January 10, 2011 Experimental Results in Charmless Hadronic B Decays from the B Factories 1 1 0 Corry L. Lee 2 n a Department of Physics J Harvard University 7 ] x e - p We report on recent measurements, from the BABAR and Belle exper- e iments, of B-meson decays to purely hadronic final states that do not h [ contain charm. The studies are based on large samples of BB pairs col- 1 lected at the Υ(4S) or B(∗)B(∗) pairs collected at the Υ(5S) by the BABAR s s v and Belle detectors at the asymmetric energy e+e− colliders at SLAC and 0 4 KEK-B, respectively. This paper includes the following results: measure- 3 ments of branching fractions and charge asymmetries of B meson decays 1 . to η′ρ, η′f , and η′K∗, where the K∗ stands for a vector, tensor, or scalar 1 0 0 strange meson; a search for B0 → K+π−K∓π±, including the K∗0 res- 1 onance; a search for B+ → a+K∗0, an axial-vector vector final state; a 1 1 measurement of B0 → hh branching fractions, where h = K+,K0,or π+; v: s S and inclusive branching fraction measurements of B+ → K+π0π0 and i X B0 → π+K0K−. S r a PRESENTED AT The 6th International Workshop on the CKM Unitarity Triangle, University of Warwick, UK, 6-10 September 2010 1 Introduction Experimental studies of charmless hadronic B decays provide a strong test of the- oretical calculations and serve as a laboratory in which to search for potential new physics effects. New physics effects can arise from new particles and couplings in the loop diagrams through which many of these decays proceed. Identifying new physics effects requires a solid theoretical description of Standard Model (SM) processes, which is complicated by the interplay of long- and short-distance QCD effects. Many theoretical predictions have been made by Perturbative QCD (pQCD), QCD Fac- torization (QCDF), Soft Colinear Effective Theory (SCET), and Na¨ıve Factorization (NF), though often with large uncertainties [1]. We report on recent results from the BABAR and Belle Collaborations. Where appropriate, we compare the results to theoretical predictions and previous measure- ments. Charge conjugate states are implied throughout this paper, and all upper limits are quoted at the 90% confidence level. The BABAR results described below use the full data sample of roughly 470×106 BB pairs; the Belle results use 657×106 BB pairs. These datasets allow access to branching fractions (BF) ∼ 10−5−10−7. The Belle measurement of B0 → hh is made s on 1.25×106 B(∗)B(∗) pairs collected at the Υ(5S). s S Experimental studies of charmless hadronic B decays employ maximum likelihood fits to discriminate between signal, the dominant background from e+e− → qq (where q = u,d,s,c), and backgrounds fromother B decays. The analyses typically make use of the fully reconstructed final state to define two primary observables: mES (BABAR) or Mbc (Belle), which uses the beam energy and reconstructed final state momentum to define an observable which peaks at the B mass [2] for signal; and ∆E, the energy difference between the reconstructed B candidate and the beam energy. Additional backgrounddiscriminationisprovidedbyevent shapevariables, asthedecayproducts of B decays tend to be spherically distributed in the Υ(4S) rest frame, while the qq background is jet-like. Event shape variables are combined into a Fisher discriminant or artificial neural network (NN). Where appropriate, resonance mass and helicity distributions are also included in the maximum likelihood fit. 2 Experimental Results Recent experimental results from BABAR and Belle are presented in this section. 2.1 B meson decays to η′ρ, η′f , and η′K∗ 0 BABAR measures branching fractions and, where appropriate, charge asymmetries for B meson decays to η′ρ, η′f , and η′K∗, where the K∗ stands for a vector K∗(892), 0 1 tensor K∗(1430), or scalar K∗(1430) interfering with the non-resonant scalar Kπ [3]. 