Alex Smith

1
Recent Advances in Charm Physics
Alex Smith University of Minnesota Physics in
Collisions June 20-22, 2002

• Why charm physics?
• Searches for new physics using D meson decays
• Mixing
• CP violation
• Measurements which provide input to QCD
• Decay processes
• Charm semileptonic decays
• Lifetimes
• Production mechanisms
• What to expect from future experiments
• Conclusions

2
Why Charm?
Charm contributes to a variety of important
topics in HEP

• Searches for new physics
• D mixing
• CP violation
• Measurements which guide QCD
• Necessary in order to extract standard model
  parameters
• Form factors and decay constants - B decay CKM
  elements
• Final state interactions, resonances in
  multi-body decays
• Lifetimes, masses, branching fractions
• Charmonium production mechanisms
• Spectroscopy of light mesons/glueball candidates
• Dalitz plot fits of D meson decays (see talk by
  Brian Meadows)
• J/y radiative decays (see talk by Shen Xiaoyan)

3
D0-D0bar Mixing
x mixing Channel for new physics
x
Two types of mixing
y (long-range) mixing SM background
Standard model prediction
y

• All mixing contributions doubly Cabibbo
  suppressed
• - Factor of tan4qc in rate
• Further GIM suppression of x possible

New physics will enhance x but not y CP
violation in mixing would be a smoking gun for
new physics
although long-distance contributions could
increase these
4
D0-D0bar Mixing
Look for mixing in wrong signed (WS) decays of
D0
Notation right-signed (RS)
Cabibbo-favored decays wrong-signed (WS)
Mixing or doubly Cabibbo-suppressed decays
Mixing is not the only way to get to wrong sign
hadronic states
Need to fit proper decay time in order to
distinguish mixing (both x and y) from doubly
Cabibbo-suppressed (DCS) decays
Complication phase difference, dKp, between CF
and DCS amplitudes can lead to observable
quantities x and y, related to x and y by a
rotation
5
Current Status of D0-D0 Mixing

• Current measurements cutting into range of some
  non-SM predictions
• Much room for improvement before we hit SM
  background

y
x
Typical upper SM predictions
Typical non-SM predictions (many higher and
lower, however)
6
X, y, and RD from D0-Kpi-

• Currently, best constraints come from this mode
  if assumptions about strong phase are made
• Unknown strong phase difference weakens these
  limits
• CLEO measurement remains the strongest
  constraint on x

7
X, y, and RD from D0-Kpi-
y
CLEO limit (still best constraint on x)
FOCUS x-y limit
x
8
RWS from D0-Kpi-

• Belle and BaBar have new WS rate measurements

9
Measurements of the Wrong Sign Rate

• Belle and BaBar
• Significant improvements in RWS
• x and y proper time fits soon!
• Information in multi-body modes not yet fully
  exploited
• x, y, CP violation

RWS in multi-body hadronic modes

• Situation more complicated
• Dalitz plot fits of RS and WS required to get
  limits on x, y, CPV
• Need lots of statistics

RWS in D0-Kp-
10
D0-K0pi0
First measurement!

• Measurement of ratio of D0 rates into K0Lp0 and
  K0Sp0 can be used to disentangle the CF and DCS
  amplitudes
• K0L content of K0 and K0 is equal
• K0L content of K0 and K0 is opposite in sign to
  K0S
• Get DCS rate from interference between the two

Very important measurement! Uncertainty still too
large to limit dKp, but more data on the way
11
D0-K0spipi Overview

• Measure x and y rather than x2 and y
• RS and WS occupy the SAME Dalitz plot
• Simultaneous measurement of relative strong phase
  between CF and DCS
• Only mode with sensitivity to sign of x!
• Doubly-Cabibbo-suppressed modes
• y sensitivity comparable to CP eigenstate (eg.,
  D0-KK-) analyses
• Better scaling of sensitivity to x with int.
  luminosity than D0-Kp- analysis
• Complicated Dalitz plot and proper time fit
  required

y
x
12
D0-K0spipi CLEO

• Time-independent Dalitz fit so far
• Rich resonance structure
• r, K-,
• Interference effects
• Fit results shown in projections

13
D0-K0spipi CLEO
Final fit with intermediate states
(Breit-Wigner resonance line shape)
Phase convention
Wrong sign
Right sign
First measurement of strong phase difference
between CF and DCS!
14
D0-K0spipi Babar
15,753 events!

