Physics with CMS

Paolo Meridiani - INFN Roma1 1

Physics with CMS

Paolo Meridiani (INFN Roma1)

http://www.roma1.infn.it/main/mappa_sito.html


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



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



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



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



MSSM physical spectrum

Mass are not predicted but usually charginos and neutralinos are lighter than squarks/spletons/gluinos



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



Producing SUSY @ LHC



SUSY signaturesIn the assumption that R-parity is conserved:



Search strategy



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



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



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



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



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



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



Example: MET SM Bkg normalization

Idea: use Zµµ + jets (>2) in data to normalize the Z (invisible) contribution and calibrate MET spectrum



Capability of observing a SUSY like excess in CMS with inclusive

searches



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



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



Endpoint analysis



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



Alternative SUSY models: GMSB

Actual phenomenology depens on neutralino life-time



And apart from SUSY?1976 NobelDiscovery of the J/PsiTing

1984 NobelDiscovery of W&ZRubbia & Van der Meer

Di-something resonances:



Di-something resonancesLeptons and photons are the cleanest signature



Extended gauge simmetries



The Randall-Sundrum model



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



Searching for Z’ resonances

Discovery potential even with less than 1fb-1



RS gravitons



Distinguishing RS graviton from Z’

Angular distribution of lepton in the boosted rest frame of the heavy mass particle



Not only resonances but also continuum...



ADD model

MD << MPl

Constraints:MD<10 TeV n>1 (Newton law tested up to 200 m)



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



High multiplicity/spherical events



Black Holes hunting...



Do not forget that... Before any discovery we need to understand SM background that we

know is there + control well our detector



CMS discovery path



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Physics with CMS