A critical review towards more accurate Monte Carlo ...cern.ch/adavid/pv-Bormio05.pdf · XLIII...

XLIII Winter Meeting on Nuclear Physics - Bormio, March 15, 2005 André David - CERN

Charm hadronization fractions

A critical review towards more accurate Monte Carlo simulations

XLIII Winter Meeting on Nuclear Physics - Bormio, March 15, 2005 André David - CERN 2

Why charm hadronization fractions?It is not easy to compare "charm production" results from different (heavy-ion) measurements / experiments∘ Some only measure D0→K+π- (easier than D+→K+π+π-)∘ Others measure "displaced vertices", more sensitive to D+ than to D0

◦ cτ(D+) ~ 3 cτ(D0)∘ Others measure "high-pt single electrons", more sensitive to D+ than to D0

◦ Br(D+→e+X) ~ 2.5 Br(D0 →e+X)

Yet, all publish "ccbar production cross-sections"

This requires knowing the relative production fractions of D+ and D0

Fixed when the system hadronizes!

A wrong understanding of these fractions leads to apparent discrepancies between values which could in reality be in very good agreement

The relative D0 and D+ fractions crucially depend on a single parameter, PV.

This talk reviews our knowledge about PV and points out the existence of a problem in most of the calculations being done in our field


Charm production in heavy-ion collisionsCharm is a hard process∘ Calculable with pQCD∘ Scales with number of binary nucleon-nucleon collisions, so…

It is abundantly produced in heavy-ion collisions∘ ~1 mb for Pb-Pb at SPS

◦ with negligible beauty feed-down∘ ~20 mb for Au-Au at RHIC∘ ~80 mb for Pb-Pb at LHC

Finally measurable in heavy-ion collisions(thanks to new silicon detectors)∘ Displaced vertices from NA60 in Indium-Indium at 17 GeV∘ Future RHIC experiments upgrades∘ Future ALICE measurements in Pb-Pb at 5.5 TeV

How do we get our baseline expectations?


We ask Pythia!Pythia is the de facto standard in Monte Carlo simulation of hard processes∘ Simulates pp, πp, ppbar, e+e-, etc∘ Nuclear effects on the PDFs are easily

included∘ Leading order calculations

How does it perform?

A few tunable handles:∘ mc, Q2, PDFs

sqrt(s) evolution properly described!∘ EPJ C1(1998)123

K-factors account for leading order approximation

What about individual cross-sections?


Individual D cross-sections from PythiaD++D0 ~ 80% of all charm∘ Sum over anti-particles is

implied throughout this talk (D+=D++D-, etc)

K-factors for charged and neutral D mesons differ by a factor 2∘ JPG 30(2004)S315

Where can this problem come from?

K-factor D0 ~ ½ K-factor D+ ?

πp

pp

D0

D0

D+

D+


Understanding the D meson hierarchyThe most abundantly produced particles with charm∘ ~80% of all charm

D*0 cannot decay into D+

∘ Because mD*0 - mD+ < mπ-

∘ This asymmetry is crucial!

2++

1+±

1--

0-+

2460

2420

2006

1864

1(P)

0(S)

cubar cdbarJPC M(GeV)L

D2*0 D2

*+

D10 D1

+

D*0 D*+

D0 D+

Beauty feed-down not shown


L=0 states: D and D*

Most abundantly produced states ∘ D* vector triplet

◦ Jz = -1, 0, +1∘ D pseudoscalar singlet

◦ Jz = 0

How to populate these L=0 states?Naïve ansatz∘ Assume that all states are equally probable∘ “Naïve spin counting”∘ Correct if states are degenerate

PV – probability of creating a vector meson∘ PV = V / (V+P)

For “naïve spin counting”∘ PV = triplet / (triplet+singlet) = 3 / (3+1) = 0.75

triplet

singlet

D*0 D*+

D0 D+

PV

1-PV


Consequences of PV in charm hadronizationNeglecting L=1 (D**) feed-down∘ ~5% effect (LEP, E691)

Assuming isospin symmetry (u = d)∘ Experimentally verified (LEP, CDF)

Direct yields are proportional to∘ D*

direct ~ PV

∘ Ddirect ~ (1-PV)

Because of the D* decay asymmetry, the measured yields are∘ D*0 = D*+ ~ PV

∘ D+ ~ (1-PV) + PV (1-B*)∘ D0 ~ (1-PV) + PV (1+B*)

