physics of hadron colliders lecture 3: m t, m w, higgs joseph kroll university of pennsylvania 17...

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


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


Recent Tevatron Stores (since 1 June 2004)

Initial luminosities of mostrecent stores in 1030 cm-2s-1


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


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


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


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


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-


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


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


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


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)


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


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


Top Mass Sample Composition

Background contribution constrained in top mass fit


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


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


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


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


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


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


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.


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


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


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)


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


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


CDF II Preliminary mt Measurements

Table courtesy of K. Yorita, Waseda University – see Fermilab W & C talk on 11 June 2004


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


From LEP Electroweak Working Group – see lepewwg.web.cern.ch/LEPEWWG/


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


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


W and Z Event Topologies

W ! e Z ! +-

W cannot be fully reconstructed Z can be fully reconstructed

figures courtesy of A. Kotwal (Duke)


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


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)


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


W! e Transverse Mass Binned in u

CDF Run Ib data80 pb-1

▲ Data

Monte Carlo

Clear broadening ofMT distributionwith increasing u


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


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


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)


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.)


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.)


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.


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)


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


Preliminary CDF II Higgs Results


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


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


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)


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


Scatter of Possible Outcomes Example for MH = 115 GeV

Limit

Signal Significance

Standard Model

4 fb-1

Remember limits arecombined CDF & DØ


Only Original Study Covered High Mass


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

physics of hadron colliders lecture 3: m t, m w, higgs joseph kroll university of pennsylvania 17...

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