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Charged Higgs Prospects with ATLAS

Bjarte Mohn University of Bergen Charged Higgs Workshop 2006 13.09.2006 UPPSALA. Charged Higgs Prospects with ATLAS. Charged Higgs Workshop, UPPSALA, September 2006. A Different Overview.

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Charged Higgs Prospects with ATLAS

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  1. Bjarte Mohn University of Bergen Charged Higgs Workshop 2006 13.09.2006 UPPSALA Charged Higgs Prospects with ATLAS

  2. Charged Higgs Workshop, UPPSALA, September 2006 A Different Overview • The first ATLAS Charged Higgs study (τ-Identification and Charged Higgs Search) was published in 1992. The second one note was published in 1994. • The bulk part of ATLAS charged Higgs notes were published between 1999 – 2003 and covered the standard channels, both above and below Mtop. • Recent years (2003 ...) notes with more refined techniques have been published, as well as studies on non Standard Model decays.

  3. Charged Higgs Workshop, UPPSALA, September 2006 The Traditional Overview • Charged Higgs searces @ ATLAS • - Below Mtop • o H+ → τυ (hadronic top decay) • o Mention leptonic top channel • - Above Mtop • o gb → tH+ → Wb tb • o Mention gg → tbH+ → Wb b tb • o gb → tH+ → Wb τυ • o Mention gb/gg → t(b)H+ → Wb(b) τυ • - Single Top sensitivity to charged Higgs • o S-channel sensitivity to H+ • - SUSY decays • o H+ in χ+χ0 decays • - Challenging Channels • o H+ → Wh0 • o H+ → WH0 • Determining MH+ • - Below Mtop • - Above Mtop

  4. Charged Higgs Workshop, UPPSALA, September 2006 Searches for charged Higgs below Mtop • In MSSM the charged Higgs may be copiously produced in top quark decays, if kinematically allowed. • If MH+ < (Mtop – Mb) the decay t → W+b together with t → H+b saturates the top decay width in the MSSM. • When assuming a heavy SUSY spectrum a charged Higgs will decay exclusively into SM particles and below Mtop the braching ratio H+ → τυ is ~100%. • The channel pp → ttbar → H+bW-bbar with H+ → τυhas been studied in ATL-PHYS-2003-038 (Biscarat & Dosil), and following figures and tables are from this note. Maximal Mixing

  5. Charged Higgs Workshop, UPPSALA, September 2006 pp → ttbar → H+bW-bbar, H+ → τυ SELECTION CHAIN: 1. Trigger selection. Assumed trigger conditions (t20xe30) or (j60,xe60) applied. 2. 3-tag cut and no isolated lepton. Require exactly 2 b-tagged jets, exactly 1 τ-tagged jet, at least 2 light jets and no isolated lepton. Samples generated with ATLAS Fast Simulation (ATLFAST) Simulated signal points

  6. Charged Higgs Workshop, UPPSALA, September 2006 pp → ttbar → H+bW-bbar, H+ → τυ 3. PTτ andETmisscuts o ETmiss > 45 GeV o PTτ > 25 GeV o PTbjet1 > 35 GeV, PTbjet2 > 20 GeV => Rejects QCD! 4. W and Top mass recontruction o |Mjj- MW| < 20 GeV o PTjet1 > 30 GeV, PTjet2 > 20 GeV o |Mjjb - Mtop| < 40 GeV => Rejects QCD!

  7. Charged Higgs Workshop, UPPSALA, September 2006 pp → ttbar → H+bW-bbar, H+ → τυ 5. ttbar event pattern o Δφ(Pt_top1, Pt_top2) o Pt_top1/Pt_top2 (hard/soft) o ΔR(bjet1, bjet2) o Δφ(Ptmiss, PtbjetW) => Reduces QCD!

  8. Charged Higgs Workshop, UPPSALA, September 2006 pp → ttbar → H+bW-bbar, H+ → τυ 5. ttbar event pattern o Δφ(PTtop1, PTtop2) o PTtop1/PTtop2 (hard/soft) o ΔR(bjet1, bjet2) o Δφ(PTmiss, PTbjetW) => Reduces QCD! 6. Cut on the charged Higgs side o Prel(τ-jet, b-jet) > 0.2 o PT τ/PTbjet > 0.8

  9. Charged Higgs Workshop, UPPSALA, September 2006 pp → ttbar → H+bW-bbar, H+ → τυ The table shows the relative efficiency for the signal and background for the cuts 1 – 6. There is still a significant ttbar background, but for the lower H+ masses this is not a problem thanks to the large branching ratio t → H+b. Masses above 113 GeV are passed through a final event selection on the transverse mass, and tightening of the PT τ cut (MT > 80 GeV && PT τ> 30 GeV).

