# Presentation: 323047

## FCNC Charm Decays

- Arnold Pompoš
- The University of Oklahoma
- For the DØ Collaboration

- Lisbon

- EPS 2005

- July 21-27

- Portugal

- 2005

## Outline

- Motivation for FCNC searches
- Analysis strategy
- Production and selection of Ds± and D± mesons
- Background reduction strategy
- Ds± & D± signal enhancing strategy
- Ds± & D± signal extraction
- Results
- Conclusion

- In this talk we report on Flavor Changing Neutral Current Charm Decays & Observation of Ds± → π → μ+μ-π

## Tevatron - pp collider

- -

**April 2001**

**Feb 2002**

**first data for analyses**

** ****data for physics**

** ****detector commissioning**

**Jul 2002**

- High data taking efficiency
- in the range 80-90%
- So far analyzed
- Above 500 pb-1
- 5x the total Run I data

- More than 1000 pb-1 delivered

## The DZero Experiment

- Beamline shielding
- Reduces accelerator background
- Silicon tracker
- Coverage up to ||<2
- Fiber tracker
- 8 double layers
- Coverage up to ||<2
- Solenoid (2 Tesla)
- Forward + central muon system
- Coverage up to ||<2
- Three level trigger system
- Outputs 50 Hz

## Motivation for FCNC searches

- For every rare SM process there is a “beyond the SM” theory in which it is enhanced.
- RPV SUSY can enhance SM suppressed FCNC processes.
- FCNC processes with _down type quarks_
- s->d type studied with kaons (K±,K0)
- b->s type studied with B mesons.
- FCNC processes with _up type quarks_
- Still lot of room for new results
- In this talk we focus on c→u transitions

## FCNC c -> u transitions with D± & Ds±

- FCNC c -> u transitions with D± & Ds±

- Penguin

- Non-resonant decays

- Resonant decay

- Box

- (GIM suppressed, Br ~ 10-8)

- (Dominant Br ~ 10-6)

**D****±**** ****→**** ****μ****+****μ****- ****π**** ******

- Non-resonant decays

**D****s****±**** ****→**** ****μ****+****μ****- ****π**** ******

- (Penguin & Box diagrams absent)

- Br(Ds± → π)
- (3.6±0.9)x10-2

- Br( → μ+μ- )
- (2.85±0.19)x10-4

- Observation = essential step to study c->ul+l- FCNC transitions

- Resonant decay

- (Analogous to b->sl+l- studies after B±->K+J/ψ->K+μ+μ- observation)

- Observe resonant Ds± and D± decays
- Search non-resonant continuum for excess events

- Analysis Strategy

## Production of D± and Ds±

- Production of D± and Ds±

- In pp collisions at √s = 1.96 TeV σ(pp → cc) ~ microbarns
- Ds± production mechanisms at the Tevatron:
- _Prompt_ production: pp -> cc -> Ds± +X
- _Secondary_ production: pp ->bb ->B+X -> Ds±+Y+X
- We are interested in Ds± → μ+μ- π            we employ dimuon triggers
- recording rate ~2Hz
- Ds± and D± decay products tend to be within a narrow jet

- -

- -

- -

- -

- -

- -

- -

- D± selection philosophy

- Look for events with μ+, μ- & a π± track located within the same jet.

- Production fraction f

## Muon, Track, D± and Ds± Selection 

- Muon selection
- Two OS μ candidate each matched to a central track of    pT > 2 GeV/c
- Muons are within the same jet and coming from the same vertex
- 0.96 < m(μ+μ-) < 1.06 GeV/c2

- π  track selection
- Track in the same jet as μ+μ- pair.
- Track from the same vertex as the μ+μ- pair.

- Ds± and D± selection
- μ+μ- & π  track form a good vertex.
- 1.3 < m(μ+μ- π  ) < 2.5 GeV/c2
- p(μ+μ- π in the SV – PV direction

**→**

**→**

**→**

- Large track multiplicity yields several Ds± or D± candidates/event

- Need better π  track selection method

## π± track selection strategy

- π± track selection strategy

- MC studies showed that the correct π  track:
- Forms a good vertex with the μ+μ- system
- Has typically higher pT
- Is close to μ+μ- in φ-η space (ΔR2= Δη2 + Δ φ2)
- We choose the π  track which minimizes
-     M = χvtx2 + 1 GeV/pT(π)2 + ΔRπ2
- This strategy picks 90% times    the right π  track (in MC events)

- Can you see the Ds± or D± ? 

