wward

Information about wward

Published on October 17, 2007

Author: Alohomora

Source: authorstream.com

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ALEPH Fragmentation Studies for mW:  ALEPH Fragmentation Studies for mW J. Jason Ward OUTLINE: 1. Introduction 2. Questions for the combined LEP mW 3. Review 3 approaches to estimate DmW 4. lqq Analysis (JETSET/HERWIG/ARIADNE) 5. qqqq Studies (Ann) 6. Suggestions for future studies 7. Conclusions For WW Physics Workshop - WWMMI Cetraro, Italy, October 12-17, 2001 wardj: wardj: wardj: Previous Presentations:  Previous Presentations This work has been presented before in ALEPH: 1. Previous presentations from myself (see attached list of references), particularly the presentation at the WW Meeting at Pisa (May 2001)  eqq and qq channels. 2. Talk in W Jamboree yesterday from Ann Moutoussi & her previous studies  qqqq channel. We have not presented recent (since October 2000) work to the LEP-wide audience yet. ALEPH Fragmentation Studies are of LEP-wide Importance:  ALEPH Fragmentation Studies are of LEP-wide Importance A long campaign to understand this systematic error in ALEPH has significantly reduced this uncertainty. The LEP experiments perform similar studies to determine DmW due to fragmentation. What we understand on ALEPH is of LEP-wide interest. Uncertainty on mW from Fragmentation:  Uncertainty on mW from Fragmentation With an infinitely precise knowledge of (E,p) of every final state particle from a single Wqq system, the mass of that W can be determined precisely, regardless of Parton Shower and hadronisation details. Fragmentation  MC Models (and Real Life) of  Detector  Analysis Parton Shower  Hadronisation Reconstruction Decays Different (E, p) spectra Different baryon production rates E or p thresholds Tracks  mp Neutrals  mg Jet finding Inter di-jet particle mixing Current Unanswered Questions:  Current Unanswered Questions What is the correlation of the fragmentation error i) between experiments? ii) between the lqq and qqqq channels? (Current assumption for each is 100%) How much do the independent ADLO tunes of JETSET/HERWIG/ARIADNE uncorrelate the systematic error? What aspect of a tune matters (JW) ? Are there detector effects? lqq and qqqq channels could have different sensitivities (e.g. 10 and 15 MeV respectively) but may still be correlated? Identification of a possible mechanism of the W mass bias due to fragmentation is the missing connection between all of these questions. Variation of Fragmentation Parameters:  Variation of Fragmentation Parameters ALEPH 172 GeV  4s for parameters including LQCD  10 MeV for all channels. JETSET, HERWIG and ARIADNE are models available to describe the W decay process. Within one model (e.g. JETSET) parameters determined from high statistics Z-peak data in each experiment e.g. ALEPH QCD = 0.292  0.007 GeV ( extent of parton branching) Q0 = 1.57  0.14 GeV ( terminate Parton Shower) Comparison of Fragmentation Models:  Comparison of Fragmentation Models Unnacceptably large mass differences motivated a lot of study HERWIG tuned at Z-peak taking into account flavour dependence. HERWIG6.2 HERWIG 5.9 ALEPH 189 GeV Eur. Phys. J. C 17(2000) 241-261 ALEPH  208 GeV ALEPH 2001-020 CONF 2001-017 March 2001 Resolved the problem with hard gluon emission Slide8:  MC/Data Comparison to Estimate DmW (ALEPH) Choose observables sensitive to fragmentation Calculate Data/MC correction for MC events in bins of the chosen variable. Back propagate to the reweighting p.d.f. and fit the data again  DmW Repeat for many variables From 189 GeV studies: DmW(4q)  10 MeV DmW(non-4q)  15 MeV Mass of smallest mass jet in lnqq Analysis Details:  Analysis Details 1 Million KinAgain JETSET / HERWIG6.2 / ARIADNE events at ECM = 188.6 GeV Nominal lqq analysis: 1. Combined eqq and qq selections. 2. For two fragmentation models being compared, common event must be selected in both. In JETSET-HERWIG comparison, 233259 common events selected. 