Upload
lysa
View
21
Download
0
Embed Size (px)
DESCRIPTION
Physics Program of the experiments at L arge H adron C ollider. Lecture 3. The LHC physics programme. Search for Standard Model Higgs boson over ~ 115 < m H < 1000 GeV. - PowerPoint PPT Presentation
Citation preview
24/10/2006Prof. dr hab. Elżbieta Richter-Wąs
Physics Program of the experiments at
Large HadronCollider
Lecture 3
24/10/20062Prof. dr hab. Elżbieta Richter-Wąs
The LHC physics programme
•Search for Standard Model Higgs boson over ~ 115 < mH < 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, B0 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 ~ 10-5 s after Big Bang
(ALICE)
24/10/20063Prof. 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
24/10/20064Prof. dr hab. Elżbieta Richter-Wąs
Keyword: large event statistics
Expected event rates in ATLAS/CMS for representative known physics processes at low luminosity (L=1033 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 15 108 104 LEP / 107 Tev.
Z ee 1.5 107 107 LEP
0.8 107 104 Tevatron
105 1012 108 Belle/BaBar
QCD jets 102 109 107 TevatronpT > 200 GeV
tt
bb
24/10/20065Prof. dr hab. Elżbieta Richter-Wąs
Precise measurements: motivation
W mass and top mass are fundamentalparameters of the Standard Model
since GF, EM, sinW are known with high precision, precise measurements of mtop and mW constrain radiative corrections and Higgs mass (weakly because of logarithmic dependence)
radiative correctionsr ~ f (mtop
2, log mH)r 3%
rW
EM
1 sin
1
G 2
m
1/2
F
W
Fermi constant measured in muondecay
Weinberg anglemeasured at LEP/SLC
Electromagnetic constantmeasured in atomic transitions, e+e- machines, etc.
24/10/20066Prof. 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 MeVpreliminary results from winter/spring conferences
integrated luminosity ~ 200 fb-1 (in June 2005 Tevatron declared collected 1fb-1)
24/10/20067Prof. dr hab. Elżbieta Richter-Wąs
Why we need precise measurement?
For equal contribution to the Higgs mass uncertaintityMw ~ 0.7 10-2 mt
LHC will measure top masswith 1 GeV precision to match it the 7 MeV erroron the wish list ?!
-> test for the Standard Model consistency-> prediction for the Higgs mass
24/10/20068Prof. 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 mW at hadron colliders
24/10/20069Prof. dr hab. Elżbieta Richter-Wąs
W production at LHC
q’
qW e,
Ex.
e, (pp W + X) 30 nb
~ 300 106 events produced~ 60 106 events selected after analysis cuts
one year atlow L, perexperiment
~ 50 times larger statistics than at Tevatron
~ 6000 times larger statistics than WW at LEP
24/10/200610Prof. dr hab. Elżbieta Richter-Wąs
W boson production at LHC
24/10/200611Prof. dr hab. Elżbieta Richter-Wąs
W boson decay
24/10/200612Prof. dr hab. Elżbieta Richter-Wąs
W boson production at Tevatron
24/10/200613Prof. dr hab. Elżbieta Richter-Wąs
Transverse momentum of the charged lepton
24/10/200614Prof. dr hab. Elżbieta Richter-Wąs
Transverse momentum of the charged lepton
24/10/200615Prof. dr hab. Elżbieta Richter-Wąs
Transverse mass of the W boson (mT)
24/10/200616Prof. dr hab. Elżbieta Richter-Wąs
Transverse mass of the W boson (mT)
24/10/200617Prof. dr hab. Elżbieta Richter-Wąs
W mass measurement at Tevatron
24/10/200618Prof. dr hab. Elżbieta Richter-Wąs
Uncertainties on mW
Come mainly from capability of Monte Carlo prediction to reproduce real life, that is:• detector performance: energy resolution, energy scale, etc. • physics: pT
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 mW wrong by 1%
24/10/200619Prof. dr hab. Elżbieta Richter-Wąs
Calibration of detector energy scale
Example : EM
CALOe- beam
E = 100 GeV Emeasured
• if Emeasured = 100.000 GeV calorimeter is perfectly calibrated• if Emeasured = 99, 101 GeV energy scale known to 1%
• to measure mW to ~ 20 MeV need to know energy scale to 0.2 ‰ , i.e.
