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W. KozaneckiLHC Beam Operations Committee, 3 Dec 2013 Slide 1 Non-factorization effects in vdM scans and their implications for ATLAS/CMS scans in 2015.

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Presentation on theme: "W. KozaneckiLHC Beam Operations Committee, 3 Dec 2013 Slide 1 Non-factorization effects in vdM scans and their implications for ATLAS/CMS scans in 2015."— Presentation transcript:

1 W. KozaneckiLHC Beam Operations Committee, 3 Dec 2013 Slide 1 Non-factorization effects in vdM scans and their implications for ATLAS/CMS scans in 2015  Introduction: scan-to-scan irreproducibility & non-factorization biases  Quantifying (and correcting for) non-factorization effects  in ATLAS, using luminosity & beamspot information: this presentation  in LHCb, by direct imaging of the 2-D beam-density distributions  see C. Barschel’s presentation, next in this meeting  Implications for the 2015 ATLAS/CMS vdM-scan request W. Kozanecki (CEA-Saclay)

2 W. KozaneckiLHC Beam Operations Committee, 3 Dec 2013 Slide 2 Absolute- L calibration challenges in 2012 (and not only then...)  Two very challenging issues in 2012 vdM scans  Scan-to-scan irreproducibility and/or systematic trend: 2-3 % (   syst, ATL ~ 3.6 %,  syst, CMS ~ 4.4 %)  Baffling in Apr 2012, real bad in Jul 2012, better in Nov 2012 (why so ≠?)  Breakdown of x-y factorization in the 3-d L distribution  aka ‘non-linear x-y correlations’  observed during vdM scans by all of ATLAS, CMS, LHCb in 2012 – but also by ATLAS in 2011 scans  These 2 issues  are clearly beam-dynamics effects, time-dependent & different fill-to-fill (instrumental drifts ruled out)  turned out to be related ATLAS ~ CMS Apr’12 vdM 2.1 %  Factorization assumes that shape of vdM scan curve during an x (y) scan is independent of the separation  y (  x) in the orthogonal plane  if this assumption is satisfied, the combination of 1 x-scan and 1 y-scan is sufficient to characterize the entire distribution L (  x,  y)  if this is violated at a “significant” level, the vdM formalism could be generalized to 2-d by performing a full 2-D grid scan (but: impractical!)

3 W. KozaneckiLHC Beam Operations Committee, 3 Dec 2013 Slide 3 Hindu goddess Durga, wife of Lord Shiva Hindu goddess Parvati, wife of Lord Shiva Avatar: in Hinduism, a manifestation of a deity or released soul in bodily form on earth; an incarnate divine teacher. an incarnation, embodiment, or manifestation of a person or idea

4 W. KozaneckiLHC Beam Operations Committee, 3 Dec 2013 Slide 4 The “beam size” and its avatars NameSymbolMeasurable using 1-Gaussian limitComments Single-beam size  x,b,  y,b b = B1, B2 BSRT, WS LHCb SMOG - Projections only Full 2-d Transverse convolved beam size  x,  y vdM scans   x =   x,1 +   x,2   y =   y,1 +   y,2 Very precise:  < 1% Transverse luminous size  x, L,  y, L 3-d vertex distribution   x, L =   x,1 +   x,2   y, L =   y,1 +   y,2 Aka ‘beamspot width’ Resolut’n-limited for  * < m Bunch length  z,b BQM Luminous length  z, L z-vertex distribution   z, L = 0.5 * f(  * )   z,1 +   z,2 ) Aka ‘beamspot length’

5 W. KozaneckiLHC Beam Operations Committee, 3 Dec 2013 Slide 5 Vertical luminous size  L (beamspot width) Testing factorization of L (  x,  y) during vdM scans Convolved beam size  (width of vdM scan curve) 12 % July 2012 vdM scans Nov 2012 vdM scans 20 %

6 W. KozaneckiLHC Beam Operations Committee, 3 Dec 2013 Slide 6 Vertical luminous size  L (beamspot width) Testing factorization of L (  x,  y) during vdM scans Convolved beam size  (width of vdM scan curve) 12 % July 2012 vdM scans Nov 2012 vdM scans 20 % The large reduction in non-linear x-y correlations, between the July & Nov 2012 scans, was achieved mainly by careful preparation of highly gaussian beams in the injectors. The elimination of  blowup by multiple scattering in a transfer line, and the reduction of the LHC octupole strength, may also have played a role. The beam-beam contribution to non- factorization effects has been calculated to be negligible by comparison.

