Physics of Hadron Colliders Lecture 3: m t, M W, Higgs Joseph Kroll University of Pennsylvania 17...
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Physics of Hadron CollidersLecture 3: mt, MW, Higgs
Joseph Kroll
University of Pennsylvania
17 June 2003
17 June 2004 Joseph Kroll University of Pennsylvania 2
“Guaranteed” Physics Goals of Run II
• Precision measurement of top mass mt
– Discuss Run I results including recent DØ update– Show preliminary Run II results
• Precision measurement of W boson mass MW
– Discuss method and report Run I results– No preliminary Run II results – discuss prospects
• Measurement of B0s flavor oscillations: ms
ms will be discussed in our 4th lecture on 21 June 2004
Today: discuss measurement of mt and MW & Higgs search
17 June 2004 Joseph Kroll University of Pennsylvania 3
Recent Tevatron Performance
14 June 2004Record luminosity:CDF + DØ average78.3 £ 1030 cm-2 s-1
CDF luminosity83.5 £ 1030 cm-2 s-1
DØ luminosity71.4 £ 1030 cm-2 s-1
Run 1 record was24 £ 1030 cm-2 s-1
17 June 2004 Joseph Kroll University of Pennsylvania 4
Recent Tevatron Stores (since 1 June 2004)
Initial luminosities of mostrecent stores in 1030 cm-2s-1
17 June 2004 Joseph Kroll University of Pennsylvania 5
Motivation for SppS: Find W§ & Z0
With known constants:
Could predict MW and MZ
At the time recently determinedfrom and anti- scattering
Too heavyfor existingaccelerators
Recall: Lecture One
This was the mid ’70’s
17 June 2004 Joseph Kroll University of Pennsylvania 6
Motivation Today:W Mass, Top Mass
Preferred known constants have changed: PDG: K. Hagiwara et al., Phys. Rev. D66 010001 (2002)
various measurements
muon lifetime
Z0 pole LEP I
17 June 2004 Joseph Kroll University of Pennsylvania 7
W Mass Depends on Top Mass & Higgs Mass
figure courtesy A. Kotwal (Duke)
Radiative corrections introduce dependence (r) on mt, MH
MSSM: SUSY in loops can cause 250 MeV shift in MW
17 June 2004 Joseph Kroll University of Pennsylvania 8
W Mass and Top Mass constrain Higgs Mass
figure courtesy PDG
top mass
W m
ass
Higgs mass
Direct: measurements of mt, MW
All: combination (90% C.L!)
Indirect: no mt, MW
includes Z pole, scattering,Cs decay etc.
n.b. with everything but mt predict
17 June 2004 Joseph Kroll University of Pennsylvania 9
LEP Electroweak Working Group (April ’04)
see http://lepewwg.web.cern.ch/LEPEWWG/
April ’04: now only use high-Q2 datano NuTeV, Parity Viol. in Cs, e-e-
17 June 2004 Joseph Kroll University of Pennsylvania 10
Measuring the Top Mass - Overview
• top quarks produced in pairs– two measurements of mt per event – constrain to one common value– three possible event topologies: can only fully reconstruct m t in two– resolve combinatorics with 2 selection
• All Hadronic: 6 jets, one or two b-tags (brief mention)– poorest signal to background– must connect observed jet 4-vector to parton 4-vector– combinations per event: 12 (2 b-tags) or 24 (1 b-tag)– constraint pairs of jets to W mass, no neutrino
• Lepton plus jets: with or without b-tags (our focus)– better signal to background– well measured lepton, but ambiguous pL
(from MW constraint)– 2 comb. (2 b-tag), 6 comb. (1-btag), 12 comb (no b-tag) (actually £ 2 –see later)
• Dilepton: with or without b-tags (not discussed due to lack of time)– best signal to background– two well measured leptons, but two cannot fully reconstruct top– two combinations
17 June 2004 Joseph Kroll University of Pennsylvania 11
All Hadronic ModeF. Abe et al., Phys. Rev. Lett. 79, p. 1992 (1997)
• Run I: based on 109 § 7 pb-1
• specialized trigger: 4 jets ET>15, ET>125 GeV• for mass analysis add ET > 200 GeV, aplanarity, ≥ 6 jets, b-tag• 136 events remain• estimated background 108±9 • kinematic fit for mt in each event
Systematics: connecting jets to partons &jet energy scale, fit model, backgrounds
2 b-tag subsample
17 June 2004 Joseph Kroll University of Pennsylvania 12
Lepton + Jets Mode (Overview)
X balances top pair in transverse plane
b jet and W add up to top yielding mt
mt from each b W pair set equal
quark pair invariant mass constrained to W mass
lepton pair invariant mass constrained to W mass two solutions to pL
( × 2 combinatorics)
