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Standard Model Physics at the LHC: the first phase

Standard Model Physics at the LHC: the first phase. M. Cobal Universita’ di Udine e INFN Gruppo Collegato di Trieste MCWS, Frascati Feb 2006. Summary of SM activities. Minimum bias and underlying event (see Bartalini’s talk)

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Standard Model Physics at the LHC: the first phase

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  1. Standard Model Physics at the LHC:the first phase M. Cobal Universita’ di Udine e INFN Gruppo Collegato di Trieste MCWS, Frascati Feb 2006

  2. Summary of SM activities • Minimum bias and underlying event (see Bartalini’s talk) • PDFs with W and Z production (see Tricoli’s talk..next time!) • Higgs (see Lari’s talk) • Physics commissioning phase • Top physics and the systematics involved • First measurements • Jet scale • ISR/FSR • New Physics with top • EW Single top production • Not covered today, but next time • W mass measurement • Top properties • WZ production • Z differential cross section measurement

  3. A new point of view: Commissioning! The game to play: Understand detector /Minimize MC dependency • Knowing the detector • Redundancy between detectors • Straight tracks, etc. • Physics: available ‘candle’ signals in physics • Presence and mass of the W±, Z0, top-quark • Presence of b-jets • Balance in transverse plane, PT Prepair with detector pessimistic scenarios Non-perfect alignment at startup, e.g. in b-tagging Dead regions in the calorimeter / noise Unknown precise jet energy scale Assess trigger dependencies Only after full understanding of these the road to discovery starts…

  4. Physics commissioning • What are we going to do with the first month of data? • Many detector-level checks (tracking, calorimetry etc) • Try to see large cross section known physics signals • But to ultimately get to interesting physics, also need to calibrate many higher level reconstruction concepts such as • jet energy scales • b-tagging • missing energy

  5. Top pairs production

  6. Top physics at LHC • Largettbar production cross section at LHC • Effect of large s at LHC  threshold for ttbar production at lower x • Production gluon dominated at LHC, quark dominated at Tevatron • About 100 times larger than cross section at Tevatron (lumi also much larger) ggtt stt(tot) = 759±100 pb Nevt ~ 700/hour qqtt

  7. Top physics topology • Decay products are 2 W bosons and two b quarks • About 99.9% to Wb, ~0.1% decay to Ws and Wd each • For commissioning studies focus on events where one W decays hadronically and the other W decays semi-leptonically • About 30% of total ttbar cross section t t

  8. What can we learn from ttbar production • Abundant clean source of b jets • 2 out of 4 jets in event are b jets  O(50%) a priori purity (need to be careful with ISR and jet reconstruction) • Remaining 2 jets can be kinematically identified (should form W mass)  possibility for further purification t t

  9. What can we learn from ttbar production • Abundant source of W decays into light jets • Invariant mass of jets should add up to well known W mass • Suitable for light jet energy scale calibration (target prec. 1%) • Caveat: should not use MW in jet assignment for purpose of calibration to avoid bias • If (limited) b-tagging is available,W jet assignment combinatoricsgreatly reduced t t

  10. Physics commissioning with top • Jet energy scales Ultimate goal for JES calibration is 1% • At startup calibration will be less known • Important –effect on Mtop measurement • Impacts many measurements, not just Mtop • Need to start data to good use for calibration purposes as quickly as possible • Top physics ideal candidate to do the job Uncertainty on light jet scale:Hadronic 1%  Mt < 0.7 GeV 10%  Mt = 3 GeV Uncertainty On b-jet scale:Hadronic 1%  Mt = 0.7 GeV 5%  Mt = 3.5 GeV 10%  Mt = 7.0 GeV

  11. R E E Use W in top events for jet calibration Effect of a mis-calibration of jet energy dominant systematics Several methods to calibrate. Simplest one: • compute R for k bins in E • apply R correction and recompute new R n times =>

  12. Results after recalibration • Use Top sample to correct jet energies of Z+jet sample • TOP 12000 jets, Z+jet 8000 jets • Apply same cuts on jets energies • Top light jet scale seems to work for all light jets • In progress: repeat exercise with backgrounds Top Z+jets After calib ‘Top’ E E

