC Distributions) Number of Entries b' b' 400 GeV/c same-sign dilepton (SSL) CMS Simulations Number of Entries b' b' 400 GeV/c trilepton threshold (3L) CMS Simulations Signal Observable: S T = MET + Σp T (jet) + Σp T (lep) Number of Entries / 0 GeV/c b' b' 400 GeV/c same-sign dilepton + trilepton Z-veto N(Jets) CMS Simulations M(ll) (GeV/c ) Number of Entries / 60 GeV b' b' 400 GeV/c same-sign dilepton + trilepton N(Jets) CMS Simulations MET is included in S T calculation MET (GeV) Number of Entries / 50 GeV - b' b' 400 GeV/c same-sign dilepton + trilepton CMS Simulations 400 GeV bʹ signal S T MC distributions are normalized to 34 pb (GeV) background, mainly

13 RESULTING FIGURES (MC Distributions) Number of Entries / 50 GeV - b' b' 300 GeV/c same-sign dilepton + trilepton Number of Entries / 50 GeV - b' b' 350 GeV/c same-sign dilepton + trilepton Number of Entries / 50 GeV - b' b' 400 GeV/c same-sign dilepton + trilepton CMS Simulations CMS Simulations CMS Simulations 300 GeV/c 350 GeV/c 400 GeV/c S T (GeV) S T (GeV) S T (GeV) Number of Entries / 50 GeV - b' b' 450 GeV/c Number of Entries / 50 GeV same-sign dilepton + trilepton same-sign dilepton + trilepton b mass 450 GeV/c 500 GeV/c CMS Simulations CMS Simulations - b' b' 500 GeV/c (GeV/c ) N S N B 0.33 [background sum] S T (GeV) S T (GeV) 3

14 EXPECTED YIELDS b Signal Background Sources b mass (GeV/c ) Cross section (pb) Yield S/N (NLO) (NLO) (NLO) (NLO) (NLO) S/N is high, up to 300~400 GeV/c b' masses. Background is dominated by the events. Process Cross section (pb) Yield 94 (CMS) 0.7 tt+w (+j) 0.44 (LO) tt+z(+j) (LO) 0.06 W+jets 9850 (CMS) <0. Z+jets 99 (CMS) <0.09 WW 43 (NLO) <0.0 WZ 8 (NLO) <0.005 ZZ 5.9 (NLO) Same-sign WW+jj 0.5 (LO) 0.00 MC background expectation QCD contributions are estimated to be small (<0.09) 4

15 BACKGROUND ESTIMATION WITH DATA Background Types Find a sign-flipped electron Find an extra (fake) lepton Select Select Opposite-sign dilepton events Normalized by electron charged mis-identification rate; measured using Z ee data. Signal region Same-sign dilepton events with loosened lepton criteria NB Normalized by lepton loose-to-tight ratio - measured from data. (GeV) Number of Entries / 50 GeV b' b' 400 GeV/c same-sign dilepton + trilepton CMS Simulations S T Use a data-driven based estimation instead of the MC expected value (ie. the 0.33 events) 5

16 BACKGROUND ESTIMATION WITH DATA Opposite-sign dilepton events (and preserve all other crieria) Number of Entries / 50 GeV pb data l control box CMS Preliminary (GeV) S T Number of Entries / 0 GeV/c pb data 6 W+jets Diboson Z+jets 5 opposite sign ee CMS Preliminary NB Number of Entries / 0 GeV/c 5 4 e + e e e M(ll) (GeV/ ) electron charged mis-id rate This estimation has little dependency on luminosity and background cross section. Systematic uncertainty from a MC closure test pb data 6 W+jets Diboson Z+jets same sign ee M(ll) (GeV/ ) Number of Entries / 50 GeV - e + e + CMS Preliminary Signal region b' b' 400 GeV/c same-sign dilepton + trilepton CMS Simulations (GeV) S T 6

