top$quark$physics$with$cms$ · cms preliminary, 36 pb-1 at s = 7 tev figure 7: likelihood as a...

Top Quark Physics with CMS

Rahmat Rahmat University of Mississippi

(on behalf of CMS Collabora?on)

The 44th Fermilab Users Mee-ng 2011

June 2, 2011 Rahmat Rahmat, University of Misssissippi 1

Outline

•  Top Physics •  t t Cross Sec?on -‐ Dilepton Channel •  t t Cross Sec?on – L+jets Channel •  Single Top Cross Sec?on •  t t Invariant Mass •  Top Mass •  Charge Asymmetry •  Summary


Top Quark Physics •  Precise Standard Model measurements

–  Heaviest known elementary par?cle –  Constraint on Higgs mass

•  A Window to new physics – Many models couple preferen?ally to top –  New par?cles may decay to top

•  Main background in many new physics scenarios(e.g. SUSY)

•  Very useful to calibrate detector –  Jet energy scale –  b-‐tagging efficiency


Top Produc?on

Freya Blekman, Vrije Universiteit Brussel5

Top pair production and decay• Pair production in 7 TeV pp collisions:

• BR(t->Wb) ! 1 in Standard Model

• Analysis strategy depends on W decay modes

~85%

~15%


Freya Blekman, Vrije Universiteit Brussel6

Compact Muon Solenoid

!!

!!

!

!


Top Quark Candidate


t t Cross Sec?on at CMS

•  First publica?on: A first measurement at 3.1 pb-‐1 in the dileptonic channel: –  TOP-‐10-‐001: «First Measurement of the Cross Sec?on for Top-‐Quark Pair

Produc?on in Proton-‐Proton Collisions at √s = 7 TeV», Phys. Le). B695 (2011) 424

•  In 2010, Results from 2010 data are based on dataset corresponding to L = 35.9 pb-‐1 of data at √s = 7 TeV. –  TOP-‐10-‐002: «Measurement of the f Pair produc?on Cross Sec?on at √s = 7

TeV using the Kinema?c Proper?es of Lepton + Jets Events» –  TOP-‐10-‐003: «Measurement of the f Pair Produc?on Cross Sec?on at √s = 7

TeV using b-‐quark Jet Iden?fica?on Techniques in Lepton + Jets Events» –  TOP-‐11-‐002 (submifed to JHEP, arXiv:1105.5661): «Measurement of the f

produc?on cross sec?on and the top quark mass in the dilepton channel in pp collisions at√s = 7 TeV»


t t Cross Sec?on -‐ Dilepton Channel (submifed to JHEP, arXiv:1105.5661)

•  Event Selec?on –  two opposite charge leptons: –  pT > 20 GeV/c, |η| < 2.5 (2.4) for e (μ), Isolated in tracker

and calorimeter –  invariant mass selec?on:

•  Mll > 12 GeV/c2, Mll ≠ [91 ± 15] –  jets selec?on:

•  corrected Jet, pT> 30 GeV/c, |η|<2.5 –  For each channel, for 2 jets no b-‐tags, 2 jets 1 b-‐tag and 1

jet no b-‐tags •  Main backgrounds aver leptonic selec?on :

–  Drell-‐Yan → ll: main background, •  rejected by Z veto, jets and ɆT, estimated from data

–  W+Jets, semi-‐lept. f, QCD: from non-‐W/Z decays, es?mated from data

–  Single top tW, diboson, Z→ττ: small cross-‐sec?ons, es?mated from MC

•  Very clean channel, thanks to b-‐tagging -‐ Cut and count experiment

•  Event coun?ng with dedicated data-‐driven techniques for the es?ma?on of background contribu?ons in e+e−, μ+μ−, and e±μ∓ channels

•  Combina?on taking correla?on into account using Best Linear Unbiased Es?mated ¾(t t) = 168 ± 18(stat) ± 14(sys) ± 7(lum) pb


5.3 Cross section measurements per decay channel 13

between the expected and observed numbers of events in all channels. A summary of the ex-pected number of background events is compared with the number of events observed in datain Table 2 for the channels used in the measurement.

