Top physics at the LHC

Paul de Jong Nikhef, Amsterdam on behalf of CMS and ATLAS

Top physics at the top factory

Top physics at the LHC

=

Outline:

The LHC, CMS and ATLAS, first data and beyond

The many hats of the top quark

Early observation: rediscovering the top (again)

Cross section measurements

Calibration of CMS and ATLAS with top quarks

Single top quark production

Top properties: mass decay spin-spin correlations

Anomalous top quark production, massive resonances

All “results” presented are of course just simulations, based on CMSPhysics TDR, and ATLAS CSC Note (under final review) 2

Last two octants started cool-downLHC cold by mid-JulyExperiments close caverns mid-JulyFirst injection late July – early AugustFirst collisions two months later?

CMS: all heavy parts installedSilicon strip tracker installedBeam pipe bake-out end of JuneThen: pixels, at least one ECAL endcapATLAS: installation complete

Combined magnet test in progressInner detector complete, commissioning coolingBeam pipe bake-out mid-July

Both experiments: adding final PS/electronicsInterfacing to central DAQ and triggerTest, test, testGet initial “day-0” calibration

3

In 2008, beam intensity and will be limited. Also, some magnets quench earlier than expected: limit beam energy to 5 TeV. Pilot physics run October and November. 143156 bunches, luminosity few times 1031

cm-2 s-1 , 40 days, 10-50 pb-1 ?

During winter shutdown commissioning to 7 TeV. Physics run May-November 2009 , 150 days, establish 75 ns operation (936 bunches), then 25 ns (2808 bunches), eventually to nominal value, lumi 1032 1-2 x 1033 cm-2 s-1 , and 2-3 fb-1 ?

2010 and after: lumi expected to keep increasing steadily to ~2 x 1034 cm-2 s-1 and eventually ~80 fb-1 per year. After 2016 a major upgrade to LHC and experiments for a super-LHC.

4

Top cross section: gluon-fusion-dominated (in contrast to Tevatron) Probes gluon pdf at x where it is relatively well known Test for advanced QCD methods beyond NLO.

~15%

~85%

At L = 1033 cm-2 s-1 produce 1 tt per second, at L = 1034 we produce 108 tt per year!Better tt/W ratio than at TevatronMore phase space for radiation of extra jets

tt bbjjjj: 45.5% bbljj: 43.5% (l = e,µ: 29%) bbll: 10.5% (l = e,µ: 4.7%)

Top at LHC

qqtt

ggtt

5

Three recent calculations of cross section: Moch and Uwer, arXiv:0804.1476: approx. NNLO Cacciari et al., arXiv:0804.2800: NLO + NLL resum, updated Kidonakis and Vogt, arXiv:0805.3844: NNLO-NNNLL+Central values agree well, error estimates differ (different methodology)

Using CTEQ6.5: = 908 ± 83 (scales) ± 30 (pdf) pb (mt = 171 GeV, Cacciari et al.) MRSTW-06: = 961 ± 90 (scales) ± 12 (pdf) pb Difference at LHC > estimated pdf error!

At 10 TeV: = 400 pbWill take longer to establish signal

More progress: spin correlations at NLO NLO corrections to tt+jet good advance towards NNLO

6

The top quark has many hats

Production: a test of QCD, resummation techniques

Tool: Calibration of detector

Background to physics beyond SM

Properties (mass, couplings), an indirect probe of new physics

Anomalous production and decay as direct probe of new physics

7

Early observation: tt bbljj with l = e or µTry to see an early signal, with no or limited b-tag

ATLAS: / = 7% (stat) ± 15% (syst) ± 3% (pdf) ± 5% (lumi) likelihood method, 100 pb-1 3% (stat) ± 16% (syst) ± 3% (pdf) ± 5% (lumi) counting method

CMS, early data, 10 pb-1 1 isolated µ, pT > 30 GeV at least 1 jet pT > 65 GeVEnough to rediscover top! (assuming QCD background under control)

ETmiss > 20 GeV

3 jets with max sum pT are hadronic topATLAS, muons onlyATLAS

CMS

ATLAS: e or µ, pT > 20 GeV, ≥ 4 jets (40,40,40,20 GeV)

