Discovery Physics at the Large Hadron Collider

Transcription

1 + / 2 GeV N evt CMS 2010 Preliminary s=7 TeV -1 L dt = 35 pb R > 0.15 R > 0.20 R > 0.25 R > 0.30 R > 0.35 R > 0.40 R > 0.45 R > [GeV] M R Discovery Physics at the Large Hadron Collider Christopher Rogan California Institute of Technology Everhart Lecture Series Interview December 12, 2011

2 + Questions of Scale 2 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

3 + Questions of Scale 3 Why do leptons have the masses they do? 1897 Electron 0.5 MeV Muon 106 MeV 1936 Tau 1.8 GeV 1974

4 + Questions of Scale 4 Why do quarks have the masses they do? Up 3MeV Down 5MeV Charm 1.5 GeV Strange 100 MeV Top 173 GeV Bottom 4.2 GeV 1977

5 + Questions of Scale 5 Why do the fundamental forces (electromagnetism, weak, strong) have different strengths? Why is gravity so much weaker than the others?

6 + Questions of Scale 6 How do these particles acquire mass? They acquire mass through interactions with the Higgs Boson

7 + Scale and the Higgs mass 7 Higgs interaction with other particles results in quantum corrections to Higgs mass m 2 H = λ F 2 8π 2 Λ 2 UV + Corrections tend to pull the mass to the UV cut-off scale BUT fits to electroweak data and measurements from Fermilab indicate that a light Higgs is strongly preferred m H < 158 (95% CL)

8 + Scale and the Higgs mass One possible solution: Supersymmetry (SUSY) 8 Undiscovered superpartners of SM particles, with spins differing by ½, cancel quantum corrections to Higgs mass m 2 H = λ F 2 8π 2 Λ 2 UV + equal couplings to Higgs λ F 2 = λ S m 2 H =2 λ S Λ 2 16π 2 UV +

9 + Supersymmetry 9 A new superpartner for each SM particles A new symmetry between fermions and bosons

10 + Supersymmetry and Scale 10 SM SUSY Predicts the unification of the strong and EWK couplings at a high scale Conserved charges (R-parity) makes the lightestsupersymmetric-particle (LSP) stable, resulting in compelling WIMP Dark Matter candidate The Bullet Cluster (1E ). Two galaxies colliding. Red shows concentration of visible matter. Blue shows dark matter inferred by gravitational lensing.

11 + Supersymmetry and Scale BUT supersymmetry predicts that SM particles and their superpartners should have the same mass. 11? which has been excluded, as we have not observed these superpartners to date

12 + Supersymmetry and Scale BUT supersymmetry predicts that SM particles and their superpartners should have the same mass. 12? If it exists, SUSY must be a broken symmetry! How does it break? At what scale?

13 + The Large Hadron Collider (LHC) 27 km circumference proton-proton collider CERN Meyrin, Switzerland TeV CM energy collisions CMS High energies allow us to probe the TeV energy scale frontier in a laboratory 13

14 + Compact Muon Solenoid (CMS) Experiment 14

15 + A Slice of CMS CMS Design High Field (4T) Compact Tracker Precise ECAL 15

16 + Scale at CMS Z(µµ) + jets production 16 jets muons m() peaks at m Z 91 GeV 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 arxiv:

17 + Scale at CMS W (eν) + jets production 17 electron Missing transverse momentum (ME T ) m T = 2p e T pν T (1 cos φ) m T (ν) has kinematic edge at m W 80 GeV 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:

18 + Scale at CMS [pb] σ tot Production Cross Section, j W 2j 3j jet E T 4j jet η CMSPublic/PhysicsResultsEWK CMS 1j 2j > 30 GeV < 2.4 Z 3j 4j Wγ γ E T Zγ > 10 GeV ΔR(γ,l) > 0.7 CMS 95%CL limit CMS measurement (stat syst) theory prediction WW WZ ZZ H(140) ZZ pb pb fb fb JHEP10(2011)132 CMS-PAS-EWK PLB701(2011)535 CMS-PAS-EWK CMS-PAS-HIG 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

19 + SUSY kinematics Example di-squark ( q ) production: q q (q χ 0 1)(q χ 0 1) 19 q q Strongly interacting sparticles (squarks, gluinos) preferentially produced q q proton proton q quarks ( ) hadronize into jets momenta measured by detector χ 0 1 χ 0 1 Neutralinos are weakly interacting (R-parity) escape detection

20 + SUSY kinematics Example di-squark ( q proton q q q ) production: proton q Scale: In squark rest frames, final state objects have momentum equal to: M = m2 q m2 χ 0 1 2m q 20 q q (q χ 0 1)(q χ 0 1) Scale of events can be used to distinguish from background χ 0 1 χ 0 1 Angle: Coming from different decays, visible particles momenta do not balance in transverse plane BUT: Christopher too Rogan many - Everhart kinematic Lecture Series Interview unknowns - December 12, 2011 to fully reconstruct the event

21 + Razor kinematics arxiv: Introduced Razor variables, R and M R, designed to discover and characterize new physics at new scales 21 M R Peaks at scale of new physics production R Measures transverse imbalance of event allows us to suppresses and model backgrounds M Simulated events W +jets SUSY R 2

22 + Razor CMS 22 n Variables R and M R 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 arxiv:

23 + 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 23 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 n Variables of scale, vector-boson and top quark production Missing transverse momentum and kinematically open final states n Razor Kinematic Variables n n Motivation and derivation Analysis design, implementation and results constraints on SUSY