Supersymmetric theories generally have new flavor violation sources in the
squark and slepton mass matrices. If significant lepton flavor violation
exists, selectron and smuon should be nearly degenerate. This leads to the
phenomenon of slepton oscillations, which is analogous to neutrino
oscillations, if sleptons are produced at the Next Linear Collider. The direct
slepton production at the Next Linear Collider provides a much more powerful
probe of lepton flavor violation than the current bounds from rare processes,
such as $\mu \to e\gamma$.Comment: 5 pages, LaTeX, 1 figure, Talk presented at the 1st Symposium on
  Flavor-Changing Neutral Currents, Feb.19-21, 1997, Santa Monica, Californi

Cheng, Hsin-Chia

English

arXiv

Supersymmetric theories generally have new flavor violation sources in the squark and slepton mass matrices. If significant lepton flavor violation exists, selectron and smuon should be nearly degenerate. This leads to the phenomenon of slepton oscillations, which is analogous to neutrino oscillations, if sleptons are produced at the Next Linear Collider. The direct slepton production at the Next Linear Collider provides a much more powerful probe of lepton flavor violation than the current bounds from rare processes, such as {mu} {r_arrow} e{gamma}. 4 refs., 1 fig

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. Fermi National Accelerator Laboratory f e o d F -  ~ 7 o a  I 1F. FERMILAB-CONF-97/089-T April, 1997 Sup ersymmet r ic Lepton Flavor Violat ion at the NLC . Hsin-Chia Cheng Fermi National Accelerator Laboratory P.O. Box 500 Batauia, IL  6051 0 Abstract Supersymmetric theories generally have new flavor violation sources in the squark and slepton m a s  matrices. If significant lepton flavor vi- olation exists, selectron and smuon should be nearly degenerate. This leads to the phenomenon of slepton osciilations, which is analogous to neutrino oscillations, if sleptons are produced at the Next Linear Collider. The direct slepton production at the Next Linear Collider provides a much more powerful probe of lepton flavor violation than the current bounds from rare processes, such as p + e7.  Talk given at the 1st Symposium on Flavor-Changing Neutral Currents -Present and Future Studies February 19-21, 1997, Santa Monica, California x . 1 1 ,  Operated by Univenltces Research Assoaalmn Inc. under Contrad No. DE-AC02-76CH03000 with the United Stales Oepariment of Energy 'iF Portions of this document may be illegible in electronic image products. Images are produced fiom the best available original dOCUment Disclaimer This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of thew employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefilness of any in&onnation, apparatus, product, or process disclosed, or represents t h  its use would not infringe privately owned rights. Reference herein to any specific comrcial product, process, or service by tra& name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Distribution Approved for public release: further dissemination unlimited. Weak scale supersymmetry (SUSY) may provide a solution to the gauge hierarchy problem and is one of the most attractive candidates beyond the standard model (SM). If SUSY is discovered, measuring the masses and cou- plings of the superpartners will become the focus of study. However, in most SUSY extensions of the SM, mass matrices of the fermions and their scalar superpartners are not diagondized in the same basis. New flavor mixing matrices W, analogous to the CKM matrix, will appear at the gaugino ver- tices. These new mixing matrices may provide important clues to the flavor structure and therefore should also be an important subject of study. Rare flavor-changing processes, such as p + ey, and neutral meson mixing already provide important constraints on these mixing matrices through the virtual effects of superpartners. However, as will be seen below, if the superpartners are produced in the future colliders, it is possible to probe these mixings di- rectly and much more powerfully at the colliders. The talk is based on work done with N. Arkani-Hamed, J.L. Feng, and L.J. Hall [l]. In this study we will concentrate on the mixing in the slepton sector. The choice is motivated by the theoretical prejudice that sleptons are probably lighter than squarks and hence more likely to be found first, and also by the absence of the lepton flavor violation in SM, which means that any lepton flavor violation should come from the supersymmetric origin. We also spe- cialize to the case of two generation mixing, in particular ER - F R  mixing for simplicity. The current experimental bound[2], B ( p  -+ er )  < 4.9 x lo'", put strong constraints on the slepton masses and mixings. If the mixing angle W12 = sin 8 is not small, the first two generation slepton masses have to be quite de- generate for the flavor-changing process ,!A + ey to be super-GIM suppressed. The bound constrains the product sin 26Am2, where Am2 = ma, - mgR, and is plotted in Fig. 1 for some choices of the SUSY parameters. If sleptons are produced at colliders, flavor mixing will lead to the inter- esting slepton oscillation phenomenon analogous to neutrino oscillation. The flavor-violating signal we hope to observe at the e+e- colliders is e+e- + slepton pairs + e*pFiogo .  