24/10/2006Prof. dr hab. Elżbieta Richter-Wąs Physics Program of the experiments at L arge H adron C ollider Lecture 3

24/10/ Prof. dr hab. Elżbieta Richter-Wąs The LHC physics programme Search for Standard Model Higgs boson over ~ 115 < m H < 1000 GeV. Search for Supersymmetry and other physics beyond the SM (q/l compositness, leptoquarks, W’/Z’, heavy q/l, unpredicted ? ….) up to masses of ~ 5 TeV Precise measurements : -- W mass, WW , WWZ Triple Gauge Couplings -- top mass, couplings and decay properties -- Higgs mass, spin, couplings (if Higgs found) -- B-physics: CP violation, rare decays, B 0 oscillations (ATLAS, CMS, LHCb) -- QCD jet cross-section and  s -- etc. …. Study of phase transition at high density from hadronic matter to plasma of deconfined quarks and gluons. Transition plasma  hadronic matter happened in universe ~ s after Big Bang (ALICE)

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Outline of this lecture Physics programme:  precise measurement of the W and top-quark mass  detector comissioning with top physics

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Keyword: large event statistics Expected event rates in ATLAS/CMS for representative known physics processes at low luminosity (L=10 33 cm -2 s -1 )  LHC is a B-factory, top factory, W/Z factory,.... also possible Higgs factory, SUSY factory, …. Process Events/s Events/year Other machines (total statistics) W  e LEP / 10 7 Tev. Z  ee LEP Tevatron Belle/BaBar QCD jets Tevatron p T > 200 GeV

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Precise measurements: motivation W mass and top mass are fundamental parameters of the Standard Model  since G F,  EM, sin  W are known with high precision, precise measurements of m top and m W constrain radiative corrections and Higgs mass (weakly because of logarithmic dependence) So far : W mass measured at LEP2 and Tevatron top mass measured at the Tevatron radiative corrections  r ~ f (m top 2, log m H )  r  3% Fermi constant measured in muon decay Weinberg angle measured at LEP/SLC Electromagnetic constant measured in atomic transitions, e + e - machines, etc.

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Precision of direct measurements 1983: UA1 5 GeV 1989: UA1 2.9 GeV 1990: UA2 900 MeV 1995: CDF 180 MeV 2000: DO 91 MeV combined Run I 59 MeV ( L =120pb -1 ) 2004: LEP 42 MeV Tevatron, Run II data : CDF: 79 MeV D0: 84 MeV preliminary results from winter/spring conferences integrated luminosity ~ 200 fb -1 (in June 2005 Tevatron declared collected 1fb -1 )

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Why we need precise measurement? For equal contribution to the Higgs mass uncertaintity  M w ~  m t LHC will measure top mass with 1 GeV precision  to match it the 7 MeV error on the wish list ?! -> test for the Standard Model consistency -> prediction for the Higgs mass

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Measurement of W mass Method used at hadron colliders different from e + e - colliders W  jet jet : cannot be extracted from QCD jet-jet production  cannot be used W  : since  + X, too many undetected neutrinos  cannot be used only W  e and W   decays are used to measure m W at hadron colliders

24/10/ Prof. dr hab. Elżbieta Richter-Wąs W production at LHC q’ q W e,  Ex. e,   (pp  W + X)  30 nb ~ 300  10 6 events produced ~ 60  10 6 events selected after analysis cuts one year at low L, per experiment ~ 50 times larger statistics than at Tevatron ~ 6000 times larger statistics than WW at LEP ~ 50 times larger statistics than at Tevatron ~ 6000 times larger statistics than WW at LEP

24/10/ Prof. dr hab. Elżbieta Richter-Wąs W boson production at LHC

24/10/ Prof. dr hab. Elżbieta Richter-Wąs W boson decay

24/10/ Prof. dr hab. Elżbieta Richter-Wąs W boson production at Tevatron

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Transverse momentum of the charged lepton

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Transverse momentum of the charged lepton

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Transverse mass of the W boson (m T )

