The Search for the Higgs at the Fermilab Tevatron:

1
The Search for the Higgs at the Fermilab
Tevatron Run 2
Higgs
Gordon Watts University of Washington,
Seattle May 10, 2001
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The Run 2b Series
UW is going to collaborate with D0 in building
the inner most two layers of Si for the Run 2b
detector
Series of talks on that topic
Archived on the Web
http//d0.epe.phys.washington.edu/Projects/layer0
So Far
Tianchis excellent talks on construction,
operation, and history of Si Vertex Detectors
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The Standard Model
u
u
Electric Charge
d
ne e
nm m
nt t
proton
q 0
leptons
q -1
everyday matter
u d
c s
t b
q 2/3
quarks
q -1/3
Color Charge (3)

• Fundamental particles
• 3 families of fundamental fermions

All Seen, nt most recent discovery
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Force Mediators
The g and the gluon are massless.
W
80.436 0.037 GeV/c2
LEPII TeV Measurments
Z0
91.1875 0.0021 GeV/c2
Where does this mass come from?
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Boson Mass
Initially Massless (Gauge Bosons)
Quantum Corrections do generate some mass


etc.
Goldstone Bosons
BUT
Predicted Mass of the Z is of MeV order!
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Generating Mass
Two common ways to generate mass in the SM
Strong-Interactions between new fermions
Adds terms to the correct calculation
Considered less likely
Weakly Interacting, self coupling scalar field
The Standard Model incorporates the second
The minimum additions
The Higgs

• Doublet of two scalars
• Vacuum expectation energy key contribution to
  W,Z Mass calculations
• One degree of freedom left over

MH is the free parameter
The MZ is now very close (91 GeV/c2)
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Current Indirect Results
Even without discovery we know quite a bit about
the Higgs
The Higgs Mass indirectly affects the measured
top and W masses
With LEP, dW 34 MeV/c2
CERN EWWG
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Direct Searches
Discovery only when direct evidence seen
Search Strategy depends on
Higgs Mass
Decay products depend upon the mass
Determines backgrounds
Collider Environment
Background production!
Production channel will determine background
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SM Higgs Decay
Know Higgs Mass
Know Branching Ratios
Excluded by LEP
Character of decay is changing rapidly in
Tevatrons Search Region
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SM Higgs Decay Modes
Topology of decay is changing rapidly
MH 110 GeV/c2
MH 130 GeV/c2
Higgs/SUSY Working group Report
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At LEP
Production
The L3 events we are waiting on
ee- HZ
ee- WWnene Hnene
Search Channels
In their mass region, H bb almost 80.
Defined by the Z decay
4 jet (qqbb), tt-bb, ee-bb, etc.
Backgrounds
Detector backgrounds, QCD production, ll-qq, qqgg
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LEP Direct Search Results
?
ee-HZ
ALEPH 4j, 2jmET, 2j2l Excess 3.9s xsec to high
by 2s
Combined LEP Result Excess of 3s
No Discovery But tantalizing hints!
Lower limit is 113.5 GeV/c2
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Direct Searches at the TeV
Light Higgs
SM Higgs Production

• QCD 2j background
• Associated HW, HZ production

Heavier Higgs

• WW (SM)
• Use gluon fusion!

Search Strategy
bb decay MH lt 120
WW decay MH gt 130
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Background Comparison
Events in one fb-1
Light Higgs
ggHbb 2 jet topology
Swamped by QCD di-jet background!
ZH,WH llbb, nnbb, lnbb, jjbb
QCD 4-jet background will make jjbb difficult!
Heavy Higgs
WW 2 lepton mET, lepton mET 2 jet, 4 jet
lepton mET 2 jet has large ttbar, WZj
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The Analysis
(light Higgs)
Raw Data
ZH,WH llbb, nnbb, lnbb, jjbb
MC Signal Background
WZZZ lljj
tt lnbbqq
WbbZbb lnbb, llbb
Trigger Isolated Lepton
QCD bb bbl (fake or b)
Same Process
Event Selection High PT Lepton, W
B Jet ID Low High Quality
JJ Mass Plot
Compare with statistical tools, look for the mass
bump, mass windows, etc.
JJ Mass Remove QCD BG
Data JJ Mass Plot
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Run 2 at the Tevatron
New Tracking, EM ID, readout
Larger pbar bunches
x10 Larger Instantaneous Luminosity
Energy now at Ös 1.98 TeV
20 Increase in associated H production
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Run 2 has started
(March 1st, 2001)
Collisions!
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Run 2 Detector Upgrades

