cms and a light higgs with tt j. r. incandela university of california santa barbara j. r. incandela...

CMS and a Light Higgs with ttCMS and a Light Higgs with tt

J. R. IncandelaUniversity of California Santa Barbara

J. R. IncandelaUniversity of California Santa Barbara

P.K.’s visit to UCSB, September 14, 2006 J. Incandela 2

Htt Group: The goal was to perform a realistic study of the feasibility of

detecting SM Higgs in this channel


CMS Htt study

Recently completed study for the CMS Physics TDR vol. 2

This is a publicly available note: (49 pages).

UCSB (JI) Led the group through the completion of this effort and note.


Overview

• Analysis involved 4 subgroups • Standard top channels: Dilepton, All-hadronic, and e/m + jets

• Largely independent over past two years

• Goal: As realistic as possible• Include backgrounds with higher order processes (Alpgen)

• Include multiple interactions (aka pile-up)

• Fully simulate the detectors

• Develop and use realistic algorithms:• Electron and muon identification

• Jet and missing Et reconstruction

• b and c jet tagging, and light quark/gluon mis-tagging

• A huge amount of work!!


Light H in conjunction with tt

Production Modes

Decay Mode


ISR

H→bb

MPI

t→W

- b

W+b←t

ET > 3 GeV


Backgrounds

•Main backgrounds are ttjj, ttbb, ttZ• ttjj dominates numerically even though a mis-tagged light quark

or gluon jet is required• The ttjj xsec is nearly 3 orders of magnitude higher than signal

• Should one get beyond ttjj, one must still confront ttbb, and ttZ with Zbb, which are irreducible


Generators and Cross sections

What’s new:

Used a more sophisticated generation scheme for the ttNj background

•PYTHIA alone under-estimates the hard radiation

•CompHEP overestimates

•Alpgen+MLM matching is “just right”


ALPGEN v.2 & MLM


Triggering

• After full CMS detector simulation and digitization, trigger simulations were run.

• Typically a bit lower efficiency than we ultimately expect• More complex and efficient triggers will likely be available. .

Trigger

offline


Leptons

•Likelihoods • Developed explicitly for Htt

•Categorized in Monte Carlo (MC)

• SIGNAL• Matched to lepton form W

• use an cone of radius 0.1 (0.01) for electrons (muons)

• BACKGROUND• All others:

• leptons from b or c hadron decays, fakes

)()(

)(

ibkg

iisig

i

isig

ii xPxP

xPL


Muon Reconstruction

• Muon likelihood: 4 “obvious” variables:

• Muon pT

• Track Isolation

• Calorimeter Isolation

• 2-D Impact Parameter significance

SIPPTCalo Iso

Track Iso


Muon Reconstruction

• 90% for signal and 1.0% for background • calculated from the semi-leptonic Htt sample

cut is –Log(L)<1.4


Electron Reconstruction

• Electron Likelihood: 5 variables:

• pT

• E/p

• Had/EM

• pT tracks inside R=0.3 cone and

outside veto cone (R=0.015)

• R between electron candidate and closest track outside veto cone

R ≡ √( ∆η2 + ∆φ2 )


Electron Reconstruction

• 84% for signal 1.5% for background

cut is –Log(L) < 1.3


•JETS• Iterative Cone Algorithm

R=0.5 (0.4 for All-had)

• ET > 20 (25 All-had)

• |η| < 2.5 (2.7 All-had)

• Use MC calibrated jets

• Remove electrons that match within R < 0.4

R ≡ √( ∆η2 + ∆φ2 )

Signal (ttH, MH = 115 GeV)

Raw (IC 0.5)

MonteCarlo calibrated

γ-jet calibrated

Jets


Jet definition: All Hadronic Case

• Different Cone Sizes Tested

• Signal and 3 most dangerous Bkg

used for testing (ttbb, tt2j, qcd170)

• 8 most energetic jets in ||<2.7 and

ET>25GeV

• Jet-Parton pairing

2 for masses of 2 W and 2 top

within 3 sigma

• Jets paired to b-parton have to be

b-tagged

2222

2

)()()()(

t

jjt

t

jjt

W

jjW

W

jjWmass m

mm

m

mm

m

mm

m

mm

Cone R=0.4 chosen


ET(ecal + hcal)

− ∑ [ET(calib)−ET(raw)]

− ∑ pT(μ)

Missing ET

Calorimeter tower measurements

Jet corrections

Muon momenta

In semi-leptonic Htt channel/

Missing Momentum


bTagging

• Combined Secondary Vertex Algorithm

• Mistagging rate as a function of b-jet efficiency for signal (left) and ttjj

(center) are shown for various types of jets

• Gluon jets include splitting to bb or cc in center plot...

