TESTING LEPTON UNIVERSALITY WITH RARE PION AND KAON DECAYS Prepared for the 2011 DOE Intensity Frontier Workshop 1

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1 TESTING LEPTON UNIVESALITY WITH AE PION AND KAON DECAYS Prpard for th 2011 DOE Intnsity Frontir Workshop 1 Douglas Bryman Univrsity of British Columbia Octobr 20, 2011 ABSTACT ar pion and kaon dcays provid highly snsitiv xprimntal probs for nw physics byond th Standard Modl via th thortically pristin / K l dcay channls (whr l= or ) which allow high prcision tsts of th hypothsis of lctron muon univrsality. Ths studis confront th Standard Modl with unrivald prcision and hav th potntial to uncovr nw intractions at th 1000 TV mass scal. Pion and kaon bams with high intnsity, high purity, and high duty factor ar ndd to pursu masurmnts to thir ultimat snsitivitis. INTODUCTION Th absnc of an xplanation for th gnration puzzl i.. why do w hav xactly thr vrsions of ach quark and lpton, is a major flaw in th highly succssful Standard Modl (SM). Elctron muon univrsality, within th contxt of th SM, rfrs to th hypothsis that chargd lptons hav idntical lctrowak gaug intractions and thy diffr only in thir masss and coupling to th Higgs. Howvr, thr could b additional Nw Physics ffcts, such as non univrsal gaug intractions, or scalar or psudoscalar bosons with couplings not simply proportional to th lpton masss which would caus apparnt violations of univrsality. On of th most snsitiv approachs to sking such nw intractions is th study of th Γ(π ( )) ratio of dcay rats of pions [1] which is prdictd with 0.02% μ Γ(π μ ( )) 1 S ar Pion and Kaon Dcays, D. Bryman, W. Marciano,. Tschirhart, T. Yamanaka, Ann. v. Nucl. Part. Sci., in prss (2011) and rfrncs thrin.

2 uncrtainty in th SM to b (2) 10 [2,3,4] and for which xprimnts can aim to approach similar snsitivitis. Nw Physics at scals as high as 1000 TV can b constraind or concivably unvild by masurmnt of this ratio. For xampl [1], considr th cas of a chargd physical Higgs boson with couplings g to th ud 2 2 u 5 d psudoscalar currnt and g l to l ( 1 s ) l, l, whr g is th SU(2) L gaug 2 2 coupling and λ rprsnts chiral braking supprssion factors. On 1 m finds m 200TV ud (λ ν ). If λν m H, as in th minimal 2 Higgs doublt mu λ ν m modl, thr is no snsitivity to nw physics. Howvr, in mor gnral multi Higgs modls such a chiral rlationship is not ncssary and may not b too supprssd. For xampl, in th cas of loop inducd chargd Higgs couplings, on finds ud m H 400GV is probd for snsitivity of 0.1%. If a discrpancy btwn thory and xprimnt is found in, som typ of chargd Higgs xplanation would b quit natural; howvr, it could also point to additional chargd axial vctor intractions or loop ffcts du to Nw Physics. It could also b intrprtd as an ffct of havy nutrino mixing damping on of th l 2 dcay mods. K Γ(K ( )) Analogous K dcays, μ can also prob high scals and hav th Γ(K μ ( )) addd appal of bing particularly snsitiv to th lpton flavor violating K dcay which might b inducd through loop ffcts [5]. EXPEIMENTAL STUDY OF and K DECAYS Th most rcnt + 2 ) branching ratio masurmnts and subsqunt dtrmination of th ratio μ wr don at TIUMF [6] and PSI [7] in th 1990s. Th rsults from th two xprimnts wr consistnt and in agrmnt with th SM xpctation prviously discussd: TIUMF (34)(44) x10 μ 4 ; and -PSI (35)(36) x10

