\ttbar and single top cross sections at the Tevatron

0.11 MB·English

by  Elizaveta Shabalina for CDF

tt¯and single top cross sections at the Tevatron
ElizavetaShabalina1,a fortheCDFandD0collaborations
Georg-August-Universita¨tGo¨ttingen,Friedrich-Hund-Platz1,D-37077Go¨ttingen,Germany
2 Abstract. We present a summary of the latest measurements of the top pair and single top cross sections
1 performedbytheCDFandD0collaborationsattheFermilabTevatroncollider.
0
2

n 1 Introduction their energy,and buildinga discriminantfromthese vari-
a ables using one of the multivariate techniques. The cross
J
TheFermilabTevatroncolliderendeditsrunonSeptember section is then extracted from a binned maximum likeli-
1 30,2011afterdeliveringmorethan10 fb 1of pp¯collision hood fit to data. The same idea is used to extract single
−
3
data per experiment at √s = 1.96 TeV. A large sample topcrosssectionwiththedifferencethatforthelatterone

of top quarks collected by the CDF and D0 experiments hasto includemanymorediscriminatingvariablesdueto
]
x allowstoperformprecisionmeasurementsoftheirproduc- averypoorsignaloverbackground(S/B)ratio.
e tionwhichispredictedtooccurwithinthestandardmodel
Thesecondapproachmakesuseofthepresenceofab-
-
(SM)eitherinpairsviastronginteractionsorassingletop
p quarkintopquarkdecaywhilethemajorbackgroundsare
e events via electroweak interactions. Such measurements dominatedby light jets production.Thus the requirement
h represent an important test of the theoretical calculations
ofatleastonejetpereventbeidentifiedasab-jetimproves
[ which predict the tt¯and single top production cross sec- significantlyS/Bandallowstoextracttt¯crosssectionfrom
tionswithaprecisionof6%to8% [1]and5%[2],respec-
1 acountingexperiment.
v tively.Precisemeasurementsoftoppaircrosssection(σtt¯) D0 performed tt¯cross section measurement with 5.3
3 indifferenttt¯finalstatesandsingletopproductionviadif-
fb 1of data by selecting events with 2 jets and split-
5 ferentproductionmechanismsarehighlydesirableasthey − ≥
tingthemintosubsamplesaccordingtothenumberofjets
6 are sensitive to the non-SM particles that may appear in
6 topquarkproductionordecays. and b-tags[3]. In the backgrounddominatedsubsamples,
. i.e., two jet events, three jet events with < 2 b-tags and
1
events with at least four jets and no b-tags, Random For-
0
est (RF) discriminant is built to separate signal from the
2 2 Top quark pair production crosssections
1 background. In the signal dominated ones, with at least
: four jets and three jets with two b-tags, the b-tag count-
v Within the SM, top quark decays to a W boson and a b-
ing is used. Cross section is extracted from the simulta-
i quark with almost 100% probability. Thus, tt¯events can
X neous fit to data of the RF discriminant and the number
create final states with two leptons (dilepton channel) if
ofb-tagsdistributionacrossdifferentjetmultiplicitiescon-
r
bothW bosonsfromtopquarkdecayleptonicallyintoeν ,
a e strainingmanysystematicuncertaintieswhichareincluded
µν or τν , single lepton (ℓ + jets channel) with one W
µ τ asnuisanceparametersinthefit.Themeasuredcrosssec-
bosondecayingleptonicallyandanotheronehadronically tionyieldsσ =7.78+0.77(stat+syst+lumi)pb,achieving
into qq¯ , or no leptons (all hadronic channel) if both W tt 0.64
′ arelativeprecisionbe−low10%.Thelatterislimitedbythe
bosonsdecayhadronically.
systematicuncertaintieswiththelargestcontributionfrom
thedeterminationofintegratedluminosity(6.1%).
