Transcript Document 7246954

Moriond QCD – March 16th, 2005
Searches for the SM Higgs
Boson at the Tevatron Collider
Tommaso Dorigo, University of Padova and INFN
Representing the CDF and D0 Collaborations
Contents:
Introduction to Run 2 at the Tevatron
Higgs Sensitivity WG predictions
Hbb decay searches
HWW decay searches
Combined limits & conclusions
Tevatron Run 2 – Quick Overview
Tevatron is performing well – delivered 800
pb-1 so far. Lstart above 1032 now common.
L has been following design curve!
Upgrades continuing – electron cooling of
antiprotons is critical.
As L increases, CDF and D0 catching up by
modifying trigger tables, improving DAQ
Design curve means 8 fb-1 by 2009!
Total Luminosity (fb-1)
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WE ARE HERE
Design
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6
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Base
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2
1
0
10/1/03
9/30/04
9/30/05
9/30/06
9/30/07
9/29/08
9/29/09
SM Higgs: Production and Decay
At the Tevatron, about five 120 GeV Higgs
bosons are produced in a typical day of
running (will be 15/day in two years).
Even the WHWWW(*)
process is promising
despite the low yield, due
to the striking signature of
missing Et plus three
leptons, two of which may
be of the same charge
but different flavor.
Excluded
Direct production occurs mostly via gluongluon fusion diagrams.
Associated production through a virtual W or Z
boson provides sensitivity in the region where
LHC will have more trouble. At higher mass,
the WW(*) final state becomes dominant.
mH (GeV/c2)
e
q
W*
l
W
n
H
q
b
b
What we know
about the Higgs
• Although they did not directly observe it,
the LEP experiments have collected a
wealth of information on the Higgs boson
through comparisons of EW observables
to EW theory + radiative corrections
• From theory we know its couplings, its
decay modes, and how its mass impacts
the W and top masses.
• If it exists, then we know its mass with
about 60 GeV accuracy, and the direct
search limit already cuts away a large part
of the allowed mass region
• Latest LEP results: MH=126+73-48 GeV,
MH<280 GeV @ 95% CL (Winter ‘05).
Higgs Sensitivity WG Predictions
In 2003 the Tevatron chances for
Higgs discovery were re-evaluated
CDF
Lum (fb-1)
Idea: with available data and operating
detectors, can better assess Tevatron
reach
Surprisingly, the new results meet or
exceed 1998 Susy/Higgs WG ones.
DESIGN
BASE
Keys to success:
-mass resolution improvements;
- optimized b-tagging;
- shape information vs counting.
Identification of High Pt Leptons
Most final states produced by Higgs decay involve
high-Pt leptons. CDF and D0 have efficient lepton
triggers and high purity ID selections
A host of electroweak measurements is being
produced at the Tevatron.
CDF
Tau leptons are also starting to contribute appreciably
to precision measurements (but no results for SM H
searches with these yet).
D0
CDF
Tagging b-jets
Identifying b-jets is of paramount importance
for low-mass Higgs boson searches.
Three methods are well-tested and used:
– Soft lepton tagging
– Secondary vertex tagging
– Jet Probability tagging
For double tag searches, efficiency factors
get squared! To retain signal, both CDF/D0
have loose and tight tagging options
Efficiency drops at low jet Et and high
rapidity but is 45-50% for central b-jets from
Higgs decay
Mistag rates are kept typically at 0.5%
SV tagging: tracks with
significant IP are used in a
iterative fit to identify the
secondary vertex inside
the jet
D0
Tight/loose SV tag eff.
CDF
I.P.
B
Can we see dijet resonances
if they are there?
A low mass Higgs search entails believing that we can:
- appropriately reconstruct hadronically-decaying objects
- accurately understand our background shapes
All of that can be proven if we see the Zbb decay in our data.
The S/N is not higher than 1/5 at the most
in the signal region
– good testing ground for H!
– can use to test/improve dijet mass
resolution with advanced
algorithms
We barely saw it in Run 1…
Can we use it in Run 2 ??
CDF sees Zbb decays in Run 2!
Double b-tagged events with no
extra jets and a back-to-back
topology are the signal-enriched
sample: Et3<10 GeV, DF12>3
Among 85,784 selected events
CDF finds 3400±500 Zbb
decays
- signal size ok
- resolution as expected
- jet energy scale ok!
This is a proof that we are in
business with small S/N jet
resonances!
CDF expects to stringently
constrain the b-jet energy
scale with this dataset
Low Mass H Searches
The only chance to see Hbb at the Tevatron
is through associated production with bosons
ZHllbb is the cleanest signature, but it yields
too few events
W/ZHjjbb has the lowest S/N but the high BR
helps at larger Higgs mass
WHlnbb is next-to-best, but CDF was
“unlucky” in Run I
The best channel is ZHnnbb
CDF has a new combination of Run 1 results
with ZHllbb, nnbb channels.
They search events with two jets with DF<2.6,
missing Et>40 GeV, no isolated track with Pt>10 GeV.
The limit is obtained by a fit to the
mass distribution of b-tagged events.
The Run 1 CDF limit is now at 7.2 to 6.6 pb for
MH=110 to 130 GeV.
New search for WH in Run 2
To search for WHlnbb events
a detailed understanding of the
composition of the W+jets sample is mandatory.
In the 2-jet bin CDF finds 62 events with a b-tag,
where 66.5±9.0 are expected, mostly from
Wbb production and mistags.
