A search for baryon-number violating Λ ml decays using JLab

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1 7 June 13 BARYONS 13 University of Glasgow, Scotland, UK M. McCracken (W&J) BNV Λ Search 7 June 13 1 / A search for baryon-number violating Λ ml decays using JLab Michael E. McCracken 1,, Matt Bellis 3 for the CLAS Collaboration 1 Washington & Jefferson College Washington, Pennsylvania USA Carnegie Mellon University Pittsburgh, Pennsylvania USA 3 Siena College Loudonville, New York USA

2 Motivation Motivation: Matter Asymmetry We observe a matter-dominated (rather than symmetric) universe η = N B N B N γ 1 9 Likely that primordial universe began in a symmetric state Sakharov Conditions (1967) To achieve current asymmetry, need: 1 Violation of C and CP invariances B violation 3 Interactions outside of thermal equilibrium Standard Model allows for B violation via sphalerons, 1 TeV Observed CP violations in K and B mesons can t account for η Explanations: 1 Undiscovered larger CP violations Undiscovered B violations M. McCracken (W&J) BNV Λ Search 7 June 13 /

3 Motivation Motivation: Baryon Decay Sakharov theorized a new boson that mixes quark and lepton flavors Such X bosons are features of many SU(5) theories and GUTs X may have q = ± 1 3, ± 3, ± 4 3 Example: p decay with X of q = 1 3 p d u u X d d π e + Relies on q X + q and q + X l Experimental upper bound on p lifetime is > 1 33 y! M. McCracken (W&J) BNV Λ Search 7 June 13 3 /

4 Motivation Motivation: Baryon Decay Non-observation of p decay: large mass of X flavor dependence of X coupling M. McCracken (W&J) BNV Λ Search 7 June 13 4 /

5 Motivation Motivation: Baryon Decay Non-observation of p decay: large mass of X flavor dependence of X coupling Parametrize X coupling (as in the CKM matrix) M. McCracken (W&J) BNV Λ Search 7 June 13 4 /

6 Motivation Motivation: Baryon Decay Non-observation of p decay: large mass of X flavor dependence of X coupling Parametrize X coupling (as in the CKM matrix) p d u u X d d π u X + d u + X e + e + X u d X c d X t d X u s X c s X t s X u b X c b X t b Xūe X ce X te Xūµ X cµ X tµ Xūτ X cτ X tτ M. McCracken (W&J) BNV Λ Search 7 June 13 4 /

7 Motivation Motivation: Baryon Decay Non-observation of p decay: large mass of X flavor dependence of X coupling Parametrize X coupling (as in the CKM matrix) Λ d s u X d u π s X + ū u + X e + e + X u d X c d X t d X u s X c s X t s X u b X c b X t b Xūe X ce X te Xūµ X cµ X tµ Xūτ X cτ X tτ M. McCracken (W&J) BNV Λ Search 7 June 13 4 /

8 Motivation Motivation: Λ ml The Λ presents a long-lived strange state (cτ = 7.89 cm) Current uncertainties on Γ allow for discovery at our sensitivities Opportunity to observe rare decays Decay mode Branching fraction Uncertainty % uncertainty pπ nπ nγ pπ γ pe ν e pµ ν µ Γ tot = 1.58 ±.7 M. McCracken (W&J) BNV Λ Search 7 June 13 5 /

9 Motivation Motivation: Λ ml The Λ presents a long-lived strange state (cτ = 7.89 cm) Current uncertainties on Γ allow for discovery at our sensitivities Opportunity to observe rare decays Differs from p decay by access to X s q CLAS has lots of them, easily separable from background Hyperon photoproduction in CLAS is understood dσ, polarization from γp K + Y published γp K + Λ PRC 81, 51 (1) γp K + Σ PRC 8, 5 (1) forthcoming papers on Y arxiv: [nucl-ex] First measurement! M. McCracken (W&J) BNV Λ Search 7 June 13 5 /

