# Presentation: 804902

## Search for CP violation in K±3 decaysby the NA48/2 experiment at CERN

**Cambridge, CERN, Chicago, Dubna, Edinburgh, Ferrara, Firenze, Mainz, Northwestern, Perugia, Pisa, Saclay, Siegen, Torino, Vienna**

***Edoardo Mazzucato***
***CEA Saclay, DAPNIA/SPP***

**For the NA48/2 Collaboration :**

**PANIC’05 Conference, Santa Fe, NM, U.S.A.**
**October 24****th**** - 28****th****, 2005**

- Edoardo Mazzucato - PANIC'05

- 24/10/2005

## Outline

- ♦  **CP-violating charge asymmetries in K******** ******** 3******** decays**
- ♦  **The NA48/2 experimental method**
- ♦  **The K****±**** ****beam line and detector**
- ♦  **Measurement of the A****g**** parameter in ****K******** ******** ****************+********-**** decays**
**♦  ****K******** ******** ****************0********0 ****decays**
- ♦  **Conclusions**

- Edoardo Mazzucato - PANIC'05

- 24/10/2005

## CP Violation in K±  3 decays

**CP Violation in K****± ******** 3******** decays**

**|M(****u****,****v****)|****2  ******** 1 + ****g****u**** + h****u****2**** + k****v****2**** +** **...                               **

_***If CP invariance holds:***_
**    ****g****K****+ ****= ****g****K****-****, h****K****+**** = h****K****-**** , k****K****+**** = k****K****-**** **

***Dalitz plot for***
***K******±****** ************ 3*********

***Only direct CP violation*** ***is*** ***involved*** ***in charged kaon decays!***

**u**** ****=**** ****2m****K****(m****K****/3-****E****3*********)/m********2**
**v**** ****=**** ****2m****K****(****E****1*********-****E****2*********)/m********2**

**If A****g**** ******** 0  ********  *****Direct CP Violation***

_***Matrix element:***_

**|h|, |k| ******** |****g****|**

_***Asymmetry parameter:***_

******3 ****(odd)**

******2 ****(even)**

******1 ****(even)**

- 

**K**

_***SM predictions:***_***     *****A****g**** ******** 10****-4**
**If A****g**** ******** 10****-4****  ********  *****SUSY, New Physics?***

**K****±********************************  ****:  g = -0.2154 ******** 0.0035**
**K****±********************************  ****:  g =  0.638 ******** 0.020**

- Edoardo Mazzucato - PANIC'05

- 24/10/2005

## The NA48/2 approach

***  ***_***NA48/2 main goal:***_
**         ****- Measure linear slope** **(****g****) asymmetries for ****K************************************ **a**nd** **K************************************ ****decays with accuracies **
**           ********A****g**** ******** 2.2 ******** 10****-4**** and ********A****g****0**** ******** 3.5 ******** 10****-4****, respectively. **
**                                     **
**                                      **********   *****significant improvement w.r.t. present measurements***
**         ****- Need more than ****2********10****9**** ****K********************************** **and ****10****8**** ****K************************************ ****reconstructed events**

***  ***_***NA48/2 method:***_
**         ****- Use of intense simultaneous ****K****+**** and ****K****-**** beams, superimposed in space, with narrow ****P****K**** spectra**
**         ****- ****Equalization of averaged ****K****+**** and ****K****-**** acceptances by frequent alternations of magnet polarities**
**           ****(magnetic spectrometer, ****K******** beam lines)**
**         ****- Measurement of asymmetry from slopes of normalized**** u****-distribution ratios (*****no MC acceptance***
**           *****correction required!***** ) :**

**R(****u****) = N****+****(****u****)/N****-****(****u****) ******** n(1+********g**** ****u****)**
**A****g**** = ********g**** / 2g**

***Induced instrumental asymmetry can only be due to ******charge-asymmetric ******and ******u-dependent****** effects*** ***that vary with time***