For simplicity, we consider the case where the 1 right-handed sleptons decay directly to the LSP. For large Am2(> mr, where r is the slepton decay width), the 2 and f i  are already out of phase when they decay and there is no interference between them. The flavor-violating cross section is given simply by multiplying the flavor conserving cross sec- tions by appropriate factors of the branching ratios B(E + p i o )  = B(F -+ ego) = 3 sin2 20, When Am2 < m r ,  the interference effects become impor- tant. One finds for uncorrelated slepton production, the flavor-violating cross section will be suppressed by the factor e = (Am2)2/[(Am2)2 + 4m2r2]. For correlated production as in the case we study, the formulae are somewhat more complicated but contain the similar suppression factor. The detailed derivation for different cases can be found in Ref.[3]. We can see that 6 N 1 for Am2 >> rnr and + 0 as Amz + 0 as we expect. When sleptons are directly produced at high energy colliders, the super-GIM suppression of the flavor-violating processes only occurs when Am < r, in constrast with the low energy flavor-violating processes which are suppressed by Am2/m2. This makes the high energy colliders a more powerful probe of the flavor violation than the low energy processes. Having discussed the flavor-violating cross sections, we consider the possi- bili ty of detecting such flavor-violating signals at future colliders. If sleptons are discovered at LEP 11, their masses should be below 85-90 GeV. The con- straint from p + ey is strong for such light sleptons. We found that the prob- ing power at LEP I1 can still be competitive with p + ey for moderate tan 0, though the regions of parameter space that can be probed are quite limited [l]. We concentrate on the study of lepton flavor violation at the Next Linear Collider (NLC) here, with assumed design energy f i  = 500GeV and luminos- ity 50 fb-l/yr in the e+e- mode. We consider a case with the following SUSY parameters: mgR,rnCR N 200GeV, MI = M2/2 = 150GeV, p = -400GeV. The LSP is almost pure bin0 and the cross section has little dependence on p and M2 if they are large. We calculate the cross section of the flavor-violating signal as a function of sin28 and Am2 and the result is shown in Fig. 1. The major backgrounds come from W+W-, e*vWF, (e+e-)W+W-, and T+T- events. Efficient cuts[4] and a right-polarized e- beam can effectively 2 isolate the flavor-violating signal. Given a year’s running at design luminos- ity, the required 5a  signal is 3.8(3.6) fb for 90%(95%) right-handed beam polarization. We can see that NLC is a powerful probe of the flavor-violating parameter space, extending to sin28 - 0.1 and much beyond the bounds from p + e7. An intriguing feature of the NLC is its ability to run in e-e- mode. Although the luminosity may be degraded somewhat from the e+e- mode, the cross section is generally higher for most regions of the parameter space and many troublesome backgrounds, for example, W pair production, are completely eliminated. In addition, both beams may be polarized and there are essentially no backgrounds for RR beam polarization. The dominant background is e-vW-, arising from imperfect beam polarization. This can be the most powerful mode for probing flavor violation. Even without additional cuts, the mixing angle may be probed down to sin28 - 0.04[1], far beyond the current bounds. We find that , if sleptons are kinematically accessible at the NLC, the e p  signal will provide either stringent limits on slepton mixing or the exciting discovery of SUSY lepton flavor violation. References [l] N. Arkani-Hamed, H.-C. Cheng, J.L. Feng and L.J. Hall, Phys. Rev. Lett. 77, 1937 (1996). [2] R.D. Bolton, et af., Phys. Rev. D 38, 2077 (1988). [3] N. Arkani-Hamed, H.-C. Cheng, J.L. Feng and L.J. Hall, LBNL-40147, Fermilab-Pub-97/078-T, hep-ph/9704205. [4] R. Becker and C. Vander Velde, in Proceedings of the European Meeting of the Working Groups on Physics and Ezperiments at Linear e+e- Colliders, ed. P.M. Zerwas, Report No. DESY-93-123C, p. 457. 3 ' *-: 1 -5 Figure 1: Contours of constant ,(e+,, + e*pFzogo) (solid) in fb for NLC, with mzR,mcR N 200GeV, MI = 150GeV, p = -400GeV, M2 = 300GeV, and tan@ = 2. The thick gray contour represents the experimental reach for one year's integrated luminosity. The dashed (dotted) contour is the current experimental bound from B(p  + e r )  = 4.9 x lo-" for the same parameters and tan @ = 2(50), mz,, mp, = 350GeV. (The cross section contours are basically unchanged for tan p = 50.) 4 