tagging b-jets reduces combinatorics
17 June 2004 Joseph Kroll University of Pennsylvania 13
Lepton + Jets Event Selection (for mt)documentation of mass analysis: CDF Collaboration, F. Abe et al., PRD 63 032003 (2001)
additional selection criteria for top mass measurement
• e or with ET (pT) > 20 GeV• ET > 20 GeV• require lepton to be isolated• remove top dilepton candidates• remove Z candidates (including ee and )• require PV within § 60 cm of nominal z = 0• require 3 jets, ET > 15 GeV, || < 2
Run I (106 pb-1): 324 candidates remain – same as cross-section analysis
• 4th jet with ET > 8 GeV, || < 2.4 (Run I: 163 candidates)• Mass reconstruction: kinematic fit 2<10
Run I: 151 events remain, 34 with b-tag (vertex tag or lepton tag)
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Classification of Events (Run I)
Class I: 4th jet satisfies ET > 8 GeV and || < 2.4 (87 of 151 events)Class II: 4th jet satisfies ET > 15 GeV and || < 2.0 (64 of 151 events)
For mass fits: put events in 4 mutually exclusive categories:
1. 2 jets with vertex tags2. 1 & only 1 jet with vertex tag3. 1 or 2 jets with soft lepton tag4. no b-tags, Class II only
Separating b-tag events into 3 categories 10% improvement on statistical error on mt
Adding category 4 7% further improvement
remaining 75 events 93% background no significant improvement from adding
17 June 2004 Joseph Kroll University of Pennsylvania 15
Signal Simulation and Backgrounds
Signal simulation based on HERWIG Monte Carlo (V5.6)• leading order QCD Matrix Element for hard scatter• coherent parton shower evolution• cluster hadronization• underlying event model based on data• used to make top mass templates for different values of mt
• also use PYTHIA and ISAJET MC as cross-checks
Background determinationW + jets uses VECBOS MC• parton level with ME for W + up to 4 jets• evolve/hadronize partons using HERWIG• normalize predictions to untagged W + jets in data
Other processes (WW, WZ, ZZ, Z→, single top)• combination of PYTHIA & HERWIG• WW, WZ, ZZ, single t normalized to theory, to data
Run II: ReplaceVECBOS withALPGEN
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Top Mass Sample Composition
Background contribution constrained in top mass fit
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Resolutions
electrons (ET in GeV)
muons (pT in GeV)
Jets (for pT > 80 GeV, non-heavy flavor)
Jets require complicated set of corrections to get back to parton
17 June 2004 Joseph Kroll University of Pennsylvania 18
Jet Energy CorrectionsJets formed from “raw”calorimeter energies
Detector Effects
Physics Effects
• nonlinear calorimeter response to low energy hadrons• B field bends low pT particles out of cone (or do not reach Cal)• cracks and transition regions of calorimeter• different response of EM & Had
• extra E from “underlying event” & multiple interactions• fragmentation effects & soft g rad.• and
Calibrate central calorimeter(||<1) in situ with spectrometer
Balance calorimeter responseout to ||=2.4 using dijet data
Check jet energy scalewith vs. jet data
Typical correction: increase raw ET by 30%
Recall from Lecture 2
17 June 2004 Joseph Kroll University of Pennsylvania 19
Jet Energy Corrections for Top Mass
Jet corrections for top mass have two steps:1. Flavor independent corrections to all jets ET > 8 GeV 2. 4 leading jets have tt specific corrections to convert jet energy to parton momentum (additional corrections applied to “X” system balancing tt
1. Flavor independent corrections: PTraw(R) PT(R) (R = 0.4 here)
frel: even out relative calorimeter response in (dijet balancing) func. of
UEM: correct for multiple interactions = (297 ± 100 MeV) per additional vertex
fabs: absolute E scale (raw E true E) – assumes flat pT spectrum – func. pT
UE: correct for energy from underlying event = 650 ± 195 MeV per jet
OC: out of cone correction – function of pT
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Relative Jet Corrections in
Technique: two jet topology (dijet) has zero net pT (balanced)
CDF II: preliminary as of Summer 2003
▲ MC ○ data
Use MC to determinemany jet corrections(e.g., top specific)
MC must reproduce datagreat effort to tune simul.
