Physics at the LHC

Part 2

Standard Model Physics

Test of Quantum Chromodynamics - Jet production

- W/Z production

- Production of Top quarks

Brief comments on electroweak measurements

QCD processes at hadron colliders

• Hard scattering processes are dominated

by QCD jet production

• Originating from qq, qg and gg scattering

• Cross sections can be calculated in

QCD (perturbation theory)

Comparison between experimental data and

theoretical predictions constitutes an important test of the theory.

Deviations? Problem in the experiment ?

Problem in the theory (QCD) ? New Physics, e.g. quark substructure ?

Leading order

some NLO contributions

LHC

Tevatron

QCD Jet cross-sections

~10 events with 100 pb-1

Jets from QCD production: Tevatron vs LHC

• Rapidly probe perturbative QCD in a new energy regime

(at a scale above the Tevatron, large cross sections)

• Experimental challenge:

understanding of the detector

- main focus on jet energy scale - resolution

• Theory challenge: - improved calculations

(renormalization and factorization

scale uncertainties)

- pdf uncertainties

High pT jet events at the LHC

Event display that shows the highest-mass central dijet event collected during 2010, where the two leading jets have an invariant mass of 3.1 TeV. The two leading jets have (pT, y) of (1.3 TeV, -0.68) and (1.2 TeV, 0.64),

respectively. The missing ET in the event is 46 GeV. From ATLAS-CONF-2011-047.

An event with a high jet multiplicity at the LHC

The highest jet multiplicity event collected, counting jets with pT greater than 60 GeV: this event has eight. 1st jet (ordered by pT): pT = 290 GeV, = -0.9, = 2.7; 2nd jet: pT = 220 GeV, = 0.3, = -0.7 Missing ET = 21 GeV,

= -1.9, Sum ET = 890 GeV.

Jet reconstruction and energy measurement

• A jet is NOT a well defined object (fragmentation, gluon radiation, detector response)

• The detector response is different for particles

interacting electromagnetically (e, ) and for hadrons

for comparisons with theory, one needs to

correct back the calorimeter energies to the „particle level“ (particle jet)

Common ground between theory and experiment

• One needs an algorithm to define a jet and to

measure its energy conflicting requirements between experiment and

theory (exp. simple, e.g. cone algorithm, vs. theoretically sound (no infrared divergencies))

• Energy corrections for losses of fragmentation products outside jet definition and underlying event or pileup energy inside

Jet measurements

Nevt

d2 / dpT d = N / ( L pT )

• In principle a simple counting experiment

• However, steeply falling pT spectra are

sensitive to jet energy scale uncertainties and resolution effects (migration between bins) corrections (unfolding) to be applied • Jet energy scale uncertainty: ATLAS: ~6% (after one year)

(similar for CMS, impressive achievements)

p

ATLAS

Test of QCD Jet production

An “early” result from the ATLAS experiment (17 nb-1, June 2010)

Inclusive Jet spectrum as a function

of Jet-PT

Very good agreement with NLO

pQCD calculations over many

orders of magnitude !

Within the large theoretical and

experimental uncertainties

[GeV]Tp

100 200 300 400 500 600

[pb/

GeV

]Tp

/dd

1

10

210

310

410

510

610

| < 2.8y|

Systematic Uncertainties

Non-pert. corr.NLO pQCD (CTEQ 6.6)

ATLAS

=0.4R jets, tanti-k

=7 TeV)s (-1 dt=17 nbL

[GeV]Tp

100 200 300 400 500 600

Dat

a/T

heor

y

0

0.5

1

1.5

2

Double differential cross sections, as function of pT and rapidity y:

[GeV]Tp

100 200 300 400 500 600

[pb/

GeV

]y

dTp

/d2 d

-310

-11010

310

510

710

910

1110

1310

1510

1710

1910

2110

2310

Systematic Uncertainties

Non-pert. corr.NLO pQCD (CTEQ 6.6)

)12 10| < 0.3 (y|

)9 10| < 0.8 (y0.3 < |

)6 10| < 1.2 (y0.8 < |

)3 10| < 2.1 (y1.2 < |

)0 10| < 2.8 (y2.1 < |

ATLAS

=7 TeVs, -1 dt=17 nbL

=0.4R jets, tanti-k

| < 0.3y|

210

=7 TeV)s (-1 dt=17 nbL =0.4, R jets, tanti-k

| < 0.8y0.3 < |210

Statistical error

| < 1.2y0.8 < |210

Systematic Uncertainties

| < 2.1y1.2 < |210

Non-pert. corr.NLO pQCD (CTEQ 6.6)

[GeV]Tp

100 200 300 400 500 600

| < 2.8y2.1 < |210

Dat

a / T

heor

y

ATLAS

Similar results from CMS, full 2010 dataset:

Invariant di-jet mass spectra:

Dijet double-differential cross section as a function of dijet mass, binned in the maximum

rapidity of the two leading jets, |y|max. The data

are compared to NLO pQCD calculations to

which soft QCD corrections have been applied.

