High Energy Tests of the Electroweak Theory

Transcription

1 High Energy Tests of the Electroweak Theory Sabine Lammers Indiana University and TU-Dresden Eleanore Trefftz GastProfessorin TU-Dresden Colloquium January 7, 2014

2 What is particle physics? not subatomic taxonomy! The known universe is effectively described by spin-1/2 fermions interacting with spin-1 or spin-0 bosons How do we distinguish new physics experimentally? We look for particles to give us clues about the underlying principles e.g. quantum mechanics, Lorentz invariance, conservation laws, symmetries, spontaneous symmetry breaking,... How do we detect new particles? resonances quantum properties: charge, spin, mass, etc. enhanced cross sections Observation of a new fundamental (not composite) charged resonance would be strong evidence for physics beyond our current understanding

3 The 4 Forces of Nature 4 known fundamental forces Gravity Electromagnetism Weak Nuclear Force Strong Nuclear Force

4 Length Scales of the Universe strong weak

5 Standard Model of Elementary Particles The SM is a Quantum Field Theory: fusion of Special Relativity and Quantum Mechanics There are 3 main ingredients: Forces: SU(3) x SU(2) x U(1) Electromagnetism(γ), Weak(W ±,Z), Strong(g) Matter 6 quarks, 6 leptons in 3 generations Spontaneous Symmetry Breaking Brout-Englert-Higgs Mechanism

6 Matter and Forces Electromagnetic Force Strong Force Weak Force electric charge isospin charge color charge For each force, there is an associated charge carried by matter particles

7 Standard Model Lagrangian All 3 forces and how they allow matter to interact is contained in the SM Lagrangian strong force and unified EW force SU(3)c x SU(2)L x U(1)Y 3 charge types red, green, blue 3x3-1 = 8 gauge bosons 8 gluons 2 charge types isospin I3 W = ±1/2 2x2-1 = 3 gauge bosons 1 charge type hypercharge 1 gauge boson W+, W-, Z, γ { The Lagrangian density (field theory analog of classical single particle L) must be invariant under the symmetries observed in the universe conservation laws Local gauge invariance is the fundamental principle on which the Standard Model is built

8 Standard Model Lagrangian There is a unique set of interaction vertices for each fundamental force Feynman graphs are built out of combinations of these vertices Summing up all possible Feynman graphs which transform a set of initial state particles to a set of final state particles gives us the interaction amplitude.

9 Spontaneous Symmetry Breaking There is a problem with the Weak Interactions SU(2) gauge invariance requires fermions and vector bosons to be massless - this is not what we observe! Spontaneous Electroweak Symmetry Breaking (EWSB) is a way of introducing mass to the theory F

10 Spontaneous Symmetry Breaking There is a problem with the Weak Interactions SU(2) gauge invariance requires fermions and vector bosons to be massless - this is not what we observe! Spontaneous Electroweak Symmetry Breaking (EWSB) is a way of introducing mass to the theory F F > F 0

11 Spontaneous Symmetry Breaking There is a problem with the Weak Interactions SU(2) gauge invariance requires fermions and vector bosons to be massless - this is not what we observe! Spontaneous Electroweak Symmetry Breaking (EWSB) is a way of introducing mass to the theory F F F > F 0

12 2013 Nobel Prize in Physics This year s physics Nobel Prize was awarded to Francois Englert and Peter Higgs for laying the theoretical foundation of the Higgs Mechanism in the Standard Model and predicting the existence of the Higgs Boson. It was awarded nearly 50 years after the work was published, probably as a result of the discovery of the Higgs Boson last year.

13 Breaking the EW Symmetry The Higgs Mechanism (or as Peter Higgs calls it, the ABEGHHK'tH Mechanism) Anderson, Brout, Englert, Guralnik, Hagen, Higgs, Kibble and 't Hooft Add a complex scalar doublet field, Φ, to the Standard Model Lagrangian with potential: Φ has 4 real components Break symmetry by choosing parameters such that the vacuum expectation value (vev) of one component of Φ is non-zero at the minimum of the potential This vacuum expectation value is the order parameter of the symmetry breaking Fluctuations of the component of Φ that acquires non-zero vev around the minimum are what we call the Higgs Boson Fluctuations of the remaining three components that don t acquire a vev become the longitudinal components of W +, W - and Z (that become massive gauge bosons) The Higgs mass is a free parameter of the theory

14 The Higgs Field

15 The Higgs Field

16 Who proposed the Higgs boson? Although Peter Higgs name is associated with the addition of a scalar field, there were several others who came up with the same idea about the same time. 10

17 Who proposed the Higgs boson? The basic method of adding a scalar field was proposed in 1962 by Philip Warren Anderson, a solid state theorist, who built on Yoichiro Nambu s work. 10

