What is the Higgs Boson And how do

What is the Higgs Boson? And how do we search for it? Jason Nielsen SCIPP / UC Santa Cruz June 25, 2007 J. Nielsen VERTEX 2004 1

Challenge of Particle Physics • Unification of the basic forces and the origin of mass for the fundamental particles • Unexpected new physics or extra dimensions not included in Standard Model • Unknown new physics (forces or particles) hinted at by cosmology Particle collisions at the energy frontier enable us to pursue these and other questions about nature J. Nielsen 2

Fundamental Particles & Forces J. Nielsen 3

Force Carrier Quanta Photon (electromagnetic) • verified 1922 • mass of photon = 0 W, Z bosons (weak force) • verified 1983 • m. W, m. Z: 80 Ge. V/c 2, 91 Ge. V/c 2 Gauge symmetry is fundamental to electrodynamics • when extended to electroweak theory, requires massless W, Z • how to accomodate their large masses? J. Nielsen 4

Higgs Mechanism in Field Theory Electroweak “Standard Model” relies on broken symmetry Additional fields with constructed potential • just like gravitational field, electric field Introduction of a pervasive Higgs field • Rotationally symmetric potential • But the stable minimum breaks the symmetry! J. Nielsen 5

Spontaneous Symmetry Breaking Came to particle physics from condensed matter physics above Tc below Tc Pencil on point Heisenberg ferromagnet Theory has rotational invariance; ground state is not invariant Symmetry has been broken by external factor J. Nielsen 6

Higgs Mechanism in Field Theory Spontaneous symmetry breaking • Lost degree of freedom -> Goldstone bosons give mass to W±, Z • One physical scalar boson: Higgs boson whose mass is unknown Discovery of the Higgs boson would help verify this approach Otherwise, much head-scratching and new theories! J. Nielsen 7

Why is the Top Quark So Massive? mt=175 Ge. V/c 2 Interaction with Higgs quantum defines mass of fermions Schwinger (1957): a coupling produces effective mass terms through the action of the vacuum fluctuations (Higgs boson) Top quark most affected by this “Higgs field molasses” Note: Higgs couplings explain fundamental fermion mass but not everyday mass! J. Nielsen 8

So What IS the Higgs boson? Higgs boson is a physical condensate of the pervasive postulated Higgs field Similar to photon, except Higgs boson is not a force carrier What kinds of particles do it couple to? • Its couplings are proportional to the fermion masses • So it couples most strongly to the most massive particles This makes it clear how to search for it, if it exists… J. Nielsen 9

Wringing Out the Higgs Condensate Physical Higgs bosons can be produced, given enough energy e+ H Z* (Here ECM > m. H + m. Z) Z e. That’s where the collider comes in But Higgs boson is fleeting: decays immediately to characteristic “final state” b H Z q b q That’s where the detector comes in J. Nielsen 10

Recent Physics Results Effects of the Higgs boson are felt via loop interactions Precision measurements are sensitive to the Higgs mass Updated winter 2007 with new Tevatron m. W=80. 4± 0. 04 Ge. V m. H < 182 Ge. V/c 2 at 95% CL (including previous searches) J. Nielsen 11

How does the Higgs Boson Decay? Notice coupling to massive particles (bb, tt, WW, ZZ) For low mass Higgs, expect decay to b quark pairs; For very high mass Higgs expect decay to ZZ J. Nielsen 12

Rare Higgs Decays (? ) (Claus Grupen) J. Nielsen 13

Identifying b Quarks from Higgs B hadrons have lifetimes of 1. 5 ps: find the decay vertex! proton-antiproton Interaction point B hadron Fit tracks together to form secondary vertex • measure flight distance of B hadron • typical flight distance is 0. 5 cm from interaction point • close, precise measurement provided by silicon is crucial J. Nielsen 14

One Provocative Candidate Event HZ bbbb selection ECM=206. 7 Ge. V 3 NN b-tagged jets Reconstructed m. H = 110 ± 3 Ge. V/c 2 J. Nielsen 15

Bumps in the Mass Spectrum Decay products of the Higgs boson form a mass resonance - similar to resonances from past discoveries of new particles Strategy for identifying Higgs boson production: 1. Excess of events in Wbb signature (or other signature) 2. Higgs decay products form a invariant mass peak J. Nielsen 16

Tevatron Cross Section Hierarchy In proton-antiproton collisions at s = 1. 96 Te. V: b-jet pairs from QCD high-energy leptons 1 Particle production rates vary widely: the Higgs is the “needle in the haystack!” J. Nielsen 0. 05 17

What kind of unit is a “barn? ” Manhattan Project physicists gave the name to the typical nuclear cross-section defined as 10 -24 cm 2 Practically “as big as a barn” where (sub)-nuclear processes are concerned Photo: Reidar Hahn, Fermilab the term “barn” wasn't officially declassified until 1948 Apparently there was also a unit called the “shed”: 10 - 48 cm 2 This summer CDF will have collected 3 giga-sheds of data! J. Nielsen 18

bb Dijet Invariant Mass Distribution J. Nielsen 19

Large Hadron Collider at CERN Next generation collider: startup scheduled for 2008 Italy p 14 Te. V Luminosity target: 1034 cm-2 s-1 p Increased production of heavy particles like Higgs, top quark CMS J. Nielsen ATLAS More particles at higher energy requires new detector design and technology 20

Higgs Decay to Photons Rare decay in SM H t t LHC detectors have been optimized to find this peak! J. Nielsen 21

Higgs Decay to ZZ Requires precise measurement of muon curvature J. Nielsen 22

ATLAS Experiment at LHC J. Nielsen 23

ATLAS Experiment at LHC ATLAS collaborator J. Nielsen 24

ATLAS DETECTOR Nov. 2005 J. Nielsen 25

Installation of inner detector end-cap J. Nielsen 26

Prospects for SM Higgs at LHC Should discover SM Higgs regardless of mass value Low-mass Higgs channels: • H ! (sm =1. 5 Ge. V/c 2) • W, Z boson fusion to Higgs: then H WW or H tt • tt. H: top quark again! High-mass Higgs channels: • golden mode 4 e/ opens >2 m. Z J. Nielsen 27

Identifying Particle Signatures ATLAS trigger system can identify specific signatures online J. Nielsen 28

“Hunt for Higgs” WWW Site One of the best I’ve seen at describing what really happens http: //www. sciencemuseum. org. uk/antenna/bigbang/huntforhiggs/index. asp Let’s have a look together at the “Hunt for Higgs” J. Nielsen 29

Future of the Higgs Search • Tevatron experiments still searching • LHC turns on in 2008 – Commissioning and calibrating detectors • Understand non-Higgs backgrounds • Find the Higgs boson peak above the bkgd! • My guess is that it will take a few years to collect enough events to convince ourselves J. Nielsen 30

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