Standard Model Physics at the LHC:the first phase

M. CobalM. CobalUniversita’ di Udine e INFN Gruppo Collegato di TriesteUniversita’ di Udine e INFN Gruppo Collegato di Trieste

MCWS, Frascati Feb 2006MCWS, Frascati Feb 2006

Summary of SM activities

Minimum bias and underlying event (see Bartalini’s talk) PDFs with W and Z production (see Tricoli’s talk..next time!) Higgs (see Lari’s talk) Physics commissioning phase

Top physics and the systematics involved First measurements Jet scale ISR/FSR

New Physics with top EW Single top production Not covered today, but next time

W mass measurement Top properties WZ production Z differential cross section measurement

A new point of view: Commissioning!

The game to play:

Understand detector /Minimize MC dependency

Knowing the detector Redundancy between detectors Straight tracks, etc.

Physics: available ‘candle’ signals in physics Presence and mass of the W±, Z0, top-quark Presence of b-jets Balance in transverse plane, PT

Prepair with detector pessimistic scenarios

Non-perfect alignment at startup, e.g. in b-tagging

Dead regions in the calorimeter / noise

Unknown precise jet energy scale

Assess trigger dependencies

Only after full understanding of these the road to discovery starts…

Physics commissioning

What are we going to do with the first month of data?

Many detector-level checks (tracking, calorimetry etc) Try to see large cross section known physics signals But to ultimately get to interesting physics, also need to

calibrate many higher level reconstruction concepts such as jet energy scales b-tagging missing energy

Top pairs production

Top physics at LHC Large ttbar production cross section at LHC

Effect of large s at LHC threshold for ttbar production at lower x

Production gluon dominated at LHC, quark dominated at Tevatron About 100 times larger than cross section at Tevatron (lumi also

much larger)

tttot = 759±100 pb

Nevt ~ 700/hour

32121 10~ ; ˆ xxxsxs

ggtt

qqtt

Top physics topology Decay products are 2 W bosons and two b quarks

About 99.9% to Wb, ~0.1% decay to Ws and Wd each

For commissioning studies focus on events where one W decays hadronically and the other W decays semi-leptonically About 30% of total ttbar cross section

t

t

What can we learn from ttbar production

Abundant clean source of b jets

2 out of 4 jets in event are b jets O(50%) a priori purity (need to

be careful with ISR and jet

reconstruction)

Remaining 2 jets can be

kinematically identified (should

form W mass) possibility for

further purification

tt

What can we learn from ttbar production

Abundant source of W decays into light jets

Invariant mass of jets should add up to well known W mass

Suitable for light jet energy scale calibration (target prec. 1%) Caveat: should not use MW

in jet assignment for purpose

of calibration to avoid bias If (limited) b-tagging is available,

W jet assignment combinatoricsgreatly reduced

t

t

Physics commissioning with top

Jet energy scales Ultimate goal for JES calibration is 1%

At startup calibration will be less known Important –effect on Mtop measurement

Impacts many measurements, not just Mtop

Need to start data to good use for calibration purposes as quickly as possible Top physics ideal candidate to do the job

Uncertainty On b-jet scale: Hadronic 1% Mt = 0.7 GeV5% Mt = 3.5 GeV10% Mt = 7.0 GeV

Uncertainty on light jet scale: Hadronic 1% Mt < 0.7 GeV10% Mt = 3 GeV

Use W in top events for jet calibration

Effect of a mis-calibration of jet energy dominant systematics

Several methods to calibrate. Simplest one:

compute R for k bins in E

apply R correction and recompute new R n times =>

jeti

parti

iWPDGW

E

EwithMMR 21/

R

E

E

Results after recalibration

E

Use Top sample to correct jet energies of Z+jet sample TOP 12000 jets, Z+jet 8000 jets Apply same cuts on jets energies Top light jet scale seems to work for all light jets In progress: repeat exercise with backgrounds

After calib ‘Top’

E

Top

Z+jets

What can we learn from ttbar production

Known amount of missing energy

4-momentum of single neutrino in

each event can be constrained

from event kinematics Inputs in calculation: Mtop from

Tevatron, b-jet energy scale

and lepton energy scale t

t

What can we learn from ttbar production

Two ways to reconstruct the top mass

Initially mostly useful in event

selection, as energy scale

calibrations must be understood

before quality measurementcan be made

Ultimately determine Mtop from

kinematic fit to complete event Needs understanding of bias

and resolution of all quantities Not a day 1 topic

t

t

How to identify ttbar events

Commissioning study Want to restrict ourselves to basic (robust) quantities Apply some simple cuts Hard pT cuts really clean up

sample (ISR). Possible because

of high production rate

Combined efficiency of requirementsis ~5% still have ~10 evts/hour

4 hard jets (PT >40 GeV)

