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Z+jets background to qqH, H-> tt from double parton and double proton hard interactions

Z+jets background to qqH, H-> tt from double parton and double proton hard interactions. Nikitenko. Hard double parton interactions can produce an additional Z+jets background to qqH, H-> tt signal. jet1. Z. q. q. g. p. p. g. jet2.

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Z+jets background to qqH, H-> tt from double parton and double proton hard interactions

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  1. Z+jets background to qqH, H->ttfrom double parton and double proton hard interactions Nikitenko

  2. Hard double parton interactions can produce an additional Z+jets background to qqH, H->tt signal jet1 Z q q g p p g jet2 sD(A,B) = (m/2) sAsB / seff, (m=2 for A=Z, B=di-jets) seff=14.5 mb from CDF: F. Abe et al., Phys.Rev. D56 (1997) 3811 Expectations at LHC: seff ~ 20 mb (T. Sjostrand, private communication) Longitudinal correlations in double-parton PDF can have sizable effect (Snigirev, Korotkikh hep-ph/0404155, Treleani et al. hep-ph/0507052)

  3. How I estimate bkg. from double parton interactions • All study is done at particle (PYTHIA) level so far • DY and di-jets were generated separately with EU switched off, MSTP(81)=0 • Di-jet events passed VBF selections at particle level • at least two jets pT > 20 GeV, h < 5.0 • Dhj1j2 > 4.2, hj1hj2 < 0 • Mj1j2 > 1 TeV • Z+jets from one pp interaction were generated with ALPGEN+PYTHIA using MLM prescription for parton-jet matching. Soft VBF pre-selections was done at ALPGEN level. UE is ON, MSTP(81)=4, DWT setting. To be repeated with T. Sjostrand option on how to mix DY and di-jet events for double-parton scattering (next slide)

  4. T. Sjostrand option • “generate DY with full UE • generate the di-jet with MSTP(81)=0 (UE is OFF). Here at first remove all low pT particles, say below 1 GeV. Then add a few of them back in, chosen to ensure that the retained event balances pT to good enough precision. (This is easy to do iteratively, each time adding the particle that reduces the length of the pTmiss vector most.) • Graft event 2 into event 1” Best, Torbjorn

  5. background from double-parton scattering is ~ 20 % of Z+jets ! * ALPGEN + PYTHIA MLM, PTDR samples; Z+2j exclusive, Z+3j inclusive; ALPGEN settings: Mll > 70 GeV, pTj > 20, h < 5, DRjj>0.5 Soft VBF preselections in ALPGEN: Dhj1j2 > 4.0, hj1xhj2 < 0, Mj1j2 > 600 GeV ** pT > 20 GeV

  6. It can be further suppressed with: • Cut on pT of Z boson (or fll) ?

  7. Z+jets bkg. from two hard pp interactions in the same crossing background from double pp interaction is ~ 18 % (7%) of Z+jets in scenario 1 (2) it can be suppressed (measured) asking at least one of two tagging jets to be in the tracker acceptance and checking that jet and lepton have the same vertex * s(DY)=2 nb and assuming spptot = 100 mb

  8. Conclusions and plans • Z+jets background from multiple parton and proton interactions is not negligible in qqH->tt analysis. ~ 20 % of dominant background Z+jets after VBF selections. • We should think how to measure it and how to reduce it. Some ideas how to reduce it • remove events with back-to-back t • require at least one jet within the tracker acceptance and check if jet and lepton have the same vertex • consider Z+j + j-j bkg. from DPS (thanks to D. Zeppenfeld for pointing it out). In qqH, H->gg study (D.Z., D. Rainwater, hep-ph/9712271) the DPS from ggj + j-j was found to be important • here the Sjostrand option to produce DPS with PYTHIA6.3 is realy needed

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