The decay of a Higgs boson into a pair of W bosons h --> W^+W^-, is a
dominant mode for Higgs boson masses above 135 GeV. At hadron colliders,
searches for this decay focus on channels in which both W bosons decay
leptonically into charged leptons, h --> W^+ W^- --> l^+ l^- plus missing
energy. We show that semileptonic decays of heavy flavors are an important
background to this signal. Lepton isolation provides too little suppression of
heavy flavor contributions, and an additional 4 to 8 orders-of-magnitude
suppression must come from physics cuts. An increase of the cut on the the
minimum transverse momentum of non-leading leptons in multilepton events is one
effective way to achieve the needed suppression, without appreciable loss of
the Higgs boson signal.Comment: 4 pages, 2 figures, 1 table. AIP style file. Paper presented by Ed
  Berger. To be published in the proceedings of the 9th Conference on the
  Intersections of Particle and Nuclear Physics (CIPANP 2006), Rio Grande,
  Puerto Rico, May 30-June 3, 200

Berger, Edmond L.

Sullivan, Zack

English

arXiv

Edmond L. Berger

Tony M. Liss

Crossref

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Higgs Boson Decay into a Pair of Leptons
Edmond L. Berger and Zack Sullivan
High Energy Physics Division, Argonne National Laboratory, Argonne, Illinois 60439
Abstract. The decay of a Higgs boson into a pair ofW bosonsh→W+W−, is a dominant mode for
Higgs boson masses above 135 GeV. At hadron colliders, search s for this decay focus on channels
in which bothW bosons decay leptonically,h →W+W− → l+l− plus missing energy. We show that
semileptonic decays of heavy flavors are an important background to this signal. Lepton isolation
provides too little suppression of heavy flavor contributions, and an additional 4 to 8 orders-of-
magnitude suppression must come from physics cuts. An increase of the cut on the the minimum
transverse momentum of non-leading leptons in multileptonevents is one effective way to achieve
the needed suppression, without appreciable loss of the Higgs boson signal.
Keywords: Higgs, W Pairs, Leptons, Backgrounds, Heavy Flavors
PACS: 14.80.Bn, 13.85.Qk, 13.85.Rm, 13.38.-b
INTRODUCTION
In hadron collisions, the cleanest signature for the Higgs boson decayh→W+W− is two
isolated opposite-sign leptons plus missing transverse energy (/ET ) from the neutrinos in
W → lν. One class of reducible backgrounds involves processes with heavy-flavor (HF)
hadrons in the final state, such asWc, Wbb̄, andbb̄, where at least one lepton comes
from the decay of a HF hadron (a hadron that includes either a bottom or charm quark).
In this report I summarize a recent study [1] in which we demonstrate that isolation does
not sufficiently suppress these HF backgrounds. Rather, thesize of the heavy-flavor
background is determined by details of the applied physics cut , and current analyses
must be improved to remove this background toh →W+W−.
We provide a full simulation of the backgrounds forh→W+W−→ l+l−/ET , following
the analysis chains of two studies: one by the D0/ Collaboration [2] from which a limit
is set onh → WW at the Tevatron, and one by the ATLAS Collaboration [3] that
estimates the reach at the LHC. The heavy-flavor background could be overwhelming
with the default cuts at the LHC, but we propose a new, more restrictive cut that would
significantly reduce the background. We emphasize the valueof direct measurements of
the magnitude and kinematic variation the heavy-flavor background in the Tevatron and
LHC data. Space restrictions limit this brief summary to ourLHC investigation.
HEAVY FLAVOR BACKGROUND AT THE LHC
The issue for the heavy flavor backgrounds is the extent to which lepton isolation and
subsequent kinematic physics cuts can suppress them. The nature of the challenge at
the LHC is illustrated by a comparison ofσ ×B(h → WW ∗ → l+l−νν̄) ∼ 0.7 pb for
mh = 150 to 190 GeV, with the size ofσ inclusivebb̄ ∼ 5×10
8 pb. Isolation inb → lX even
at the 0.5% level leaves a HFl+l−/ET background that is 104 greater than the signal. We
must address questions of both the magnitude and the shape ofthe backgrounds.
In order to make statements regarding experimental issues,we require a detailed sim-
ulation of reconstructed events. We run events through the PYTHIA 6.322 [4] shower-
