Higgs Searches with taus in the final state in ATLAS

Transcription

1 Higgs Searches with taus in the final state in ATLAS (University of Washington) Terascale Workshop: Interpreting emerging Higgs Uni of Oregon With many thanks to Anna Goussiou, J. Keller, M. Beckingham, K. Nakamura, T. Vickey, M. Flechl and the rest of the ATLAS Higgs group 1

2 Overview Introduction: Motivation to look for Higgs using taus Taus with ATLAS Overview of ATLAS Higgs searches with Taus Neutral Higgs Z ττ and Mass reconstruction SM Higgs search MSSM Higgs search Charged Higgs Summary 2

3 Higgs with taus: Introduction (I) There is excellent motivation from theory to look for Higgs decaying to taus Standard Model Higgs Branching ratio H τ τ is ~ 2nd highest for low Higgs masses and cleaner than bb SM Higgs branching ratios as a function of Higgs mass arxiv:

4 Higgs with taus: Introduction (II) MSSM Higgs 5 Higgs bosons: CP-even h,h; CP-odd A; Charged H+/- hep-ph/ For large parts of the parameter space h/a/h ττ and H+/- τv are enhanced but experimentally challenging FeynHiggs Neutrinos in tau decay products soft leptons, ττ mass reconstruction Large backgrounds to hadronically decaying taus from jets 4

5 Taus in ATLAS 5

6 Taus in ATLAS Taus: the only leptons with also hadronic decay modes τ μνν and τ eνν (BR: 35.3%) Leptonic tau decays (τlep) 1-prong decays (BR: 49.5%) 3-prong decays (BR: 15.2%) τ+ π+ν (BR: 10.9%) τ+ π0π+ν (BR: 25.5%) τ+ π+π0π0ν (BR: 9.5%) τ+ π+π+π-ν (BR: 9.3%) τ+ π+π+π-π0ν (BR: 4.6%) Hadronic tau decays (τhad) Tau reconstruction refers to hadronically decaying taus For ATLAS a tau is a jet: Anti-kT jet clustering (ΔR=0.4); Jet pt>10 GeV, eta <2.5; track association DR=0.2 of the tau jet axis 6

7 Taus in ATLAS Reconstructed taus suffer from huge backgrounds from jets Low track multiplicity and narrow hadronic shower width 3-prong tau BDT discriminant against jets Electrons can also fake taus Transition radiation & shower shape High to Low threshold TRT hits ratio 1-prong tau-id performance against jets electrons taus Latest public ATLAS tau performance result (to be updated soon for the full 2011 data) 7

8 Taus in ATLAS Energy scale From MC, but using a data-driven uncertainty based on single particle response Tau identification vs jets and electrons is measured with data Jet discrimination: using Z ττ and W τν Electron discrimination: using Z ee 8

9 ATLAS Higgs Searches with Taus: Overview Latest public results: SM Higgs search: Neutral MSSM Higgs search Moriond Conf. Note (4.6fb-1) Lepton-Photon Conference result (1fb-1), paper with the full 2011 data in preparation Charged MSSM Higgs search Moriond Conf. Note result (4.6fb-1) 9

10 Neutral Higgs Searches (H τ τ) 10

11 Common Analysis Features in H ττ H ττ searches suffer from the following problem Huge (compared to signal) Z ττ background, which peaks very close to the signal due to the poor di-tau mass resolution Phys.Lett. B705 (2011) For this reason special techniques have been developed for the Z ττ estimation and the di-tau mass reconstruction Semi-data-driven tau-embedding technique Several approaches trying to use tau and event properties in order to improve mass resolution 11

12 Z ττ Background Estimation A fully data-driven sample is difficult to be selected Nevertheless we can start from a Z μμ sample from data tau embedding samples Select Z μμ events from data Remove muon tracks and associated calorimeter cells simulate stand-alone Z ττ decays with simulated τ momenta same as the removed muon momentum (after mass correction) Merge into a single hybrid event re-reconstruct objects and missing ET Fully data-driven jet spectrum, underlying event, pile-up, Di-tau mass for tau embedding and Alpgen simulation 12

