Early Physics results from LHC

Early Physics results from LHC 

1 

Ludwik Dobrzynski - LLR Ecole Polytechnique - CNRS-IN2P3 

jeudi 25 novembre 2010

Early Physics results from LHC 

Physics @ LHC 

Some of the physicists’ jargon 

Evolution of the Cross-Sections 

LHC physics Objects 

W an Z study 

Top at the LHC 

Multijet event in CMS in first run at 7 TeV 

Conclusion 

jeudi 25 novembre 2010 

1 

Ludwik Dobrzynski - LLR Ecole Polytechnique - CNRS-IN2P3

Physics @ LHC 

The Standard Model has been deeply tested at HERA, LEP and Tevatron and with 

success! … but the essential physics motivations and questions remain. They 

concern : 

Electroweak Symmetry Breaking 

Hierarchy of Fundamental Interactions 

Unification and Extended Symmetries 

The absence of BSM discoveries up to now, induce that everything is possible 

and LHC must be ready for surprises 

During the last decades, the universe has become much complicated (dark Matter, 

dark energy, neutrino masses … !) 

2 


Objectifs for the LHC collaborations 

Find new particles/new symmetries/new forces? 

Origin of Mass - Higgs boson(s) 

Supersymmetric particles - a new zoology of particles, dark 

matter particle? … 

Extra space-time dimensions: gravitons, Z’ etc. ? 

The Unexpected !! 

Studies of CP Violation and Quark Gluon Plasma 

3 



3 



• Cross section (σ) 

– A measure of ‘frequency’ of the physical process 

– Units: barns (10 -28 cm 2 ) 

• Typical values: femtobarns (fb), picobarns (pb) 

3 







• Luminosity (L) 

– Or instantenous luminosity 

– A measure of collisions ‘frequency’ 

• Typical (at Tevatron/Early LHC): L = 10 32 cm -2 s -1 3 










• Typical (at Tevatron/Early LHC): L = 10 32 cm -2 s -1 

• Integrated luminosity (L = ∫Ldt) 

– A measure of number of accumulated collisions after a certain time period 

– Units: (cross section) -1 …. E.g. 1 fb -1 = 1000 pb -1 

• Tipical (Tevatron/Early LHC): few fb -1 3 










• Typical (at Tevatron/Early LHC): L = 10 32 cm -2 s -1 

• Integrated luminosity (L = ∫Ldt) 

– A measure of number of accumulated collisions after a certain time period 

– Units: (cross section) -1 …. E.g. 1 fb -1 = 1000 pb -1 

• Tipical (Tevatron/Early LHC): few fb -1 

• Number of events (N) 

– Number of (expected) events (N) after a certain time of running 

N = σ · L 

3 


Signal vs background(s) 

4 



• Signal: an event coming from the physical process under study 

– Example: H ZZe + e - e + e - 4 




– Example: H ZZe + e - e + e - 

• Background: any other event 

– ‘Dangerous’ background is any other process giving at least 4 electrons in the final state 

• But be careful: electrons seen by detector are reconstructed objects and in some cases 

when some other objects (f.g. jets) are miss-reconstructed as electrons 

– ‘Trivial’ backgrounds are all other backgrounds and are easily rejected by a simple 

requirement of having at least 4 electrons in the final state 

4 











Z 

e 

e 

q 

Z 

e 

e 

Z 

e 

e 

q 

Z 

e 

e 

4 











Z 

e 

e 

q 

Z 

e 

e 

Z 

e 

e 

q 

Z 

e 

e 

Signal: ppHZZ4e 

4 











Z 

e 

e 

q 

Z 

e 

e 

Z 

e 

e 

q 

Z 

e 

e 

Signal: ppHZZ4e 

‘Dangerous’ background: ppZZ4e 

4 


Measurements vs predictions 

5 



Predictions/Simulation 

5 




Measurements 

5 




Measurements 

Event Generation 

Tools: MC generators (PYTHIA, ...) 

Output: final state particles 

5 







Measurements 

Collisions 

Tools: Accelerator (LHC, Tevatron ...) 


5 







Measurements 

Collisions 



Detector simulation 

Tools: MC simulators (GEANT) 

Output: simulated detector response 

5 







Measurements 

Collisions 






Data acquisition 

Tools: Detectors (CMS, ATLAS,...) 