if E electron = 100 GeV then 99.98 GeV < Emeasured < 100.02 GeV
24/10/200620Prof. 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 mee = mZ
reconstructedknown to 10-5
from LEP
cell out
inin
check that all cells give same response: if not correct
24/10/200621Prof. dr hab. Elżbieta Richter-Wąs
Use Z events to calibrate recoil in W events
24/10/200622Prof. dr hab. Elżbieta Richter-Wąs
Expected precision on mW at LHC
Source of uncertainty mW
Statistical error << 2 MeVPhysics uncertainties ~ 15 MeV(pT
W, W, W, …)Detector performance < 10 MeV(energy resolution, lepton identification, etc,)Energy scale 15 MeVTotal(per experiment, per channel) ~ 25 MeV
Combining both channels (e) and both experiments (ATLAS, CMS), mW 15 MeV should be achieved.However: very difficult measurement
24/10/200623Prof. dr hab. Elżbieta Richter-Wąs
Recent (summer 2005) results from CDF
24/10/200624Prof. dr hab. Elżbieta Richter-Wąs
Measurement of mtop
• Top is most intriguing fermion: -- mtop 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 events
S+BS+B
BB
-- u c t d s b
m (t-b) 170 GeV radiative corrections
24/10/200625Prof. dr hab. Elżbieta Richter-Wąs
Why is top quark so important
24/10/200626Prof. dr hab. Elżbieta Richter-Wąs
Top production at LHC
e.g.g
g
t
t
t
t
q
q
(pp +X ) 800 pbtt
107 t tbar pairs produced in one year at low L
~ 102 times more than at Tevatron
measure mtop, tt, BR, Vtb, single top, rare decays (e.g. t Zc), resonances, etc.
production is the main background to new physics (SUSY, Higgs)
tt
24/10/200627Prof. 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 + ET
miss final states. BR 10 %. Little kinematic constraints to reconstruct mass.
-- semileptonic channel: one W jj , one W l 4 jets + 1l + ET
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
24/10/200628Prof. dr hab. Elżbieta Richter-Wąs
Tagging a b-quark
24/10/200629Prof. dr hab. Elżbieta Richter-Wąs
Example from D0: tt Wb Wb b bjj event
24/10/200630Prof. dr hab. Elżbieta Richter-Wąs
Expected precision on mtop at LHC
Source of uncertainty mtop
Statistical error << 100 MeVPhysics 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 : mtop ~ 1 GeV at LHC
From mtop ~ 1 GeV, mW ~ 15 MeV indirect measurement mH/mH ~ 25% (today ~ 50%)
24/10/200631Prof. 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 Mt, cross section σt
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
24/10/200632Prof. dr hab. Elżbieta Richter-Wąs
Learning from ttbar production
t
t
Abundant clean source of b jets2 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
24/10/200633Prof. dr hab. Elżbieta Richter-Wąs
Learning from ttbar production
Abundant source of W decays into light jetsInvariant 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 jetassignment for calibration purposeto avoid bias
If (limited) b-tagging is available,W jet assignment combinatoricsgreatly reduced
t
t
24/10/200634Prof. dr hab. Elżbieta Richter-Wąs
Learning from ttbar production
t
t
Known amount of missing energy4-momentum of single neutrino in eachevent can be constrained from eventkinematics
Inputs in calculation: m(top) from Tevatron, b-jet energy scale and lepton energy scale
Calibration of missingenergy relevant forall SUSY and mostexotics!
24/10/200635Prof. dr hab. Elżbieta Richter-Wąs
Learning from ttbar production
t
t
Two ways to reconstruct the top massInitially mostly useful in event selection,as energy scale calibrations must be understood before quality measurementcan 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
24/10/200636Prof. dr hab. Elżbieta Richter-Wąs
Various scenarios under study
pp collisionsWhat 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 scenariosPartly 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
24/10/200637Prof. dr hab. Elżbieta Richter-Wąs
UE / min-bias determination
LHC?
