Download Qu`attendre des premières données du LHC

Qu’attendre des premières
données du LHC ?
Réunion expérimentateursthéoriciens du 13 Janvier 2008
Transparents empruntés à M. Cobal, I. Hinchliffe, K. Lassila-Perini, A. Moraes,
A. Tricoli
Environ deux mois de retard par rapport à ce calendrier…
Cross-sections and rates
At luminosity 1032 cm-2 s-1
Inelastic:
 bb production:
 W ℓ:
 Z  ℓℓ:
 tt production:

107 Hz
104 Hz
1 Hz
0.1 Hz
0.1 Hz
Tout d’abord, mesurer avec précision les
sections efficaces Modèle Standard 
permettra de s’assurer de la compréhension
du détecteur et de la physique.
LHC: physics roadmap
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 Wjj, 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)
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
Look for New Physics potentially accessible in first year(s)
e.g. Z’, SUSY, Higgs ?
Charged particle density at  = 0
(Only need central inner tracker and a few thousand pp events)
LHC?
Multiple interaction model in PHOJET predicts a ln(s) rise in energy dependence.
PYTHIA suggests a rise dominated by the ln2(s) term.
Vital for understanding : Detector backgrounds, Energy scales, Detector occupancy
Minimum Bias
– Non-single diffractive evts, s ≈ 60-70 mb
– Soft interactions
• Low PT, low Multiplicity.
 Soft tracks: pTpeak~250MeV
 Approx flat distribution in  to ||~3 and in f
 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.
Initial Tracking & Alignment
• 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
pT (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 layers
150MeV
Underlying Event
– All the activity from a single particle-particle interaction
High PT scatter
on top of the “interesting” process.
Beam remnants
• Initial State Radiation (ISR).
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.
• Phenomenological study of Multiplicity & PT of charged
tracks.
UE: measurement plan at the LHC
From charged jet using MB and jet triggers
Topological structure of p-p collision from charged tracks
The leading Ch_jet1 defines a direction in the f plane
The transverse region is particularly sensitive to the UE
Main observables:
+ dN/ddf, charged density
+ d(PTsum)/ddf, energy density
From D-Y muon pair production
(using muon triggers)
observables are the same but
defined in all the f plane
(after removing the  pairs everything else is UE)
LHC predictions: pp collisions at √s = 14 TeV
Central Region
Charged particles:
pt>0.5 GeV and |η|<1
Transverse < Nchg >
(min-bias dNchg/dη ~ 7)
ATL-PHYS-PUB-2005-007
LHC
dNchg/dη ~ 30
x3
Cone jet finder:
R     f   0.7
2
2
dNchg/dη ~ 15
UE particles come from
region transverse to the
leading jet.
x1.5
Tevatron
Pt (leading jet in GeV)
Why PDFs are vital at LHC?
•
On Hadron Colliders every Cross-Section
calculation is a convolution of the cross-section at
parton level and PDFs:
PDFs are vital for reliable predictions for
new physics signal (Higgs, SuperSymmetry, Extra Dimensions etc.)
and background cross-section
at LHC.
[at CDF ΔσHiggs,SUSY (CTEQ) ~ 5%]
pA
fa
x1
sˆ
pB
x2
fb
X
How do we want to constrain PDFs?
• W total and differential cross sections theoretical calculations are
very robust: known to NNLO in QCD pert. theory
input E.W. param. known to high accuracy
Main Theoretical
uncertainty comes from PDFs

EXP.: Clean measurement
Abundance of W’s
(300M evt/y at LHC at low Lumi.)
5
MRST2002-NLO
LHC
1  E  pz 

yW  ln 
2  E  pz 
dsW/dyW . Bl
(nb)
4
x1 = 0.12
x2 = 0.0003
x1 = 0.006
x2 = 0.006
x1 = 0.0003
x2 = 0.12
3
x1, 2 
2
W±
Symmetric
1
0
-6
-4
-2
0
yW
2
4
6
MW
exp  yW  Q  M W
s
Kinematic regime for LHC much broader
than currently explored
Study the effect of including the W Rapidity distributions
in global PDF Fits by how much can we reduce the PDF errors?
Generate data with CTEQ6.1 PDF, pass through ATLFAST detector simulation and then
include this pseudo-data in the global ZEUS PDF fit.
Central value of prediction shifts and uncertainty is reduced
BEFORE including W data
AFTER including W data
~1day of data-taking
at low Lumi
W+
to lepton rapidity spectrum
data generated with CTEQ6.1 PDF
compared to predictions from
ZEUS PDF
W+ to lepton rapidity spectrum
data generated with CTEQ6.1 PDF
compared to predictions from
ZEUS PDF AFTER these data are
included in the fit
Specifically the low-x gluon shape parameter λ, xg(x) = x –λ , was λ = -.187 ± .046
for the ZEUS PDF before including this pseudo-data. It becomes λ = -.155 ± .030
after including the pseudo-data
Top physics in the early phase
The LHC will be a top-factory !
 sNLO~830 pb : 2 tt events per second !
 more than 10 million tt /year
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)
ds
ds
,
dp t d(M tt )
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
W CANDIDATE
 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
Top physics during commissioning
m (topjjj)
m(Wjj)
m (topjjj)
L=300 pb-1
S
|mjj-mW | < 10 GeV
S/B = 1.77
B
S/B = 0.45
S : MC @ NLO
B : AlpGen x 2 to account for W+3,5 partons (pessimistic)
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)
Luminosity (pb-1)
W+jets x 4
W+jets x 8
Siginficance (s)
Luminosity (pb-1)
W+jets x 2
Nominal W+jets
Siginficance (s)
Fitted #signal events
Top signal significance vs
luminosity
Luminosity (pb-1)
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 e
- 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 stt.
Events
Use W boson
mass to
enhance purity
Miscalibrated
detector or escaping
‘new’ particle
Perfect detector
Missing ET (GeV)