Important for: - Test of QCD - Search for new resonances decaying into two jets (see later)

In addition to QCD test: Sensitivity to New Physics

• Di-jet mass spectrum provides large

sensitivity to new physics

e.g. Resonances decaying into qq,

excited quarks q*, .

• Search for resonant structures in the

di-jet invariant mass spectrum 1000 2000 3000

Eve

nts

-110

1

10

210

310

410 DataFit

(1000)q*(1700)q*(2500)q*

[GeV]jjReconstructed m1000 2000 3000

B(D

- B

) /

-2

0

2ATLAS

-1 = 36 pbdt L

= 7 TeVs

CDF (Tevatron), L =1.13 fb-1: 0.26 < mq* < 0.87 TeV

ATLAS (LHC), L = 0.000315 fb-1 exclude (95% C.L) q* mass interval 0.30 < mq* < 1.26 TeV

L = 0.036 fb-1: 0.60 < mq* < 2.64 TeV

QCD aspects in W/Z (+ jet) production

QCD at work

• Important test of NNLO Drell-Yan QCD prediction for the total cross section

• Test of perturbative QCD in high pT region (jet multiplicities, pT spectra, .)

• Tuning and „calibration“ of Monte Carlos for background predictions in searches at the LHC

How do W and Z events look like ?

As explained, leptons, photons and missing transverse energy are key signatures at hadron colliders

Search for leptonic decays: W (large PT ( ), large PT

miss)

Z

A bit of history: one of the first W events seen; UA2 experiment

W/Z discovery by the UA1 and UA2 experiments at CERN

(1983/84)

Transverse momentum of the electrons

Electrons:

• Isolated el.magn. cluster in the calorimeter

• PT> 25 GeV/c

• Shower shape consistent with expectation for electrons

• Matched with tracks

Z ee

• 70 GeV/c2 < mee < 110 GeV/c2

W e

• Missing transverse momentum > 25 GeV/c

Trigger:

• Electron candidate > 20 GeV/c

Today’s W / Z e / ee signals

CDF Experiment, Fermilab CDF W e

missing transverse momentum PTmiss (GeV/c)

First measurements of W/Z production at the LHC -CMS data from 2010: 36 pb-1 -

Distributions of the missing transverse energy, ETmiss,

of electron candidates for data and Monte-Carlo simulation,

broken down into the signal and various background components.

Distributions of the invariant di-electron mass, mee, for events passing the Z selection. The data are compared to

Monte-Carlo simulation, the background is very small.

num

ber

of e

vent

s / 1

GeV

0

0.2

0.4

0.6

0.8

1

1.2

310

data-e+ e Z

CMS preliminary

= 7 TeVs at -136 pb

) [GeV]-e+M(e60 80 100 120

-505

W and Z production cross sections at LHC

) [nb] l B( W WX ) ( pp 0 2 4 6 8 10 12

= 7 TeVs at -136 pbCMS

[with PDF4LHC 68% CL uncertainty]NNLO, FEWZ+MSTW08 prediction

0.52 nb± 10.44

eW nblumi 0.42± syst 0.17± stat 0.03±10.48

μ W nblumi 0.41± syst 0.16± stat 0.03±10.18

(combined) lW nblumi 0.41± syst 0.13± stat 0.02±10.31

) [nb] l B( W WX ) ( pp 0 2 4 6 8 10 12

ll ) [nb] B( Z ZX ) ( pp 0 0.2 0.4 0.6 0.8 1 1.2

= 7 TeVs at -136 pbCMS

[with PDF4LHC 68% CL uncertainty]NNLO, FEWZ+MSTW08 prediction, 60-120 GeV

0.04 nb± 0.97

eeZ nblumi 0.040± syst 0.024± stat 0.011±0.992

μμ Z nblumi 0.039± syst 0.020± stat 0.008±0.968

(combined) ll Z nblumi 0.039± syst 0.019± stat 0.007±0.975

ll ) [nb] B( Z ZX ) ( pp 0 0.2 0.4 0.6 0.8 1 1.2

Good agreement with NNLO QCD calculations C.R.Hamberg et al, Nucl. Phys. B359 (1991) 343.