ing Monte Carlo, and feed the output through a heavily modifiev rsion of PGS [5]
that reproduces the results of the relevant full ATLAS detector simulations to within
10% [6]. This code has a more accurate treatment of geometriceffe ts and efficiencies
than ATLFAST. The ATLAS physics cuts described in the ATLAS Technical Design Re-
port (TDR) [3] are then applied to the objects found in PGS. Weus MadEvent 3.0 [7] to
generate hard events, and we match the cross sections after showering to the differential
next-to-leading order (NLO) cross sections. ForW j j and the relevant single-top-quark
process, aK factor times a leading-order (LO) distribution is sufficient to retain all an-
gular correlations. ContinuumW+W− andh → W+W− are evaluated using PYTHIA
routines withK factors.
TABLE 1. Cross sections (in fb) for opposite-sign leptons as a functio of cuts for the 160 GeV
Higgs boson ATLAS analysis.bb̄ j⋆ production is a lower limit based on limited phase space. A
dash indicates statistics are too small to estimate.
Cut level h →WW WW bb̄ j⋆ Wc Single-top Wbb̄ W cc̄
Isolatedl+l− 336 1270 > 35700 12200 3010 1500 1110
ET l1 > 20 GeV 324 1210 > 5650 11300 2550 1270 963
/ET > 40 GeV 244 661 > 3280 2710 726 364 468
Mll < 80 GeV 240 376 > 3270 2450 692 320 461
∆φ < 1.0 136 124 > 1670 609 115 94 131
|θll |< 0.9 81 83 > 1290 393 68 49 115
|ηl1 −ηl2|< 1.5 76 71 > 678 320 48 24 104
Jet veto 41 43 > 557 175 11 12 7.4
130< MllT < 160 GeV 18 11 — 0.21 1.3 0.04 0.09
In Table 1 we show the cross sections we compute for opposite-sign dileptons. The
second column shows the signal processh → WW , and the remaining columns dis-
play the backgrounds that we examine, beginning with continuumWW production and
followed by several heavy flavor backgrounds. The first levelof cuts requires two iso-
lated leptons, each withpT l > 10 GeV and|ηl|< 2.5. Isolation of electrons and muons
replicates recent ATLAS descriptions [3, 6], and it is applied within the modified PGS
detector simulation. Next, a cut is placed on the transversenergy of the reconstructed
highest-ET leptonl1 of ET l1 > 20 GeV. A fairly high missing energy of 40 GeV is then
required. Spin correlations inh→W+W− → l+l−/ET tend to send the leptons in the same
direction. Consequently, their invariant mass is low, and ATL S requiresMll < 80 GeV.
Likewise, the angle between the leptons should also be small, and an aggressive cut is
made on the azimuthal angle between the leptons,∆φ < 1.0. The next-to-last cut is a
veto of any event having a jet withET j > 15 GeV, and|η j| < 3.2. It serves to reject
background fromtt̄ production. Finally, a tight cut is made on the transverse massMllT
of the dilepton and missing energy. It appears naively to remove most of the heavy-flavor
background.
The final cut onMllT is the key to the ATLAS sensitivity. In Fig. 1(a) we see a com-
parison of the Higgs boson signal, the continuumWW background, and the heavy-flavor
backgrounds. The HF backgrounds are more than an order of magnitude larger than the
previously calculated backgrounds forMllT < 110 GeV. As a result of the physics cuts
and lepton isolation, thebb̄ background has been suppressed by 11 orders of magnitude.
It is unlikely that the tail of this distribution cuts off sharply at 125 GeV. It would be dif-
ficult to believe an excess observed in the regionMllT < 160 GeV without a measurement
of this HF background. Even for a 200 GeV Higgs boson, the median transverse mass is
below 140 GeV, leading to poor mass resolution if events are obs rved.
18014010060
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FIGURE 1. (a) Opposite-sign dilepton transverse mass distribution for a 160 GeV Higgs boson, the
continuumWW background, and the sum of heavy-flavor backgrounds (HFB) atATLAS. The inset shows
a blow-up of the signal region. (b) Same as (a) but after the cut on the next-to-leading leptonpTl2 is raised
from 10 GeV to 20 GeV.
Fortunately, we can reduce the HF background to a manageablel vel by pushing the
MllT mass peak associated with the heavy-flavors below 110–120 GeV. Th distribution
in ET of the leading lepton in theW+jets and single-top samples is fairly insensitive
to small increases in theET threshold near 20 GeV. This feature is not surprising since
the leptons come from realW decays. The leading lepton fromb or c decay falls faster,
