Charged Higgs search with top : example of single-top events in ATLAS

A. Lucotte / LPSC Grenoble Uppsala 15-SEP-2006

Charged Higgs search with top :

example of single-top events in ATLAS

Ketevi Assamagan (BNL)Ketevi Assamagan (BNL)

Arnaud Lucotte (LPSC Grenoble)Arnaud Lucotte (LPSC Grenoble)

Outline Outline

Motivation Motivation

Single-top production @ LHCSingle-top production @ LHC

Single-top cross-section measurementsSingle-top cross-section measurementsEvents selectionEvents selection

Results and performanceResults and performance

Charged Higgs contribution measurementsCharged Higgs contribution measurementsCross-sections in (mCross-sections in (mH±H±,tan ,tan ) )

Efficiency and event yields in (mEfficiency and event yields in (mH±H±,tan ,tan ))

ConclusionConclusion


Single top Production @ LHC

Production at the LHC Production at the LHC 3 contributing mechanisms in SM

Cross-section theoretical uncertainties are significant :

– NLO/NLL available for s-, t- and (new) W+t channels

– Main uncertainty due to the choice for the (b,g) PDF

– Choice of factorization/renormalization scale

– Top mass uncertainty : ∆mtop

t-channelt-channel

W+t channelW+t channel

s-channels-channel

∆∆σσtheotheo//σσ= ± 4-8%= ± 4-8%

σσNLONLO(qb(g)(qb(g)tb(g)) = 150±6 pbtb(g)) = 150±6 pb

σσNLONLO(qb(g)(qb(g)tb(g)) = 88.5±4 pbtb(g)) = 88.5±4 pb- dominant at the LHCdominant at the LHC- used for Vused for Vtbtb, , tt, FCNC …, FCNC …

σσLOLO(gb(gbtW) = 60±15 pbtW) = 60±15 pb

- high level of bcgkd: tthigh level of bcgkd: tt- sensitive to extra W, Hsensitive to extra W, H±±……

σσNLONLO(qq’(qq’tb) = 6.1±0.3 pbtb) = 6.1±0.3 pb

σσNLONLO(qq’(qq’tb) = 3.8±0.2 pbtb) = 3.8±0.2 pb

- high level of bckgd: tt, W+jets- high level of bckgd: tt, W+jets

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Single top Production @ LHC

Production at the LHC Production at the LHC 3 contributing mechanisms in SM + Higgs in 2HDM

Our strategy in charged Higgs searches :

– Measure precisely single-top cross-section

estimate stat. + systematic uncertainties

– Measure deviation from single-top cross-section

interpretation in (mH±,tan) plane

t-channelt-channel

W+t channelW+t channel

s-channels-channel

σσNLONLO(qb(g)(qb(g)tb(g)) = 150±6 pbtb(g)) = 150±6 pb

σσNLONLO(qb(g)(qb(g)tb(g)) = 88.5±4 pbtb(g)) = 88.5±4 pb- dominant at the LHCdominant at the LHC- used for Vused for Vtbtb, , tt, FCNC …, FCNC …

σσLOLO(gb(gbtHtH±±) in (mH±,tan) in (mH±,tan))

- Up to a few pb Up to a few pb - High level of tt + t backgds High level of tt + t backgds

σσLOLO(H(H±±tb) in (mtb) in (mH±H±,tan,tan))

- Up to a few pb…- Up to a few pb…- High level of t + tt + W+jets- High level of t + tt + W+jets

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HH±±

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HH±±

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W* single top event selection

Analysis StrategyAnalysis StrategySelect and tag event

– 1 high-pT lepton (25 GeV/c)

– High missing ET (25 GeV/c)

– at least 2 high-pT central jets

with at least 1 b-tagged jet

Discriminate from non-W background

– Reconstruct the (leptonic) mWT

Discriminate from top-pair background

– Veto of a 3rd high-pT jet

– Keep low total energy events (HT or MTOT)

Discriminate from W+jets background

– Require a 2nd high-pT b-tagged jet

– Reconstruct a Top mass Mlvb

– Use event shape & highσσ x BR (pb x BR (pb))