Output: detector response 

5 







Measurements 

Collisions 









Event reconstruction 

Tools: Detectors’ software packages (custom made; MC used in algorithms) 

Output: reconstructed physical objects (electrons, muons, jets ...) 

5 







Measurements 

Collisions 









Event reconstruction 

Tools: Detectors’ software packages (custom made; MC used in algorithms) 

Output: reconstructed physical objects (electrons, muons, jets ...) 

Data analysis 

Tools: Statistics (ROOT, ...; MC used in algorithms; f.g. Toy MC) 

Output: new knowledge (parameter/interval estimates, hypothesis tests, article, talks ...) 

5 


Simulation example 

6 


Transverse slice through CMS detector 

21 

I. 

P 



Detector commissioning

Detector commissioning 

Understanding the detectors is a MAJOR task. 

➡LHC was eagerly awaited by a large community, theorists… 

➡Pressure for early results exist 

➡Strong internal competition is there 

But must not compromise quality! 


Detector commissioning 

Understanding the detectors is a MAJOR task. 

➡LHC was eagerly awaited by a large community, theorists… 

➡Pressure for early results exist 

➡Strong internal competition is there 

But must not compromise quality! 

Blind analyses: desirable, practical? 

Look at 10 7 bins, see three 5σ peaks even if no new physics! 


Major Commissioning Challenges 

10 9 10 5 102 

form the base for the “commissioning of physics tools” 

like b tagging, electrons/photons, muon, jets, missing E 

18 

T … 


The ATLAS/CMS detectors at LHC 

σ/p T = 4.5-7 % (1 TeV µ), if comb. with TK 

116 

gaus 

σ/E = 2.8 %/√E ⊕ 0.3 % (barrel) 




116 

gaus 

σ/E = 5.3 %/√E +0.36 % (endcap) 



116 

gaus 

σ/E = 5.3 %/√E +0.36 % (endcap) 




116 

gaus 

σ/E = 5.3 %/√E +0.36 % (endcap) 




116 

gaus 

σ/E = = 5.3 2.8 %/√E +0.36 ⊕ % (barrel) (endcap) 



Increase in luminosity 

and consequent increase 

in LHC physics reach 

during the running in 

2010/2011 at 7 TeV 

center of mass energy 

Initial running, 

hadronic 

resonances 

W, Z 

In April 2010, first 

W,Z events 

In July 2010, 

first tt events 

tt 

Higgs 

Expected level at 

end of 2011…. 

12 

LHC running in 

2010/2011 at 7 TeV 




Increase in luminosity 

and consequent increase 

in LHC physics reach 

during the running in 

2010/2011 at 7 TeV 


The first goal of LHC 

is to “rediscover the 

Standard Model”. 

Initial running, 

hadronic 

resonances 

W, Z 

In April 2010, first 

W,Z events 

In July 2010, 

first tt events 

tt 

Higgs 

Expected level at 

end of 2011…. 

12 

LHC running in 

2010/2011 at 7 TeV 



LHC physics Objects 

13 


Physics Objects 

The CMS detectors are performing very well 

Commissioning of Physics Objects is continusely progressing 

– Charged track reconstruction, electrons, photons, muons and taus 

– Jets & MET 

• Refine noise filters, cleaning algo’s 

• Optimization of jet algorithms for resolution, scale, lepton and γ fakes, etc. 

– Commission higher level algo’s 

• B tagging 

• Particle Flow 

Also calibrate with known objects 

– Study candles for leptons and photons 

• π 0 ,η,..ϒ,ψ,… initially to understand the detector, tracking, object id’s 

• Extend to W, Z →leptons 

14 


Particle ID 

Φ Κ + Κ − using dE/dX 

K 0 s, Λ, φ … 

Validate Tracking 

Reconstruction 

Secondary Vertex Finding 

Magnetic Field 

Material 

Momentum scale 

Validation of MC 

15 

CMS DP 2010-001 


ECAL energy scale 

in the barrel the scale is now set by the π 0 calibration 

16 

EB ~ 1% ….. EE ~ 3% 


EM Shower Energy Scale … 

Results from first 1 million π 0 γγ 

E T (cluster)>0.4 GeV, 

p T (γγ)>0.9 GeV 

± 2% 

ϕ 

17 

η 

Uniformity of EM calorimeter response: 

from π 0 EM energy scale known to better than 2% 

over full η range, uniformity in ϕ

Missing transverse energy 

in the calorimeters 

Sensitive to calorimeter performance 

(noise, coherent noise, dead cells, 

mis-calibrations, cracks, etc.), and 

cosmics and beam-related backgrounds 

Calibrated E miss T from 

minimum-bias events 

Measured over ~ full calorimeter coverage 

(360 0 in φ, |η| < 4.5, ~ 200k cells) 