2.1 ℓ+jetschannel The CDF collaboration significantly reduces the de-
pendence on the luminosity measurement and its associ-
The ℓ +jets channel provides the most precise measure- ated large systematic uncertainty by exploiting the corre-
mentsofthett¯productioncrosssectionduetoitsrelatively lation between the luminosity measurementsfor Z boson
andtt¯production[4].CDFanalysiscomputestheratioof
large branching and manageable background dominated
the tt¯to Z boson cross section, measured using the same
by the productionof W bosonsin association with heavy
andlightflavorjets(W+jets).Twoapproachesareusedto triggers and dataset, and multiplies this ratio by the pre-
discriminate tt¯signal from the background.The first one ciselyknowntheoreticalZcrosssection,thusreplacingthe
exploits differences in kinematic properties of signal and luminosityuncertaintywiththesmallertheoreticalandex-
W+jetsbackground.Itreliesonselectinganumberofdis- perimentaluncertaintiesonZ crosssection.Usingthisap-
criminatingvariables,typicallyrelatedtodifferentfeatures proach in the dataset of 4.6 fb−1CDF collaboration mea-
sures cross section using two methods: by constructing a
oftheevents,suchasangulardistributionsoftheobjectsor
neuralnetwork(NN)discriminantbasedonkinematicin-
a e-mail:[email protected] formationandbycountingb-tags.Statistical combination
EPJWebofConferences
Events / 0.1Events / 0.1Events / 0.1223223223050050050000000000000000000 DØ, L=DtOW5t.at3+ht jaeefbrts -1 ber of eventsber of eventsber of events456456456000000000000000000 DØ, L=5.3 fb-1 / binNevents 450000 dtWQtaC+taDje tMsultijet
111555000000 mmm 300
Multijets NuNuNu333000000
111000000000 200
222000000
555000000 111000000 100
000000 000...111 000...222 000...333 000...444 000...555 000...666 000...777 000...888 000...999 111 000 222 333 ≥≥≥444
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
RRRFFF dddiiissscccrrriiimmmiiinnnaaannnttt NNNuuummmbbbeeerrr ooofff jjjeeetttsss NN output
Fig.1.RFdiscriminantoutputforeventswiththreejetsandnob-tags(left)andjetmultiplicityspectrumofeventswithatleast2b-tags
(middle)fromD0analysis,NNoutputforeventswith 3jetsfromCDFanalysis(right).
≥
ofthesetwomeasurementsyieldsthemostprecisetopquark
CDF Run II Preliminary (5.1 fb-1)
crosssectionmeasurementtodateofσ = 7.70 0.52pb
tt ± 990000 data
witharelativeuncertaintyof7%.Allcrosssectionsquoted Bkgd ± 1σ uncertainty
880000 tt (σ = 7.4 pb)
above assume top quark mass mt = 172.5 GeV. Figure 1 770000 WW/WZ/ZZ
DY
(left)showstheRFdiscriminantdistributionforeventswith 660000 Fake
three jets and no b-tags and Fig. 1 (middle) shows the ntsnts550000
jetmultiplicitydistributionforeventswithatleasttwo b- veve
EE440000
tags in D0 data compared to the background model pre-
330000
dictionandtt¯signalcontributioncalculatedusingthemea-
220000
suredcrosssection.Figure1(right)showstheNNoutput
110000
usedtoextractthett¯crosssectioninthekinematicanalysis
byCDF, forsimulatedsignalandbackgroundevents,and 00 00 jjeett 11 jjeett ≥≥ 22 jjeett HHTT>>220000++OOSS
data.
Fig.2.Jetmultiplicityspectrumofpretagevents.
2.2 Dileptonchannel
of 12% resulting in σ = 7.36+0.90(stat+syst+lumi)pb.
