Standard Model Physics at the LHC: the first phase

1
Standard Model Physics at the LHCthe first phase

• M. Cobal
• Universita di Udine e INFN Gruppo Collegato di
  Trieste
• MCWS, Frascati Feb
  2006

2
Summary of SM activities

• Minimum bias and underlying event (see
  Bartalinis talk)
• PDFs with W and Z production (see Tricolis
  talk..next time!)
• Higgs (see Laris 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

3
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
4
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

5
Top pairs production
6
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)

gg?tt
stt(tot) 759100 pb
Nevt 700/hour
qq?tt
7
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
8
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

t
t
9
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
10
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 light jet scale Hadronic
1 ? ?Mt lt 0.7 GeV 10 ?
?Mt 3 GeV
Uncertainty On b-jet scale Hadronic
1 ? ?Mt 0.7 GeV 5 ? ?Mt
3.5 GeV 10 ? ?Mt 7.0 GeV
11
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 gt
  

12
Results after recalibration

• Top
• Zjets

After calib Top
E
E

• Use Top sample to correct jet energies of Zjet
  sample
• TOP 12000 jets, Zjet 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

13
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
14
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
15
How to identify ttbar events

• Commissioning study ? Want to restrict ourselves
  to basic (robust) quantities
• Apply some simple cuts
• Hard pT cuts really clean upsample (ISR).
• Possible becauseof high production rate

?
?
?
Combined efficiency of requirementsis 5 ?
still have 10 evts/hour
1 hard lepton (Pt gt20 GeV)
4 hard jets (PT gt40 GeV)

• Selecting ttbar with b-tagging expected to be
  easy S/BO(100)
• But we would like to start without b-tagging

Missing ET (ET gt20 GeV)
?
16
Backgrounds that you worry about

• W4jets (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 Z4 jets rate and
  MC (Z4j)/(W4J) 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

17
Standard top analysis

• First apply selection cuts
• Assign jets to W, top decays

Missing ET gt 20 GeV
Selection efficiency 5.3
1 lepton PT gt 20 GeV
4 jets(R0.4) PT gt 40 GeV
W CANDIDATE
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.
18
Generation tools

• Summary of R. Chierici

Pythia Herwig ME MC_at_NLO
Hard scattering LO tt LO tt LO ttn 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 Pythia or Herwig interface (double counting problem, can be fixed by the CKKW) Herwig
beam-beam remants, MPI all No MPI (Yes, v605) Pythia or Herwig interface (double counting problem, can be fixed by the CKKW) Herwig
Spin corr NO Yes Yes No
Comments Good for inclusive tt but poor in ttnjets Good for inclusive tt but poor in ttnjets Good for multi-jets, but still LO Good for tt multi jets

• ME ALPGEN/MadGraph/ComHep/TopRex etc

19
MC samples
ttbar (signal)
Wjets (background)

• Generator MC_at_NLO
• Includes all LO NLO m.e.

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

Hard Process
CPU intensive!
Fragmentation, Hadronization Underlying event
Herwig (Jimmy) no pileup
Atlas DetectorSimulation
ATLAS Full Simulation 10.0.2 (30 min/ev)
T1 Sample 175K event 300 pb-1
A7 Sample 145K event 61 pb-1
20
Signal-only distributions (Full Sim)

• 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

m(tophad)
m(Whad)
MW 78.10.8 GeV
mtop 162.70.8 GeV
L300 pb-1 (1 week of running)
Jet energy scalecalibration possible fromshift
in m(W)
S
B
S/B 0.5
S/B 1.20
21
Signal Wjets background (Full Sim)

• Plots now include Wjets background
• Background level roughly triples
• Signal still well visible
• Caveat bkg. cross section quite uncertain

m(tophad)
m(Whad)
Jet energy scalecalibration possible fromshift
in m(W)
S
L300 pb-1 (1 week of running)
B
S/B 0.27
S/B 0.45
22
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)lt10 GeV

m(tophad)
m(tophad)
L300 pb-1 (1 week of running)
m(Whad)
S
S/B 1.77
B
S/B 0.45
23
Signal Wjets background (Full Sim)

• Can also clean up sample with requirement on
  Mtop(jln) 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

TOP CANDIDATE
SEMI LEPTONIC TOP CANDIDATE
m(tophad)
m(tophad)
L300 pb-1 (1 week of running)
S
m(jln)-mtlt30 GeV
B
S/B 1.11
S/B 0.45
24
Effect if increasing realism

• Evolution of Mtop resolution, yield with
  improving realism

m(top) (GeV) resolution (GeV) s(N) stat
Effect ofdetectorsimulation
Truth jets 171.1 0.4 7.0 0.2 6.0
Full simulation 162.7 0.8 15.8 0.8 6.3
Effect ofincreasingWjets bkg.
50 164.1 1.0 17.0 1.5 10
100 165.9 1.4 19.8 2.8 17
Hadronic MW80.410 GeV 160.0 1.0 15.4 1.2 8.3
Effect ofmW cut
25
Exploiting ttbar as b-jet sample (Full Sim)

• 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)

Wjets (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!
26
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 4 partons (32 pb)
W 3 partons (80 pb)
W 5 partons (15 pb)
W ? l n
W ? l n
W ? l n
2 parton reconstructed as single jets
parton is reconstructed as 2 jets
These are the cross sections with the analysis
cuts on lepton and jet pT applied at the truth
level
27
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 Wjets
  background.

