LLP Simple Monte Carlo CMS Far Forward Spectrometer
- Slides: 15
LLP Simple Monte Carlo CMS Far Forward Spectrometer Dan Green Fermilab 9/13/2021 SUSY, light stop 1
Figure of Merit ● FOM – what defines a competitive search? Aperture of beam pipe is R distance from IP is d -> solid angle is ~ (R/d)^2. Assume R set by LHC machine. Compared to FASER; d ~ 100 m to 480 m > factor ~ 25. Short lifetimes are lost as they decay upstream. Factor is ~ exp(-d/γτc). That implies that a shorter lifetime region can be explored with a smaller d. This makes us unique. For the same lifetime and γ factor, d/γτc ~ 1, and asking not to decay upstream, the gain in rate is ~ d or another factor ~ 5. The detector decay volume has a length ~ L. A longer decay pipe with L ~ 20 m is also a decay rate advantage. There will also be many and interesting backgrounds. The detector has calorimeter timing w. r. t. CMS. There is e ID in a compact shower detector. Perhaps muon ID is added. There is tracking to get the parent mass, to find a good vertex, and to point back to the IP. All of these tools will suppress the backgrounds which will, in fact, define the limits that can be achieved. The actual rejections are hard to quantify. 9/13/2021 SUSY, light stop 2
FOM, d = 100 m defined = 1 The short LLP decay length. The long decay vacuum tank and the large solid angle vacuum tank make CMS very competitive at the LHC 9/13/2021 SUSY, light stop 3
Limits and Boundaries ● Too short a lifetime and all decay away. Too long a lifetime and no decays in the experiment. Too small a cross section and not enough events detected. to discover. ● CMS is nearer to the IP (100 m) with a long vac tank (20 m) with a large aperture (r = 40 cm). Limit boundaries are larger in CMS LLP detector. 9/13/2021 SUSY, light stop 4
Initial Layout Concept 9/13/2021 SUSY, light stop 5
Tracking Assume Si tracking with 20 um resolution. Six x and 6 y planes. Fit to lines and find vertex (red *). Tie to CMS IP. All LLP ->e+e angles are known. For now cut at |y| = 12 cm. 9/13/2021 SUSY, light stop 6
Reconstruction ● The ratios p 1/po, p 2/po and po/mo can be found without a magnet. Two body decays, po → p 1 + p 2, are in a plane. 9/13/2021 SUSY, light stop 7
Reconstruction - 2 Decay vertex must be in the vacuum tank. Resolution Is ~ OK ~ few cm. Needed to remove backgrounds. 9/13/2021 SUSY, light stop 8
Kinematics - Reconstructed Even without a magnet one can find the CM decay angle (2 body) and the parent p/m ratio. Cut on coplanarity for 2 body. BSM models would predict p/m of parent and CM decay angle 9/13/2021 SUSY, light stop 9
Rates and Backgrounds ● There are many charged and neutral particles traversing the vacuum tank. ● For now cut at |y| = 12 cm. Tank radius is 40 cm. 9/13/2021 SUSY, light stop 10
Efficiency ● Generate events: fixed m and grid of pt and y of the LLP. 9/13/2021 SUSY, light stop 11
Efficiency - 2 ● Look at m = 2, 4, 8, and 16 Ge. V. Cover m values above those accessible in fixed target beam dump experiments. ● Cut on |y| < 12 cm to avoid secondaries for now. Some alternatives mentioned later. 9/13/2021 SUSY, light stop 12
Efficiency (pt, y), fixed LLP mass 9/13/2021 SUSY, light stop At low LLP mass, opening angle is small. Large pt, small y is needed to be seen in tracking. As LLP mass increases lower pt becomes efficient but at higher y so as to fit in the vac tank. Max efficiency Is ~ 60% for all masses. 13
Aperture Effects, m = 4 Ge. V The real backgrounds are unclear UL – cut |y| < 12 cm. UR – cut R < 12 cm BR – no tracking aperture cut Studies and measurements are needed to optimize the layout 9/13/2021 SUSY, light stop 14
Next Steps ● More studies of the tracking aperture are needed – optimization. Potential large gains in efficiency. ● Theoretical input is needed to set limits on particular models and compare to other experiments (PR) both LHC and beam dump – fixed target. ● An EOI to CMS needs to be drafted leading to a TDR proposal. ● Accelerator and detector technology needs to be quantified – cost, real layout, etc. ● A cadre of folks < 75 years old needs to step up and push us “forward”. 9/13/2021 SUSY, light stop 15
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