  13. What can we learn from ttbar production • Known amount of missing energy • 4-momentum of single neutrino in each event can be constrained from event kinematics • Inputs in calculation: Mtop from Tevatron, b-jet energy scale and lepton energy scale t t

  14. What can we learn from ttbar production • Two ways to reconstruct the top mass • Initially mostly useful in event selection, as energy scale calibrations must be understood before quality measurementcan be made • Ultimately determine Mtop from kinematic fit to complete event • Needs understanding of bias and resolution of all quantities • Not a day 1 topic t t

  15. How to identify ttbar events • Commissioning study  Want to restrict ourselves to basic (robust) quantities • Apply some simple cuts • Hard pT cuts really clean upsample (ISR). • Possible becauseof high production rate    Combined efficiency of requirementsis ~5%  still have ~10 evts/hour 1 hard lepton (Pt >20 GeV) 4 hard jets (PT >40 GeV) Selecting ttbar with b-tagging expected to be easy: S/B=O(100) But we would like to start without b-tagging Missing ET(ET >20 GeV) 

  16. W+4jets (largest bkg) Problematic if 3 jets line up Mtop and W + remaining jet also line up to Mtop Cannot be simulated reliablyby Pythia or Herwig. Requires dedicated event generator AlpGen Ultimately get rate from data Z+4 jets rate and MC (Z+4j)/(W+4J) ratio Vast majority of events can be rejected exploiting jet kinematics. QCD multi-jet events Problematic if one jets goes down beampipe (thus giving ETmiss) and one jets mimics electron Cross section large and not well known, but mostly killed by lepton ID and ETmiss cuts. Rely on good lepton ID and ETmiss to suppress e-,p0 W  l n Backgrounds that you worry about

  17. ‘Standard’ top analysis • First apply selection cuts • Assign jets to W, top decays Missing ET > 20 GeV Selection efficiency = 5.3% 1 lepton PT > 20 GeV 4 jets(R=0.4) PT > 40 GeV W CANDIDATE TOP CANDIDATE 1 Hadronic top: Three jets with highest vector-sum pT as the decay products of the top 2 W boson: Two jets in hadronic top with highest momentum in reconstructed jjj C.M. frame.

  18. Summary of R. Chierici Generation tools • ME: ALPGEN/MadGraph/ComHep/TopRex etc

  19. MC samples ttbar (signal) W+jets (background) • Generator: MC@NLO • Includes all LO + NLO m.e. • Dedicated Generator: AlpGen • Includes all LO W + 4 parton m.e. Hard Process CPU intensive! Fragmentation, Hadronization & Underlying event Herwig (Jimmy) [ no pileup ] Atlas DetectorSimulation ATLAS Full Simulation 10.0.2 (30 min/ev) ‘T1’ Sample 175K event = 300 pb-1 ‘A7’ Sample 145K event = 61 pb-1

  20. W CANDIDATE TOP CANDIDATE Signal-only distributions (Full Sim) • Clear top, W mass peaks visible • Background due to mis-assignment of jets • Easier to get top assignment right than to get W assignment right • Masses shifted somewhat low • Effect of (imperfect) energy calibration m(tophad) m(Whad) MW = 78.1±0.8 GeV mtop = 162.7±0.8 GeV L=300 pb-1 (~1 week of running) Jet energy scalecalibration possible fromshift in m(W) S B S/B = 0.5 S/B = 1.20

  21. W CANDIDATE TOP CANDIDATE Signal + Wjets background (Full Sim) • Plots now include W+jets background • Background level roughly triples • Signal still well visible • Caveat: bkg. cross section quite uncertain m(tophad) m(Whad) Jet energy scalecalibration possible fromshift in m(W) S L=300 pb-1 (~1 week of running) B S/B = 0.27 S/B = 0.45