D*0 D*+

D0 D+

B*=0.677 ± 0.005

PV

1-PV


Experimental handles on PVUsually measured from “easy” fully-charged decays∘ D0 → K-π+

∘ D+ → K-π+ π+

∘ D*+ → D0 (K-π+) π+

∘ (D*0 hardly used because of neutral decay modes)◦ D*0 → D0 π0 and D0 γ

Particle ratios provide information on PV

For instance:∘ D+/D0 = R1 PVB* = (1-R1) / (1+R1)∘ (D0 from D*+) / D0 = R2 PVB* = R2 / (1-R2)∘ D*+/D0 = R3 PV = R3 / (1 - B* R3)


Pythia vs. pp and πp data on D+/D0

From Pythia∘ D+/D0 ~ 0.32, compatible with PV = 3/4

From pp and πp data∘ D+/D0 ~ 0.4-0.6, indicates PV < 0.75

What about measurements of PV?

JPG 30(2004)S315

0.60±0.090.40±0.03

0.32

πppp


First PV measurementsCLEO ∘ PR D37(1988)1719∘ Combined measurement from D*+, D+ and D0

∘ PV = 0.85 ± 0.11 ± 0.17∘ Large errors forced an alternative way of extracting PV

D*+ polarization analysis∘ Independent of branching fractions∘ Values on the 3/4 target

90’s conclusion:“Naïve spin counting works!”


Problems in determining PV from D*+ polarizationAll such analysis assume a statistical model∘ from PR D19(1979)2806

CLEO II abandoned the method∘ Because “the validity of the statistical model was assumed when

deriving” PVCLEOII PR D58(1998)052003

OPAL did not even extract a PV this way

Eventually PV cannot be derived from D*+ polarization informationPL B427(1998)356

But the CLEO D*+/D0 measurement (0.85±0.20) was on the 0.75 ballpark, right?


The CLEO PV measurement revisitedPV = 0.85 ± 0.11 ± 0.17CLEO quotes the partial result∘ PVB* = 0.44 ± 0.04 ± 0.05

So, is the PDG B* still the same as in 1988 ? No !∘ Although now stable for 10 years∘ It was 30% lower in 1988 !

As B* goes up, PV goes downBy 2005 standards, CLEO measured PV = 0.65 ± 0.09


Checkpoint: where are we?D+/D0 data from pp and πp would be better described by a lower PV∘ Pythia points at 0.75∘ While averages of pp (πp) data point to ~0.4 (0.6)

PV results from D*+→D0π+ published from 1986 to 1994 are 25–40% too high∘ But we can correct for the B* evolution∘ CLEO PV in 2005 is 0.65±0.09, not the original 0.85±0.11±0.17

D*+ polarization results are wrong∘ Some authors acknowledged that and stopped using it (CLEO-II)∘ Others did not even try (OPAL)

D*+ polarization “measurements” were the main source of support for the “naïve spin counting” picture

So, is there more experimental evidence that PV < 0.75?∘ “Doctor, can I have a second opinion?”∘ “Sure! Even 63 of them!


PV measurements: Data sources and procedureMeasurement types∘ D+/D0

∘ D*+/D0

∘ D*+/D+

∘ D*0/D0

Production mechanisms∘ pp, ppbar, pA, πA, KA and AA∘ γp and DIS∘ e+e- annihilation∘ e+e-→Z0 hadronic decays∘ neutrinos

Collision energies∘ From below threshold up to TeV

Method∘ Correct cross-section data for

changes in branching ratios∘ Form possible ratios∘ Extract PV values∘ All errors are added in quadrature

Collected data from∘ Hadroproduction (24 values)

◦ NA32, NA27, WA92, E769, HERA-B, CDF and STAR

◦ From 10s of GeV to 2 TeV∘ Photoproduction (8 values)

◦ E691, NA14/2 and ZEUS◦ 15 GeV to 200 GeV

∘ Z0 hadronic decays (4 values)◦ DELPHI, OPAL, ALEPH and SLD at 91.2

GeV∘ Electroproduction at 10.5 GeV (11 values)

◦ CLEO and ARGUS∘ Neutrinos (1 value)

◦ A combination of NOMAD and CHORUS measurements at ~40 GeV

∘ DIS (2 values)◦ H1 at 319 GeV

∘ Electroproduction below 7 GeV (13 values)◦ MARK-I, MARK-II and BES◦ From 3.7 to 7 GeV