8

With working b-tagging, selection much cleaner, at the cost of some efficiency

ATLAS, 1 or 2 b-tags

CMS: uncertainty on tt for 1, 5 and 10 fb-1 in the muon channel

ATLAS ATLAS

CMS

Issues: luminosity, b-tagging efficiency, pile-up, ISR/FSR 9

Dilepton channel: tt bbllSmall branching ratio, but clean

CMS: / = 0.9% (stat) ± 11% (syst) ± 3% (lumi) in 10 fb-1 Systematics: pdf’s, b fragm., underlying events, b-tag, JES

ATLAS: / = 4% (stat) +5-2 % (syst) ±2% (pdf) ± 5% (lumi) cut and count, 100 pb-1

4% (stat) ± 4% (syst) ± 2% (pdf) ± 5% (lumi) template method 5% (stat) +8

-5 % (syst) ± 0.2% (pdf) ± 5% (lumi) likelihood method

CMS, 10 pb-1 isolated e or µ, pT > 30 GeV ET

miss > 20 or 30 GeV incompatible with Z mass stat error 9-13%

ATLAS

ATLAS

CMS

Also looking at e and µ

10

Top as a Tool: Calibration with top events

b-quark-tagging:

Jet energy scale

Count nr of events with n-tagged jets, 100 pb-1, ATLAS uncertainty 5% on b, 8-14% on tt

Selection of b-enriched jet sample, 200 pb-1 l+jets ATLAS 40-120 GeV jets, 6% (stat) ± 3% (syst) on b

Selection of b-enriched jet sample, 1 fb-1, CMS 30-200 GeV jets, 6 (barrel)-10% (endcaps) on b

Use MW to measure light JES, 50 pb-1, ATLAS 2% stat, 1% syst, jets > 40 GeV

Use kinematic fit to measure light and b JES, 100 pb-1, CMS 0.9% stat, small syst, jets > 40 GeV

A clean sample of top events is a great tool to understand the detectors:

Take care not to measure Vtb with a b-tag whose efficiency has been determined assuming Vtb ≈ 1…

ATLAS

Also: check lepton triggers, ETmiss reconstruction

CMS

11

Single top

t-channel:

1 fb-1 : / = 5.7% (stat) ± 22% (syst) 10 fb-1 : 2% (stat), 10% (syst)

ATLAS: cut-based analysis (basis), and multivariate (BDT)

1 fb-1 : Vtb / Vtb = ± 11% (stat+syst) ± 4% (theory)

CMS: cut-based analysis 10 fb-1 : / = 2.7% (stat) ± 8% (syst) ± 8.7% (lumi)

= electroweak top quark production

ATLASATLAS

CMS

BDT > 0.6

12

ATLAS results summary:

tW channel (associated W production) Looks just like tt, but with one less b-jet ATLAS: cut-based analysis, and boosted decision trees CMS: cut-based analysis, with smart b-W pairing / = 8-9% (stat) ± 16-24% (syst) in 10 fb-1

s-channel: very difficult… very low cross section Could be mediated by new physics (H± ) ATLAS: Likelihood analysis CMS: cuts, with genetic algorithm In 10 fb-1 : / ≈ 35% (CMS) – 50% (ATLAS)

13

Top properties: if the top is indeed the Q=2/3, spin=1/2, heavy partner of the b, then:

Production: top essentially unpolarised, but spin-spin correlations Decay should be V-A, dominated by tbW

Top charge: Q=2/3 vs Q=4/3

Top spin: top must be fermion cross section measurement will rule out s = 3/2 more info from spin-spin correlations

Decays into a b or into an anti-b?Measure with b-jet charge, or semileptonic tag5 separation with 1 fb-1 (ATLAS)Systematics: ISR/FSR, MC model, pile-upOther suggestions: look at extra ’s in ttbar

14

Top massOne of the most crucial parameters of the Standard ModelStatistical error will soon be negligibleVery careful treating of systematic errors neededAlso theoretical interpretation not unambiguous: pole-mass?

Golden channel: tt bbljj good branching ratio W mass constraint from jj can be reconstructed

ATLAS: with b-tag, either geometric association of jets to W and top/anti-top, or association by minimizing 2 ; mass from peak fit, or full kinematic fit.