Calorimeter transition (crack) regions
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Jet Corrections and Systematics
Typical top jet has 30 < pT < 90 GeV; typical correction: 1.45
Systematic uncertaintyon jet correction forcorrected jet PT
4%
Corrected Jet PT
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2. Top Event Specific Jet Corrections
Three sources – evaluated with HERWIG assuming mt = 170 GeV
(a) Assumed jet P_T spectrum: ► flavor independent corrections used flat P_T spectrum ►corrects for different spectrum of jets from top decay
(b) Heavy flavor jets b (mainly) and c ►harder fragmentation than light quark & gluon jets ►semileptonic decays produce neutrinos – undetected ► from semileptonic decay deposit 2 GeV on average
(c) Top multijet environment ►dijet environment used to derive corrections
Only applied to the four highest PT jets
17 June 2004 Joseph Kroll University of Pennsylvania 23
Top Event Specific Jet Corrections (cont.)
Procedure to determine & apply correction
Associate partons to jets in - space
Compare: jet PT (after flavor indep. corr.) to parton PT
Correction is median of distribution:
Spread (for kinematic mt fit) of distribution characterized by = ½ difference in PT between 16th and 84th percentile of dist.
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Top Dependent Corrections and Resolution
Fractional Correction Fractional Spread
(A) W-jet(B) b-jet(C) b! e(D) b!
(A) W-jet(B) b-jet(C) b! e(D) b!(E) other
Correction function of jet PT after flavor independent correction
17 June 2004 Joseph Kroll University of Pennsylvania 25
Kinematic Fit
2 is formed from measured quantities for each combination in each event
Combination with minimum 2 determines p and Mt for event
notation: UE is unclustered energy, W = 2.06 GeV, t = 2.5 GeV
BTW: jet directions measured much better than energy
partons are assigned to jets – jet 4-vectors adjusted according to resolutions i,j
require 2min<10
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Likelihood Fit Determines Top Mass
mt distribution from top candidates is compared to expected distributionfrom top at different masses and from background processes Likelihood
176.1 § 5.1(stat.) § 5.3 (syst.) GeV
Result
Systematics
n.b. statistical error is “lucky” (low)
17 June 2004 Joseph Kroll University of Pennsylvania 27
CDF II Top Mass Results
CDF has several preliminary measurements of mt from Run II data
Same Run II b-tagged samplementioned at end of Lecture 2
Only top candidates with≥ 1 b jet (vertex tags only)no soft lepton or class II yet
Systematic error dominatedby jet energy scale, which isnot understood as well as in Run I
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Dynamic Likelihood Method
Selection identical to previous lepton + jets (b-tag) analysis exceptRequire exactly 4 jets ET > 15 GeV, || < 1 (reduce ISR and FSR)
22 eventsin 162 pb-1
The same data – just a different way of extracting information (mt) from data
originally proposed in K. Kondo, J. Phys. Soc. Jpn. 57, p. 4126 (1988)
• Use matrix element (M) for top production and decay• Integrate over structure functions ( f ) and transverse pT due to ISR• transfer function w(x,y), x = parton, y = observed (jets)• sum over all possible parton – jet assignments (and ambiguities)
preliminary CDF II result see www-cdf.fnal.gov/physics/new/top/top.html
17 June 2004 Joseph Kroll University of Pennsylvania 29
CDF II Preliminary mt Measurements
Table courtesy of K. Yorita, Waseda University – see Fermilab W & C talk on 11 June 2004
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Recent Revision of World Average of mt
Average of CDF and DØ published Run I meas.from the Tevatron Electroweak Working Groupsee hep-ex/0404010
Significant change due torecent update by DØ – a reanalysis of Run I data
Updated DØ measurementnow the single most precisemeasurement of top massV. M. Abazov, Nature, 429, p. 638 (2004)
Increase in mt has asignificant effect onconstraints on MH
17 June 2004 Joseph Kroll University of Pennsylvania 31
From LEP Electroweak Working Group – see lepewwg.web.cern.ch/LEPEWWG/
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What about the W Mass?
World average top mass is mt = 178.0 § 4.3 GeV/c2 – a 2.4% measurementRun II goal: reduce error to 2 – 3 GeV/c2, i.e., approach a 1% measurement
Measurement of MW much more straightforward experimentally & theoretically• use leptonic modes – no issue of converting jet momenta to parton momenta• less particles in the final state – just e or and “”
However, world average isMW = 80.425 § 0.034 GeV/c2 (0.042 %)
CDF II Goal: MW = 15 MeV in 2 fb-1 (0.02%)
This may be achievableif systematics continue toscale with statistics.