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Transverse mass of the W boson (m T )

24/10/ Prof. dr hab. Elżbieta Richter-Wąs W mass measurement at Tevatron

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Uncertainties on m W Come mainly from capability of Monte Carlo prediction to reproduce real life, that is: detector performance: energy resolution, energy scale, etc. physics: p T W,  W,  W, backgrounds, etc. Dominant error (today at Tevatron, most likely also at LHC): knowledge of lepton energy scale of the detector: if measurement of lepton energy wrong by 1%, then measured m W wrong by 1%

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Calibration of detector energy scale Example : EM CALO e - beam E = 100 GeV E measured if E measured = GeV  calorimeter is perfectly calibrated if E measured = 99, 101 GeV  energy scale known to 1% to measure m W to ~ 20 MeV need to know energy scale to 0.2 ‰, i.e. if E electron = 100 GeV then GeV < E measured < GeV  one of most serious experimental challenges

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Calibration strategy detectors equipped with calibration systems which inject known pulses: calorimeter modules calibrated with test beams of known energy  set the energy scale inside LHC detectors: calorimeter sits behind inner detector  electrons lose energy in material of inner detector  need a final calibration “ in situ ” by using physics samples: e.g. Z  e + e - decays 1/sec at low L constrain m ee = m Z reconstructed known to  from LEP cell out in  check that all cells give same response: if not  correct

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Use Z events to calibrate recoil in W events

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Expected precision on m W at LHC Source of uncertainty  m W Statistical error << 2 MeV Physics uncertainties ~ 15 MeV (p T W,  W,  W, …) Detector performance < 10 MeV (energy resolution, lepton identification, etc,) Energy scale 15 MeV Total (per experiment, per channel) ~ 25 MeV Combining both channels ( e  ) and both experiments (ATLAS, CMS),  m W  15 MeV should be achieved. However: very difficult measurement

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Recent (summer 2005) results from CDF

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Measurement of m top Top is most intriguing fermion: -- m top  174 GeV  clues about origin of particle masses ? Discovered in ‘94 at Tevatron  precise measurements of mass, couplings, etc. just started Top mass spectrum from CDF tt  bl bjj eventsS+B B -- u c t d s b  m (t-b)  170 GeV  radiative corrections

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Why is top quark so important

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Top production at LHC e.g. g g t t t t q q  (pp  +X )  800 pb 10 7 t tbar pairs produced in one year at low L ~ 10 2 times more than at Tevatron measure m top,  tt, BR, V tb, single top, rare decays (e.g. t  Zc), resonances, etc. production is the main background to new physics (SUSY, Higgs)

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Top decays t W b BR  100% in SM -- hadronic channel: both W  jj  6 jet final states. BR  50 % but large QCD multijet background. -- leptonic channel: both W  l  2 jets + 2l + E T miss final states. BR  10 %. Little kinematic constraints to reconstruct mass. -- semileptonic channel: one W  jj, one W  l  4 jets + 1l + E T miss final states. BR  40 %. If l = e,  gold-plated channel for mass measurement at hadron colliders. In all cases two jets are b-jets  displaced vertices in the inner detector

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Tagging a b-quark

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Example from D0: tt  Wb Wb  b  bjj event

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Expected precision on m top at LHC Source of uncertainty  m top Statistical error << 100 MeV Physics uncertainties ~ 1.3 GeV (background, FSR, ISR, fragmentation, etc. ) Jet scale (b-jets, light-quark jets) ~ 0.8 GeV Total ~ 1.5 GeV (per experiment, per channel) Uncertainty dominated by the knowledge of physics and not of detector. By combining both experiments and all channels :  m top ~ 1 GeV at LHC From  m top ~ 1 GeV,  m W ~ 15 MeV  indirect measurement  m H /m H ~ 25% (today ~ 50%) If / when Higgs discovered, comparison of measured m H with indirect measurement will be essential consistency checks of EWSB

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Detector commissioning with top physics  Top one of ‘easier’ bread and butter Cross section 830±100 pb 1. Used as calibration tool 2. Rich in ‘precision and new physics’ Top mass M t, cross section σ t 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 and missing energy

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Learning from ttbar production t t 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

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Learning 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 t t

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Learning from ttbar production t t 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 Calibration of missing energy relevant for all SUSY and most exotics!

