LEP-style counting in luminosity measurement at CepC

Transcription

1 LEP-style counting in luminosity measurement at CepC S. Luki Vin a institute of nuclear sciences, University of Belgrade Lumical meeting, online, Sep. 29, 2017 S. Luki, Lumical meeting, online, Sep. 29, 2017 LEP-style counting 1/22

2 Outline 1 Introduction LEP-style Bhabha counting 2 Scope and method 3 Results 4 Summary and outlook S. Luki, Lumical meeting, online, Sep. 29, 2017 LEP-style counting 2/22

3 Section 1 Introduction LEP-style Bhabha counting S. Luki, Lumical meeting, online, Sep. 29, 2017 LEP-style counting 3/22

4 Left-right asymmetry of the detected Bhabha angles Δθ e + θ e - Common feature of several systematic eects in Bhabha counting: Left-right (LR) asymmetry of the nal Bhabha particles in the LumiCal frame. LR asymmetry: Measured polar angles biased in the opposite directions by approx. the same amount. Caused by: Beam energy spread and asymmetry, axial and radial oset of the scattering location (mean and per-event) At LEP, Bhabha counting biases due to LR asymmetries were cancelled to a large extent by using counting cuts designed for this purpose (next slide) [EPJC 14 (2000), 373]. S. Luki, Lumical meeting, online, Sep. 29, 2017 LEP-style counting 4/22

5 LEP-Style counting r Legend: Geometrical Wide (fiducial) Narrow z Two radial acceptance ranges: R wide = (r in,f, r out,f ), Rnarrow = (rin,n, rout,n) Two Bhabha counts: N1 accepts R wide on the left and Rnarrow on the right } N Bh = N2 accepts Rnarrow on the left and R wide on the right N1 + N2 2 S. Luki, Lumical meeting, online, Sep. 29, 2017 LEP-style counting 5/22

6 Working assumptions on the CepC LumiCal geometry (reminder) LumiCal tentative location (Suen, CepC MDI meeting May 19th 2017) Geometrical coverage: rin = 25 mm, rout = 100 mm Fiducial coverage: r in,f = 50 mm, r out,f = 75 mm Distance of the front plane from IP: d IP = 950 mm In this study: LumiCal centered on the outgoing beam axis S. Luki, Lumical meeting, online, Sep. 29, 2017 LEP-style counting 6/22

7 Section 2 Scope and method S. Luki, Lumical meeting, online, Sep. 29, 2017 LEP-style counting 7/22

8 Ongoing study Eects presently studied at the generator level: Uncertainty of the beam energies undetected biases E1 and E2 of the two beams, studied via the linear combinations: Symmetric bias: E+ = E1 + E2( ECM) Causes an eect on the acceptance AND on the xs. Antisymmetric bias: E = E1 E2( ECMβ z ) More straightforward analysis and interpretation of the eects. Energy spread of the beams Oset (x IP, y IP, z IP) of the relative position of LumiCal vs. the IP Bias of the LumiCal inner radius r in,f Spread of the measured radial shower coordinate r shower w.r.t. the particle radial coordinate Distance of LumiCal to the IP, d IP Fluctuations (σ,, x IP σy IP σz IP, ) of the relative position of LumiCal vs. the IP, including uctuations due to the beam size. S. Luki, Lumical meeting, online, Sep. 29, 2017 LEP-style counting 8/22

9 Procedure (update) 1 Samples are generated using BHLUMI Bhabha event generator: ECM = 240 GeV Final particle theta range from 45 to 85 mrad (includes a 7 mrad margin outside of the FV to allow non-collinear FSR to contribute) Statistics events 2 Smearing applied to the nal particle vertices and momenta, according to the nominal CepC parameters. 3 Bias or additional smearing is applied to the nal particles, or to the counting cuts, according to one systematic eect at a time 4 Particle paths are projected to the front Lumical plane. 5 Close-by particles are summed to imitate cluster merging 6 Condition to count an event: two methods Full ducial acceptance (r in,f < r < r out,f) LEP-style counting with rin,n r in,f = r out,f rout,n = rcut Both scenarios require one particle on each side of the IP with E > 0.5E0 (E0 is the nominal beam energy), within the radial cuts specic for the counting method. S. Luki, Lumical meeting, online, Sep. 29, 2017 LEP-style counting 9/22

10 Section 3 Results S. Luki, Lumical meeting, online, Sep. 29, 2017 LEP-style counting 10/22

11 Antisymmetric beam energy bias Denition Asymmetry of the incident beam energies E 0,e + E 0,e 0 produces a longitudinal boost β z of the event. Eects on the Luminosity measurement: Fiducial cuts: 10 3 acceptance loss per beam energy asymmetry. LEP-style cuts: 10 3 acceptance loss per beam energy asymmetry. N Bh /N Bh Fiducial r cut = 1.0 mm r cut = 2.0 mm r cut = 3.0 mm = 5.0 mm r cut β z S. Luki, Lumical meeting, online, Sep. 29, 2017 LEP-style counting 11/22

12 β z distribution due to the beam energy spread Denition Beam energy spread causes a distribution of β z event-by event. Eects on the Luminosity measurement: Fiducial cut: 10 3 bias at 20% uncertainty of the beam energy spread. LEP-style cuts: Beam energy spread may be safely ignored! N Bh /N Bh Fiducial r cut = 1.0 mm r cut = 2.0 mm r cut = 3.0 mm = 5.0 mm r cut σ β z S. Luki, Lumical meeting, online, Sep. 29, 2017 LEP-style counting 12/22

