thesis lpnhe

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Information about thesis lpnhe

Published on February 7, 2008

Author: Perrin


Charm quark fragmentation and Search for Popcorn Mesons in Events with a Lc and an anti-Lc: Charm quark fragmentation and Search for Popcorn Mesons in Events with a Lc and an anti-Lc Brandon Hartfiel 3 Nov-2005 Hadronization and Popcorn Phenomenology The BaBar Detector c quark  Lc Momentum Spectrum Data vs.Models Search for Popcorn This is not B physics: This is not B physics BB threshold 10 fb-1 200 fb-1 Most BaBar analyes study B decays coming from Y(4S) ~10% of BaBar data taken below BB threshold allows us to understand continuum background test non-perturbative QCD Meson Formation: Meson Formation a positron and electron collide at high energy e+ e- Meson Formation: Meson Formation creating a virtual photon g* Meson Formation: Meson Formation which decays into a quark-antiquark pair R R Meson Formation: Meson Formation As the quarks move apart, they radiate bremsstrahlung gluons. This process is calculable with pQCD for momentum transfers above about 1 GeV. a(2 GeV)=.3 a(1 GeV)=.5 a(.67 GeV)=1. R R Meson Formation: Meson Formation We model the soft gluons by a string of constant linear energy density (~1 GeV/fm). The hard gluons can be modeled by kinks in the string R R Meson Formation: Meson Formation New quark-antiquarks pairs can form from the energy in the string. We now have two independent mesons R R R R String energy going into the creation of the new quark anti-quark pair Lund Model Symmetric Fragmentation Function: Lund Model Symmetric Fragmentation Function First – particle type chosen. Suppression factors for heavy quarks, vector particles, bayrons etc. Then the particle momentum is calculated using the Lund symmetric fragmentation function. Massless quarks imply b and c are not causally connected. So any iterative procedure for choosing particles should be independent of order. This gives us the function for UCLA Area Law: UCLA Area Law Lattice QCD suggests that the probability of any particular event should be proportional to exp(-bA) where A is the total space time area swept out by the string. We divide the area into regions taken up by a particular hadron and a remainder area A’ which gives us a fragmentation function which can be used iteratively. chooses mh and z simultaneously Kartvelishvili Fragmentation Function: Kartvelishvili Fragmentation Function In deep inelastic scattering,the probability of the red quark having almost all of the proton momentum goes like f(x)  (1-x)2n-1 where n is the number of spectator quarks x→1 We expect the same behavior in the crossed diagram meson f(z) = za(1-z) baryon f(z) = za(1-z)3 The other fragmentation functions do not take into account the difference between mesons and baryons space time e- e+ e- e- Other Fragmentation Functions: Other Fragmentation Functions Peterson – transition amplitude q  q + hadron  1/DE Bowler – Lund with hyperbolic massive quark paths Collins and Spiller – Heavy quark mass expansion BCFY – Heavy quark mass long equation- see thesis expansion HERWIG - clusters instead of strings, decay by phase space. Baryon Formation (Diquarks): Baryon Formation (Diquarks) Sometimes a “wrong color” pair forms inside the string. Because two independent color singlets are not formed, there is still a flow of color from one side of the event to the other. The blue quark (transverse) masses come from borrowed energy, so this is a tunneling state, with a limited lifetime. R B B R Lifetime ~ .197 GeV/mB fermi Baryon Formation (Diquarks): Baryon Formation (Diquarks) The most probable kind of baryon formation involves a green-antigreen pair being formed as quickly as possible in order to minimize the blue quarks’ flightlengths R B G G B R Blue-Green Diquark Shortest possible anti-B flight length String energy going into BG quark-antiquark pair Baryon Formation (Popcorn): Baryon Formation (Popcorn) In order to make room for a meson between the two baryons, the blue quarks must live a long time. R B B R Long flight length means these types of events are less probable Baryon Formation (Popcorn): Baryon Formation (Popcorn) the probability suppression factor for this event, compared to baryon-antibaryon-meson is R B G G G G B R Popcorn Meson Numerical Examples: Numerical Examples One Pion mB ~ .100 GeV M = .140 GeV probability = .87 Two pions with one unit rapidity difference mB ~ .100 GeV probability = .67 M = .388 GeV assuming pt is ~ .1 GeV for quarks and mesons Three