2 0 Such measurements test predicted η/η′ mixing as well as provide access to potential new physics effects. Theoretical predictions for the B+ → η′ρ+ branching fraction in pQCD and QCDF (6−9×10−6) disagree with those from SCET (∼ 0.4×10−6). Few theoretical predictions exist for decays involving the K∗(1430) or K∗(1430). 2 0 First observations are presented for four modes, including B+ → η′ρ+, which has a measured branching fraction of (9.7+1.9 ± 1.1) × 10−6. This measurement favors −1.8 the pQCD and QCDF predictions and is in poor agreement with the upper limit from Belle (< 5.8×10−6) [4]. The branching fractions involving the tensor K∗(1430) 2 are substantially higher than those involving the K∗(892), a pattern also observed in B → ωK∗ decays [5]. Plots of the ππ and Kπ invariant masses are shown in Fig. 1, including a cut on the likelihood function to enhance the visibility of signal. (a) 1155 (c) (b) (d) 88 8800 V V V V Me Me Me Me4400 5 66 5 1100 0 6600 0 2 2 5 5 ents / 44 ents / ents / 4400 ents / 2200 Ev Ev 55 Ev Ev 22 2200 00 00 00 00 00..66 00..88 11..00 00..66 00..88 11..00 00..88 11..00 11..22 11..44 00..88 11..00 11..22 11..44 mmππππ ((GGeeVV)) mmππππ ((GGeeVV)) mmKKππ ((GGeeVV)) mmKKππ ((GGeeVV)) Figure 1: B-candidate ππ invariant mass projections for: (a) η′ρ0/η′f , (b) η′ρ+; and 0 Kπ invariant mass for: (c) η′K∗0, (d) η′K∗+. The solid curve is the fit function, black long-dash-dot is the total background, and the blue dashed curve is the total signal contribution. In (a) the ρ0 component (red dashed) is separated from the f 0 (green dotted). In (c,d), the K∗(892) (red dashed) is separated fromthe (Kπ)∗ (green 0 dotted) and the K∗(1430) (magenta dot-dashed) components. 2 2.2 Search for B0 → K+π−K∓π± Belle searches for B0 → K+π−K∓π± where a Kπ pair can be a resonant K∗0 or K∗0, where K∗0 represents either the vector K∗(892)0 or scalar K∗(1430)0 [6]. The 0 B0 → K∗0K∗0 decay is dominated by a b → d penguin, and is expected to have a BF of ∼ 10−7 − 10−6 in the SM. The decay B0 → K∗0K∗0 is suppressed in the SM, the expected BF ∼ 10−15. No significant signals are observed. BF upper limits are placed: BF(B0 → K∗0K∗0) < 0.8 ×10−6 and BF(B0 → K∗0K∗0) < 0.2 ×10−6. The former limit is slightly below the BABAR measurement of BF(B0 → K∗0K∗0) = (1.28±0.34)×10−6 [7]. Projections of the K∗0K∗0 fit results on the signal-enhanced data sample are given in Fig. 2. 2 50 20 (a) 20 (b) 45 (c) 17.5 17.5 40 Events/ 10 MeV17211..50555 Events/ 2 MeV17211..50555 Events/ 50 MeV 112233050505 2.5 2.5 5 0 0 0 -0.1 -0.05 0 0.05 0.1 5.25 5.26 5.27 5.28 5.29 5.3 0.7 0.9 1.1 1.3 1.5 1.7 ∆E (GeV) M (GeV/c2) M (K+π-) (GeV) bc 1 Figure 2: Projections of (a) ∆E, (b) Mbc, and (c) K+π− invariant mass for B0 → K∗0K∗0. The black curve shows the overall fit result; the red shaded region represents thesignalcomponent; thebluedottedcurverepresents qq background, thedot-dashed blue is b → c background and the green dashed is B → charmless background. 2.3 Search for B+ → a+(1260)K∗0 1 BABARpresentsasearchfortheaxial-vectorvectordecayB+ → a+(1260)K∗0(892)[8]. 1 Theoretical predictions of the BF differ greatly between methods, QCDF predicting ∼ 11×10−6 and NF calculating a BF an order of magnitude smaller. No significant signal is observed, and BABAR sets a 90% confidence level upper limit on the BF < 3.6×10−6, assuminganequalBFfora+(1260) → π+π−π+ anda+(1260) → π+π0π0, 1 1 and that the BF for a+(1260) → 3π is 100%. 