but an increase in the cut will not improve the overall significance. On the other hand,
the next-to-leading lepton has an exponentially falling background as a function of
ET . An increase of the minimum transverse energy cut on additional leptons from 10
GeV to 20 GeV reduces the background by roughly a factor of 20,while maintaining
about 2/3 of the signal and continuumWW backgrounds. In particular, the dangerous
bb̄ background drops by a factor of 30, theW j +X backgrounds go down a factor of
10, and single-top-quark production goes down a factor of 5.Such a cut is nearly a
“magic bullet” for Higgs boson masses above 140 GeV. An estimate of the effect of
this one change in the cuts is shown for the signal and total backgrounds in Fig. 1(b).
The leading edge of the heavy-flavor transverse-mass peak is20 GeV lower than with
the default cuts. The downward shift of this leading edge, along with the lower overall
magnitude of the background, protects the Higgs boson signal region from uncertainties
in the modeling of the heavy-flavor background. The residualHF background will still
be measurable at lowerMllT , and it provides a control sample.
Our analysis demonstrates that despite small efficiencies,h avy-flavor decays into
leptons are a potentially serious background. An increase in the transverse-energy cut of
secondary leptons is effective at reducing the background,but every level of cuts is sig-
nificant. Some of the proposed cuts are sensitive to actual detector performance, noise,
and the underlying event — none of which will be known until data re accumulated.
Extrapolations of the magnitude and shape of the HF background using Monte Carlo
techniques have large inherent uncertainties. We recommend that the HF background be
measured with cuts as close as possible to the final sample. Atthe LHC the HF back-
ground is large enough that it can be studied in theMllT distribution and fully controlled.
DISCUSSION
Although our study focuses onh →W+W−, it raises a broader question of the potential
danger of heavy-flavor leptons in multi-lepton analyses. For example, trilepton searches
for supersymmetry typically have soft additional leptons.There could be a significant
impact on analyses of these types of signals if lepton transverse momentum cuts must
be raised to remove the heavy-flavor leptons.
In investigations at linear colliders, the Higgs boson decay h →W+W− can be recon-
structed fully from hadronic decays of theW in the Higgs-strahlung processe+e− →
hZ →W+W−Z, with Z → qq̄ or Z → l+l− [8]. The branching fractionBR(h → WW ∗)
can be measured to∼ 4% accuracy ine+e− → hZ → WW ∗Z, with WW ∗ → 4 jets or
WW ∗ → lν + 2 jets. Studies of the decay modeh → l+l−Emiss do not appear neces-
sary for access to theh → W+W− coupling or branching fraction, but there may be
interesting additional information to be gained. Ine+e− → hZ, with Z → l+l−, and
h → W+W− → l+l− + Emiss, there should be interesting kinematic signatures in the
four-charged-lepton final state.
ACKNOWLEDGMENTS
Work in the High Energy Physics Division at Argonne is supported by the U. S. Depart-
ment of Energy, Division of High Energy Physics, Contract No. W-31-109-ENG-38.
REFERENCES
1. Zack Sullivan and Edmond L. Berger, Phys. Rev. D74, 033008 (2006), arXiv:hep-ph/0606271.
2. D0 Collaboration, V. M. Abazov,et al., Phys. Rev. Lett.96, 011801 (2006).
3. ATLAS Collaboration, ATLAS Technical Design Report Vol.II, CERN-LHCC-99-15, p. 704.
4. T. Sjostrand,et al., Comput. Phys. Commun.135, 238 (2001); T. Sjostrand, L. Lonnblad, S. Mrenna,
and P. Skands, arXiv:hep-ph/0308153.
5. Higgs Working Group Collaboration, M. Carena,et al., in Physics at Run II: the Supersymmetry/Higgs
Workshop, Fermilab, 1998, edited by M. Carena and J. Lykken (Fermilab, Batavia, 2002), p. 424.
6. ATLAS Collaboration, ATLAS Technical Design Report Vol.I, CERN-LHCC-99-14; with updates
from B. Mellado, S. Paganis, W. Quayle, and Sau Lan Wu, ATL-CAL-2004-002, unpublished; F.
Derue, C. Serfon, ATLAS-PHYS-PUB-2005-016, unpublished.
7. F. Maltoni and T. Stelzer, JHEP0302, 027 (2003).
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A. Raspereza, Eur. Phys. J. C44, 481 (2005); ECFA/DESY LC Physics Working Group, J. A. Aguilar-
Saavedra,et al., arXiv:hep-ph/0106315.


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