Unknown a& Fermi National Accelerator Laboratory FERMILAB-Conf-96/438-E CDF Beauty Baryons: Recent CDF Results J. Tseng For the CDF Collaboration Fermi National Accelerator Laboratory I? 0. Box 500, Batavia, Illinois 60.510 Massachusetts Institute of Technology Cambridge, Massachusetts 02139 December 1996 Published Proceedings of the 2nd International Conference on Hyperons, Charm and Beauty Hadrons, Montreal, Quebec, August 27-30,1996 Operated by Universities Research Association Inc. under Contract No, DE-AC02-76CH03000 with the United States Department oi Energy Disclaimer This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for th e accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Distribution Approved for public release; further dissemination unlimited. Beauty Baryons: Recent CDF Results J. Tseng,a for the CDF Collaboration ‘Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA Using data collected between 1992 and 1995 at the Fermilab Tevatron, CDF has searched for the hb baryon through both semileptonic and hadronic decay channels. This presentation reviews measurements of the & mass, lifetime, and production and decay rates performed with this data. The Fermilab Tevatron Pp collider. with center- of-mass energy fi = 1800 Gel’, produces copious numbers of b quarks-some eighty thousand mil- lion between 1992 and 199.5-and is thus a useful tool for the study of b-quark physics. It is natural: therefore, having found and studied mesons con- taining b quarks, to search the data for the pres- ence of baryons containing such quarks. This arti- cle describes recent studies concerning the lowest lying baryon, the ,ib, conducted at the Collider Detector at Fermilab (CDF). [l] 1. Full Reconstruction: .I6 --+ J/+.1’ One of the signatures proven most useful for b hadron reconstruction at CDF is the decay J/p5 + pL+p-, because the two muons readily distinguish the event from the far more com- mon light-quark processes that occur at a hadron collider. The muons are identified by matching tracks in the Central Tracking Chamber (CTC) with track stubs in wire chambers placed out- side the electromagnetic and hadronic calorime- ters. These outer chambers cover the pseudora- pidity range 1~1 < 1 relative to the nominal in- teraction point. The tracks are required to have momentum, pi, measured in the detector plane transverse to the beam! of at least 2 GeV’/c. The two muons are vertex constrained as well as mass constrained to the world average J/q4 mass. [2] and the resulting probability is required to ex- ceed 0.5%. The search for Ah in the resulting data sam- ple is for its decay to J/$:1’. [3] (In this article, charge conjugate decays are implied.) The de- cay B” + J/$Kf, followed by Kf + R+T-, is topologically very similar to A* --+ J/$:1’ and is used to check the mass measurement as well as normalize the rate measurement. The ho is reconstructed through its decay to PC. Because of the large mass difference be- tween the proton and pion, the proton mass is assigned to the higher-m track. The ho vertex is required to have a transverse displacement, pro- jected along its momentum, of at least 1.0 cm beyond the primary vertex. The total pi of the A0 must be at least 1.5 GeV/c. The two tracks are also required to have &Y/&c measurements in the CTC within 2a of their predicted energy loss. Combinations with mass within 4 MeV/c2 (about 20) of the world average for A0 are taken to be ,1’ candidates. The event is removed from considera- tion if assigning the proton candidate a pion mass results in a combined mass within 12 MeV/c2 of the Kf mass. The J/T) and A0 candidates are then com- bined, constraining the dimuon pair to the J/q mass and the A0 to point back to the dimuon vertex. The combined pi is required to exceed 6.0 GeV/c’ and have 1~1 < 1 relative to the pri- mary interaction point. In addition, a lifetime cut, CT > 100 pm, is applied to reduce non-b backgrounds. The resulting mass distribution is shown in Figure 1. 