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Version 26 (modified by Gustavo Gil Da Silveira, 12 years ago) ( diff )

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LPAIR Analysis

  1. LPAIR Analysis
    1. Introduction
    2. Set up
      1. Code changes
      2. Event samples
      3. Structure functions
      4. Variables
      5. Comparison between W1 and W2 Structure Functions
    3. Results
      1. No experimental cuts
        1. Cross sections
        2. Distributions
      2. Experimental cuts
        1. Cross sections
        2. Distributions
      3. Ratio
    4. LPAIR++

Introduction

This investigation is meant to study the parton distribution functions (PDF) being used in the ​LPAIR Monte Carlo generator w.r.t. the inelastic collision when the incoming proton breaks up into a hadronic final state. This analysis was motivated by the observation of a deficit on the number of events in the transverse momentum (pT) distributions of dimuons from data when compated to the predictions from LPAIR (Figure 6 in ​CMS-FSQ-12-010). A possible reason for such disagreement may be related to the PDF being employed by LPAIR to compute the contribution of proton dissociation.

There are currently two versions of this Monte Carlo generator (which can be found ​here) studying two different kind of processes:

  1. DESY version: developed for electron-proton collisions in HERA, with built-in option for proton-proton collisions as test. This version allows one to study elastic and inelastic (single dissociation) processes for the production of dileptons; and
  1. CDF version: improved version which allows one to study elastic and double dissociation processes with more phase-space cuts available.

In this current analysis, we focus in the exclusive dimuon production, γγ -> μ+μ-, in inelastic collisions (single dissociation). LPAIR makes use of the ​JetSet library present in ​Pythia in order to perform the hadronization of the proton remnants.

The built-in options for proton dissociation in LPAIR make use of two parametrizations for the parton densities inside the proton, namely:

  1. SURI-YENNIE ​Ann. of Phys. 72 (1972) 243 with option 11; and
  1. BRASSE ​Nucl. Phys. B110 (1976) 413 with option 12 in the LPAIR card. This parametrization is restricted: MX < 2 GeV and Q2 < 5 GeV2.

Due to the larger hadronic masses and pT distributions of the dimuon pair probed in the LHC kinematical regime, we employ SURI-YENNIE in this analysis.

Following the implementation made by Dariusz Bocian (IFJ-PAN, Krakow) ​Acta Phys. Polon. B35 (2004) 2417, the current LPAIR code (DESY version) was modified in order to account for the proton-nucleus collisions from previous studies. Then, LPAIR samples are produced as HBOOK files, which have to be converted to ROOT files using the ​h2root script provided by ​ROOT.

In case one has several ROOT files to merge, the ​hadd script is employed.

Set up

Code changes

A few changes are made in the code to have access to more information about the particles in the event sample produced in LPAIR, especially to distinguish the muons from the exclusive production from the remnants from the proton break up. Then, two changes are made in the file ilpair-cms-pp.f in order to store information from JetSet in the HBOOK files:

  1. particle status code, ks, at line 246; and
  1. particle parentage, icode, at line 248.

Also, taking advantage for the fact that LPAIR performs the theoretical calculation in terms of Q2, the pT cut is internally applied by means of a Q2 cut in file peripp.f at line 29.

Event samples

The samples are produced with 1 million (1M) events, to check the shape of all kinematic distributions, and 5 million (5M) events in order to have enough statistics for high-pT dimuons. For the former, the samples are produced by merging ten sub-samples containing 100k events, which are obtained by running LPAIR at a time. while for the latter, five samples of 1M events by applying cuts on Q2: 0-1, 1-5, 5-20, 20-50, and 50-100 GeV2. The production of each of the sample with 1M events follows the same procedure as for 1M events for the whole Q2 range.

It is worth to notice that the histograms in the distributions with 5M events are stacked, and each individual set of events can be distinguished by the lines crossing vertically the histogram.

Structure functions

The W1 and W2 structure functions are defined in terms of the parton densities in the proton:

F2 = (4/9)*(uval + 2*usea) + (1/9)*(dval + 2*dsea) + (1/9)*2*ssea

F1 = F2/2x

with Q2 = Q2min for the region below the Q2min of each parametrization.