7 W. KozaneckiLHC Beam Operations Committee, 3 Dec 2013 Slide 7 Production of (more) gaussian beams in the injector chain SPS WS profiles 18 Jul 12 LHCb vdM fill SPS WS profiles 02 Nov 12 Injector MD Data g + p0 fit See H. Bartosik’s LBOC presentation of 26 Nov 2013

8 W. KozaneckiLHC Beam Operations Committee, 3 Dec 2013 Slide 8 Comparison of vdM scan profiles in Jul & Nov 2012 ATLAS scans Fit functions: g + g + constant g + constant July 2012 scan Nov 2012 scan g+g+p0:  2 /DOF = 2.8 g+p0 :  2 /DOF = 84.5 g+g+p0 fit collapses into 2 identical gaussians, i.e. g+p0

9 W. KozaneckiLHC Beam Operations Committee, 3 Dec 2013 Slide 9 Vertical luminous size  L (beamspot width) Quantifying factorization of L (  x,  y) during vdM scans Convolved beam size  (width of vdM scan curve) 12 % July 2012 vdM scans 20 %  Non-factorization manifests itself via:   &  L different in centered & off-axis scans   y L varying w/  x during horizontal scan (and vice-versa)  non factorizable single-beam distributions (LHCb beam-gas imaging)  systematic scan-to-scan  vis inconsistencies  IP 1, 5, 8 can be studied quantitatively only * by fitting evolution of beamspot position & luminous width during scans  in particular: variation of  y L (resp.  x L ) w/  x (  y) during horizontal (resp. vertical) scans (both centered & off-axis)  possible only if the vertex resolution does not dominate the luminous width * LHCb: additional input from BGI, needs  b >>  vertexing

10 W. KozaneckiLHC Beam Operations Committee, 3 Dec 2013 Slide 10 Luminous-region modelling x y L  Aim: reproduce the evolution, during vdM scans, of  the luminosity-scan curves L (  x,  y)  the beamspot position,,  the luminous widths (  x L,  y L ) and length (  z L )  Procedure  Assumption: each beam (1, 2) = non- factorizable double gaussian (or other model)  1,2 (x, y, z) = w 1,2 G A (x, y, z)+ (1-w 1,2 ) G B (x, y, z) described (in this case) by ~ 11 parameters/beam  Fit these beam parameters to the measured beam-separation (  x,  y) dependence of: L,,,,  x L,  y L,  z L Beam Parameters for each of B1, B2 σ x,a = narrow gaussian x-width σ y,a = narrow gaussian y-width σ z,a = narrow gaussian z-width κ a = x-y correlation in narrow gaussian w a = weight of narrow gaussian σ x,b = broad gaussian x-width σ y,b = broad gaussian y-width σ z,a = broadgaussian z-width κ b = x-y correlation in broad gaussian α xz beam crossing angle in x-z plane α yz beam crossing angle in y-z plane

11 W. KozaneckiLHC Beam Operations Committee, 3 Dec 2013 Slide 11 Luminous-region observables during a (symmetric) vdM scan  The evolution of the beamspot position and luminous width during a scan gives information on the shape of the beams  Example 1: different x widths  Example 2: same x and y widths, but with opposite x-y correlation The luminous region moves in the same direction as the narrower of the two beams The luminous region moves in the direction transverse (y) to the scan direction (x)

12 W. KozaneckiLHC Beam Operations Committee, 3 Dec 2013 Slide 12 Luminous-region observables during a (symmetric) vdM scan (2)  Example 3: evolution of the shape of the luminous region  each beam is modelled as a double gaussian Luminous region (x projection) Beam 1 Beam 2

13 W. KozaneckiLHC Beam Operations Committee, 3 Dec 2013 Slide 13  For each scan (x & y) and each [available] colliding bunch [3-4/30], the fit results are compared to the measurements  this example: F 2855 (July 2012), BCID 1, horizontal scan 6 Typical result of combined fit to L & luminous-region scan data L (a.u.) Beamspot x-positionBeamspot y-positionBeamspot z-position x luminous width y luminous width Luminous length x-y correlation coeff.

