[Hps] HPS Meets Wednesday May 11

[Nelson, Timothy Knight]

Dear All,

After digging through my notes, I wanted to offer a brief comment in answer to Natalia's question today: "Why is the LHCb vertexing reach so powerful at larger couplings."  It's actually not that the vertex detector is so exquisite: the *planned* material budget in the active region is 50% more per 3-d measurement than that of the HPS SVT and the single hit resolution is roughly double our single-hit resolution in y, which is what really matters for us. John is correct that their larger boost for A' leads to less multiple scattering of the A' daughters so that you do win, albeit not that quickly, as you boost the A'. 

I believe the bigger factor is simply the job that the vertexing has to do.  Because of the limited phase space for their decay sequence, they count on having essentially no background from random coincidences. So, unlike HPS, where we need to cut far out on the vertexing tails to eliminate enormous prompt backgrounds, at LHCb, the vertex cut is designed only to eliminate D*->Dee, which is a rare process that I don't believe has been observed (Natalia: about how rare?) and for which the electrons have a larger range of opening angles due to the 3-body phase space.  So, their vertex cut is certainly set much tighter, perhaps an order of magnitude tighter than in HPS, which translates directly into the same factor more reach at the top of the vertexing region.  In fact, I am not even sure they need to set their vertex cut for zero background: since they anticipate something like 5 trillion D*->Dgamma in Run 3, they can have some pretty large signal yields at the top of the vertexing region where epsilon^2 is ~10^-8 - 10^-7!

Best,
Tim

> On May 10, 2016, at 10:02 AM, Jaros, John A. <john at slac.stanford.edu> wrote:
> 
> Dear HPS,
> 
> Back in the saddle. HPS will meet Wednesday, May 11, at 8:30 am PDT/11:30 am EDT/5:30 pm DST. 
> 
> Let me know if you'd like to present something at this meeting. 
> 
> -   News? 
>     PPC?
>     EC
>     JLAB?     
> 
> -  Pass 6 Update                                                                       Rafo/Nathan
> 
> -  HPS Technical Publications                                               Stepan
> 
> -  Overview of the Dark Sectors 2016 Workshop         Tim, Maurik, John, others?
>    *  Intro Talks
>    *  Possible signal in Be8 Decays?
>    *  e+e- Collisions?
>    *  Competition for HPS                                                     
>            LHCb
>            SeaQuest
>            SHIP   
> 
> -  AOB        
> 
> Talk to you tomorrow.
> 
> John
> 
> 
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