To battle: (b) jet energy scale influence of extra jets

15

CMS: Semileptonic tt decay, via event-by-event likelihood (ideogram) in 10 fb-1 : mt = 0.2 GeV (stat) ± 1.1 GeV (syst) if b-JES known to 1.5%

Also fully leptonic and fully hadronic decay channels

Alternative method: look for J/ from b-decay, plot mass with lepton from W

Dilepton: mt = 4.5 GeV (<1 fb-1 ) 1.2 GeV in 10 fb-1

All hadronic: mt = 4.2 GeV in 1 fb-1

Error dominated by theory, not jet energy scales

mt = 0.5 ± 1.5 GeV (20 fb-1 )

CMS

CMS

CMS

CMS

16

Top decay

Top decay to b quarks: R = Br(tWb)/Br(tWq) = |Vtb |2 / (|Vtb |2 + |Vtd |2 + |Vts |2 )

Measure by counting b-tags in tt events, dominated by b-tag efficiency systematicsAlternative non-SM decays may be possible: t bH+ , but also FCNC decays

Top decay analysis: polarization of W’s is a good analyzer of V-A structure

In 730 pb-1 : F0 = 0.70 ± 0.04 ± 0.02 FL = 0.29 ± 0.02 ± 0.03 FR = 0.01 ± 0.02 ± 0.02

: angle between lepton in W frame, and W in top frame

Systematics: b-jet energy scale top mass hadronization scheme

ATLAS:

17

anomalous couplings

Generally, the tbW vertex can be written as:

(F0 , FL , and FR depend on these couplings)

ATLAS: measure FL /F0 , FR /F0 and angular (forward/backward) asymmetriesUse TopFit to extract couplings:

Dominating systematics: ISR/FSR, difference between MC generators, pile-up

(PR/L = (1 ± 5 )/2)

18

Spin correlations:

Top is produced unpolarized, but two spins are correlated. At threshold: ggtt produces 1 S0 state, qq tt produces 3 S1 state Since t = 1.4 GeV > QCD top decays before hadronization: correlations preserved

: spin analysing power (~1 for lepton or d-type quark, ~0.5 for b or lowest energy jet)

CMS, 10 fb-1 : A = 0.38 ± 0.03 (stat) +0.06

-0.08 (syst)

ATLAS, 200 pb-1 : a = 0.17 (stat) ± 0.15 (syst)

Systematics: ISR/FSR jet energy scale b-tag efficiency

@LHC: if qq: A ~ -0.3 if gg: A ~ 0.3

CMS(before detector simulation)

Enhance Awith Mtt cut

CMS(before detector simulation)

10 fb-1 : 0.02 (stat) ± 0.03 (syst)

19

Flavour-changing neutral current (FCNC) decays are suppressed in the SM but can appear in new physics (e.g. SUSY)

Select tt blqX (X=g,,Zll), l = e or µ

CMS: X = , Z, cut-based analysisIn 10 fb-1 : detect at 5 level: Br(tq) > 8.4 x 10-4 Br(tqZ) > 15 x 10-4

ATLAS: likelihood analysis with kinematics Derive 95% CL limits for 1 fb-1 Systematics: ISR/FSR, pile-up, luminosity, backgrounds, MC generator

CMS, 5 discovery

20

Anomalous top quark production: a direct sign of new physics

Anomalous single top quark rate from anomalous Vtb , or FCNCH± tbGluino stop topResonances in tt invariant mass: general heavy Z’ bosons KK gravitons, gluons ZH in little Higgs models

In many cases: production of high pT top quarks decay products overlap: special reconstruction! generally efficiency decreases with top pT

ATLAS: limits for a general resonance and for KK gluon

21

SummaryAt a top factory, statistics are not an issue for top properties measurements… (exception: rare decays)

Systematics are an issue: Detector systematics: (b) jet energy scale b-tagging pile-up luminosity object identification and trigger Theoretical uncertainties: ISR/FSR and extra jets b-decay modeling differences between MC’s pdf’s fragmentation and hadronization

22

5. Multiple Interactions

SummaryAt a top factory, statistics are not an issue for top properties measurements… (exception: rare decays)

Systematics are an issue: Detector systematics: (b) jet energy scale b-tagging pile-up luminosity object identification and trigger Theoretical uncertainties: ISR/FSR and extra jets b-decay modeling differences between MC’s pdf’s fragmentation and hadronization

Provided these are under control, the LHC promises to test the nature of the top quark to excellent precision, and use the top quark as a tool, and as a probe of new physics.

Lots of hard, but eventually rewarding work for experimentalists!

Lots of hard, but eventually rewarding work for theorists and experimentalists !

22