produced via Vector Boson Fusion in the channel
H ! WW ! l
jj with the ATLAS detector
V. Cavasinni, D. Costanzo, E.Mazzoni,I. Vivarelli
INFN and University of Pisa, Italy
Abstract
The discoverypotentialof aStandardModelHiggsbosonof130200 GeVpro-
ducedvia thevector bosonfusionmechanism and decayinginto Wpairsis studied
inthisnote. TheWs aredetected studyingone leptonicand one hadronic decay.
ThisstudywasperformedusingATLFASTtosimulatethedetectorandusingthe
PYTHIAMonteCarloto generate boththesignal and thebackgroundevents. The
mostimportantbackgroundistheQCDproductionofW+jetswhichwassimulated
also usingtheVECBOSparton levelMonteCarlogenerator interfacedto PYTHIA
for the showering and fragmentation of quarks. A signal can be observed after 3
years of data taking at low luminosity in the mass interval considered, providing
a complementary evidence to that given by other channels studied for this mass
range.
ATL-PHYS-2002-010 02/04/2002
1 Introduction 2
2 Signal and Background Simulation 2
2.1 SignalGeneration. . . 2
2.2 Background Generation . . . 3
2.2.1 VECBOS GeneratorImplementation . . . 3
2.3 WW+jetsEWGenerator Implementation . . . 4
3 Event Selection 5 3.1 GeneralCuts( lter level) . . . 5
3.2 Cutson Leptons . . . 6
3.3 Cutson theForward (tagging) Jets . . . 6
3.4 Cutson theCentralJets . . . 8
3.5 CentralJetVeto . . . 9
3.6 Cutson r min andr max . . . 9
4 Results 10
5 Higgs Mass Reconstruction 12
6 A Di erent Jet Tagging Algorithm 12
7 Conclusions 14
8 Acknowledgements 14
The Vector BosonFusionmechanism(VBF)toproduce the Higgsbosonat theLHC was
already identi ed as the most suitable one to search for a heavy SM Higgs boson with
M
H
>600GeV ([1],[2], [3]). ThefavoriteHiggsdecay channelinthis massregionis into
W pairs which can be detected asa dileptonic decays or one into hadrons and the other
into alepton pair.
Recently D. Rainwater and D. Zeppenfeld [5]suggested that alsoin the intermediate
massrangeregion(130GeV<M
H
<200GeV)VBFisacompetitiveproductionchannel
compared to the channels previously investigated such as the gluon fusion, the t
tH or
the WH(ZH) associated production. The key features of the VBF mechanism are the
presenceof twoenergeticforward jetsinthe nalstate(taggingjets)and thelackofcolor
exchangebetweentheinitialstatequarks. Thislastfactlimitsthehadronicactivityinthe
centralrapidity region incontrast with most background processes wherecolor exchange
yieldsenhanced hadron productionbetween the forward jets.
The Higgsdecayproducts underinvestigationinref.[5]wereW +
W whereboth W's
decay leptonicallyora direct decay of Higgsinto pairs.
In this work a study of VBF is presented studying the case with two Ws in the
nal state with one W that decays leptonically and the other one into jet pairs. The
advantagesofthis nalstate,ascomparedtotheleptonicchannel,arethelargerbranching
fraction and the possibility to impose tighter constraints to the decay products, i.e., the
invariant mass of the jet pair and the angular correlations between the charged lepton
and the jets. On the other hand, studying this channel it is necessary to contrast an
additional severe background source, the QCD production of W+jet events. In spite
of this new background, this channel provides a signi cant contribution to the Higgs
discovery, complementaryto the full leptonic one, in the interval 130 GeV < M
H
<200
GeV.
2 Signal and Background Simulation
TheATLAS detector hasbeensimulatedusing theATLFAST package [4]with ajetclus-
tering cone of 0.4. Jets are reconstructed within this cone using the standard ATLFAST
clustering routine and, to achieve the correct energy scale calibration, a multiplicative
correctionfactorasafunctionoftherawjetP
T
hasbeenapplied,asprovidedinthestan-
dard ATLFAST package. Thiscalibration factor corresponds towhat willbeavailable to
ATLAS for single jet calibration with Z+jet events. The default settings of ATLFAST
havebeen used everywhere in this note.
