Proposal for chiral bosons search at LHC via their unique new signature

0.16 MB·English

by  M. V. Chizhov

Proposal for chiral bosons search at LHC via their unique new signature
M. V. Chizhov
Centre for Space Research and Technologies, Faculty of Physics,
University of Sofia, 1164 Sofia, Bulgaria
V. A. Bednyakov and J. A. Budagov
Dzhelepov Laboratory of Nuclear Problems,
Joint Institute for Nuclear Research, 141980, Dubna, Russia
Theresonance productionof newchiralspin-1bosonsandtheirdetection throughtheDrell–Yan
process at the CERN LHC is considered. Quantitative evaluations of various differential cross-
sections of the chiral bosons production are made within the CalcHEP package. The new neutral
8 chiral bosons can be observed as a Breit–Wigner resonance peak in the invariant dilepton mass
0 distribution, as usual. However, unique new signatures of the chiral bosons exist. First, there is
0 no Jacobian peak in the lepton transverse momentum distribution. Second, the lepton angular
2 distributionintheCollins-Soperframeforthehighon-peakinvariantmassesoftheleptonpairshas
a peculiar “swallowtail” shape.
n
a
PACSnumbers: 12.60.-i,13.85.-t,14.80.-j
J

8
2
I. INTRODUCTION alternative proposal for the search for new spin-1 heavy

bosons.
]
h The gauge interactions are the only well-established In what follows we will concentrate only on resonance
p fundamental interactions in Nature. Nevertheless, the production of the excited neutral heavy bosons at the
-
p additional Yukawa interactions of the Higgs bosons are CERN LHC and their searchvia the very clean dilepton
e also necessary for a self-consistent construction of the Drell–Yan process. To be more concrete in our predic-
h Standard Model (SM). They are still waiting for their tions we will use a simple phenomenological model of
[ experimental verificationas a priority part of the CERN excited bosons with the chiral couplings as in eq. (2).

