Transcript SEARCHES FOR THE HIGGS BOSON AT THE TEVATRON Daniela Bortoletto Purdue University D. Bortoletto Moriond QCD.

SEARCHES FOR THE HIGGS BOSON
AT THE TEVATRON
Daniela Bortoletto
Purdue University
D. Bortoletto
Moriond QCD
1
THE STANDARD MODEL HIGGS
SM unifies
weak
and
electro-magnetic
interactions
Experimentally:
weak gauge bosons
are massive
 EWK symmetry
breaking
BEH
mechanism
●
●
●
●
Finding the Higgs boson is essential to confirm the validity of the BEH
mechanism
The search is difficult since mH is not predicted in SM
Since the Higgs decays very quickly (10-24 s)  it can be observed only
through its decays into other particles
The Higgs couples to mass and decays preferentially to the heaviest objects
kinematically allowed
D. Bortoletto
Moriond QCD
2
Higgs boson phenomenology
 Higgs decay modes and searches in 1975:
D. Bortoletto
D. Bortoletto, RPM,
Moriond
Berkeley
QCD
3
THE TEVATRON
 Proton-antiproton collider with 1.96 TeV center-of-mass energy
 396 ns between bunches
Peak instantaneous luminosity
L=4.31 1032 cm-2 sec-1
End of operation September 2011
Fermilab
Tevatron
CDF
1 km
D0
≈ 12 fb-1 delivered
≈ 10 fb-1 acquired by the experiments
D. Bortoletto
Moriond QCD
4
HIGGS PRODUCTION AND DECAY
 Four main production mechanisms at hadron colliders
ggH
-
qqZH
qqWH
135
100
GeV LOW GeV
MASS
s  1.96GeV
qq'qq' H (VBF)
s  7TeV
HIGH MASS
s  14TeV
1 TeV
Branching fraction too
small for discovery at the
TEVATRON
D. Bortoletto
Moriond QCD
5
The Higgs challenge S/B
• Many of the background
processes have cross section
orders of magnitude larger
than the Higgs
Potential Higgs signal is TINY

Maximize signal acceptance

Excellent modeling of
background processes

Use multivariate analysis
techniques (MVA) to fully
exploit all kinematic differences
H ®WW
W
Z
Wγ Zγ WW tt WZ t
ZZ
Expect 167 SM Higgs events (reconstructed and selected) and
~200,000 events from SM backgrounds for mH=125 GeV/c2
D. Bortoletto
Moriond QCD
6
Main Higgs channels at the Tevatron
Low Mass MH < 135 GeV/c2
ZHllbb
Select:
WHlnbb
 0,1,2 leptons and/or missing Et
 Two or three high Et jets
Strategy:



ZHnnbb
WHl)nbb
Maximize lepton reconstruction and selection efficiencies
Maximize efficiency for tagging b-quark jets
Optimize dijet mass resolution
High Mass MH > 135 GeV/c2
• Main channel: ggH WW which is
also important at low mass
High PT leptons and Missing transverse energy
D. Bortoletto
Moriond QCD
77
Tevatron Higgs searches
Total
WH  l n bb
H  WWl n l n
ZH nnbb
ZH  llbb
D. Bortoletto
Moriond QCD
8
Higgs analyses strategies
Select data
sample
Apply loose
selections
Signal region
Verify modeling of
background
Control
regions
Multivariate techniques
Improve S/B
Improve S/B
Channel 1
Template 1
Template 2
Channel 2
…….
Separate into
channels based on S/B
Limits or signal
significance
D. Bortoletto
Moriond QCD
…….
Systematics and
correlations
9
Improvements since summer 2011
 Both experiment are:
 Validating the Higgs search
techniques in WZ/ZZ→ X + bb
searches (talks on Thursday)
Light Jets
b-jets
 Cross section is ~5 times higher
 Using 25% more luminosity in
many analysis
 New techniques, improved MVA
and modeling to increase the
sensitivity
 Additional triggers and leptons
HOBIT
SECVT
X
 CDF
 New multivariate b-tagger
optimized for H bb jets (HOBIT)
with ~20% more acceptance
D. Bortoletto
mistag
rate
SecVtx
efficiency
HOBIT
efficiency
~1%
39%
54%
Moriond QCD
10
ZHllbb
• MVA Improvements
• Many backgrounds processes are
present the llbb selection
• The individual processes have
different kinematics
• We utilize the three expert
networks to assign events to
distinct regions in the final event
discriminant used in the
extraction of upper limits.
tt-like
Z+qq - like
WZ, ZZ - like
D. Bortoletto
other
other
ZH - like
Moriond QCD
11
ZHllbb
tt
WZ,
ZZ
Z+
qq
Identify
events
with
enhanced
S/B
ZH
s/b=1/1
Tagged
events
Is the
event
tt-like?
YES
Region 1
D. Bortoletto
NO
Z+qq
like?
YES
Region 2
NO
WZ/ZZ
like?
YES
NO
Region 4
Region 3
Moriond QCD
12
MET+bb
 50% of signal is
fromWH with lost
leptons
Increasing purity
Medium b
Tight b
s/ b=0.3%
s/ b=1.5%
• Add together b-tagger outputs for both
jets
• Cut on the sum instead of per jet cuts