E T 

miss spectrum from SUSY searches: 

events with ≥ 3 high-p T jets, p T (j1) > 70 

GeV 

Calibrated 

SUSYx10 

(m~400 GeV) 


Progress in MET 

Excellent resolution and small non-gaussian tails. 

Understanding all sources of erratic noise is very important 

for cleaning the distributions. MET ready for physics. 

19 


M,'%&,H,D05.-3. 

#$%&'()%*'+,(-+).*/%).,-%,*0),12)&-.3/4)256 /7,*0),8*'%&'2&,9/&)+,'*,.,:,;,/?)27$+,*//+.,*/,3/%.*2'-%,@AB &-.*2-C$*-/%.,-%.-&),D2/*/%,E>F#G 

H, ++ -.,B/+&ID+'*)&,D2/3)..,*/,3'+-C2'*),*0),&)*)3*/2,*/,*0),$+*-('*),D2)3-.-/% 

J(/%B,,C'3KB2/$%&.,*/,.)'230).,7/2,L)?,>05.-3., 

20 

!" 


#$ %!" &%'()*%&%+,( 

-'.+$(%/%0,.1+($2$#$ % 

3 4$ 5%6$7$!8$9%:;$

W transverse mass 

• e or µ with p T >20 GeV, E T 

miss 

>25GeV 

• MC normalised to data 

• 119k electron and 135k muon candidates 

22 


#$ %%!" &%'()*%&%+,( 

-'.+$(%/%0,.1+($2$#$ %% 

!$1331(.,%4(.5+$%/%0,*1+( 

6 7$ 8$!9$:%;!?@A 

!"#$%!&%/%0,*1+$.B%+,.C.0',.1+$0*.,%*.' 

DD$>$-$E% F % 4 G$>$HHD$:%; 

I00%3,'+0%$J$%CC.0.%+0K$2$L$M9N 

6J3%0,%B$OPQ$L$H99$ 

-'.+$R'0S5*1)+B2$TUV$W%,( 

\\]^$ E_P#$ //G$L$9?"D$+R 3%*$C'&./K$ 

C1*$DD$>$-E//G$8$HHD$:%; 

-'.+$(%/%0,.1+($2$#$ 

!$1331(.,%4(.5+$#$%&' 

3 7 8$!9$:%;!?@ 

=3 7 EXV4-OG=$>$HY$:%; 

$'()*+"#Z$=# 4# [,J =>H$0& 

DD$>$-$E F 4 G$>$HHD$:%; 

I00%3,'+0%$J$%CC.0.%+0K2$L$MYN 

6J3%0,%B$OPQ$8$H99$ 

-'.+$R'0S5*1)+B($2$,,

Z mass peaks 

• Invariant mass of Z candidates with first pass processing 

• MC normalised to data 

• 9k electron and 14k muon pair events 

24 


W and Z transverse momentum 

• Distributions of p T 

W 

and p T 

Z 

25 


Missing transverse energy 

• E T 

miss 

distribution for events with e or µ with p T >20 GeV 

26 


First W → eν events in CMS, April 2010 

W + _ → e + _ + ν e 

q 

W + _ 

e + _ 

q _ 

27 

ν e 


First W → µν events in CMS, April 2010 

W + _ → µ + _ + ν µ 

28 


First Z → e + e - event in CMS, April 2010 

q 

Z 

e + _ e + _ 

q _ 

29 


Z → µ + µ − event in CMS 

q 

Z 

µ + _ µ + _ 

q _ 

30 


W and Z 

W → µν 

W → eν 

← W (35pb -1 ) 

Z→ ττ 

Z→ ττ 

↓ 

Z →µµ 

Z →ee 

← Z (35 pb -1 ) 

31 


Electroweak: Z & W cross sections 

Z and W cross sections and ratios 

32 


W + and W - and theory 

Clearly Lumi the area 

with largest potential 

of improvement 

Lumi 

error 

33 


W and Z cross section with e and µ 

W 

W + 

Z 

W - 

• Dominant lumi uncertainty (11%) should be reduced by a factor 2 soon. 

