physics with cms
DESCRIPTION
Physics with CMS. Paolo Meridiani (INFN Roma1). Outline. 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 - PowerPoint PPT PresentationTRANSCRIPT
Paolo Meridiani - INFN Roma1 1
Physics with CMS
Paolo Meridiani (INFN Roma1)
Paolo Meridiani - INFN Roma1 2
Outline 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 can be done at the beginning? SM physics with CMS: known 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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Hunting new physics Recall:
Why do we think about extending the SM?• Gravity is not incorporated • Hierarchy problem• Unification of couplings (GUT)• Flavour/ number of families beg for explanation• ...
Many candidate theories:• Supersimmetry• Extradimensions• .....
I do not have the time to go deeply into all of them, but I would like you to have a feeling of the strategy and things to be controlled for new physics discoveries in CMS
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Features of SuperSimmetry SuperSimmetry: achieved in theory where lagrangian is invariant with
respect to What this brings: Cancellation of quadratic divergences
Unification of couplings
Provides a candidate for cold dark matter (in case of R-parity conservation)
Easy to accomodate EW precision data
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MSSM Minimal Supersimmetric extension of the SM Introduce super-partners s = ½ for each SM particle Two Higgs doublets with <vev> and superpartners. After EW simmetry
breaking 5 Higgs bosons: h, H, A, H remain Supersimmetry should be broken: no superpartner observed to date Additional ingredient: R-parity a new conserved quantum number
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MSSM physical spectrum
Mass are not predicted but usually charginos and neutralinos are lighter than squarks/spletons/gluinos
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Breaking SUSY In MSSM supersimmetry is broken with explicit terms (105 free
parameters) then reduced to 15-20 imposing phenomenological constraints
Since SUSY cannot be broken spontaneously, idea is to postulate an hidden sector of interactions
Most of the following analysis will be shownin the mSUGRA scenario
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SUSY signaturesIn the assumption that R-parity is conserved:
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Benchmark points
Basis of detailed studies in the mSUGRA context
Low mass points for early LHC running but outside Tevatron reach
High mass points for ultimate LHC reach
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Inclusive search: jets + MET
Most powerfull way to observe SUSY excess CMS Example:
MET>200 GeV + Clean-up 3 jets:
• ET> 180, 110, 30 GeV Indirect lepton veto Cuts on between jets and MET HT/Meff=ET1+ET2+ET3+MET>500 GeV
CMS Results: LM1 efficiency is 13%, S/B ~ 26 ~6 pb-1 for 5 discovery
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Same sign muons Even cleaner signature with low background
due to same-sign requirement Concentrate here on isolating the SUSY
diagrams giving prompt muons with strong muon isolation & tight quality cuts
Cuts (optimize @ LM1): 2 SS isolated muons
• pT > 10 GeV MET > 200 GeV 3 jets:
• ET1>175 GeV • ET2>130 GeV • ET3>55 GeV
65% efficient at identifying SUSY diagrams, 90% pure
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Inclusive MET + Z0
Catch Mostly from q, g decays Z0 gives extra handle against non-resonant dilepton bkg
Cuts (optimize @ LM4): MET > 230 GeV 2 OS SF leptons
• pT(e) > 17 GeV, or• pT(µ) > 7 GeV
81 < Mll < 96.5 GeV < 2.65 rad
Background (10 fb-1) SM: 200 40 (t-tbar+diboson) Systematic uncertainty 20%
LM4 Signal (10 fb-1) 1550 30
0 0 02 1 Z
e+e–CMS
Sensitive here
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Convince yourself that you have observed something
If SUSY is there should be relatively easy to get an excess of events over the SM
But the real problem is convince yourself that this is a real excess MET: key signature for SUSY in the assumption of R-parity
conservation But tuning MET will not be easy:Lesson from Tevatron:
All the instrumental garbage go in MET
Long and painstaking polishing phase is required
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MET in CMS Sum mom. over calorimeter towers
MET is magnitude of imbalance MET Resolution
Measure from data Use min-bias and prescaled
jet triggers to measure resolution CMS stochastic term ~0.6–0.7
Jet calibration crucial to improve resolution and reduce systematic uncertainty
Variety of techniques possible -Jet balancing, di-jet balancing, W mass constraint in tt events
CMS GOAL: Achieve <3% JES uncertainty for ET>50 GeV with 1–10 fb-1
QCD Minbias
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Example: MET SM Bkg normalization
Idea: use Zµµ + jets (>2) in data to normalize the Z (invisible) contribution and calibrate MET spectrum
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Capability of observing a SUSY like excess in CMS with inclusive
searches
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The fun begins... Assuming (hoping) to have observed an excess within 10
fb-1 what happens? Need to demonstrate that:
Every particle has a superpartner with s=1/2 and same gauge quantum numbers
Mass relations predicted by SUSY holds Observables:
Masses BR’s Production cross-sections Angular decay distributions
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SUSY spectroscopy With stable 1
0 cannot fullyreconstruct squark or gluinodecay chains in general
Kinematical endpoints in invariant massdistributions give accesson sparticle masses
Start from dileptons from 20 (mll)
Add quark jet to get squark (mllq) Add another jet to get gluino (mllqq)
These and other combinations (e.g. mlq) have endpoints thatare functions of the sparticle masses
p
p
g~
b~
b
b
01
~02
~~
q~
Kinematic end points (MC)in Dalitz plot of Mll and Mllq
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Example m(l+l-) at CMS Measure invariant mass distribution of same-flavor opposite-sign (SFOS) leptons as evidence for
or Striking signature: probably first and clearest signal of SUSY
LM1 with 1 fb-1, fit result:
0 02 1 0 0
2 1
max 80.4 0.5 (stat) 1.0 (misalign) 0.8 (EM scale) GeVm
Subtract different flavor leptons
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Alternative SUSY models: GMSB
Actual phenomenology depens on neutralino life-time
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And apart from SUSY?1976 NobelDiscovery of the J/PsiTing
1984 NobelDiscovery of W&ZRubbia & Van der Meer
Di-something resonances:
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Di-something resonancesLeptons and photons are the cleanest signature
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Extended gauge simmetries
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The Randall-Sundrum model
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Experimental facts in reconstructing high energy objects in CMS
Very High Energy Electrons and Photons Saturation in CMS ECAL (limited electronics
range)above 1.7 TeV barrel, 3 TeV endcaps; mass
resolution barrel 0.6 % (7%) with (without) saturation correction (based on non saturating adjacent crystals)
Very High Energy Muon Misalignment + multiple scattering dominate Muon bremsstrahlung
In general resolution of high energy electrons/photons better than muonsE/E constant 0.5% p/p p
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Searching for Z’ resonances
Discovery potential even with less than 1fb-1
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Distinguishing RS graviton from Z’
Angular distribution of lepton in the boosted rest frame of the heavy mass particle
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Not only resonances but also continuum...
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ADD model
MD << MPl
Constraints:MD<10 TeV n>1 (Newton law tested up to 200 m)
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ADD graviton
Signature:Single high pT in central regionHigh missing pT back-to-back photon
Main SM bkg: Z+→ + Normalization from Z+→ +
Discovery reach MD< 3 TeV for 30fb-1
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High multiplicity/spherical events
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Black Holes hunting...
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Do not forget that... Before any discovery we need to understand SM background that we
know is there + control well our detector