Summary of ILD performance at SPS1a

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1 Summary of ILD performance at SPSa Mikael Berggren, Nicola d Ascenzo, Peter Schade, and Olga Stempel DESY -FLC Notekstr. 85, 67 Hamburg -Germany arxiv:9.434v [hep-ex] 4 Feb 9 The performance of the ILD detector at the ILC for the analysis of µ and τ channels at the SUSY benchmark-point SPSa has been studied with full detector simulation. It is concluded that if 5 fb is delivered to the experiment, (M χ = 9 MeV/c, (M µl = MeV/c, (M χ =.38 GeV/c, and (σ(e + e µ L µ L =.35 fb can be achieved from the µ channels alone. The preliminary results from the τ channels, indicates that (P τ = 3 % is also achievable. Introduction The SUSY benchmark point SPSa [] offers a rich phenomenology at the ILC. It is point with quite low mass-spectrum in the slepton sector, and heavy squarks. Bosinos up to χ 3 (in e+ e χ χ 3 would be produced at E CMS = 5 GeV. It is a pure msugra model, hence R-parity and CP is conserved. The unification scale parameters are: M / = 5 GeV,M = 7GeV,A = 3GeV,tanβ =, andsign(µ = +. Thepointisnotin contradiction with any experimental limits [3]. The τ is the NLSP, and M τ = 7.9GeV/c and M χ = 97.7 GeV/c, so (M =. GeV/c. At E CMS = 5 GeV, this yields P τ,min =. GeV/c hence γγ events will pose a problem. As SPSa is a point with an important co-annihilation contribution to the dark-matter relic density, the M τ is a most important quantity to determine. An other consequence of the τ being the NLSP, is that τ:s are present in large fraction of the SUSY decays, so that SUSY itself will be a mayor background source for τ channels. On the other hand, the M µl (M µr is 89.9(5.3 GeV/c, so that the minimum µ energy is 3.(6.6 GeV. As, in addition, the branching ratios to µ in bosino decays are quite low, the µ final states offer cleaner conditions, and are well suited for doing the most precise measurements. The present note reports on the status of the analysis of of the µ and τ channels of the SPSa scenario in the LDC detector. SPheno [4] was used to run the unification-scale modeltotheewscale,andwhizard[5]wastheusedtogenerateevents. TheLDCPrime Sc detector model was fully simulated using MOKKA[6], and the events were reconstructed with MarlinReco [7]. The same chain was used to produce background events. Analysis of µ channels Two channels containing only µ:s in the final state was chosen as a first study [8]: µ L µ L µµ χ χ and χ χ µ µ R χ µµ χ χ. As mentioned in the introduction, the SUSY background problem is not too severe in the µ channels, and it is advantageous to run the ILC at the polarisation giving the largest signal. Hence, these channels were studied assuming 8 % left e polarisation and 6 % right e + polarisation. Under these conditions, the µ L µ L process has a large cross-section, and is well suited to determine M µl and M χ. χ χ has a small cross-section BR, but can be used to determine M χ, without the need Support by DFG through SFB 676 is acknowledged LCWS/ILC 8

2 - Yield (5 fb Standard Model Background ( SUSY background( + - e e χ χ µ µ µµχ ( + e + e - µ µ - µ χ µ χ ( L L Yield (5 fb 6 Standard Model Background ( SUSY background( + - e e χ χ µ µ µµχ ( + e + e - µ µ - µ χ µ χ ( L L m µµ [GeV] Figure : The distributions of P µ (left, and M µµ (right to scan over the threshold. The main background processes are other SUSY giving two µ:s, mainly µ R µ R, χ χ with one χ going to µµ, the other to νν and τ τ with τ µν µν τ Standard model background is mainly from WW and ZZ. Finally, each of the two processes is background to the other one. The following kinematic variables were used to disentangle signal and background, and to separate the two signal channels: The momentum of µ:s (P µ, the acolinearity angle between the µ:s (θ acol, the acoplanarity angle between them,defined as the acolinearity in the projection perpendicular to the beam-axis (θ acop, the total missing transverse momentum (P Tmiss in the event, the invariant mass of the two µ:s (M µµ, the total missing energy (E miss, the polar angle of the missing momentum (θ missing p, and the velocity β of the µ system. The distributions of P µ and M µµ are shown in Fig., and that of β in Fig. - Yield (5 fb 5 Standard Model Background ( 4 3 SUSY background( + - e e χ χ µµ µµχ ( - - e + e µ + µ µ χ µ χ ( L L β Figure : The distribution of the velocity β The µ L µ L channel was selected by demanding that E miss [,43] GeV, M µµ / [8,]GeV and < 3 GeV/c, and θ missing p [.π,.9π]. Assuming an integrated luminosity of 5 fb, this leaves 3 events of SM background and events of SUSY background, while 63 signal events were selected, corresponding to an efficiency of 6 %. The µ L and χ masses were then extracted by fitting the edges of the P µ distribution, see Fig. 3. The errors on the fitted masses are (M µl = MeV/c and (M χ = 9 MeV/c, respectively. The beam-energy spread dominates these numbers. The production cross-section was determined using the extended likelihood formed by L(p Tµ,θ acol, as these two variables were not used in selecting the signal. The uncertainty on the observed value is (σ(e + e µ L µ L =.35 fb. The χ χ channel was selected by demanding that θ missing p [.π,.8π], β >.6, E miss [355,395]GeV, p Tmiss > 4GeV/c, M µµ [4,85]GeV/c and E viss > 4GeV/c. LCWS/ILC 8

