The energy dependence of the production cross section of a light Higgs boson
is studied at threshold and compared to the expectations of several spin
assumptions. Cross section measurements at three centre-of-mass energies with
an integrated luminosity of 20 fb-1 allow the confirmation of the scalar nature
of the Higgs Boson.Comment: 4 pages (Latex), 4 figures (Postscript

Dova, M. T.

Garcia-Abia, P.

Lohmann, W.

English

arXiv

Garcia-Abia, Pablo

DESY

Determination of the Higgs boson spin with a linear $e^+ e^-$ collider

The energy dependence of the production cross section of a light Higgs boson is studied at threshold and compared to the expectations of several spin assumptions. Cross section measurements at three centre-of-mass energies with an integrated luminosity of 20 fb-1 allow the confirmation of the scalar nature of the Higgs Boson.Facultad de Ciencias Exacta

Dova, María Teresa

El Servicio de Difusión de la Creación Intelectual

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hep-ph/0302113
Determination of the Higgs boson spin
with a linear e+e− collider
M.T. Dova†, P. Garcia-Abia∗ and W. Lohmann‡1
†Universidad National de La Plata, CC67, 1900 La Plata, Argentina.
∗CIEMAT, Avda. Complutense 22, E–28040 Madrid, Espan˜a.
‡DESY Zeuthen, Platanenallee 6, D–15738–Zeuthen, Germany
Abstract. The energy dependence of the production cross section of a light Higgs
boson is studied at threshold and compared to the expectations of several spin assump-
tions. Cross section measurements at three centre-of-mass energies with an integrated
luminosity of 20 fb−1 allow the confirmation of the scalar nature of the Higgs Boson.
INTRODUCTION
Spontaneous symmetry breaking in the Standard Model leads to one remnant
scalar particle, the Higgs boson [1]. A light Higgs boson is expected to be produced
at an e+e− collider of a few hundred GeV via the Higgs-strahlung process, e+e− →
ZH. However, if a particle produced in association with the Z is detected, the
confirmation of its scalar nature will be essential to its identification with the
Higgs boson. Here we study the production of a Standard Model Higgs boson with
a mass mH = 120 GeV near the kinematic threshold and compare the cross section
to predictions for bosons of spin 0, 1 and 2 produced in association with the Z [2].
EXPERIMENTAL CONDITIONS AND DETECTOR
SIMULATION
The study is performed for a linear collider operated at a centre-of-mass energies
of 215, 222 and 240 GeV. The simulated data statistics corresponds to an integrated
luminosity of 20 fb−1 at each energy point. The detector used in the simulation
follows the proposal presented in the TESLA Technical Design Report [3]. The
simulation of the detector is done using SIMDET [4].
1) Talk given at the International Workshop on Linear Colliders, August 26-30, 2002, Jeju Island,
Korea, to appear in the Proceedings.
SIGNAL AND BACKGROUND PROCESSES
Events of the signal, e+e− → ZH, are generated using PYTHIA [5]. Only Z
decays into electrons and muons are considered. For the Higgs boson all decay
modes are simulated as expected in the Standard Model. The values of the cross
section in the Standard Model and the expected numbers of events corresponding
to a luminosity of 20 fb−1 are given in Table 1 at centre-of-mass energies of 215,
222 and 240 GeV. The following background processes are considered: e+e− →
√
s( GeV) 215 222 240
σ(ZH→ ℓ+ℓ−X) 7.2 12.6 16.8
events 144 252 336
TABLE 1. The cross sections and numbers of events
expected in the Standard Model for the production of
a Higgs boson of mH = 120 GeV at centre-of-mass en-
ergies of 215, 222 and 240 GeV and a luminosity of 20
fb−1
(γγ) → e+e−f f¯, e+e− → qq(γ) , e+e− → W+W− and e+e− → ZZ. Initial state