Precision is already dominated by systematic uncertainties [The error bars represent successively the statistical, the statistical plus systematic and the total

uncertainties (statistical, systematic and luminosity). All uncertainties are added in quadrature.]

Measured cross section values in comparison to NNLO QCD predictions:

W cross sections at the LHC, charge separated

[nb]+W

5.5 6 6.5 7

[nb]

-W

3.5

4

4.5

= 7 TeV)sData 2010 (MSTW08HERAABKM09JR09

total uncertaintyuncorr. exp. + stat.uncertainty

-1 L dt = 33-36 pb

ATLAS Preliminary

[nb]+W

5.5 6 6.5 7

[nb]

-W

3.5

4

4.5

||0 0.5 1 1.5 2

A

0.15

0.2

0.25

0.3

0.35

||0 0.5 1 1.5 2

A

0.15

0.2

0.25

0.3

0.35=7 TeV)sData 2010 (

MC@NLO, CTEQ 6.6MC@NLO, HERA 1.0MC@NLO, MSTW 2008

-1 L dt = 31 pb

W

ATLAS

Provides important constraints on parton distributions (u, d-quark)

Summary of W/Z cross section results -comparison between theory and CMS measurements-

Ratio (CMS/Theory)0.6 0.8 1 1.2 1.4

= 7 TeVs at -136 pbCMS

B ( W ) theo. 0.051± exp. 0.009±0.987

)+ B ( W theo. 0.049± exp. 0.009±0.982

)- B ( W theo. 0.056± exp. 0.010±0.993

B ( Z ) theo. 0.047± exp. 0.010±1.003

W/ZR theo. 0.016± exp. 0.010±0.981

+/-R theo. 0.037± exp. 0.011±0.990

Ratio (CMS/Theory)0.6 0.8 1 1.2 1.4

lumi. uncertainty: 4%±

Good agreement between data and NNLO QCD predictions for all measurements

W and Z production cross sections at hadron colliders

[TeV] s1 10

) [n

b]

l B

r(W

W

-110

1

10

)pW (p

W (pp)

(pp)+W

(pp)-W

= 7 TeV)sData 2010 (

-1 L dt = 310-315 nb

lW+ l+W- l-W

(l/e) CDF W/

) (e/D0 W/

l UA1 W

e UA2 W

)-/e+ (ePhenix W/

NNLO QCD

ATLAS

[TeV] s1 10

ll)

[nb

] *

Br(

Z/

*Z

/

-210

-110

1

)p* (pZ/

* (pp)Z/

= 7 TeV)sData 2010 (

-1 L dt = 316-331 nb

< 116 GeV)ll

ll (66 < m*Z/

< 116 GeV)ee

ee (66 < m*CDF Z/

< 110 GeV)ee

ee (70 < m*D0 Z/

< 116 GeV)ee

(66 < m ee/*CDF Z//

< 105 GeV)ee

ee (75 < m*D0 Z/

> 70 GeV)ee

ee (m*UA1 Z/

> 50 GeV) (m *UA1 Z/

> 76 GeV)ee

ee (m*UA2 Z/

NNLO QCD

ATLAS

Theoretical NNLO predictions in very good agreement with the experimental measurements (for pp, ppbar and as a function of energy)

QCD Test in W/Z + jet production

Jet multiplicities in W+jet production

- LO predictions fail to describe the data; - Jet multiplicities and pT spectra in agreement with NLO predictions within errors; NLO central value ~10% low