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Figure 3: Number of events passing the full dilepton selection criteria without a b tag (points),as a function of the jet multiplicity for e±µ∓ (left) and all dileptons (right). There is no E/Trequirement for the e±µ∓, and a requirement of E/T > 30 GeV for the e+e− and µ+µ−. The ex-pected distributions for the tt signal and the background sources are shown by the histograms.The Drell–Yan and non-W/Z lepton backgrounds are estimated from data, while the otherbackgrounds are from simulation. The total uncertainty on the background contribution isdisplayed by the hatched region.


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Figure 4: Jet multiplicity for events passing full dilepton selection criteria with at least oneb-tagged jet, otherwise the same as in Fig. 3.

The tt production cross section is measured using:

σ(pp → tt) =N − BAL

, (1)

5.3 Cross section measurements per decay channel 13

between the expected and observed numbers of events in all channels. A summary of the ex-pected number of background events is compared with the number of events observed in datain Table 2 for the channels used in the measurement.


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Figure 3: Number of events passing the full dilepton selection criteria without a b tag (points),as a function of the jet multiplicity for e±µ∓ (left) and all dileptons (right). There is no E/Trequirement for the e±µ∓, and a requirement of E/T > 30 GeV for the e+e− and µ+µ−. The ex-pected distributions for the tt signal and the background sources are shown by the histograms.The Drell–Yan and non-W/Z lepton backgrounds are estimated from data, while the otherbackgrounds are from simulation. The total uncertainty on the background contribution isdisplayed by the hatched region.


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Figure 4: Jet multiplicity for events passing full dilepton selection criteria with at least oneb-tagged jet, otherwise the same as in Fig. 3.

The tt production cross section is measured using:

σ(pp → tt) =N − BAL

, (1)

t t Cross Sec?on -‐ Lepton+Jets Channel (CMS TOP-‐10-‐002)

•  Measurement without b-‐tagging •  Simultaneous binned likelihood fit to

–  Missing ET with 3 jet •  discriminate QCD from true W decay

–  M3 with ≥ 4 jets •  M3 is invariant mass of 3 jets with max § PT •  separates top from other events with real W

decays •  MT(W) shows good agreement with data •  Combined measurement

•  ¾tt = 173 (stat+sys) ± 7(lum) pb pb •  Compa?ble with other measurement

17

the asymmetry between W+ boson and W− boson production due to the proton-proton initial527

state to determine the templates for the W+jets background. A fourth method measured the tt528

cross section from events containing a high-pT isolated muon and at least three jets. For this529

analysis, we relax the relative isolation requirement to Irel < 0.1 but introduce a requirement on530

the missing transverse energy in the event of /ET > 20 GeV to keep the amount of QCD multijet531

background small. A method based on Ref. [52, 53] is used to estimate the amount of QCD532

multijet background separately for events with three jets and events with at least four jets. The533

number of top-pair and W+jets events is extracted from a fit to the M3 distribution. All four534

of these methods gave results similar to our previously quoted main result, but with slightly535

larger combined statistical and systematic uncertainties in each case.536

Complementary to the top-quark-pair production measurements, the production cross section537

of exactly one muon in association with additional hard jets has been measured. In all pro-538

cesses considered as signal for this measurement the muon originates from a W boson. Both539

single top quark decays and decays of top-quark pairs in the lepton+jets channel, including540

decays via tau leptons in the intermediate state, are contributors to this signature. An addi-541

tional component of this signal comes from the production of a W boson with additional jets,542

which is the most prominent background for the analysis of tt “lepton+jets” decays. The same543

event selection as in the main analysis has been applied. In addition to the main analysis all544

jets in data have been corrected for event pileup leading to reduced JES and pileup uncertain-545