4q channel analysis details to be inserted here. Take identical 4-vectors of primary 4-fermions from WW and apply different models (JETSET/HERWIG/ARIADNE) to each event Reduction of statistical uncertainty on DmW compared to producing independent 4-fermion samples Kinematical Differences of Fragmentation Models at KINGAL Level:  Kinematical Differences of Fragmentation Models at KINGAL Level EDiJet unchanged These quantities pDiJet unchanged unchanged by mDiJet unchanged KinAgain construction <qq> = 2.3010  0.0005 rad unchanged <DiJet>JETSET = 2.2290  0.0006 rad <DiJet>HERWIG = 2.2269  0.0006 rad JETSET jets have a wider opening angle than HERWIG jets (by 2.1  0.3 mrad) The qq opening angle is wider than the Jetset DiJet opening angle (by 71.9  0.2 mrad) <mJet>JETSET = 14.206  0.010 GeV <mJet>HERWIG = 14.230  0.011 GeV JETSET jets have smaller masses than HERWIG jets (by 23.5  14.8 MeV) Bigger jet masses  smaller opening angle mW remains unchanged JETSET-HERWIG biases in these observables at Reco. level are purely due to detector  reconstruction Definition of the Double Difference in a KinAgain Event:  Definition of the Double Difference in a KinAgain Event More care is needed to assess Jetset-Herwig differences at Reco. Level that have come about due to a corresponding change at KINGAL level. For such variables, V, can construct {(VRECO)J - (VRECO)H} - {(VKINGAL)J - (VKINGAL)H} The “Double Difference”. The “Double Difference” can be calculated on an event-by-event basis for KinAgain events. All correlations are implicitly accounted for in the errors on the means. If the “Double Difference” is significantly non-zero, then there is a residual bias of interest. Maximal use of the KinAgain philosophy; event-by-event comparison has more information than sample-by-sample comparison. Previous Results with the Double Difference :  Previous Results with the Double Difference First four quantities are identical at Kingal level. The remaining quantities are a double difference of (Reco. - Kingal). Comments on the Double Difference Results :  Comments on the Double Difference Results There are many very significant discrepancies in JETSET-HERWIG. This event-by-event approach at the pre-kinematic fit level provides powerful leverage to find the sources of the discrepancies that cannot be obtained at a later analysis level (post-kinematic fit, mass parameter fitting). If argument applied to JETSET-ARIADNE; Conclude that Ariadne is identical to JETSET and there is no leverage to find any source in the JETSET-ARIADNE framework. (Modulo multiplicities, but mass reconstruction not affected). Slide14:  Fragmentation Models HERWIG JETSET ARIADNE Parton Shower (PS) Based on LLA Parton branching probability from DGLAP Decreasing evolution of parton virtual mass Decreasing evolution of parton angular order Colour Dipoles Successive gluon emission increases # of independent dipoles. Decreasing pt in dipole emission Hadronisation (Partons  Hadrons) Cluster Model End of PS, all gqq. Colour neutral clusters from qq pairs. HeavyLight clusters Clusters  2 hadrons Lund String Model Break string to create virtual qq pair in a flux tube Diquark-antiquark pairs  Baryon production All models tuned at the Z peak Numbers for the Following Calculation :  Numbers for the Following Calculation JETSET-HERWIG: <EDiJet>J - H = - 166  25 MeV (-6.7 ) <pDiJet>J - H = - 81  18 MeV (-4.6 ) JETSET only: <EDiJet>J = 90.917  0.021 GeV <pDiJet>J = 45.444  0.019 GeV <mDiJet>J = 78.233  0.020 GeV Ebeam = 188.6/2 = 94.3 GeV The Impact of Rescaling the Raw DiJet System :  The Impact of Rescaling the Raw DiJet System Raw DiJet Mass mJ2 = EJ2 - pJ2 m = (1/mJ).(EJ E - pJ p) EJ E = -15.092 GeV2 pJ p = -3.681 GeV2 (EJ E - pJ p) = -11.411 GeV2 m = (-11.411) / (78.233) = -146 MeV (c.f. -142  22 MeV from direct observation). Rescaled Raw DiJet Mass mR2 = (EJ2 - pJ2).(ELEP / EJ) mR = (1/mJ).(pJ ELEP / EJ2).(pJ E - EJ p) pJ E = -7.544 GeV2 EJ p = -7.364 GeV2 pJ ELEP / EJ2  0.518 (pJ E - EJ p) = -0.180 GeV2 The Impact of Rescaling the Raw DiJet System :  The Impact of Rescaling the Raw DiJet System Explains why mResc results in this presentation are not significant, even when other discrepancies are huge. Choice of mass estimator can suppress differences due to fragmentation. (statement could be generalised to all variables used in reweighting fit). Do different kinematic fits re-organise the biases differently? If so, is one kinematic fit less sensitive to fragmentation than another? JETSET Particle Production Rates at KINGAL Level :  JETSET Particle Production Rates at KINGAL Level Standard Particle Number (SPN) integer identifies particle. Many ’s (1), ’s (8,9) produced. The particles exclude anything associated with the lepton. Particle Production Rate Differences at Kingal Level:  Particle Production Rate Differences at Kingal Level Most discrepant rates in JETSET-HERWIG6.2 are the baryons (neutrons followed by protons). JETSET-ARIADNE particle rate discrepancies are considerably smaller than those