13 Mass Reconstruction Visible mass: Effective mass invariant mass of the visible tau decay products invariant mass of the visible tau decay products + MET Collinear mass Assume that neutrinos are emitted parallel to the visible products direction 2 equations and 2 unknowns mvis m collinear = x1 x2 x1,2 : momentum fractions carried away by the visible tau products 13

14 Mass Reconstruction Missing Mass Calculator technique A step beyond collinear mass : assume the angle between the neutrinos and the visible hadronic taus Δθ non-zero ending up having more unknowns than equations NIM A654 (2011) 481 Solve the equations in a grid of angles Δθ and choose the best one 14

15 SM Higgs Search SM Higgs production mechanisms considered in the search gluon fusion vector boson fusion (VBF) Higgsstrahlung or VH Final States are categorized according to the tau pair decay lep-lep : H τ τ eμ / μμ / ee lep-had : H ττ μτhad / eτhad had-had : H ττ τhadτhad and according to jet properties to distinguish amongst the different production mechanisms 15

16 SM Higgs Search Event selection Trigger: single lepton (pt: e GeV;mu 18 GeV) and 2 hadronic tau (pt: 29, 20 GeV) triggers in use opposite sign requirement on τ -τlep/had lep/had 16

17 SM Higgs Search: Lep-Lep 4 event categories (VBF, VH, 1jet and 2 jets) Fake lepton background estimated using a template of subleading lepton pt from a control region with inverted lepton isolation Final mass distributions for the various categories: 17

18 SM Higgs Search: Lep-Had 7 event categories: 2nd most important background from (multi-)jet production VBF; 1 jet e/μ; 0 jet, MET<20GeV e/μ; 0 jet MET>20 GeV, e/μ; Estimated using a control region where tau+lepton have same sign charge Mass distributions: 18

19 SM Higgs Search: Had-Had Single event category: τhadτhad + 1 jet 2nd most important background: jet production Estimated using a 2-D template fit: number of tracks in a wide code around each tau candidate Mass distribution 19

20 SM Higgs: Limits 95% CLs exclusion limits Expected: 3-8 Observed: 3-12 Mass range GeV All channels combined Each channel separately 20

21 Neutral MSSM H ττ Production mechanism possible to separate the b associated production by using b-tagged jets, but you will see it with the full 2011 dataset Event selection and analysis methods very similar to what has been already presented for SM Lep-lep (eμ) lep-had had-had 1 isolated e with pt > 25 GeV isolated e / μ with pt > 25 / 20 GeV 2 τhad with pt > 45 GeV and pt > 30 GeV 1 isolated μ with pt > 20 GeV exactly one with pt > 20 GeV Opposite sign Opposite sign MET > 20 GeV, MT<30 GeV Sum of lepton pt and MET < 120 GeV, ΔΦ(e,μ)>2 had Opposite sign MET > 25 GeV, 21

22 Neutral MSSM H ττ Mass distributions 22

23 Neutral MSSM H ττ: Limits Limit on σ BR(φ ττ) ma-tanβ space limit mhmax 23

24 Neutral MSSM H ττ: Limits Exclusion limits: All channels combined Limit on σ BR(φ ττ) ma-tanβ space limit mhmax Currently analysis with 4.7 fb-1 ongoing 2 categories per channel: b-tag and b-veto samples analyzed separately and limits combined Sensitivity very similar to 4.7 fb-1 result 24