pions, each with one units rapidity difference mB ~ .100 GeV probability = .48 M = .722 GeV These probabilities do not include the reduced phase space for the rest of the event Rapidity Correlations at DELPHI: Rapidity Correlations at DELPHI L- L rapidity correlations in Z0 decays suggest that there are .5 popcorn mesons per baryon-antibaryon pair Analyses at Babar: Analyses at Babar Continuum Lc Momentum Spectrum Search for popcorn The BaBar Detector: The BaBar Detector 9 GeV e- 3.1 GeV e+ Asymmetry allows B meson flightlength/lifetime measurements Silicon Vertex Tracker 5 layers of 300mm silicon 60mm vertex resolution Drift Chamber 40 Stereo Layers 7.5% dE/dx resolution (.45 + .13 pt)% pt resolution The BaBar Detector: The BaBar Detector Detector of Internally Reflected Cherenkov Radiation (DIRC) 144 fused silica radiators 10,000 PMTs 2.5 mrad resolution Electromagnetic Calorimeter 6580 CsI crystals energy resolution (1.85+2.32/ )% Instrumented Flux Return 19 layers of resistive plate counters DIRC: DIRC DIRC complements particle identification of the drift chamber: DIRC complements particle identification of the drift chamber dE/dX Cherenkov angle momentum GeV There is no cherenkov radiation from kaons below .5 GeV or protons below .9 GeV, but dE/dx from the drift chamber provides good information for low momenta. The dE/dx curves begin to cross at about 1 GeV, but the cherenkov angle separation is excellent in this range pions kaons protons momentum GeV PID Efficiencies: PID Efficiencies dotted lines are Cherenkov thresholds for k and p efficiency/missID tradeoff in the transition region missID above diagonal less important because of lower multiplicities LcpKp Acceptance and Efficiency: LcpKp Acceptance and Efficiency many particles boosted out of the front end. black line shows acceptance -.8 < cos qcm < .3 and kinematic limit pcm < 4.75 GeV/c Continuum B-decay acceptance chosen to eliminate low and fast varying efficiency regions Continuum Spectrum: Continuum Spectrum Efficiency corrections done in lab frame, then each particle is boosted to CM Nineteen .25 GeV/c center of mass momentum bins Variable invariant mass bin sizes. One or two Gauss, linear or parabolic background. Consistency Checks: Consistency Checks Invariant mass plots fitted five different ways. Largest deviation taken as systematic Data divided into 6 center of mass angular regions which correspond to detector regions with different efficiencies and systematic errors Total c2/DoF = 63.8/65 Continuum Spectrum Results: Continuum Spectrum Results smooth curve with expected shape correlated efficiency errors 3 largest errors statistical constant errors not shown tracking 2.5% luminosity 1% MC Dalitz structure .9% Comparing to MC and CLEO: Comparing to MC and CLEO JETSET MC with BaBar tuning MC spectrum harder doesn’t match peak 9.46 fb-1 vs. 101 pb-1 Model Comparisons: Model Comparisons BCFY, Collins and Spiller, and Kart. fail to reproduce the peak. Modified Kart. performs much better. Can the other functions be modified for baryons? Majority of Herwig error in the highest bins. Two parameter models are generally better. Physics or curve fitting? 131/18 35/17 124/18 106/18 29/18 29/17 50/18 338/19 43/17 c2/DoF Potential Source of Discrepancy - Decay of Heavy Charmed Baryons: Potential Source of Discrepancy -Decay of Heavy Charmed Baryons Heavier baryons have a lower pmax and decay into Lc softens the spectrum event more Undiscovered charmed baryons will soften the Lc spectrum Search for Popcorn in Events with a Lc and a Lc-bar : Search for Popcorn in Eventswith a Lc and a Lc-bar Slide 33:  Rapidity -2 0 2 4 Lc L L DELPHI High multiplicity + string rapidity ordering not preserved => popcorn hard to identify BaBar c quarks must be at the ends of the strings => if there are no other baryons in the event then all mesons must be either popcorn or the decay products of heavy charmed baryons popcorn decay was this pion produced between the Lambdas ?? Event Selection: Event Selection Lc and Lc with pcm > 2.3 Global likelihood PID 5 SVT 15 DCH hits Neutrals Flightlength > 2.5mm Cos q > .97 Vertex prob. > .001 % Addition of last 3 modes gives only 4% reduction in statistical error Only use first two modes Lc+ Sp+ Lg combine Lp+ Xc  X- p+ X-  L p- combine Lp+ Signal Region: Signal Region Keep events within 12 MeV radius of Lc mass Subtract single and double fake rates Event Variables: Event Variables Lc produced back to back – excludes very heavy resonances which would decay isotropically missing mass < 2 limits