Discovering the Higgs Boson J. Pilcher Talk for Graduate Students January 9, 2004

Page 2 Introductions l Research in experimental high energy physics n Work with Kelby Anderson, Ed Blucher, Frank Merritt, Mark Oreglia, Mel Shochet n Graduate students Francesco Spano, Martina Hurwitz l Upcoming experiment motivated by the previous one n High precision tests of the electroweak theory n OPAL experiment at the LEP facility at CERN

January 9, 2004Page 3 Previous Work l Reaction studied l The collider n CERN, Geneva n e + e - collisions n E cm to ~200 GeV

January 9, 2004Page 4 Physics Prejudice l The three families of “point-like” fermions n Unclear why there is this replication l Interactions via the gauge bosons

January 9, 2004Page 5 Physics Prejudice l Weak boson mass splittings from interaction with the Higgs boson l Leading order predictions of the electroweak theory are very simple n 3 “degrees of freedom” n If M W, M Z, and  em are known, many observables are predicted s Expected accuracy a few % l Higher order effects involve unseen states n Top quark, Higgs boson n Predictions are a function of the variables M T and M H n Fit observables to obtain the unknowns s Cross sections, angular distributions, forward-backward asymmetries

January 9, 2004Page 6 How well does it work? l The observed cross section n Extraction of properties of Z and W bosons

January 9, 2004Page 7 Properties of the Z l Fit to resonance gives mass and width

January 9, 2004Page 8 Properties of the Z l Quality of the measurements?

January 9, 2004Page 9 Properties of the Z l Mass results

January 9, 2004Page 10 Properties of the W boson l Cross section and mass determinations

January 9, 2004Page 11 Properties of the W boson l W boson mass

January 9, 2004Page 12 Other observables

January 9, 2004Page 13 Determination of Top Quark Mass l Compare with direct measurements

January 9, 2004Page 14 Determination of Higgs Boson Mass l Fit gives M H = /-33 GeV n M H < 193 GeV (95% CL) l Limit from direct search n M H > 114 GeV (95% CL) l Overall n 114 < M H < 193 GeV

January 9, 2004Page 15 The end of the LEP Era l OPAL finished data taking in fall 2000 n 1000 pb -1 of data collected n LEP collider was pushed to the highest possible energy s To E CM ~ 207 GeV n Many high precision measurements n The electroweak model works remarkably well n Prediction of the Higgs mass n No direct observation of the Higgs

January 9, 2004Page 16 What’s the Next Step? l The CERN Large Hadron Collider (LHC) n New project approved in 1995 s Proton-proton collider n E CM = 14 TeV ( GeV) s Actually this is for protons s Each internal quark has only a fraction of this energy s Effective E CM for qq collisions is ~ 14/3=4.7 TeV –Factor of ~20 higher than LEP s Cross sections fall like 1/E 2 –Luminosity of collider must be 400 times larger than LEP s Design luminosity /cm 2 /sec n Expected to start operation in ~ 3 years n The Chicago group is part of the ATLAS experiment for this facility

January 9, 2004Page 17 What about Fermilab? l Fermilab collider has E CM for proton- antiproton collisions of 2 TeV n E CM for qq collisions of 0.7 TeV n Luminosity ~ /cm 2 /sec s A little low l It is operating NOW

January 9, 2004Page 18 What is the LHC? l Fill the 27 km circumference LEP tunnel with superconducting magnets n m-long dipole magnets with 8.3T field

January 9, 2004Page 19 Where is it? l Just outside Geneva

January 9, 2004Page 20 What is the LHC? l Build new state-of-the-art detectors n ATLAS and CMS for high P T physics n Also ALICE for heavy ion physics s Search for quark-gluon plasma n Also LHCb for studying the physics of the b quark s VERY large data samples l Chicago has been working on the ATLAS experiment since 1995 n Also 30 other US institutions

