Fermilab experiment E791 studied weak decays of D+, D_s, and D^0 mesons
produced in collisions of 500 GeV/c negative pions with Pt and C targets. The
experiment collected over 200,000 fully reconstructed charm decays. Four recent
results are discussed here: (a) measurement of the form factor ratios V/A_1,
A_2/A_1, and A_3/A_1 in D->K*-l-nu and Ds->phi-l-nu decays; (b) measurement of
the difference in decay widths $\Delta\Gamma$ between the two D^0/D^0-bar mass
eigenstates; (c) search for rare and forbidden D decays to dilepton final
states; and (d) search for a ``Pentaquark,'' a bound state of c-bar-suud.Comment: 8 pages, 2 figures, contribution to XXXIVth Rencontres de Moriond,
  Electroweak Interactions and Unified Theories, March, 199

Schwartz, A. J.

English

arXiv

We report the results of some recent E791 charm analyses. They include 1) a search for rare and forbidden decays, 2) measurements of form factors for D{sup +} {yields} K{sup *0}l{sup +}{nu}{sub l} and D{sup +}{sub s} {yields}{phi}l{sup +}{nu}{sub l}, and 3) D{sup +}{sub s} and D{sup 0} lifetime measurements including the lifetime difference between D{sup 0} {yields} K{sup -}{pi}{sup +} and D{sup 0} {yields} K{sup -}K{sup +}. The latter is the first direct search for a possible lifetime difference that could contribute to D{sup 0} - {anti D}{sup 0} mixing

Sanders, David A.

