Top Properties @ 4fb -1

Top Properties @ 4fb-1

Amnon HarelAmnon [email protected]@fnal.gov

University of RochesterUniversity of Rochester

DØ Collaboration Week, 19th June 2007

Stop searchStop search

Top charge asymmetryTop charge asymmetry

Amnon HarelAmnon Harel 22D0 collaboration week 6/19/20076/19/2007

A Stop SearchA Stop SearchThe illustrated decay chain is relevant for:

W+

b

b

-

W-

νµ

qq’

χ1-

χ10

χ10~

~

~

µ+

t

t

W+

-

b

b

-W-

µ+

qq’

νµ

The MET will not be higher - the neutralinos are back to back.

Searching in the l+jets channel, with at least one tight b-tag.

The event signature is very similar to standard top l+jets decays.

tt MM 1

~ GeVM 4501~

GeVMGeV 1051351

~

The jets are softer:• achieved good data vs. MC agreement for 15GeV jets, used to get more signal acceptance• the b-tagged jet’s pT is a good separating variable

Subtler event kinematics are also different:Use a kinematic fitter to identify top pair events.

t1~-t1~

χ1+~

Su-Jung Park


A Stop SearchA Stop Search

Bayesian approach to limit setting

Includes partial systematics

[GeV] [GeV] [GeV] [pb]

175 135 50 0.58

175 120 50 0.58

175 105 50 0.58

160 120 50 1.00

160 105 50 1.00

145 105 50 1.80

1~m 0

1~m1tm~ Mass Points Studied:

Signal

, l+jetstt

Expected Limits

0.9 fb-1

Su-Jung Park

Note is in group review – aiming at summer conferences (SUSY07)

0.9 fb-1


@ 1 fb@ 1 fb-1-1

Limits a bit stronger when assuming the top cross section:

0.9 fb-1

Observed

Expected

Prospino NLO


@ 4 fb@ 4 fb-1-1

Under the assumption that the newer data is exactly as useful as the current data

4.0 fb-1

Observed

Expected

Prospino NLO


4th jet pT

Afb

Top Charge AsymmetryTop Charge AsymmetryMeasuring the top charge asymmetry in the l+jets channel, using b-tagging and a kinematic fitter to reconstruct the tops. Probes new physics, e.g.

On the face of it, it’s a trivial measurement: count and publish.

NN

NNA fb

The top asymmetry varies greatly throughout phase space

t

t

q

q

Several contributions to Afb, all are interference terms of at least NLO:

t

t

q

q

NNLO terms, qgtt, etc.

Analysis Strategy

• Statistics limited:• kinematic fitter• integrated measurement

uncorrected Afb

• Keeps the acceptance simple:• hard cuts on pTs, • loose complicated cuts• fitting procedure

MC@NLO

Harel

}{#

}{#

tt

tt

YYN

YYN

ttZ '


Top Charge AsymmetryTop Charge AsymmetryA likelihood that discriminated between top and W+jets events without biasing |ΔY|

MultijetAmount and asymmetry taken from dataW+jets•Asymmetric production•Reconstruction as a top pair washes out the asymmetry

D

tt YY

%)()( systfitA fb 1812 Analysis in EB review.Measured observable asymmetry:SM prediction: (1±1(syst))%

tt YY

D

0.9 fb-1 0.9 fb-1


Z’ limits @ 1 fbZ’ limits @ 1 fb-1-1

%)()( systfitA fb 1812

Production through a Z’ resonance is asymmetric in most models• Assume Z-like couplings• Unique probe for a wide resonance

0.9 fb-1ObservedExpected


@ 4 fb@ 4 fb-1-1

Just assumed 4 times the statistics. Ignoring 0.9 != 1, trigger issues, etc.

Measurement still completely statistics dominated.

Can we observe the difference between Afb in tt and Afb in ttj ?NLO predictions are 6% and -7%. This was my original motivation

Expect to measure Afb4 to within 5%, Afb

5+ to within 7%, and Δ to within 8.5%.

Of course, the 4 jet bin also contains ttj contributions, and so on.