• Measurement of the W→lν and Z/γ* →ll production cross sections in p-p collisions 

• at sqrt(s) = 7 TeV with the ATLAS detector, Submitted to JHEP (11 Oct 2010) 

34 


Z→ee invariant mass 

• Calibrate EM scale with constrained fit to the Z lineshape in 28 calorimeter regions. 

Typical corrections 2%, consistent with precision of cryostat temperature 

measurement in test beam. 

• This calibration is being applied in the reprocessing. 

Fit to a Breit-Wigner 

convolved with 

crystal ball function. 

Stat errors only. 

35 


Study of the top quark: 

one of the main goals of the LHC 

time 

top 

At nominal energy and luminosity LHC will 

produce about 1 top-antitop pair second! 

36 

anti-top 


LHC is a tt factory 

Total production cross section 

(90%) 

+ 

(10%) 

tt production cross section at LHC: 

~833 pb 

tt production cross-section 

at Tevatron: 

6.7 pb 

34 


LHC is a tt factory 

Total production cross section 

(90%) 

+ 

(10%) 

tt production cross section at LHC: 

~833 pb 

tt production cross-section 

at Tevatron: 

6.7 pb 

2 tt events per second ! 

> 8 millions tt events expected per year 

34 


Lepton+Jets loose selection 

• Triggers: µ+X (p T > 9 GeV/c) or e/γ+X (E T 

> 15 GeV) 

– Ask for exactly 1 prompt, isolated electron 

(muon) of good quality 

Detected energy around the lepton 

Rel.isol. = 

€ 

track 

∑ p T 

+ 

ECAL 

p T 

R 30 GeV/c 

≥ 4 jets is typical for ttbar 

38 


Lepton+Jets loose selection 

• Triggers: µ+X (p T > 9 GeV/c) or e/γ+X (E T 

> 15 GeV) 

– Ask for exactly 1 prompt, isolated electron 

(muon) of good quality 

Rel.isol. = 

€ 

Detected energy around the lepton 

track 

∑ p T 

+ 

ECAL 

p T 

R 30 GeV/c 

≥ 4 jets is typical for ttbar 

p 

jet 

38 


Lepton + jets top selection, September 2010 

Using the full statistics currently validated (0.84pb -1 ) and requiring at least 1 jet b- 

tagged (secondary vertex tagger with ≥2 tracks; high efficiency with ~1% fake rate) 

e/µ + jets final states 

L=0.84pb -1 

tt → bWbW → blvbqq 

For N(jets) ≥3 we count 30 

signal candidates over a 

predicted background of 5.3 

39 

t-tbar events are observed in CMS at 

a rate consistent with NLO cross 

section 


e+Jets candidate event on July 18 

Event passes all cuts: 

1 high-momentum electron 

significant MET ≈ 44 GeV 

4 high-p T jets, 

two of which with good/clear b-tags 

(with reconstructed 2 nd ary vertices) 

m T (W) ≈ 77 GeV/c 2 

Mass of 2 untagged jets ≈ 102 GeV/c 2 

m(jjj) ≈ 208, 232 GeV/c 2 

(for the two 3-jet combinations) 

40 


µ+Jets candidate event on July 14 

Event passes all cuts 

of full selection 

1 high-momentum muon 

significant MET > 100 

m T (W) = 104 GeV/c 2 

4 high-p T jets, 

one of which with good b-tag 

µ 

reconst. top mass around 210 GeV/ 

c 2 

masses of 2 untagged jets (3 possible 

comb.): 104, 105, 151 GeV/c 2 

41 

µ 


eμ candidate 

DL2 

p T (tracks) > 1 GeV 


p T (μ)= 48 GeV p T (e)=23 GeV 

E T 

miss=77 GeV, H T =196 GeV 

p T (b-tagged jet) = 57 GeV 

Secondary vertex: 