1 LHC program. Besides this, many other programs and
v searcheshavebeen approved[1, 2] to explorefurther the
5 LHC potential. One of them consists in searching of hy- II. THE MODEL
3 ∗
pothetical excited fermions f and their magnetic mo-
2
4 ment (Pauli) type couplings to ordinary matter Newheavyneutralgaugebosonsarepredictedinmany
. extensions of the Standard Model (SM). They are asso-
1 f∗ = g f¯∗σµνf(∂ Z ∂ Z )+h.c., (1) ciated with additional U(1)′ gauge symmetries and are
0 Lexcited 2Λ µ ν − ν µ ′
8 generically called Z . The gauge interactions of these
0 where the parameter Λ is connected to the composite- bosons with matter lead to a specific angular distribu-
: tionoftheoutgoingleptoninthedileptoncenter-of-mass
v ness mass scale of the new physics. Up to now their
reference frame with respect to the incident parton
i searcheshavebeenfulfilledatthepowerfulcolliders,such
X
as LEP [3], HERA [4] and Tevatron [5]. Due to their
ar anomalous type of couplings they lead to a unique ex- ddcσosZθ′∗ ∝1+ASYM·cosθ∗+cos2θ∗, (3)
perimental signature for their detection.
Inthispaperwewouldliketointerprettheinteractions which at present is interpreted as a canonical signature
(1)fromadifferentpointofview,introducingtheexcited for the intermediate bosons with spin 1. The coeffi-
boson states cient ASYM defines the backward-forward asymmetry,
′
LZex∗cited = 2gΛf¯σµνf(cid:0)∂µZν∗−∂νZµ∗(cid:1) (2) depInenaddidnigtioonn,Pa-npoatrhiteyr otyfpZe ocfousppilnin-1gsbtoosomnastmtear.y exist,
whichleads to a different signaturein the angulardistri-
instead of the fermionic ones. It has been shown [6] bution. This follows from the presence of different types
that these states with analogousinteractions are present ofrelativisticspin-1fermionchiralcurrentsψ¯γµ(1 γ5)ψ
amonghadronresonances. Suchtypeofnew electroweak and ∂ν[ψ¯σµν(1 γ5)ψ], which can couple to th±e cor-
heavybosonsZ∗ couldalsobe interestingobjects forex- responding boso±ns. The hadron physics of the quark-
perimental searches due to their different couplings to antiquark meson states provides us with an example of
the ordinary fermions in comparison with the gauge Z′ such kind of interactions and a variety of spin-1 states.
couplings. Alas, this proposal is not popular now and It was pointed out [6] that three different quantum
is not present in the experimental programs, yet. With numbers JPC of existing spin-1 mesons, 1−−, 1++
this note we wouldlike to establishthe status quo of our and 1+−, cannot be assigned just to two vector q¯γµq
2
and axial-vector q¯γµγ5q quark states. So, the addi- III. THE EXPERIMENTAL SIGNATURE
tional antisymmetric quark tensor currents ∂ (q¯σµνq)
ν
and∂ (q¯σµνγ5q)arerequired,whichalsodescribevector
ν
Up to now, the excess in the Drell–Yan process with
and axial-vector meson states, but with different trans-
high-energy invariant mass of the lepton pairs remains
formation properties with respect to Lorentz group and
with different quantum numbers 1−− and 1+−, respec- the clearest indication of the heavy boson production at
the hadron colliders. So, using only a modest integrated
tively. This example demonstrates that both the pure
luminosity of 200 pb−1 collected during RUN II, the D0
tensorstates,b mesons,andmixedcombinationsofvec-
1 ′
′ Collaboration puts tight restrictions on the Z masses
tor and tensor states, ρ and ρ mesons, exist.
for the different models from the dielectron events [10]:
The mesons coupled to the tensor quark currents are
some types of “excited” states as far as the only orbital MZS′M < 780 GeV, MZη′ < 680 GeV, MZψ′ < 650 GeV,
angular momentum with L = 1 contributes to the total MZ′ < 640 GeV and MZ′ < 575 GeV. Comparable
χ I
statistics in the dimuon channel leads approximately to
angular moment, while the total spin of the system is
zero. This property manifests itself in their derivative the same constraint MZ′ < 680 GeV [11]. The CDF
SM
constraintsfromthedielectronchannelarebasedonmore
couplings to matter and a different chiral structure of
data, 1.3 fb−1, which leads to tighter restrictions [12]:
the interactions in comparison with the gauge ones. In
contrastwiththegaugecouplings,whereeitheronlyleft- MZ′ < 923 GeV, MZ′ < 891 GeV, MZ′ < 822 GeV,
SM η ψ
handed or right-handed fermions participate in the in- MZ′ < 822 GeV and MZ′ < 729 GeV. Therefore, our
χ I
teractions, the tensor currents mix both left-handed and mass choice (5) for the new bosons does not contradict
right-handedfermions. Therefore,liketheHiggsparticles the Tevatron constraints. At the same time such bosons
the corresponding bosons carry a nonzero chiral charge. couldbeobservedasresonancepeaksontheZ bosontail
To our knowledge, such bosons were first introduced by in the invariant dilepton mass distribution at the LHC
Kemmer [7] and they naturally appear in the extended (Fig.1[14])alreadyinthefirstdaysofthephysicalruns.
conformal supergravity theories [8].
There are searches for the excited lepton and quark
states, but not for the boson ones. This paper fills this
gap considering properties of the neutral heavy chiral
bosons which help us to disentangle their production at
the hadroncolliders fromother particles. In orderto de- V]
e
tect them in the Drell-Yan processes they should couple G 100
b/
to the down type of fermions p
M [ee 10
g
Lexcited = 2√2Λ(cid:0)ℓ¯σµνℓ+d¯σµνd(cid:1)(cid:0)∂µZν∗−∂νZµ∗(cid:1). (4) d/d 1
0,1
Let us assume for simplicity that Λ is equal to the mass
∗
of the new Z boson 0,01
M 1 TeV (5) 1E-3
≈
1E-4
and g being the coupling constant of the SU(2) weak
W
gauge group. For comparison we will consider topologi- MZ 200 400 600 800 1000 1200
′
cally analogous gauge interactions of the Z boson Mee [GeV]
g
Lgauge = 2(cid:0)ℓ¯γµℓ+d¯γµd(cid:1)Zµ′ (6) FIG. 1: The invariant dilepton mass distributions for the
gauge Z′ boson (blue) and the excited chiral Z∗ boson (red)
with the same mass M. The coupling constants are cho- with theDrell–Yan SMbackground at the CERN LHC.
sen in such a way that all fermionic decay widths in the
Born approximation of the both bosons are identical. It
means that their total production cross-sections at the Thepeaksintheinvariantmassdistributionsoriginate
hadron colliders are nearly equal up to next-to-leading from the Breit–Wigner propagator form, which is the
order corrections. Their leptonic decay width same both for the gauge and chiral bosons in the Born
approximation. However, the common wisdom, that a
g2 peak in the invariant mass distribution of the two final
Γ = M 2.8 GeV. (7)
ℓ particles must correspondto the Jacobianpeaks in their
48π ≈
transverse momentum distributions p , is not valid for
T
issufficientlynarrowsothattheycanbeidentifiedasres- the chiral bosons due to the following fact. The main
onancesatthehadroncollidersintheDrell–Yanprocess. feature of the interactions (4) consists in different angu-
3
lar distribution of final fermions [9]

ddcσoZs∗θ∗ ∝cos2θ∗ (8) [fb]e0,25
d
/
in comparison with the distribution (3) for the gauge d0,20
interactions. Itleadsto astepwiseleptontransversemo-
mentum distribution, rather than to the Jacobian peak
0,15
at the kinematical endpoint M/2 for the gauge bosons

