Charm with ZEUS HERA-II data

Transcription

1 Charm with ZEUS HERA-II data Falk Karstens on behalf of the ZEUS collaboration University of Freiburg/ Germany Outline: DIS 26 21th April 26 HF-2 D ± mesons in deep inelastic scattering D decay length significance D ± mesons in deep inelastic scattering

2 1 Charm search with D ± (21) D (K + π ± ) + π s p e + d u u 2/3-1/3 1/3-2/3 g c u c t d s b d s b u c t c d D *- Boson Gluon Fusion is the dominant process in DIS gluon sensitivity d - open charm pair production s u u D fragmentation F(c, c D ± ) = W - s.197 ±.12 K + d u - charm deexcitation: Zweig rule - strong interaction, short lifetime B(D ± D π s ± ) =.677 ±.5

3 2 Charm search with D ± (21) D (K + π ± ) + π s p e + d u u 2/3-1/3 1/3-2/3 g c u c t d s b d s b u c t c d D *- d s - u u D F B = (.51 ±.13) % W - s K + d u - charm ground state to strange state: weak interaction, longer lifetime B(D ± D π ± s ) B(D K π ± ) =.38 ±.9 all elementary forces relevant for particle physics covered small phase space of about 6 MeV for slow π s ± limited by mass difference

4 HERA II 3 HERA I , HERA II new elements are micro vertex detector (MVD) and straw tube tracker (STT) in HERA II gated ZEUS data L(e + p) = 41 pb 1 and L(e p) 16 pb 1 in HERA II Integrated Luminosity (pb -1 ) HERA delivered days of running

5 4 Selection criteria for D ± (21) D (K + π ± ) + π ± s mass calculations done with all track combinations and mass assignment primary vertex tracks mass cuts: 1.8 GeV < M(D ) < 1.92 GeV.143 GeV < M <.148 GeV M = M(K π ± π ± s ) M(K π ± ) DIS cuts: 5 GeV 2 < Q 2 < 1 GeV 2.2 < y <.7 D ± kinematic cuts: 1.5 GeV < p T (D ± ) < 15 GeV p T (K) >.4 GeV p T (π) >.4 GeV p T (π s ) >.12 GeV 1.5 < η D ± < 1.5

6 Event display high p t, Q 2 from π s π K + π K + π s XY View ZR View Q 2 = 918 GeV 2 y =.39 p T (D ) = 1.3 GeV η(d ) =.6 p T (K + ) = 1.5 GeV p T (π ) = 7.9 GeV p T (π s ) = 1.1 GeV

7 Positron scattering: e + + p D ± + X 6 Combinations ZEUS + -1 ZEUS (prel.) 3-4 e p (4 pb ) Wrong Charge M (GeV) subtraction of correlated errors gives a narrow signal invariant mass difference peaks around the rest mass of π s (139 MeV) plus its kinematic energy background subtraction method to get the number of candidates N rec = N sig N back luminosity L = 4 pb 1

8 Electron scattering: e + p D ± + X 7 Combinations ZEUS ZEUS (prel.) 4-5 e p (33 pb Wrong Charge - -1 ) luminosity L = 33 pb 1 only a fraction of the current data M (GeV)

9 Ratio between σ(e p) and σ(e + p) cross-sections 8 p) + p)/σ(e - σ (e ZEUS 3 lower limit in Q 2 shifted from ZEUS GeV 2 to 5 GeV 2 ZEUS (prel.) Q (GeV ) 1 3 HERA I data in Phys. Rev. D69: 124, 24 (hep-ex/3868) given in blue L(e p) = 17 pb 1 ; L(e + p) = 65 pb 1 HERA II data confirms that the excess in HERA I data was a statistical fluctuation charm is produced equally in e + p and e p collisions

10 Micro Vertex Detector 9 two/ three barrel layers and four forward wheels 2 µm intrinsic hit resolution impact parameter resolution of about 1 µm is required for efficient charm tagging alignment accuracy of 2 µm needed

11 MVD alignment impact parameter w.r.p. beam-spot in microns vs φ for high momentum tracks p T > 3 GeV in central region η < 1 of good quality min. 4 MVD hits natural spread of tracks in interaction region covered cosmic alignment in yellow ep collision tracks used for alignment in blue good agreement with MC given in red

12 D decay length significance 11 Decay length significance secondary vertex s - s beam-spot p - decay length L = s P decay length L primary vertex P K + y x projection onto momentum in 2D L xy = ( s P ) p(d ) p T (D ) significance s L = L σ L P = beam-spot gives higher precision than P = primary vertex

13 More data Candidates ZEUS + D* D D K π+ s - π + (+ c.c.) -1 ZEUS (prel) 12 pb Fit Wrong-charge background current 25 data added for the signal L(e p) = 12 pb 1 signal width improved from.93 MeV to.76 MeV because of improved track fits M(Kππ s ) - M(Kπ) (GeV)

14 D decay length significance 13 Decay length significance II Candidates D* D D K D π+ s - π + from D* S L (+ c.c.) -1 ZEUS (prel) 12 pb Wrong-charge background (x 1.9) ZEUS D s taken from D ± s wrong-charge background normalized to signal shift seen because of decay length cτ = 123 µm Decay Length Significance (S = L xy /σ Lxy ) L exponential decay length distribution hidden by resolution effects, what is not a surprise

15 D ± K + π ± + π ± selection criteria 14 Combinations ZEUS(prel.) 12 pb ± D p T > 2.5 GeV D + K - π + π + + (c.c.) ZEUS same DIS cuts as for D ± s p T (D ± ) > 2.5 GeV p T (K ) >.65 GeV p T (π ± ) >.45 GeV additional quality cut min. 4 MVD hits 15 hardly any signal visible 1 lifetime longer: cτ = 314 µm M(Kππ)(GeV)

16 D ± K + π ± + π ± decay length significance 15 Entries ZEUS ZEUS(prel.) 12 pb Mirror negative L positive σ(l) D + K - π + π + + (c.c.) -1 L σ(l) negative side mirrored to the left enhancement signed decay length L = L sign( L p D ±) 1 2 cut signal L σ(l) > 3 is used to enrich the Significance L σ(l)

17 D ± K + π ± + π ± signal 16 Combinations ZEUS(prel.) 12 pb ± D p T > 2.5 GeV Fit (Gaussian + p2) D + K - π + π + + (c.c.) ± N(D ) = 231 ± 43 L > 3 σ(l) ZEUS big improvement background reduced by a factor of M(Kππ)(GeV)

18 Summary and outlook 17 D ± cross-section ratio: HERA II data confirms that the excess in HERA I data was a statistical fluctuation decay length of D s from D ± s visible in decay length significance plot and could be used to clean up the signal further use of decay length significance for untagged D s decay length significance cut useful to reduce s b for D ± production