Lowbackground,characteristicofthedileptonchanneland tt 0.79
Theresultisdominatedbythes−ystematicuncertaintywith
dominatedbythedibosonandZbosonproductioninasso-
thelargestuncertaintyfromluminosityof0.57pb.Figure3
ciationwithjets,allowstousesimplecountingmethodsto
shows the outputof the b-taggingdiscriminantfor events
extract tt¯cross section. The disadvantage of this channel
withatleasttwojetsineµchannelindataandsimulation.
isitslowbranchingfractionwhichmadeσ measurement
tt¯
statistically limited until recently.The CDF collaboration
uses5.1 fb 1ofdatatomeasureσ inthedileptonchannel
− tt¯
using 343 candidate events in the pretag sample and 137 ntsnts (c) DØ, L=5.4 fb-1
eventsafterb-jetidentificationrequirement[5].Bothmea- veve115500 eµ+2jet
surementsareingoodagreementwitheachotherandyield EE
Z(+jets): 30
σ =7.40 0.58(stat) 0.63(syst) 0.45(lumi)pbforthe
tt ± ± ± 110000 WW/WZ/ZZ: 9
pretaganalysisandσ = 7.25 0.66(stat) 0.47(syst) Instrumental: 23
tt ± ± ±
0.44(lumi)pbfortheb-taggedone.Statisticalandsystem- t t: 192
aticuncertaintiesarecomparableforbothapproaches.The DATA: 281
5500
b-taggedanalysishasasmallersystematicuncertaintydue
toasignificantlylowerbackgroundswhichhaverelatively
large uncertainties and achieves a precision below 13%. 00
00 55 1100
Figure2showsjetmultiplicityspectruminthepretagsam-
bb--ttaaggggiinngg NNNN ddiissccrriimmiinnaanntt
ple.
The D0 collaboration uses a more complicated new Fig. 3. Expected and observed distributions for the smallest b-
techniquetomeasureσ in5.4 fb 1ofdata[6].Inaddition tagging NN discriminant output of the two leading jets for eµ
tt¯ −
to events with at least two jets this analysis uses one-jet eventswith 2jets.
≥
events in eµ channel and extracts σ from the likelihood
tt¯
fittothedistriminantbasedontheD0NNb-taggingalgo-
rithm.Thealgorithmcombinesinformationabouttheim- 2.3 Combinationofthecrosssections
pactparametersofthetracksandvariablesthatcharacter-
ize the propertiesof the reconstructed secondary vertices Up to now tt¯productioncross sections were measured at
inasinglediscriminant.Thecrosssectionismeasuredby the Tevatron in all channels except for the one with two
simultaneously fitting the distributions of the smallest of hadronic taus in the final state using a variety of meth-
thetwob-taggingNNoutputvaluesofthetwoleadingjets ods.Allmeasurementsare inagreementwiththe theoret-
inthefourchannelswithsystematicuncertaintiesincluded icalcalculations.Figure4(5)showsasummaryofthere-
inthefit.Thisapproachallowstoconstraintheuncertain- sults from the D0 (CDF) experiment. A recent combina-
tiesandachievethebestprecisioninthedileptonchannel tion of dilepton and ℓ + jets cross section measurements
2011HadronColliderPhysicssymposium(HCP-2011),Paris,France,November14-182011
with 5.4fb 1byD0 yieldsσ = 7.56+0.63pbwhichcorre- ingSMrelativeproductionratesfort-ands-channels)us-
− tt 0.56
spondstoa relativeuncertaintyof(+−8.3-7.4)%[6].Com- ing three different multivariate discriminants constructed
binationofthebestCDFmeasurementsindifferentchan- usingBoostedDecisionTrees[10],BayesianNeuralNet-
nelsachievesrelativeuncertaintyof6.4%andyieldsσ = work(BNN)[11]andNeuroevolutionofAugmentedTopolo-
tt¯
7.50 0.48pb[7]foratopmassof172.5GeV. gies [12]. The outputs of individual methods have corre-
