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

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