1 hard lepton (Pt >20 GeV)

Missing ET (ET >20 GeV)

Selecting ttbar with b-tagging expected to be easy: S/B=O(100)

But we would like to start without b-tagging

Backgrounds that you worry about

W+4jets (largest bkg)

Problematic if 3 jets line up Mtop and W + remaining jet also line up to Mtop

Cannot be simulated reliablyby Pythia or Herwig. Requires dedicated event generator AlpGen

Ultimately get rate from data Z+4 jets rate and MC (Z+4j)/(W+4J) ratio

Vast majority of events can be rejected exploiting jet kinematics.

QCD multi-jet events

Problematic if one jets goes down beampipe (thus giving ETmiss) and one jets mimics electron

Cross section large and not well known, but mostly killed by lepton ID and ETmiss cuts.

Rely on good lepton ID and ETmiss to suppress

W l

e-,0

‘Standard’ top analysis

First apply selection cuts

Assign jets to W, top decays

1 lepton PT > 20 GeV

Missing ET > 20 GeV

4 jets(R=0.4) PT > 40 GeV

Selection efficiency = 5.3%

TOP CANDIDATE

1 Hadronic top:Three jets with highest vector-sum pT as the decay products of the top

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

W CANDIDATE

Generation tools

ME: ALPGEN/MadGraph/ComHep/TopRex etc

Pythia Herwig ME MC@NLO

Hard scattering

LO tt LO tt LO tt+n NLO tt

(hard gluon)

PS shower (ISR/FSR)

coherent branching

(LO DGLAP)

coherent branching

(LO DGLAP)

Pythia

or Herwig interface

(double counting

problem, can be fixed

by the CKKW)

Herwig

Hadronization LUND string cluster model

beam-beam remants, MPI

all No MPI

(Yes, v605)

Spin corr NO Yes Yes No

Comments Good for inclusive tt

but poor in tt+njets

Good for multi-jets, but still LO

Good for tt multi jets

Summary of R. Chierici

‘T1’ Sample175K event = 300 pb-1

‘A7’ Sample145K event = 61 pb-1

MC samples

• Generator: MC@NLO• Includes all LO + NLO m.e.

• Dedicated Generator: AlpGen• Includes all LO W + 4 parton m.e.

HardProcess

Fragmentation,Hadronization &Underlying event

Herwig (Jimmy) [ no pileup ]

ATLAS Full Simulation 10.0.2 (30 min/ev)

ttbar (signal) W+jets (background)

Atlas DetectorSimulation

CPU intensive!

Signal-only distributions (Full Sim)

MW = 78.1±0.8 GeVmtop = 162.7±0.8 GeV

S/B = 1.20 S/B = 0.5

S

B

m(tophad) m(Whad)

TOP CANDIDATE

W CANDIDATE Clear top, W mass peaks visible Background due to mis-assignment of jets

Easier to get top assignment right than to get W assignment right

Masses shifted somewhat low Effect of (imperfect) energy calibration

Jet energy scalecalibration possible fromshift in m(W)

L=300 pb-1

(~1 week of running)

Signal + Wjets background (Full Sim)

S/B = 0.45 S/B = 0.27

S

B

m(tophad) m(Whad)

TOP CANDIDATE

W CANDIDATE Plots now include W+jets background

Background level roughly triples Signal still well visible Caveat: bkg. cross section quite uncertain

Jet energy scalecalibration possible fromshift in m(W)

L=300 pb-1

(~1 week of running)

TOP CANDIDATE

W CANDIDATE

Signal + Wjets background (Full Sim)

Now also exploit correlation between Mtop(had) and MW(had) Show Mtop(had) only for events

with |m(jj)-m(W)|<10 GeV

m(tophad) m(tophad)

B

S

S/B = 0.45

S/B = 1.77

m(Whad)L=300 pb-1

(~1 week of running)

Signal + Wjets background (Full Sim)

TOP CANDIDATE

Can also clean up sample with requirement on Mtop(jl) [semi-leptonic top] NB: There are two Mtopsolutions for each candidate

due to ambiguity in reconstruction of pZ of neutrino Also clean signal quite a bit

MW cut not applied here

m(tophad) m(tophad)

B

S

S/B = 0.45 S/B = 1.11

SEMI LEPTONIC TOP CANDIDATE

|m(jl)-mt|<30 GeV

L=300 pb-1

(~1 week of running)

