First Physics at the LHC M. Cobal (1), E. Migliore (2) (1) University of Udine and INFN Trieste (2)...

First Physics at the LHC

M. Cobal (1), E. Migliore (2) (1) University of Udine and INFN Trieste

(2) University of Torino and INFN Torino

IV Workshop Italia ATLAS-CMS

Bologna, 23-25 November 2006

Thanks to:

G. Polesello

F. Giannotti

R. Chierici

R. Tenchini

P. Bartalini

IV Workshop Italiano ATLAS-CMS

2008 should look something like…

Hardware commissioning to 7 TeV

Machine Checkout 1 month

Commissioning with beam 2 months

Pilot Physics 1 month

Provisional Reach

1031

Running at 75 ns L~ 1032 cm-2s-1

~ 3 months of running+some optimism ~ 1 fb-1


Prepare the road to discovery: measure backgrounds to New Physics : e.g. tt and W/Z+ jets look at specific “control samples” for the individual channels:

e.g. ttjj with j b “calibrates” ttbb irreducible background to ttH ttbb

Understand/calibrate detector and trigger in situ using “candles” samples e.g. - Z ee, tracker, ECAL, muon chamber calibration and alignment, etc. - tt bl bjj jet scale from Wjj, b-tag performance, etc.

Understand basic SM physics at s = 14 TeV measure cross-sections for e.g. minimum bias, W, Z, tt, QCD jets (to ~20 %), start to tune Monte Carlo measure top mass give feedback on detector performance

Note : statistical error negligible with O(10 pb-1)

Look for New Physics potentially accessible in first year(s)e.g. Z’, SUSY, Higgs ?

LHC: physics roadmap


How many events at the beginning ?

10 pb-1 1 month at1030 and < 2 weeks

at 1031, =50%

100 pb-1 few days

at 1032 , =50%

similar statisticsto CDF, D0 today

Assumed selection efficiency:W l, Z ll : 20%tt l+X :1.5% (no b-tag, inside mass bin)

+ lots of minimum-bias and jets (107 events in 2 weeks of data taking if 20% of trigger bandwidth allocated)

1 fb-1


Which detector performance on day one ?

Based on detector construction quality, test-beam results, cosmics, simulation

Ultimate statistical precision achievable after few weeks of operation. Then face systematics…. E.g. : tracker alignment : 100 m (1 month) 20m (4 months) 5 m (1 year) ?

Expected performance day 1 Physics samples to improve

ECAL uniformity ~ 1% Minimum-bias, Z eee/ scale ~ 2 % Z ee

HCAL uniformity ~ 3 % Single pions, QCD jetsJet scale < 10% Z ( ll) +1j, W jj in tt events

Tracking alignment 20(100)-200 m in R? Generic tracks, isolated , Z m


LHC ?

What we will know @ LHC start?W, Z cross-sections: to 3-4% (NNLO calculation dominated by PDF)

tt cross-section to ~7% (NLO+PDF)

Lot of progress with NLO matrix elementMC interfaced to parton shower MC(MC@ NLO, AlpGen,.. )

<Nch> at =0 for generic pp collisions (minimum bias)

Candidate to very early measurement:

tuning of MC models understand basics of pp collisions, occupancy, pile-up, …

— AlpGenTra

nsvers

e <

Nch

g >

PYTHIA6.214 - tuned

PHOJET1.12

x 3

LHC

x1.5


Minimum Bias

Non-single diffractive evts, ≈ 60-70 mb Soft interactions

Low PT, low Multiplicity. Soft tracks: pT

peak~250MeV Approx flat distribution in to ||~3 and in Nch~30; ||<2.5

Rate: R~700kHz @ L=1031cm-2s-1, For dN/d require ~10k

What we would observe with a fully inclusive detector/trigger.

Several MB interactions can take place in a single beam crossing. MB seen if “interesting” triggered interaction also produced. Pile-up effect. Tracking detectors help to separate the different primary vertices. Possible overlap of clusters in calorimeters. Need energy flow.


Very first alignment will be based on: Mechanical precision Detailed survey data Cosmic data Minimum bias events and inclusive bb

Studies indicate good efficiencies after initial alignment

~ 80% down to PT = 500 MeV Precision will need Zs and

resonances to fix energy scales,

constrain twists, etc.

Even lower PT accessible with reduced tracking ?

PT = 400 MeV - tracks reach end of TRT

PT = 150 MeV - tracks reach last SCT layer

PT = 50 MeV - tracks reach all Pixel layers150MeV

Initial Tracking & Alignment

pT (MeV)


Underlying Event

All the activity from a single particle-particle interaction on top of the “interesting” process.

