physics with cms
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Paolo Meridiani - INFN Roma1 1
Physics with CMSPhysics with CMS
Paolo Meridiani (INFN Roma1)
Paolo Meridiani - INFN Roma1 2
OutlineOutline
Lecture 1 Is SM satisfactory? Open questions in the SM? LHC: the answer to unanswered questions? CMS Detector: a challenging detector for a challenging machine CMS Commissioning: how much time is required to make it work?
Lecture 2 CMS early physics: what should be done at the beginning? SM physics with CMS: known SM physics can be done better in
CMS? Higgs Physics with CMS: if it’s there we will catch it!
Lecture 3 Beyond the SM physics at CMS: hunting new theories
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How 2008 should look like...
How 2008 should look like...
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What we should do at the begin?
What we should do at the begin?
New territory to be explored. 14 TeV is just an extrapolation from what we already know...
Pre-operations Synchronization (all subdetectors) Pre-Alignment (Tk + Muon) Pre-calibration (HCAL & ECAL)
Next triggering on collision events Next high rate events: from 10 mb to 1b
Minbias Jets, dijet imbalance Direct photons
Then from b to nb W, Z, W/Z + jets, diphoton, dilepton
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Pre-operationsPre-operations
Synchronization Set relative timings to better than 1ns using lasers and pulsers
checking with cosmic muons (should be achieved in cosmic global runs)
Pre-calibration ECAL: intercalibration with cosmics (1.5%)+ TB intercalibrated SM
(one quarter of EB at 0.3%) HCAL: radioactive sources + TB 5%
Pre-alignment Muon: Alignment with cosmic muons + optical alignement system
(also MB w.r.t ME). Track motion when field on (already tested in Magnet test 2006)
Tracker: survey + optical alignment + cosmic muons
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The collisions start...The collisions start...
We should first understand the trigger table
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How to demonstrate that we are looking at collisions?
How to demonstrate that we are looking at collisions?
First thing than one wants to do is to demonstrate that trigger is selecting real beam-beam interactions. How to do this?
Look at position of the reconstructed vertex. z position will give information on the bunch lenght. RMS should
be compatible with the expected bunch lenght / 2. From tails background can be estimated
Transverse position will give information on transverse beam size and its stability
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Measure dN/d, dN/dpTMeasure dN/d, dN/dpT
Minbias are the events with the largest xsec
But minimum bias charge multiplicity known only at 50%
Few 104 events needed to get preliminary measurement of dN/d & dN/dpT
Acquire them with the special prescaled trigger
Probably less than half-hour of good data will be sufficient
Useful also to look for beam background First inputs to start tuning MonteCarlo
for pileup and set final trigger strategies (e.g. Isolation)
Probably first article published by CMS...
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What else one can do with minimum bias events?
What else one can do with minimum bias events?
Useful to improve ECAL & HCAL calibrations
Charged pions will be used to improve tracker alignment
Use the assumption that energy deposit is uniform in . Possibility to calibrate rings at same . Precision limited by tracker material which is not completely uniform in
Examples with ECAL
Neutral pions will be another source for ECAL intercalibration
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Next step: dijetsNext step: dijets Start using prescaled trigger to try to measure jet-cross section For example look at angular distribution, ratio 2J/J to extract trigger and
reconstruction efficiency Also possibility to intercalibrate the HCAL rings, using the 2J balance
After that start investigating MET in dijets MET is dijet is due to energy mismeasures If tails are under control than b and t pairs should dominated for large (>100 GeV)
MET values
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Electrons, muons Electrons, muons
Then move to nb processes at LHC, W/Z production. NLO Xsec known at 4-5% level
Many different uses: Lepton energy scale from W &
Z• Goal 0.1% can be achieved
with 1fb-1
Tune detector simulation (model Z mass and W transverse mass)
Efficiencies from Z (tag/probe)
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We are starting to undestand detector, beam what we do...We are starting to undestand detector, beam what we do...
We can start measuring SM xsec (W/Z), W mass, top mass but fundamental ingredients for precise meaurements are:
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Precise measurement of MW
Precise measurement of MW
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How to measure W mass?How to measure W mass?