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Learning from ttbar production t t Two ways to reconstruct the top mass  Initially mostly useful in event selection, as energy scale calibrations must be understood before quality measurement can be made  Ultimately determine m(top) from kinematic fit to complete event Needs understanding of bias and resolutions of all quantities Not a day 1 topic

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Various scenarios under study pp collisions  What variations in predictions of t-tbar – which generator to use?  Underlying event parameterization  Background estimation from MC  Aim to be as independent from MC as possible. Detector pessimistic scenarios  Partly or non-working b-tagging at startup  Dead regions in the LArg  Jet energy scale  Get good ‘feel’ for important systematic uncertainties – use data to check data  Estimate realistic potential for top physics during the first few months of running

24/10/ Prof. dr hab. Elżbieta Richter-Wąs UE / min-bias determination LHC? at  = 0 M top for various UE models Top peak for standard analysis using 2 b-tags Difference can be as large as 5 GeV Extremely difficult to predict the magnitude of the UE at LHC  Will have to learn much more from Tevatron before startup  Energy dependence of dN/d  ?  Really need data to check data on UE – Only few thousand events required

24/10/ Prof. dr hab. Elżbieta Richter-Wąs QCD Multijet Background Not possible to realistically generate this background  Crucially depends on Atlas’ capabilities to minimize mis-identification and increase e/  separation of This background has to be obtained from data itself  E.g. method developed by CDF during run-1: e-,0e-,0 Use missing ET vs lepton isolation to define 4 regions: A. Low lepton quality and small missing E T Mostly non-W events (i.e. QCD background) B. High lepton quality and small missing E T Observation of reduction in QCD background by isolation cut C. Low lepton quality and high missing E T W enriched sample with a fraction of QCD background D. High lepton quality and high missing E T W enriched sample The QCD reduction factor B/A can be applied to the “W enriched sample “ (region C and D). The non-W candidate in D will therefore be (B/A)xC. Therefore, the fraction of non-W events in the region D will be: (B.C)/(A.D)

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Top Mass: physics TDR reconstruction Gold-plated channel : single lepton p T (lep) > 20 GeV p T miss > 20 GeV ≥ 4 jets with p T > 40 GeV ≥ 2 b-tagged jets | m jjb - |< 20 GeV Uncertainty on light jet scale: Hadronic 1%   M t < 0.7 GeV 10%   M = 3 GeV B-tagging essential, JES dominate Uncertainty On b-jet scale: Hadronic 1%   M t = 0.7 GeV 5%   M t = 3.5 GeV 10%   M t = 7.0 GeV

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Pessimistic scenario: LArg dead Regions EM clusters Jets m top (without ) – m top (with) ATLAS Preliminary Argon gap (width ~ 4 mm) is split in two half gaps by the electrode  ~ 33 / 1024 sectors where we may be unable to set the HV on one half gap  multiply energy by 2 to recover Simulated tt events (~ 1.5 days at LHC at low L) If 33 weak HV sectors die (very pessimistic), effects on the top mass measurement, after a crude recalibration, are:  Loss of signal: < 8 %  Displacement of the peak of the mass distribution: -0.2 GeV  (Increase in background not studied)

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Pessimistic scenarios: b-tagging & JES Algorithms benefiting from early top-sample for calibration  B-tagging Identify jets originating from b quarks from their topology Select a pure t-tbar sample with tight kinematical cuts Compare 0 vs 1 vs 2 b-tagged jets in top events Can expect the b-tagging efficiency different in data from MC  Jet energy scale calibration Relate energy of reconstructed jet to energy of parton Dependent of flavor of initial quark  need to measure separately for b jets Observation of hadronic W for calibrating JES In most pessimistic scenario b-tag is absent at start Can we observe the top without b-tagging?

