standard model physics at the lhc: the first phase
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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…
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