14 W. KozaneckiLHC Beam Operations Committee, 3 Dec 2013 Slide 14 For the set of single beam parameters output by the fit:  define the non-factorization correction R  the “true” luminosity (i.e. not assuming factorization) is determined by computing numerically the 4-d overlap integral:  the “vdM” luminosity (assuming factorization) is computed from the integral under the x- & y- scan curves (which is proportional to 1/  x  y ) Quantifying (non-)factorization – at last!

15 W. KozaneckiLHC Beam Operations Committee, 3 Dec 2013 Slide 15 Consistency of L calibrations, before factorization correction Jul 2012 F 2855 Jul 2012 F 2856 Apr 2012 F 2520 Nov 2012 F 3311 Nov 2012 F 3316

16 W. KozaneckiLHC Beam Operations Committee, 3 Dec 2013 Slide 16 Consistency of L calibrations, before & after factorization correction Jul 2012 F 2855 Jul 2012 F 2856 Apr 2012 F 2520 Nov 2012 F 3311 Nov 2012 F 3316

17 W. KozaneckiLHC Beam Operations Committee, 3 Dec 2013 Slide 17 Consistency of L calibrations, before & after factorization correction Jul 2012 F 2855 Jul 2012 F 2856 Apr 2012 F 2520 Nov 2012 F 3311 Nov 2012 F 3316 ? ? The best correction is... no correction at all!

18 W. KozaneckiLHC Beam Operations Committee, 3 Dec 2013 Slide 18 Parameters for first vdM IP1 at √s = 13 TeV ParameterNov 2012Apr-May ‘15Comments  * [m] / E b [TeV] 11 / / 6.5 Optimum btwn  L & statistics Transverse phase-space tailoring in injector chain + LHC Gaussian (no tails!)  minimize NLC Gaussian (no tails!)  minimize NLC No screens Minimize LHC octupoles  N (  m-rad) 2.5 – 3.5~ 3  L  beam-beam  Luminous size  L (  m) ~ 57 RQ same  L as in 2012 Vertex resolution (  m) Bunch pattern ~ b > 1  s sep’rtn No trains ~ b > 1  s sep’rtn No trains No parasitic crossings Weaker tails Bunch intensities0.7 –  beam-beam Nominal crossing angle0Negotiable Review after 1st scan session

19 W. KozaneckiLHC Beam Operations Committee, 3 Dec 2013 Slide 19 Conclusions  One of the most challenging issues in vdM scans is the control of non- factorization biases  only known countermeasure: beam-tailoring in injector chain  quantitative understanding (if only to set a systematic uncertainty) entirely dependent on luminous-region (aka beamspot) analysis  imperatively requires that  vertexing <<  L  The reduction in x/y beam size (during vdM scans) brought about by  the higher beam energy in 2015  the envisaged lower  * after ramp/queeze  the lower emittances (that also exacerbate beam-beam corrections) very severely impacts the ability to exploit beamspot information very severely impacts the ability to exploit beamspot information  ATLAS & CMS  request scan conditions instrumentally equivalent to those in Nov 2012  maintaining the same luminous size  L implies increasing  * to m  tailoring transverse phase space in the injectors  need the scans (vdM + LSC) asap in 2015 (for ATLAS, need BCM stabilized)  luminometer diamonds must have ‘seen’ > 10 pb -1 shortly before scan (pumping)  for 1 st scan of 2015, Xing angle at IP1 OK for ATLAS (compatibility w/ LHCf)

20 W. KozaneckiLHC Beam Operations Committee, 3 Dec 2013 Slide 20 Additional material

21 W. KozaneckiLHC Beam Operations Committee, 3 Dec 2013 Slide 21  A key assumption of the vdM scan method as currently applied is that the luminosity factorizes in x & y: factorizes in x & y:  This is equivalent to assuming that the shape of the scan curve during an x (y) scan is independent of the separation  y (  x) in the orthogonal plane  if this is the case, the combination of 1 x-scan and 1 y-scan is sufficient to characterize the entire distribution L (  x,  y)  if this is violated at a “significant” level, the vdM formalism can be generalized to 2 dimensions by performing a grid scan (impractical!)  Although linear x-y coupling does violate this assumption, the induced bias is typically very small (  L / L ~ 0.1%) with present LHC optics (small x-y coupling coeff.,  x ~  y,   x ~   y ) A fundamental assumption: x-y factorization of L (  x,  y)