W* W* (lv)b b (lv)b b 2.22.2

W+jetsW+jets lv+jets lv+jets 3,8503,850

W+QQW+QQ lv+QQ lv+QQ 66.766.7

WZ WZ lv+jetslv+jets 3.43.4

WW WW lv + jets lv + jets 17.117.1

tt tt (lv)b (lv)b (lv)b (lv)b 38.238.2

tt tt (lv)b (jj)b (lv)b (jj)b 242.8242.8

bb

bbe,µe,µ

qq

vv

““l+jets” final statel+jets” final state


W* single top selection : jet multiplicity

Jet selectionJet selectionJet mulitplicity

– At least two high-pT central jets

useful against W+jets…

– Veto of 3rd high-pT jet (difficult...but crucial)

needed against top pair events

b-tagged jet

– Exactly two high-pT central b-jets (35-45 GeV/c)

effb = 60% for rejection uds=100 eff

c=10%

needed against QCD, W+jets, etc…


W* single top selection : b-tagged jets

Jet selectionJet selectionJet mulitplicity

– At least two high-pT central jets

useful against W+jets…

– Veto of 3rd high-pT jet (difficult...but crucial)

needed against top pair events

b-tagged jet

– Exactly two high-pT central b-jets (35 GeV/c)

effb = 60% for rejection uds=100 eff

c=10%

needed against QCD, W+jets, etc…


W* single top selection :leptonic top mass

Top mass reconstruction Top mass reconstruction 1) Determination of longitudinal pZ()

Interpret of mET as pT()

Compute pz() using W-mass constraint

2-fold ambiguities for pz() : choose best M(lvb)

If no solution (MWT > MW) take real part

2) Recontruction of M(lb)

Choose b-jet resulting in the highest pTtop

- Optimization of the window on M(lb)

lower bound : W+jets , WZ, etc..


W* single top selection :total energy

Total transverse energy HTotal transverse energy HTT

HT Definition

HT = pT(jet) + pT(lepton) + mET

Note : can also use Mtot, Ptot…

Discriminating power

Optimization of the window on HT:


upper bound : top pair

Depend upon :

– jet energy scale determination

– Lepton and mET determination


W* single top selection :total energy

Total transverse energy HTotal transverse energy HTT

HT Definition

HT = pT(jet) + pT(lepton) + mET

Note : can also use Mtot, Ptot…

Discriminating power

Optimization of the window on HT:


upper bound : top pair

Depend upon :

– jet energy scale determination

– Lepton and mET determination


W* single top selection : event yields

Performance Performance Efficiency and rejection :

– Efficiency ε ≈ 1.7% and N(30fb-1) ~ 600 events

– Main backgrounds in 30 fb-1:

top pair: ~2,500 evts [lbjjb(60%) llbb(30%) bb(10%)]

WQQ, W+jets: ~1,400 evts

Wg channel : ~1,100 evts

Statistical sensitivity :

Statistical precision : from 12% to 7%

Separating tb from tb final state helps:

against top pair events (charge symmetric production)

S/B ~ 0.14

√(S+B)/S ~ 12%

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W* single top selection : statistical precision

Performance Performance Efficiency and rejection :

– Efficiency ε ≈ 1.7% and N(30fb-1) ~ 600 events

– Main backgrounds in 30 fb-1:

top pair: ~2,500 evts [lbjjb(60%) llbb(30%) bb(10%)]

WQQ, W+jets: ~1,400 evts

Wg channel : ~1,100 evts

Statistical sensitivity :

Statistical precision : from 12% to 7%

Separating tb from tb final state helps:

against top pair events (charge symmetric production)

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S/B ~ 0.14

√(S+B)/S ~ 12%


W* single top selection : main systematics

Systematic uncertaintiesSystematic uncertaintiesMain experimental biases :

– Knowledge of b-tagging efficiency (&rejection rate)

important since double-tag evts

consider 3% uncertainty in b

– Gluon Radiation Modeling :

ISR’s affect the jet multiplicity

FSR’s affect the jet energy & multiplicity

– Determination of Jet Energy Scale

affects the reconstructed masses, HT…

Theoretical biases:

– Use of MC to estimate backgrounds …

we will have to use the data !