10 Analysis Overview Analysis Overview We search for B-violating Λ ml decays in nine final states: K + e K + µ K e+ K µ + π+ e π + µ π e+ π µ + Ks ν Chosen for: B = 1 L = ±1 (B L) =, Reconstructability in CLAS Conservation of angular momentum Conservation of charge M. McCracken (W&J) BNV Λ Search 7 June 13 6 /

11 Analysis Overview Analysis Overview We search for B-violating Λ ml decays in nine final states: K + e K + µ K e+ K µ + π+ e π + µ π e+ π µ + Ks ν X charge quark lepton-quark Reaction B L (B L) (e) couplings couplings Λ K + l 1 +1 d s, s s sl, dl 3 Λ K l , 1 uū, dū s l, d l 3 3 Λ π + l 1 +1 d d, s d sl, dl 3 Λ π l , 4 sū, uū u l, s l 3 3 Λ K ν 1 ±1, 1, u s, s s sν, uν 3 3 d d, u d dν M. McCracken (W&J) BNV Λ Search 7 June 13 6 /

12 Analysis Overview CLAS JLab CEBAF Large Acceptance Spectrometer Toroidal B Wire tracking chambers Scintillator timing detectors Tagged γ from bremsstrahlung of 4.3 GeV e beam Dataset collected during May-June 4 M. McCracken (W&J) BNV Λ Search 7 June 13 7 /

13 Analysis Overview Blind Analysis Overview 1 Identify number of γp K + Λ, Λ pπ events Fiducial, particle ID, and analysis cuts applied Acceptance correct to get the number of Λ pπ produced Develop cuts to select bnv signal optimize cuts using signal MC and blinded data 3 Investigate effect of bnv cuts on background MC 4 Un-blind data, identify Nbnv 5 Calculate Γbnv or set upper limit M. McCracken (W&J) BNV Λ Search 7 June 13 8 /

14 Analysis Overview Blind Analysis Overview We use a naïve estimate of the significance of a measurement ς = N s / N s + N b (1) We set confidence levels for bnv observation to 4σ ɛ is the efficiency for a given bnv channel (acceptance and cuts) Γ pπ =.639 is the Λ pπ branching fraction N pπ is the number of Λ pπ decays in dataset N bnv is the observed number of bnv events after unblinding Then we can calculate Γ bnv N bnv Γ bnv = (Γ pπ ) ɛ N pπ () M. McCracken (W&J) BNV Λ Search 7 June 13 9 /

15 Λ pπ Counting Λ pπ Counting Identify data SM decays using a K + pπ final state Generate MC γp K + Λ K + pπ distributed according to brem. spectrum and published dσ/dω Process with detector simulation Apply loose missing mass and timing cuts, detector cuts Extract number of events from (p p + p π ) histograms, binned in cos θ cm K events after all cuts Acceptance correct for analysis cuts and detector acceptance N pπ = Λ pπ produced during data taking M. McCracken (W&J) BNV Λ Search 7 June 13 1 /

16 Λ pπ Counting 3 Counting Λ pπ events Invariant mass of pπ system for all data events, binned in cos θ cm K 9 cm -1. cosθ < -.8 K cm -.8 cosθ < -.6 K cm -.6 cosθ < -.4 K cm -.4 cosθ < -. K cm 4 -. cosθ <. K cm. cosθ <. K 1 cm. cosθ <.4 K 1 cm.4 cosθ <.6 K 1 cm.6 cosθ <.8 K 1 1 cm.8 cosθ < 1. K INV(p,π -) 1.4 (GeV/c ) M. McCracken (W&J) BNV Λ Search 7 June /

17 Λ ml event selection Λ ml event selection Begin with timing-based PID cuts specific to final-state particles Cutting out proton is a strong constraint Tune additional cuts on MM and MM(K + ) Use inv(m d, l) spectrum to identify signal Select optimal cuts based on (reduced) Punzi figure of merit arxiv:physics/3863 [data-an] For a given set of cuts, C, P(C) = ɛ(c) a/ + B(C) (3) Attractive features: a is confidence level (sigmas) finite as B(C) does NOT depend on amount of signal or production cross section M. McCracken (W&J) BNV Λ Search 7 June 13 1 /