Initial State Radiation (ISR). Final State Radiation (FSR). Spectators. … Multiple Interactions ? (These models are certainly very successful!).

The UE is correlated to its “interesting” process. Share the same primary vertex. Events with high PT jets or heavy particles have more underlying activity

Pedestal effect.

UE ≠ MB but some aspects & concepts are similar. Phenomenological study of Multiplicity & PT of charged tracks.

High PT scatter

Beam remnants

ISR


UE: measurement plan at the LHC

Topological structure of p-p collision from charged tracks

The leading Ch_jet1 defines a direction in the planeThe transverse region is particularly sensitive to the UE

From charged jet using MB and jet triggers)

From D-Y muon pair production(using muon triggers)

observables are the same but defined in all the plane

(after removing the pairs everything else is UE)

Main observables:+ dN/dd, charged density+ d(PTsum)/dd, energy density


Good RECO/MC agreement in shape

Differences compatible with the expected corrections from charged jet PT calibration, charged tracks innefficiencies and fake rate

MCMBJET60JET120

PT>0.9, ||<1

dNch/dd VS PT_ch_jet1

RECO/MC Differences absorb in the ratio, no need to apply corrections!Emphasis on the reconstruction of soft tracks…

PT_ch_jet1

Main observables:- dN/dd, charged density- d(PTsum)/dd, energy density

Njets > 1, |ηjet| < 2.5, ETjet >10 GeV,

UE with charged jets


Charged Particle Ratio: PTmin = 900 & 500 MeV/c

0.0

0.2

0.4

0.6

0.8

0 250 500 750 1000 1250 1500

Lepton-Pair Invariant Mass (GeV)

Ch

arg

ed

Pa

rtic

le R

ati

o

Generator Level14 TeV

Charged Particles (||<1.0, PT>0.5 & 0.9 GeV/c)(excluding lepton-pair )

PY-ATLAS

PY Tune DW

HERWIG

Charged PTsum Ratio: PTmin = 900 & 500 MeV/c

0.4

0.6

0.8

1.0

1.2

0 250 500 750 1000 1250 1500

Lepton-Pair Invariant Mass (GeV)

Ch

arg

ed

PT

sum

Rat

io

Generator Level14 TeV

Charged Particles (||<1.0, PT>0.5 & 0.9 GeV/c)(excluding lepton-pair )

PY-ATLAS

PY Tune DW

HERWIG

dN/dd dPTsum/dd

Ratio PT>0.9GeV/PT>0.5GeV (PT tracks threshold)

Ratio observables are sensitive to differences between models !!!

M(,) M(,)

UE with Drell-Yan


Production of W and Z boson

Large W (Z) cross section: 10 nb (1 nb) and clean leptonic signatures Compare to theo. prediction or assume prediction and use to measure

luminosity Example : uncertainties with 1 fb-1 in the muon channel in detector

fiducial volume


PDFs:LHC Kinematic regime

ys

Mx exp2,1 MQ

Kinematic regime for LHC much broader than currently explored

z

z

pE

pEy ln

2

1

Test of QCD:

Test DGLAP evolution at small x: Is NLO DGLAP evolution sufficient

at so small x ? Are higher orders important?

Improve information of high x gluon distribution

xmns log~

At TeV scale New Physics ’s predictions are dominated by high-x gluon uncertainty(not sufficiently well constrained by PDF fits)

At the EW scale theoretical predictions for LHC are dominated by low-x gluon uncertainty (i.e. W and Z masses) => see later slides

How can we constrain PDF’s at LHC?


We rapidity distributions

HERWIG MC Simulations with NLO Corrections

At y=0 the total PDF uncertainty is ~ ±5.2% from ZEUS-S ~ ±3.6% from MRST01E ~ ±8.7% from CTEQ6.1MZEUS-S to MRST01E central value difference ~5% ZEUS-S to CTEQ6.1 central value difference ~3.5%

CTEQ61 MRST02 ZEUS-S

CTEQ61 MRST02 ZEUS-S

e- rapidity e+ rapidity

Generator Level

ATLASDetector Levelwith sel. cuts

Error boxes are the Full PDF Uncertainties

GOAL: syst.syst. exp. error ~4%

ys

Mx exp2,1

W production over |y|<2.5 at LHC involves 10-4 < x1,2 < 0.1 region dominated by g qq


Generate 1M “data” sample with CTEQ6.1 PDF through ATLFAST detector simulation and then include this pseudo-data (with imposed 4% error) in the global ZEUS PDF fit (with Det.->Gen. level correction).Central value of ZEUS-PDF prediction shifts and uncertainty is reduced:

ZEUS-PDF BEFORE including W data

e+ CTEQ6.1 pseudo-data

low-x gluon shape parameter λ, xg(x) ~ x –λ

BEFORE λ = -0.199 ± 0.046AFTER λ = -0.181 ± 0.030

40% error reduction

ZEUS-PDF AFTER including W data

e+ CTEQ6.1 pseudo-data

In few day stat. of LHC at low Luminosity

Including ATLAS data on PDF fits

Systematics (e.g. e acceptance vs ) can be controlled to few % with Z ee (~ 30000 events for 100 pb-1)


Measure total ttbar cross section:• test of pQCD calculations (predicted at ~ 10%)• sensitive to top mass

Measure differential cross sections •sensitive to new physics

Make initial direct measurement of top mass Measure single top production (t-channel)

Top physics in the early phase

)d(M

d ,

dp

d

ttt

The LHC will be a top-factory ! NLO~830 pb : 2 tt events per second ! more than 10 million tt /year


Top physics during commissioning

Several months to achieve pixel alignment

Study separation of top from background without b-tagging• Use high multiplicity in final states• High Pt cuts to clean sample• Use kinematical features

Even with a 5% efficiency 10evts/hour at 1033

Hadronic top: Three jets with highest PT

W boson: Two jets in hadronic top with highest PT in reconstructed jjj C.M. frame

TOP CANDIDATE

W CANDIDATE


m (topjjj)

B

S

S/B = 0.45

m(Wjj)

S/B = 1.77

L=300 pb-1

m (topjjj)

Expect ~ 100 events inside mass peak with only 300 pb-1 top signal observable in early days with no b-tagging and simple analysis W+jets background can be understood with MC+data (Z+jets)

S : MC @ NLOB : AlpGen x 2 to account for W+3,5 partons (pessimistic)

|mjj-mW| < 10 GeV

Top physics during commissioning


Top signal significance vs luminosityFit

ted

#sig

nal even

ts

Sig

infi

can

ce (

)

Luminosity (pb-1) Luminosity (pb-1)

Sig

infi

can

ce (

)

Luminosity (pb-1)

Nominal W+jets W+jets x 2 W+jets x 4 W+jets x 8


What can you do with early tops?

Calibrate light jet energy scale - impose PDG value of the W mass (precision < 1%)

Estimate/calibration b-tagging - From data (precision ~ 5%)

- Study b-tag (performance) in complex events

Study lepton trigger

Calibrate missing transverse energy - use W mass constraint in the event - range 50 GeV < p T < 200 GeV

Estimate (accuracy ~20%) of mt and tt.

Use W boson mass to enhance purity

Missing ET (GeV)

Even

ts

Perfect detector

Miscalibrated detector or

escaping ‘new’ particle


Systematic effects for top physics

Almost all SM measurements at LHC dominated by systematic errors. Can be divided into instrumental and from theory/modeling Dominant instrumental uncertainties for top physics:

Luminosity: Reasonable goal is 3-5%

measure number of interactions/bunch crossing (HF) and (pp) (TOTEM)

Reconstruction related: Jet energy scale

need calibrated calorimetry (beam tests, MB, single particles, Z, W…)need jet energy calibration to a few % (with Z()+jet)

need excellent energy flow (association tracker+calo+muon system) b-tagging efficiency+fake rate

use tt for calibration: to 4-5% with 10/fb

Lepton identification and energy scale use Z, other mesons. Less crucial than for the W mass measurement

Theory related systematics are as important as instrumental ones !


pdfsconstrain using LHC data

hard scattered partonsprocess description (signal AND backgrounds)

scale dependency

final state radiation vary QCD and Q2

max consistently ?

hadronization b+light jets: is LEP good enough?

initial state radiation vary QCD and Q2

max consistently ?

minimum bias and underlying event energy dependent. Do our own tuning !

Jets

Jets

UE

UE

PDFsPDFsMEsMEs

UE, MPIUE, MPI

fragmentationfragmentation

PSPS

PDF tuning PDF tuning UE tuning UE tuning radiation tuning radiation tuning fragmentation fragmentation tuningtuning

We model most of our description of reality at the LHC: Need to quote a confidence on the description of our simulations Need to avoid non realistic scenarios (ex: FSR OFF) Need to avoid to double count errors

LHC must learn the best way to use its own data to constrain LHC must learn the best way to use its own data to constrain models models

Learning from data


X-section of tt semileptonics at 1 fb-1

Single lepton selection

Lepton with cuts on ET and Missing ET>40 GeV Hadronic activity (4 jets) b-tagging of 2 jets


