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Measurement of the Underlying Event and Minimum Bias at LHC

Outcoming parton. ISR. Proton. Proton. FSR. Outcoming parton. Measurement of the Underlying Event and Minimum Bias at LHC. Filippo Ambroglini (University of Perugia) On behalf of ATLAS and CMS. Thanks to: Conference organization!

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Measurement of the Underlying Event and Minimum Bias at LHC

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  1. Outcoming parton ISR Proton Proton FSR Outcoming parton Measurement of the Underlying Event and Minimum Bias at LHC Filippo Ambroglini (University of Perugia) On behalf of ATLAS and CMS Thanks to: Conference organization! CMS QCD group, Livio Fanò, Paolo Bartalini, Florian Bechtel, Rick Field, Klaus Rabbertz, Nikos Varelas, Ferenc Sikler, Richard Hollis, Craig Buttar, Arthur Moraes, Iacopo Vivarelli

  2. Outlook • Motivation for Minimum Bias and Underlying Event study • Introduction to the problematic • Minimum Bias • Trigger • Measurement • Underlying Event • Strategy description • Measurement Filippo Ambroglini - Moriond QCD 2008

  3. Motivation Exploring Fundamental aspects of hadron-hadron collisions Describe QCD@LHC in the best way Not enough to rescale conclusions from Tevatron to 14 TeV [different Q^2, x range and energy dependence of the cut-offs] Structure of hadrons Factorization of interactions spin offs on other relevant physics Impact on the search of new physics The understanding of the MB and transition of pQCD and soft QCD Important for Higgs in VBF and in general for all the channels that using the the jet veto and fwd jet tag Calibration of major physics tools Low, medium and high-PT QCD affected by “surrounding” processes which affect: Pile up understanding, jet energy, isolation performances, vertexing, detector response, High-PT background… Tuning of Monte Carlo Models Both not-perturbative and perturbative aspects Remnants, I-FSR radiation,MPI… (UEactivity, mini-jet, hard scattering) Understanding the detector occupancies, background, … Filippo Ambroglini - Moriond QCD 2008

  4. Minimum Bias e Underlying Event Underlying Event Minimum Bias • All the activity of a single parton-parton interaction that is on top of the “interesting” process. • Initial State Radiation (ISR). • Final State Radiation(FSR). • Spectators. • MPI multiple parton interaction • UE is correlated with the “interesting” process. • Share the same vertex. • The UE activity grow up with the energy scale of the main interaction • Pedestal effect. • Some times is also useful ! • Vertex reconstruction for Hgg. • Generic proton-proton interaction • Elastic + Inelastic ~ 100 mb @ LHC. •  Soft. Low PT, low Multiplicity. • At LHC, many MB interaction can take place in the same bunch crossing. <Nint> = Linst * s/f . •  MB can recorded together with other interaction that can activate the trigger. •  Pile-Up. • What can be observed with a detector/trigger fully inclusive. proton proton UE != Minimum Bias but phenomenological aspects are similar Filippo Ambroglini - Moriond QCD 2008

  5. Theory/Model of pp interaction Inspired by observations of double high PT scatterings The Pythia solution: Multiple Parton Interactions (MPI) (now available in other general purpose MCs: Herwig/Jimmy, Sherpa, etc.) ISR, FSR, SPECTATORS… Not enough to account for the observed multiplicities & PT spectra !!!  Main Parameter: PT cut-off PT0 (dampening also describes quarkonia x-sections) • Cross Section Regularization for PT -> 0 • PT0 can be interpreted as inverse of effective colour screening length • Controls the number of interactions hence the Multiplicity: < Nint > = sparton-parton /sproton-proton Emphasis on the Energy-dependence of the parameters. CDF, UA5 MB Phenomenology favors exponent behavior CGC Theory favors constant behavior [G.Gustafson & G.Miu] Models with Varying impact parameter between the colliding hadrons better describe shapes Filippo Ambroglini - Moriond QCD 2008