2.1 Signal Generation
Samples of 100,000events in the Higgs mass range between 130 GeV and 200 GeV have
been generated using Pythia 6.1, ISUB=123 and ISUB=124(VectorBoson Fusion). The
corresponding cross section for those samples times branching ratio and corresponding
integrated luminosity are shown in table 1. These leading order cross sections are in a
Mass (GeV) BR (pb) LuminosityProduced (fb )
130 0.42 238
140 0.69 145
150 0.94 106
160 1.23 80
170 1.28 78
180 1.17 85
190 0.90 110
200 0.78 128
Table 1: Valueof BR andproduced luminosity for theHiggs signalas a functionof the Higgs
mass. Thesevalues have been obtained withPYTHIA.
goodagreementwiththosefromtheanalysisinwhichbothWsdecayleptonicallyrescaled
by a factor 4 toaccount for a di erent branching ratio [8].
2.2 Background Generation
Alistofthemostimportantbackgroundstothischannel,togetherwiththeircrosssection,
is given intab. 2 ascompared to the signal froma Higgsboson with M
H
=160 GeV.
Due to the large production cross section, one of the most important background is
the t
t production, whose inclusive cross section is three orders of magnitude larger than
that of the signal.
The WW+jets background has alsobeenincluded, groupingthe events asWW from
EW contribution (WW bremsstrahlung in quark{(anti)quark via t{channel boson ex-
change) and WW from QCD contribution backgrounds (QCD radiation of jets from
WW production). WW+jets QCD background has been generated using PYTHIA 6.1
ISUB=25 resulting in a cross section about 30 times greater than that of the signal.
A Matrix Element generator, provided by Zeppenfeld et al., has been used to generate
WW+jets EW background events and modi ed to allowthe hadron decay of one of the
two Ws.
Di erently to the analysis presented in [5], the QCD production of W+4 jet events
as to be taken into account as another source of background to this channel. The cross
section for W+4 jet events is two order of magnitude larger than that of the signal. As
a rst attempt, PYTHIA has been used to simulate this background. However, since
PYTHIA cannot generate one W plus 4 jets as a leading order calculation, but only W
plus one jet followed by showering, to cross check the PYTHIA results the VECBOS [6]
parton levelgenerator has been used. VECBOS predicts a cross sectiontwo timeslarger
than PYTHIA.
2.2.1 VECBOS Generator Implementation
VECBOSisamatrixelementgenerator,whichcan generateW+njets,withnuptofour.
Detailsabout VECBOS can be found in [6].
Signal(M
H
=160GeV) 1.23 PYTHIA
t
t 1610
3
PYTHIA
W +4jets 260 VECBOS
WW+jetsQCD 28 PYTHIA
WW+jetsEW 0.325 ME generator
Table 2: List of the signal (M
H
=160 GeV) and background processes together with the cross
section times BR and the Monte Carlo used.
Due to the procedure of generation, events from VECBOS have a weight W, which
accounts for the matrix element calculations in di erent regions of the phase space. In
ordertoproduceunweighted events, astandardhit{and{missunweighting procedurewas
applied:
Chooseamaximum weightvalue (W
MAX
),close tothe real maximumweightfound
inthe weightdistribution;
Foreach event compare its weight W with W
MAX
;
If W < W
MAX
generate a random number R between the minimum of the weight
distribution and W
MAX
, and compare it with W. If W > R then keep the event
and rede ne W =1, otherwise the event is rejected;
IfW >W
MAX
the event is kept with weightW=W
MAX .
Using this procedure, a large number of events with weight one is obtained together
with a small number of events with W > 1. Thus, the unweighting technique is not
complete, and we still have to take care of some weighted events. It is clear from the
procedurethat asW
MAX
isclosertothemaximumweightofthedistribution,thenumber
of weighted events become smaller. If W
MAX
is chosen as the maximum weight distri-
bution the unweighting would be complete. However, in such a case a large number of
events would be rejected. The choice of W
MAX
is a trade{o between the total number
of weighted eventsobtained and the CPU time needed to generate the sample.