- Sideband data for background studies

**D****s****± ****and D****± ****signal region**

- DØ ∫Ldt = 508 pb-1

## Background Suppression

- ID: tracking isolation of Ds±
- ID=p(D)/Σp(cone), cone ΔR < 1
- SD: transverse flight length significance of Ds±
- SD = Lxy/σ(Lxy)
- θD: collinearity angle
- Angle between SV-PV and p(Ds±)
- RD: ratio of π  impact parameter significance to SD
- RD= (IPπ/σ(IPπ))/SD

**→**

**→**

**→**

- To minimize background, we employ 4 variables:

## Backg. Suppression Variables-Example

- DØ ∫Ldt = 508 pb-1

**prompt D****s****±**

**D****s****± ****from B**

- Isolation ID
- (MC)

- Isolation ID
- (sideband data)

- Decay flight length signific. SD
- (MC)

- Decay flight length signific. SD
- (sideband data)

- DØ ∫Ldt = 508 pb-1

**total D****s****±**

**prompt D****s****±**

**D****s****± ****from B**

**total D****s****±**

**Arbitrary Units**

**Arbitrary Units**

**Number of events / 0.02**

**Number of events**

- Prompt signal better isolated than background

- Signal has higher significance than background

## Combined Likelihood Variable

- No box cuts, use likelihood variables to extract Ds± and D±
- Isolation ID is independent of SD,θD & RS
- SD,θD & RS are independent if SV well separated from PV

- Combined likelihood  “L”  reflecting correlations:
-  L=L(ID)xL(SD,θD,RS)            if SD <20
-  L=L(ID)xL(SD)xL(θD)xL(RS) if SD >20

- Plot likelihood ratio “d” :
-  d=L(Signal)/(L(Signal)+L(Background))

- θD vs SD

- Likelihood Ratio “d”

- Background

- Signal

## Results: Ds± observation

- Good agreement of “d” between MC and Data
- Keep events with d>0.9
- optimizes ε(signal)/√ε(backg)

- In the Ds± region 1.91 <m(μ+μ- π ) <2.03 GeV/c2
-  51 events found
-  18±4 background events expected

- Likelihood Ratio “d”

**MC**

**Data**

- Result I.

- 31±7 Ds±  events observed

- (corresponds to >7σ significance)

- Nice, but how to extract the D±? 

## Results: D± extraction

- Relax cut on likelihood ratio to 0.75
- Fit 2 Gaussians (signal) & exponential (background)
- Floating μ(Ds±), σ(Ds±)
- Fix μ(Ds±)-μ(D±) to 1.969-1.864 GeV
- Fix σ(D±) to m(D±)/m(Ds±) x σ(Ds±)

- Use the above found μ(Ds±), σ(Ds±) and fit the d>0.9 distribution

- Nice, but need more data for observation . Let’s set a limit on D± production!

- Result II.

- In 1.91 <m(μ+μ- π ) <2.03 GeV/c2, d>0.75

- 51+12-11 Ds±  events observed

- Result III.

- (corresponds to >2.7σ significance)

- 13.2+5.6-4.9 D±  events observed

## Result: Br ( D± → π± μ+μ- ) limit 

- Result: Br ( D± → π± μ+μ- ) limit

- PDG

- From MC

- Measured or
- Limit Setting

- To determine

- Br(D±  →π→μ+μ- π)

- Br(Ds± →π→μ+μ- π)

-  f(D±)

- f(Ds±)

- ε(Ds±;d>0.75)

- ε(D±;d>0.9)

- N(D±;>0.9)

- N(Ds±;>0.75)

- =

- x

- x

- Literature

- Result V.

- Br(D±  →π→μ+μ- π)

- Br(Ds± →π→μ+μ- π)

- < 0.28

- Result IV.