25 Charged Higgs Searches: H τ ν 25

26 H τν: Introduction MSSM (but also other BSM scenaria) predict charged Higgs bosons In MSSM a light (m<mtop) charged Higgs boson will be produced in top decays and it will decay most often as H τν (and less often as H cs) Search channels are organized according to the decay mode of W and H+ tau+lepton: tt bbwh bb (lv) (τhadv) tau+jets: tt bbwh bb (qq) (τhadv) lepton+jets: tt bbwh bb (qq) (τlepv) lepton+lepton: tt bbwh bb (lv) (τlepv) jets+jets tt bbwh bb (qq) (cs) Relevant for this talk Search with 36pb-1 (no taus here) ATLAS-CONF

27 H τν: Selection for Channels Lepton+jets Tau + jets Tau + lepton Trigger: single lepton Trigger: tau + MET Trigger: single lepton 1 isolated e/μ, pt > 25/20 GeV 1 isolated e/μ, pt > 25/20 GeV 1 τhad with pt > 40 GeV 1 τhad with pt > 20 GeV At least 4 jets (pt>20 GeV) with at least 2 b-tagged At least 4 jets (pt>20 GeV) with at least 1 b-tagged At least 2 jets (pt>20 GeV), with at least 1 b-tagged MET & Topological cuts MET & Topological cuts vertex ΣpT > 100 GeV Most sensitive channel for this search is tau+jets 27

28 H τν : Some Highlights Some highlights of the lepton+jets analysis discrimination between W lv and H τv lvvv: very similar signatures: we must rely on kinematics mlb = invariant mass of a b-jet + lepton associated with the same top quark (cosθ* ~ mlb) Charged Higgs Transverse mass (mth): maximization of the lepton+v invariant mass using as constraints missing energy and top mass Both variables need b-jet-top association, which is done with a χ2 minimization: 28

29 H τν : Results & Prospects Limits Branching ratio t bh MH+ vs tanβ (mhmax) assuming BR(H τν)=100% Either discovery or exclusion of MSSM with low mass H+ with timescale ~ Winter 2012/2013 Search on heavy H τv with timescale ~ this summer 29

30 Summary ATLAS Higgs searches with taus in the final states H ττ Interpreted in the context of SM, MSSM or generic Higgs production With ~5fb-1 sensitivity is ~3-8 times the SM cross section for Higgs masses in the region GeV Complementary to SM searches in other channels Prospects are good, since we expect lots of data this year H τv Low mass (m<mtop) searches so far high mass studies will follow soon 30

31 Extra Slides 31

32 Extra Slides Branching ratio with tanβ MMC vs Collinear mass 32

33 SM Higgs Event Yields per category 33

34 MSSM Higgs Searches: Events per channel 34

35 SM Higgs: Event category definition lep-lep : 0 jets : eμ events only, Δφleplep > 2.5, pt1+pt2+met < 120 GeV 1 jet : ee,μμ,eμ; ptjet > 40 GeV, m(ττjet)>225 GeV, MET>40 (20 for eμ) GeV VBF : ptjet > 40 GeV, 25 GeV; Δη(jj)>3, m(jj)>350 GeV, reject event if jet with pt>25 GeV found between the 2 jets VH : ptjet > 40 GeV, 25 GeV; Δη(jj)<2, 50 GeV < mjj<120 GeV lep-had VBF : 2 jets pt>25 GeV, MET>20 GeV, Δη(jj)>3 & opposite hemispheres, m(jj)>300 GeV, lepton+tau in the η range between the jets 1 jet : MET > 20 GeV, at least one jets pt > 25 GeV and fail VBF ; τe and τμ separately 0 jets, low MET : no jet pt>25 GeV, MET<20GeV ; τe and τμ separately 0 jets, high MET : no jet pt>25 GeV, MET>20GeV ; τe and τμ separately had-had : 1-jet : jet pt > 40 GeV, MET>20GeV, ΔR(ττ)<2.2, m(ττjet)>225 GeV 35

36 Taus in ATLAS: Trigger ATLAS tau trigger provides single tau and tau combined triggers with (another) tau, electron, muon, missing energy (MET) or missing energy significance For higgs searches so far single lepton triggers, double tau triggers and tau+met significance triggers have been used Till summer 2011 After summer