1 2.4 B0 → hh, where h = K+,K0,or π+ s S Belle presents results for B0 → hh, where h = K+,K0,or π+ [9]. Understanding s S these channels could help understand the “Kπ puzzle” [1] in B0 decays, and com- paring charge asymmetries between B and Bs decays could provide a window on new physics. Belle measures BF(B0 → K+K−) = (38± 12)× 10−6, in good agree- s ment with (though with larger errors than) CDF [10], and presents upper limits on B0 → K+π−, π+π−, and K0K0. This is the first search for B0 → K0K0, and Belle s s places the upper limit of < 66×10−6. 2.5 Inclusive B+ → K+π0π0 BABAR reports a preliminary measurement of the inclusive B+ → K+π0π0 branching fraction [11]. Understanding B → K∗π decays could help shed light on the “Kπ puzzle” [1], and B+ → K∗+π0 is poorly measured, with the three-body state having never been previously investigated. This analysis employs mES and a event-shape 3 neural network discriminant, yielding 1220±85 signal events. The measured BF is (15.5±1.1±1.6)×10−6 with a significance > 10σ. Projections of the fit results on the signal-enhanced data sample are given in Fig. 3. 2)c BABAR 81)3000 BABAR V/ preliminary 0 preliminary Me400 (0. 6 s / 2000 2. nt s / (200 Eve1000 t n e v E 0 5.26 5.27 5.28 0.4 0.6 0.8 1 m (GeV/c2) NN ES out Figure 3: Projections of B+ → K+π0π0 candidates onto (a) mES and (b) neural network, including cuts to enhance signal visibility. The solid blue curve represents the total fit result, the dot-dashed black curve gives the signal contribution, the red dotted curve the qq, and the green dashed curve the total background contribution. 2.6 Inclusive B0 → π+K0K− S BABAR observes the inclusive B0 → π+K0K− decay with 5.2σ significance [12]. This S decayproceedsthroughab → dpenguinandab → utree, socouldaccessnewphysics effects. Additionally, this channel could be used to search for anisospin partner of the fX(1500), observed in the K+K− spectrum of B+ → K+K−π+ [13]. BABAR measures BF(B0 → π+K0K−) = (3.2 ± 0.5 ± 0.3) × 10−6. A qualitative investigation of the S K0K− spectrum reveals no evidence for an isospin partner to the fX(1500). S 3 Conclusion Charmless hadronic B meson decays provide an excellent laboratory in which to test Standard Model predictions. As many of these decays are governed by loopdiagrams, they also present an exciting opportunity to search for new physics effects. To date, around 100 charmless hadronic B decay branching fractions have been measured with greater than 4σ significance [1]. The physics output in charmless hadronic B physics by the BABAR, Belle, and CDF Collaborations continues to be strong. 4 References [1] H. Y. Cheng and J. G. Smith, Ann. Rev. Nucl. Part. Sci. 59, 215 (2009) [arXiv:0901.4396 [hep-ph]]. [2] K. Nakamura et al. [Particle Data Group], J. Phys. G 37, 075021 (2010). [3] P. del AmoSanchez et al.[BaBarCollaboration], Phys. Rev. D82, 011502(2010) [arXiv:1004.0240 [hep-ex]]. [4] J. Schumann et al. [Belle Collaboration], Phys. Rev. D 75, 092002 (2007) [arXiv:hep-ex/0701046]. [5] B. Aubert et al. [BABAR Collaboration], Phys. Rev. D 79, 052005 (2009) [arXiv:0901.3703 [hep-ex]]. [6] C. C. Chiang et al. [Belle collaboration], Phys. Rev. D 81, 071101 (2010) [arXiv:1001.4595 [hep-ex]]. [7] B. Aubert et al. [BABAR Collaboration], Phys. Rev. Lett. 100, 081801 (2008) [arXiv:0708.2248 [hep-ex]]. [8] B. Aubert et al. [BABAR Collaboration], Phys. Rev. D 82, 091101 (2010) [arXiv:0808.0579 [hep-ex]]. [9] C. C. Peng et al. [Belle Collaboration], Phys. Rev. D 82, 072007 (2010) [arXiv:1006.5115 [hep-ex]]. [10] M. Morello [CDF Collaboration], Nucl. Phys. Proc. Suppl. 170, 39 (2007) [arXiv:hep-ex/0612018]. A. Abulencia et al. [CDF Collaboration], Phys. Rev. Lett. 97, 211802 (2006) [arXiv:hep-ex/0607021]. [11] P. del Amo Sanchez et al. [BABAR Collaboration], arXiv:1005.3717 [hep-ex]. [12] P. del Amo Sanchez et al. [BABAR Collaboration], Phys. Rev. D 82, 031101 (2010) [arXiv:1003.0640 [hep-ex]]. [13] B. Aubert et al. [BABAR Collaboration], Phys. Rev. Lett. 99, 221801 (2007) [arXiv:0708.0376 [hep-ex]]. 5