  6. pQCD models Generators setup used (details in backup slides) Pythia Tune DW (from Tune A) OLD MPI model, IP CORRELATIONS Pythia Tune DWT DW and default PT-cut-off evolution Pythia Tune S0 New MPI, more correlations Charged multiplicity dN/d vs  dN/dPt vs Pt <Pt> vs multiplicity All these Pythia Tunes describe the UE@Tevatron, but show severaldifferences extrapolating to LHC energynot enough to re-scale conclusions to 14 TeV. Filippo Ambroglini - Moriond QCD 2008

  7. Minimum Bias Trigger • Using forward detectors • Random trigger (inefficient for Nint<<1) • Pile up from other streams (jet or lepton triggers as example) Best solution, at the moment, seems to be the one based on forward detectors The use of pile up from other streams could introduce biases (under study) Filippo Ambroglini - Moriond QCD 2008

  8. Trigger description (ATLAS) η=2.0 interaction point η=3.8 Beam-pipe MBTS Trigger scintillation counters mounted on the front face of the end-cap cryostats covering same radii as the inner detector MBTS (minimum bias trigger scintillator) Noise spectrum fits well to a Gaussian with σ=2.52pC Beyond 9pC (almost 4σ), non-Gaussian behavior is possible # of backward-forward coincidences Accidental rate from noise must be suppressed to ~Hz, limited by EF output-rate of 100Hz Suppression required of (10-6)10-7 at (900 GeV) 14 TeV Filippo Ambroglini - Moriond QCD 2008

  9. Trigger description (CMS) J. Phys. G: Nucl. Part. Phys. 34 (2007) 2307-2455 Based on Hadron Forward Calorimeter 3<||<5 18 wedges/side 0.175x0.175 towers All E>1GeV ET>1GeV Using towers or single cells fired Cut on the number of calorimeter towers hit >10 → 90% Filippo Ambroglini - Moriond QCD 2008

  10. MB prediction (ATLAS) Eur.Phys.J.C50:435-466,2007 • Starting from KNO scaling on past experiment: • Observables are defined • Models are exploited • Tune based on MB and UE observables are chosen E735 1.8 TeV UA5 900 GeV CDF 1.8 TeV Filippo Ambroglini - Moriond QCD 2008

  11. MB prediction (ATLAS) Eur.Phys.J.C50:435-466,2007 Extrapolating to LHC PHOJET that don’t have the MPI predict less particles than PYTHIA • Foreseen measurements: • NSD events • charged spectra (pt, ) • fragmentation Filippo Ambroglini - Moriond QCD 2008

  12. MB prediction (CMS) Charged hadrons pions kaons protons PAS QCD_07_001 From Tsallis fit and expected performances on tracking, particle corrections are calculated and applied (PID performed with dE/dx) Filippo Ambroglini - Moriond QCD 2008

  13. MB prediction (CMS) PAS QCD_07_001 LHC expected multiplicity and average particle PT PT distribution and particle multiplicity Filippo Ambroglini - Moriond QCD 2008

  14. Underlying Event observables “Away” Region 2 “Transverse” Region “Toward” Region jet1  “Transverse” Region “Away” Region 0 -2 2  From jet Topology of p-p collision from charged jet (CMS) calorimetric jet (ATALS) Charged jet definition -> ICA algorithm with mass-less tracks as input The leading jet defines: + a direction in the  plane + the PT/ET is used as reference for the energy scale of the interaction The transverse region is particularly sensitive to the UE • Main observables: • dN/dd, charged density • d(PTsum)/dd, energy density Filippo Ambroglini - Moriond QCD 2008

  15. Feasibility @ LHC Jet #1 Direction Df “Toward” “Transverse” “Transverse” “Away” UE Observables VS Leading Charged Jet Uncorrected distributions from 10pb-1 PAS QCD_07_003 Toward Tracks: PT>0.9 |h|<2 Away Away Transverse Filippo Ambroglini - Moriond QCD 2008

  16. UE estimation (ATLAS) Reconstructed tracks √s=14TeV √s=900GeV Results in the “transverse” region Multiplicity of charged particles with pT > 0.5 GeV and ||< 1 in region transverse to leading jet ~ 15 days of data taking enough to cover up to pT(leading jet) ~ 40 GeV Filippo Ambroglini - Moriond QCD 2008

