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Top quark mass

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Top quark mass

For DØ collaboration

Regina Demina

University of Rochester

Wine and Cheese seminar at FNAL, 07/22/05

- Introduction
- Top quark mass measurement in Run II
- Matrix element method description
- In situ jet energy scale calibration on hadronic W-mass
- Sample composition
- Result
- Systematics

- Tevatron combined top mass
- Top quark production
- Update on cross section in l+jets channel
- Search for resonance production

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

t

H

W

W

W

W

MW mt2

MW ln(MH)

b

CDF&D0

RUNII

- Fundamental parameter of the Standard Model.
- Important ingredient for EW precision analyses at the quantum level:
which were initially used to indirectly determine mt.

After the top quark discovery, use precision measurements of MW and mt to constrain MH.

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

At √s=1.96 TeV top is produced in pairs via quark-antiquark annihilation 85% of the time, gluon fusion accounts for 15% of ttbar production

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

Since the top lifetime top ~ 1/ M3top~10 -24 sec

qcd ~ -1 ~10 -23 sec

BR(tWb)

Both W’s decay via Wl

final state: llbb -

DILEPTON

One W decays via Wl

final state: lqq bb - LEPTON+JETS

Both W’s decay via Wqq

final state: qqqq bb

ALL HADRONIC

the top quark does not hadronize. It decays as a free quark!

Lepton provides a good trigger, all jets are tough

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

- 2 b-jets
- Lepton: electron or muon
- Neutrino (from energy imbalance)
- 2 q’s – transform to jets of particles
- Note that these two jets come from a decay of a particle with well measured mass – W-boson – built-in thermometer for jet energies

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

- Electrons are identified as clusters of energy in EM section of the calorimeter with tracks pointing to them
- Muons are identified as particles passing through entire detector volume and leaving track stubs in muon chambers. Track in the central tracking system (silicon+SciFi) is matched to track in muon system
- Jets are reconstructed as clusters of energy in calorimeter using cone algorithm DR<0.5

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

- Method developed by DØ (F. Canelli, J. Estrada, G. Gutierrez) in Run I

Single most precise measurement of top mass in Run I

Mt =180.1±3.6(stat) ±4.0(syst) GeV/c2

Systematic error dominated by JES

3.3 GeV/c2

With more statistics it is possible to use additional constraint on JES based on hadronic W mass in top events – in situ calibration

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

- Goal:measure top quark mass
- Observables:measured momenta of jets and leptons
- Question:for an observed set of kinematic variables x what is the most probable top mass
- Method: start with an observed set of events of given kinematics and find maximum of the likelihood, which provides the best measurement of top quark mass
- Our sample is a mixture of signal and background

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

W(x,y) is the probability that a parton

level set of variables y will be measured

as a set of variables x

probability to observe a set of kinematic variables x for a given top mass

dnσ is the differential cross section

Contains matrix element squared

f(q) is the probability distribution than a parton will have a momentum q

Normalization depends on mt

Includes acceptance effects

Integrate over unknown q1,q2, y

q

b

q’

t

t

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

- Partons (quarks produced as a result of hard collision) realize themselves as jets seen by detectors
- Due to strong interaction partons turn into parton jets
- Each quark hardonizes into particles (mostly p and K’s)
- Energy of these particles is absorbed by calorimeter
- Clustered into calorimeter jet using cone algorithm

- Jet energy is not exactly equal to parton energy
- Particles can get out of cone
- Some energy due to underlying event (and detector noise) can get added
- Detector response has its resolution

- Transfer functions W(x,y) are used to relate parton energy y to observed jet energy x

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

- h dependence of JES is derived on g+jet data, but the overall scale is allowed to move to optimize MW

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

- All jets are corrected by standard DØ Jet energy scale (pT, h)
- Overall JES is a free parameter in the fit – it is constrained in situ by mass of W decaying hadronically
- JES enters into transfer functions:

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

Normalization

e+jets

μ+jets

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

- Set of observables – momenta of jets and leptons: x
- Integrate over unknown
- Kinematic variables of initial (q1,q2) and final state partons (y: 6 x3 p) = 20 variables
- Integral contains 15 (14)d-functions for e(m)+jets
- total energy-momentum conservation: 4
- angles are considered to be measured perfectly: 2x4 jet +2 lepton
- Electron momentum is also considered perfectly measured, not true for muon momentum: 1(0)

- 5(6) dimensional integration is carried out by Vegas
- The correspondence between parton level variables and jets is established by transfer functionsW(x,y) derived on MC
- for light jets (from hadronic W decay)
- for b-jets with b-hadron decaying semi-muonically
- for other b-jets

- Approximations
- LO matrix element
- qqttprocess only (no gluon fusion – 15%)

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

- W+jets is the dominant background process
- Kinematics of W+jets is used as a representation for overall background (admixture of multijet background is a source of systematic uncertainty)
- Contribution of a large number of diagrams makes analytical calculation prohibitively complex
- Use Vecbos
- Evaluate MEwjjjj in N points selected according to the transfer functions over phase space
- Pbkg- average over points

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

Lepton+jets sample

- Isolated e (PT>20GeV/c, |h|<1.1)
- Isolated m (PT>20GeV/c, |h|<2.0)
- Missing ET>20 GeV
- Exactly four jets PT>20GeV/c, |h|<2.5 (jet energies corrected to particle level)
Use “low-bias” discriminant to fit sample composition

- Used for ensemble testing and normalization of the background probability.
- Final fraction of ttbar events is fit together with mass