-- distance from primary: 3.8 mm 

-- 3 tracks p T > 1 GeV 

43

Dileptonic channels: ee, µµ, eµ + X 

• Triggers: µ+X (p T > 9 GeV/c) or e/γ+X (E T >15 

GeV) 

• 2 isolated, prompt, oppositely charged leptons 

(l = e,µ) of good quality 

p T (l) > 20 GeV/c 

|η µ | < 2.5 , |η e | < 2.4 

Relative isolation 30 (20) GeV (in eµ+X) 

• Z-boson veto: 

ν 

76 < M ee,µµ < 106 GeV/c 2 

• Count additional jets: 

anti-k T jets, R = 0.5 

using calorimeter⊕tracking info 

|η| < 2.4, p T > 30 GeV/c 

l - 

b-jet 

€ 

b-jet 

W - 

b 

t 

t 

b 

W + 

ν 

l + 

p 

≥ 2 jets typical for ttbar 

€ 

43 


The “golden” µµ+jets candidate July 18 

44 


µµ +Jets Candidate Event 

Event passes all cuts of full selectio 

2 muons with opposite charge 

2 jets, both w/ good/clear b-tags 

(and secondary vertices!) 

significant MET (>50 GeV) 

2 nd ary- vertex 

6σ ellipse 

m(µµ) = 26 GeV/c 2 

y [cm] 

Preliminarily reconstr. mass is in the 

range 160–220 GeV/c 2 (consistent with 

m top ) 

x [cm] 

45 


µµ +Jets Candidate … cont’d 

Multiple primary vertices → multiple 

pp collisions (“pile-up”). Jets & 

muons originate from same vertex. 

y [cm] 

0.84 pb -1 of data. 

46 

z [cm] 

Very clean candidate sitting in a region where we expect very 

little background! The event appeared in the fist 0.25 pb -1 of 

data analyzed for ICHEP (together with other 8 candidates 

lepton+jets+b-tag). Here we present updated results relative to 


Di-lepton + jets top selection, ~1pb-1, Sept. 2010 

• Full selection applied: Z-bosonVeto, |M(ll)-M(Z)| >15 GeV 

• MET > 30 (20) GeV in ee,µµ, (eµ); N(jets) ≥ 2 

ee/eµ/µµ 

L=0.84pb -1 

ee/eµ/µµ 

L=0.84pb -1 

4 ttbar candidates (1eµ, 1ee, 2µµ) over a negligible background. 

Top signal at LHC established. 

47 


µµ +Jets Candidate … cont’d 

Multiple primary vertices → 

multiple pp collisions (“pile-up”) 

Jets & muons originate from 

same primary vertex 

y [cm] 

Very clean candidate sitting in a 

region where we expect very 

little background! 

z [cm] 

48 


!"#$%&'()*+,-',.'/ 

-./&0&,#12$#&03405670!$89%)3:;'000 

1,0'$#$"')1?@*+&#,0 

!$89%)3:;',0$)&0#$9&;04)320A3;#&0 

6$)=3B0A6CDEF0$;'0GEHIJD 

K&#,0$)&0!"#$%%&'0!L0#/&0MNO0 

$=%3)1#/2P 

&*+&#, 

*+&#, 

49 


Measurement of top cross section 

• Complete set of ingredients to investigate production of ttbar, which is the 

next step in verifying the SM at the LHC: 

• e, µ, E T 

miss 

, jets, b-tag 

• Assume all tops decay to Wb: event topology 

then depends on the two W decays. 

• Of interest: 

• lepton (e or µ), 

E T 

miss 

, jjbb (37.9%) 

• dilepton (ee, µµ or eµ), 

E T 

miss 

, bb (6.46%) 

• Data-driven methods to control QCD and W+jets backgrounds 

• Counting experiment, with simultaneous likelihood fit to all channels to 

derive the combined cross section. 

50 


• Combining all channels, 

Top production cross section 

• Significance of ~4.8σ w.r.t. background only hypothesis. 

51 


Top 

Full selection applied: Z-Veto, |M(ll)-M(Z)|>15 GeV 

MET >30 (20) GeV in ee,µµ, (eµ); N(jets)≥2 

σ(pp → t t) ¯ = 194 ± 72(stat.) ± 24(syst.) ± 21(lumi.) pb 

L=3.1pb -1 

ee/eµ/µµ 

52 

Accepted by PL-B arXiv:1010.5994 


Conclusions 

• The full ATLAS & CMS Detector were operational for 1 st LHC beams in 2008. Cosmic Data taking 

campaigns in 2008 and 2009 was used for commissioning. 