Finally the Jetset MonteCarlo has been used to fragment the partons produced by
VECBOS. The color uxes were assigned ina randomway, compatible with QCDrules.
2.3 WW+jets EW Generator Implementation
To produce the WW+jets EW background a matrix element generator provided by
Zeppenfeld and his group has been used. This ME generator was written to generate
WW+jets events where both W decay leptonically. We simply substituted one leptonic
pair with an hadronic one (u
d) after the generation, but before the event is treated by
Pythia for initialand nal state radiationand hadronization. The color ux was de ned
taking into account that the hadronic pair is a color singlet. The cross section has been
obtained by rescalingthatof the process with afullyleptonic nal state by afactor four.
Figure1: P
T
distribution of lepton. Thesignal iscompared withWW+jets EWand ttevents.
3 Event Selection
3.1 General Cuts ( lter level)
A rst event selection is applied to reduce the size of the ntuples used for this analysis,
applying some loose cuts. The rst requirement is the presence of a standard trigger
signal:
Trig >1 (1)
Aleptonhas tobepresentinthe nalstate(P
t
>20GeV, <2:5) whilethepresence
of a neutrino assures asizable amount of 6E
T (6E
T
>20 GeV).
The presence oftwo forward jets,duetothe VBFproductionmechanism, isexploited
to reject the backgrounds. Di erent algorithms have been tried to tag these jets with
the nal result depending on the algorithmchosen. In this paperwe willproceed in this
analysis following two di erent choices. At rst, the algorithmwhichhas been optimized
to provide the best results is discussed while in sec. 6 the results obtained with the
algorithmused in the analysis where both Ws decays leptonic[8] are alsopresented.
In the rst algorithm, tagging jets are those with the largest P
t
one in the positive
and one in the negativeforward (2<jj<5)regions.
AsearchisthenmadeforthejetsproducedfromtheW decay. Thesejetsareexpected
tobecentralinrapidity,thereforetwojetsinthecentral(-2<<2)regionofthe detector
are required.
Figure2: distributionforthe forward taggingjets,signaliscompared withttandWW+jets
QCDbackgrounds.
3.2 Cuts on Leptons
The most important backgrounds in this analysis are t
t and W+jets, due to their very
largeBR(comparedtothe signal). However,they canbestronglyreducedby applying
a forward jet tagging and a veto on centralhadronic activity. Di erently the WW+jets
EW background has kinematicaldistributions and color structure similar tothose of the
signal [5], and therefore it is an important background despite the small BR . As it
can be seen in g. 1 EW processes and t
t production are characterized by a large P
t of
the decayed lepton while the signal (M
H
= 160 GeV in the gure) has a lepton with a
smaller P
t
. The same features apply also for 6E
T
. Therefore leptons are selected within a
P
t
window:
30GeV<P
tl ep
<100 GeV 30 GeV<P
tmiss
<100 GeV (2)
As it can be seen in tab. 3 this cut is e ective against WW+jets EW while it does
not reject signi cantly the other backgrounds.
3.3 Cuts on the Forward (tagging) Jets
The presence of two forward-emitted hard jets is the distinctive signature of VBF Higgs
production. The presence of these tagging jets provides a powerful tool to reduce the
backgroundssigni cantlyasinmostofthebackgroundeventsforwardjetsarenotpresent.
A rst selectionon forward jets has been done at lter level requiring at least one jet in
the positive forward eta direction( >2)and one inthe negativebackward eta direction
( <2).
The di erence in rapidity between the two tagging jets is plotted in g. 2: jets are
T
compared with t
t, W+jets and WW+jets QCDbackgrounds.
more separated inrapidity for the signalthan for the backgrounds. Thus, a cut on is
appliedby requiring:
>5 (3)
The energy of the forward jets isexpected tobeverylarge as they originatefromthe
valence quarkof theincomingprotons. LookingattheP
t
distributions( g.3) ofthe two
jets,it can benoted that thesignal ischaracterizedby harderdistributions. Thus ahigh
cut onP
t (P
t
>60GeV)stronglyreduces the W+jetsand WW+jetsQCDbackgrounds,
but with aloweÆciency (5 10%) onthe signal. A cut onthe invariant massof the two
forward jets is suggested by the fact that QCD processes usually have smaller invariant
masses than EW processes, as mainlysmall x gluonsare produced inQCD interactions.