3 - Yield (5 fb B+A/(+exp(x-E/S signal χ / ndf 9.73 / 6 Amplitude(A 48.9 ±.94 Edge (E 3.5 ±.4 Slope (S.349 ±.65 Background (B 38. ± Yield (5 fb B+A/(+exp((x-E/S Signal χ / ndf = 8.39 / 4 Amplitude(A ± 3.5 Edge (E 5.5 ±. Slope (S.3775 ±.33 Background (B 5.7 ± Figure 3: The fit to the lower (left and upper (right edges of the P µ distribution At the assumed integrated luminosity, 5 events of SM background and 4 events of SUSY background is expected, while 7 signal events were selected, corresponding to an efficiency of 34 %. Assuming that the χ mass is known from the previous channel, the mass of χ can be extracted by a fit to the edge in the invariant mass spectrum, Fig. 4, and an uncertainty of (M χ =.38 GeV/c was found. - Yield (5 fb 4 χ / ndf =.57 / 6 Background(B 55. ±.93 8 Edge (E 8.5 ±. Width (S.747 ±.4 Signal Amplitude (A 47. ± 5.8 Signal Tail (T.73 ±.785 Background Exp (BE.9554 ±.647 Background Slope (BS ± 5.6 Standard Model Background SUSY background χ µ µ µµχ Total signal Invariant Mass [GeV] 3 τ channels Figure 4: The fit to the M µµ distribution As mentioned in the introduction, SUSY itself poses a background problem in the τ analysis, and it is therefore needed to run the ILC at the polarisation that minimises the background. For % left e polarisation and % right e + polarisation, the crosssections for χ χ and χ + χ are several hundred fb, and the branching ratios to τ is above 5 %. With the opposite polarisation, however, these cross-sections will almost vanish. Hence, these channels were studied assuming 8 % right e polarisation and 6 % left e + polarisation. As the γγ background poses another challenge for the τ channels, quite strong criteria must be applied: A correlated cut in ρ (the transversemomentum of the jets wrt. the thrust axis, in the projection perpendicular to the beam was also done: ρ > 3sinθ acop +.7. To further reduce the γγ background to acceptable levels, it was demanded that there be no significant activity in the BeamCal, and that the φ angle of the missing momentum was not in the direction of the incoming beam-pipe. The τ mass can be extracted from the end-point of the spectrum of E τ, which is equal to E τ,max, and the χ mass, known eg. from the µ L analysis above. In principle, the maximum of the spectrum spectrum is at P τ,min, so that the τ can be used to find M χ as well, but due to the large γγ background, the maximum is quite hard to observe. LCWS/ILC 8

