PHYSICS WITH LHC EARLY DATA

Transcription

1 PHYSICS WITH LHC EARLY DATA ONE OF THE LAST PROPHETIC TALKS ON THIS SUBJECT HOPEFULLY We may have some two month of the Machine operation in 2008 LONG HISTORY... I will extensively use: Fabiola GIANOTTI CERN, ATLAS 2004 Wouter VERKERKE NIKHEF, AOW Paris 2005 Fabiola GIANOTTI CERN, ATLAS / /9/9/332 Fabrice HUBAUT CPPM/IN2P3, ATLAS & CMS I lepsza referencja Pamela FERRARI ATLAS & CMS asxiv hep ex v2 Albrert De ROECK CERN, KITF Seminar, 1 Apr 2008 Anna KACZMARSKA IFJ PAN Institute Seminar 16 Feb / 34

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5 What energy at the start? 900 GeV 10 TeV 12 TeV 14 TeV 5 / 34

6 Peter Jenni ATLAS Coll. Meeting 7 Apr. 08 LHC machine status and schedule A rather extensive meeting took place on 3rd March between the DG R Aymar, J Engelen and ATLAS represented by M Nessi and PJ During this meeting we were informed about the machine status, we had a chance to explain our schedule, and to give input to the LHC start-up strategy discussions The main conclusions, which are still valid, were: - The current planning of the LHC is such that the machine is expected to be cold by by mid-june, and first injections could start soon after, from end June onwards - CERN Management will give ATLAS a warning signal 2 months before we have to start closing, currently expected by mid-april (around RRB) when the progress on the machine will be known from the cooldown status of further sectors - Experience from power tests in Sector 4-5 showed that several magnets need training, starting somewhere above 5 TeV equivalent - The first physics run (typically 2 months) in 2008 will be at 10 TeV (or slightly above), the energy will be defined in April when experience from Sector 5-6 will also be available Similar statements were also made during this Council Week, in addition it was said that 10 TeV collisions could be reached sometime in August 6 / 34

7 One can follow cooling states of sectors almost on line that must take still some time 7 / 34

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9 Some LHC parameters COLLISION ENERGY INJECTION ENERGY PROTONS PER BUNCH NUMBER OF BUNCHES NOMINAL BUNCH SPACING NOMINAL LUMINOSITY REVOLUTION TIME (REVOLUTION FREQUENCY STORED BEAM ENERGY BUNCH LENGTH LUMINOSITY LIFETIME ACCELERATION TIME BEAM RMS at IP 450GeV/7TeV 7 TeV 450 GeV ns 34 cm-2s µs khz) * 336 MJ 75.5 mm 10 h 20 min.375mm/16.63 µm 9 / 34

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11 WHAT IS THIS ALL FOR? Flagships: Higgs searches physics beyond the SM supersymmetry extra dimensions precision physics (W, top, Higgs mass,...) b physics quark gluon plasma BUT it is NOT the first day physics. These goals and unprecedented complexity of detectors (ATLAS and CMS) require first: commissioning of detectors and triggers, first calibrations and alignment with minimum bias and QCD jet production, measure of some exclusive channels (e.g. Z > lepton lepton) to set the absolute electron and mion ECAL energy scale measure tt events to determine the absolute jet energy scale and understand b tagging in the inner detectors 11 / 34

12 Recently, prof. Peter W. Higgs has visited the ATLAS detector. Some say, that it is going to be the only Higgs seen in ATLAS 12 / 34

13 ATLAS Inner Detector: (Silicon pixels + strips +TRT particle ID (e/π ) ) B=2T, σ /pt ~ 4x10-4 pt 0.01 (e.g. H bb) Height ~ 22 m Length ~45m weight ~7000 t Muon system: (precision chambers + triggers in air core toroids B=4T mean value), σ /pt ~ 7 % at 1 TeV standalone (e.g. H,A µµ, H 4µ) Electromagnetic calorimeter : (Pb, liquid argon) σ /E ~ 10%/ E, Uniform longitudinal segmentation Provides: e/γ identification, energy and angular resolution, γ/jet, γ /π0 separation (e.g. H γγ) Hadronic Calorimeters: (Fe scint Cu-liquid argon) σ /E ~ 50%/ E 0.03 Jet, ETmiss performance (e.g. H τ τ, H bb) 13 / 34

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16 business plan L= / 34

17 A solid candidate for the very early valuable result: it is enough to collect events to measure dnch/ dƞ 17 / 34

18 Considerable statistics easily vailable to study and estimate absolute scales for electron and mion ECAL measurements as well as for the absolute jet energy scale 18 / 34