  10. Charged Higgs Workshop, UPPSALA, September 2006 pp → ttbar → H+bW-bbar, H+ → τυ • A high significance is obtainable for all tanB and mH+ < 150. • The hadronic channel extends the discovery region down to very low branching ratios: • BR(t → H+b) ~1.5% with 10 fb-1 • BR(t → H+b) ~1.0% with 30 fb-1 • The dip between tanβ ~ 5 and tanβ ~10 is fully covered for the first time. In ATL-PHYS-94-053 (Cavalli et al.) the so-called leptonic channel was studied where the W-boson decays leptonically. This channel features a high Pt lepton for triggering, but the presence of two neutrinoes exludes any mass extraction possibilities. The study was later restudied, and it was found that the results can be improved significantly.

  11. Charged Higgs Workshop, UPPSALA, September 2006 Searches for charged Higgs above Mtop • The region MH+ > Mtop was at first thought to be difficult due to large QCD backgrounds to the main decay mode H+→ t b. • Efficient b-tagging has however enabled good discovery potentials. • Above Mtop the main charged Higgs production mechanism is gb → tH+ and at ATLAS the following has been studied: • gb → tH+ → Wb tb → lυb jjbb(Assamagan ATL-PHYS-99-013) • gb → tH+ → Wb τυ → jjb τ(hadr)υ(Assamagan & Coadou ATL-PHYS-2000-031) • Recently a new study has been performed with a matched production discription • gb/gg → t(b)H+ → Wb (b) τυ → jjb (b)τ(hadr)υ • All plots and number on the following slides are from these publications.

  12. Charged Higgs Workshop, UPPSALA, September 2006 gb → tH+ → Wb tb → lυb jjbb Selection chain: a) Search for an isolated lepton, three b-tagged jets and minimum 2 non b-jets b) Reconstruct Wlep by taking the missing transverse momentum as the neutrino momentum, and fix the longitudinal component by MW. => 2 solutions! c) Reconstruct Whad by |Mjj – MW| < 25 GeV. Combine the 3 b-jets and W-candidates to two top candidates => Many combinations! d) Require | Mlυb –Mtop|< 25 GeV && |Mjjb – Mtop| < 25 GeV. Rescale top-candidates to Mtop. Combine both top-candidates with last b-jet to get two H+ candidates => Combinatorical background. e) Final cut on PTH+ > 80 GeV and PTtop > 60 GeV X-sections

  13. Charged Higgs Workshop, UPPSALA, September 2006 gb → tH+ → Wb tb → lυb jjbb The channel suffers from large combinatotorial background and the efficiency for a correctly reconstructed tb combination is between 48% and 65%. Finding cuts to increase the fraction of correct tb pairs would significantly improve the mass resolution and discovery possibilities.

  14. Charged Higgs Workshop, UPPSALA, September 2006 gb → tH+ → Wb tb → lυb jjbb 30 fb-1 • Above MH+ ~ 300GeV the reduced signal rate and cominatorial background render the observation of this channel difficult. • Below 300 GeV it is possible to observe a clear signal peak above the background.

  15. Charged Higgs Workshop, UPPSALA, September 2006 gg → tbH+→lυb b tb →lυb b jjbb • Large background and large combinatorial effects gave rise to a new study where the gg → tbH+ process with 4 b-tags was used. (SN-ATLAS-2004-042, Gollub et al. ) • A combinatorial likelihood with 9 variables was used to find the correct tb combinations, and enhance performance.

  16. Charged Higgs Workshop, UPPSALA, September 2006 gg → tbH+→lυb b tb →lυb b jjbb • A selection likelihood was used to distinguish between signal and background and selection efficiencies between 1.4 – 2.7% were obtained. • Results show that a reach down to tanβ = 35 is possible with low luminosity for MH+ = 250 GeV. For high luminosity the reach is extended down to tanβ = 28. • The discovery possibilities of the channel is hampered by the low signal x-section and the requirement of having 4 tagged b-jets. • Theoretical uncertainties on the signal and background x-sections lead to a sizeable uncertainty on the signal x-section.