22 W. KozaneckiLHC Beam Operations Committee, 3 Dec 2013 Slide 22  To estimate (roughly) the magnitude of a potential NLC-induced bias, ATLAS routinely compared the visible cross-sections (i.e. the L calibration scales) obtained by fitting the x- & y- vdM-scan curves using either  an uncorrelated model (= baseline): g+g (can simplify to g, or to g+p0)  a correlated double-gaussian model (naïve & by no means unique) that reduces to the uncorrelated model at  x =  y = 0 (but with f x = f y )  Observed impact on visible cross-sections at √s = 7 TeV (ATLAS)   vis /  vis ~ 3%, 2%, 0.9%, 0.5 % for Apr ’10, May ’10, Oct ’10, May ’11  The more single-gaussian the scan curves, the smaller the potential bias (a property of this model – but probably not a general property?)  As the effect looked small for the two main 7 TeV scan sessions, and for lack of manpower, didn’t look much further until large 2012 signal A complementary approach: correlated fits to vdM scan curves L (x,y)

23 W. KozaneckiLHC Beam Operations Committee, 3 Dec 2013 Slide 23 Comparison of uncorrelated & correlated fits to vdM scan curves 0.4 % 1.8 % 1.6 % 3.0 % 2.8 % Uncorrelated (= factorizable) L ~ G 1 (x) G 2 (y) Correlated (non-factorizable) L ~  g N (x, y) + (1 –  g W (x,y)

24 W. KozaneckiLHC Beam Operations Committee, 3 Dec 2013 Slide 24 Comparison of uncorrelated & correlated fits to vdM scan curves 0.4 % 1.8 % 1.6 % 3.0 % 2.8 % Uncorrelated (= factorizable) L ~ G 1 (x) G 2 (y) Correlated (non-factorizable) L ~  g N (x, y) + (1 –  g W (x,y) Notes the true bias may be larger than the difference between uncorrelated & correlated fits (coupling-model dependence?) there may be other coupling models which also yield a stable central value, but significantly different from the present one.

25 W. KozaneckiLHC Beam Operations Committee, 3 Dec 2013 Slide 25 A detour: beam-gas & luminous-region imaging Measured (  1,2 ~ 90  for   m, E b = 4 TeV) Vertex resolution ~ 30  Beam-gas imaging (LHCb only)  measure  1 x,y,  2 x,y separately  independent absolute L calibration Luminous-region ima ging  msre  L x,y + their dependence on  x,y during vdM scan Resolution systematics critical: 10% on resolution  1.2 % on  1,  2  2.4 % on  L with 2012 conditions The basic problem...   b scales like sqrt (  * /E b )  with 2012 (2015) conditions, a 10% uncertainty on the vertexing resolution implies a 2-3 % (4 – 5%) error on  L (for the same  )  the vertex resolution won’t get any better!

26 W. KozaneckiLHC Beam Operations Committee, 3 Dec 2013 Slide 26 Do’s and don’t ‘s: some of the lessons learnt from 2012 scans  Don’t...  use small  *  reconstructed luminous width  L (= beamspot width) becomes resolution-dominated and very difficult to analyze   ~ 5 too high for comfort: potential detector non-linearities  push for small emittances  the smaller , the more  L is resolution-dominated  set nominal crossing angle  complicates measurement/characterization of satellites notable exception: LHCb needs large Xing-angle for beam-gas enhanced ghost-charge measurement  use nominal bunch intensities  beam-beam corrections: 2% for Nov N ~ 0.9 p/bunch !  Do favor...  large  *  make  L ALAP (  resolution)   * = 11,  = 3  8 TeV  “nominal LHC” emittances  make  L ALAP (  resolution)  BUT avoid anything that creates non-gaussian tails (e.g.  blowup by screen in transfer line) Large enough  L critical for (a) non-factorization systematics (b) L calibration by beam-gas imaging  zero crossing angle  optimize satellite reconstruction  beams as gaussian as possible in SPS + LHC (+ moderate bunch N)  tailor injected phase space (still an art more than a science...)  avoid strong octupoles


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