1.1. Mass Measurement The above reconstruction method has been tested in parts on several established signals such as $(2S) 4 J/I)T+x-. xcr,z --+ J/$7 with pho- ton conversion 7 + efe- , f- -+ Aor-, and, as b b .3 f : 1. F I a 5.1 5.5 5.6 5.7 5.8 Mass(J/* A) GeV/c’ UAl 5640k50f30 i DELPHI - 5668f16k8 ALEPH - 5614521 f4 CDF 5621i4f3 I.45 53 536 1.6 5.65 1.7 1.7, la 545 5.s mass (GeV/c’) Figure 1. Invariant mass distribution of J/$-1’ candidates. The fit results are from an unbinned maximum likelihood fit to a gaussian signal over a linear background. Figure 2. Recent -lb mass measurements. systematic uncertainties. That difference is mhb - mg0 = 340 k S(stut) h l(sys) ?deV/c2. mentioned above, the topologically similar B” --t J/$X,0. ?I:0 systematic mass effects were uncov- ered with these checks. The statistical significance of the peak is es- timated by the probability that a linear back- ground could fluctuate up to the observed num- ber of events in five consecutive bins, which cor- responds to the mass resolution of the .I* deter- mined by Monte Carlo. The search window was defined in several ways, and all methods yield a significance of about 30. The mass of this signal is 562114 MeV/c2 from an unbinned maximum likelihood fit. Systematic uncertanties due to detector effects and nonuni- formities are measured to be 3 MeV/c2. The final mass result is mhb = 5621 * 4(stat) i 3(sys) MeV/c2. 1.2. Production and Decay Rate As mentioned above, the Aa production and decay rate can be measured relative to that of B” --t J/$X:. The reconstruction algorithms are essentially the same, and hence many systematic uncertainties largely cancel in the ratio. Additional quality cuts are imposed upon the data in order to perform the measurement where efficiencies are well understood. These cuts in- clude the requirement that all tracks have pi > 400 MeV/c and the muons must be reconstructed using precision measurements in the Silicon V’er- tex Detector (SVX). [6] The ratio of efficiencies of these cuts is measured in Monte Carlo to be eBOBr(X: -+ 7r+a-) chbBr(.10 + p?r-) = 2.02 k 0.05. After these cuts the signals are as shown in Fig- ure 3. There are 7.7k3.4 ,4b events and 57.6f8.7 Boas. The ratio of production and decay rates is therefore U(j$J + .jbx)Br(&, + J/&4”) a(Fp -+ B”X)Br(Bo -+ J/+X:) Figure 2 shows this value in comparison with recent measurements from U.41. DELPHI, and ALEPH. [4] :5] Calculating the mass difference between the Ab and the B” almost eliminates the 1. ~ N ,,=7.7*3.4 A 2 2. - j ‘s.. - - \ - -- c- - _ LO ;: O. 5.1 $5 5.8 1.7 5.1 P Mass (J/q A) W/C’ 0 30. ~ z _ N.,=57.6f8.7 ; D 20. - E : Ifi’ rl IO. - -I---- _ r-L-L __ -- -- -__ ---- -- _ --- _ __- -- 0. - 5 5.2 5.1 5.6 Mass (J/C K.) G&/c’ Figure 3. & --+ J/$:1’ and B” + J/$JK~ mass distributions for the rate measurement. = 0.27 41 O.l2(stut) f O.Oj(sys). The largest systematic uncertainties are due to the fragmentation and decay dynamics of the &lb. 2. Semileptonic Decay: .b, + .i$L-ClX Partial reconstruction of the semileptonic de- cay of the !Ib is another approach to the study of the b baryon. and it offers several advantages. among them a large branching fraction on the or- der of lo%, and, because of the high thresholds on the single lepton triggers at CDF. favorable kine- matics for distinguishing the high energy signal from the relatively low energy backgrounds. The signature for Lib semileptonic decay used in the present analyses is a -*right sign” charm- lepton pair (.2$e- and :i;e+) with invariant mass consistent with & decay. The .I: is fully recon- structed through its decay to ~K-T+. “VVrong sign” pairs (AZ.!+ and ,I;C-), which cannot come from semileptonic hb decay, are used for back- ground studies. The CDF trigger selects electrons with electro- magnetic energy deposition in the central detec- tor region of at least 9 GeV and pointed to by a CTC track. The electron candidates are further purified by examining their deposition profiles in the calorimeter as well as in strip chambers em- bedded within the calorimeter. Muon candidates are selected as described in Section 1 but with pi > 9 GeV/c. The ,I: is reconstructed as a three-prong ver- tex with all tracks in the SVX. The pi cuts on the proton, kaon. and pion are set at 2, 1, and 0.45 GeV/c. Since both the .ib and :I$ are long- lived particles, cuts on the apparent flight dis- tance of the vertex and the impact parameters of the tracks further suppress the background while keeping most of the signal. The proton dE/dx is required to be within 2a of expectation. The A, candidate is then combined with the lepton. and the invariant mass of the two particles is calculated and required to be between 3.5 and 6 GeV/c2. The lower limit cuts out part of the kinematically allowed region but suppresses back- grounds from B meson decays. 