**            ****( -70 ******** 53) ******** 10****-4****   *****BNL AGS (1970)*****             **
**A****g ****= **
**            ****(  22 ******** 40) ******** 10****-4     *****Hyper CP (2000)******  ******prelim.***

- {

**              ****(19 ******** 125) ******** 10****-4    *****CERN PS (1975)*****       **
**A****g****0**** =**
**              ****(  2 ********   19) ******** 10****-4    *****Protvino****** ******IHEP (2005)***

- {

- Edoardo Mazzucato - PANIC'05

- 24/10/2005

## The simultaneous K± beams

**The simultaneous K****±**** beams**

***focusing beams***

**K****+**

**K****+**

**K******

**K**−

**~7********10****11**
**p/spill**

**Front-end **
**achromat**

**Front-end **
**achromat**

**Second**
** ****achromat**

**Beams coincide within ****~****1mm**
**all along the ****114m**** decay volume,****always in vacuum**

**vacuum **
**tank**

**He  tank**
**+ spectrometer**

***not to scale!***

**20 M****Hz****  (****π****/K ~ 12)**** ****π**** decay products stay inside pipe**

**Quadrupole**
** ****quadruplet**

**Magnet**

-  **Cleaning**
** ****Beam spectrometer**

-  **Focusing**
** ******** ****sweeping**

-  **Momentum**
**  ****selection**

**400 GeV/c**

- Edoardo Mazzucato - PANIC'05

- 24/10/2005

## The NA48 detector

**♦ ****Magnetic spectrometer**
**    **4 high-resolution DCHs
**♦ ****Scintillator hodoscope**
**    **150 ps time resolution - fast trigger
**♦ ****LKr electromagnetic calorimeter**** **
**   **Quasi-homogeneous,** **high granularity
-     e/ discrimination
**♦ ****Hadron calorimeter, **** ****veto counters, **** ****vetos**

**The NA48 detector**

- Edoardo Mazzucato - PANIC'05

- 24/10/2005

## NA48/2 data taking completed

**2003**** ****run:  ~ 50 days **
**2004**** ****run:  ~ 60 days**
**Total statistics in 2 years:**
**K******** ******** ************ ********0**** ********0**** : ******** 1********10****8  ****decays**
**K******** ******** ************ ********+**** ********−  ****: ~ 4********10****9  ****decays**
** **~ **200 TB of data recorded**
***Preliminary result based on 2003 K******************+************-****************** ******sample***

- Edoardo Mazzucato - PANIC'05

- 24/10/2005

## Selection of K events

**K****+**

**K****-**

**1.03 ******** 10****9**** events**

**0.58 ******** 10****9**** events**

******m**** = 1.7 MeV/c****2**

****

****

**Even pion**
** ****in beam pipe**

**Odd pion**
** ****in beam pipe**

**u**

**|v|**

***1.6 ************ 10******9****** events ***
***in 2003 data sample***

** ****2-level trigger:**
**      ****- L1 : fast signals from hodoscope**
**      ****- L2 : at least one good vertex made of two tracks in DCHs**** **
** ****Offline reconstruction based on magnetic spectrometer only**
-       **-** **3-track vertex in decay volume**
**      ****- time, quality cuts, ghost track suppression, etc...**
** ****Acceptance limited mostly by beam pipe through DCHs**

-  ***Negligible ************ ************ ************ background***

***K******+ ******/ K******-****** ************ 1.8***

***Based only on Hodoscope and Magnetic Spectrometer***

- Edoardo Mazzucato - PANIC'05

- 24/10/2005

## Acceptance equalization for K+ and K-

-  **Jura **
**(Left)**

**Acceptance equalization for K****+**** and K****-**

- A+

- A-

**Detector Left-Right asymmetry**
** ****cancellation in the 4 K****+****/K****­**** ratios:**

**Salève**
**(Right)**

- ♦
- ♦
- ♦
- ♦

**R(u) = R****US****(u)********R****U****J****(u)********R****D****S****(u)********R****DJ****(u) ********(1 ****+ 4 ****Δ****g**** u)**

