Experimental Sensitivity for Majorana Neutrinos Produced via a Z Boson at Hadron Colliders

0.16 MB·English

by  Arvind Rajaraman

Experimental Sensitivity for Majorana Neutrinos
Produced via a Z Boson at Hadron Colliders
A. Rajaraman1 and D. Whiteson1
1University of California, Irvine, Irvine, California 92697
Wepresentexperimentalsensitivityfor theproductionof afourth-generation Majorana neutrino
(N) via an s-channel Z boson Z NN WℓWℓ at hadron colliders. This channel has not been
studied for the Tevatron dataset →with R →= 5 fb−1, where we find that a single experiment could
significantlyextendcurrent95%C.L.masLslowerlimits,tomN >175GeV/c2,orreport3σevidence
for the N if mN < 150 GeV/c2. With 5 fb−1, a single LHC experiment at √s = 10 TeV could
expect to set a 95% C.L. mass lower limits of mN >300 GeV/c2, or report 3σ evidence for the N
if mN <225 GeV/c2.
0
PACSnumbers: 12.60.-i,13.85.Rm,14.80.-j
1
0
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INTRODUCTION or to muons, and about 80 GeV if it decayed to taus.

n
Theoretical analyses of fourth generation neutrinos at
a
J The study of a fourth generation of quarks and lep- hadron colliders [16–19] have focused on the process
qq¯′ → W± → Nl± where the fourth generation neu-
8 tons has undergone a renaissance. While earlier studies
[1] claimed that the fourth generation was ruled out by trino is producedin associationwith a lightchargedlep-

] experimental data, a recent analysis [2] has shown that ton. Thisprocesshasthesignificantadvantagethatonly
h
oneheavyparticle is produced,whichincreasesthe mass
p the constraintscanbe satisfiedby appropriatechoicesof
- mass splittings between the new fourth generation par- reach considerably. Furthermore, the neutrino will de-
p cay through N → l±W∓ which will produce the low-
ticles. Furthermore, the existence of the fourth gener-
e
backgroundlike-signdileptonsignatureinhalftheevents.
h ation can help to address certain discrepancies between
[ the Standard Model and experimental results in the b- However, the production cross-section for this process