Top mass measurement at fb-1

•Should be able to measure top mass at ~ 1%in both dileptonic and semileptonic channel•Need control of the jet energy scale !•Larger error ~2-3% in the hadronic channel

ttbar semileptonics


LHC startup will require a long period of development and understanding

With first data measure

detector performance in situ physics particle multiplicity in minimum bias top signal with ~ 30 pb-1

(tt) to 20% and Mtop to 7-10 GeV with 100 pb-1 ? PDF (low-x gluons !) with W/Z (O(100) pb-1 ?) first tuning of MC (MB, UE, tt, W/Z+jets, QCD jets,…)

Goal is to arrive @ high statistics (few fb-1) data-taking ready to go for early discovery physics

BUT….as soon as interactions @ 14 TeV happen interesting and new physics will appear in the data. Surprises? ….see discussion of Ernesto!

Conclusions

IDEE PER LA DISCUSSIONE

capacità di fare misure accurate/difficili possibilità di fare scoperte

… con 100-1000 pb-1


Cross-sections and rates

At luminosity 1032 cm-2 s-1

Inelastic: 107 Hz bb production: 104 Hz W ℓ: 1 Hz Z ℓℓ: 0.1 Hz tt production: 0.1 Hz


Sensitive tests of understanding of detector properties learn how to do b-physics in exclusive channels w/o particle-ID test of tracking: alignment, material budget, B field test of trigger: L1+HLT

b-physics with 100 pb-1: control channels

Need to be measured at low luminosity to fully exploit the high luminosity LHC run

AT

LA

S

Statistics

100 pb-1

Stat. error

on lifetime

World today (stat+syst)

B+ B+→J/K+ 17000 1.5 % 0.4 % Reference/control channel (ie. B0→ μ+μ-)

B0 B0→ J/K0* 8700 2.2 % 0.5 % Control channel for masses, lifetimesBs Bs→ J/ 900 6 % 2 %

b b→ J/ 260 8 % 5%


BsJ/

Sensitivity to s/s (SM10%) in the “no-tag” mode BsJ/ +-K+K- (SM: σ75 pb)

Trigger selection: L1: di-μ (pT>3 GeV) HLT: J/ and reconstruction using only 5 Tracker hits and w/o μ-ID

|M(J/) |<150 MeV (L2) Mass resolution: σ(J/) 50 MeV

Transverse decay length (Lxy/>3) |M () |<20 MeV, |M(Bs )|<190 MeV (L3)

Mass resolution: σ() 5 MeV, σ(Bs) 60 MeV sig20% bgd=10-5

HLT rate(sig+bgd): 0.1 Hz @ 1033 cm-2s-1, 15000 evts at 1 fb-1

Offline selection: μ-ID+Full Tracker reconstruction Kinematic fit

sig75% (Offline/HLT) Mass resolution: σ(Bs) 15 MeV

CMS


BsJ/

Likelihood for s/s

diff. decay rate as a function of (cos,,cos;t) [PLB 369(1996) 144] + K+

Accurate knowledge of (t) “c distorsion” (ie. track seeds at HLT level) determine (t) from B0J/ K0* +-K+-

K/ mis-id: narrow MBs range (±36 MeV) low stat. included in the likelihood with higher stat.

20% accuracy at 1.5 fb-1

HLT Offline

Plots for 30 fb-1 CMS


Bsμ+μ-

Sensitivity to new physics MSSM: BR ~ (tan) 6

Selection: Trigger

L1: 1-μ (di-μ) 1031(1033) pT>6 GeV L2: di-μ (pT>6 GeV) μ/tracker matching

mass cut Offline

pT> 6 GeV, ||<2.5, Rμμ<0.9

m(μμ)=MBs+140 -70 MeV

Isolation Transverse decay length (Lxy/>11)

+ pointing (<1°) flight direction

Affected by bgd Bsμ+- (BR10-4)

with / mis-id x10 larger than SM predictions for Bsμ+μ-

ATLAS

SM: 3.5 10-9

S.Sivoklokov ICHEP06

90% upper limit

8 10-8


MW with “Scaled Observables Method”

Exploit large Z→ℓℓ statistics available at LHC (104-105 evts with 100 pb-1) measurement of MW from the lepton pT spectrum (no MT No MET)

R(x) different detector acceptance to leptons from W and Z

“template” MT

Scaled lepton pT

CMS

V

VT

W

Z

meas

WT

W

ZZTZ

Tpred

WTmeas

ZT M

pX

M

MXRp

M

Mp

dp

d

dp

d

dp

d ,,,

,,, : where)(

PR

D 5

7 (1

998)