(Fig. 2). Therefore, already the lepton transverse mo-
0,10
0,05

V]100
e 0,00
G -3 -2 -1 0 1 2 3
b/ 10
p e
[T
p 1
d
d/ FIG.3: Thedifferentialcross-sectionsforthegaugeZ′ boson
0,1
(blue)andtheexcitedchiralZ∗boson(red)decayingtoalep-
0,01 tonpairwiththeinvariantmass800GeV<Mℓℓ <1200GeV
as functions of the lepton pseudorapidity at the Fermilab
1E-3 Tevatron.
1E-4
1E-5

MZ/2 100 200 300 400 500 600 b]0,4
pT [GeV] [pe
d
/
FIG. 2: The differential cross-sections for the gauge Z′ bo- d0,3
son (blue) and the excited chiral Z∗ boson (red) with the
Drell–Yan SM background as functions of the lepton trans-
verse momentum at theCERN LHC.
0,2
mentum distribution demonstrates a difference between
the gaugeandchiralbosons. Inordertomakemoresub-
0,1
stantialconclusions,letusinvestigateotherdistributions
selecting only “on-peak”events with the invariantdilep-
ton masses in the range 800 GeV <M < 1200 GeV.
ℓℓ
According to the eq. (8), there exists a characteristic 0,0
-3 -2 -1 0 1 2 3
plane, perpendicular to the beam axis in the parton rest
e
frame, where the emission of the final-state pairs is for-
bidden. Thenonzeroprobabilityintheperpendiculardi-
rectioninthelaboratoryframeisdue tothe longitudinal FIG.4: Thedifferentialcross-sectionsforthegaugeZ′ boson
boostsofcollidingpartons. So,attheFermilabTevatron (blue)andtheexcitedchiralZ∗boson(red)decayingtoalep-
the production of such heavy bosons occurs almost at tonpairwiththeinvariantmass800GeV<Mℓℓ <1200GeV
the threshold with approximately zero longitudinal mo- asfunctionsofthelepton pseudorapidityattheCERNLHC.
menta. Hence,theleptonpseudorapiditydistributionfor
the chiral bosons has a minimum at η =0 (Fig. 3). On
ℓ
the other hand the CERN LHC is sufficiently powerful (the Collins-Soper frame). In the Fig. 5 we compare the
to produce heavy bosons with a mass M = 1 TeV with differentialcross-sectionsforthe gaugeZ′ bosonandthe
high longitudinal boosts. Therefore, the pseudorapidity excitedchiralZ∗ bosondecayingtotheleptonpairswith
distributionsforthe gaugeandchiralbosonsatthe LHC the invariantmass 800 GeV <M < 1200GeV as func-
ℓℓ
look similar (Fig. 4). tions of cosθ∗ . Instead of a smoother angular distribu-
CS
Crucialconfirmation of the existence of the new inter- tion for the gauge interactions, a peculiar “swallowtail”
actions (4) should come from the analysis of the angular shape of the chiral boson distribution occurs with a dip
∗
distributionofthefinalleptonswithrespecttotheboost at cosθ = 0. It will indicate the presence of the new
CS
directionoftheheavybosonintherestframeofthelatter interactions. Neither scalars nor other particles possess
4
such a type of angular behavior.

b]
p
[S 1 IV. CONCLUSIONS
C
*
s
o
dc Inthis paperwehaveconsideredthe experimentalsig-
/ ∗
d natures of the excited chiral heavy bosons Z and com-
0,1 pared them with the gauge Z′ bosons. It has been

stressedthatthechiralbosonshaveanewangulardistri-
bution, yet unknown for experimentalists. It leads to an
absence of the Jacobian peak in the transverse momen-
0,01 tum distribution and to a profound dip in the angular
distribution at the rest frame of the heavy chiral boson.
Thesefeaturescouldhelptodiscriminatethechiralboson
production from a resonance production of other parti-
-1 0 1
cles at the hadron colliders.
cos *CS
FIG.5: Thedifferentialcross-sectionsforthegaugeZ′ boson
(blue)andtheexcitedchiralZ∗boson(red)decayingtoalep- Acknowledgements
tonpairwiththeinvariantmass800GeV<Mℓℓ <1200GeV
as functions of cosθC∗S at theCERN LHC. We are grateful to V. G. Kadyshevsky,M. D. Mateev,
N. A. Russakovich, I. R. Boyko and R. V. Tsenov for
support and fruitful cooperation.
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