σσ//σσ = 5% = 5%

σσ//σσ =6.6% =6.6%

σσ//σσ =8.7% =8.7%

σσ//σσ = 8% = 8%


W* single top with 30 fb-1 : where we talk about H± …

Charged Higgs & single-topCharged Higgs & single-top

Production mode in 2 HDM :

5 higgs: 3 neutral (A,h,H) + 2 charged (H±)

Mass spectrum predicted in MSSM

(H+tb) couplings depends on mH± and tanβ

Cross-section x BR(H±tb):

May be as large as 1/3 of W* in (mH± ,tan) plane

– Increase with tan

– Decrease with mH±

mmH+H+ (GeV/c (GeV/c22))

Cross-section in pbCross-section in pb


Charged Higgs sensitivity : cross-section and efficiency

Performance in (mPerformance in (mH±H±,tan,tan) plane ) plane

Strategy :

Use the very same analysis developed for the s-channel

**No specific analysis performed for a H± search**

– no use of spin-0 property of H±

– no use of other H± decays (tau..)

Limited by the precision on σ(single-top), σ(top pair)..

Selection efficiency:

Increases with mH± up to 250 GeV/c2

higher pT of H±decay products with mH±

Then, decrease with higher HT values (out of window)

Cross-section in pbCross-section in pbEfficiency in %Efficiency in %


Charged Higgs sensitivity: event yields…(1)



– Cross-section increases with tan

– Cross-section decreases with mH±

– Efficiency increases with mH±

Event yields : mH±=220 GeV/c2 ,tan=50

– efficiency ~ 0.3%

– Event yield : ~140±10 evts (after Mlvb and HT window cuts)

mH±= 220, tanβ=50S/B ~ 1.3










– Event yield : ~285±15 evts (after Mlvb and HT window cuts)

mH±= 250, tanβ=50S/B ~ 5.7










– Event yield : 190±10 evts (after Mlvb and HT window cuts)

mH±= 300, tanβ=50S/B ~ 3.9


Charged Higgs sensitivity : significance

Performance in (mPerformance in (mH±H±,tan,tan) plane) plane

Significance :

A 5σ-discovery can be reached in a (mH±,tan) region

– provided σ(single-top), σ(top pair) are well determined

– provided we can control experimental systematics

Perspectives :

Many improvements expected:

– specific analysis for H± still to be done

– more sophisticated tools to be used

– extension to other H± decays

– complementarity with W+t measts &direct searches

Significance in NSignificance in Nσσ


Charged Higgs sensitivity : significance

Performance in (mPerformance in (mH±H±,tan,tan) plane) plane

Significance :

A 5σ-discovery can be reached in a (mH±,tan) region

– provided σ(single-top), σ(top pair) are well determined

– provided we can control experimental systematics

Perspectives :

Many improvements expected:

– specific analysis for H± still to be done

– more sophisticated tools to be used

– extension to other H± decays

– complementarity with W+t measts & direct searches

Significance in NSignificance in Nσσ

tantanββ=30, m=30, mH+H+=300 GeV=300 GeV

ATL-PHYS-2001-017ATL-PHYS-2001-017

direct gb H± tb search


Conclusion & perspectives

Single-top cross-section measurementsSingle-top cross-section measurementsEvent Selection

S/B ~ 12-15% and about 1,000 evts / 10 fb-1

Significant level of background contamination:

top pair, W+jets events, QCD…other single-top

Performance

Statistical precision of 7-12% for W* channel

Measurements dominated by systematics : ~10%

experimental : b-tag, JES, ISR/FSR

theoretical : use of MC for backgrounds

Charged Higgs with top eventsCharged Higgs with top eventsIndirect/direct measurements:

Low Higgs mass : top-pair production

– disappearance of semi-leptonic e/ events

– appearance of extra tau contributions

Low and High Higgs mass : single top production

– deviation in σ(Wt) measurement

– deviation from σ(W*) in (e,) final state

5σ discovery possible with 30 fb-1

All these measuremetns are All these measuremetns are Complementary to other direct searchesComplementary to other direct searches

Documents

Charged Higgs search with top : example of single-top events in ATLAS