Bremsstrahlung is simulated by PYTHIA. Beamstrahlung is taken into account
using the CIRCE program [6].
CROSS SECTION MEASUREMENT
The cross section determination is based on the e+e− → ZH → ℓ+ℓ− + 2-jet
final state, where ℓ is an electron or muon. The signature is two isolated energetic
electrons or muons and two jets. The identification of electrons and muons and
the formation of jets are performed as described in Reference [7]. The selection
efficiencies for the processes e+e− → ZH → e+e− 2-jet and e+e− → ZH→ µ+µ−2-
jet are about 50%. Each event is subject of a kinematic fit imposing energy and
momentum conservation [8]. The distributions of the two jet invariant mass are
shown in Figure 1 for centre-of-mass energies of 215, 222 and 240 GeV. The dijet
mass spectra are fitted with the superposition of signal and background distribu-
tions. The signal is parametrised with a gaussian and the background is fixed to
the Monte Carlo expectation.
RESULTS
The cross sections obtained for the process e+e− → ZH→ ℓ+ℓ−+2-jet at centre-
of-mass energies of 215, 222 and 240 GeV are shown in Figure 2. The results of fits
using the predictions from models with s=0, 1 and 2 [2] with the normalisation
as free parameter are also shown. The s=0 case is clearly distinguished from the
s=1 and 2 cases. The fit with the s=0 prediction has a good χ2 probability and
2
010
20
100 120 140 160
Mass of the 4C fit  [GeV]
Ev
en
ts
 / 
G
eV
Data
Fit result
m H  =  120  GeV
0
10
20
30
40
100 120 140 160
Mass of the 4C fit  [GeV]
Ev
en
ts
 / 
G
eV
Data
Fit result
m H  =  120  GeV
0
20
40
60
100 120 140 160
Mass of the 4C fit  [GeV]
Ev
en
ts
 / 
G
eV
Data
Fit result
m H  =  120  GeV
FIGURE 1. The dijet invariant mass from the ZH → ℓ+ℓ−qq final state after a 4C kinematic
fit for
√
s = 215 (top left), 222 (top right) and 240 GeV (bottom).
its normalisation factor equals to unity. The other fits have a χ2 probability of
less than 10−5. The study of the Higgs boson production just above the kinematic
threshold shows that the measurement of the cross section at three centre-of-mass
energy points using a luminosity of 20 fb−1 allows confirmation of the scalar nature
of the Higgs boson. Bosons with other spins can be disfavoured2.
ACKNOWLEDGMENTS
The Authors thank Fondation Antorchas, Argentina, for support.
2) There are particular scenarios for s=1 and 2 [2], which show a threshold behaviour similar in
shape to the s=0 one. This can be disentangled using angular information in addition.
3
s, GeV
cr
os
s s
ec
tio
n,
 fb
0
5
10
15
210 220 230 240 250
s = 2
s = 0
s = 1
FIGURE 2. The cross sections determined at
√
s = 215, 222 and 240 GeV (dots) and the
predictions for s=0 (full line), s=1 (dashed line) and s=2 (dotted line).
REFERENCES
1. P.W. Higgs, Phys. Lett. B 12, 132 (1964),Phys. Rev. Lett. 13, 508 (1964), and Phys.
Rev. 145, 1156 (1966); F. Englert and R. Brout, Phys. Rev. Lett. 13, 321 (1964);
G.S. Guralnik, C.R. Hagen and T.W.B. Kibble, Phys. Rev. Lett. 13, 585 (1964).
2. D.J. Miller et al., Phys. Lett. B 505, 149-154 (2001).
3. TESLA Technical Design Report, DESY 2001-011, ECFA 2001-209.
4. SIMDET V3.2, M. Pohl and H.J. Schreiber, DESY-99-030 (1999).
5. PYTHIA V6.136, T. Sjo¨strand, Comp. Phys. Comm. 82, 74 (1994).
6. CIRCE V6, T. Ohl, Comp. Phys. Comm. 94, 53 (1996).
7. P. Garcia-Abia and W. Lohmann,EPJdirect C2 (2000) 1-6, P. Garcia-Abia et al.,
LC-PHSM-2001-054, DESY 2001.
8. V. Blobel, “Constrained Least Squares and Error Propagation”, Hamburg (1997).
4


Determination of the Higgs boson spin with a linear e+e− collider

Determination of the Higgs boson spin with a linear e+e- collider

Determination of the Higgs boson spin with a linear e+e- collider

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