18 Λ ml event selection Example: Λ π + µ event selection γp K r + Λ K r + π + µ Choose positive track assignment using inv(π +, µ ) Generate 1 6 γp K + Λ K + π + µ MC events timing cuts vetted on MC; skim dataset hdeltatof1_p_a hdeltatof_p_a hdeltatof3_p_a K + π + µ tof v p for final-state tracks M. McCracken (W&J) BNV Λ Search 7 June /

19 Λ ml event selection Λ π + µ event selection Apply cuts on MM and MM(K + r ) Vary widths, choose pair with optimal P MM : w 1 [.1,.4] GeV /c 4 MM(K + r ): w [.1,.] GeV/c mmoffk width mm width MM(K ) (GeV/c ) P(C) for all cut combinations M. McCracken (W&J) BNV Λ Search 7 June /

20 Λ ml event selection Λ K s ν event selection Apply cuts on MM and -d cut based on π + π opening angle and p K Vary widths, choose pair with optimal P Due to missing p ν, background persists ). (GeV/c w w (GeV /c ) 1 P(C) for all cut combinations mm(k ) (GeV/c ) M. McCracken (W&J) BNV Λ Search 7 June /

21 Assessing background contributions Assessing background signatures Assess whether cuts give background some signal-like shape Generate MC events for the following final states pe + e pπ + π pπ+ π π pµ + µ pk + K K + Λ K + pπ Apply optimal cuts to each reaction. Very few events pass INV(π,l) (GeV/c ) INV(π,l) (GeV/c ) inv(π, e + ) before and after analysis cuts M. McCracken (W&J) BNV Λ Search 7 June /

22 Unblinding and preliminary results Unblinding Effects of cuts on signal and background MC are understood Unblind the signal region of identification plots M. McCracken (W&J) BNV Λ Search 7 June /

23 8 6 Unblinding 4 Unblinding and preliminary results INV(π,l) (GeV/c ) INV(π,l) (GeV/c ) INV(π,l) (GeV/cINV(π,l) ) (GeV/c ) inv(π +, µ ) before cuts INV(π,l) (GeV/c INV(π,l) ) (GeV/c )... and after optimal cuts A single event in signal region... 1 M. McCracken (W&J) BNV Λ Search 7 June / 1

24 Unblinding and preliminary results Unblinding In all but the K s ν channel, there are or 1 events in signal region We estimate the upper-bound on N bnv using the Feldman-Cousins method PRD 57, (1998) At 95% confidence, what is the maximum N bnv given expected background N B and observed N O? Example: for Λ π + µ N B =.5 N O = 1 N bnv < 4.64 M. McCracken (W&J) BNV Λ Search 7 June /

25 Unblinding and preliminary results Unblinding In all but the K s ν channel, there are or 1 events in signal region We estimate the upper-bound on N bnv using the Feldman-Cousins method PRD 57, (1998) At 95% confidence, what is the maximum N bnv given expected background N B and observed N O? Example: for Λ π + µ N B =.5 N O = 1 N bnv < 4.64 We use this upper bound and calculate a maximum Γ bnv Γ bnv = (Γ pπ ) N bnv 4.64 < (.639) ɛ N pπ = (4) PRELIMINARY M. McCracken (W&J) BNV Λ Search 7 June /

26 Unblinding and preliminary results Preliminary results We use the FC method for all of the charged decay modes FC N decay N B N bnv Γ bnv upper O upper bound bound ( 1 7 ) K + e K + µ K e K µ π + e π + µ π e π µ Ks ν to come to come PRELIMINARY M. McCracken (W&J) BNV Λ Search 7 June /

27 Conclusions Conclusions Λ ml is useful as a test of B conservation Provides search similar to that for nucleon decay CLAS data set provides 1 7 sensitivities We find no appreciable signal at these sensitivities MC studies are necessary to estimate Γ for Λ Ks ν Many thanks to BARYONS organizers and the CLAS Collaboration! M. McCracken (W&J) BNV Λ Search 7 June 13 /