V 0 production studies at LHCb. Mathias Knecht, EPFL , joint SPS-ÖPG-ÖGA 2 meeting, Innsbrück, Österreich, September 2-4, 2009

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1 V 0 production studies at LHCb Mathias Knecht, EPFL , joint SPS-ÖPG-ÖGA 2 meeting, Innsbrück, Österreich, September 2-4, 2009

2 Outline One of the first measurements to be done with the LHCb detector... because we can! Motivations: physics of hadronization / fragmentation processes Experimental needs: minimal detector (tracking only) Data: minimal sample (10 8 minimum bias events) V 0 selection, results of analysis and expected errors

3 The LHCb detector Single-arm forward spectrometer dedicated to the study of CP violation and rare decays in the b-quark sector Peaked b-quark production at low p T justifies this layout y Tracking stations RICH (Particle Identification «PID») Ring Imaging CHerenkov muon stations 5m Vertex Locator (VeLo) Vertex Locator TT Magnet T1 T2T3 RICH2 ECAL SPD/PS M1 HCAL M2 tracks M3 M4 M5 pp interaction 5m calorimeters 10m 15m 20m z

4 V 0 particles (strange) K S (meson ds), Λ (baryon uds) and Λ ( u d s ) Decay (60% of the time) to p + π - (Λ), p - π + (Λ), and π + π - (K S ) named after the tracks they produce: a V (twobody decay) and 0 because it is neutral π - V 0 (not seen) (bubble chamber-like) p + (Λ) or π + (K S )

5 Hadronization The way quarks are created and recombine after a high-energy collision When 2 quarks are moving away from each other, the increasing potential energy creates new quarks p + p +

6 V 0 and hadronization Nowadays, hadronization is badly known: only phenomenological models tuned on SPS, LEP and TEVATRON data The s quarks in K S and Λ are produced in the hadronization/ fragmentation processes studying these particles helps us understanding hadronization The phenomenological models show divergences when extrapolated to LHC energies, especially in the kinematical range covered by LHCb The study of V 0 production at LHCb could give a valuable input to understand the hadronization processes and tune the phenomenological models It will also help tuning the Monte Carlo generators

7 Observables of interest A few observables have been identified to compare between models, among which: differential cross-sections in η (pseudo-rapidity) and p T (transverse momentum) K S multiplicity meson/baryon ratio anti-baryon/baryon ratio... η = ln tan 1 2 arcsin P T P «pseudo-rapitdity» see, for instance, P. Skands, arxiv/hep-ph: We will focus on the Λ/Λ ratio as an example

8 Λ/Λ!'"( Λ/Λ ratio for different MC tunings prompt Λ/Λ ratio as a function of the pseudo-rapidity η, for several different phenomenological models («tunings») no diffractive events (next slide) ATLAS, CMS and LHCb coverages are shown! ATLAS/CMS LHCb LHCb lies within a range of η which is more sensitive to the various models (order ~5%)... "')( "') "'#( tunings *+,- *+,-./"0123 *+,-.1" *+,-.14 *+,-.15 *+,-.1$ *+,-.167,8 *+,-.1/39: *+,-.1+;2< "'#!!"!#!$!%!& " & % $ #!" η Because ATLAS and CMS cover a η region in which models have been tuned to TEVATRON data! (beware y-scale)

9 Diffractive events Hadronic diffraction is defined as a reaction in which no quantum numbers are exchanged in a high energy collision (a «Pomeron» is exchanged) Not typically what we are interested in Because of their topology, they are easy to remove taken from Marco Pacheco, LISHEP09

10 Data sample We plan to use the very first 100 million (10 8 ) minimum bias events collected with the LHCb detector in the following conditions: stable detector stable colliding beams with center of mass energy > 4TeV Our study relies only on the Vertex Locator and the tracking stations working no PID, no energy minimal detector! K S /Λ samples will also be used for RICH (PID) calibration (very pure p and π samples)