  17. Charged Higgs Workshop, UPPSALA, September 2006 gb → tH+ → Wb τυ → jjb τ(hadr)υ • Above the top-quark and for high tanβ the tb and τυ are the dominant decay channels, while tb is almost excusively the lone decay channel for low tb. • Above tanβ = 10 there might be a good possibility for detecting a charged Higgs through the tau nu channel, which could be from large irreducible backgrounds. • The tau polarization gives another handle for further suppression of the background, in particular ttbar.

  18. Charged Higgs Workshop, UPPSALA, September 2006 gb → tH+ → Wb τυ → jjb τ(hadr)υ Selection chain a) One τ-jet, three non τ-jets, at least one of them a b-jet, and no more then one hard b-jet (|η| < 2.0, PT > 50 GeV) b) Reconstruct Wcand by |Mjj – MW| < 25 GeV, rescale to MW and reconstruct top by χ2 = (Mjjb - Mtop)2. c) Raise cut on PT τ, PT τ > 100 GeV. d) Apply cut on PTmiss, PTmiss > 100 GeV, and cut on the azimuthal opening angle (Δφ) between the τ-jet and Missing Pt. Require Δφ > 0.5 e) Tighten cut on Δφ, Δφ > 1.0.

  19. Charged Higgs Workshop, UPPSALA, September 2006 gb → tH+ → Wb τυ → jjb τ(hadr)υ • The missing transverse momentum makes it impossible to reconstruct the charged Higgs mass, but the transverse mass distribution is bounded from above by MH+. • The table shows that the background is relatively small, and in fact it is the size of the signal which limits the discovery potential. • Further improvement in the signal-to-background ratio is possible by a cut on the tau polarization effects; • Pπ+/Pτ-jet > 80% • But for the relevant analysis it resulted in too few statistics.

  20. Charged Higgs Workshop, UPPSALA, September 2006 gb → tH+ → Wb τυ → jjb τ(hadr)υ • The missing transverse momentum makes it impossible to reconstruct the charged Higgs mass, but the transverse Mass distribution is bounded from above by MH+. • The table shows that the background is relatively small, and in fact it is the size of the signal which limits the discovery potential. • Further improvement in the signal-to-background ratio is possible by a cut on the tau polarization effects; • Pπ+/Pτ-jet > 80% • But for the relevant analysis it resulted in too few statistics.

  21. Charged Higgs Workshop, UPPSALA, September 2006 gb/gg → t(b)H+ → Wb (b) τυ → jjb (b)τ(hadr)υ • In the new Monte Carlo generator Matchig the gb → tH+ and gg → tbH+ production processes are matched for the first time. • This enables continuous scans over the transition region, which has been difficult to access before. • In a new ATLAS charged Higgs Full Simulation study, this has been done been done for the first time looking at tau nu decays. • The results are promising, although a slight degradiation from the previous work. • More details in the talk by Martin Flechl on Saturday morning. Results for tanβ= 35 and 30 fb-1

  22. Above the Mtop the H+→ tb decay becomes domiant, and the single top S-channel becomes competative discovery channel with the gb → tH+. H+ • The contribution from a charged Higgs boson can be as large as 2/3 of the SM S-channel for high tanβ and masses above 200 GeV. (top right plot.) • In ATL-PHYS-2006-003 (Lucotte & Chevallier) this sensitivity was investigated by applying the same set of cuts as in the S-channel analysis. • A 5 sigma sensitivity seems possible only for high tanβ and MH+ ~ 250 GeV. It is though expected that these results could be improved by cuts using the specific properties of the charged Higgs. Charged Higgs Workshop, UPPSALA, September 2006 Single Top sensitivity to the existence of H+

  23. Charged Higgs Workshop, UPPSALA, September 2006 Charged Higgs in chargino-neutralino decay • In the intermediate tanβ region ( 4 < tanβ < 10) charged Higgs decays into Standard Model particles have been found to be undetectable at ATLAS. • Among the new massive sparticles predicted by MSSM are charginos (χ+) and neutralinos (χ0) • In hep-ph/0504021 (C. Hansen et al.) it has been shown that H+ decays into chargino and neutralino can be a viable method for charged Higgs discovery in the intermediate tanβ region. • The process gb → tH+ with H+→χ+ χ0 can be discovered over the SM and MSSM background using 3 hard leptons ans substantial amounts of missing energy. H+ production Possible H+ decay

  24. Charged Higgs Workshop, UPPSALA, September 2006 Charged Higgs in chargino-neutralino decay • A MSSM parameter point which maximises the BR(H+→χ+ χ0 ) was chosen for the analysis. • Detector simulation was done with ATLFAST and signal events were generated with HERWIG. ISAJET was used for BR calculations. • Simulation samples: Event Selection: 1) Exactly 3 isolated leptons with PT > 7 GeV, and at least one with PT > 20 GeV.