2.1. Production and Decay Rate The reconstruction program described above was first carried out in the single electron data of the first 19.3 pb-‘. The pKx mass distri- bution is shown in Figure 4 for the “right sign” combinations, and Figure 5 for the “wrong sign.” Monte Carlo studies show that with the above cuts, backgrounds from ‘1: duplicates (switch- ing the proton and pion), reflections from other charm hadrons. B meson and non-semileptonic iib decays, and pairs from different quarks do not contribute significantly to the size of the “right sign” signal. The product of cross section and branching fractions is measured with this signal to be ub(p& > lo.5 GeV/c, IyI < 1)x fhbBr(.db --+ Ad$e-c,X)Br(A, + pK-x+) = (1.9 * O.jl(stat) & 0.36(sys)+~:~~(th)) nb where f~, is the probability that the fragmen- tation of a b quark produces a b baryon which then yields the “right sign” signature; here “:I+,” designates any such baryon. the hb itself being expected to contribute the largest part. The sys- tematic uncertainties are mostly those due to de- tector simulation: whereas the theoretical uncer- 19.3pb.’ electron data w- c : > J I, : CDF Preliminary E Bd+ N = 33.7f9.0 f . JJi m = 2286.4f3.1 MeV/c’ . 0 = 0.5 MeV/c’ m_ 25 ; ,“I I 20 1 ‘.’ ” E--- , I (0 Lo.. $A.-, 0 ;L---- 1.1 1.15 21 2 2, 2.3 2.35 2.4 2.43 m(pKn) RS Lw/,’ 19.3pb-’ electron data CDF Preliminary N = 2.1 5~5.3 Q2.r 2 1, 22 1.25 2.1 1.35 24 2.45 m(pKn) WS wJ/d Figure 4. “Right sign” pKx mass distribution. Figure 5. “1Vrong sign” pKa mass distribution. tainties are mostly from uncertainties in fragmen- tation and the decay model used in the efficiency calculation. The above rate measurement can be converted into a product of branching fractions by dividing it by the b quark cross section measured at CDF for the specified kinematic region, [ij ab(p$ > lo..5 Ge\:/c, IyI < 1) = 1.99 i 0.30 i 0.41 pb. Removing common systematic uncertainties. the product of branching fractions is Such a short lifetime is difficult to accomodate theoretically. [ll] The :ib lifetime is measured at CDF with elec- trons and muons from 110 pb-’ of data collected from 1992 to 1995. [12] The analysis uses cuts sim- ilar to those used in the rate measurement. The “right sign” and “wrong sign” pKr mass distri- butions are shown in Figure 7. There are 197 % 2.5 “right sign” events, and again no discernible peak among the ‘*wrong sign” combinations. The lifetime measurement is performed by measuring the transverse distance from the pri- mary interaction point to the A,1 vertex. The distribution of “pseudo-proper decay lengths,” M CT’ = L,, ___ PT (M) ’ so called because the neutrino momentum is not included. is shown in Figure 8. The effect of the missing neutrino is modeled in Monte Carlo and is seen to be largely insensitive to p~(:l,e) and lep ton pi. The correction is convoluted with the life- time fit to give the true ~7. The lifetime fit simul- taneously fits the signal and background shapes and gives fA,Br(hb -+ A:e-v,X)Br(.i, + ~K-T+) = (9.5 It 2.5(sM) * 2.9(sys):;:;(th)) x 10-4, which is higher than but consistent with previous results from LEP. [8] 2.2. Lifetime Measurement The hb lifetime has been measured using semileptonic decays at LEP and is notable for be- ing unexpectedly short. QCD predictions place the lifetime ratio TA,/TBO at not much less than 0.9: [9] as shown in Figure 6, the observed ratio is 0.73 k 0.06, averaging over analyses based upon sign correlations in AI,e, .I.& and pt pairs. [lo] TAb = 1.32 i O.l5(stat) & O.O7(sys) ps 0’ *.62*0.06 B0 1.56kO.06 - 1.61:;:;; 4 - 1.14*0.06 -, -- 1.41f0.51 Inclusive - 1.5673~0.020 0.1 0.1, ! 1.2 1.5 I’, 2 2.25 Id 2.1, 3 lifetime (ps) M(PK~) G&/c’ Figure 6. Comparison of b hadron lifetimes. Figure 7. “Right-sign” (points) and “wrong-sign” (hatched histogram) pKx mass distributions. where the systematic uncertainties include varia- tions in the background shape and fitting proce- dure, possible bias due to the event selection cuts, and uncertainties in .\b production and decay, in- eluding the effect of additional daughter hadrons. The lifetime ratio is therefore be c7@p - &x)Br(:jb - +&do) CT& -+ B”X)Br(Bo -+ J/q!~rKf) = 0.2i * 0.12(3tat) i 0.0.5(sys) TAb - = 0.8.5 f O.lO(stat) * O.O3(sys). n30 The CDF lifetime measurement is compared in Figure 9 with other measurements which fully re- construct the A$ and is seen to be consistent with them. [13] Considered alone. it is also in good agreement with the theoretical expectation. 3. Conclusion In data collected at the Tevatron between 1992 and 1993, CDF has observed the -lb through its hadronic decay to J/$;1’ and semileptonic de- cay to A$.!-?lX. Its mass is measured to be 5621&4(stut) & S(sy3) Mel’/ca and its lifetime to be 1.32 =t O.l~(sta.t) i O.O’i(sys) ps. The lifetime ratio, 7~~ /rg0 = 0.85 zt O.lO(stat) 31 O.OS(sys), is in agreement with theoretical expectations as well as recent LEP measurements. 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