• CLEO will have RD, x, y, and ACP measurements
  very soon
• BaBar analysis is in progress
• Difficult analysis
• Many systematic uncertainties will scale with
  statistics

15
D0-KK- and D0-pipi-
y can be determined by measuring the lifetime
difference between D0 decays to CP-even and
CP-odd final states
Experimentally, it is easier to measure the
lifetime difference of a CP-even decay relative
to the non-CP final state D0-K-p (assumes no CP
violation)
Many systematic errors will cancel in the ratio
These were some of the first D mixing results to
come out of the B factories
16
D0-KK- and D0-pipi-

• Technique, resolution, and systematics are quite
  different at fixed target experiments (FOCUS,
  E791) and ee- (Belle, BaBar, CLEO)

17
D0-KK- and D0-pipi-

• FOCUS measurement is high relative to both CLEO
  and FOCUS D0-Kp- limits
• Unknown strong phase difference
• New BaBar and Belle measurements pull yCP back
  towards D0-Kp- limits

y
yCP from D0-KK-/pp-
x
18
Measurements of the Mixing Rate Using Wrong Sign
Semileptonic D0 Decays

• Sensitive to mixing only (no DCS decays)
• Will need separate measurement of y if that turns
  out to be larger or comparable to x
• Measurements from
• E791 (D0-Kln)
• CLEO (D0-Kln)
• Sensitivity estimate from FOCUS
• B factories should have results soon
• Accessible to future experiments
• Hadron machines
• Lepton helps triggering
• CLEO-c
• Opposite side tag

y
x
19
Three Types of CP Violation
2
Decay (AD) Af ? Af
D
?
f
2
2
D0 D0
D0 D0
?
Mixing (AM)
f
f
2
2
D0
D0
f
f
Interference between mixing and decay (f)
?


D0 D0
D0 D0
f
f
20
Searches for CP Violation

• Ingredients for observing non-standard model
  physics through CPV in D decays
• Decay amplitude with contributions from at least
  two diagrams with different weak phases
• Non-negligible strong phase shift
• Likely to be non-zero in charm decays, since
  SU(3) flavor symmetry is badly violated
• SM predictions
• O(10-3) or below in SCS modes
• Due to interference of tree and penguin
  amplitudes
• No SM CPV in DCS and CF modes
• Any observation is new physics
• Non-SM predictions
• Up to O(10-2)

21
Charm Semileptonic Decay Rates and Form Factors

• Measurements of charm semileptonic branching
  fractions and form factors can be used to improve
  estimates of corresponding quantities in the B
  sector
• Leads to improved estimates of Vub and Vcb
• Several experiments are working on other
  semileptonic modes
• D0-pln , rln , Kln
• Ds- fln

(FOCUS also sees first evidence for an S-wave
component)
New FOCUS result is a dramatic improvement (tiny
backgrounds)!
22
Motivation to Measure Charmed Particle Lifetimes

• Non-perturbative QCD effects are important in
  weak decays of charmed particles

External Spectator
Internal Spectator
W Annihilation
W Exchange
Helicity and Wavefunction Suppressed
Color-suppressed

• Which processes are important in charmed meson
  and baryon decays?

Challenge for theory is to reproduce the observed
lifetime hierarchy in charmed baryons and mesons
23
D Meson Lifetimes
Large observed ratio is understood to be due
to destructive interference in diagrams
contributing only to D decays
New precise measurements of t(D0) and t(D) from
FOCUS
24
Charmed Baryon Lifetimes

• Unlike charmed mesons, decays of charmed baryons
  are not color or helicity suppressed
• W-exchange diagrams may be important

25
Charmed Particle Lifetimes

• Theory now describes most of the observed
  lifetime hierarchy
• Still some notable discrepancies with theory,
  however
• Further measurements will help guide theory
• New and more precise lifetimes
• Further analyses of charmed hadron decays (like
  Lc)
• Tuning with data will yield better theoretical
  tools

26
Prompt charmonium production I

• Tevatron Run 1A CDF and D0 observe O(10-100)
  surplus in charmonium production cross section
  above NRQCD predictions
• Something is missing in the model. Color-octet?
  Gluon splitting?
• Can test NRQCD using ee- collisions at lower
  energies

Dominant
Dominant at py endpoint
O(10)
Small
27
Prompt charmonium Production
PRL 88, 052001, (2002)