Use Missing pTTRK to suppress
multijet background
 Exclude isolated tracks from
Missing pTTRK to improve WH
acceptance by 10%
D. Bortoletto
25% improvement in
sensitivity expected from
additional data: 6%
Moriond QCD
13
Met +bb


Signal
mass
resolutio
n
Improve jet energy resolution with
Neural network which correlates jetrelated variables and returns most
probable jet energy based on bottom
quark hypothesis
Jet energy is currently used only to
determined corrected MET. Selection
improves S/B separation
b-targeted
corrections
Multijet
S/B=1/5
Higgs
Analysis does not yet use HOBIT.
Further improvements expected
D. Bortoletto
Moriond QCD
14
Limits for Hbb
Limits at MH = 115 GeV:
Exp: 1.71 x σ(SM)
Obs: 1.79 x σ(SM)
Limits at MH = 125
GeV:
Exp: 2.49 x σ(SM)
Obs: 3.29 x σ(SM)
TEVATRON
CDF
MH=135
Channel
Local Pvalue
Global Pvalue
H->bb
2.9σ
2.7σ
D. Bortoletto
Broad excess observed in H→bb
Largest Excess: 135 GeV
LEE of 2 for range from 100 to 150 GeV/c2
Moriond QCD
15
Limits for Hbb
Limits at MH = 115 GeV:
Exp: 1.71 x σ(SM)
Obs: 1.79 x σ(SM)
Limits at MH = 125 GeV:
Exp: 2.49 x σ(SM)
Obs: 3.29 x σ(SM)
TEVATRON
CDF
Channel
Local Pvalue
Global Pvalue
MH=135
H->bb
2.9σ
2.7σ
Broad excess observed in H→bb
Largest Excess: 135 GeV
LEE of 2D.for
range from 100 to 150 GeV/c2
Bortoletto
Moriond QCD
16
Tevatron combination: WZ and ZZ



same final state
same set of tagged events
different MVA optimized for WZ and ZZ events
W/Z+Z→bb: σobs = (1.01 ± 0.21) x σSM
D. Bortoletto
Moriond QCD
17
TEVATRON COMBINATION SM HIGGS
TEVATRON 95% C.L. upper limits on SM Higgs boson production
− Expected exclusion: 100 < MH < 120 GeV, 141 < MH < 184 GeV
− Observed exclusion: 100 < MH < 106 GeV, 147 < MH < 179 GeV
D. Bortoletto
Moriond QCD
18
MH=125 GeV
MH=125 GeV
High s/b region
MH=125 GeV
Log 10(S/B)
Right-to-left
Integral of S/B distribution
MH=165 GeV
Fits to data, with background
subtraction
MH=165 GeV
High s/b region
MH=165 GeV
D. Bortoletto
Moriond QCD
19
The excess

•
Simple overlay of H→bb signal
prediction for the dijet invariant
mass (MH = 120 GeV)
 Data and diboson prediction
from Tevatron low mass
WZ/ZZ measurement
 Additional signal is not
incompatible
Local p-value distribution for
background only expectation
• Minimum local p-value: 2.7
standard deviations
• Global p-value with LEE
factor of 4 range from 100
to 200 GeV/c2 : 2.2
standard deviations
D. Bortoletto
Moriond QCD
20
The excess

•
Simple overlay of H→bb signal
prediction for the dijet invariant
mass (MH = 120 GeV)
 Data and diboson prediction
from Tevatron low mass
WZ/ZZ measurement
 Additional signal is not
incompatible
Local p-value distribution for
background only expectation
• Minimum local p-value: 2.7
standard deviations
• Global p-value with LEE
factor of 4 range from 100
to 200 GeV/c2 : 2.2
standard deviations
D. Bortoletto
Moriond QCD
21
Conclusions
Thank you to Michelle Stancari, Joe Haley, Homer Wolfe,
Satish Desai, Wade Fisher, Tom Junk, Eric James, Karolos
Potamianons, Quiguna Liu, and many others
 Tevatron experiments are now
analyzing full data set in most
channels
 More improvements are
expected in the near future
 The data appears to be
incompatible with the
background, with a global
P-value of 2.2 s.d. ( 2.7 local )

