Searching for the Lightest Neutralino at Fixed Target Experi

1、searching for the lightest neutralino at fixed target experi 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 re

2、gions complementa a r x i v :h e p - p h / 7 1 9 5v 5 1 n o v 2 searching for the lightest neutralino at fixed target experiments l.borissov,j.m.conrad,m.shaevitz columbia university,new york,ny,10027(february 1,2021)most ongoing supersymmetry searches have concentrated on the high-energy frontier.h

3、igh-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 l

4、ightest 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 (?01)is the lightest supersymmetric particle (lsp).if the lsp

5、is allowed to decay,r -parity violation (r p )is required via the superpotential:w r p =ijk l i l j e k +ijk l i q j d k +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 ijk ,ijk ,a

6、nd 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 );a mass term mixing the two higgs doublets ()and th

7、e values of the -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 oppor

8、tunity 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,?01s can be pair

9、-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 bou

10、nds8.thus the only relevant production parameters are m1,m2,and tan.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( 1923 (3) where k is an e?ective four-fermion coupling in?01decays (fig.3)proportional

11、to the?01f?f coupling and the r p couplingijk.for a large region of susy parameter space k0.1 100gev 100gev (5) 1330.006 100gev(6) which come from current universality requirements and limits one mass13. eq.(4)can be rewritten in terms of the average decay length in the lab frame: l(cm)=0.3() m?f m?

12、01 51 d? d? (1?e?z/l) d? 1?e ?3.5103cm e1.5105 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 compl

13、ementa using a monte carlo event generator14,15,we per-form a scan of umssm parameter space for m1= 1,10,100gev/c2,m2=0,.,400gev/c2,=?200,.,200gev/c2and tan=1.5,.,40.we set m? f800gev/c2and m3=1tev.we found no strong ,as long as m1m2so that dependence on m1and m? f ?01is mostly bino.we consider m1=1

14、gev/c2as indica-tive for the case accessible to nutev and similar?xed target experiments and present results for two represen-tative decay lengths:l=1.5105cm,where sensitivity is optimal,and l=1.5106cm,far past the detector, where sensitivity is signi?cantly reduced.the results for small and large t

15、anare shown on fig.5and fig.6 respectively.the exclusion regions in these plots are sim-ilar to the ones obtained by the lep experiments9with the di?erence that the latter address the case when the ?01decays inside the lep detectors while we consider the case of large?01decay length,well beyond any

16、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 energy8,9. we argue that nutev and similar?xed target experi-ments,such as ktev,may be i

17、n 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 production23. iv.acknowledgements we would like to thank to c.quigg,j.lykken,m. carena,v.barger,l.dilella,p.nienaber and the nutev

18、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 particle

19、s with long lifetimes and low cross-sections in regions complementa decay length 106cm 5 10 1 10 1.5 102 10 d d m b s r a d #e v e n t s fig.4.contour plots of ?01 pair-production di?erential cross-section seen by the nutev detector versus ?01decay length in the lab frame.the shaded contours represent the number of ?0 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-g