UNT Digital Library

F Fermi National Accelerator LaboratoryFERMILAB-Conf-99/109-EE791Recent Results from Fermilab Charm Experiment E791David A. SandersFor the E791 CollaborationUniversity of MississippiFermi National Accelerator LaboratoryP.O. Box 500, Batavia, Illinois 60510May 1999Published Proceedings of DPF ’99 American Physical Society Division of Particles and Fields,University of California, Los Angeles, January 5-9, 1999Operated 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.Copyright NoticationThis manuscript has been authored by Universities Research Association, Inc. under con-tract No. DE-AC02-76CH03000 with the U.S. Department of Energy. The United StatesGovernment and the publisher, by accepting the article for publication, acknowledges thatthe United States Government retains a nonexclusive, paid-up, irrevocable, worldwide licenseto publish or reproduce the published form of this manuscript, or allow others to do so, forUnited States Government Purposes.hep-ex/9903067 v2   12 Apr 1999Recent Results from Fermilab Charm Experiment E791David A. SandersUniversity of MississippiRepresenting the Fermilab Experiment E791 CollaborationWe report the results of some recent E791 charm analyses. They include 1) a search for rareand forbidden decays, 2) measurements of form factors for D+ ! K0`+̀ and D+s ! `+̀ , and3) D+s and D0 lifetime measurements including the lifetime dierence between D0 ! K + andD0 ! K K+. The latter is the rst direct search for a possible lifetime dierence that couldcontribute to D0  D0mixing.I. INTRODUCTIONE791 is a high statistics charm experiment that acquired data at Fermilab during the 1991-1992 xed-target run.The experiment combined a fast data acquisition system with an open trigger. Over 2  1010 events were collectedwith the Tagged Photon Spectrometer [1] using a 500 GeV   beam. There were ve target foils with 15 mm center-to-center separations: one 0.5 mm thick platinum foil followed by four 1.6 mm thick diamond foils. The spectrometerincluded 23 planes of silicon microstrip detectors (6 upstream and 17 downstream of the target), 2 dipole magnets,10 planes of proportional wire chambers (8 upstream and 2 downstream of the target), 35 drift chamber planes, 2multi-cell Cerenkov counters that provided =K separation in the 6-60 GeV/c momentum range [2], electromagneticand hadronic calorimeters, and a muon detector.II. RARE AND FORBIDDEN DECAYS OF D+, DS AND D0We report the preliminary results of a search for avor-changing neutral-current (FCNC), lepton number violating(LNV) and lepton family violating (LFV) decays of D+, D+s and D0, into modes1 containing muons and electrons.This analysis is an extension of our previous work [3]. The search for rare and forbidden decays provides another testof the Standard Model. The contribution, within the Standard Model, of short distance electroweak processes to thebranching ratios of the FCNC-like decay modes described here is expected [4] to be less than 10 8. The LNV andLFV decay modes are totally forbidden by the Standard Model. Therefore, the search for these rare and forbiddendecay modes allows tests for violations of the Standard Model from FCNC, neutrino oscillations or other even moreexotic processes. We are currently examining all the modes described in Table 1; however, we are only presenting thefollowing searches in this paper: D+ ! +e+e , D+ ! +e, D+ !  e+e+, D+ !  +e+, D0 ! e+e , andD0! e.TABLE I. Rare and Forbidden Decay Modes.FCNC LFV LNVD+ ! ++  D+ ! +e D+ !  ++D+ ! +e+e  D+ !  +e+ D+ !  e+e+D+s ! K++  D+s ! K+e D+s ! K ++D+s ! K+e+e  D+s ! K +e+ D+s ! K e+e+D0 ! +  D0 ! eD0 ! e+e 1all references to D+, D+s and D0 and their decay modes also imply the corresponding charge-conjugate states.1To separate charm candidates from background, we require the secondary vertex to be well-separated from theprimary vertex and located well outside the target foils and other solid material, the momentumvector of the candidateD+ or D0 to point back to the primary vertex, and the decay track candidates to pass closer to the secondary vertexthan the primary vertex. Specically, the secondary vertex must be separated by > 20L for D+, (> 12L for D0)from the primary vertex and by > 5:0L from the closest material in the target foils, where L in each case is thecalculated longitudinal resolution in the measured separation. The sum of the momentum vectors of the three tracksfrom this secondary vertex must miss the primary vertex by < 40 m in the plane perpendicular to the beam. Weform the ratio of each track's smallest distance from the secondary vertex to its smallest distance from the primaryvertex, and require the product of these ratios to be less than 10 n where n is the number of tracks in the secondaryvertex. There is also a cut on the lifetime of the D+ or D0 candidate; it is < 5 picoseconds for the D+ and < 3picoseconds for the D0. Finally, the net momentumof the D+ candidate transverse to the line connecting the primaryand secondary vertices must be less than 250 MeV/c (300 MeV/c for the D0).For this analysis we use a \blind" analysis technique. This method entailed the following steps: rst covering thesignal region with a \box", then optimizing ALL cuts using Monte Carlo simulated signal events and data from thewings outside the \box" before opening the \box" and only then opening the \box" that covers the signal region. Theclosed \box" mass windows are: 1:84 < M (D+) < 1:90 GeV/c2 for D+ ! , 1:78 < M (D+) < 1:90 GeV/c2 forD+! ee and e, 1:95 < M (D+s ) < 1:99 GeV/c2 for D+s ! K, 1:91 < M (D+s ) < 1:99 GeV/c2 for D+s ! Keeand Ke, 1:83 < M (D0) < 1:90 GeV/c2 for D0 ! , and 1:76 < M (D0) < 1:90 GeV/c2 for D0 ! ee and e. Oneshould note the asymmetric windows for the decay modes containing electrons. This is to account for the electronbremsstrahlung tail. The upper limit of the branching ratio is calculated using the following formulae:BRX =NX="XNnorm="normBRnorm =NXNnorm"norm"XBRnorm (1)where NX is the number of observed events for a given decay mode and " is the detection eciency. The ratio of thedetector eciencies for the normalization decay mode versus the unknown decay mode is"norm"X=NMCnormNMCX: (2)NX is corrected, using the method of Feldman and Cousins [5], for background. We have not yet corrected forsystematic errors. The number of normalization events for the D+ decay modes is Nnorm = 25330  169, D+!K ++ events, and for the D0 decay modes is Nnorm = 26980 185, D0! K + events.In addition to the cuts made on the kinematic variables described above, particle identication cuts are used toreduce the background. These cuts tagged the dilepton candidates, thereby greatly reducing the background. Eventswhich could also be reconstructed as a reection, due to particle misidentication, from other purely hadronic decaymodes into a region outside the \boxes", but within the overall mass window, were removed. Examples of this wouldinclude D+ ! K ++ reconstructed as D+ !  `+`+ or D0 ! K + reconstructed as D0 ! `+` . In Figure 1,we present preliminary plots of the data for the D+ and D0 decay modes described in this paper. The yellow boxesare the part of the plot that were originally covered by the \boxes". The events observed within these \boxes" arethe total number of observed events, signal plus background. In the plots of the D+ decay modes the lower yellowbox is the D+ \box" and the upper yellow box is the D+s \box".2FIG. 1. The preliminary data for the D+ and D0 decay modes presented here.We estimate the mean background assuming a at distribution calculated from the events seen in the regions thatare outside the \boxes". Then the Poisson signal mean  for the total number of observed events for a 90% condencelevel upper limit are calculated, using the method of Feldman and Cousins [5], and this number, NX , is substitutedinto Equation 1 thus giving a 90% condence level upper limit branching ratio. The estimated background, totalnumber of events within the \box", the number NX for a 90% condence level upper limit, the calculated upper limitbranching ratios and the previously measured upper limits [6] are given in Table 1.3TABLE II. Preliminary Results 90% C.L. Upper LimitsMode Estimated Background Observed Calculateda NX Branching Ratio 1998 PDG BRD+ ! +e+e  0.6 1 3.76 < 4:3 10 5 < 6:6 10 5D+ ! +e 0.0 1 4.36 < 3:2 10 5 < 1:2 10 4D+ !  e+e+ 0.0 2 5.91 < 7:7 10 5 < 1:1 10 4D+ !  +e+ 0.6 1 3.76 < 3:6 10 5 < 1:1 10 4D0 ! e+e  1.8 0 1.30 < 5:2 10 6 < 1:3 10 5D0 ! e 2.6 3 4.80 < 1:0 10 5 < 1:9 10 5aBy Method of Feldman and CousinsA comparison between our calculated 90% condence level upper limit branching ratios and the previous results [6]is shown by the plot of the 90% C.L. upper limits in Figure 2.10−610−510−410−3Rare and Forbidden Decay Branching RatiosE791 Value1998 PDG ValueBR - 90% CL Upper LimitPreliminary ResultsD0e+e−µ+e−π+e+e−π+µ±e±D+π−e+e+π−µ+e+FIG. 2. The 90% C.L. upper limit branching ratios for E791 data and for the 1998 PDG values.4III. DS LIFETIME MEASUREMENTSAccurate measurements of the lifetimes of the weakly decaying charm mesons are useful for understanding thecontributions of various weak decay mechanisms. Despite the fact that they are all tied to the charm quark decay,the decays of the ground state charm mesons can have dierent contributions from the four rst-order processes (twospectator, W-annihilation, and W-exchange) and their lifetimes are, in fact, quite varied [6]. D+:  D0:  (Ds) = 2:5 : 1 : 1:1 (3)We have made a new precise measurement of the Ds lifetime [7] using 1662 56 fully reconstructed D+s ! +decays. The Ds lifetime is measured using an unbinned