17 DATA RESULTS Only two events failed with the last step selection Number of Entries / 50 GeV pb data b'b' 400 GeV/c same-sign dilepton + trilepton CMS Preliminary S T (GeV) Number of Entries / 0 GeV/c pb data b'b' 400 GeV/c same-sign dilepton + trilepton CMS Preliminary M(ll) (GeV/c ) Zero event found in the signal region b (400 GeV/c ) MC expected background Yields Number of Entries 34. pb data b'b' 400 GeV/c same-sign dilepton CMS Preliminary Number of Entries 34. pb data b'b' 400 GeV/c trilepton CMS Preliminary Data (34 pb ) Estimated background (data-driven) ± N(Jets) N(Jets) Full systematic uncertainties included 7

18 CMS Preliminary ST = 363 GeV MET = GeV µ (pt = 55.3 GeV/c) µ (pt =.8 GeV/c, η = 0.6) Jet (pt = 7.8 GeV/c) e + (pt = 63.8 GeV/c) e + (pt = 69.9 GeV/c) CMS Preliminary Trilepton channel Jet (pt = 54. GeV/c) e (pt = 34.7 GeV/c, η =.6) MET = 48.6 GeV ST = 65 GeV Jet (pt = 5.5 GeV/c) Jet (pt = 33.7 GeV/c) Jet (pt = 45.8 GeV/c) Same-sign dilepton channel Two interesting events, but failed with the last step selection 8

19 EXCLUSION LIMITS No signal observed in data: we set the exclusion limit at 95% C.L. We use a Bayesian limit for null hypothesis tests, with all the systematic uncertainties included: pp! b'b' cross section (pb) CMS Preliminary Limit: M(b')>357 GeV/c SM + " SM + " Observed (34. pb ) 7 TeV (Berger and Cao) b' mass (GeV/c ) Observed limit is consistent with the (median) expected limit b' production cross section as a function of its mass. 9

20 SUMMARY We report the search of heavy bottom-like 4th generation quark in tw final state at 7 TeV LHC pp collisions. The analysis with 34. pb CMS data is presented: - The expected signal yield for bʹ (300~500 GeV/c ) is 7.7~0.5 events with this data set. - No event found in the signal region. - A limit for bʹ tw signal is set: M(bʹ )>357 GeV/c at 95% C.L., comparing to the NLO production cross sections. This result extends the current CDF published limit based on an analysis of the same decay signature. [M(bʹ )>338 GeV/c, PRL 4, 0980 (0)] 0

21 BACKUP SLIDES

22 TEVATRON LIMITS CDF t lepton+jets CDF b same-sign dilepton Observed limit is slightly above the expected limit for t data. D0 t lepton+jets CDF b lepton+ jets Tevatron limits on 4 th generations: b lepton+jets: M(b ) >385 GeV b same-sign dilepton: M(b ) >338 GeV t lepton+jets: M(t ) >335 GeV (CDF) / >96 GeV (D0)

23 SYSTEMATIC UNCERTAINTIES Signal Efficiencies: Δ ε/ε Source JES JE resolution MET Leptons Pile-up jets Input M(b ) =300 M(b ) =350 M(b ) =400 M(b ) =450 M(b ) =500 JME--0.%.8%.%.3%.% % of pt 0.3% 0.6% 0.% 0.% 0.4% ±σ.% 0.5% 0.% 0.% 0.% e: 5.8%, µ: 5.4% 3% 3% 3% 3% 3% 0 PU vs 5 PU.%.%.%.%.0% Background Estimation: Source Input ΔB/B Luminosity Method error background cross sections QCD JES JE resolution ±% 0% - 56% tt ±39%, etc. 5.7% ±0% 9% JME--0.0% % of pt.5% PDF MC Statistics Sum CTEQ6 uncertainty sets 5% 7% 0% % % - 3.0%.7%.7%.5%.4% 0% % 4% 4% 4% MET Leptons pile-ups PDF ±σ 5.6% e: 5.8%, µ: 5.4%.5% 0 PU vs 5 PU <0.% CTEQ6 uncertainty sets.% Control reg. statistics - 3% Sum 65% 3