January 9, 2004Page 21 The ATLAS Detector

January 9, 2004Page 22 The ATLAS Detector

January 9, 2004Page 23 How to see the Higgs? l Decays very rapidly n Observe decay products and calculate the mass of their parent n H decays to heaviest states accessible s Specific modes depend on mass n Low probability but excellent mass resolution s Can see signal as a narrow peak above background n Important for 80 < M H < 120 GeV Requires excellent EM calorimeter for  energy

January 9, 2004Page 24 How to see the Higgs? l Signal shown corresponds to integrated luminosity of 100 fb -1 n 1 year at design luminosity (but first year only 10 fb -1 ) n Peak corresponds to mass resolution of ~ 2%

January 9, 2004Page 25 How to see the Higgs? n Important for 120 < M H < 170 GeV s One W could be “off shell” are not detected but appear as “missing energy” Requires good ability to detect e and  n Requires good calorimeter to see “missing energy” n No distinctive mass peak s Broad excess of events over background

January 9, 2004Page 26 How to see the Higgs? m H = 160 GeV ATLAS qqH  qqWW  qq  e qqH  qq   qq  e +X

January 9, 2004Page 27 How to see the Higgs? l This is the “golden” mode n All final state particles directly detected with good resolution s Narrow mass peak l Important for 150 < M H < 700 GeV

January 9, 2004Page 28 How to see the Higgs? l Signal for 300 GeV Higgs with 10 fb -1 of luminosity n First year of operation

January 9, 2004Page 29 Charged Lepton Detection Muon detection Toroid magnets High precision drift chambers material to shield against hadrons Electron detection LAr EM calorimeter Magnetic tracker Compare E and p to reject hadrons

January 9, 2004Page 30 How to see the Higgs? l Must be sensitive to many decay modes n A combination of channels may be needed

January 9, 2004Page 31 Can we build all this stuff? Most of the surface buildings handed over to ATLAS last year Underground civil construction complete and detector being installed

January 9, 2004Page 32 Muon Toroid Design 8 superconducting coils

January 9, 2004Page 33 Muon Toroid Construction

January 9, 2004Page 34 Muon Toroid Construction

January 9, 2004Page 35 Calorimetry EM calorimeter measures energy of e and  n EM showers develop in 1.5-mm Pb sheets n Ionization sampled with liquid Argon layers l Hadron calorimeter measures energy of quarks and gluons n Hadronic showers develop in 4-mm Fe plates n Ionization sampled with plastic scintillator and photomultiplier tubes

January 9, 2004Page 36 Calorimetry Had Tiles Had LAr EM LAr Forward LAr Solenoid Barrel cryostat End-cap cryostat

January 9, 2004Page 37 Barrel EM Calorimeter

January 9, 2004Page 38 Hadron Calorimeter (TileCal) Had Tiles Had LAr EM LAr Forward LAr Solenoid Barrel cryostat End-cap cryostat Chicago involved here With signal processing electronics With mechanical construction

January 9, 2004Page 39 Hadron Calorimeter (TileCal) Mechanical concept Chicago Sub-module Construction

January 9, 2004Page 40 TileCal Module Construction

January 9, 2004Page 41 TileCal Readout l Electronics drawer in each module carries photomultiplier tubes and electronics n Each cell of calorimeter viewed by 2 phototubes n Electronics needs very wide dynamic range (65,000:1)

January 9, 2004Page 42 TileCal Electronics

January 9, 2004Page 43 Current Chicago Activity l Assembling calorimeter n Surface assembly this year n Start underground installation 2004 l Integrating electronics systems n Calibration, control, signal processing l Preparing software to process the data n Especially for Tile Calorimeter n Studying physics signals n Developing methods for in-situ calibration s Using physics signals

January 9, 2004Page 44 Opportunities for Students l In the past mainly undergraduates n Assisting with construction work n Getting research experience n Senior theses on ATLAS physics l PhD theses need to be on publishable physics n In past years has been a bit too long before data s We expect to start data taking in 2007 l Now excellent time to get involved in commissioning the detector and developing analysis software