A new measurement of $R_b$ with the ALEPH detector at LEP using five mutually exclusive hemisphere tags is presented. The preliminary result is $R_b = 0.215

Bazarko, A O

A new measurement of $R_b$ with the ALEPH detector at LEP using five mutually exclusive hemisphere tags is presented. The preliminary result is $R_b = 0.2158A new measurement of $R_b$ with the ALEPH detector at LEP using five mutually exclusive hemisphere tags is presented. The preliminary result is $R_b = 0.2158 \pm 0.0009 (stat.) \pm 0.0011 (syst.)$

Bazarko, Andrew O.

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DPF965a1/A05004A MEASUREMENT OF Rb USING MULTIPLE TAGSANDREW O. BAZARKOCERN, CH-1211 Geneva 23, Switzerlandrepresenting the ALEPH COLLABORATIONA measurement of Rb using ve mutually exclusive hemisphere tags is presented.The preliminary result is Rb = 0:2158 0:0009(stat.)  0:0011(syst.).1 IntroductionAmong the measurements available at the Z0 pole, Rb, the ratio of the Z0partial width into b quarks and its total hadronic partial width, is currentlyexciting particular interest. Most electroweak and QCD radiative correctionscancel in the ratio, leavingRb sensitive to corrections that couple preferentiallyto b quarks, like the large CKM coupling to top quarks. As the parameters ofthe Standard Model become better constrained, a precise measurement of Rbtests the presence of novel vertex corrections.This paper presents a new measurement of Rb using data recorded byALEPH in 1992-95, consisting of nearly 4 million hadronic events. The mea-surement is performed using ve hemisphere tags.1 The most important of thetags is a lifetime-mass tag, oering high-purity b tagging.2 Two additional btags are used, together with tags designed to select charm and u; d and s fla-vors. The latter permit the experimental determination of the backgroundeciencies of the additional b tags. The ve tags are mutually exclusive, sothat a hemisphere is tagged at most by one tag.2 The MethodEvents are divided into hemispheres by the plane perpendicular to the thrustaxis. The fraction of tagged hemispheres fs;I with tag I isfs;I = Rbb;I + Rcc;I + (1− Rb − Rc)uds;I (1)where a;I are the hemisphere tagging eciencies for flavor a using tag I.The fraction of doubly tagged events fd;IJ with tags I and J is:fd;IJ = [Rbb;Ib;J(1 + b;IJ ) +Rcc;Ic;J(1 + c;IJ)+(1− Rb −Rc)uds;Iuds;J(1 + uds;IJ)] (2− IJ) (2)Presented at DPF96, American Physical Society Division of Particles and Fields 1996Divisional Meeting, Minneapolis, Minnesota, 10-15 August 1996.1where a;IJ are the hemisphere-hemisphere eciency correlations for flavor aand tags I and J .Twenty quantities are measured: 5 single tag fractions and 15 double tagfractions. Of the 62 unknown parameters in the above two equations, Rb and13 eciencies are tted to the data. The two remaining eciencies, c;Q andx;Q (the Q tag is described below), and the 45 correlations are calculated usingsimulation. The systematic error reflects the uncertainties in these calculations.Rc is taken as 0:171 from electroweak theory.3 Event selectionUsing the data sample obtained on and near the Z0 resonance with the ALEPHdetector during the period 1992 to 1995, events must satisfy the followingrequirements: at least ve reconstructed tracks; at least two jets clusteredusing the JADE algorithm 3 with energy greater than 10 GeV; j cos T j < 0:65,where T is the angle between the beam and the thrust axis; and y3 < 0:2,where y3 is the value of ycut that sets the transition from 2 to 3 jets.3 Theremaining 2,059,066 events have a selection bias in favor of b quarks relativeto the lighter quarks of 0:23  0:04% and a contamination of tau events of0:30 0:01%.4 The Five Hemisphere TagsIn order to keep the hemisphere{hemisphere correlations small and their cal-culation transparent, the tag variables are constructed from quantities limitedto the particular hemisphere only. A new primary vertex nder is employed,which reconstructs the Z0 decay point seperately in each hemisphere.The Q tag is a high purity b tag formed from the combination of twotagging algorithms. Both algorithms make use of 3-dimensional track impactparameters, dened as the distance of closest approach of the track to theprimary vertex. The rst algorithm provides the probability that all of thehemisphere tracks originate from the primary vertex.4 The second algorithmexploits the b/c hadron mass dierence. Tracks in a hemisphere are orderedinversely to their probability to originate from the primary vertex and arecombined, in this order, until the invariant mass of the combination exceeds1.8 GeV=c2. The probability of the last track added to have originated fromthe primary vertex is used as a tagging variable.The S tag uses the same lifetime-mass information as the Q tag but in aless pure b tagging range, together with a neural net variable that optimizes25 event shape parameters for the selection of Z0 decays to b quarks.52The L tag requires an identied muon or electron, with momentum greaterthan 3 GeV/c, and transverse momentum with respect to its jet greater than1.4 GeV/c.6The X tag requires high probability that all tracks originate from theprimary vertex, together with a