pT spectrum of leading jet

jets

) [p

b]je

tN

(W +

1

10

210

310

410 + jetseW

=7 TeVsData 2010, ALPGENSHERPAPYTHIABLACKHAT-SHERPAMCFM

-1Ldt=33 pb

ATLAS Preliminary

jetInclusive Jet Multiplicity, N

0 1 2 3 4 5

The

ory/

Dat

a

0

1

jetInclusive Jet Multiplicity, N

0 1 2 3 4 5

The

ory/

Dat

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0

1

[pb/

GeV

]T

/dp

d

-610

-510

-410

-310

-210

-110

1

10

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310 + jetseW

=7 TeVsData 2010, ALPGENSHERPABLACKHAT-SHERPAMCFM

-1Ldt=33 pb

ATLAS Preliminary

1 jetsW +

-1

2 jets, x10

W +

-2

3 jets, x10

W +

-34 jets, x10

W +

The

ory/

Dat

a

0.5

1

1.5

2 1 jetW +

[GeV]T

First Jet p100 200 300

The

ory/

Dat

a0.5

1

1.5

2 2 jetsW +

Top Quark Physics

• Discovered by the CDF and DØ collaborations at the Tevatron in 1995

• Tevatron top physics results are consistent

with expectations from the Standard Model, however, often limited by statistics

• Tevatron achieved an impressive precision on

the measurement of the top quark mass

• LHC: huge production rates (for s = 7 TeV: about a factor 25 larger

cross sections than at the Tevatron)

- Better precision

- Search for deviations from Standard Model expectations

Why is Top-Quark so important ?

• We still know little about the properties of the top quark:

mass, spin, charge, lifetime, decay properties (rare decays), gauge couplings, Yukawa coupling,

• A unique quark: decays before it hadronizes, lifetime ~10-25 s no “toponium states”

remember: bb, bd, bs .. cc, cs .. bound states (mesons)

The top quark may serve as a window to New Physics related to the

electroweak symmetry breaking;

Why is its Yukawa coupling ~ 1 ??

Top Quark Production

Pair production: qq and gg-fusion

Tevatron

1.96 TeV

LHC

14 TeV

qq

gg

85%

15%

5%

95%

(pb) 7.0 pb 887 pb

• NLO corrections completely known • NNLO partly known

approximate NNLO results:

For LHC running at s = 7 TeV, the cross section is reduced by a factor of ~5, but it is still a factor 25 larger than the cross section at the Tevatron

BR (t Wb) ~ 100%

Both W’s decay via W ( =e or μ; 4%)

One W decays via W ( =e or μ; 30%)

Both W’s decay via W qq (46%)

Top Quark Decays

Important experimental signatures: : - Lepton(s)

- Missing transverse momentum

- b-jet(s)

Dilepton channel:

Lepton + jet channel:

Full hadronic channel:

First measurements of Top Quark production at the LHC

Event display of a top pair

e-μ dilepton candidate with

two b-tagged jets. The

electron is shown by the

green track pointing to a

calorimeter cluster, the muon

by the long red track

intersecting the muon

chambers, and the missing

ET direction by the dotted

line on the xy-view.

The secondary vertices of

the two b-tagged jets are

indicated by the orange

ellipses on the zoomed

vertex region view.

First results on top production from the LHC

Number of jets

1 2 3 4

Eve

nts

0

20

40

60

80

100

120

Number of jets

1 2 3 4

Eve

nts

0

20

40

60

80

100

120 data

ttsingle topZ + jetsW + jetsQCDuncertainty

ATLAS

-1 L = 2.9 pb

+jetstagged e/ Event Selection:

• Lepton trigger

• One identified lepton (e,μ) with pT > 20 GeV

• Missing transverse energy: ETmiss > 35 GeV

(significant rejection against QCD events)

• Transverse mass: MT (l, ) > 25 GeV (lepton from W decay in event)

• One or more jets with pT > 25 GeV and < 2.5

Tagging of b-quarks

B mesons travel ~ 3 mm before decaying:

– Search for secondary vertex

Silicon Vertex tag

Description of the invariant mass distributions in the l-had channel

[GeV]jjjm0 200 400 600 800 1000

Eve

nts

/ (25

GeV

)

0

20

40

60

80

100

120

140+jets][

Data

Model

Background

3 jets / 0 b-tag

-1 L dt = 35 pb

ATLAS Preliminary

[GeV]jjjm0 200 400 600 800 1000

Eve

nts

/ (25

GeV

)

0

5

10

15

20

25

30 +jets][

Data

Model

Background

3 jets / 1 b-tag

-1 L dt = 35 pb

ATLAS Preliminary

[GeV]jjjm0 200 400 600 800 1000

Eve

nts

/ (25

GeV

)

0

2

4

6

8

10+jets][

Data

Model

Background

2 b-tag3 jets /

-1 L dt = 35 pb

ATLAS Preliminary

[GeV]jjjm0 200 400 600 800 1000

Eve

nts

/ (25

GeV

)

