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TRANSCRIPT
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Discovery Physics at the Large Hadron Collider
Christopher Rogan California Institute of Technology
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410R > 0.15R > 0.20R > 0.25R > 0.30R > 0.35R > 0.40R > 0.45R > 0.50
=7 TeVsCMS 2010 Preliminary
-1 L dt = 35 pb∫
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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+ Questions of Scale
Christopher Rogan - Everhart Lecture Series Interview - December 12, 2011
Muon − 106 MeV
Tau − 1.8 GeV
Electron − 0.5 MeV
Why do leptons have the masses they do?
1897
1936
1974
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+ Questions of Scale
Christopher Rogan - Everhart Lecture Series Interview - December 12, 2011
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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+ Questions of Scale
Christopher Rogan - Everhart Lecture Series Interview - December 12, 2011
Why do the fundamental forces (electromagnetism,
weak, strong) have different strengths?
Why is gravity so much weaker than the others?
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+ Questions of Scale
Christopher Rogan - Everhart Lecture Series Interview - December 12, 2011
How do these particles acquire mass?
They acquire mass through interactions with
the Higgs Boson
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Christopher Rogan - Everhart Lecture Series Interview - December 12, 2011
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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Christopher Rogan - Everhart Lecture Series Interview - December 12, 2011
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
Christopher Rogan - Everhart Lecture Series Interview - December 12, 2011
A new superpartner for each SM particles
A new symmetry between fermions and bosons
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+ Supersymmetry and Scale
Christopher Rogan - Everhart Lecture Series Interview - December 12, 2011
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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+ Supersymmetry and Scale
Christopher Rogan - Everhart Lecture Series Interview - December 12, 2011
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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+ Supersymmetry and Scale
Christopher Rogan - Everhart Lecture Series Interview - December 12, 2011
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)
Christopher Rogan - Everhart Lecture Series Interview - December 12, 2011
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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Christopher Rogan - Everhart Lecture Series Interview - December 12, 2011
Compact Muon Solenoid (CMS) Experiment
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+ A Slice of CMS
Christopher Rogan - Everhart Lecture Series Interview - December 12, 2011
CMS Design High Field (4T) Compact Tracker Precise ECAL
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+ Scale at CMS
Christopher Rogan - Everhart Lecture Series Interview - December 12, 2011
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
Christopher Rogan - Everhart Lecture Series Interview - December 12, 2011
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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Christopher Rogan - Everhart Lecture Series Interview - December 12, 2011
[p
b]to
tσ
Prod
uctio
n C
ross
Sec
tion,
-110
1
10
210
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510
CMS
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
Christopher Rogan - Everhart Lecture Series Interview - December 12, 2011
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
proton proton q
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+ SUSY kinematics Example – di-squark ( ) production: q̃
q̃ q̃
χ̃01
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
Christopher Rogan - Everhart Lecture Series Interview - December 12, 2011
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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
Christopher Rogan - Everhart Lecture Series Interview - December 12, 2011
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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
Christopher Rogan - Everhart Lecture Series Interview - December 12, 2011
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