A fit to the dijet mass
distribution allows to
extract a 95% CL limit
of 5 pb to SM
WH production.
The obtained limit
is consistent both
with a priori
predictions and with
expectations based
on HSWG results.
WH Search in D0
D0 also study their W+2jet bin with btagging in 174 pb-1 of high-Pt leptons from
Run 2 data.
They find 76 events with one b-tag (exp.
72.6±20.0), 6 with two b-tags (exp. 4.4±1.2).
The dijet mass distributions show no
anomaly with 1 b-tag. The 2-tag distribution
is divided in search windows to set limits to
Higgs production.
95%CL limits on
sWH*B(Hbb) are set
at 9-12 pb for
MH=115-135 GeV
By-product: a 95% CL
limit is set to Wbb
production (DR>0.75,
Pt>20 GeV) at 6.6 pb.
High Mass Searches: HWW(*)
The SM production of WW pairs has been
measured by CDF in Run 1 and by both
CDF and D0 in Run 2: excellent
agreement with NLO.
To search for Higgs boson decays, events
with two high-Pt leptons (e,m) and large
missing Et are selected; the tt background
is rejected with a jet veto.
Then both experiments use the
helicity-preferred alignment of
charged leptons in F to
discriminate known backgrounds.
n
W+
e+
n
W-
e-
CDF results on HWW
CDF searches for HWW events by selecting two
tight leptons (ee,em,mm) with Ete(Ptm)>20 GeV and
missing Et>25 GeV (50 GeV if DFll<20°).
A strict jet veto (Et<15 GeV if |h|<2.5) rejects top
candidates.
Finally, a small dilepton mass is required (Mll<5580 GeV for MH=140-180 GeV).
8 events are observed in 184 pb-1 of Run 2 data with
the Mll <80 GeV cut, with an expected background of
8.9±1.0.
A likelihood fit to the DFll distribution is performed to
extract a limit on the HWW cross section as a
function of its mass.
The result is sHWW*B(WWllnn)>5.6 pb for MH=160
GeV.
D0 Results on HWW
D0 also searches for HWW decays by selecting events with two
oppositely charged leptons (ee,em: Pt>12,8 GeV; mm: Pt>20,10 GeV),
missing Et>20 GeV (30 for mm), and imposing a loose jet veto
(Et<90 GeV, or Et1,Et2 <50,30 GeV).
The azimuthal angle DFll between the
two leptons is then required to be less
than 1.5 for electron pairs (2.0 for the
em,mm combinations).
Combining the three channels they
find 9 events, when 11.2±3.2 are
expected from background sources
in 177 pb-1 of Run 2 data.
They can thus exclude s*B>5.7 pb
at 95% C.L. for MH=160 GeV.
WHWWW(*) Search
CDF also searches for the striking signature
of three W bosons in 193.5 pb-1 of Run 2 data.
First, the dataset with a lepton with Pt>20 GeV
and a second with Pt>6 GeV of same charge
is analyzed and found in agreement with
expectations.
Then, optimized cuts are applied to the
second lepton (e.g. Pt>18 GeV for MH>160
GeV) and on the vector sum of leptons
transverse momenta (Ptll>35 GeV).
Zero events are observed, when 0.95±0.61±
0.18 are expected from known sources.
95% CL limits are thus set at 12 (8) pb for
MH=110 (160) GeV.
Putting it all together…
Summary and Outlook
The Higgs boson is being hunted at the Tevatron in all
advantageous search channels. D0 and CDF are competing –
that’s good! – but will soon start to also combine their results.
No surprises with the analyzed 200 pb-1 samples, but we have
already three times more data on tape to look at!
We are on track to supersede the LEP2 lower limit on MH in time
for Moriond QCD 2007!
By the end of 2009, the Tevatron might be able to see a MH=115
GeV Higgs at 5s, or exclude it all the way to 180 GeV.
…but that will require both cunning and the Tevatron delivering
according to the design plan!
What I feel I can promise at 95% CL: exclusion up to 135 GeV,
3s evidence at 115 GeV.
CDF and D0 in Run 2
The CDF Detector
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•
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CDF significantly upgraded from Run 1:
New L00+SVX+ISL silicon detector
New central tracker
Extended muon coverage to |h|<1.5
New end-plug calorimeters
SVT measures IP to 45 mm at Level 2!
The challenge is now a smooth
operation for many years of running…
The D0 Detector
Massively upgraded from Run 1 to
include:
• 77,000 ch scintillating fiber tracking
• 2.0 Tesla solenoid
• 800,000 channel silicon detector
(4 barrel layers, 2-sided disks)
• Extended muon coverage (MDT)
Tracker working well despite low
volume (R=1/3 RCDF)
High performance b-tag to |h|<2.0
Integrated Weekly Luminosity (pb-1)
60
400
stacktail bandwidth upgrade
Design
350
Base
Actual
Integrated Luminosity (pb-1)
Design
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40
300
electron cooling
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200
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Base
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10/1/03
50
0
10/01/03
12/31/03
03/31/04
06/30/04
9/30/04
9/30/05
09/29/04
Date
2005 : so far
along Design
9/30/06
9/30/07
9/29/08
9/29/09
Design
for 2005
Base
for 2005
2004
2003
2002
CDF: a
WWeenn
candidate
CDF: a
WWemnn
candidate