PV measurements: sqrt(s) dependenceAbove the D*D* threshold

PV = 0.591 ± 0.007

D*D

* thr

esho

ld

CDF

ZEUS

H1

STAR

LEP

ARGUS/CLEO

Hadro

BES/MARK-I & IIBES/MARK-I & II


PV measurements: system dependence

PV = 0.591±0.007


So, can we change PV in Pythia?Yes! The different PV values are∘ PARJ(11) = 0.5

◦ for mesons consisting solely of u and d quarks∘ PARJ(12) = 0.6

◦ for mesons with s quarks∘ PARJ(13) = 0.75

◦ for mesons with c or b quarks

By default Pythia uses “naïve spin counting” for charmed and beauty mesons


JPG 30(2004)S315

0.60±0.090.40±0.03

0.32

Let’s try it!For instance∘ Set PARJ(13) to 0.591∘ Generate charm (and only charm)∘ Count the produced D+ and D0

∘ Extract D+/D0 = 0.44 !Much better agreement with the pp and πp data∘ where D+/D0 = 0.42 ± 0.03

πppp


What about beauty?Much less experimental data∘ σB*/(σB+σB*) from ALEPH,

DELPHI, OPAL and L3

DELPHI corrects forB** feed-down and finds∘ PV = 0.75 ± 0.10

Naïve spin counting seems to hold for B mesons !


Could this be driven by mass differences?Mass differences∘ mD* - mD ~ 141 MeV ( 7% mD )∘ mB* - mB ~ 46 MeV ( 0.9% mB )

For instance, Rapp & Shuryak use effective thermal weights∘ Heavier states are suppressed by

◦ (M/m)3/2 exp(-(M-m)/T), where T=170-175 MeV∘ For charmed mesons PV ~ 0.595

Statistical hadronization models also predict PV around 0.6∘ ZP C69(1996)485 (Becattini)

◦ Fits light and strange meson and baryon data ◦ Predicts D+/D0 = 0.397 (PV ~ 0.64)

∘ PL B571(2003)36 (Andronic et al.)◦ Takes Ndir

ccbar and dNch/dy as inputs◦ Predicts D+/D0 = 0.456 (PV ~ 0.56)


An important effect: individual yields changeAbsolute yields change∘ D+(PV=0.6) = 1.20

D+(PV=0.75) ∘ D0(PV=0.6) = 0.93

D0(PV=0.75) ∘ D*+(PV=0.6) = 0.80

D*+(PV=0.75)

Particle ratios change∘ D+/D0(PV=0.6) = 1.29

D+/D0(PV=0.75) ∘ D*/D0(PV=0.6) = 0.86

D*/D0(PV=0.75)

But the total cross-section stays the same


Can this have consequences on RHIC Au-Au charm?If individual D meson yields change, then lepton yields from D semileptonic decays also change∘ B(D+→e+X) = (17.2±1.9) %∘ B(D0→e+X) = (6.87±0.28) %

e+X←D(PV=0.6) = 1.13 e+X←D(PV=0.75)

Charm at RHIC in Au-Au relies exclusively on electron yield measurements!


Summary“Naïve spin counting” was always thought to rule the D meson state population∘ Expected from HQET (assumes charm is heavy enough)∘ And “confirmed” by early PV studies in the early 1990s

◦ The CLEO measurement (using a poorly measured branching ratio)◦ And the results from D*+ polarization (using a wrong analysis)

During the 1990s many data samples were accumulated with PV values smaller than 0.75Even so, “naïve spin counting” is the Pythia default in 2005

We showed that∘ PV is the same across a wide-range of energies away from threshold∘ PV is universal for different production mechanisms∘ PV is well below the “naïve spin counting” expectation of 0.75∘ We obtained an average PV of 0.591±0.007


ConclusionsThere are plenty of D meson hadroproduction data∘ D+/D0 = 0.42 ± 0.03∘ Pythia’s default PV (0.75) predicts D+/D0 ~ 0.32∘ Tuning Pytia with our PV average (0.591) yields D+/D0 ~ 0.44

Pythia can (and should) be tuned to reflect this∘ PARJ(13) controls both charm and beauty mesons

There are sizeable consequences for charm electron yields at RHIC∘ e+X←D(PV=0.6) = 1.13 e+X←D(PV=0.75)

A critical review towards more accurate Monte Carlo ...cern.ch/adavid/pv-Bormio05.pdf · XLIII...

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