quark sector[3–8], leading to a revival of interest in this depends on the mixing between the fourth generation
1
v possibility (see [9] for a review). with the first three generations due to the W →lN ver-
9 In view of the imminent arrival of LHC data, it is of tex. Inmany models, this mixing angle canbe small; for
2 example, if the mixing is generated by GUT or Planck-
particular interest to search for signals of these fourth-
2 scale suppressed operators,the angle may be as small as
generation fermions at hadron colliders. Such analyses
1.1 hseaavrechbeedenfopretrhfoertm′ eqduafrokr[t1h0e] tu′sianngdtbh′eqCuKarMks.suCpDprFeshseads 1100−−161, t[h12e].neFuotrrinmoixpirnogdupcatriaomnertaetresssimnatlhleisr cthhaannnaebloauret
0 decay t′ → qW and for the b′ quark [11] using the pro- too small to be observable at colliders [16]. In models
0
1 cessb′b′ →tt¯W−W+ whichcanyield like-signdileptons. withsmallmixingangles,thedominantproductionmech-
: These searches have set limits of 311 GeV for the t′ [10], anism becomes pair production through an s-channel
v and 338GeV for the b′ [11] (but see [12]). The LHC will Z, for which the production rate is model-independent.
i
X discoverorexcludefourthgenerationquarksuptoabout These signals have been studied at various benchmark
pointsfortheLHC[20]andforfuturelinearcolliders[21].
r a TeV [13–15].
a However, the analysis of the s-channel Z process has
However,fewdetailedanalyseshavebeenperformedin
not been performed for the Tevatron. In this Letter, we
the lepton sector. By analogy with the first three gener-
present a sensitivity study for the Tevatron and argue
ations,one might expect the leptons ofthe fourth gener-
that the LEP limits on Majorana neutrinos can be sig-
ationtobelighterthanthefourthgenerationquarksand
nificantly improved with an analysis of the data already
in particular, the fourth generation neutrino might be
taken. It would be of great interest to perform a full
expected to be the lightest new particle. It is of interest
analysisofthisdata. Wealsoperformasimilarstudyfor
to see if this neutrino can be found at colliders.
the LHC, which can probe the parameter space to much
Past searches for fourth generation neutrinos have
higher energies.
mainlybeen performedatleptoncolliders. Inparticular,
LEP II has looked for neutrinos produced in the process
e+e− → Z → NN, where the neutrinos subsequently
decay via the process N → l±W∓. No excess of such PRODUCTION AND DECAY
eventswasfound,whichplacedalimitofabout100GeV
forDiracneutrinosdecayingtoelectrons,muonsortaus. We consider an extension to the standard model by
For Majorana neutrinos the corresponding limits were a fourth generation of fermions and a right-handed neu-
about 90 GeV if the neutrino decayedeither to electrons trino. The mass term for the neutrinos can be written
2
as 4 jets. If one decays leptonically, we expect approxi-
mately 2 jets and a third lepton. If both decay leptoni-
1 0 m Q
L =− (Qc Nc) D R +h.c. (1) cally,weexpectapproximatelyzerojetsbutfourleptons.
m 2 R R (cid:18)md M (cid:19)(cid:18)NR (cid:19) All but the fully leptonic mode, the smallest fraction,
contribute to the ℓ±ℓ±jj signature and allow for direct
where ψc = −iγ2ψ∗. This theory has two mass eigen- reconstruction of the N.
states of masses m1 = −(M/2)+ m2D+M2/4,m2 =
(M/2) + m2 +M2/4. In additpion, the mass of the
D
fourthgenperationleptonisconstrainedtobecloseinmass EXPERIMENTAL SENSITIVITY
to the neutrinos by precision electroweakconstraints [2].
We consider processes where only the lightest neu- We select events with the ℓ±ℓ±jj signature:
trinoisproduced,providingthemostmodel-independent
bound on this theory. In future work, we will study the • twolike-signedreconstructedleptons(eorµ),each
effect of the second neutrino and fourth generation lep- with pT >20 GeV and |η|<2.0
ton. For this analysis, we treat the lepton and second
• at least two reconstructed jets, each with p > 15
neutrino as infinitely massive, corresponding to a limit T
where M,m go to infinity with m2D fixed. GeV and |η|<2.5
D M
The lighter neutrino mass eigenstate is the Majorana Inthecasethattwojetsarereconstructed,themassof
fermionN =Nc+N . Thecouplingforpairproduction the N can be reconstructedfrom the two jets and either
L L
via the Z is through the coupling L =Z Jµ where of the leptons. In the case that three jets are recon-
Z µ
structed, we form the mass of the N from the invariant
Jµ = e (N¯1γµγ5N1) mass of the two jets which are closest to the W mass,