28
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 pT
• Efficiency fraction of events passing
• all present analysis cuts that are triggered
• Includes effects of untriggerable
• events due to cracks etc

29
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 Wjets estimate underway
• Incorporate and estimate trigger efficiency to
  few ()
• Also continue to improve jet assignment
  algorithms
• Estimate of s(ttbar) with error lt 20 in first
  running period
• One of the first physics measurements of LHC?

30
ISR and FSR in top events

• Mtop 172.72.9 GeV/c2 (current world average)
• by end of Run II reduce uncertainty on Mtop to
  lt1.5 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), LQCD and
  PDF (for ISR).

31
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

Challenge determine Mtop around 1 GeV accuracy
in 1 year of LHC
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
32
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)

ISR
33
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

1) Logarithmic slope is fitted 2) Fit results
used to define a 1s 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) ?QCD in ISR shower PARP(64) K
factor for the starting Q2 scale of the ISR
shower
hep-ex-0510048
34
Evaluation of the ISR/FSR effects

• To evaluate the uncertainty due to ISR/FSR, the
  relevant parameters are varied by 1s, and new
  178 GeV/c2 tt 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!

35
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).

36
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..

37
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
38
The LHC case III

• We tried the gg?bbar 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.

gg?bbar
39
Future Plans

• Identify all the tunable parameters in PYTHIA 6.3
• Compare also to what we can do with MC_at_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!

40
EW single top production
41
Why single top at the LHC?
3 production modes in SM
t (Wg) channel
Wt channel
s (W) channel
Wg and W can be identified at the Tevatron ALL
can be precisely measured at LHC
42
Studies

• Properties of the Wtb vertex
• Determination of s(p?Wt) and G(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

43
Single top and New Physics
T.Tait, C.-P.Yuan, Phys.Rev. D63 (2001) 0140018
44
Cross sections
45
Theoretical errors at the LHC
Process PDF m-scale (m/2-2m) Dmtop (at LHC)
s-channel 4 2 2
t-channel lt2 3 1
Wt ? lt5 1
(Z.Sullivan, Phys.Rev. D70 (2004) 114012)
Less than at TeV, since the x-region for the
gluon PDFs is better known
Should be very similar to t-channel and a gg?tt
46
t-channel

• Selection
• Exactly 2 jets with high PT 1 central jet from b
  at high PT
• 1 forward jet , ?gt2.5
• Top reconstructed with b-jet. Solution which
  minimize mlvb mtgen
• Resolution in Mtop gt 25 GeV
• Window in HT or Mtop
• Performance
• e 1.3, N(30fb-1) 7,000 eventi
• Background Wjets , ttbar
• Sys lum, e b-tag, JES

ATLFAST
S/B 3 v(SB)/S 1.4 _at_ 30 fb-1
47
s-channel

• Selection
• Separated analysis for tbbar and tbarb
• Asym. For single top, sym. for ttbar and Wjets
• 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
48
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
• exp11, th 8, lumi 5

Uncertainty (30 fb-1) d?/? ()
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
More realistic estimation ongoing
Data needed
49
s-channel, CMS
Preliminary

• Preselection
• 1 high-PT lepton
• Exactly 2 high-PT jets, both b-tagged
• Missing Energy
• Topological selection
• Reconstruct Top w/ the lowest-Pz n 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
• ST cut
• Other topological variables are used, most
  notably M(tb) (directly related to s)

After preselection
After preselection
M(tb)
50
Wt-channel

• Background for gb?Ht
• 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, s/Vtb
  measurements
• Work with theoreticians to identify new physics

51
Wt-channel

• Selection
• At least 3 jets at high pT, only 1 b-jet
• W? jj 60 ltmjj lt 90 GeV/c2
• Reconstruction of top len min mlvbmt
• Windows in HT o Mtop
• Performance
• e 0.90, N(30fb-1) 4,700 events
• Main background ttbar, t-channel
• Sys lum, b-tag e, JES

ATLFAST
S/B 1/7 v(SB)/S 4 _at_ 30 fb-1
52
b-tagging efficiency Results
b-tag efficiency vs jet pT
b-tag efficiency vs jet ?
53
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

54
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