Overview of FOCUS and a report on charmed mesons in FOCUS

ELSEVIER Nuclear Physics B (Proc. Suppl.) 75B (1999) 105-111

I m l l l ~ l ~ W l

P R O C E E D I N G S SUPPLEMENTS

Overview of FOCUS and a report on charmed mesons in FOCUS

D. Pedrini a On behalf of the FOCUS(E831) collaboration*

aI.N.F.N, sezione di Milano, Via Celoria 16, 20133 Milano, Italy

An overview of Fermilab experiment FOCUS and of its physics goals will be presented. During the 96-97 fixed target run at Fermilab we collected more than 6.3 billion events with the expectation of in excess of 1 million fully reconstructed charmed particles. Preliminary results on charmed meson decays will be reported.

1. Introduct ion

FOCUS means Photoproduc t ion of Charm with an Upgrade Spectrometer with a lexical li- cence. The word upgrade refers to the upgrade of the E687(the predecessor experiment) spectrometer.

The charmed particles were produced by the interaction of high energy photons (obtained by means of bremsstrahlung of electron and positron beams) with a Beryllium Oxide target. The data

*Coauthors: J.Link, M.Reyes, P.M.Yager (UC DAVIS); J.Anjos, I.Bediaga, C.Gobel, J.Magnin, I.Pepe, A.Reis, A.S&nchez-Herngndez,F.Sim&o(CPBF,Rio de Janelro); S.Carrillo, E.Casimiro, H.Mendez, M.Sheaff, C.Uribe, F.Vaaquez (CINVESTAV, M6xico City); L.Cinquini,J.P.Cumedat,B .O 'P~i l ly , J.E.Ramircz, E.Veumderlng (CU Boulder); J.N.Butler, H.W.K.Cheung, I.Gainea, P.H.Garbinciua, L.A.Garren, S.A.Gourlay, P.H.Kemper, A.E.Kreymer, R.Kusc.hke (Fermilab); S.Bianco, F.L.Fabbrl, S.Sarwar, A.gaUo (INFN Frascati); C.Cawlfield, R.Gardner, E.Gottechalk, K.S.Park, A.Rahimi, J.Wies (UI Cham- paign); Y.S.Cheung, J.S.Kemg, B.R.Ko, J.W.Kwak, K.B.Lee, S.S.Myung, H.Park (Korea Univer- sity, Seoul); G.Alimonti, M.Boechini, B.Caccianiga, A.Calandrino, P.D'Angelo, M.DiCorato, P.Dini, M.Giammarchi, P.Insani, F.Leveraro, S.Malvezzi, D.Menemce, M.Meszadri, L.Milazzo, L.Moroni, F.Prelz, A.Sala, S.Sala (INFN and Milano); T.F.Davenport III (UNC Asheville); V.Arena, G.Boca, G.Bonomi, G.Gianini, G.Liguori, M.Merlo, D.Pemtea, S.P.Ratti, C.Riccardi, L.Viola, P.Vitulo (INFN and Pavia); A.Fernandez (Puebla, M6xico); A.M.Lopez, A.Mirlea, E.Montiel, D.Olaya, C.River6, Y.ghang (Mayaguez,Puerto Rico); N.Copty, W.E.Johns, M.PvLrohlt, J.R.Wihton (USC Columbia); K.Cho,T.Handler (UT Knoxville); M.Hoaack, M.Nehring, P.D.Sheldon, M.Webster (Vanderbilt); K.Stenson (Wisconsin, Madison); Y.Kwon (Yonsei Universlty, Korea)

were collected at Fermilab during the 1996-97 fixed target run. More than 6.3 billion triggers were collected. Our initial goal was to increase the statistics of E687 of a factor 10.

2. Physics Goals

A large sample of charmed particles reconstructed will allow a systematic investigation of the charm properties and could open a window onto new physics beyond the Standard Model. Some of these studies are discussed below.

The world averages for lifetime measurements are dominated by results from E687, which is the only experiment which has measured all the charmed hadron lifetimes [1]. The results are internally consistent and the ratios of lifetimes are to a large extent unbiased by systematic effects. FOCUS should have at least 16 times the statistics of E687, therefore we expect to reduce the charm lifetime statistical errors by a factor 4. While there is no theoretical need to measure r(D+)/r(D °) more precisely, it is desireable to determine r(D+)/r(D °) with an accuracy of ,-~ 1%, this will allow us to quantitatively address some aspects of the W-annihilation mecha- nism[2]. FOCUS should reach this level of accuracy for this measurement.

Semileptonic decays are easier to interpret the- oretically than hadronic decays. From measurements of semileptonic branching ratios, FOCUS could determine the ratio of CKM matrix ele- ments V~/Vcd to 2%. In addition the form factors for the charmed particles will be determined more accurately[3].