H→bb only: 2.6 s.d. ( 2.8 local )
 Higgs mass range of
115 < MH < 135 continues to
be very interesting
 Let us hope that 2012 is the year of the Higgs boson
 For additional details see
 Tevatron: http://tevnphwg.fnal.gov/results/SM_Higgs_Winter_12/
 CDF: http://wwwcdf.fnal.gov/physics/new/hdg/Results.html
 D0: http://wwwd0.fnal.gov/Run2Physics/WWW/results/higgs.html
D. Bortoletto
Moriond QCD
22
BACKUP
D. Bortoletto
Moriond QCD
23
CONSTRAINTS ON THE HIGGS
• SM parameters ( MW , Mt , Z pole
measurements etc)
New CDF 2012 W mass
MW = 80387 ± 12 stat ± 15 syst MeV/c2
New World Average
MW = 80390 ± 16 MeV/c2
MH<145 GeV @ 95% CL
MH = 90+29-23 GeV
• Many direct searches at the
Large Electron Positron
Collider, TEVATRON proton
anti-proton collider, nd the
LHC
D. Bortoletto
Exclusions of MH:
− LEP < 114 GeV (arXiv:0602042v1)
− Tevatron [156,177] GeV ( arXiv:1107.5518)
− LHC [~127, 600] GeV
arXiv:1202.1408 (ATLAS)
arXiv:1202.1488 (CMS)
Moriond QCD
24
Modeling
llbb final discriminant in the pretag region
which is background dominated
D. Bortoletto
Moriond QCD
25
H → WW
Limits at MH = 125 GeV:
Exp: 3.14 x σ(SM)
Obs: 3.50 x σ(SM)
Limits at MH = 125 GeV:
Exp: 3 x σ(SM)
Obs: 3 x σ(SM)
D. Bortoletto
Moriond QCD
26
Limits for H->WW
Final states: ee, μμ and eμ
• Exploit spin correlations to control
backgrounds
• Z → ll is major background for ee and μμ
channels
• Use Boosted Decision Trees to control
backgrounds from Z → ee, μμ
• Signal and background composition vary
with jet multiplicity
• Consider multiple signals: Gluon fusion,
Vector boson fusion,H → ZZ...
D. Bortoletto
Moriond QCD
27
CDF and D0 Individual results
Winter
2012
Summer
2011
D. Bortoletto
Moriond QCD
28
ZHnnbb



21% additional luminosity
Small improvements in background rejection
Limits show same basic behavior with 0.5 to 1.0σ increases in
significance of excess
Summer 2011
D. Bortoletto
Winter 2012
Moriond QCD
29
D. Bortoletto
Moriond QCD
30
WHlnbb




26% (69%) additional luminosity for 2-jet (3-jet) channels
5-10% level lepton acceptance/trigger efficiency improvements
New HOBIT b-tagger equivalent to adding another 20% in additional luminosity
Limits show same basic behavior with 1.0 to 1.5σ increases in significance of excess
Summer 2011
D. Bortoletto
Winter 2012
Moriond QCD
31
ZHllbb




23% additional luminosity
More gain from HOBIT in this analysis than WH (original tagging not as
sophisticated)
56% of data events in current analysis were not included in previous analysis!
37% sensitivity improvement (4.67 2.95 at mH=120 GeV/c2)
Summer 2011
D. Bortoletto
Winter 2012
Moriond QCD
32
32
ZHllbb


Electron channels
Here we observe a significant change
Summer 2011
D. Bortoletto
Winter 2012
Moriond QCD
33
ZHllbb

ZHllbb channel has . . .
lowest backgrounds
 smallest expected
signal yields (9 events
for mH=120 GeV/c2)
Some discriminant bins
with large S/B
 Low probability for
observing events in
these bins
 A few such events can
have substantial
effects on observed
limits


D. Bortoletto
S = 0.16 events,
B= 0.06 events
Moriond QCD
34
34
H → WW



18% additional data
Small signal acceptance improvements (0.1 < ΔRll < 0.2)
No appreciable change in behavior of limits
Summer 2011
D. Bortoletto
Winter 2012
Moriond QCD
35
H->ZZ
D. Bortoletto
Moriond QCD
36
D. Bortoletto
Moriond QCD
37
D. Bortoletto
Moriond QCD
38
Measurement of WZ and ZZ
WZ and ZZ events
 same final state
 same set of tagged events
 different MVA optimized for WZ and ZZ events
s(WZ+ZZ)= 4.08 ± 1.32 pb
Significance 3.2σ
D. Bortoletto
s(WZ+ZZ)= 5.0±1.0±1.3 pb
Significance: 3.3σ
s(WZ+ZZ): Theory= 4.4±0.3
pb
Moriond QCD
39