Experimental measurement of a new Asymmetry at Belle

1. 08 June 2009 Experimental measurement of a new Asymmetry at Belle Manmohan Dash work done at Virginia Tech & Belle by the generous support of Professor Leo Piilonen.

2. “Experimental study of a new form of asymmetry in the high energy charm Physics at Belle” Seminar given at Dept. of High Energy Physics, Tata Institute of Fundamental Research, Mumbai, India

3. Gratitude I am thankful • To Dr Gagan Mohanty for inviting me to TIFR • To Professor Naba Mondal for kindly arranging this seminar. • To the friendly people of TIFR for their support in my stay at TIFR.

4. Symmetry is central to understanding of nature • The basic quest of any High Energy Physics experiment is to explore the physical realities at the most fundamental level accessible to present day particle accelerators. • The concepts of symmetry are central to such an endeavor and our knowledge.

5. Violations of symmetry can be measured experimentally • Experimentally we try to measure the small deviations from symmetries and this often lead to deeper consequences in our understanding of nature. • My measurement is an experimental investigation of asymmetry in neutral charmed meson’s (D0, D-zero) decay.

6. There “is” an asymmetry in the decay of D0 to (KS/KL)0 • I have performed a measurement at KEK e+e- collider to determine the asymmetry in the rates at which the neutral charmed meson D0 decays into its final states of KS00 and KL00 • No other measurement is known yet to determine this asymmetry.

7. Its becoming clear that this asymmetry does exist. • My signals are large and clear especially for the notorious KL but the measurement is still on the tenterhooks. • The size of the analyzed data sample needs to be upgraded. • Final verdict on the asymmetry needs very careful analysis.

8. High Energy Accelerator Research Organization or KEK for [“Kō Enerugī Kasokuki Kenkyū Kikō”] Presently I am a member at Belle

9. Belle detector cavity e+ source Accelerator-Collider-Detector

10. The Belle-Detector

11. The Event View at Belle Detector Side view End view

12. The Belle “brouhaha” Since summer of 2002, when I joined as a graduate student researcher at Belle, Belle has broken and established many records for unprecedented amount of data and luminosity. This has also resulted in hundreds of Physics results coming out of Belle many of which made international news. New particles, resonances and “unknown” particle states were discovered along with many important asymmetry measurements.

13. Further in this talk… • Elementary particles • An introduction to the concept of symmetry and its violations • Introduction to this measurement, its motivation • Details of approach • Results of this study • Future path

14. Elementary Particles

15. What’s an elementary particle? In the study of the structure and interaction of “very very very very small” particles “beyond” the nucleusif we can’t probe the size of particles with the highest resolution then we call them elementary.

16. Examples of an elementary particle? Electrons are elementary. “Protons” were thought to be elementary. It is known that proton’s “mass and charge” are spread over a length, in 3-D space, whose radii is something like 10-15 meters. [1femto-meter]

17. What is Resolution !! it’s the smallest extent of something that we can measure, beyond that it would be like a “point” something whose extent we can’t measure. Wavelength of beam Resolution of an optical microscope Angular Aperture of the light beam Larger “angle” and smaller “wavelength” gives better resolution, therefore ultraviolet is better than visible light.

18. Resolution in particle physics For elementary particles, which obey Quantum Mechanics [Laws of Physics for extremely small objects where they exhibit both “wave” and “particle” behavior] Where q = “momentum” transferred to the “particles” of the incident beam

19. Proton is not an elementary particle From the equation just explained “momentum” corresponding to an “energy” of 10 GeV “gives us” a resolution of 10-16 meters, which is one-tenth the size of a proton. So now we can make 10 marks on the body of the proton, not just ONE. “Ichi-ni-san-yon-go-roku-hachi-nana-kya-ju”

20. Why High Energy in “High Energy Physics” We saw that with higher energy of incident beam we are equipped better to probe the smaller size and know what’s elementary and what’s not. There is yet another reason why Particle Physics needs very high energies. There are elementary particles which are hundreds of times massive than extended particles like protons. And to produce these particles in the Lab in order to study them means production of higher energies.

21. How high is high energy ? Total energy of a beam in accelerator where 1 particle would have 1 TeV of energy: “per” bunch of 1013 particles, “per” second equals to energy of 30,000 light bulbs which again equals to energy of a 15 tonne truck moving at 30 miles per hour

22. Symmetry and its Violations

23. Concepts of symmetry • Symmetry is important at quantum level. At a classical level its not manifested very exactly. At quantum level this is very precise and must be dealt with the principles of Quantum Mechanics. • Examples of symmetry: All transformation laws in Physics. Here the state vector is conserved [unchanged] under the transformation, hence the symmetry.