18 Who proposed the Higgs boson? A relativistic model was developed in 1964 by three independent groups: Francois Englert and Robert Brout; Peter Higgs; Gerald Guralnik, C. R. Hagen, and Tom Kibble 10

19 Higgs Production at LHC Production dominated by gluon fusion Second most copious production - Vector Boson Fusion (WW, ZZ fusion) Decay to pairs of fermions or bosons, with different BRs

20 Higgs Measurement Phys. Lett. B 726 (2013), pp The Higgs Boson discovery was announced July 4, 2012 by the ATLAS and CMS experiments, and was made by combining data from all production and decay channels. With the full Run1 dataset, the Higgs resonance is visible in some individual production/decay channels testing compatibility of the resonance properties in different channels is important to gain confidence that this is, indeed, the SM Higgs Higgs properties of interest: mass, coupling, spin, parity Example: mh (H γγ) = ± 0.2 (stat) ± 0.7 (sys) GeV mh (H ZZ) = ± 0.6(stat) (sys) GeV slight difference in preferred mass

21 Higgs Measurement Phys. Lett. B 726 (2013), pp Measurements of compatibility between observed Higgs rates and SM expectation for mh = several channels show a slight excess above SM expectation (μ=1), but nothing significant enough to generate excitement -- will be interesting to see how this discrepancy is resolved in RunII of the LHC Support for scalar nature of the resonance - testing different spin hypotheses

22 Testing EW Theory - Fusion and Scattering of EW Bosons EW Theory is non-abelian! gauge bosons possess weak charge! they interact with each other Therefore, self - interactions between EW Bosons in the form of triple and quartic gauge couplings should exist.

23 Experimental Tests of EW Theory EW Interactions were measured extensively by LEP experiments Apart from few features that were experimentally unaccessible (including observing Higgs Boson), SM theory has been confirmed at very high precision LEP EW Working Group Physics Reports, 532 (2013) arxiv:

24 Experimental Tests of EW Theory Triple gauge boson couplings validated through e + e - W + W - cross section measurements Only performed limits on anomalous quartic gauge couplings, which have meanwhile been superseded by the LHC LEP EW Working Group Physics Reports, 532 (2013) arxiv:

25 WW Scattering - Role of the Higgs Without the Higgs, the WW scattering cross section rises unbounded with increasing center-of-mass energy unphysical! violates unitarity - integrated probability density cannot be larger than 1 By including additional diagrams: where H is the SM Higgs, an exact cancellation unitarizes the high energy behavior of σ(ww WW) Alboteanu, Kilian and Reuter, JHEP 0811:010,2008, arxiv: 0806:4145

26 EW gauge couplings at LHC Single Boson final states a.k.a Vector Boson Fusion Protons in LHC serve as source for beams of Vector Bosons Final state bosons accompanied by two final state quarks which manifest as jets of hadronic particles W, Z Bosons most easily triggered/measured through their leptonic decay products Colorless exchange region between two quarks free of hadronic activity Two Boson final states a.k.a Vector Boson Scattering These processes are just beginning to become accessible at LHC energies and luminosities

27 CERN

28 Large Hadron Collider

29 Proton Collisions Beams: 2 x protons per bunch stored in 16 μm diameter Each proton bunch: ~ 20 cm long 2000 bunches circulating simultaneously in each direction 20 million crossings per second, resulting in collisions per crossing

30 An LHC Collision ATLAS Experiment 2013 CERN

31 ATLAS

32 A Toroidal LHC Apparatus

33 Compact Muon Solenoid

34 CERN - Building 40

35 Particle Identification

36 Every LHC collision produces a plethora of particles that are created through a variety of mechanisms, including the hard interaction. We study each of these mechanisms by simulating them with Monte Carlo methods and comparing the parameters of the simulation with data. For a given signal process, there are typically several other types of background processes with the same final state which need to be carefully accounted for. Analysis Flow in HEP Overview of HEP analysis: 1. Record (a subset of) LHC collisions which have the features of the hard process in question. 2. Simulate signal process and all possible background processes using theoretical inputs for cross. sections and kinematic distributions 3. Validate the simulation in a kinematic region close to but not overlapping the signal region. 4. Compare the number of events in data to those in simulation. ΣN = Σσ L processes

37 VBF W, Z Production - Search Strategies Challenges and handles distinguish EW production from strongly produced jets accompanied by W, Z bosons tune selection cuts to optimize signal-to-background ratio use multivariate techniques to exploit kinematical differences between signal and irreducible backgrounds measure the energy of jets accurately use data measurements in control regions to constrain uncertainties on jet energy and resolution accurately simulate the contribution of purely hadronic events in which one of the jets mimics the signature of a final state lepton use data in orthogonal sample to simulate kinematic shape of fake backgrounds, fit background sample to data in signal region to determine overall normalization