ties. To obtain the cross section, the observed number of events in data is corrected for the546

remaining background processes. These are QCD multijet production, the production of a Z547

boson with additional jets, single-top-quark decays, and other tt decays. The number of QCD548

multijet events is determined from data using a template fit to the missing-transverse-energy549

distribution in each inclusive (or exclusive) jet-multiplicity bin. The normalization and shape550

of the other backgrounds is taken from the simulation. Figure 8 shows the cross section for the551

production of a single muon with pT > 20 GeV and |η| < 2.1 and additional jets as a function552

of the inclusive and exclusive multiplicity of jets with pT > 30 GeV within |η| < 2.4. The tran-553

sition from a phase space dominated by W+jets events (in the 1-jet and 2-jet bins) towards a554

region dominated by the production of top-quark pairs (in the 4-jet bin) is clearly visible. The555

comparison of data and simulation indicates a good understanding of this transition, while the556

overall normalization seems to be slightly underestimated. This is consistent with the main557

analysis, which also found a W+jets cross section larger than the theoretical prediction.558

7 Conclusions559

A measurement of the cross section for top-quark pair production in proton-proton collisions560

at a center-of-mass energy of 7 TeV has been performed at the LHC with the CMS detector.561

The analysis uses a data sample corresponding to an integrated luminosity of 36 pb−1 and562

is based on the reconstruction of the final state consisting of one isolated, high transverse-563

momentum muon or electron and hadronic jets. The measured cross section for the combined564

electron+jets and muon+jets channels is 173+39−32(stat. + syst.) ± 7(lumi.) pb. This measure-565

ment agrees with current theoretical values, e.g., [10–12, 32, 33], which agree amongst them-566

selves within their quoted uncertainties. The approximate NNLO calculation from Ref. [12]567

yields 163+7−5(scale) ± 9(PDF) pb, while a similar calculation using HATHOR1 [10, 11] yields568

160+5−9(scale) ± 9(PDF) pb. Our measurement also agrees with the earlier CMS measurement569

1The HATHOR calculation was performed using the MSTW2008 NNLO PDF [34] and mtop = 173 GeV as thechoice for both the factorization and renormalization scales. The scale uncertainty is evaluated by independentlyvarying the scales by factors of 2 and 0.5, and the PDF uncertainty is calculated using the 90% confidence levelenvelope of the PDF.


t t Cross Section - Lepton+Jets Channel without b-tag

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e+jets measurement� 180+45

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µ+jets measurement� 168+42


Combined measurement� 173+39


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Holger Enderle (Uni Hamburg) PHENO 11 - Recent Top Results from CMS 10th May 2011 18 / 13

t t Cross Sec?on in the Lepton+Jets Channels with b-‐Tagging (TOP-‐10-‐003)

•  Measurement with b-‐tagging •  Event Selec?on:

–  one lepton (with second lepton veto): pT > 30 (20) GeV/c, |η| < 2.5 (2.1) for e (μ), Isolated in tracker and calorimeter

–  jets selec?on: corrected Jet, |η|<2.4 –  ɆT and b-‐tag selec?on: ɆT > 20, Secondary Vertex

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σtt = 150 ± 9 (stat.) ± 17 (syst.) ± 6 (lumi.) pb


t t Cross Sec?on Combined

•  Combined measurement has precision of 12%

•  Very good agreement with approxima?on NNLO theory

•  Comparable to world average

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GeV) •  S?ll rather small signal to background

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–  Exploit two characteris?c features of Single top quark produc?on (2D analysis)

–  Use MVA technique Boosted Decision Trees for further separa?on (BDT analysis)

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BDT, e channel 37.837.8 ±59.2

channelµBDT, 40.440.4 ±89.8

2D, e channel 73.173.1 ±154.2

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-1=7 TeV, L=35.9 pbsCMS Preliminary, CMS combined result� σ(t) = 84 ± 30 pb