Figure courtesy LEP EWWG April 2004
direct
indirect
17 June 2004 Joseph Kroll University of Pennsylvania 33
W and Z Production at Tevatron
Tree Level Production (LO): W and Z have no pT (except for parton Fermi motion ~ few 100 MeV)
NLO: ISR produces W, Z + jets – can be significant pT(W,Z)
Production cross-section enhanced by 1 + 8/9s(MW2) » 1.3
17 June 2004 Joseph Kroll University of Pennsylvania 34
W and Z Event Topologies
W ! e Z ! +-
W cannot be fully reconstructed Z can be fully reconstructed
figures courtesy of A. Kotwal (Duke)
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W Reconstruction at the Tevatron
Only leptonic decays are used: hadronic decays lost in QCD backgroundy
y special trigger on displaced tracks at L2 being used to collect Z ! bb.
Reminder (see Lecture One): Only lepton directly observed, must be inferredLarge & varying amount of E undetected down the beampipeMeasure transverse quantities only.
Rest frame of W: distribution of lepton ET has a singularity (Jacobean peak)
singularitysee A. Gordon, Fermilab-thesis-1998-10 @ www-lib.fnal.gov/archive/thesis/index.shtml
Breit-Wigner Shape
17 June 2004 Joseph Kroll University of Pennsylvania 36
Figure 1.5 from A. Gordon Thesis
Jacobean peak
Integrate over BW with W = 2.06 GeV
Lepton ET
Arb
itra
ry s
cale
MW/2
▲ add pT(W) & Longitudinal acceptance
Affect of Breit Wigner & Transverse Motion
Integrating over BW & including pT(W) (higher order QCD) smears singularity
Singularity is removedbut sharp edge remains
Shape of edge dependson pT(W) distribution
If we use ET (pT) distributionto measure MW must modelpT(W) correctly (systematic)
17 June 2004 Joseph Kroll University of Pennsylvania 37
Transverse Mass (MT)
Lecture One: MT removes dependence on pT(W) (to 1st order)
ET (pT) vector of
u is vector ET in calorimeter not associated with lepton
MT has worse resolution than lepton ET tradethis systematic for pT(W) modelling systematic
17 June 2004 Joseph Kroll University of Pennsylvania 38
W! e Transverse Mass Binned in u
CDF Run Ib data80 pb-1
▲ Data
Monte Carlo
Clear broadening ofMT distributionwith increasing u
17 June 2004 Joseph Kroll University of Pennsylvania 39
CDF W Mass Data Sample & Selection
References: (all are CDF Collaboration)Run Ia (20 pb-1): F. Abe et al., PRL 75, p. 11 (1995), PRD 52, p. 4784 (1995)Run Ib (80 pb-1): T. Affolder et al., PRD 64, 032001 (2001)
Only Run I results – no preliminary Run II results yet Run Ib W! e(84 pb-1)Strict criteria to reducesystematics in MW
Trigger sample
well measured electron
kinematic criteria toreduce background& systematics
well measured electron
remove ! e+e-
Final sample (avoid W!, etc.)
{{
|| < 1
17 June 2004 Joseph Kroll University of Pennsylvania 40
CDF W Mass Data Sample & Selection
References: (all are CDF Collaboration)Run Ia (20 pb-1): F. Abe et al., PRL 75, p. 11 (1995), PRD 52, p. 4784 (1995)Run Ib (80 pb-1): T. Affolder et al., PRD 64, 032001 (2001)
Only Run I results – no preliminary Run II results yet Run Ib W! (80 pb-1)Strict criteria to reducesystematics in MW
is min. ion.
well measured
same kinematiccriteria as W! e {
{
|| < 1
17 June 2004 Joseph Kroll University of Pennsylvania 41
W Mass Analysis Strategy
• Use p for muons and E for electrons– E has better resolution & is less sensitive to bremsstrahlung
• Calibrate p and E scale in situ using , Z0
– Run 1a: absolute W scale set by spectrometer (will try again in Run II)
– Run 1b: absolute W scale set by Z mass (MZ x-check failed in Z! e+e-)
• Measure lepton resolution using measured Z0 width
• Study recoil (u) with Z0 data
• Measure pT(Z) with Z0 data, use theory to extrapolate to W (cs ! W)
• Use W charge asymmetry to constrain parton distributions– longitudal acceptance affects transverse mass shape
• Likelihood fit of MC model to MT distribution in data
– model includes affects of QED radiative corrections
• Study systematics by varying MC model and refitting data– many systematics scale with statistics (e.g., # Z’s, pdf constrains)
17 June 2004 Joseph Kroll University of Pennsylvania 42
Fits to the Transverse Mass Distributions
W! eRun Ib84 pb-1
W! Run Ib80 pb-1
MW = 80.473 § 0.065 § 0.092 GeV/c2 MW = 80.465 § 0.100 § 0.103 GeV/c2
Combined (e & ) Run Ia & Run Ib (105 pb-1)MW = 80.433 § 0.079 GeV/c2 (stat. © syst.)