0102030405060708090

+jets][

Data

Model

Background

4 jets / 0 b-tag

-1 L dt = 35 pb

ATLAS Preliminary

[GeV]jjjm0 200 400 600 800 1000

Eve

nts

/ (25

GeV

)

0

10

20

30

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50 +jets][

Data

Model

Background

4 jets / 1 b-tag

-1 L dt = 35 pb

ATLAS Preliminary

[GeV]jjjm0 200 400 600 800 1000

Eve

nts

/ (25

GeV

)

02468

10121416182022 +jets][

Data

Model

Background

2 b-tag4 jets /

-1 L dt = 35 pb

ATLAS Preliminary

• Top fractions increase with number of b-tags • Good description for all jet-multiplicity and b-tag combinations

• Data are consistent with top quark production with mass of 173 GeV

CMS tt signals in the di-lepton channel

Top cross section measurements based on 2010 data from ATLAS and CMS

• Results between the two experiments are consistent • Perturbative QCD calculations are in agreement with the obtained results

[TeV]s

1 2 3 4 5 6 7 8

[pb]

tt

1

10

210

18 pbATLAS 180 , Prelim.)-1(35 pb

19 pbCMS 158 , Prelim.)-1(36 pb

NLO QCD (pp)

Approx. NNLO (pp)

)pNLO QCD (p

) pApprox. NNLO (p

CDF

D0

6.5 7 7.5

100

150

200

250

300

SM/tt

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

) tt(

-log

0

0.5

1

1.5

2

2.5

3

3.5

4

ATLAS Preliminary

-1L dt = 35 pb

= 7 TeVs

Best fit (ATLAS) gives a slightly higher cross-section

than the expected approx.

NNLO QCD value,

but consistent within 1

(red: likelihood, stat errors only; blue: stat + syst. uncertainties)

Top-quark mass measurement Top-Quark Mass [GeV]

mt [GeV]160 170 180 190

2/DoF: 6.1 / 10

CDF 173.0 ± 1.2

D 174.2 ± 1.7

Average 173.3 ± 1.10

LEP1/SLD 172.6 + 13.3172.6 10.2

LEP1/SLD/mW/ W 179.2 + 11.5179.2 8.5

July 2010

First top quark mass measurements from CMS

• Use lepton + jet channel

• Full 2010 data set

• 637 candidate events selected

Top quark mass after the fit of the e+jets selected sample for an integrated luminosity of 36/pb

after applying the event selection and requesting

a good fit quality.

Fitted Top Mass [GeV]0 100 200 300 400 500 600

Nu

mb

er o

f E

ven

ts /

20 G

eV

0

20

40

60

Data

t tl W

Single-Top-l+l* Z/

QCD

-1CMS Preliminary, L = 36 pb

e + jets channel

0 100 200 300 400 500 6000

20

40

60

0 100 200 300 400 500 6000

20

40

60

Already impressive precision reached at that early stage of the experiment !

CMS, 06.06.2011

Relation between mW, mt, and mH

The W-mass measurement

Ultimate test of the Standard Model: comparison between the direct Higgs boson mass and predictions from radiative corrections .

rW

EM=1 sin

1

G 2

m

1/2

F

W

mW (from LEP2 + Tevatron) = 80.399 ± 0.023 GeV

mtop (from Tevatron) = 173.3 ± 1.1 GeV

0.8%

3•10-4

A light Higgs boson is favoured by present

measurements

80.3

80.4

80.5

150 175 200

mH [GeV]114 300 1000

mt [GeV]

mW

[G

eV]

68% CL

Δα

LEP1 and SLD

LEP2 and Tevatron (prel.)

March 2009

160 165 170 175 180 185mt [GeV]

80.20

80.30

80.40

80.50

80.60

80.70

MW

[GeV

]

SM

MSSM

MH = 114 GeV

MH = 400 GeV

light SUSY

heavy SUSY

SMMSSM

both models

Heinemeyer, Hollik, Stockinger, Weber, Weiglein ’10

experimental errors 68% CL:

LEP2/Tevatron (today)

Tevatron/LHC

ILC/GigaZ

Predictions for future precision (including LHC), compared to the Standard Model and its Minimal Supersymmetric Extension (MSSM)

Ultimate test of the Standard Model: compare direct prediction of Higgs mass

with direct observation

Final cross section summary