• Data taking with LHC Pilot runs in December 2009 was a great success Performances corresponds 

to expectations. Analyses were performed within hours, O(day)’s ! 

• The experiments have been running in a “minimum bias” mode with LHC collisions at √s = 7 TeV 

for ~ one month … First EWK Boson candidates were observed … Beam squeezing brought a L 

increase in May ! 

• Di-boson observation and first significant constraints (or hints) on the SM Higgs boson are 

expectedend of 2011 

• The experiment has a some discovery reach beyond the SM already in 2010-2011 (e.g. scalar sector 

at high tan β, sparticles, TeV Resonances ....) 

Finally… 

if you want to learn more about the possible physics with LHC 

look between others to Beate Heinemanne lectures 

53 

http://www-atlas.lbl.gov/~heinemann/homepage/publictalk.html 


• m µµ 94 GeV, E T 

miss 

= 161 GeV 

Candidate for ZZ→µµνν 

54 


First CMS ZZ4µ Event 

Only tracks with pT>1 GeV are displayed 


First CMS ZZ4µ Event 

56 


Heavy Ions: e + e - Z candidate 

M e+e- =96 GeV 

( E e not corrected for underlying event) 

From Tizianoʼs talk 

57 


Backup 

58 


Multijet event in CMS in first run at 7 TeV 

59 


Jets 

• Measurement of inclusive jet and dijet cross sections in proton-proton 

• collisions at 7 TeV centre-of-mass energy with the ATLAS detector 

• Accepted by EPJC arXiv:1009.5908 

60 


Dijet mass & angular distributions: 3pb -1 

0.50 < m(q*) < 1.53 TeV @ 95% CL 

Quark contact interactions with 

scale Λ < 3.4 TeV @ 95% CL 

• Search for New Particles in Two-Jet Final States in 7 TeV Proton-Proton Collisions with the ATLAS Detector at 

the LHC, Phys. Rev. Lett. 105, 161801 with 315 nb -1 

• Search for Quark Contact Interactions in Dijet Angular Distributions in in 7 TeV Proton-Proton 

Collisions with the ATLAS Detector at the LHC, Accepted by PLB 

61 


Inclusive jets 

• Combine a range 

• of triggers to cover 

• the full p T spectrum 

• In all cases, jets 

• corrected to 

• hadronic scale 

• (JES uncertainty 7%) 

• Jet p T >60GeV 

• Highest p T jet 1.3 TeV 

• Shape comparison with MC PYTHIA (LO + parton shower) 

62 


Highest pT jet event 

p T jet1=1.3 TeV (also 

p T jet2=1.2 TeV, m jj =2.6 TeV) 

63 


Dijets and multi jets 

• Leading jet p T >60 GeV, 

• Subleading p T >30 GeV 

• Highest dijet mass 3.7 TeV 

Count jets with p T >60 GeV 

One event with 8 jets 

64 


Highest mass di-jet 

p T jet1=670 GeV, 

p T jet2=610 GeV, m jj =3.7 TeV 

65 


8-jet event 

8 jets with p T >60 GeV 

66 


Jet measurement 

! Jets are the experimental signature of 

quarks and gluons, observed as highly 

collimated sprays of particles. 

! A jet algorithm is a set of mathematical 

rules that reconstruct unambiguously the 

properties of a jet. 

- Fixed cone algorithms. 

- Successive recombination algorithms. 

! Different inputs to the jet algorithm 

lead to different types of jets: 

- Calorimeter energy depositions. 

- Tracks. 

- Particle or energy flow objects. 

- Simulated particles. 

1 

Jet 1 Jet 2 

0 

-1 

! 

E T 

CMS 

Calorimeter 

Simulation 

" 

67 

oriond/QCD, 18 March 2009 


K. Kousouris 

4

Possible early discovery: 

anomalies in high E T QCD jets, di-jet masses 

• 1 fb -1 : jets up to 3-3.5 TeV, di-jet masses up to 6 TeV: new territory! 

