Search for invisible Higgs in tth0 production Motivation Review of preliminary analysis Event generation: Comparison of Pythia 6.3-6.4 Ricardo Gonçalo, RHUL

Introduction Motivation: Previous analyses: Several scenarios for physics BSM predict a significant invisible branching ratio: The simplest is the Higgs decay to neutralinos (probably disfavoured by the MSSM) New scenario proposed: D.Cerdeño, A.Dedes and T.Underwood (hep-ph/0607157) which includes a light, invisibly decaying Higgs) Complementary to the Vector Boson Fusion and associated ZH production with invisible Higgs in case of suppressed coupling of Higgs to vector bosons (MSSM “intermediate regime” mA < 500 GeV; small tan) Aim to contribute to the Invisible Higgs CSC note Previous analyses: Within Atlas (that I’m aware of): I must review these Paper by E.Richter-Was, B.Kersevan and M.Malawski M.Malawski - MSc Thesis Dietrich Schroff - PhD Thesis Frank Meisel – Diplomarbeit My own preliminary analysis presented in Higgs WG meeting (3 Nov.04) and in local meetings (1 Nov.04, 22 Apr.05): overview and main results shown below Ricardo Gonçalo ATLAS-UK - Durham Sep.06

Overview of preliminary analysis Cut-based analysis Event simulation with ATLFAST Only signal and tt background Difficulties: Two components of missing momentum (from Higgs and neutrino)  can’t reconstruct tbl without further assumptions tt is the most significant background and is similar to signal: most other backgrounds reducible ttZ irreducible but cross section small (well…comparable to signal, must be included!) Signal/Background ~10-3 Process xBR tth 330 (*) fb tt 490 000 fb bbW, W  l 73 000 fb bbZ, Z  l+l- 61 400 fb ttW, W  l 420 fb ttZ, Z   190 fb (*) =520 fb in reference analysis, with SM couplings Ricardo Gonçalo ATLAS-UK - Durham Sep.06

Signal and tt background Transverse mass, ET and ETmiss are good discriminating variables mT has sharp edge for tt background at ~mW But background xsection ~1000 times higher than signal Tails of background distributions very large ttbar tth mT(GeV) ETmiss (GeV) ET (GeV) Ricardo Gonçalo ATLAS-UK - Durham Sep.06 No cuts; Lumi = 1fb-1

Preliminary analysis: cuts Reference analysis cuts: Leptons: 1 electron (pT>25GeV; ||<2.5) or 1 muon (pT>20GeV; ||<2.5) Veto on additional electron (pT>10GeV) or additional muon (pT>6GeV) Jets: 2 b-tagged jets 2 or more un-tagged jets tbjj reconstruction: |mjj-mW|<15GeV; ||<2.0 for jets in Wjj |mbjj-mt|<25GeV tbl reconstruction: mT > 120 GeV (using lepton and full ETmiss to calculate mT) Other: Missing ET > 150GeV Scalar sum of pT of reconstructed l j j b b: ET>250GeV In reconstructed W  jj: Rjj =(2jj+2jj) < 2.2 (to reject lep-tau decays)  Ricardo Gonçalo ATLAS-UK - Durham Sep.06

Preliminary analysis: comparison with reference analysis (Richter-Was et al) Cumulative efficiency of cuts: ~10-20% agreement with reference analysis for signal (see “cross check” column) Slightly worse agreement wrt tt (factor 1.35 – 2.8) for cuts after mT cut Testing all tbjj combinations against mW and mt gives efficiency ~30% better wrt “cross check” (“this analysis” column) both for signal and background  (small) net gain in significance Cut Reference tth Cross check tth this analysis tth Reference tt Cross check tt this analysis tt Lepton 22% 23.6% 23.7% Jets/b jets 5.0% 5.7% 4.9% 5.01% 5.02% t bjj 2.6% 2.9% 3.4% 2.4% 2.60% 2.98% mT 0.87% 1.12% 1.33% 0.041% 0.055% 0.063% ETmiss 0.41% 0.51% 0.67% 2.0x10-5 3.8x10-5 ±0.6 4.9x10-5 ET 0.40% 0.50% 3.7x10-5 ±0.6 4.6x10-5 Rjj 0.28% 0.33% 0.44% 7.5x10-6 2.1x10-5 ±0.5 2.8x10-5 Ricardo Gonçalo ATLAS-UK - Durham Sep.06