38 VBF - Distinguishing Topology = ln(tan 2 ) jet 1 η = 0 jet 2 η =- η =+ VBF events are distinguished by jets widely separated in rapidity (polar angle) and a harder jet pt spectrum J. High Energy Phys. 10 (2013) 062

39 VBF - W Production Requirements imposed on data events: Inclusive W+jets Phase Space ATLAS work in progress Signal region (includes preselection cuts) ATLAS work in progress Signal region cuts maximize signal contribution data blinded Data are intentionally invisible (blinded) until analysis techniques are finalized

40 VBF - W Backgrounds Probability to find 3rd jet in rapidity gap spanned by two highest pt jets in W+ 2 jet events at DZero (pp- experiment) Systematic uncertainties on the background simulation can bury the signal e.g. theoretical imprecision on the production rates or Probability of third jet emission DØ, 3.7 fb, W( e )+ 2jets+X (leading p T NLO Blackhat+Sherpa HEJ Sherpa jets, rapidity gap emission) Alpgen+Pythia Alpgen+Herwig Pythia Herwig Backgrounds can be measured in other phase space regions or even other experiments (to validate the MC simulation) Theory/Data p e T R cone =0.5, p >15 GeV, e <1.1, M T W T jet >20 GeV, y <3.2 >40 GeV, p >20 GeV T W( e )+ 2jets+X y ~ η y(j,j jet Phys. Rev. D 88, (2013) 1 ) 2

41 VBF - Z results First measurement of EW Zjj at a hadron collider performed by CMS Phase space: mll > 50 GeV, pt,jets > 25 GeV, ηjets < 4, mjj > 120 GeV σll(l=e,μ)= 154 ± 24 (stat) ± 46 (exp. syst.) ± 27 (th.syst.) ± 3 (lumi.) fb Theory prediction (NLO) = 166 fb Significance of 2.6 standard deviations EW Zjj contributions: VBF component (left) cannot be calculated independently without breaking gauge invariance J. High Energy Phys. 10 (2013) 062

42 Vector Boson Scattering Complementary approach for studying Electroweak Symmetry Breaking Verify if the newly discovered Higgs Boson fully restores the unitarity in VLVL scattering Search for alternative mechanisms for the unitarity restoration: new resonances or strong VV interactions or... Several channels: same sign WW - very good 1:1 s/b opposite sign WW - larger 1:60 σ, but smaller s/b WZ - larger σ, but smaller 1:140 s/b 1:140 ZZ - process available only after several 1: 1400 more years of LHC running Measurement would be the first observation of quartic gauge couplings Large concentration of effort from TU-Dresden driving this analysis

43 VBS Signal and Backgrounds Signal Processes Background Processes Extremely small signal cross section (depends on cuts) 1:1 1:60 1:140 1:140 1: 1400

44 VBS W ± W ± and WZ searches VBS Searches currently focus on same-sign WW and WZ final states, where W, Z decay leptonically. Same kinematic features in jet distributions as VBF. Pure signal process too small to observe with RunI data. Focus is now on measuring combined EW and QCD VVjj C.Gumpert, DPG, DD 03/2013 Felix Socher talk DPG 03/2013

45 Candidate WZjj event

46 Anomalous Quartic Gauge Couplings Generic extension of quartic couplings of SM: SM prediction: gi =1, h ZZ = 0 Different values for couplings break gauge structure of the SM Non-SM couplings give hints for new physics in the form of new particles and/or interactions

47 Conclusions Tests of the high energy behavior of the electroweak theory are in full swing at the LHC The Higgs Boson discovery is the most remarkable achievement in high energy physics in decades - another feather in the cap of the Standard Model There are no indications from Run I of the LHC that the Standard Model is deficient Vector Boson Scattering will be an area of concentration in RunII, in addition to studying Higgs properties As always, we hope to find something beyond the Standard Model

48 Maybe a 5th force?

49 Maybe a 5th force? Thanks for listening!

50 Extra Slides

51 Calorimeter

52 ATLAS Muon Spectrometer Momentum resolution ΔpT/pT< 10% up to 1 TeV Coverage: η < 2.7 Optical alignment goal: 30μm

53 LHC Schedule Current Schedule: TeV run (always subject to change) shutdown TeV run 2018 shutdown TeV high luminosity run 2022 shutdown

54 Spontaneous Symmetry Breaking in Superconductivity

55 LHC - Fun Facts

56 Anomalous Triple Gauge Couplings

57 fermions vector bosons