443

3


Diffractive physics

SD~ 15 mb DPE ~ 1 mb 1032 cm-2s-1 → no/low pile-up → selection of events based on

Large Rapidity Gap between scattered proton and X Measurement of SD and DPE cross-sections in presence of

hard scale (dijets, vector bosons, heavy quarks) Test of the scale where the factorization is violated

Handle for understanding parton-parton interactions (UE)

SD

DPE

ND

SD

N

N

N

N

2 gluon exchange with vacuum quantum numbers “Pomeron”

X

Double Pomeron exchange:

X

Single diffraction:


Di-lepton resonances

100 pb-1

Z’→ μ+ μ- single-μ OR di-μ trigger opposite charge μ brems. accounted

Z’: M=1 TeV

σ*BR=150 fb

(σ*BR=370 fb with /Z/Z’ interf.)

initial alignmentRMS±30% 120 GeV

MC truth

±1σ

Unbinned max likelihood for bgd and sig+bgd Improve alignment: ie. CMS long term alignment (1fb-1) RMS±30% 60 GeV

DY

CMS


Z’ discovery reach (5)

CMSATLAS


Di-jet resonances

CMS Physics interest in the high mass tail

W’, Z’ excited quarks RS gravitons

Limits from CDF and DØup to 1 TeV

Narrow resonances ie. Z’: /M1-3%

Crucial knowledge of the physics of jets

absolute jet energy scale modeling of jets

10 100 pb-1

Luminosity for 10 evts above threshold

1

|jet |<1

dσ/dM (pb/GeV)=exp (- M/486 GeV)


Di-jet azimuthal decorrelation

2 dijet

dijet 2

dijet=

dijet~ 2

dijet sensitive to higher orders

QCD radiation w/o measuring

the 3rd or the 4th jet

ATLAS


300 < ETMAX < 600 GeV

600 < ETMAX < 1200 GeV

Di-jet azimuthal decorrelation

Di-jet event selection: Cone jet algorithm (R=0.7) Njets = 2 |ηjet| < 0.5

ETjet #2 > 80 GeV

ATLAS


The Higgs BosonF.

Gia

nott

i IC

HE

P06


σ2.4 pb No narrow m(WW) peak

exploit the spin-0 of SM Higgs

Counting exp. in (ℓℓ) distribution

It requires low and well known background (ggtt (tt and qqWW)

Selection: 2 isolated opposite charge ℓ Jet veto (ET>15 GeV and ||<2.5) MET>50 GeV ℓ kinem.*: ||<2, 12 <m(ℓℓ)<40 GeV, (ℓℓ)<45˚

30< pTmax< 55 GeV, 25< pTmin< 55 GeV

H WW(*)ℓℓ MH=165 GeV

W+ W-

+ -

* Optimized for 1fb-1

S/B=1.66

σ/σ bgd=20% (syst)

CMS


SUSY with 100-1000 pb-1

If s-particles have the same couplings as SM particles the production is dominated by s-quark and gluinos (if light enough) Dominant signatures: Jets and MET

Hard at the beginning!

DØ (V.Shary CALOR04)

ATLAS


Low Mass SUSY

MET

m(ll)High pT jets

“Early discovery” selection 2 isolated leptons pT>10 GeV 24 jets: pTjet1>100 GeV, …, pTjetN>50 GeV MET>200 GeV

SM bgd: tt (10-4), W/Z+jets (10-5), WW/ZZ+jets

SameFlavourOppositeSign (ie. μ+μ-+ e+e- from °2 chain) Bgd subtraction with DifferentFlavourOppositeSign (ie. eμ)

SameFlavourSameSign (ie. μ±μ± + e±e± from two ±) No tt and DY bgd


Low Mass SUSY: di-lepton endpoint

ATLAS “SU3” mSUGRA point m(~g)= 720 GeV = 19 pb

CMS “LM1” mSUGRA point m(~g)= 600 GeV = 50 pb

1 fb-1

Prelim.

CMS ATLAS


Outlook

Se siamo “bravi”… Possibilità di fare misure precise

Mtop, MW, canali esclusivi del b

… ed anche “fortunati” Canali dileptonici (ma dopo avere capito tt !):

Z’ Low Mass SUSY Higgs (ma solo per MH=165 GeV)

Credits:

M.Arneodo, P.Bartalini, U.De Sanctis, F.Gianotti, T.Lari, G.Polesello, R.Tenchini

First Physics at the LHC M. Cobal (1), E. Migliore (2) (1) University of Udine and INFN Trieste (2)...

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Transcript of First Physics at the LHC M. Cobal (1), E. Migliore (2) (1) University of Udine and INFN Trieste (2)...