<nch> at = 0 Mtop 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
24/10/200638Prof. 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 10-5
This background has to be obtained from data itself
E.g. method developed by CDF during run-1:
e-,0
Use missing ET vs lepton isolation to define 4 regions:
A. Low lepton quality and small missing ET
Mostly non-W events (i.e. QCD background)
B. High lepton quality and small missing ET
Observation of reduction in QCD background by isolation cut
C. Low lepton quality and high missing ET
W enriched sample with a fraction of QCD background
D. High lepton quality and high missing ET
W enriched sampleThe 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)
24/10/200639Prof. dr hab. Elżbieta Richter-Wąs
Top Mass: physics TDR reconstruction
Gold-plated channel : single lepton
• pT (lep) > 20 GeV
• pTmiss > 20 GeV
• ≥ 4 jets with pT > 40 GeV
• ≥ 2 b-tagged jets
• | mjjb-<mjjb>|< 20 GeV
Uncertainty on light jet scale: Hadronic 1% Mt < 0.7 GeV10% M = 3 GeV
B-tagging essential,
JES dominate
Uncertainty On b-jet scale: Hadronic
1% Mt = 0.7 GeV5% Mt = 3.5 GeV10% Mt = 7.0 GeV
24/10/200640Prof. dr hab. Elżbieta Richter-Wąs
Pessimistic scenario: LArg dead Regions
EM clusters
Jets
mtop(without ) – mtop(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 100 000 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)
24/10/200641Prof. 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?
24/10/200642Prof. dr hab. Elżbieta Richter-Wąs
Top analysis w/o b-tag
• First apply selection cuts
• Assign jets to W, top decays
1 lepton PT > 20 GeV
Missing ET > 20 GeV
4 jets(R=0.4) PT > 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
24/10/200643Prof. dr hab. Elżbieta Richter-Wąs
Signal-only distributions
MW = 78.1±0.8 GeVmtop = 162.7±0.8 GeV
S/B = 1.20 S/B = 0.5
S
B
m(tophad) m(Whad)
TOP CANDIDATE
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 scalecalibration possible fromshift in m(W)
L=300 pb-1
(~1 week of running)
24/10/200644Prof. 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
24/10/200645Prof. dr hab. Elżbieta Richter-Wąs
Signal + Wjets background
S/B = 0.45 S/B = 0.27
S
B
m(tophad) m(Whad)
TOP CANDIDATE
W CANDIDATE • Plots now include W+jets background– Background level roughly triples– Signal still well visible– Caveat: bkg. cross section quite
uncertain
Jet energy scalecalibration possible fromshift in m(W)
L=300 pb-1
(~1 week of running)
24/10/200646Prof. dr hab. Elżbieta Richter-Wąs
Signal + Wjets background
TOP CANDIDATE
W CANDIDATE
• 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)
B
S
S/B = 0.45
S/B = 1.77
m(Whad)L=300 pb-1
(~1 week of running)
24/10/200647Prof. 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 pZ of neutrino
• Also clean signal quite a bit– m(W) cut not applied here
m(tophad) m(tophad)
B
S
S/B = 0.45 S/B = 1.11
SEMI LEPTONIC TOP CANDIDATE
|m(jl)-mt|<30 GeV
L=300 pb-1
(~1 week of running)
24/10/200648Prof. 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 = Eparton / Ejet
– Takes into account variation of rescaling parameter with energy and correlation between energies and opening angle
M E E Mjj j j j j W 2 11 2 1 2( cos )
jeti
parti
iWPDG
WE
EwithMM 21
before
after
2
,,
2,1
2
2χ
EX
W
Wjj
X
iXi
EiXMm
24/10/200649Prof. dr hab. Elżbieta Richter-Wąs
Exploiting ttbar as b-jet sample
TOP CANDIDATE
W CANDIDATE
• 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)‘random jet’,
no b enhancement expected
Clear enhancementobserved!
ttbar (signal)‘always b jet if all jet assignment are OK’
b enrichment expected and observed
24/10/200650Prof. dr hab. Elżbieta Richter-Wąs
Use b-tag of 4th 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
24/10/200651Prof. 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 l 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
24/10/200652Prof. 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 measurementsCan 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, ET-miss, event topology
Many useful spinoffsHadronic W sample light quark jet energy scale calibrationKinematically identified b jets useful for b-tag calibration
Continue to improve realism of study and quality of analysisImportant improvement in W+jets estimate underwayIncorporate and estimate trigger efficiency to few (%)Also continue to improve jet assignment algorithms
Expect estimate of (ttbar) with error ~20% in first running periodOne of the first physics measurements of LHC?