• Some NRQCD calculations predict a large yield in
  the endpoint region due to color-octet ee-?J/yg
• This was not observed!
• By comparing on/off resonance
• Cross sections for 2.0 GeV/c

ee-?J/yg not observed in endpoint region
28
Prompt_charmonium Production
Clear threshold At 2mc
J/y
Side band
29
Prompt Charmonium Production
ee-?J/yDX
ee-?J/yD0X

• Use D-D0p, D0-Kp, KK, Kppp, Ks0pp, Kpp0

• Use D0-Kp, KK modes

30
Prompt Charmonium Production

• Use JETSET rates to convert s(ee-?J/yD()X) to
  s(ee-?J/ycc)
• Compare with measured s(ee-?J/yX)
• Recall that prediction was only O(0.1)!

31
The Near Future in Charm Physics

• D0 mixing
• New measurements of x and y from Belle and
  BaBar using D0-Kp-
• dKp from Belle by measuring different D-Kp
  isospin states
• Can get x and y from D0-Kp-
• Time-dependent CP asymmetry measurements in
  D0-KK-, pp-
• Dalitz analyses
• D0-Ks0pp-
• Best sensitivity to x with B factory samples
  (including its sign)
• RS and WS interfere since they have the same
  final state
• CLEO will have mixing/CPV limits soon
• Babar is working on this mode with 3X the CLEO
  statistics
• QCD input from charm
• Necessary ingredient to improve measurements of
  standard model parameters
• Semileptonic branching ratios and form factors
  (several new FOCUS results imminent)
• fD and fDs measurements (Belle, BaBar)
• More lifetime measurements and spectroscopy at B
  factories
• Further understanding of charmonium production
  puzzle

32
The Near Future in Charm Physics D Mixing

• Does not include B factory results, for which
  sensitivity estimates have not been shown
• D0-K0spp-
• D0-Kp-
• D0-Kl-n
• Can expect great improvement when these
  measurements are added

y
x
33
Future Experiments in Charm Physics

• ee- machines
• Belle, BaBar--- running
• CLEO-c 2003 L(1-4)x1032/cm2s
• BESIII 2005-6 L1033/cm2s
• Clean environment
• Easy triggering
• Lower cross section than in hadronic collisions
• Hadron machines
• CDF, D0, BTeV , LHCb, Compass, Hera-b
• Difficult triggering on hadronic final states
• Large cross section for charm (also 10X that for
  bs)

34
CLEO-c Experiment

• 2003 3 fb-1 at y(3770)
• L3.6x1032/cm2s
• 30M events, 6M tagged D decays
• 310 times MARK III
• 2004 3 fb-1 at sqrt(s)4100 MeV
• L3.0x1032/cm2s
• 1.5M Ds pairs, 0.3M tagged Ds decays
• 480X MARK III, 130X BESII
• 2005 1 fb-1 at the J/y(3100)
• L1.0x1032/cm2s
• 1 billion J/y decays
• 170 times MARK III, 20X BESII

35
CLEO-c Experiment
CLEO-c reach for some key measurements

• Absolute branching fractions
• Semileptonic form factors
• D mixing searches
• CP violation searches
• Rare D decays

36
Future Charm Physics at Hadron Machines

• CDF
• Up to 107 D0-Kp
• 15,000 WS D0-Kp-
• Assumes current trigger rate holds up
• Assumes same RS/WS efficiency ratio as B
  factories
• CPV reach of 10-3?
• LHCb
• Trigger not optimal for charm
• BTeV
• Up to 108 D0-Kp
• 150,000 WS D0-Kp-
• Many assumptions in this number
• CPV reach down to 10-4?

37
Summary and Outlook

• Many exciting new results from existing data
• Including many other important results I did not
  have time to cover
• Several new results expected within a year or
  less
• B factories
• Data is coming in fast
• Eagerly awaiting results from analyses in
  progress
• CDF
• SVX triggers are taking charm!
• Great potential if charm stays within the trigger
  bandwidth budget
• Future experiments
• Funding decision soon on CLEO-c, first data in
  2003
• Many uncertainties in charm physics potential at
  hadron machines, however
• Potential for huge gains in sensitivity
• Preliminary Run II CDF charm plots show that it
  can be done!
• BTeV trigger should be quite good for charm
• We can look forward to great advances in charm
  physics which will improve our understanding of
  the standard model and beyond