11 Prompt-V 0 selection A prompt V 0 is a particle produced at the primary vertex (PV) or coming from the electromagnetic or strong decay of a particle produced at the PV V 0 from long-lived decaying particle are not interesting for the study of the hadronization processes Selection: we have no PID we take all tracks and combine them, and ask: 2 oppositely charged particles we calculate the mass under pion or proton hypothesis By taking only tracks that go through all the tracking system («long tracks») we remove many V 0 s that are coming from the decay of long-lived particles

12 Main cut: ν 1 variable We apply a simple cut on a custom variable using impact parameters of the pion, proton, and K S /Λ candidates, w.r.t the primary vertex ν 1 = log10 IP IP proton pion IP Λ Λ candidate ν 1 = log10 IP IP pion + pion IP K K S candidate It is based on geometrical topology of this 2-body decay very good V 0 v.s. background separation entries Entries Mean RMS entries 2000 Entries Mean RMS background background background true V 0 entries Entries Mean RMS true V 0 true V ! 1 K s ! " ! " 1 We can find the best cut value by estimating the significance as a function of the cut value

13 Significances v.s. ν 1 cut Based on Monte-Carlo we can get the maximum significance σ for our selection S= nb. of signal events in mass window B= nb. of background events in mass window The best cut value is ν 1 >2.0 σ = S S + B Significance 150 Significance Significance nu1 cut value, K s nu1 cut value,! nu1 cut value,! yellow: maximum values

14 Invariant mass distributions Very clean signal Main background for Λ is K S contribution Small contamination of diffractive events ~2% (red) K S π + π - Λ p + π - Λ p - π +

15 Signal composition example with Λ Main component is Λ from decay of shortlived particles Very small contamination of decay of long-lived (non-prompt), ~0.5% Λ from hadronic interaction ~0.1% Λ from the primary vertex 40.75% Λ from decay 59.15% Λ from hadronic interaction with the detector Short lived: 0.1% Σ 0 42% Σ* - 17% Σ* 0 16% Σ* + 17% Ξ - 3.9% Ξ 0 2.3% Long-lived: Ω % Λ c %

16 Efficiencies K S Λ anti-λ decays in pπ or ππ (PDG) «2 prong» 69.2% 63.9% 63.9% in acceptance (geometrical topology of the detector) 43% of above 41% 41% reconstructed as long tracks 10% 4% 4% selection efficiency 94% 82% 81% total efficiency 2.76% 0.89 % 0.89% The main components of efficiency are the geometry of the detector the fact that we require long tracks, whereas K S and Λ are long-lived particles (i.e. they tend to decay after VeLo and make downstream tracks)

17 Λ/Λ ratio from the analysis We perform a fit in each bin of η, and we extract the signal and background We can evaluate the ratio r=λ/λ and its statistical error based on significance in each bin of η

18 Yields η bins Λ anti-λ σ(r), 9x10 6 S B S B σ(r): significance on the ratio events simulation σ(r) scaled to 10 8 events 2.5<η< % expected rel. error 3.0<η< % 3.5<η< % 4.0<η< % 4.5<η< % We see that the statistical errors are around 1% Since phenomenological hadronization models differ up to 5%, LHCb will be a very useful instrument to disentangle various models.

19 Conclusion We plan to use the very first data of the LHCb detector, possibly available end 2009, to perform an analysis of V 0 production The LHCb detector lies conveniently within a η range complementary to ATLAS and CMS, which will allow to probe unique features of phenomenological models Our analysis showed that with 10 8 events LHCb has the required sensitivity to disentangle models Our results will be important also for detector calibration

20 Backup slides

21 Decay geometry IP(Λ) π - PV IP(π ) p +

22 Errors on Λ/Λ ratio We calculate the statistical error on the ratio r=λ/λ, with σ being the significances for Λ and Λ σ Λ = S Λ S Λ + B Λ σ Λ = S Λ S Λ + B Λ Δ r = r 1 σ Λ σ Λ 2

23 Total efficiency Total efficiencies for K S, Λ and Λ as a function of η ε total = n selected n generated Errors: statistical (simulation with 10 7 minbias events) " eff " eff eff. K S ! 3 4 5! 3 4 5!