Significance observed (expected)� 2D fit: 3.7 (2.1)� BDT: 3.5 (2.9)

Atlas: σ(t) = 53+46−36 pb

Holger Enderle (Uni Hamburg) PHENO 11 - Recent Top Results from CMS 10th May 2011 9 / 13

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•  Reconstruc?on of mt t done in 3 steps –  reconstruc?on of leptonic W (with MET as pT (μ))

•  2 real solu?ons → keep both •  imaginary solu?ons → modify MET

–  jet-‐parton associa?on by χ2 minimisa?on •  5 quan??es used: mlep (top), mhad (top), mhad (W ), pT (t t ), HT

frac?on •  correct associa?on in ∼ 80% (in simula?on)

–  kinema?c fit to improve resolu?on •  mtop = 172.5 GeV/c2 •  mW = 80.4 GeV/c2

•  Looking for narrow resonances –  Model independent

•  Lepton+jets channels –  e± + jets –  µ± + jets

•  No significant signal observed

t t Invariant Mass (TOP-10-007)

Search for narrow resonances� model independent

Lepton+jets channel� e±+jets� µ±+jets

Reconstruction of the t t system� with help of kinematic fit

Good agreement in mtt with SM� limits set for Z’ production at 95% CL

Already competitive with Tevatron,particularly at higher masses

� for gg induced resonances(at mtt � 550 GeV)

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Holger Enderle (Uni Hamburg) PHENO 11 - Recent Top Results from CMS 10th May 2011 12 / 14June 2, 2011 Rahmat Rahmat, University of Misssissippi 13

Top Quark Mass Measurement in the Dilepton Channels (submifed to JHEP, arXiv:1105.5661)

•  Event Selec?on: –  Inclusive single lepton trigger

•  muon with pT>15 GeV/c (µµ/eµ) or electron with ET>17 GeV (ee/eµ) –  ≥ 2 leptons, pT>20 GeV/c |η|<2.5

•  Isolated and promptly produced –  ≥ 2 jets, pT>30 GeV/c |η|<2.5

•  An?-‐kT (R=0.5), par?cle flow based algorithm –  MET > 30 (20) GeV for the ee/µµ (eµ) channel

•  Evaluated using 2 methods –  fully kinema?c analysis (KINb) –  analy?cal matrix weigh?ng technique(AMWT)

•  Largest systema?cs from jet energy scales •  CMS combined result

–  mtop = 175.5 ± 4.6(stat) ± 4.6(syst) GeV/c2

•  Good agreement with world average –  mtop = 173.1 ± 1.1 GeV/c2


20 6 Measurement of the top quark mass

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Figure 8: Reconstructed top quark mass distributions from the KINb (left) and AMWT (right)methods. Also shown are the total background plus signal models, and the background-onlyshapes (shaded). The insets show the likelihoods as functions of mtop.

where xi are the Bjorken x values of the initial-state partons, F(x) is the PDF, the summationis over the possible leading-order initial-state partons (uu, uu, dd, dd, and gg), and the termp(E∗|mtop) is the probability of observing a charged lepton of energy E∗ in the rest frame of thetop quark, for a given mtop. For each value of mtop, the weights w are added for all solutions.Detector resolution effects are accounted for by reconstructing the event 1000 times, each timedrawing random numbers for the jet momenta from a normal distribution with mean equalto the measured momentum and width equal to the detector resolution. The weight is aver-aged over all resolution samples for each event and mtop hypothesis. For each event, the mtophypothesis with the maximum averaged weight is taken as the reconstructed top quark massmAMWT. Events that have no solutions or that have a maximum weight below a threshold valueare discarded. Based on simulations, we expect this requirement to remove about 9% of the ttand 20% of the Z+jet events from the sample.