To reject W+jets background as much as possible a cut at 60 GeV both for the leading
and next{to{leading forward jets is imposed, as well as a cut on M
jfwinv
> 1200 GeV.
The nal cuts on forward jets are:
P
tjfw1
>60GeV P
tjfw2
>60 GeV M
jfwinv
>1200 GeV (4)
After these cuts are applied, the WW+jets QCD background becomes negligible if
compared withthe signal and itwillbenolonger considered. These cuts (especiallythat
on the invariant mass) are also a powerful tool against t
t. However, the cuts on the
forward region are non-e ective on the EW background due to its topological similarity
with the Higgs signal.
The last requirement applied in the forward region is a veto against an excessive
hadronic activity. In factt
tproduction has anextremelyrichhadronic activitywhile the
signalisessentiallyanEWprocess. Therefore,alimitinthenumberofjetsintheforward
region is imposed ( g. 4),extending the jet veto discussed in sec. 3.5 alsointhe forward
region.
Figure4: Number of forward jets for signal (left) andtt backgrounds. Thehadronic activity is
moreenhanced for t
t.
N
jfw
<5 (5)
3.4 Cuts on the Central Jets
TheW decayproductsare identi ed asthe leadingand next-to-leadingjetsinthecentral
region. These twojets are expected to bemostly in the centralregionand this feature is
exploited tofurther distinguish the signal from the backgrounds.
The P
t
distributions for the leading and the next-to-leading jet in the central region
are plotted in g. 5 for the signal and the backgrounds. The P
t
of jet forsignal events is
usually smallerthanfor W+jetsandt
tevents. Thisisa consequenceof the factthatWs
are producedalmost atrest if M
H
is about160 GeV, sothat the centraljets donot have
a booste ect. The cuts appliedon the P
t
of central jetsare:
30GeV<P
tjetce1
<100 GeV 25 GeV<P
tjetce2
<75 GeV (6)
For M
H
160 GeV the dijet invariantmass peaks at about the mass of the W while
the backgrounds yield a wider distribution (with the exception of WW+jets, where a
W ! jj decay is present). As g. 6 shows, the distribution for signal and WW+jets
have a peak, while the distribution for t
t events is wider with a smallpeak coming from
hadronic W decays int! Wb processes.
A cut onthe invariant mass of the centraljets (M
invjce
)is thusapplied as:
65GeV<M
invjce
<90GeV (7)
When M
H
<160 GeVisconsidered,the Wbosonsproducedfromthe Higgsdecayare
nolongeronmass-shellandalowercutontheinvariantmass ofcentraljetswould reduce
the total event yield and bias the Higgs mass reconstruction. Hence this cut is released
when dealingwith smaller Higgsmasses.
t
t
t, W+jets andWW+jets EWbackgrounds.
The optimal strategy was to apply a slightly di erent set of cuts in the mass region
M
H
< 160 GeV by releasing the lower bound for invariant mass of central jets and
tightening the cuts on the jet angular separation tokeep the backgrounds under control
as discussed inthe following sections.
3.5 Central Jet Veto
AfterthecutsoncentraljetsareappliedthemostimportantbackgroundsareW+jetsand
t
t production. As the hadronic activity of QCDprocesses (inparticular t
t backgrounds)
is usually higherthan for EW processes, ajet veto can be appliedas ithas been done in
every analysis studying VBF signature.
The algorithm chosen is to veto any event with a third jet with P
t
> 20 GeV in the
centralregion.
P
t (3
rd
central jet)<20 GeV (8)
As it can beseen on table3 this cut has a 90% eÆciency on signaland on WW+jets
EW while itreduces t
t by afactor of 2=3.
