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Monte Carlo Reweighting: an Alternative to 10TeV Production?

Monte Carlo Reweighting: an Alternative to 10TeV Production?. Claire Gwenlan, UCL. Recently investigated reweighting existing FullSim samples to 10 TeV

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Monte Carlo Reweighting: an Alternative to 10TeV Production?

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  1. Monte Carlo Reweighting: an Alternative to 10TeV Production? Claire Gwenlan, UCL • Recently investigated reweighting existing FullSim samples to 10 TeV • My motivation was to look at the potential for SUSY discovery with small amounts of 10 TeV, BUT given the decisions currently being made on which samples to produce at 10 TeV, perhaps method is relevant and/or interesting to this audience? • Just a couple of slides shown here, taken from a recent talk I gave in the ATLAS-UK SUSY meeting (full talk appended in Backups)

  2. Reweighting : 14 TeV  10 TeV • If no 10 TeV Monte Carlo … can re-weight existing 14 TeV samples • hadronic cross section given by: For a particular set of MC events – with particular incoming parton momenta (p1, p2) – the hard cross section is *fixed*. To change to a different beam energy, need to adjust the *probability* that partons of those momenta were found in the proton in the first place (i.e the PDFs) just a PDF re-weight proton PDFs hard sub-process cross section What you need to know: • incoming parton momenta for: • x = |pz|/Ebeam (different for different beam energies) • Q2, the fact. scale of the process (same for both beam energies) • incoming parton flavours Re-weight 14 TeV MC to 10 TeV using: xi : x of parton i at 14 TeV; x i’: x of parton i at 10 TeV 2

  3. Check of Reweight Method EG: 50000 SU3 SUSY events generated in ATLFAST • Good agreement between reweighted and 10 TeV samples Effective Mass PDF weight pTJet,1,2 >150,100 GeV Meff = pTJet,1 + pTJet,2 + MET example weights (all < 1) 3

  4. Other example checks  a few other example checks (currently low statistics, except for J8MET)good agreement CSCID: 008094 CSCID: 005985 CSCID: 008190 CSCID: 008270

  5. Results: SU3 SUSY vs. Bkg Example: CSC5 0lepton + MET dijet analysis • Integrated L = 100 pb-1 • BEFORE reweighting  • AFTER reweighting  14 TeV  10 TeV:  SU3 signal reduced by ~ 2.9  SM bkg reduced by ~ 2.3 5

  6. Summary • The PDF re-weight method seems to work • Note some caveats however  method is not adjusting the amount of energy left in the beam remnant, so the underlying event treatment is not quite right • however, all checks done so far indicate this isn’t a big effect • Could this method be useful, given the large amount of 10 TeV simulation we are about to set going? • At the very least, it could be used by individuals whose favourite samples are not planned to be produced at 10 TeV • But perhaps it could even be more helpful than this? i.e. an alternative to some 10 TeV production, saving lots of time, CPU, … ? Presumably this would all have to be verified and validated 6

  7. BackUps - (Full Talk)

  8. SUSY Discovery at 10 TeV ? ATLAS-UK SUSY/Exotics Meeting 8th May 2008 Claire Gwenlan

  9. Introduction • 10TeV run coming soon (2-3 months? 100 pb-1 possible?) • Studies show that, if SUSY is relatively light, it could be discovered very early at the LHC (based on 14TeV Monte Carlo) – so what about at 10TeV – could we hope to see something? “Commissioning to 10 TeV should be fast, no quench being anticipated, giving us confidence that the experiments will be recording data at record high energies by the summer. In 1989, it was only a matter of weeks before LEP produced its first profound result – a measurement of the number of light neutrino families. In this respect at least, history will not be repeating itself. The LHC is a discovery machine, and the discoveries it is chasing will require a little more patience.” Robert Aymar, on the 10TeV run 9

  10. PDFs  from 14TeV  10TeV • Currently no MC… BUT can re-weight existing 14TeV samples • hadronic cross section given by: For a particular kinematic configuration with s, t, u, should just need to change the probability that incoming partons *had* that configuration i.e. just a PDF re-weight proton PDFs Plot from James Stirling: DIS08 ^ ^ ^ Re-weight MC to 10TeV using: xi : x of parton i at 14TeV; x i’: x of parton i at 10TeV 10

  11. Check of Reweight Method EG: SU3 SUSY events generated in ATLFAST • Good agreement between reweighted and 10TeV samples (bkgs also checked) Effective Mass PDF weight pTJet,1,2 >150,100 GeV Meff = pTJet,1 + pTJet,2 + MET example weights (all < 1) 11

  12. SUSY analysis • EG: CSC5-style, inclusive 2- and 3-Jet analyses (full details in note) • results shown in main part of talk are for the 2-Jet case (3-Jet results in backups) “Generic search for R-parity conserving SUSY in 2-Jet + MET+0-lepton channel” Effective Mass: Meff = ∑ pTJet,i + MET [sum runs over two highest-pT jets] 12

  13. Results: SU3 vs. SM bkg EG: CSC5 2Jet + MET + 0lepton SUSY analysis: • Integrated L = 100 pb-1 • BEFORE reweighting  13

  14. Results: SU3 vs. SM bkg EG: CSC5 2Jet + MET + 0lepton SUSY analysis: • Integrated L = 100 pb-1 • BEFORE reweighting  • AFTER reweighting  14 TeV  10 TeV:  SU3 signal reduced by ~ 2.9  SM bkg reduced by ~ 2.3 14

  15. Results: other mSUGRA points EG: CSC5 2Jet + MET + 0lepton SUSY analysis: • Integrated L = 100 pb-1 • Comparison of some other SUSY benchmark points (SUX) • all are mSUGRA, but cover quite a wide range of phenomenologies 15

  16. Statistical Significances 100 pb-1 • Numbers are maximum significances (taken above some Meff threshold) (don’t take too much notice of the actual values – it’s just to get a rough feeling) 16

  17. Discovery Significances 100 pb-1 • Numbers are maximum significances (taken above some Meff threshold) (don’t take too much notice of the actual values – it’s just to get a rough feeling) * Zn is a measure of the significance (as used in CSC5) which tries to take into account systematic uncertainties on the bkg measurements. The numbers in the table above are calculated assuming 50% uncertainty on QCD and 20% on all other bkgs – these are not necessarily the “right” numbers – dedicated bkg studies needed for those! 17

  18. Summary • Is there potential for discovery with small amounts of 10 TeV data?  YES – there does seem to be potential! • A 5x increase in centre-of-mass energy compared to previous experiments is still a lot!!! – and the discovery of light SUSY may not need much data (it doesn’t take much to give large S/B values for the models considered here) • BUT that data still needs to be understood • This was really just a quick look for fun – to see if anything is even potentially feasible! It looks like it could be, but the limiting factor will of course be how well we can determine and understand the backgrounds with the small amount of data we expect. 18

  19. BackUps

  20. Results: other mSUGRA points EG: CSC5 3Jet + MET + 0lepton SUSY analysis: • Integrated L = 100 pb-1 20

  21. mSUGRA Points • More details on the mSUGRA points considered 21

  22. MC Bkg Samples 22

  23. Sensitivity to choice of Q2 Reweighting technique is not very sensitive to choice of factorisation scale s, t, u are the usual Mandelstam variables

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