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Top Properties @ 4fb -1. Stop search Top charge asymmetry. Amnon Harel aharel@fnal.gov University of Rochester. DØ Collaboration Week, 19 th June 2007. Su-Jung Park. µ +. ν µ. µ +. b. W +. ν µ. W +. t. ~. -. χ 1 0. t. ~. -. χ 1 +. b. W -. b. ~. t 1. q. -. q’. ~. - PowerPoint PPT Presentation
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Top Properties @ 4fb-1
Amnon HarelAmnon Harelaharel@fnal.govaharel@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
Amnon HarelAmnon Harel 33D0 collaboration week 6/19/20076/19/2007
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
Amnon HarelAmnon Harel 44D0 collaboration week 6/19/20076/19/2007
@ 1 fb@ 1 fb-1-1
Limits a bit stronger when assuming the top cross section:
0.9 fb-1
Observed
Expected
Prospino NLO
Amnon HarelAmnon Harel 55D0 collaboration week 6/19/20076/19/2007
@ 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
Amnon HarelAmnon Harel 66D0 collaboration week 6/19/20076/19/2007
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 '
Amnon HarelAmnon Harel 77D0 collaboration week 6/19/20076/19/2007
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
Amnon HarelAmnon Harel 88D0 collaboration week 6/19/20076/19/2007
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
Amnon HarelAmnon Harel 99D0 collaboration week 6/19/20076/19/2007
@ 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
Amnon HarelAmnon Harel 1010D0 collaboration week 6/19/20076/19/2007
Back up slides
Amnon HarelAmnon Harel 1111D0 collaboration week 6/19/20076/19/2007
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.
Amnon HarelAmnon Harel 1212D0 collaboration week 6/19/20076/19/2007
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)
Amnon HarelAmnon Harel 1313D0 collaboration week 6/19/20076/19/2007
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.
Amnon HarelAmnon Harel 1414D0 collaboration week 6/19/20076/19/2007
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
Amnon HarelAmnon Harel 1515D0 collaboration week 6/19/20076/19/2007
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)
Amnon HarelAmnon Harel 1616D0 collaboration week 6/19/20076/19/2007
The DThe DØØ Detector Detector
Amnon HarelAmnon Harel 1717D0 collaboration week 6/19/20076/19/2007
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.
Recommended