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Top analysis w/o b-tag First apply selection cuts Assign jets to W, top decays 1 lepton P T > 20 GeV Missing E T > 20 GeV 4 jets(R=0.4) P T > 40 GeV Selection efficiency = 5.3% 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. W CANDIDATE

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Signal-only distributions M W = 78.1±0.8 GeV m top = 162.7±0.8 GeV S/B = 1.20 S/B = 0.5 S B m(top had )m(W had ) TOP CANDIDAT E W CANDIDATE 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 Jet energy scale calibration possible from shift in m(W) L=300 pb -1 (~1 week of running)

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Consistency checks m(t) Subset of events where chosen 3-jet combination does not line up with top quark Empirical background shape describes combinatoric background well under peak

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Signal + Wjets background S/B = 0.45 S/B = 0.27 S B m(top had )m(W had ) TOP CANDIDAT E W CANDIDATE Plots now include W+jets background –Background level roughly triples –Signal still well visible –Caveat: bkg. cross section quite uncertain Jet energy scale calibration possible from shift in m(W) L=300 pb -1 (~1 week of running)

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Signal + Wjets background TOP CANDIDAT E W CANDIDATE Now also exploit correlation between m(top had ) and m(W had ) –Show m(top had ) only for events with |m(jj)-m(W)|<10 GeV m(top had ) B S S/B = 0.45 S/B = 1.77 m(W had ) L=300 pb -1 (~1 week of running)

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Signal + Wjets background TOP CANDIDATE Can also clean up sample by with requirement on m(jl ) [semi-leptonic top] –NB: There are two m(top) solutions for each candidate due to ambiguity in reconstruction of p Z of neutrino Also clean signal quite a bit –m(W) cut not applied here m(top had ) B S S/B = 0.45 S/B = 1.11 SEMI LEPTONIC TOP CANDIDATE |m(jl)-m t |<30 GeV L=300 pb -1 (~1 week of running)

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Exploiting ttbar for JES calibration Use the W mass constraint to set the JES –Rescale jet E and angles to parton energy  = E parton / E jet –Takes into account variation of rescaling parameter with energy and correlation between energies and opening angle before after

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Exploiting ttbar as b-jet sample TOP CANDIDAT E W CANDIDATE Simple demonstration use of ttbar sample to provide b enriched jet sample –Cut on m(W had ) and m(top had ) masses –Look at b-jet prob for 4 th jet (must be b-jet if all assignments are correct) W+jets (background) ‘random jet’, no b enhancement expected Clear enhancement observed! ttbar (signal) ‘always b jet if all jet assignment are OK’ b enrichment expected and observed

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Use b-tag of 4 th jet ttbar W+jets Standard analysis (for comparison) Cut on b signal probability > 0.90 on 4th jet ttbar W+jets Use b-tag of 4th jet to clean up hadronic top

24/10/ Prof. dr hab. Elżbieta Richter-Wąs How well we understand W+jets bgd.? 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 –MLM matching prescription to explicit elimination of double counting. W  l W + 4 partons (32 pb * ) W + 3 partons (80 pb * ) W + 5 partons (15 pb * ) parton is reconstructed as 2 jets 2 parton reconstructed as single jets * These are the cross sections with the analysis cuts on lepton and jet pT applied at the truth level

24/10/ Prof. dr hab. Elżbieta Richter-Wąs Summary on the top mass Understand the interplay between using the top signal as tool to improve the understanding of the detector (b-tagging, jet E scale, ID, etc..) and top precision measurements Can reconstruct top and W signal after ~ one week of data taking without using b tagging  Can progressively clean up signal with use of b-tag, E T -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 and quality of analysis  Important improvement in W+jets estimate underway  Incorporate and estimate trigger efficiency to few (%)  Also continue to improve jet assignment algorithms Expect estimate of  (ttbar) with error ~20% in first running period  One of the first physics measurements of LHC?