Direct di- Tevatron On behalf of the & Collaborations Liang HAN University of Science & Technology of China (USTC)
2010/03/ st motivation: SM Higgs 2 Searching for light Higgs(~130GeV) “inaccessible” gg H bb, S/B(QCD) ~ “common” qqbar W(l )/Z(ll, ) + H(bb) “extra” gg H , with Br(H ) ~ 0.2 % Direct photon pair production at O(1) fb Gluon-gluon fusion W/Z associate W fusion (VBF) 135GeV Production 1. Gluon-gluon fusion gg H 2. W/Z associate qq W/Z+H 3. W fusion (VBF) qq qq+H
2010/03/ SM prediction on direct diphoton production LO ( 2 EM ): Quark annihilation: NLO ( s 2 EM ): virtual + real emission infra-safe (Box) (ISR) Gluon fusion: LO ( s 2 EM ): gluon PDF density enhancement at low mass + Fragmentation: FSR collinear singularity + photon isolation p T ( )<M( ) Suppressed by: (no theory) 3
2010/03/ nd motivation: precise test of QCD predictions RESBOS, Phys. Rev. D 76, (2007) : + Quark Scattering qqbar and Gluon Fusion gg up to NLO + Fragmentation at LO, with additional NLO approximation + Resummation of soft/collinear terms of initial gluons up to all orders, cancelling divergence at NLO as p T ( ) 0 DIPHOX, Eur. Phys. J. C 16, 311 (2000) : + qqbar up to NLO + gg at LO + Fragmentation up to NLO + asymmetry di-photon p T ( p T ( ) PYTHIA, Comp. Phys. Comm. 135, 238(2001) : + qqbar and gg at LO + Resummation via parton shower 4
2010/03/ CDF results First di- Tevatron: 207pb -1, PRL 95, (2005) p T ( 1)>14GeV, p T ( 2)>13GeV; | 1,2 |<0.9; E T iso <1GeV Bump of p T ( )~30GeV dominated by events of /2 and p T ( )>M( ), described in DIPHOX as final state radiation + Fragmentation on the same quark Reasonable agreements between data and QCD predictions in different region : Low p T ( )~0GeV and , DIPHOX unstable due to the lack of resummation 5 DIPHOX RESBOS PYTHIA M (GeV) p T (GeV) (rad)
2010/03/ p T ( 1)>21GeV, p T ( 2)>20GeV, | 1,2 | 0.4 Isolation requirement(jet and Fragmentation) + track veto(electron) p T ( )<M( ) remove Fragmentation, reduce theoretical uncertainty Neutral Network discriminator O NN to separate from EM-like jet D0 analysis based on 4.2fb -1 data: D0 di-photon measurement 6 Loose
2010/03/ Background composition Electron misidentified in Drell-Yan Z/ * ee : Estimated with GEANT simulation, normalized up to NNLO and 4.2fb -1 Jet-misidentified in +jet and jet+jet : Split data(Z ee deducted) into 4 groups based on tighter O NN normalization + Npp : both pass + Npf : leading passes, trailing fails + Nfp : vice-versa + Nff : both fail 4×4 /j O NN >0.6 efficiency matrix Line shapes estimated by reversing O NN <0.1 7 LooseTight
2010/03/ Theoretical predictions: + RESBOS and DIPHOX, with CTEQ6.6M, R = F = f =M + PYTHIA with CTEQ5L Data.vs.MC comparison: RESBOS with resummation demonstrates better agreement with data data shows harder p T ( ) and excess in low ( ) Differential cross section 8 M (GeV) (rad) p T (GeV) cos * =tanh[( )/2]
2010/03/ Double-differential cross section: The p T ( ) inconsistence occurs in M < 50GeV region, where the gluon fusion is significant. NNLO correction to gg at low mass? 9 distributions tell the same story
2010/03/ Systematic uncertainty Dominated by uncertainty of di-photon purity, ~10-15%, followed by luminosity ~ 6% the accuracy is around O(1)fb, statistics are close to systematic 10
2010/03/ Impact on Higgs search Reducible background (Z ee, +j, jj)subtracted, sideband fitting into signal region Combine all signal channels (gg H, W/Z+H, VBF) to increase sensitivity D0: CDF: 11
2010/03/ Summary Direct di- production at Tevatron has been studied both by CDF and D0 DIPHOX treats the Fragmentation better; impose p T ( )<M( ) would reduce the discrepancy to RESBOS; RESBOS, with NLO gg , gives the best agreement with data; hints the need of NNLO corrections for low mass region (<50GeV) Data are compared with theoretical predictions, RESBOS, DIPHOX and PYTHIA. None of these calculations provides full description of data in all kinematic regions. Provide extra the sensitivity to SM Higgs search in the most interested mass region ~130GeV 12
2010/03/ Backup Slides 13
2010/03/ CDF & DØ Preshower: to distinguish vs. neutral jets Calorimeter : fine granularity and good energy resolution CDF : D0 : Isolation requirement to suppress fragmentation D0 : lead + scintillating strip Central Preshower (CPS) CDF : Preshower detector + shower maximal CES Shower shape difference between single and multi- from neutral hadron (e.g. 0 ) 14
2010/03/ FSR ISR 15 Loose + Input : Preshower & Calorimeter shower shapes + tracker activities + Training : MC EM-like jet vs. + Validation : data Z ll FSR
2010/03/ Double-differential cross section: 16
2010/03/