• Tag Rates for b-, c- and uds-jets vs discriminator cut for ttjj sample (right)

c

udsuds

c

g

cb

uds


Semi-Leptonic Selection

• Preselection• HLT + Isolated Lepton + 6 or 7 Jets• ET > 20GeV • 4 bTagged jets

• D>0.5 (70% bTag efficiency)

• Veto events with two leptons, or wrong lepton.

• Jet Pairing• Likelihood method: Levent=LmassxLbTagxLkinematics

• Mass refers to likelihoods for the obtained masses for hadronic W and tops

• Kinematics • Takes into account b jets from top quarks slightly more

energetic than those from H or jj (rather complicated formula…)

• LbtagLbsele=Dh1xDh2xDbTopHadxDbTopLep


Semi-Leptonic Selection

Example of performance for muon selection

Chosen working points are: 0.55 and 0.72


Final results 60 fb-1

Muon Channel

A bit less S/ √B cause mass constraintbut betterS/N

Electron Channel

No discrepancies at Just less efficiency in e channel for HLT and isolation.

S

√B

= 2.35

Muon + electron, no systematics


di-Lepton Selection

• Signal• H forced to bb

• One W forced to (e,,); the other free • Find large contribution from single lepton events (1 real + 1 fake

lepton)

• Preselection• 3 b-jets and D>0.7

• Selection• 2 leptons (e,) passing Likelihood criteria

• –Log(Lmu)<1.8 and –Log(Lele)<1.3 with pT>20 GeV

• At least 4 jets with ET> 20 GeV• No additional tagging requirement

• Corrected ETmiss > 40 GeV


Full Hadron Selection• Same analysis as Jet Algo choice and same mass for jet pairing

• 8 most energetic jets in ||<2.7

• Centrality Cuts

mass for 2W and 2tops within 3 sigma from expected values

• 2 working points

• Low S/N Higher Significance

• ET(7th)>30GeV – ET(8th)>20GeV ordered ET jet

• 3 out of Dh1 Dh2 DbTopHad1 DTopHad2 > 0.80

• CentH>0.55

• High S/N Lower Significance

• ET(7th)>30GeV – ET(8th)>20GeV ordered ET jet

• Dh1 Dh2 DbTopHad1 DTopHad2 orderd in D; D(3th)>0.85 and D(4th)>0.70

• CentH>0.55 – CentAll>0.80

jet

jetT

E

E

8

8All Centrality

jet

jetT

E

E

2

2Higgs Centrality


Full Hadron Selection: Results 60 fb-1

Analyzed Ev. Sel.Eff . Opt.1(%) Nexp 60fb- 1 Sel.Eff . Opt.2(%) Nexp 60fb- 1

ttH (115) 49636 0.294 44 2.317 347

ttH (120) 163494 0.366 45 2.550 314

ttH (130) 43254 0.358 27 2.797 214

ttbb 203135 0.0645 109 0.7015 1187

tt1j 1031551 0.0005 49 0.0084 860

tt2j 559111 0.0009 54 0.0333 1996

tt3j 68015 0.0015 35 0.0794 1905

tt4j 97334 0.0021 75 0.1818 6656

ttZ 80226 0.0312 11 0.3577 121

q170 264310 0.0004 76 0.0238 4805

q120 55128 0.0000 0 0.0018 83

Total 410 17614

M(H)=115GeV S/sqrt(N) 2,17 S/sqrt(N) 2.61

S/N 10.7% S/N 2.0%

M(H)=120GeV S/sqrt(N) 2,23 S/sqrt(N) 2,37

S/N 11.0% S/N 1.8%

M(H)=130GeV S/sqrt(N) 1.33 S/sqrt(N) 1.61

S/N 6.5% S/N 1.2%

An example: tables of this type are in the note for all channels


Speculation on Mature Experiments

• The mature CMS working point is taken to have the following systematic uncertainties:

• Flat 3% JES

• 10% Jet Resolution

• 4% in bc-Jet tagging efficiencies

• 10% in uds-Jet tagging efficiencies

• 3% in luminosity


Single Lepton Table

• Uncertainties from JES and uds-Tagging efficiencies

• Most dangerous BKGD are tt1j and tt2j


Single Leptons 60 fb-1

•Significances are drastically reduced once reasonable systematic uncertainties are included

•Led to re-optimization with a looser selection

• Still pretty grim

Muon

Electron


Di-Lepton Table

• Again - big uncertainties from JES and uds-tagging

• Most dangerous BKGDs are ttNj


Hadron Table

• Again big uncertainties from JES and uds-Tagging efficiencies but…Surprise! The most dangerous background is QCD (look at JES) and then tt4j, tt3j and tt2j…


The message!

Jet Energy Scale

and

light quark jet mis-tagging

are major systematics

Furthermore, when signal and background production

systematics are included, it gets worse.


Cross Section uncertainties


CMS-CDF comparison

• Exercise performed with the diLepton channel• CMS, CDF tag uncertainties taken to be the same (4% bc and 10% uds)

• Some indication we’re not too far off the mark.


Is there any hope?

• A couple of possible mitigating factors:1. Backgrounds from Data: 60fb-1 of integrated luminosity will

provide plenty of data for which detailed studies can be performed to understand the detector and algorithms

2. The availability of large control samples of top events will enable b tagging of high energy jets to be very well understood. This will probably enable some further suppression of light quark and charm jet tagging relative to b tagging. Similarly, experience with real data will likely improve jet reconstruction and energy measurements.

• Will they be enough?

• How much work and how much data will it take?


Summary

• Statistical Significance for the 4 sub-channels lower than previous studies but still combine to greater than 2.0 in 60 fb-1.

• But the systematic uncertainties: JES and uds-tagging are major problems

• They eliminate all sensitivity! (Combined significance of less than 0.2 for all channels)

• Real data will give the final answer on the feasibility of this channel, but all indications are that it will be very difficult.

More Information


Muon Selection: Results 60 fb-1

Analyzed Ev. PreSel. Eff .(%) Nexp 60fb- 1 Sel.Eff . Opt.1(%) Nexp 60fb- 1 Sel.Eff . Opt.2(%) Nexp 60fb- 1

ttH (115) 27768 5.66 271 2.3 111 1.13 54

ttH (120) 91447 5.66 223 2.3 91 1.1 43

ttH (130) 19467 6.3 145 2.0 65 1.23 30

ttbb 372737 0.92 1573 0.30 503 0.13 221

tt1j 393000 0.08 8616 0.007 675 0.0013 130

tt2j 569000 0.15 8878 0.014 843 0.0021 126

tt3j 110000 0.14 3493 0.009 218 0.0009 22

tt4j 86697 0.05 1857 0.005 169 0.0000 0

ttZ 50000 0.27 90 0.078 26 0.034 12

Total 24507 2433 511


S/N 4.5% S/N 10.3%


S/N 3.7% S/N 8.5%


S/N 2.7% S/N 5.9%


Electron Selection: Results 60 fb-1

Analyzed Ev. PreSel. Eff .(%) Nexp 60fb- 1 Sel.Eff . (%) Nexp 60fb- 1

ttH (115) 27500 4.25 204 1.26 60

ttH (120) 100000 4.28 170 1.26 50

ttH (130) 24500 4.50 111 1.25 31

ttbb 420000 0.7 1188 0.17 286

tt1j 528000 0.08 8500 0.0038 386

tt2j 286000 0.14 8266 0.0056 336

tt3j 110000 0.12 2924 0.0045 109

tt4j 102000 0.10 3732 0.0039 143

ttZ 50000 0.20 75 0.05 16

Total 24685 1276

M(H)=115GeV S/sqrt(N) 1.7

S/N 4.7%


S/N 3.9%


S/N 2.4%

cms and a light higgs with tt j. r. incandela university of california santa barbara j. r. incandela...

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