Discovering the Higgs Boson

J. PilcherTalk for Graduate Students

January 9, 2004

January 9, 2004Page 2

Introductions Research in experimental high energy

physics Work with Kelby Anderson, Ed Blucher, Frank

Merritt, Mark Oreglia, Mel Shochet Graduate students Francesco Spano, Martina

Hurwitz Upcoming experiment motivated by the

previous one High precision tests of the electroweak theory OPAL experiment at the LEP facility at CERN

January 9, 2004Page 3

Previous Work

Reaction studied

The collider CERN, Geneva e+e- collisions Ecm to ~200

GeV

January 9, 2004Page 4

Physics Prejudice The three families of “point-like” fermions

Unclear why there is this replication

Interactions via the gauge bosons

g8 γ W± Z0

νe

e−

⎛ ⎝ ⎜

⎞ ⎠ ⎟

νμ

μ−

⎛ ⎝ ⎜

⎞ ⎠ ⎟

ντ

τ−

⎛ ⎝ ⎜

⎞ ⎠ ⎟

u

′ d ⎛ ⎝ ⎜

⎞ ⎠ ⎟

c

′ s ⎛ ⎝ ⎜

⎞ ⎠ ⎟

t

′ b ⎛ ⎝ ⎜

⎞ ⎠ ⎟

January 9, 2004Page 5

Physics Prejudice Weak boson mass splittings from interaction

with the Higgs boson Leading order predictions of the electroweak

theory are very simple 3 “degrees of freedom” If MW, MZ, and em are known, many observables are

predicted Expected accuracy a few %

Higher order effects involve unseen states Top quark, Higgs boson Predictions are a function of the variables MT and MH Fit observables to obtain the unknowns

Cross sections, angular distributions, forward-backward asymmetries

January 9, 2004Page 6

How well does it work? The observed cross section

Extraction of properties of Z and W bosons

January 9, 2004Page 7

Properties of the Z Fit to resonance gives mass and width

January 9, 2004Page 8

Properties of the Z Quality of the measurements?

January 9, 2004Page 9

Properties of the Z Mass results

January 9, 2004Page 10

Properties of the W boson Cross section and mass determinations

January 9, 2004Page 11

Properties of the W boson W boson mass

January 9, 2004Page 12

Other observables

January 9, 2004Page 13

Determination of Top Quark Mass

Compare with direct measurements

January 9, 2004Page 14

Determination of Higgs Boson Mass

Fit gives MH = 81 +52/-33 GeV MH < 193 GeV (95% CL)

Limit from direct search MH > 114 GeV (95% CL)

Overall 114 < MH < 193 GeV

January 9, 2004Page 15

The end of the LEP Era

OPAL finished data taking in fall 2000 1000 pb-1 of data collected LEP collider was pushed to the highest

possible energy To ECM ~ 207 GeV

Many high precision measurements The electroweak model works remarkably

well Prediction of the Higgs mass No direct observation of the Higgs

January 9, 2004Page 16

What’s the Next Step? The CERN Large Hadron Collider (LHC)

New project approved in 1995 Proton-proton collider

ECM = 14 TeV (7000 + 7000 GeV) Actually this is for protons Each internal quark has only a fraction of this energy Effective ECM for qq collisions is ~ 14/3=4.7 TeV

– Factor of ~20 higher than LEP Cross sections fall like 1/E2

– Luminosity of collider must be 400 times larger than LEP Design luminosity 1034/cm2/sec

Expected to start operation in ~ 3 years The Chicago group is part of the ATLAS experiment for

this facility

January 9, 2004Page 17

What about Fermilab? Fermilab collider has ECM for proton-

antiproton collisions of 2 TeV ECM for qq collisions of 0.7 TeV Luminosity ~ 1031/cm2/sec

A little low

It is operating NOW

January 9, 2004Page 18

What is the LHC? Fill the 27 km circumference LEP tunnel

with superconducting magnets 1200 14-m-long dipole magnets with 8.3T

field

January 9, 2004Page 19

Where is it? Just outside Geneva

January 9, 2004Page 20

What is the LHC? Build new state-of-the-art detectors

ATLAS and CMS for high PT physics Also ALICE for heavy ion physics

Search for quark-gluon plasma Also LHCb for studying the physics of the b

quark VERY large data samples

Chicago has been working on the ATLAS experiment since 1995 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? Decays very rapidly

Observe decay products and calculate the mass of their parent

H decays to heaviest states accessible Specific modes depend on mass

€

H → γγ

Low probability but excellent mass resolution Can see signal as a narrow peak above background

Important for 80 < MH < 120 GeV Requires excellent EM calorimeter for

energy

January 9, 2004Page 24

How to see the Higgs?

Signal shown corresponds to integrated luminosity of 100 fb-1

1 year at design luminosity (but first year only 10 fb-1) Peak corresponds to mass resolution of ~ 2%

January 9, 2004Page 25

How to see the Higgs?

Important for 120 < MH < 170 GeV One W could be “off shell”

are not detected but appear as “missing energy”

Requires good ability to detect e and Requires good calorimeter to see “missing

energy” No distinctive mass peak

Broad excess of events over background

€

H 0 → W +W − → l +νl −ν

January 9, 2004Page 26

How to see the Higgs?

) cos-(1 Ep 2 m missT

TT ϕΔ= ll

mH = 160 GeV ATLAS

qqH qqWW qq e

qqH qq qqe +X

January 9, 2004Page 27

How to see the Higgs?

This is the “golden” mode All final state particles directly detected with

good resolution Narrow mass peak

Important for 150 < MH < 700 GeV

€

H 0 → Z 0Z 0 → l + l −l + l −

January 9, 2004Page 28

How to see the Higgs?

Signal for 300 GeV Higgs with 10 fb-1 of luminosity First year of operation

January 9, 2004Page 29

Charged Lepton DetectionMuon 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? Must be sensitive to many decay modes

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 EM showers develop in 1.5-mm Pb sheets Ionization sampled with liquid Argon layers

Hadron calorimeter measures energy of quarks and gluons Hadronic showers develop in 4-mm Fe plates Ionization sampled with plastic scintillator

and photomultiplier tubes

January 9, 2004Page 36

CalorimetryHad Tiles

Had LAr

EM LAr

Forward LArSolenoid

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 LArSolenoid

Barrel cryostat

End-cap cryostat

Chicago involved hereWith signal processing electronicsWith 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

Electronics drawer in each module carries photomultiplier tubes and electronics Each cell of calorimeter viewed by 2

phototubes Electronics needs very wide dynamic range

(65,000:1)

January 9, 2004Page 42

TileCal Electronics

January 9, 2004Page 43

Current Chicago Activity Assembling calorimeter

Surface assembly this year Start underground installation 2004

Integrating electronics systems Calibration, control, signal processing

Preparing software to process the data Especially for Tile Calorimeter Studying physics signals Developing methods for in-situ calibration

Using physics signals

January 9, 2004Page 44

Opportunities for Students In the past mainly undergraduates

Assisting with construction work Getting research experience Senior theses on ATLAS physics

PhD theses need to be on publishable physics In past years has been a bit too long before data

We expect to start data taking in 2007

Now excellent time to get involved in commissioning the detector and developing analysis software