Qu’attendre des premières données du LHC ? Réunion expérimentateurs-théoriciens du 13 Janvier 2008 Transparents empruntés à M. Cobal, I. Hinchliffe, K. Lassila-Perini, A. Moraes, A. Tricoli

Environ deux mois de retard par rapport à ce calendrier…

Cross-sections and rates At luminosity 1032 cm-2 s-1 Inelastic: 107 Hz bb production: 104 Hz W ℓ: 1 Hz Z  ℓℓ: 0.1 Hz tt production: 0.1 Hz Tout d’abord, mesurer avec précision les sections efficaces Modèle Standard  permettra de s’assurer de la compréhension du détecteur et de la physique.

LHC: physics roadmap Prepare the road to discovery: Understand/calibrate detector and trigger in situ using “candles” samples e.g. - Z  ee,  tracker, ECAL, muon chamber calibration and alignment, etc. - tt  bl bjj jet scale from Wjj, b-tag performance, etc. Understand basic SM physics at s = 14 TeV measure cross-sections for e.g. minimum bias, W, Z, tt, QCD jets (to ~20 %), start to tune Monte Carlo measure top mass  give feedback on detector performance Note : statistical error negligible with O(10 pb-1) Prepare the road to discovery: measure backgrounds to New Physics : e.g. tt and W/Z+ jets look at specific “control samples” for the individual channels: e.g. ttjj with j  b “calibrates” ttbb irreducible background to ttH  ttbb Look for New Physics potentially accessible in first year(s) e.g. Z’, SUSY, Higgs ?

Charged particle density at  = 0 (Only need central inner tracker and a few thousand pp events) LHC? Multiple interaction model in PHOJET predicts a ln(s) rise in energy dependence. PYTHIA suggests a rise dominated by the ln2(s) term. Vital for understanding : Detector backgrounds, Energy scales, Detector occupancy

Minimum Bias Non-single diffractive evts, s ≈ 60-70 mb Soft interactions Low PT, low Multiplicity. Soft tracks: pTpeak~250MeV Approx flat distribution in h to |h|~3 and in f Nch~30; |h|<2.5 Rate: R~700kHz @ L=1031cm-2s-1, For dN/dh require ~10k What we would observe with a fully inclusive detector/trigger.

Initial Tracking & Alignment Very first alignment will be based on: Mechanical precision Detailed survey data Cosmic data Minimum bias events and inclusive bb Studies indicate good efficiencies after initial alignment ~ 80% down to PT = 500 MeV Precision will need Zs and resonances to fix energy scales, constrain twists, etc. pT (MeV) Even lower PT accessible with reduced tracking ? PT = 400 MeV - tracks reach end of TRT PT = 150 MeV - tracks reach last SCT layer PT = 50 MeV - tracks reach all Pixel layers 150MeV

Underlying Event All the activity from a single particle-particle interaction on top of the “interesting” process. Initial State Radiation (ISR). Final State Radiation (FSR). Spectators. … Multiple Interactions ? (These models are certainly very successful!). The UE is correlated to its “interesting” process. Share the same primary vertex. Events with high PT jets or heavy particles have more underlying activity Pedestal effect. Phenomenological study of Multiplicity & PT of charged tracks. High PT scatter Beam remnants ISR

UE: measurement plan at the LHC From charged jet using MB and jet triggers Topological structure of p-p collision from charged tracks The leading Ch_jet1 defines a direction in the f plane The transverse region is particularly sensitive to the UE Main observables: + dN/dhdf, charged density + d(PTsum)/dhdf, energy density From D-Y muon pair production (using muon triggers) observables are the same but defined in all the f plane (after removing the m pairs everything else is UE)

LHC predictions: pp collisions at √s = 14 TeV x1.5 x 3 dNchg/dη ~ 30 dNchg/dη ~ 15 Central Region (min-bias dNchg/dη ~ 7) ATL-PHYS-PUB-2005-007 Transverse < Nchg > LHC Charged particles: pt>0.5 GeV and |η|<1 Cone jet finder: UE particles come from region transverse to the leading jet. Tevatron Pt (leading jet in GeV)

Why PDFs are vital at LHC? On Hadron Colliders every Cross-Section calculation is a convolution of the cross-section at parton level and PDFs: PDFs are vital for reliable predictions for new physics signal (Higgs, Super- Symmetry, Extra Dimensions etc.) and background cross-section at LHC. [at CDF ΔσHiggs,SUSY (CTEQ) ~ 5%] pA pB fa fb x1 x2 X

How do we want to constrain PDFs? W total and differential cross sections theoretical calculations are very robust: known to NNLO in QCD pert. theory input E.W. param. known to high accuracy EXP.: Clean measurement Abundance of W’s (300M evt/y at LHC at low Lumi.) Main Theoretical uncertainty comes from PDFs Tevatron W± Symmetric Kinematic regime for LHC much broader than currently explored

Study the effect of including the W Rapidity distributions in global PDF Fits by how much can we reduce the PDF errors? Generate data with CTEQ6.1 PDF, pass through ATLFAST detector simulation and then include this pseudo-data in the global ZEUS PDF fit. Central value of prediction shifts and uncertainty is reduced BEFORE including W data AFTER including W data ~1day of data-taking at low Lumi W+ to lepton rapidity spectrum data generated with CTEQ6.1 PDF compared to predictions from ZEUS PDF AFTER these data are included in the fit W+ to lepton rapidity spectrum data generated with CTEQ6.1 PDF compared to predictions from ZEUS PDF Specifically the low-x gluon shape parameter λ, xg(x) = x –λ , was λ = -.187 ± .046 for the ZEUS PDF before including this pseudo-data. It becomes λ = -.155 ± .030 after including the pseudo-data

Top physics in the early phase The LHC will be a top-factory !  NLO~830 pb : 2 tt events per second !  more than 10 million tt /year Measure total ttbar cross section: test of pQCD calculations (predicted at ~ 10%) sensitive to top mass Measure differential cross sections sensitive to new physics Make initial direct measurement of top mass Measure single top production (t-channel)

Top physics during commissioning Several months to achieve pixel alignment Study separation of top from background without b-tagging Use high multiplicity in final states High Pt cuts to clean sample Use kinematical features Even with a 5% efficiency 10evts/hour at 1033 Hadronic top: Three jets with highest PT W boson: Two jets in hadronic top with highest PT in reconstructed jjj C.M. frame W CANDIDATE TOP CANDIDATE

Top physics during commissioning m (topjjj) B S S/B = 0.45 S/B = 1.77 L=300 pb-1 m (topjjj) m(Wjj) |mjj-mW| < 10 GeV S : MC @ NLO B : AlpGen x 2 to account for W+3,5 partons (pessimistic) Expect ~ 100 events inside mass peak with only 300 pb-1 top signal observable in early days with no b-tagging and simple analysis W+jets background can be understood with MC+data (Z+jets)

Top signal significance vs luminosity Siginficance (s) Fitted #signal events Luminosity (pb-1) Luminosity (pb-1) Siginficance (s) Luminosity (pb-1) Nominal W+jets W+jets x 2 W+jets x 4 W+jets x 8

What can you do with early tops?  Calibrate light jet energy scale - impose PDG value of the W mass (precision < 1%)  Estimate/calibration b-tagging e - From data (precision ~ 5%) - Study b-tag (performance) in complex events  Study lepton trigger  Calibrate missing transverse energy - use W mass constraint in the event - range 50 GeV < p T < 200 GeV  Estimate (accuracy ~20%) of mt and tt. Use W boson mass to enhance purity Events Perfect detector Miscalibrated detector or escaping ‘new’ particle Missing ET (GeV)