A likelihood L is computed for values of mtop between 151 and 199 GeV/c2 in steps of 3 GeV/c2,using data in the range 100 < mAMWT < 300 GeV/c2. A unique shape determined from MC isused for each b-tag category, where the peak mass distribution of each individual contributionis added according to its expected relative contribution. For the Z+jet background, both thedistribution and its relative contribution are derived from data in the Z-boson mass window(c.f. Section 5.1.1). For the other contributions (signal, single top production, non-dileptonicdecays of tt pairs), the distributions predicted by the simulation are used. Further backgroundcontributions are negligible and are not taken into account in the fit.

We determine the bias of this estimate using ensembles of pseudo-experiments based on theexpected numbers of signal and background events, as shown in Fig. 9. A small correctionof 0.3 ± 0.1 GeV/c2 is applied to the final result to compensate for the residual bias introducedby the fit (Fig. 9, left). The width of the pull distribution is on average about 4% smaller than1.0, indicating that the statistical uncertainties are overestimated (Fig. 9, right). The statisticaluncertainty of the measurement is therefore corrected down by 4%.

Figure 8 (right) shows the predicted distribution of mAMWT summed over the three b-tag cate-gories for the case of simulated mtop = 175 GeV/c2, superimposed on the distribution observed

20 6 Measurement of the top quark mass


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Figure 8: Reconstructed top quark mass distributions from the KINb (left) and AMWT (right)methods. Also shown are the total background plus signal models, and the background-onlyshapes (shaded). The insets show the likelihoods as functions of mtop.

where xi are the Bjorken x values of the initial-state partons, F(x) is the PDF, the summationis over the possible leading-order initial-state partons (uu, uu, dd, dd, and gg), and the termp(E∗|mtop) is the probability of observing a charged lepton of energy E∗ in the rest frame of thetop quark, for a given mtop. For each value of mtop, the weights w are added for all solutions.Detector resolution effects are accounted for by reconstructing the event 1000 times, each timedrawing random numbers for the jet momenta from a normal distribution with mean equalto the measured momentum and width equal to the detector resolution. The weight is aver-aged over all resolution samples for each event and mtop hypothesis. For each event, the mtophypothesis with the maximum averaged weight is taken as the reconstructed top quark massmAMWT. Events that have no solutions or that have a maximum weight below a threshold valueare discarded. Based on simulations, we expect this requirement to remove about 9% of the ttand 20% of the Z+jet events from the sample.

A likelihood L is computed for values of mtop between 151 and 199 GeV/c2 in steps of 3 GeV/c2,using data in the range 100 < mAMWT < 300 GeV/c2. A unique shape determined from MC isused for each b-tag category, where the peak mass distribution of each individual contributionis added according to its expected relative contribution. For the Z+jet background, both thedistribution and its relative contribution are derived from data in the Z-boson mass window(c.f. Section 5.1.1). For the other contributions (signal, single top production, non-dileptonicdecays of tt pairs), the distributions predicted by the simulation are used. Further backgroundcontributions are negligible and are not taken into account in the fit.

We determine the bias of this estimate using ensembles of pseudo-experiments based on theexpected numbers of signal and background events, as shown in Fig. 9. A small correctionof 0.3 ± 0.1 GeV/c2 is applied to the final result to compensate for the residual bias introducedby the fit (Fig. 9, left). The width of the pull distribution is on average about 4% smaller than1.0, indicating that the statistical uncertainties are overestimated (Fig. 9, right). The statisticaluncertainty of the measurement is therefore corrected down by 4%.

Figure 8 (right) shows the predicted distribution of mAMWT summed over the three b-tag cate-gories for the case of simulated mtop = 175 GeV/c2, superimposed on the distribution observed

KINb

AMWT

Top Quark Mass Measurement in the Lepton+jets Channels (CMS-‐PAS TOP-‐10-‐009)

•  Event Selec?on: –  Exactly one high pT isolated lepton (electron pT > 30

GeV, μ pT > 20 GeV), loose 2nd lepton veto and four or more PF jets (pT > 30 GeV).