4.0 fb-1Expected (given 1 fb-1)

Expected (MC@NLO)Z’ limits


Back up slides


Top Charge AsymmetryTop Charge AsymmetryThis one’s mine, so I’m probably going into too many details…

Measuring the top charge asymmetry (Afb) in the l+jets channel, requiring an L4 b-tag (NN>0.2) and using HITFIT to reconstruct the tops.

On the face of it, it’s a trivial measurement: count and publish.

NN

NNA

Let’s slow down and first understand the top pair and W+jets asymmetries.The top charge asymmetry has several origins, all at NLO interference terms:1. An interference between a simple qq->g->tt diagram and the corresponding gluonic box

diagram2. An interference between that simple diagram with an added ISR gluon & with an FSR gluon3. Interferences in qg->tt diagrams (e.g. Flavor creation)4. Higher order terms (have not been evaluated)5. Interferences with mixed (electroweak neutral current + gluon) box diagramsSome of these effects tend to cancel out.

The top asymmetry varies greatly throughout phase space (see next slide). Which “A” did we measure?

The main background is also asymmetric! The W+jets asymmetry is much simpler and comes directly from the V-A nature of the W vertex and the proton anti-proton initial state (u&dPDFs). Only exception is Wc(+jets) production, which probes the sPDF and is thus almost symmetric.


AAfbfb: The Asymmetries: The Asymmetries

4th Leading jet pT

5th Leading jet pT

So the top asymmetry is unstable, and the W+jets asymmetry is stable.

Not a surprise: The latest theory calculation is that the tt and ttj asymmetries are approximately 6% and -7% (jet acceptance cuts: pT>20GeV, |η|<3)


AAfbfb: Analysis Strategy: Analysis StrategyFrom a hitfit study: the best observable is to combine the two top rapidities into ΔY.

This also gets the small benefit of reducing the effect of boosts along the z-axis.

As the phase space were the measurement is done must be specified:• Keep it simple

• Jet pT>20GeV

• Use only a loose cut on a likelihood discriminant that does no bias us towards low |ΔY|.

• Study jet efficiencies from particle level to selected jets (jet smearing, JESμ, jet ID S.F., EM removal).

• Closure test, using a dedicated PMCS on W+jets and signal MC@NLO events, showing that a factorized box cuts are sufficient.

There are also reconstruction biases:• Lepton charge misidentification and asymmetries are small and suppressed by

using the entire effect for the sign of ΔY• Geometric misreconstruction described with event-by-event dilution factors

parametrized in |ΔY|.

Time constraints: CDF Ph.D. thesis in July. Expected blessing 8/06, Moriond.


AAfbfb: Fitting: FittingNsig and Afb are correlated in two ways:

•A larger Nsig implies the observed asymmetry must be assigned to more events

•Since Nsig=N++N-, a statistical fluctuation in, e.g., N+ would increase both.

Trying to untangle this by doing a simultaneous fit.

Currently doing a simultaneous fit over:•Likelihood discriminant – identified signal fraction•Leptonic W mass – cross checks matrix method’s determination of multijet fraction

•Sign of events passing (loose) likelihood cut – gives Afb

Fit not as stable as expected: adding the asymmetry information changed fitted fractions significantly (higher multijet fraction). More fit stability tests needed.

Fit results: Nsig=261±20, NW=78±20, NQCD=39±4, Afb=0.16±0.08


AAfbfb: W+jets Asymmetry: W+jets AsymmetryA pleasant surprise was that the W+jets asymmetry is washed out by the reconstruction as a top pair. Some asymmetry remains and a detailed breakdown is required to evaluate systematics (e.g. TRFs).

Reconstruction as a top pair washes

it out

Breakdown needed for systematics (e.g. b-tagging RFs)


The DThe DØØ Detector Detector


Matched MCMatched MC

Matrix element generator

Alpgen / Sherpa

Parton shower generator

Pythia / Herwig

Hard scatter partons

Particles

Detector simulation

LO calculations for 2N hard processes

Only 2->2 hard processes

Partons are matched to parton-shower jets

to avoid double counting of

equivalent phase space

configurations.

Good at generating hard, large-angle

processes (calculates interference)

Weak on the “texture” of the QCD radiation

Good at generating the details within a jet

Multijet events don’t describe data well

(and are hard to generate)

Resummed soft, collinear radiation.

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Top Properties @ 4fb -1