  22. W CANDIDATE TOP CANDIDATE Signal + Wjets background (Full Sim) • Now also exploit correlation between Mtop(had) and MW(had) • Show Mtop(had) only for events with |m(jj)-m(W)|<10 GeV m(tophad) m(tophad) L=300 pb-1 (~1 week of running) m(Whad) S S/B = 1.77 B S/B = 0.45

  23. Signal + Wjets background (Full Sim) • Can also clean up sample with requirement on Mtop(jln) [semi-leptonic top] • NB: There are two Mtopsolutions for each candidate due to ambiguity in reconstruction of pZ of neutrino • Also clean signal quite a bit • MWcut not applied here TOP CANDIDATE SEMI LEPTONIC TOP CANDIDATE m(tophad) m(tophad) L=300 pb-1 (~1 week of running) S |m(jln)-mt|<30 GeV B S/B = 1.11 S/B = 0.45

  24. Effect if increasing realism • Evolution of Mtop resolution, yield with improving realism m(top) (GeV) resolution (GeV) s(N) stat Effect ofdetectorsimulation Effect ofincreasingWjets bkg. Effect ofmW cut

  25. W CANDIDATE TOP CANDIDATE Exploiting ttbar as b-jet sample (Full Sim) • Simple demonstration use of ttbar events to provide b-enriched jet sample • Cut on MW(had) and Mtop(had) masses • Look at b-jet prob for 4th jet (must be b-jet if all assignments are correct) W+jets (background) ‘random jet’,no b enhancement expected ttbar (signal) ‘always b jet if all jet assignment are OK’ b enrichment expected and observed AOD b-jet probability AOD b-jet probability Clear enhancementobserved!

  26. Improving the analysis • We know that we underestimate the level of background • Only generating W + 4 partons now, but W + 3,5 partons may also result in W + 4 jet final state due to splitting/merging W + 3 partons (80 pb*) W + 4 partons (32 pb*) W + 5 partons (15 pb*) W  l n W  l n W  l n 2 parton reconstructed as single jets parton is reconstructed as 2 jets * These are the cross sections with the analysis cuts on lepton and jet pT applied at the truth level

  27. Improving the analysis • Improving the W + 4 jets background estimate • Need to simulate W + 3,5 parton matrix elements as well • But not trivial to combine samples: additional parton showering in Herwig/Jimmy leads to double counting if samples are naively added • But new tool available in AlpGen v2.03: MLM matching prescription. • Explicit elimination of double counting by reconstructing jets in event generator and killing of ‘spillover’ events. • Work in progress • To set upper bound: naïve combination of W + 3,4,5 parton events would roughly double W+jets background.

  28. #triggered events / # events 73.5% Nominal analysis cut Electron pT (GeV) Effect of trigger • Look at Electron Trigger efficiency • Event triggered on hard electron • Triggering through 2E15i, E25i, E60 channels • Preliminary trigger efficiency as function of lepton pT Efficiency = fraction of events passing all present analysis cuts that are triggered • Includes effects of ‘untriggerable’ events due to cracks etc

  29. Summary • Can reconstruct top/W signal after ~ 1 week of data takingwithout using b tagging • Can progressively clean up signal with use of b-tag, ET-miss, event topology • Many useful spinoffs • Hadronic W sample  light quark jet energy scale calibration • Kinematically identified b jets useful for b-tag calibration • Continue to improve realism of study & quality of analysis • Important improvement in W+jets estimate underway • Incorporate and estimate trigger efficiency to few (%) • Also continue to improve jet assignment algorithms • Estimate of s(ttbar) with error < 20% in first running period • One of the first physics measurements of LHC?

  30. ISR n ISR and FSR in top events • Mtop = 172.7±2.9 GeV/c2 (current world average) • by end of Run II: reduce uncertainty on Mtop to <1.5 GeV • Extra jets originating from the incoming partons and outgoing partons affect the measurement of Mtop when they are misidentified as jets from the final state partons or change the kinematics of the final state partons. Systematic uncertainty due to this effect was usually assigned using MC events where ISR (and FSR) are switched ON and OFF. Non physical! • ISR and FSR are controlled by the same DGLAP evolution equation that tells us the probability for a parton to branch (splitting function) and is driven by Q2 (factorisation scale), LQCD and PDF (for ISR).

