Searching for the Lightest Neutralino at Fixed Target Experiments

Most ongoing supersymmetry searches have concentrated on the high-energy frontier. High-intensity fixed target beamlines, however, offer an opportunity to search for supersymmetric particles with long lifetimes and low cross-sections in regions complementa

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Searching for the Lightest Neutralino at Fixed Target Experiments L.Borissov,J.M.Conrad,M.Shaevitz Columbia University,New York,NY,10027(February 1,2008)Most ongoing supersymmetry searches have concentrated on the high-energy frontier.High-intensity ?xed target beam-lines,however,o?er an opportunity to search for supersym-metric particles with long lifetimes and low cross-sections in regions complementary to the ones accessible to collider ex-periments.In this paper,we consider R -parity violating su-persymmetry searches for the lightest neutralino and use the NuTeV experiment as an example for the experimental sensi-tivity which can be achieved.I.MOTIV ATION A review of sypersymmetric models can be found in Ref.[1].We consider models where the lightest neu-tralino (?χ01)is the Lightest Supersymmetric Particle (LSP).If the LSP is allowed to decay,R -parity violation (R p )is required via the superpotential:W R p =λijk L i L j ¯E k +λ′ijk L i Q j ¯D k +λ′′ijk ¯U i ¯D j ¯D k (1)where i ,j and k are generation indices,L and Q are the lepton and quark SU (2)super?eld doublets,E ,U ,and D are the lepton and quark singlets and λijk ,λ′ijk ,and λ′′ijk are Yukawa-type couplings between the ?elds.This model is speci?ed by the squark and slepton masses;mass terms for the gauginos at the electroweak scale (M 1,M 2and M 3),the ratio of vacuum expectation values of the two neutral Higgses (tan β);a mass term mixing the two Higgs doublets (µ)and the values of the λ-couplings.

In the Minimal Supersymmetric Standard Model (MSSM),uni?cation is imposed at the GUT-scale,which leads to the relation:

M 1=

5

Most ongoing supersymmetry searches have concentrated on the high-energy frontier. High-intensity fixed target beamlines, however, offer an opportunity to search for supersymmetric particles with long lifetimes and low cross-sections in regions complementa

gion consisting of three large helium bags and six drift chambers(Fig.1).The decay channel is shielded by an upstream veto wall.

A.Production and decay

At the NuTeV target,?χ01’s can be pair-produced in the s-channel via a Z,or in the t-channel through an exchange of a squark(Fig.2).If the squark mass is suf-?ciently small,production can be enhanced.For our es-timates,however,we have chosen to work with sfermion masses of the order of800GeV,conservatively above present experimental bounds[8].Thus the only relevant production parameters are M1,M2,µ,and tanβ.Pro-duction is insensitive to M3,which is responsible for the gluino mass.

NuTeV uses a high intensity proton beam,but its center-of-mass energy(

√

192π3

(3) where K is an e?ective four-fermion coupling in?χ01decays (Fig.3)proportional to the?χ01f?f coupling and the R p couplingλijk.For a large region of SUSY parameter space

K～0.1 100GeV

100GeV (5)

λ133<0.006× 100GeV(6)

which come from current universality requirements and

limits onνe mass[13].

Eq.(4)can be rewritten in terms of the average decay

length in the lab frame:

l(cm)=0.3(βγ) m?f m?χ01 51

d?

d? ×(1?e??z/l)

d? ×1?e

?3.5×103cm

e1.5×105

cm

Most ongoing supersymmetry searches have concentrated on the high-energy frontier. High-intensity fixed target beamlines, however, offer an opportunity to search for supersymmetric particles with long lifetimes and low cross-sections in regions complementa

Using a Monte Carlo event generator[14,15],we per-form a scan of uMSSM parameter space for M1= 1,10,100GeV/c2,M2=0,...,400GeV/c2,µ=?200,...,200GeV/c2and tanβ=1.5,...,40.We set m?

f≈800GeV/c2and M3=1TeV.We found no strong

,as long as M1<<M2so that dependence on M1and m?

f

?χ01is mostly bino.We consider M1=1GeV/c2as indica-tive for the case accessible to NuTeV and similar?xed target experiments and present results for two represen-tative decay lengths:l=1.5×105cm,where sensitivity is optimal,and l=1.5×106cm,far past the detector, where sensitivity is signi?cantly reduced.The results for small and large tanβare shown on Fig.5and Fig.6 respectively.The exclusion regions in these plots are sim-ilar to the ones obtained by the LEP experiments[9]with the di?erence that the latter address the case when the ?χ01decays inside the LEP detectors while we consider the case of large?χ01decay length,well beyond any detector placed at the production vertex.

III.CONCLUSIONS

The work presented here is an attempt to motivate R p searches at?xed target experiments,since collider exper-iments run into sensitivity problems at low energy[8,9]. We argue that NuTeV and similar?xed target experi-ments,such as KTeV,may be in a unique position to complement collider searches.Recently,the work pre-sented in this paper has been applied by the NuTeV col-laboration to set a limit on neutralino production[23].

IV.ACKNOWLEDGEMENTS

We would like to thank to C.Quigg,J.Lykken,M. Carena,V.Barger,L.DiLella,P.Nienaber and the NuTeV collaboration.This research was supported by the U.S.Department of Energy and the National Science Foundation.

Most ongoing supersymmetry searches have concentrated on the high-energy frontier. High-intensity fixed target beamlines, however, offer an opportunity to search for supersymmetric particles with long lifetimes and low cross-sections in regions complementa

decay length 106cm

5 10

1 10

1.5 102 10

d Σ d m b s r a d

#e v e n t s

FIG.4.Contour plots of ?χ01

pair-production di?erential cross-section seen by the NuTeV detector versus ?χ01decay length in the lab frame.The shaded contours represent the number of ?χ0

1decays occuring inside the detector assuming perfect detector e?ciency.The four con?dence level contours represent exclusion regions (above the contours)following a Feldman and Cousins approach for 0signal events and 0back-ground [20].

-200-150-100-50050100150

200

050

100150200250300350400 l=1.5·105 cm

l=1.5·106 cm M 2 (GeV/c 2)

µ (GeV/c 2)

FIG.5.Sensitivity for the NuTeV experiment for small tan β=1.5

at M 1=1GeV /c 2for two representative decay lengths.Regions in uMSSM space inside the contour lines are excluded at 90%CL using a Feldman and Cousins approach (see Fig 4).

l=1.5·105 cm

400350

30025020015010050

0200

150

100500-50

-100-150-200 l=1.5·106 cm M 2 (GeV/c 2)

µ (GeV/c 2)

FIG.6.Sensitivity for the NuTeV experiment for large

tan β=30at M 1=1GeV /c 2.Regions in uMSSM space inside the contour lines are excluded at 90%CL using a Feld-man and Cousins approach (see Fig 4).

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