- Br(D±  →π→μ+μ- π)

- Br(Ds± →π→μ+μ- π)

- = 0.17

- (at 90% C.L.)

- Br(D±  →π→μ+μ- π) < 3.14 x 10-6

- (at 90% C.L.)

- Br(D±  →π→μ+μ- π) = (1.70           )x10-6

- +0.08 +0.06

- -0.07 -0.07

- +0.79 +0.76

- -0.73 -0.82

## Conclusion 

- We searched 508 pb-1 of DØ’s dimuon sample and observed clear signal in Ds± →π→μ+μ- πchannel
- We set a 90% C.L. upper limit Br(D± →π→μ+μ- π3.14x10-6

- DØ has best sensitivity to study FCNC in charm decays and we now continue to search the non-resonant continuum for signs of physics going beyond the Standard Model.

- Tevatron is doubling the luminosity each year  MORE GREAT PHYSICS RESULTS are coming soon & even more in few years .

## Flavor Changing Neutral Currents 

- (for non-Experts)

## Weak Interactions - Part I. 

- Charged Current (W± mediated)

- Lepton flavors:

- Quark flavors:

- W± couples within the same generation

- NO flavor changing

- where                           mixes generations

- W± couples up with _primed_ down

- INDUCES flavor changing

- Example:
-   ud coupling
-   strength ~cosθc
-   95% of GF

- n  →  p+ e- νe

-   us coupling
-   strength ~sinθc
-   5% of GF

- K-  →  π0 e- νe

- -

- -

## Weak Interactions - Part II.

- Neutral Current (Z0 mediated)

- If the world had only u & d’= dcosθc +ssinθc, then neutral current would be:           (u,d’)x(u,d’)T = uu+d’d’ = uu + ddcos2 θc +sssin2 θc + (ds+sd)cosθcsinθc  **(non zero!)**

- If Z0 coupled to uu or d’d’ then K0→μ+μ-

- But in the Standard Model it does not.

- What about higher order corrections, such as:

- must exist.

- But this process does not occur in experimental data. Why?

- K0→μ+μ-

- non zero.

- The amplitude M~ +cosθcsinθc

## Weak Interactions - Part III.

- GIM mechanism

- Glashow, Iliopoulos & Maiani proposed the existence of the “c” quark.
- Then the neutral current out of two doublets                 is:

- Cancelation = explains low rate in data

- NOTE: Cancellation not exact due to different u and c masses. This allows to
-             predict the c mass based on observed rates.

- J0 =uu + d’d’ + cc + s’s’ = uu + dd  + cc + ss

- No flavor changing terms exist

- M~ +cosθcsinθc

- M~ - cosθcsinθc

- Due to the proposed “c” quark, the K0→μ+μ- process has two diagrams:

## Backup Slides

## Result: Br ( D± → π± μ+μ- ) limit 

- Result: Br ( D± → π± μ+μ- ) limit

- PDG

- From MC

- From Limit Setting

- To determine

- 90% C.L. upper limit on
-    Br(D±  →π→μ+μ- π) found by:

| Statistical | +47,-43 % |
| --- | --- |
| Fitting | +14%,-24% |
| f(Ds±) | 26% |
| f(D±) | 8% |
| ε(Ds±)/ε(D±) | 19% |
| Br(Ds± →π) | 25% |
| Br(→μ+μ-) | 7% |

- Uncertainties summary

- Br(D±  →π→μ+μ- π)

- Br(Ds± →π→μ+μ- π)

-  f(D±)

- f(Ds±)

- ε(Ds±;d>0.75)

- ε(D±;d>0.9)

- N(D±;>0.9)

- N(Ds±;>0.75)

- =

- x

- x

- Literature

- = 0.9, where r=

- N(D±;>0.9)

- N(Ds±;>0.75)

- Result IV.

- Br(D±  →π→μ+μ- π)

- Br(Ds± →π→μ+μ- π)

- < 0.28 (at 90% C.L.)

- Br ( **D****± ****→**** ****π****± ****μ****+****μ****-** ) < 3.14 x 10-6 (at 90% C.L.)