requirement on the same neural net variableas the S tag, but in the opposite sense, in order to tag uds events.The C tag, which is the most dicult to construct, makes use of lifetimeand event shape properties. The tag uses a neural net variable that optimizesone lifetime and 19 event shape parameters for the selection of Z0 decays toc quarks. In addition, tracks are divided into two groups with rapidity withrespect to their jets greater or less than 5.1, chosen to get equal numbersof tracks for b hemispheres. The probabilities that the two groups of tracksoriginate from the hemisphere primary vertex are used for tagging.The Monte Carlo expectations for the 35 eciencies are shown in Table1.Tag uds (%) c (%) b (%)Q 0:029 0:20 19.55S 0.200 1.40 17.57L 0.156 0.69 4.25X 11.70 3.96 0.23C 7.93 16.20 2.59Table 1: Tagging eciencies as given by Monte Carlo simulation.5 ResultThe t of Rb and 13 of the 15 eciencies to the 20 measured tag fractionsyieldsRb = 0:21582 0:00087 (3)with a 2 of 8.1 for the 6 degrees of freedom, where the error is statistical. Thisresult has been corrected for the event selection bias and tau contaminationdiscussed in section 3.6 Systematic uncertaintiesThe impact of a particular correlation on Rb is given by Rb=(Rb a;IJ),i.e. the relative uncertainty in Rb is the impact times the uncertainty in thecorrelation. The impacts vary a great deal, and uncertainties in only a dozen3or so of the correlations contribute signicantly to the uncertainty in Rb. Thecorrelations with the biggest impacts are b;QQ, with impact 0.45, and b;QS ,with impact 0.42. The impacts of the two Q tag background eciencies aregiven byRbRbb;Qc;Q= −1:5;RbRbb;Quds;Q= −5:6A brief description of the sources of systematic uncertainty is given below.6.1 Detector simulation uncertaintyThe Monte Carlo predictions for the Q tag background eciencies dependon the impact parameter resolution and the eciency for tracks to record avertex detector hit. These quantites are measured in the data and remaininguncertainties produce corresponding errors in c;Q and uds;Q.6.2 Systematics from b and c physics uncertaintiesUncertainties in the physics charmed and light flavored hadrons propagatedominantly through the Q tag background eciencies, whereas uncertaintiesin bottom physics propagate through the correlations. To assess these uncer-tainties the physics inputs to the Monte Carlo simulation are varied withintheir allowed experimental ranges.7 The largest contribution found with thisprocedure is due to the rate of gluon splitting into heavy quarks.6.3 Hemisphere-hemisphere correlation uncertaintiesThe correlations have three origins: detector inhomogeneities, the size andposition of the interaction region, and the coupling of the B hadron momentathrough gluon radiation and hadronization. The Monte Carlo simulation’sability to reliably calculate these sources of correlation is checked against databy studying the correlation due to variables linked to these correlating eects.Uncertainties are assigned that are in addition to those due to b and c physicsthat have been considered above.The correlation contributions in the following four variables are comparedbetween data and simulation: the cosine of the angle between the thrust andthe beam axes, the component of the error in the reconstructed hemisphereprimary vertex transverse to the thrust axis, the momenta of the two jets(because of their correlation with the B hadron momenta), and y3 (to checkthe gluon radiation-induced correlations). The larger of either a measureddierence between data and simulation or the precision of the comparison isassigned as an uncertainty in the correlation. The uncertainty inRb is found by4propagating and adding their contributions linearly, so as to take correlationsbetween the correlations into account.A summary of the systematic errors is given in Table 2.source Rbdetector simulation 0.00050Monte Carlo statistics 0.00047event selection 0.00010physics uncertainties 0.00083hemisphere correlations 0.00033Table 2: Systematic uncertainties.7 ConclusionsThe fraction of hadronic Z0 decays to b quarks, Rb, has been measured usingve mutually exclusive tags. The preliminary value is:Rb = 0:2158 0:0009(stat.) 0:0011(syst.)− 0:019 (Rc − 0:171) (4)where the rst error is statistical, the second is systematic, and the explicitdependence on Rc is given.This measurement includes the 1992 data used in the earlier ALEPH Rbmeasurement with a lifetime tag4 and therefore supercedes it.1. ALEPH Coll., A Measurement of Rb using Mutually Exclusive Tags,Contributed paper to the ICHEP Conference, Warsaw, July 1996, PA10-015.2. ALEPH Coll., A Measurement of Rb using a Lifetime-Mass Tag, Con-tributed paper to the ICHEP Conference, Warsaw, July 1996, PA10-014.3. W. Bartel et al., JADE Coll., Z. Phys. C33 (1986) 23.4. D. Buskulic et al., ALEPH Coll., Phys. Lett. B313 (1993) 535.5. D. Buskulic et al., ALEPH Coll., Phys. Lett. B313 (1993) 549.6. D. Buskulic et al., ALEPH Coll., Nucl. Instr. Methods A360 (1995) 481.7. Presentation of LEP Electroweak Heavy Flavour Results for Summer1996 Conferences, LEPHF/96-01, ALEPH 96-099.5

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