17 June 2004 Joseph Kroll University of Pennsylvania 43
Summary of Systematic Errors on MW
Comments:• Z ! e+,e- & +- used separately in W ! e & independent systematics• main systematics scale with statistics (again the Z samples)• assuming this continues for 2 fb-1 total error of 15 MeV (CDF & DØ comb.)
17 June 2004 Joseph Kroll University of Pennsylvania 44
What About Direct Detection of the Higgs?
Disclaimer: In the late 90’s – optimistic projections of the total integratedluminosity for Run II (15 – 30 fb-1) led to over selling the SM Higgsdiscovery potential of the Fermilab Tevatron.
The Tevatron is not a SM Higgs machine (that’s why we are building the LHC).The primary motivation of Run II is to study top. We are confident we willcollect at least 10 times the integrated luminosity of Run I with better detectorsand a higher center of mass energy (increasing the top cross section by 30%).Tens of top candidates will become hundreds or maybe even a couple thousand.
Nevertheless, from an experimental point of view, it is extremely interestingto study the SM Higgs signature at the Tevatron. Already preliminary limitson SM Higgs production (albeit well above the expected SM cross-section)are coming out of Run II data analysis.
17 June 2004 Joseph Kroll University of Pennsylvania 45
SM Higgs Production and Decay
T. Han, S. WillenbockPhys. Lett. B 273, p. 167 (1991)
A. Djouadi, K. Kalinowski, M. SpiraComp. Phys. Commun. 108C, p. 56 (1998)
17 June 2004 Joseph Kroll University of Pennsylvania 46
Higgs Signature at the Tevatron
110 < MH < 140 GeV
bb decay dominates• gg! H, H! bb overwelmed by QCD bb background• Associated production with W and Z with leptonic (e and ) decays viable• Must reconstruct Higgs signal in bb invariant mass distribution• Use “sidebands” to constrain backgrounds• Associated production with W and Z to may work too
140 GeV < MH
WW(*) decay dominates• W pair production – clean signature with known SM background• Very little constraint on background (angular distributions)• Must rely on theory to predict SM WW background
17 June 2004 Joseph Kroll University of Pennsylvania 47
Preliminary CDF II Higgs Results
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WH: W! l, H! bb
Standard CDF W! l detection following Run II top analysis discussed earlierRestrict signal to two jet bin, require b-tag using lifetime for both jets
W + 2 jets – no b-tags W + 2 jets with 2 b-tags (A£» 2%)
!(16%)
Observed: 62 EventsExpected: 60.6 § 4.4
Before tagging: 2072 events observed
17 June 2004 Joseph Kroll University of Pennsylvania 49
What is the Sensitivity with More Data?
Joint CDF & DØ study carried out in 2003 – follow up original study 1998 – 2000Reference to original study: M. Carena, J.S. Conway, H.E. Haber, J.D. Hobbs et al., hep-ph/00010338Reference to updated study: B. Klima, J. Kroll, C. Tully, B. Winer et al., FERMILAB–PUB–03/320–E
Low mass region: 110 < MH < 130 GeV This study includes ZHwith Z!, l+l-
The bb signatureis particularly powerfuln.b., a significant fractionof WH, with W! l& lepton not detectedadd to “bb” sample
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Typical Experiment – with 10 fb-1!
These plots have 10% bb mass resolutionControl of systematics will be crucialSingle top a particularly insidious (same topology as WH)
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What Does That Plot Really Mean?
frac
tion
of
exp
erim
ents
th
at s
atis
fy c
rite
ria
4 fb-1
(a more realistic goal)
These curves correspond to50% of pseudoexperimentsmake the statistical statementor better (or worse)
MH = 120 GeV
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Scatter of Possible Outcomes Example for MH = 115 GeV
Limit
Signal Significance
Standard Model
4 fb-1
Remember limits arecombined CDF & DØ
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Only Original Study Covered High Mass
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Conclusion on SM Higgs @ Tevatron
Finding the SM Higgs in Run II very challengingEspecially with reduced expectations for integrated Laside: if we had not become set on optimistic projectionswould be very happy with what we are getting now
From the experimental viewpoint, l bb, bb, llbbare very interesting signatures – we should pursue them