Preliminary analysis: initial results Most background comes from lep-tau and lep-lep decays of tt, as concluded by E.Richter-Was et al.  decays increase the missing ET Wjj reconstructed from ISR/FSR jets in lep-tau and lep-lep events For L=30 fb-1, scaling tth cross section to SM value (as in reference analysis): tth: = 520 fb; 44.3 events tt: = 490x103 fb; 812 events S/B = 1.55 lep-lep lep-tau tau-tau lep-had had-had Ricardo Gonçalo ATLAS-UK - Durham Sep.06

Preliminary analysis: kinematic fit Tried something different: Assume pTmiss comes from  and h0 only Build grid of points in pT and (l,) and calculate pZ pZ can be found from pT and plep assuming W on mass-shell From pb , p and plep, calculate mt for each point Propagate errors in mt from grid spacing to obtain mt and calculate 2 mW W b t e/  mt Ricardo Gonçalo ATLAS-UK - Durham Sep.06

Preliminary analysis: kinematic fit Kinematic fit ~works for signal: (pTh) ~ 85GeV Will try to use fit results for discrimination against tt background The hope was that this would allow other cuts to be relaxed Correlations to mT and ETmiss may be important Ricardo Gonçalo ATLAS-UK - Durham Sep.06

New variables to use in selection? Higgs pT from the kinematic fit Signal tt background Ricardo Gonçalo ATLAS-UK - Durham Sep.06

Cut on l,Etmiss Naively expect lepton to be mainly away from ETmiss direction in signal and close to ETmiss direction in background (lepton and  from W decay) In fact, most tt background which passes standard cuts comes from lep-tau with ETmiss from both  and W decay and one detected lepton ttbar tth Truth from angular distance between true lepton and missing ET Ricardo Gonçalo ATLAS-UK - Durham Sep.06

Preliminary analysis: optimisation Varied several cuts over a few threshold values to optimise significance = S/(S+B) All cut combinations considered Cuts varied: mT (standard cut: > 120 GeV) ETmiss (standard cut: > 150GeV) Scalar sum of pT of reconstructed l j j b b: ET (standard cut: >250GeV Added pTH obtained from kinematic fit Added  between reconstructed lepton and ETmiss Also tried varying electron and muon pT cuts Significance increased from ~1.6 to ~3 Ricardo Gonçalo ATLAS-UK - Durham Sep.06

Preliminary analysis: optimisation Some cuts have more impact than others Results point to optimal thresholds: mTW cut ~ 115GeV ETmiss cut ~ 220 GeV l,Etmiss cut < 2.8 rad No clear solution for ET or pTh,fit But: correlations between variables are large Ricardo Gonçalo ATLAS-UK - Durham Sep.06

CSC analysis Aim to re-do and extend preliminary analysis for Invisible Higgs CSC note Signal simulation: Fully simulated tth0 (h0inv.) sample requested – to be simulated with Pythia 6.4 Using Pythia and forcing invisible decay of Higgs: h0  ZZ  4 Background sample must be produced independently on the grid Background rejection factor ~10-5  need O(10-100 M) events or efficient Generator level filtering New PtmissAndOrLepton filter written and in 12.0.3 Based on Pauline Gagnon’s PtmissPlusLeptonFilter Request missing ET and/or one lepton (configurable) Different pT thresholds for e and  Expected efficiency for tt ~10% : 1 e (pT>12.5, ||<2.7) or 1  and missing ET > 100 GeV It may have problems: will have to revisit this carefully Ricardo Gonçalo ATLAS-UK - Durham Sep.06

Study of Pythia versions/PS Pythia parton-showers (PS) and multiple-interactions changed dramatically since Pythia 6.2 Using validation samples (generated by Bruce Mellado, thanks!) to evaluate the differences Single lepton filter with pT>10GeV and ||<2.7 Pythia ggtth0 + qqtth0 Nr.events Passed filter Filter efficiency Equivalent luminosity 6.323, old PS 350.6 fb 20000 11618 58.1% 57.0 fb-1 6.323, new PS 350.8 fb 11486 57.4% 6.403 (new) 351.4 fb 30000 16879 56.3% 85.4 fb-1 Ricardo Gonçalo ATLAS-UK - Durham Sep.06