24. Symmetry in the Physical world Continuous: rotation of a circle • Continuous • Discrete Discrete: rotation of a polygon 900 +900

25. Symmetry leads to invariance and vice versa • Invariance of physical laws • Invariance of force: infinitely long electrically charged wire: cylindrical symmetry of the electric force field E E E Rotation of a system of charges => rotation of the Electric force

26. Examples of symmetry operations • “Time” • Space transformations • Scaling, reflection, rotation etc • “Functions”

27. Continuous symmetries Space-time symmetries • Time translation • Spatial translation • Spatial rotation: Proper:square matrix det =1, Improper:square matrix det =-1 • Poincare transformation:distances in Minkowski space-time are invariants

28. Discrete symmetries • Time reversal: Upon reversing the “sign” of time some physical laws/quantities are unchanged

29. Discrete symmetries • Spatial inversion • Symmetries in Crystals • C,P,T symmetries are discrete, used in Particle Physics

30. CPT invariance Hypothesis: Under C, P or T the universe is invariant • “charge conjugation” or particle-to-antiparticle transformation • Mirror reflection or space parity • Time reversal • Collectively CPT is supposed to be an invariant in nature Individually C, P or T do not provide a good symmetry

31. CP Violation, Super-symmetry • Violation of CP symmetry is in consonance with amount of baryonic matter in the universe which in turn is necessary for existence of life. • Super-symmetry is an advancement to Standard Model of Particle Physics and assigns a super-partner to Bosons and Fermions.

32. Motivations/Introduction “my measurement”

33. Motivations • As mentioned in the early slides: There “is” an asymmetry in D0 to(KS/KL)0, which has been proposed in phenomenological works. • Independent of that, this asymmetry can be “measured” with recourse to the present sensitivity of our detector at Belle.

34. The signal channels • Explicitly, there is a natural asymmetry in the Branching Fraction of • These decays are identified experimentally (a.k.a. tagged) by their parent decay

35. Asymmetry because of interference • The asymmetry arises supposedly because of interference in the amplitudes of the Cabibbo Favored (CF) and Doubly Cabibbo suppressed (DCS) modes of the D0 meson.

36. Detector bias • Apart from this asymmetry the detector can provide a bias in the measurement of KL vis-à-vis KS. The complete KL info is not available from the detector. A “missing energy method” is used to tackle this. • The KL reconstruction efficiency is strongly momentum dependent

37. Reconstruction efficiency KL, Strong momentum dependence KS efficiency “flat”

38. Calibration channels • We need to have calibration modes of the D0. When the D0 decays via the K*- (or K*+) resonance to the (KS and KL final states) there is no natural asymmetry here. The asymmetry is detector induced. • Most systematics of the detector cancel out due to same final state particles, (same reconstruction efficiency).

39. Details of approach of my measurement

40. The KL is partially reconstructed KL direction only Improved direction

41. K-Long missing energy is recovered Detector does not give pKL, “quadratic solutions” are obtained from relativistic kinematics

42. KS is reconstructed pretty well “V for Vertex”

43. Signal and calibration modes

44. Event Selection • To do this analysis an integrated luminosity of 32 fb-1 belle data was used. The corresponding monte carlo used is roughly 3 times of this. • The D* candidates were selected by choosing their reconstructed scaled momentum xp between 0.6 to 1.0 • this new variable is defined as xp = p*/sqrt(Ebeam2 - M2) where p* is the cms momentum. • This rejects D* from B-decays and suppresses Combinatorics.

45. Event selection methods • Apart from standard selection procedure for Pions and KS, the daughter gammas of pi0 are rejected if their energy is less than 50 MeV • For the KS its pi-pi vertex should be separated from the IP in the transverse plane by more than 500 micro-meters • KS’s pi-pi tracks should not be separated by more than 1 cm, at the KS vertex, along the beam axis • Angle between assumed KS trajectory from IP to vertex and the reconstructed one should be very small, i.e. cosine(of this angle)>0.95

46. Event Selection • KL in the KLM are contaminated by unreconstructed charged particles which are rejected by vetoing associated energy in the ECL in the range of 0.15 to 0.3 GeV which corresponds to minimum ionization energy

47. Event Selection • A large chunk of background is gotten rid of by applying a selection on the K0 flight angle wrt D0 boost as it peak towards forward direction as shown in next slide. For signal this angle is isotropic with a slight tilt to the forward direction due to detector efficiency. This happens when arbitrary soft pions as opposed to our signal slow pion combines with high momentum K and forms a good signal.

48. The sharp peak is caused by the reverse situation when K is soft

49. Event Selection • Invariant mass of K*- (K0pi-) is required to be within 50 MeV/c2 of nominal K*(892) mass, also the pi+pi- pair in these modes are required to be less than 0.7 GeV/c2 • Thismakes the signal and calibration kinematically similar and reduces contribution from K0Rho

50. Modes studied in monte-carlo