13 Radial oset of the IP w.r.t. LumiCal Denition Radial distance between the LumiCal axis and the beam axis. Caused by Lumical misplacement or beam oset. Eects on the Luminosity measurement: Fiducial cuts: 10 3 bias per ca. 100 µm oset. LEP-style cuts: < 10 3 bias for a > 1 mm oset (with rcut 3 mm) N Bh /N Bh 1 10 Fiducial r cut = 1.0 mm 2 r 10 cut = 2.0 mm r cut = 3.0 mm r cut = 5.0 mm x IP S. Luki, Lumical meeting, online, Sep. 29, 2017 LEP-style counting 13/22

14 Axial oset of the IP w.r.t. LumiCal Denition: IP is not at the center along z between the Lumical halves. Eects on the Luminosity measurement: Fiducial: 10 3 acceptance loss per 1.4 mm axial oset. LEP-style: 10 mm axial oset easily tolerated. Implies a requirement on the synchronisation of the colliding beams of better than 1 ps (Fiducial) or 7 ps (LEP style) N Bh /N Bh Fiducial r cut = 1.0 mm r cut = 2.0 mm r cut = 3.0 mm = 5.0 mm r cut z IP S. Luki, Lumical meeting, online, Sep. 29, 2017 LEP-style counting 14/22

15 Transverse uctuations of the scattering location Denition Bhabha events see dierent angles to LumiCal due to radial uctuations of the relative position of LumiCal w.r.t. the IP. Eect on the luminosity measurement Fiducial: Radial uctuations LumiCal w.r.t. IP must remain within 100 µm. LEP-style: Radial uctuations up to 1 mm are acceptable. N Bh /N Bh Fiducial r cut = 1.0 mm r cut = 2.0 mm r cut = 3.0 mm = 5.0 mm r cut (mm) σ x S. Luki, Lumical meeting, online, Sep. 29, 2017 LEP-style counting 15/22

16 Longitudinal uctuations of the scattering location Denition Eective longitudinal IP location varies event by event due to: Longitudinal position within the bunch Axial uctuations of the relative position of the IP w.r.t. LumiCal. Eect on the luminosity measurement Fiducial: Axial uctuations must remain within 1 mm. σ z beam size not negligible. LEP-style: Axial uctuations up to 10 mm are acceptable. N Bh /N Bh Fiducial r cut = 1.0 mm r cut = 2.0 mm r cut = 3.0 mm = 5.0 mm r cut (mm) σ z S. Luki, Lumical meeting, online, Sep. 29, 2017 LEP-style counting 16/22

17 Inner radius of the LumiCal ducial volume Denition Precision of the LumiCal dimensions determines the precision of the local coordinate system in which the hit locations are measured. Eects on the Luminosity measurement: Fiducial: 10 3 acceptance bias per 13 µm bias of r in,f. Marginal deterioration of bias when LEP-style cuts are used. N Bh /N Bh Fiducial r cut = 1.0 mm r cut = 2.0 mm r cut = 3.0 mm = 5.0 mm r cut r in (mm) S. Luki, Lumical meeting, online, Sep. 29, 2017 LEP-style counting 17/22

18 Precision of the reconstructed radial shower coordinate Denition Spread of the measured radial shower coordinate r shower vs. the true r particle. Impacts the Bhabha count because of the θ 3 slope of the cross section. Eect on the luminosity measurement Fiducial: σ precision of r shower must be better than 150 µm. LEP-style: r shower precision of 1 mm ok. Comfortable. N Bh /N Bh Fiducial r cut = 1.0 mm r cut = 2.0 mm r cut = 3.0 mm = 5.0 mm r cut σ r (mm) S. Luki, Lumical meeting, online, Sep. 29, 2017 LEP-style counting 18/22

19 Distance between the LumiCal and the IP Denition Distance uncertainty between the IP and each of the LumiCal halves. Eects on the Luminosity measurement: 10 3 acceptance bias per ca. 500 µm distance bias. Not aected by LEP-style cuts. N Bh /N Bh Fiducial r cut = 1.0 mm r = 2.0 mm cut r cut = 3.0 mm r cut = 5.0 mm d IP (mm) S. Luki, Lumical meeting, online, Sep. 29, 2017 LEP-style counting 19/22

20 Section 4 Summary and outlook S. Luki, Lumical meeting, online, Sep. 29, 2017 LEP-style counting 20/22

21 Summary of the results Requirements on beam delivery, MDI and LumiCal installation, needed to limit individual systematic eects to Parameter unit limit (Fiducial) limit (LEP style) ECM MeV Ee + E e MeV δσ Ebeam 20% Eect cancelled σ Ebeam x IP mm 0.1 >1 z IP mm Beam synchronisation ps 1 7 σ x IP mm σ z IP mm 1 10 r in µm σ rshower mm d IP µm S. Luki, Lumical meeting, online, Sep. 29, 2017 LEP-style counting 21/22

22 Conclusions and outlook Systematic eects in the luminosity measurement at CepC are studied using generator samples of Bhabha events. First order estimates of various systematic eects are given Pure geometrical acceptance eects (+ xs eect of ECM) Interaction with the detector is not simulated (yet) Merging of close-by nal particle showers (FSR) is approximated. Two counting methods are considered: full ducial volume and LEP-style. LEP-style counting eectively cancels most eects. Precision of the inner acceptance radius remains the most stringent geometrical requirement. S. Luki, Lumical meeting, online, Sep. 29, 2017 LEP-style counting 22/22