Supersymmetric lepton flavor violation at the NLC

Supersymmetric theories generally have new flavor violation sources in the squark and slepton mass matrices. If significant lepton flavor violation exists, selectron and smuon should be nearly degenerate. This leads to the phenomenon of slepton oscillations, which is analogous to neutrino oscillations, if sleptons are produced at the Next Linear Collider. The direct slepton production at the Next Linear Collider provides a much more powerful probe of lepton flavor violation than the current bounds from rare processes, such as {mu} {yields} e{gamma}

F Fermi National Accelerator LaboratoryFERMILAB-Conf-97/089-TSupersymmetric Lepton Flavor Violation at the NLCHsin-Chia ChengFermi National Accelerator LaboratoryP.O. Box 500, Batavia, Illinois 60510April 1997Presented at the 1st Symposium on Flavor-Changing Neutral Currents - Present and Future Studies,Santa Monica, California, February 19-21, 1997Operated by Universities Research Association Inc. under Contract No. DE-AC02-76CH03000 with the United States Department of EnergyDisclaimerThis report was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither the United States Government nor any agency thereof, nor any oftheir employees, makes any warranty, expressed or implied, or assumes any legal liability orresponsibility for the accuracy, completeness, or usefulness of any information, apparatus,product, or process disclosed, or represents that its use would not infringe privately ownedrights. Reference herein to any specic commercial product, process, or service by tradename, trademark, manufacturer, or otherwise, does not necessarily constitute or imply itsendorsement, recommendation, or favoring by the United States Government or any agencythereof. The views and opinions of authors expressed herein do not necessarily state or reectthose of the United States Government or any agency thereof.DistributionApproved for public release; further dissemination unlimited.hep-ph/9704289   10 Apr 97FERMILAB-CONF-97/089-Thep-ph/9704289April, 1997Supersymmetric Lepton Flavor Violationat the NLCHsin-Chia ChengFermi National Accelerator LaboratoryP.O. Box 500Batavia, IL 60510AbstractSupersymmetric theories generally have new avor violation sourcesin the squark and slepton mass matrices. If signicant lepton avor vi-olation exists, selectron and smuon should be nearly degenerate. Thisleads to the phenomenon of slepton oscillations, which is analogousto neutrino oscillations, if sleptons are produced at the Next LinearCollider. The direct slepton production at the Next Linear Colliderprovides a much more powerful probe of lepton avor violation thanthe current bounds from rare processes, such as ! e.Talk given at the 1st Symposium on Flavor-Changing Neutral Currents{Present and Future StudiesFebruary 19-21, 1997, Santa Monica, CaliforniaWeak scale supersymmetry (SUSY) may provide a solution to the gaugehierarchy problem and is one of the most attractive candidates beyond thestandard model (SM). If SUSY is discovered, measuring the masses and cou-plings of the superpartners will become the focus of study. However, in mostSUSY extensions of the SM, mass matrices of the fermions and their scalarsuperpartners are not diagonalized in the same basis. New avor mixingmatrices W , analogous to the CKM matrix, will appear at the gaugino ver-tices. These new mixing matrices may provide important clues to the avorstructure and therefore should also be an important subject of study. Rareavor-changing processes, such as ! e, and neutral meson mixing alreadyprovide important constraints on these mixing matrices through the virtualeects of superpartners. However, as will be seen below, if the superpartnersare produced in the future colliders, it is possible to probe these mixings di-rectly and much more powerfully at the colliders. The talk is based on workdone with N. Arkani-Hamed, J.L. Feng, and L.J. Hall [1].In this study we will concentrate on the mixing in the slepton sector. Thechoice is motivated by the theoretical prejudice that sleptons are probablylighter than squarks and hence more likely to be found rst, and also by theabsence of the lepton avor violation in SM, which means that any leptonavor violation should come from the supersymmetric origin. We also spe-cialize to the case of two generation mixing, in particular ~eR  ~R mixing forsimplicity.The current experimental bound[2], B(! e) < 4:910 11, put strongconstraints on the slepton masses and mixings. If the mixing angle W12 =sin  is not small, the rst two generation slepton masses have to be quite de-generate for the avor-changing process ! e to be super-GIM suppressed.The bound constrains the product sin 2m2, where m2 = m2~eR m2~R, andis plotted in Fig. 1 for some choices of the SUSY parameters.If