Effect if increasing realism

Evolution of Mtop resolution, yield with improving realism

Hadronic MW=80.4±10 GeV

160.0 ± 1.0 15.4 ± 1.2 8.3%

+50% 164.1 ± 1.0 17.0 ± 1.5 10%

+100% 165.9 ± 1.4 19.8 ± 2.8 17%

Truth jets 171.1 ± 0.4 7.0 ± 0.2 6.0%

Full simulation 162.7 ± 0.8 15.8 ± 0.8 6.3%

m(top) (GeV) resolution (GeV) (N) stat

Effect ofdetector

simulation

Effect ofincreasingWjets bkg.

Effect ofmW cut

Exploiting ttbar as b-jet sample (Full Sim)

TOP CANDIDATE

W CANDIDATE Simple demonstration use of ttbar

events to provide b-enriched jet sample Cut on MW(had) and Mtop(had) masses Look at b-jet prob for 4th jet (must be b-jet if

all assignments are correct)

W+jets (background)‘random jet’,

no b enhancement expected

ttbar (signal)‘always b jet if all jet assignment are OK’

b enrichment expected and observed

AOD b-jet probability AOD b-jet probability

Clear enhancementobserved!

Improving the analysis

We know that we underestimate the level of background Only generating W + 4 partons now, but W + 3,5 partons may also result

in W + 4 jet final state due to splitting/merging

W l W l W l

W + 4 partons(32 pb*)

W + 3 partons (80 pb*)

W + 5 partons(15 pb*)

parton is reconstructed as 2 jets

2 parton reconstructed as single jets

* These are the cross sections with the analysis cuts on lepton and jet pT applied at the truth level

Improving the analysis

Improving the W + 4 jets background estimate Need to simulate W + 3,5 parton matrix elements as well But not trivial to combine samples: additional parton showering in

Herwig/Jimmy leads to double counting if samples are naively added

But new tool available in AlpGen v2.03: MLM matching prescription. Explicit elimination of double counting by reconstructing

jets in event generator and killing of ‘spillover’ events.

Work in progress To set upper bound: naïve combination of W + 3,4,5 parton events

would roughly double W+jets background.

Effect of trigger

Look at Electron Trigger efficiency Event triggered on hard electron

Triggering through 2E15i, E25i, E60 channels

Preliminary trigger efficiency as function of lepton pTEfficiency = fraction of events passing

all present analysis cuts that are triggered

Includes effects of ‘untriggerable’

events due to cracks etc

Nominal analysis cut

Electron pT (GeV)#tr

igg

ere

d e

ven

ts /

# e

ven

ts73.5%

Summary

Can reconstruct top/W signal after ~ 1 week of data taking without using b tagging Can progressively clean up signal with use of b-tag, ET-miss, event topology

Many useful spinoffs Hadronic W sample light quark jet energy scale calibration Kinematically identified b jets useful for b-tag calibration

Continue to improve realism of study & quality of analysis Important improvement in W+jets estimate underway Incorporate and estimate trigger efficiency to few (%) Also continue to improve jet assignment algorithms

Estimate of (ttbar) with error < 20% in first running period One of the first physics measurements of LHC?

ISR and FSR in top events

Mtop = 172.7±2.9 GeV/c2 (current world average) by end of Run II: reduce uncertainty on Mtop to <1.5 GeV

Extra jets originating from the incoming partons and outgoing partons affect the measurement of Mtop when they are misidentified as jets from the final state partons or change the kinematics of the final state partons.

Systematic uncertainty due to this

effect was usually assigned using MC events where ISR (and FSR) are switched ON and OFF.

Non physical!

ISR and FSR are controlled by the same DGLAP evolution equation that tells us the probability for a parton to branch (splitting function) and is driven by Q2 (factorisation scale), QCD and PDF (for ISR).

ISR

What we have been doing

In determining Mtop, biggest

uncertainties are on: Jet energy calibration FSR: ‘out of cone’ give

large variations in mass B-fragmentation

ISR/FSR systematics evaluated by looking at the shift in Mtop obtained by switching radiation ON and OFF in Pythia and taking the 20% of it

Source of uncertainty

Hadronic Mtop

(GeV)

Fitted Mtop (GeV)

Light jet scale

0.9 0.2

b-jet scale 0.7 0.7

b-quark fragm

0.1 0.1

ISR 0.1 0.1

FSR 1.9 0.5

Comb bkg 0.4 0.1

Total 2.3 0.9

Challenge:

determine Mtop around 1 GeV

accuracy in 1 year of LHC

Drell Yan processes

Currently, CDF determines the systematic uncertainty due to ISR, using Drell-Yan events.