2sinθW cosθW and either of the leptons. In the case that four jets are
reconstructed,the massofthetwoNscomefromthe ljj
The heavy neutrino will decay through N → W±l∓ assignments that give the best W masses and the small-
(the neutralcurrentprocessis suppressed.) Note that N est difference between the two reconstructed m s. See
N
candecaytoeithersignoflepton,givinglike-signleptons Figure 2.
inhalfoftheevents,seeFig1. Weassumethattheheavy
neutrinodecayspromptly;thiswillbethecaseunlessthe
mixing between the fourth generationand the first three
s 450
is extremely small [12]. nt
ve 400 2j
WeconsiderthepossibledecaymodesN →W(e,µ,τ). E 3j
In a hadron collider, backgrounds to τ leptons are much 350 4j
larger and efficiencies are much lower than for e and µ, 300
giving the τ decay mode little power. We consider two
250
cases,(a)µµ, inwhichthe non-τ decaysappearsolelyas
200
muons: BR(N →Wµ) = 1 - BR(N →Wτ); and (b) ℓℓ,
with ℓ = e,µ in which the non-τ decays appear as both 150
electrons and muons: BR(N → Wµ) + BR(N → We) 100
= 1 - BR(N → Wτ). The µ±µ± mode has significantly
50
smaller backgroundrate than ℓ±ℓ±.
80 100 120 140 160 180 200
Neutrino Mass [GeV/c2]
FIG.2: ReconstructedMajorananeutrino(N)massinevents
with 2, 3 or at least 4 jets for mN =150 GeV/c2.
Backgrounds
FIG. 1: Pair production of the heavy Majorana neutrino N At the Tevatron, the largest backgrounds to the
via a Z boson, and subsequentdecay W±l∓. ℓ±ℓ±jj signature come from Wγ or WZ production or
misidentifiedleptons[22]eitherfromsemi-leptonictt¯de-
If the N decays to ℓW, then the decay of NN → cays or direct W+ jets production.
ℓWℓW canbecategorizedbythedecaysoftheW bosons. For the Tevatron, we extrapolate the number of ex-
If both Ws decay hadronically,we expect approximately pected backgrounds events from Ref. [22] to a dataset
3
with 5 fb−1, use madgraph [23] to model the kinemat- struct frequentist intervals. If the N does not exist and
icsoftheevents,pythia[24]forshoweringandaversion no excess is seen, the median expected upper limits on
ofpgs[25]tunedtodescribetheperformanceoftheCD- the cross-section are given in Table I and Fig. 4. In
FII detector. 5 fb−1, with BR(N → µW) = 100%, a single Tevatron
At the LHC, the diboson contribution includes an ad- (LHC) experiment could expect to set a 95% lower limit
ditional process, qq → W±W±q′q′, which directly pro- of m > 175 (300) GeV. The limits as a function of
N
duces the ℓ±ℓ±jj signature. For the LHC, we calculate BR(N → τW) are given in Fig. 5. If the N does exist,
the size and kinematics of each contribution using mad- a 3σ excess would be observed in the regions shown in
graph, use pythia for showering and a version of pgs Fig. 5.
tuned to describe the performance of the ATLAS detec-
tor.
Figure 3 shows the reconstructed mass shape for N n [fb] n [fb]102
pair production and for the backgrounds in the µ±µ±jj ctio 10 ctio
e e
case. oss-s oss-s 10
Cr 1 Cr
1
1.4 Zfi NN 10-1 100 15n0 Mass [2G00eV/c2] 10-1 100 200n Ma3s00s [GeV4/0c02]
4 4
1.2
Diboson
FIG. 4: Theoretical cross-section for N production and de-
1
caytoℓ±Wℓ±W andmedianexpected95%C.Lexperimental
0.8 Fakes cross-section upperlimitsin 5fb−1 ofTevatrondata(left) or
LHCdata (right), assuming BR(N µW)=100%.
0.6 →
0.4
) 1 ) 1
0.2 tW tW
0.8 0.8
fi 1 fi 1
80 100 120 140 160 180 200 nR( 0.6 nR( 0.6
Neutrino Mass [GeV/c2] B B
0.4 0.4
0.2 0.2
0 100 150 0 200 250 300
25 Neutrino Mass [GeV/c2] Neutrino Mass [GeV/c2]
Zfi NN
) 1 ) 1
20 tW tW
diboson 0.8 0.8
fi 1 fi 1
15 fakes nR( 0.6 nR( 0.6
B B
0.4 0.4
10
0.2 0.2
0100 120 140 0100 150 200
5
Neutrino Mass [GeV/c2] Neutrino Mass [GeV/c2]
0 FIG. 5: Median expected 95% C.L experimental exclusion
100 150 200 250 300 350 400 450 500
Neutrino Mass [GeV/c2] (top) or 3σ evidence (bottom) in 5 fb−1 of Tevatron (left)
LHCdata(right)asafunctionofBR(N Wτ). Twodecay
cases are shown: µ±µ± (black) or ℓ±ℓ±→(red), as defined in
FIG.3: ExpectedreconstructedneutrinomassforN produc- thetext.
tionwithmN =150GeV/c2,andbackgroundstotheµ±µ±jj
signature in 5 fb−1 of Tevatron data (top) or 10 TeV LHC
data (bottom). CONCLUSIONS
Thes-channelpairproductionofheavyMajorananeu-
Expected Limits and Discovery Potential trinos via a Z boson (Z →NN →WℓWℓ) is a powerful
discoverymodeathadroncolliders. With5 fb−1ofdata,
Weperformabinnedlikelihoodfitinthereconstructed the Tevatron can significantly extend the limits on such
N mass,andusethe unifiedorderingscheme[26]tocon- neutrinosto175GeV/c2,anda3σevidenceispossibleif
4
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