± D Run II July 2011
lation of 70% and are combinedusing second BNN. The
le5p.4t ofbn 1+jets + dileptons (PLB) 7.40 + 00 . .11 99 + 00 .. 55 07 pb measuredcrosssectionis[9]σ((s+t)) = 3.43+00..7734pband
le5p.3t ofbn 1+jets (topo + b-tagged, PRD) 7.65 + 00 . .22 55 + 00 .. 57 75 pb hthaesd2i1sc%rimreilnaatinvtefoprreacicsoiomnb.inFeigdusre+6tcshhoawnnsetlh(−eleoftu)tapnudt os-f
d5il.e4 pfbt o 1ns (topo + b-tagged, PLB) 7.27 + 00 ..44 55 + 00 . .76 63 pb channel(middle) for the signal region.In these plots, the
le1p.0t ofbn 1+track (b-tagged)* 5.0 + 11 ..64 + 00 .. 89 –0.3 pb binsaresortedasafunctionoftheexpectedS/Bratiosuch
ta2u.2+ flbe 1pton (b-tagged)* 7.32 + 11 .. 32 44 + 11 ..2 0 06 –0.45 pb that S/B increases monotonically within the range of the
ta1u.0+ fjbe 1ts (b-tagged, PRD) 6.30 + 11 ..10 59 + 0 0 .. 67 72 –0.40 pb discriminant.Figure6(right)showsthedistributionofone
al1lj.0e tfbs 1(b-tagged, PRD) 6.9 ( +s 1t1 a ..3t3 ) ( + s 11 y ..s 44t ) (–lu0m.4i ) pb omfatshse, fmorosthteporewgeiorfnulodfilsacrrgime vinaalutiengfovrasriigabnlaelsd,itsocprimquinarak-
mtop = 175 GeV MN.. KCiadcocniaakrii se ta anld., RJH. VEoPg 0t,8 P0R9,D 1 7287, (027040080)5 (2008) tion (S/B > 0.24).Combined s+t contributionis clearly
CTEQ6.6M S. Moch and P. Uwer, PRD 78, 034003 (2008) visible in the plotswhile the signalpresensein s-channel
0 2 4 6 8 10 12 discriminantisnotsignificant.
* = preliminary
red = 2011 result σ(pp→ tt + X) [pb] Given that potential contributions from new physics
blue = 2010 results
scenarios can affect only one of the channels it is impor-
Fig.4.Summaryoftheσ measurementsbytheD0experiment.
tt¯ tanttoperformsingletopcrosssectionmeasurementsins-
and t-channels separately. Using same data set as for the
combined s + t measurement D0 collaboration performs
model-independent measurement of t-channel single top
production [13]. In this analysis, two-dimensional poste-
7.2
CKiadcocniaarki ise t& a lV.,o agrtX, iavr:X08iv0:048.20850.308 (4240 0(280)08) Assume mt=172.5 GeV/c2 rior probability density is constructed as a function of t-
Moch & Uwer, arXiv:0807.2794 (2008) and s-channelcrosssectionswithoutconstraintontherel-
Dile(pLt=o n5.1 fb-1) 7.40 ± 0.58 ± 0.63 ± 0.45 ative rate of these contributions. Posterior distribution is
integrated over s-channel axis to obtain one dimentional
Lep(tLo=n +4j.e6t sfb (-t1o)pological) 7.82 ± 0.38 ± 0.37 ± 0.15 posteriorprobabilitydensityusedtoextractt-channelcross
Lep(tLo=n +4j.e3t sfb (-b1)-tagged) 7.32 ± 0.36 ± 0.59 ± 0.14 section.Thelatterismeasuredtobeσ(t)=2.90±0.59pb.
The observed (expected) significance of the result is 5.5
All-h(La=d r2o.n9i cfb-1) 7.21 ± 0.50 ± 1.10 ± 0.42 (4.6)standarddeviations.Crosssectionins-channelisob-
ME(TL+=> 23.je2t sfb-1) 7.99 ± 0.55 ± 0.76 ± 0.46 tainedinasimilarwaybyintegratingovert-channelaxis:
σ(s)=0.98 0.63pb.
ME(TL+=2 5/3.7je tfsb-1) 7.11 ± 0.49 ± 0.96 ± 0.43 Themos±tprecisemeasurementinthe s-channelisob-
(stat) ± (syst) ± (lumi)
tained by the CDF collaborationσ(s) = 1.8+0.7pb in the
4 5 6 7σ(pp →8 tt) (pb)9 10 11 12 single top observation analysis using 3.2fb−1−o0.f5data [14]
with the significance of the signal exceeding 3 standard
Fig. 5. Summary of the σ measurements by the CDF experi-
tt¯ deviations.
ment.