Advantages of Drell-Yan events are twofold: Due to the dilepton final states, there are no FSR jets DY dileptons are produced by the qqbar annihilation process, as are

most (85%) ttbar pairs at the Tevatron (not the LHC case).

Very similar to top production process Dominated by q-qbar annihilation,

but in lower mass region Z decays into lepton pairs (e, , or pair)

q

q l+

l-

Z/γl = ,e

ISR

New ISR evaluation

The logarithmic dependence of some observables (average PT of the dilepton system, number of soft jets etc) on the scale (Mll is measured and it is found that both PYTHIA and HERWIG describe the ISR activity well over a wide range of DY mass regions

hep-ex-0510048

1) Logarithmic slope is fitted

2) Fit results used to define a 1σ

range of uncertainty

3) This used to generate different

MC samples (ISR up/ISR down)

using some tunable physics

MC parameters that can be varied:

PARP(61) = ΛQCD in ISR shower

PARP(64)= K factor for the starting

Q2 scale of the ISR shower

Evaluation of the ISR/FSR effects

To evaluate the uncertainty due to ISR/FSR, the relevant parameters are varied by ±1σ, and new 178 GeV/c2 t¯t signal and background Monte Carlo templates have to be produced by performing event selection and mass reconstruction on the modified samples. The FSR systematics is a bit more suspect: The Tevatron argument is that

the mechanism of the ISR and FSR is the same, which is true from theoretical point (DGLAP) but not true from the implementation point (in Pythia the two differ..)

The signal and background p.d.f.’s used in the analysis remain unchanged.

The shift in the fitted top quark mass is taken as the systematic uncertainty associated with the FSR effect.

The bottom line we should keep in mind is that the procedure works since they have achieved a good fit of Pythia predictions to the DY data!

The LHC case

The use of the slope found using DY events can be applied at LHC when the qq initial state is the dominant process (i.e: W’ and leptoquarks production?)

However we can claim that

if we tune the ISR parameters on Z0 (and maybe another process for cross-check) the ISR is ok for all processes in general

what is different are the evolution kernels which carry no sys error and the PDFs which have to be tuned separately;

we just cannot use the fitted DY curve as such but have to vary the parameters a bit on a process basis (as they did in Tevatron).

The LHC case

Where other initial states give dominant contributions, like gg in top production a similar approach might be possible: One should look for the easily controllable processes with high statistics and equal initial states as a cross check.

We should think of a more consistent way to tune FSR on the data instead of assuming that if the ISR works so does FSR (as done in Tevatron). In Pythia (old 6.2 or new 6.3 mechanism) the FSR is much more

advanced than ISR and uses some different parameters as well

The ISR is governed also by the PDFs which is not the case for FSR, for example;

The coherence effects which are implemented differently etc..

The LHC case II

Tuning of Pythia to the DY pT(ll) vs. Q2 distribution is of paramount importance – direct tuning of ISR parameters on the first data!

To achieve this we have to identify all the tunable parameters in the new Pythia 6.3 ISR/FSR – they have changed considerably

We also have to think more about the distributions and processes we can tune ISR/FSR on. To this extent a good understanding of the planned triggers is needed to have the data necessary to span various mass regions

DY events

The LHC case III

We tried the ggbbar process (same initial state as top).

The Q2 distribution falls rapidly, which means that we can get a fit only on very low values (then no overlap with the Z0 region)

The jet content different in the two processes (number of jets vs jet energy) We should also look for a process closer to the top scale.

ggbbar

Future Plans

Identify all the tunable parameters in PYTHIA 6.3 Compare also to what we can do with MC@NLO. Here, in

the hard event there is already part of the radiation, so we cant do as with PYTHIA. We can instead: Play with the scale of the shower in HERWIG.

This option is what corresponds more closely to the naif idea of studying the systematics of radiation

Look not only to Ptll but also to Njet vs Q2 (important for top: from DY Ptll don t know whether there are many soft jets or few hard jets, and this is crucial for the final effect on Mtop!

Look at top events themselves!

