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Future e+e- Collider Sensitivity to the
HZ Coupling
M. Dubinin, H.J. Schreiber and A. Vologdin
Standard Model-like Higgs couplings to
fermions
studied
W, Z boson
and
in
detail
trilinear Higgs self-coupling
Hgg, H and HZ vertices
- another important set of couplings
induced by loops of heavy particles
So far,
Hgg in e+e- collisions and at LHC investigated
H either in e+e- interactions and at LHC
investigated
or  fusion process   H  X
H. Jürgen Schreiber, ECFA/DESY Prague, Nov. 2002
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We explore the potential of linear e+e- collider
 HZ coupling through rare decay H  Z
Reaction (2-to-4 particle reaction):
e  e    e  e Z
 H signal events (H  Z) are imbedded
 account for complete irreducible background
Assuming
MH = 120, 140, 160 GeV
at
s 500 GeV and
 Ldt = 1 ab
-1
Event generation: CompHEP
PYTHIA (for some reducible
background reactions)
Higgs boson width and H  Z branching fractions
imported from HDECAY
H. Jürgen Schreiber, ECFA/DESY Prague, Nov. 2002
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Typically, we expect
~ 100 H  Z signal events
and
~ 50.000 irreducible background events
 principal cuts (at generation level)
E  10 GeV
5o    175o
96 %
Higgs events
survive
 ~ 50 % background discarded
Even after these cuts
small H event rate vs. huge irreducible background
and
background events have similar final state signatures
as H  Z events
So far, Z  qq
 signal event topology:
2 hadronic jets orginating
from the Z boson together
with an energetic photon
and large missing energy
H. Jürgen Schreiber, ECFA/DESY Prague, Nov. 2002
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Possible reducible background
ee 
WW ( )
eW ( )
WWZ ( )
millions
of
events;
small part
might mimic
ZZZ ( )
the signal
e  e  Z ( )
tt ( )
 tt ( ), WWZ ( ), ZZZ ( )
could be effectively removed
by selection procedures applied
 some surviving contributions
from
e  e   WW ( )
 e W ( )
 e  e  ( * / Z ) ( )
which are included in the final M(Z)
and in
BF(HZ) / BF (H  Z)
H. Jürgen Schreiber, ECFA/DESY Prague, Nov. 2002
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Large background and tiny signal event rates
 different strateties for extracting signal events
 consecutive cuts
 jet finder(s)
„Higgs likeness“
Independent of the selection technique chosen
 event (quality) cuts:
- Evis < 240 GeV
- ET < 210 GeV
- # of charged particles > 3
- missing mass: 180 - 400 GeV
ensures clean elimination of
e  e   HZ  ( Z ) ( )
 jet (quality) cuts:
• Ejet > 8 GeV
• |cos jet| < 0.95
• (between 2 jets) > 20o
• # of particles/jet > 3
( M qq ~ M Z )
H. Jürgen Schreiber, ECFA/DESY Prague, Nov. 2002
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Event selection using consecutive cuts

e+
H
Z
H

e-
Out of all photons
• E > 20 GeV with ET > 15 GeV
• no further particle in a cone of
half-angle 10o
• if more than 1 photon, the one
with largest energy is selected
• all remaining particles
 2 jets
• Mjj  MZ (84 - 105 GeV)
Finally, M(Z)
• Higgs signals clearly established
• accounting for all Z decays and irreducible and
reducible background (~ 17 % of the irreducible)
 
and uncertainty of  ( e e  e e H )
B( H  Z )
 52 % (23 %, 41 %)
B( H  Z )
for MH = 120 (140, 160) GeV
H. Jürgen Schreiber, ECFA/DESY Prague, Nov. 2002
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Event selection using jet finders
All particles reconstructed by SIMDET were passed
through a jet cluster algorithm
aim:
isolate  from H  Z;
one of the jets compatible with an
isolated high-energy photon and
all remaining jets should be
compatible with Z
Two jet finder:
PUCLUS (PYTHIA)
Results very similar;
DURHAM (ycut = 0.004)
PUCLUS somewhat
less reducible
background
Resulting M(Z)|PUCLUS
• Higgs signal clearly visible
•
B( H Z )
 61 % (28 %, 52 %)
B( H  Z )
for MH = 120 (140, 160) GeV
H. Jürgen Schreiber, ECFA/DESY Prague, Nov. 2002
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Event selection by ‚Higgs-likeness‘
Probability PH is constructed giving a measure that an
event is part of the signal
PH : [0, 1 ]
background events have preferably low PH
signal events PH close to 1
Variables taken into account:
E() , polar cms() , polar cms (Z)
-decay angels in H rest frame
angles between (, Z)|cms
collinearity  (beam, )
coplanarity  (beam, , Z)
coplanarity  (beam, H, )
transverse mass (, missing system)
transverse mass (H, missing system)
Finally
PH > 0.98
H. Jürgen Schreiber, ECFA/DESY Prague, Nov. 2002
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• Higgs bosons clearly established
• ‚Higgs-likeness‘ method
 best signal-to-background ratios
(due to best reducible background rejection)
• BF ( H  Z )
 44 % (23 %, 39 %)
BF ( H  Z )
for MH = 120 (140, 160) GeV
Significant improvement by longitudinally
polarized beams:
e.g.
e- polarisation:
- 80 %
e+ polarisation:
+ 60 %
(ambitious)
BF ( H  Z )
 26 % (15 %, 23 %)
BF ( H  Z )
( H  Z )
( H  Z )
 g HZ
g HZ
 27 % (16 %, 24 %)
if tot (H) has an error of 5 %
 14 % (8 %, 12 %)
H. Jürgen Schreiber, ECFA/DESY Prague, Nov. 2002
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•Overlap of   hadrons to H signal events after
passing selection procedures
- no loss or distortions
• Inclusion of leptonic Z decays, Z 
ll ,
or Z Z fusion events,
neglected so far
 only slight
improvement on
BF ( H  Z )
BF ( H  Z )
• Due to our present knowledge, Higgs-strahlung
e  e   HZ  ( Z ) Z
including huge reducible and irreducible backgrounds
at e.g. s  300 GeV
 less precision on BF (H  Z )
• Completely open whether full luminosity of 1 ab-1
at s = 500 GeV with desired beam plarisation is
delivered to this measurement
 lower bounds on
g HZy
g HZy
H. Jürgen Schreiber, ECFA/DESY Prague, Nov. 2002
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