3 for TIUMF and PSI rspctivly, whr th first and scond uncrtaintis wr du to xpt 4 statistical and systmatic ffcts. Th PDG avrag valu is 1.230(4) 10 [8] including rsults from [9]. Two nw xprimnts ar undrway at TIUMF [10] and PSI xp [11] which aim to improv th prcision of / by a factor of 5 or mor, thrby confronting th SM prdiction to bttr than ±0.1%. At that lvl, nw physics ffcts could appar as a dviation from xpctations or in th absnc of a dviation strong nw constraints on nw physics hypothss will b placd. Th PIENU xprimnt [10] is a rfinmnt of th tchniqu usd in th prvious TIUMF xprimnt[6]. Th branching ratio will b obtaind from th ratio of positron yilds from th dcay and from th dcay chain. By masuring positrons from th dcays and in a non magntic spctromtr many normalization factors, such as th solid angl of positron dtction, cancl to first ordr and only small nrgy dpndnt ffcts, such as thos for multipl Coulomb scattring (MCS) and positron annihilation, nd to b corrctd for. Major improvmnts in prcision stm from th us of an improvd gomtry, a suprior calorimtr, high spd digitizing of all pulss, Si strip tracking, and highr statistics. Th improvmnts lad to an xpctd prcision on xp th / branching ratio <0.06 %, which corrsponds to a 0.03 % uncrtainty in th ratio of th gaug boson lpton coupling constants g /g μ tsting lctron muon univrsality. At PSI, th PIBETA CsI spctromtr [12] built for a dtrmination of th 0 branching ratio and othr masurmnts [13] has bn upgradd and nhancd for th PEN [11] masurmnt of th branching ratio. Th PEN tchniqu is similar to that mployd in th prvious PSI xprimnt [7] which usd a narly 4π sr BGO spctromtr. PEN bgan opration sinc 2007 and has obsrvd >10 7 dcays. PEN compltd xp data acquisition 2010 and xpcts to obtain an improvd prcision masurmnt of / at th lvl <0.05%. cnt progrss on K μ has bn mad by KLOE [14] and NA62 [15] with currnt fforts at NA62 aimd at raching 0.4% prcision. KLOE collctd a data st of 3.3 billion K + K pairs, obsrving dcay products in a drift chambr in a 0.52 T axial magntic fild xp surroundd by an lctromagntic calorimtr. Th masurmnt of K / consistd of comparing th corrctd numbrs of dcays obsrvd from th K ( ) and K ( ) channls. Th rsult found was = (2.493±0.025(stat) ±0.019(syst) 10 prdiction at th 1% lvl. xp KKLOE -5 / [14] in agrmnt with th SM NA62 at CEN using th stup from NA48/2 has mbarkd on a sris of K 2 /K μ2 masurmnts [15]. A K + bam is producd by th 400 GV/c SPS. Positivly chargd particls within a narrow momntum band of (74.0 ± 1.6) GV/c ar slctd by an

4 achromatic systm of four dipol magnts and a muon swping systm, ntr a fiducial dcay volum containd in a 114 m long cylindrical vacuum tank producing a scondary bam. Th K ( ) and K ( ) dtction systm includs a magntic spctromtr, a plastic scintillator hodoscop (HOD) and a 27 X 0 liquid krypton lctromagntic calorimtr (LKr). As in KLOE, th xprimntal stratgy is basd on counting th numbrs of rconstructd K ( ) and K ( ) vnts concurrntly liminating dpndnc on th absolut bam flux and othr potntial systmatic uncrtaintis. Th rsult was xp KNA / = (2.487 ± 0.011stat. ± 0.007syst.) 10 = (2.487 ± 0.013) 10 [15] in agrmnt with th SM prdiction. This rsult is basd on 40% of th data sampl acquird in Th full data sampl may allow a statistical uncrtainty of 0.3% and a total uncrtainty of %. FUTUE POSPECTS If PIENU and PEN achiv thir snsitivity goals thr will still b a considrabl window for nw physics to appar without complication from SM prdiction uncrtaintis. To rach th ultimat lvls of uncrtainty ~0.02% prsntd by th SM calculations would rquir a nw gnration of xprimnts and xprimntal tchniqus capabl of providing control of systmatic uncrtaintis at th 0.01% lvl or blow. High prcision masurmnts of / K μ would b important to confront thoris which may aris from discovris of high mass ffct at th LHC. High intnsity bams with 100% duty factor and ultra high intnsity and purity may b th ky lmnts nabling ultra high prcision xprimnts on / K dcays rsulting in brakthroughs in undrstanding of univrsality and lucidation of subtl non SM ffcts. Such bams would b availabl for pions and kaons at Projct X and for pions at PSI or TIUMF.

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