• Sensitive to substructure, contact interactions, high mass resonances 

10 8 

CMS 






Jet cross section 

Tevatron 

10 8 

LHC 

CMS 

E T (TeV) 







Tevatron 

10 8 


CMS 

E T (TeV) 







Tevatron 

10 8 


CMS 

E T (TeV) 

Challenges 

•Jet energy scale, 

•Parton density function (PDF) 

•underlying event, trigger, 

•scale variation, hadronization 


Jets and Missing E T 

The highest mass dijet event in the first 120nb -1 of data 

69 


Inclusive jet cross section 

Inclusive jet p T spectra have ben produced for all three jet approaches used in CMS. 

All results are in good agreement with NLO theory. 

With the new Particle Flow approach the distributions can be extended to a low p T value 

of 18 GeV. 

70 


Jets 

Event with 6 jets with 

p T in the range 30-70 GeV 

Uncalibrated 

energies 

17 


Jets 

Event with 6 jets with 

p T in the range 30-70 GeV 

Uncalibrated 

energies 

Leading jet p T > 80 GeV 

Other jets p T > 40 GeV 

17 


Observed event with hardest jet 

p T (jet) > 1.1 TeV 


Second leading p T > 40 GeV 

p T (j 1 )= 1120 GeV 

p T (j 2 )= 480 GeV 

p T (j 3 )= 155 GeV 

p T (j 4 )= 95 GeV 

Event display shows 

uncalibrated energies 

22 


Observed event with hardest jet 

p T (jet) > 1.1 TeV 


Second leading p T > 40 GeV 

p T (j 1 )= 1120 GeV 

p T (j 2 )= 480 GeV 

p T (j 3 )= 155 GeV 

p T (j 4 )= 95 GeV 



22 


p T (j 1 )=420 GeV 

p T (j 2 )=320 GeV 



1 


Highest-mass di-jet 

event observed so far: 

M jj = 2.55 TeV 

p T (j 1 )=420 GeV 

p T (j 2 )=320 GeV 



1 


!"" #$%$&'()'*+',-(./0&$()' 

>-K$@L7;;MCNOP7 

1 234/$-3%3)&'()',56"78'9'7:;'

Beginning to explore new territory 

Search for narrow resonances in di-jet final states 

We have measured, in 0.84pb -1 of data, the dijet mass differential cross section for 

|η 1 ,η 2 | < 2.5 and |Δη 2 | < 1.3. The distribution is sensitive to the coupling of any new 

massive object to quarks and gluons. 

L= 0.84pb -1 

95% CL mass limits for String resonances > 2.1TeV; Excited quarks >1.14TeV; 

Axigluons/Colorons >1.06TeV; E 6 Diquarks > 0.58 TeV. 

75 


Inclusive jet cross-section 

p T 

j > 60 GeV, |y j |< 2.8 

Measured jets corrected to 

particle-level using partonshower 

MC (Pythia, Herwig): 

justified by detailed comparison 

studies and good agreement with 

data 

NLOJET++ 

Results compared to NLO QCD 

prediction after corrections for 

hadronization and underlying event 

Theoretical uncertainty: ~20% 

(up to 40% at large |y j |) 

from variation of PDF, α s , scale (μ R , μ F ) 

Experimental uncertainty: ~30-40% 

dominated by Jet E-scale 

(known to ~7%) 

Luminosity (11%) not included 


Good agreement data-NLO QCD over 5 orders of magnitude 

24

Single lepton channel 

• 1 e or µ with p T >20 GeV, E 

miss T >20 GeV, E 

miss T +m T (W)>60 GeV 

• N jets with p T >25 GeV, with no b-tag requirement or at least one b-tag 

• Signal defined to have 4 or more jets, and at least 1 b-tag 

77 


Cross check of 3-jet mass 

• Invariant mass of the highest p T 3-jet combination 

• for tagged 3 and 4 jet events used in cross-check 

• analyses. Agrees with top hypothesis. 

78 


Dilepton analysis 

• ee, (µµ): require E T 

miss 

>40 (30) GeV, veto m Z region. 

• eµ: scalar sum of transverse energy H T >150 GeV 

79 


Dilepton events 

• Count events with two or more jets: 2 ee, 3 µµ, 4 eµ 

• b-tag is not used in the analysis, but is a cross-check 

80

Early Physics results from LHC

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