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

Calibration on Full MC

lepton+jets

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

Mt=169.5±4.4 GeV/c2

JES=1.034±0.034

calibrated

calibrated

DØ RunII Preliminary

expected: 36.4%

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

B-jet energy scale

- Relative data/MC b/light jet energy scale ratio
- fragmentation:+-0.71 GeV/c2
- different amounts of p0, different p+ momentum spectrum
- fragmentation uncertainties lead to uncertainty in b/light JES ratio
- compare MC samples with different fragmentation models:
- Peterson fragmentation with eb=0.00191
- Bowler fragmentation with rt=0.69
- calorimeter response: +0.85 -0.75 GeV/c2
- uncertainties in the h/e response ratio
- + charged hadron energy fraction of b jets > that of light jets
- corresponding uncertainty in the b/light JES ratio
- Difference in pT spectrum of b-jets and jets from W-decay: 0.7 GeV/c2

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

- The effect is reduced by
- Requiring four and only four jets in the final state
- High PT cut on jets

- Yet in ~20% of the events there is at least one jet that is not matched (DR(parton-jet)<0.5) to top decay products
- These events are interpreted as background by ME method

- We study this systematic by examining ALPGEN ttj sample and varying its relative fraction between 0 and 30% (verified on our data by examining the fraction of events with the 5th jet)
- Final effect on top mass0.34 GeV/c2

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

Signal/Background Modeling

- QCD background:+-0.67 GeV/c2
Rederive calibration includingQCD events from data(lepton anti-isolation)

(note: sample statistics limited) can be reduced in the future

- W+jets modeling:+-0.32 GeV/c2
study effect of a differentfactorization scalefor W+jets events

(<pT,j>2 instead of mW2 + SpT,j2)

- PDF uncertainty:+-0.07 GeV/c2
CTEQ6M providessystematic variations of the PDFs

reweight ensembles to compare CTEQ6M with its systematic variations

(by default the measurement uses CTEQ5L throughout:

use a LO matrix element, and for consistency with simulation)

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

Signal fraction

- Signal fraction:+0.50 -0.17 GeV/c2
Fitted top mass depends slightly

on true signal fraction (if signal

fraction is smaller than expected):

=> Vary signal fraction within uncertainties

from topological likelihood fit

- Note: ftop fit yields identical result

with factor √2 smaller uncertainties

Cross check on data: cut on log10(pbkg)<-13

Ftop=31%46±6%

Mtop=170.2±4.1 GeV/c2

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

- Run II top quark mass based on lepton+jets sample: Mt=169.5 ±4.4(stat+JES) +1.7-1.6 (syst) GeV/c2
- JES contribution to (stat+JES) 3.3 GeV/c2
- Break down by lepton flavor
- Mt(e+jets)=168.8 ±6.0(stat+JES) GeV/c2
- Mt(m+jets)=172.3 ±9.6(stat+JES)GeV/c2

- Cross check W-mass

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

DØ Run II preliminary

- Statistical uncertainties are partially correlated for all l+jets Run II results

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

Assumptions:

- only lepton+jets channel considered
- statistical uncertainty normalized at L=318 pb-1 to performance of current analyses.
- dominant JES systematic is handled ONLY via in-situ calibration making use of MW in ttbar events.
- remaining systematic uncertainties:include b-JES, signal and background modeling, etc (fully correlated between experiments) Normalized to 1.7 GeV at L=318 pb-1.
- Since most of these systematic uncertainties are of theoretical nature, assume that we can use the large data sets to constrain some of the model parameters and ultimately reduce it to 1 GeV after 8 fb-1.

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

JES is treated as a part of systematic uncertainty, taken out of stat error

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

- Mt=172.7±2.9 GeV/c2
- Stat uncertainty: 1.7GeV/c2
- Syst uncertainty: 2.4GeV/c2
- hep-ex/0507091
- Top quark Yukawa coupling to Higgs boson
- gt=Mt√2/vev=0.993±0.017

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

- MH=91+45-32GeV/c2
- MH<186 GeV/c2 @95%CL

68% CL

MW,GeV/c2

MH,GeV/c2

Mt,GeV/c2

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

And now for something

completely different...

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

DØ RunII Preliminary, 363pb-1

- Isolated lepton
- pT>20 GeV/c, |he|<1.1, |hm|<2.0

- Missing ET>20GeV
- Four or more jets
- pT>15 GeV/c, |h|<2.5
s=8.1+1.3-1.2(stat+syst)±0.5(lumi) pb

- pT>15 GeV/c, |h|<2.5

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

DØ RunII Preliminary

Submitted for publication

Updates

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

- Check ttbar invariant mass for possible resonance production

DØ RunII Preliminary, 363pb-1

sNNLO(tt)=6.77±0.42

- Events are kinematically constrained
- mT=175GeV/c2
- Leptonic and hadronic W masses

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

- Limit M(Z’)>680 GeV/c2 with G=1.2%MZ’ at 95%CL

DØ RunII Preliminary, 363pb-1

*

*R. Harris, C. Hill, S. Parke hep-ph/9911288

Run I limit 560 GeV/c2

Run II limit 680 GeV/c2

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

- First DØ RunII top mass measurement in l+jets channel to surpass Run I precision
- Mt=169.5 ±4.4(stat+JES) +1.7-1.6 (syst) GeV/c2

- Developed method for in situ jet energy scale calibrationusing hadronic W-mass constraint
- Combined Tevatron top mass measurement reaches a precision of 1.7%
- ttbar production cross sections updated for l+jets channel
- Invariant mass of ttbar system probed for resonance production, exclusion limit for M(Z’)>680 GeV/c2 at 95%CL

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

Backup slides

Parton Level Tests

Text

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL

DØ RunII Preliminary, 363pb-1

Regina Demina, Joint Theoretical and Experimental Seminar at FNAL