***Fit of ******R(u)****** is sensitive only to the ******time variation****** of ******asymmetries****** in experimental conditions***
***with characteristic time smaller than corresponding field alternation periods (******beam************ week******,***
***detector************ day******)  ************  ******2003 data taken in 4 SUPER-SAMPLES of  2 weeks each***

**Quadruple ratio:**

***Cancellation of systematic biases:***
**    ****- Detector**** ****L****-****R**** ****asymmetries ( K****+**** ******** ****Salève**** ****/ K****-**** ******** ****Salève****  and  K****+**** ******** ****Jura**** / K****-**** ******** ****Jura**** ****)**
**    ****- Beam line biases ( K****+**** beam ****Up**** / K****-**** beam ****Up****  and  K****+**** beam ****Down**** / K****-**** beam ****Down****) **
**    ****- Global time-variable biases  (K****+**** and K****-**** simultaneously recorded)**

- Edoardo Mazzucato - PANIC'05

- 24/10/2005

## Time variations of beam geometry

**Beam (x, y) -vs- K******** momentum**

**Sample beam profile at DCH1**

**Beam widths:**
**~ 5 mm**

**5 mm**

**X [cm]**

**Y [cm]**

** ****Acceptance mostly defined by ******** ******** 20 cm central beam pipe**
** ****Apply larger acceptance cut centered on averaged beam **
**  ****positions as a function of time, K******** charge and momentum**

**Beam movements:**
**~ 2 mm**

**2 mm**

**X [cm]**

**Y [cm]**

**K****-**

**K+**

- Edoardo Mazzucato - PANIC'05

- 24/10/2005

## Time variation in the calibration of  the Magnetic Spectrometer

**e.g. sensitivity to DCH4 horizontal shift: **
******M/********x ******** 1.5 keV/c****2****/********m **

******M****3******

**Relative alignment of DCHs fine tuned by imposing mean reconstructed invariant M****3********  masses to be equal for K****+ ****and K****-**

**~ 420 ****keV/c****2**

***Momentum scale***** ****variation due to limited **
**control of spectrometer magnet current **
**(10****-3****) cancels  (simultaneous beams)**

**Sensitivity to 10****-3**** error in field integral:**
******M ******** 100 keV/c****2**

***Momentum scale***** ****adjusted by imposing **
**PDG value of M****K******

- Edoardo Mazzucato - PANIC'05

- 24/10/2005

## Δg: 2003 result

| Source | Δg
(10-4) |
| --- | --- |
| Acceptance and beam geometry | 0.5 |
| Spectrometer alignment | 0.1 |
| Spectrometer magnetic field | 0.1 |
|    decay | 0.4 |
| Calculation of « u » and fitting | 0.5 |
| Accidental activity | 0.3 |
| Source of statistical nature |  |
| L1 trigger efficiency | 0.4 |
| L2 trigger efficiency | 0.8 |
| Total  | 1.3 |

**SS0**

**u**

**SS1**

**SS2**

**SS3**

| Sample | K+++-
(106) | K---+
(106) | Δg
(10-4) |
| --- | --- | --- | --- |
| SS0
SS1
SS2
SS3 | 431
258
253
  95 | 240
144
141
  53 | 0.5 ± 2.4
2.2 ± 2.2
-3.0 ± 2.5
-2.6 ± 3.9 |
| Total |          1036 |            577 | -0.2 ± 1.3 |

**R(u)**

***Asymmetry fits (from one of the 3 independent analyses)***

**u**

**u**

**u**

**R(u)**

**R(u)**

**R(u)**

******g=(0.6 ******** 2.4)********10****-4**
** ********2/ndf=39.7/38**

******g=(-3.1 ******** 2.5)********10****-4**
** ********2/ndf=29.5/38**

******g=(2.3 ******** 2.2)********10****-4**
** ********2/ndf=38.1/38**

******g=(-2.9 ******** 3.9)********10****-4**
** ********2/ndf=32.9/38**

***Combined result contains L2 trigger correction with conservative systematic uncertainty (obtained from downscaled control events)***