0920-5632/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. Pll S0920-5632(99)00335-7

106 D. Pedrini /Nuclear Physics B (Proc. Suppl.) 75B (1999) 105-111

Studies of charm production dynamics test leading order and next-to-leading order pertur- bative QCD[4]. There are still open issues in the charm photoproduction dynamics. For example the asymmetries in the photoproduction of charmed mesons and bayons. The data can be compared to the predictions from models of charm quark fragmentation, like the Lund string fragmentation model[5]. The double charm events will be particulary useful for this study.

In the baryon sector only singly charmed baryons have been observed so far and the ex- istence of the ~2e has been established definitely only a few years ago. A large charmed baryon sample will allow, finally, a systematic study and a more accurate determination of the charm baryon lifetime hierarchy.

The study of the excited states llke D** and A~ can be compared to the the predictions of HQET (Heavy Quark Effective Theory).

For hadronic decays sophisticated Dalitz plot analyses will be feasible due to the high statistics. In these analyses a very high SIN ratio is crucial in order to properly determine the subresonant structure of the decay modes. These studies, in association with the isospin decomposition of the ~-body decay modes, will help in clarifying the importance of FSI (Final State Interactions) in the charm decays.

D mesons are the only examples of heavy quark systems in which Cabibbo-favoured, singly Cabibbo-suppressed and Doubly Cabibbo Sup- pressed Decay (DCSD) decays have all been measured[l]. FOCUS has already the largest signal (79 + 14 events) of the decay mode D + K+Tr-~r + obtained with only 16% of our data (see figure 1) where both the decay modes D + ---* K+Tr-~r + (DCSD) and D + --* K+Tr-~r + (singly Cabibbo-suppressed) are clearly visible (throught this paper the charge conjugate state is implied when a decay mode of a specific charge is stated).

FOCUS will have the chance to look at the lep- tonic decay D + ~ /~+v which allows the measurement of fD, the pseudoscalar decay constant. The method requires determining the direction of the D + by seeing the parent D + in the target microstrips (see below).

The study of charrn particles could become

High Impact Physics in the areas where it is possible to search for new physics beyond the Stan- dard Model. These areas are rare and forbidden decays, study of D O _~-6 mixing and CP violation in the charm sector.

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Figure 1. K+~r-v + invariant mass.

3. FOCUS Spectrometer

An overview of the FOCUS spectrometer is shown in figure 2. A bremsstrahlung photon beam of mean energy approximately 200 GeV im- pinges on a beryllium oxide segmented target; the particles from the interaction are detected in a large aperture magnetic spectrometer with excel- lent vertex measurement, particle identification and calorimetric capabilities.

D. Pedrini/Nuclear Physics B (Proc. Suppl.) 7517 (1999) 105-111 107

B e a m }irection

S Spectrometer

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Electromagnetic Counter Calorimeter P,W.C.'s Calorimeter Muon

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T,i e, / " ' " Be/m t Straw Hodoscope / Tubes Cerenkov Calorimeter Muon Filter

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Figure 2. FOCUS spectrometer.

The FOCUS spectrometer is an upgrade of the E687 spectrometer decribed in detail in refer- ence[6].The detector upgrades were the replacement of the hadron calorimeter allowing the inclusion of the hadronic energy measurement in the first level trigger of the experiment, the insertion of straw tubes to increase the efficiencies of the tracking in the central region (pair region), the replacement of the thick target with a segmented one (four BeO slabs) to increase the D decays in air, the inclusion of four microstrip planes(target silicon) in the target region (see figure 2) to increase the vertex resolution, a new lead glass elec- t romagnetic calorimeter and an upgrade of the existing outer electromagnetic calorimeter to increase the reconstruction efficiency for photons and electrons, two new muon systems to increase both the efficiency and quality of the muon iden-

tification which is critical for semileptonic decays and finally a faster DAQ[7].

4. Onl ine m o n i t o r u s i n g t h e I n t e r n e t

One of the reasons for the success of FOCUS was the use of a Web browser to monitor the experiment during the data taking. Web based tools allowed a continuous remote monitoring of every detector at a level not possible in the past. This was not simply the possibility to connect remotely to the experimental hall (this was feasible also in the past), but actually to monitor and control the experiment from one's home institution as if one was in the control room. Thanks to the new software tools, which allowed easy production of timelines and correlation plots between the different detectors and even enabled us to monitor

the beam, it was pretty easy to detect and fix the problems. We were able to monitor the spectrometer performance by tracking the charm yield for each run.

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5. P A S S 1 a n d S K I M S t a t u s

The PASSl(event reconstruction) is underway at Fermilab (Feynman Computer Center), but the monitoring and control of this process is allocated to different collaborating institutions worldwide. Using Web based tools a remote institution can manage the reconstruction of the data. At the end of June 1998 50% of the total sample (6.3 billion events) has been reconstructed. We antic- ipate that PASS1 will finish in October of 1998.