  25. Charged Higgs Workshop, UPPSALA, September 2006 Charged Higgs in chargino-neutralino decay Event Selection: 2) From the neutralino decay there are two same flavour, oppopsite charge leptons which invariant mass is kinematically constrained from above by: 3) Top Cut: - At least 3 jets with PT > 20 GeV - Select those from the top-quark decay by minimizing | Mjjj – Mtop| and further require |Mjjj – Mtop| < 35 GeV.- Among these three jets find the two jet most likely from the W boson by minimizing |Mjj – MW| and require |Mjj - MW| < 15 GeV. 4) Jet Cut: The scalar sum of the PT of all jets not used for the top-quark is not allowed to exceed 300 GeV.

  26. Charged Higgs Workshop, UPPSALA, September 2006 Charged Higgs in chargino-neutralino decay • Selection efficiencies between 1% and 23% is obtained depending on tanβ and MH+. • A discovery potential is calculated by linear extrapolation between the simulated points, and is shown in the lower left figure. • The left edge follows the limiting edge of the BR(H+→χ+ χ0 ) while the right and lower edge follow the pattern of the NLO signal x-sec. • Since no discriminating signature is found, the search is done as a counting experiment, hence relying on that the various backgrounds are measured through other channels. • Many more MSSM points must be investigated before general conclusions can be made, but as a preliminary result the study shows that for a specific MSSM point charged higgs through SUSY decays may be detected with ATLAS.

  27. Charged Higgs Workshop, UPPSALA, September 2006 Looking for a challange? Try these! • Searches involving decays to neutral Higgs bosons (h0, H0) have also been performed with ATLAS. • In ATL-PHYS-99-025 (Assamagan) it was showed that despite good mass reconstruction and ttbar supression the discovery potential of the channel (H+→ Wh0, h0→bb) is limited by the low signal rate. • In ATL-PHYS-PUB-2005-017 (Mohn et al.) a large mass splitting MSSM scenario is assumer in which MH+ >> MH0 opening the H+→ WH0 decay channel. • Despite large branching ratio the signal is found to be too much background like and impossible to extract.

  28. Charged Higgs Workshop, UPPSALA, September 2006 MH+ determination with H+→τυ Both below and above Mtop the H+→τυ decay channel is the most sensitive channel for a charged Higgs discovery. However the decay mode does not allow for the observation of a resonance peak above the background, and only the transverse mass can be detected. A maximum likelihood method is used both below and above Mtop to extract the mass. Precision down to a few % can be acheived. HIGH MASS (300 fb-1) LOW MASS (10 fb-1)

  29. Charged Higgs Workshop, UPPSALA, September 2006 MH+ determination with H+→t b • In the tb channel the full invariant mass can be reconstructed but the channel suffers from the large irreducibel background and also from signal combinatorial background. • It is possible to extract the mass using a likelihood method or by fitting the signal and background. • One assumes that the background shape and normalisation can be found be fitting outside the signal region • A Gaussian shape is assumed for the signal and an exponential for the background. • Results show the precision on the mass determination from the likelihood and the fitting methods are comparable. 100 fb-1

  30. Charged Higgs Workshop, UPPSALA, September 2006 Summary and Conclusion • If MH+ < 165 GeV the charged Higgs boson can be discovered at ATLAS within 3 years at low lumi. • A heavier charged Higgs would have to wait for the high luminosity mode, and depending on tanB be found within 1 or 3 years. • The intermediate tanβ region is not covered by any SM decay at present, but has been shown to be reachable with specific MSSM scenarious. • Charged Higgs searches require powerful tools and in particular: • τ-tagging • B-jet tagging • Well understood Ptmiss performance • Good knowledge of the ttbar x-section • +++

  31. Charged Higgs Workshop, UPPSALA, September 2006

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