The built-in PDFs in LPAIR are replaced by two paramatrizations for the parton densities:

  1. Glueck-Reya-Vogt (GRV) ​Eur. Phys. J. '''C5''' (1998) 461: version GRV95 at leading order (LO) in αs.
  • 0.4 < Q2/GeV2 < 1.E6;
  • 1.E-5 < x < 1.

README

  1. Parametrization by Fiore et al ​Eur. Phys. J. '''A15''' (2002) 505.

Abstract: An explicit model realizing parton-hadron duality and fiting the data is suggested. Complex nonlinear Regge trajectories are important ingredients of the model. The inclusion of ∆ and N∗ trajectories should account for all resonances in the direct channel. The exotic trajectory is responsible for the smooth background.

This parametrization is meant to fit the data for the low Q2 region, reproducing the resonances of low-mass mesons. The figure below shows the distribution F2 vs. x for Q2 = 0.5 GeV2:

source:/trunk/plots/FIORE_F2_x.png

  1. Parametrization by Martin-Ryskin-Stirling-Thorne (MRST 2001) at leading order ​Phys. Lett. '''B531''' (2002) 216
  • 0.4 < Q2/GeV2 < 1.E6;
  • 1.E-5 < x < 1.

README

  1. Parametrization by Martin-Stirling-Thorne-Watt (MSTW 2008) at leading order ​ Eur. Phys. J. '''C63''' (2009) 189
  • 1.25 < Q2/GeV2 < 1.E7;
  • 1.E-5 < x < 1.

README

Variables

The final-state kinematics is used to make the distributions in terms of the variables of the interacting particles, like the photon virtuality, Q2, by:

Q2 = [Pp(E)-prem(E)]2-[Pp(x)-prem(x)]2-[Pp(y)-prem(y)]2-[Pp(z)-prem(z)]2

and the momentum fraction of the proton carried by the photon, x, by

x = Q2/(MX2-Mp2+Q2),

with ξ given by

ξ=log10(x)

Comparison between W1 and W2 Structure Functions

Distributions

Results

No experimental cuts

Cross sections

The predicted cross sections using each PDF for the parton densities are:

PDF Cross section (pb)
SURI-YENNIE 9262.
FIORE 10557.
GRV 7749.

and when producing the sub-samples by Q2 cuts:

PDF Cross section (pb)
Q2 (GeV2) 0-1. 1.-5. 5.-20. 20.-50. 50.-100.
SURI-YENNIE 3379.41 387.51 73.95 13.10 3.86
FIORE 4140.11 747.88 40.18 0.477 0.0124
GRV 2439.74 484.31 105.41 19.22 5.51

Distributions

The kinematic distributions are presented below for the sets of 1M and 5M events considering the possibilities for the parton densities.

OneMillionEvents

FiveMillionEvents

One should notice that the samples with 5M events show different distributions w.r.t. the fact that the five regions are produced with Q2 cuts, which comnpletely changes the pT distributions of the dileptons.

Experimental cuts

In order to have results closer to the observed results in the data analysis, event samples are produced within the experimental acceptance used for at trigger level in CMS:

  1. pT(μ) > 10. GeV;
  1. |η(μ)| < 2.5;
  1. M(μ+μ-) > 10. GeV.

Cross sections

PDF Cross section (pb)
MX,max 100 GeV 300 GeV
SURI-YENNIE 0.82 0.87
GRV 0.92 1.01

Distributions

The first set of plots shows the events restricted to a hadronic system with mass lower than 100 GeV, while the second consider masses up to 300 GeV.

SURI-GRV: Distributions with Mx > 100 GeV

SURI-GRV: Distributions with Mx > 300 GeV

SURI-GRV: Distributions with Mx > 300 GeV

SURI-GRV-MRST-MSTW: Distributions with Mx > 300 GeV

Ratio

The comparisons are performed by the ratio between the distributions with experimental acceptance for the SURI-YENNIE and GRV parametrizations:

Ratio between SURI and GRV

Ratio between SURI, GRV, MSTW and MRST

LPAIR++

to be added

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