• Both CDF D0 have had extensive upgrades
• Quick Scan of the final states
• B-quark tagging
• Increased coverage, layers of Si
• 2jet mass resolution
• Preshower, tracking, larger samples for jet
  energy calibration
• Lepton ID
• Increased coverage
• Missing ET
• No large improvements expected

What can you Expect?
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Current Upgrade Status At the TeV

• Run 2 has started
• Detectors are mechanically complete
• Still installing and building electronics
• Had a store of 36x36 (bunches).
• Final configuration.
• We have seen tracks!
• Focus is now on improving the luminosity

Radiation Losses At D0 several weeks ago
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Tracks!
Si Only
xy view
20 of our Si tracker cabled
Andy Haas Elastic Tracker
Tracks whole detector at once
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Combined Track
First 36x36 Store
Run 119679 Event 238687
Fiber Tracker
Silicon Tracker
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Run 2 Predictions

• Predictions from a series of Run 2 workshops
• EW, Top, Higgs/SUSY
• Work of almost 100 people
• Monte Carlos used were not Run 2 detector
  simulators
• Parameterized MC
• Next Task for CDF D0
• Bring up experiments
• Calibrate detector simulations with real data
• Start searches
• Repeat efficiency acceptance studies
• Invent new techniques!
• Higgs/SUSY was optimistic already

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Run 2 Predictions

• Detector efficiencies were extrapolated from Run
  1
• Increased detector coverage was accounted for
• 3d tracking where we once had 2d (b-tagging)
• Average Calorimeter/jet resolution for the two
  detectors (D0 is better)
• Some assumptions are optimistic (jj resolution).
• No careful accounting for Multiple Interactions
• Calculate efficiencies, acceptances
• Luminosity required for discovery, 95 CL limit,
  etc.

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Jet-Jet Resolution
Crucial for Light Higgs Discovery

• Better Energy Scale
• Better Jet Algorithms
• m corrections
• A lot of work to do

Higgs/SUSY Report
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B Jet Tagging

• Displaced tracks are most powerful indicator
• Use likelihood and NN to combine other event
  features
• Jet PT
• Track charge/multiplicity
• Width
• Relative PTs/SLT

Primary
Secondary
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B Tagging
Recent BID review Got tagging rate Background
rate needs work
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Backgrounds
Effort underway to calculate background rates to
NLO

• No QCD background
• No detector effect

Ellis, Cambel, Veseli hep-ph/0006304
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SM By Channel

• Two Search Stradegies

Put it together
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SM Higgs Search
(2006)
(2003)
The two experiments combined
Does not include ttH.
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Detector Effects
Achieving this will require a long-term effort
Accelerator will change over the course of time
L1 L2 will die
Due to construction, and length of shutdown,
faster/cheaper to replace whole detector!
Consequences

• Dead Silicon (radiation damage)
• Large effort underway to replace

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Instantaneous Luminosity
Steady increase in delivered luminosity
Instantaneous luminosity increases
Tevatron Collides bunches
Number of minimum bias events
Can separate in z

• Increase bunch size
• Increase of bunches

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Multiple Interactions
Tevatron Collides bunches
5E32
1 Hard Scatter (Higgs production)
Number of minimum bias events
Can separate in z
LHC
Confusion in the tracker, trigger
Better Tracking (pitch, etc.)
Beam Tilt in Z
Can simulate a displaced vertex!
3D Tracking
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Conclusions

• The Higgs Search is a long term one
• It will take us 2 years to rule out MH 115
  GeV/c2 at 95 CL.
• 5s discover of 115 GeV/c2 is just within our
  reach, according to study
• Will have things to say about MSSM
• Light sparticle searches
• Indirect measurements
• Pushing edge of promised luminosity
• Will press for more luminosity
• Will look for improved experimental techniques!

First Conference Results Spring 2002
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Run 2 Indirect Results
With 2 fb-1 of data
(2 years)
dMW 40 MeV/c2 dMt 2.5 GeV/c2
(per experiment)
dMH 80 MH

• Top will not by statistics limited
• Large Top dilepton samples
• Large W statistics
• Split W samples to study systematics
• Better detectors -gt better energy scale
  measurement

2-3 Years
(continuous)