  17. UE estimation (ATLAS) Njets > 1 |jet| < 2.5 ETjet >10 GeV |track | < 2.5, pTtrack > 1.0 GeV/c Filippo Ambroglini - Moriond QCD 2008

  18. UE estimation (CMS) jet events 100 pb-1 • Effect from correction: • get back the DWT • Good discrimination power: • DW/DWT (from 900 MeV) • S0/DWT using tracking from 500 MeV PAS QCD_07_003 Filippo Ambroglini - Moriond QCD 2008

  19. UE estimation (CMS) PAS QCD_07_003 Uncorrected data. Ratios of observables using minimum PT of 0.9 GeV/c and 1.5 GeV/c No need to apply corrections, absorbed in the ratio Additional discrimination between tunes Filippo Ambroglini - Moriond QCD 2008

  20. UE estimation (CMS) RECO RECO MC MC 2 J. Phys. G: Nucl. Part. Phys. 34 995-1579 UE region of interest From D-Y muon pair production (using muon triggers) observables are the same but defined in all the plane (after removing the  pairs everything else is UE)  0 -2 2  Filippo Ambroglini - Moriond QCD 2008

  21. Conclusions • Minimum Bias measurement plans exist for both experiments • Measuring charged hadrons spectra will allow to calibrate and understand soon the detectors and establishing a solid basis for exclusive physics: • Monte Carlo predictions, based on Tevatron data, greatly differ if extrapolated to LHC energy (MPI component) • Underlying Event activity is studied in the transverse region of charged jet events (studies exist in CMS on DY events - not presented here) • Measuring UE will allows us to: • tune the energy dependence models (largely related to the MPI) • improve the QCD understanding in pp collisions • fundamental for all the LHC measurements (“old” and “new” physics) • Strategy proposed by CMS: • 1 pb-1 -> tools calibration (tracking, triggers, correction and response function…) • 10 pb-1 -> start control of systematic and discriminating between models • 100 pb-1 -> deeper discrimination, enhanced considering ratio distribution Filippo Ambroglini - Moriond QCD 2008

  22. Back Up Filippo Ambroglini - Moriond QCD 2008

  23. Pythia Tuning PT0= PT0(Ecm/E0)PARP(90) Filippo Ambroglini - Moriond QCD 2008

  24. MB Trigger (ATLAS) Shown below for each coincidence logic:Required counter threshold to suppress accidental rate to 1HzCorresponding trigger efficiency for each type of event Filippo Ambroglini - Moriond QCD 2008

  25. Start-up scenario (Tracking) PAS QCD_07_003 Mb and Jet events 1 pb-1 Ideal aligned detector Misaligned Misaligned + APE • Efficiency and fake performances are recovered using APE (additional error to the hit taking into account alignment precision) • Transverse momentum resolution partially recovered • Higher efficiency with APE is due to the MS effect recovered by a larger search window Filippo Ambroglini - Moriond QCD 2008

  26. Start-up scenario (UE) PAS QCD_07_003 Uncorrected data. Recovering the tracking at startup assure the possibility to build UE observables from first days of data taking and start building correction functions Filippo Ambroglini - Moriond QCD 2008

  27. Compact Muon Solenoid CMS Central Detectors T1 T1 RP RP T2 T2 HF HF ZDC ZDC CASTOR CASTOR CMS Forward detectors: Hadron Forward Calorimeter HF: 3 ≤|| ≤ 5 Castor Calorimeter: 5.2 ≤|| ≤ 6.5 Beam Scintillation counters BSC Zero-Degree Calorimeter ZDC TOTEM detectors: T1 (CSC) in CMS endcaps, T2 (GEM) oltre HF T1 + T2: 3 ≤ || ≤ 6.8 Roman Pots con Si det. fino a 220 m 1000 100 10 pT (GeV) 1 Extended Tracking 0.1 -12 -10 -8 -6 -4 -2 0 +2 +4 +6 +8 +10 +12 h Filippo Ambroglini - Moriond QCD 2008

  28. Quarkonia x-section Total Color octet Color-singlet CTEQ6L NRQCD PYTHIA pT0=2.85 GeV CDF data [M. Bargiotti, P. Bartalini] Filippo Ambroglini - Moriond QCD 2008

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