maximum-likelihood t to be 0:518 0:014 0:007 ps. Thisvalue is somewhat higher than the world average D0 lifetime [6] of 0:467 0:017 ps. Using our result and the worldaverage D0 lifetime [6], we nd that the ratio of the Ds lifetime to the D0 lifetime to be (Ds) (D0)= 1:25 0:04 (a 6 dierence from unity) (4)showing, for the rst time, signicantly dierent lifetimes for the Ds and D0. This result may be used to constrainthe contributions of various decay mechanisms to charm decay and further rene our quantitative understanding ofthe hierarchy of charm particle lifetimes.IV. MEASUREMENT OF THE LIFETIME DIFFERENCE BETWEEN D0 ! K + AND D0 ! K K+We report the rst directly measured constraint on the decay-width dierence   for the mass eigenstates of theD0 D0system [8]. We obtain our result from lifetime measurements of the decays D0 ! K + and D0 ! K K+,under the assumption of CP invariance, which implies that the CP eigenstates and the mass eigenstates are thesame. The lifetime of the CP -even nal state D0 ! K K+, as calculated from 6683  161 weighted events, isKK = 0:410 0:011 0:006 ps. The lifetime of an equal mixture of CP -odd and CP -even nal states D0! K +,as calculated from 60581353 weighted events, is K = 0:4130:0030:004 ps. We nd that   = 2 ( KK    K)= 0:04 0:14 0:05 ps 1, leading to a limit of  0:20 <   < 0:28 ps 1 at 90% C.L. The value of   is consistentwith zero and thus, at our level of sensitivity, is consistent with the Standard Model.V. MEASUREMENTS OF D+ ! K0`+̀ AND D+S! `+̀ FORM FACTORSThe Vector and Axial form factors are used to extract elements of the Cabibbo-Kobayashi-Maskawa mixing matrix.Here we present measurements of the form factor ratios, evaluated at q2 = 0 (GeV=c)2, rV = V (0) =A1 (0), r2 =A2 (0) =A1 (0), for both the muon and electron channels, and r3 = A3 (0) =A1 (0) in the muon channel. This is therst simultaneous measurement of both the muon and electron channels.A. D+ ! K0`+̀ Form Factor RatiosWe have already reported the form factor ratios rV and r2 for the decay D+! K0e+e, K0! K + [9] . Theform factor ratios in the decay D+ ! K0+, K0! K + were measured using 3034(595) signal (background)events and yield [10] rV = 1:840:110:09, r2 = 0:750:080:09, and as a rst measurement r3 = 0:040:330:29.The combined electron and muon form factor ratios, for approximately 6000 events, are rV = 1:87 0:08 0:07 andr2 = 0:73 0:06 0:08.5B. D+s ! `+̀ Form Factor RatiosWe have measured the form factor ratios rV and r2 for the decay D+s ! `+̀ , ! K K+ in both the electron andmuon channels [11]. The form factor ratios in the electron channel were measured using 144(22) signal (background)events and yield rV = 2:24 0:47 0:21 and r2 = 1:64 0:34 0:20. In the muon channel there were 127(34) signal(background) events, yielding rV = 2:32 0:54 0:26 and r2 = 1:49 0:36 0:20. The combined electron and muonform factor ratios, for approximately 271 events, are rV = 2:27 0:35 0:22 and r2 = 1:57 0:25 0:19.We gratefully acknowledge the assistance of the stas of Fermilab and of all the participating institutions. Thisresearch was supported by the Brazilian Conselho Nacional de Desenvolvimento Cientco e Tecnologico, CONACyT(Mexico), the Israeli Academy of Sciences and Humanities, the U.S. Department of Energy, the U.S.-Israel BinationalScience Foundation, and the U.S. National Science Foundation. Fermilab is operated by the Universities ResearchAssociation, Inc., under contract with the United States Department of Energy.[1] J. A. Appel, Ann. Rev. Nucl. Part. Sci. 42 367-399 (1992), and references therein;D. J. Summers et al., Proceedings of the XXVII th Rencontre de Moriond, Electroweak Interactions and Unied Theories,Les Arcs, France 417-422 (15-22 March, 1992);S. Amato et al., Nucl. Instr. Meth. A324 535-542 (1993);S. Bracker et al., IEEE Trans. Nucl. Sci. NS-43 2457-2464 (1996);E.M. Aitala et al. (E791), FERMILAB-PUB-98-297-E and hep-ex/9809029, submitted to Eur. Phys. J. C.[2] D. Bartlett et al., Nucl. Instr. Meth. A260, 55-121 (1987).[3] E.M. Aitala et al. (E791), (hep-ex/9507003) Phys. Rev. Lett. 76, 364-367 (1996).[4] A.J. Schwartz, Mod. Phys. Lett. A 8, 967-977 (1993).[5] Gary J. Feldman and Robert D. Cousins, Phys. Rev. D57, 3873-3889 (1998).[6] C. Caso et al. (Particle Data Group), Eur. Phys. J. C3, 1 (1998).[7] E.M. Aitala et al. (E791), (hep-ex/9811016) Phys. Lett. B445, 449-454 (1999).[8] E.M. Aitala et al. (E791), FERMILAB-PUB-99/036-E and hep-ex/9903012, submitted to Phys. Rev. Lett.[9] E.M. Aitala et al. (E791), (hep-ex/9710216) Phys. Rev. Lett. 80, 1393-1397 (1998).[10] E.M. Aitala et al. (E791), (hep-ex/9709026) Phys. Lett. B440, 435-441 (1998).[11] E.M. Aitala et al. (E791), (hep-ex/9812013) Phys. Lett. B450, 294-300 (1999).6

Recent Charm Results from Fermilab Experiment E791

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