H. Jürgen Schreiber, ECFA/DESY Prague, Nov. 2002
Future e+e- Collider Sensitivity to the HZ Coupling
M. Dubinin, H.J. Schreiber and A. Vologdin
Standard Model-like Higgs couplings to
studied
in
detail
fermions
W, Z boson
and
trilinear Higgs self-coupling
- 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
Hgg, H and HZ vertices
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H. Jürgen Schreiber, ECFA/DESY Prague, Nov. 2002
We explore the potential of linear e+e- collider
HZ coupling through rare decay H Z
Reaction (2-to-4 particle reaction):
Zee ee
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
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H. Jürgen Schreiber, ECFA/DESY Prague, Nov. 2002
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 eventssurvive
~ 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, qqZ
2 hadronic jets orginating
from the Z boson together
with an energetic photon and large missing energy
signal event topology:
3
H. Jürgen Schreiber, ECFA/DESY Prague, Nov. 2002
Possible reducible background
ee
)(
)(
)(
)(
)(
)(
ZZZ
WWZ
tt
Zee
We
WW
millionsofevents;small part might mimic the signal
)(),(),( ZZZWWZttcould be effectively removedby selection procedures applied
some surviving contributions from
)()/(
)(
)(
*
Zee
We
WWee
which are included in the final M(Z) and in
BF(HZ) / BF (H Z)
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H. Jürgen Schreiber, ECFA/DESY Prague, Nov. 2002
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
)()( ZHZee
• Ejet > 8 GeV• |cos jet| < 0.95 (between 2 jets) > 20o
• # of particles/jet > 3
)~( Zqq MM
jet (quality) cuts:
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H. Jürgen Schreiber, ECFA/DESY Prague, Nov. 2002
Event selection using consecutive cuts
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 )( Hee ee
%)41%,23(%52)(
)(
ZHB
ZHB
for MH = 120 (140, 160) GeV
e+
e-
ZH
H
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H. Jürgen Schreiber, ECFA/DESY Prague, Nov. 2002
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 andall remaining jets should be compatible with Z
Two jet finder:
PUCLUS (PYTHIA)
DURHAM (ycut = 0.004)
Results very similar;
PUCLUS somewhat less reducible background
Resulting M(Z)|PUCLUS
• Higgs signal clearly visible
• %)52%,28(%61)(
)(
ZHB
ZHB
for MH = 120 (140, 160) GeV
Event selection using jet finders
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H. Jürgen Schreiber, ECFA/DESY Prague, Nov. 2002
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
Event selection by ‚Higgs-likeness‘
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H. Jürgen Schreiber, ECFA/DESY Prague, Nov. 2002
• Higgs bosons clearly established
• ‚Higgs-likeness‘ method
best signal-to-background ratios
(due to best reducible background rejection)
%)39%,23(%44)(
)(
ZHBF
ZHBF•
for MH = 120 (140, 160) GeV
Significant improvement by longitudinally
polarized beams:
e.g. e- polarisation: - 80 %
e+ polarisation: + 60 % (ambitious)
%)23%,15(%26)(
)(
ZHBF
ZHBF
%)24%,16(%27)(
)(
ZH
ZH
if tot (H) has an error of 5 %
%)12%,8(%14
HZ
HZ
g
g
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H. Jürgen Schreiber, ECFA/DESY Prague, Nov. 2002
•Overlap of hadrons to H signal events after passing selection procedures
- no loss or distortions
ZZHZee )(
including huge reducible and irreducible backgrounds
at e.g.
less precision on BF (H Z )
• Due to our present knowledge, Higgs-strahlung
• Inclusion of leptonic Z decays, Z ,llor Z Z fusion events,
neglected so far
only slight
GeVs 300
• Completely open whether full luminosity of 1 ab-1 at = 500 GeV with desired beam plarisation is
delivered to this measurements
)(
)(
ZHBF
ZHBF
improvement on
lower bounds on HZy
HZy
g
g
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