4 Measurementsof V
tb
| |
SingletopproductionallowstoprobeWtbinteractionsince
3 Production of single top quarks
itsproductionrateisproportionaltothe V 2 mixingma-
tb
| |
trix element. The latter can be measured directly without
AttheTevatronsingletopquarksareproducedeitherbya
any assumption on the number of quark families and the
t-channelexchangeofavirtualW bosonwhichproducesa
unitarityof the CKM matrix.D0 collaborationusescom-
topquarkviainteractionwithab-quark,orbyans-channel
bineds+tcrosssectionmeasurementtoextract V assum-
exchangeofanoff-shellW bosonwhichdecaysintoatop tb
| |
ingthattopquarksdecayexclusivelyintoWbandthatWtb
andabquark.Thethirdprocess,associatedproductionofa
interactionisCP-conservingandofV Atype.Theseas-
W bosonandatopquark,hasanegligibleproductionrate
−
sumptionsallowananomalousstrengthoftheleft-handed
at the Tevatron. Approximate NNLO calculation [2] pre-
Wtbcoupling(fL).Assumingthemeasuredsingletopcross
dictst-channel(s-channel)crosssectiontobe2.26 0.12 1
pfibcu(l1t.b0e4c±au0s.0e4opfbth).eOreblsaetirvvealtyiosnmoafllthcerossinsgsleecttioopn[a8n]±disvdeirfy- tsreicbtuiotinon[s9]fr|Vomtbft1Lh|e=th1e.o0r2e+−ti00c..11a01l.uUnncceertratainintiteiessoinncthluedes-caonnd-
largebackground,andrequirestousepowerfulmultivari- t-channelproductioncrosssections.If fL isassumedtobe
1
atetechniques. unityand Vtb in [0,1]region,a limit Vtb > 0.79at 95%
| | | |
C.L.isextracted.
A measurementof σ can be used to study V indi-
tt¯ tb
| |
3.1 Crosssectionmeasurements rectlybymeasuringtheratioofbranchingfractions
RecentanalysisoftheD0collaborationuses5.4fb 1ofdata (t Wb) V 2
andmeasurescombineds+tchannelcrosssection−(assum- R= B(t→Wq) = V 2 +|Vtb |2 + V 2
tb ts td
B → | | | | | |
EPJWebofConferences
4]4] (f) DØ, 5.4 fb -1 2]2] 3300 (b) DØ, 5.4 fb -1 V]V] (d) DØ, 5.4 fb -1
00 00 ee
0.0. 2200 Data 0.0. Data GG
nts/nts/ 1155 tWb++tjeqtbs nts/nts/ 2200 ttbq b ts/5ts/5 110000
veve tt veve W+jets enen
[E[E 1100 Multijets [E[E tt EvEv
Yield Yield 55 Yield Yield 1100 Multijets Yield [Yield [ 5500
00 00 00
00..88 00..8855 00..99 00..9955 11 00..88 00..8855 00..99 00..9955 11 114400 116600 118800 220000 222200
RRaannkkeedd BB ddiissccrriimmiinnaanntt RRaannkkeedd BB ddiissccrriimmiinnaanntt TToopp QQuuaarrkk MMaassss [[GGeeVV]]
ttbb++ttqqbb ttbb
Fig. 6. Distributionsof the s+t discriminant (left) and sdiscriminant (middle) in the signal region [0.8, 1] and distribution of the
reconstructedtopquarkmassinadatasamplewithS/B>0.24(right).