4 To extract the signal in order to determine the end-point the following cuts were applied: E miss [43,49] GeV, M jet < GeV/c, θ jet above degrees, θ acop < 6 degrees, θ acol [8,7] degrees, cosθ missingp <.9, and charge of each jet = ±. In addition, the anti-γγ cut described above was applied. After these cuts, the SM background was events, the SUSY background was 747, while 86 signal events remained (. % efficiency. Fig. 5 shows that the end-point is almost background free, and also that the turnover point (expected to be at P τ,min = Events /.53 GeV energy of jet[gev] Figure 5: E jet distribution after all cuts in the decay-mode independent analysis. GeV/c is too distorted by the cuts to be useful. The τ mass-eigenstates are expected to be different from the chiral ones, and the offdiagonal term of mass-matrix is M τ (A τ µtanβ. The diagonal terms in the mass matrix are known from M µl and M µr, so a measurement of θ mix gives A τ µtanβ. If χ is purely bino - it is in SPSa - the τ polarisation (P τ depends only on θ mix. P τ can be extracted from spectrum for exclusive decay-mode(s. In this analysis, the τ π + ν τ mode has been studied. The spectrum of π:s in the decay-chain τ τ π + ν τ is shown in Fig. 6, with and without ISR and beam-spread. The highest sensitivity to the polarisation is in the region with P π < P τ,min. The τ τ π + ν τ signal is selected with a set of cuts that intend to distort the spectrum as little as possible. The following pre-selections were first applied: The events should pass the anti-γγ cut, E vis should be < GeV, the number of reconstructed particles <, and at least one of the two jets should contain a single particle. This single particle should be identified as a π, and have E < 43 GeV. Finally, the total charge should be. Events passing this preslection should then also fulfil the following criteria: The mass of the rest of the event (ie. after removing the signal pion should be below.5 GeV/c cosθ of both jets should be <.9, and θ acop should be Ratio [%] / GeV without beam spread or ISR with beam spread and ISR E π [GeV] Figure 6: The P π distribution, at generator level. Shaded: fixed beam-energy, open: ISR and beam-spread included above 85 degrees. Finally, the sum over the two jets of the p T of one jet wrt. the direction of the other should be below 3 GeV/c. With these cuts, 34 SM jets remain, and 373 SUSY jets, while 3 signal-jets are retained (3 %. The initial and final π spectra are shown in Fig. 7. The procedure to extract the polarisation in the presence of background is to first fit the simulated background alone to a heuristic function. a The signal selections cuts are then applied to the a When real data is available, the simulation of the background can be verified by reversing cuts to select LCWS/ILC 8

5 entries / [GeV] e^+e^- e+e- + τ τ e+e- e+e- + X SM background other SUSY Signal type backg. Signal π entries / [GeV] Signal π Reconstructed data points Fit to data MC spectrum E signal candidate [GeV] E π [GeV] Figure 7: Distribution of P π before (left and after (right cuts. The right-hand plot also shows the spectrum after background-subtraction and efficiency correction (dots, and the final fit. signal+background sample, and the function is subtracted from the observed distribution. An efficiency correction function, determined from signal-only simulation, is applied. The resulting distribution is then fitted with the theoretical spectrum, corrected for ISR and beam-spread, and the polarisation is obtained, see Fig. 7, right. Assuming an integrated luminosity of 5 fb, the value found is P τ = (93±3 %, where the error also includes the uncertainty of the background parametrisation. 4 Conclusions A study of some channels in SPSa SUSY scenario fully simulated in the LDC detector at the ILC was presented. By analysing the channel e + e µ L µ L, it was concluded that (M χ = 9MeV/c, (M µl = MeV/c and (σ(e + e µ L µ L =.35 fb, could be attained with an integrated luminosity of 5 fb with 8 % left e polarisation and 6 % right e + polarisation. In the channel χ χ µ µ R χ µµ χ χ, (M χ =.38 GeV/c, was found, under the same conditions. It should be noted that this value is comparable to what a dedicated scan of the χ χ threshold would give. In addition, a progress report on τ production was given. The preliminary result on the measurement of the τ polarisation gives (P τ = 3 %. Note, however, that this requires that the beam-polarisations are opposite to what the was used in the µ channel. References [] Presentation: [] J. A. Aguilar-Saavedra & al., Eur.Phys.J.C46 (6 43, arxiv:hep-ph/5344. [3] C. F. Berger, J. S. Gainer, J. L. Hewett, T. G. Rizzo, arxiv:8.98. [4] W. Porod, Comput. Phys. Commun. 53 (3 75, arxiv:hep-ph/3. [5] W. Kilian, T. Ohl, J. Reuter, arxiv [6] P. M. Freitas, MOKKA [7] [8] N. d Ascenzo, PhD thesis, DESY Thesis-9-4 (9 a signal-free, but SUSY-dominated region in the parameter-space. LCWS/ILC 8