Multi-parton interactions & the underlying event - Zeus - Desy

Underlying event in γp - multi-jets• Here we study: γp → 3+ or 4+ jets(ETjet> 6 GeV)• γ may act like a point-like (direct) or composite object (resolved)• MPIs only present in resolved (hadron-hadron-like) process• Multi-jets generated by QCD processes (see figure right)...• ...and hard-MPIs? Note: soft underlying event changes jetenergy scale and so, given some E jetT• Variables looked at:criteria, affects jet rates.– M nj : invariant mass of the n-jet system. Compared to MCs with and withoutMPIs and LO pQCD.– x obs γ : which approximates x γ , the fraction of γ’s momentum transferred to thehard interaction (i.e. the jets). At LO, x γ = 1 (direct) & x γ < 1 (resolved) -however ambiguous at HOs. Compared to MCs with and without MPIs• events studied in two M nj regions: (25 ≤ M nj < 50 GeV) & (M nj ≥ 50 GeV)T. Namsoo4EDS07 (Blois Workshop)

Underlying event in γp - multi-jets(nb/GeV)dσ/dM 3j1010101-1-2-3Three-jets-1ZEUS 121 pbHERWIG+MPI (×1.7)HERWIG (×1.7)PYTHIA+MPI (×3.1)PYTHIA (×3.8)HERWIG (×1.7) - direct(nb/GeV)dσ/dM 4j-110Four-jets101010-2-3-4-1ZEUS 121 pbHERWIG+MPI (×2.1)HERWIG (×3.1)PYTHIA+MPI (×5.3)PYTHIA (×9.2)HERWIG (×2.1) - direct(nb/GeV)dσ/dM 3j101010-1-2-3Three-jets-1ZEUS 121 pbO(αα 2s ) ⊗ had. corr. ⊗ MPI corr.1010-4-550 100 150(GeV)M 3j1010-5-650 100 150(GeV)• both 3- and 4-jet mass distributions fall exponentially• MC without MPIs fails to describe low M nj regions• adding MPIs helps description of M nj (see M 4j )• highest order pQCD in γp only LO for 3-jet process• shown here corrected for hadronisation and MPI effects• largely describes M 3j data but theo. uncertainty large• description greatly improved by MPI corrs.M 4jCorr. Factortheorydata ⁄1010-4-521.51210MPI corr.had. corr.3 E-scale uncertaintytheo. uncertainty50 100 150M 3j(GeV)T. Namsoo5EDS07 (Blois Workshop)

Underlying event in γp - multi-jets(nb)4 Three-jets (25≤M 3j

Underlying event in DIS - mini-jets• resolved processes suppressed by virtuality, Q 2 .• are we even sensitive to MPIs/underlying event in DIS?• Strategy:– select DIS events (5 < Q 2 < 100 GeV 2 )– select hardest jet in HCM (PTjet > 5 GeV)– define 4 regions (see figure right)– ∑ of particle E T defines low/high activity regions– measure average mini-jet multiplicity, < N minijet >.– where mini-jets have P jet > 3 GeV...T∑ NeventsN minijet– ...and < N minijet >=N events• transverse regions - sensitive to incoherent energy flow.• can further reduce coherent radiation by requiring backto-backsubleading jet (see figure right)– select dijet events (PTjet > 5 GeV)– with subleading jet in “away region”– repeat procedure...T. Namsoo7EDS07 (Blois Workshop)

Underlying event in DIS - mini-jets (inclusive)Toward1.41.2Mini Jet Production. Inclusive Sample. 0.5 < η lab < 2.79225 < Q < 10 GeV(a)H1HERA I(Preliminary)210 < QCDM2< 25 GeV(b)225 < QljRapgap (dir+res)2< 100 GeV(c)Toward1.41.2Mini Jet Production. Inclusive Sample. 0.5 < η lab < 2.79225 < Q < 10 GeV(a)H1HERA I(Preliminary)210 < QPythia2< 25 GeV(b)225 < QljPythia MI2< 100 GeV(c)111.5(d) (e) (f)1.5(d) (e) (f)Away1Away1< N MiniJetHigh>>0.50.30.2(g) (h) (i)< N MiniJetHigh0.50.30.2(g) (h) (i)0.10.1(j)(k)(l)(j)(k)(l)Low0.05Low0.05010 2010 20* (GeV) P T, lj10 20010 2010 20* (GeV) P T, lj10 20• < N minijet > (P jet1T) in the 4 regions. Shown for high η jet1 region in 3 Q 2 bins.• expect larger resolved contribution in high η jet1 (forward) region.• all MC models describe the “towards” and “away” regions reasonably well.• MPIs improve description of “low” and “high” regions at low Q 2 but not at mid Q 2T. Namsoo8EDS07 (Blois Workshop)

Underlying event at the LHC?• What will the underlying event be like at the LHC? Can we say anything presently?• Clearly, LHC extrapolations based on tunes to current data disagree• certainly first LHC data will provide an interesting test for the current models• but beyond just being a background for physics it will be interesting if MPI eventscan be used constructively to gain further insight into e.g. proton structure.T. Namsoo11EDS07 (Blois Workshop)

Summary• the topic of MPIs is presently very relevant. From practical considerations:– they interfere with what physics analyses can be done– they interfere with what triggering strategies can be employed• at HERA, MPIs are possible in resolved photon interactions• resolved processes suppressed with increasing Q 2 and x γ .• low-Q 2 multi-jet γp data suggestive of large MPI contribution at low M nj & low x γ .• furthermore, influence of MPIs predicted to grow with jet multiplicity.• HERA DIS mini-jet data also suggestive of MPIs at lowish-Q 2 (upto O(10) GeV 2 )• however, always question whether MPIs or HO effects/soft physics?• at the Tevatron, the picture is the same.• particle P T sums are in excess of MC prediction without MPIs• description can be remedied by the inclusion of MPIs• But as for the LHC, extrapolations to the relevant energies have large uncertainties• LHC data will provide an interesting test of the models• but beyond just being a background for physics, it will be interesting if MPI eventscan be used constructively to gain further physical insightsT. Namsoo12EDS07 (Blois Workshop)