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Ingredients for precise W mass spectrum prediction
Ingredients for precise W mass spectrum prediction
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Rediscover the topRediscover the top
With 840pb LHC is a top pait factory
But also single top has a huge xsec 250 pb t-channel, 62pb tW,
10 pb s-channel
•
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Top physics: early analysisTop physics: early analysis Top as “commissioning tool”
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Top: semileptonicTop: semileptonic
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Top: leptonic + hadronicTop: leptonic + hadronic
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Triple gauge boson couplings
Triple gauge boson couplings
ZZ also irreducible bkg for H→4l searches
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Higgs production @ LHCHiggs production @ LHC
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Higgs SearchesHiggs Searches
“Benchmark” channels: Strongly tied to
detector performance H, HZZ(*)4l Narrow peaks
Event counting No peak Need good control of
background normalization HWW(*)
VBF Take advantage of the
special topology HWW(*), H
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HH
very clean signature in mH<140GeV/c2 regionlow branching ratio (0.002)
signature: two isolated high pT photons narrow peak in di-photon invariant mass
backgrounds: pp→gg (irreducible) pp→ g+jets, pp→jets (reducible)
experimental requirements: very good g identification and isolation aiming at 0.5% ECAL energy resolution
signal:mH = 115 GeV/c2 σxBR = 99.3fbmH = 140 GeV/c2 σxBR = 65.5fbbackgrounds: pp → gg σ = 82pb pp → g +jets σ = 5x104pb pp → jets σ = 2.8x107pb
photons (clusters in ECAL)
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HH
signalx10
background events normalized to 1fb-1
two approaches:cuts based analysis andneural network analysis
signal: very small contribution to the total number of events (signal efficiency at 120 GeV/c2 ~ 30%)
30fb-1: discovery possible for masses < 140 GeV/c2
using 0.5% resolution
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H→ZZ→4lH→ZZ→4l
GOLDEN CHANNEL: cleanest discovery channel over mH>140GeV/c2 range
signature: 2 pairs of opposite-charge, same flavour isolated leptons from primary vertex dileptons invariant mass ~ mZ
backgrounds: pp → ZZ(*) (irreducible, dominant) pp→tt, pp→Zbb (reducible)
main experimental challenges: lepton identification with high efficiency and resolution down to low (~ 5 GeV/c) pT
selection criteria: requirements on vertex, pT(l), isolation, m(ll)
after cuts
2e2μ final state
beforeselections
afterselections
1fb-1
1fb-1
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An event H→4e at CMS...An event H→4e at CMS...
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discovery channel in 2mW < mH < 2mZ
signature: 2 charged leptons and missing energy no jet activity in the central region
2 neutrinos in the final state: no mass peak, counting experiments →accurate background estimate from data needed
main backgrounds: WW(*) (irreducible, dominant) pp→ tt, pp→ Wtbpp→ W+jets, pp→ Z+jets
crucial for the analysis:reconstruction tools for charged leptons, missing energy and jet veto understanding !!!
H→WW→2l2H→WW→2l2
2 opposite charge leptonsno jet with ET > 15GeV, |η|<2.5MET > 50 GeV12 < m(ll) < 40 GeV30 < pT
max < 55 GeVpT
min > 25 GeVΔΦ(ll) < 45º
cuts and counts analysis
} (reducible)
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H→WW→2l2H→WW→2l2
large S/B, 5σ with L<1fb-1 mH=165 GeV/c2
WW control region, no ΔΦ(ll) cut
10fb-1, e
critical: precise background knowledge→ control regions using data ie. WW: inverted kinematic cuts on ΔΦ(ll) and m(ll) ie. tt: extra b-tagged jets
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Higgs in VBF and associated productionHiggs in VBF and associated production
associated ttH, WH production: additional leptons/jets in the final statevector boson fusion: two tagging jets, large Δηjj (>4.5), large m(jj) (>1TeV)
despite lower cross section wrt gg fusion increased discriminating power against QCD jets background better main vertex reconstruction
with large statistics: enhance the significance, measure of Higgs couplings some examples in CMS:
VBF with H→ →l+tjet+ ETmiss (5σ with L=60fb-1 if mH<140GeV/c2) VBF with H→(3σ with L=60fb-1 if mH<150 GeV/c2) ttH, WH with H→ (3σ with L=100fb-1 if mH<150 GeV/c2)
ttH with H→ VBF with H→→l+tjet+ ETmiss
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CMS Higgs discovery potential: if it’s there we will catch it
CMS Higgs discovery potential: if it’s there we will catch it
all Higgs mass range: significance larger than 5σ with 30 fb-1
mH < 140 GeV/c2 discovery with L < 10 fb-1
mH > 140 GeV/c2 discovery with L < 5 fb-1
5fb-1 enough140<mH<450GeV/c2 discovery with 30fb-
1 in the full range
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Measure Higgs mass and width
Measure Higgs mass and width
Higgs mass precision:better than 0.1% if mH<200 GeV/c2
better than 2% up to 600 GeV/c2
Higgs width precision: detector effects dominate if mH < 200 GeV/c2
if mH > 200 GeV/c2 possible measurement with precision better 30% in ZZ channel
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End of lecture 2End of lecture 2
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