events with two neutrons very few events with two Lc and nothing else. Suggests exclusive baryon-antibaryon production is small Black – sideband subtracted signal Red - sideband Other Tracks: Other Tracks require 5 SVT and 10 DCH hits. DOCAxy< 5mm very few protons implies long range baryon number compensation. very few kaons lots of pions – need to estimate how many are from heavier charmed baryon decay Ks w r: Ks w r maybe something No Ks signal a large number of the charged kaons were probably missID Background from Decay: Background from Decay Xc (usc,dsc) and Wc (ssc) decay to strange baryons because of larger available phase space and Cabbibo suppression. Six particles observed to decay to Lc: Six particles observed to decay to Lc Red – Signal Black – single Lc sidebands weighted to match Lc p and q distributions Heavier Lc States: Heavier Lc States because it’s close to Sc(2455) + p threshold, the theoretical Lc(2593) lineshape looks funny. We assign a 25% error. We double the fit errors on the Lc(2765) and Lc(2880) Sc: Sc we subtract the Lc(2625) reflection the Lc(2593) lineshape is wrong so the reflection ends up in the wrong place. We leave it out and double the Sc(2455) error. Total Decay Pions: particle observed per Lc obs. p per baryon total Sc(2455) .0815 +/- .0037 .98 +/- .04 .0799 +/- .0049 Sc(2520) .0571 +/- .0026 1.14 +/- .08 .0650 +/- .0054 Lc(2593) .0146 +/- .0037 2.47 +/- .10 .0362 +/- .0091 Lc(2625) .0355 +/- .0011 2.66 +/- .24 .0946 +/- .0058 Lc(2765) .0346 +/- .0043 2.82 +/- .21 .0977 +/- .0142 Lc(2880) .0069 +/- .0015 2.76 +/- .14 .0185 +/- .0042 double count correction -.0130 +/- .0027 for Lc(2593) -> Sc(2455) .379 +/- .028 vs. 2.7 total p per event Total Decay Pions includes pions from baryons that are not reconstructed Results: Results Measured Pions 2.651 +/- .078 Pions from Decay -.758 +/- .066 4 Baryon Events .016 +/- .016 Double Counted Tracks -.011 +/- .000 Kaons .061 +/- .007 Subtotal 1.959 +/- .104 / Tracking (71.7 +/- 3.8)% 2.732 +/- .201 p  m -.052 +/- .026 g e -.055 +/- .028 Total 2.625 +/- .205 What about charmed Baryons that haven’t been discovered yet? : What about charmed Baryons that haven’t been discovered yet? Lc charmed baryon rest frame Daughter pions from strong decays should be front/back symmetric. boost Lc Lc + popcorn rest frame boost Popcorn mesons appear in the direction of the center of the event Angle Between (Lc + p) and p Momenta: Angle Between (Lc + p) and p Momenta if charmed baryons this heavy exist, their decay pions are distributed like popcorn from efficiency loss Where does the asymmetry come from?: Where does the asymmetry come from? If the pions decay in front, the Lc is less likely to pass the 2.3 GeV/c momentum cut. So we cannot use the decay angle to exclude the possibility of very heavy charmed baryons ….. Lc (3300) Lc p p p Lc(3300) rest frame However, the pions from heavy baryons would sometimes decay transversely: However, the pions from heavy baryons would sometimes decay transversely and this would create a large angle between the Lc and Lc-bar which is not seen in the data Lc (3300) Lc p p p Lc(3300) rest frame Angle between Lc daughters in exclusive baryon-antibaryon events: Angle between Lc daughters in exclusive baryon-antibaryon events very heavy charmed baryons can be excluded because of the high transverse momentum of the daughter particles. Arguments in favor of a large amount of popcorn: Arguments in favor of a large amount of popcorn We see 2.6 additional charged pions per event which are not coming from decays of previously observed charmed baryons. We do not observe any new charmed baryons. In order to produce the front/back decay angle asymmetry we observe, the undiscovered states would have to be heavy, and produce a small number of pions as they cascade down to the Lc(2285) state. But heavy charmed baryons can be ruled out as they would produce Lc’s with a higher transverse momentum than is seen in the data if the production rates of the undiscovered baryons are the same as the average rate of the discovered baryons, we would need 16 new resonances since we expect the rates to go down with increased mass, we actually need many more than 16. Summary: Summary BaBar continuum Lc momentum spectrum is a great improvement over previous measurements – allows us to rule out some of the fragmentation models We find 2.6 charged “popcorn mesons” per Lc + anti-Lc event (plus 1-2 neutral mesons) compared to ~.5 in the current tuning of JETSET.

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