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20 First picks: rediscovery of the known physics to demostrate the detector understanding J/ψ µ µ 1 pb-1 After all cuts: ~ 4200 (800) J/ψ (Y) µ µ ev./day, L = 1031 Y µ µ (assume 30% efficiency of data taking) 10 pb-1 ATLAS preliminary After all cuts: ~ 160 Z µ µ ev./day at L = 1031 (assume 30% data taking efficiency) alignment of mion spectrometer, momenta calibrations for the ID and mion system, trigger optimization, test of the Kraków, 6 May 2008of mion reconstruction efficiency ATLAS preliminary 20 / 34

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22 FASCINATING ROLE OF ttbar EVENTS : ONE GETS TO INTERESTING PHYSICS SIMULTANEOUSLY CALIBRATING MANY HIGHER LEVEL RECONSTRUCTION CONCEPTS SUCH AS JET ENERGY SCALES, B TAGGING AND MISSING ENERGY I have learned that from Wouter Verkerke at Paris AOW in 2005! Top production and decay at LHC LHC σ ~850 pb 10% qq, 90% gg Tevatron σ ~7 pb 85% qq, 15% gg LHC will be a top factory: 107 events already with 10 fb-1 22 / 34

23 Top physics at LHC Large ttbar production cross section at LHC sˆ = sx1 x2 ; x1 x2 ~ 10 3 gg tt σ tt( tot) = 759±100 pb Nevt ~ 700/hour qq tt Production: gluon dominated at LHC, quark dominated at Tevatron About 100 times larger than cross section at Tevatron (and lumi also) 23 / 34

24 Top topology Decay products are 2 W bosons and two b quarks. About 99.9% decays to Wb For commissioning studies consider events with one hadronic and one semi-leptonic W decay (about 30% of the total ttbar cross section 24 / 34

25 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 25 / 34

26 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 W mass in jet assignment for calibration purpose to avoid bias If (limited) b tagging is available, W jet assignment combinatorics greatly reduced 26 / 34

27 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: m(top) from Tevatron, b jet energy scale and lepton energy scale 27 / 34

28 How to identify ttbar events? Restrict ourselves to basic (robust) quantities Apply some simple cuts Hard pt cuts really clean up sample (ISR). Possible because of high production rate 4 hard jets (PT >40 GeV) Combined efficiency of requirements is ~5% still have ~10 evts/hour 1 hard lepton (Pt >20 GeV) Missing ET (ET >20 GeV) 28 / 34

29 Signal only distributions (Full Simulation) W CANDIDATE TOP CANDIDAT E Clear top, W mass peaks visible Background due to mis assignment of jets Masses shifted somewhat low mtop = 162.7±0.8 GeV Easier to get top assignment right than to get W assignment right m(tophad) Effect of (imperfect) energy calibration MW = 78.1±0.8 GeV L=300 pb-1 (~1 week of running) m(whad) Jet energy scale calibration possible from shift in m(w) S B S/B = / 34

30 Signal + Wjets background (Full Simulation) W CANDIDATE TOP CANDIDAT E 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 scale calibration possible from shift in m(w) L=300 pb-1 (~1 week of running) S/B = 0.45 S/B = 0.27 S/B = / 34

31 Signal + Wjets background (Full Simulation) W CANDIDATE TOP CANDIDAT E Now also exploit correlation between m(tophad) and m(whad) Show m(tophad) only for events with m(jj) m(w) <10 GeV m(tophad) m(tophad) m(whad) L=300 pb-1 (~1 week of running) S/B = 1.77 S/B = / 34

32 Signal + Wjets background (Full Simulation) Can also clean up sample by with requirement on m(jlν) [semi leptonic top] NB: There are two m(top) solutions for each candidate due to ambiguity in reconstruction of pz of neutrino Also clean signal quite a bit m(w) cut not applied here m(tophad) m(tophad) L=300 pb-1 (~1 week of running) m(jlν )-mt <30 GeV S/B = 0.45 S/B = / 34

33 Exploiting ttbar as b jet sample (Full Simulation) W CANDIDATE TOP CANDIDAT E Simple demonstration use of ttbar sample to provide b enriched jet sample Cut on m(whad) and m(tophad) masses Look at b jet prob for 4th jet (must be b jet if all assignments are correct) W+jets (background) ttbar (signal) random jet, no b enhancement expected always b jet if all jet assignment are OK b enrichment expected and observed AOD b-jet probability AOD b-jet probability Clear enhancement observed! 33 / 34

34 If we are lucky we may have some interesting use of the LHC machine already by this winter. Any way, come to Kraków EPS Conference next year 34 / 34