between JETSET-HERWIG6.2 Neutron Rate Differences:  Neutron Rate Differences This plot was shown at the Sesimbra LEP-wide meeting Strong dependence of mass from kinematic fit versus number of neutrons (identified at truth level of particle generation) and ( neutron)JETSET-HERWIG = 0.271  0.003 This is consistent with the LEP-wide mass-fit studies that tended to show negative JETSET-HERWIG trends Reconstructed Raw DiJet Mass :  Reconstructed Raw DiJet Mass Horizontal axis: n is J-H neutron difference (“neutron” = n + n#) Vertical axis: m is J-H raw DiJet mass For whole sample: <m> = -142  22 MeV When n=0 is imposed: <m> = +106  36 MeV Indicates other sources exist. G = Gradient = -1.042  0.014 GeV/n <m>n=0 + ( <n>  G) = 106 + (0.2706  -1042) = 106 - 282 MeV = -176  36 MeV Internally consistent. Neutron rate differences in Jetset-Herwig6.2 cause large systematic differences in the raw DiJet mass measurement. Reconstructed DiJet Energy and Momentum :  Reconstructed DiJet Energy and Momentum Same vertical scale in both plots. DiJet energy discrepancy is bigger than DiJet momentum discrepancy as a function of n (as demonstrated with the means of those quantities) Interjet Angle and Jet Mass at KINGAL Level :  Interjet Angle and Jet Mass at KINGAL Level The neutron rates very much drive the kinematics of Jet Mass and Interjet Angle. Larger jet masses  smaller opening angle. Interjet Angle and Jet Mass Double Difference :  Interjet Angle and Jet Mass Double Difference Those interjet angle differences are followed at Reco. Level, to within  2 mrad as a function of n. Jet Mass is not reconstructed well for non-zero n. Comments on the Neutron Rate Difference :  Comments on the Neutron Rate Difference Observations are consistent with: 1. For JETSET alone, the 2C mass from the Kinematic Fit depends upon the number of neutrons 2. n for JETSET-ARIADNE is considerably smaller than that for JETSET-HERWIG6.2, and there is no observable significant difference between JETSET and ARIADNE. The neutron production rate difference is one dramatic source of reconstruction difference between JETSET and HERWIG. Other sources must exist to explain the residual offset of <m> = +106  36 MeV at n = 0. Natural now to examine the other particles... Constraints on Particle Rate Differences (1) :  Constraints on Particle Rate Differences (1) The quantities change so much from when the n = 0 constraint is not applied. This implies that the n discrepancy between J and H is a source of serious bias. Significant discrepancies still exist, so there must be other sources. Constraints on Particle Rate Differences (2) :  Constraints on Particle Rate Differences (2) All “high level” observables agree perfectly. The charged multiplicity and the neutral energy stand out as being discrepant…not too worrying since the important “high level” observables agree, but irritating non-the-less. This implies that the p discrepancy between J and H plays an important part. Constraints on Particle Rate Differences (3) :  Constraints on Particle Rate Differences (3) All observables agree perfectly. The K0L discrepancy seems to play a role in the description of the charged and EM sectors. JETSET and HERWIG6.2 can be brought into agreement by imposing three constraints: n = 0 p = 0 K0L = 0 This suggests that their differences without these constraints are the sources of the biases observed. Conclusions (1) :  Conclusions (1) 1. Double Difference on an event-by-event basis provides excellent sensitivity to understand biases in the KinAgain framework. 2. Interjet angles at the Reco. level change in accordance with how kinematics has changed them at KINGAL level. 3. Raw DiJet mass, energy, momentum and jet masses contain the biases of interest. 4. ARIADNE looks like JETSET. Exit ARIADNE. 5. Raw vs. rescaling m understood. One kinematic fit might be less sensitive to fragmentation than another. 6. Neutron and proton rate discrepancies resolve the JETSET-HERWIG difference. The K0L rate difference can describe effects in the charged and EM sectors. 7. Gluon radiation and particle rates are obviously connected. 8. Suggest consistency between baryon rates, JETSET-HERWIG trends and sign of slopes in QCD studies. Conclusions (2) :  Conclusions (2) We have new insights into what can be a source of error on mW due to fragmentation… Approaching the questions with a possible source in mind should help to answer some of our long-standing questions.

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