sleptons are produced at colliders, avor mixing will lead to the inter-esting slepton oscillation phenomenon analogous to neutrino oscillation. Theavor-violating signal we hope to observe at the e+e  colliders is e+e  !slepton pairs ! e ~0~0. For simplicity, we consider the case where the1right-handed sleptons decay directly to the LSP. For large m2( m , where  is the slepton decay width), the ~e and ~ are already out of phase whenthey decay and there is no interference between them. The avor-violatingcross section is given simply by multiplying the avor conserving cross sec-tions by appropriate factors of the branching ratios B(~e ! ~0) = B(~ !e~0) = 12sin2 2, When m2 < m , the interference eects become impor-tant. One nds for uncorrelated slepton production, the avor-violating crosssection will be suppressed by the factor  = (m2)2=[(m2)2 + 4m2 2]. Forcorrelated production as in the case we study, the formulae are somewhatmore complicated but contain the similar suppression factor. The detailedderivation for dierent cases can be found in Ref.[3]. We can see that  ' 1for m2  m  and  ! 0 as m2 ! 0 as we expect. When sleptons aredirectly produced at high energy colliders, the super-GIM suppression of theavor-violating processes only occurs when m <  , in constrast with thelow energy avor-violating processes which are suppressed by m2=m2. Thismakes the high energy colliders a more powerful probe of the avor violationthan the low energy processes.Having discussed the avor-violating cross sections, we consider the possi-bility of detecting such avor-violating signals at future colliders. If sleptonsare discovered at LEP II, their masses should be below 85-90 GeV. The con-straint from ! e is strong for such light sleptons. We found that the prob-ing power at LEP II can still be competitive with ! e for moderate tan ,though the regions of parameter space that can be probed are quite limited[1]. We concentrate on the study of lepton avor violation at the Next LinearCollider (NLC) here, with assumed design energyps = 500GeV and luminos-ity 50 fb 1=yr in the e+e  mode. We consider a case with the following SUSYparameters: m~eR;m~R ' 200GeV; M1 = M2=2 = 150GeV;  =  400GeV.The LSP is almost pure bino and the cross section has little dependence on andM2 if they are large. We calculate the cross section of the avor-violatingsignal as a function of sin 2 and m2 and the result is shown in Fig. 1.The major backgrounds come fromW+W , eW, (e+e )W+W , and+  events. Ecient cuts[4] and a right-polarized e  beam can eectively2isolate the avor-violating signal. Given a year's running at design luminos-ity, the required 5 signal is 3.8(3.6) fb for 90%(95%) right-handed beampolarization. We can see that NLC is a powerful probe of the avor-violatingparameter space, extending to sin 2  0:1 and much beyond the boundsfrom ! e.An intriguing feature of the NLC is its ability to run in e e  mode.Although the luminosity may be degraded somewhat from the e+e  mode,the cross section is generally higher for most regions of the parameter spaceand many troublesome backgrounds, for example, W pair production, arecompletely eliminated. In addition, both beams may be polarized and thereare essentially no backgrounds for RR beam polarization. The dominantbackground is e W , arising from imperfect beam polarization. This can bethe most powerful mode for probing avor violation. Even without additionalcuts, the mixing angle may be probed down to sin 2  0:04[1], far beyondthe current bounds. We nd that , if sleptons are kinematically accessible atthe NLC, the e signal will provide either stringent limits on slepton mixingor the exciting discovery of SUSY lepton avor violation.References[1] N. Arkani-Hamed, H.-C. Cheng, J.L. Feng and L.J. Hall, Phys. Rev.Lett. 77, 1937 (1996).[2] R.D. Bolton, et al., Phys. Rev. D 38, 2077 (1988).[3] N. Arkani-Hamed, H.-C. Cheng, J.L. Feng and L.J. Hall, LBNL-40147,Fermilab-Pub-97/078-T, hep-ph/9704205.[4] R. Becker and C. Vander Velde, in Proceedings of the European Meetingof the Working Groups on Physics and Experiments at Linear e+e Colliders, ed. P.M. Zerwas, Report No. DESY-93-123C, p. 457.3sin 2θ∆mm22___10 10101010101-2-2-1-1-3-4501010.10.01Figure 1: Contours of constant (e+e R ! e ~0~0) (solid) in fb for NLC,with m~eR;m~R ' 200GeV, M1 = 150GeV;  =  400GeV; M2 = 300GeV,and tan = 2. The thick gray contour represents the experimental reach forone year's integrated luminosity. The dashed (dotted) contour is the currentexperimental bound from B(! e) = 4:9 10 11 for the same parametersand tan  = 2(50), m~eL;m~L = 350GeV. (The cross section contours arebasically unchanged for tan  = 50.)4

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Supersymmetric Lepton Flavor Violation at the NLC

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