EW single top production

Why single top at the LHC?

t (Wg) channels (W*) channel Wt channel

Wg and W* can be identified at the Tevatron

ALL can be precisely measured at LHC

3 production modes in SM

Studies

Properties of the Wtb vertex Determination of (p→Wt) and (t→Wb) Direct determination of |Vtb| Top polarization

Precision measurements → Test for new physics Anomalous couplings, FCNC W’ extra-gauge boson (GUT, KK) Extra Higgs Boson (2HDM)

Single top is background for.. Higgs physics with jets

Single top and New Physics

T.Tait, C.-P.Yuan, Phys.Rev. D63 (2001) 0140018

FCNCkZtc=1

4th generation,|Vts|=0.55, |Vtb|=0.835(extreme values allowed w/o the CKM unitarity assumption)

SMTop-flavorMZ’=1 TeVsen2f=0.05

Top-pionMp±=450 GeVtR-cR mixing ~ 20%

s-channel

t-ch

annel

Cross sections

Theoretical errors at the LHC

Process PDF-scale

(/2-2)

mtop

(at LHC)

s-channel 4% 2% 2%

t-channel <2% 3% 1%

Wt ? <5% 1%

(Z.S

ulli

van

, Phys.

Rev. D

70

(2

00

4)

11

40

12

)

Should be very similar to t-channel and a gg→tt

Less than at TeV, since the x-region for the gluon PDFs is

better known

t-channel

Selection Exactly 2 jets with high PT: 1 central jet from b at high PT 1 forward jet , |η|>2.5 Top reconstructed with b-jet. Solution which minimize |mlvb –

mtgen| Resolution in Mtop > 25 GeV Window in HT or Mtop

Performance : ε ≈ 1.3%, N(30fb-1) ~ 7,000 eventi Background: W+jets , ttbar Sys: lum, b-tag, JES

S/B ~ 3

√(S+B)/S ~ 1.4% @ 30 fb-1

ATLFAST

s-channel

Selection Separated analysis for t+bbar and tbar+b Asym. For single top, sym. for ttbar and W+jets 2 and only 2 central b-jets with

high PT

Top reconstructed with the lvb

combination with the highest PT Windows in HT or Mtop

Background T-channel, ttbar

ATLFAST

s-channel Performance

Standard Sel. + topological Sel. (HT, Mtop)

optimization upper/lower limits of HT

Results Statistical sensitivity : 7% to 12% (corresponds to 15%-10% syst.) according to the topological sel.(and of S/B)

Measurements dominated by sistematics: exp:11%, th: 8%, lumi: 5%

Uncertainty

(30 fb-1)

δ/ (%)

Scala in energia 3.4

ISR/FSR modelling

7.3

b-tag & mistag 6.4

bckgd theoretical 8.0

Total Systematics 13

Total Statistics 9

Data needed

More realistic estimation ongoing

s-channel, CMS

Preselection– 1 high-PT lepton

– Exactly 2 high-PT jets, both b-tagged

– Missing Energy

Topological selection– Reconstruct Top w/ the lowest-|Pz|

solution and the b-jet with “jet charge” opposite to the lepton (if both opposite, the one giving the highest-PT top is chosen)

– Window in Mtop

– T cut

– Other topological variables are used, most notably M(tb) (directly related to ŝ)

bjets

TmissTTT PEP

,,

PreliminaryPreliminary

M(tb)

i

κ

i

i

κ

ii

bjet

pj

pjqq

After preselection

After preselection

Wt-channel

Background for gbH±t Very difficult channel:

ttbar hide the signal (a ttbar event with a b-jet outside the acceptance is perfextly simulating a W event)

Not even trivial to define exactly what is the signal (at NLO: mixed with the ttbar diagrams!)

Goal: Identification, sys evaluation, /Vtb measurements Work with theoreticians to identify new physics

Wt-channel

Selection :

– At least 3 jets at high pT, only 1 b-jet

– W jj: 60 <mjj < 90 GeV/c2

– Reconstruction of top le: min |mlvb–mt|

– Windows in HT o Mtop

Performance : –ε ≈ 0.90%, N(30fb-1) ~ 4,700 events

– Main background: ttbar, t-channel

– Sys : lum, b-tag , JES

S/B ~ 1/7

√(S+B)/S ~ 4% @ 30 fb-1

ATLFAST

b-tagging efficiency: Results

b-tag efficiency vs jet b-tag efficiency vs jet ppTT

b-tag efficiency vs jet b-tag efficiency vs jet ηη

Wt for new physics

First complete calculation one-loop for Wt in the MSSM performed with the MSUGRA mechanism of simmetry breaking

Different observables proposed to isolate the purely SUSY effect

M. Beccaria, M. Macorini, F.M. Renard, C. Verzegnassi

hep-ph/0601175

Experimental study just started

Other SM benchmarks…

Expected cross section for useful processes Inclusive jet production Photon/diphoton DY cross section W/Z as luminosity measurements W/Z + jets…