****

******2****/ndf = 3.2/3**

***Systematic uncertainties***

***Statistics and result***

- Edoardo Mazzucato - PANIC'05

- 24/10/2005

## Ag preliminary result (2003 data)

**A****g**** preliminary result ****(2003 data)**

**A****g**** = (0.5 ****±**** 2.4****stat.**** ****± ****2.1****stat. (trig.) ****±**** 2.1****syst.****) ******** 10****-4**

**A****g**** = (0.5 ****±**** 3.8) ******** 10****-4**

**Δ****g = (-0.2 ****±**** 1.0****stat.**** ****± ****0.9****stat. (trig.) ****±**** 0.9****syst.****) ******** 10****-4**

***Result stability***

***Time stability***

**K****+****/K****-**

** ****Detector**
** ****asymmetry**

**K****Left****/K****Right**

**K****Up****/K****Down**

**Δ****g**

** ****Beam line**
** ****asymmetry**

**♦**** ****4 super-samples  **
**   ****give consistent   **
**   ****results**
**♦ ****Detector and beam**
**   ****line asymmetry **
**   ****effects at a few 10****-4 **
**    ****level, reproduced **
**   ****by MC**** **

- Edoardo Mazzucato - PANIC'05

- 24/10/2005

## Ag0 asymmetry in K 00 decays

-  **The**** ****« u »**** ****variable can be reconstructed from LKr calorimer only  (very good resolution**** ****at low**** ****u****)**
-  **Statistics available in 2003:****  ****31 ******** 10****6**** K****+**** ****and ****17 ******** 10****6**** K****-**** ****decays****  ********  ********A****g****0**** (stat)**** ******** 1.7 ******** 10****-4**
-                               ***Lower statistics compensated by higher slope value ( g = 0.638 ) ***
***                                             ******and more favourable Dalitz plot distribution***
-  **Extrapolation to 2003 + 2004 data****:**  ******A****g****0**** (stat)**** ******** 1.3 ******** 10****-4**
-  **Negligible background (******** ******** ******** decay, wrong ******** pairings)**
** ****Different sources of systematics w.r.t.**** ****K************ ********** ******+********-** **decays** **(radial acceptance, trigger ...)**

******g fit for SS1**

******g = 4.8********10****-4**
******2****/ndf =45.3/50**

**u**

**R(u)**

***Preliminary result on A******g******0****** with 2003 data will be presented at CERN on Nov. 1******st****** , 2005***

********** ****= 1.1 MeV/c****2**

**M****3******** (GeV/c****2****)**

- Edoardo Mazzucato - PANIC'05

- 24/10/2005

## Conclusions

- ♦  **2003 preliminary result on the CP-violating charge asymmetry in**
**    ****K******** ******** ************************** **decays:**
**                        ****A****g**** = (0.5 ****±**** 2.4****stat.**** ****± ****2.1****stat. (trig.)**** ****±**** 2.1****syst.****) ******** 10****-4**
- ♦  **10 times better precision than previous measurements**
- ♦  **Result consistent with SM predictions**
- ♦  **Still room to decrease systematic uncertainties (e.g. trigger)**
- ♦  **A****g****0**** ****measurement in**** ****K******** ******** ****************0********0**** ****decays has ****comparable sensitivity**
**         *****Preliminary result will be presented at CERN on Nov. 1******st******, 2005***** **
**♦  ****Inclusion of 2004 data will more than double total statistics**
**         *****Better quality of 2004 data due to more frequent alternation***
***          ******of magnets polarity***
- ♦  **Design goal is within reach for both decay modes**

- Edoardo Mazzucato - PANIC'05

- 24/10/2005