–  Par?cle flow jets and missing transverse energy are used to achieve the best expected mass resolu?on.

•  Using Ideogram method –  A constrained kinema?c fit is used to reconstruct the

complete kinema?cs of the event under the hypothesis that the event is a f lepton+jets event

–  A likelihood is calculated for each event in the data sample from the output of the kinema?c fit

–  The likelihood calcula?on takes into account all the possible assignments of jets to quarks in the f lepton+jet event hypothesis, and considers the possibility that the event is a f event or a background event

–  A joint likelihood fit over all events in the data sample is then used to extract the value of the top quark mass

6/2/11 Rahmat Rahmat, University of Misssissippi 15

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'Fresh from the oven: approved yesterday!’

•  Combina?on e+jets and µ+jets

The most precise top mass measurement outside Tevatron

•  Combina?on of dilepton and lepton+jets top mass using the BLUE (Best Linear Unbiased Es?mate) method. mt = 173.4 ± 1.9(stat) ± 2.7 (syst) GeV

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Figure 7: Likelihood as a function of the top quark mass from the fit to the data (left) and theestimated uncertainty compared to the expectation from MC pseudo-experiments (right), forthe electron+jets (top), muon+jets (middle), and the combined lepton+jets channel (bottom).

mt = 178.2 ± 3.7(stat) GeV

and from 334 events in the muon+jets channel:335

mt = 170.2 ± 2.6(stat) GeV.

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ence corresponds to 1.8 σ), and the combined likelihood of the two channels yields the follow-337

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mt = 173.1 ± 2.1(stat)+2.4−2.1(JES)± 1.4(other syst) GeV.

The estimated statistical uncertainties observed in data agree well with the expectation from339

the MC pseudo-experiments, as shown in Fig. 7(right).340


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Charge Asymmetry (CMS-‐PAS TOP-‐10-‐010)

•  Tevatron –  Valence (an?-‐)quarks from certain direc?on –  Forward-‐backward asymmetry

•  LHC –  gg fusion symmetric –  asymmetry only from small qq frac?on

•  AC = (N+ -‐ N-‐)/ (N+ + N-‐ ) N+ / N-‐ are the number of events with posi?ve /nega?ve values of |´top|-‐ |´antitop| Predicted in SM AC = 0.0130(11)

•  Would indicate BSM if there is devia?on e.g. axigluon

•  Measured at CMS AC = 0.060 ± 0.134(stat) ± 0.026(sys)

Charge Asymmetry (TOP-10-010)Tevatron: proton-antiproton collider

� valence (anti-)quarks from certain direction� forward-backward asymmetry

LHC: proton-proton collider� gg fusion symmetric� asymmetry only from small qq fraction� no valence antiquarks� quarks have higher x on average� asymmetry in |ηt |− |ηt |=: AC

Predicted in SM� AC = 0.0130(11)

Deviation would indicate BSM physics� e.g. axigluon

Measured at CMS (unfolded)� AC = 0.06 ± 0.13(stat)± 0.03(syst)

Competitive in sensitivity with Tevatronend of 2011 (� 1 fb−1) |

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Holger Enderle (Uni Hamburg) PHENO 11 - Recent Top Results from CMS 10th May 2011 13 / 14June 2, 2011 Rahmat Rahmat, University of Misssissippi 17

Summary

•  CMS is a new Top Factory •  With only 36 pb-‐1 we have: –  t t cross sec?on (±12%) –  Single top cross sec?on (±36%) –  The most precise top mass measurement outside Tevatron –  t t invariant mass (-‐> limits for Z’ produc?on) –  Charge asymmetry (compe??ve with Tevatron end of 2011?)

•  Today, June 2, 2011 we have more than 500 pb-‐1 and many in Top Physics


top$quark$physics$with$cms$ · cms preliminary, 36 pb-1 at s = 7 tev figure 7: likelihood as a...

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