Discovery Physics at the Large Hadron Collidercrogan/files/Everhart_Rogan_12_12_11.pdf · Large Hadron Collider Christopher Rogan California Institute of Technology M R [GeV] 100150200250300350400

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Discovery Physics at the Large Hadron Collider

Christopher Rogan California Institute of Technology

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Everhart Lecture Series Interview – December 12, 2011

+ Questions of Scale

Christopher Rogan - Everhart Lecture Series Interview - December 12, 2011

The Standard Model (SM) of particle physics A collection of parameters including masses, couplings and complex phases Has successfully predicted our experimental observations at previously accessible energy scales to extremely high precision BUT still many fundamental questions remain unanswered, many related to SCALE

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Muon − 106 MeV

Tau − 1.8 GeV

Electron − 0.5 MeV

Why do leptons have the masses they do?

1897

1936

1974

3



Why do quarks have the masses they do?

1967

1995

1977

Up ∼ 3 MeV

Down ∼ 5 MeV

Charm 1.5 GeV

Strange 100 MeV

Top 173 GeV

Bottom 4.2 GeV

1967

1974

1964

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Why do the fundamental forces (electromagnetism,

weak, strong) have different strengths?

Why is gravity so much weaker than the others?

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How do these particles acquire mass?

They acquire mass through interactions with

the Higgs Boson

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Higgs’ interaction with other particles results in quantum corrections to Higgs mass

∆m2H

= − |λF |2

8π2

�Λ2UV

+ · · ·�

mH < 158(95% CL)

Corrections tend to pull the mass to the UV cut-off scale

Scale and the Higgs mass

BUT fits to electroweak data and measurements from

Fermilab indicate that a light Higgs is strongly preferred

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One possible solution:

Scale and the Higgs mass

∆m2H

= 2× λS

16π2

�Λ2UV

+ · · ·�

∆m2H

= − |λF |2

8π2

�Λ2UV

+ · · ·�

Supersymmetry (SUSY)

Undiscovered superpartners of SM particles, with spins differing by ½, cancel quantum corrections to Higgs mass

|λF |2 = λSequal couplings to Higgs

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+ Supersymmetry


A new superpartner for each SM particles

A new symmetry between fermions and bosons

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+ Supersymmetry and Scale


The Bullet Cluster (1E 0657-56). Two galaxies colliding. Red shows concentration of visible matter. Blue shows dark matter inferred by gravitational lensing.

SM SUSY Predicts the unification of

the strong and EWK couplings at a high scale

Conserved charges (R-parity) makes the lightest-

supersymmetric-particle (LSP) stable, resulting in compelling WIMP Dark Matter candidate

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BUT supersymmetry predicts that SM particles and their superpartners should have the same mass….

… which has been excluded, as we have not observed these superpartners to date

?

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BUT supersymmetry predicts that SM particles and their superpartners should have the same mass….

? If it exists, SUSY must be a broken symmetry!

How does it break? At what scale?

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+ The Large Hadron Collider (LHC)


27 km circumference proton-proton collider

3.5 + 3.5 TeV CM energy collisions

High energies allow us to probe the TeV energy scale frontier in a laboratory

CERN Meyrin, Switzerland

CMS

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Compact Muon Solenoid (CMS) Experiment

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+ A Slice of CMS


CMS Design High Field (4T) Compact Tracker Precise ECAL

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+ Scale at CMS


Di-lepton invariant mass is used to identify Z bosons

SM scale ‘candle’ used to calibrate detector, commission object reconstruction and study backgrounds to new physics

m(��) peaks at mZ ∼ 91 GeV

Z(µµ) + jets production

muons

jets

arXiv:1110.3226

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+ Scale at CMS


W (eν) + jets production

electron

mT (�ν) has kinematic edge at mW ∼ 80 GeV

mT =�2peT p

νT (1− cosφ)

Missing transverse

momentum (MET)

Weakly interacting particles (ex. neutrinos) escape detection – their transverse momentum can be inferred from conservation of momentum (colliding partons can have different z-momenta)

arXiv:1110.3226

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[p

b]to

tσ

Prod

uctio

n C

ross

Sec

tion,

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W

1j≥

2j≥

3j≥

4j≥

Z1j≥

2j≥

3j≥

4j≥

> 30 GeV jetTE

| < 2.4 jetη|

γW

> 10 GeVγ TE

,l) > 0.7γR(Δ

γZ

WWWZ

ZZ ZZ→

(140)H

-136 pb -136 pb -11.1 fb -11.7 fbJHEP10(2011)132

CMS-PAS-EWK-10-012PLB701(2011)535 CMS-PAS-EWK-11-010 CMS-PAS-HIG-11-015

theory predictionsyst)⊕CMS measurement (stat

CMS 95%CL limit

Exploiting our knowledge of the standard model scales has allowed us to measure the properties of these processes

We can use this same knowledge to search for new physics at an unknown scale

https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsEWK Scale at CMS 18

+ SUSY kinematics


Neutralinos are weakly interacting (R-parity) – escape detection

Example – di-squark ( ) production: q̃ q̃q̃ → (qχ̃01)(qχ̃

01)

Strongly interacting sparticles (squarks, gluinos) preferentially produced

quarks ( ) hadronize into jets – momenta measured by detector

q̃ q̃

χ̃01χ̃0

1

qq

proton proton q

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+ SUSY kinematics Example – di-squark ( ) production: q̃

q̃ q̃

χ̃01

qq

proton proton

M∆ =m2

q̃ −m2χ̃01

2mq̃Scale:

Angle:

In squark rest frames, final state objects have momentum equal to:

Coming from different decays, visible particles’ momenta do not balance in transverse plane

q̃q̃ → (qχ̃01)(qχ̃

01)

Scale of events can be used to distinguish from background

χ̃01

BUT: too many kinematic unknowns to fully reconstruct the event

Φ


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+ Razor kinematics Introduced “Razor” variables, R and MR, designed to discover and characterize new physics at new scales

arXiv:1006.2727

MRR

Peaks at scale of new physics production

Measures transverse imbalance of event – allows us to suppresses and model backgrounds

M∆

Simulated events

R2

W + jets SUSY


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+ Razor Searches @ CMS

https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsSUS11008 https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsSUS10009

Christopher Rogan - Everhart Lecture Series Interview - December 12, 2011 arXiv:1107.1279

n  Variables R and MR used to control the shapes of backgrounds in the phase-space of scale beyond the standard model

n  2-dimensional analytical modeling of backgrounds allow us to use advanced statistical analysis techniques to identify peaking signatures of SUSY or other new physics

n  Multiple finals states (categorized by lepton multiplicity) and these variables also allow for characterization of potential discoveries

n  At the moment: analysis used to set the some of the world’s most stringent limits on SUSY

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+ Talk Outline n  Motivation: Open questions of scale in high energy physics

n  Parameter ‘fine-tuning’ and the Higgs mass hierarchy problem n  Supersymmetry (SUSY) and its implications

n  The Large Hadron Collider (LHC) n  Design and scope n  ‘Physics of scale’ at the LHC

n  The CMS experiment n  Design and commissioning n  Physics event reconstruction

n  Standard model scale ‘candles’ and measurements n  ‘Variables of scale’, vector-boson and top quark production n  Missing transverse momentum and kinematically ‘open’ final states

n  Razor Kinematic Variables n  Motivation and derivation n  Analysis design, implementation and results – constraints on SUSY


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Documents

Discovery Physics at the Large Hadron Collidercrogan/files/Everhart_Rogan_12_12_11.pdf · Large Hadron Collider Christopher Rogan California Institute of Technology M R [GeV] 100150200250300350400