  31. In determining Mtop, biggest uncertainties are on: Jet energy calibration FSR: ‘out of cone’ give large variations in mass B-fragmentation ISR/FSR systematics evaluated by looking at the shift in Mtop obtained by switching radiation ON and OFF in Pythia and taking the 20% of it What we have been doing Challenge: determine Mtop around 1 GeV accuracy in 1 year of LHC

  32. q l- Z/γ q l+ l = ,e Drell Yan processes • Currently, CDF determines the systematic uncertainty due to ISR, using Drell-Yan events. • Advantages of Drell-Yan events are twofold: • Due to the dilepton final states, there are no FSR jets • DY dileptons are produced by the qqbar annihilation process, as are most (85%) ttbar pairs at the Tevatron (not the LHC case). • Very similar to top production process • Dominated by q-qbar annihilation, but in lower mass region • Z decays into lepton pairs (e, , or  pair) ISR

  33. New ISR evaluation • The logarithmic dependence of some observables (average PT of the dilepton system, number of soft jets etc) on the scale (Mll is measured and it is found that both PYTHIA and HERWIG describe the ISR activity well over a wide range of DY mass regions 1) Logarithmic slope is fitted 2) Fit results used to define a 1σ range of uncertainty 3) This used to generate different MC samples (ISR up/ISR down) using some tunable physics MC parameters that can be varied: PARP(61) = ΛQCD in ISR shower PARP(64)= K factor for the starting Q2 scale of the ISR shower hep-ex-0510048

  34. Evaluation of the ISR/FSR effects • To evaluate the uncertainty due to ISR/FSR, the relevant parameters are varied by ±1σ, and new 178 GeV/c2 t¯t signal and background Monte Carlo templates have to be produced by performing event selection and mass reconstruction on the modified samples. • The FSR systematics is a bit more suspect: The Tevatron argument is that the mechanism of the ISR and FSR is the same, which is true from theoretical point (DGLAP) but not true from the implementation point (in Pythia the two differ..) • The signal and background p.d.f.’s used in the analysis remain unchanged. • The shift in the fitted top quark mass is taken as the systematic uncertainty associated with the FSR effect. • The bottom line we should keep in mind is that the procedure works since they have achieved a good fit of Pythia predictions to the DY data!

  35. The LHC case • The use of the slope found using DY events can be applied at LHC when the qq initial state is the dominant process (i.e: W’ and leptoquarks production?) • However we can claim that • if we tune the ISR parameters on Z0 (and maybe another process for cross-check) the ISR is ok for all processes in general • what is different are the evolution kernels which carry no sys error and the PDFs which have to be tuned separately; • we just cannot use the fitted DY curve as such but have to vary the parameters a bit on a process basis (as they did in Tevatron).

  36. The LHC case • Where other initial states give dominant contributions, like gg in top production a similar approach might be possible: One should look for the easily controllable processes with high statistics and equal initial states as a cross check. • We should think of a more consistent way to tune FSR on the data instead of assuming that if the ISR works so does FSR (as done in Tevatron). • In Pythia (old 6.2 or new 6.3 mechanism) the FSR is much more advanced than ISR and uses some different parameters as well • The ISR is governed also by the PDFs which is not the case for FSR, for example; • The coherence effects which are implemented differently etc..