3.6 Cuts on r
min
and r
max
After the cuts described in the previous sections, the W+jets background is still three
times larger than that of the signal, while the t
t and WW+jets EW backgrounds are
almostas largeasthe signal. To reducethe backgroundstoa levellowerthan thesignal,
the angular correlations between the lepton and jets characterizing the Higgs events are
exploited.
TheHiggsbosonisaspin{0particledecayingintospin{1W bosons. Asaconsequence,
Figure6: Invariant mass distributionfor central jetssignal is compared withtt and WW+jets
EWbackgrounds.
enhanced ifthere isabooste ect. Moreover, the lepton-dijetsystem shouldhaveasmall
invariant mass, as a remembrance of the parent Higgs mass. As the invariant mass of
three particlesgoesto zero if they are all close inangle, this feature is tightly correlated
with the angular separation between the leptonand the dijetsystem.
These Higgsdecay characteristicsare exploited usingthe variablesr
min
and r
max ,
de ned as the minimumand the maximumof the angular distance of the charged lepton
fromeach of the twojets.
A plot of r
max
vs. r
min
for the signal (di erent M
H
) and various backgrounds,
shows that the signal events cluster in the bottom left corner of the picture. Moreover,
as the Higgsmass decreases, r
max
becomes smaller.
Thus the followingcuts for M
H
160 GeV are applied:
r
min
<1 r
max
<2 (9)
For masses M
H
< 160 GeV, due to the release of the lower bound of the central jet
invariant mass, atighter cut isapplied, i.e.:
r
min
<0:8 r
max
<1:4 (10)
As shown intab. 3(atM
H
=160 GeV)these tightcuts stillhavea higheÆciency for
the signal while they are e ectiveto reduce backgrounds.
4 Results
The results ofthe analysis are summarizedintab. 3 forM
H
=160 GeV.The corrections
due to the detector eÆciency and fake veto rate [7], have been applied, but they turn
max min H
for backgrounds. Signal clusters in the left{bottom corner.
out to be negligible, due to the value of P
t
chosen for the veto and tagging thresholds.
From tab. 3 itis clear that the most importantbackground is W+jets. Both the results
obtained generatingthis backgroundswith PYTHIA andVECBOS are included,and the
two predictions di er by a factor 4 due to the shower approach used in PYTHIA, which
produces softer jets and lower jet multiplicity. WW+jets QCD background is totally
negligiblewhile t
t and WW+jets EW together amount to30% of the signal.
Tab.4givestheacceptedcrosssectionsandthestatisticalsigni canceforanintegrated
luminosity of 30 fb 1
in the mass range 130 GeV < M
H
< 200 GeV, compared with
backgrounds. The most realistic prediction (i.e. VECBOS) for the W+jets background
has been used. After three years of data-taking at low luminosity, a signal is expected
to be visible at least in the 140GeV < M
H
<190GeV mass range. The Higgs discovery
in awider mass range is limited mostly by the decrease of H !WW branching ratio at
low masses and by the decrease of the cross section for VBF productionat high masses.
Moreover, the cut on r
max
is less eÆcient onthe signalas M
H
increase.
Cut Signal tt W+4jets W+4jets WW+jets WW+jets
(M
H
=160 GeV ) VECBOS PYTHIA EW QCD
Inclusive 1.23 10 3
19010 3
54010 3
- 325 27.910
3
Filterlevel 82 8.410
3
44.410 3
3310 3
66.6 130
Cutson leptons 40.5 4.010 3
20.010 3
1610 3
22.3 65.3
Cutson forwardjets 5.8 165 690 96 11.6 0.1
Cutson centraljets 1.6 11 14.9 7.7 2.5 < 1.510
2
Centraljetveto 1.5 3.0 7.7 4.4 2.2 < 1.510
2
Cutson r
min
and 0.8 0.1 0.4 0.1 0.1 < 1.510
2
r
max
Table 3: Signal (m
H
=160 GeV) and background events expected after each cut. All numbers
are expressed in fb.
M
H
(GeV) Backgrounds Acceptedcross
S
p
B
(fb) (fb) (30fb
1
)
130 0.2 0.15 1.8
140 0.25 3.0
150 0.35 4.2
160 0.6 0.8 5.6
170 0.8 5.6
180 0.6 4.2
190 0.5 3.5
200 0.3 2.1
Table 4: Signal and background for various Higgs masses. The two di erent values for back-
grounds are obtained applying the two di erent sets of cuts for M
H
160 GeV and M
H
<160
GeV.