Comparison Pythia 6.3/6.4/new/old PS Following plots compare Pythia 6.403 and 6.323 with “new” and “old” parton shower (PS) and multiple interactions (MI) algorithms The label “new PS” is used for pT-ordered parton showers and “interleaved” PS-MI model (PS and MI compete for available phase space) “old PS” refers to the Pythia < 6.3 algorithm; still available but not default in 6.3 and 6.4 Bjorken x calculation uses first daughters of interacting protons in ev.record using barcode: not always right!! Code in: Generators /GenAnalysisTools /CBNT_Truth/src/CBNT_SpclMC.cxx Different parton shower algorithms clearly changed the ordering of outcoming partons Ricardo Gonçalo ATLAS-UK - Durham Sep.06

Observable quantities: Missing ET truth Differences don’t look that huge (well...not compared to VBF!) But the cuts on ETmiss and ET will be in poorly understood regions Ricardo Gonçalo ATLAS-UK - Durham Sep.06

tth truth: transverse momenta pTtth probably shows the largest differences between versions The tth system recoils against ISR and multiple interactions Ricardo Gonçalo ATLAS-UK - Durham Sep.06

Conclusions First steps under way to generated data for CSC analysis Signal will be produced with Pythia 6.4 (new PS) Background (ttbar, ttZ (Z), ?…) will be done in own production New generator filter to provide necessary ttbar background sample Looking at differences between Pythia versions for this channel Differences not huge, but may be significant Could provide estimate of systematic uncertainty? Should try other generators? Signal sample requested Will be produced with Pythia 6.4 (new PS) A lot of work to be done to revive this analysis… Ricardo Gonçalo ATLAS-UK - Durham Sep.06

Backup slides Ricardo Gonçalo ATLAS-UK - Durham Sep.06

The truth! At parton level: tops back-to-back in background more “mercedes star”-like in signal Most missing pT from Higgs decay (especially in had-had channel) (,ETmiss)  (h,ETmiss) Ricardo Gonçalo ATLAS-UK - Durham Sep.06

Preliminary analysis: simulation Channel: Higgs (mh = 120 GeV) decaying to neutralinos in MSSM (tan= 5, mA= 1 TeV; M1= 44 GeV, M2= 220 GeV, M3= 1 TeV) PYTHIA 6.203 for signal and background Generated 60 M tt + 10 M tth Atlfast simulation, ATLAS release 7.0.2 Low luminosity setting Cone jets (Rcone = 0.4) Jet tagging: b jets 60%; c mistag 10%; u,d,s, mistag 1% CTEQ5L PDFs mtop = 175 GeV Interfaced own code to Atlfast within Athena to produce dedicated ntuple Ricardo Gonçalo ATLAS-UK - Durham Sep.06

Azimuthal angle difference between 3-momenta of tops and Higgs Small differences between Pythia 6.3/6.4 and PS versions (right) Compare to previous plot from Pythia 6.2 (left) Labels: t1 = leading top (highest pT) t2 = sub-leading top Ricardo Gonçalo ATLAS-UK - Durham Sep.06

Observable quantities: Missing ET methods ETmiss estimated using different methods Reconstruction/MissingET/src/CBNT_MissingETtruth.cxx Plotting angle between ETmiss estimates…not sure I understand the differences between methods But they are similar for all Pythia/PS versions Ricardo Gonçalo ATLAS-UK - Durham Sep.06

Observable quantities: lepton pT & isolation Looks ok Left plot: with isolation Right plot: with sub-leading lepton (note lepton filter on leading lepton) Ricardo Gonçalo ATLAS-UK - Durham Sep.06

Observable quantities: transverse mass Plot on the right: events with a single lepton (e or ) Compare with plots below Cut on mT at ~115GeV Ricardo Gonçalo ATLAS-UK - Durham Sep.06

tth truth: angles between true momenta Some difference between versions; easier to see in polar angle Pythia 6.4 seems to give an intermediate result between 6.3 new/old PS Ricardo Gonçalo ATLAS-UK - Durham Sep.06

Optimised cuts Signal… … and tt background Ricardo Gonçalo ATLAS-UK - Durham Sep.06

Sum ET Signal… … and tt background Ricardo Gonçalo ATLAS-UK - Durham Sep.06

Transverse W mass Signal… … and tt background Ricardo Gonçalo ATLAS-UK - Durham Sep.06

Lepton and jet variables Signal… … and tt background Ricardo Gonçalo ATLAS-UK - Durham Sep.06

Not using Rjj as selection variable Signal… … and tt background Ricardo Gonçalo ATLAS-UK - Durham Sep.06

Higgs pT from fit Signal… … and tt background Ricardo Gonçalo ATLAS-UK - Durham Sep.06

l,Etmiss Signal… … and tt background Ricardo Gonçalo ATLAS-UK - Durham Sep.06