The SKIM for the different physics analyses proceeds in parallel with PASS1. It allows us to use real physics results to obtain sophisticated feedback on the performance of the reconstruction algorithms, for example by looking at Cabibbo suppressed decays. The SKIM started in April of 1998 and is expected to finish early in 1999.

A fun way to see how the PASS1 proceeds is shown in figure 4, where the PASS1 progress at the end of June 1998 is shown.

6. H a d r o n i c D e c a y s o f c h a r m e d m e s o n s

The hadronic decays of charmed mesons are re- markable because they address important ques- tions about the non-spectator contributions to the charm decays and FSI.

We designate as golden, modes the following hadronic decays of D o and D+: D o --* K - v +, D O --+ K - r - r + r + and D + --+ K - ~ + r +. These are the decay modes (obtained using loose cuts) used to monitor the performances of the spectrometer. Figure 3 shows the two golden modes for D O . There are approximately 171000 D o ---* K - r + candidates and 159000 D o --* K - r - ~ r + r + from 3 x l09 triggers.

We reconstructed approximately half a million golden mode decays from 50% of our data. There- fore a reasonable estimate is 1 million fully reconstructed charmed particles, or 16 times the E687

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108 D. Pedrini /Nuclear Physics B (Proc. SuppL) 75B (1999) 105-111

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Figure 3. K - r + and K - r - ~ r + r + invariant masses.

statistics. With this sample an extensive study of the charm hadronic decays can be undertaken and modes barely visible before can be easily studied.

In figure 5 two Cabibbo-suppressed decays of D °, D o ---+ K - K + and D o ---* r - r +, out of 16% of our data are shown.

The ratio of these decay modes disagreed with the theoretical prediction for a long time. The theory predicted the ratio to be less than 1.4,

D. Pedrini /Nuclear Physics B (Proc. Suppl.) 75B (1999) 105-111 109

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Figure 4. Web page of the PASS1 progress at the end of June.

whereas the experimental value was 2.7. Nowa- days this difference is ascribed to the inelastic FSI[1], but it is impor tant to have the most accurate measurement of this ratio to try to under- stand the importance of FSI in charm decay.

The decay mode D + ~ ~r-Tr+Tr + is particulary intriguing because it could be due to the W- annihilation diagram. To study this decay mode a Dalitz plot analysis is necessary in order to take account of interference effects. We can determine the resonant substructure of the decay mode and make a meaningful comparison of experimental da ta and theoretical models. In figure 6 the ~r-Tr+~r + invariant mass and the Dalitz plot under the D + mass region, together with the X and Y projections, are shown. The data corresponds

to 16% of the full sample. For the moment this is just a qualitative study. No sideband subtraction is performed to properly handle the background component under the D + signal and no fit is made to the Dalitz plot. However looking at the projection plots, where the positions of some of the expected resonances are indicated, we see no ev- idence of the p resonance. The presence of this resonance could be due to the W-annihilation diagram.

7. C o n c l u s i o n s

We have shown the incredible quality of the data collected during the the 96-97 fixed target run at Fermilab.

110 D. Pedrini/Nuclear Physics B (Proc. Suppl.) 75B (1999) 105-111

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The initial goal of the experiment was to reach 10 times the statistics of the predecessor experiment E687 and this has been achieved. A very reasonable estimate is 1 million fully reconstructed charmed particles, or 16 times the E687 statistics.

This huge sample will allow a comprehensive and detailed study of charm physics that is still interesting and could still produce some surprises 24 years after the discovery of the first charm particle, the J /¢ .

The first physics results are expected by the end of 1998.

the Italian Istituto Nazionale di Fisica Nucle- are and Ministero dell'Universitgt e della Ricerca Scientifica e Tecnologica, the Brazilian Conselho Nacional de Desenvolvimento Cientffico e Tec- nol6gico, CONACYT-M~xieo, the Korean Min- istry of Education and the Korean Science and Engineering Foundation.

8. Acknowledgements

We wish to aknowledge the assistance of the staff of Fermi National Accelerator Laboratory, the INFN of Italy and the physics departments of the collaborating institutions. This research was supported in part by the National Science Foundation, the U.S. Department of Energy,

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D. Pedrini/Nuclear Physics B (Proc. Suppl.) 75B (1999) 105-111 111

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:i . • .'..'.~ .. '.~_ - " ' , ' " " ' ' " " - '" " "trY.

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4. J. Wiss, " C h a r m at Fixed ta rge t" to be pub- lished in Proceedings of the Varenna School 8-18 Ju ly (1997)

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Documents

Overview of FOCUS and a report on charmed mesons in FOCUS