5 Higgs Mass Reconstruction
ToreconstructtheHiggsmasstheneutrinolongitudinalmomentumhastobedetermined.
To extract this information the W mass constraint has been imposed to the lepton{
neutrino pair . This leads to a second order equation and therefore to two solutionsfor
the neutrino longitudinalmomentum. The lowest solution has been chosen (assuggested
by the Monte Carlo) and then the invariant mass of the lepton+neutrino+central jets
system can becalculated.
Fig 8shows the sumof signalandbackground(fullline)and backgroundonly(dotted
line) for M
H
=160 GeV,for 30 fb 1
.
6 A Di erent Jet Tagging Algorithm
To compare these results with those obtained by other collaborators who studied other
decays of the Higgs boson produced via VBF ([8],[9],[10],[11]), a common tagging jet
algorithm has been also tried. In this case the tagging jets are identi ed as the two
leadingjetsrespectivelyinthe positiveandnegativerapidityhemispheres. Moreover,jets
line is the signal plus the background while dotted line is the background only. Results after 30
fb 1
areshown. M
H
is 160 GeV.
coming fromthe decay of the W have been identi ed as the two jets with the largest P
t
in the region between the tagging jets.Following this new algorithm, also the ltering
procedure has been modi ed accordingly.
The remaining criteria followed were the same as discussed in the previous sections.
Optimizingthe cuts, the overallresults areworse thanthose ofthe previousanalysis (see
tab. 5) as the rejection factor for the W+jets (a factor four larger than before) and for
the t
t background (a factor 2 larger than before) is not big enough. The tagging jets,
de ned using this algorithm,canbealsointhe centralregion which iswheremost of the
high P
t
jets producedinassociation withthe W are emittedby the W+jetsbackground.
Tab. 5 shows the results obtained using the new tagging algorithm for a Higgs with
M
H
=160 GeV.The selectionhas been made usingthe cuts discussed in sec.3. Correc-
tions for detector eÆciency and fakeveto rate have alsobeen applied.
Cut Signal tt W+4jets WW+jets WW+jets
(M
H
=160 GeV ) EW QCD
Inclusive 1.2310 3
19010 3
54010 3
325 27.910 3
Filterlevel 127 6.910
3
10010 3
74.8 120
Cutson leptons 64 4.410
3
44.010 3
26.4 60.0
Cutson forwardjets 7.6 140 1090 13.4 0.1
Cutson centraljets 2.7 21 71 4.7 <1.510
2
Centraljetveto 2.1 5.7 38.5 3.9 <1.510
2
Cutson r
min
and 0.9 0.3 1.6 0.15 <1.510
2
r
max
Table 5: Signal (M
H
= 160 GeV) and background events expected after each cut using the
forward jet algorithm discussed in sec. 6. All numbers are expressed in fb.
7 Conclusions
TheHiggsdetectionwiththe ATLAS experimentusing VBFH !WW !jjl has been
studied. Byexploitingthedistinctivefeatures ofVBFlikeforwardjettagging,centraljet
veto and angular correlations, we were able to reduce the number of background events
below the number of expected signal events.
A range of Higgs mass from 130 GeV to 200 GeV was investigated, proving that a
signal can be observed in the 140 GeV < M
H
< 190 GeV mass interval after 30 fb 1
with a signi cance as large as 5, depending on the Higgs mass. Compared to the fully
leptonicchannel,thesearchinthissemileptonicdecaychannelrequiresspeci calgorithms
forjettaggingasitisa ectedby anadditionalW+jetsbackground. Althoughthe results
obtained are worse than those obtained for the di{lepton channel, the study of this nal
state is useful to cross check the discovery of the Higgs boson after three years of data
taking at lowluminosity.
8 Acknowledgements
We would like to thank Marcus Klute for providing us the code to generate WW+jets
EW backgroundsand Karl Jakobs for useful discussions.
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