  37. The LHC case II • Tuning of Pythia to the DY pT(ll) vs. Q2 distribution is of paramount importance – direct tuning of ISR parameters on the first data! • To achieve this we have to identify all the tunable parameters in the new Pythia 6.3 ISR/FSR – they have changed considerably • We also have to think more about the distributions and processes we can tune ISR/FSR on. To this extent a good understanding of the planned triggers is needed to have the data necessary to span various mass regions DY events

  38. The LHC case III • We tried the ggbbar process (same initial state as top). • The Q2 distribution falls rapidly, which means that we can get a fit only on very low values (then no overlap with the Z0 region) • The jet content different in the two processes (number of jets vs jet energy) We should also look for a process closer to the top scale. ggbbar

  39. Future Plans • Identify all the tunable parameters in PYTHIA 6.3 • Compare also to what we can do with MC@NLO. Here, in the hard event there is already part of the radiation, so we cant do as with PYTHIA. We can instead: • Play with the scale of the shower in HERWIG.This option is what corresponds more closely to the naif idea of studying the systematics of radiation • Look not only to Ptll but also to Njet vs Q2 (important for top: from DY Ptll don t know whether there are many soft jets or few hard jets, and this is crucial for the final effect on Mtop! • Look at top events themselves!

  40. EW single top production

  41. Why single top at the LHC? 3 production modes in SM t (Wg) channel Wt channel s (W*) channel Wg and W* can be identified at the Tevatron ALL can be precisely measured at LHC

  42. Studies • Properties of the Wtb vertex • Determination of s(p→Wt) and G(t→Wb) • Direct determination of |Vtb| • Top polarization • Precision measurements → Test for new physics • Anomalous couplings, FCNC • W’ extra-gauge boson (GUT, KK) • Extra Higgs Boson (2HDM) • Single top is background for.. • Higgs physics with jets

  43. FCNC kZtc=1 t-channel 4th generation,|Vts|=0.55, |Vtb|=0.835 (extreme values allowed w/o the CKM unitarity assumption) SM Top-flavor MZ’=1 TeV sen2f=0.05 Top-pion Mp±=450 GeV tR-cR mixing ~ 20% s-channel Single top and New Physics T.Tait, C.-P.Yuan, Phys.Rev. D63 (2001) 0140018

  44. Cross sections

  45. Theoretical errors at the LHC (Z.Sullivan, Phys.Rev. D70 (2004) 114012) Less than at TeV, since the x-region for the gluon PDFs is better known Should be very similar to t-channel and a gg→tt

  46. t-channel • Selection • Exactly 2 jets with high PT: 1 central jet from b at high PT • 1 forward jet , |η|>2.5 • Top reconstructed with b-jet. Solution which minimize |mlvb – mtgen| • Resolution in Mtop > 25 GeV • Window in HT or Mtop • Performance: • ε ≈ 1.3%, N(30fb-1) ~ 7,000 eventi • Background: W+jets , ttbar • Sys: lum, e b-tag, JES ATLFAST S/B ~ 3 √(S+B)/S ~ 1.4% @ 30 fb-1

  47. s-channel • Selection • Separated analysis for t+bbar and tbar+b • Asym. For single top, sym. for ttbar and W+jets • 2 and only 2 central b-jets with high PT • Top reconstructed with the lvb combination with the highest PT • Windows in HT or Mtop • Background • T-channel, ttbar ATLFAST

  48. s-channel • Performance • Standard Sel. + topological Sel. (HT, Mtop)  optimization upper/lower limits of HT • Results • Statistical sensitivity : 7% to 12% (corresponds to 15%-10% syst.) according to the topological sel.(and of S/B) • Measurements dominated by sistematics: exp:11%, th: 8%, lumi: 5% More realistic estimation ongoing Data needed

  49. s-channel, CMS Preliminary • Preselection – 1 high-PT lepton – Exactly 2 high-PT jets, both b-tagged – Missing Energy • Topological selection – Reconstruct Top w/ the lowest-|Pz| n solution and the b-jet with “jet charge” opposite to the lepton (if both opposite, the one giving the highest-PT top is chosen) – Window in Mtop –ST cut – Other topological variables are used, most notably M(tb) (directly related to ŝ) After preselection After preselection M(tb)

  50. Background for gbH±t Very difficult channel: ttbar hide the signal (a ttbar event with a b-jet outside the acceptance is perfextly simulating a W event) Not even trivial to define exactly what is the signal (at NLO: mixed with the ttbar diagrams!) Goal: Identification, sys evaluation, s/Vtb measurements Work with theoreticians to identify new physics Wt-channel

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