with q being a d, s, or b quark. In the SM R = 1, con-
strained by the unitarity of the CKM matrix. R < 1 can Nevent103 DØ, L=5.4 fb-1 Dtta Rta=1
indicateapresenceofnewphysics,forexample,existence tt R=0.5
tt R=0
ofadditionalquarkfamilies.D0experimentmeasuresRsi- 102 Background
multaneouslywith the tt¯cross section using dilepton and
ℓ+jetseventsin5.4fb 1ofdata[15].Intheℓ+jetschan-
− 10
nel,countingofb-taggedeventsin dataandsimulationis
used to distunguish between different decay modes of tt¯
1
pair. In the dilepton channel, the shape of the output of
theNNb-taggingalgorithmservesthesamepurpose.Fig-
10-1
ure 7 demonstrates a change of the predicted number of 0 5 10
eventswithdifferentb-tagmultiplicityinℓ+jetschannel NN output
while Fig. 8 shows a change of the shape of NN output
Fig.8.Distributionoftheminimumb-tagNNoutputofthejets
in the dilepton channelfor variousvalues of R compared
ofhighest-p indileptonchannel.
T
to data. Simultaneous measurement of σ and R yields
tt¯
σ = 7.74+0.67pb and R = 0.90 0.04, the latter being
tt¯ 0.57 ±
themostpre−cisemeasurementofRtodate.Assuminguni-
References
tarity of 3 3 CKM matrix V > 0.88 at 99.7% C.L.
tb
× | |
1. S. MochandP. Uwer Phys.Rev.D78,(2008),034003;
U. Langenfeld, S. Moch and P. Uwer, 80, (2009),
054009;N.Kidonakis,Phys.Rev.D82,(2010),114030.
NNNNNNNNNNNNeventeventeventeventeventeventeventeventeventeventeventevent 232323232323232323232323505050505050505050505050000000000000000000000000000000000000000000000000 DØ, L=5.3 fb-1 Dtta Rta=1 23.. NV..KAibdaosnoavkiest,Pahly(sD.R0evc.oDll7a4b,o(r2at0i1o1n)),,1P1h4y0s1.R2.ev.D 84,
tt R=0.5
tt R=0 (2011),012008.
222222222222000000000000000000000000000000000000 Background
4. T. Aaltonen et al (CDF collaboration), Phys.Rev.Lett.
111111111111555555555555000000000000000000000000 105,(2010),012001.
5. CDFCollaboration,CDFpublicnote10163.
111111111111000000000000000000000000000000000000
6. V. Abasov et al (D0 collaboration), Phys.Lett.B 704,
555555555555000000000000000000000000 (2011),403.
7. CDFCollaboration,CDFpublicnote9913.
000000000000
000000000000 111111111111 ≥≥≥≥≥≥≥≥≥≥≥≥222222222222 8. T. Aaltonen et al (CDF collaboration), Phys.Rev.Lett.
NNNNNNNNNNNN
ttttttttttttaaaaaaaaaaaagggggggggggg 103,(2009),092002;V.M. Abazovetal(D0collabora-
Fig.7.Numberofb-taggedjetsinℓ+jetseventswith 4jets. tion),Phys.Rev.Lett.103,(2006),092001.
≥
9. V. Abasov et al (D0 collaboration), Phys.Rev.D 84,
(2011),112001.
10. L.Breimanetal,ClassificationandRegressionTrees
(Wadsworth,Stamford,1984).
11. R. M. Neal, Bayesian Learning for Neural Networks
5 Conclusions
(Springer-Verlag,NewYork,1996).
12. K. O. Stanley and R. Miikkulainen, Evolutionary
Computation10,(2002)99.
The Tevatron experiments provide precise measurements
13. V. Abasov et al (D0 collaboration),Phys.Lett.B705,
ofthetoppairproductioncrosssections.Recentmeasure-
(2011),313.
ments of the electroweak single top production improve
14. T.Aaltonenetal(CDFcollaboration),Phys.Rev.D82,
theuncertaintyonthecombineds+tchannelcrosssection
(2010),112005.
andallowetoobservet-channelsingletopproduction.All
15. V.M.Abazovetal(D0collaboration),Phys.Rev.Lett.
results agree with the SM and challenge the precision of
107,(2011),121802.
thetheoreticalcalculations.
Candidates
CDF Run II Preliminary (4.8 fb-1)
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